Effect of annealing on the mobility and morphology of thermally activated pentacene thin film transistors

Pentacene thin film transistors (TFTs) were fabricated by the organic molecular beam deposition method. The TFTs were characterized in order to study the effect of thermal annealing on the morphology and carrier mobility of the transistors. For all the TFT samples the mobility exhibited an Arrhenius relationship with temperature, indicating a thermally activated transport that could be explained by the carrier trap and thermal release transport mechanism. Therefore, in order to investigate the annealing effect, we tested the data for a significant period of time after annealing until the temperature recovered to room temperature, so that the thermal activation effect was screened and possible effects of thermal expansion and stress were also ruled out. As a result, we found that only with a temperature below a critical temperature of approximately 45°C could annealing improve the mobility, while annealing with T>50°C would decrease the mobility compared to the value before annealing. Atomic force microscopy observation and x-ray diffraction (XRD) data indicated that annealing caused decreased grain size and decreased XRD peak intensity for all samples. Increasing the annealing temperature to 70°C caused obvious desorption because of the low van der Waals intermolecular forces in the organic film. The mobility deterioration after high temperature annealing may be ascribed to the deteriorated microstructure, while the improved mobility may result from the increased crystallinity in the bottom several layers near the substrate film interface. The results also suggested that the influence of possible structure evolution should be distinguished when investigating temperature dependent transport properties.