Shallow vs. Deep Charge Localization in Hybrid Lead Halide Perovskites

Fascinating properties and startling photovoltaic performance of hybrid organic-inorganic pervoskites have brought quite a bit of attention to this class of halide semiconductors. Their all-inorganic analogs are also under considerable scrutiny for potential optoelectronic applications. Here, we shall discuss the electronic structure of a hybrid perovskite CH₃NH₃PbI₃, its connection to preferential orientation of the organic cations CH₃NH₃⁺ and tilting of the PbI₆ octahedra. We also compare its charge localization behavior at shallow levels and associated lattice relaxation versus those at deep polaronic states. The shallow level originates from screened Coulomb interaction between the perturbed or distorted host and an excited electron or hole. The host lattice has a tendency towards forming these shallow traps while deep polarons remain energetically unfavorable. Hence, active shallow traps are expected to shape the carrier dynamics in CH₃NH₃PbI₃,working as temporary trapping-detrapping centers which may be instrumental in reduced quenching of electron-hole pairs, adding to its defect tolerant behavior.