Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CHNHPbI: Implications on Solar Cell Degradation and Choice of Electrode.

Solar cells based on methylammonium lead triiodide (MAPbI) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long-term stability of MAPbI-based solar cells has yet to be achieved. Besides the well-known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current-voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI. In addition to Au, many other metals have been used as electrodes in MAPbI solar cells. However, how the external metal impurities introduced by electrodes affect the long-term stability of MAPbI solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI based on first-principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI while having low resistivities and suitable work functions for carrier extraction.