Effects of Different Ring‐Expanded Strategies for Nonfullerene Acceptors in Organic Photovoltaics under Donor and Acceptor Excitation.

The ring‐expanded strategy in nonfullerene acceptors (NFAs) with the acceptor–donor–acceptor backbone has been reported to be an effective method to improve the fill factor (FF), open circuit voltage (VOC), and short circuit current (JSC) simultaneously in organic photovoltaics. However, design control is still missing in the ring‐expanded strategy, and is urgently needed to further develop the origins and rules. To give insight into this strategy, a detailed theoretical study of the ring‐expanded mechanism is performed on the systems comprising different 9,9′‐bifluorenylidene‐based cores and 1,1‐dicyanomethylene‐3‐indanone group. Some main parameters involved in photoelectric conversion mechanism under the donor excitation (DE) and/or acceptor excitation (AE) are assessed by changing the position and size of ring‐expanded modes. The results show that the external ring‐expanded modes can not only maintain the original advantage as much as possible, variations in sizes and positions also offer them an opportunity to regulate the aforementioned parameters systematically, leading to better improvement regardless of AE or DE. Thus, the steady improvement in performance mentioned previously is the key to overcoming the negative correlation among FF, VOC, and JSC. This insight and discovery of the ring‐expanded strategy provides new design approaches for the next generation of NFAs. [ABSTRACT FROM AUTHOR]