Organic-inorganic perovskites are attracting increasing attention for their use in high-performance solar cells. Nevertheless, detailed understanding of charge generation, interplay of excitons and free charge carriers, and recombination pathways, crucial for further device improvement, are still incomplete. In this work we present a very generic yet analytically solvable model describing both equilibrium properties of free charge carriers and excitons in the presence of electronic sub-gap trap states, and their kinetics after photo-excitation, in the perovskite CH3NH3PbI3-xClx. At low fluences the charge trapping pathways limit the photoluminescence quantum efficiency whereas at high fluences the traps are predominantly filled and recombination of the photo-generated species is dominated by efficient radiative bimolecular processes. The model is able to reproduce the time-resolved photoluminescence decays and photoluminescence quantum efficiencies, which we show approach 100% at low temperatures and at high fluences. The results from the model strongly indicate that the trap concentration increases with increasing temperature, suggesting an intrinsic origin of trap states. Our work provides an understanding of how to further enhance the material performance for high-efficiency perovskite solar cells and light-emitting diodes.
University of Oxford, Massachusetts Institute of Technology
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