We studied pentacene thin film field-effect transistors to characterize their behavior as organic phototransistors.
The shift in turn-on voltage (Von), 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 Von during bias stress experiments in the dark and both processes can be described with the same
Von 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 Von. 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.