Temperature dependence of the Hall effect in pentacene field-effect transistors: Possibility of charge decoherence induced by molecular fluctuations

Temperature dependence of the Hall effect is measured for both single-crystal and thin-film field-effect transistors of pentacene. At room temperature, the inverse Hall coefficient 1/R(H) exceeds field-induced charge density Q by a factor of 2 at each gate-electric field for all the samples, regardless of their charge-carrier mobility and detailed subthreshold properties. Violating the band-transport model that 1/R(H) equals to Q, the excess 1/R(H) does not measure the charge amount anymore so that the result possibly indicates insufficient electromagnetic coupling due to somewhat reduced charge coherence. At lower temperatures, the deviation of 1/R(H) from Q gradually diminishes to approach the band-transport behavior. Interestingly, the degree of the deviation has been universal for the measured five samples, including both polycrystal and single-crystal pentacene films. The result suggests that significant molecular fluctuation near room temperature can affect the fundamental electronic state.