Organic thin film transistors (TFTs) have attracted much attention due to their wide and potential applications in, e.g., flexible displays, radio frequency identification cards, and sensors. Pentacene TFTs, in which gold (Au) source/drain electrodes are commonly used as the charge-injecting metal, are particularly attractive on account of their large mobility (~ 1 cm2 V-1 s-1) and high on/off ratios (>105), particularly when compared to those for amorphous silicon TFTs.