P.T. Herwig

Two-dimensional charge transport in self-organized, high-mobility conjugated polymers

Self-organization in many solution-processed, semiconducting conjugated polymers results in complex microstructures, in which ordered microcrystalline domains are embedded in an amorphous matrix. This has important consequences for electrical properties of these materials: charge transport is usually limited by the most difficult hopping processes and is therefore dominated by the disordered matrix, resulting in low charge-carrier mobilities (⩽10-5u2009cm2u2009V-1u2009s-1). Here we use thin-film, field-effect transistor structures to probe the transport properties of the ordered microcrystalline domains in the conjugated polymer poly(3-hexylthiophene), P3HT. Self-organization in P3HT results in a lamella structure with two-dimensional conjugated sheets formed by interchain stacking. We find that, depending on processing conditions, the lamellae can adopt two different orientations—parallel and normal to the substrate—the mobilities of which differ by more than a factor of 100, and can reach values as high as 0.1u2009cm2u2009V-1u2009s-1 (refs 3, 4). Optical spectroscopy of the field-induced charge, combined with the mobility anisotropy, reveals the two-dimensional interchain character of the polaronic charge carriers, which exhibit lower relaxation energies than the corresponding radical cations on isolated one-dimensional chains. The possibility of achieving high mobilities via two-dimensional transport in self-organized conjugated lamellae is important for applications of polymer transistors in logic circuits and active-matrix displays.

Bias-stress induced instability of organic thin film transistors

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.

The Meyer–Neldel rule in organic thin-film transistors

We have measured and analyzed the temperature and gate voltage dependencies of the field-effect mobility in organic thin-film transistors. We find that the mobility prefactor increases exponentially with the activation energy in agreement with the Meyer–Neldel rule. This behavior is demonstrated in the mobility data of solution-processed pentacene, poly(2,5-thienylene vinylene) and in mobility data reported in literature. Surprisingly, the characteristic Meyer–Neldel energy for all analyzed materials is close to 40 meV. Possible implications for the charge transport mechanism in these materials are discussed.

Organic field-effect transistors and all-polymer integrated circuits

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.

Low-cost all-polymer integrated circuits

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.

The isokinetic temperature in disordered organic semiconductors

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) 1/2 and the characteristic temperature T 0 =5.2.10 2 K. A similar value (T 0 =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.

Materials optimization for semiconducting polymer LEDs and FETs

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 cm 2 /V-sec) and ON-OFF ratio (4 × 10 4 ) makes this material a suitable candidate for the development and large-scale manufacturing of all-polymer integrated circuits.