OPV Tandems with CNTS: Why Are Parallel Connections Better Than Series Connections

Abstract

The efficiency of organic photovoltaic cells can be increased in tandem OPV structures with complementary light absorption in top and bottom sub-cells. We demonstrate that strong transparent CNT sheets can be used as an effective charge collector interlayer in OPV and hybrid tandem solar cells. Most importantly we show that CNT sheets can be used in monolithic parallel tandems (P-T) as common a electrode interconnect between top and bottom sub-cells. For achieving good performance one of these subcells in P-T should be of inverted type. We achieved good inversion in OPV, using ZnO nanoparticles, which act as hole barrier layers and invert a typical anode ITO into a cathode. With this inverted bottom cell the efficiency of P-T is significantly improved, as compared to our earlier results. We briefly discuss the modeling analysis of OPV tandems and derive an optimal set of parameters, for highest efficiency P-T. Our simple model shows that for tandems with unbalanced photocurrents but similar open circuit voltages the optimized P-T architecture is always better than conventional series tandem (S-T) geometry. Indeed the experimental comparison of P-T with S-T for hybrids of OPV and dye sensitized solar cells demonstrate the imporved efficiency of the former.