Solution processed organic bulk heterojunctions (BHJs) are promising for enabling low-cost, green, and flexible photovoltaics. The BHJ architecture, with its nanoscale interpenetrating donor-acceptor structure, allows for ample light absorption in a thin (hundreds of nanometer thick) active layer, while satisfying the need for a high density of donor-acceptor interfaces for exciton dissociation. Nevertheless, the highly heterogeneous nanostructured active layer, with its locally varying composition, crystallinity, and miscibility between components, leads to a complex landscape for charge transport. Little is known about the impacts of local BHJ structure on charge percolation across the active layer (out-of-plane).
In this presentation, we will delineate key structural features that impact out-of-plane charge transport in small molecule:fullerene BHJ active layers comprising p-DTS(FBTTh2)2:PC71BM. A series of 15 BHJ films with varying compositions and degrees of phase separation were characterized electrically by conductive atomic force microscopy (C-AFM) and structurally by grazing incidence x-ray diffraction (GIXD). C-AFM was used to quantify and map local hole mobility, while GIXD provided the crystallite size and population along π-π and alkyl stacking directions. These experiments show that the strongest predictor of out-of-plane charge carrier mobility across all morphologies is the in-plane π-π crystallinity within a film. This result is further supported by nanoscale hole mobility maps that show a halo of moderate hole mobility regions concentrated around high hole mobility hot spots, indicating lateral access of charge from the surrounding regions to the hot spots. Additional insight into the dependence of hole mobility on active layer composition and phase separation is provided by percolation theory. This analysis provides clues about the differences in domain connectivity that affect hole transport efficiency when the degree of phase separation is increased.