ESR studies of field-induced polarons in MIS diode structures with self-organized regioregular poly(3-hexylthiophene)

We report electron spin resonance (ESR) studies on field-induced charge carriers in regioregular poly(3-hexylthiophene) (RR-P3HT) using metal-insulator-semiconductor (MIS) diode structures with self-organized RR-P3HT as the active semiconductor layer and Al2O3 as an insulating layer. The field-induced ESR signals (g ∼ 2.002) were clearly observed; their intensities monotonically increased as the absolute value of the gate bias increased in the accumulation mode with a saturation behavior at higher voltages. The ESR signals were consistent with those of the photogenerated positive polarons in RR-P3HT, demonstrating that the carriers are polarons. The transient responses of the field-induced ESR intensity show a fast component with a time constant of an order of less than 20 ms. The self-organized lamellar molecular orientation was confirmed by the anisotropic ESR signals due to π-electrons.