Conversion efficiency limits and bandgap designs for multi-junction solar cells with internal radiative efficiencies below unity

We calculated the conversion-efficiency limit eta(sc) and the optimized subcell bandgap energies of 1 to 5 junction solar cells without and with intermediate reflectors under 1-sun AM1.5G and 1000-sun AM1.5D irradiations, particularly including the impact of internal radiative efficiency (eta(int)) below unity for realistic subcell materials on the basis of an extended detailed-balance theory. We found that the conversion-efficiency limit eta(sc) significantly drops when the geometric mean eta(int)* of all subcell eta(int) in the stack reduces from 1 to 0.1, and that eta(sc) degrades linearly to log eta(int)* for eta(int)* below 0.1. For eta(int)*<0.1 differences in eta(sc) due to additional intermediate reflectors became very small if all subcells are optically thick for sun light. We obtained characteristic optimized bandgap energies, which reflect both eta(int)* decrease and AM1.5 spectral gaps. These results provide realistic efficiency targets and design principles. (C) 2016 Optical Society of America.