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

Abstract 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.