Understanding thermal effects on band gap and absorption in MAPbI3 perovskite solar cells

As a key parameter governing the optoelectronic properties of semiconductors, the band gap plays a crucial role in determining the performance of solar cells. In this study, we investigate the temperature dependence of the band gap and absorption coefficient of methylammonium lead iodide (MAPbI₃) perovskite. Unlike conventional semiconductors such as silicon, where the band gap decreases with increasing temperature, MAPbI₃ exhibits the opposite trend, with the band gap increasing as the temperature rises. By analyzing the structural properties of perovskites and other materials with similar behavior, we propose a theoretical framework to explain this phenomenon. We attribute this behavior to the presence of group-14 atoms, and the influence of their s-orbital electrons. Furthermore, a discontinuity is observed in the temperature dependence of the MAPbI₃ band gap, which we link to changes in the density of states associated with the I–Pb–I bending mode during the orthorhombic-to-tetragonal phase transition. Based on these findings, we present a theoretical model that accurately describes the temperature dependence of the MAPbI₃ band gap, showing excellent agreement with experimental data. Using this model, we further derive the temperature dependence of the absorption coefficient, providing valuable insights into the optical properties of perovskite-based solar cells.