Effects of deposition methods and processing techniques on band gap, interband electronic transitions, and optical absorption in perovskite CH₃NH₃PbI₃ films

Despite intensive studies on the improvements of conversion efficiencies in solar cells, many questions regarding the effects of deposition techniques on optical properties and electronic band structures of CH₃NH₃PbI₃ (MAPbI₃) remain unresolved. Here, perovskite MAPbI₃ films were prepared using different deposition methods and processing techniques. The effects of deposition and processing parameters on dielectric functions and optical absorption were investigated by fitting the reflectance spectra in the photon energy range of 0.5-5.16 eV. It is found that the bandgap (E_g) of the films deposited by two-step spinning (1.591 eV) is larger than that prepared by evaporations (1.514 eV), due to different Pb-I orbital hybridization and spin-orbit coupling. Moreover, the E_g value of the films increases from 1.543 eV to 1.591 eV after toluene solution dripping. Five interband electronic transitions (Ep1, Ep2, Ep3, Ep4, and Ep5) are observed, and the origins of Ep2, Ep3, and Ep4 are assigned to the direct transitions between the highest valence band and the lowest lying conduction band at the R, M, and X symmetry points. Further, the transition energies of the films deposited by evaporation are less than those prepared by two-step spinning. The present results shed light on preparing more reliable and reproducible high performance MAPbI₃-based solar cells.