Characterization of the field-effect conductivity distribution in pentacene thin-film transistors by a near-field scanning microwave microscope

Direct probing of the conductivity distribution in organic materials is motivated by the need to obtain a deeper understanding of carrier behavior in organic thin-film transistors (OTFT), organic electro-luminescent devices, organic photoconductors, and organic biosensors. Here we used a near-field scanning microwave microscope to visualize conductivity profiles in OTFT channel. Applying this technique to pentacene field-effect transistors has revealed changes of the conductivity distribution in the channel arising from the development and exhaustion of an accumulated charge region. The electric field profiles, the complementary image of conductivity profiles, which are visualized by using an optical second harmonic generation method, support the results. We anticipate that direct observation by this microwave method will find wide application in measurement of carrier conductivity in organic and nonorganic materials.

► We visualized the conductivity profiles in the active layer of a pentacene FET by a NSMM. ► The 3D image of microwave reflection coefficient clearly distinguishes the conductivity profiles caused by holes and electrons, though motion of electrons from the drain generates the same trace of transient current as that of holes moving from source to drain. ► The electric field profiles, the complementary image of conductivity profiles, which are visualized by using an optical second harmonic generation method, support the results.