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

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, which speaks against the assumption of negligible thermal expansion effects. Here we 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. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts.