Efficiency limits for single-junction and tandem solar cells

Basic limitations of single-junction and tandem p–n and p–i–n diodes are established from thermodynamical considerations on radiative recombination and semi-empirical considerations on the classical diode equations. These limits are compared to actual values of short-circuit current, open-circuit voltage, fill factor and efficiency for amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon solar cells. For single-junction cells, major efficiency gains should be achievable by increasing the short-circuit current density by better light trapping. The limitations of p–i–n junctions are estimated from recombination effects in the intrinsic layer. The efficiency of double-junction cells is presented as a function of the energy gap of top and bottom cells, confirming the ‘micromorph’ tandem (a-Si:H/μc-Si:H) as an optimum combination of tandem solar cells.