Optimized short-circuit current mismatch in multi-junction solar cells

Multi-junction photovoltaic devices include two or more component sub-cells which are electrically interconnected in series. At any power point, the current output of the total device is limited by the sub-cell with the smallest current density. Therefore, the maximum efficiency is reached when the sub-cells have equal current densities at their respective maximum power points. In this case the sub-cells are so called “power matched”. We report an experimental procedure in which the current-voltage characteristics of tandem solar cells can be measured under various irradiance spectra, i.e. under various short-circuit current matching conditions. This permits the probing of the optimized short circuit current mismatch, where the sub-cells are power matched, which is essential to define design rules for the tandem stack. The method applies well to devices where one of the sub-cells is metastable. We show that, in the case of thin-film silicon tandem cells, the optimum mismatch changes significantly after light induced degradation. Consequently, the degradation factor of such devices is shown to depend not only on material quality but also on the initial short circuit current matching. This experiment also provides relative quantification of the fill factors of each sub-cell. Our example suggests that a high bottom cell deposition rate can be detrimental to the fill factor of the top cell in the case of thin-film silicon tandem cells deposited in superstrate configuration.