Phase‐Pure 2D Interfacial Perovskite Passivation for Stable and Efficient Photovoltaics

ABSTRACT Phase‐pure two‐dimensional (2D) interfacial passivation has emerged as an effective strategy for addressing the intrinsic instability and interfacial defects of three‐dimensional (3D) perovskite absorbers. However, conventionally formed 2D layers often suffer from mixed‐ n phases, heterogeneous quantum‐well distributions, and disordered orientation, which impede charge transport, distort energy‐level alignment, and accelerate structural degradation. In this review, we elucidate the thermodynamic and kinetic origins of mixed‐phase formation and discuss how dimensional heterogeneity adversely impacts carrier dynamics and device stability. We then summarize recent advances in achieving phase‐pure 2D perovskite interlayers that enable precise n ‐value control, favorable crystal orientation, and optimized interfacial energetics. These strategies yield highly ordered 2D/3D heterostructures that effectively suppress ion migration, mitigate non‐radiative recombination, and significantly enhance long‐term operational stability. Finally, we outline the remaining challenges and emerging opportunities for scalable, phase‐pure engineering toward high‐efficiency and stable perovskite photovoltaic technologies. Overall, this review provides a unified framework linking phase purity, interfacial ordering, and device stability, offering guidance for the development of next‐generation robust perovskite photovoltaics.