Electron Spin Resonance Observations of Field-Induced Polarons in Regioregular Poly(3-octylthiophene) Metal-Insulator-Semiconductor Diode Structures

Electron spin resonance (ESR) measurements are performed on field-induced carriers in regioregular poly(3-octylthiophene) (RR-P3OT) using metal-insulator-semiconductor (MIS) diode structures with RR-P3OT and Al2O3 as the active semiconductor and insulating layers, respectively. Clear ESR signals (g similar to 2.002) are observed, which increase as the absolute value of the gate bias increases in the accumulation mode. The ESR signal is consistent with that of the photogenerated positive polarons in RR-P3OT detected by the light-induced ESR of RR-P3OT/C-60 composites, providing direct evidence that the field-induced carriers are polarons. Moreover, the molecular orientation of RR-P3OT's in the diode structures, which is consistent with the self-organized lamellae reported, is confirmed by the anisotropy of the ESR signal.