We demonstrate organic thin film transistor (TFT) technique can be used as a low-sample-demand tool for probing bulk transport properties of these amorphous organic semiconductors. Some examples are hole transporting materials Spiro-TPD, 2TNATA, NPB and TPD. To do this, the gate dielectric layer for the TFT structure needs to be non-polar. The non-polar surface can be realized through deposition of suitable non-polar polymers or organic insulators. On such a surface, we observe that bulk-like behaviors can be realized for amorphous organic semiconductors. We highlight that only a thin film down to ~10 nm is sufficient for TFT measurements. Our approach will be of special relevance to the evaluation of transport properties of newly synthesized materials in which the yield is yet to be optimized. On polar gate dielectric, we observe strong interactions between molecules and the dielectric layer. Temperature dependent measurements were used to study the gate dielectric /organic induced energetic disorder. We also discovered that there may be preferred orientations of organic molecules on polar surfaces.
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