Microscopic characterization of printable low-voltage electrolyte-gated transistors by electron spin resonance

We have investigated the microscopic properties of printable low-voltage electrolyte-gated transistors using electron spin resonance (ESR). The utilized devices were ion gel-gated regioregular poly(3-hexylthiophene) (RR-P3HT) thin-film transistors. We performed simultaneous mesurements of field-induced ESR (FI-ESR) and device characteristics using the same device. Clear FI-ESR signals due to hole carriers (positive polarons) were observed by applying a negative gate voltage. The anisotropy of the ESR linewidth indicated two-dimensional magnetic interactions between high density charges. The magnetism of the majority of hole carriers at a high charge density was found to be nonmagnetic from the simultaneous measurements of FI-ESR and transfer characteristics. The anisotropy of the g value of the ESR signal at a high gate voltage indicated the disordered molecular orientation of RR-P3HT where paramagnetic holes exist. These results provide insight into the charge transport mechanism of RR-P3HT polymer semiconductors with high charge densities.