Quasi‐2D Morphologies of a Non‐Fullerene Acceptor Y6 by Interfacial Assembly via Langmuir–Schaefer Technique

Abstract Supramolecular organization governs the structure and optoelectronic properties of organic thin films. This study shows that films based on the non‐fullerene acceptor Y6 can be precisely structured via assembly at the air‐water interface. Theoretical cross‐sectional areas, Langmuir isotherms, and Brewster angle microscopy reveal that Y6, despite its complex structure, is sufficiently amphiphilic to form well‐defined 2D layers. Mechanical annealing through repeated compression‐expansion cycles systematically improves structural uniformity, as evidenced by narrower in situ detected fluorescence spectra, while simultaneously shifting the maximum of the compressional modulus toward denser packing. Compared to spin‐cast films, Langmuir–Schaefer (LS) layers exhibit a significantly reduced Stokes shift, suggesting less reorganization after photoexcitation and thus a higher supramolecular order. Organic thin‐film transistors (OTFTs) fabricated using the LS technique achieve mobilities comparable to those of spin‐cast films, despite being substantially thinner (≤ 3 nm, determined by atomic force microscopy), thus requiring considerably less material. Notably, Y6‐LS OTFTs outperform previously reported polymer‐based LS‐OTFTs by one order of magnitude in charge carrier mobility. This work highlights the potential of interfacial assembly for thin film fabrication and underscores the advantages of mechanical annealing and in situ spectroscopy to enhance the performance of organic optoelectronic devices.