Dirk Heithecker

Inverted top-emitting organic light-emitting diodes using sputter-deposited anodes

We demonstrate vacuum-sublimed topside-emitting inverted organic light-emitting diodes (IOLEDs) employing low-power radio-frequency magnetron sputter-deposited indium tin oxide (ITO) anodes. The device introduces a two-step sputtering sequence to reduce damage incurred by the sputtering process, paired with a buffer- and hole-transporting material Pentacene. Systematic optimization of the organic growth sequence focused on device performance characterized by current and luminous efficiencies, suggest the incorporation of rather thick Pentacene layers. The optimized thickness is obtained as a trade-off between light absorption and protective properties of Pentacene. The optically and electrically undoped organic multilayer devices capped with 90-nm ITO exhibit high current efficiencies of 3.9 cd/A at a raised luminance level of 1.500 cd/m2, combined with luminous efficiencies of 0.7 lm/W. The inverted configuration allows for integration of organic light-emitting diodes (OLEDs) with preferentially used n-c...

Low-voltage organic electroluminescence device with an ultrathin, hybrid structure

We have prepared organic light-emitting diodes with a narrow recombination zone confined by an organic double-heterojunction structure using both polymer and small molecules (a hybrid structure). In these light-emitting diodes, we used very thin small molecule layers, down to a total thickness of 40 nm, to achieve an exponential forward characteristic. These layers were evaporated on a highly conductive layer of PEDT:PSS for a high-yield process and for good charge injection at the anode. Although no doping processes were applied during device fabrication, either at the injecting electrodes or in the Alq3 layer, the diodes attained high brightness at very low voltage, for instance, 10.000 cd/m2 at voltage of 4.7 V.

OLED matrix displays: in-line process technology and fundamentals

Abstract For more than a decade considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. To date, the properties of matrix displays consisting of organic light emitting diodes (OLEDs) basically meet automotive and consumer product requirements. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. Due to the sensitivity of organic thin films, device structuring by conventional etching techniques is not feasible and alternative structuring techniques were developed. To meet industrial demands for device fabrication an in-line-OMBD was developed. For the first time ever, a fully in-line processed OLED is presented. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal–organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode setups. Further insight in the energy band alignment at organic heterointerfaces is gained by ultraviolet photoelectron spectroscopy (UPS). In addition to widely employed electrical ( I – V , C – V ) and optical ( P – I ) measurements, thermally stimulated current (TSC) and luminescence (TSL) allow the characterization and a more detailed understanding of carrier traps and charge transport in organic devices. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.

OLED matrix displays: technology and fundamentals

For more than a decade, considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. To date, the properties of matrix displays consisting of organic light emitting diodes (OLEDs) basically meet automotive and consumer product requirements. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. In contrast to polymer devices, OLEDs are processed in ultrahigh vacuum systems. The organic source materials are sublimated from effusion cells. Due to the sensitivity of organic thin films, device structuring by conventional etching techniques is not feasible and alternative structuring techniques were developed. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal-organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode set-ups. Further insights into the energy band alignment at organic heterointerfaces are gained by ultraviolet photoelectron spectroscopy (UPS). In addition to widely employed electrical (I-V, C-V) and optical (PI) measurements, thermally stimulated current (TSC) and luminescence (TSL) allow the characterization and a more detailed understanding of carrier traps and charge transport in organic devices. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.

Spatially selective flash sublimation of small organic molecules for organic light-emitting diodes and display applications

A flash-sublimation technique for the spatially selective deposition of small organic molecules is presented. Single-pulse electrically heated copper stripes (width 100 μm) serve as heating elements. The relevant time scale of our technique is on the order of milliseconds. Under high vacuum conditions, the heating elements are used to locally flash-sublimate small-molecule material from a previously coated polyimide foil onto a (ITO-)glass substrate, positioned at a vertical distance of 60 μm. The spatial resolution in our nonoptimized experiments was 250 μm. Organic light-emitting diode (OLED) devices with flash-deposited tris-(8-hydroxyquinoline)aluminum (Alq3) as the active material are demonstrated with satisfying electrical and optical properties. Flash sublimation of a stacked nonintermixed Alq3 (30 nm) [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5Hbenzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) (1 nm)/Alq3 (30 nm) layer is shown to yield a red-emitting (λ=624 nm) DCM2-doped ...

Modellkonsistente Bestimmung des LGD im IRB-Ansatz von Basel II

ZusammenfassungGemäß den im Juni 2004 durch den Baseler Ausschluss endgültig verabschiedeten Kapitalstandards (Basel II) sind Kredite in Höhe des so genannten unerwarteten Verlusts mit Eigenkapital zu unterlegen. Für erwartete Verluste hat das jeweilige Kreditinstitut Rückstellungen zu bilden, wobei hier etwaige Differenzen in der Eigenkapitalunterlegung zu berücksichtigen sind. Damit die folglich relevanten Größen des unerwarteten und des erwarteten Verlusts ermittelbar sind, benötigt man für einen Kredit die Kenntnis der erwarteten Ausfallwahrscheinlichkeit (PD) und der erwarteten Verlustquote bei Ausfall (LGD). Während in Basel II für die Größe PD konkrete Vorschriften zur Bestimmung vorliegen, die auf dem Kreditrisikomodell von Vasicek (1987; 1991) basieren, sind die Vorgaben zur Ermittlung des LGD noch ungenau und bilden allenfalls einen Rahmen, der für die Umsetzung zu beachten ist. Inhalt dieses Beitrags ist daher die Entwicklung einer auf dem Modell von Vasicek basierenden Berechnungsvorschrift für die Größe LGD, die den Rahmenbedingungen von Basel II genügt und darüber hinaus eine sachgerechte Berücksichtigung von Kreditsicherheiten vorsieht.SummaryAccording to the new capital adequacy framework (Basel II) finally adopted by the Basel Committee on Banking Supervision in June 2004 the eligible regulatory capital amounts to the unexpected losses of credits. For expected losses the bank has to make provisions, whereby differences between provisions and expected losses have to be included in the regulatory capital. To determine the relevant variables of unexpected and expected losses, the knowledge of the credit specific probability of default (PD) and the appropriate loss given default (LGD) is necessary. While the determination of PD is regulated in Basel II and based on the credit risk model of Vasicek (1987/1991), statements according to the calculation of the LGD are imprecise and can only act as a framework. Therefore this paper deals with the development of a calculation rule for the LGD, which is based on the model of Vasicek, and meets the basic conditions of Basel II. Furthermore, credit collaterals are considered.

Credit Risk of Collaterals: Examining the Systematic Linkage between Insolvencies and Physical Assets in Germany

According to the new capital adequacy framework (Basel II) the Basel Committee on Banking Supervision (BCBS) strongly advices banks to investigate the relationship between default rates and values of collaterals of secured loan portfolios. This is caused by the fact that the values of collaterals are expected to decline with rising defaults. However, the literature on modelling and examining this effect is rather rare. Therefore, we present a framework based on the Internal Ratings Based (IRB) approach of Basel II in order to examine such dependencies using standard econometric tests. We apply it to insolvency rates and empirical data for physical assets.

Optimizing Credit Risk Mitigation Effects of Collaterals Under Basel II

One of the main differences in measuring capital requirements for credit risk due to the Internal Ratings Based Approach (IRBA) of the new regulatory Capital Standards (Basel II) is the enlarged acceptance of collaterals in order to reduce (supervisory) credit risk. Whereas in the Advanced IRBA own models for estimating the effects of collaterals can be used, in the Foundation IRBA concrete formulas are given. However, these regulations only explain the case of a single claim collateralized by a single asset or guarantee. If numerous collaterals are available for multiple loans, we receive an allocation problem of collaterals to loans with the objective of keeping regulatory capital low. In this article we model the corresponding optimization problem. Since numerical standard procedures often converge towards the wrong local minimum, we show how to apply evolutionary algorithms as well as simulated annealing.

Systematic credit cycle risk of financial collaterals: modeling and evidence

According to the new capital adequacy framework (Basel II) finally adopted by the Basel Committee in June 2004 the eligibility of collaterals, especially financial collaterals, is extended in comparison to the existing rules. However, financial assets are valued conservatively in the credit context which suggests a strong correlation between collaterals and credit default rates. This paper discusses the impact of the dependency of financial collaterals and default rates on credit risk. Therefore, a general calculation framework for the loss rate of collateralized loans is given and an analytical solution for the valuation of financial collaterals is presented. Finally, the model is applied on empirical data of German insolvencies and German capital markets.