Fullerene derivative induced morphology of bulk heterojunction blends: PIPCP:PC₆₁BM

The performance of organic solar cells (OSCs) depends crucially on the morphology in bulk heterojunctions (BHJs), including the degree of crystallinity of the polymer and the amount of each material phase: aggregated donor, aggregated acceptor, and molecular mixed donor : acceptor phase. In this paper, we report the BHJ morphology of as-cast blend films incorporating the polymer PIPCP as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as the acceptor. Tracking the scattering intensity of PC₆₁BM as a function of PC₆₁BM concentration shows that PC₆₁BM aggregates into donor-rich domains and there is little to no phase where the PC₆₁BM and PIPCP are intimately mixed. We further find that on blending the scattering peak due to PIPCP ordering along the backbone increases with decreasing PIPCP fraction, which is attributed to improved ordering of PIPCP due to the presence of PC₆₁BM. Our results suggest that the improved ordering of PIPCP along the backbone (consistent with an increased conjugation length) with blending contributes to the observed low open-circuit voltage energy loss.