Homogenizing Cesium Distribution via Rubidium Incorporation Enables Pure-Iodide 1.67 eV Bandgap Perovskite Solar Cells with Efficiency Exceeding 22%

Cs-rich multiple-cation pure-iodide wide-bandgap perovskite materials with excellent photostability are promising candidates for stable tandem solar cells. However, these Cs-rich perovskites often suffer from vertical cation inhomogeneity, which compromises device performance and operational stability. Herein, we report that incorporating rubidium (Rb) accelerated the phase transition and promoted better crystallization of the CsDMAMAFA perovskite, thereby ensuring a more uniform vertical distribution of Cs. In addition, Rb + incorporation relieved lattice strain, reduced iodide-vacancy defects, and optimized the interfacial energy levels. As a result, the Rb-doped pure-iodide wide-bandgap perovskite solar cells achieved an efficiency of 21.62% with a bandgap of 1.67 eV, which can be further increased to 22.51% via an additional 1,3-diaminopropane dihydroiodide (PDAI 2 ) surface treatment. The Rb-doped devices also exhibited enhanced photostability, maintaining 88% of the initial efficiency after 400 h under ISOS-L-1 conditions (ambient air, 23 ± 2 °C), even without encapsulation. This work provides a simple and effective route to efficient and stable pure-iodide wide-bandgap perovskite solar cells.