Cation Engineering for Efficient and Stable Wide‐Bandgap Perovskite Solar Cells

Large voltage deficit and photoinduced halide segregation are the two primary challenges that hinder the advancement of wide‐bandgap (WBG) (Eg ≥ 1.65 eV) perovskite solar cells (PSCs). Herein, a cation engineering approach to enhance the optoelectronic properties of formamidine–cesium (FA‐Cs) WBG perovskites by incorporating methylamine (MA) as the third cation is presented. Three perovskite species with a bandgap of 1.68 eV, abbreviated as Cs0.05, Cs0.15, and Cs0.25, are systematically studied by optimizing the MA content. The incorporation of MA is found to effectively enhance the crystallinity and improve the carrier lifetimes of the three perovskite species. Moreover, the microstrain in the FA‐MA‐Cs perovskite films is significantly reduced due to the buffer effect of MA between the size‐mismatched FA and Cs, a benefit derived from the cascade cation design. The optimized compositions for the three species are Cs0.05MA0.2FA0.75PbI2.58Br0.42, Cs0.15MA0.1FA0.75PbI2.68Br0.32, and Cs0.25MA0.03FA0.72PbI2.73Br0.27, respectively. Among these, Cs0.25MA0.03FA0.72PbI2.73Br0.27 perovskite stands out due to its high crystallinity, low microstrain, and low trap density, giving rise to the highest efficiency of 20.64% with the lowest voltage loss. This perovskite also exhibits superior air, light, and thermal stability. These findings underscore the importance of rational cation design in achieving efficient and photostable WBG PSCs.