First-Principles Study of Electron Injection and Defects at the TiO/CHNHPbI 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-TiO(001) and a tetragonal CHNHPbI(110) (MAPbI(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI and the TiO 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 TiO layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI layer produced no additional states in the MAPbI gap, a Pb vacancy (V) at the interface created an energy level below the MAPbI CB edge and had a lower energy of formation than the V defect in bulk because of the interaction with the TiO surface.