Stable dinitrile end-capped closed-shell non-quinodimethane as a donor, an acceptor and an additive for organic solar cells

Non-fullerene acceptors exhibit great potential in improving the photovoltaic performances of organic solar cells. However, it is important to further enhance their chemical stability and device durability for future commercialization, especially for Y6-series small molecule acceptors with 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) as the ending group. In this work, an IC-free photovoltaic material YF-CN consisting of the 2-fluoren-9-ylidenepropanedinitrile terminal was designed and synthesized by Stille coupling. YF-CN has a closed-shell chemical structure with enhanced photostability and improved morphological compatibility with the binary PCE10:Y6 blend. The moderate energy level enables YF-CN to serve as a multifunctional material, such as a donor, an acceptor and a third component. When adding YF-CN as a second donor into the PCE10:Y6 system, an improved power conversion efficiency of 12.03% was achieved for the as-cast device. Importantly, the ternary PCE10:YF-CN:Y6-devices showed enhanced storage durability maintaining 91% of their initial PCE after storing for 360 h. This work provides a new perspective to understand the open-shell character of the donor and the closed-shell structure of the acceptor, as well as a promising design concept of stable IC-free acceptors for organic solar cells.