Christian Schildknecht

Vertical channel all-organic thin-film transistors

Technologically simple and cost-effective processes are essential for the fabrication of organic electronic devices. In this letter, we present a concept for making vertical channel all-organic thin-film transistors on glass substrate. This concept avoids the need for patterning processes with high lateral resolution by defining the channel length through the thickness of an insulating layer. Our devices are based on commercially available poly(ethylene dioxythiophene)/poly(styrene sulfonate) dispersion for source, drain, and gate electrodes, photoresist as the insulating layer and photosensitized poly(vinyl alcohol) as the gate insulator. Pentacene was used as the organic semiconductor. Functional devices with channel length of 2.4 μm and width of 1 mm have been realized, and we report electrical characteristics of these devices.

Transition metal oxides as charge injecting layer for admittance spectroscopy

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 6.3eV. These results considerably enhance the application range of the admittance spectroscopy method. The measured data are in excellent agreement with data obtained by the time-of-flight technique.

11.2: Efficient Deep Blue Triplet Emitters for OLEDs

Cyclometallated iridium carbene complexes are introduced as efficient blue triplet emitters. Quantum mechanical calculations have been used to design and to optimize this class of materials predominantely with respect to color coordinates and luminescence quantum yield. To complete the set of materials required for deep blue OLED devices we engineered suitable host and blocker materials for the use in combination with large triplet energy carbene emitters. These tailor-made materials were applied to develop deep blue electroluminescent devices with excellent efficiency.

Novel deep-blue emitting phosphorescent emitter

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.

Inverted topside-emitting organic light-emitting diodes

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/m2 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/m2 around 10 V at efficiencies of 1.4 lm/W and 4.4 cd/A.

Host Materials for Blue Phosphorescent OLEDs

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.

IR studies on the interaction of Ca and Mg with the blue emitter material Ir(cn-pmbic)3

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)3 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.

Focus-Induced Photoresponse: a novel way to measure distances with photodetectors

We present the Focus-Induced Photoresponse (FIP) technique, a novel approach to optical distance measurement. It takes advantage of a universally-observed phenomenon in photodetector devices, an irradiance-dependent responsivity. This means that the output from a sensor is not only dependent on the total flux of incident photons, but also on the size of the area in which they fall. If probe light from an object is cast on the detector through a lens, the sensor response depends on how far in or out of focus the object is. We call this the FIP effect. Here we demonstrate how to use the FIP effect to measure the distance to that object. We show that the FIP technique works with different sensor types and materials, as well as visible and near infrared light. The FIP technique operates on a working principle, which is fundamentally different from all established distance measurement methods and hence offers a way to overcome some of their limitations. FIP enables fast optical distance measurements with a simple single-pixel detector layout and minimal computational power. It allows for measurements that are robust to ambient light even outside the wavelength range accessible with silicon.

Controlling the radiative rate of electrophosphorescent organometallic complexes by engineering the singlet-triplet splitting

We address the role of singlet-triplet splitting in controlling the radiative rate for deep-blue electrophosphorescent metal complexes. An enhanced radiative rate correlates with a small splitting, highlighting the road map to efficient electrophosphorescence.

Charge injecting layers for admittance spectroscopy

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.