Exciton and Free Charge Dynamics of Methylammonium Lead Iodide Perovskites Are Different in the Tetragonal and Orthorhombic Phases

The small exciton binding energy of perovskite suggests that the long-lived photoluminescence and slow recovery of the ground state bleaching of the tetragonal phase at room temperature results primarily from the decay of free charges rather than the decay of the initially created excitons. Here we demonstrate the ground state bleaching recovery of the orthorhombic phase of methylammonium lead iodide (CH3NH3PbI3) is much faster than that of the tetragonal phase using temperature dependent transient absorption spectroscopy. The distribution in orientation of the methylammonium group which is disordered in the tetragonal phase and ordered in the orthorhombic phase results in smaller dielectric constant and larger exciton binding energy in the latter phase. We observe the recovery of the ground state bleaching in the orthorhombic phase to be comprised of decays of both excitons and free charges. Our findings suggest CH3NH3PbI3 behaves like a nonexcitonic semiconductor in the tetragonal phase and an excitonic semiconductor in the orthorhombic phase.