Differing Impacts of Blended Fullerene Acceptors on the Performance of Ternary Organic Solar Cells

Organic photovoltaic (OPV) devices offer the ability to tune the electronic and optical properties of the active layer by selection of a wide range of molecules; however, their power conversion efficiencies currently lag other competing photovoltaic technologies. One method to enhance their performance is to add a further active material into the absorber layer, resulting in a ternary OPV. However, selecting appropriate ternary blend components to yield an improvement in performance is challenging due to the multitude of materials properties and physical processes that ternary blends can display. Here, we perform a systematic set of experiments on OPV ternary blends incorporating either of the donor polymers P3HT or PTB7 and the fullerene acceptors PCBM and ICBA. Some combinations of ternary blends are shown to outperform the reference binaries in terms of open-circuit voltage or short-circuit current; however, this was not observed for all combinations. Improvements in internal quantum efficiency of the order of 25% were observed for PTB7-based ternaries compared to the reference binary, which is attributed to a reduction in charge recombination. All blends showed some improvement in open-circuit voltage with addition of ICBA due to alloying of the fullerene components, but to differing degrees which is argued to be due to molecular morphology. These findings demonstrate that the benefits one can obtain using a ternary OPV approach vary depending on the materials system to an extent that depends upon the ternary blend morphology.