  We show that it is possible to reach one of the ultimate goals of organic
electronics: producing organic field-effect transistors with trap densities as
low as in the bulk of single crystals. We studied the spectral density of
localized states in the band gap (trap DOS) of small molecule organic
semiconductors as derived from electrical characteristics of organic
field-effect transistors or from space-charge-limited-current measurements.
This was done by comparing data from a large number of samples including
thin-film transistors (TFT's), single crystal field-effect transistors
(SC-FET's) and bulk samples. The compilation of all data strongly suggests that
structural defects associated with grain boundaries are the main cause of
"fast" hole traps in TFT's made with vacuum-evaporated pentacene. For
high-performance transistors made with small molecule semiconductors such as
rubrene it is essential to reduce the dipolar disorder caused by water adsorbed
on the gate dielectric surface. In samples with very low trap densities, we
sometimes observe a steep increase of the trap DOS very close (<0.15 eV) to the
mobility edge with a characteristic slope of 10-20 meV. It is discussed to what
degree band broadening due to the thermal fluctuation of the intermolecular
transfer integral is reflected in this steep increase of the trap DOS.
Moreover, we show that the trap DOS in TFT's with small molecule semiconductors
is very similar to the trap DOS in hydrogenated amorphous silicon even though
polycrystalline films of small molecules with van der Waals-type interaction on
the one hand are compared with covalently bound amorphous silicon on the other
hand.
