Copper indium gallium sulfur selenide (Cu(In1-xGax)SeS, CIGS) thin film solar cells are fabricated using a solution-based process, and their defect models are studied through a computer-aided design method. Cu(In1-xGax)SeS is structured with a graded bandgap by controlling the ambient gas and precursor composition, during the fabrication process. The defects in the CIGS are modeled as two donor-like defects, which are differently distributed as per the CIGS grain size (large and small grains at upper and bottom layers, respectively), whereas those in the cadmium sulfide (CdS)/CIGS interface are modeled as a complex model of both donor- and acceptor-like defects in the CdS, near the interface. By measuring the external quantum efficiency and current density-voltage characteristics, the best-fitting match of the simulated values with the measured values are obtained. The simulation results demonstrate that the defects (defect density of ~7 × 1018) in the CdS interface are more serious, compared to the CIGS defects (defect density of ~2 × 1015 in the bottom), which were initially expected to be more severe because of grain nonuniformity. For increasing the cell efficiency, we establish that the process and material quality need to be further improved not only during CIGS formation using a multistep spin-coated precursor but also during the initial deposition of the CdS buffer. This numerical approach can enable better understanding of the defect behavior in solar cells, and indicate directions for improvement in the fabrication process and device structure, for developing high-efficiency solution-based CIGS solar cells.