Direct Observation of Field-Induced Carrier Dynamics in Pentacene Thin-Film Transistors by Electron Spin Resonance Spectroscopy

Abstract

We report our recent studies on interfacial charge transport in organic transistors on the basis of electron spin resonance (ESR) spectroscopy of field-induced carriers. We successfully observed motional narrowing effects in field-induced ESR spectra for relatively high-mobility (∼0.6 cm2 V-1 s-1) pentacene thin-film transistors (TFTs); Lorentz-type resonance spectra become narrower as the carrier motion becomes more active with increasing temperature or gate voltage. The variation of spectral linewidth is within the range of 30–200 µT, from which we conclude that the charge transport is dominated by the traps with trap residence time of 4 ns. Meanwhile, it was also reported for lower-mobility (∼0.01 cm2 V-1 s-1) pentacene TFTs that ESR spectra do not show motional narrowing and that the linewidth remains unchanged by gate voltage or temperature at about 200 µT. On the basis of these examinations, we discuss the microscopic nature of field-induced carrier dynamics in organic TFTs associated with the origin of linewidth in ESR spectra.