Terminal Groups of Nonfullerene Acceptors: Design and Application

Materials design plays a critical role in improving the power conversion efficiency (PCEs) of organic solar cells (OSCs). Recent advances in nonfullerene acceptors (NFAs) have achieved great success and made a large contribution to the rapid increase in PCEs. The current state-of-the-art OSCs have PCEs of >19%, demonstrating their great potential for use in practical applications. All high-efficiency NFAs adopt an A–D–A or A–DA′D–A (A = acceptor and D = donor) structure. Modulating the electron push–pull effect using an alternating D–A structure has proven to be an effective molecular design method. Specially, for NFAs, the design and application of terminal groups are of great significance. In this Perspective, a brief introduction is given to the development of terminal groups and representative materials. We highlight the critical role of the molecular electrostatic potential in evaluating the electron-withdrawing strength. The applications of terminal groups in designing wide-, middle-, and low-bandgap NFAs are discussed. Then, an outline of the other functions of terminal groups in affecting the intermolecular packing, blend morphology, and energy loss is presented. Furthermore, insights into the key issues that should be considered when developing new terminal groups, including efficiency, cost, and stability, are discussed.