PEDOT:PSS is a conductive polymer blend widely used as a hole transport and electrode interlayer in solar cells. It is attractive because it is highly transparent, allowing efficient light to reach the active layer, and it has good hole conductivity along with a suitable work function that enables efficient charge extraction at the electrode interface. In addition, it can be processed from solution to form smooth, uniform films, which improves device quality and reproducibility.
However, in tandem solar cells it can become problematic because its acidic and hygroscopic nature can degrade sensitive layers such as perovskites. It can also contribute to interfacial instability and parasitic losses, which ultimately reduce long-term efficiency and operational stability.
With this in mind, a group of researchers from the Hong Kong University of Science and Technology (HKUST) has designed a PEDOT:PSS-free all-perovskite tandem solar cell that utilizes a phenothiazine-functionalized phosphonic acid monolayer as the hole transport layer (HTL).
“We designed two-terminal monolithic all-perovskite tandem solar cells that stack two perovskite absorbers with complementary bandgaps in one structure, offering a promising route to surpass the efficiency limits of single-junction solar cells while retaining the advantages of lightweight and potentially low-cost manufacturing,” the research’s corresponding author, Fengzhu Li, told pv magazine. “A major challenge lies at the buried interface of the narrow-bandgap tin-lead perovskite subcell. Many high-performance devices rely on PEDOT:PSS as a hole-transport material, but this polymer can absorb moisture, interact unfavorably with perovskite precursors and promote phase segregation during crystallization. These issues can undermine both device performance and stability.”
In the study “Interface-mediated crystallization enables PEDOT:PSS-free all-perovskite tandems with 29.1% efficiency and enhanced durability,” published in Joule, Li and his colleagues explained that they used in-situ characterization to reveal how PEDOT:PSS induces an unstable crystallization pathway in mixed tin-lead perovskite films. “We then replaced PEDOT:PSS with a phenothiazine-functionalized self-assembled monolayer, known as 4PAPT, which promotes direct phase transition, improves crystal orientation and suppresses non-radiative recombination losses,” he went on to say.
Compared to PEDOT:PSS, 4PAPT reportedly enables faster and more direct perovskite crystallization, suppresses intermediate tin iodide–dimethyl sulfoxide (SnI₂–DMSO) phase formation, promotes preferred orientation, and improves interfacial stability, resulting in reduced defect density and enhanced carrier transport.
The researchers also found that that PEDOT:PSS is more susceptible to DMSO-induced degradation and instability during processing, while 4PAPT maintains stable wetting behavior, preserving interfacial integrity throughout deposition. Overall, 4PAPT was found to promote faster, more uniform crystallization and higher-quality mixed tin–lead perovskite films compared to PEDOT:PSS. In situ ultraviolet–visible (UV–vis) spectroscopy also showed faster absorption evolution and phase transition on 4PAPT, while PEDOT:PSS exhibited slower kinetics.
The team built the all-perovskite tandem solar cell using a stacked device architecture on indium tin oxide (ITO) transparent electrodes. The bottom cell consisted of a wide-bandgap (WBG) perovskite absorber, interfaced with a carbazole-based naphthalene derivative (CbzNaph) as the hole-selective layer, followed by fullerene (C60) as the electron transport layer and atomic layer deposited tin dioxide (ALD-SnO₂) as the recombination layer, completed with a gold (Au) electrode. The top cell was built with the SAM as the hole transport interface, combined with a narrow-bandgap (NBG) perovskite absorber, C60 electron transport layer, bathocuproine (BCP) as the exciton blocking layer, and a silver (Ag) back electrode.
“Our molecular interface strategy enabled a narrow-bandgap single-junction perovskite cell with 23.2% efficiency,” Li further explained. “We the translated the strategy into monolithic all-perovskite tandem solar cells by developing the SAM combining thiol and phosphonic acid anchoring groups on SnO2/Au surfaces. The resulting dense molecular interlayer maintained efficient charge transport while avoiding the instability associated with PEDOT:PSS.”
“The PEDOT:PSS-free all-perovskite tandem solar cell achieved a reported efficiency of 29.1%, the highest reported efficiency to date for PEDOT:PSS-free all-perovskite tandem configurations,” Li added. “Encapsulated devices retained 90% of their initial efficiency after more than 800 hours of maximum power point tracking under simulated one-sun illumination at around 40 C.”
“The instability of PEDOT:PSS is not only a materials problem; it also affects how the perovskite film forms at the buried interface. By replacing this polymer with molecularly designed self-assembled monolayers, we were able to control crystallization from the start and carry that benefit into high-efficiency tandem devices,” co-author Yen-Hung Lin emphasized. “Perovskite tandem solar cells have reached a stage where every interface matters. Our study shows a critical principle: molecular interfaces can be designed as active platforms to control crystallization, reduce energy loss, facilitate charge transport and improve long-term stability across different tandem architectures.”
This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: [email protected].
Comments
Please login to comment
Thursday, July 9, 2026
11:00 am – 12:30 pm CEST, Berlin, Paris, Madrid
Thursday, June 18, 2026
2:00 pm – 3:00 pm CEST, Berlin, Paris, Madrid
Be part of the high-level European conference on solar and energy storage, exploring bankable BESS projects, warranties, and energy management for residential and C&I sectors
The new pv magazine Global May issue is now available!
Mountains to climb
Available in print and digital formats.
Entries open in seven categories: Modules, Inverters, BoS, BESS, Manufacturing, Sustainability, Projects.
April 01 – August 31, 2026
A two-day conference in Austin, Texas, bringing together leaders in US solar manufacturing, equipment specification, and factory execution.
Saudi Arabia is accelerating its clean energy transition—join the SunRise Arabia Clean Energy Conference 2026 in Riyadh to explore how solar PV and energy storage are powering its digital economy.
Showcase your brand across all our platforms: from 13 websites in 7 languages to our magazines, daily newsletters, industry events and more. Reach your audience the right way!
We are participating in Intersolar 2026 again this year! Visit us at our Booth Hall 2 A2.250 to discuss the latest trends within the photovoltaic industry with the pv magazine team.
June 23-25, 2026 | MUNICH, GERMANY
You have no items in your basket.
