Polarons in CH₃NH₃.PBI₃: Formation, transport and recombination

Hybrid halide perovskites are soft, polar, semiconductors[1]. We propose that low energy (9 meV) optical phonons limit room temperature mobility. We have written open source codes to solve the finite-temperature Feynman polaron state. This provides a temperature-dependent calculation of mobility[2], in good agreement with experiment.

This model suggests a mechanism to explain recent data on slow cooling of photo-excited carriers. The polaron state is stable at high temperature, and has a limited phonon density of states which the hot-electron is in thermal contact with. The low lattice thermal conductivity slows dissipation of this transient hot-spot[3].

We construct a multi-scale model for the formation of the polaron, and its migration through the material. We quantify the beneficial decrease in recombination rate due to segregation of electrons and holes in the 'ferroelectric highways' and relativistic spin-split of the Rashba effect, versus the detrimental decrease in mobility due to disorder. We quantify the contribution of short-range ferroelectric order on carrier stability and electron-hole recombination in this unique class of materials.

[1] JM Frost et al. Acc.Chem.Res. 49 (3) pp 528–535 (2016)
[2] JM Frost. ArXiv:1704.05404
[3] JM Frost et al. ACS Energy Letters (2017)