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Illuminating Engineering OLED for lighting – ready for the market?

Autor / Redakteur: Benjamin Krummacher, Karsten Diekmann, Stefan Seidel und Steven Rossbach * / Dipl.-Ing. (FH) Hendrik Härter

Flexible or rigid, transparent or glossy: With the OLED can achieve many designs. Compared to classical lighting many disciplines are required for light with OLED.

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Image 2: An OLED is flexible when the substrate is made of a flexible plastic.
Image 2: An OLED is flexible when the substrate is made of a flexible plastic.
(photo: OSRAM)

In the past years, along with the rapid spread of smart phones, OLED (organic light emitting diode) technology has matured from the pre-development stage and at the beginning of the millennium has left the early application phase behind. OLEDs have now established themselves in the displays of many hand-held products and the technology is currently being driven forward in wide-area displays for tablets and televisions.

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OLEDs supplement classic light emitting diodes

This success story is also now being pushed forward for use in general lighting applications and the automotive sector: OLEDs supplement classic light emitting diodes (LEDs) as an additional light source in so-called solid state lighting, abbreviated to SSL, meaning efficient lighting with light sources based on semiconductors. A modern AMOLED display consists of many tiny OLED pixels, but for lighting purposes, luminous surfaces from ten to several hundred square centimetres in size are standard.

Wide-area OLED light sources can be produced in differing variants and can be opaque, reflective or transparent. The components have an extremely low thickness of approximately 2 mm and the luminous surfaces can also be designed to be pliant and flexible. The production of wider luminaires with OLEDs will lead to cost savings because the light sources with their wide light characteristic are inherently similar to luminaires, and components such as light guides or diffusers and complex lamping scenarios are then no longer required.

An OLED consists of several organic semiconductor layers between two electrodes, at least one of which is transparent. Image 1 shows two different OLED types: A component with single-sided light distribution (left) and a transparent component emitting light in two directions (right). During OLED production the organic layers are applied consecutively onto a substrate which has been coated with a conductive coating , followed by an additional conductive electrode. A typical construction is based on a glass substrate coated with indium tin oxide (ITO) as a transparent anode and a light-proof metal layer as the cathode. If elastic or pliable substrate materials are used, then flexible or malleable components can be produced (image 2).

How the color of an OLED is generated

The stack of organic layers including electrodes is usually thinner than 1 μm, corresponding to one hundredth of a human hair, and during production, thermal evaporation processes are usually applied under high vacuum.

If DC voltage of a few volts is applied to the electrodes, the injected electrons and holes in the organic layers recombine and emit light of a specific color. The light color depends on the composition of the organic emitter material and can be specifically tuned via its chemical structure; combinations are also possible, emitting white light for example can be acccopmlished by combining red, green and blue emitters.

The principle of light generation, so-called electroluminescence, and the level of influence of the emission color characterise the SSL light sources and determine their efficiency: light emission, in contrast to standard lamps such as incandescent and halogen lamps, only occurs in the wavelength range perceived by the human eye. OSRAM has already produced white OLED laboratory samples with a luminous efficacy of 87 lm/W (lumens per watt). The efficiency of a typical halogen lamp in comparison is approximately 25 lm/W, and energy saving lamps have around 55 lm/W. The brightness of OLEDs is seamlessly dimmable and their design and manufacture are mercury free.

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OLED as a design object

OSRAM has recently developed the prototype of a hand-held application that exploits the combined characteristics of transparent and reflective OLEDs. As with a fold-out mobile phone, transparent and reflective OLEDs are positioned one above the other and bring together their brightness in the function of a searchlight. If the transparent OLED is folded upwards it can be used for face lighting, and the other switched-off OLED functions as a make-up mirror. The transparent OLED can also be used as a reading light. If an image printed on a gecko foil for example is slid between the OLEDs in their folded-in-state, then a portable image holder or personalised electronic item is the result.

OLED for gerneral lighting – an all market acceptance

Essential technical challenges that have to be resolved before wide use for general lighting is possible include a further increase in efficiency, the exploiting of cost reduction potential in production processes and materials used and the further scaling of the luminous surfaces. One factor that limits the size of the luminous surface is the limited lateral conductivity of currently used transparent electrodes.

Apart from this and an analog to the base-socket concepts for existing forms of technology such as E14 or E27 incandescent lamp sockets, it is also a matter of identifying and establishing solutions for the simple integration of wide-area OLED components into various lighting applications. Dr. Karsten Diekmann, Head of OLED Product Development at OSRAM: "Existing infrastructures are not completely designed for OLEDs. At OSRAM we're working according to a concise roadmap on all market acceptance topics so that OLEDs will be completely competitive to the end of this decade and so that they're able to establish themselves as part of general lighting."

As part of the OLYMP project, subsidised by the Federal Ministry for Education and Research, OSRAM is cooperating on such aspects with established companies such as the LED connection specialists BJB and the luminaire producer Trilux. Trilux is also busy analyzing how flat OLED luminaires in general lighting can be used both functionally and effectively. The luminaire designer Michael Jurisch has presented a suspended luminaire with a cord pull system and three levels of OLED segments to achieve lighting conditions that can be adapted optimally to current situational needs.

To enable the early development of new forms of luminaires, OSRAM will be presenting the next OLED component generation at the leading trade fair for the lighting industry, Light & Building 2014.

OLED find one's way into automobile

The tiles, with a size of 12 cm x 12 cm, will have an efficiency of 75 lm/W, a service life of 18,000 h and a luminance of 3000 cd/m². While costs, efficiency and operating duration are the three dominating key parameters for market penetration in general lighting, light design is of significant importance for future use in automotive applications. Just a few years ago, manufacturers concentrated on the design of the vehicle itself, but most brands now have their branded light appearance. Until now LEDs were in focus, for example with dynamically flowing or highly striking point-based daytime running lights, and today OSRAM LEDs are used in 50 per cent of all new vehicles. OLEDs will link up to this and lead to other unique luminous characteristics. Because most eye-catchers with automotive lighting applications are wide-area lights such as indicators, tail lamps and interior lighting, the suitability of OLEDs for such applications is obvious.

Integration into cars by a least 2016

Resistance to temperature must be improved though before bringing organic light-emitting diodes into automotive applications, and in order to fulfill the specifications of manufacturers for use in vehicles. A component, for example, used in tail lights must be able to resist temperature peaks of 85 centigrade and more. This was only the case for two or three hours with OLED products for lighting applications introduced in 2009, but since that time, OLEDs have progressed rapidly, and with OSRAM this parameter now consists of several thousand hours.

This level of reliability was achieved by using more stable materials and a sophisticated arrangement as part of a highly complex organic thin-film system. The aim is to continue optimizing automotive OLEDs to enable their integration into cars by a least 2016, but even now OLEDs can definitely become a part of planning for the next generation of vehicles.

In September this year, OSRAM presented a concept study for a tail lamp that displays completely new design options with OLEDs. The extremely flat construction needing neither heat sink nor additional optic enables installation that saves space and weight and that decisively influences chassis construction.

Brand-typical look of light of a car

A further feature of the concept is segmented OLEDs, where the luminous surface of an OLED is split into three areas that can be independently controlled to increase the level of functionality. For rear light functionality, only a third of the segments of a total of 10 OLEDs are needed to achieve the required brightness. If all 30 segments are switched on then even the brightness stipulations of the ECE (Economic Commission for Europe) for braking light are in compliance.

The segmentation also enables individual signatures such as greeting scenes to be displayed that in turn generate a recognition factor. In addition to both rear lights and brake lights, indicators were also implemented exclusively with OLEDs. Use of OLEDs in vehicles will not only be driven by the design factor on the exterior but also in vehicle interiors. The new possibilities made available by wide-area light sources will certainly characterise the brand-typical look of light of a car.

* Benjamin Krummacher, Karsten Diekmann, Stefan Seidel and Steven Rossbach work for OSRAM in Munich.

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