The high efficiency of thin Cu(In1-xGax)Se-2 solar cells is investigated using the Atlas SILVACO-TCAD tools. The CIGS model is based on an experimental study and takes into account the physical properties, dimensions, and thicknesses of the different layers. The main features model used consists of the thermionic emission concept that was used to describe the carrier transport model across the buffer/absorber heterojunction. Thermionic emission was found to be an important mechanism for carrier transport across the hetero-interface since it produced a matching behaviour for the cell under study with experimental behaviour, thus validating the considered CIGS model for further investigation. To improve efficiency, magnesium fluoride (MgF2) is used as the preferred anti-reflective coating to maximize the photocurrent from the incident light intensity. The effect of varying the single-layer antireflective coating thickness variation, surface recombination velocity at the hetero-interface between CdS and CIGS layers, absorber thickness variation, and gallium content in the Cu(In1-xGax)Se-2 on cell performance are studied under AM 1.5G, 1 sun illumination. The novelty of this research includes the configuration of the p-CIGS/n-CdS cells with thin CIS material used for the back surface field layer to improve efficiency. The result has shown that a cell performance of 25% efficiency can be achieved using a 1 mu m thick absorber and an MgF2 anti-reflective layer. This result can further aid the development of ultrathin, flexible, and tandem solar cell devices.