Heinz-Georg Nothofer

Improving the Performance of Doped pi-Conjugated Polymers for Use in Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) represent a promising technology for large, flexible, lightweight, flat-panel displays. Such devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode into the lowest unoccupied molecular orbital of the adjacent molecules (simultaneously, holes are injected by the anode into the highest occupied molecular orbital). The two types of carriers migrate towards each other and a fraction of them recombine to form excitons, some of which decay radiatively to the ground state by spontaneous emission. Doped π-conjugated polymer layers improve the injection of holes in OLED devices; this is thought to result from the more favourable work function of these injection layers compared with the more commonly used layer material (indium tin oxide). Here we demonstrate that by increasing the doping level of such polymers, the barrier to hole injection can be continuously reduced. The use of combinatorial devices allows us to quickly screen for the optimum doping level. We apply this concept in OLED devices with hole-limited electroluminescence (such as polyfluorene-based systems), finding that it is possible to significantly reduce the operating voltage while improving the light output and efficiency.

BLUE POLARIZED ELECTROLUMINESCENCE FROM A LIQUID CRYSTALLINE POLYFLUORENE

demonstrated an organic EL device based on aligned con-jugated polymers which directly emitted polarized light,and realized that such devices would be particularly usefulas backlights for conventional liquid crystal displays(LCDs). However, typical supertwisted nematic LCDs re-quire a contrast ratio of 40 to 50,

Circularly Polarized Electroluminescence from Liquid‐Crystalline Chiral Polyfluorenes

Circularly polarized electroluminescence (CPEL) has potential application in optical data storage and processing. It was recently discovered that CPEL could be generated from chiral-substituted, conjugated polymers. These authors have synthesized a range of polyfluorenes (e.g., see Figure) that form liquid crystals and emit bluish light with a significant (200 times greater than in the first report) circularly polarized component

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.

Efficient electronic coupling and improved stability with dithiocarbamate-based molecular junctions

Molecular electronic devices require stable and highly conductive contacts between the metal electrodes and molecules. Thiols and amines are widely used to attach molecules to metals, but they form poor electrical contacts and lack the robustness required for device applications. Here, we demonstrate that dithiocarbamates provide superior electrical contact and thermal stability when compared to thiols on metals. Ultraviolet photoelectron spectroscopy and density functional theory show the presence of electronic states at 0.6 eV below the Fermi level of Au, which effectively reduce the charge injection barrier across the metal-molecule interface. Charge transport measurements across oligophenylene monolayers reveal that the conductance of terphenyl-dithiocarbamate junctions is two orders of magnitude higher than that of terphenyl-thiolate junctions. The stability and low contact resistance of dithiocarbamate-based molecular junctions represent a significant step towards the development of robust, organic-based electronic circuits.

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid-crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra-thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid-crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole-conducting alignment layers in polymer light-emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole-transporting molecule (HTM) have been mixed into the PAP layer. These hole-conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.-%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m2 and an efficiency of 0.3 cd/A could be realized.

Efficient blue organic light-emitting diodes with graded hole-transport layers.

Using a derivative of the blue-emitting polyfluorene (see the graph for the output spectrum and the inset picture for the color), efficient devices exhibiting a power efficiency close to 2.7 cd A(-1) at 100 cd m(-2) light output were realized.

Effects of singlet and triplet energy transfer to molecular dopants in polymer light-emitting diodes and their usefulness in chromaticity tuning

Efficient, white emission with Commission Internationale de L’Eclairage coordinates of 0.33, 0.42 is demonstrated from polymer light-emitting diodes operating at high brightness. An amino endcapped polyfluorene derivative doped with rubrene was used as the active layer, in combination with a poly(3,4-ethylenedioxythiophene) hole transport layer. Luminescence reaches 14950 Cd/m2 at a current density of 150 mA/cm2, and the maximum external quantum efficiency measured 1.05% for a bias voltage of 9.0 V. We observe a fall in external quantum efficiency relative to the undoped device, which we ascribe to an increased singlet–triplet annihilation rate at the dopant sites.

Polarized Electroluminescence from Highly Aligned Liquid Crystalline Polymers

Two classes of polymer materials, hexyl-dodecyl copoly(phenylene-ethynylene) and poly(2,7-(9,9-di(ethylhexyl)-fluorene)), respectively, were aligned by two different alignment techniques. For the copolymer, the crystalline and liquid crystalline state were found to differ only slightly and alignment was achieved in the crystalline state, whereas the two phases differ substantially for the polyfluorene and alignment could be obtained in the liquid crystalline state. Light emitting diodes of differing structures were developed using the two materials as emissive layers. In the case of the polyfluorene, a state-of-the-art polarization ratio of 21 was realized.

Fabrication of asymmetric molecular junctions by the oriented assembly of dithiocarbamate rectifiers.

The oriented assembly of molecules on metals is a requirement for rectification in planar metal-molecule-metal junctions. Here, we demonstrate how the difference in adsorption kinetics between dithiocarbamate and thioacetate anchor groups can be utilized to form oriented assemblies of asymmetric molecules that are bound to Au through the dithiocarbamate moiety. The free thioactate group is then used as a ligand to bind Au nanoparticles and to form the desired metal-molecule-metal junction. Besides allowing an asymmetric coupling to the electrodes, the molecules exhibit an asymmetric molecular backbone where the length of the alkyl chains separating the electrodes from a central, para-substituted phenyl ring differs by two methylene units. Throughout the junction fabrication, the layers were characterized by photoelectron spectroscopy, infrared spectroscopy, and scanning tunneling microscopy. Large area junctions using a conducting polymer interlayer between a mercury-drop electrode and the self-assembled monolayer prove the relationship between electrical data and molecular structure.