Efficient white OLEDs are becoming attractive as large area light sources for illumination and in future also for general
lighting. We discuss device concepts for white OLEDs and their potential to achieve high efficacy and good lumen- and
color-maintenance at the same time. We focus on OLEDs using a combination of fluorescent blue and phosphorescent
red and green emitters (hybrid OLEDs). Hybrid OLEDs have high efficacy and lifetime in the white to warm white color
region (color points B and A on the black-body-curve). Near illuminant A efficacy values of 28-29 lm/W without optical
out-coupling can be achieved with a hybrid OLED. The external quantum efficiency (EQE) is 14%. A typical color
rendering index (CRI) is 84. Recent results for monochrome OLEDs and for hybrid OLED stacks are presented.
Organic light emitting diodes (OLEDs) provide potential for power-efficient large area light sources that combine
revolutionary properties. They are thin and flat and in addition they can be transparent, colour-tuneable, or flexible. We
review the state of the art in white OLEDs and present performance data for three-colour hybrid white OLEDs on indexmatched
substrates. With improved optical outcoupling 45 lm/W are achieved. Using a half-sphere to collect all the light
that is in the substrate results in 80 lm/W. Optical modelling supports the experimental work. For decorative applications
features like transparency and colour tuning are very appealing. We show results on transparent white OLEDs and two
ways to come to a colour-variable OLED. These are lateral separation of different colours in a striped design and direct
vertical stacking of the different emitting layers. For a striped colour tuneable OLED 36 lm/W are achieved in white with
improved optical outcoupling.
Today, organic light emitting diodes are used in small to medium displays in portable electronic equipment like MP3
players and mobile phones. Their thin form factor, together with good readability due to low angular dependence of the
emission makes them attractive for these applications.
The rapid progress in the last years has lifted the performance of OLEDs to a level where one can seriously start to
consider applications in lighting markets. Whereas it is obvious that first applications will be in less demanding niche
markets, clearly the most interesting target is the general illumination market. In this report, first applications
requirements will be described, followed by a brief review of state of the art monochrome OLEDs. The main part deals
with the various ways in which monochrome devices can be combined into white ones, giving examples of existing
solutions. The conclusion is that for the white OLED design, there no clear winner yet. Given the rapid progress in
material and device development, one can expect that within a few years white OLEDs will be available which can start
to penetrate the general lighting market.
The use of organic light-emitting diodes (OLEDs) for large area general lighting purposes is gaining increasing interest during the recent years. Especially small molecule based OLEDs have already shown their potential for future applications. For white light emission OLEDs, power efficiencies exceeding that of incandescent bulbs could already be demonstrated, however additional improvements are needed to further mature the technology allowing for commercial applications as general purpose illuminating sources. Ultimately the efficiencies of fluorescent tubes should be reached or even excelled, a goal which could already be achieved in the past for green OLEDs.1 In this publication the authors will present highly efficient white OLEDs based on an intentional doping of the charge carrier transport layers and the usage of different state of the art emission principles. This presentation will compare white PIN-OLEDs based on phosphorescent emitters, fluorescent emitters and stacked OLEDs. It will be demonstrated that the reduction of the operating voltage by the use of intentionally doped transport layers leads to very high power efficiencies for white OLEDs, demonstrating power efficiencies of well above 20 lm/W @ 1000 cd/m2. The color rendering properties of the emitted light is very high and CRIs between 85 and 95 are achieved, therefore the requirements for standard applications in the field of lighting applications could be clearly fulfilled. The color coordinates of the light emission can be tuned within a wide range through the implementation of minor structural changes.
We present results and a discussion of highly efficient polymer Light-Emitting Diodes (polymer LEDs, PLEDs). The external quantum efficiency in current standard devices reaches up to 2-4% only. We have explored two routes to enhance this value. In the first route, PEDOT/PSS is replaced with a novel anode or hole injection layer. The efficiency with some Light Emitting Polymers (LEP) is improved significantly, resulting in an efficacy of 35 cd/A for a yellow emitting poly-(para-phenylene-vinylene) and 20 cd/A for a blue emitting poly-(spirobifluorene). We attribute the major improvement compared to standard devices, where about 10 and 5 cd/A are obtained, respectively, to a combination of improved exciton formation efficiency and light out-coupling efficiency, and to less quenching of the radiative decay under actual device operating conditions. In the second route, we developed a new host polymer with high triplet energy such that transition metal-based green-emitting phosphorescent dyes can be used without significant back transfer of triplet excitons to the polymer host. First results using this system showed about 25 cd/A using a soluble green Ir-based emitter. Importantly, all data are obtained in a standard two-layer device of a hole transport/injection layer and the LEP.
This paper reports the synthesis as well as electrochemical and optical characterization of six highly luminescent TPA- based molecules, TPA-X-TPA, where X represent the vinylene and vinylene-arylene-vinylene units and examines their use as the light-emitting dopant of a composite emission layer in trilayer OLEDs. The device configuration ITO/HTL/EML/ETL/Ca/Al is based on a double heterostructure, where HTL is the hole-transport layer, ETL is the electron- transport layer, and EML is a composite film of the TPA-X- TPA dopant dispersed in an ET matrix. This composite EML is prepared by a vacuum codeposition technique. A variety of TPA-X-TPA doped OLEDs were fabricated and characterized by EL spectra and luminous efficiencies. We found that the EL- emission arise from the dopant molecule, blue EL was obtained with a luminous efficiency of 0.3 1m/W and a luminance of 50 cd/m2 at 7V device voltage. In comparison with 1,4-bis(4'-diphenylamino-styryl)-2,5- dimethoxybenzene as the dopant molecule strong green EL was observed.
The signal and noise propagation through a generalized x-ray image intensifier/TV camera chain is modeled in terms of the modulation transfer function, two dimensional noise power spectrum, and detective quantum efficiency. The model covers effects of energy dependent x- ray absorption in the cesium iodide entrance screens, K-fluorescence and K-reabsorption, electron optics, output screens, lenses and also sensing with electronic pick-up tubes (plumbicons) or CCD sensors. Several Philips x-ray image intensifiers in combination with plumbicon TV-chains and with laboratory type CCD cameras are compared with the model. Based on the model results we further present an image simulation tool. The image simulation allows a direct evaluation of the impact of the individual components of the imaging chain on image quality. It is shown that future high resolution CCD systems can exhibit a superior image quality as compared with electronic pick-up tube systems.