Formation of Color Centers in Lead Iodide Perovskites: Self-Trapping and Defects in the Bulk and Surfaces

Self-trapping of excitons or of free charges associated with the formation of color centers is typical of conventional halides. By analogy, lead halide perovskites could in principle show self-trapping of photogenerated charge carriers, possibly leading to defect formation and long-term material instability. Here we investigate the energetics of hole self-trapping in methylammonium lead iodide (MAPbI3) by performing first-principles electronic structure calculations. The thermodynamics and kinetics for the formation of bridging I2 – dimers and iodine vacancy/I3 – trimer Frenkel defects, originated by self-trapping of one and two holes, respectively, are investigated both in the bulk and at selected surfaces, in both pristine and defective systems. Our results indicate that hole self-trapping is unlikely to occur in the bulk, being thermodynamically unfavorable with associated high-energy barriers. Self-trapping remains unfavorable at surfaces, though it is significantly stabilized compared to the bulk. The inclusion of typical hole-trapping defects, such as the lead vacancy and the interstitial iodine, further stabilizes the formation of color centers, which eventually become stable for the PbI2-terminated MAPbI3 surface. Overall, our results clearly indicate that surfaces and grain boundaries are the main instability sources in lead iodide perovskites and that tailoring surface passivation is crucial for improving the performance and long-term stability of devices based on lead halide perovskites.