Unique Trapped Dimer State of the Photogenerated Hole in Hybrid Orthorhombic CH3NH3PbI3 Perovskite: Identification, Origin, and Implications.

Revealing the innate character and transport of the photogenerated hole is essential to boost the high photovoltaic performance in the lead-based organohalide perovskite. However, knowledge at the atomic level is currently very limited. In this work, we systematically investigate the properties of the photogenerated hole in the orthorhombic CH3NH3PbI3 using hybrid functional PBE0 calculations with spin-orbit coupling included. An unexpected trapping state of the hole, localized as I2- (I dimer), is uncovered, which was never reported in photovoltaic materials. It is shown that this localized configuration is energetically more favorable than that of the delocalized hole state by 191 meV and that it can highly promote the diffusion of the hole with an energy barrier as low as 131 meV. Furthermore, the origin of I dimer formation upon trapping of the hole is rationalized in terms of electronic and geometric effects, and a good linear correlation is found between the hole trapping capacity and the accompanying structural deformation in CH3NH3PbX3 (X = Cl, Br, and I). It is demonstrated that good CH3NH3PbX3 materials for the hole diffusion should have small structural deformation energy and weak hole trapping capacity, which may facilitate the rational screening of superior photovoltaic perovskites.