Simulations Show Graded Lead-Free Double Perovskites Can Reach Over 24 Percent Efficiency
A comprehensive numerical modeling investigation has demonstrated that lead-free vacancy-ordered double perovskite solar cells can achieve theoretical power conversion efficiencies exceeding twenty-four percent when engineered with optimized carrier transport layers and graded absorber bandgaps. Conducted by researchers Farhana Bari Sumona and Nahid-Al Mahmud and published in the journal Discover Electronics, the computational study evaluated the photovoltaic potential of cesium platinum iodide and rubidium platinum iodide absorbers. Conventional perovskite photovoltaics rely heavily on lead-halide compositions, which have achieved certified laboratory efficiencies above twenty-five percent but continue to face persistent commercialization hurdles due to environmental toxicity and structural vulnerability under ambient conditions. Vacancy-ordered double perovskites have emerged as compelling, non-toxic alternatives capable of addressing chemical degradation and heavy metal contamination. To establish a rigorous comparative roadmap, the investigators deployed one-dimensional device simulations to explore how selective electron and hole extraction materials, absorber defect densities, compositional bandgap tuning, and elevated operating temperatures govern device performance.
The research team utilized the Solar Cell Capacitance Simulator, widely known as SCAPS-1D, to execute one-dimensional semiconductor transport modeling under standard AM1.5G solar illumination at an incident power of one thousand watts per square meter. The numerical framework simultaneously solves the Poisson equation for spatial electrostatic potential alongside electron and hole continuity and drift-diffusion transport equations. To capture realistic recombination kinetics across diverse device junctions, the simulator incorporates the Shockley-Read-Hall non-radiative recombination model alongside specified minority carrier lifetimes, carrier diffusion lengths, and dielectric properties. Across the investigation, the researchers modeled a total of eighteen distinct device architectures by pairing the two double perovskite absorbers with three separate electron transport layers and three hole transport layers. The candidate electron transport layers included zinc selenide, niobium pentoxide, and silver sulfide, while the evaluated hole transport candidates comprised copper iodide, magnesium-doped copper chromium oxide, and graphene oxide, each evaluated on fluorine-doped tin oxide substrates with gold rear contacts.
Comparative evaluations revealed that the architectural pairing of zinc selenide as the electron transport layer and copper iodide as the hole transport layer delivered the highest baseline photovoltaic performance for both double perovskite absorbers. In this optimal baseline configuration, designated as fluorine-doped tin oxide, zinc selenide, perovskite absorber, copper iodide, and gold, the rubidium platinum iodide device generated a baseline power conversion efficiency of 20.96 percent. This device demonstrated a short-circuit current density of 34.97 milliamperes per square centimeter, an open-circuit voltage of 0.843 volts, and a fill factor of 71.10 percent. In contrast, the baseline cesium platinum iodide device produced a baseline power conversion efficiency of 20.29 percent, delivering a short-circuit current density of 28.41 milliamperes per square centimeter, an open-circuit voltage of 1.111 volts, and a fill factor of 64.29 percent. These distinct output profiles highlight how chemical composition directly shapes charge collection and interfacial electrical characteristics.
Detailed examination of the baseline current density-voltage characteristics and quantum efficiency spectra illuminated stark operational trade-offs between the two perovskite absorber formulations. The rubidium platinum iodide cell demonstrated substantially superior charge extraction efficiency, manifested in a noticeably higher fill factor and an elevated short-circuit current density. Spectral quantum efficiency curves indicated that both materials exhibit broad optical responses spanning three hundred to nine hundred nanometers, yet rubidium platinum iodide demonstrated enhanced absorption within the near-infrared portion of the solar spectrum, directly boosting photocurrent generation. The authors attributed this performance advantage to favorable band alignment and the larger ionic radius of rubidium, which diminishes carrier recombination and supports smoother charge extraction across transport interfaces. Conversely, the cesium platinum iodide device exhibited a considerably higher open-circuit voltage, reflecting a wider built-in potential and distinct band edge positioning that preserves photogenerated electrical driving force despite lower overall extracted current density.
The study also analyzed why non-optimal transport layer combinations produced significantly depressed photovoltaic metrics across the eighteen simulated architectures. Specifically, devices constructed using niobium pentoxide or silver sulfide alongside magnesium-doped copper chromium oxide or graphene oxide exhibited elevated interface recombination losses and reduced power conversion metrics. The investigators noted that suboptimal transport pairings introduce severe energy band alignment mismatches at the absorber boundaries, creating interfacial energy barriers that impede smooth carrier injection into external electrodes. Furthermore, variations in intrinsic carrier mobilities among alternative transport films failed to suppress charge accumulation near contact interfaces. In contrast, zinc selenide presents an ideal electron affinity and bandgap that closely matches both double perovskite absorbers, while copper iodide exhibits high hole mobility and favorable valence band alignment, ensuring rapid charge separation and suppressing parasitic back-recombination before extracted carriers reach the electrical contacts.
To overcome fundamental carrier collection limitations and minimize bulk recombination, the authors modeled a continuous bandgap grading strategy across the absorber layer. In these graded configurations, the absorber bandgap was modeled as a linear transition expanding from 1.2 electron volts at the electron transport layer interface to 2.0 electron volts at the hole transport layer interface. This gradual variation alters the conduction and valence band edges across the device interior, establishing a built-in quasi-electric field that accelerates photogenerated electrons toward the front contact while driving holes toward the rear contact. The modeled band bending significantly enhanced carrier separation dynamics and reduced localized carrier accumulation. The results demonstrated substantial performance gains for both materials, elevating the simulated efficiency of the rubidium platinum iodide cell to 22.67 percent, while the cesium platinum iodide cell reached a peak power conversion efficiency of 24.35 percent, demonstrating an exceptional response to bandgap engineering.
In the bandgap-graded architecture, the cesium platinum iodide cell attained an open-circuit voltage of 0.999 volts, a short-circuit current density of 34.39 milliamperes per square centimeter, and a fill factor of 70.95 percent, positioning its overall efficiency of 24.35 percent remarkably close to values traditionally achieved by toxic lead-halide perovskite devices. Meanwhile, the bandgap-graded rubidium platinum iodide device achieved a short-circuit current density of 38.27 milliamperes per square centimeter, an open-circuit voltage of 0.802 volts, and an enhanced fill factor of 73.84 percent. The authors explained that while cesium platinum iodide exhibited a greater percentage efficiency improvement through compositional bandgap tuning, rubidium platinum iodide retained its commanding advantage in current output. From a fabrication perspective, the researchers suggested that such continuous gradients could theoretically be achieved through controlled precursor stoichiometry, sequential deposition techniques, or precision vapor deposition, although preserving crystallographic phase stability across compositionally graded layers presents an ongoing challenge for experimental synthesis.
Because defect states within the semiconductor bandgap act as primary non-radiative recombination centers, the investigators performed a sensitivity analysis across absorber defect densities ranging from 10 to the 12th power to 10 to the 17th power defects per cubic centimeter. The simulation demonstrated that device efficiency degrades severely as defect concentrations increase, driven by Shockley-Read-Hall recombination processes that drastically curtail carrier diffusion lengths and operational carrier lifetimes. For the rubidium-based baseline device, elevating the defect concentration caused the power conversion efficiency to plummet from 23.24 percent down to 6.07 percent, accompanied by a drop in fill factor from 76.88 percent to 39.95 percent. The cesium-based device exhibited greater resilience under equivalent conditions, though its efficiency still declined from 29.54 percent to 20.29 percent as defect density escalated. The authors highlighted that while defect densities near 10 to the 12th power represent idealized theoretical targets, experimental approaches such as additive treatments, ligand engineering, and surface passivation are essential to approach clean defect regimes.
Thermal resilience represents another vital benchmark for real-world photovoltaic deployment, prompting the researchers to model steady-state device performance across an elevated temperature window extending from 300 to 350 Kelvin, corresponding to standard ambient up to seventy-seven degrees Celsius. Both device configurations demonstrated remarkable electronic stability across this range, avoiding catastrophic thermal degradation. The rubidium platinum iodide cell experienced a moderate efficiency reduction from 20.96 percent down to 19.87 percent as thermal energy accelerated carrier scattering and stimulated intrinsic recombination. Remarkably, the cesium platinum iodide device maintained an almost entirely flat efficiency profile, shifting marginally from 20.29 percent at 300 Kelvin to 20.28 percent at 350 Kelvin. Intriguingly, the open-circuit voltage of the rubidium-based cell exhibited an initial rise at moderate temperatures due to thermally excited carrier mobility and expanded quasi-Fermi level splitting, before thermal agitation and defect-driven recombination dominated at higher temperatures, causing subsequent voltage reduction.
To provide clear perspective for solar engineering researchers, the study explicitly delineated several boundaries and structural limitations inherent to its numerical methodology. The authors emphasized that the SCAPS-1D software relies on one-dimensional drift-diffusion formulations that cannot resolve spatial variations in internal light intensity, three-dimensional optical reflection losses, or non-uniform photon absorption profiles. Furthermore, the simulation evaluates steady-state electrical transport and does not model time-dependent physical degradation phenomena, such as continuous halide ion migration, lattice expansion, interfacial chemical reactions, delamination caused by thermal expansion mismatches, or phase transitions under persistent environmental moisture. Consequently, the observed thermal tolerance reflects the intrinsic electronic tolerance of the ideal semiconductor band structures rather than a guarantee of indefinite physical longevity. Practical experimental devices must also contend with series contact resistances, pinhole formation, and parasitic absorption within transparent conductive substrates that were not fully captured in the idealized numerical framework.
The comparative findings offer a structured, data-driven framework for experimentalists seeking to fabricate environmentally sustainable, high-efficiency perovskite solar cells without toxic lead. The authors concluded that choice of absorber should align directly with specific photovoltaic deployment targets: rubidium platinum iodide provides an exceptional platform for applications requiring superior photocurrent generation, robust charge extraction, and high fill factors, whereas cesium platinum iodide represents the preferred candidate when elevated operating voltage and bandgap tunability are prioritized. To translate these numerical predictions into physical hardware, future experimental efforts must focus on optimizing thin-film crystallization methods, developing effective anti-reflective surface coatings, and deploying interface passivation molecules to suppress interfacial trap density. By systematically mapping charge transport dynamics, bandgap engineering benefits, and thermal boundaries, this numerical study provides actionable theoretical guidance for advancing non-toxic double perovskite solar technology toward commercial feasibility.
Subject of Research: Numerical modeling and comparative optimization of lead-free Cs2PtI6 and Rb2PtI6 double perovskite solar cells
Article Title: Utilizing numerical modeling to optimize lead-free double perovskite solar cells based on Cs2PtI6 and Rb2PtI6 with graded bandgaps for enhanced photovoltaic performance and thermal response
Article References: Sumona, F. B., & Mahmud, N.-A. (2026). Utilizing numerical modeling to optimize lead-free double perovskite solar cells based on Cs2PtI6 and Rb2PtI6 with graded bandgaps for enhanced photovoltaic performance and thermal response. Discover Electronics, 3(1), Article 165. https://doi.org/10.1007/s44291-026-00316-6
Image Credits: AI Generated
DOI: 10.1007/s44291-026-00316-6
Keywords: Cs2PtI6, Rb2PtI6, Double Perovskites, Lead-Free Solar Cells, SCAPS-1D Simulation, Bandgap Grading, Photovoltaic Performance, Power Conversion Efficiency, Electron Transport Layer, Hole Transport Layer, Zinc Selenide, Copper Iodide, Defect Density Analysis, Shockley-Read-Hall Recombination, Thermal Response Modeling, Open-Circuit Voltage, Short-Circuit Current Density
News Source: Denise Maddox. (November 15, 2026). Simulations Show Graded Lead-Free Double Perovskites Can Reach Over 24 Percent Efficiency. Scienmag.
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