We have studied the current-voltage characteristics and threshold voltage behaviors in polycrystalline pentacene-based organic thin-film transistors (OTFTs) with gold top-contact electrodes for different thickness of the pentacene films. The study uses a number of techniques to measure threshold voltage including: square-root method, constant-current method, maximum transconductance method and capacitance-voltage measurements. The results of our experiments suggest that the space-charge and trapping effects of the pentacene bulk film plays an important role in current-voltage characteristics. In the presence of trapping centers within the polycrystalline pentacene film, we have provided an analytical model explaining the effect of bulk traps on the thickness-dependent threshold voltage. Square-law of threshold voltage versus pentacene thickness in OTFTs was derived during our study providing consistent results with experimental data. Furthermore, using the square-law, we estimated trap density of about 1017 cm-3 in experimental pentacene films. The orders of magnitude in trap density of polycrystalline pentacene films are similar to the values found in other literature source.
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