  We present the first thermoelectric measurements on pentacene field effect
transistors. We report high values of the Seebeck coefficient at room
temperature between 240 and 500 micro V/K depending on the dielectric surface
treatment. These values are independent of the thickness of the channel and of
the applied gate voltage. Two contributions to the Seebeck coefficient are
clearly identified: the expected contribution that is dependent on the position
of the transport level and reflects the activated character of carrier
generation, and an unexpected intrinsic contribution of 265 plus minus 40 micro
V/K that is independent of the temperature and the treatment of the oxide
surface. This value corresponds to an unusually large lattice vibrational
entropy of 3 kB per carrier. We demonstrate that this intrinsic vibrational
entropy arises from lattice hardening induced by the presence of the
charge-carrier. Our results provide direct evidence of the importance of
electronic polarization effects on charge transport in organic molecular
semiconductors.
