Publication date: 15th December 2025
Lead-free perovskite-inspired materials are emerging as attractive absorbers for sustainable photovoltaics in outdoor and indoor Internet-of-Things applications (1-2). Among them, Cs2AgBi2I9 is a particularly promising candidate owing to its wide bandgap (~1.78 eV), strong blue-rich absorption, low exciton binding energy (~40 meV), weak electron–phonon coupling and large-polaron formation, which together favor efficient charge generation, transport and defect tolerance (1). Here we show that targeted hole-transport-layer (HTL) engineering is key to unlocking the photovoltaic potential of Cs2AgBi2I9 under both 1-sun and indoor white LED (WLED) illumination.
We study mesoscopic n–i–p devices with the architecture glass/FTO/c-TiO2/mp-TiO2/ Cs2AgBi2I9/HTL/Au and compare the conventional small-molecule HTL Spiro-OMeTAD with conjugated polymer HTLs, focusing on a fluorinated benzothiadiazole-based polymer (FBT, PPDT2FBT). FBT provides more favourable energetic alignment with the Cs2AgBi2I9 valence band, as well as improved film coverage and interfacial contact. Consequently, FBT-based devices exhibit enhanced short-circuit current density, open-circuit voltage and fill factor compared with Spiro-OMeTAD- and PCPDTBT-based references. The champion FBT device delivers a power conversion efficiency (PCE) of 3.96% under AM 1.5G illumination, representing a substantial advance over earlier Cs2AgBi2I9 solar cells without HTL optimisation.
The benefit of FBT HTLs is even more pronounced under indoor conditions. Under 1000 lux WLED illumination (6500 K), FBT-based devices achieve an indoor PCE close to 9%, demonstrating efficient harvesting of low-intensity blue-rich spectra. High fill factors and open-circuit voltages are retained as the light intensity decreases, underscoring their suitability for practical indoor operation. These efficiencies remain below the spectroscopically limited maximum predicted for Cs2AgBi2I9 under WLED spectra, indicating substantial headroom for further performance gains via combined absorber and interface engineering.
To elucidate the origin of enhanced performance, we employ electrochemical impedance spectroscopy (EIS), transient photovoltage (TPV), transient photocurrent (TPC) and Capacitance-Voltage (CV). EIS shows higher recombination resistance and reduced low-frequency capacitance in FBT-based cells, evidencing suppressed non-radiative recombination and slower ion migration at the Cs2AgBi2I9/HTL interface. TPV and TPC reveal longer carrier lifetimes, faster charge extraction and lower series resistance, while CV confirms improved energetic alignment, a larger built-in potential and reduced interfacial trap density.
Overall, combining the favourable bulk properties of Cs2AgBi2I9 with application-driven HTL engineering enables a clear step-change in performance for lead-free perovskite-inspired photovoltaics. By translating interface design principles from lead-halide perovskites to this bismuth-based analogue, we outline a general blueprint for engineering charge-selective contacts in perovskite-inspired absorbers, with FBT emerging as a powerful HTL platform to bridge materials design and device application and to narrow the gap to the theoretical indoor efficiency limit of this lead-free absorber.
M.K. and P.V. thank the SPINIP-HORIZON-MSCA-2023-PF-1 (Project Grant No: 101150357) for funding.M.K. and P.V. acknowledge the support of the SPOT-IT project founded by the CETPartnership, the Clean and Energy Transi-tion Partnership under the 2022 CET Partnership joint call for researchproposal, co-founded by the European Commission (GA 101069750) and the founding of the organizations detailed on https://cetpartnership.eu/funding-agencies-and-call-modules.
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