Temperature-dependent band gap of halide perovskite CsPbBr₃with first-principles methods

The electronic band gap is an important physical quantity for photovoltaic materials. Different from common semiconductors, band gaps of halide perovskites (HaPs) tend to slightly increase with temperature. It has been discussed that both lattice expansion and electron-phonon coupling influence the temperature dependence of the band gap. Meanwhile, as an anharmonic system, structural dynamics are expected to play an important role for optoelectronic properties of HaPs. In order to investigate this relationship, we perform molecular dynamics and Monte-Carlo calculations based on density functional theory and compare the temperature evolution of the band gap with results from reflectance measurements. Using CsPbBr₃ as a model system, we find that dynamic fluctuations impact the band gap in these materials in a profound way. In particular, we demonstrate that thermal characteristics of it cannot be rationalized using the average crystal symmetry at a given temperature. Furthermore, by comparing different levels of vibrational theory, we find anharmonic vibrational contributions to have strong impacts on the magnitude and temperature-dependence of the band gap. Our work sheds light on the connections between structural dynamics and optoeletronic properties of HaPs.