Rational Molecular Engineering of NiO_x Interfaces for High-Performance Inverted Perovskite Solar Cells

Nickel oxide (NiO_x) is among the most widely used hole-transport materials (HTMs) for inverted perovskite solar cells (PSCs), yet its substantial surface defects compromise the device's performance and long-term stability. Despite the development of various surface engineering strategies, the underlying mechanism governing interfacial dynamics is incompletely understood. Herein, we systematically investigate the structural roles of molecular passivators in tailoring NiO_x properties, with a focus on elucidating the distinct mechanisms of two structurally analogous modifiers: the polymer polyvinylpyrrolidone (PVP) and the small-molecule N-methylpyrrolidone (NMP). The results demonstrate that the pronounced steric hindrance arising from the long polymer chains of PVP constructs a physical barrier, which detrimentally impacts charge transport and perovskite crystallization. Conversely, NMP capitalizes on its small molecular size and chemical reactivity to achieve directional selective passivation. This chemical modification not only effectively optimizes interfacial properties but also facilitates the crystallization of perovskite films. As a result, the NMP-modified PSCs achieve a power conversion efficiency (PCE) of 20.89%, in contrast to 18.52% for their PVP-modified counterparts. Notably, the unencapsulated NMP-modified device retains 93% of its initial efficiency following 1800 h of storage at 25 °C under a nitrogen atmosphere. This work sheds light on the intrinsic correlation between molecular structure and device performance, thereby offering valuable guidance for further optimization of both the efficiency and long-term stability of PSCs.