If silicon-based LEDs represent the state of the art in lighting, OLEDs will be the next generation. The article gives an overview on what is possible with this type of illumination.
Luminescent carpets and ceilings. Windows that provide bright daylight even when it's dark outside. Glimmering luminescent clothes. When designers and scientists start talking about organic light emitting diodes (OLEDs), their imagination knows no bounds. Indeed, the future holds in store infinite ways of using OLEDs. But even today, these ultra-flat light sources have already managed to raise light to a brand new level. So how are they different from existing light sources? OLEDs are surface light sources, rather than spotlights. Just a mere 1.8 millimeter thick, OLEDs diffuse a warm, pleasant and homogeneous light over the entire surface. More precisely, OLEDs are composed of ultra-thin layers of organic semiconductors and color molecules, which are embedded between two layers of glass. When a voltage is applied, the organic layers begin to light up.
Manufacturing OLEDs involves one of the most high-tech processes around and can be compared with the manufacture of PC chips. It begins with an extremely thin, transparent and electroconductive oxide layer composed of indium tin oxide (ITO) being applied to a glass slide. This layer forms the anode. The subsequent stages of the process involve the application of the organic layers, although the term 'organic' has nothing to do with vegetables or animals in this case. Contrary to LEDs, OLEDs are manufactured using chemically organic material – in other words, carbonate-based components. The final stage incorporates an aluminium cathode, whose prime function is ensuring that the OLED works like a mirror when it is turned off. When voltage is now applied to the OLED, electricity flows from the anode to the cathode and makes the layers in the middle light up. The color that we see depends on the voltage, as well as – more importantly – the material that has been embedded in the organic layers.
Fig. 1: Structure of an OLED. Surface light sources versus spotlights
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The main difference between LEDs and OLEDs is that OLEDs, contrary to LEDs and other common light sources, diffuse their light from the source right across the entire surface, hence the term 'surface light sources'. Light from OLEDs is characterized by a natural softness, whilst diffusing nicely and not dazzling. Due to their extreme flatness – below 1.8 millimeter –, OLEDs can be integrated in many different surfaces and products and allow light sources to be designed in a wide range of shapes and sizes. OLEDs can be fully dimmed right across the spectrum – less electricity means less light, more electricity makes it brighter. No sophisticated electronics are required – standard potentiometers are all that they need.
OLEDs are available in virtually all colors, not to mention in the sphere of high quality white light. Alongside standard shapes, companies such as Philips also provide sophisticated shapes as well as structural OLEDs. As a result, dreams of technical light designs can be given wings very easily. In terms of size, OLEDs have also been gaining ground in recent years, with dimensions exceeding 140 cm² becoming increasingly common. However, for many consumers, the homogeneity of the light is more important than size. With big OLEDs, the "halo" effect (the OLED's darker center and lighter edge) is the restricting factor. This effect occurs because the electricity is always fed in at the edge, with the resistance in the OLED causing the power of the light to weaken as it reaches the center.
Figure 2: OLEDs generate homogenous light from a flat light source