H. Okamoto

To clarify the limiting factor of carrier transport in organic molecular semiconductors, we performed charge modulation spectroscopy of a field-effect transistor with a 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW) single crystal, which showed a hole-carrier mobility of 8.4 cm2 V−1 s−1 at 295 K. The terahertz absorption of electric-field-induced hole carriers increases with decreasing frequency down to 150 cm−1 (4.5 THz). However, it is not reproduced by the simple Drude model but tends to be suppressed with decreasing frequency. The spectral shape of the absorption and the mobility value were simultaneously reproduced by the Drude–Anderson model, which incorporates carrier scattering due to thermal molecular fluctuations. The frequency of the intermolecular vibration that dominates carrier scattering is estimated to be approximately 8 cm−1, which is in good agreement with the theoretically predicted value. Moreover, analyses of the absorption spectra at low temperatures reveal that the mobility increases to 14 cm2 V−1 s−1 at 240 K. These results demonstrate that thermal molecular fluctuations limit the mobility.