Enhanced stability and optical absorption in the perovskite-based compounds MA$_{1-x}$Cs$_x$PbI$_{3-y}$Br$_y$.

Organometal halide perovskites have been outstanding from enormous amount of functional materials thanks to their highly cost-effective processability and prominent light harvesting capacity. Unfortunately, poor long-term stability seriously hinders their further development. The recent experimental observations suggest that Cesium is a promising candidate to enhance the stability of MAPbI$_3$. To explore the inherent mechanism, a first-principles investigation based on density functional theory, including hybrid functional, has been performed to analyze the electronic and optical properties of perovskite series MA$_{0.75}$Cs$_{0.25}$PbI$_{3-y}$Br$_y$. The results indicate that perovskite compound MA$_{0.75}$Cs$_{0.25}$PbI$_2$Br is significantly superior to the other doped series in terms of optical absorption within the visible-light range. In the meanwhile, both Bader charge analysis and charge density distribution show that the compound of MA$_{0.75}$Cs$_{0.25}$PbI$_2$Br is the most stable among all the doped perovskite series. Moreover, it is clearly manifested that the impact of cesium is mainly embodied in the enhancement of the stability rather than in the improvement of optical absorption. Our study sheds a new light on screening new-type light harvesting materials, and provides theoretical insight into the rationale design of highly efficient and stable photovoltaic devices based on these functional materials.