A mobility model for organic thin-film transistors (OTFTs) has been considered that fully accounts for the effect of grains and grain boundaries of the organic layer. The model has been applied to a top contact pentacene OTFT. Comparison between simulation results and experimental data shows a strong dependence of mobility as a function of grain size. The field-effect-extracted mobility is not linearly related to the grain size, but presents a rather abrupt reduction for a grain size smaller than 2 μm

We have fabricated a large body of pentacene thin films on different organic and inorganic dielectric materials at four substrate temperatures with different nominal film thicknesses ranging from the submonolayer over the multilayer to the "thick" film regime. These films were characterized by atomic force microscopy and analyzed quantitatively by means of scaling and rate equation theory in order to deduce the molecular growth dynamics of pentacene films on organic substrates that are used as gate dielectrics in organic thin film transistors. We found that on all substrates and in the substrate temperature range 25°C ഛ T s ഛ 70°C the growth can be well described as diffusion-limited aggregation. A critical island size of 3 ഛ i ഛ 4 was deduced from the scaling of the distribution density of the pentacene grain areas and the power-law dependence of the saturated nucleation density on the deposition rate. This is valid for all substrates in the investigated temperature regime and is also found to be true for 50 nm thick pentacene films thus emphasizing that the molecule-molecule interaction itself is independent of the underlying surface.

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Generalized infrared spectroscopic ellipsometry over the wave-number range from 300 to 2000 cm−1 is used for precise determination of the dielectric function, frequency, amplitude, and broadening parameters of 27 infrared active modes for polarization parallel to the growth surface of pentacene thin films obtained by molecular-beam deposition on glass. No in-plane anisotropy was detected, which is indicative for a random orientation of crystallites around the growth direction supporting previous x-ray diffraction results.

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