03 Jun 2003
Bright, efficient displays made from organic light-emitting diodes are starting to appear in mobile phones and digital cameras. Olaf Gelsen from German OLED maker Covion Organic Semiconductors charts the rise of this exciting new technology.
From Opto & Laser Europe June 2003
While much of the action in OLED materials is being driven by large Asian firms, Europe is also well represented in the market. One of the most influential players is Frankfurt-based Covion Organic Semiconductors. Part of the Avecia group, Covion now offers development, pilot-line and full commercial-scale manufacturing of the two main types of OLED materials - small molecule and polymer (see below).
Key characteristics The key to OLED technology was the development of organic semiconducting materials - also a crucial starting point for so-called "plastic electronics". During the 1980s, Kodak and UK-based start-up Cambridge Display Technology (CDT) developed displays that formed images by passing electric currents through thin layers of organic material to generate light. In effect, each pixel in the display behaves in the same way as a miniature LED. Today the technology can be used to create displays of all sizes and performances, ranging from simple monochrome versions to video-capable, full-colour graphic displays.
One of the advantages of OLEDs is that because their pixels directly emit light, the displays boast a higher brightness and resolution at wider viewing angles than backlit LCDs. They are also thinner, lighter and more power-efficient. OLEDs are expected to be easier and cheaper to make, because they avoid the need for the polarizers and filters inherent in LCD technology.
In principle, OLED displays can be constructed on any substrate. Although glass or silicon is often used at present, plastic substrates will ultimately enable roll-to-roll processing and bring the benefits of cost-effective mass-production. Experts have estimated that such OLEDs could be one-third less expensive than LCDs manufactured on similar production scales.
To date, the main commercial applications for OLED displays have been mobile phone screens, car radios and digital cameras. Product milestones include Pioneer's use of the first small-molecule, multi-colour OLED display in a car stereo (1999), and the Philips "Spectra" razor (featured in the Bond film Die Another Day), which has a polymer OLED display based on material supplied by Covion.
Commercial potential In March, Kodak launched the first digital camera to incorporate a full-colour active-matrix OLED display. The 2.2-inch OLED display is 30% larger than the LCD screen found in the camera's predecessor.
When it comes to the commercial potential for OLED displays, market estimates have been climbing steadily since the technology first attracted serious attention in the mid-1990s. A recent study by DisplaySearch, a US-based research firm, estimates that OLED sales for small display devices such as mobile phones and PDAs will reach $2.5bn (€2.14bn) by 2007.
OLED technology is also expected to gain a foothold in the market for medium and large-sized flat-panel displays. This is potentially a highly lucrative arena; the market for screens for personal computers and notebook computers is estimated to be worth around $15bn today, and is forecast to reach $40bn by 2007. Although LCDs are behind almost 90% of the flat-panel displays produced today, OLEDs are set to strongly challenge this domination in the future. Over the next 10-15 years, technologists estimate that OLEDs may capture up to 50% of the flat-panel display market.
Successful market penetration of today's LCD markets will depend on the willingness of manufacturers to invest in this new-generation technology in spite of the falling price of LCDs. Two of the keys to unlocking the high business expectations for OLEDS and their cousins, polymer light-emitting displays (PLEDS) are improvements in device operating lifetimes and the perfection of full-colour performance. Both are particularly important for larger-screen applications, such as PCs.
OLED displays still have some way to go to achieve good colour, high efficiency and long lifetimes - for the colour blue in particular. Better lifetimes are the big issue if OLEDs are to conquer continuous "vision" based flat-panel market sectors such as PCs and TV, which are presently the almost exclusive preserve of LCD technology.
Small-screen products such as cellphones have acceptable display lifetimes of up to 10,000h at normal brightness. But for bigger screens, a much longer life is needed - say 30,000h for a typical PC flat-screen application - and the industry is still a couple of years away from realizing that. Looking further ahead, if OLEDs are to be used in TV displays as cathode-ray tube replacements, at least 50,000h operating lifetimes will be required.
Flexible future To compete with LCDs, both OLED technologies (small molecule and polymer) must combine high resolution and full colour with highly competitive production costs. Recently, a research project on a new variant of PLED technology - partnering Covion with Cologne University's photonics group and the Technical University, Munich - has taken an important step forward.
The group has developed a production process that enables RGB (red-green-blue) deposition at high resolution, using a photolithographic technique. The process exploits a new class of electroluminescent polymers that can be spin-coated and cured by ultraviolet light, but maintain their electrical and optical properties. Combined with a simple patterning process, the development enables the creation of high-resolution pixelated matrix displays.
This new production technology overcomes the resolution limitations encountered in previous fabrication techniques, such as ink-jet deposition. Instead, it uses standard photolithography that is well established in the volume production of many electronic components, such as colour filters for LCD monitors. When fully commercialized, it promises to combine the high performance and competitively lower costs of PLEDs with a higher resolution.
Core fabrication technologies Making the OLED materials
OLED materials for displays can be classified into two main types, each of which has its own distinct fabrication process and a different set of advantages and limitations.
1) Small molecule This approach involves the vacuum batch deposition of small-molecule organic material layers onto a glass or silicon backplane. The technology is well proven but currently only suits mass-production of small or medium-sized displays up to about 15 inches in diameter. The fabrication of larger displays is hindered by the performance of the shadow masks that are used to define (pattern) the pixels of the display.
2) Polymer The big advantage of polymer light-emitting materials (PLEDs) is that they are soluble and can be deposited onto a glass or flexible plastic substrate under ambient conditions. Polymer technology enables the fabrication of larger screen sizes than small-molecule OLEDs, as there is no need for the shadow masks required by vacuum deposition processing. PLED displays also operate at a lower voltage and are more power-efficient than those based on small molecules.
Making the displays
OLED displays pass through several manufacturing steps:
1) Substrate fabrication The first step is the production of the substrate, also known as the backplane. During this stage, thin-film transistor circuits that drive each pixel are placed onto a glass or silicon substrate. In the future, the substrates may be made from plastic. The deposition and patterning takes place in a cleanroom and is similar to that used to make integrated circuits.
2) OLED deposition The next step is the fabrication of the OLED part of the display. This involves the deposition of the active light-emitting layers by either vacuum or wet process deposition techniques, depending on the type of OLED material. Finally, the cathode electrode is deposited by a vacuum or sputtering process.
3) Encapsulation To protect the electronics and active OLED layer from exposure to water vapour and oxygen, these parts are hermetically sealed in a protective package. This is essential to maximize the display's performance and lifetime.
4) Assembly Finally, all the parts of the display are assembled to create a complete module.