Oxalate Pushes Efficiency of CsPb(0.7)Sn(0.3)IBr(2) Based All‐Inorganic Perovskite Solar Cells to over 14%

All‐inorganic CsPbIBr(2) perovskite solar cells (PSCs) have recently gained growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the relatively low device efficiency hinders its further development. Herein, highly efficient and stable CsPb(0.7)Sn(0.3)IBr(2) compositional perovskite‐based inorganic PSCs are fabricated by introducing appropriate amount of multifunctional zinc oxalate (ZnOX). In addition to offset Pb and Sn vacancies through Zn(2+) ions incorporation, the oxalate group can strongly interact with undercoordinated metal ions to regulate film crystallization, delivering perovskite film with low defect density, high crystallinity, and superior electronic properties. Correspondingly, the resulting device delivers a champion efficiency of 14.1%, which presents the highest reported efficiency for bromine‐rich inorganic PSCs thus far. More importantly, chemically reducing oxalate group can effectively suppress the notorious oxidation of Sn(2+), leading to significant enhancement on air stability.