Precise End‐Group Conjugated‐Site Isomerization of Wing‐Typed Dimeric Acceptors Enables 20.32% Efficiency Organic Solar Cells

Molecular isomerization has been proven as an effective strategy to optimize the backbone geometry of photovoltaic materials and improve power‐conversion‐efficiency (PCE) of organic solar cells (OSCs). However, the precise isomerization of dimeric acceptors and their effect on optoelectronic and photovoltaic properties have been rarely studied. Herein, four isomeric wing‐typed dimeric acceptors (named Dimer‐1, Dimer‐2, Dimer‐3, and Dimer‐4) with an alkylated linker are developed to investigate how different conjugated‐sites of end‐groups (methoxy‐substituted EG1 and methoxy‐free EG2) manipulate molecular packing and optoelectronic properties. Among them, Dimer‐2, with two inside EG1 and two outside EG2 groups, show superior molecular packing, crystallinity, and photoluminescence. Therefore, the PM6:Dimer‐2‐based OSC achieves a higher PCE of 15.12% with obviously boosted photovoltage and fill factor, mainly due to its optimized morphology and reduced energy loss. Building on this success, Dimer‐2 is introduced into the classic PM6:BTP‐eC9 and D18:L8‐BO pairs to prepare ternary systems with complementary absorption and favorable vertical phase distribution, leading to impressive PCEs of 20.06% and 20.32%, respectively. Notably, 20.32% efficiency is the highest value among ternary OSCs with non‐fully conjugated dimeric acceptors. This work indicates that the precise isomerization of end‐group conjugated‐sites is a feasible approach to developing high‐performance dimeric acceptors and constructing efficient OSCs.