Free-electron-like Hall effect in high-mobility organic thin-film transistors

Gate-voltage-dependent Hall coefficient R(H) is measured in high-mobility field-effect transistors of polycrystalline dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene films. The value of R(H) evolves with density of accumulated charge q, precisely satisfying the free-electron formula R(H) = 1/q near room temperature. The result indicates that the intrinsic charge transport inside the grains is bandlike in the vacuum-deposited high-mobility organic-semiconductor thin films that are of significant interest in industry. At lower temperatures, even Hall-effect mobility averaged over the whole polycrystalline film decreases due to the presence of carrier-trapping levels at the grain boundaries while the free-electron-like transport is preserved in the grains. With the separated description of the intergrain and the intragrain charge transport, it is demonstrated that the reduction in mobility with decreasing temperature often shown in organic thin-film transistors does not necessarily mean mere hopping transport.