NEXAFS Spectroscopy Reveals the Molecular Orientation in Blade-Coated Pyridal[2,1,3]thiadiazole-Containing Conjugated Polymer Thin Films

The characterization of the microstructure and molecular orientation is critical to understanding the performance of conjugated polymer semiconductors. In this work, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to study the molecular orientation of blade-coating thin films of regioregular PCDTPT (poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine]) on substrates with and without uniaxial nanogrooves. The prediction of NEXAFS spectra through density functional theory calculations allowed for the interpretation of the experimental spectral features and provided information about molecular orientation. Using the polarization dependence of the Nitrogen 1s to pi*-resonance signals, the molecular orientation was quantified through calculations of the order parameters S (out-of-plane) and eta (in-plane) for both the top side and bottom side of the film. All films have out-of-plane orientation where the conjugated backbones have a preferential edge-on alignment relative to the substrate surface. On the other hand, with increasing blade-coating rates, the greatest degree of in-plane polymer-chain orientation occurs on the bottom side of a film deposited on a nanogrooved substrate. The results demonstrate the utility of the nanogroove method to induce alignment of solution-processed semiconducting polymers.