First-Principles Investigation on the Electronic and Mechanical Properties of Cs-Doped CH₃NH₃PbI₃.

Methylammonium lead iodide, CH₃NH₃PbI₃, is currently a front-runner as light absorber in hybrid solar cells. Despite the high conversion efficiency, the stability of CH₃NH₃PbI₃ is still a major obstacle for commercialization application. In this work, the geometry, electronic structure, thermodynamic, and mechanical property of pure and Cs-doped CH₃NH₃PbI₃ have been systematically studied by first-principles calculations within the framework of the density functional theory (DFT). Our studies suggest that the (CH₃NH₃)⁺ organic group takes a random orientation in perovskite lattice due to the minor difference of orientation energy. However, the local ordered arrangement of CH₃NH₃⁺ is energetic favorable, which causes the formation of electronic dipole domain. The band edge states of pure and Cs-doped CH₃NH₃PbI₃ are determined by (PbI₆) group, while A-site (CH₃NH₃)⁺ or Cs⁺ influences the structural stability and electronic level through Jahn⁻Teller effect. It has been demonstrated that a suitable concentration of Cs can enhance both thermodynamic and mechanical stability of CH₃NH₃PbI₃ without deteriorating the conversion efficiency. Accordingly, this work clarifies the nature of electronic and mechanical properties of Cs-doped CH₃NH₃PbI₃, and is conducive to the future design of high efficiency and stable hybrid perovskite photovoltaic materials.