In organic photovoltaics (OPVs), determining the energy-level alignment of a donor and an acceptor is particularly important since the interfacial energy gap between the highest occupied molecular orbital (HOMO) level of a donor and the lowest unoccupied molecular orbital (LUMO) level of an acceptor (EDHOMO-EALUMO) gives the theoretical maximum value of the open-circuit voltage (VOC). To increase the EDHOMO-EALUMO, non-fullerene acceptors, which have a lower electron affinity (EA) than C60, are receiving increasing attention. In this study, we investigated the energy-level alignment at the interface of a boron chloride subphthalocyanine (SubPc) donor and a halogenated SubPc (Cl6SubPc) acceptor using soft X-ray spectroscopy techniques. The estimated EDHOMO-EALUMO of Cl6SubPc/SubPc was 1.95 eV, which was significantly higher than that of 1.51 eV found at the interface of C60/SubPc. This increased EDHOMO-EALUMO was the origin of the enhanced VOC in OPVs. Additionally, we studied the molecular orientation of Cl6SubPc using angle-dependent X-ray absorption spectroscopy. The highly disordered Cl6SubPc molecules result in low carrier mobility, which contributes to the lower short-circuit current density of the Cl6SubPc acceptor OPVs than the C60 acceptor OPVs.
Bibliographical noteFunding Information:
This research was supported by the Basic Science Research Programs through the National Research Foundation of Korea (Grant No. NRF-2015R1C1A1A01055026 and 2017R1D1A3B03034867). The NSLS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
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All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry