Brominated Small-Molecule Acceptors with a Simple Non-fused Framework for Efficient Organic Solar Cells

Recently, non-fullerene organic solar cells (OSCs) have experienced a rapid increase in efficiency, benefiting from the development of small-molecule acceptors (SMAs). The widely studied SMAs for high-performance OSCs mostly contain a generally large fused-ring core and fluorination of the end group. In this work, two brominated SMAs with a simple non-fused framework, named as F8IDT-Br and C8IDT-Br, have been designed and synthesized in only two synthetic steps, in which the fluorene or carbazole ring acts as an electron-donating central core (D) by employing thiophene as the π bridge and brominated 1,1-dicyanomethylene-3-indanone as the end group (A). Optical, electrochemical, and thermal properties and crystalline behaviors of these two acceptors were characterized. The two acceptors are directly compared to explore the effects of the core moiety on the intrinsic and photovoltaic properties. The widely used polymer PM6 was selected as a donor to fabricate the devices. As a result, a power conversion efficiency (PCE) value of 9.70% is obtained in PM6/F8IDT-Br-based devices, outperforming the previously reported results (PCE: <8%) from OSCs based on non-fused fluorene core-based acceptors. It means that bromination of non-fused SMAs can further improve relevant device performance. The best PCE value of 9.85% is achieved in PM6/C8IDT-Br-based devices, which is noticeable efficiency in non-fused SMA-based OSCs. Moreover, the devices based on these two brominated acceptors exhibit excellent storage stability. The results demonstrate that these two acceptors are excellent materials among simple non-fused SMAs.