Temperature-Dependent Optical Band Gap in CsPbBr3, MAPbBr3, and FAPbBr3 Single Crystals

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr 3 , CsPbBr 3 and for the first time FAPbBr 3 ) by spectroscopic ellipsometry in the range of 1-5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic-tetragonal and tetragonal-cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.