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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 shifts 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 4×1012/cm2. 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 1×1012/cm2. [ABSTRACT FROM AUTHOR]

The authors report on the modeling of the water related trap state in pentacene single crystal field-effect transistors that is created by a prolonged application of a gate voltage [C. Goldmann et al., Appl. Phys. Lett. 88, 063501 (2006)]. The authors find a trap state narrow in energy to be appropriate to explain the steplike feature measured in the subthreshold region of the transfer characteristic. The trap state forms in an interface layer next to the gate insulator and is centered at 430±50 meV above the valence band edge. The density increases from (2 to 10.5)×1018/cm3 during gate bias stress. The knowledge of the details of this defect state can help to identify the physical and chemical origin of the created trap state. [ABSTRACT FROM AUTHOR]

The surface conductivity is measured by a four-probe technique for pentacene and rubrene single crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ∼1012 cm-2 at the surface of the organic single crystals. [ABSTRACT FROM AUTHOR]