Absence of Mixed Phase in Organic Photovoltaic Active Layers Facilitates Use of Green Solvent Processing

We examine the morphology of a donor molecule:fullerene organic photovoltaic (OPV) active layer, processed from the “green solvent” 2-methyl-THF (m-THF), as well as the conformation of the two OPV constituent components in the casting solution. We observe that the small molecule (X2) has a weak association with itself in chloroform solvent, whereas it does not self-associate in m-THF. Despite this difference, the morphologies of the final processed films are extraordinarily similar: there is negligible molecularly mixed phase in the layer, and the domain sizes of the pure X2 and pure fullerene are 15–20 nm. We attribute this similarity between the final films to the strong aggregation behavior of X2 upon drying; changes in solvent or solvent additive have therefore only a minor effect on the final bulk heterojunction morphology. This contrasts with the majority of other OPV molecular donor semiconductor systems, which need careful tuning of solvent and/or solvent additive to achieve the optimal morphology and photovoltaic performance. We argue that the absence of a mixed phase is a result of the strong self-aggregation behavior of X2 and a key property of this material combination that makes it robust to a change in processing solvent.