Ryousuke Matsubara

Analysis of barrier height at crystalline domain boundary and in-domain mobility in pentacene polycrystalline films on SiO2

Crystalline domain size and temperature dependences of the carrier mobility of commonly used pentacene polycrystalline films on SiO2 have been studied by four-point-probe field-effect transistor measurements. The mobility is found to be proportional to the crystalline domain size and thermally activated. This behavior is well explained by a polycrystalline model with the diffusion theory, and thereby the barrier height at boundary and the mobility in domain are calculated to be 150meV and 1.0cm2∕Vs, respectively. The in-domain mobility is lower than those expected in single crystals, which suggests that there exist some other limiting factors of carrier transport than the domain boundaries.

Conductivity fluctuation within a crystalline domain and its origin in pentacene thin-film transistors

Surface topography and high-resolution potential images in a thin-film transistor with a polycrystalline pentacene active layer have been measured by atomic-force-microscope potentiometry. A potential fluctuation independent of topographic features was found in large flat molecular terraces. The origin of the potential fluctuation was concluded to be the fluctuation of the top level of the highest-occupied-molecular-orbital band, which results in the variation of local carrier concentration. The full width at half maximum of the band fluctuation was estimated to be 12meV, which might reduce the mean carrier velocity in crystalline domains.