Conducting Channel Formation in Poly(3-hexylthiophene) Field Effect Transistors: Bulk to Interface

The most critical step in fabricating solution-processed organic field effect transistors (OFETs) is the solidification process of the polymer solution at the polymer/gate dielectric interface, where the charge transport properties of the resultant thin film are determined by the structural organization of the polymer molecules. In this paper, the conducting channel formation process in poly(3-hexylthiophene) (P3HT) FETs is described via in situ sheet conductance measurements using four-contact geometry field effect devices. In the in situ measurements, the onset of the drain current was achieved by percolation between P3HT chains resulting from the increase in P3HT concentration in solution during solvent evaporation. The P3HT channel sheet conductance and P3HT/Au contact resistance extracted from the gated four-contact measurements varied during channel formation. After percolation, the four-contact mobility increased up to a peak value of 0.17 cm(2)/(Vs) after which it continuously decreased. This decrease was coupled with a sharp drop in the threshold voltage from 182 to 22 V. We suggest that the variation in electrical properties result from a competition between the evolution of P3HT microstructure involving potential phase transitions between isotropic, liquid crystalline, and polycrystalline phases and atmospheric degradation, both of which are critical factors that should be considered in optimizing device performance.