Optical considerations in the performance of hybrid four-terminal tandem photovoltaic modules

The authors present measurement results of the top and bottom cell performance in a four-terminal hybrid tandem structure optically similar to modules made by laminating together thin-film Si:H alloy and copper indium diselenide (CIS) plates. The performance was found as a function of the layer thicknesses of the cell. The performance of the bottom cell is shown to be affected by higher than expected losses in the front cell in the range of 22-50%. These losses are due to the absorption and scattering of radiation in the CIS band by the rough top cell front transparent conductor. These losses account for the fact that the optimal efficiency is found at a ratio of front-cell-to-back-cell efficiency of 2.5 rather than 2.0. A method is presented to predict the performance of hybrid tandem devices using averaged optical quantities.