by
Geoff Archenhold
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After returning from a recent European workshop on Solid-State
Lighting organised by the Joint
Research Centre, a Directorate
General of the European Commission it is clear that Europe is keen to support and promote the adoption of LED and OLED lighting in the future. Although, many of us within the lighting industry will not recognise the name of the JRC the advice they provide will become a significant factor in the adoption of LEDs and SSL throughout Europe. In a nutshell the JRC is a research based policy support organisation providing the scientific advice and technical know-how to support
EU policies ie; they will help EU commissioners, regulators and eventually partner Governments to determine the size and scale of the future lighting (and hence SSL) industry through policies, regulation and standardisation.
During the workshop it became clear that the emphasis was not on “when” LEDs were going to displace conventional light sources but more on “how” they were going to do it. Many of the
LED fixture manufacturers and lighting designers presented their latest products or projects and were demonstrating why LEDs provide extra functionality that increases quality of life through a variety of advances such as correlated colour temperature control and digital functionality.
Therefore, I thought I would
What are Organic Light
Emitting Diodes (OLEDs)?
An Organic Light Emitting Diode is, as the name refers, an electroluminescence or light emitting device based on organic compounds whose molecules in-turn are based on carbon. OLEDs are similar to electroluminescent lighting, in which a sheet of material is excited using electrical energy so that it emits light. Depending on the organic material used within an OLED the colour of light emitted can change and today virtually any colour within the visible spectrum can be reproduced including white.
OLEDs first attracted a great deal of attention from researchers in the 1960’s as they showed potentially high light output efficiency and the ability to generate a wide variety of colours. However, it was not until the development of new “small molecule” materials and structures in 1987 by researchers at Eastman Kodak laboratories that OLEDs became a potentially useful light source. In
1990, a new type of OLED device was discovered by a research group at Cambridge University that used conjugated polymers and since then, OLED research has achieved, in terms of device dedicate this month’s article to the future trends of Solid-State Lighting including the possibilities of new technologies such as Organic
Lighting Emitting Diodes (OLEDs). efficiency, levels that exceed both incandescent and Halogen lighting.
Therefore, OLEDs essentially exist in two forms:
• Small molecule – SMOLED
• Polymer – PLED or Light
Emitting Polymer – LEP
The production of small-molecule
OLED devices require specialist vacuum deposition manufacturing capabilities which makes the production process more expensive than other processing techniques.
Since SMOLED are typically produced by evapourating layers of low molecular weight materials on glass substrates, they are generally not flexible. However, “long chain” polymer-based OLEDs use a thin film for full-spectrum colour lighting and require a relatively small amount of power for the light produced. PLEDs do not require a vacuum in order to be produced and the emissive materials can be applied directly on the substrate by a technique derived from commercial inkjet printing. This offers the potential for polymer-based OLEDs to be produced inexpensively and on substrates that can also be flexible offering exciting design opportunities.
Generally, an OLED device is no more than 100 to 500 nanometers thick or about 200 times smaller than a human hair enabling them to be easily flexed as shown in Figure 1.
Figure 1: A picture of a flexible OLED display
The Structure of an Organic
Light Emitting Diode
OLEDs typically employ either two or three layers of organic material within a device structure.
Figure 2 shows a three layer
OLED device consisting of the following parts:
• Substrate (clear plastic, glass, foil) - The substrate supports the
OLED;
• Anode (transparent) - The anode removes electrons when a current flows through the device;
• Organic layers - These layers are made of organic molecules or polymers;
• Conducting layer - This layer is made of organic plastic molecules that transport charge from the anode;
• Organic Emitters layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made;
• Electron transport layer - helps transport electrons from the cathode to the emissive layer;
• Metal Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device.
As an electrical current is passed
Figure 2: A typical three layer OLED Structure from the cathode to the anode through the organic layers light is emitted and the intensity or brightness of the light depends on the amount of electrical current applied. As with LEDs, the more current passing through an OLED device, the brighter the light emitted.
OLEDs are now being commercialised in small-size displays used in devices such as mobile phones, PDAs, MP3 players and digital cameras. Although OLEDs had offered significant advantages over TFT displays which require backlighting, the technology has not taken off as the time taken to develop stable polymer materials with sufficient light output and lifetimes was longer than it took for
TFT technology to improve (and reduce in price) limiting OLEDs impact on the general displays market.
Although OLEDs have not had the anticipated impact within the displays market the materials technology developed may benefit the lighting industry in the long term. Again, this will depend on how quickly and successfully
LEDs will displace conventional light sources – if LEDs manage to displace CFL’s in a short space of time and achieve high volumes then potentially they could assign
OLEDs to niche lighting applications.
However, there is considerable investment in worldwide OLED research to create high-brightness, high efficiency and long life white
OLEDs for lighting and signage.
White light can be generated by several approaches including:
• The down-conversion of a blue emitting OLED by organic or inorganic phosphors. Similar to a White LED that uses a Blue LED with a Phosphor;
• The use of a stack of blue, green and red emitting layers to create a white output;
• The use of intrinsically white emitting layers. Superior light quality with very high Color
Rendering Index (CRI>90) can be realised.
OLED devices can also exist in three types these being Transparent, Top-emitting and flexible.
1. Transparent OLED : Transparent OLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate. When a transparent OLED is turned on, it allows light to pass in both directions.
2. Top-emitting OLED : Top-emitting OLEDs have a substrate that is either opaque or reflective.
3. Foldable OLED: Foldable
OLEDs have substrates made of very flexible metallic foils or plastics. Foldable OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage.
Potential Advantages of Organic Light Emitting Diodes as light sources
There are several advantages that
OLEDs possess in terms of being used as a light source including:
• Since OLEDs can be printed onto any suitable substrate using inkjet printer or even screen printing technologies, they can theoretically have a significantly lower cost;
• Printed OLEDs onto flexible substrates opens the door to new applications such as roll-up lighting and lighting embedded into textiles;
• OLEDs enable a greater range of colours, brightness, and viewing angle (see figure 3);
• OLED pixel colours appear

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Geoff Archenhold correct and unshifted, even as the viewing angle approaches 90 degrees from normal;
• OLEDs have large beam angles and can emit light over large areas providing diffuse and low glare lighting;
• Have the long-term potential to be energy efficient compared to traditional white light sources;
• They are low-voltage devices
(typically 2V - 10V);
• Lightweight;
•Rugged and vibration resistant.
Potential Disadvantages of Organic Light Emitting
Diodes as light sources
Unfortunately, as OLEDs are still a new technology there are a number disadvantages that will need to be resolved in order to ensure their future success within the lighting industry. The following are a few of the disadvantages of
OLED lighting:
• Low device efficiency;
• Uniformity of light emiting layers over large areas;
• Difficulty of high electrical currents evenly required over large areas;
• Low Luminance outputs not yet suitable for lighting;
• Poor operational lifetimes for various emitter materials;
• Poor storage life due to degradation on moisture and oxygen;
• Complexity of design due to limited material selections for electrodes;
• Low cost manufacturing technology for large OLED devices not established;
• Heat dissapation may also be a limiting factor;
• Degredation if exposed to UV;
• A new lighting infrastructure is required.
Currently, there is a tremendous amount of research work being undertaken by the major lighting manufacturers in order to overcome many of the disadvantages outlined above. Several noticeable advances in recent times include:
• Konica Minolta : In June 2006, they developed a small white
OLED panel with a world record power efficiency of 64 lumens per watt at 1,000 cd/m2 - a brightness which is appropriate for lighting applications. They achieve over 10,000 hours lifetime (defined as 50% initial intensity) by developing an ultra-high barrier film fabrication technology to enable high productivity, as well as coating technology that is derived through their experience of developing photographic film and display materials.
Konica Minolta and GE recently announced that they have signed a strategic alliance agreement to accelerate the development and commercialisation of OLED
(Organic Light Emitting Diode) devices for lighting applications.
The goal is to bring OLED lighting to market within the next three years.
• Yamagata University : The research team led by professor Kido has developed a white OLED light source with an efficiency of 48 lumens per watt at 1,000 cd/m2.
• Universal Display Corporation : In 2006, UDC developed a white OLED device with 24 lumens per watt efficiency at an output of 500 cd/m2 although the lifetime was not disclosed.
• Philips / Novaleds : In July
2006, the partnership announced a white OLED lighting panel
(shown in figure 4) obtaining 32 lumens per watt with a CRI of 88
Figure 3: Coloured OLED lighting panels from Philips at 1000 cd/m2 and an estimated lifetime of 20,000 hours.
• OLLA : In May 2007 the
European funded OLLA project developed a large area (15cm by 15cm) OLED tile with an efficiency of 25 lumens per watt at an initial brightness of 1,000 cd/m2 with a lifetime of over
5,000 hours.
The recent advances in white
OLED lighting research show considerable promise for the future of the technology especially
Figure 4: A high efficiency white OLED from Philips/
Novaled within lighting. Although it is predicted to take between 5 and
10 years for OLED lighting to start penetrating certain areas of the general lighting market its impact will be measured by how quickly the technology can get to market and whether the improvements and costs of standard LEDs proves too stiff a competition.
What is important for the lighting industry is that we do not begin to believe the hype which surrounds OLEDs at the current time. For example, at the recent
JRC conference many suggested that OLED lighting would be seen everywhere within the next few years and that we would even have OLED wallpaper enabling you to change the colour of a room instantly. Although not wishing to appear sceptical (as I do believe OLEDs will have a place in the general lighting market) I did ask the audience what would happen if I had OLED wallpaper at home and I wanted to hang a picture from the wall. The answer of course would be that I would break the OLED panel and probably receive a nasty shock from the high current required to power it. My point was that sometimes we need to be practical about new lighting technologies and their uses.
In terms of early application adoption for OLED technologies, I would suggest the following in no particular order:
• Emergency exit signs;
• Signage that requires even and consistent illumination;
• Ceiling tiles within offices;
• Lighting integrated within furniture;
• Lighting within textiles eg; for high visibility and safety work.
It is important to remember that the current trend in residential lighting is to increase the number of localised lighting points within a room which would put general large area diffuse lighting such as that offered by OLEDs at a distinct disadvantage.
However, what is certain is that the UK, US and European governments are supporting OLED research in order to continue to improve the materials, processes and manufacturing technologies necessary for creating high brightness, long lasting white OLED light sources.
Recent LED technology advances
Currently the lighting industry is witnessing an unprecedented level of innovation and each month there is a new advancement in LED technology. Since my previous article we have seen announcements of the increase of white LED efficiency, records for single LED lumens outputs and a variety of new LED multi-die packages offering fixture manufacturers greater flexibility in designing lighting products.
For example, Philips Lumileds recently announced in research a white LED device delivering 136 lumens at 350mA (115 lm/W) whilst at 2000mA it produces
502 lumens (61 lm/W). Figure 5 illustrates what a significant leap in performance such a device would be compared to the current high powered white LEDs available on the market today. As the performance of LEDs at various forward currents becomes difficult to compare, figure 5 shows how modern high powered LEDs are becoming scalable enabling the fixture designer to choose any desired forward current to deliver the required fixture light output.
The ability to produce scalable light at higher forward currents has been achieved by utilising new LED manufacturing processes which overcome inherent limitations with previous LED chips

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Geoff Archenhold which make them more inefficient when operating at high forward currents. Philips Lumileds have managed to overcome such issues by ensuring their latest generation of LEDs operate effectively at high currents as shown by figure 6.
Recently, Philips Lumileds also launched a new range of compact LED products known as the
Luxeon Rebel family as shown in
Figure 7. Based on both InGaN and AlInGaP materials, the Rebel
LEDs feature Philips Lumileds latest production technologies and are available in a wide range of colours and various whites.
The new cool white Rebel can operate between 350mA and
1000mA with a typical luminous efficacy of 73 lm/W at 350mA and 61 lm/W at
750mA forward current. A significant advantage of the Rebel family is its very small form factor making it significantly easy to small package coupled with high luminous flux enables the Rebel to claim the highest light density (at
1185 lumens per cm2 equivalent) and the highest packing density.
The compact size of the Rebel emitters allows excellent colour mixing for RGB fixtures without significant colour fringing providing significant advantage over fixtures with larger LED emitters.
In an interesting departure from standard LED manufacturer’s information, the Rebel datasheet provides useful lifetime data at
Figure 5: The typical light output from various single chip LED emitters
Figure 6: The latest efficiency gains from Philips Lumileds achieved at higher forward currents place multiple Rebels close together unlike many recent high power LEDs.
Figure 8 compares the Rebel with other popular high power LEDs and its 3mm x
4.5mm size makes it the smallest surface mount LED of its type. The
Figure 7: The new compact Luxeon Rebel and its schematic view various forward current and LED junction temperatures enabling a fixture design engineer to easily determine the performance of fixtures based upon the Rebel
(see figure 9). It is hoped that by providing such data other leading
LED manufacturers will follow thereby enabling engineers to design fixture to real life operating parameters.
Further advances in
Figure 8: A scaled comparison of the Rebel against similar high power LEDs multi-chip emitters within a single package are starting to deliver high

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Geoff Archenhold power solutions to the lighting designer. With its new version of
OSTAR Lighting, OSRAM Opto
Semiconductors has just announced its first cold white LED to achieve more than 1000 lumens.
This means it is brighter than a 50
W halogen lamp, making it ideal for a wide range of applications in the general lighting sector. The high-flux LED is equipped with six closely packed 1 mm2 highpower chips enabling a single
OSTAR emitter combined with a
38° reflector to illuminate a desk with more than 500 lux from a height of two meters. OSRAM also claims that the OSTAR sets a new standard for a multi-chip solution by producing 75 lm/W from an operating current of 350 mA.
Figure 10 shows the new OSTAR
LED emitter from OSRAM.
PerkinElmer Optoelectronics have also recently announced its new
ACULED Very High Lumen (VHL)
LED which is offered in standard monochromatic (UV, Blue, Green,
Yellow, Red, IR) as well in an
RGBY (red-green-blue-yellow) version. With a luminous flux of up to 325 lumens, the ACULED

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Geoff Archenhold
VHL offers excellent brightness, improved luminous efficacy and a package thermal resistance as low as 4.5 K/W. Figure 11 shows a multi chip, RGBY version of the
ACULED VHL.
Improved white LED light
– just what the lighting designer ordered
Over the past few months the
LED industry has responded to the requests from the lighting industry to produce white LEDs that are consistent in colour temperature. For example, Lexidis
Lighting, a joint venture company between TridonicAtco of Austria and Toyoda Gosei of Japan have recently announced that the new
XED LED will be free from any colour binning issues. LEXEDIS claim to have eliminated once and for all the cumbersome colour binning by improving its production process capabilities.
LEXEDIS defines the quality of its chromatic products using the
MacAdam metric implemented worldwide by fluorescent manufacturers with the XED being rated as 6-step MacAdam. A
MacAdam ellipse is the region on a chromaticity diagram which contains all colors which are indistinguishable, to the average human eye, from the color at the center of the ellipse as shown in
Figure 12.
Although, other LED manufacturers such as Philips Lumileds do not specify the consistency of their white LEDs using MacAdam ellipses they have responded to the industry need by significantly improving the resolution of their binning structure as shown in
Figure 13.
Conclusions
The recent breakthroughs in LED emitter performance is leading to increased performance of
White LEDs and more importantly overcoming many of the previous barriers that held them back from the significant general lighting market. The increase in total luminous flux, reduction in package size and high quality consistent white LEDs are now set to ensure that LEDs displace fluorescents.
I predict that the end of compact fluorescents will be much sooner than the conventional lighting companies may think (within the next 5 years) due to several reasons:
• Consumers do not like the light generated by CFL’s;
• Currently, consumers have to choose CFL’s as there is no significant other choice of energy efficient light sources;
• CFL’s take a significant time to get to a standard operating condition;
• LED based systems are already more efficient than CFL’s making them more environmentally friendly in terms of electricity consumed to light on task metrics;
• Although LED systems are significantly more expensive than their CFL counterparts I believe consumers would pay more for quality and functionality of light.
However, it is clear that LEDs must not become complacent as the near-term light source of the future because the nascent OLED technology may just prove to be a compelling competitor within the next decade. OLED lighting research has accelerated significantly in recent years with some astounding results and providing the scientists and engineers can translate their lab results into production practice we may see
100 lumen per Watt OLEDs within five years.
What is certain is that Solid-State
Lighting is here to stay and it has excellent growth prospects both in the short and long term through a variety of technologies including LEDs and OLEDs.
Importantly the UK government via organisations such as the
Carbon Trust, may soon support the introduction of LED based lighting fixtures through initiatives such as the Enhanced Capital
Allowance scheme and this will provide a significant boost to UK lighting manufacturers that meet the criteria. However, minimum lighting standards for LED fixture measurement need to be agreed in order to place them on government approved technology lists
- a challenge still to be overcome.
Figure 12: A chromaticity chart with various MacAdam ellipses plotter
Figure 13:
The new Philips
Lumileds white
LED binning structure
The very latest technology trends for both OLEDs and LEDs will be discussed and demonstrated at the euroLED 2007 conference and lighting exhibition in
Birmingham between the 5th-7th
June. In the next edition, I shall endeavour to highlight many of the new product launches taking place at euroLEDs.