Electron spin resonance observation of charge carrier concentration in organic field-effect transistors during device operation

Hisaaki Tanaka*, Masataka Hirate, Shun Ichiro Watanabe, Kazuaki Kaneko, Kazuhiro Marumoto, Taishi Takenobu, Yoshihiro Iwasa, Shin Ichi Kuroda

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    25 Citations (Scopus)


    Charge carrier concentration in operating organic field-effect transistors (OFETs) reflects the electric potential within the channel, acting as a key quantity to clarify the operation mechanism of the device. Here, we demonstrate a direct determination of charge carrier concentration in the operating devices of pentacene and poly(3-hexylthiophene) (P3HT) by field-induced electron spin resonance (FI-ESR) spectroscopy. This method sensitively detects polarons induced by applying gate voltage, giving a clear FI-ESR signal around g=2.003 in both devices. Upon applying drain-source voltage, carrier concentration decreases monotonically in the FET linear region, reaching about 70% of the initial value at the pinch-off point, and stayed constant in the saturation region. The observed results are reproduced well from the theoretical potential profile based on the gradual channel model. In particular, the carrier concentration at the pinch-off point is calculated to be β/(β+1) of the initial value, where β is the power exponent in the gate voltage (Vgs) dependence of the mobility (μ), expressed as μâ̂Vgsβ-2, providing detailed information of charge transport. The present devices show β=2.6 for the pentacene and β=2.3 for the P3HT cases, consistent with those determined by transfer characteristics. The gate voltage dependence of the mobility, originating from the charge trapping at the device interface, is confirmed microscopically by the motional narrowing of the FI-ESR spectra.

    Original languageEnglish
    Article number045309
    JournalPhysical Review B - Condensed Matter and Materials Physics
    Issue number4
    Publication statusPublished - 2013 Jan 18

    ASJC Scopus subject areas

    • Condensed Matter Physics
    • Electronic, Optical and Magnetic Materials


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