End-Group Engineering of Nonfullerene Acceptors for High-Efficiency Organic Solar Cells

In recent years, organic solar cells (OSCs) have made significant advancements due to a deeper understanding of molecular design and device technology. One area of molecular design that has contributed to these advancements is the emergence of nonfullerene small-molecule acceptors (SMAs) and polymerized SMAs. The molecular design strategy of state-of-the-art SMAs focuses on two aspects: the electron-rich central core unit and electron-deficient end groups. Different from the manipulation of the central cores, end-group engineering is a direct and efficient means to adjust the physicochemical properties and crystallization/aggregation behavior of acceptors, leading to enhanced photovoltaic performance. On the basis of our recent research advances, herein we focus on the topic of end-group engineering of nonfullerene acceptors, aiming to provide a comprehensive understanding of the optimization of end groups for the design of high-performance acceptor materials.In this Account, first, we systematically compare the difference between thiophene-fused and benzene-fused end groups in synthetic routes and molecular energy levels. Unlike the centrosymmetric benzene, the axisymmetric thiophene-fused end groups have two different fusion modes, resulting in their different frontier orbital energy levels. Second, we offer a wrought review of SMAs with thiophene-fused or thiophene derivatives-fused end groups, emphasizing the important role of thiophene derivatives-fused end groups in enhancing molecular packing, improving exciton bonding energy, and reducing energy loss in OSCs. Additionally, we reveal the specific reason why the thiophene-fused end group with an alpha/beta fusion site and the thiophene-fused end group with a beta/gamma fusion site have significantly different molecular energy levels. Third, we summarize the photovoltaic parameters and conventional physicochemical properties of polymerized SMAs based on monobromobenzene-fused end groups and fluorobromine (or chlorobromide) cosubstituted benzene-fused end groups. We demonstrate that regioregular polymerized SMAs show great prospects in realizing high-performance all-polymer solar cells by eliminating the disorder of molecular backbone structure with pure monobromobenzene-fused end groups. Furthermore, the halogenation strategy (fluorination and chlorination) is also an effective method for designing high-performance PSMAs with large electron mobility induced by the intermolecular noncovalent interactions of halogen<middle dot><middle dot><middle dot>H, halogen<middle dot><middle dot><middle dot>S, and halogen<middle dot><middle dot><middle dot>halogen. Finally, we analyze the role of asymmetric end group substitution for developing high-performance SMAs. In comparison with symmetric SMAs, the asymmetric one achieves low energy loss while ensuring sufficient charge separation. As a summary and perspective, we discuss the current questions regarding end groups and propose our insights into the future development of nonfullerene acceptors with novel end groups toward low-cost and high-performance OSCs.