Substituting Cs for MA on the surface of MAPbI₃ perovskite: A first-principles study

Recently, mixed cation/anion perovskite solar cells (PSCs) have attracted considerable attention due to their higher stability and better photovoltage performance than pure ABX₃ perovskite solar cells. However, the fundamental mechanism of such an improvement has not yet been fully investigated. In this paper, substituting cesium cation for methylammonium cation of the (1 0 0) surface of CH₃NH₃PbI₃ perovskites was made to study the improvement of stability and electronic properties of MAPbI₃ perovskite. The results show that the decomposition energies of perovskite increase with the increased concentration of doping Cs⁺, which indicates that the thermodynamic stability can be enhanced. By analyzing the Bader charge and charge density difference of the perovskite/TiO₂ heterostructures, we find that the system of doping Cs⁺ transfers more electrons to TiO₂ since the distance between I and Ti reduces. Doping Cs⁺ can make the PbI₆ octahedral frame tilt and the tilting orientation is different for different concentration of Cs⁺. Meanwhile, the band gap increases with the increased concentration of Cs⁺, which leads to the increase in open circuit voltage V_OC. This study can give some guidance for improving stability and PCE of the perovskite solar cells.