Current matching optimization in high-efficiency thin-film silicon tandem solar cells

In thin-film silicon “micromorph” tandem photovoltaics devices, the fill factor (FF) of the microcrystalline silicon (μc-Si:H) bottom cell is typically higher than that of the amorphous silicon (a-Si:H) top cell. Consequently, in order to maximize the efficiency, the bottom cell should be slightly limiting the current in short-circuit condition. Using an intermediate reflector layer (IRL) is a good solution to increase the photocurrent in the top cell without changing the thickness of the active layer. In this contribution, we evaluate different IRL by measuring current-voltage (I-V) characteristics of tandem cells under various irradiance spectra (from red-rich to blue-rich), to vary the short-circuit current matching conditions (from top-limited to bottom-limited). This technique is applicable to multi-junction device of any other photovoltaic technology. With this experiment, a fair appreciation of the IRL relevance can be made, regardless of the sub-cells current matching under AM 1.5 G. By this study, we show that a non-appropriate deposition regime for the silicon-oxide-based IRL can affect the stability of the bottom cell of micromorph devices. We then show that with appropriate deposition regime the degradation rate of micromorph devices can be drastically reduced. Combining such an IRL with all recent development in our laboratory, we obtained a tandem efficiency of 11.9 % under AM 1.5G.