Interfacial Ring Orientation in Polythiophene Field-Effect Transistors on Functionalized Dielectrics

Abstract

The impact of surface chemistry on interfacial molecular orientation is studied for polymeric organic field-effect transistor (oFET) assemblies using vibrational sum frequency generation (VSFG) spectroscopy. The carbon–carbon vibrational modes on the backbone of side-chain deuterated poly(3-hexylthiophene) (DP3HT) are demonstrated to be an excellent handle for molecular orientation. DP3HT is utilized to avoid overlap in this spectral region with alkyl CH bending vibrations. Raman and FTIR spectroscopies are used to characterize the vibrational spectra of the thin films, and band assignments are confirmed by DFT calculations. Organosilane self-assembled monolayers are used to prepare oFET dielectrics with a range of surface energies, as measured by their water contact angles. The surface chemistry is found to have a profound influence on the field-effect carrier mobilities with lower surface energies producing higher mobilities. Polarization selective VSFG spectroscopy is then used to determine the relative orientation of the C═C symmetric stretching mode with respect to the surface normal. High field-effect mobilities for DP3HT on low surface energy functionalized dielectrics are directly correlated with the relative orientations of this vibrational transition dipole moment as measured by VSFG. The connection between this nonlinear spectroscopic observable and molecular structure enables this approach to confirm that low surface energy dielectrics lead to edge-on orientation of DP3HT conjugated chains with consequently higher mobilities.