Effect of fullerene acceptor on the performance of solar cells based on PffBT4T-2OD.

We have studied bulk-heterojunction (BHJ) solar cells composed of the polymer PffBT4T-2OD as electron donor and three different electron accepting fullerenes, namely PC71BM, PC61BM and indene-C60-bis-adduct (ICBA) in order to understand the impact of different fullerenes on the morphology and efficiency of the corresponding photovoltaic devices. Despite PffBT4T-2OD:ICBA devices being characterised by higher values of Voc, they display the lowest power conversion efficiency (PCE) due to their lower Jsc and FF values. We find that although all blend films have similar morphologies, X-ray scattering indicates a reduced degree of order within the fullerene domains in the ICBA-based film. Due to the high LUMO level of ICBA, the corresponding blends are characterised by a lower initial exciton dissociation and this associated with the reduced ordering within the ICBA domains results in increased geminate recombination of the photogenerated electrons in the fullerene-rich domains and a consequently reduced PCE of the corresponding devices.