Buried Interface Modification Toward Efficient CsPbI _2.2 Br _0.8 Based Monolithic Perovskite/Organic Tandem Solar Cells

Abstract Wide‐bandgap perovskite sub‐cells (WPSCs), one of the most crucial components of perovskite‐based tandem solar cells (PTSCs), play a critical role in determining the performance of tandem devices. However, confined by the compromised crystallization properties of wide‐bandgap perovskites, WPSCs exhibit significantly lower efficiency than their theoretical limit. In particular, for n‐i‐p structured all‐inorganic WPSCs (AIWPSCs), severe nonradiative recombination due to the buried interface defects severely decreases the photovoltaic performance. Herein, an efficient propionate group (PA ⁻ ) based ionic liquid, methylamine propionate (MAPA), is introduced into the perovskite/electron‐transport layer (ETL) interface to passivate the buried interface of AIWPSCs. The intense interaction between the PA ⁻ and Pb–Pb dimer effectively heals the defects at the buried interface and facilitates a more homogeneous elemental distribution in the perovskite film. As a result, CsPbI _2.2 Br _0.8 ‐based AIWPSCs with a high power conversion efficiency (PCE) of 18.29% and open‐circuit voltage ( V _OC ) of 1.33 V are obtained, which illustrates the superiority of MAPA in optimizing the performance of AIWPSCs. Moreover, by integrating these AIWPSCs with small‐bandgap organic solar cells (SOSCs), high performance n‐i‐p structured all‐inorganic perovskite/organic tandem solar cells (AIPOTSCs) with a high PCE of 23.19% and V _OC of 2.08 V are also achieved.