Metal-semicoductor contact effects and temperature-dependence of carrier transport in large-grain organic semiconductor thin film transistors

The pen-writer solution deposition method is used to deposit 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) organic semiconductor thin films and also polymer dielectric layers. Organic transistors with source/drain contact layers either below or on top of the semiconductor layer and with or without surface treatment are investigated. Contact resistance produces a pronounced non-linearity in the output characteristics at low drain voltage, which is found to be consistent with a Schottky Barrier model. By treating the Au electrode with pentafluorobenzenethiol (PFBT) in a bottom contact geometry, the contact resistance is greatly reduced. An optimized geometry is obtained by using pen-written Cytop^TM dielectric in a top-gate/bottom-contact structure, which exhibits a near-intrinsic average mobility, up to 9.0 cm²/V-s for C₈-BTBT thin films deposited at high writing speed (25mm/s) and deposition on a heated substrate (60°C). We will report temperature-dependent carrier transport results for individual devices with both a top gate (with cytop dielectric) and bottom gate (with silicon dioxide dielectric) to compare results at both interfaces of the same C₈-BTBT thin film.