Direct observation of the charge carrier concentration in organic field-effect transistors by electron spin resonance

Charge carrier concentration in operating field-effect transistor (FET) of regioregular poly(3-hexylthiophene) has been directly determined by electron spin resonance (ESR). ESR signals of field-induced polarons are observed around g=2.003 under the application of negative gate-source voltage (V(gs)). Upon applying drain-source voltage (V(ds)), ESR intensity decreases linearly in the low V(ds) region, reaching to about 50% of the initial intensity at the pinch-off point (V(ds)congruent to V(gs)). For larger absolute values of V(ds), it becomes nearly V(ds) independent. These behaviors are well explained by the change in the carrier concentration obtained by the FET theory using gradual channel approximation.