Assessing Optical Properties of CH₃NH₃PbBr₃ Single Crystals across the Structural Phase Transitions by Spectroscopic Ellipsometry

Lead halide perovskites have drawn considerable attention on account of their potential applications in solar cell and optoelectronic devices. To realize such technological possibilities, the optical characterizations of these materials must be assessed. This paper presents a comprehensive study of the temperature-dependent optical properties of CH₃NH₃PbBr₃ single crystals between 4.5 and 400 K within an energy range of 0.73 to 6.42 eV determined through spectroscopic ellipsometry. The profile of the room-temperature refractive index was typical of a semiconductor and decreased markedly with an increase in temperature with thermo-optic coefficients of approximately −4.18 × 10^–4 and −3.29 × 10^–4 K^–1 at 600 and 1200 nm, respectively. The cooling–heating cycles of the extinction coefficient reflected the thermal hysteresis of the first-order orthorhombic–tetragonal structural phase transition. Moreover, the Urbach energy, fundamental band gap, exciton peak position, exciton binding energy, and interband transition energies were considerably affected during the structural phase transitions of orthorhombic–tetragonal I at 130 K, tetragonal I–tetragonal II at 150 K, and tetragonal II–cubic at 240 K. These results provide crucial information for the design and fabrication of lead halide perovskite-based devices at various temperatures.