Double-caged fullerene acceptors: effect of alkyl chain length on photovoltaic performance

We report the synthesis and spectroscopic and electrochemical characterization of the benzyl, C₄, C₆, C₈, C₉, C₁₀, and C₁₈n-alkyl esters of double-caged fullerene compounds (dFR). Counterintuitively, their solubility depends nonmonotonically on the alkyl chain length with a minimum at n-octyl. The series of dFR compounds were tested in organic solar cells (OSCs) as an acceptor material blended with the poly(3-hexylthiophene-2,5-diyl) (P3HT) donor. With the exception of the n-octadecyl derivative, the performance of the P3HT/dFR solar cells correlates with the solubility of the dFR component, likely due to the improved bulk heterojunction morphology and its favorable effect on J_SCvia lower series resistance. A peak PCE of 3.0% was found for the n-nonyl compound, thus exceeding other known fullerene oligomers with fused fullerene cages or cross-linked fullerene dimers. The poor performance of compounds with too long alkyl chains like n-octadecyl reflects larger series resistance that results from a higher degree of phase segregation.