Rubidium Induced Phase Regulation for High-Performance Quasi-2D Perovskite Solar Cells

Quasi-two-dimensional (2D) perovskites are notable for their diverse formulations and environmental stability. However, solution-processed quasi-2D perovskites often exhibit inherent multiple-quantum-well structures with broad phase distributions, limiting their efficiency in photovoltaic applications. Here, we demonstrate that incorporating rubidium ions effectively narrows the phase distribution in quasi-2D perovskite by accelerating the formation of the n = 1 2D perovskite phase during the initial crystallization stage. This leads to a decrease in the remaining free organic spacer cations, which in turn limits the transition to medium- n phases ( n = 3, 4) and promotes the formation of high-quality 3D-like or large- n perovskites, ultimately enhancing charge transport of quasi-2D perovskite. Consequently, we achieve quasi-2D perovskite solar cells with a champion power conversion efficiency of 21.9%. Furthermore, the thermal stability of the unencapsulated devices containing rubidium is significantly improved, with the T80 lifetime under continuous 60 °C stress increasing from 1150 to over 3000 h.