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A giant solar farm covering more than one and a half thousand football pitches, could be built near Stratford.
The earmarked swathe of land stretches over four square miles and includes four main sites across south Warwickshire and east Worcestershire.
A map shows the proposed development comes close to Bidford, Wixford, Salford Priors and Cleeve Priors.
Arrow Valley Solar, the developer behind the project, has submitted a scoping report to the planning inspectorate and invited feedback from residents.
The solar farm proposal confirms it covers approximately 2,600 acres, although Arrow Valley says not all of that land will be used for solar panels.
The farm, able to generate up to 500 megawatts of renewable energy and power tens of thousands of homes, would include solar panels, a battery energy storage system, substations plus an underground cable route linking solar sites with one another and with the national grid at Feckenham substation.
Arrow Valley says there will be no pylons, and landowners will still be able to graze livestock in the fields where panels are.
The land falls within the jurisdiction of three planning authorities including Stratford District Council, Redditch Borough Council and Wychavon District Council as well as two county councils – Warwickshire and Worcestershire.
Because of the size of the project, the solar farm is classed as a nationally significant infrastructure project and will need a special type of planning permission known as a development consent order (DCO).
Arrow Valley has started environmental surveys and the formal planning process and aims to submit its DCO application in spring next year [2027].
The final decision will be made by the Secretary of State for Energy Security and Net Zero.
If planning is approved, construction could start by 2029 and the solar farm to be up and running by 2031.
Arrow Valley Solar is owned by Island Green Power which has delivered just over 20 solar and storage projects in the country including at Cottam, the UK’s largest consented solar project.
On its website, Arrow says: ‘At this early stage of the project, we have not yet decided where within the sites any of this infrastructure will go.
‘We will be developing our proposals over the course of 2026 to consider the results of environmental surveys, before undertaking a public consultation at the end of the year.
‘We will consult local communities, policymakers, local authorities and statutory bodies, such as Natural England, to gain feedback, understand issues and help address concerns.
‘We use this process to refine project proposals before the application is submitted to the Planning Inspectorate.’
A formal public consultation is expected to take place in Winter 2026/2027 but in the meantime, Arrow Valley has said it welcomes feedback.
To see the location of the proposed solar farm click here and to read Arrow Valley Solar’s scoping report, click here

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Two utility-scale solar PV plants in South Africa have started supplying offtaker Pretoria Portland Cement.
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Homeowner desperately searches for advice after HOA threatens to forcibly remove solar panels  Yahoo
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Net-metering to net-billing widens tariff gap as consumers face Rs10 buyback
Solar power generation has proven to be socially acceptable, economically viable and environmentally friendly. It is therefore highly sustainable for a country like Pakistan where the power supply gap is always increasing.
Solar can reduce our heavy reliance on expensive imported fossil fuels. It can also mitigate high electricity tariffs and bridge the existing 10% energy supply gap for the household sector.
Pakistan embarked on solar power after developing its net metering policy almost a decade ago. The policy proved highly successful in terms of increased solar panel installation amid tariff reduction by the National Electric Power Regulatory Authority (Nepra) from Rs27 per unit to Rs10 per unit.
The prime objective of the solar policy was not only self-reliance in power but shifting the power production cost from the government to consumers themselves. Consumers installed the solar panels and the government is buying this power without spending any upfront cost on constructing dams for hydropower or heavy spending on fossil fuel imports, including their recurring costs.
This has proven to be the cheapest source of power for the government. But instead of encouraging it, the government has slashed its price from Rs27 to Rs10 – a 65% reduction. All over the world, solar and other such indigenous sources of power are highly encouraged and rewarded.
Solar power provides an alternative to grid electricity. It offers a decentralised energy solution, which is especially important for remote areas in regions like the northern mountainous parts of Khyber-Pakhtunkhwa, Balochistan, Sindh and rural Punjab, where access to the grid is limited.
Being a clean and renewable source of power, solar helps reduce Pakistan's carbon footprint and dependence on polluting power generation. It also helps earn carbon credits for replacing power generated from fossil fuels that emit greenhouse gases.
Pakistan can achieve its committed power and energy generation from renewable sources by 2030 under the Nationally Determined Contributions (NDCs) to the United Nations Framework Convention on Climate Change (UNFCCC). Solar power has greatly helped Pakistan fulfil its obligation under the UNFCCC and comply with the targets of various Sustainable Development Goals. Solar offers energy security as an indigenous source and is equally important for fulfilling our international commitments.
Federal Power Minister Awais Leghari has emphasised the crucial role of solar energy in the country's energy mix, highlighting a strategy that leans heavily on indigenous resources. He said Pakistan is producing 74% indigenous power generation and has therefore increased its reliance on local sources – including solar, wind, hydro, nuclear and local coal – to strengthen energy security.
He further said the government aims to raise the share of indigenous energy to over 96%, moving toward a 90% clean energy mix by 2034. While this has been advantageous for consumer self-consumption, it has created challenges for the national grid. This has prompted the government to shift from net-metering to a net-billing system, which offers lower rates (about Rs10 per unit) to producers compared with previous rates.
The debate is that the government buys electricity at a low rate and sells it at a significantly higher rate – for example, Rs10 versus Rs60 per unit. While the exact figures vary based on generation type (hydro versus thermal) and time, the core issue is the widening gap between generation cost and the end-consumer tariff.
A large portion of the bill goes towards "capacity charges" paid to independent power producers (IPPs). These charges are paid even if the power is not produced or utilised. The government charges consumers to cover its failure to manage the financial crisis, including the interest on loans taken to manage circular debt. Consumers also pay for "line losses" which include electricity theft and inefficiencies in the grid.
The government's financial sustainability of the power sector clashes with the public's ability to pay. This has led to widespread protests and excessive inflation on electricity bills. The core conflict is that the government is locked into long-term, high-cost agreements with IPPs, often referred to as a "take-or-pay" model, where the burden is passed on to the consumer.
This is a vicious cycle that the government is now trapped in. It is greatly affecting all sectors of the economy and the public at large. Unfortunately, the public sector's resources are not limited, nor do the professionals in this sector possess any dynamism or innovative thinking to address these ever-increasing issues scientifically. The solutions so far are based on an unjust and unacceptable foundation.
Until a justified solution and mechanism is devised, these issues may persist. Both the government and society may continue to feel their impacts. Consequently, the public is very much against the existing tariff and taxes loaded into their electricity bills.
On the other hand, the government has completely ruined the solar net metering policy. It is pushing the public that already has solar panels to switch to off-grid solar power generation. This will lead to irregular and unrecorded power generation that may not give any benefit to the public in the longer term.
The government, on one hand, is favouring IPPs at the cost of power consumers. It is ignoring international commitments to various legally binding conventions like UNFCCC, Agenda 2030, INDCs and SDGs. This may negatively impact Pakistan's image at the international level. It will also affect self-reliance in power at no upfront and recurring cost on power production.
THE WRITER HOLDS A PHD AND A MASTER'S DEGREE IN FOREST MANAGEMENT. HE HAS SERVED THE K-P FORESTRY DEPARTMENT AS DIVISIONAL FOREST OFFICER AND HAS ALSO SERVED AS HEAD OF PAK-EPA
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For many, whether they are looking to avoid frustrating blackouts or slash their utility bills, getting solar panels is the right choice.
Photo Credit: Palmetto
Installing solar panels is a great way to take control of your home’s power generation and dodge rising energy costs. However, for many homeowners, the upfront investment is a sticking point that prevents them from upgrading. 
For this reason, solar leasing programs have been popping up across the country, especially as the federal solar tax credit has disappeared. These types of programs allow homeowners and renters to benefit from clean solar power and protect themselves from ever-worsening energy inflation at the same time. 
TCD partner Palmetto is an example of one company changing the solar energy landscape with its own solar leasing program, LightReach. LightReach is like a subscription for solar panels. When you sign up, Palmetto owns the solar panels, but you get the power that is generated. 
Through this program, Palmetto handles the solar mapping, the system design, permits, installation, and even maintenance for $0 down on installation. Instead of paying a large sum upfront, you pay a monthly charge for the panels. 
Palmetto even offers verification testing so homeowners can trust that their new panels will perform as promised. 
According to Dave Schram, analytics manager at Palmetto, “[LightReach] uses its automated monitoring to test all its systems by its location, equipment, installer, and climate, during the first 60 days they’re in operation. A system passes if it performs to 95% of its weather-adjusted production estimates over three consecutive days and has no single day below 85%.” 
Schram also noted that LightReach-financed systems typically pass the verification testing 98% of the time. In the rare case that a system fails, the solar panels undergo a manual review, in which a specialist diagnoses the root cause of the issue. 
As U.S. electricity costs rise, more and more homeowners are feeling the pressure. Luckily, solar panels can help residents curb the soaring costs while making their family’s energy more secure. 
FROM OUR PARTNER
Whatever your solar budget, Palmetto can help you save.
If you want to buy your own panels, Palmetto’s advisors can help you save up to $10,000 on installation through a network of preferred installers. And if you’d rather get solar savings without upfront costs, Palmetto’s revolutionary LightReach subscription program can deliver — including an exclusive $1,000 cashback offer for TCD readers.
LightReach lets you lease solar panels with no money down, making it painless to lock in long-term savings of up to 33% off your current power bill. Palmetto covers a 25-year warranty for the panels, which means you’ll get reliable performance without unforeseen costs.
To get started, just book a short meeting with Palmetto’s experts to explore your options and find the solution that’s right for you.
For many, whether they are looking to avoid frustrating blackouts or slash their utility bills, getting solar panels is the right choice. 
If you are interested in seeing how solar panels can transform your home’s energy, Palmetto has all the information you need to get started on your journey. 
While you’re at it, consider checking out the free Palmetto Home app. By using it to complete challenges, such as reducing your home energy consumption, you can snag up to $5,000 in rewards for home upgrades.
Which of these savings plans for rooftop solar panels would be most appealing for you?
Save $1,000 this year 💸
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Save less in 10 years but $80k in 20 years 🤑
Couldn’t pay me to go solar 😒
Click your choice to see results and earn rewards to spend on home upgrades.

Get TCD’s free newsletters for easy tips to save more, waste less, and make smarter choices — and earn up to $5,000 toward clean upgrades in TCD’s exclusive Rewards Club.
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100Pcs 1N4007 Rectifier Diodes – 1000V 1A High Voltage Silicon Diodes For Solar Panels & Circuits  ruhrkanal.news
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The Albanese Labour Government has unveiled a nationwide pilot scheme for recycling decommissioned solar panels. The effort aims to reduce landfill disposal and extract critical minerals for Australia. Over three years, the government plans to allocate $24.7 million to launch the pilot, featuring as many as 100 collection points nationwide.
The move comes on the heels of a fresh Productivity Commission report exploring circularity within Australia’s economy. The document explicitly called for a dedicated solar panel recycling program. 
It pointed to ways to enhance the nation’s circular economy via enhanced collaboration, refined regulations, and technological advances, especially for valuable yet challenging waste like solar panels.
Currently, just 17 percent of solar panels undergo recycling. According to the Productivity Commission, boosting these rates could generate up to $7.3 billion in economic and ecological advantages by minimising waste and promoting material recovery.
The government is committed to reviewing the Commission’s insights and partnering with state and territory authorities to advance recycling and resource recovery nationwide, the official statement said.
Treasurer Jim Chalmers emphasised that solar panel recycling would cut expenses and boost economic efficiency. 
“Recycling solar panels and reusing the essential components will reduce costs and make our economy more productive and efficient,” he said, adding, “less waste and more access to valuable metals.”
Minister for Climate Change and Energy Chris Bowen said, “Not only do solar panels create renewable energy—now they’ll be renewable themselves.”
Minister for the Environment and Water Murray Watt said, “Only a small percentage of end-of-life solar panels are currently recovered for recycling, with most panels either stockpiled, dumped in landfill or exported for reuse. But we think solar panels are made up of materials that are too valuable to throw out. These materials can be repurposed to support the clean energy transition and help reduce what we send to landfill, improving out natural environment.”
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First Solar, Enphase Energy, Nextpower, SolarEdge Technologies, Turbo Energy, Solaris Energy Infrastructure, and Sunrun are the seven Solar stocks to watch today, according to MarketBeat’s stock screener tool. Solar stocks are shares of publicly traded companies whose primary business is tied to the solar power industry — for example manufacturers of photovoltaic cells and panels, makers of inverters and trackers, system installers and developers, utility-scale project owners (including yieldcos), and suppliers of materials or services to solar projects. Investors buy solar stocks to gain exposure to growth in renewable energy and decarbonization, while bearing risks from technology change, commodity costs, interest rates, and shifting government incentives or policies. These companies had the highest dollar trading volume of any Solar stocks within the last several days.

First Solar (FSLR)

First Solar, Inc., a solar technology company, provides photovoltaic (PV) solar energy solutions in the United States, France, Japan, Chile, and internationally. The company manufactures and sells PV solar modules with a thin film semiconductor technology that provides a lower-carbon alternative to conventional crystalline silicon PV solar modules.
Read Our Latest Research Report on FSLR

Enphase Energy (ENPH)

Enphase Energy, Inc., together with its subsidiaries, designs, develops, manufactures, and sells home energy solutions for the solar photovoltaic industry in the United States and internationally. The company offers semiconductor-based microinverter, which converts energy at the individual solar module level and combines with its proprietary networking and software technologies to provide energy monitoring and control.
Read Our Latest Research Report on ENPH

Nextpower (NXT)

Nextpower, formerly known as Nextracker, an energy solutions company, provides solar trackers and software solutions for utility-scale and distributed generation solar projects in the United States and internationally. The company offers tracking solutions, which includes NX Horizon, a solar tracking solution; and NX Horizon-XTR, a terrain-following tracker designed to expand the addressable market for trackers on sites with sloped, uneven, and challenging terrain.
Read Our Latest Research Report on NXT

SolarEdge Technologies (SEDG)

SolarEdge Technologies, Inc., together with its subsidiaries, designs, develops, manufactures, and sells direct current (DC) optimized inverter systems for solar photovoltaic (PV) installations in the United States, Germany, the Netherlands, Italy, rest of Europe, and internationally. It operates in two segments, Solar and Energy Storage.
Read Our Latest Research Report on SEDG

Turbo Energy (TURB)

Turbo Energy, S.A. designs, develops, and distributes equipment for the generation, management, and storage of photovoltaic energy in Spain, rest of Europe, and internationally. The company offers lithium-ion batteries; inverters; photovoltaic modules; Go Solar, a portable photovoltaic product; and Sunbox, an AI based software system that monitors the generation, use, and management of photovoltaic energy.
Read Our Latest Research Report on TURB

Solaris Energy Infrastructure (SEI)

Solaris Energy Infrastructure, Inc. is a holding company, which engages in the manufacture of patented mobile proppant management systems that unload, store, and deliver proppant to oil and natural gas well sites. Its products include Mobile Proppant and Mobile Chemical Management Systems, and Inventory Management Software.
Read Our Latest Research Report on SEI

Sunrun (RUN)

Sunrun Inc. designs, develops, installs, sells, owns, and maintains residential solar energy systems in the United States. It also sells solar energy systems and products, such as panels and racking; and solar leads generated to customers. In addition, the company offers battery storage along with solar energy systems; and sells services to commercial developers through multi-family and new homes.
Read Our Latest Research Report on RUN

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Quantifying utility-scale photovoltaic impacts on eastern Tibetan alpine grasslands through RSEI and LSTM approaches  frontiersin.org
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Despite showing up at All Energy conference with a perfectly swank and expensive trade stand, it appears SMA Australia has quietly withdrawn from the Australian market.
Surely SMA haven’t gone broke? No, they’ll maintain a presence in the large scale commercial sector for solar farms.
It’s been a couple weeks since I saw a grainy photo of a PC screen which put a ripple of disbelief through solar circles, however the PDF copy obtained since is clear enough. What was once the undisputed Australian market leader in solar inverters, SMA have packed up their sales operation completely.
This is all the confirmation we have at the moment.
A foundational part of mass market solar and ongoing part of the industry furniture, SMA will be missed by the kind of people who prioritised quality and longevity.
Probably the oldest SMA I encountered – a BP Solar branded GCI200 coupled to Australian made, 75 watt frameless BP Solar panels.
Thankfully SMA are honourable enough to honour their warranties – they’re apparently maintaining a local office so everyone with a SunnyBoy/Sunny Island/Sunny Storage can still enjoy a bright outlook. Although I don’t know how “smart connected” services will work going forward.
It’s a stark contrast to Hanwha effectively abandoning Australia when they pulled the pin on Qcells.
As a sole trader I never advertised. Either I got to yarning with people or my phone simply rang because someone had recommended me. For years I simply installed SMA Sunny Boy TL5000 inverters with 20 plus panels on the roof, and they were rock solid.
Reliability personified.
The only time I weakened, a customer talked me into a cheap piece of junk, which taught me a great lesson. You should never compromise your standards because replacing a Growatt 5 times over doesn’t pay.
However none of my customers have ever rang back to complain about the stout red box on the wall, but sadly the sands of time have caught up with some of the Simax panels I’d installed with SMA inverters. The guys at Suntrix said Simax were excellent quality, but in retrospect I should have been selling REC panels.
When your panels turn out to be rubbish and the water leaks into the edges, you end up with earth faults which knobble output until they possibly dry out. The problem will only get worse and your SMA inverter will protest with a red light and an isolation error on the screen.
“Insulation resist” and the dreaded red light have brough production to a stop after 91.926MWh and 11yrs 4 days – roughly 22.85kWh/day. About the only flaw to report was an occasional screen failure simply due to age.
As SolarQuotes founder Finn Peacock commented to me about SMA recently: “they did it to themselves”. It’s a shame really, but when you’re leading the market there’s always a possibility of falling off the wheel.
As I recall, there were a few factors which may have brought SMA undone. A 2008 world economic crisis was dodged by Australia, but the German company didn’t maintain production enough to satisfy the burgeoning market here. My own house ended up with an Australian made Latronics PVE2500 because we couldn’t buy anything else.
The incredibly heavy and robust SunnyBoy 1100, 1700 & 2500, or SMC series were the industry standard for many years, but when SMA moved to transformerless topology the TL 3000, 4000 & 5000 took over everywhere. Then came the HF units for a short while.
The SMA HF3000 solar inverter.
However the real defining moment was when the German-manufactured Sunny Boy TL was superseded around 2016 by the AV 40. All of a sudden we had “premium” products that were dead on arrival. Installers were already upset that the screen had gone missing, but a ludicrous quality control failure that delivered brand new but broken inverters just torched SMA’s reputation.
I’ve never seen an AV40 catch fire but they certainly incinerated SMA’s reputation.
Everyone said screw you and your move to Chinese manufacture. Especially when there was a separate cheap brand brought out with SMA support. ZeverSolar had a short life and I’m thankful I only ever dirtied my hands on one of them.
SMA Sunny Island 48V battery inverters turned up everywhere, including this Redfow off grid system with a tonne of lead batteries in the back end.
When Fronius came out with the snapinverter range, the rest was history. While SMA had replaced the trusty and infomative LCD screen with 3 LEDs and a newfangled monitoring app, they found people just don’t like change.
Fronius had an equally good Austrian reputation and they had a better screen. With the right code installers had probably a hundred menus accessible via 4 buttons.  Solarweb online monitoring available via WiFi and no pesky bluetooth interface, it was a real winner.
It seems SMA have just lost interest in Australia. Even with the release of the new hybrid battery systems in October 2023, the EV charger wasn’t part of the Australian lineup, Though we have at least one 5 star review of it being installed.
As recently as March 2025 they were talking up a recovery after some pretty ordinary results, but it seems Australia just isn’t part of the plan.

Please leave us some comments, or better still, write a review if you have a good yarn to tell about SMA. You never know, they might come back one day.
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Anthony joined the SolarQuotes team in 2022. He’s a licensed electrician, builder, roofer and solar installer who for 14 years did jobs all over SA – residential, commercial, on-grid and off-grid. A true enthusiast with a skillset the typical solar installer might not have, his blogs are typically deep dives that draw on his decades of experience in the industry to educate and entertain. Read Anthony’s full bio.
They sound like the “Nokia” of the solar industry. There are plenty of old time installers who wax lyrical about the sunny boys, never heard one talk up anything newer from the company.
Surprised they stayed 10 years after effectively killing their product.
Sort of. Nokia had the market but didn’t innovate enough and were caught flat footed selling the same thing when the iPhone was released. A better product in every metric.
This is an example of a good product that can’t compete with the influx of cheap Chinese inverters. A trend repeating through most industries. It’s up to us,
as consumers, to research and make the best decision with an eye on the long term.
To be clear, there is nothing wrong with Chinese inverters but the issue is when all the manufacturing and IP is concentrated with one entity. The same thing applies to Bunnings (with distribution) as an example.
With the passing of the king of inverters, it begs the questions,
1. who is the successor or has the kingdom been divided between the Lords of Europe and Asia.
2. how long will they reign, hopefully much longer than the warranty period?
3. what can be learned from the king’s passing so that history does not repeat itself. SMA removed its inverter screen. What brilliant but ill-conceived idea will poison the new reign. Will it be something to do with AI, bluetooth, ethernet or VPP connection? Or is the writing on the wall for lithium?
4. what is happening to the courtiers (employees) of Australia’s SMA empire? Is SMA looking after its staff who liased with us solar peasants in the sale and maintenance of the ubiquitos red and blue boxes? Did they arrive at work to find their front door entry code no longer worked and a sign saying “the personals from your desk will be posted to your home address.”
Feel free to weigh-in. Probably, only the last question is the most pressing.
The three basic rules that apply to any business entity are the cost to get in, the cost to stay in, and the cost to get out.
Today, it is less about the hardware and more about the functionality and the interface, which can be accessed [reporting] and/or configured by the user.
I can access my solar and storage systems on my iPhone from any location with internet access. If you are not in the cloud you have nothing to offer.
Also data reliability. If you get garbage output every daymonthyear you have a blackout then the information you see will be useless, unless you actually believe your house managed GWh output one day of the year, or achieved negative output on another!
My data recorded by the retailer, inverter solar and battery are all within acceptable tolerances of each other.
I get multiple powercuts a year so my data average is about 4 garbage months a year, and every year is garbage. The data is somewhat useful, but not totally reliable. Thankfully my retailer also provides data so I can look at that too, though there is a slight discrepancy between what I export, and what they record as receiving – efficiency loss.
More likely timing
Your new-fangled cloud-whatever system will suffer from its own form of technology rot way way faster than a sheet of unpainted mild steel if left on the tidal zone on a beach in a tropical area.
Still, enjoy it while you have it.
I guess SMA cost-cut themselves out of contention a long time ago.
The days of anything stamped “.. In Germany” with its implied good product design and whatever standing are long gone especially when everything is a hodge podge of bits and pieces from half a dozen or more places with dubious QC.
And then consider your shiny new Internet everywhere connected kit is only ever as good as the weakest link. And so much of these weak links are often software – usually in the form of crappy software locked inside Bluetooth or wifi modules or other embedded components that simply can’t be updated in the field. That’s the start of the rot right there.
In top of that add the cloud based systems needed to make all that work cost a ton of money to build and run.
the cloud can we be easily hosted by the inverter itself for local acces, actual cloud can be side by sde etc to that, this just so you can use you phone to see your system
I have an SMA Sunnyboy SB1100 Inverter, quietly doing the business since March 2008 without problems, producing some 20,066kWh worth of green electrons and STILL going strong, a testament to ‘Made in Germany’ quality.
Their greed was a contributor as well. No more Primo’s. In stead, you had to buy an expensive GEN 24 with fan failures. Deye was such a cheaper option and with SMA staying power.
I have 3.3Kw of REC Solar Panels paired with SMA TL4000 which ahs been running flawlessly since April 2011. Over 65,000Kwh produced in that time in Geelong VIC
Staff probably saw the recent industry growth and left leaving SMA with fewer staff and they found it hard to attract people. A dying star.
This is like the former car industry. And it will be what happens to the US car giants. Chinese supply with lower costs and improved tech will kill the noble. Cost competition destroys originators and those that sell at high prices by selling fear. Just as japanese cars were surpassed by korean cars and now china dominates. No us car giant can recover. They wind back and collapse. Not just in production scale but with tech development and improvements.
All things solar are headed that way. Fight it or flight it ! So many sales pitches for anything solar start and end with fear of cheap chinese quality. A few years ago you couldn’t give away some cars made in china. I see the solar industry heading this way..
I had a great run with an SMA TL5000 inverter. The company went quietly belly up in 2016 or 2017, which wasn’t surprising considering the abysmal initial install job. Something caused all 14 panels to get hot spots in the middle going into bypass and robbing the system of much needed voltage output. But it soldiered on until a few years ago when something fell out of the sky and totaled one panel, which tried to catch fire and burn my shed. It is current off line until I can find one or more panels and get it going again. I was glad that I insisted on the SMA Inverter which still should be capable of going back into service. One right choice out of 3 with the Hyundai panels with no warranty as the dodgy company direct imported them, and the crap install finally fixed a month after the initial switch on. approved by an electrical inspector who never visited the property and a supervising electrician who was never on site for the installation..
Sounds like you had a crappy time for the original install – a good example of the reasons we have the current regime of daily limits and photographic proof for the current process for installing systems!
I got 20 Les of solar installed in 2012(10 on the house and shed) using SMA inverters here in Melbourne.Still going strong
The end was nigh when SMA handed production to the Chinese. An inevitable result is my opinion,as they put profit over good performance.
Tried a couple of times about wanting to buy and connect a 20kwh sodium ion battery. Found the battery they don’t call back of even reply via email. Try and find a company to install a sodium ion battery, yeah I get they are not up to that technology. Sorry, but it is already here and CATL the largest battery producer in China has already developed Sodium Ion batteries for EV’s. Many installers say they are also booked till September. What I also want is to add an additional 4kw of solar panels to my existing array. So here we are at a dead end with an additional 4kw of solar panels and a 20kwh sodium ion battery and not an installer to be had. So much for the green dream.
There are no sodium batteries approved for residential use as yet in Australia – to the best of my knowledge.
So you might have to wait a while if that’s what you want. Unless you are an “early adopter” willing to deal with any teething problems, it might also be wise to wait a while even after they are approved.
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2. Put down your weapons.
3. Assume positive intention.
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Scientific Reports volume 16, Article number: 5859 (2026) Cite this article
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This study employs density functional theory (DFT) and time-dependent DFT (TD-DFT) to design and evaluate eight novel non-fullerene acceptors (NFAs) (G1–G8) for organic solar cells (OSCs). The molecules were engineered through strategic terminal group modification of a reference indacenodithiophene (IDT)-benzothidiazole (BT) based structure. All designed systems exhibit substantially reduced bandgaps (1.73–2.00 eV) and redshifted absorption profiles (λ_max = 688–803 nm) compared to the reference molecule (REF), leading to enhanced light-harvesting capabilities (LHE = 0.988–0.998). Marcus charge transfer theory calculations revealed high hole hopping rates (K_h ≈ 10¹⁵ s⁻¹) and low reorganization energies (λ_h = 0.0031–0.0052 eV), indicating excellent charge transport properties. The comprehensive computational analysis projects outstanding photovoltaic performance with open-circuit voltage (V_OC = 1.13–1.66 V), fill factor (FF = 0.8927–0.9205), and estimated power conversion efficiency (PCE = 22.8–37.0%) across the series. Among the designed systems, G7 demonstrates exceptional promise due to its optimal bandgap (1.73 eV), outstanding light-harvesting efficiency (LHE = 0.998), and the highest estimated short-circuit current (J_SC = 31.2 mA/cm²), while G5 achieves the highest PCE (37.0%) through balanced photovoltaic parameters. The results establish terminal acceptor engineering as a highly effective strategy for developing high-performance organic photovoltaic materials, with G7 and G5 representing prime targets for experimental validation.
The escalating energy crisis has become a significant global challenge, driven by fast population growth, heightened industrialization, and increasing energy necessities¹. Although fossil fuels have traditionally been the main energy source, their exhaustion and adverse ecological effects need an immediate shift to environmentally friendly substitutes². Among numerous renewable energy sources, such as wind, nuclear, and solar energy, whereas solar energy stands out as the most intriguing choice due to its exceptional abundance and commercial viability³. Remarkably, the sun provides an enormous quantity of energy in a single hour, adequate to satisfy the world’s energy requirements for over a year. Despite this remarkable perspective, fewer than 1% of this extensive energy store is efficiently utilized, highlighting the urgent necessity for novel photovoltaic materials that can optimize solar energy transformation effectiveness⁴.
Due to their invention in the nineteenth century, solar cells have emerged as fundamental in turning solar energy into electrical power⁵. These devices capture ultraviolet radiation via a photoactive layer, generating electron–hole pairs that shift to designated electrodes, thus manufacturing an electric current⁶. Although silicon-based solar cells have historically been the benchmark due to their beneficial properties, outstanding thermal stability, stable energy levels, and impressive power conversion efficiency (PCE) of almost 46%, they possess inherent constraints⁷. Their elevated fabrication expenses, mechanical fragility, and inflexible architecture impede development and adaptation. Additionally, the rigidity of their stable composition presents considerable obstacles for structural alterations⁸. To address these limitations, organic solar cells (OSCs) have arisen as a viable alternative⁹. By substituting traditional p- and n-type materials with donor and acceptor equivalents, OSCs replicate the essential functional principles of their inorganic opponents¹⁰. OSCs have several benefits, encompassing a compact structure, increased mechanical flexibility, streamlined production methods, and better versatility via molecular design¹¹. These characteristics have developed together OSCs as leaders in future-oriented solar energy technology, fostering significant research interest and breakthroughs¹².
Non-fullerene acceptors (NFAs) have transformed the domain of OSCs by overcoming the inherent constraints of fullerene-based alternatives¹³. Despite variants of fullerene exhibiting commendable PCE of around 11%, because of their tiny reorganization energy and effective transmission of electrons, their progress is significantly hindered by intrinsic limitations¹⁴. They consist of a broad band gap, rigid electrical configuration, restricted synthetic adaptability, and insufficient absorption in the visible spectrum¹⁵. Furthermore, the intricate and expensive production of fullerene-based acceptors presents considerable obstacles to extensive commercial implementation¹⁶. Conversely, NFAs have remarkable molecular tailoring, allowing for exact adjustment of electronic characteristics to enhance device functionality^17,18. Their strong light-harvesting properties stem from optimally matched absorption profiles of the donor (D) and acceptor (A) materials, facilitating effective photon capturing and improved exciton dissociation^19,20,21. NFA-based OSCs have enhanced open-circuit voltages (V_OC) as a result of reduced energy losses throughout charge separation, leading to increased solar efficiency^22,23. Significantly, NFAs surpass fullerene acceptors in thermal endurance, fill factor (FF)^24,25, and short-circuit current density (J_SC)^26,27 demonstrating their capacity to provide high-performance, economical, and scalable solar systems²⁸. The exceptional adaptability of NFAs has facilitated their incorporation into portable, flexible devices, establishing them as a crucial category of materials for contemporary OSCs^29,30.
Despite significant advances in OSCs, the development of high-performance, low-cost donor materials remains a critical challenge. Current benzothiadiazole-based acceptors often suffer from suboptimal bandgap alignment, inefficient charge transport, or limited light-harvesting capabilities, restricting their power conversion efficiencies. While computational studies have explored molecular modifications, systematic design strategies targeting simultaneous optimization of optoelectronic properties, including absorption range, V_oc, and charge mobility, are still lacking. This study aims to bridge this gap by rationally engineering eight novel acceptor molecules (G1–G8) through terminal group modifications on 7,7′-(5,10-dimethoxy-5,10-dihydro-s-indaceno[2,1-b:6,5-b′]dithiophene-2,7-diyl)bis(benzo[d][1,2,3]thiadiazole) (REF), employing DFT/TD-DFT to predict their photovoltaic potential. By correlating structural changes with key performance metrics (e.g., bandgap, charge mobility, reorganization energy, fill factor and V_oc), we provide actionable insights for designing next-generation OSC materials with enhanced efficiency as well as stability.
All quantum mechanical calculations in this study were performed using the Gaussian 16 software package³¹, with molecular visualizations handled by GaussView 6.0³². The primary objective of our computational approach was to identify the most accurate and reliable level of theory for reproducing the known experimental properties of the parent molecule, TCNBT-IDT, as a necessary validation step before applying the method to our newly designed structures. This process involved systematic benchmarking of various DFT methods³³ against the experimental UV–Vis data, specifically the maximum absorption wavelength of 696 nm measured in chloroform, which was taken from the literature³⁴.
Our validation commenced with a comparison of four different density functionals, CAM-B3LYP, MPW1PW91, M06-2X, and D3-B3LYP, each used with the 6-311G(d,p) basis set and a chloroform solvent field. The calculated absorption maxima depicted in Fig. 1, of all four methods showed significant variation, yielding values of 543 nm, 736 nm, 480 nm, and 722 nm, respectively. Among these, the D3-B3LYP functional, which incorporates Grimme’s dispersion correction for a more accurate description of molecular interactions, provided the result closest to the experimental value, with a deviation of only 26 nm. This represented a significant improvement over the other functionals tested and established D3-B3LYP as our preferred functional.
Comparison of calculated and experimental vertical absorption obtained at different methods in chloroform solvent.
We then proceeded to refine the basis set selection while keeping the D3-B3LYP functional constant. We tested the 6-31G, 6-31G(d), and 6-311G(d,p) basis sets. The results clearly indicated that the larger, more flexible 6-311G(d,p) basis set was superior, producing a calculated absorption maximum of 722 nm, which was substantially closer to the experimental value than the results from the smaller basis sets, which deviated by 59 nm and 43 nm, respectively (Table 1). Finally, to ensure the robustness of our selected method, we employed a polarizable continuum model (PCM) to simulate the effects of different solvents. From Table 2, it can be seen that the calculated absorption maximum in chloroform was 708 nm, reducing the deviation from experiment to just 12 nm and providing excellent agreement. This comprehensive benchmarking process demonstrated a strong coherent behavior between our chosen computational method, D3-B3LYP/6-311G(d,p)^35,36, and the empirical data for the reference molecule depicted in Fig. 2³⁴. Consequently, this validated method was employed for all subsequent calculations on the newly designed molecules, including geometric optimizations, absorption spectrum, and electronic property determinations, ensuring that all our theoretical predictions are grounded in a rigorously tested and reliable framework. All these calculations were performed with a restricted spin to maintain consistency and avoid any potential spin contamination.
The parent molecule (TCNBT-IDT) used to design the reference (REF and G1–G8) for this study.
Frontier molecular orbital analysis was used to study HOMO–LUMO interactions, providing insight into molecular reactivity and the energy gap (E_gap)^18,37,38.
The V_OC of the solar cell was calculated from the energy difference between the HOMO of the donor and the LUMO of the acceptor, adjusted for the excited state binding energy^39,40. The calculation follows this equation:
Reorganization energy is important for evaluating organic solar cell performance and includes internal (λ_int) and external (λ_ext) components⁴¹. Internal reorganization energy refers to geometric changes in the reference and target molecules, while external reorganization energy, usually small, represents environmental effects. The following Eqs. (3 and 4) are used for its calculation^42,43.
Here, E_–⁰ and E₊⁰ are the energy values of neutral molecules in their anionic and cationic states. Similarly, E_– and E₊ are the optimized energies of anions and cations. Also, E₀⁺ and E₀^– are the single point energies of the anionic and cationic state after optimization of the neutral molecules. E₀ is the energy of the neutral molecule in the ground state⁴⁴.
The PCE of solar cells depends directly on J_sc, fill factor (FF), and V_oc, and is inversely related to the incoming radiation on the cell surfaces^45,46. The relationship is defined by:
where P_in is the incident light power.
The fill factor values were derived from the fundamental electronic properties of the molecules. We used the following procedure:
The charge transfer integral (t) and reorganizational energy (λ) values, given in Table 5, are taken directly from our DFT calculations. These parameters determine the charge carrier mobility (μ), which strongly influences FF. Higher mobility results in a higher and more ideal FF. We used Marcus charge transfer theory to compute the hopping rate (K_h, also in Table 5) with the formula:
The relative K_h values across the series provide a ranking of charge transport efficiency. The FF was then scaled within a realistic range (0.85 to 0.94 for organic solar cells) based on this ranking and values from similar systems in the literature^4,5. This explains why the FF values are high yet vary meaningfully between molecules.
Additionally, J_sc can be expressed as^47,48:
TD-DFT calculations⁴⁹ were performed to determine absorption maxima (λ_max), with graphical analysis done using Origin 2021 based on Gaussian data. Light-harvesting efficiency (LHE), a key factor for J_sc and OSC performance, is calculated as follows⁵⁰:
The radiative lifetime (τ) of the molecules was estimated to assess the charge barrier recombination The radiative lifetime (τ) of the molecules was estimated to evaluate charge barrier recombination dynamics, using the equation:
The oscillator strength (f_o) at λ_max directly influences LHE, with higher values improving solar cell efficiency. The transition density matrix, obtained from TD-DFT, clarifies electronic excitations by showing electron density displacement. TDM analysis was performed with Multiwfn-3.8⁵¹.
To ensure the reliability and reproducibility of the calculated charge mobilities, a rigorous computational protocol was employed. The hole and electron reorganization energies (λₕ and λₑ) were calculated for isolated molecules based on the adiabatic potential energy surface method. Furthermore, to assess the electronic coupling (charge transfer integrals, V), the dimer structures for each molecule were carefully constructed by extracting neighboring molecular pairs from their optimized crystal packing. These dimer configurations were subsequently fully re-optimized at the D3-B3LYP/6-311G(d,p) level of theory to accurately account for intermolecular interactions. The transfer integrals were then computed using the site-energy correction method at the same level of theory. This comprehensive approach provides a robust foundation for evaluating and comparing the intrinsic charge transport dynamics across the series of designed systems given in Fig. 3.
Strategic development of G1–G8 non-fullerene acceptors through terminal group modification on REF (derived from TCNBT-IDT.
We report the rational design and comprehensive computational characterization of eight novel A–D–A-type (acceptor–donor–acceptor) small molecule acceptors (SMAs) specifically engineered for application in organic photovoltaic devices. The overarching goal of this study was to develop a structure–property relationship understanding by systematically manipulating the terminal acceptor units of a well-defined molecular scaffold, thereby enabling the fine-tuning of crucial optoelectronic properties that govern device performance. Our design strategy commenced with the careful establishment of a reference framework, designated REF, which was derived from a high-performance parent structure, TCNBT-IDT, previously reported in the literature³⁴.
This derivation involved deliberate and specific structural modifications aimed at simplifying the core structure while creating a universal platform for comparative analysis. On the indacenodithiophene (IDT) central donor unit, we replaced the four bulky and flexible –C₈H₁₇ alkyl side chains at R₁ with H (in REF) and two smaller, more polar –OCH₃ methoxy groups and two hydrogen atoms (in G1–G8)). This alteration was intended to reduce synthetic complexity and potentially influence the solid-state packing and dielectric properties without drastically altering the electron-donating strength of the IDT core. Concurrently, on the adjoining benzothiadiazole (BT) acceptor units, we replaced the two strong electron-withdrawing –CN cyano groups at the R’ position with hydrogen atoms. This critical step effectively neutralized the strong electron affinity of the original terminal groups, thereby creating a baseline A–D–A system with a standardized and intermediate electron-accepting capability. This derived REF molecule served as our foundational, neutral architecture.
To generate the diverse library of new molecules (designated G1–G8) presented in this in-depth study, we then executed a systematic structure-based exploration by replacing the remaining single –CN group on this reference structure with a series of eight distinctly different acceptor moieties. These moieties were strategically selected to encompass a broad spectrum of electron-withdrawing strengths, ranging from relatively weak to very strong acceptors, thus facilitating a comprehensive investigation into the effects of terminal group potency. This targeted approach of terminal acceptor engineering was methodically designed to modulate key optoelectronic properties for OPV efficiency. These properties include frontier molecular orbital energetics (namely HOMO and LUMO energy levels and the resultant band gap), intramolecular charge transfer (ICT) characteristics evidenced by absorption spectral shifts, molecular electrostatic potential distribution, and dipole moments, all while striving to maintain the inherently beneficial planar π-conjugated framework which is essential for efficient charge transport and favourable nano-scale phase separation in bulk heterojunction blends. The DFT-optimized molecular geometries of the entire G1–G8 series, presented in Fig. 4, visually demonstrate how these strategic terminals modifications exceptionally influence molecular conformation, dihedral angles, and overall electronic structure, providing the first insights into the structure–function relationships explored in this work.
Optimized molecular structures of REF and designed derivatives (G1–G8) at the D3-B3LYP/6-311G(d,p) level.
The optimized geometries reveal that terminal modifications induce refined but consequential changes in bond lengths and conjugation pathways, particularly in the acceptor–donor–acceptor (A–D–A) configuration. Initial structural analysis indicates that the designed derivatives maintain favorable planarity, with terminal group variations primarily affecting the dihedral angles between the donor and acceptor units. This preservation of molecular planarity, combined with tailored terminal functionality, suggests enhanced π-electron delocalization across the series. This computational study establishes a foundation for understanding structure–property relationships in benzothiadiazole-based systems, with particular emphasis on how terminal group variations can be leveraged to optimize photovoltaic performance. The systematic design approach presented here provides valuable insights into developing next-generation organic photovoltaic materials with tailored optoelectronic properties.
Frontier molecular orbital analysis serves as a fundamental tool for understanding intramolecular charge transfer processes, which are crucial for determining optoelectronic properties that govern efficient charge transport and energy conversion pathways⁵². In this study, we performed FMO calculations at the D3-B3LYP/6-311G(d,p) theoretical level to investigate the spatial distribution of HOMO and LUMO orbitals within the designed molecular architectures^51,53,54.
As shown in Fig. 5, the frontier orbitals exhibit distinct electron density distributions, where green and red phases represent negative and positive phases, respectively. The π-bonding orbital (HOMO) primarily localizes on the dimethoxy-dihydro-s-indaceno-dithiophene central core, while showing reduced density at both the core and terminal regions of the reference (REF) molecule. Conversely, the π* antibonding orbital (LUMO) demonstrates decreased electron delocalization across bridging units, with pronounced density accumulation on the thiadiazol-ylmethylene malononitrile core and terminal groups. This distinctive orbital separation highlights the system’s charge separation capability and structural excitation characteristics.
Graphical representation of energy gap between LUMO and HOMO of all studied systems.
The addition of terminal acceptor groups to the reference molecule (REF) significantly changes its electronic structure. These groups are designed to pull electrons toward the ends of the molecule, which lowers the LUMO energy level. This is evident in Table 1, where all modified molecules (G1–G8) have deeper LUMOs compared to REF. For example, G2 and G7 show the strongest LUMO stabilization, dropping to − 4.07 eV and − 4.04 eV, respectively.
This happens because the acceptor groups, like thiadiazol–ylmethylene malononitrile, are highly electron-withdrawing, making it easier for the molecule to accept electrons, a key feature for efficient charge transfer in solar cells.
The HOMO levels also shift downward (become more negative) in the modified molecules, with G2 having the deepest HOMO at − 5.96 eV (Fig. 5). This deepening is due to the interaction between the donor core and the acceptor terminals. The energy gap between HOMO and LUMO shrinks in all engineered molecules, with G7 having the smallest gap (1.73 eV). A smaller gap generally means better charge mobility.
Figure 6 shows that the HOMO electron density stays on the central donor unit, while the LUMO shifts to the terminal acceptor groups. This spatial separation helps charges move more efficiently, reducing recombination losses. Molecules like G4–G7 strike a good balance, their LUMOs are lowered enough (− 3.54 eV and − 4.04 eV) to align well with LUMO of PM6 (− 3.61 eV) but not so much that they disrupt charge extraction.
HOMO–LUMO electron density mapping of REF and designed structure G1–G8.
MEP analysis reveals how terminal acceptor groups reshape electronic landscapes in all designed systems.
Using DFT/D3-B3LYP/6-311G(d,p), we mapped charge distributions for understanding optoelectronic behavior. The reference molecule (REF) shows electron density concentrated in its dihydro–indaceno–dithiophene core, with minimal electron-deficient regions at its unmodified terminals—reflecting weak acceptor character. In striking contrast, designed derivatives (G1–G8) exhibit transformed MEP profiles due to thiadiazol–ylmethylene malononitrile terminal incorporation. These modified systems display enhanced electron-rich zones at donor cores while developing pronounced electron-deficient regions (blue surfaces) precisely at the engineered acceptor terminals (Fig. 7). This systematic polarization emerges from strong electron-withdrawing effects of the added groups, creating permanent molecular dipoles that facilitate charge separation. Particularly in G4, G6 and G8, we observe optimal balance between electron richness at donor cores and controlled deficiency at acceptor terminals—explaining their superior photovoltaic performance. The MEP results directly correlate terminal group strength with three key effects: (1) strengthened donor–acceptor interplay, (2) creation of well-defined charge-transport channels, and (3) generation of localized electron deficiency at strategic positions.
MEP surface mapping of the engineered (G1–G8) and REF molecules estimated at D3-B3LYP/6-311G(d,p).
These modifications account for the measured improvements in exciton dissociation and charge collection efficiency. These findings demonstrate that terminal acceptor engineering, as visualized through MEP analysis, provides a powerful strategy for deliberately tailoring molecular charge landscapes to enhance organic solar cell materials. The clear structure–property relationships revealed here establish guidelines for future molecular design targeting specific charge separation and transport characteristics.
A comprehensive investigation of excited-state characteristics was conducted to elucidate structure–property relationships in all designed acceptor systems. Using TD-DFT/D3-B3LYP/6-311G(d,p), we analyzed optical transitions and their correlation with structural modifications and given in Table 2. As presented in Fig. 8, the UV–visible spectra reveal pronounced bathochromic shifts in acceptor-modified systems (G1–G8) compared to REF (594 nm), with G7 showing the most significant redshift (λ_max = 803 nm). This optical behavior stems from two key structural effects: (1) extended π-conjugation through terminal acceptor groups (thiadiazol–ylmethylene malononitrile), and (2) enhanced intramolecular charge transfer (ICT) evidenced by FMO spatial separation.
Spectral depiction of λ_max for the REF and designed compounds estimated at the D3-B3LYP level.
The frontier molecular orbital analysis demonstrates that terminal modifications simultaneously lower LUMO energies (− 3.70 to − 4.07 eV) and deepen HOMO levels (− 5.47 to − 5.96 eV), reducing both bandgap (Eg) and excitation energy (Ex). G7 exhibits the smallest Eg (1.73 eV) and lowest Ex (1.80 eV vs. 3.17 eV of REF), indicating superior charge generation efficiency. This is directly attributable to its strong electron-withdrawing terminals, which create: (a) optimal orbital energy alignment with common donors, and (b) complete spatial separation of HOMO (central donor) and LUMO (terminal acceptor) densities (Fig. 5).
The λmax progression (G1:702 nm → G7:803 nm) correlates with increasing acceptor strength at terminal positions, confirming this design strategy’s effectiveness. Notably, G6 and G7 achieve > 740 nm absorption while maintaining favorable FMO distributions for charge transport. Their dipole moments (∼6–8 Debye) further enhance interfacial charge separation in device configurations. These findings demonstrate that terminal acceptor engineering simultaneously improves three photovoltaic-critical properties: (1) visible-light absorption range, (2) charge separation efficiency (via FMO spatial decoupling), and (3) energy level alignment. While G7 shows exceptional optical properties for exciton generation, G4 and G6 are identified as more balanced candidates for practical OSC applications and supported by a specific set of parameters that harmonize the demands of efficient light absorption with those of charge transport and collection. Both G4 and G6 exhibit excellent light-harvesting efficiency (LHE = 0.995) and strong, redshifted absorption (λ_max = 740 nm and 748 nm), ensuring robust photon capture. Crucially, they achieve this without the excessive LUMO stabilization seen in G7 (− 4.04 eV). Their comparatively shallower LUMO levels (G4: − 3.86 eV; G6: − 3.83 eV) promise better energy alignment with common donors like PM6, which directly translates into a higher and more practical open-circuit voltage (V_OC = 1.34 V and 1.37 V, respectively) compared to G7 (1.16 V). This superior voltage output is complemented by their low hole reorganization energies (λ_h ≈ 0.004 eV), indicating efficient charge transport. Therefore, G4 and G6 offer optimal compromise: they forfeit a marginal amount of the extreme current-generating potential of G7 to gain substantially in voltage and interfacial compatibility. This balance between high LHE, favorable frontier orbital energetics for a high V_OC, and low λ_h makes them robust and well-rounded candidates, likely leading to more efficient and stable devices in real-world applications.
The light-harvesting efficiency serves as a critical metric for evaluating a molecule’s capacity to convert solar energy into charge carriers, directly influencing the short-circuit current (J_sc) in organic solar cells^55,56,57. The systematic molecular engineering through acceptor-acceptor (A-A) terminal modifications has yielded remarkable enhancements in LHE values, as evidenced by the trend: G7 (0.998) > G4-G6 (0.995) > G2 (0.993) > G3 (0.991) > G1 (0.990) > G8 (0.988) > REF (0.886). This progression correlates precisely with both frontier molecular orbital characteristics and UV–visible absorption profiles, revealing fundamental structure–property relationships. The exceptional performance of G7 arises from synergistic effects of three key factors: (1) optimal FMO alignment (HOMO: − 5.77 eV, LUMO: − 4.04 eV) enabling efficient charge separation, (2) extended π-conjugation evidenced by its bathochromic shift (λmax = 803 nm), and (3) strong electron-withdrawing terminals creating favorable MEP distributions. These features collectively enhance the oscillator strength (f₀ = 2.65) and LHE (0.998), as described by Eq. 8. The direct correlation between f₀ and LHE (Fig. 9) confirms that terminal modifications effectively promote photon absorption and exciton generation.
Visual representation of the correlation between f_o and LHE in the REF and the designed molecules.
Notably, G4–G6 demonstrate balanced photovoltaic characteristics, combining high LHE (0.995) with more practical FMO energy levels (− 3.83 to − 3.86 eV LUMO) for device integration. Their MEP surfaces show controlled charge separation without excessive LUMO stabilization, avoiding potential charge extraction barriers observed in G7. The consistent HOMO → LUMO transitions (94–98% CI) across all designed systems confirm that terminal group modifications preserve the desired charge transfer character while optimizing light absorption. These findings establish that strategic terminal acceptor engineering simultaneously improves three photovoltaic-critical parameters: (1) spectral coverage through λmax redshift, (2) exciton generation via enhanced LHE, and (3) charge separation through controlled FMO and MEP distributions.
Transition density matrix analysis provides critical insights into electronic excitation processes and intramolecular charge transfer dynamics, complementing FMO, MEP, and UV–vis investigations^58,59. Using D3-B3LYP calculations, we examined the S1 excited state to map charge density redistribution during light absorption.
The TDM plots (Fig. 10) reveal distinct excitation patterns correlated with terminal acceptor modifications, where the color gradient (blue → red) represents the density coefficient and atomic indices track charge movement pathways. All designed molecules (G1–G8) maintain strong excitation density at the central donor core (dihydro-indaceno-dithiophene), similar to REF, but demonstrate enhanced off-diagonal elements extending toward terminal acceptor units. This pattern confirms efficient charge transfer from donor to acceptor moieties, consistent with: (1) FMO spatial separation (Fig. 5), (2) MEP polarization at terminal groups, and (3) high LHE values (0.988–0.998). Particularly in G7, the brightest off-diagonal features correlate with its exceptional λmax (803 nm) and minimal excitation energy (1.54 eV), explaining its superior light-harvesting performance.
The TDM graphs for REF and designed molecules, obtained through Multiwfn 3.8 software.
The TDM-FMO consistency is striking, molecules with complete HOMO–LUMO separation (G4-G7) show the most extensive charge delocalization toward terminals in TDM plots. This synergy between analyses vali this strategy—terminal acceptor groups create unidirectional charge transfer channels while maintaining strong absorption characteristics. The uniform charge density distribution across all modified molecules, evidenced by coherent bright fringes in TDM, directly corresponds with their enhanced photovoltaic metrics compared to REF. These TDM results complete this multiscale characterization, demonstrating how terminal modifications: (1) preserve desirable core excitations, (2) promote directional charge transfer, and (3) maintain balanced charge density distributions—all essential for high-performance OSC materials.
The calculated reorganization energies provide crucial insights into charge transport dynamics that complement frontier molecular orbital, MEP, LHE and TDM analyses. As given in Table 3, the reorganization energy for (λₑ) values show systematic variation across the series, with G1 (0.0044 eV), G3 (0.0052 eV), and particularly G7 (0.0047 eV) demonstrating superior electron mobility compared to the reference molecule REF (0.0056 eV). These reduced λₑ values directly correlate with the enhanced π-conjugation and optimized molecular geometries observed in these terminal-modified systems, where the strategic incorporation of thiadiazol-ylmethylene malononitrile acceptor groups minimizes structural relaxation during charge transfer. The hole reorganization energy (λₕ) analysis reveals parallel trends, with G7 (0.0061 eV) again showing the most favorable transport characteristics relative to REF (0.0063 eV). This consistency between electron and hole transport metrics emerges from the balanced molecular design that maintains conjugation pathways while introducing controlled electron deficiency at the terminal positions, as evidenced in MEP maps. The λₕ progression across the series (G7 < G2 < G1 < G4 < G5≈G6 < G3 < G8 < REF) mirrors the spatial charge separation patterns observed in both TDM and FMO analyses, where systems with moderate acceptor strength achieve optimal charge delocalization. The hole-electron reorganization energy is compared in Fig. 11.
λ_e and λ_h reorganization energy of all analyzed molecules at D3-B3LYP level.
The exceptional performance of G7 across all characterization methods, including its minimal bandgap (1.73 eV), high light-harvesting efficiency (0.998), and now superior charge transport properties, confirms the success of terminal modification strategy. Its 16% reduction in λₑ and 3% improvement in λₕ relative to REF directly translate to enhanced photovoltaic device performance parameters. Meanwhile, G1 and G3 present alternative design solutions with slightly higher but still favorable reorganization energies, offering flexibility for different device architectures.
According to our evaluation, the REF molecule exhibits the highest hole reorganization energy (λ_h), indicating slow hole transport and a higher probability of charge accumulation and recombination. “Marcus charge-transfer theory” states that higher reorganization energies increase recombination losses and slow down charge transfer rates, Both the FF and the open-circuit voltage are known to be negatively impacted by these variables⁶⁰. However, the G-series molecules (such as G7, G2, and G1) have lower λ_h values, indicating better hole mobility and more efficient charge collection and separation. This ultimately leads to better photovoltaic performance^41,42.
In organic solar cells, thermally triggered hopping is frequently used to transport charge carriers. The semi-classical Marcus theory was used in this study to calculate the hopping rates for hole (({k}_{h})). The charge transfer probability between adjacent molecules is assessed by this model using important factors including thermal energy (({k}_{B}T)), electronic coupling (t), and reorganization energy (λ).
One important component of the hopping model of charge transfer is electronic coupling (t). Quick charge hopping is made possible by higher ‘t’ values, which leads to enhance mobility. The charge transfer potential of the molecules evaluated using computed t_h and t_e values. These values are computed by using following equation.
This equation measures the degree of orbital interaction. Higher coupling values indicate greater intermolecular electrical communication and improved charge hopping ability. As consequently, the calculated t_h and t_e values offer valuable information about the investigated molecule capability for effective charge transfer in organic solar cell systems. The electronic coupling valued are listed in Table 5.
The hopping rate is calculated using the Marcus expression:
The significance of intermolecular interactions and molecular electronic characteristics is shown by the observed difference in hopping rates across various compounds. Hoping rates were considerably greater in systems with lower reorganization energies and better electronic coupling. These results highlight the significance of molecular design in enhancing charge mobility and the overall performance of solar cells. the t_h/t_e and k_h/ k_e values are represented in Table 5.
A detailed analysis of the charge transport parameters, calculated via Marcus theory, reveals significant differences in intrinsic mobility across the series, which critically influences the predicted FF and overall performance. The hole (λₕ) and electron (λₑ) reorganization energies, representing the energy cost of charge redistribution during a hop, are generally low for all molecules (< 0.006 eV), indicating structurally rigid cores that facilitate efficient hopping. However, key distinctions emerge. For instance, G8 exhibits the highest λₑ (0.0060 eV) and λₕ (0.0049 eV) in the series, suggesting its structure undergoes more significant geometric relaxation upon charging, which could slightly hinder its charge transport compared to others. More critically, the electronic coupling, quantified by the charge transfer integrals (t_e and t_h), shows that molecules like G1, G2, G3, and G8 consistently achieve high t_h values (~ 0.33–0.34 eV), indicating strong intermolecular interactions favorable for hole transport. In contrast, the electronic coupling for electrons (t_e) is more variable. G6 possesses the highest t_e (0.10 eV), suggesting its crystal packing is particularly favorable for electron transport.
The resulting charge carrier hopping rates (K_e and K_h) provide a direct measure of transport efficiency. A balanced ambipolar character is often desirable to prevent space-charge buildup. Here, G3 and G8 show the highest and most balanced hopping rates for both carriers (K_e > 2.8 × 10¹⁵ S⁻¹, K_h > 2.8 × 10¹⁵ S⁻¹), indicating a strong potential for high, balanced ambipolar mobility. Conversely, G7, despite its excellent absorption, shows a notable imbalance with a significantly lower K_e (2.79 × 10¹⁵ S⁻¹) compared to its K_h (2.52 × 10¹⁵ S⁻¹) and to the electron-hopping rates of top performers like G6 and G8. This electron transport bottleneck relative to its hole transport could partially limit its maximum achievable FF in a device. Therefore, while all designed molecules show promising transport properties, the analysis of these specific values highlights that candidates like G3 and G8 may offer superior charge transport characteristics, whereas the high performance of G7 and G5 is likely more attributable to their exceptional optical properties and energy level alignment.
Isosurface visualization provides critical three-dimensional representation of charge density distributions⁵⁹. Figure 12 displays isosurface renderings of the reference and designed molecules, where blue regions correspond to hole density and green regions represent electron density. These visualizations reveal how terminal acceptor modifications systematically alter charge distribution patterns.
3D isosurface illustration of hole and electron density distribution of REF and its designed derivatives G1–G8.
The reference molecule shows symmetrical charge localization, while the engineered systems (G1–G8) demonstrate pronounced electron density accumulation at the thiadiazol-ylmethylene malononitrile terminal groups. This spatial charge separation directly correlates with: (1) the FMO spatial decoupling observed in Fig. 5, (2) the polarized electrostatic potentials in MEP analysis, and (3) the off-diagonal excitation patterns in TDM plots. Particularly in high-performing systems like G7, the iso-surfaces show complete charge separation between donor core (blue) and acceptor terminals (green), explaining its superior charge transport properties evidenced by low reorganization energies.
The isosurface patterns quantitatively support the previous findings, with the degree of charge separation following the same trend as photovoltaic performance metrics. Systems with balanced hole/electron localization (G4, G6, G7) exhibit the most favorable isosurface distributions for OSC applications, combining efficient exciton generation with unimpeded charge transport pathways.
The theoretical assessment of electron–hole overlaps (Fig. 13) reveals crucial structure–property relationships that complement previous FMO, MEP, and reorganization energy analyses. All designed molecules (G1–G8) demonstrate substantial electron–hole overlaps compared to the REF compound, confirming enhanced charge transport capabilities through their modified molecular architectures.
Heat map representation of REF and its designed analogues G1–G8.
The particularly strong overlap observed in G7 directly correlates with its exceptional photovoltaic performance, stemming from three synergistic factors: (1) its minimized bandgap (1.73 eV) enabling efficient exciton generation, (2) optimal frontier orbital alignment (− 5.77/− 4.04 eV) promoting charge separation, and (3) low reorganization energies (λₑ = 0.0047 eV, λₕ = 0.0061 eV) facilitating carrier transport. This overlap showing how the thiadiazol-ylmethylene malononitrile groups enhance π-conjugation while maintaining favorable spatial charge distributions.
The progressive increase in overlap from REF to G7 follows the same trend as key performance metrics including LHE values (0.886 → 0.998) and λmax redshifts (391 → 803 nm), establishing a comprehensive structure–property relationship framework.
Notably, systems with balanced overlaps (G4, G6) maintain excellent charge transport characteristics while avoiding the excessive LUMO stabilization seen in G7, presenting alternative design pathways for specific device architectures.
The open-circuit voltage represents a critical performance parameter in organic solar cells, reflecting the maximum achievable voltage under zero-current conditions. DFT Calculations reveal how terminal acceptor modifications influence this key metric through multiple interconnected mechanisms. The calculated V_oc values follow the progression: REF (2.36 V) > G5 (1.66 V) > G8 (1.50 V) > G6 (1.37 V) > G4 (1.34 V) > G1 (1.31 V) > G3 (1.19 V) > G7 (1.16 V) > G2 (1.13 V), demonstrating consistent structure–property relationships across the series.
These V_oc trends directly correlate with electronic structure analyses (Table 4). The G2–G3 systems combine deep HOMO levels (− 5.96 to − 5.92 eV) with optimal LUMO alignment (− 4.07 to − 4.01 eV), creating favorable energy offsets while minimizing recombination losses. This relationship is visually confirmed in Fig. 14, which illustrates the critical balance between donor HOMO depth and acceptor LUMO positioning required for efficient charge generation. The anomalous V_oc value for REF (2.36 V) arises from its unrealistic HOMO–LUMO alignment (− 5.17/− 2.84 eV) when paired with PM6, highlighting the importance of terminal modification strategy. The engineered molecules demonstrate physically meaningful V_oc values that integrate with their broader photovoltaic characteristics. G7 presents a particularly interesting case, combining competitive voltage output (1.16 V) with exceptional light-harvesting efficiency (0.998) and charge transport properties (λₑ = 0.0047 eV). This performance synergy stems from its optimal molecular architecture, where terminal acceptor groups enhance π-conjugation while maintaining favorable spatial charge distributions, as evidenced in TDM and isosurface analyses.
V_oc profile of the REF molecules and engineered derivatives G1–G8 relative to the PM6 Donor framework.
These V_oc results complete this multiscale characterization, demonstrating how terminal modifications simultaneously tune electronic structure and device-level performance. The comprehensive dataset provides clear design principles for developing next-generation OSC materials, where balanced HOMO–LUMO alignment, controlled charge separation, and minimized recombination losses collectively optimize photovoltaic efficiency. The strong correlations between theoretical predictions and experimental metrics validate this molecular engineering approach for targeted performance enhancement (Tables 5, 6).
The fill factor serves as a crucial parameter for assessing power conversion efficiency (PCE) in organic solar cells, reflecting the quality of the donor–acceptor interface and charge collection efficiency⁶¹. The simulations performed using Eq. (6) with standard physical constants (Boltzmann constant = 8.713304 × 10⁵ eV/K, elementary charge = 1, temperature = 298 K), reveal systematic variations in FF across the molecular series that correlate with their normalized V_oc values (43.69–91.32 eV) summarized in Table 4. The engineered molecules demonstrate FF values compared to the reference compound, following the progression: REF (0.9402) > G5 (0.9205) > G8 (0.9141) > G6 (0.9078) > G4 (0.9059) > G1 (0.9045) > G3 (0.89.68) > G7 (0.8951) > G2 (0.8927).
This trend aligns precisely with electronic structure analyses, where systems exhibiting optimal HOMO–LUMO alignment and balanced charge transport properties achieve the highest FF values.
G7 emerges as the standout performer, (0.8951) reflecting exceptional interfacial properties and minimized recombination losses. This performance superiority stems from three synergistic factors: (1) favorable energy level alignment (− 5.77/− 4.04 eV), (2) efficient charge separation evidenced in TDM analysis, and (3) low reorganization energies (λₑ = 0.0047 eV, λₕ = 0.0061 eV). The strong correlation between FF and normalized V_oc, visualized in 3D plot given in Fig. 15.
3D visualization of FF and V_oc for the REF and designed molecules G1–G8.
Power conversion efficiency is the paramount benchmark for evaluating the photovoltaic capability of a material to convert incident solar radiation into usable electrical energy⁶². This parameter holistically integrates the three fundamental performance characteristics of a solar cell, open-circuit voltage, fill factor, and J_sc), through the fundamental relationship defined in Eq. (5). Our theoretical estimation of these parameters under standard AM 1.5G illumination (100 mW/cm²) demonstrates a profound and universal enhancement in the projected performance of all newly designed molecular systems (G1–G8) compared to the reference structure (REF).
The calculated PCE values, detailed in Table 7, provide a compelling validation of our molecular design strategy. The reference molecule is estimated to have a PCE of 12.0%. This performance is significantly surpassed by every engineered derivative, with efficiencies ranging from 22.8% to a remarkable 37.0%. This substantial leap underscores the effectiveness of terminal group engineering in tailoring key optoelectronic properties. The hierarchy of performance is not dictated by a single parameter but by a complex interplay between them. For instance, molecule G7 achieves the highest estimated J_sc value (31.2 mA/cm²), a direct consequence of its superior light-harvesting efficiency (LHE = 0.998) and redshifted absorption, which enables capture of a broader range of solar photons. However, its overall PCE of 32.4% is tempered by its comparatively lower V_oc of 1.16 V.
In contrast, the top-performing system, G5, achieves the highest PCE of 37.0% through an optimal balance of all three parameters. It possesses the second-highest V_oc (1.66 V) of the designed set, driven by a favorable HOMO–LUMO alignment, coupled with an excellent fill factor (0.9205) indicative of efficient charge transport and collection, and a very high estimated J_sc (24.2 mA/cm²) due to its strong oscillator strength. This relationship between high voltage, good current, and minimal electrical losses is the hallmark of any efficient photovoltaic material. Other standout performers include G6 (31.1%) and G4 (30.6%), which also exhibit this balanced combination of properties.
The progression of PCE values across the series (22.8–37.0%) aligns with trends from our computational analyses, including enhanced charge separation, improved light absorption, and favourable charge transport dynamics. This correlation confirms that our approach provides a robust theoretical framework for molecular screening. The designed architectures, particularly G5 (PCE = 37.0%) and G7 (PCE = 32.4%), demonstrate exceptional promise, significantly outperforming the reference system (PCE = 12.0%) and experimentally reported acceptors ITIC (11.41%) and Y6 (2.4%). Future work must focus on the synthesis of these leading candidates and experimental validation of their performance. Further optimization could explore these structures in ternary blends or tandem cells to push efficiencies beyond the values predicted here.
The progression of PCE values across the series shows a strong correlation with the positive trends observed in these foundational computational analyses, including enhanced charge separation, improved light absorption, and favourable charge transport dynamics. This consistent alignment confirms that our screening approach provides a reliable theoretical framework for identifying promising candidates. The designed architectures, particularly G5 and G7, demonstrate exceptional promise for application in high-performance organic photovoltaics. To gain deep insight into the practical donor–acceptor charge transfer characteristics, a representative complex between the top-performing acceptor G7 and the well-known donor polymer PM6 was investigated. G7 was selected due to its potent electron-withdrawing geometry and desirable optoelectronic characteristics. The PM6/G7 complex was also optimized using the D3-B3LYP/6-311G(d,p) level of theory. As shown in Fig. 16, the optimized geometry reveals stable interfacial contacts between the donor backbone of PM6 and the electron-deficient unit of G7, facilitating efficient charge transfer. Future work must focus on the synthesis of these leading candidates and the experimental validation of their performance in fabricated devices.
Optimized PM6/G7 complex at D3-B3LYP/6-311G(d,p) level.
The analysis indicates that LUMO is concentrated over the G1 acceptor unit, while the HOMO is primarily located along the conjugated backbone of PM6. The potential for effective photoinduced charge transfer from PM6 to G7 is highlighted by the distinct spatial separation of HOMO and LUMO concentrations. The HOMO–LUMO distribution pattern, shown in Fig. 17, clearly demonstrates that hole density resides on PM6, while electron density is shifted toward G7. Such orbital localization favors exciton dissociation, minimizes charge recombination, and establishes a suitable pathway for charge separation at the donor–acceptor interface.
HOMO and LUMO distribution on PM6/G7 complex at D3-B3LYP/6-311G(d,p) level.
The systematic variation in end-group structures across the G1–G8 series reveals profound structure–property relationships governed by the specific chemical nature of the functional groups. The theoretical PCE is determined by a complex interplay between the electron-withdrawing strength, conformational rigidity, and conjugation length imparted by each unique end-group, which directly modulates the fundamental processes of charge generation, recombination, and transport.
This analysis establishes two distinct design paradigms exemplified by G5 and G7. The outstanding predicted PCE of G5 (37.0%) is driven by its exceptional Voc (1.66 V) and high FF (0.9205). The high Voc is a direct result of its optimal energy level alignment. G5 possesses the highest-lying LUMO level (− 3.54 eV, Table 3) among the high-performing candidates, which minimizes energy loss. This is facilitated by its specific end-group, which provides substantial electron-withdrawing capability without overly deepening the LUMO, as reflected in its small Δ_LUMO value of − 0.07 eV (Table 6). Concurrently, high FF in G5 points to efficient charge transport, underpinned by its well-balanced electron and hole hopping rates (K_e and K_h, Table 3) and moderate reorganization energies. This balanced charge-dynamic suggests a lack of significant transport bottlenecks, leading to the efficient extraction of both carriers.
In contrast, G7 achieves a high theoretical PCE (32.4%) through a different mechanism, dominated by an exceptionally high predicted J_sc (31.2 mA/cm²). This is facilitated by its exceptionally low band gap (1.73 eV, Table 3) and a strong, red-shifted absorption peak at 803 nm (Table 4). The origin of this superior light-harvesting can be traced to R₂ end-group of G7, which features a fused, planar heterocyclic architecture incorporating strong electron-withdrawing nitro (–NO₂) and cyano (–CN) groups. This structure enables extensive π-conjugation and intense intramolecular charge transfer, yielding a near-unity LHE (0.998). However, the powerful electron-withdrawing nature of these groups, attributable to their combined inductive (− I) and mesomeric (− M) effects, also results in a deeper LUMO level (− 4.04 eV) for G7 compared to G5. This deeper LUMO is the fundamental reason for its lower Voc (1.16 V) and larger Δ_LUMO (0.43 eV), indicating energy loss during charge generation.
The influence of specific chemical motifs is further evident across the series. The potent − I/− M effects of the nitro group in G2, G3, and G4 substantially stabilize their FMOs. Conversely, the hydroxyl (–OH) group in G6 introduces a unique push–pull character, being inductively withdrawing (− I) yet resonantly donating (+ M), which fine-tunes intramolecular charge transfer. Consequently, the MEP surfaces visually corroborate this, with the most potent acceptor end-groups exhibiting the most pronounced electron-deficient regions.
In conclusion, the regulatory effect of end-groups is intrinsically governed by their distinct electron-withdrawing strength and ability to promote planarity. The G5-type strategy focuses on maximizing Voc and FF through careful energy level tuning and balanced transport, while the G7-type strategy prioritizes maximizing J_sc via extreme band gap narrowing enabled by strongly withdrawing, planar, and conjugated structures. This nuanced understanding provides a foundational principle for the targeted molecular engineering of non-fullerene acceptors. While G7 demonstrates exceptional light-harvesting capability (LHE = 0.998, λ_max = 803 nm) and the highest predicted short-circuit current (J_sc = 31.2 mA/cm²), its relatively lower open-circuit voltage (V_oc = 1.16 V) presents a classic trade-off in solar cell design. We explicitly analyzes this and positions other candidates for specific applications:
G7 exhibit unparalleled absorption in the near-infrared region (803 nm) makes it an ideal candidate for use as the bottom-layer acceptor in tandem solar cells, where its ability to harvest long-wavelength photons that penetrate through the top cell would be maximized. Its high J_sc would also be beneficial in low-light or diffuse light conditions.
G5 achieves the highest predicted PCE (37.0%) due to its superior balance of all parameters. It possesses the highest V_oc (1.66 V) among the designed systems, which is critical for minimizing energy losses and achieving high efficiency in standard single-junction devices. Coupled with an excellent FF (0.9205) and a very high J_sc (24.2 mA/cm²), G5 represents the most well-rounded candidate for general-purpose, high-performance OSCs.
G4 and G6 offer a compelling balance and combine very high LHE (0.995), excellent J_sc (~ 25 mA/cm²), and good V_oc (1.34 V and 1.37 V, respectively) with more moderate LUMO energy levels (− 3.86 eV and − 3.83 eV). This positions them as potentially more manufacturable and stable alternatives to G7, whose very deep LUMO (− 4.04 eV) might lead to interfacial energy barriers or stability issues in a real device. Their properties suggest they would be easier to integrate into robust device architecture without sacrificing performance.
The calculated V_oc values (using the PM6 donor) show that G2 and G3 have the lowest V_oc (1.13 V and 1.19 V) in the designed series due to their very deep LUMO levels. This makes them less suitable as acceptors. However, their deep HOMO levels could make them interesting candidates for exploration as donor materials in a different device context, a point we now mention as a direction for future work.
Despite these promising results, there are certain limitations of this study, experimental validation leading to device fabrication is required to verify the higher highest PCE of G7. Similarly certain polar and non-polar solvents could modify the optoelectronic properties and PV responses of studied materials.
This computational study successfully designed eight novel acceptor molecules (G1–G8) by strategically modifying a thiophene-thiadiazole core with various terminal acceptor groups. DFT and TD-DFT calculations at the D3-B3LYP/6-311G(d,p) level, benchmarked from experimental data, demonstrated that all engineered molecules exhibit significantly reduced bandgaps, ranging from 1.73 to 2.00 eV compared to the reference molecule E_gap 2.33 eV, and bathochromically shifted absorption maxima between 688 and 803 nm. Among the designed systems, G7 emerged as the most promising candidate due to its optimal bandgap of 1.73 eV, outstanding light-harvesting efficiency of 0.998, and minimized reorganization energies that facilitate efficient charge transport. The strategic incorporation of strong electron-withdrawing terminal groups enhanced intramolecular charge transfer through extended π-conjugation, as confirmed by transition density matrix analysis. While G7 shows exceptional promise for its current generation capabilities, other candidates like G5 offer a superior balance of high open-circuit voltage and efficiency. These findings validate terminal group engineering as a powerful strategy for tailoring optoelectronic properties at the molecular level. The study provides strong theoretical foundation and specific design rules for developing high-performance organic photovoltaic materials, with G7 and G5 identified as prime targets for subsequent experimental synthesis and device integration.
No datasets were generated or analysed during the current study.
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The researchers would like to thank the Deanship of Scientific Research, Qassim University for funding the publication of this study.
The authors declare that no funding was received to support this research.
Institute of Chemistry, The Islamia University of Bahawalpur, Baghdad-ul-Jadeed Campus, Bahawalpur, Pakistan
Abdul Ghaffar, Muhammad Arif Ali & Muhammad Arshad
Department of Chemistry, The Government Sadiq College Women University, Bahawalpur, 63100, Pakistan
Afifa Yousuf
Department of Environment and Natural Resources, College of Agriculture and Food, Qassim University, 51452, Buraidah, Qassim, Saudi Arabia
Muhammad Zahid Qureshi
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Net-metering to net-billing widens tariff gap as consumers face Rs10 buyback
Solar power generation has proven to be socially acceptable, economically viable and environmentally friendly. It is therefore highly sustainable for a country like Pakistan where the power supply gap is always increasing.
Solar can reduce our heavy reliance on expensive imported fossil fuels. It can also mitigate high electricity tariffs and bridge the existing 10% energy supply gap for the household sector.
Pakistan embarked on solar power after developing its net metering policy almost a decade ago. The policy proved highly successful in terms of increased solar panel installation amid tariff reduction by the National Electric Power Regulatory Authority (Nepra) from Rs27 per unit to Rs10 per unit.
The prime objective of the solar policy was not only self-reliance in power but shifting the power production cost from the government to consumers themselves. Consumers installed the solar panels and the government is buying this power without spending any upfront cost on constructing dams for hydropower or heavy spending on fossil fuel imports, including their recurring costs.
This has proven to be the cheapest source of power for the government. But instead of encouraging it, the government has slashed its price from Rs27 to Rs10 – a 65% reduction. All over the world, solar and other such indigenous sources of power are highly encouraged and rewarded.
Solar power provides an alternative to grid electricity. It offers a decentralised energy solution, which is especially important for remote areas in regions like the northern mountainous parts of Khyber-Pakhtunkhwa, Balochistan, Sindh and rural Punjab, where access to the grid is limited.
Being a clean and renewable source of power, solar helps reduce Pakistan's carbon footprint and dependence on polluting power generation. It also helps earn carbon credits for replacing power generated from fossil fuels that emit greenhouse gases.
Pakistan can achieve its committed power and energy generation from renewable sources by 2030 under the Nationally Determined Contributions (NDCs) to the United Nations Framework Convention on Climate Change (UNFCCC). Solar power has greatly helped Pakistan fulfil its obligation under the UNFCCC and comply with the targets of various Sustainable Development Goals. Solar offers energy security as an indigenous source and is equally important for fulfilling our international commitments.
Federal Power Minister Awais Leghari has emphasised the crucial role of solar energy in the country's energy mix, highlighting a strategy that leans heavily on indigenous resources. He said Pakistan is producing 74% indigenous power generation and has therefore increased its reliance on local sources – including solar, wind, hydro, nuclear and local coal – to strengthen energy security.
He further said the government aims to raise the share of indigenous energy to over 96%, moving toward a 90% clean energy mix by 2034. While this has been advantageous for consumer self-consumption, it has created challenges for the national grid. This has prompted the government to shift from net-metering to a net-billing system, which offers lower rates (about Rs10 per unit) to producers compared with previous rates.
The debate is that the government buys electricity at a low rate and sells it at a significantly higher rate – for example, Rs10 versus Rs60 per unit. While the exact figures vary based on generation type (hydro versus thermal) and time, the core issue is the widening gap between generation cost and the end-consumer tariff.
A large portion of the bill goes towards "capacity charges" paid to independent power producers (IPPs). These charges are paid even if the power is not produced or utilised. The government charges consumers to cover its failure to manage the financial crisis, including the interest on loans taken to manage circular debt. Consumers also pay for "line losses" which include electricity theft and inefficiencies in the grid.
The government's financial sustainability of the power sector clashes with the public's ability to pay. This has led to widespread protests and excessive inflation on electricity bills. The core conflict is that the government is locked into long-term, high-cost agreements with IPPs, often referred to as a "take-or-pay" model, where the burden is passed on to the consumer.
This is a vicious cycle that the government is now trapped in. It is greatly affecting all sectors of the economy and the public at large. Unfortunately, the public sector's resources are not limited, nor do the professionals in this sector possess any dynamism or innovative thinking to address these ever-increasing issues scientifically. The solutions so far are based on an unjust and unacceptable foundation.
Until a justified solution and mechanism is devised, these issues may persist. Both the government and society may continue to feel their impacts. Consequently, the public is very much against the existing tariff and taxes loaded into their electricity bills.
On the other hand, the government has completely ruined the solar net metering policy. It is pushing the public that already has solar panels to switch to off-grid solar power generation. This will lead to irregular and unrecorded power generation that may not give any benefit to the public in the longer term.
The government, on one hand, is favouring IPPs at the cost of power consumers. It is ignoring international commitments to various legally binding conventions like UNFCCC, Agenda 2030, INDCs and SDGs. This may negatively impact Pakistan's image at the international level. It will also affect self-reliance in power at no upfront and recurring cost on power production.
THE WRITER HOLDS A PHD AND A MASTER'S DEGREE IN FOREST MANAGEMENT. HE HAS SERVED THE K-P FORESTRY DEPARTMENT AS DIVISIONAL FOREST OFFICER AND HAS ALSO SERVED AS HEAD OF PAK-EPA
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July was the month with the highest solar photovoltaic power generation in Spain in 2024. In that month, the country’s solar PV production amounted to 5.8 terawatt hours. January had the lowest production at 1.9 terawatt hours.
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Scientific Reports volume 16, Article number: 8611 (2026) Cite this article
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Tandem perovskite solar cells (TPSCs) have attracted considerable attention due to their potential for achieving high efficiency, low production cost, and excellent scalability. In this study, a two-terminal monolithic tandem solar cell combining a lead-free Methylammonium Bismuth Iodide ((CH₃NH₃)₃Bi₂I₉, abbreviated as MBI) perovskite top sub-cell (Eg = 1.9 eV, absorber thickness 320 nm) and an thin CIGS bottom sub-cell (Eg = 1.68 eV, absorber thickness 500 nm) was designed and comprehensively optimized using Silvaco Atlas TCAD. To eliminate the use of scarce and expensive indium, fluorine-doped tin oxide (FTO) was deliberately selected as the front transparent conductive oxide (TCO) instead of the conventionally used indium tin oxide (ITO). The superior thermal stability of FTO (stable up to 600 °C versus 350 °C for ITO), its higher tolerance to physical abrasion, and its direct deposition capability on glass without an intermediate passivation layer make it a more robust and cost-effective choice for large-scale manufacturing and for subsequent high-temperature processing steps required in CIGS deposition. The standalone optimized MBI-perovskite single-junction cell using FTO achieved a power conversion efficiency of 15.13%. After individual calibration and optimization of both sub-cells, the fully coupled two-terminal monolithic tandem device delivered a realistic and reproducible efficiency of 35.67% (Voc = 4.53 V, Jsc = 29.23 mA/cm², FF = 77.88%) under standard AM1.5G illumination. These results highlight the feasibility of high-performance, indium-free, lead-free perovskite/CIGS tandem architectures.
Fossil fuels, the primary energy source currently in use, have terrible environmental effects and contribute to global warming through the production and increase of greenhouse gases. One of the most difficult problems of the 21 st century is combating global warming and keeping the rise in global temperature to 2 degrees Celsius. The demand for energy has grown significantly over the past century, with projections indicating that by 2050, the average energy demand will reach 28 TWh and by 2100, it will reach 46 TWh¹.
Photovoltaics and other clean energy devices have great potential in the commercial market because of their many benefits, such as producing power from sunshine without pollutants and requiring little maintenance over the long run². Crystalline silicon (Si) solar cells in all their varieties currently rule the photovoltaic industry. The efficiency of silicon solar cells can exceed 25%³. With a simulated efficiency of roughly 26.4%, copper indium gallium selenide (CIGS), which has a thin window layer of tungsten disulfur (WS2), is regarded as one of the most promising and effective thin-film solar cells⁴. With an efficiency gain of over 30% in recent years, perovskite solar cells have also demonstrated rapid progress and created new opportunities for photovoltaics⁵. Perovskite cells perform well because of their broad absorption spectrum, extended emission length, high open circuit voltage, bandwidth adjustment, and low recombination rate.
The outstanding inherent optical characteristics and lower temperature processability of lead-free metal halide perovskites make them ideal light harvesting materials for photodetector applications. For non-conductive hole-free perovskite solar cells, a novel lead-free and air-stable absorber called methylammonium iodide bismuthate ((CH₃NH₃)₃Bi₂I₉) (MBI) was presented⁶.
Only photons with energies greater than or equal to the material’s energy gap may be absorbed by this type of solar cell, which limits its effectiveness. Additionally, a portion of the incident spectrum is lost, and even higher energy photons are lost. Heat is the result of the energy loss. Because tandem (multi-junction) photovoltaic systems are made up of sub-cells that individually absorb distinct wavelengths of the incident spectrum, they can get around this restriction⁷.
To absorb light across the solar spectrum, multi-junction solar cells made of various compositions use multiple layers of semiconductors with varying band gaps and lattice constant matching. Consequently, these cells can convert solar energy into electricity in a wide range of situations⁸. In comparison to the subsequent layers, the first one has the highest band gap energy, allowing it to absorb photons with a high energy and a high frequency. The following layers will absorb lower-energy photons because their gap energies are smaller. When contrasted with single-junction cells, this arrangement improves the solar cell’s efficiency⁹.
Multiple materials with varying band gaps are grown on top of one another in tandem solar cells. A variety of solar spectrum wavelengths can be absorbed by each semiconductor layer, which is arranged as a distinct layer inside the overall cell structure, and transformed into electrical energy. In order to absorb the portion of the spectrum with more energy, namely short wavelengths, the semiconductor sub layers can be stacked so that the first layer of that semiconductor has a larger energy gap than the other semiconductors used in the entire structure¹⁰. Higher wavelength, lower energy waves travel through the structure’s initial layer and strike the bottom layers, which are composed of semiconductors with smaller energy gaps, where they are absorbed¹¹.
To enhance absorption in the absorber layers and minimize optical losses in tandem solar cell stacks, effective light control is important¹². To achieve integrated tandem solar cell functionality, it is necessary to use electrodes that are both transparent and conductive. One type of transparent and conductive electrode that is commonly employed is transparent conductive oxides (TCOs), which have excellent electro-optical characteristics and are easy to produce¹⁷. Indium tin oxide (ITO) is one of the most popular TCOs because of its excellent stability and electro-optical characteristics. Sputtering is suitable for industrial-scale applications and is frequently used as the deposition technique for TCOs. However, because undesirable optical features—like higher parasitic absorption with increasing free carrier density—are linked to the desired electrical qualities, careful sputtering process optimization is required. Moreover, the possibility of destroying the organic layers and the underlying perovskite layer by altering their chemical bonding makes direct sputtering of TCOs onto perovskite solar cells difficult^13,14,15. A typical strategy to lessen the possibility of sputtering damage is to include a buffer layer beneath the TCO. One example of this is ALD-deposited SnO_x in the p-i-n perovskite solar cell stack¹⁶.
Recent advances in tandem solar cells have shown that this technology has significant potential to overcome the Shockley–Queser limit by splitting the spectrum between upper cells with a wide band gap (WBG) and lower cells with a narrow band gap. Optical simulations based on the transfer matrix method for perovskite/CI(G)S structures have identified sources of unwanted absorption and have shown that efficiencies of around 30% can be achieved by optimizing the transport layers¹⁸. In experimental work, the addition of BaTiO₃ to the electron transport layer in α-FAPbI₃ phase photovoltaic cells has improved charge separation and achieved efficiencies of 11%¹⁹. Silvaco TCAD modelling for all-thin perovskite/c-Si stacks has also shown that n–p fusion structures without charge transport layers can achieve efficiencies of 36.37% under optimal conditions; this highlights the essential role of internal electric fields²⁰. Furthermore, optimizing the electro-optical properties of ITO electrodes produced by direct current sputtering in perovskite/silicon back-to-back cells was able to improve the Jsc uniformity (from 19.3 ± 0.4 to 19.8 ± 0.2 mA cm2) and the efficiency from 22% to 25%²¹.
Lead-free perovskites such as CsSn(I₁₋ₓBrₓ)₃, which can be tuned, have been simulated for single-junction cells and, together with a CdS electron transport layer, have been able to achieve efficiencies of 18.5%²². In SCAPS-1D modelling of Bi₂FeCrO₆ cells, an efficiency of 7% was reported for a 150 nm absorber layer, and it was shown that by reducing the defect density to less than 10¹³ cm⁻³, efficiencies higher than 10% can be achieved²³. Lead-free CsSn₀.5Ge₀.5I₃/CIGS inorganic tandem structures have also achieved efficiencies of 38.39% by tuning the layer thickness²⁴. In another example, CIGS/CIGS tandems optimized using Silvaco-Atlas and silver electrodes with thicknesses of 0.17/6.3 μm achieved an efficiency of 27.12%²⁵. Lead-free all-perovskite tandems (MGeI₃/FASnI₃) also achieved an efficiency of 30.85% by matching the thicknesses of 983 and 1600 nm for proper current delivery²⁶. Furthermore, replacing CIGS with GeTe in perovskite/GeTe tandems significantly increased the efficiency, reaching more than 41%²⁷.
Numerical modelling of perovskite/u-CIGS tandems, validated with experimental data, showed that optimizing the thickness of the CH₃NH₃PbI₃ layer and improving the antireflection coatings can increase the single-junction efficiency to 16.13%, an achievement that increases the efficiency to 20.84% ​​in the tandem configuration²⁸. This body of research emphasizes the necessity of using indium-free TCOs such as FTO, employing realistic defect modelling, and also accurate current matching between subcells to achieve scalable efficiencies above 30% in lead-free MBI/CIGS tandems, and indeed forms the main motivation for presenting a comprehensive electro-optical optimization approach in this work. In order to overcome the fundamental Shockley-Queser limitation in single-junction cells and achieve higher efficiencies without significantly increasing the manufacturing cost, tandem two-junction architectures that combine wide- and narrow-bandgap subcells have been introduced as a leading and attractive solution²⁹. By spectrally splitting sunlight and reducing thermal losses through the use of materials with different band gaps, two-terminal tandem cells can provide superior performance and cost-effectiveness compared to conventional single-junction cells or more complex multi-junction structures (more than two subcells)²⁹. CIGS thin-film technology also has the potential to achieve higher efficiencies, lower processing costs than crystalline silicon, and is a suitable option for ultrathin and flexible applications, with its high absorption coefficient, direct band gap, and ability to be manufactured at very low thicknesses³⁰. On the other hand, metal halide perovskites have been introduced as the best options for wide bandgap subcells in tandem architectures due to their bandgap tunability, long carrier diffusion length, high defect tolerance, and the possibility of fabrication at low temperatures by low-cost solution or evaporation methods⁵.
This work presents the creation of a TiO₂ stack film with enhanced electrical carrier contact and an electro-optical Fluorine-doped Tin Oxide (FTO) thin film with low sheet resistance and low absorption. Tandem perovskite solar cells with MBI-CIGS have the optimal films. A PN junction structure is utilized in this structure to enhance the effectiveness of a lead-free perovskite absorber material. Additionally, the lower layer utilizes an improved CIGS thin film to tailor the energy bands and absorber material thickness, enhancing the performance of this solar cell. A metallic substance composed of gold on ZnO has also been employed to link the upper perovskite and bottom CIGS solar cell layers. We were able to attain a respectable efficiency of 35.67 at an open circuit voltage of 4.32 V by utilizing these advancements in the tandem solar cell structure. Additionally, the FF coefficient was enhanced to 80% with the assistance of adjusting the CdS thickness.
The suggested approach and the advancements of each kind of solar cell are then examined and addressed in Sect. 2. Section 3 reviews the simulation results of each cell separately and together. Section 4 concludes with the presentation of the simulation findings.
All simulations in this study were performed using Silvaco Atlas (version 5.34.0.R) under the standard AM1.5G spectrum (ASTM G173-03, 100 mW/cm²). The fully coupled electro-optical modelling approach implemented the Luminous beam propagation method (BPM) with full complex refractive indices (n, k) for accurate to 5 nm wavelength steps between 300 and 1200 nm, enabling simultaneous solution of Poisson’s equation, drift-diffusion carrier transport, and continuity equations across the entire monolithic two-terminal stack in a single deck^20,25,28. No external transfer-matrix method or post-processing script was used; optical generation rate G(x,λ) was calculated directly inside Atlas and automatically inserted into the drift-diffusion solver.
The device structure was defined as a continuous monolithic stack (lower than 2.5 μm total thickness) consisting of glass/FTO (200 nm)/TiO₂ (30 nm)/MBI-perovskite (optimized 420 nm)/Spiro-OMeTAD (150 nm)/IZO (60 nm)/SnO₂ (8 nm)/ultra-thin Au (5 nm)/ZnO: Al (100 nm)/intrinsic ZnO (50 nm)/CdS (50 nm)/u-CIGS (500 nm, Ga/(In + Ga) = 0.4)/Mo back contact. Current continuity was strictly enforced by defining only two electrical contacts (front FTO and rear Mo), ensuring true two-terminal tandem behaviour without artificial external current matching. Material parameters, defect models (SRH, Auger, interface defects Dit = 10¹⁰–10¹⁵ cm⁻²), mobility models (Poole-Frenkel for organic layers, concentration-dependent for inorganic layers), and complex refractive indices were adopted directly from experimentally validated sources^6,42as provided in Appendix A (Supplementary Material) and Table 2. Current matching was achieved by iterative optimization of the MBI-perovskite absorber thickness while keeping the u-CIGS thickness fixed at 500 nm, until the integrated photocurrent of both sub-cells differed by less than 0.1 mA/cm².
The optical and electrical interactions between the layers make it challenging to design an appropriate model to replicate the tandem arrangement. We initially only looked at the two cells in order to create a realistic representation. A titanium dioxide (TiO₂) layer serves as the electron transport layer (ETL) in the tandem perovskite-based component’s traditional planar architecture. Additionally, it employs a Spiro-OMeTAD layer as the hole transport layer (HTL), which guarantees improved photostability and carrier mobility. The top sub-cell employs a lead-free methylammonium bismuth iodide ((CH₃NH₃)₃Bi₂I₉, MBI) perovskite absorber with an optical bandgap of 1.9–1.95 eV. The single-junction structure used for initial calibration and optimization is glass/FTO (200 nm)/compact-TiO₂ (30 nm)/MBI-perovskite (100–600 nm)/Spiro-OMeTAD (150 nm)/Au, identical to the experimentally reported device by Shah et al.⁶. In that validated structure, the optimized MBI-perovskite cell delivered an open-circuit voltage of Voc = 1.02–1.05 V (average 1.03 V) under standard AM1.5G illumination, which is the highest Voc reported to date for a solution-processed (CH₃NH₃)₃Bi₂I₉-based solar cell⁶. This relatively high Voc (for a lead-free bismuth halide perovskite) originates from the low bulk defect density (Nt ≈ 10¹⁴cm⁻³) and effective passivation of TiO₂/MBI and MBI/Spiro-OMeTAD interfaces achieved in the reference device. All subsequent tandem simulations inherit the same layer sequence, doping concentrations, defect densities, and interface recombination velocities reported in ref⁶. for the top sub-cell. The activation energy and operating temperature have an impact on the mobility of carriers in the absorber layer (CH₃NH₃)₃Bi₂I₉, which is crucial to overall efficiency. In this work, we first used SilvacoTCAD to simulate the SiO₂/FTO/TiO₂/(CH₃NH₃)₃Bi₂I₉/Spiro-OMeTAD/Gold setup. To ensure that the model accurately reflects the impacts of perovskite thickness, validation data was also used for calibration. Step two involved running simulations of the u-CIGS solar cell independently and then re-calibrating the model using validation data. In reference²⁵, the optical and electrical properties of thin CIGS (u-CIGS) solar cells with a 500 nm absorber thickness are studied layer by layer; this work serves as an inspiration for the suggested model. There is a good agreement between the simulation and measurement data. Ultimately, a successful simulation of the two-terminal perovskite/u-CIGS tandem device was obtained, with an efficiency of up to 30.84%. The greatest option for low-cost solar cells is thought to be tandem solar cells, which have efficiencies of about 30%. The connection between the cells in this work is made using a metal electrode structure on a ZnO substrate. Because the best metal electrode has been studied, gold is employed in this investigation. Due to the extremely thin thickness of the metals and materials, this results in a very slight cost savings, but it also raises the expense of designing the tandem structure. The tandem solar cell’s characteristics will be improved by the installation of a metal connector.
The top and bottom sub-cells are simulated separately in order to examine the performance of tandem solar cells. Electrical and optical losses at each contact are disregarded, and the ohmic junction is taken to be perfect, in accordance with the commonly used method in tandem cell simulation^7,12.
Figure 1 shows a schematic cross-sectional view of the simulated integrated two-terminal MBI-perovskite/u-CIGS tandem solar cell. No additional anti-reflection coating or surface texturing is applied in this structure; therefore, all reflection losses are attributed to the smooth front surface of the FTO. The reported reflection is solely due to the air/FTO interface, which accounts for about 10–12% in the visible region, and no other ARC or texture is considered. By varying the thickness of the upper subcell and xing the thickness of the lower subcell, the current matching condition is accomplished. Figure 1 depicts the construction of the CIGS lower subcells with Cu(In_1-x Ga_x)Se₂ absorber material, the perovskite (MBI), and the perovskite ((CH₃NH₃)₃Bi₂I₉) top subcells.
The electron transport layer (ETL) in the perovskite subcell (MBI) is titanium oxide (TiO₂), the hole transport layer (HTL) is HAT₆ Hexakis(hexyloxy)triphenylene, and the active layer is (CH₃NH₃)₃Bi₂I₉.
Fluorine-doped tin oxide (FTO) is used as a substitute for indium tin oxide (ITO). As a transparent conductive electrode, FTO allows photons to penetrate the cell while transporting the generated electrons to the external terminals. A comparison between ITO and FTO glass is presented in Table 1³¹.
On the other hand, the IDL interface defect layer is employed to form a tandem device between the cell’s ohmic junction layers. Prior to the absorption layer, zinc oxide (ZnO) and cadmium sulfide (CdS) are utilized in the CIGS subcell. Table 2 lists the top and bottom subcell simulation parameters, which are based on the models used in review papers^{18,19,20,21,22,23,24,25,26,27,28}.
Cross-section of perovskite (MBI)/CIGS solar cell.
According to (1)³², S(λ) (W/m²) is the power density of the optical spectrum that is transmitted from the upper subcell to the lower subcell.
where AM 1.5 is the incident spectrum, x is the layer number, n is the total number of subcell layers, d is the thickness of each layer (cm) and alpha is the absorption coefficient (cm*1) which is for each material (with the prefactor A alpha) with Eq. (2) given by³³;
Where Eg is the energy gap of the material (eV), h is the Planck constant (eV.sec), and V is the spectral frequency.
A modified numerical method based on the idea put out by paper³⁴is suggested in this section. In order to optimize the thickness (TS) of the top sub-cell for current matching and maximum efficiency, the suggested algorithm modification employs two phases: Thick First Search (TFS) with a fine step of 5 nm and Thin Coarse Search (TCS) with a period step of 50 nm. In Fig. 2, the suggested algorithm’s flowchart is displayed. Based on the final thickness, all connection performance metrics are computed at each stage. Because fewer computations are required overall, this suggested improvement results in a quicker response for determining the ideal upper subcell thickness for the tandem-bonded cell. Additionally, by reducing the second phase step to 5 nm, it is able to determine a more precise optimum thickness. The overall thickness of the tandem structure should not be greater than 50 μm, but the bottom subcell layer should be thick to absorb as many of the transmitted photons from the upper subcell as feasible. This was not taken into account throughout the optimization process. To guarantee free charge transfer to the electrodes, a fictitious diffusion length is incorporated³⁴.
Flowchart of the upper subcell thickness optimization technique for tandem-junction solar cells (η₁ = best thickness found in coarse sweep (50 nm step); η₂ = best thickness found in fine sweep (5 nm step); hopt = final optimal thickness; ΔJ = |Jsc, top − Jsc, bottom|.).
To achieve strict current matching in the two-terminal monolithic tandem device (Jsc, top = Jsc, bottom ± 0.1 mA/cm²), the thickness of the MBI perovskite top absorber (Ttop) was systematically optimized while keeping the thin CIGS bottom absorber fixed at 500 nm.
A fast and accurate two-phase iterative algorithm was developed (flowchart in Fig. 2):
Phase 1 – Thick First Search (TFS): A coarse thickness sweep from 100 nm to 800 nm with 50 nm steps is performed. At each step, the short-circuit current densities of the top (Jsc, top) and bottom (Jsc, bottom) sub-cells are extracted from the fully coupled tandem simulation. The thickness that yields the minimum |Jsc, top − Jsc, bottom| is identified and denoted η₁.
Phase 2 – Fine Local Search (FLS): Starting from η₁, a fine sweep is performed in both directions (± 150 nm around η₁) with 5 nm steps. The new thickness that minimizes |Jsc, top − Jsc, bottom| is denoted η₂. If the improvement in current mismatch is greater than 0.05 mA/cm², an additional very fine sweep (± 20 nm around η₂, 1 nm step) is executed to obtain the final optimal thickness hopt.
This hierarchical approach typically converges in fewer than 60 total simulations while achieving sub-0.1 mA/cm² precision. The final optimized top absorber thickness was determined to be 420 nm, delivering perfectly current-matched Jsc = 19.8 mA/cm² for both sub-cells.
Silvaco-Atlas was used to fully design the structure of the solar cell. Using organic and inorganic charge transport layers, we initially created a model of a single-junction perovskite solar cell ((CH₃NH₃)₃Bi₂I₉). The model under investigation is predicated on validation evidence that has been documented in the literature³⁴. Since the front glass of the device serves as the front contact, the first layer is a transparent conductive oxide (TCO). In this simulation, air or vacuum is used in place of that container. FTO conducting glass, which has the structural formula SnO₂ with a work function of 4.7 eV, is among the most popular and affordable conductive glasses made. After the TCO, the electron transport layer (ETL) is a doped titanium dioxide (TiO₂) layer (n-type, Eg = 3.20 eV, χ = 4.21 eV, and Nd = 1 × 10¹⁸ cm⁻³). The hole transport layer (HTL) consists of a Spiro-OMeTAD layer (p-type, Eg = 3.0 eV, χ = 2.2 eV, and Na = 1 × 10¹⁷ cm⁻³) and a perovskite absorber layer (undoped, Eg = 1.9 eV, and χ = 3.9 eV). The structure is completed by a back gold contact (work function = 5.1 eV). Figure 2 displays the measured J-V curves and schematic cross-section of the PSC model under investigation. Figure 2 shows the J-V characteristics of the original model (redline) and the suggested one (black dotted line) under AM1.5 light. The material properties of the structure’s various layers are displayed in Table 2 and were taken from previous research^{18,19,20,21,22,23,24,25,26,27,28}. The final tandem performance parameters are extracted from a single, fully coupled two-terminal simulation enforcing strict current continuity and potential continuity across the entire device, rather than from external addition of independently simulated sub-cell characteristics.
In this work, all optical and electro-optical simulations were performed exclusively using the built-in Luminous module of Silvaco-Atlas (version 5.34.0.R or higher), which solves the full complex-index beam propagation method (BPM) with incoherent multi-beam interference and fully coupled drift-diffusion equations in a single deck. No external transfer-matrix method (TMM) or post-processing script was employed. The complete two-terminal monolithic tandem structure (front FTO through back contact, total near 2.5 μm thickness) was defined in one single structure file with continuous mesh and region numbering. At each wavelength (300–1200 nm, 5 nm step), the Luminous module calculates the complex refractive index-based generation rate G(x,λ) throughout the entire stack, automatically accounting for interference, reflection, parasitic absorption in all layers, and spectral filtering by the top sub-cell. This generation profile is directly inserted into the Poisson and carrier continuity equations, which are solved simultaneously with the drift-diffusion transport model under the Newton–Richardson method using Fermi–Dirac statistics. Because only two electrical contacts (front and back) are defined, current continuity is strictly enforced across both sub-cells and the recombination junction at every bias point, guaranteeing physically rigorous two-terminal tandem behaviour without any artificial external current-matching or separate sub-cell summation.
All simulations were performed using Silvaco Atlas by self-consistently solving Poisson’s equation and the electron/hole continuity equations together with the drift-diffusion transport model. The governing equations are:
where ψ is the electrostatic potential, n and p are carrier concentrations, G is the optical generation rate, R is the net recombination rate, and Jn, p are the current densities. Fermi–Dirac statistics, concentration-dependent lifetime/doping models, and the Newton–Richardson method with Gummel/block iterations were employed for convergence.
Optical generation was calculated internally using the Luminous module with the full complex refractive index (n, k) of every layer and the beam propagation method (BPM) at 5 nm wavelength steps (300–1200 nm) under the standard AM1.5G spectrum (ASTM G173-03, 100 mW/cm²). No external transfer-matrix method was used; the spatially resolved generation rate G(x,λ) was directly injected into the continuity equations.
The following recombination and mobility models were activated according to material type (detailed parameters in Table 2):
Inorganic layers (FTO, TiO₂, CdS, CIGS, ZnO, etc.): SRH, radiative (coefficient B), Auger, band-gap narrowing (Schenk), concentration-dependent mobility (ConMOB), and thermionic emission/tunnelling at hetero interfaces.
Organic/perovskite layers (MBI, Spiro-OMeTAD): SRH, Langevin recombination, radiative recombination, and field-dependent Poole–Frenkel mobility.
Ultra-thin Au recombination junction: thermionic field emission and tunnelling models.
These identical models and parameters were first validated on standalone single-junction MBI-perovskite and thin CIGS cells before being applied to the fully coupled monolithic two-terminal tandem structure. The numerical solution of the above equations and the implementation of the described physical models in Silvaco Atlas follow the standard drift-diffusion framework widely adopted for thin-film and perovskite solar cell simulations^43,44.
The band structure of the interfaces must be the main emphasis in order to manage the interlayer interfaces and create an efficient model. Thermionic diffusion physics governs carrier transport through the TiO₂/Perovskite heterogeneous junction, while a drift-diffusion transition governs the (CH₃NH₃)₃Bi₂I₉/Spiro-OMeTAD interface. There are two Schottky connections between the ITO and gold layers. Both the anode and cathode have fixed work functions of 5.1 eV and 4.7 eV, respectively. Both electrodes’ Schottky characteristics allow surface recombination in the simulation. Inorganic and organic materials were found to have different modes of defects. For both the acceptor and donor traps, it was assumed that the interface defect density (Dit) between the TiO₂/Perovskite and (CH₃NH₃)₃Bi₂I₉/Spiro-OMeTAD materials was 1010 cm⁻². Organic materials allow for the use of Poole-Frenkel and Langevin recombination models²⁸. To facilitate the interchange of charge carriers, singlet, and triplet excitons, the Langevin recombination model is triggered²⁸. Singlet excitons are created by a portion of the absorbed photons and make their way to the interface, where they are further separated by an energy level offset. The contact terminals separate and gather the carriers when they slide down due to the built-in electric fields when they are detached from the singlet. The model statement takes into account the separation²⁸. The Poole-Frenkel mobility model^28,35,36,37is used to determine the carrier mobility based on the permittivity of the organic materials (CH₃NH₃)₃Bi₂I₉ and Spiro-OMeTAD:
Here, E is the electric field, Δ_{n, p} is the activation energy in zero electric field for electrons and holes, β_{n, p} is the electron and hole Poole-Frenkel factor, and µ_{nPF, pPF} (E) are the Poole-Frenkel mobilities and µ_{n0, p0} are the zero-field mobilities for electrons and holes, respectively. The following formula will be used to determine βn, p in Eq. (6)^28,35,36,37:
q is the electron charge, while ε is the permittivity. The physical processes of the organic and inorganic layers must be combined in order to get a precise match with the validation results (simulation results). Due to their dominance in the severely doped ETL layer (n-type, TiO₂), the simulation program takes into account Shockley Read Hall (SRH) and Auger recombination for inorganic materials. Additionally, the concentration dependent mobility (ConMOB) model and the Schenk band gap narrowing (BGN) model were taken into consideration²⁸. The recently enhanced mobility and doping concentration of spiro-OMeTAD material, which can enhance both FF and Voc cell features, respectively, are utilized by the suggested cell. As seen in Fig. 3, current density-voltage (J-V) curves were acquired using the typical AM 1.5G solar spectrum. We’ll take the suggested model for additional research. Figure 4 shows the spectral photocurrent density (mA.cm⁻²) across the optimized standalone MBI-perovskite single-junction cell (FTO/TiO₂/MBI/Spiro-OMeTAD/Au) under AM1.5G illumination. High absorption and high current is observed in the MBI layer for wavelengths below ~ 650 nm (consistent with its 1.9 eV band gap), while longer wavelengths are minimally absorbed, confirming the suitability of the MBI perovskite as a wide-band gap top cell in the tandem architecture.
Figure 5 shows Equilibrium energy band diagram, electric field distribution, and electron/hole concentrations across the calibrated standalone MBI-perovskite single-junction cell under AM1.5G illumination at short-circuit condition. A strong built-in electric field of approximately 1–2 × 10⁵ V/cm is confined almost entirely within the 420 nm-thick MBI absorber layer owing to the p-i-n-like configuration (n-type TiO₂ ETL and p-type Spiro-OMeTAD HTL). This field efficiently separates photogenerated carriers, resulting in electron accumulation (> 10¹⁶ cm⁻³) near the TiO₂/MBI interface and hole accumulation of similar magnitude near the MBI/Spiro-OMeTAD interface. The quasi-Fermi levels for electrons and holes split by ~ 1.18 eV inside the absorber, which is consistent with the obtained open-circuit voltage of 1.21 V. The sharp drop of the electric field in the transport layers and the negligible carrier concentration outside the absorber confirm excellent charge selectivity and minimal recombination losses, validating the reliability of the calibrated single-junction model before its integration into the monolithic tandem structure.
Schematic cross-section and measured J-V curves of the investigated PSC structure.
Spectral photocurrent density (generated current density per wavelength interval) of the calibrated standalone MBI-perovskite single-junction cell (glass/FTO/TiO₂/MBI 420 nm/Spiro-OMeTAD/Au) under AM1.5G illumination for A = 1 m².
The simulation results of the perovskite cell under illumination show the electron/hole concentration and electric field distribution throughout the entire structure.
It is important to pay attention to the back contact’s work function, or the metal used to make it, since it can enhance the cell’s overall performance and design. We determined the PSC’s J-V properties in two scenarios, concentrating on employing gold and silver as the back contact. It has been possible to model the thermionic emission mechanism at the absorber/ETL interface through simulations. At the interface between the perovskite and TiO₂ layers, quantum mechanical reflections and the tunneling effect are also taken into account, enabling the thermionic emission model. With a work function of 4.64 eV, gold produces the best results in terms of FF, according to the simulation findings for the two metals, silver and gold. This proposes using gold instead of silver. Simulation of the standalone MBI-perovskite single-junction cell with different back-contact work functions revealed that gold (φ = 4.64 eV) yields the highest fill factor of 85.3% and efficiency of 15.13%, compared to 81.7% FF (14.2% PCE) for silver (φ = 4.26 eV). This improvement arises primarily from the higher built-in potential and reduced Schottky barrier at the Spiro-OMeTAD/Au interface, which suppresses back-surface recombination and enhances field-assisted carrier collection; consequently, gold was selected as the optimum back contact for both the calibrated single-junction and the tandem device.
The thickness of the perovskite absorber has a significant impact on cell performance. Finding the ideal value for this parameter is therefore necessary for the cell design. The relationship of cell performance for perovskite thicknesses between 100 and 600 nm is depicted in Fig. 6. Voc and FF deteriorate as the thickness of the perovskite increases. Degradation in Jsc is known to occur when the absorber layer is reduced, which can place restrictions on the depletion region^38,39. The short-circuit current density (Jsc) decreases at very low MBI absorber thicknesses (lower than 300 nm) primarily because of incomplete light absorption in the long-wavelength region near the 1.9 eV bandgap (λ = 600–650 nm), where the absorption coefficient of MBI is relatively modest (α = 2–4 × 10⁴ cm⁻¹). Although a thinner absorber also slightly reduces the depletion width, the dominant loss mechanism is optical rather than electrical: a significant fraction of near-band gap photons passes through the layer without being absorbed, leading to lower photocurrent generation, as clearly evidenced by the EQE roll-off in the red/infrared region for thicknesses below 350–400 nm (see Fig. 7). It was discovered that the ideal perovskite thickness was approximately 400 nm, offering a maximum conversion efficiency of roughly 16.13%. The EQE of the models under consideration with varying perovskite material thicknesses is displayed in Fig. 7, with minor differences between the 100 and 600 nm range. Since green/blue photons are nearly all absorbed by the perovskite layer at a thinner thickness, the influence of absorber thickness on the EQE is, as predicted, much more noticeable in the red/infrared portion of the spectrum.
In Fig. 7, the external quantum efficiency (EQE) of the single-junction MBI-perovskite top cell (FTO/TiO₂/(CH₃NH₃)₃Bi₂I₉/Spiro-OMeTAD/Au) was calculated for absorber thicknesses ranging from 100 nm to 600 nm under AM1.5G illumination (300–1200 nm, 5 nm wavelength step) using the fully coupled beam propagation method in Silvaco-Atlas. The complex refractive indices (n and k) were directly extracted from the experimental data reported in the references cited in this study and other relevant references. The optical constants of FTO were taken from¹⁷, the values ​​for TiO₂ from the Silvaco library, and the optical data for (CH₃NH₃)₃Bi₂I₉ from⁶. For Spiro-OMeTAD, the data of Listorti et al. and the calibrated data sets used in^20,28were used. Finally, the standard optical constants of Palik were used for the gold back-bonding layer. Parasitic absorption in the 200 nm FTO layer and reflection at the air/FTO interface (approximately 10–12% in the visible range) were fully included without any artificial anti-reflection assumption. The results reveal that EQE exceeds 85% throughout the 400–600 nm region even at the lowest thickness, while significant enhancement occurs in the 550–650 nm region as thickness increases from 100 nm to 400 nm, beyond which saturation is observed, confirming 400 nm as the optimum absorber thickness for the standalone MBI-perovskite sub-cell.
The effect of changes in MBI perovskite layer thickness on cell performance.
Simulated EQE with different thicknesses of MBI perovskite.
The Silvacoillust tool was used to calibrate a CIGS thin solar cell with the following configuration: ZnO: Al (300 nm)/ZnO (100 nm)/CdS (50 nm)/CIGS (500 nm)/Al₂O₃ (25 nm)/Ag in Fig. 8. The numerical models and physical parameters are identical to those employed in earlier research^38,40. The simulated J-V curves and power characteristics of thin CIGS cells are shown in Fig. 9, which uses a back contact resistance of Rc = 0.1 Ω.cm² to model the series resistance³².
CIGS thin device structure.
J–V curves and power of the proposed thin CIGS model (dCIGS = 500 nm).
The electrical circuit of the u-CIGS cell, which was depicted in this study using ATLAS without shunt resistors, is shown in Fig. 10a. A contact resistor is utilized to simulate the series resistance in the reduced circuit model shown in Fig. 10b³². The complete characteristic equation of the two-diode model under light is obtained from the equivalent circuit and utilizing KVL and KCL:
where q is the electron charge, k is the Boltzmann constant, T is the temperature, J is the measured output current density, JPH is the photocurrent density, and V is the applied voltage. To differentiate the various contributions to the overall current density, each diode is assessed in the proper bias areas in this model. Since the non-ideality factors in CIGS PV cells differ greatly from those in silicon cells, the G/R and diffusion currents may not be entirely separated at first. Diode 1 (D1), which is determined by the current density J₀₁ and the non-ideality factor n1, represents the diffusion current connected to the main PN junction. The generation/recombination (G/R) current, represented by the second diode (D2), is defined by its non-ideality factor (n₂) and current density (J₀₂). Nonetheless, the J-V curves clearly show their contribution to the total cell dark current, and simulations verify that G/R phenomena (D2) predominate in reverse forward operation and low voltage or diffusion (D1). The simulation parameters vary under time-transport situations and at higher voltages. The final term in (5) displays the investigated shunt leakage current density (Jsh) in the reverse bias zone. In order to increase the consistency between the simulation and experimental results, this method was utilized to describe and calibrate the material models and derive the dark electrical characteristics from the ATLAS constraint (i.e., without R_sh).
(a) Equivalent electrical circuit for the dual diode model of the u-CIGS cell, (b) reduced ATLAS model with contact resistance.
A thorough simulation and analysis of the Perovskite/CIGS double-junction solar cell is provided, taking into account the upper and lower cells that were examined in the preceding two sections. Given that the impact of various metal networks on the CGS/CIGS tandem solar cell has been previously investigated¹⁹, in this instance, the top cell’s absorber layer is made of perovskite material, and the upper junction is made of gold due to the material’s work function. It is actually possible to say that a PN layer is formed in the intrinsic layer of the homogenous perovskite junction. The voltage-current characteristic of the Perovskite/CIGS tandem solar cell structure is displayed in Fig. 11.
Schematic cross-section of perovskite/CIGS tandem solar cell structure and simulated J-V curves of calibrated thin CIGS, optimized perovskite, tandem perovskite/CIGS solar cells.
The aforementioned findings demonstrate that the manufacture of two-terminal cells is technically more difficult due to the requirement that the sub-cells be matched to one another. It is possible to think of two tandem cells as two diodes connected in series. Consequently, the open-circuit voltage of the tandem cell is equal to the total of the Voc of the individual sub-cells, and the short-circuit current for the entire tandem cell is constrained by the lowest Jsc of the sub-cell. Two transparent conducting layers are necessary for a two-terminal solar cell, which reduces parasitic absorption and boosts efficiency. Silvaco techniques have been used to electrically and optically model the structure of a two-terminal perovskite/thin CIGS tandem cell with a back passivation layer, 200 nm aperture width, and 2 μm cell pitch. According to the investigation, the power conversion efficiency is actually higher than 30%. For both cells, the band gap remains stable at 1.9 eV for the perovskite cell and 1.65 eV for the thin CIGS cell. Figure 11 displays the two-terminal perovskite/CIGS tandem device’s whole structure. Figure 11 displays the top, bottom, and tandem solar cells’ simulated J-V curves under AM 1.5. The EQE for the CIGS cell as a function of wavelength is simulated in Fig. 12. A very thin gold metal interface between the two cells allowed for the successful simulation of the two-terminal perovskite/CIGS tandem device, with efficiencies of up to 35.76%. To evaluate our research, we compare the PV output parameters of the simulated and validation models with other recently published works under standard lighting, as indicated in Table 3. This table shows that the efficiency is also boosted in the tandem mode due to an increase in the open circuit voltage.
Figure 12 shows the spectral photocurrent density of the bottom CIGS subcell in a fully coupled tandem structure under AM1.5G irradiation. The photocurrent generation starts to increase significantly at around 650 nm—the absorption range of the MBI layer—and remains above 85% until near 1050 nm, eventually integrating to Jsc = 19.8 mA cm2, a value that confirms perfect current matching with the top subcell. This behavior indicates a very favorable spectral splitting and minimal parasitic absorption in the upper layers. In Fig. 12, the external quantum efficiency of the bottom thin CIGS sub-cell in the fully coupled two-terminal tandem structure is presented after optical filtering by all overlying layers (FTO/TiO₂/MBI-perovskite/Spiro-OMeTAD/recombination junction). The calculation was performed over the same 300–1200 nm spectral range (5 nm step) using the identical beam propagation model in Silvaco-Atlas. The complex refractive indices of the top-cell layers were identical to those used for Fig. 7 (refs. 6, 17, 20, 28). For the CIGS stack, experimentally measured optical constants were employed: ZnO: Al and intrinsic ZnO from referenced via refs^25,38., CdS from silvaco library, and Cu(In₀.₆Ga₀.₄)Se₂ (Eg = 1.68 eV) in refs^25,30,38. The ultrathin recombination junction (IZO/SnO₂/Au = 15 nm total) introduces negligible parasitic absorption. The resulting EQE curve exhibits a sharp onset at approximately 650 nm (complementary to the MBI absorption edge) and remains above 85% until approximately 1050 nm, demonstrating excellent spectral utilization of the transmitted sub-band gap photons and confirming successful current matching at Jsc = 19.8 mA/cm² for both sub-cells under the optimized top-absorber thickness of 420 nm. The external quantum efficiency of the bottom thin CIGS sub-cell presented in Fig. 12 was obtained from the complete monolithic two-terminal tandem device simulation, thereby inherently including spectral filtering and parasitic absorption by the entire top-cell stack (FTO/TiO₂/MBI-perovskite/Spiro-OMeTAD/recombination junction). This ensures that only the fraction of the AM1.5G spectrum transmitted through the wide-band gap MBI-perovskite top cell (cut-off = 650 nm) reaches the CIGS absorber, accurately reflecting real tandem operating conditions and confirming current-matched performance at Jsc = 19.8 mA/cm².
Spectral photocurrent density (generated current density per wavelength interval) of the calibrated standalone u-CIGS bottom solar cell (glass/FTO/TiO₂/MBI 420 nm/Spiro-OMeTAD/Au) under AM1.5G illumination for A = 1 m².
Table 4 presents the validation of the individual sub-cell models employed in this study. The MBI-perovskite top cell was constructed using the exact layer thicknesses, doping concentrations, and defect densities reported by Shah et al.⁶, yielding simulated photovoltaic parameters that deviate by less than 2% from the measured values. The ultra-thin CIGS bottom cell was calibrated against the certified 20.4% benchmark device of Chirilă et al.⁴², which is widely accepted in the CIGS community and repeatedly reproduced in subsequent simulation studies. The resulting deviations of less than 1% in all key metrics (Voc, Jsc, FF, PCE) confirm the physical realism and high predictive accuracy of the material parameters, mobility, recombination, and optical models used throughout this work, thereby providing a reliable foundation for the two-terminal perovskite/CIGS tandem optimization and the reported efficiency of 35.67%.
The achieved power conversion efficiency of 35.67% and open-circuit voltage of 4.53 V in the simulated two-terminal MBI-perovskite/u-CIGS tandem structure originate from the nearly ideal additive voltage and extremely low non-radiative recombination losses enabled by the numerical modeling framework. In Silvaco-Atlas, when a highly conductive recombination junction (thin Au/ZnO/Au stack < 15 nm total thickness) is implemented with negligible series resistance and near-perfect tunnel/recombination characteristics, the open-circuit voltage of the tandem device approaches the arithmetic sum of the individual sub-cells (Voc, perovskite = 1.38 V + Voc, CIGS = 3.15 V optimized independently). Additionally, the defect density in the MBI absorber was intentionally set to a very low value (Nt = 1 × 10¹⁰ cm⁻³) based on the most optimistic values reported for high-quality lead-free bismuth-based perovskites processed under controlled conditions, which minimizes non-radiative voltage loss (ΔVnr < 50 mV per sub-cell). These idealized conditions, while challenging to fully replicate experimentally at present, are physically valid within the simulation environment and represent an upper theoretical limit for this material combination.
It is important to emphasize that the reported 35.67% efficiency and 4.53 V Voc constitute a theoretical upper bound under the following idealized assumptions: (i) near-unity internal quantum efficiency in both sub-cells, (ii) perfect current matching achieved by precise thickness optimization, (iii) negligible parasitic absorption and reflection losses due to optimized anti-reflection coating and textured FTO, and (iv) an almost lossless transparent recombination junction. In real devices, sputtering damage, interface recombination at the tunnel junction, higher defect density in MBI layers (typically 10¹⁴–10¹⁶ cm⁻³), and optical losses in the thick FTO substrate would reduce the tandem Voc to 3.2–3.6 V and efficiency to the 28–32% range, which is consistent with the best certified perovskite/CIGS or perovskite/silicon tandems reported by 2025. Therefore, the presented results serve as a roadmap highlighting the theoretical potential of lead-free MBI-based tandems when future materials and interface engineering challenges are overcome.
The simulated EQE spectra presented in Figs. 7 and 12 are fully consistent with the physical properties of the materials and the tandem configuration. For the single-junction MBI-perovskite cell, the sharp absorption onset at 640–650 nm corresponds exactly to the reported optical band gap of (CH₃NH₃)₃Bi₂I₉ of 1.9–1.95 eV (ref⁶), while the near-90% EQE plateau between 400 and 600 nm and gradual roll-off toward longer wavelengths are typical for lead-free bismuth-based perovskites due to their slightly indirect character and moderate carrier diffusion length. In the bottom u-CIGS sub-cell, the EQE exhibits a very sharp rise at near 650 nm (perfectly complementary to the MBI top-cell cut-off) and remains > 85% up to 1050 nm, dropping steeply thereafter, which matches the calibrated band gap of 1.68 eV (Ga/(In + Ga) = 0.4) used in our model and validated against validation CIGS cells in refs^25,30,38., and⁴⁰. All spectra were calculated over the wavelength range 300–1200 nm with 5 nm resolution using the AM1.5G (ASTM G173-03) spectrum;
In a realistic two-terminal (2T) monolithic perovskite/CIGS tandem solar cell, the total open-circuit voltage is fundamentally limited by several unavoidable loss mechanisms that are not fully captured under highly idealized simulation conditions. First, even in state-of-the-art lead-based perovskites, non-radiative recombination typically causes a voltage deficit of 0.25–0.40 V per sub-cell relative to the radiative limit, and this deficit is significantly larger (0.45–0.70 V) in lead-free bismuth-based perovskites such as (CH₃NH₃)₃Bi₂I₉ due to higher bulk and interface defect densities (typically 10¹⁴–10¹⁶ cm⁻³). Second, the recombination/tunnel junction—depending on its design (using heavily doped TCO, ultrathin metal layers, or stacks of highly doped semiconductors) and the quality of the junction—inevitably causes an additional voltage drop in the range of 0.1–0.6 V. Third, the misalignment of the band levels and the pinning of the Fermi level in these junctions also lead to a further reduction in the effective internal potential. As a result, the perovskite/CIGS and perovskite/silicon tandems experimentally reported up to November 2025 have only achieved Vocs in the range of 2.80–3.05 V, despite the fact that the total theoretical band gap of these structures is between 2.8 and 3.1 eV.
To reflect these physical constraints, we have recalibrated the tandem model in the revised manuscript by incorporating more realistic parameters: (i) MBI absorber defect density increased to 1 × 10¹⁵ cm⁻³, (ii) interface recombination velocity of 10³–10⁴ cm/s at both ETL/absorber and HTL/absorber interfaces, (iii) a practical recombination junction consisting of 80 nm IZO/8 nm SnO₂/5 nm Au with measured sheet resistance and moderate tunneling resistance, and (iv) experimentally derived optical constants (n, k) for thick FTO substrates. Under these conditions, the simulated two-terminal tandem device delivers a realistic Voc of 2.94 V (1.21 V from the MBI top cell + 1.73 V from the u-CIGS bottom cell), Jsc of 19.8 mA/cm² (current-matched), FF of 82.4%, and PCE of 30.2%. This performance is now fully consistent with the best certified perovskite-based tandems reported in 2024–2025 and represents an achievable target for future lead-free MBI/CIGS tandems once interface passivation and junction engineering reach the level of lead-halide systems.
In the present work, the two-terminal monolithic tandem device was constructed and solved as a single, fully coupled structure within a single Silvaco-Atlas deck, ensuring strict series interconnection and current continuity between the sub-cells. All layers – from the front FTO substrate through the MBI-perovskite top cell, the intermediate recombination/tunnel junction (IZO/SnO₂/ultra-thin Au stack), the thin CIGS bottom cell, and finally the back metal contact – were defined sequentially in one structure file with continuous mesh and shared region numbering. The drift-diffusion equations, Poisson equation, and carrier continuity equations were solved simultaneously across the entire stack (total thickness ~ 2.5 μm) using the Newton–Richardson method with full Fermi–Dirac statistics and lattice heating disabled. Because only two electrical contacts (front and back) were defined, current continuity is automatically enforced by the solver: at every bias point, exactly the same current density J flows through both sub-cells and the recombination junction, exactly replicating the physical behavior of a real two-terminal monolithic device. No external post-processing or manual addition of independently simulated sub-cell characteristics was performed.
Current matching was achieved by iterative thickness optimization of the MBI-perovskite top absorber while monitoring the photocurrent generated in each sub-cell under the AM1.5G spectrum filtered by the upper layers. The optical generation rate throughout the entire structure was calculated using the beam propagation method with complex refractive indices (n, k) taken from validation data for all layers (FTO, TiO₂, MBI, Spiro-OMeTAD, IZO, CIGS, ZnO, etc.). The thickness of the MBI layer was varied between 300 and 550 nm until the integrated photocurrent of the top cell equaled that of the bottom u-CIGS cell within ± 0.1 mA/cm². The final optimized configuration yielded Jsc = 19.8 mA/cm² for both sub-cells, corresponding to the operating current of the complete tandem device. Figure 12 (updated) now shows the generation rate profile across the entire stack, clearly demonstrating that nearly all photons with λ < 650 nm are absorbed in the wide-bandgap MBI top cell, while longer-wavelength photons efficiently reach and are absorbed in the narrow-bandgap u-CIGS bottom cell. This rigorous coupled electro-optical simulation, combined with enforced current continuity, guarantees physically valid two-terminal tandem performance and eliminates any possibility of artificial overestimation.
In this work, a high-performance, lead-free, and indium-free two-terminal monolithic perovskite/CIGS tandem solar cell was successfully designed and optimized using Silvaco Atlas TCAD, achieving a realistic power conversion efficiency of 35.67% (Voc = 4.53 V, Jsc = 19.8 mA/cm², FF = 82.4%) under AM1.5G illumination. This was accomplished by combining a wide-band gap methylammonium bismuth iodide (MBI, Eg = 1.9 eV) top sub-cell with a standard-thickness CIGS (Eg = 1.68 eV, 500 nm) bottom sub-cell interconnected through a low-resistance IZO/SnO₂/ultra-thin Au recombination junction. The key enabling factors were the replacement of ITO with thermally stable and indium-free FTO as the front electrode, precise optimization of the MBI absorber thickness to 420 nm for perfect current matching (ΔJ < 0.1 mA/cm²), minimization of parasitic absorption and reflection losses via careful layer selection, and, most importantly, a substantial reduction of non-radiative carrier recombination through low defect densities, effective interface passivation, and optimized band alignment. The resulting strong suppression of recombination losses, together with efficient spectral splitting and excellent charge collection, directly accounts for the high open-circuit voltage, fill factor, and overall tandem efficiency. These results clearly demonstrate the promising potential of environmentally friendly, lead-free MBI-based perovskite/CIGS tandem architectures for low-cost, scalable, and ultra-high-efficiency next-generation photovoltaic technology.
The data used in the paper will be available upon request. Please contact shayesteh.compu@gmail.com.
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Department of Electrical Engineering, Ya.C., Islamic Azad University, Yazd, Iran
Reza Mosalanezhad, Mohammad Reza Shayesteh & Majid Pourahmadi
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All authors contributed to the study conception and design. Data collection, simulation and analysis were performed by Reza Mosalanezhad, Mohammad Reza Shayesteh and Majid Pourahmadi. The first draft of the manuscript was written by Mohammad Reza Shayesteh and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Correspondence to Mohammad Reza Shayesteh.
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Mosalanezhad, R., Shayesteh, M.R. & Pourahmadi, M. Silvaco TCAD modeling, optical simulation, and optimization for high-current perovskite and u-CIGS tandem solar cells with efficiencies above 30%. Sci Rep 16, 8611 (2026). https://doi.org/10.1038/s41598-026-39816-6
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The remote province of Qinghai in northwest China is emerging as a crucial hub in the country’s ambitious drive to develop high-tech green energy and achieve carbon neutrality. Amid the windswept Gobi Desert, a massive solar farm is not only generating vast amounts of power but also fostering unexpected ecological benefits.
China, which has announced plans to reach peak emissions by 2030 and become carbon neutral by 2060, is increasingly relying on green energy initiatives. The challenging conditions of the Gobi Desert have paradoxically created an ideal environment for harnessing solar power.
At the heart of this effort is the Talatan Solar Park, a sprawling facility home to millions of solar panels.
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The park integrates hydro-power, wind-power, and geothermal energy, with solar panels being the primary energy source.
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The energy generated at the park is transmitted over hundreds of kilometers through China’s network of ultra-high voltage power lines to major cities where demand is high. This advanced transmission system ensures minimal power loss over long distances.
Beyond the large-scale solar farms, renewable energy solutions are also appearing in more localized, innovative forms across Qinghai. One state-operated service station along a busy highway stands out as a model of true carbon neutrality, generating all the energy it needs to operate.
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This pilot program, featuring solar panels, heated exterior walls for energy efficiency, and lots of EV chargers, is being studied for replication. The prevalence of electric vehicles in this rural region, with nearly 60 percent running on electricity, underscores the local commitment to green transportation.
Intriguingly, the vast array of solar panels at Talatan has had an unforeseen positive impact on the desert environment.
“Once the panels were cleaned, it allowed water from precipitation to trickle off — irrigation in the desert. Slowly the vegetation was restored. The wind decreases cut down on sand storms, and the vegetation naturally recovered,” said An.
This led to an unexpected transformation: the desert began to turn into grasslands, with vegetation growing over a meter in height. However, this new growth presented a challenge: increased risk of grassland fires, particularly at the end of winter. To address this, a low-tech, yet highly effective solution was devised.
“Once the grass grew, the biggest hidden danger came at the end of winter — grassland fire prevention. So we allowed grazing in this large field of solar panels, helping the local economy and reducing the risk of grass fires,” said An.
This initiative has seen sheep herders, a traditional part of the landscape for generations, now guiding their flocks through the solar fields. This win-win approach allows the sheep to graze freely among the panels. It keeps the vegetation at a manageable level and reduces fire hazards while simultaneously supporting the local pastoral economy.
As the sheep meander through this high-tech landscape, they help ensure that the sun’s powerful rays continue to flow efficiently through the nation’s power lines, bridging the gap between ancient traditions and a carbon-neutral future.
Solar Park in Qinghai’s Gobi Desert generates ample clean energy, transforms desert into lush ecosystem
More than 1,000 coal mines in China have adopted intelligent systems, as their application expands from pilot projects to large-scale deployment, the China National Coal Association said recently.
Statistics show that by the end of 2025, a total of 1,066 coal mines nationwide have introduced smart systems, with such technologies now supporting more than 65 percent of the country’s coal production capacity. The number of autonomous mining trucks in operation surpassed 4,000 units, roughly doubling on an annual basis.
The rapid adoption of smart mining is driven by robust domestic capabilities in intelligent equipment and technology. In Beijing, a newly deployed underground Internet of Things (IoT) precision positioning and management system links workers, positioning cards and operating zones, while also enabling health monitoring. Its core technologies and components are fully domestically developed and have been applied in coal mines and coal preparation plants. “This underground positioning system we’ve developed has a positioning deviation of less than 20 centimeters when a person or device is stationary. Even when a person or device is moving at high speeds, the margin of error remains minimal. A single device can cover a radius of 800 meters,” said Wu Fengdong, general manager of China Coal Beijing Coal Mining Machinery Co., Ltd., a subsidiary of the state-owned China National Coal Group Corporation.
Since the start of the 14th Five-Year Plan period (2021–2025), cumulative investment in smart mining has exceeded 107.1 billion yuan (about 15.6 billion U.S. dollars), with intelligent technologies now widely applied, accelerating the shift from traditional mining to modern, technology-driven extraction.
Over 60 pct of China’s coal production capacity uses smart technology by end of 2025
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Domestic PV module capacity reaches 144 GW, but raw material dependence persists
Indian PV manufacturers outperformed Chinese peers in profitability during 2024–2025
TaiyangNews successfully concluded its flagship Solar Technology Conference India 2026 (STC.I 2026), held in New Delhi on February 5th & 6th, discussing the rapid evolution of India’s giga-scale solar manufacturing ecosystem and its path ahead. Among the highlights was a dedicated session titled Market Overview – Supply, Demand and Price Dynamics, where Indian analysts from EUPD Research (Rajan Kalsotra), Rystad Energy (Sushma​ Jagannath), Wood Mackenzie (Sureet Singh), and S&P Global (Abhyuday Tewari) shared their views on the latest developments in the world’s third-largest solar market, which has quickly emerged as a major module manufacturer as well. We have summarized the findings of this session in 10 key takeaways, listed below.  
The session opened with a clear message of India’s strong PV installation demand in the upcoming years. Looking at historical data from the Ministry of New and Renewable Energy (MNRE), India’s yearly PV installation demand was hovering around 8-12 GW per year until 2022. However, the situation has changed drastically since 2024, and we now see 35 GW of PV already installed in FY 2025-26 (Apr-Jan). This momentum is forecast to continue, with the country expected to install approximately 50 GW per year on average and reach around 252 GW DC by 2030, according to EUPD Research. 
Takeaway: India targets 280 GW AC of solar PV installations by 2030, and the country has already installed 140 GW AC (as per MNRE) by January 2026. Drivers of this demand are the government’s reverse PV auction programs, high-cost conventional electricity, rapid expansion of the commercial and industrial (C&I) sector, and residential PV subsidy schemes. While this means India would need less than the current annual averages to reach its target, it’s likely that the country will exceed it. With India’s plans to commission around 8 GW of data centers by 2030, power demand is expected to increase starting in 2027, necessitating additional power generation capacity. Moreover, the rooftop market is also quickly developing now. However, grid infrastructure development and monitoring PV curtailment issues will also be important.
Grid constraints are becoming a critical challenge both globally and in India. According to Rystad data, India curtailed over 250 GWh of solar PV in 2025, while China’s curtailment rose 60% year on year. This underscores the urgent need for expanded transmission capacity, energy storage, and grid flexibility to accommodate growing renewable generation.
Takeaway: With rising demand for PV installations, there has also been a rise in PV curtailment issues. To address the issue, the Government of India has initiated the ‘Green Energy Corridor’ project, aimed at connecting solar-rich states with the national grid. This project has already entered Phase II, wherein 20 GW of renewable energy capacity will be integrated to the grid by the end of 2026. Under Phase II, the target is to develop 2,800 km of new corridors and 35 substations within the given timeline.
With just 12 GW of local module manufacturing capacity in 2020, the country has expanded to 144 GW by 2025. This exponential growth has largely been driven by supportive government policies, including the Approved List of Models and Manufacturers (ALMM) mandate, the Production Linked Incentive (PLI) scheme, and the Basic Customs Duty (BCD) framework. According to industry estimates, local module manufacturing capacity could exceed 279 GW by 2030, and cell capacity could grow from 27 GW in 2025 to 171 GW by 2030.
Takeaway: As domestic PV module supply grows faster than annual installation demand, an oversupply situation will develop in the near term. The forecast holds true for cells as well, which is expected to reach 171 GW by 2030. However, in contrast to the above graph, we believe wafer supply may increase drastically post-2028, accounting for ALMM List-III for wafers coming into effect by then.
Even though India achieved 144 GW of module manufacturing capacity by 2025, it continues to depend on China for raw materials. In 2025 alone, India imported 49.5 GW of cells and 30 GW of wafers from China, as per EUPD Research.
Takeaway: A review of historical data shows that in 2023, India imported approximately 16 GW of PV modules, despite a domestic manufacturing capacity of 8-10 GW. Module imports began to decline following the implementation of ALMM List I in April 2024. Cell imports are expected to follow a similar trend, with volumes declining after the implementation of ALMM List II in June 2026. Wafer imports are likely to continue until 2028, after which the government plans to introduce its ALMM III list (see also figure #3). However, for upstream components such as ingots and polysilicon, the supply gap is projected to persist even beyond 2030.
As per Wood Mackenzie data, 97% of Indian module exports went to the US in 2024. However, exports dropped from 4.5 GW in 2024 to 2.9 GW in Q1-Q3 2025, due to tariff pressures and ongoing investigations. High US tariffs have already reduced export volumes, which are expected to decline further in 2026 unless both governments reach a mutual agreement. The US government has initiated Anti-Dumping (AD) and Countervailing Duty (CVD) investigations in August 2025 against India, Indonesia, and Laos. Whereas the preliminary CVD rate for India was set at 126% in February and a final determination is expected for July, the preliminary AD determination is expected for April (see India, Indonesia, Laos Solar Imports Face High US CVD Rates).
Takeaway: India’s export concentration in the US is its biggest near-term vulnerability, unless the manufacturers diversify their export markets. Europe and Southeast Asia could be the next target markets for Indian suppliers. However, maintaining high quality standards, R&D in PV model upgrades, and price parity with their Chinese counterparts will be crucial for local manufacturers.
Indian solar module manufacturers have been narrowing the long-standing cost gap with China, driven by expanding domestic production capacity, improved manufacturing efficiencies, and supportive government policies, according to a Fraunhofer report, presented by EUPD.
Takeaway: While India is becoming more competitive, most manufacturers still use Chinese equipment, and Chinese manufacturers continue to sell at significantly lower prices. However, as major markets such as the US and potentially Europe implement protectionist measures against Chinese products, India will be well-positioned to benefit, provided it can consistently deliver high-quality products.
From a pricing perspective, Indian module manufacturing costs remained approximately 11% higher than China in Q2 2025, underscoring China’s continued structural cost advantage driven by scale, supply-chain integration, and financing efficiencies. However, market pricing dynamics have shifted as well. The India-China spot price gap narrowed from 9 euro ¢/W in Q1 2024 to 4.9 euro ¢/W by February 2026. This has been possible only because of the Indian government’s policy support, market protection, and incentives for local manufacturing.
Takeaway: Indian module prices have quickly decreased, narrowing the price gap to their Chinese competitors. But this was only possible because of government support. Another challenge will arise when the ALMM for cells will be implemented in June 2026, so that only domestic cells can be used for module production. The crucial question at that time will be whether the ratio of module performance to cost will remain sufficient to maintain, or further reduce, the price gap. However, as long as the Indian market is protected and resilience criteria in other markets offer export opportunities for Indian modules, prices will not be determined by cost.
While leading Chinese module manufacturers mostly reported losses in 2024-2025 amid intense price competition and prolonged oversupply, Indian PV manufacturers demonstrated stronger profitability trends during this period. However, according to EUPD Research, this advantage could be short-term because of an aggravating overcapacity situation in India.
Takeaway: Indian PV manufacturers are reporting higher profit margins due to import restrictions and PLI incentives compared with their Chinese competition. At the same time, Indian manufacturers are cautious, operating at only 50-60% of their capacity and accepting orders only on ‘made to order’ terms. However, the future well-being of Indian companies will depend largely on the development of trade barriers in the US, Europe, and other markets against Chinese incumbents.
As the European Union intensifies its focus on supply-chain resilience, policy developments are creating opportunities for alternative manufacturing hubs. Under the EU’s Net-Zero Industry Act (NZIA), resilience-based procurement criteria could unlock an annual demand of approximately 3 GW, favoring diversified and non-dominant suppliers, according to EUPD Research.
Takeaway: Indian PV manufacturers have an advantage in emissions intensity of module shipments to Europe over their Chinese competition. If India succeeds in keeping pace with next-generation PV technology, the European market could emerge as a stable and long-term export destination for its products.
The session concluded with all 4 speakers sharing a common view that India’s PV growth is no longer centered on mere capacity expansion. Instead, the focus has shifted toward deeper upstream integration, continuous efficiency upgrades, stronger ESG transparency, diversification of export markets, alignment with energy storage solutions, and a more strategic, long-term positioning in the global landscape. And the common message that resonated across each presentation: Indian manufacturers must move from volume-driven growth to value-driven competitiveness.
At STC.I 2026, the Indian market was framed not just as a growth narrative, but as a pivotal strategic turning point.
Demand is strong
Manufacturing is booming
Global prices are under pressure
Trade tensions are reshaping flows
Europe is opening doors
Profitability is fragile
India is on the way to becoming the world’s second-biggest PV manufacturing hub. Export opportunities, price competitiveness, and keeping pace with next-generation PV technology will determine the future of this country’s export ambitions.
TaiyangNews 2024

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Accelerating distributed solar PV and battery energy storage deployment will support Ukraine in establishing energy security.
In the year following the Russian Federation’s full-scale invasion of Ukraine in 2022, available dispatchable power generation capacity halved from roughly 38 GW to 19 GW. After severe attacks in spring 2024, capacity declined further, down to 12 GW. Towards the end of 2024 Ukraine was able to restore 3 GW and has worked to restore and add additional capacity throughout 2025, despite ongoing attacks.
Distributed solar PV has played a key role, providing cost effective and rapid increases in electricity generation capacity, contributing to system resilience and overall energy security. The move towards a greater level of decentralisation in power generation can also support Ukraine in meeting its long-term decarbonisation goals, as set out in the 2030 National Energy and Climate Plan and the 2050 Energy Strategy.  
This report explores the current policy landscape for distributed solar PV in Ukraine and outlines three potential policy options to accelerate the deployment of this technology. It focuses on expanding the capacity of distributed solar PV to achieve the modelled results from IEA report Empowering Ukraine through a Decentralised Energy System, which outlines a pathway to rebuild and modernise Ukraine’s power sector amid ongoing attacks on energy infrastructure
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IEA (2025), Policy options to accelerate distributed solar PV in Ukraine, IEA, Paris https://www.iea.org/reports/policy-options-to-accelerate-distributed-solar-pv-in-ukraine, Licence: CC BY 4.0
This report explores the current policy landscape for distributed solar PV in Ukraine and outlines three potential policy options to accelerate the deployment of this technology. It focuses on expanding the capacity of distributed solar PV to achieve the modelled results from IEA report Empowering Ukraine through a Decentralised Energy System, which outlines a pathway to rebuild and modernise Ukraine’s power sector amid ongoing attacks on energy infrastructure.
The IEA estimates Ukraine would need to add around 4 GW of distributed PV per year until 2030 (over 24 GW in total) to create a more decentralised and secure power system and achieve the objectives laid out in its national energy and climate plan (NECP). Ukraine will also need an additional 5.6 GW of new BESS by 2030 (0.9 GW per year). In this context, a fast buildout of distributed solar PV and BESS is needed.
Ukraine had more than 9 GW of installed solar PV capacity prior to the Russian Federation’s full-scale invasion in 2022. Most of the capacity was from distributed installations. Utility-scale capacity advanced rapidly from 2018 to 2020, driven by the feed-in tariff (“Green Tariff”) policy.
However, this capacity was affected by Russia’s invasion of Ukraine, with roughly 1 GW of (mostly utility-scale) solar PV capacity damaged, destroyed or inaccessible due to occupation. Combined with hydropower (-1.5 GW) and wind power (-1.3 GW) losses, the drop in capacity resulted in a nearly 4 TWh decline in total renewable generation from 2021 to 2022. The war also had an impact on the pace of new utility-scale installations.
Solar power was a leader in renewables deployment in 2024. According to various sources, including the Ukrainian Ministry of Energy, around 300 MW (and up to 900 MW, depending on the source) of new solar PV was installed in 2024, including behind-the-meter installations1, while 20 MW of new onshore wind power came online. Thus, reaching the required 24 GW of new distributed solar PV for a distributed energy system by 2030 implies that the total installed capacity more than quadruples from the estimated capacity of around 7 GW in 2024.
2023 values are estimated = estimated capacity for 2030. 2030 modelling results from IEA (2024), Empowering Ukraine Through a Decentralised Electricity System.
IEA (2025), Renewables 2025; IEA (2024), Empowering Ukraine Through a Decentralised Electricity System.
As Russia continues to strategically target Ukrainian energy infrastructure ahead of the fourth winter since the beginning of the war, large-scale plants, both conventional and renewable, are particularly vulnerable to attacks given their size and location. The areas of highest solar PV resource potential in the country are in the south. Other plants were also affected by flooding after the destruction of the Kakhovka hydropower dam in 2023. Damaged installations can be rebuilt (and will receive the same compensation they qualified for when commissioned), but the risk in these locations remains high from developer perspective.
Distributed solar PV applications can provide a solution to the challenges threatening infrastructure in Ukraine. First, given their small capacity, distributed solar PV applications can be deployed more rapidly (in a couple of months) than utility scale solar PV (in around one year) or wind power (two to three years). In addition, given the proximity to demand centres, the needs of distributed resources for repairs, upgrades or new build of transmission infrastructure would be minimal, reducing both cost and time required to provide power to the grid. Ukraine recognises the positive impact distributed energy resources can have, with distributed plants potentially removing vulnerabilities faced by large-scale installations, as they have no single points of failure, are more difficult to target in an attack, less complicated and faster to repair and less dependent on the overall health of the grid.
The amount of distributed solar PV in Ukraine has grown significantly since 2019, with installations by around 70 000 households (under a fixed tariff scheme) seeking resilience and cost savings. Interest in installing solar PV on rooftops has remained strong while attacks on energy infrastructure and the frequency of outages have increased. Ukraine’s international partners and the government have prioritised the installation of solar PV on hospitals and schools. The Government of Ukraine has introduced several incentives to help reduce the financial burden on individuals. Zero interest loans (for up to EUR 10 000) are available to households for a 10-year period to spur continued investment, and there is an exemption on value-added tax (VAT) and import duty relating to solar PV systems, batteries and other technologies. The GreenDim programme helps fund rooftop solar PV on apartment buildings run by homeowners’ associations and the “5-7-9% Affordable Loans” programme supports commercial and industrial customers. Ukraine’s “Decarbonisation Fund” provides low interest loans for decarbonisation projects in schools, hospitals, industry and small and medium-sized enterprises.
Two programmes provide further incentives to generate power. Distributed solar PV systems can qualify for the Green Tariff: a programme that enables residential systems of up to 30 kW generating electricity from renewable sources to sell it to the Guaranteed Buyer. The Green Tariff provides a fixed payment for net energy provided to the grid and has an official end date of December 2029. Distributed energy systems can also qualify for the Net Billing Programme introduced in late 2023, which provides cost reduction based on the wholesale market price. Payment for each of these programmes depends on system size, with small systems receiving benefits from electricity suppliers and larger systems receiving payment from the Guaranteed Buyer, a publicly-owned organisation. In both cases, the end consumer relies either fully or partially on the state, with revenues for payment coming from wholesale market sales and a transmission tariff.
Additionally, many residential and industrial consumers installed solar PV capacities behind-the-meter (BTM) to ensure the security of electricity supply before winter 2024/2025. The distribution system operator is not informed in most cases (e.g. installation of 6-15 kW by residential consumers), as these consumers do not install meters and do not sign supply contracts. In addition, most solar PV systems, since 2023, have been installed with battery energy storage systems (BESS). Pairing distributed solar PV systems with BESS can provide power for longer and help with system integration and flexibility. Several hospitals and schools are already benefiting from this joint approach.
While Ukraine’s programmes for solar PV and other renewable technologies have been effective in attracting investments, there have been concerns with the implementation of these programmes such as retroactive reductions of awarded feed-in tariff levels and developers not receiving payment. The Green Tariff programme prompted a boom in renewable energy capacity in Ukraine. Despite its success in attracting investment in renewables, this feed-in tariff has so far resulted in developers going without payment due to financial challenges faced by the off taker. As a result, the tariff was reduced by 15% for solar installations in 2020. Despite these measures, the transmission system operator (TSO) Ukrenergo still owes the Guaranteed Buyer, the state-owned off taker of electricity from renewable sources around EUR 335 million, to compensate those that produced electricity under the Green Tariff programme from 2022 to 2025. Payments are being made, but as a result of this persistent indebtedness of the Guaranteed Buyer, most wind power producers decided to switch from the Green Tariff to market electricity sales when the opportunity arose in 2023.
Persistent power cuts due to Russian attacks on generation and transmission infrastructure continue to drive demand for distributed solar PV, as consumers intend to partly produce their consumed electricity themselves. However, the challenges of the existing programmes are reducing the uptake in Ukraine. The outstanding debts owed to renewable developers have decreased investor confidence, potentially reducing the number of developers willing to build projects that qualify for a programme receiving payment from the government. This was evident in the December 2024 pilot auction, when the 11 MW available received no bids. In addition to improving the financial situation of the off taker, alleviating, streamlining and removing barriers to system installation could facilitate increased deployment.
The Solar Energy Association of Ukraine (SEAU) estimates 800-850 MW of new solar PV capacity was added in 2024.

The Solar Energy Association of Ukraine (SEAU) estimates 800-850 MW of new solar PV capacity was added in 2024.
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To help implement its commitment to provide 100 percent renewable power for operating the high-speed rail system, the California High-Speed Rail Authority (Authority) intends to build a series of photovoltaic (PV) solar systems and battery energy storage system (BESS) facilities in the Central Valley.
In 2008, the Authority prepared a study indicating that the high-speed rail system would be supplied with energy from the California grid. In 2012 and 2014, the Authority approved construction of the Merced to Fresno project section and the Fresno to Bakersfield project section, respectively, as part of the statewide system.
To provide electric power to the system, traction-power substations (TPSS) were approved for construction at approximately 30-mile intervals along the alignment. Traction-power substations (TPSS) are responsible for converting power for multiple resources, to ensure availability and quality of traction power supplied to the Overhead Contact System (OCS). The OCS delivers electricity directly to the trains via overhead wires, enabling their operation on the rail network. To reduce dependency on third-party infrastructure improvements, optimize the project’s operational cost structure, and align with the goal to power the system with 100 percent renewable energy, the Authority intends to integrate PV solar fields and BESS units as part of the overall system.
The PV sites will generate energy, which will be transmitted to the TPSS and stored in co-located BESS units. The BESS units will dispatch energy during peak demand periods to optimize costs and provide backup power, ensuring up to six hours of continuous operation during electric utility service disruptions. While PV sites will serve as the primary power source, each TPSS will remain connected to the grid to supplement energy during periods of limited solar production, ensuring uninterrupted service. This configuration enhances traction power reliability while enabling the Authority to (1) advance its sustainability commitments, (2) achieve greater energy resilience, and (3) minimize reliance on third-party led grid infrastructure upgrades.
To learn more about the environmental process, including opportunities for public input, please visit the PV/BESS environmental planning webpage.

Green Practices & Sustainability
(916) 324-1541
info@hsr.ca.gov
Copyright © State of California

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You can charge pretty much anything with a solar panel, given the right setup and enough time. For example, certain smaller panels have USB ports incorporated so you can plug your mobile devices right in, like you would a conventional power adapter. Many smart home gadgets you might have around your outdoor areas use solar to charge, like speakers, cameras, lights, and beyond. But does that also mean you can charge more substantial devices like, say, a Tesla or another electric vehicle? You absolutely can, but it’s not a plug-and-play solution. You may need to do a few things first.
For instance, you cannot simply run cables straight from solar panels to your EV battery or its charging system. Not to mention, solar energy levels fluctuate based on a number of factors, and you’ll need a consistent energy source to charge your Tesla’s battery. The correct way to do it is to use a battery backup solution, which the panels would charge first, and then you plug that into your EV. Tesla’s Powerwall is an excellent example. The Powerwall includes lithium-ion battery packs, where the energy is stored, as well as a built-in inverter, so if you opt to make your own system, you’d need a similar installation. An inverter converts the direct current (DC) electricity produced by solar panels into alternating current (AC) so it’s usable by an EV. It’s definitely possible to create your own solar backup system, but you’d need several components, including a charge controller, a suitable battery, an inverter, and, of course, solar panels. Ultimately, though, it is possible to charge your Tesla or EV with solar, even though it’s not as straightforward as it might have seemed at first.
Read more: SpaceX’s Raptor Engine Vs. Blue Origin’s BE-4 – What’s The Difference In These Rocket Engines
Every EV has different energy requirements. It depends on how far and how often you drive, which affects how often you’re recharging your vehicle’s batteries. Moreover, electric vehicles have battery capacities of varying sizes, some bigger than others. The larger the battery, the more power they hold, and the more energy needed to recharge it.
Edmunds estimates the average electric vehicle consumes 394 kilowatt-hours (kWh) per month. Annually, that’s more than 4,700 kilowatt-hours. You can find the number of panels needed to charge an EV by dividing that total by a local production ratio (a measure of how efficiently a system converts DC to AC power), then dividing again by the wattage of panels you’re considering. If your production ratio is 1.5 and you’re considering 350-watt panels, that works out to about nine panels to charge an EV reliably, but that’s not entirely accurate. Sure, some of the best solar panels you can buy are rated for 400 watts or above, but panels hardly ever deliver at their full capacity on a consistent basis. All of which means that you’ll probably need more than nine panels to charge an EV on the regular.
That doesn’t include additional panels you’d be using to power your home’s energy needs, either. The typical home requires between 16 and 23 panels to offset utility bills with solar. Taking more than half of those away to charge your EV isn’t ideal. It might be better to use net metering to sell excess renewable energy back to the grid for credits, and instead charge your EV the traditional route while using the credits to lower your utility bill. Net metering is something you should consider before installing solar panels on your home, anyway.
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“This is a huge moment for our Northern California flock.”
Photo Credit: iStock
California condors have already come back from the brink. Now, it appears they’re on the verge of a major milestone in the northern part of the state.
A pair of the birds appear to be caring for the region’s first wild condor egg in over 100 years, the San Francisco Chronicle detailed. Recent observations indicate incubation duties, which will last just under two months.
Scientists from the Yurok Tribe broke the news in a Facebook post, relaying cautious optimism around the chick’s survival prospects. 
“I have been waiting for this moment since the first condors arrived in 2022,” Yurok Wildlife Department Director Tiana Williams-Claussen said. “As a scientist, I know I shouldn’t get my hopes up too high, but that doesn’t mean I can’t cheer for these young parents’ success.”
The news provides another hopeful moment for the critically endangered California condor. Their population reached a perilous low in 1982, with only an estimated 22 individuals.
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To save the species, a captive breeding program took the birds out of the wild, leading to a resurgence across the state. Southern areas were first to reintroduce the birds, which have massive almost 10-foot wingspans. 
For the 26 California condors released in Humboldt County, this is a seminal moment. The pair has been given names that reflect their cultural significance: A0, also known as Ney-gem’ ‘Ne-chweenkah (“She carries our prayers”), and A1, or Hlow Hoo-let (“At last I fly”).
At nearly seven years old, they are experienced birds and are equipped with GPS trackers. Since the birds hit sexual maturity at age six, these two are right on schedule. 
Condors live over 50 years, so there is hope that this is far from the only crack at offspring they will get. The parents share the responsibilities of incubation and chick-rearing.
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Despite continued concern about the birds’ vulnerability despite California’s ban on lead ammo, the prospects for natural reproduction in the wild harbors great potential.
“This is a huge moment for our Northern California flock,” said Chris West, Northern California Condor Restoration Program manager and Yurok Wildlife Department senior biologist. 
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Monday, April 6, 2026
Victoria’s Swinburne University of Technology will lead a $3 million international solar panel recycling project focused on extracting and reusing precious and critical minerals.
Backed by grant funding from the Australian government, the new Silicon Zero Emission Recycling, Refining and Production program, is investigating new methods of removing impurities and salvaging precious and critical minerals from end-of-life solar panels, with a particular focus on silicon.
“The Si-Zero Research Program is the first of its kind in the world,” said Dr Bintang Nuraeni, a Swinburne researcher involved in the program.
“It brings together international expertise to develop zero-carbon processes for recovering high-purity silicon and other valuable materials from end-of-life solar panels, strengthening the foundation for a sustainable and circular solar industry.”
The project will be spearheaded by Swinburne’s Professor Akbar Rhamdhani, who says very high-grade silicon is needed to produce more solar panels, and other technologies.
“In a traditional process, we use carbon and extremely high temperatures to reduce raw silica to metallurgical-grade silicon. It’s very energy intensive and takes a lot of time.
“Recycling can bypass this.”
But even recycling has its challenges, often requiring a lot of time and energy, while the silicon extracted from solar panels must also be refined to extreme purity – up to 99.99999 per cent.
That is why Professor Rhamdhani and his team are developing a process where much of the work is done in bulk by robots and processing is powered by green energy and electricity.
“We are developing a process that is quite clean, with a no or very low carbon footprint,” he said.
Swinburne’s leadership is well placed, considering Australia is very quickly set to become one of the planet’s largest contributors to solar waste, with one million tonnes expected by 2050.
With more solar panels per person than any other country in the world, early Australian solar installations are starting to reach their end-of-life, necessitating new and improved recycling methods.
Globally, solar waste is expected to reach 78 million tonnes by 2050.
Another country that is likely to contribute even more to the global solar waste issue is India, which is also represented in the Si-Zero Research Program by participants from the Indian Institute of Technology, Hyderabad (IIT Hyderabad).
It says that by 2050, India’s cumulative solar waste may exceed 19 million tonnes, posing a major environmental risk if unmanaged.
Other participants include researchers from the Badan Riset dan Inovasi Nasional (BRIN), Indonesia’s National Research and Innovation Agency, as well as the country’s Universitas Gadjah Mada (UGM), as well as the Sadoway Labs Foundation in the United States.
The project will also provide for 10 PhD students and five research fellows across the four countries.
Joshua S. Hill is a Melbourne-based journalist who has been writing about climate change, clean technology, and electric vehicles for over 15 years. He has been reporting on electric vehicles and clean technologies for Renew Economy and The Driven since 2012. His preferred mode of transport is his feet.
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Amaury Perez Sanchez
The director of the National Load Dispatch Center of the Cuban Electric Union, Engineer Félix Estrada Rodríguez, reported that Cuba generated more than 800 MW of photovoltaic (PV) power during a midday period on Tuesday, Feb. 10, 2026. This achievement follows the installation of more than 1,000 MW of solar energy capacity throughout 2025, in line with the government’s efforts to diversify the country’s energy mix.
Cuban President Miguel Díaz-Canel recently emphasized the importance of advancing toward renewable energy sources. In his remarks, he highlighted that the use of clean energy not only contributes to environmental sustainability but also strengthens Cuba’s energy sovereignty.
Díaz-Canel stressed that these types of projects aim to improve energy supply and reduce costs in the long term. Authorities continue working to increase the country’s renewable energy capacity, with the goal of meeting the targets set in the national energy policy, amidst increasing U.S. sanctions against the country.
Cuba’s Ministry of Energy and Mines announced via X (see below) that, at midday of Tuesday, Feb. 10, the country generated more than 800 MW of PV energy for the first time, but the next day, this figure was surpassed, reaching more than 900 MW. This increase in solar energy production occurs in the context of a tightened blockade, emphasizing Cuba’s commitment to continue advancing toward energy sovereignty.
En #Cuba se generaron por primera vez más de 800 MW con energía fotovoltaica, en un segmento del medio día de ayer, martes 10 de febrero, en el mismo horario hoy, se generaron más de 9️⃣0️⃣0️⃣ MW.
En medio del recrudecido bloqueo, seguimos impulsando nuestra soberanía energética. pic.twitter.com/bVhDEr5dbe
— Ministerio de Energía y Minas de Cuba 🇨🇺 (@EnergiaMinasCub) February 11, 2026

The Ministry emphasizes that these efforts are crucial to guaranteeing a more autonomous and sustainable energy supply, despite external difficulties. Cuban authorities and specialists continue working on expanding renewable energy sources, reaffirming their goal of diversifying the country’s energy mix.
—Amaury Pérez Sánchez (amauryps@nauta.cu) is a chemical engineer based in Cuba with the University of Camagüey.
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Scientific Reports volume 16, Article number: 9842 (2026) Cite this article
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Recent advances in the non-fullerene acceptors (NFAs) have achieved remarkable attention owing to their significant photovoltaic and optoelectronic characteristics in organic solar cells. Herein, new A–π–A configuration-based NFAs (TPBD-Cl to TPBD-CF₃) were designed by incorporating the strong electron withdrawing acceptor units at their terminals. Theoretical investigation was performed for these chromophores by using DFT/TD-DFT methods at M06/6-311G(d, p) level. The results demonstrate that the end-capped acceptor substitution has promoted the charge transfer and efficiently reduced the energy gap (2.25–2.13 eV). Further, it was revealed that the structural modifications elevated that absorption wavelengths in the studied chromophores (721.97–754.58 nm). The transition density matrix (TDM) and density of states (DOS) analyses demonstrated an efficient charge transfer from the donor (π–spacer) moieties towards terminal acceptors. Notably, TPBD-NO₂ compound exhibited the least energy gap (2.13 eV), highest absorption wavelength (λ_max = 754.58 nm) and correspondingly least excitation energy (1.64 eV). Moreover, it showed the minimal exciton binding energy (0.50 eV) among all other derivatives which highlight its potential for the organic solar cell materials. Photovoltaic parameters were calculated by blending the donor (PBDB-T) with designed acceptors showed the good open-circuit voltages (V_oc). These findings suggest that the end-capped engineering in NFAs chromophores can significantly enhance the optoelectronic performance, offering promising materials for next-generation OSC devices.
The growing demand of energy worldwide and decreasing reserves of fossil fuels prompted the need to establish effective energy sources. Solar energy is one of the most promising technologies of all renewable energy sources due to its abundance and environmentally friendly nature. Organic photovoltaic (OPV) systems have particularly received great researchers’ interest owing to their affordability, flexibility and adjustable optoelectronic characteristics^1,2,3,4. Nevertheless, to obtain high power conversion efficiency (PCE) in OPV devices, one must design new organic semiconductor materials with an optimal set of electronic and optical characteristics.
Several OSCs employed the fullerene-based acceptors, which have significant benefits with a reported PCE of about 10%⁵. Even though the fullerene-based OSCs achieved great success, they faced a number of challenges, including high purification, low stability, and limited absorption in the visible wavelength region⁶. Considerable study has been devoted to the non-fullerene acceptors, especially the non-fullerene small-molecule acceptors (NF-SMAs), in an attempt to overcome these challenges⁷. Compared to fullerene derivatives, NFAs have several advantages, including cost effectiveness, adjustable energy levels and efficient absorption of visible light⁸. These materials have been analyzed in a variety of conformations, including the acceptor–donor–acceptor (A–D–A), acceptor–π–donor–π–acceptor (A–π–D–π–A), acceptor–donor–acceptor–donor–acceptor (A–D–A–D–A) and donor–acceptor–donor (D–A–D) reported in literature^9,10.
Recent work on the A–D–A type NFAs has resulted in remarkable progress in the organic solar cells. Structural modifications of the donor (D) core, acceptor end groups, and side chains are shown to effectively tune the energy levels, absorption spectra, molecular packing, and charge transport¹⁰. The photovoltaic efficiency in such compounds is improved by the asymmetric central core to achieve directional intramolecular transfer of charges between the donor center and the terminal acceptors. This asymmetry causes unequal distribution of charges which enhances the electric field strength inside and promotes effective dissociation of excitons. Furthermore, a decrease in the molecular symmetry decreases over-aggregation resulting in optimized molecular packing and fewer losses of recombination. The extended π-conjugation facilitates the reduction of energy gaps and high absorption shifts. The combination of these effects leads to better charge separation, transport and performance of the photovoltaic materials¹¹.
Literature shows that side-chain engineering in a series of non-fullerene acceptors (TTCn-4 F) demonstrated a high PCE of 15.34% in the TTC8-4 F molecule blending with PM6 due to favorable morphology and balanced charge mobility¹². Recently, an ortho-benzodipyrrole (o-BDP) based non-fullerene acceptors (CFB and CMB) demonstrated significant performance. The PM6:CFB and PM6:CMB blends achieved 16.55% and 16.46% PCEs, respectively, owing to reduced energy losses and efficient packing¹³. These results highlight that systematic optimization of NFAs via the central core design, end-group tuning, and side-chain modification continues to drive OSC’s performance towards higher efficiencies and practical device applications¹⁴.
In recent years, the A–D–A configuration-based NFA molecules are developed by isomerization to study its influence on the ICT. Comparison showed that TPBTT-4 F is much better than its counterpart in exhibiting the red-shifted absorption and elevated HOMO levels, which lead towards reduced energy gap¹⁵. Moreover, it showed a high power conversion efficiency (15.72%) due to reduced nonradioactive energy loss (E_nonrad) for organic solar cells. These properties impelled the authors to design new NFAs using the above-mentioned compound by utilizing the end-capped tailoring strategy. So, to improve the charge transfer, the designed compounds have strong electron withdrawing groups in their acceptors as shown in the Fig. 1. Moreover, these compounds are not reported previously in the literature and are unique in the field of solar cells. The objective of this research work is to investigate the impact of various acceptor moieties on the photovoltaic and optoelectronic properties of the newly designed derivatives. For this purpose, DFT and TD-DFT methods are employed which determined various parameters including the optical, electronic and photovoltaic properties. The results provide useful insights into the design and optimization of small-molecule NFAs, hence leading to more efficient solar cell materials.
To conduct the quantum chemical calculations, the Gaussian 16 suite program^16,17 was utilized in this work. The designed compounds (TPBR and TPBD-Cl toTPBD-CF₃) were optimized at the ground state using the M06 functional¹⁸ and 6-311G(d, p) basis set¹⁹. The true minima structures with no imaginary frequency were obtained and visualized via the GaussView 6.0 software²⁰. The frontier molecular orbitals (FMOs), global reactivity parameters (GRPs), density of states (DOS), transition density matrix (TDM), open circuit voltage (V_oc), and UV-Visible absorption were accomplished at the above-mentioned functional to investigate the impact of end-capped acceptors on the designed chromophores. The solvent effect was studied using the conductor like polarizable continuum (CPCM) model in the UV-Visible spectra²¹. For the calculation of data from the output files, Avogadro²², Multiwfn 3.8²³, PyMOlyze 2.0²⁴, GaussSum²⁵, Chemcraft²⁶ and Origin 8.5 software²⁷ were utilized.
The electron push-pull effects on NFAs are significant to enhance the intramolecular charge transfer (ICT) and intrinsic photophysical properties to develop efficient OSCs. In this study, TPBTT-4 F compound is selected from the literature to design new NFAs chromophores which possess strong end-capped acceptor groups to achieve efficient solar cell materials. TPBTT-4 F exhibits elevated HOMO level (-4.19 eV) owing to which impressive power conversion efficiency was achieved (15.72%)¹⁵. Herein, TPBTT-4 F is utilized as a reference compound (TPBR) which is further used to develop different derivatives, such as TPBD-Cl, TPBD-Br, TPBD-NO₂, TPBD-SO₃H, TPBD-CN and TPBD-CF₃ having electron withdrawing functional groups (–Cl, –Br, –NO₂, –SO₃H, –CN and –CF₃, respectively) attached to benzene rings of their terminal acceptors. A schematic representation is shown in the Fig. 1 to understand the designing approach in these derivatives. The Fig. S1 shows their chemical structures, while Fig. 2 represents the optimized structures for the designed chromophores. The Table S1 shows IUPAC names of the reference compound and its derivatives, while their Cartesian coordinates are shown in the Tables S2–S8.
Schematic representation of the designed compounds.
True minima geometries of TPBR and TPBD-Cl – TPBD-CF₃.
Frontier molecular orbitals study is an important tool for estimating the stability, light absorption, and electrical characteristics of organic systems^28,29. The highest occupied molecular orbital (HOMO) evaluates the electron donating tendency, while the lowest unoccupied molecular orbital (LUMO) estimates the electron withdrawing capability³⁰. To characterize the distribution of electronic cloud in the HOMOs and LUMOs, frontier molecular orbitals study is the key analysis³¹. The reactivity and stability of a chemical can be assessed by the energy gap = E_LUMO−E_HOMO calculation^32,33. The FMOs analysis for the reference and designed derivatives (TPBR and TPBD-Cl – TPBD-CF₃) is performed using the M06/311G(d, p) level and the results are presented in Table 1. The Fig. 3 shows the visual representation of electronic cloud density in these orbitals.
Results indicate that the reference compound (TPBR) shows the highest energy gap (2.27 eV) along with corresponding HOMO/LUMO energies of -5.79 and − 3.52 eV. The calculated HOMO level energies for TPBD-Cl to TPBD-CF₃ derivatives are − 5.82, -5.81, -6.01, -6.01, -5.99, and − 5.91 eV, whereas their LUMO energy values are − 3.57, -3.56, -3.87, -3.85, -3.83, and − 3.69 eV, respectively. Similarly, they exhibit the following energy gap values: 2.25, 2.25, 2.13, 2.15, 2.15, and 2.21 eV, respectively. Previous similar studies show that greater electron attracting capability of acceptor groups results in narrowing of energy gap in the organic chromophores³⁴.
For the reference compound (TPBR), a wider energy gap (2.27 eV) which might be due to the presence of weak electron withdrawal effect at the terminal acceptors, hence resulting in less LUMO stabilization and charge transfer as compared to its derivatives. The tuning of HOMO and LUMO levels as a result of electron withdrawing substituents is a determinant of E_gap in the organic chromophores³⁵. With the introduction of chlorine substituent (TPBD-Cl), the energy gap (2.25 eV) is reduced significantly. Chlorine has a strong –I (inductive) effect but has a moderate π-donation capability via the dπ–pπ delocalization, which increases the electronic communication of the terminal acceptor to the conjugated backbone. This two-fold act stabilizes the two frontier orbitals although the LUMO is more stabilized giving a smaller HOMO-LUMO gap.
In contrast, TPBD-Br results in almost similar but slightly less pronounced energy gap reduction (2.25 eV). Although, the bromine is heavier than chlorine and also induced more polarization, its orbital overlap efficiency with the π-system is reduced due to which it does not significantly increases the conjugation. The most prominent reduction in energy gap is observed for TPBD-NO₂ (2.13 eV), which contains the –NO₂ substituents. Nitro groups are substituents which possess strong − I effect and − M (mesomeric) effect due to which it effectively lowers the LUMO levels owing to efficient π-electron delocalization³⁶. So, the ICT is elevated from the π-spacer to the terminal acceptor, substantially lowering the energy gap. In TPBD-SO₃H, the presence of –SO₃H groups slightly shift the energy gap (2.15 eV) compared to TPBD-NO₂, despite their strong electron-withdrawing nature. This behavior can be attributed to their reduced planarity and increased steric hindrance, which partially disrupt conjugation along the π-framework.
The TPBD-CN compound exhibits a moderate energy gap (2.15 eV) that is governed by the cumulative − I effect of multiple –CN groups. These groups effectively lower the LUMO level to retain the molecular planarity. However, the absence of strong resonance interaction compared to nitro groups results in a slightly wider energy gap in TPBD-CN than TPBD-NO₂. This indicates the phenomena of controlled tuning of energy gap which is favored by a balanced inductive withdrawal without excessive structural distortion. In last derivative (TPBD-CF₃), the incorporation of –CF₃ groups show 2.21 eV as the E_gap. Although –CF₃ group possess strong − I effect, it does not participate in the π-conjugation. So, the LUMO in this compound is stabilized via the inductive effect, however, its ICT is limited. This results in comparatively larger E_gap than nitro- or cyano-substituted analogues. Overall, the designed chromophores are listed in the following decreasing order of E_gap: TPBR > TPBD-Cl = TPBD-Br > TPBD-CF₃ > TPBD-CN > TPBD-SO₃H > TPBD-NO₂. This order highlights that the energy gap tuning requires not only strong electron withdrawing substituents, rather it is also favored by effective delocalization of orbitals and molecular planarity³⁷.
FMOs analysis also clarifies ICT among the orbitals of the organic chromophores in addition to their corresponding orbital energies³⁸. The pictograms of HOMO and LUMO for the designed compounds are shown in the Fig. 3. All the studied compounds show an electron density on the π-spacer for the HOMO, but for the LUMO, the charge is shown on the acceptor region. In order to further investigate the charge transfer propensity including intramolecular charge transfer, density distribution analysis is done. The isodensity amplitude plots of HOMO and LUMO are plotted for the studied compounds as shown in the Fig. 4. It is noticed that for LUMO, the electronic coud is spread over the acceptor units, while in HOMO it is localized on the π-spacers. Thus, the obtained HOMO-LUMO energy gaps indicate a suitable balance between molecular stability and optical absorption, suggesting that the designed molecule can effectively facilitate charge separation and enhanced photovoltaic performance³⁷.
HOMOs and LUMOs for the investigated compounds.
Percentage contribution of the molecular orbital density corresponding to the partitioned segments obtained from the HOMO and LUMO of the entitled chromophores.
The number of accessible electronic states of a molecule at a given energy level is indicated by the density of states. A higher DOS value at a specific energy indicates that more states exist at that particular energy level³⁹. The goal of this study is to determine how each molecule fragment contributes to the formation of different energy levels, particularly HOMO and LUMO. DOS analysis shows the percentage contributions of each individual fragment in determining the HOMO and LUMO charge densities in the studied chromophores which are displayed in the Table S19, while the Fig. 5 shows the graphical representations of these results. In this research, the designed chromophores (TPBR and TPBD-Cl to TPBD-CF₃) are divided into two fragments (π–spacer and acceptor) to establish the A–π–A configuration. The HOMO (valence band) is shown by the energy values to the left side along the x-axis, while the LUMO (conduction band) is represented by energy values towards the right side. The energy gap is the distance between these valence and conduction bands which is represented as the region in between HOMO and LUMO peaks showing negligible intensity (see Fig. 5). Each individual fragment is shown by separate peaks i.e., red curves show the π–spacer, while, the green peaks represent the acceptor unit. The overall DOS peaks are shown by black curves which represent the combine contribution of all the fragments in determining the charge distribution on the HOMO and LUMO.
The terminal acceptors exhibited the charge distribution percentages as 26.5, 27.5, 27.5, 29.8, 29.6, 29.7 and 28.5% to HOMO for TPBR and TPBD-Cl to TPBD-CF₃, respectively. However, 62.6, 63.0, 63.0, 71.8, 67.4, 66.8 and 63.8% are contributions for the LUMO formation in TPBR and TPBD-Cl to TPBD-CF₃ chromophores, respectively. Similarly, the π-spacer contributes 73.5, 72.5, 72.5, 70.2, 70.4, 70.3 and 71.5% of the charge density towards HOMO and 37.4, 37.0, 37.0, 28.2, 32.6, 33.2, and 36.2% to LUMO in the studied compounds, correspondingly. These results demonstrate that the π-spacer in all the designed derivatives occupy maximum charge density for HOMO, whereas the charge in LUMO is mostly found over the terminal acceptors. Further, the DOS spectra in Fig. 5 corroborates the above-mentioned results. Since the green curves indicate the maximum peak intensity in LUMO for all chromophores, which is close to -3.5 to -3.8 eV, the charge is significantly concentrated over the acceptor in all the studied molecules. While, in HOMO, the maximum charge is situated above the π-spacer unit that is indicated by the red maximum peaks close to -6.0 eV for these chromophores. These results validate the FMOs analysis (see Table 1) which mark the significance of this analysis in determining the electronic characteristics of the designed solar cell materials.
DOS representation for the investigated compounds.
The global reactivity parameters of the studied compounds are evaluated using the energies of the frontier molecular orbitals, since the HOMO-LUMO energy gap obtained from FMOs analysis directly determines the chemical reactivity and stability descriptors. The global softness (σ)⁴⁰, hardness (η)⁴¹, global electrophilicity index (ω)⁴², chemical potential (µ)⁴³, ionization potential (IP)⁴⁴, electron affinity (EA)⁴⁵, electronegativity (X)⁴⁶ and charge transfer (CT) within a molecule (ΔN_max) are some of the GRPs⁴⁷ which are calculated through HOMO/LUMO energies. Furthermore, the Koopman’s theorem⁴⁸ is used to compute the above-mentioned parameters for the TPBR and TPBD-Cl to TPBD-CF₃.
Table 2 displays the computed values of GRPs for the studied compounds which indicate that efficient charge transfer relies on the EA and IP; their high values indicate the stability of a compound. In descending order, the computed ionization potential (IP) values are as follows: TPBD-SO₃H (6.01) = TPBD-NO₂ (6.01) > TPBD-CN (5.99) > TPBD-CF₃ (5.91) > TPBD-Cl (5.82) > TPBD-Br (5.81) > TPBR (5.79) in eV. A molecule is said to be softer if its energy gap is less, which denotes greater reactivity and less stability. The descending order of global softness is as follows: TPBD-NO₂ (0.46) > TPBD-SO₃H (0.46) > TPBD-CN (0.46) > TPBD-CF₃ (0.45) > TPBD-Cl=TPBD-Br (0.44) > TPBR (0.43) in eV^− 1. The utmost electronegativity value (4.940 eV) is displayed by TPBD-NO₂ compound, which indicates its acceptor nature. Furthermore, the most negative chemical potential (µ) obtained is -4.94 eV obtained for TPBD-NO₂ which confirmed its strong acceptor nature, emphasizing its improved CT capabilities. Reactivity is dependent on two fundamental properties that are inversely proportional: softness (σ) and hardness (η). Molecules with higher σ and lower η values exhibit improved reactivity, decreased stability, and the least energy gap⁴⁹. Moreover, the enhanced polarizability of all the proposed compounds is also demonstrated by their lower hardness and higher softness values⁵⁰. Interestingly, in the studied series of chromophores listed above, TPBD-NO₂ displayed the greatest σ value (0.46 eV^− 1) and the least η as 1.06 eV. The obtained values reveal a consistent relationship between the HOMO-LUMO energy gap and the calculated descriptors, confirming the reliability of FMOs-based reactivity predictions.
UV-Visible spectroscopy is an essential tool to analyze the electronic transitions in a molecule as well as their opto-electronic characteristics for the OSCs^51,52. The wavelength and intensity of absorbed light provide useful information about the electronic structure, conjugation, π–π* and n–π* transitions, and the overall optical properties of the compounds⁵³. This analysis is commonly applied to estimate absorption maxima (λ_max), optical band gap, and light-harvesting ability of organic molecules, which is particularly valuable in designing materials for the optoelectronic and photovoltaic applications^54,55. Herein, the study is performed at the afore-mentioned TD-DFT level in the gaseous and chloroform phases. The CPCM is utilized for analyzing the solvent effects in this study. The chloroform solvent acts as a continuous dielectric medium surrounding the solute molecule. This approach accounts for bulk solvation effects on the electronic excitation energies and provides more realistic absorption spectra compared to gas phase calculations⁵³. The main results are presented in the Table 3, while detailed analysis is represented in the Tables S20-S33. The Fig. 6 displays their absorption spectra obtained in both gas and solvent media. The results show that lower excitation energy values are obtained by substitution of the strong electron accepting moiety in derivatives which elevates their λ_max values⁵⁶.
The above data shows that the TPBD-Cl to TPBD-CF₃ have much higher λ_max than the TPBR molecule. The λ_max of these investigated chromophores are situated in the visible region in chloroform (709.86-754.57 nm) as well as in the gaseous phases (653.20-684.57 nm). The solvent environment has a significant impact on absorption maxima, oscillator strengths, and other key optical characteristics. A red-shift happens when the absorption peak shifts to longer wavelengths, which is caused by polar solvents stabilizing chromophores’ excited states more than nonpolar solvents. Aprotic solvents, on the other hand, cause a blue shift, in which the absorption peak shifts to shorter wavelengths owing to the excited states’ inferior stability. A crucial element in regulating the transition probabilities within a molecule is the interplay between the polarity and viscosity of the solvent, which influences the oscillator strengths (f_os)⁵⁷.
In chloroform solvent, the λ_max values for the investigated compounds (TPBR and TPBD-Cl to TPBD-CF₃) are noted as follows: 709.86, 721.97, 723.74, 754.57, 748.33, and 750.28 and 730.82 nm, respectively. Contrarily, they have lower excitation energy values as 1.74, 1.71, 1.71, 1.64, 1.65, 1.65, and 1.69 eV for TPBR and TPBD-Cl to TPBD-CF₃, respectively. Correspondingly, their oscillation strengths are 2.63, 2.75, 2.78, 2.42, 2.60, 2.63, and 2.61. Among all derivatives, TPBR, TPBD-Cl and TPBD-CF₃ possess maximum H→L transition of 91% as compared to other derivatives. TPBD-CF₃ displays the second highest H→L transitions as 90%. Rest of the derivatives, such as TPBDNO₂, TPBD-SO₃H and TPBD-CN exhibit 84, 88 and 89% of the H→L transitions, respectively. In the case of chloroform solvent, the absorption maxima (λ_max) in solvent phase in nm are observed in following decreasing trend: TPBD-NO₂> TPBD-CN> TPBD-SO₃H> TPBD-CF₃> TPBD-Br> TPBD-Cl> TPBR.
In the gaseous phase, TPBR shows the λ_max = 653.20 nm along with excitation energy (E) of 1.89 eV. The oscillation strength (f_os) of TPBR is 2.40, and the H→L transition is noted as 93%. The derivatives (TPBD-Cl-TPBD-CF₃) show the following λ_max values: 663.05, 664.65, 684.35, 683.52, 684.57, and 669.57 nm, respectively. Contrarily, they exhibit reduced E values of 1.87, 1.86, 1.81, 1.81, 1.81, and 1.85 eV. Furthermore, their respective f_os are indicated as 2.55, 2.59, 2.37, 2.46, 2.48, and 2.42. Derivatives: TPBR, TPBD-Cl, TPBD-Br, TPBD-CN and TPBD-CF₃ exhibit a 93% of H→L contributions, while, TPBD-NO₂ and TPBD-SO₃H exhibit about 92% H→L percentages. The absorption maxima (λ_max) in nm for the gaseous phase among the studied chromophores exhibit the following decreasing trend: TPBD-CN> TPBDNO₂> TPBD-SO₃H> TPBD-CF₃> TPBD-Br> TPBD-Cl> TPBR. Hence, data shows that maximum λ_max is obtained for TPBD-NO₂ in both phases along with the least transition energy as compared to other compounds. The red-shifted absorption in TPBD-NO₂ is in correspondence with reduced energy gap as shown in its FMOs study. This might be due to the synergistic electronic effects of –NO₂ group in its acceptor unit. As, the nitro group exhibits both strong − I effect and resonance (− M) electron-withdrawing behavior, it causes an efficient π-conjugation and charge delocalization in the molecule. Its high electron affinity and low-lying π*-orbitals significantly stabilize the LUMO and increase the ICT³⁶. In contrast, limited resonance is shown by –CN groups, while –CF₃ acts mainly through an inductive effect with negligible conjugation. Thus, –NO₂ more effectively lowers the HOMO–LUMO gap and promotes bathochromic absorption shifts.
To further assess the robustness of the TDDFT simulation results of UV-Visible analysis, the studied compounds are additionally evaluated using the CAM-B3LYP, M06-2X and ωB97XD functionals. This is done to check the validity of the selected level of theory i.e., M06/6-311G(d, p). The calculated results are depicted in the Tables S34-S36 which indicate almost similar results with TPBD-NO₂ and TPBD-CN revealing the highest λ_max and least excitation energies with only minor variations (≤ 30 nm). While, the nature of the dominant transitions and charge-transfer characteristics remained unchanged, confirming the reliability of the chosen methodology. Moreover, these results are in close correspondence with its FMOs and GRPs data as they collectively state these compounds are the most suitable candidates for solar cells.
UV-Visible spectra of the investigated compounds in gaseous and solvent phases.
The mobility and transition of an electronic charge density in an organic system is observed using their transition density matrix plots. It is a three-dimensional representation of electron-hole pair distribution and delocalization which help to clarify the location of excited electrons, holes, and electrons within the organic solar cells⁵⁸. Identifying the improved exciton dissociation in the excited states of molecules is most easily accomplished with the TDM heat maps, which is essential for the development of solar cells⁵⁹. Owing to least contribution of hydrogen atoms, they overlooked in this analysis. All the designed derivatives are A–π–A configuration-based structures with two key components: (i) the π-spacer and (iii) the acceptor. As depicted in the Fig. 7, the local excitations (LE) are indicated by bright diagonal regions, where the electron and hole remain on the same molecular fragment. In contrast, off-diagonal intensity is represented by the charge-transfer (CT) excitations, which are particularly present between π-spacer and A regions. This reflects sufficient electron migration from the donor (π-conjugated core) to the acceptor terminal units.
Herein, the TDM heat maps confirm that the designed compounds exhibit strong intramolecular charge transfer characteristics, because the excitation is not localized at one site. Rather, it involves electron transition between donor and acceptor fragments of the molecule. In these molecules, the bright regions are located between the π-spacer and acceptor segments, confirming donor → acceptor charge transfer. All compounds have strong electrical charge coherence, according to the transition density maps. Overall, this behavior is desirable for organic solar cell chromophores and supports their photovoltaic efficiency.
TDM maps of the entitled compounds.
Hole-electron analysis refers to the migration of an excited electron from the hole area to the electron. In case of the studied OSCs, charge transfer (CT) happens at the interface of the π-spacer and acceptor which is necessary for the separation of charges⁶⁰. This study is frequently used to determine the location of electron density within a chemical compound⁶¹. It is a practical method for identifying the characteristics of charge transfer and electron excitations⁶². Figure 8 shows that the reference compound (TPBR) has the highest electron intensity located at C35 atom, and maximum hole intensity at the C24 atom. The highest electron intensity in the TPBD-Cl is found at C43 atom, while the highest hole intensity is found at C24 atom. The highest electron intensity of TPBD-Br is found at C43, while the highest hole intensity is found at C24 atom. C24 atom shows the maximum hole intensity in the TPBDNO₂, whereas the electron intensity is the highest at C37 atom. The highest electron intensities in the TPBD-SO₃H are found at C37 atom, while major hole intensity found at C24 atom. The electron intensity in the TPBD-CN peaks at C35, O49, and C52 atoms and maximum hole intensity at C24 atom. The TPBD-CF₃ exhibits a maximum electron intensity at C35, S49, and C52 atoms and a maximum hole intensity at C33 atom.
Overall, the hole electron analysis clearly demonstrates that electron density is mainly localized on specific carbon atoms depending on the molecular structure and substituent pattern. In all designed derivatives, the electron and hole distributions are well separated, indicating an efficient charge transfer character. In particular, most molecules show maximum hole accumulation at the C24 atom, while the positions of maximum electron density vary among different substituent types. This spatial separation of hole and electron densities supports the effective ICT behavior, which is a desirable feature for enhancing exciton dissociation and improving photovoltaic performance for the organic solar cell applications.
Hole electron analysis representation for the studied compounds.
One important factor affecting a compound’s optoelectronic characteristics and charge separation efficiency is the exciton binding energy (E_b)⁶³. It is inversely proportional to charge mobility and exciton dissociation, and directly related to the energy gap and optimization energy values⁶⁴. Greater exciton dissociation in the compound is indicated by a lower E_b, while decreased dissociation is reflected by a higher E_b⁶⁵. As seen in Eq. (1), the binding energy can be computed by subtracting the energy gap (E_gap) from the optimization energy⁶⁶.
In this case, E_H−L stands for HOMO-LUMO energy gap, E_b for binding energy, and E_opt for the first excitation energy⁶⁷.
According to the data mentioned in the Table 4, TPBD-SO₃H has the lowest exciton binding energy (0.49 eV) as compared to other derivatives. While, TPBD-NO₂ has the second least value for E_b as 0.50 eV which corresponds with its structural properties and charge transfer ability. Moreover, TPBD-NO₂ is the compound obtained with the least energy gap (2.13 eV) and highest λ_max (754.57 nm) due to its strong electron-withdrawing and high charge transfer nature, TPBD-SO₃H shows a lower exciton binding energy. This can be attributed to the highly polar nature of –SO₃H group, which enhances dielectric screening and reduces electron-hole Coulombic attraction. Overall, the examined chromophores in relation to their E_b values are arranged as follows: TPBD-Br> TPBR> TPBD-Cl> TPBD-CF₃> TPBD-CN> TPBD-NO₂> TPBD-SO₃H. As a result, it is clear that TPBD-NO₂ has possessed a higher degree of dissociation into free electrons and more photo-electronic properties, making it an effective material for the OSCs⁶³.
Another parameter that aids in assessing the degree of charge transfer between the donor and acceptor sites is molecular electrostatic potential⁶⁸. It is a three-dimensional depiction of the charge density at different points in the molecules under study. The MEP, which corresponds with a molecule’s reactive potential, highlights its electrophilic and nucleophilic centers⁶⁹. In this case, colors like green, blue, and yellow are utilized to show where different charges (positive, negative, or neutral) are placed over a molecule. The sequence of potential reductions is blue > yellow > dark blue > red. The blue color of the molecule indicates a region of positive charge, primarily over the central core. Figure 9 shows that the blue color of π-spacers in all the studied compounds (TPBR and TPBD-Cl – TPBD-CF₃) showing a nucleophilic propensity, whereas, the dark blue color near hydrogen atoms connected to aromatic carbon groups indicates the strongest positive potential. On the other hand, substituent groups exhibit the yellow hue over heteroatoms and have a somewhat negative potential (electrophilic). These results indicate that every molecule under study has substantial electrostatic potential.
ESP plots for the studied chromophores.
To understand the improved optoelectronic properties in the studied chromophores, their charge transfer analysis is performed⁷⁰. As know from previous studies, the molecular orbital distributions effectively show an electronic interaction among the donor and acceptor components⁷¹. This involves the formation of donor–acceptor complex to reveal the intermolecular charge transfer characteristics. The FMOs investigation shows that the derivative (TPBD-NO₂) exhibits the least energy gap as 2.13 eV. Similarly, the UV-Visible absorption wavelength is also maximum for TPBD-NO₂ as 754.57 nm. So, it is considered as the most suitable candidate for complex formation owning to its good charge transfer nature and promising optoelectronic properties. Among various donor, the PBDB-T is utilized in this study owing to its favorable properties. In order to explore the behavior of intermolecular charge transfer, the PBDB-T donor polymer and the TPBD-NO₂ acceptor are coupled in this case to form a donor–acceptor complex. As shown in Fig. 10, the in HOMO, the charge cloud is mostly centered on the PBDB-T, signifying it as the principal electron donor. While, the TPBD-NO₂ has the highest concentration in LUMO, indicating that it effectively accepts the electrons. Moreover, the electronic excitation and charge transfer from the donor towards the acceptor molecule are shown by the arrow pointing from HOMO to LUMO. Thus, this investigation shows good charge separation between them which essential in their optoelectronic functionality.
Charge transfer analysis of PBDB-T: TPBD-NO₂ donor: acceptor complex.
Open circuit voltage (V_oc) is important measure to demonstrate the performance of an organic solar cell material⁷². It accounts for the maximum amount of current obtained from any optical substance at zero voltage conditions⁷³. Presence of charge carriers, light source, external fluorescence, electrode function, OSC’s temperature, light intensity, and a variety of environmental conditions can influence the value of V_oc. The energy difference between the HOMO and LUMO of donor and acceptor molecules (HOMO_donor−LUMO_acceptor) is directly related to their open circuit voltages⁷⁴. The V_oc values in the current study are calculated using the well-known donor (PBDB-T) with a E_HOMO of -5.401 eV obtained from its optimization at the afore-mentioned level of theory. In theory, the V_oc of the OSCs is calculated using the Eq. (2), as described by Scharber and colleagues⁷⁵.
The Table S35 shows the energy difference for the HOMO_{PBDB T}−LUMO_acceptor for TPBR and TPBD-Cl to TPBD-CF₃ as 1.89, 1.83, 1.83, 1.53, 1.55, 1.57 and 1.70 eV, respectively. Similarly, their V_oc values are obtained as follows: 1.59, 1.53, 1.53, 1.23, 1.25, 1.27 and 1.40 V, respectively. The TPBR compound shows the utmost V_oc value of 1.59 V as compared to the designed derivatives. For these titled compounds, the V_oc findings are decreasing in the following order: TPBR> TPBD-Br> TPBD-Cl> TPBD-CF₃> TPBD-CN> TPBD-SO₃H> TPBD-NO₂. As mentioned before, the difference for the HOMO_{PBDB T}−LUMO_acceptor determines the V_oc value. Better optoelectronic properties and a higher V_oc value are the outcomes of lower acceptor origin LUMO. A low-lying LUMO of the acceptor increases the migration of electrons from the HOMO of D molecules, improving the photovoltaic characteristics. The orbital energy diagram of the aforementioned chromophores with respect to PBDB-T donor polymer is displayed in the Fig. 11 which shows that the donor polymer has a greater LUMO level than the designed acceptor chromophores. It should be emphasized that the molecular-level parameters presented here provide upper-limit estimates, since the actual device performance (such as V_oc) might be influenced by the morphology and solid-state packing. Nevertheless, these findings suggest that the proposed organic molecules possess promising characteristics and can be considered as strong potential candidates for future OSC’s applications.
Graphical representation of V_oc for the investigated chromophores with respect to PBDB-T.
In summary, a series of NFA-based organic chromophores (TPBR and TPBD-Cl to TPBD-CF₃) are designed using the end-capped structural modeling with malononitrile-based acceptors. The impact of these acceptors on the optoelectronic and photovoltaic properties is explored using the quantum chemical approach. A thorough examination of frontier molecular orbitals, optical characteristics, and photovoltaic factors reveals that TPBD-NO₂ is the most promising candidate for use in OSCs. It exhibits the smallest HOMO-LUMO energy gap (2.13 eV), red-shifted λ_max values in both the solvent (754.57 nm) and gas (684.35 nm) phases. Moreover, the DOS and TDM visual analyses further validated the above-discussed findings. Further, the designed compounds are blended with the PBDB-T donor to examine their photovoltaic features. Notably, all the designed systems exhibited favorable values of V_oc, which inferred that these chromophores can be regarded as the promising candidates for high-performance photovoltaic materials.
All data generated or analyzed during this study are included in this published article and its supplementary information files.
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This Project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Saudi Arabia under grant no. (IPP:455-961-2025). The authors, therefore, acknowledge with thanks DSR for technical and financial support.
Institute of Chemistry, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan, 64200, Pakistan
Mashal Khan, Fatima Sarwar, Khansa Gull, Memoona Arshad, Iqra Shafiq & Rifat Jawaria
Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia
Muhammad Nadeem Arshad & Khalid A. Alzahrani
Chemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia
Khalid A. Alzahrani
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Mashal Khan: Investigation; Formal analysis; Writing – original draft; Visualization.Fatima Sarwar: Formal analysis; Data curation; Validation.Khansa Gull: Literature review; Methodology; Validation.Memoona Arshad: Visualization; Figure preparation; Data curation.Iqra Shafiq: Investigation; Formal analysis; Writing – original draft; Visualization, supervision Muhammad Nadeem Arshad: Methodology; Software; Resources; Technical support.Khalid A. Alzahrani: Resources; Project administration; Supervision; Funding acquisition.Rifat Jawaria: Conceptualization; Supervision; Validation; Writing – review & editing.
Correspondence to Iqra Shafiq or Rifat Jawaria.
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Ratepayers are scrambling to avoid high electricity bills, and lawmakers in some US states are responding with new legislation to allow balcony solar panels. These plug-in panels make solar power accessible to households that don’t have the opportunity, or the inclination, to install their own rooftop solar panels. But, why leave it up to the states? It’s only a matter of time before the federal government steps in to help support the spread of this new money-saving technology, which will benefit millions of Americans…hold please…
Unlike solar-powered generators and other portable solar devices, balcony solar panels plug into a simple household outlet and feed electricity into household wiring, reducing the amount of electricity sucked from the grid. Ukraine is credited with popularizing the devices soon after Russia launched its unprovoked war against the country in 2022 and began destroying its energy infrastructure. The movement soon took off in Germany with strong support from the nation’s policy makers (see lots more plug-in solar background here).
The “balcony” in balcony solar refers to the ripe opportunity for hanging plug-in solar panels from apartment balconies, but a balcony is not required. The panels can be hung or propped up anywhere around a property where a household outlet is available.
The movement has been achingly slow to develop in the US. Utilities can take some of the blame because they generally treat a simple plug-and-play solar panel like a full blown rooftop solar installation, requiring costly connection agreements and fees. Landlords and homeowner associations can also shoulder responsibility for limiting use of the devices or prohibiting them altogether. Additionally, safety considerations and the age of the US housing stock come into play.
Still, the case for balcony solar is a compelling one. Back in 2017, for example, researchers at Michigan Technological University estimated the US demand for “plug and play PV” at 57 gigawatts, potentially saving ratepayers $13 billion per year while creating a substantial new industry market of $14.3 billion – $71.7 billion.
The cost of solar panels has dropped substantially since 2017 while electricity rates have gone up, so it’s no surprise to see legislators in at least two dozen states considering bills to cut down the hurdles to balcony solar, addressing safety concerns alongside utility red tape and other obstacles.
Utah was first out of the starting gate in March of 2025. Last month, Virginia also stepped up to the plate, and now Maine has chimed in. On April 2 the state legislature approved LD 1730, a bill that puts safety front and center.
“Given the varied electrical wiring in homes across Maine, LD 1730 emphasizes safety for consumers and line workers by requiring systems be installed by a licensed electrician,” observes the Natural Resources Council of Maine.
That’s not exactly the full plug-and-play vision, but the cost of hiring an electrician could be offset, and then some, within a year. “For the average household in Maine, a 1,200-Watt plug-in solar system could cut electricity bills by nearly 20% or $388 a year, according to the Office of the Public Advocate,” NRCM notes.
NRCM further observes that electricity rates have increased in Maine by 68% over the past five years. The upward trend predates US President Donald Trump’s rolling disaster of a war on Iran, and the war has only made things worse by ratcheting up the cost of natural gas as well as fuel oil for power plants.
“When oil and gas prices jump, like they are right now, Maine’s electricity prices follow, hitting the poorest households hardest,” NRCM emphasizes.
There being no such thing as a free lunch, the up-front cost of a balcony solar kit can put the panels beyond the reach of those who need it most. Gosh, those Biden-era federal tax credits for household renewable energy improvements would sure come in handy right now. Meanwhile, perhaps some enterprising startup will develop a subscription or leasing service, similar to those emerging in the rooftop solar industry.
The non-profit organization Bright Saver has been advocating for balcony solar in the US, and the results from Utah, Virginia, and Maine show that their efforts have been gaining traction. The organization has also identified a sort of back door for balcony solar through existing rooftop solar arrays that are already part of a net metering program.
The organization’s Net Metering Expansion Kits are already available in California, where state regulations allow rooftop solar owners to expand an existing array by up to 1 kilowatt. The panels can be placed anywhere sunlight is optimal, if not on the roof then on the side of a house, or in the yard.
Bright Saver’s “NEM Go” kit includes four 250-watt solar panels. Kits are also available in 800 and 1600 watt configurations.
If all goes according to plan, balcony solar kits could be as easy to pick up as a week’s worth of groceries. The German-based multinational supermarket chain Lidl, for example, is reportedly planning to carry plug-in solar panels at its UK stores for around £400 (about $460), following the UK government’s decision to lift regulatory hurdles.
“The government estimates that a typical UK home could save between £70 and £110 a year on their energy bills. At an upfront cost of around £400, that means the panel will pay for itself in around four years,” The Independent reported on April 2.
“The Department for Energy Security and Net Zero says the kits will be available ‘within months,’ with brands like EcoFlow hoping to have stock ready in time for the summer,” The Independent added.
No word yet on whether or not the company plans to stock the shelves of its US stores with balcony solar panels, but that could happen. Lidl (not to be confused with Aldi) is relatively new to the US supermarket scene, but it has already established a foothold in solar-friendly East Coast states including New York and New Jersey as well as Virginia.
The company also has a footprint in the Carolinas, Delaware, Georgia, Maryland, Pennsylvania, and Washington, DC.
Image: Balcony solar kits can be placed anywhere sun is available, feeding clean electricity directly into household wiring while reducing monthly utility bills (cropped, courtesy of EcoFlow).
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Storm damage at St. Croix after Hurricane Maria (top) and after Hurricane Fiona (bottom).
The General Services Administration (GSA) contracted with the National Renewable Energy Laboratory and Lawrence Berkeley National Laboratory to investigate damage to five government-owned solar photovoltaic (PV) arrays at GSA buildings in the Caribbean. The PV arrays suffered extensive damage from Hurricanes Irma and Maria in 2017. The rebuilding process GSA undertook with its energy service company and engineering firm, Jacobs Engineering, provides helpful lessons learned that can be applied in planning solar PV projects in locations with severe wind and rain events. One of the five sites offered a particularly high-value opportunity for lessons learned due to the innovative rebuilding process utilized by GSA managers and the in-depth analysis performed by the project engineers. Several findings from this analysis are applicable to other arrays across the country.
Inadequate frame stiffness and fastener strength led to failures.
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This solar company is helping homeowners install rooftop panels without the upfront costs  finance.yahoo.com
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AUSTRALIA’S NATIONAL SCIENCE AGENCY
Tomorrow’s energy system doesn’t just need to be renewable, it needs to be reliable too. Smart inverters might hold the key to safely connecting more household solar, batteries and EV chargers.
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By  Julio Braslavsky ,  Scott Walker 20 October 2025 5 min read
Australia is leading the world in rooftop solar adoption. By June 2025, 4.2 million rooftop solar systems had been installed nationwide representing 26.8 gigawatts of clean power generation capacity connected directly to local electricity networks that serve homes and businesses.
This surge in solar power is helping reduce carbon emissions, but it’s also creating new challenges for electricity grid operators where the traditional burning of fossil fuels for electricity generation is being replaced by cleaner and sustainable sources of energy.
Dr Julio Braslavsky, Senior Principal Research Scientist at CSIRO, says one of the biggest challenges in local networks with high levels of solar-powered homes is the emergence of large ‘tidal’ swings in power flows between power utilities and homes. 
“In the evenings homes typically consume power, which flows from large power generation plants, such as wind farms, hydroelectric, coal and gas-fired power plants. Let’s call this the ‘low-tide’ time in local electricity grid operation,” he said.
“In the middle of a sunny day, however, today about 40% of Australian homes generate their own power from rooftop solar panels. When the rooftop solar generation exceeds the power consumption in the houses, the excess power is ‘exported’, flowing back to the grid, supplying other homes and beyond. Let’s call this the ‘high-tide’ time in local electricity grid operation.”

Since Australia leads the world in rooftop solar installations, such reversal in power flows between low and high ‘tides’ in Australian electricity grids can be dramatic, pushing the operation of local electricity networks to their capacity limits – in South Australia the entire state electricity demand is often 100% supplied solely by rooftop solar during ‘high-tide’ times.
Dr Braslavsky says the capacity of local networks to safely sustain such tidal swings in power is fundamentally constrained by a phenomenon technically known as ‘phase imbalance’.
“Phase imbalance is a normal characteristic of local network operation and represents the unevenness in powers flowing through the three-phase utility poles and wires systems at any given point in time,” he said. 
Phase imbalance arises during normal operation because most homes are connected to a single phase (one of three) and have different consumption/generation patterns through the day. The imbalance, however, is aggravated during the large tidal swings in power produced by lots of rooftop solar and can lead to increased inefficiencies, power quality problems and network congestion – think of it like a three-lane highway where one lane is jammed while the others are nearly empty. Severe phase imbalance can lead to potential safety issues.
To tackle the challenge of phase imbalance, researchers from CSIRO—Australia’s national science agency—teamed up in 2023 with X, the innovation lab formerly known as Google X, through a project called Tapestry. Their goal? To investigate smart inverter technology to help balance grid operation in real time, mitigating the impacts of ‘tidal’ power swings, and expanding the capacity of the network to safely connect not only more rooftop solar, but also home batteries and charging of electric vehicles. 
Inverters are versatile electronic devices that convert the direct current (DC) electricity generated by solar panels into alternating current (AC) electricity used in homes and on the grid. But modern inverters can do much more than just convert power—they can also help manage how electricity flows through the network.
“Stemming from the foundational collaboration with X, CSIRO moved to developing and testing innovative designs of smart grid inverters that can tackle phase imbalance in real time and increase network utilisation in local electricity distribution networks,” Dr Braslavsky said.
This innovation isn’t just relevant to Australia. In October 2024, CSIRO presented its smart inverter technology to Southeast Asian energy leaders, including Indonesia’s state-owned utility PLN. Indonesia faces similar challenges, with high inefficiencies, safety concerns and congestion issues caused by phase imbalance in its local distribution networks. 
PLN expressed strong interest in collaborating with CSIRO to develop and test a prototype inverter that could help unlock more capacity for rooftop solar and other consumer energy resources.
“Through its development initiatives in Southeast Asia, Australia’s Department of Foreign Affairs and Trade is funding a CSIRO-PLN partnership that aims to demonstrate how smart inverters can reduce congestion, improve infrastructure efficiency and support electricity decarbonisation,” Dr Braslavsky said.
The joint project involves designing and testing a solid-state inverter that can dynamically rebalance electricity flows, then simulating performance using real-world data from Indonesian and Australian networks. The project team is also conducting workshops and lab visits to share knowledge and refine the technology, and are exploring field trials to validate the inverter in live network conditions
. 

To make sure these smart inverters work as intended, the team is developing detailed computer models and running lab tests. These models simulate how the inverters behave in real-world conditions, including how they respond to uneven solar generation or sudden changes in demand.
One exciting result from the simulations shows the potential of simplified inverter architectures to reduce phase imbalance by supplying corrective currents. This opens the door to cost-effective solutions that could be deployed widely across the grid.
The team has already completed low-power lab tests and is preparing for high-power trials at CSIRO’s Energy Centre in Newcastle. The next phase includes real-time hardware-in-the-loop testing using Indonesian network scenarios, followed by a joint technical report and potential field demonstrations.
If successful, this innovation could help Australia and Indonesia integrate more rooftop solar without compromising reliability. By 2050, rooftop solar capacity in Australia is expected to reach 72 gigawatts, with nearly 80% of detached homes in the National Electricity Market (NEM) having solar panels (doubling current levels). Smart inverters like the ones being developed in this project will be essential to making that future work. 
This article was first published online in Energy Magazine.

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Hildisrieden, LU – April 03, 2026 – PRESSADVANTAGE –
Oekoboiler Swiss AG, a Swiss manufacturer specializing in energy-efficient heat pump boilers, continues to expand its sustainable hot water solutions that seamlessly integrate with photovoltaic systems across Switzerland. The company’s advanced systems combine heat pump and solar technology to deliver significant energy savings while reducing CO2 emissions in residential and commercial buildings.
The company’s innovative approach addresses the growing demand for sustainable building technologies as Switzerland moves toward stricter energy-efficiency standards. Oekoboiler’s systems utilize a dual-energy approach that draws approximately 75 percent of the required energy from ambient air and only 25 percent from electricity, resulting in up to an 80 percent reduction in energy consumption compared to traditional water-heating methods.

The company’s heat pump boilers operate independently from central heating systems, making them particularly suitable for both new construction and retrofitting existing buildings. This flexibility has positioned Oekoboiler as a key provider of sustainable hot water solutions throughout Switzerland, where the company plans, installs, and maintains systems tailored to individual building requirements. Learn more here: https://pressadvantage.com/organization/oekoboiler-swiss-ag.
As Switzerland prepares for the implementation of EnEV 2025 energy efficiency standards, Oekoboiler’s technology offers building owners a pathway to compliance while maintaining comfort and reliability. The systems feature smart controls that optimize energy usage based on demand patterns and available solar energy, ensuring maximum efficiency throughout the year.
The integration capabilities extend beyond basic functionality, with WiFi-enabled models allowing remote monitoring and control. This connectivity enables property owners and facility managers to track energy consumption, adjust settings, and receive maintenance alerts, contributing to long-term system efficiency and reliability.
Oekoboiler’s product range includes storage capacities from 150 to 450 liters, accommodating various building sizes and hot water demands. Each system undergoes rigorous testing in Switzerland, ensuring quality and performance standards that meet the country’s stringent building regulations.
The environmental benefits of Oekoboiler’s technology extend beyond energy savings. The heat pump operation naturally dehumidifies basement spaces where units are typically installed, preventing mold formation and eliminating the need for separate dehumidification equipment. This dual functionality adds value for property owners while contributing to healthier indoor environments.

Oekoboiler Swiss AG maintains its commitment to Swiss engineering excellence through continuous product development and comprehensive service support. The company’s focus on quality consultation and customized solutions has established its reputation as a trusted partner for sustainable building projects throughout Switzerland. Additional information about Oekoboiler Swiss AG can be found at https://oekoboiler-swiss-ag.localo.site.
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For more information about Oekoboiler Swiss AG, contact the company here:
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Greenskies Clean Energy filed a petition last year to build a 1.2-megawatt solar facility on mostly vacant land along Lake Street in Manchester, as pictured on Jan. 15, 2026.
Greenskies Clean Energy filed a petition last year to build a 1.2-megawatt solar facility on mostly vacant land along Lake Street in Manchester, as pictured on Jan. 15, 2026.
Greenskies Clean Energy filed a petition last year to build a 1.2-megawatt solar facility on mostly vacant land along Lake Street in Manchester, as pictured on Jan. 15, 2026.
MANCHESTER — State officials have approved a Lake Street solar facility that has proved controversial among some neighbors.
The Connecticut Siting Council approved a petition from North Haven-based solar developer Greenskies Clean Energy to build a 1.2-megawatt solar photovoltaic electric generating facility at 81 and 93 Lake St., two largely vacant agricultural properties totaling close to 30 acres.
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The project consists of two separate but connected solar arrays, with a 750-kilowatt facility on 93 Lake St. and a 450-kilowatt facility on 81 Lake St. taking up a total footprint of 6.3 acres with a combined 2,136 modules.
The original petition filed by Greenskies in August billed the facility as providing "multiple benefits" to the town, state, and region through production of renewable energy, and the Siting Council's decision echoes that sentiment.
The draft decision and order, dated March 27, states that the Siting Council finds there is a "public benefit" for the construction of the facility and that it would not have a "substantial adverse environmental effect," and that the council will therefore issue a declaratory ruling for the proposed facility.
The Siting Council's draft opinion, dated March 27, states that pursuant to Connecticut General Statutes, the council has "exclusive jurisdiction" over the facility proposed by Greenskies and shall approve by declaratory ruling any such project as long as it "meets the air and water quality standards of the Department of Energy and Environmental Protection and the Council does not find a substantial adverse environmental effect."
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The opinion states that the proposed facilities would be remotely monitored on a 24/7 basis and comply with relevant building, electrical, and fire protection codes, and Greenskies would work with local emergency responders and file an emergency response plan. Noise generation and air quality would comply with state standards, and DEEP would need to issue a stormwater permit prior to construction.
The opinion further states that Greenskies has expressed a willingness to install landscape plantings and implement best management practices for stormwater in response to neighborhood concerns about visibility of the facility and water quality.
Members of the Siting Council approved approved the plan in a 7-0 vote Thursday, with one member recusing themselves. Few spoke about the project in detail during the meeting, though one member briefly discussed his issues with the plan.
Bill Syme said the proposal from Greenskies was not "one of (his) favorites" due to prime farmland being taken out of production, but that he could foresee minimal impact to neighbors and the environment.
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Khristine Hall said she was happy that a condition for a post-construction noise study was included, though noted it is typical for the Siting Council to do so.
"Even though the host parcel owner was not concerned about the noise, which may be above the noise limits, I think it's important to have that study and see what the compliance is once the facility is started," Hall said.
Chance Carter thanked staff members for working on the documents, and said he was pleased to see that the approval requires Greenskies to work with the town's fire department to ensure emergency services can reach the site.
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Joseph Villanova is a reporter with the Journal Inquirer and CT Insider, primarily covering Manchester and East Hartford. He joined the newsroom in July 2021.
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In 2024, China accounted for over 85 percent of the global photovoltaic (PV) module production. The country representing the second-largest share of PV production was India, accounting for just 3.3 percent. 

Solar is one of the fastest-growing energy technologies in the global market, as the average cost of using solar PV has decreased over the years. Recent years have seen impressive annual growth in the global production volumes of solar modules. At the same time, the average installed cost for solar photovoltaics has consistently decreased every year since 2010. Investments in solar photovoltaic energy worldwide have grown rapidly in the last few years. 

In addition to dominating the PV module production market, China is also the global leader in installed PV capacity. What’s more, most of the leading solar companies worldwide headquartered in China.
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How a ‘perfectly symmetrical’ 2D perovskite could boost tandem solar cells  techxplore.com
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Sunday, April 5, 2026
Large scale solar farms grabbed a record share of Australia’s main grid over the weekend, peaking at more than 30 per cent for the first time – before being shunted aside as the nation’s huge array of rooftop PV took precedence.
It’s a familiar story for large scale solar. Rooftop PV has eaten into the coal power generator’s midday lunch, but it is also having an impact on large scale solar, forcing heavy levels of curtailment during the day that can reach up to 50 per cent for some installations, depending on their location.
On Sunday morning, at 8.10 am (AEST) large scale solar – according to data from GPE NEMLog – reached a record 30.6 per cent share of supply on Australia’s main grid, the National Electricity Market. That smashed the previous record of 27.9 per cent by 10 per cent.
That previous record had only just been set four days prior, also at 8.10am, breaking a milestone that had stood since December last year.
The early morning records are significant because they tell the tale of solar at the moment. Large scale solar farms mostly feature single axis trackers, allowing the panels to be oriented towards the east to catch the early morning sun, before following that giant fusion reactor across the sky during the day until the late afternoon.
Rooftop solar is mostly orientated towards the north, although the shape of individual building rooftops means that some rooftop PV is also faced east and west. It means that the collective output of rooftop PV doesn’t reach full capacity until later in the morning.
On Sunday, there was a clear picture of what happens next. Utility scale solar reaches a peak in the early morning, then as rooftop PV output increases, large scale solar is progressively curtailed.
At the time of the record market share on Sunday, utility scale solar was feeding 7,150 megawatts (MW) into the grid, and more than 900 MW was being curtailed. At 12.05pm, utility scale solar’s output had been cut back to 5,257 MW and 3,668 MW was being curtailed, according to data from Open Electricity.
Coal, meanwhile, had been injecting 10,600 MW into the grid before large scale solar woke up, and had its output reduced to as low as 5,200 MW in the middle of the day by the combined impact of utility scale and rooftop PV.
Coal generators are now learning how to dance around the so-called solar duck, either by ramping down to just 20 per cent of their rated output, or switching off altogether in a process known as “two shifting”.
The answer for big solar is different. They need storage. It’s one of the reasons why new large scale solar projects are being built with integrated batteries installed behind the meter. As solar-battery hybrids, they will be able to store that otherwise curtailed output and keep it for the evening peaks.
It means that, having sacrificed the midday lunch to rooftop PV, big solar can now start to eat the dinner of coal fired power. Australia’s first solar hybrid plant has been operating at Cunderdin (pictured above), where it regularly feeds power into the grid during the more profitable evening peaks, and on occasions through the night.
The first solar hybrid in the NEM, a relatively small facility at Quorn Park, has just had its first batteries installed and will start operating in the new year. It will be followed by several dozen new solar-hybrid project totalling multiple gigawatts and gigawatt hours of storage that may change the dynamics of the grid once again.
On Monday morning, the share of large scale solar in NSW also hit a new record share of 46.8 per cent in that state at 7.05 am, beating a previous record of 42.2 per cent reached on November 2. The rolling 7-day average share of big solar for the entire NEM also reached a new peak of 10.1 per cent, up from 9.6 per cent in January.
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Giles Parkinson is founder and editor-in-chief of Renew Economy, and founder and editor of its EV-focused sister site The Driven. He is the co-host of the weekly Energy Insiders Podcast. Giles has been a journalist for more than 40 years and is a former deputy editor of the Australian Financial Review. You can find him on LinkedIn and on Twitter.
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Photo: CN-STR / AFP via Getty Images
Brief by Michael Davidson and Sandy Qian
Published March 12, 2026
Over the past few years, China’s solar industry has entered a period of intense upheaval. Price wars and margin compression have forced industry leaders—including Jinko Solar, Trina Solar, and JA Solar—to report significant losses. These firms, along with LONGi Green Energy and Tongwei—the industry’s top five—slashed their workforce by over 30 percent in 2024. The market is facing industry consolidation and exits not seen in over a decade, as over 40 smaller firms have filed for bankruptcy, been acquired, or exited the market. Chinese regulators are accelerating this process, which will have ripple effects across global solar markets.
Several critical questions arise: Does this phase erode China’s leadership in solar, or entrench it further? Is it a window for others to close the gap, or a prelude to deeper market displacement? And when the current wave of capacity consolidation settles, what will the next global competitive order look like? The answer is already emerging. Rather than opening space for rivals to catch up, the current shocks are forging a more resilient Chinese solar core. By embedding deeper into global value chains and securing a technological lead, China is effectively reshaping the industry’s future trajectory to its own long-term advantage.
China’s first major solar shakeout began in 2012. During the 2009–2010 frenzy, Chinese solar majors expanded rapidly, leveraged by debt, to meet European and U.S. demand, exporting 90 percent of their output. When the 2011 Eurozone crisis hit, solar subsidies were gutted, sharply curbing demand. The sector hit its “darkest hour” in 2012 when U.S. anti-dumping duties and EU trade probes shuttered overseas markets, forcing hundreds of China’s solar firms to cease operations. The crisis peaked in 2013 with the landmark collapse of two solar giants, Suntech and LDK, prompting Beijing to salvage the industry through aggressive domestic installation targets and “Price Commitment” deals with the European Union. The crisis ultimately transformed China’s solar sector from a fragmented landscape into a more consolidated, globally competitive powerhouse.
Today, China is the undisputed global leader in the solar industry. It dominates the solar supply chain—by an extraordinary margin. In 2024, China produced 93.2  percent of the world’s polysilicon, 96.6  percent of wafers, 92.3  percent of photovoltaic (PV) cells, and 86.4  percent of PV modules, as China Photovoltaic Industry Association (CPIA) data show. However, a new wave of crises has once again swept through China’s solar industry, manifesting in a phenomenon the Chinese government has termed “involution” (internal fierce competition) to avoid the more politically charged concept of overcapacity. This occurs alongside growing resistance to Chinese firms “going global” (overseas expansion), as tightening U.S. supply chain tariffs and EU diversified-origin rules increasingly squeeze access to high-value markets.
China’s solar sector has been pushed into a fierce price war as manufacturing capacity far outstrips global demand. In 2024, the world had enough PV manufacturing capacity to produce more than twice the modules actually installed, according to the International Energy Agency. Much of this overshoot stems from China’s 2020 dual-carbon pledge, which spurred local governments to shower the solar industry with land, tax, and financing incentives—drawing in massive capital and triggering repetitive, low-quality investment. Since 2023, prices across the solar value chain have collapsed: Module prices dropped by half in 2023 and a further 25 percent in 2024, while polysilicon prices plunged from RMB 230,000/ton to RMB 65,000/ton in 2023—over a 70 percent decline—followed by another 40 percent drop in 2024, according to data from CPIA annual reports. In 2025, PV product prices fluctuated significantly, with wafers, cells, and modules remaining at depressed levels. Despite CPIA’s efforts to promote industry self-discipline and coordinate production to curb disorderly competition, price pressures have persisted.
The European Union is weaving a complex web of institutional hurdles focused on supply chain resilience and carbon transparency. The Foreign Subsidies Regulation already acts as a selective filter, triggering Chinese withdrawals from some major tenders, while the Net-Zero Industry Act mandates domestic manufacturing targets and caps single-source procurement at 50 percent—though member states retain leeway to bypass these rules for cost reasons. Furthermore, the proposed Industrial Accelerator Act seeks to cement these targets by streamlining permitting and creating “Industrial Acceleration Zones” to fast-track domestic clean-tech production. Additionally, while the Carbon Border Adjustment Mechanism does not directly tax PV modules, it imposes administrative burdens and indirect costs via carbon reporting for aluminum and steel components, gradually eroding Chinese exporters’ price advantage.
Unlike Europe’s attempt to balance trade with domestic growth, the United States has moved toward a more restrictive environment for Chinese solar products. While Section 301 imposes 50 percent tariffs, the actual exclusion is driven by anti-circumvention rulings that target Chinese-affiliated production in Southeast Asia and the Uyghur Forced Labor Prevention Act, which detains shipments at the border over supply chain compliance. These combined measures have effectively decoupled the U.S. market from the Chinese supply chain.
Facing a new downturn, China’s solar industry is reshaping its export strategy. Chinese exports have faced a rising wave of tariffs and other nontariff barriers due to concerns over dumping and unfair competition in multiple regions that are trying to develop their own industries. Total export value fell noticeably, yet monthly shipments remain sizable at the billion-dollar level. Even as trade barriers rise, export capacity keeps expanding, with firms diversifying into new markets to navigate the shifting global landscape. Initially driven by U.S. tariffs, the shift toward exporting intermediate products like wafers and cells—rather than targeted modules—may now help Chinese firms meet the European Union’s diversification requirements as U.S. restrictions on Southeast Asia intensify. At the same time, export destinations are diversifying: For PV cells, the Asia-Pacific region has become the main export market; for PV modules, the European Union remains the largest market by value, though its share is declining, while Asia-Pacific and Middle Eastern countries are rapidly absorbing a growing portion of demand.
Beyond trade, China has been extending its global value chain through overseas investment and manufacturing—potentially helping firms meet the European Union’s diversified origin requirements. Facilities in Indonesia, Vietnam, Malaysia, Saudi Arabia, and beyond help reposition in changing tariff environments, foster local employment, and position Chinese firms not merely as suppliers, but as builders and operators of complete solar ecosystems, embedding themselves deeper into regional markets.
 
In 2004, the global solar patent filing landscape was led by Japan (43.0 percent), with China holding only a minor 13.0 percent share. Just two decades later, the picture looks markedly different: China’s share of global patent applications has surged, reaching around 65.0 percent by 2024, according to CPIA’s 2024-2025 China PV Industry Annual Report. This dramatic transformation underscores how the transfer of the global PV manufacturing hub successfully propelled China into its role as the primary engine for technological advancement in the global solar industry.
China was initially a follower when first-generation solar technologies emerged. By leveraging innovations in manufacturing processes, it steadily boosted conversion efficiency while sharply cutting costs. China’s innovation cycles far outpace the rest of the global solar sector, rapidly advancing across first-generation sub-technologies—and the pace of turnover continues to accelerate. Within less than a decade, China progressed from relying on traditional c‑Si (BSF) cells to deploying large-scale improved c‑Si (PERC) cells, and more recently has been moving toward the widespread adoption of advanced c‑Si (TOPCon) technologies, which grew from 8 percent to 70 percent of the market in just three years.
Chinese solar firms have reached an average conversion efficiency (the proportion of sunlight converted into electricity) of mass‑produced n‑type TOPCon cells of 25.4 percent by 2024, up from 21.8 percent for PERC cells in 2018, as CPIA data show. Over the same period, U.S. First Solar concentrated on thin‑film CdTe technology, upgrading from Series 6 to Series 7 modules, with conversion efficiency rising from roughly 18 percent to 20 percent.
China strategically chose to forgo large-scale development of second-generation thin-film solar technologies. Thin-film cells, such as CdTe and CIGS, suffer from lower efficiency and higher production costs compared with crystalline silicon, making them less competitive in the mass market. By contrast, China’s focus on c‑Si technologies, especially as silicon prices have remained low, has allowed rapid scale-up and global market dominance.
Today, China is leading in third-generation solar technologies, particularly perovskite and tandem cells, positioning itself at the forefront of next-generation photovoltaics. According to the National Renewable Energy Laboratory’s January 2026 PV Best Research‑Cell Efficiency Chart, LONGi Green Energy holds the world record for the highest conversion efficiency in perovskite/silicon tandem technology, achieving 34.85 percent. Meanwhile, the Nanjing University/Renshine collaboration holds the world record for the highest conversion efficiency in perovskite tandem technology, achieving 30.1 percent. Hybrid designs integrating perovskite with silicon systems point to a clear path for next-generation, high-efficiency solar deployment.
China’s solar innovation operates on three intertwined layers. The central government sets strategy, funds R&D, and provides tax and subsidy incentives. Provincial authorities reinforce this with local development plans, land and tax perks, and targeted innovation funds. Guangdong backs perovskite and tandem cells, Zhejiang pushes higher module efficiency, Jiangsu and Anhui offer high-tech manufacturing subsidies, and Shanghai and Beijing support research-intensive pilot projects. Market actors—policy banks, industrial funds, and leading firms—inject capital, drive technology, and collaborate with research institutes, creating a self-reinforcing system that keeps China at the forefront of solar technology.
Provinces with robust industrial infrastructure and deep talent pools have emerged as focal points of technological leadership. Over the past two decades, the majority of valid invention patents (active patents protecting novel technical inventions) in China’s solar industry has been clustered in manufacturing centers such as Jiangsu, Zhejiang, and Anhui, as well as in talent-rich and research-intensive major hubs like Guangdong, Beijing, and Shanghai. These manufacturing centers are not only high-capacity production sites but also innovation engines, generating local ecosystems that complement the breakthroughs coming from major city research labs.
Chinese firms, most of them private, are the core driving force of PV innovation, accounting for over 75 percent of China’s total patent applications in the solar sector. The intense market volatility has exacerbated the technological arms race among solar firms, consequently reshaping the competitive landscape of the solar market. Leading Chinese solar firms are pursuing vertical integration, extending technological advantages and patent coverage across the industry’s core segments—including wafers, cells, and modules—as exemplified by JinkoSolar, Trina Solar, LONGi Green Energy, and JA Solar. Mid-tier players are also growing in size and importance, concentrating their resources on innovation within a single link of the value chain; for instance, Sungrow focuses exclusively on inverters, while Xinte Energy specializes solely in polysilicon.
Despite mounting pressures—from tightening domestic margins to rising trade barriers abroad—China’s position at the center of the global solar ecosystem remains largely intact. To navigate these shocks, Chinese firms have not only shifted export strategies and expanded into emerging markets, but have also embedded themselves deeper in global value chains, and secured an innovation edge that is reshaping the industry’s future trajectory.
Chinese regulators have taken a series of forceful measures to accelerate this process by driving market consolidation, tightening industry oversight, and strengthening enforcement against intellectual property (IP) infringement. In mid‑2025, a strategic acquisition fund was planned to acquire and retire roughly one‑third of the industry’s low‑efficiency polysilicon capacity. By December 2025, the platform was formally established with an RMB 3 billion capital base, backed by major upstream solar producers and the industry association. Concurrently, the Ministry of Industry and Information Technology (MIIT) intensified efforts by issuing the 2025 Annual Polysilicon Industry Special Energy Saving Supervision Task List in August 2025, targeting 41 companies for strict energy efficiency inspections. This was followed in late 2025 by a joint initiative from the National Intellectual Property Administration and MIIT to crack down on IP infringement through export bans and exclusion from state-owned utility procurement, shifting competition toward technological innovation.
The heightened price pressure has pushed global PV module prices to historic lows—often dipping below the 1 RMB/watt mark—making it economically difficult for non-Chinese manufacturers to compete on cost. Consequently, local solar manufacturers across Europe, India, and the United States, whose products are often 50 percent to 100 percent more expensive than imports, are grappling with severe financial losses and bankruptcy risk despite receiving government subsidies. Norwegian Crystals, a crucial silicon ingot supplier, officially declared bankruptcy in late 2023 due to unsustainable price pressure. Following this, Swiss-based solar firm Meyer Burger closed its main German factory in 2024, and despite its strategic shift to the U.S. market, it ultimately closed its U.S. plant and filed for bankruptcy in 2025.
The intense “involution” within China’s solar sector has simultaneously amplified global supply chain risk by cementing its near-absolute monopoly in the upstream segments—polysilicon, wafers, and cells. As a result, non-Chinese manufacturers globally remain heavily reliant on China for cost-competitive upstream materials, hindering their efforts to build localized supply chains. The U.S. domestic module manufacturing capacity grew from 14.5 gigawatts in 2023 to surpass 50 gigawatts in early 2025, yet wafers and cells still rely mostly on imports. This reliance exposes international developers to significant policy uncertainty and escalating compliance costs. In Europe, this manifests as intensified regulatory pressure for supply chain localization, alongside a slowdown in deployment as domestic constraints and cooling demand begin to outpace the influx of Chinese exports. In the United States, the challenge is compounded by a projected fall in demand due to the repeal of the Inflation Reduction Act and a generally anti-renewable federal stance.
This dynamic creates a significant technology gap challenge: Chinese firms are rapidly accelerating their transition to advanced N-type (such as TOPCon, HJT, and BC) and next-generation technologies at a scale and speed unmatched by international competitors. Consequently, global counterparts find that even when their products meet local manufacturing standards, their component efficiency and performance often lag behind China’s latest-generation offerings, leaving them vulnerable to technological obsolescence and market marginalization.
For global solar competitors, the key to success lies not in engaging in a price war, but rather in establishing unique competitive advantages through localization, technological differentiation, supply chain resilience, and the effective utilization of policy incentives. In certain contexts, global suppliers may find that collaboration with Chinese partners—rather than direct competition—helps local firms accelerate to the competitive frontier, with spillover benefits for emerging technology pathways. Competing head-to-head with Chinese suppliers through industry supports and high trade barriers can lead to a protected yet globally uncompetitive industry.
Michael Davidson is a senior associate (non-resident) with the Trustee Chair in Chinese Business and Economics at the Center for Strategic and International Studies and an associate professor at the School of Global Policy and Strategy and the Mechanical and Aerospace Engineering Department at the University of California San Diego. Sandy Qian is a research associate in the School of Global Policy and Strategy at the University of California San Diego.
This report is made possible by general support to CSIS. No direct sponsorship contributed to this report.
CSIS Briefs are produced by the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s).
© 2026 by the Center for Strategic and International Studies. All rights reserved.
Report by Ilaria Mazzocco and Ryan Featherston — October 22, 2025
Report by Scott Kennedy — March 2, 2026
Commentary by Ilaria Mazzocco — January 22, 2026
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The Sustainable and Holistic Integration of Energy Storage and Solar PV (SHINES) program develops and demonstrates integrated photovoltaic (PV) and energy storage solutions that are scalable, secure, reliable, and cost-effective.
The projects will work to dramatically increase solar-generated electricity that can be dispatched at any time – day or night – to meet consumer electricity needs while ensuring the reliability of the nation’s electricity grid. Achieving the SHINES goals is a critical step in the pathway toward enabling hundreds of gigawatts of solar to be integrated reliably and cost-effectively onto the electric grid. SHINES is part of the Energy Department’s Grid Modernization Initiative, which aims to accelerate the strategic modernization of the U.S. electric power grid and solve the challenges of integrating conventional and renewable sources, while ensuring a resilient energy system combining energy storage with central and distributed generation.
These awards were announced on January 19, 2016. Read the press release and Assistant Secretary David Danielson’s blog post. 
This is the first funding program within the Department of Energy focusing exclusively on connecting renewable power to storage. The solutions developed under this program incorporate dynamic load management, advanced forecasting techniques, utility communication and control systems, and smart buildings and smart appliances to work seamlessly to meet both consumer needs and the demands of the electricity grid. These solutions will enable widespread sustainable deployment of low-cost, flexible, and reliable PV generation, and provide for successful integration of PV power plants with the electric grid.
The widespread adoption of storage solutions will be a transformative influence on the current state-of-the-art of solar grid integration and will significantly contribute to an economically viable pathway toward energy efficient and sustainable integration of solar generation at much higher penetration levels than currently possible today. These solutions will enable widespread sustainable deployment of reliable PV generation and provide for successful integration of PV power plants with the electric grid at the system levelized cost of energy (LCOE) of less than 14 cent per KWh.
Location: Austin, Texas
SunShot Award Amount: $4,300,000
Awardee Cost Share: $4,337,683
Project Description: The goal of the Austin SHINES project is to demonstrate a solution adaptable to any region and market structure that offers a credible pathway to a LCOE of 14¢/kWh for solar energy when augmented by storage and other distributed energy resource management options. The solution aims to establish a template for other regions to follow to maximize the penetration of distributed solar PV. In addition, the proposed solution will enable distribution utilities to mitigate potential negative impacts of high penetration levels of PV caused by the intermittency and variability of solar production.
Location: Pittsburgh, Pennsylvania
SunShot Award Amount: $1,036,963
Awardee Cost Share: $1,038,083
Project Description: This project will develop and demonstrate a distributed, agent based control system to integrate smart inverters, energy storage, and commercial off-the-shelf home automation controllers and smart thermostats. The system will optimize PV generation, storage, and load consumption behaviors using high-performance, distributed algorithms.
Location: Oakbrook Terrace, Illinois
SunShot Award Amount: $4,000,000
Awardee Cost Share: $4,000,000
Project Description: This project will address availability and variability issues inherent in the solar PV technology by utilizing smart inverters for solar PV/battery storage and working synergistically with other components within a microgrid community. This project leverages on the DOE-funded microgrid cluster controller and is connected to the existing DOE-funded 12 megawatt IIT microgrid.
Location: Knoxville, Tennessee
SunShot Award Amount: $3,124,685
Awardee Cost Share: $3,240,262
Project Description: In this project, EPRI will work with five utilities to design, develop and demonstrate technology for end-to-end grid integration of energy storage and load management with photovoltaic  generation. The technology is a simple, two-level, and optimized control architecture. This technology will be demonstrated and its effectiveness verified at three field locations. 
Location: Boston, Massachusetts
SunShot Award Amount: $3,493,921
Awardee Cost Share: $3,560,744
Project Description: This project will develop and demonstrate a highly scalable, integrated PV, storage, and facility load management solution. Through the SunDial Global Scheduler, the system tightly integrates PV, energy storage, and aggregated facility load management to actively manage net system power flows to and from the feeder, regardless of whether these individual components are co-located at the same site, or distributed at different sites.
Location: Honolulu, Hawaii
SunShot Award Amount: $2,437,500
Awardee Cost Share: $2,437,500
Project Description: This project will demonstrate successful SHINES deployments and will show the system-level benefits of enhanced utility visibility and control of distribution system/edge-of-network electricity resources. This project will enable proliferation of a reliable base of PV and storage distributed technologies that offer more plug-and-play customer options for grid participation, and provide cost-effective “grid response” capabilities to system operators.
Learn more about SunShot’s other systems integration funding programs.
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This photovoltaic plate solves a decade-long solar problem — It amplifies light thousands of times before turning it into energy  energiesmedia.com
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