OLEDs gain more and more interest in the field of lighting applications. The OLED technology provides striking
advantages and covers completely new application fields offering a new freedom in design for next generation lighting.
Large area OLEDs might act as a 2-dimensional light source which is thin, flat and lightweight generating diffuse, nonglaring illumination.
In the first part of our report we investigate small scale inhomogeneities of polymer based OLEDs. Devices were
monitored during operation by taking pictures of the active area at constant periods of time. These pictures were
analyzed by a software tool with respect to the occurrence and evolution of defects. Initially induced inhomogeneities are
growing and dominate the performance with increasing operation time. Within the error margin of the setup no
additional spots are generated during operation.
The voltage drop inside the ITO anode due to a high resistivity plays an important role for the brightness homogeneity of
large area devices. The voltage drop causes a brightness fall-off towards the center of the device. It is maintaining with
increasing average current density and luminance, respectively. At a brightness of 1000cd/m2 the deviation at the center
exceeds 30%. The homogeneity of luminance is improved by incorporation of additional metal lines on the anode layer.
The best results were achieved with 200nm thick aluminum structures with a pitch of 1mm and a width of 60μm of each
line. At an average current density of 45mA/cm2 the decay towards the center of the device is only half of the decrease
without any additional metallization.
The transport of charge carriers in polymer-based Organic Light-Emitting Diodes (OLEDs) as determined by the hopping mobility is an important factor influencing both lifetime and performance of OLED devices. It is strongly dependent on the density and energetic distribution of trap states in the polymer material. Especially in multi-component copolymers single functional groups can act as hole or electron traps determining the optical and electrical characteristics of the device. Transient measurements of the charge carrier mobility together with steady-state current-voltage characteristics are used to investigate the behavior of three blue polyspiro-based light-emitting polymers (LEP) with varying compositions. The first material is a simple homopolymer, the second adds a hole transporting component which is copolymerized into the backbone and the third, most complex, additionally includes a blue chromophore. With some of the added components acting as charge carrier traps the electrical behaviour of the diodes changes significantly.
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.
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.
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.