Cation redistribution enables low-temperature stabilization of MA-free FAPbI₃

Metal halide perovskites have reshaped photovoltaic research, with formamidinium lead triiodide (FAPbI₃) standing out as a top contender for single-junction solar cells due to its ideal band gap. However, obtaining its photoactive black phase, especially at low processing temperatures, remains a key challenge, as it tends to degrade into the undesirable yellow δ-phase. While conventional strategies to stabilize the cubic FAPbI₃ phase rely on methylammonium (MA)-assisted formulations, which can compromise long-term stability and inevitably increase the band gap, we present an MA-free approach that combines a small amount of cesium (2%) with a binary antisolvent system. The addition of a polar alcoholic component promotes FA⁺ redistribution during film formation, enabling uniform incorporation of the limited Cs⁺ content throughout the perovskite layer. As a result, the α-phase is stabilized at 100 °C, while maintaining a narrow band gap close to 1.5 eV, demonstrating a promising route toward the low-temperature processing of FA-based photovoltaic devices.