The photovoltaic characteristics of a double junction CGS/CIGS tandem solar cell, based on copper gallium diselenide (CGS) and copper indium gallium diselenide (CIGS) structures as top-cell and bottom-cell respectively, were numerically simulated under AM1.5 G spectral illumination using the two-dimensional device simulator Silvaco-Atlas. The performance of single CGS and CIGS solar cells with fixed thicknesses at 0.26 mu m and 3.5 mu m, respectively, were simulated first. They present conversion efficiencies of 18.92% and 20.32% respectively, in good agreement with experimental high record efficiencies found in literature. Maintaining the thickness of the bottom CIGS cell at 3.5 mu m, the top-cell CGS thickness dependence of the tandem cell performance is then investigated within the range from 0.10 mu m to 0.28 mu m. The optimal thickness corresponding to maximum conversion efficiency of 26.21% is 0.19 mu m. This thickness of the top-cell coincides well with the point of "current matching" where the short-circuit current densities of the top, bottom and tandem cells are exactly equal to one another and equal to 18.06 mA/cm(2), which is also maximum short-circuit current density of the tandem cell at the optimal top-cell thickness 0.19 mu m. Thickness dependent light absorption of the top-cell CGS layer is the main reason for this efficiency improvement. (C) 2015 Elsevier Ltd. All rights reserved.