Benzylhydrazinium Chloride Interface Engineering Boosts Performance of Wide‐Bandgap Perovskite and Tandem Solar Cells

Wide‐bandgap (WBG) perovskites have gained great attention as promising top‐cell absorbers with the potential of enabling efficient perovskite/crystal silicon tandem solar cells (TSCs). However, defects generated during the deposition of perovskite substantially degrade device performance and operational stability. Here, we demonstrate an interfacial engineering strategy for efficient defect passivation of Cs 0.05 (FA 0.77 MA 0.23 ) 0.95 Pb(I 0.77 Br 0.23 ) 3 WBG perovskite (1.68 eV) films based on small organic molecule, benzylhydrazine monohydrochloride (BHC). The BHC, incorporating both primary amine (–NH 2 ) and protonated ammonium (–NH 3 + ) functional groups, enables bifunctional synergistic passivation of undercoordinated Pb 2+ and cation vacancies on perovskite surface. This strategy boosts the power conversion efficiency (PCE) of WBG perovskite solar cells from 19.01% to 20.87%, with simultaneous improvements in all photovoltaic parameters including open‐circuit voltage, short‐circuit current density, and fill factor. Unencapsulated devices retain 80% of initial PCE after 960 h storage under ambient conditions. When integrated into perovskite/silicon TSCs, a PCE of 29.56% is achieved.