Detailed balance limit for the series constrained two terminal tandem solar cell

Tandem stacks of solar cells have clearly shown their ability to increase the efficiency of solar energy conversion. This paper investigates the limiting efficiency of unconstrained and the series constrained two-terminal tandem solar stacks under blackbody and AM 1.5 G radiation for stacks containing up to six cells. The efficiency limit under maximally concentrated blackbody radiation for the unconstrained device which contains a large number of cells has previously been calculated to be 86.8%. Surprisingly, the same limit has been shown to apply even when the cells are constrained in series connected two-terminal configuration. This represents the upper limit for conversion of the Sun's energy to electricity. Low-dimensional structures may offer advantages for two-terminal tandem stacks in the future, for example, by using superlattices of different periodicity to engineer the band gap while maintaining constant lattice spacing. In such a way, a generic approach to the design of tandem cells may be developed that would make it much simpler to increase the number of cells in a tandem stack then at present.