Abstract
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 0.26μm and 3.5μm, 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 3.5μm, the top-cell CGS thickness dependence of the tandem cell performance is then investigated within the range from 0.10μm to 0.28μm. The optimal thickness corresponding to maximum conversion efficiency of 26.21% is 0.19μm. 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 18.06mA/cm2, which is also maximum short-circuit current density of the tandem cell at the optimal top-cell thickness 0.19μm. Thickness dependent light absorption of the top-cell CGS layer is the main reason for this efficiency improvement.
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