Morphology Dependent Hole Transfer Under Negligible HOMO Difference in Non-Fullerene Acceptor Based Ternary Polymer Solar Cells.

In the field of organic solar cell (OSCs), it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photo-induced hole transfer, due to the large binding energy of excitons in organic materials. In this work we investigate polymeric donor : nonfullerene acceptor junctions in binary and ternary blend polymer solar cells (PSCs) which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is morphology-dependent. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC70BM. High external quantum efficiency (EQE) was achieved at long wavelengths, despite a 1:9 ITIC to PC70BM ratio, which indicates efficient photo-induced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra (TAS) further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend, compared to that of donor:ITIC binary blend.