Design of Highly Efficient CZTS/CZTSe Tandem Solar Cells

A CZTS/CZTSe tandem solar cell with copper zinc tin sulfide (CZTS) and copper zinc tin selenide (CZTSe) absorber layers for the top and bottom cell, respectively, is proposed. The effects of different interconnect layers such as graphene, fluorine-doped tin oxide (FTO), indium tin oxide (ITO), and a p(+)-n(+) tunnel junction consisting of CZTS and CdS on the performance of the cell are investigated, revealing that the p(+)-n(+) tunnel junction results in the best performance. The impact of adding a thin SnS layer under the absorber layers to reduce recombination is also examined, revealing a significant improvement in cell performance. The optimized thicknesses of the buffer and absorber layers are obtained in all cases. The maximum efficiency of 26.21% is achieved in the optimum tandem device including the SnS layer and p(+)-n(+) tunnel junction, being much higher than results for previous structures. Finally, the effect of the absorber bandgap energy on the performance of the tandem cells is investigated.