Solution processable, precursor based zinc oxide buffer layers for 4.5% efficient organic tandem solar cells

In this work we present regular and inverted organic tandem solar cells from poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole): [6,6]-phenyl C70-butyric acid methyl ester (PCDTBT:PC71BM) with power conversion efficiencies of up to 4.5%. The recombination zone comprises an electron conducting, precursor based zinc oxide buffer layer that was applied from solution under ambient conditions and at moderate processing temperatures. Optimized active layer thicknesses in both subcells were derived from optical Transfer Matrix simulations. The short circuit current density of the tandem cell exceeds half the short circuit current density of the single absorber cells indicating a real gain in quantum yield when utilizing the tandem architecture.

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► Regular and inverted PCDTBT based tandem solar cells are simulated and fabricated accordingly.
► The recombination zones comprise solution deposited ZnO processed at low temperatures.
► Intermediate charge recombination is enhanced by incorporating gold nano-clusters.
► Tandem cells exhibit higher quantum yield as compared to a reference single cell.