[Show abstract][Hide abstract] ABSTRACT: Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.
Journal of the American Chemical Society 03/2006; 128(7):2336-45. · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nine different α,ω-phenyl-endcapped bithiophenes were synthesised, and the effect of the different side chains on the liquid crystalline properties, alignment ability and charge carrier mobility have been studied. An increase in chain length leads to a decrease in the liquid crystalline–isotropic phase (clearing) transition temperature. Remarkably, introduction of an asymmetric carbon centre close to the π-conjugated segment resulted in the loss of the liquid crystalline phase. Alignment on rubbed polyimide was obtained for the liquid crystalline thiophene derivatives lacking heteroatoms in the side chain and for the chiral α,ω-phenyl-endcapped bithiophene. Some bithiophenes showed bipolar charge transport in time-of-flight (TOF) measurements, with mobilities up to 3 × 10−3 cm2 V−1 s−1 in the liquid crystalline state. Field effect transistors revealed mobility for holes up to 1 × 10−3 cm2 V−1 s−1 (crystalline state). From the data set obtained, it can be concluded that the use of linear hydrocarbon chains as solubilising tails in these types of π-conjugated building blocks gives the best overall electronic performance.
Journal of Materials Chemistry 01/2006; 16(44). · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a systematic study of the influence of the processing conditions on the charge-carrier mobility in hole-only diodes and field-effect transistors (FETs) based on alkoxy-substituted poly(p-phenylene vinylene) (PPV). It is demonstrated that by chemical modification from asymmetrically to fully symmetrically substituted PPVs the mobility in both types of devices can be significantly improved. Furthermore, for symmetrical PPVs the mobility is strongly dependent on processing conditions, such as choice of solvents and annealing conditions. The increase in mobility is accompanied by a strong enhancement of the anisotropy in the charge transport. Ultimately, mobility of up to 10−2 cm2/Vs in FETs and 10−5 cm2/Vs in hole-only diodes have been achieved.
Journal of Applied Physics 06/2005; 97(12):123703-123703-6. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The influence of the substitution pattern (unsymmetrical or symmetrical), the nature of the side chain (linear or branched), and the processing of several solution processable alkoxy-substituted poly(p-phenylene vinylene)s (PPVs) on the charge-carrier mobility in organic field-effect transistors (OFETs) is investigated. We have found the highest mobilities in a class of symmetrically substituted PPVs with linear alkyl chains (e.g., R1, R2 = n-C11H23, R3 = n-C18H37). We have shown that the mobility of these PPVs can be improved significantly up to values of 10-2 cm2 V-1 s-1 by annealing at 110°C. In addition, these devices display an excellent stability in air and dark conditions. No change in the electrical performance is observed, even after storage for thirty days in humid air.
[Show abstract][Hide abstract] ABSTRACT: —Dye-doped semiconducting polymers are used as active layers in polymer light-emitting diodes (polyLEDs). The emission color can be tuned by doping the active polymer with certain dyes. This concept of energy transfer is demonstrated for a green matrix doped with a red-emitting dye, suitable for use in LEDs. An absolute PL efficiency of 39% is observed for this system. Another very attractive development is taking place in the area of all-polymer transistors. This may lead to a (partial) replacement of the driving electronics by all-plastic circuits. A new precursor route toward poly(thienylenevinylene)s (PTVs), suitable as active material in all-polymer integrated circuits, is presented. Synthesis of the precursors is reproducible and fast, and can readily be scaled for manufacture. Quantitative conversion of the precursor polymer can be accomplished by heating at 150°C for 20 min. The resulting mobility (6 × 10−3 cm2/V-sec) and ON-OFF ratio (4 × 104) makes this material a suitable candidate for the development and large-scale manufacturing of all-polymer integrated circuits.
Journal of The Society for Information Display - J SOC INF DISP. 01/2002; 10(1).
[Show abstract][Hide abstract] ABSTRACT: We have investigated the field dependence of the in-plane conductivity in poly(2,5-thienylene vinylene) thin films. The conductivity is found to have a square root dependence on the lateral electric field with values of the activation energy, Δ=0.46 eV, B=2.3·10-5 eV(m/V)^½ and the characteristic temperature T0=5.2·10^2 K. A similar value (T0=4.9·10^2 K) is found for the isokinetic temperature in Meyer-Neldel experiments on poly(2,5-thienylene vinylene) field-effect transistors. Based on these results, we argue that entropy changes due to hopping of charge carriers should be incorporated in theoretical descriptions of the field dependent mobility in disordered organic semiconductors.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the stability of polythienylene vinylene field-effect transistors under gate bias stress. On time scales up to 1000 s and temperatures up to 140 °C, we only observe reversible charge relaxation effects and no degradation. We show the time dependence of the threshold voltage shift at different temperatures. Furthermore, we discuss the influence of water and oxygen on the relaxation process.
[Show abstract][Hide abstract] ABSTRACT: Electrical properties of field-effect transistors made of different solution processable organic semiconductors are described. The temperature and gate-voltage dependence of the mobility is shown and theoretically described using a model based on the variable-range hopping of charge carriers in an exponential density of states. Furthermore, a technology has been developed to make all-polymer integrated circuits. It involves reproducible fabrication of field-effect transistors on flexible substrates, where the semiconducting, conducting and insulating parts are all made of polymers. Integrated circuits consisting of more than 300 field-effect transistors are demonstrated.
[Show abstract][Hide abstract] ABSTRACT: A technology has been developed to make all-polymer integrated circuits. It involves reproducible fabrication of field-effect transistors in which the semiconducting, conducting and insulating parts are all made of polymers. The fabrication on flexible substrates uses spin-coating of electrically active precursors and patternwise exposure of the deposited films. In the whole process stack-integrity is maintained. Vertical interconnects are made mechanically. As a demonstrator for the technology functional 15-bit programmable code generators are fabricated. These circuits still operate when the foils are sharply bent. Due to the limited number of process steps the technology is potentially inexpensive.
Solid-State Circuits Conference, 1998. ESSCIRC '98. Proceedings of the 24th European; 10/1998