Georg Jakopic

Organic diodes as monolithically integrated surface plasmon polariton detectors

The authors show that organic p∕n heterojunction diodes enable the direct electric detection of surface plasmon polaritons (SPPs). The organic diodes are built from two organic semiconductor thin film layers with an area of 150×500μm2 deposited on an extended silver thin film. Besides serving as the bottom electrode of the diode this silver thin film is as well a waveguide feeding SPPs to the diode area. The authors visualize the direct SPP detection by a spatially resolved induced current map.

Integrated organic electronic based optochemical sensors using polarization filters

A compact, integrated photoluminescence based oxygen and pH sensor, utilizing an organic light emitting device (OLED) as the light source and an organic photodiode (OPD) as the detection unit, is described. The main challenge in such an integrated sensor is the suppression of the excitation light at the detector, which is typically by many orders of magnitude higher in intensity than the emitted fluorescence. In our approach, we refrain from utilizing edge filters which require narrow band excitation sources and dyes with an adequate large Stokes shift. We rather developed an integrated sensor concept relying on two polarizers to separate the emission and excitation light. One polarizer is located right after the OLED, while the other one, oriented at 90° to the first, is placed in front of the OPD. The main advantage of this solution is that any combination of excitation and emission light is acceptable, even if the two signals overlap spectrally. This is especially important for the use of OLEDs as the ...

Fabrication of n‐ and p‐Type Organic Thin Film Transistors with Minimized Gate Overlaps by Self‐Aligned Nanoimprinting

[∗] U. Palfi nger , C. Auner , H. Gold , A. Haase , J. Kraxner , G. Jakopic , J. R. Krenn , B. Stadlober Institute of Nanostructured Materials and Photonics Joanneum Research GmbH Franz-Pichlerstrasse 30, A-8160 Weiz (Austria) Fax: 0043-316-876-2710 Telephone: 0043-316-876-2721 E-mail: barbara.stadlober@joanneum.at T. Haber , M. Sezen , W. Grogger Institute for Electron Microscopy Graz University of Technology Steyrergasse 17, A-8010 Graz (Austria) G. Domann Fraunhofer-Institut für Silicatforschung ISC Neunerplatz 2, D-97082 Würzburg (Germany)

Submicron pentacene-based organic thin film transistors on flexible substrates

The authors demonstrate the fabrication of organic thin film transistors (OTFTs) based on pentacene with submicron channels on flexible substrates. Nanoimprint lithography is used for the patterning of the source and drain electrodes and processed directly on the spin-on gate dielectric, the structured gate electrode, and the flexible substrate. The use of sub-100-nm thin organic gate dielectrics enables full drain current saturation for devices with channel lengths down to 500nm. The submicron OTFTs exhibit negative threshold voltages with an absolute value well below 5V and have subthreshold swings around 0.5V/decade. This demonstrates the possibility to fabricate fully structured and miniaturized OTFTs operating at low voltages and paves the way for a low-cost fabrication of downscaled high performance organic electronic circuits.

Sexithiophene films on clean and oxidized Si(111) surfaces: Growth and electronic structure

The growth and the electronic properties of sexithiophene thin films on silicon surfaces have been studied by angle resolved ultraviolet photoelectron spectroscopy while morphology and crystalline order have been assessed by atomic force microscopy and x-ray diffraction. The influence of the surface modification of the substrate as well as of the growth temperature and the growth rate were investigated in ultrahigh vacuum through controlled multistep growth sequences ranging from (sub)monolayer coverage up to thick films. Depending on the preparation, two electronically distinct sexithiophene (6T) films could be produced, as revealed by their ionization potentials which differed by ∼0.7eV. The evolution of the electronic energy levels with film thickness is considered in terms of the concepts of interface dipole and band bending, with the latter being argued to be inappropriate. Irrespective of substrate or growth conditions the thick 6T films were found to all have the same work function of ∼0.4eV, which...

Application of two-photon 3D lithography for the fabrication of embedded ORMOCER ® waveguides

The idea of applying the two-photon 3D lithography (2P-3DL) to an industrial printed wiring board (PWB) fabrication process is quite pioneering. Taking advantage of the unique rapid prototyping properties of 2P-3DL--its particularly inherent true 3D capability and its high flexibility in processing- this lithographic method can be adapted and optimized concerning the direct laser-writing of integrated optical interconnects with tens of microns in diameter. This will push the method forward towards industrial fabrication of next generation PWBs with integrated optical layers, and put it on the leading edge of printed circuit board (PCB) technology. In this context, the concept of a direct laser-written embedded waveguide is based on the local increase of the refractive index of the exposed material, which is triggered by two-photon absorption (TPA) at the laser focus. The laser induced refractive index difference forms the core of the waveguide, whereas the unexposed surrounding material forms the cladding. Thus, only one optical material is required to form the waveguide using true 3D lithographic process compared to other devices, which significantly simplifies processes. The material is subject to stringent requirements concerning the PWB production process: beside its high refractive index change, a low optical loss of the fabricated optical interconnect is required. The integration of the waveguide into the volume of the material also requires thick films up to 500 microns on the PWB substrate, and the material has to withstand the complete PWB fabrication process, where the board is chemically treated and exposed to high temperatures as well as high pressure during the lamination processes of subsequent metal layers. For this application, an inorganic-organic hybrid polymer (ORMOCER) film is applied, casted onto a PWB substrate, and the two-photon 3D lithography system parameters and optics are tuned such that waveguides with a diameter of approx. 30 microns can be inscribed. The board is equipped with laser- and photodiodes, which are totally covered by the thick ORMOCER film. The integration of the waveguide in such a preconfigured board requires precise 3D registration of the sample prior to the waveguide writing in order to align the waveguide relative to the optoelectronic components. By means of the 3D registration, the waveguide alignment is an inherent part of the fabrication process. The 3D capabilities of the 2P-3DL permit not only the fabrication of single embedded waveguides with a simple geometry, but also more complex waveguide structure (e.g. bundles) with largely arbitrary waveguide configurations. In this paper, we present the development and realization of the two-photon 3D lithography for the fabrication of integrated optical interconnects on PWBs. The ultimate goal of this approach is the large-scale fabrication of leadingedge PWBs with an integrated optical layer for additional functionality. The functioning of the fabricated and embedded waveguides is demonstrated by measurements of the essential parameters of such an optoelectronic system (photocurrent, optical loss, throughput, etc).

Refractive index modulation in polymers bearing photoreactive phenyl and naphthyl ester units using different UV wavelengths

Tuning and patterning of the refractive index in polymeric films is of importance for modern optical applications such as wave guiding and optical data storage. In this contribution, we report on the synthesis of new photoreactive polymers bearing aryl esters which undergo the photo-Fries rearrangement to give aromatic hydroxyketones. Poly(norbornene)-based polymers bearing naphthyl ester groups (poly-2) and copolymers bearing both phenyl and naphthyl ester groups (poly(1-co-2)) were prepared by ring opening metathesis polymerization (ROMP). The photochemistry of these polymers under irradiation with UV light of either 254 nm or 313 nm was investigated. The photoreaction causes an increase of the refractive index in poly-2 between Δn589 = +0.022 and +0.048 depending on the irradiation wavelength. The copolymer poly(1-co-2) offers the possibilities of selectively exciting either the naphthyl ester with UV light of 313 nm causing an increase in the refractive index of Δn589 = 0.010, or both the naphthyl and phenyl chromophores with UV light of 254 nm (Δn589 = 0.036). When thin films of poly(1-co-2) already irradiated with 313 nm light are subjected to a second illumination step with 254 nm light, a significant decrease of the refractive index (Δn589 = −0.043) is observed. The reason for this behaviour seems to arise from the formation of different photoproducts depending on the irradiation sequence. Thus the final refractive index can be adjusted to values between n589 = 1.564 to n589 = 1.633. The two step illumination procedure provides the possibility of erasing and even inverting the index contrast generated during the first illumination step.

Combined XPS, AFM, TEM and ellipsometric studies on nanoscale layers in organic light emitting diodes

We report on combined investigations on indium–tin oxide (ITO) coated glass substrates in order to reveal the influence of chemical and physical treatments on the morphology and the elemental distribution at the ITO surface. The latter was studied by X-ray photoelectron spectroscopy while a combination of atomic force microscopy (AFM) and transmission electron microscopy was used to study the morphologic aspects. We find that etching ITO with chromosulphuric acid significantly reduces the amount of carbon contamination present at the surface of as received ITO substrates whereas the morphology of ITO remains almost the same. Additionally, we present ellipsometric studies of an organic semiconducting oligomer, para-hexaphenyl (PHP), grown on different substrates by physical vapour deposition. This method enables the determination of the optical parameters and the film thickness. In order to understand the growth mechanism of PHP in more details, its growth on silicon (Si) was also studied. We present AFM images of the first two as well as of several monolayers of PHP on Si giving evidence for the formation of PHP islands on the surface. 2002 Elsevier Science B.V. All rights reserved.

Structural and electrical properties of polymorphic pentacene thin films

Due to its outstanding carrier transport capabilities the aromatic hydrocarbon pentacene is still one of the most promising out of all organic semiconducting materials investigated so far. Pentacene appears in several polymorphic structures that significantly differ with respect to the d(001) spacing. It is shown, that precise control of the epitaxial growth process of thin films enables not only to adjust the formation of the polymorphic phases, but also to influence grain size and shape. The relative volume fraction of the pentacene polymorphs is determined by several parameters which are substrate material, deposition rate, film thickness and substrate temperature. A comparison of X-ray diffraction and Raman measurements reveals that the phase with the smaller layer-by-layer spacing grows on top of the other]. Moreover, there is a strict correlation between evaporation rate and maximum grain size. In addition to structural we also investigated the electrical properties of pentacene thin films focussing on polymorphism and its influence on the transport properties. Apart from the fact that the charge carrier mobility is strongly influenced by the grain size it turned out that the bulk phase is related to a lower intrinisic mobility than the thin film phase.

Carrier redistribution in organic/inorganic (poly(3,4-ethylenedioxy thiophene/poly(styrenesulfonate)polymer)-Si) heterojunction determined from infrared ellipsometry

Carrier redistribution in organic/inorganic (poly(3,4-ethylenedioxy thiophene/poly(styrenesulfonate)polymer)-Si) heterojunction determined from infrared ellipsometry