We studied pentacene thin film field-effect transistors to characterize their behavior as organic phototransistors.
The shift in turn-on voltage (V<sub>on</sub>), 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<sub>on</sub> during bias stress experiments in the dark and both processes can be described with the same
V<sub>on</sub> 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<sub>on</sub>. 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.
Now organic thin film transistor technology gains growing maturity, high performance organic photodetectors are
the missing link towards full organic photosensitive sensor arrays, needed for the realization of applications like
organic scanners and organic cameras. In the borderline of the research in organic solar cells, research in organic
photodetectors has mainly been limited to organic photodiodes. However, phototransistors offer the possibility to
reach higher sensitivities, thanks to the internal current gain of a transistor structure. This document focuses on
organic field-effect phototransistors (organic photoFETs) where illumination can be used as an optical gate signal
in addition to the electrical gate signal. An experimental description of pentacene photoFETs during broadband
illumination is given. A distinction between the direct photocurrent and the additional current enhancement by
a thresholdshift can be made, and a relation for this thresholdshift in function of time is revealed. Consequently,
the diffculty to compare different organic photoFETs without any information about the illumination time is
highlighted and a comparison based on this power law is proposed. Subsequently it is pointed out that by the
stability of this thresholdshift after illumination, the pentacene photoFET does not only act as a light detector
but also as a memory element, capable to store information about illumination power and duration for an
extended amount of time.