ESR observations of electric-field-induced polarons in regioregular poly(3-alkylthiophene)s in MIS diode structures

Electron spin resonance (ESR) measurements have been performed on metal-insulator-semiconductor (MIS) diodes formed with a regioregular poly(3-alkylthiophene) (RR-PAT) film as a semiconductor in order to study the spin states of the field-induced charge carriers in the RR-PAT film. The structure of the MIS diodes fabricated is Al/Al2Al2O3(insulator)/RR-PAT(semiconductor)/Au. We have successfully observed an electric-field-induced ESR signal by applying voltage across the Al and Au electrodes of the MIS diode. The ESR intensity increases under negative bias applied to the Al electrode owing to the accumulation of holes at the interface between Al2O3 and RR-PAT. The observed ESR signal is consistent with that of positive RR-PAT polarons detected by light-induced ESR measurements on RR-PAT/C60 composites. Therefore, the present results directly show that the field-induced carriers are polarons.