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

Abstract

A series of soluble C₇₀-derivatives (C₇₀-PCBX) was synthesized by varying the alkyl chain length of the adduct (X = R₁, R₃, R₅, R₇, R₉) attached to the C₇₀-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 C₇₀-PCBX domains in the P3HT : C₇₀-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 C₇₀-PCBX as a result of the longer alkyl chains, as was predicted by molecular simulations. The nanomorphologies of the P3HT : C₇₀-PCBX active layers offered an explanation for the trend in solar cell performance: as the series progressed from C₇₀-PCBR₁ to C₇₀-PCBR₇, the open circuit voltage and the short circuit current gradually increased, followed by a slight drop for C₇₀-PCBR₉. An optimal overall power conversion efficiency was observed for C₇₀-PCBR₇ with a P3HT : C₇₀-PCBR₇ blend ratio of 1 : 0.8 to 1 : 0.9.