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

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 relativ...