Abstract
To overcome the intrinsic limitations of fullerene-based organic photovoltaic (OPV) devices, research on OPV devices based on non-fullerene acceptors (NFAs) has been actively conducted in recent years. It is important to understand the relationship between the structure of the NFAs and photovoltaic properties to create high-performance OPV devices. In this study, we have designed and synthesized a series of NFAs (DFDO-RC2 and DFDE-RC2) based on electron-rich dithienosilole (D) and electron-deficient difluorobenzodiathiazole (F), benzodiathiazole-connected 3-ethylrhodanine (RC2) units, and alkyl chains of 2-ethylhexyl (E) and octyl (O) groups. The PTB7-Th:DFD-RC2 devices showed low PCEs mainly due to the highly located highest occupied molecular orbital (HOMO) energy levels of the DFD-RC2 acceptors compared to the PTB7-Th polymer donor. To lower the HOMO levels of the DFD-RC2 NFAs, the backbone structures were modified by replacing difluorobenzodiathiazole core moiety with difluorobenzene (FBz) to obtain DFBz-RC2 molecules (DFBzO-RC2 and DFBzE-RC2). PTB7-Th:DFBz-RC2 devices exhibited significantly improved PCEs compared to PTB7-Th:DFD-RC2 devices. The DFBzO-RC2 and DFBzE-RC2 molecule-based OPVs exhibited remarkably high Vocs of 1.03 and 1.07 V, respectively, which characteristic is associated with the very low energy loss (Eloss) of 0.51 eV in both PTB7-Th:DFBzO-RC2 and PTB7-Th:DFBzE-RC2 devices. Overall, our investigation of the various synthesized molecules reveals the structure-to-photovoltaic properties, which guide the design of new high-performance NFAs to advance in the field of organic solar cells.
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