Precursor Engineering for Ambient-Compatible Antisolvent-Free Fabrication of High-Efficiency CsPbI2Br Perovskite Solar Cells

High temperature stable inorganic CsPbX 3 (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high-efficiency tandem solar cells. Unfortunately, the current high-efficiency CsPbX 3 perovskite solar cells (PSCs) are prepared in vacuum, a moisture-free glovebox or other low-humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI 3 , and HPbBr 3 ) is developed to replace the traditional precursors (CsI, PbI 2 , and PbBr 2 ) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH•Cs + ) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI 2 Br film deposition. The CsPbI 2 Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI 2 Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.