Effect of the alkyl chain length of C70-PCBX acceptors on the device performance of P3HT: C70-PCBX polymer solar cells

A series of soluble C-70-derivatives (C-70-PCBX) was synthesized by varying the alkyl chain length of the adduct (X R-1, R-3, R-5, R-7, R-9) attached to the C-70-polyhedron. After blending with P3HT, bulk heterojunction polymer solar cells were fabricated, and their performances were evaluated. As the alkyl chain length increased, phase segregation between the P3HT and C-70-PCBX domains in the P3HT : C-70-PCBX blend active layer increased, which enhanced the device performance to a moderate degree. The performance enhancement may have been due to the increased solubility and self-aggregation of C-70-PCBX as a result of the longer alkyl chains, as was predicted by molecular simulations. The nanomorphologies of the P3HT : C-70-PCBX active layers offered an explanation for the trend in solar cell performance: as the series progressed from C-70-PCBR1 to C-70-PCBR7, the open circuit voltage and the short circuit current gradually increased, followed by a slight drop for C-70-PCBR9. An optimal overall power conversion efficiency was observed for C-70-PCBR7 with a P3HT : C-70-PCBR7 blend ratio of 1 : 0.8 to 1 : 0.9.