Intermediate Bands in Hybrid Perovskites

To further improve energy conversion efficiencies, we studied substitutions in MAPbX₃ perovskites (where MA = methylammonium and X = Br/Cl) to obtain mid-gap states which can act as intermediate bands (IBs) for additional absorption of sub-gap photons. Density functional theory (DFT) calculations revealed that substitution of Pb by Co in MAPbBr_yCl_3-y perovskites creates mid-gap energy states, which was experimentally confirmed via absorption and photoluminescence spectroscopy.¹ The lack of a second sub-gap absorption feature indicated that the mid-gap states may be unfilled. We report the electronic structure changes upon substitution of Pb by other metals, specifically noting trends in parent band gap and substituent energy states within the band gap. We further determined the thermodynamic equilibrium growth conditions necessary for creating a stable external substitution that compensates for dominant intrinsic defects. Based on this work, promising candidates were identified as Pb-substituents in lead-based hybrid perovskites to create IB photovoltaic materials.

¹M. D. Sampson, J. S. Park, R. D. Schaller, M. K. Y. Chan, A. B. F. Martinson, “Lead Halide Perovskite-Based Intermediate Band Absorbers,” Journal of Materials Chemistry A 5, 3578-3588 (2017).