Crystalline Phases Regulate Electronic Trap States at Defective Surfaces of Lead Halide Perovskites

The role of organic cations in A-sites of lead halide perovskites (LHPs) on carrier dynamics has been debated in an effort to understand the remarkable properties in these materials. However, the change of A-site species in LHPs often leads to the variation of crystalline phases at room temperature. Herein, we combine density functional theory (DFT) and time-dependent DFT methods to study electron traps in CH₃NH₃PbI₃ which exhibits different structural phases with temperature and in APbBr₃ [A = CH₃NH₃, CH(NH₂)₂, or Cs] with their crystalline phases at room temperature. Regardless of halide species, electron traps arising from halide vacancies at surfaces are spatially localized in tetragonal phase and turn to be rather delocalized in orthorhombic and cubic phases. The reason is revealed by analyzing the projected p orbitals of Pb atoms at conduction band edges, providing a novel strategy of healing surface defects to improving the performances of the LHP solar cells.