First-Principles Analysis of Radiative Recombination in Lead-Halide Perovskites

Abstract

Slow radiative recombination due to a slightly indirect band gap has been proposed to explain the high efficiency of lead-halide perovskite solar cells. Here, we calculate the radiative recombination rate from first principles for the prototypical lead-halide perovskite, MAPbI 3 (MA=CH 3 NH 3 ). Since the structure is dynamic, with the MA molecule rotating even at room temperature, we determine the momentum mismatch between the band edges as a function of the orientation of the MA molecule. Our results demonstrate that the indirect nature of the band gap suppresses the radiative recombination rate by less than a factor of two, and that the radiative recombination coefficient is as high as in traditional direct-gap semiconductors. Our study provides a rigorous assessment of the radiative recombination mechanisms and their relation to the high efficiency of lead-halide perovskite solar cells, and will provide a sound basis for accurate modeling.

First‐Principles Simulation of Carrier Recombination Mechanisms in Halide Perovskites

“…i) The calculated room‐temperature radiative recombination coefficient in MAPbI 3 for three representative orientations of the MA molecule. Adapted with permission . Copyright 2018, American Chemical Society.…”

“…Quantitatively, we also calculated the radiative recombination coefficient at room temperature at a function of carrier density for three representative high‐symmetry MA orientations . Since the extrema of the momentum and energy splitting always occur at the high‐symmetry orientations (see Figure d–g), an inspection of the high‐symmetry configurations captures the upper and lower bounds of the radiative recombination coefficient.…”

Polarons in Metal Halide Perovskites

“…The low bimolecular recombination rate and the modest mobility of charge carriers have not a clear origin and several hypotheses have been advanced. Among others, Rashba–Dresselhaus effects have been proposed as a possible mechanism preventing the recombination due to the decreased recombination probability induced by the splitting in reciprocal space of valence and conduction band (CB) edges, turning the bandgap from direct to partly indirect . These effects, however, require breaking the crystal inversion symmetry, which may be possibly accessible on fairly small nanodomains, of the order of few nm …”

Ab initiocalculations of exciton radiative lifetimes in bulk crystals, nanostructures, and molecules

“…Different from the 0D, 1D and 2D cases, this result implies that excitons with zero center of mass momentum cannot decay radiatively, since the resulting photon would possess zero momentum and energy. In contrast, the equations employed so far to compute from first principles radiative decay in bulk employ either simplified independent-particle approaches [10,11], which neglect excitons altogether, or formulas appropriate for 0D or 1D [32], which fail to take into account momentum conservation. Note that neglecting excitons does not merely change the transition dipole and energies, but rather, the exciton momentum and dispersion are essential to accurately computing the temperature and polarization dependence of light emission.…”

“…In our approach, exciton recombination is still described using the Fermi Golden rule in Eq. (11). However, due to the lower dimensionality, the transition dipole is restricted to the 2D plane containing the material:…”