Controllable Phase Separation with Star‐Shaped Y‐Type Electron Acceptors for High‐Efficiency and Stable Organic Solar Cells

Abstract Star‐shaped conjugated materials exhibit monodisperses, well‐defined structures akin to small molecules while possessing the high molecular weights typical of polymers, making them appealing for organic electronics. Herein, a series of star‐shaped electron acceptors, labeled SP1‐Ph, SP2‐Ph, SP3‐Ph, SP4‐Ph, and SP6‐Ph correspond to one to six number of arms, have been synthesized for use in organic solar cells (OSCs). The acceptors have been synthesized through Williamson ether synthesis, utilizing OH‐substituted Y‐type precursors and (multiple‐substituted bromomethyl)benzene. An increase in the number of arms results in weakened crystallinity and different aggregation behavior. Therefore, different number of arms can efficiently tune phase separation sizes between the acceptors and donor polymer blended films, which dominates the charge generation process in OSCs. As a result, SP3‐Ph and SP4‐Ph, with modest phase separation size in binary blends, have achieved optimal efficiencies of 16.10% with high stability. However, SP6‐Ph‐based OSC exhibits oversized phase separation and low efficiency of 8.87%. Furthermore, the use of SP3‐Ph and SP4‐Ph in ternary OSCs leads to an impressive efficiency of 19.3%. These results highlight the ability of star‐shaped electron acceptors with varying arm numbers to precisely control the phase separation of photoactive layers, thereby advancing the development of highly efficient and stable OSCs.