Electron spin resonance of field-induced polarons in regioregular poly(3-alkylthiophene) using metal-insulator-semiconductor diode structures

We report electron spin resonance (ESR) studies on field-induced charge carriers in regioregular poly(3-hexylthiophene) (RR-P3HT) and regioregular poly(3-octylthiophene) (RR-P3OT) for metalinsulator-semiconductor (MIS) diode structures with Al2O3 as an insulating layer. The field-induced ESR signals (g similar to 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 and RR-P3OT, demonstrating that the carriers are polarons. The transient responses of the field-induced ESR intensity show fast and slow components. The self-organized lamellar molecular orientation was confirmed by the anisotropic ESR signals due to pi-electrons, which is semiquantitatively reproduced by an ESR simulation.