Fe-doped CuGaS2 (CuGa1-xFexS2) - Detailed analysis of the intermediate band optical response of chalcopyrite thin films based on first principle calculations and experimental studies

We have theoretically and experimentally studied the tailoring of intermediate bands in the CuGa1-xFexS2 chalcopyrite system. An ab initio density functional theory calculations within GGA + U approach have been performed to model pristine and Fe-doped CuGaS2 to understand their structural and electronic properties and thereby understand the origin of intermediate states produced with Fe doping. Thin films of the pristine and Fe-doped CuGaS2 of various compositions are deposited using chemical spray pyrolysis for the experimental studies. The crystal structure, morphology, and topography of the deposited thin films are examined using Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), respectively. The binding energy and chemical composition of CuGa0.95Fe0.05S2 thin films are determined using X-ray Photoelectron Spectroscopy (XPS). The direct and intermediate band optical responses have been probed through UV–Vis–NIR Spectroscopy. Pristine CuGaS2 exhibits a direct bandgap of 2.43 eV, with formation of intermediate bands in Fe-doped CuGaS2 at 1.82 eV, 1.75 eV, 1.54 eV and 1.49 eV with varying Fe atomic percentage (X = 0.025, 0.05, 0.075, and 0.1 at. %). The optimum Fe concentration is determined to be X = 0.05 (CuGa0.95Fe0.05S2) as it exhibits the highest photocurrent (1.46 mA), electrical conductivity, and intermediate band optical response. The IB formation is further supported by the calculated density of states of the Fe doped CuGaS2 structure.