Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps

Abstract

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase [1], which speaks against the assumption of negligible thermal expansion effects. Here I will show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling [2]. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts. As an example, I will also present recent results obtained for MA rich FAxMA1-xPbI3 solid solutions, where FA stands for formamidinium.

[1] A. Francisco-López, B. Charles, O. J. Weber, M. I. Alonso, M. Garriga, M. Campoy-Quiles, M. T. Weller, and A. R. Goñi, J. Phys. Chem. C 122, 22073-22082 (2018). [2] A. Francisco-López, B. Charles, O. J. Weber, M. I. Alonso, M. Garriga, M. Campoy-Quiles, M. T. Weller, and A. R. Goñi, J. Phys. Chem. Lett. 10, 2971-2977 (2019).