Orientation-Tuned Piperazinium Derivatives for Efficient and Stable Inverted Perovskite Solar Cells

Interfacial defects remain a major bottleneck to simultaneously achieve high efficiency and long-term stability in inverted perovskite solar cells (PVSCs). While recent studies have highlighted the importance of the orientation of passivators at the perovskite surface, the systematic control and quantification of orientation preference have received far less attention. Here, we present an orientation-tuned molecular design strategy using piperazinium derivatives in which substituent engineering modulates steric hindrance to favor face-on alignment at the perovskite surface. Among the series, N-formylpiperazinium iodide (NFPI) exhibits the strongest face-on preference, enabling dual-site anchoring to undercoordinated Pb²⁺ and FA⁺ vacancies. Density functional theory and spectroscopy confirm that NFPI delivers the highest binding energy, reduced trap density, prolonged carrier lifetimes, and enhanced interfacial band bending. Consequently, NFPI-passivated PVSCs achieve a champion power conversion efficiency (PCE) of 25.82% with a high open-circuit voltage of 1.182 V and minimal hysteresis. Moreover, devices retain 90% of the initial PCE after 1500 h of thermal aging at 65 °C (ISOS-D-2), and encapsulated devices reach a T₈₀ of 477 h under continuous operation (ISOS-L-1). This work establishes adsorption orientation control as a key design principle for efficient and durable PVSCs.