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

Abstract

In the field of organic solar cells, 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 photoinduced hole transfer due to the large binding energy of excitons in organic materials. In this work, we investigate polymeric donor:non-fullerene acceptor junctions in binary and ternary blend polymer solar cells, which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is dependent on morphology. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC 70 BM. High external quantum efficiency was achieved at long wavelengths, despite ITIC-to-PC 70 BM ratio of 1:9, which indicates efficient photoinduced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra 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.

Ultrafast Hole Transfer and Carrier Transport Controlled by Nanoscale-Phase Morphology in Nonfullerene Organic Solar Cells

“…The photoinduced charge transfer at the interface and the subsequent carrier transport lies at the heart of the operation and performance of OSCs. ,,− These dynamic processes are controlled by donor/acceptor molecular properties (e.g., structures and energetics) and phase morphology properties (e.g., domain size and phase segregation). ,,,− Compared to fullerene OSCs, the hole transfer (HT) from photoexcited NFAs is important to device performance but still largely unexplored. Efficient HT has been observed from NFAs to polymer donors with a small energy offset. ,,,,, It has been demonstrated that by tuning molecular energy levels and structures, HT efficiency and thus PCE of OSC devices can be enhanced. ,,,,, Despite these intense researches from molecular aspects, a correlation between HT process and macroscopic NFA phase properties is still lacking.…”

Enhanced Short-Wavelength Absorption and Effective Exciton Dissociation in NC70BA-Based Ternary Polymer Solar Cells

“…In contrast, by incorporating a blend of IT-4F and NC 70 BA into PBDB-T-2F (the optimized ternary ratio), the η value is further decreased (the η value is 0.87 ns), which is attributed to the more efficient exciton dissociation within the optimized PBDB-T-2F:IT-4F:NC 70 BA ternary film . The exciton dissociation efficiency ( E ) can be estimated by the following equation where η neat and η blend are the fluorescence lifetimes of the neat and blend films, respectively. The exciton dissociation efficiency is 78.78% for the optimized ternary film and is higher than that of the PBDB-T-2F:IT-4F binary film (the efficiency is 75.37%) .…”

Enhanced Performance and Stability of Ternary Organic Solar Cells Utilizing Two Similar Structure Blend Fullerene‐Free Molecules as Electron Acceptor

“…That result can be ascribed to smooth LUMO level interface and small barrier at the m ‐ITIC/ITIC‐Th interface. Thus, current injected at the two direction for the electron‐only device exhibited clearly demonstrated bilateral symmetry (from the ITO to Al electrode and from Al to ITO electrode) and favor to suppress the electron recombination at the m ‐ITIC:ITIC‐Th interfaces …”