Charge-Transport Networks via Small-Molecule Self-Assembly in Conjugated Polymer Bulk Heterojunctions

The formation of a charge-transporting network inside a confining polymer matrix is of immense importance in organic optoelectronic devices, such as organic photovoltaics (OPVs). OPV devices based on polymer/small-molecule bulk heterojunctions have recently achieved record power-conversion efficiencies, which has rejuvenated broad interest in the field. However, it remains an outstanding challenge to relate the small-molecule chemical structure to the formation of a hierarchical device morphology, which exerts a large influence on optoelectronic processes. We aimed to answer the question, are there molecular assembly motifs that can lead to a well-defined mesoscale small-molecule network within a crowded polymer matrix without sacrificing efficient exciton dissociation? To do so, we interrogated two small molecules with the same peripheral interacting subunits but different linkages to the central core. We find that a triphenylamine core leads to robust self-assembly into nanowires that percolate through a polymer matrix but do not overly phase-separate, retaining efficient exciton dissociation. In contrast, a flourene core results in fractal, tortuous networks in the same polymer blend, which have substantially lower effective charge mobilities compared to nanowires. Our results have significant implications for electronic polymer blends, with particular relevance for nonfullerene organic photovoltaic devices.