Defect physics of CH3NH3PbX3 (X=I, Br, Cl) perovskites

Abstract

The properties of defects, including point defects, grain boundaries, and surfaces in photovoltaic absorbers play critical roles in determining the nonradiative recombination behavior, and, consequently, the performance of solar cells made of these absorbers. Here, we review our theoretical understanding of the defect properties of organic-inorganic methylammounium lead halide perovskites, CH3NH3PbX3 (X=I, Br, Cl), using density-functional theory calculations. We show that CH3NH3PbI3 perovskites exhibit unique defect properties-point defects with low formation energy values only create shallow levels, whereas point defects with deep levels have high formation energies. Surfaces and grain boundaries do not produce deep levels. These unique defect properties are attributed to the antibonding coupling between Pb lone-pair s and I p orbitals, the high ionicity, and the large lattice constants. We further show that CH3NH3PbI3 exhibits an intrinsic ambipolar self-doping behavior with electrical conductivity tunable from p-type to n-type via controlling the growth conditions. However, CH3NH3PbBr3 exhibits unipolar self-doping behavior. It demonstrates a preference for p-type conductivity if synthesized under thermal equilibrium growth conditions. CH3NH3PbCl3 may exhibit a compensated self-doping behavior due to its large bandgap. Doping CH3NH3PbI3 using external dopants may improve the p-type conductivity, but not the n-type conductivity due to compensation from intrinsic defects.