Influence of Thermal Energy and Electronic Structure on the Refractive Index and Extinction Coefficient of MAPbI<sub>3</sub> Perovskite Solar Cell Materials

The optical performance of perovskite solar cells depends strongly on the refractive index (n), extinction coefficient (k), temperature, and electronic structure of the absorber material. This study investigates the influence of thermal energy and bandgap variation on the optical constants of MAPbI3 perovskite absorbers using temperature-dependent spectroscopic ellipsometry data obtained over 298.15–348.15 K and bandgap energies of 1.58–1.77 eV. Linear regression was used to determine the thermo-optic coefficients (dn/dT and dk/dT) and bandgap-sensitivity coefficients (dn/dEg and dk/dEg), while derivative spectroscopy identified wavelength regions with enhanced optical sensitivity. The results show that temperature causes only modest changes in optical dispersion and absorption, indicating good thermal stability. In contrast, bandgap variation significantly reduces the refractive index and shifts the absorption edge toward shorter wavelengths, demonstrating that electronic-structure modification has a stronger influence on optical behaviour than thermal effects. The highest sensitivities occur near the absorption edge, where interband transitions dominate. These findings highlight bandgap engineering as the primary strategy for tuning the optical properties of perovskite absorbers and provide useful guidance for the design of efficient and thermally stable perovskite solar cells.