Phonon-Assisted Trapping and Re-excitation of Free Carriers and Excitons in Lead Halide Perovskites

Abstract

Despite the advances in solar cells based on lead halide perovskites, the nature of photogenerated charges and trap states within these materials remains unclear. A model describing recombination in CH3NH3PbI3−xClx has been developed that accounts for phonon-assisted free-exciton and free-carrier trapping. We utilize optical spectroscopies and observe significant co-existence of the tetragonal and orthorhombic structural phases at low temperatures. From these measurements, we evaluate the longitudinal−optical phonon energy, exciton binding energy, and temperature-dependent electronic bandgap. We use these parameters to model the temperature- and fluence-dependent time-resolved photoluminescence decays, enabling us to demonstrate how shallow traps from which carriers can be re-excited can account for the delayed recombination in lead halide perovskites. The trap-state density reaches a maximum at the tetragonal to orthorhombic phase transition at ∼140 K, suggesting the formation of disorder-induced trap states, which are shown to dominate the recombination dynamics in CH3NH3PbI3−xClx.