Progress in environmental stability and processability, and the increase of the field-effect mobility of organic semiconductors has triggered their use as the active element in microelectronic devices. The advantages of their application are the easy processing, for example, spin-coating and ink-jet printing, without a temperature hierarchy, and their mechanical flexibility. Applications are foreseen in the field of large-area electronics where numerous devices are integrated on low-cost substrates such as plastics. The first flexible, even rollable, quarter video graphics array (QVGA) active matrix displays based on organic semiconductors have already been reported. In present commercial displays, amorphous silicon, a-Si, is used as the active semiconductor. In order to be competitive, organic transistors should exhibit the same performance with respect to current modulation and electrical reliability. The field-effect mobility of organic transistors is already compar-able to that of a-Si-based transistors. Values of unity have been demonstrated not only for evaporated organic semiconductors, but also using solution-processed semiconductors. In this paper we discuss the electrical instability of organic transistors. We observe that the threshold-voltage shift shows a stretched-exponential time dependence under an applied gate bias. The relaxation time is observed to be in the order of 10 7 s (ca. 4 months) at room temperature and is comparable to the best values reported for a-Si-based transistors. The activation energy is common for all other organic transistors reported so far. The constant activation energy supports charge trapping by residual water as the common origin. Quantitative analysis shows that differences in reliability of organic transistors are due to differences in the frequency prefactor.
The electrical instability of practical transistors is a device parameter. It can be due to ionic displacements in the gate dielectric; photo-oxidation under applied bias in an ambient atmosphere; or charge trapping at interfaces or at impurities in the bulk, due to defect creation or water at the gate-dielectric-semiconductor interface. Here we focus on the intrinsic electrical instability. We use thermally grown SiO 2 as gate dielectric and determine the dynamics of the electrical instability of organic transistors as a function of time and temperature in a vacuum and in the dark.
We used polytriarylamine (PTAA) as a model compound. This organic semiconducting polymer is amorphous and air stable, with a highest occupied molecular orbital (HOMO) energy level of about -5.1 eV (1 eV = 1.602 × 10 -19 J), and yields reproducible transistors with a mobility of about 10 -3 -10 -2 cm 2 V -1 s -1 . The chemical structure is depicted in the insert of Figure , where X and Y are short chain alkyl groups. The transistors were fabricated using heavily doped p-type Si wafers as the common gate electrode with a 200 nm thermally oxidized SiO 2 layer as the gate dielectric. Gold source and drain electrodes were defined by using photolithography with a channel width (W) and length (L) of 1000 lm and 10 lm, respectively. A 10 nm titanium layer was used for adhesion. The SiO 2 layer was passivated with hexamethyldisilazane (HMDS) prior to semiconductor deposition. PTAA films were spin-coated from toluene with a layer thickness of 80 nm. To compare the reliability of PTAA transistors with that of other organic semiconductors we investigated regioregular poly(3-hexylthiophene) (P3HT) (Merck, UK), poly(9,9′-dioctyl-fluorene-co-bithiophene) (F8T2) (American Dye Source, Canada) and 3-butyl a-quinquethiophene (3-BuT5) (Syncom B.V., The Netherlands) transistors. These