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We report on contact effects in polymeric thin film transistors based on poly(3-octylthiophene) and poly(3-hexadecylthiophene) with gold contact electrodes and in the bottom contact configuration. A method to extract the intrinsic channel mobility from the measured extrinsic mobility over a broad range of gate voltage is presented. This method uses the I-V characteristics of the transistor in its reverse mode operation. The results show that the intrinsic mobility in the channel is gate voltage dependent and increases almost linearly with voltages at biases above the threshold voltage. By applying a model based on the theory of space-charge-limited conduction, the dependence of the threshold voltage on the contacts and the shifts observed in this parameter with different polymer film thicknesses are explained. We also apply this model to explain the effects of light in reducing the contact effects and changing the device parameters from extrinsic in the dark to intrinsic under illumination.

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