Hydrazide Derivatives for Defect Passivation in Pure CsPbI 3 Perovskite Solar Cells

All‐inorganic CsPbI 3 perovskite presents preeminent chemical stability and a desirable band gap as the front absorber for perovskite/silicon tandem solar cells. Unfortunately, CsPbI 3 perovskite solar cells (PSCs) still show low efficiency due to high density of defects in solution‐prepared CsPbI 3 films. Herein, three kinds of hydrazide derivatives (benzoyl hydrazine (BH), formohydrazide (FH) and benzamide (BA)) are designed to reduce the defect density and stabilize the phase of CsPbI 3 . Calculation and characterization results corroborate that the carboxyl and hydrazine groups in BH form strong chemical bonds with Pb 2+ ions, resulting in synergetic double coordination. In addition, the hydrazine group in the BH also forms a hydrogen bond with iodine to assist the coordination. Consequently, a high efficiency of 20.47 % is achieved, which is the highest PCE among all pure CsPbI 3 ‐based PSCs reported to date. In addition, an unencapsulated device showed excellent stability in ambient air.