Single-crystal growth of complex non-fullerene acceptor molecules via cocrystallization.

The growth of high-quality organic single crystals is essential for probing intrinsic optoelectronic properties and molecular packing. However, the conventional vapor- and liquid-phase methods fail for structurally complex molecules like the non-fullerene acceptor (NFA) Y6, where thermal instability and steric hindrance from branched sidechains inhibit crystallization. Here, we report an additive-directed cocrystallization strategy to grow Y6-additive cocrystals (YACs) with controlled morphology and tunable thicknesses (18 nm to 341 nm). The single-crystal structure is determined by Micro Electron Diffraction Technology at first time. Growth mechanism studies reveal that additive molecules mitigate sidechain interference by enabling configuration coupling of π-π stacking, yielding YACs with central length of 450 μm and largest lengths of 1.5 mm. Generalizability is demonstrated across 10 kinds of Y6-like NFAs with axial/central symmetry and 2 kinds of effective additives. Most of YACs exhibit strong second harmonic generation (SHG) response. This work establishes a paradigm of single-crystal growth for structurally hindered complex molecules and provides a crystallographic basis for investigating the optoelectronic properties.