Experimental verification of optically optimized CuGaSe2 top cell for improving chalcopyrite tandems

An efficient tandem solar cell requires a top cell which is highly transparent below the energy gap of its absorber. Previously we had reported on a theoretically optimized CuGaSe 2 top cell stack based on realistic material properties. It promised a significant increase in optical transparency and, consequently, enhanced CuGaSe 2 /Cu(In,Ga)Se 2 tandem efficiency. Here we present the first steps taken towards the experimental realization of this optimized tandem. We started with a mechanically stacked device which achieved 8.5% efficiency. Optical measurements of the improved top cells and corresponding photo current densities of the filtered bottom cell are reported. The experimental findings are in agreement with the optical modeling. These data are used to assess the level of tandem performance that could be accomplished in the near future and to discuss the priorities of further research.