Conversion efficiency limits and optimized designs for tandem solar cells with realistic sub-cell material quality

摘要

To examine practical efficiency limits of tandem solar cells, we calculated conversion efficiency (η) and optimized sub-cell band-gap energies (E) in 1-, 2-, 3-, 4-, and 5-junction cells via a detailed-balance theory, taking account of the realistic internal luminescence quantum yields (y) of the sub-cell materials. We found that η drastically decreases as the geometric mean y* of all sub-cells' y drops from 1 to 0.1, and decreases proportionally to logy * in the low-y * region. Even with moderate material quality with y * = 0.001, η above 30% (35%) should be still available in 2-junction (3-junction) tandem cells, though we need optimized designs with much higher E than those known for the ideal cases with y * = 1.