Observation of Multiple-Electronic-Domain Bulk Heterojunctions in Non-Fullerene Organic Photovoltaic Active Layers using Cross-Sectional Scanning Tunneling Microscopy and Spectroscopy

It is widely believed that the active layers of the state-of-the-art organic photovoltaic cells (OPVCs) exhibit bulk heterojunctions (BHJs). While new non-fullerene molecular combinations are explored in the applications of OPVC, whether if the molecular domain structures still follow the BHJ architecture is an important question to answer. Here, with cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S), isotropic electronic domains are observed using the d$I$/d$V$ mapping. In addition to the electron donor (PBDB-T-2F rich) and acceptor (IT-4CL rich) domains, an intermediate electronic domain is observed. Furthermore, the evolution of the isotropic molecular domain random structures as a function of the depth of the film is presented. The confirmation of the intermediate domains and the depth dependent nano-structural evolution may provide important insights on improving the OPVC performance.