Asymmetric Dual-Interface Passivation with Functionalized Ammonium Halides for High-Performance Inverted CsPbI2Br Perovskite Solar Cells

Interfacial defect passivation has emerged as a critical strategy for mitigating non-radiative recombination losses in inorganic perovskite solar cells (PSCs). However, the distinct chemical environments at the bottom (hole-transport layer) and top (electron-transport layer) interfaces demand passivation agents with tailored functionalities—a principle that remains largely underexplored. Herein, we systematically employed two organic ammonium iodide salts, phenylethylammonium iodide (PEAI) and 2-thiophenemethylammonium iodide (ThMI), to separately modulate the bottom NiOx/CsPbI2Br and top CsPbI2Br/PCBM interfaces of inverted PSCs with a configuration of ITO/NiOx/CsPbI2Br/PCBM/BCP/Ag. We reveal different interfacial modulation effects: bottom-interface modification by both PEAI and ThMI dramatically improves the fill factor (FF), with PEAI delivering a more pronounced enhancement due to improved interfacial contact and reduced series resistance. However, top-interface passivation effectively boosts the open-circuit voltage (Voc), where ThMI exhibits superior voltage elevation capability over PEAI by neutralizing undercoordinated Pb2+ defects via its thiophene moiety. Capitalizing on this complementary selectivity, we construct an asymmetric dual-interface passivation architecture with PEAI at the bottom and ThMI at the top (ITO/NiOx/PEAI/CsPbI2Br/ThMI/PCBM/BCP/Ag), which synergistically enhances both FF and Voc. Consequently, the optimized PEAI/ThMI device achieves a champion power conversion efficiency (PCE) of 15.44%, with a Voc of 1.15 V, a Jsc of 16.34 mA/cm2, and an FF of 82.15%, significantly outperforming the control device (11.79%). This work establishes a rational design paradigm for interface-specific passivation in inverted inorganic PSCs, highlighting the importance of molecular functionality in addressing distinct interfacial recombination pathways.