First-Principles Study of Electron Injection and Defects at the TiO₂/CH₃NH₃PbI₃ Interface of Perovskite Solar Cells

We investigated electron injection rates and vacancy defect properties by performing first-principles calculations on the interface of an anatase-TiO₂(001) and a tetragonal CH₃NH₃PbI₃(110) (MAPbI₃(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI₃ and the TiO₂ conduction band (CB) minimum is negligibly small, the indication being that electron-injection times for low-energy excited states are quite long (more than several tens of picoseconds). We also found that higher-lying CB states coupled more strongly; injection was expected to take place on a femtosecond time scale. Furthermore, we found that vacancy defects in the TiO₂ layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI₃ layer produced no additional states in the MAPbI₃ gap, a Pb vacancy (V_Pb) at the interface created an energy level below the MAPbI₃ CB edge and had a lower energy of formation than the V_Pb defect in bulk because of the interaction with the TiO₂ surface.