We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self assembled monolayer (SAM). The observed shift of the transfer characteristics range from -2 to 50 V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4 × 10 12 /cm 2 . In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1 × 10 12 /cm 2 .

Central to the operation of organic electronic and optoelectronic devices is the transport of charge and energy in the organic semiconductor, and to understand the nature and dynamics of charge carriers is at the focus of intense research efforts. As a basic transport property of solids, the Seebeck coefficient S provides deep insight as it is given by the entropy transported by thermally excited charge carriers and involves in the simplest case only electronic contributions where the transported entropy is determined by details of the band structure and scattering events. We have succeeded for the first time to measure the temperature- and carrier-density-dependent thermopower in single crystals and thin films of two prototypical organic semiconductors by a controlled modulation of the chemical potential in a field-effect geometry. Surprisingly, we find the Seebeck coefficient to be well within the range of the electronic contribution in conventional inorganic semiconductors, highlighting the similarity of transport mechanisms in organic and inorganic semiconductors. Charge and entropy transport is best described as band-like transport of quasiparticles that are subjected to scattering, with exponentially distributed in-gap trap states, and without further contributions to S.

-We report on single crystal high mobility organic field-effect transistors (OFETs) prepared on prefabricated substrates using a "flip-crystal" approach. This method minimizes technique employed in this study shows potential as a general method for studying charge transport in field-accumulated carrier channels near the surface of organic single crystals.2

Influence of surface states on the two-dimensional electron gas in AlGaN/GaN heterojunction field-effect transistorsA method has been developed to inject mobile charges at the surface of organic molecular crystals, and the dc transport of field-induced holes has been measured at the surface of pentacene single crystals. To minimize damage to the soft and fragile surface, the crystals are attached to a prefabricated substrate which incorporates a gate dielectric (SiO 2 ) and four probe pads. The surface mobility of the pentacene crystals ranges from 0.1 to 0.5 cm 2 /V s and is nearly temperature independent above ϳ150 K, while it becomes thermally activated at lower temperatures when the induced charges become localized. Ruling out the influence of electric contacts and crystal grain boundaries, the results contribute to the microscopic understanding of trapping and detrapping mechanisms in organic molecular crystals.

The density of trap states in the bandgap of semiconducting organic single crystals has been measured quantitatively and with high energy resolution by means of the experimental method of temperature-dependent space-charge-limited-current spectroscopy (TDSCLC). This spectroscopy has been applied to study bulk rubrene single crystals, which are shown by this technique to be of high chemical and structural quality. A density of deep trap states as low as ∼ 10 15 cm −3 is measured in the purest crystals, and the exponentially varying shallow trap density near the band edge could be identified (one decade in the density of states per ∼ 25 meV). Furthermore, we have induced and spectroscopically identified an oxygen-related sharp hole bulk trap state at 0.27 eV above the valence band.