Jeanine Chrobak-Kando, Business Development Manager for LED lighting at Verbatim considers how OLEDs are evolving as complementary sources of indoor lighting
Organic light emitting diodes (OLEDs) now appear in a host of commercial electronics applications, most commonly in displays for radio displays in high-end cars, tablet PCs, mobile phones and other consumer gadgets.
An understanding of OLED technology has been with us for over half a century since researchers at Nancy-Université, France, first observed electroluminescence in organic materials in the 1950s. The affect was only apparent with relatively high voltages applied to the materials. The technology that’s currently employed is attributed to Mr. W. Tan and Mr A. VanSlyke and was invented while they were working at KODAK. The breakthrough was to produce a technology that operated from low voltages and was relatively economical to manufacture. Today’s OLED construction is based upon a KODAK patent and in November of 1997 that Touhoku-Pioneer started the first mass production of OLEDs, initially for car dashboard displays.
The first OLED screens in personal digital assistants (PDAs) appeared in 2004 and by 2008 consumer electronics companies were demonstrating large-screen televisions with high resolution, high contrast ratio and peak luminance of 600cd/m2.* The drivers of OLED development in these applications have been the need to reduce costs, reduce weight, reduce power consumption and provide a better user experience through improved contrast and viewing angle. But what about OLEDs as general light sources? To understand the potential for OLEDs in lighting, let’s start with a little technical background.
How OLEDs work
OLEDs work by sandwiching a layer of organic material between two electrodes, an anode and a cathode, and depositing the whole thing onto a substrate, typically glass or plastic. When a low DC voltage is applied to the electrodes (positive to the anode, negative to the cathode), light is emitted as electrically charged particles (holes and electrons) which combine within the organic film. The characteristics and intensity of light emitted, and how it is extracted from the OLED assembly, determine its suitability for lighting applications. One challenge for OLED development engineers has been how to create large panels. In general, the larger the panel that can be printed in one process, the lower the cost of the OLED light – there’s no need to fix a lot of individual panels together to achieve sufficient luminous intensity. Processes and materials limitations determine how large a panel can be made. Another important consideration is the operating life of OLEDs. The higher the light output, the more limited the life of the device, so the most appropriate trade-off for each application has to be made.
OLEDs can be made in one of 3 ways, as shown in Figure 1.
Figure 1: The striped pattern maximizes light output and makes colour tuning possible
Some leading manufacturers favour the multi-layer type on the left, and it’s the simplest process. The tandem construction is a little more complex but delivers more intense light output and larger formats can be created. Tandem construction is sometimes combined with the multi-layer structure. The approach taken by Mitsubishi Chemical, which markets OLEDs under the Verbatim brand, is shown in the centre of Figure 1. Narrow stripes of adjacent red, green and blue OLEDs are deposited. This method of production is more complicated but yields two advantages. Each layer within an OLED has a different refractive index. This causes internal reflections within the device that limit out-coupling – the proportion of light generated that reaches the outside environment. The RGB stripe construction is the most effective in maximizing the light output for a given energy consumption. The second advantage is that the OLED is not only dimmable, as with other types of construction, but also colour tunable and white-tone tunable. Dimming does not affect colour.
Manufacturing processes for OLEDs seek to maximize light output and minimize pixel defects. To achieve these aims, in place of the dry process of vapour deposition of the layers that make up the OLED, Mitsubishi Chemical uses a wet process. This avoids defects in the OLED structure when microscopic particles of foreign material are encountered. The liquid simply flows around the offending particles, reducing the roughness of the substrate.
Desirable characteristics for OLED lighting
As mentioned earlier, OLEDs need a large emission surface to be suitable for lighting applications so that they emit sufficient light to be useful. The quality of light, usually expressed as its Colour Rendering Index (CRI) is important in rendering colours accurately. Low power operation, meaning high efficiency in converting electricity to light, is vital in a world focused on reducing energy consumption and CO2 emissions. Also, in common with the requirements of OLED displays, OLED lights should not contain hazardous substances, need to be simple to operate, and must exhibit fast on/off response.
Shining the light onto eight different coloured tiles, numbered R1 to R8, and analyzing the spectrum of the light reflected from the tiles determines the CRI of a light source. Perfect colour rendition equates to a CRI of 100 for each colour. In general CRI is quoted as the average figure across all of the test colours, the CRI Ra figure. R9, the colour red, is not used in the calculation of the general rendering index value Ra, but is important within the spectrum of human vision so a high R9 figure is also desirable in OLED lighting. The wavelength of light above R9 (approximately 650nm) contributes little to human vision. Today’s OLED panels exhibit a CRI of 84 at R9 and a CRI Ra of greater than 80.
CRI or CQS?
CRI was originally developed as a way of evaluating the light quality from incandescent lamps. An alternative method of evaluating LEDs and OLEDs is sometimes used. Called the CQS system, this uses 15 coloured tiles, rather than the 8 used in the CRI system.
What’s available today?
The latest dimmable, colour tunable and white tone tunable OLED panels are available in sizes up to about 140mm x 140mm, as shown in Figure 2.
Figure 2: OLED panels up to 140mm square are dimmable, colour tunable and white tone tunable
They offer luminance (light output) of approximately 1000 cd/m2 at a colour temperature of 3000k (within the “warm white” category), enhanced by a light extraction film on the luminous surface. Power consumption is about 2 Watts. Panels are typically between 3.6 and 8.65mm thick and have an operating life of over 8000 hours. Operating life, which equates to over 5 years at 4 hours-per-day before the output has fallen to 70% of its initial value.
The white tone is tunable from 2700K, a typical “warm white” figure, to about 6500K, equivalent to bright sunlight.
Using a simple 3-channel electronic controller located on the back of each panel, the colour can be tuned virtually instantaneously. Using this feature, together with dimming, the emotional impact of a lighting scheme based on OLED panels can be changed to reflect the mood required for the environment. For example, bright, white light may be desirable in the morning but more subdued, relaxing lighting with muted colours may be preferable towards the end of the day. The technical protocols for RGB colour tuning (DMX) and dimming (DALI) are well established and low cost controllers are widely available. Panels are easily calibrated and matched using the controllers to compensate differences between panels cased by manufacturing process variations. In the near future, there is an expectation that the DALI protocol will be extended to include all aspects of colour control, as well as dimming functions.
The possibilities for creative lighting schemes in domestic and commercial environments are unlimited. Figure 3 shows a Verbatim exhibition display of OLED lighting as a simple example.
Figure 3: A Verbatim dispaly designed in collaboration with Uchihara Creative Lighting Design Inc. (Photography by Toshio Kaneko).
OLEDs are not yet ready to replace general indoor lighting, as has been suggested by some enthusiasts. However, their development has reached the stage where they complement ambient lighting and task lighting, to produce beautifully balanced lighting schemes both in places of work and in the home. Their potential in retail environments and other public spaces is unlimited and their low power requirements meet the demands of the most ardent environmentalists.
*cd/m2 is Candellas per square metre, a measure of luminance. A common candle emits light with an intensity of around 1 Candella.