Abstract
Single heterojunction organic photovoltaic cells based on co-evaporated donor–acceptor layers with power conversion efficiencies (η) above 5.5% are demonstrated, using either high (1.8 eV) or low (1.4 eV) optical gap materials. The high energy absorbing cell utilizes a high fullerene-C70 content, in combination with a high mobility amorphous donor, while the low energy absorbing cell consists of a donor–acceptor molecule paired with C60 as the acceptor. The integration of the two cells in an optimized tandem configuration leads to η =7.2%, verified by external quantum efficiency measurements of the subcells. Notably, the fill-factor of the tandem stack is higher than either one of the sub-cells.
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