Nonlinear spectroscopic markers of structural change during charge accumulation in organic field-effect transistors

Vibrational sum frequency generation (VSFG) is used to characterize the buried polymer-dielectric interface in poly(triarylamine) (PTAA) organic field-effect transistors (oFETs) over a spectral range of more than 1100 cm ^-1. The FTIR and Raman spectra are presented for the neutral and chemically oxidized thin films of this polymer as a starting point for identifying the potential vibrational changes that are induced by doping. In the VSFG spectra collected as a function of applied gate bias, we find evidence for interfacial electric fields, polaronic absorbances, and a strong VSFG-active band that is a spectral signature of the interfacial molecules that are perturbed by electrical doping. In most cases, the unipolar electrical behavior of PTAA oFETs is directly correlated with intensity changes in this structural perturbation band regardless of the dielectric (SiO₂) surface functionalization. However, in a few selected examples, the VSFG measurements demonstrate that accumulation of either holes or electrons can be achieved with this organic semiconductor, even in the absence of measurable source-drain currents. These results highlight the potential for PTAA to achieve ambipolar device operation and the power of nonlinear spectroscopy to provide the feedback needed to optimize this performance when electrical measurements cannot.