Impact of sub-cell internal luminescence yields on energy conversion efficiencies of tandem solar cells: A design principle

To develop a realistic design principle, we calculated the maximum conversion efficiency η_sc and optimized sub-cell band-gap energies E_g in double-junction tandem solar cells via a detailed-balance theory, paying particular attention to their dependence on internal luminescence quantum yields y_int of the top and bottom sub-cell materials. A strong drop in the maximum η_sc occurs when y_int slightly drops from 1 to 0.9, where the drop in yint of the bottom cell causes a stronger effect than that of the top cell. For low values of y_int, the maximum η_sc has a simple logarithmic dependence on the geometric mean of the two sub-cells'y_int.