Optimum Design of Thin-Film-Based Tandem-Type Solar Cells

The conversion efficiency of a thin-film-based tandem solar cell is theoretically calculated and the best material combinations are discussed. As constituent materials, not only crystalline but also amorphous materials are considered. Attention is given especially to the electrical performances of both the top and bottom cells, which have strong effects on the optimum conditions and attainable conversion efficiency. It is found that a conversion efficiency of above 30% can be expected when utilizing an amorphous Si-based cell as the top cell with a single crystalline Si bottom cell, and above 20% with CuInSe 2 or CdTe as a bottom cell. In order to obtain a conversion efficiency of above 40%, insolation concentration with effective cell cooling is essentially required in tandem solar cells of both single crystalline materials.