Understanding the temperature dependence of hybrid perovskite band gaps in terms of thermal expansion and electron-phonon interaction

The fundamental gap of hybrid lead halide perovskites decreases linearly with decreasing temperature at ambient conditions. Using MAPbI3 (MA, methylammonium) as example, I will show that this atypical gap renormalization with temperature is due to an equal footing of thermal expansion and electron-phonon interaction effects. I will also present recent results obtained for FAxMA1-xPbI3 solid solutions (FA, formamidinium). Strikingly, the temperature dependence of the gap of a phase stable at intermediate compositions and below ca. 250 K exhibits a quadratic bowing with temperature. Band-structure and lattice-dynamics calculations provide crucial insights into this intriguing temperature behavior of the gap.