Theory of the organic field-effect transistor

A method is described to derive the mobility of organic field-effect transistors (OFETs) by using the current-voltage characteristics at low drain voltage. After correction for the contact series resistance, it appears that the mobility of sexithiophene (6T) and octithiophene (8T) is gate bias dependent, which actually corresponds to charge concentration dependence. Temperature dependent measurements show that the mobility is thermally activated, which is interpreted in terms of polaron hopping transport. At temperature lower than around 100K, the activation energy is considerably reduced, and the mobility becomes almost temperature independent below 25K. These features mirror a change in the transport regime, from thermally activated hopping at high temperatures, to a more coherent mechanism in the intermediate regime, and eventually band-like transport at very low temperatures. The charge concentration dependence is attributed to the presence of traps, which can be identified to grain boundaries.