An Investigation of Planar Interface-Induced Structural Features in Conjugated Polymer Thin Films Using...

The structural organization of conjugated polymers at buried interfaces plays a decisive role in determining their optical and electronic properties. However, experimental tools to probe such interfacial features remain limited. In this work, ultraviolet-visible attenuated total reflectance (UV-vis ATR) spectroscopy was applied to thin films of poly(3-hexylthiophene-2,5-diyl) to enable selective characterization of the polymer-substrate interface. ATR spectra revealed a distinct absorption peak near 690 nm, which was absent in corresponding transmission spectra. Time-dependent density functional theory (TD-DFT) calculations demonstrated that this band originates from polymer chains with a planar backbone with a small C-C-C-S dihedral angle, which is distinct from conventional aggregate structures. Thermal treatment and solvent-dependent film preparation further confirmed the interfacial origin and structural sensitivity of the 690 nm feature. These findings establish UV-vis ATR spectroscopy as a powerful approach to uncover interface-specific molecular arrangements in conjugated polymer films and provide new insights into structure-property relationships that are critical for organic electronic devices.