First-Principles Study of Electronic Structure and Optical Properties of Chalcopyrite Semiconductors for Photovoltaic Applications in Nigeria

Chalcopyrite semiconductors based on copper indium gallium diselenide and copper zinc tin sulphide offer theoretical power conversion efficiencies exceeding 30 percent for multi-junction configurations, making them attractive candidates for advanced photovoltaic devices in Nigeria's high-insolation environment. The electronic structure and optical properties of these quaternary and ternary chalcopyrites remain incompletely understood at the level of ab initio theory, particularly for compositions and defect configurations relevant to thin film device fabrication. This study conducts a first-principles investigation of the electronic structure and optical properties of CuIn-1-x-Ga-x-Se2 and Cu2ZnSnS4 using hybrid HSE06 functional DFT calculations in the Vienna Ab initio Simulation Package. The optical dielectric function, absorption coefficient, and reflectance spectrum are computed from the electronic structure using the random phase approximation with local field effects. Defect formation energies and charge transition levels are computed for the dominant intrinsic defects including copper vacancies and antisite defects. The computed band gap of CIGS varies from 1.02 eV for pure CIS to 1.67 eV for pure CGS, in agreement with experimental bowing parameter data. Optical absorption coefficients exceeding 10 to the fifth per centimetre above the band edge confirm excellent light-harvesting capability. The study provides theoretical design parameters for CIGS thin film solar cell optimisation applicable to Nigerian solar manufacturing. Keywords: chalcopyrite, first-principles, electronic structure, CIGS, photovoltaic