Revealing Unusual Bandgap Shifts with Temperature and Bandgap Renormalization Effect in Phase-Stabilized Metal Halide Perovskite Thin Films

Hybrid organic–inorganic metal halide perovskites are emerging materials in photovoltaics, whose bandgap is one of the most crucial parameters governing their light-harvesting performance. This work presents the temperature and photocarrier density dependence of the bandgap in two phase-stabilized perovskite thin films (MA_0.3FA_0.7PbI₃ and MA_0.3FA_0.7Pb_0.5Sn_0.5I₃) using photoluminescence and absorption spectroscopy. Contrasting bandgap shifts with temperature are observed between the two perovskites. Using X-ray diffraction and in situ high-pressure photoluminescence spectroscopy, it is shown that thermal expansion plays only a minor role in the large bandgap blueshift, which is attributed to the enhanced structural stability of the samples. The first-principles calculations further demonstrate the significant impact of thermally induced lattice distortions on the bandgap widening. It is proposed that the anomalous trends are caused by the competition between static and dynamic distortions. Additionally, both the bandgap renormalization and band-filling effects are directly observed for the first time in fluence-dependent photoluminescence measurements and are employed to estimate the exciton effective mass. The results provide new insights into the basic understanding of thermal and charge-accumulation effects on the band structure of hybrid perovskite thin films.