Ultrasound-Assisted Zwitterion Grafting on NiO_x for Suppressing Self-Assembled Monolayer Migration in Perovskite Solar Cells

Self-assembled monolayers (SAMs) are widely used as hole-selective materials in inverted perovskite solar cells (PSCs), yet their performance and stability are often limited by poor molecular ordering and interfacial incompatibility. Here, we present a novel ultrasonic chemical strategy to functionalize NiO_x nanoparticles via anchoring the piperazine-1,4-bisethanesulfonic acid (PIPES) zwitterionic molecule. Ultrasonic cavitation generates hydroxyl radicals that oxidize Ni²⁺ to Ni³⁺, enhancing the electrical conductivity and hole mobility of NiO_x. The exposed sulfonate groups of PIPES further promote the uniform assembly and hydrogen bonding anchoring of the subsequent SAM layer [(4-(9'-phenyl-9H,9'H-[3,3'-bicarbazole]-9-yl)butyl)phosphonic acid, 4PABCz]. This integrated NiO_x+PIPES/4PABCz hole transport layer fosters stronger dipole formation and interfacial polarization, facilitating charge separation and transport. Consequently, the optimized devices achieve a champion power conversion efficiency (PCE) of 27.03% (with a certified steady-state efficiency of 26.47%). Remarkably, the devices exhibit exceptional operational and thermal stability, retaining 88.2% of their initial PCE after 1000 h of continuous illumination at 85°C and 91.0% after 1200 h of thermal aging at 85°C. This work introduces a robust and effective NiO_x modification strategy, providing profound insights into interfacial design for high-performance, stable inverted PSCs.