Influence of postannealing on polycrystalline pentacene thin film transistor

We studied systematically the influence of in situ postannealing treatment of ultrahigh vacuum grown polycrystalline pentacene thin film transistor. The gradual grain growth with the elimination of defects and misoriented crystallites is confirmed in x-ray diffraction (XRD) data and the atomic force microscopy image as the annealing temperature increases. The XRD data reveal that the pentacene molecules are packed parallel to each other in an upright position with a tilting angle of 15.5°. The postannealing results in the enhanced field effect mobility of pentacene organic thin film transistors increases from 0.19±0.04 to 0.49±0.05 cm2/V s after annealing at 90 °C. We suggest that the abnormally small on/off current ratio (∼103) due to the large leakage current is attributed to the conduction via impurity levels originated from the structural isomers of pentacene.