Phase separation and affinity between a fluorinated perylene diimide dye and an alkyl-substituted hexa-peri-hexabenzocoronene

Abstract

Fluorination of alkyl groups is a known strategy for hindering miscibility, thus promoting phase separation, when blends are prepared with a hydrocarbon compound. A new perylene bis(dicarboximide) derivative functionalized with branched N-perfluoroalkyl moieties (BPF-PDI) has been synthesized as electron acceptor to be potentially used in conjunction with the electron donor hexakis(dodecyl)hexabenzocoronene (HBC-C12) in bulk heterojunction solar cells. Aiming at controlling self-assembly between the two components at the supramolecular level, stoichiometric blends in CHCl3 have been prepared either by spin- or drop-casting onto silicon surfaces, and further subjected to solvent vapour annealing (SVA) treatment in a CHCl3-saturated atmosphere. Spectroscopic investigation in solution shows the formation of supramolecular BPF-PDI–HBC-C12 dimers, with an association constant Kass = (2.1 ± 0.3) × 104 M−1, pointing to a strong and unexpected affinity between the two species within the mixture. Characterization through optical and atomic force microscopies of the deposited samples revealed that the self-assembly behaviour of the blends on SiOx is remarkably different from mono-component films, thus confirming the absence of a macroscopic phase-separation between the two components featuring isolated domains of the neat acceptor or donor compound. In addition, X-ray studies provided evidence for the existence of a local-scale phase separation. These findings are of importance for organic photovoltaics, since they offer a new strategy to control the phase separation at different scales in electron acceptor–donor blends.