Direct Observations of Uniform Bulk-heterojunctions and the Energy Level Alignments in Non-Fullerene Organic Photovoltaic Active Layers*

State-of-the-art organic photovoltaics cells (OPVCs) relies on the engineering of the energy levels of the organic molecules as well as the bulk-heterojunction nanomorphology to achieve high performance. However, both are difficult to measure inside the active layer where the electron donor and acceptor molecules are mingled. While the energy level alignments of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) between the electron donors and acceptors may be altered in the mixed active layer compared to their pure forms, the nanomorphology of the donor and acceptor molecular domains are mostly studied in indirect means. Here, we present the direct observations of the nanomorphology of the molecular domains as well as the energy level alignments in the active layer of a non-fullerene based OPVC (donor: PBDB-T-2F and acceptor: IT-4Cl) using cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). It is revealed that (1) the bulk-heterojunction (BHJ) structures are homogeneous and uniform throughout the ~1.2 μm thick active layer; (2) the energy alignments between the donor-rich and acceptor-rich domains are directly observed; (3) there exists the intermixing domains at the boundaries of the donor-rich and acceptor-rich domains with thickness in the nm scale; (4) the exciton binding energies in PBDB-T-2F and IT-4Cl are estimated to be 0.74 eV and 0.32 eV, respectively; and (5) there is a ~0.7 V loss in the open circuit voltage. The results provide nano-scale understanding of the OPVC active layers to guide further improvement of the OPVC performance.