  The photo-induced electron and hole transfer across the
semiconductor-dielectric interface in trap dominated p-type organic
field-effect transistors (OFETs) has been investigated. It has been observed
that the transfer of electrons into the dielectric results in a decrease of the
field-effect mobility of polarons, suggesting that additional shallow traps are
generated in the conduction channel. Using this effect, the dependence of the
field-effect mobility on the density of shallow traps, mu(N), has been
measured, which allowed to estimate the average polaron trapping time, tau_tr =
50 +- 10 ps, and the density of shallow traps, N_0 = (3 +- 0.5)*10^11 cm^-2, in
the channel of single-crystal tetracene devices.
