Anja Haase

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)

Controlling energy level offsets in organic/organic heterostructures using intramolecular polar bonds

The impact of intramolecular polar bonds (IPBs) on the energy level alignment in layered systems of rodlike conjugated molecules standing on the substrate was investigated for pentacene (PEN) and perfluoropentacene (PFP) on SiO2 using ultraviolet photoelectron spectroscopy. A remarkably large energy offset of 1.75 eV was found between the highest occupied molecular orbital (HOMO) levels of PEN and PFP caused by IPBs at the surface of standing PFP layers. This large HOMO-level offset results in a narrow intermolecular energy gap of approximately 0.4 eV at the interface between PEN and PFP layers. However, the absence of significant spatial overlap of PEN and PFP electron wave functions across the layers suppresses interlayer optical transitions.

Tuning the threshold voltage in organic thin-film transistors by local channel doping using photoreactive interfacial layers.

In recent years, organic thin-film transistors (OTFTs) have attracted a great deal of attention due to their potential applications in low cost sensors,1 memory cards,2 and integrated circuits.3 Great efforts are under way to design OTFTs with high performance, high stability, high reproducibility, and low cost.4 Two of the most crucial device parameters are the charge carrier mobility and the threshold voltage (VTh). Concerning the mobility, the main goals for most applications is its maximization.5 For VTh, the situation is more complex: for example, for integrated circuits it would be desirable to tune VTh over a broad range,6 e.g., for inverter applications. In silicon technology, complementary circuits that consist of p-channel and n-channel transistors are typically used.7 There have been many attempts to adapt this technology to OTFTs and fabricate organic complementary inverters.2,8,9 They, however, suffer from poor n-type transistor performance and/or air instability of n-type semiconductor materials. An alternative approach is the adaptation of unipolar depletion-load inverters enabling simplified processing, even if they do not provide the low power consumption and the simple circuit design intrinsic to complementary logic.10,11 Depletion-load inverters consist of an enhancement-mode driver transistor and a depletion-mode load transistor and can be realized using only p-type OTFTs. So far there have been attempts to achieve this target by using a level shifter12,13 or a dual gate structure.14 The main objective is to find a reproducible method to realize driver and load transistors with equivalent device characteristics (in particular mobilities), but different VTh values.

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...

Cellulose as biodegradable high-k dielectric layer in organic complementary inverters

We report on the natural source based and biodegradable material cellulose on Al2O3 as ultrathin hybrid high-k dielectric layer for applications in green electronics. Dielectric films of 16 nm cellulose (e ≈ 8.4) and 8 nm Al2O3 (e ≈ 9) exhibit low leakage currents up to electric fields of 1.5 MV/cm. Pentacene and C60 based organic thin film transistors show a well-balanced performance with operation voltages around 2 V. They are implemented in complementary inverters with excellent switching behavior, a small-signal gain up to 60 and with exceptionally high and balanced noise margin values of 82% at ultralow operation voltage (VDD = 2.5 V).

Temperature stability of the pentacene thin-film phase

This work presents the influence of temperatures above 300 K on the crystal structure and morphology of pentacene thin films. The thermal expansion of the unit cell and the relative amount of different phases are investigated via grazing incidence x-ray diffraction. Geometrical considerations about the specific molecular packing of the thin-film phase explain the anisotropic non-linear expansion. Furthermore, around 480 K, a phase transformation of the thin-film phase to the bulk phase is observed. In contrast, only a weak influence of the temperature on the height distribution of the thin-film phase crystallites is found.

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.

Photochemical control of the carrier mobility in pentacene-based organic thin-film transistors

We use a photoreactive interfacial layer of poly(endo,exo-bis(2-nitrobenzyl) bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PBHND) between the gate dielectric and the pentacene active layer to tune the effective hole mobility in organic thin film transistors over approximately one order of magnitude. This can be explained by the photoconversion of the PBHND layer changing its surface energy and thus strongly impacting pentacene growth. A linear relationship between the mobility and the grain size is observed.