Impact of lattice distortions on unusual bandgap shift in organic-inorganic lead and mixed-lead-tin halide perovskites

Metal halide perovskites exhibit unusual bandgap shift with temperature and understanding its origin is essential in improving the thermal stability of perovskite-based photovoltaic devices. Here, we measure the temperature dependence of the bandgap in two hybrid organic-inorganic perovskite thin films (MA_0.3FA_0.7PbI₃ and MA_0.3FA_0.7Pb_0.5Sn_0.5I₃) using photoluminescence and absorbance spectroscopy. We show that the thermal expansion effect is negligible in these systems and, supported by first-principles calculations, thermally induced lattice distortions are the possible cause for their anomalous bandgap shift with temperature. We find that the static and dynamic distortions likely have opposite effects on the bandgap with temperature, as shown by the contrasting shifting trends of the bandgap in the lead and mixed-lead-tin perovskites. In addition, bandgap renormalization and band-filling effects are observed in fluence dependent photoluminescence measurements. Our study sheds new light on the thermal effects on the band structures of hybrid perovskites.