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

High temperature stable inorganic CsPbX3 (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 CsPbX3 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, HPbI3, and HPbBr3) is developed to replace the traditional precursors (CsI, PbI2, and PbBr2) 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 CsPbI2Br film deposition. The CsPbI2Br 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 CsPbI2Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air. Herein, a novel precursor (HCOOCs and HPbX3) for deposition of high‐quality CsPbI2Br films, irrespective of humidity is presented. CsPbI2Br cells prepared in an atmosphere with 30% and 91% relative humidity exhibit efficiencies of 16.1% and 15.1%, respectively, which are the highest among all inorganic CsPbX3 (X: I, Br, or mixed halides) PSCs prepared in a medium or high humid atmosphere.