MACl as a Multirole Additive in Equimolar I/Br Wide‐Bandgap Perovskites: Controlling Halide Segregation and Ensuring Device Stability for Indoor Photovoltaics

Indoor photovoltaics (IPVs) based on metal halide perovskites offer a promising power source for self‐sustaining Internet of Things (IoT) devices under artificial lighting. However, Br‐containing wide‐bandgap perovskites are prone to halide segregation both immediately after film formation and under illumination, posing challenges for long‐term compositional stability. In this study, we develop FA 0.85 Cs 0.15 PbI 1.5 Br 1.5 perovskite thin films with methylammonium chloride (MACl) additives, employing a wide‐bandgap composition (I/Br = 1:1, ~1.86 eV) well aligned with the visible spectrum of white light‐emitting diodes. Devices incorporating MACl exhibited enhanced open‐circuit voltage and trends toward reduced leakage current and suppressed trap‐assisted recombination. Comprehensive material analyses revealed MACl‐induced enhancements in grain connectivity and optoelectronic quality, as well as improved initial halide homogeneity and reduced light‐induced halide segregation. The MACl‐treated devices showed no degradation over 35 days of ambient storage without encapsulation, whereas some MACl‐free devices exhibited temporary shorting, likely caused by irregular macro voids formed through solvent outgassing during aging.