  We studied the influence of oxygen on the electronic trap states in a
pentacene thin film. This was done by carrying out gated four-terminal
measurements on thin-film transistors as a function of temperature and without
ever exposing the samples to ambient air. Photooxidation of pentacene is shown
to lead to a peak of trap states centered at 0.28 eV from the mobility edge,
with trap densities of the order of 10(18) cm(-3). These trap states need to be
occupied at first and cause a reduction in the number of free carriers, i.e. a
consistent shift of the density of free holes as a function of gate voltage.
Moreover, the exposure to oxygen reduces the mobility of the charge carriers
above the mobility edge. We correlate the change of these transport parameters
with the change of the essential device parameters, i.e. subthreshold
performance and effective field-effect mobility. This study supports the
assumption of a mobility edge for charge transport, and contributes to a
detailed understanding of an important degradation mechanism of organic
field-effect transistors. Deep traps in an organic field-effect transistor
reduce the effective field-effect mobility by reducing the number of free
carriers and their mobility above the mobility edge.
