Influence of chloride on the crystallization and photostability of mixed-halide FAPbBr3-x Cl x perovskites for photovoltaics.

Widening the bandgap of bromide-based metal-halide perovskites by incorporating chloride allows for tunable absorption onsets in the 2.3-3.0 eV range. While mixed Br/Cl perovskites have been extensively studied for light-emitting diodes and photodetectors, their photovoltaic performance remains mostly unexplored. Here, formamidinium lead halide (FAPbBr3-x Cl x ) perovskites, with x = 0.0-2.4, are studied to understand how chloride incorporation shapes structural, optoelectronic, and photovoltaic properties. Chloride is fully incorporated, as evidenced by near-linear bandgap increase and lattice contraction, but it profoundly alters the crystallization process. Higher chloride concentrations retard the nucleation, leading to larger grains with more irregular shapes and increased grain merging. The mixed Br/Cl perovskites experience light-induced halide segregation, with faster initial segregation kinetics at higher chloride fractions. The bromide- and chloride-rich domains remix in the dark with time. For x = 0.6, solar cells benefit from incorporating small amounts of cesium, which reduces hysteresis and significantly improves light stability. Top passivation by 4-fluorophenethylammonium chloride to suppress interfacial recombination, combined with a ternary-fullerene blend electron-transport layer (PCBM, CMC, ICBA), raises the open-circuit voltage (V OC) to 1.66 V and increases the power conversion efficiency (PCE) from 5.5% to 7.3% at an optical bandgap of 2.38 eV. Unlike lower-bandgap perovskites where losses are dominated by the electron-selective interface, mixed Br/Cl perovskites show comparable recombination losses at the electron- and hole-selective interfaces. At higher chloride fractions (x > 0.6), nonradiative recombination increases and the open-circuit voltage does not improve despite the widening of the bandgap, limiting device performance. PCEs of 5.1% and 3.0% are obtained for x = 1.2 and 1.8, making optimization of the perovskite/transport-layer interfaces essential for such wide-bandgap absorbers.