Fluorine‐Isomerized and Alkoxylated Y‐Series Acceptors for Indoor and Ternary Organic Photovoltaics

Designing nonfullerene acceptors with tailored absorption and energy levels is critical for indoor and ternary organic photovoltaic (OPV) applications. Here, we report two blueshifted Y‐series acceptors, Y6‐iso and Y6‐O‐iso, derived from the benchmark Y6 molecule through fluorine isomerization on the IC‐type terminal group and alkoxy side‐chain substitution. Fluorine isomerization from the conventional 5,6‐ to the 4,5‐positions reduces the intramolecular charge transfer (ICT) strength, inducing a blueshift while maintaining molecular planarity and charge transport properties. The alkoxy side chain enables a resonance between the oxygen atom and the terminal group, presumably disrupting the original electronic communication between the central fused‐ring core and the terminal group. This effect further diminishes ICT, producing a pronounced blueshift and elevating the lowest unoccupied molecular orbital (LUMO) level, effectively aligning the absorption with indoor light‐emitting diode (LED) spectra. Consequently, the PM6:Y6‐O‐iso binary achieves a power conversion efficiency of 25.3% under 2000 Lx and 23.7% under 500 Lx LED illumination. When incorporated as a guest molecule in PM6:Y6 ternary devices, both acceptors improve V _OC and complement the absorption of Y6, yielding power conversion efficiency of 18.0% and 18.4%, respectively. These results demonstrate a rational molecular design strategy, highlighting fluorine isomerization and alkoxy substitution as effective approaches to optimize nonfullerene acceptors for versatile indoor and ternary OPV applications.