Theoretical and Practical Conversion Efficiency Limits of Solar Cells at Orbital Operating Temperatures

In this study, we analysed the correlation between bandgap energy, solar absorptance, and the orbital operating temperature of the solar cells, and derived both theoretical and practical conversion efficiency limits for single- and multi-junction solar cells at that temperature. Since the temperature dependence of solar cell properties vary with the external luminescence efficiencies, we calculated the conversion efficiency limits under various conditions. Our results show that the operating temperature of an InGaP/GaAs/Ge triple-junction solar cell exceeds that of an InGaP/GaAs dual-junction solar cell by more than 30°C due to the higher solar absorptance of triple-junction solar cell. Furthermore, we clarified that a dual-junction solar cell can achieve a higher conversion efficiency than a triple-junction solar cell at the orbital operating temperature, depending on the external luminescence efficiency of each subcell, i.e., its temperature dependence, even when the InGaP and GaAs subcell properties in the dual- and the triple-jucntion solar cells are identical. Our calculation results will provide a new design guideline for space solar cells that incorporates orbital operating thermal conditions into conversion efficiency.