Role of Individual Bands in the Unusual Temperature-Dependent Band Gap of Methylammonium Lead Iodide

The optical band gap of methylammonium lead iodide (CH₃NH₃PbI₃) is known to increase with temperature; such a dependence is referred to as an unusual behavior when compared to conventional semiconductors. In addition, the band gap exhibits a sharp decrease in a narrow temperature range during the orthorhombic-to-tetragonal phase-transition of the material. In this work, we investigate the conduction and valence band energies to deliberate on the role of individual bands in yielding the unusual temperature dependence of the transport gap and also its sharp decrease at the transition temperature. Scanning tunneling spectroscopy (STS) of CH₃NH₃PbI₃ films performed at different temperatures allowed us to derive the band energies across the two structural phases and at the transition temperature as well. The temperature dependence of band energies has been substantiated by the changes in the lattice parameters (from synchrotron radiation X-ray diffraction studies at different temperatures) vis-à-vis the orbital overlaps responsible in forming the conduction and valence bands. The STS studies have provided useful information in gaining deeper insights into the unusual temperature dependence of hybrid halide perovskite solar cells.