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.