Correlations between SFG spectra and electrical properties of organic field effect transistors

Simultaneous sum frequency generation (SFG) vibrational spectra and electrical measurements were obtained on organic field effect transistors (OFETs) fabricated with the semiconductors: 5,5'-bis(4-hexylphenyl)-2,2'bithiophene (6pttp6), 5,5'-bis(4-ethylphenyl)-2,2'-bithiophene (2pttp2), and pentacene. In-situ measurements during gating of the OFETs showed strong correlations between vibrational spectra and electronic properties. One correlation involved structural changes in the hexyl and ethyl groups, of 6pttp6 and 2pttp2, respectively, and saturation source-drain current; the correlation was observed only at negative gate voltages (when carr ier injection was possible) and was more pronounced for 6pttp6, with the introduction of gauche defects in the longer hexyl chains. A second correlation between the dependence of SF nonresonant background on gate voltage and electronic mobility was observed on OFETs of all three semiconductors, at both positive and negative gate voltages. This correlation suggests that a common molecular structural packing element may determine the magnitude of both the electronic mobility and higher order nonlinear optical susceptibilities in oligomeric thin films. These results also demonstrate the utility of SFG in probing molecular structural and electrical field effects at the buried semiconductor- dielectric interface of OFETs.