Regioisomer effects of [70]PCBM on film structures and photovoltaic properties of composite films with a crystalline conjugated polymer P3HT

Despite the wide prevalence of [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) as an electron acceptor in high-performance organic photovoltaic (OPV) devices, [70]PCBM has been generally used as a mixture of regioisomers. In this study, we utilized regioisomerically pure α- and β-[70]PCBM in combination with a crystalline conjugated polymer donor, poly(3-hexylthiophene) (P3HT) to investigate systematically the regioisomer effects of [70]PCBM on structures and electronic and photovoltaic properties of the composite films. Notably, the β-isomer induced a face-on P3HT packing in the blend film, whereas an edge-on alignment of P3HT was observed in the composite films with the α-isomer as well as the regioisomer mixture (mix-[70]PCBM). The hole mobility in a vertical direction to the substrate in the P3HT:β-[70]PCBM film was higher than those in P3HT:α-[70]PCBM and P3HT:mix-[70]PCBM due to the face-on P3HT packing. Reflecting the superior hole mobility, the OPV device based on the P3HT:β-[70]PCBM film showed a higher power conversion efficiency of 3.69% than the devices based on the other composite films (3.11–3.21%). The results obtained here demonstrate that the use of regioisomerically pure [70]fullerene mono-adducts can modulate the polymer packing direction in the blend film, which is an unprecedented approach to improve the device performances of OPVs.