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 two-dimensional layers. Mechanical
annealing through compression-expansion (CE) cycles systematically improves structural uniformity, as evidenced
by narrower in-situ detected fluorescence spectra. Repeated CE-cycles also shift the maximum of the
compressional modulus towards 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.