Structural engineering using rubidium iodide as a dopant under excess lead iodide conditions for high efficiency and stable perovskites

The highest efficiency perovskite solar cells reported so far are based on mixtures of formamidinium lead triiodide (FAPbI₃) and methylammonium lead tribromide (MAPbBr₃), where MAPbBr₃ acts as a stabilizer to improve the formation of the black perovskite phase. In this work, we dope the perovskite precursor mixture with rubidium iodide (RbI) and study the interplay between the doping substituent and the PbI₂ excess on the perovskite phase formation and film morphology. The addition of 5% RbI in combination with excess PbI₂ eliminates the formation of yellow non-perovskite phase and enhances the crystallinity of the films. However, the addition of more than 10% RbI results in the formation of a Rb-rich phase, which is detrimental for the cell performance. The findings are confirmed by cathodoluminescence measurements, which also reveal the spatial distribution of different phases on the perovskite films. The performance of RbI-doped perovskite cell is reported for the first time with the highest efficiency of 18.8% and improved thermal/photo stability compared to the undoped cells. The study demonstrates the potential of Rb as an alternative cation for use in high efficiency perovskite cells.