Jacek Ulanski

Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules

Adding low-molecular-weight hole-transporting molecules (HTM) with different oxidation potentials to the polyfluorene emission layer of single-layer light-emitting diodes causes significant changes in the device properties. The pronounced increase in luminance efficiency combined with a decrease in current is attributed to significant hole trapping, as further suggested by thermoluminescence experiments. Using a oligo-triphenylamine HTM with an ionization potential of ∼4.9 eV, light-emitting diodes with stable blue emission, a brightness of 800 cd/m2 and an efficiency of 0.87 cd/A were realized. Further, the red-emitting contribution to the spectra as observed in the pure polymer devices could be fully suppressed.

High‐Performance Single Crystal Organic Field‐Effect Transistors Based on Two Dithiophene‐Tetrathiafulvalene (DT‐TTF) Polymorphs

Solution prepared single crystal organic field-effect transistors (OFETs) combine low-cost with high performance due to structural ordering of molecules. However, in organic crystals polymorphism is a known phenomenon, which can have a crucial influence on charge transport. Here, the performance of solution-prepared single crystal OFETs based on two different polymorphs of dithiophene-tetrathiafulvalene, which were investigated by confocal Raman spectroscopy and X-ray diffraction, are reported. OFET devices prepared using different configurations show that both polymorphs exhibited excellent device performance, although the -phase revealed charge carrier mobility between two and ten times higher in accordance to the closer stacking of the molecules.

Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye: A deep blue light-emitting composite for light-emitting diode applications

We investigated the spectral properties of light-emitting diodes based on a deep blue-emitting pyrazoloquinoline dye doped into a poly(N-vinylcarbazole)-based matrix. Even though the electroluminescence (EL) of the host is redshifted and broadened with respect to the emission of the dye, the EL spectrum becomes fully dominated by the dye emission at concentrations of ca. 2wt%. This is attributed to a competition of exciplex formation on the matrix and exciton formation on the dye.

Superconductivity in reticulate doped polycarbonate films, containing (BEDT-TTF)2I3

Abstract Electric properties of conducting reticulate doped polymeric films, containing ET polyiodide crystalline network, have been investigated. In order to improve their conducting properties they were annealed in iodine vapors. It was found, that certain conditions of thermal treatment lead to metal-like state, stable down to helium temperatures. The overall decrease of resistivity R 300K /R 4.2K reaches 45. At some conditions of the thermal treatment an accelerated decrease of resistivity is observed below 7 K, vanishing with application of magnetic field, thus giving the first evidence of the (incomplete) superconducting transition in polymeric organic composite material.

Influence of SiO2 surface energy on the performance of organic field effect transistors based on highly oriented, zone-cast layers of a tetrathiafulvalene derivative

In this paper we present that the surface energy of silicon dioxide employed as the dielectric in bottom gate organic field effect transistors has large impact on the device performance. By the use of the zone-casting simple solution processing technique, we ensured reproducibility of active layer preparation confirmed by the atomic force microscopy and x-ray diffraction that showed high crystalline quality. Electrical measurements revealed that charge carrier mobility based on highly ordered zone-cast tetrakis-(octadecylthio)-tetrathiafulvalene layer was increased 30 times to 0.2 cm2/V s, when dielectric surface energy decreased from 51.8 to 40.1 mN/m.

Photogeneration and Transport of Charge Carriers in Hybrid Materials of Conjugated Polymers and Dye-Sensitized TiO2

Steady state photoconductivity and current–voltage (I–V) experiments are performed on solid films of organic/inorganic composites of dye-sensitized TiO2 in combination with poly(N-vinylcarbazole) (PVK), a ladder-type PPP and a soluble PPV derivative. The I–V characteristics of the composites in the dark are explained by the formation of percolation networks of nanoparticles between the electrodes. Photoaction spectra of the devices prove that the photogeneration of charge carriers is significantly enhanced and spectrally broadened only if electron transfer from the polymer to the dye is possible. Increasing the concentration of the nanoparticles in the hybrid materials changes the spectral shape of the photoresponse. For high TiO2 contents signatures due to the absorption of the Ruthenium dye can be observed. The different electronic properties of anatase/brookite TiO2 and rutile TiO2 have only minor effects on the generation of charge carriers and on the shape of the photoaction spectra. Indeed, photocur...

4 Electrical properties of organic materials

Recent research into the electrical properties of low-molecular weight organic solids and polymers is reviewed. Conditions for an effective injection from electrodes into organic solids, and recent views on the photogeneration of charge carriers in photoactive polymers are briefly discussed. The transport of charge carriers in organic materials is reviewed, focusing on disordered (polycrystalline and amorphous) materials. Particular attention is paid to materials potentially applicable as elements of electronic devices: thin film field-effect transistors, and light-emitting and photovoltaic devices.

Dielectric and viscoelastic studies of curing epoxy-amine model systems

SummaryThe curing of different epoxy-amine model systems which present only gelification and/or vitrification processes has been followed with the evolution of their viscoelastic and electrical behaviours during the network formation. Some attempts are performed to correlate the significant evolution of the mechanical and electrical parameters with the physico-chemical changes of these well defined chemical systems during their hardening and complete curing.

Is a percolation threshold in conductive systems below 0.003 possible

Reticulate crystallization of a charge-transfer (CT) complex in polymeric solution leading to conducting polymer films with a very low amount of conductive filler is considered. The previously published model concept of such crystallisation is now supported by measurements of current flowing through a cast solution during film solidification and CT complex crystallisation. The results obtained enabled one to select appropriate conditions under which polymers can be made conductive by doping with only 0.3 wt.% of CT complex.

Molecular relaxation in anisotropic composites based on (hydroxypropyl)cellulose and acrylic polymer

The birefringence and molecular dynamic of the anisotropic polymer composites obtained by photopolymerisation of the lyotropic liquid-crystalline systems composed of (hydroxypropyl)cellulose and photopolymerisable acrylic acids are studied. Molecular relaxations in such composites were investigated by means of dielectric and mechanical spectroscopies. The molecular relaxation processes, characteristics of pure components, are modified in the composites due to strong intermolecular interactions between the macromolecules. These interactions are also responsible for stabilisation of the structure and the optical properties of the composites. In dielectric spectroscopy two representations were used: temperature dependences of dielectric loss e″ and of electric modulus M″. This latter one was especially useful in a high temperature range where the ionic conductivity dominates the dielectric response.