PCTrz, a new bipolar host material containing a phenoxy-carbazole separated from a biscarbazolyl-triazine by a non-conjugated
ether bond is presented. Computational calculations demonstrated the separation of PCTrz into an oxidation
and a reduction site. A phosphorescent OLED with PCTrz as host and FIrpic as blue emitter yielded high current
efficiencies of up to 16.2 cd/A. Additionally two electron transporting host materials DBFTaz and DBFTazC, both
containing 1,2,4-triazole moieies, were synthesized and characterized. The triazole moiety in DBFTaz was formed by a
classical ring closure reaction between a N-benzoylbenzimidate and a hydrazine. For DBFTazC we used another
synthetical pathway which involves subsequent coupling of a carbazole and a triazole moiety to a dibenzofuran core.
Both triazoles posses high triplet energies of 2.95 eV for DBFTaz and 2.97 eV for DBFTazC, which make the
compounds interesting as matrix materials for blue phosphorescent OLEDs.
Corresponding to their relative low work function, Ca and Mg are interesting metals for cathodes in organic light emitting devices. In this study, the interaction of these metals with the blue phosphorescent emitter material Ir(cnpmbic)<sub>3</sub> is investigated by in-situ infrared spectroscopy. Thin films of the organic material are deposited by vapour sublimation under ultra-high vacuum conditions. Further deposition of Ca on the organic layer at room temperature gives rise to new features in the infrared spectrum of the sample. The new features indicate diffusion of Ca into the organic layer and do not appear at much lower temperature (110 K). They are attributed to dynamic charge transfer processes
that may occur at rough metal surfaces. On the other hand, Mg deposited at room temperature does not stick on the organic material.
Admittance spectroscopy is a simple yet powerful tool to determine the carrier mobility of organic compounds. One requirement is to have an Ohmic contact for charge injection. By employing a thin interfacial layer of tungsten oxide or molybdenum oxide we have found a possibility to efficiently inject holes into organic materials with a deep highest occupied molecular orbital level down to 6.3 eV. These results considerably enhance the application range of the admittance
spectroscopy method. The measured mobility data are in excellent agreement with data obtained by the time-of-flight technique. To efficiently inject electrons into materials with an ionization potential of up to 2.7 eV we thermally evaporated an intermediate layer of cesium carbonate and discuss the extracted electron mobilities.
Cyclometallated iridium <i>N</i>-heterocyclic carbene (NHC)-complexes have become known as efficient deep blue triplet
emitters in OLEDs. With these emitters suitable CIE color coordinates of CIE x ~ 0.15 and CIE y = 0.1...0.2 can easily
be reached. To keep the expensive and tedious synthetic and laboratory screening effort for new emitters and
complementary materials as efficient as possible a good computational pre-screening method based on quantum
chemical theory is used. In this paper, data will be presented which show a good correlation between calculated and
measured values of for example triplet energy, ionization potential and electron affinity. Only by having good control of
these parameters it is possible to design efficient and long lasting devices. Based on this, we will show our progress in
the deep-blue color region by optimizing the device setup and by employing a new, much more stable complementary
Currently, one of the most challenging applications for OLEDs is
the full color display. The most energy-efficient way to realize
light generation in OLEDs is by using phosphorescent emitters.
Green and red emitters have already been demonstrated, but the
search for blue emitting organic phosphorescent emitters with good
color purity is still ongoing with arduous effort. Here we present
our work with a new material developed at BASF which allows
phosphorescent emission in the deep-blue spectral range. The
emitter has an emission maximum at 400 nm, which gives CIE color
coordinates of x = 0.16 and y = 0.06. An OLED device made with
this new material shows a maximum external quantum efficiency of
1.5 %. The OLED was built in a three layer structure, with the
emitting zone being a hybrid guest-host system. As host material
we used the optically and electronically inert polymer
poly-methyl-methacrylate (PMMA). Because of its lack of charge
transport abilities we doped the host material with a high
concentration of the triplet emitting material, i.e. the emitter
itself is also used as charge transport material.
Organic field effect transistors are expected to be applicable for low-cost, large-area electronic applications, e.g. the incorporation as active-matrix into displays based on organic light emitting diodes (OLED). There are two major challenges which have to be tackled. As the low charge carrier mobility allows only for comparatively low saturation currents, the ratio of channel width and length has to increase by several orders of magnitude, compared to poly-Si-technology. Furthermore, as organic semiconductor devices usually degrade upon exposure to solvents, standard photolithography cannot be applied once the organic materials have been deposited. Therefore, the definition of single pixels has to occur before the deposition of organic materials.
We prepared OFETs employing a bottom-Al-gate, an 50 nm thick anodized Al-oxide gate dielectric and a inter-digital drain-source-structure (Au), topped with 30 nm of pentacene as active layer. By applying an inter-digital structure we increased the W/L-ratio to 4340. For the given configuration, a saturation current of 4 mA could be observed at -20 V drain-source- and -20 V gate-source-voltage.
The drain-source-contacts enclosed a predefined ITO-anode shorted to drain and acting as OLED-anode. For preventing shortcuts between the OLED-cathode and the OFET, poly-vinyl-alcohol (PVOH) was spin-coated from an aqueous solution and structurized by photolithography. When the OFET characteristics were measured afterwards the field-effect- mobility dropped by two orders of magnitude but recovered due to desorption of residual water. Afterwards, the organic layers and a Al/LiF-cathode were deposited. The area covered by the OLED was 1.33mm<sup>2</sup>. Applying an operating bias of 11 V between cathode and source, allows for switching of the OLED by changing the gate-source-voltage from +2.5 V to -5 V. The on-state-brightness is 850 cd/m<sup>2</sup> and the on-off-ratio 950. Considering a realistic filling factor of 40% the values observed may be sufficient for active-matrix display-applications.
Optically pumped organic semiconductor thin-films have been processed on first and second order distributed feedback gratings. The organic thin-films were made by co-evaporation of tris-(8-hydroxy quinoline)aluminium (Alq<sub>3</sub>) and the laser dye 4-(Dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2). The DFB laser wavelength varied depending on the grating period between 647.8 nm and 668.6 nm for first order operation and between 626.7 nm and 640.1 nm for second order operation. By evaporating the same organic film on both resonator designs we could compare first and second order laser parameters. We measured laser output characteristics and determined threshold energy values for different wavelengths and for first and second order of the Bragg grating. The laser threshold energy of the first order organic DFB laser was reduced by a factor 8 compared to the second order laser. Minimum threshold energy density was measured for a first order sample with 13.8 μJ/cm<sup>2</sup>. Reducing the laser threshold value is especially important for future applications like electrically driven organic solid-state lasers, where it will be more difficult to reach the laser threshold excitation.
Top-emitting organic light-emitting diodes (OLEDS) for next-generation active-matrix OLED-displays (AM-OLEDs) are discussed. The emission of light via the conductive transparent top-contact is considered necessary in terms of integrating OLED-technology to standard Si-based driver circuitry. The inverted OLED configuration (IOLED) in particular allows for the incorporation of more powerful n-channel field-effect transistors preferentially used for driver backplanes in AM-OLED displays. To obtain low series resistance the overlying transparent electrode was realized employing low-power radio-frequency magnetron sputter-deposition of indium-tin-oxide (ITO). The devices introduce a two-step sputtering sequence to reduce damage incurred by the sputtering process paired with the buffer and hole transporting material pentacene. Systematic optimization of the organic growth sequence focused on device performance characterized by current and luminous efficiencies is conducted. Apart from entirely small-molecule-based IOLED that yield 9.0 cd/A and 1.6 lm/W at 1.000 cd/m<sup>2</sup> a new approach involving highly conductive polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS) as anode buffers is presented. Such hybrid IOLEDs show luminance of 1.000 cd/m<sup>2</sup> around 10 V at efficiencies of 1.4 lm/W and 4.4 cd/A.