Reducing Open‐Circuit Voltage Losses in Wide‐Bandgap FAPbBr ₃ Perovskite Solar Cells for Continuous Unassisted Light‐Driven Water Splitting

ABSTRACT Wide‐bandgap perovskite solar cells (PSCs) based on formamidinium lead bromide (FAPbBr 3 ) hold promise for unassisted solar‐driven water splitting using a single‐junction absorber. To meet the required thermodynamic voltage and overpotentials, nonradiative recombination in the PSC must be suppressed. Herein, a dual‐passivation strategy is used that combines formamidinium thiocyanate (FASCN) for bulk passivation and 1,3‐propane diammonium iodide (PDAI 2 ) for top surface passivation. FASCN promotes the growth of larger perovskite grains, while PDAI 2 mitigates surface defects, together leading to suppressed nonradiative recombination and improved charge extraction. Further optimization of the electron‐transport layer (ETL) using a ternary fullerene blend (PCBM, CMC, ICBA) in a 1:1:1 weight ratio improves energetic alignment and suppresses interfacial energy losses, resulting in an increase in open‐circuit voltage from 1.41 to 1.60 V and a power conversion efficiency of 9.4% in small‐area devices. After scaling up to a 1.0 cm 2 active area, the PSC is integrated into a coupled photovoltaic‐electrochemical (PV‐EC) system for continuous solar‐driven water splitting employing platinum (Pt) as the hydrogen evolution catalyst and ruthenium dioxide (RuO 2 ) as the oxygen evolution catalyst. The system achieves a solar‐to‐hydrogen (STH) efficiency of 6.5%, outperforming single‐absorber PV‐EC systems reported to date.