Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors J. Appl. Phys. 97, 083714 (2005); 10.1063/1.1895466Pentacene organic field-effect transistor on metal substrate with spin-coated smoothing layer This paper reports on improvements of the field-effect mobility in regioregular head-to-tail coupled poly͑3-hexylthiophene͒ based transistors by mechanically induced alignment of polymer chains in the active layer. It is demonstrated that mechanical rubbing perpendicular to the source drain contacts can increase the field-effect mobility up to 800% whereas rubbing parallel to the source drain contacts results in a reduced mobility. The polymer alignment is thereby deduced from optically polarized transmission spectroscopy on polymer-coated quartz glass substrates and is shown to directly correlate with the electrical behavior of a bottom-gate field-effect transistor. The influence of layer thickness on rubbing is investigated and it is shown that annealing after mechanical rubbing at high temperature can further increase the alignment. Differences between thick drop-cast and thin spin-coated films are explained in terms of different solvent evaporation rates, allowing the material to order to a different degree. This interpretation is deduced from characteristic optical and electrical features of the differently prepared poly͑3-hexylthiophene͒ films.

A concept for the suppression of resonant vibration of an elastic system undergoing forced vibration coupled to electroactive polymer (EAP) actuators based on dielectric elastomers is demonstrated. The actuators are utilized to vary the stiffness of the end support of a clamped beam, which is forced to harmonic vibration via a piezoelectric patch. Due to the nonlinear dependency of the elastic modulus of the EAP material, the modulus can be changed by inducing an electrostrictive deformation. The resulting change in stiffness of the EAP actuator leads to a shift of the resonance frequencies of the vibrating beam, enabling an effective reduction of the vibration amplitude by an external electric signal. Using a custom-built setup employing an aluminum vibrating beam coupled on both sides to electrodized strips of VHB tape, a significant reduction of the resonance amplitude was achieved. The effectiveness of this concept compared to other active and passive concepts of vi bration reduction is discussed

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This paper reports on the influence of the field-effect mobility of transistors based on regioregular head-to-tail coupled poly (3-hexylthiophene) by mechanically induced alignment on differently treated insulator surfaces. It is demonstrated that on hydrophilic insulator surfaces mechanical rubbing of the polyhexylthiophene layers perpendicular to the source drain contacts can increase the field-effect mobility whereas rubbing parallel to the source drain contacts results in a reduced mobility. In contrast it is shown that in transistors with a hydrophobic insulator surfaces, which show much higher mobilities no further improvement can be achieved. The rubbing induced polymer alignment is deduced from optically polarized transmission spectroscopy on polymer-coated quartz glass substrates. The different behavior of the field-effect mobility will be explained in terms of different degrees of crystallinity.

Dielectric elastomers are a promising material for solid-state actuators due to the high obtainable strain. Their action bases mainly on the Maxwell effect of electrostatics: The attraction between the electrodes in a condensor arrangement causes a compressing stress σ z in field direction as σ z = -εε 0 E 2 .( E q . 1 a ) Provided that the electrodes are compliant, a dielectric material deforms then under the influence of an electric field to an extent that the sum of deformation energy and electric field energy assumes a minimum, and the resulting deformation s x of the electrode area, assuming linear stress-strain relation, isHere, ε is the dielectric permittivity, ε 0 is the permittivity of the vacuum, and Y is Young's modulus. κ M is the corresponding electro-mechanical coupling coefficient. A quantitative evaluation of Eq. 1 proves that the Maxwell effect yields only in elastomers a verifiable or technically exploitable deformation. The effect has the same electric field dependence as electrostriction but is ruled by the ratio between dielectric permittivity and Young's modulus of the material. In elastomers, the electromechanical response is even ruled by the Maxwell effect, and true electrostriction plays almost no role. A route to increase the electro-mechanical performance of a dielectric elastomer is consequently to increase its dielectric constant by blending with a high dielectric constant filler while attempting not to increase its stiffness. In doing that, compounds of polydimethylsiloxane (PDMS) filled up to 10 wt-% with nano-scaled titanium dioxide (ε ≈ 80) were prepared. Pure PDMS yields a contraction s z of 10 % at 25 MV/m. The influence of the filler on the mechanical, dielectric, and electro-mechanical behaviour was investigated. The dielectric constant increased linearly with filler content (Fig. 1a). At the other hand, the elastic modulus of the compunds increased slightly in that composition range, too (Fig. 1b). The overall electromechanical response κ exp exhibited consequently a maximum for a filler level of 6 wt-% where it increased by 140 % with respect to that of the unfilled material (Fig. 1c). Considering the data of Figs. 1a,b and use of Eq. 1 allows moreover to split the experimental κ exp in the contributions of Maxwell effect (κ M ) and true electrostriction (κ E ). The approach enables to tune the mechanical and electromechanical properties of a dielectric elastomer to a desired ratio.