Electron spin resonance observation of gate-induced charge carriers in organic field-effect devices fabricated on silicon substrates

Electron spin resonance (ESR) measurements have been performed on metal-insulator-semiconductor (MIS) diode structures of regioregular poly(3-hexylthiophene), RR-P3HT, fabricated on silicon substrates with SiO2, layers as gate insulators. The conductivity of substrates was chosen so that it does not significantly lower the quality factor of the ESR cavity. Clear ESR signals due to field-induced polarons have been observed at g-values of around 2.002, consistent with those observed in the MIS devices of RR-P3HT fabricated on Al2O3 gate insulators. Carrier spins tend to saturate above the charge concentration of about 0.3%. suggesting the conversion of polarons with spin 1/2 to spinless bipolarons for higher carrier concentrations. The angular dependence of ESR signals exhibits distinct anisotropy, reflecting the fact that the surface of SiO2 is flatter than that of Al2O3. These results demonstrate that ESR can be performed on organic field-effect devices fabricated on silicon substrates.