Photoinduced doping effect of pentacene field effect transistor in oxygen atmosphere studied by displacement current measurement

It is widely accepted that atmospheric oxygen can work as an electron-accepting dopant mainly to p-type organic semiconductors. We have examined the effect of oxygen on a pentacene field effect transistor (FET) with and without exposure to light using the displacement current measurement. Under vacuum conditions, the change in the displacement current due to hole injection from the source and drain electrodes to the pentacene layer is clearly observed, suggesting that the origin of the mobile carriers in the pentacene FET is carrier injection. When the FET is exposed to oxygen under dark conditions, a very small change in the threshold gate voltage for hole injection is observed. In contrast, with exposure to both oxygen and light, we observed that the threshold voltage is lowered and shifted across the zero bias and even to the polarity against hole injection. This photoinduced doping effect induces a distinct increase in the drain current of the FET, and it is maintained for at least several hours even after the irradiation is turned off. This finding suggests that the performance of organic semiconductor devices is affected not only by atmospheric oxygen but also by ambient light even during the processing and storage of the devices.