A Novel 4,4'-Bipiperidine-Based Organic Salt for Efficient and Stable 2D-3D Perovskite Solar Cells

The efficiency of metal halide perovskite solar cells (PSCs) has dramatically increased over the past decade (formerly 3.8%, now 25.5%). It has been widely demonstrated that the defects passivation of perovskite photo-active layer plays a vital role in increasing the efficiency and improving the stability of PSCs. In this study, we developed a novel 4,4'-bipiperidine (BiPi)-based organic salt with good stability and successfully introduced this ligand into perovskite for the first time. The embedded BiPi-based organic salt in the 3D perovskites facilitated the formation of two-dimensional-three-dimensional (2D-3D) perovskite materials that passivated the perovskite layer, with a constructive consequence in both photovoltaic performance and device stability. Incorporating this ligand improved the crystallinity of the perovskite materials with reduced defect states, prolonged resolved carrier lifetime, and improved stability. An optimized PSC device exhibited substantially improved device stability and an outstanding power conversion efficiency of 20.03%, with the aid of the BiPi-based organic salt [open-circuit voltage (V_OC), 1.10 V; current density (J_SC), 23.51 mA/cm²; and fill factor (FF), 0.77], which are 13.0% higher than the original device. Our study provides a ligand design protocol for developing next-generation, highly efficient, stable PSCs.