Efficient Ternary Blend Bulk Heterojunction Solar Cells with Tunable Open-Circuit Voltage

To explore the potential of ternary blend bulk heterojunction (BHJ) photovoltaics as a general platform for increasing the attainable performance of organic solar cells, a model system based on poly(3-hexylthiophene) (P3HT) as the donor and two soluble fullerene acceptors, phenyl-C61-butyric acid methyl ester (PC61BM) and indene-C60 bisadduct (ICBA), was examined. In all of the solar cells, the overall ratio of polymer to fullerene was maintained at 1:1, while the composition of the fullerene component (PC61BM:ICBA ratio) was varied. Photovoltaic devices showed high short-circuit current densities (Jsc) and fill factors (FF) (>0.57) at all fullerene ratios, while the open-circuit voltage (Voc) was found to vary from 0.61 to 0.84 V as the fraction of ICBA was increased. These results indicate that the Voc in ternary blend BHJ solar cells is not limited to the smallest Voc of the corresponding binary blend solar cells but can be varied between the extreme Voc values without significant effect on the Jsc or FF. By extension, this result suggests that ternary blends provide a potentially effective route toward maximizing the attainable JscVoc product (which is directly proportional to the solar cell efficiency) in BHJ solar cells and that with judicious selection of donor and acceptor components, solar cells with efficiencies exceeding the theoretical limits for binary blend solar cells could be possible without sacrificing the simplicity of a single active-layer processing step.