Suitable top cell partners for copper indium gallium selenide-based tandem solar cells to achieve >30% efficiency

A four-terminal tandem structure based on the copper indium gallium selenide (CIGS) solar cell is a viable approach to achieve high efficiency. For this purpose, a dual-junction configuration, consisting of the experimental CIGS cell with a record efficiency of 23.35% as the bottom cell and wide bandgap thin-film solar absorbers as the top cell, is evaluated by modeling. Herein, the effects of luminescence efficiency, optical absorption, bandgap, thickness, and light trapping of the top cell on the designed tandem performance are investigated. The results show that the optimum bandgap and the thickness of the absorber layer for the top cell are between 1.7 and 2 eV and 0.5 μm, respectively. Light trapping for less than 1 μm-thick top cell has a strong impact on cell performance. The analytical model for the tandem solar cell is provided to recognize the suitable materials as the partner for the CIGS cell. The analysis shows that the gallium arsenide (GaAs), copper zinc tin sulfide (CZTS), cadmium telluride (CdTe), and copper gallium selenide (CGS) thin-film solar cells can be considered suitable top cells to achieve a tandem cell with more than 30% efficiency. The Al_0.31Ga_0.69As/CIGS tandem solar cells with 35.9% efficiency are also proposed.