Characterization of Charge Transport and Microstructure of Non-Fullerene Molecular Networks in Bulk Heterojunction Thin Films

A large effort exists to explore non-fullerene small-molecules for organic photovoltaic devices, however, there is a fundamental lack of understanding of how the chemical structure of polymer and small-molecule influence the hierarchical microstructure of bulk-heterojunction (BHJ) thin films. Two photophysical processes that dictate device performance – charge separation and transport – depend intimately on the hierarchical length scales of phase separation and interpenetrating polymer/small molecule networks. To elucidate the relationship between BHJ microstructure and chemical structure, we interrogate a series of bay-annulated isoindigo dye molecules with varied connectivity of pi-stacking faces in polymer blends and couple transmission electron microscopy with real-space pair distance distribution functions generated from the generalized indirect Fourier transform of resonant elastic X-ray scattering data. To connect the nanoscale morphology with charge transport, we use dark current injection measurements in diode configuration devices to interrogate the formation of percolated charge transfer channels. We find that the geometry of the small-molecule frontier electronic wavefunctions has a very large influence on the molecular network that forms inside the polymer matrix.