Organic phototransistor behavior and light-accelerated bias stress

Abstract

We studied pentacene thin film field-effect transistors to characterize their behavior as organic phototransistors. The shift in turn-on voltage (V_on), responsible for the high sensitivity of these devices to illumination, is proved to be dependent on the illumination time and applied gate voltage during illumination, a relationship which was, until now, completely neglected in the description of these devices. Moreover, we show this behavior to be similar to the shift in V_on during bias stress experiments in the dark and both processes can be described with the same V_on vs time relationship, already previously reported for dark bias stress experiments on organic transistors. By comparing these characteristics in devices with a different treatment of the gate dielectric, trapping of electrons by OH-groups at the gate dielectric/organic semiconductor interface is indicated as a main origin for these shifts in V_on. In this way we do not only reduce organic phototransistors behavior to light-accelerated bias stress in unstable thin film transistors, but also pin-point one major cause of organic transistor instability.