Ralph Paetzold

Permeation rate measurements by electrical analysis of calcium corrosion

Highly sensitive permeation measurements are crucial for the characterization and development of polymeric substrates for flexible display applications. In particular, organic light-emitting devices require substrates with extremely low permeation rates for water and oxygen. Here we demonstrate a concept for measuring ultralow permeation rates. The amount of oxidative degradation in a thin Ca sensor is monitored by in situ resistance measurements. The benefits of this technique are demonstrated for polyester foils with single- and double-sided barrier coatings. A sensitivity limit is imposed by the quality of the encapsulation. The resulting base line contribution to the water vapor transmission rate of a glass reference is below 10−6 g/m2 day at accelerated test conditions.

Performance of flexible polymeric light-emitting diodes under bending conditions

Organic light-emitting diodes were fabricated on a 125-μm-thick polyethylene terephthalate substrate covered with 100 nm indium tin oxide. The luminance–current–voltage performance and the emission spectrum of the devices are investigated in the bent state under mechanical stress at different bending radii. Down to a curvature of 15 mm, no significant decrease in the device performance is found compared to the relaxed state, as well as to conventional devices on glass substrates.

Impact of Joule heating on the brightness homogeneity of organic light emitting devices

Joule heating and its impact on the brightness homogeneity are investigated since the luminance distribution is a key issue in large area organic light emitting diodes. In addition to previous reports, it is also important to consider the device temperature as a main factor for determining the luminance homogeneity. At a luminance of 1000cd∕m2 the active area reaches an average temperature of 40.6°C and a peak of 46.2°C. The increased device temperature is leading to higher local current densities resulting in a reduced brightness homogeneity. Modeling confirms these results and can be used for further device layout optimization.

High-sensitivity permeation measurements on flexible OLED substrates

We describe a novel method to measure permeation rates for oxidizing agents with very high sensitivity. The technique is based on monitoring the resistance of a degrading Ca sensor in situ, inside a climate chamber. A sensitivity limit below 10-6 g/m2 day is reported for accelerated measurement conditions of 38°C and 90% relative humidity. The benefits of the method are demonstrated for single- and double-sided barrier foils, and the temperature and humidity dependence of the transport through PET is analyzed in detail. The method is also applied to obtain permeation rates for a barrier-coated substrate after as well as during bending. Theoretical simulations are used to evaluate the influence of a defect-dominated transport mechanism on the experimental results and to model the time evolution of the concentration profile in a double-barrier stack. Implications for the development of barrier-enhanced substrates for flexible OLED applications are discussed.

Organic Thin Film Devices for Displays and Lighting

Organic materials can be used for fabrication of, e.g., electronic circuits, solar cells, light sensors, memory cells and light emitting diodes. Especially organic light emitting diodes (OLEDs) are increasingly attractive because of their huge market potential. The feasibility of efficient OLEDs was first shown in 1987 [3]. Only about ten years later the first product, a display for car radios, entered the market. Today monochrome and full colour OLED-displays can be found in many applications replacing established flat panel display technologies like TFT-LCDs. This substitution is a consequence of the outstanding attributes of OLED technology: Organic light emitting displays are self-emissive, thin, video capable and in addition they show a wide temperature operation range and allow a viewing angle of nearly 180 degree in conjunction with a low power consumption. As performance has steadily increased over the last years, today OLEDs are also under investigation as next generation light source. In contrast to inorganic LEDs, they can be built as flat 2-dimensional light sources that are lightweight, colour tunable, and potentially cheap. This will open up new degrees of freedom in design leading also to completely new applications. In this contribution we will have a brief view on the history of organic electroluminescent materials before we introduce the basic principles of OLEDs with a focus on the physical processes leading to light generation in thin organic films. Along with an overview of different concepts and technologies used to build OLEDs, the current status of OLED development will be illustrated. The last part focuses on the challenges that have to be overcome to enable a sustainable success in the display and lighting markets.

OLED lighting based on white broadband copolymer emitters

OLEDs for lighting applications are gaining increasing attention due to the possibility to produce large area, 2-dimensional light sources. In contrast to the existing technology e.g. based on white inorganic LEDs this offers a completely new freedom in design for applications of next generation lighting. Today, different approaches to achieve white broadband emission for organic lighting solutions are investigated ranging from devices with blue emission in combination with conversion layers to RGB-color by lateral patterning with the support of active color tunability. Within this contribution we present results of broadband emitting copolymers to achieve white emission. New requirements arising from the shift of OLEDs in a display configuration to those for lighting applications are discussed with focus on the electro-optical behavior. Furthermore, we describe challenges that result from using large active areas and investigate ways to improve large area lighting tiles.

High Efficiency PPV-Based Polymer Light Emitting Diodes With Cs 2 CO 3 Cathode

The thin electron injection layers between the cathode and the light emitting polymer layer in polymer light emitting diodes (PLEDs) have been shown to have a big impact on the final device performance. Usually, in PLEDs low work function metals like Ba, Mg or Ca are used to reduce the energy barrier between the cathode and the polymer thus providing a better electron injection from the cathode. Also salts like LiF, NaF, Cs 2 CO 3 and CsF have recently been shown to function as electron injection layers in light emitting devices. From these, especially caesium carbonate (Cs 2 CO 3 ) results into high efficiency diodes both as a solution processed electron injection layer in PLEDs, as well as an n-dopant in the electron transport layer in vacuum deposited small molecule based OLEDs. The functional mechanism of Cs2CO3 as a pure interlayer is not yet fully understood. The proposed mechanisms include the n-doping of the organic layer with Cs 2 CO 3 , the thermal decomposition of Cs 2 CO 3 and following formation of caesium metal or the formation of an n-doped CsO 2 layer. In this study the phenomena resulting from the combination of a hole-dominant alkoxy-phenyl-substituted poly(phenylene vinylene) (PPV) based light emitting polymer with a highly efficient electron injection layer of Cs 2 CO 3 in light emitting diodes has been investigated. As a result, diodes with about 35 % higher efficiency were achieved with PPV-Cs 2 CO 3 structure in comparison to the traditional PPV-Ba structure. Additionally to the increased efficiency, also the lifetime of the Cs 2 CO 3 -diodes is comparable to the Ba-diodes implying that the long-term stability of the diodes is not affected by the optimized Cs 2 CO 3 -cathode. The strong increase in the electron injection of the Cs 2 CO 3 diodes is apparently caused by a highly conductive, n-doped layer resulting from the charge transfer reaction between Cs 2 CO 3 and PPV, where the magnitude of the reaction and resulting effects strongly depend on the amount of the applied Cs 2 CO 3 . The conclusion of the n-doped layer can be drawn from the LIV, impedance and photoluminescence measurements of the diodes with Ba and Cs 2 CO 3 cathodes before, during and after electrical stressing.

69.2: Laser-Microstructuring of Cathodes for OLED-Displays

We introduce an innovative approach for structuring the metal cathode of passive matrix OLED displays based on laser ablation of the thin metal film. Laser patterned displays are presented and compared to conventional photolithographically processed devices with regard to their LIV-characteristic and rectification ratio.

Flexible polymeric light-emitting diodes

Flexibility is one of the most frequently mentioned advantages if organic light emitting diodes are compared to other display technologies. In this contribution we show how the different functional layers respond to applied mechanical stress. To characterize the intrinsic flexibility of the stacked layers in an organic light emitting diode separately, samples with anode and cathode layers on flexible plastic substrates are investigated separately first. We observe that the ITO can withstand more than 30 000 bending cycles, concave as well as convex, down to a radius of curvature of 8 mm without apparent damage. Furthermore, the operational characteristics of completed flexible organic light-emitting devices built on indium-tin oxide coated poly(ether sulfone) under single bending cycles are investigated. Performance data taken at 15 mm radius of curvature show no influence compared to the non-planar conditions.