OLEDs have unique advantages compared to LEDs for flexible large-area and semi-transparent light sources. However, it's biggest enemies are water and oxygen. Therefore OLEDs need to be encapsulated.
They are already used in mobile phones: organic light-emitting diodes, called OLED. They provide natural colors, a very high contrast and low electric power consumption in displays since no background lighting is needed.
Current research activities concentrate more and more on the use of organic light-emitting diodes for general lighting. OLEDs offer unique properties compared to inorganic LED devices. The OLED itself is an area light source, which can be scaled to larger areas > 100 cm² and more. Moreover, the OLED offers new design possibilities since it can be manufactured as transparent device. Special electronic properties like color variability or the use of the light area as touch sensor for switching-on/switching-off or dimming are some of the advantages of the OLED. The luminous efficiency of commercial OLED panels is already comparable to compact fluorescent lamps and the difference to the luminous efficiency of inorganic LEDs decreases continuously.
A particularly interesting aspect of the OLED is that – as thin film setup – it can be applied even to flexible substrates. That enables the lightweight construction of OLED modules for the integration into furniture, windows or even clothing. Flexible OLED provide completely new possibilities for designers to create exceptionally shaped light sources.
Water and oxygen are enemies for OLED
A big challenge on the way to the OLED on flexible substrates is its protection against corrosive gases – particularly water vapor and oxygen. Glass plates, which have been used as substrate for OLED so far, are impermeable to water vapor and oxygen, whereas this is not the case with flexible substrates, mostly plastic films. Typical polymer films, e. g. polyethylene terephthalate – PET, allow a diffusion of 1 g to 50 g water per day on a surface of one square meter. But just a few milligram of water on the same area can damage an OLED irreparably. In order to achieve a sufficient long lifetime of 10 000 hours, only 10-6 g, i. e. 0,000001 g water per day and square meter may impact the OLED, which corresponds approximately to a drop of water per month on a surface as large as a football field.
Abovementioned, plastic films are coated with barrier layers or multi-layer systems in order to encapsulate flexible OLED devices effectively against water vapor and oxygen. Afterwards, they are used either as substrate for the setup of the OLED or are bonded as encapsulation films to the OLED using a barrier adhesive.
Transparent metal oxide layers or multi-layer stacks based on alternating oxide layers and polymer layers are used as coating on the polymer webs to achieve a low water vapor permeation. The lowest water vapor permeability is achieved with multilayer systems. An essential property of the layer systems is that they cover and compensate surface irregularities and defects on the substrate surface as shown in image 1.
A water vapor permeability of 5 x 10-5 g/(m²d) was achieved at room temperature and 50% humidity in a productive roll-to-roll process using a system of two zinctinoxide (Zn2SnO4, ZTO) barrier layers created by reactive sputtering and an ORMOCER interlayer. The term ORMOCER describes an inorganicorganic hybrid polymer, which is synthesized in a sol-gel process and whose properties can be tailored according to the respective application. The system was developed within a collaboration of the Fraunhofer Institutes FEP, ISC and IVV.
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