PLED displays technology moves out of the lab, into mainstream

Polymer light emitting diode (PLED) technology is set to transform the display industry with its superior imaging performance, compact, lightweight properties and versatile form factor. It looks set to replace liquid crystal displays (LCDs) and cathode ray tubes (CRTs) in many existing applications, as well as open up exciting possibilities for new product forms and applications.



Discovered at Cambridge University in the UK, and now championed by Cambridge Display Technology Ltd, PLEDs are often regarded as the biggest advance in display technology.



PLEDs are semi-conducting materials that can emit any part of the full spectrum of light when electrically stimulated. The light produced is dependent on the precise chemical composition of the polymer. Polymer OLEDs are an advanced form of Organic Light Emitting Diode (OLED) in that they are solution-processable and can be applied in solution using ink jet printing, resulting in fast, cost effective manufacture.



This property also makes PLED technology highly scaleable – perfect for the move towards large panel TV displays.



Products incorporating PLED displays are expected to offer distinct advantages over other display solutions such as thinner and more compact displays, with significantly higher contrast and brightness.



PLEDs emit their own light, eliminating the space, weight and power consumption required by backlights in LCDs. PLEDs also offer more vibrant, high contrast images and a wide viewing angle approaching 180 degrees.



PLEDs have a very fast response time that does not change with temperature – typically a thousand times faster than LCDs – making PLEDs ideal for fast moving video display applications. Their durability should make them a good choice in the automotive, military and medical industries where high performance is essential.



In the long run, PLEDs are expected to have a 20 - 40 per cent cost advantage over LCDs. The simple structure of a PLED display significantly reduces the cost of materials and manufacturing tools. PLED’s adaptability to a variety of patterning techniques and manufacturing processes allow for further cost savings.





Article Continues below...

Potentially, developments such as wristwatch televisions, flexible display screens and even displays on clothing are future possibilities.



Part of the growth of PLEDs is attributable to the fact that they can be applied in liquid form which allows for displays to be ink-jet printed, offering a potentially lower manufacturing cost.



CDT expects PLED technology to really come into its own with TV displays, where the scalability benefits of manufacture using ink jet printing and other printing techniques under development will be vital in large panel production.



Breaking through the barriers of technology

Improving the performance of a PLED device requires the development of materials with high photoluminescence (PL) efficiency and good choice and design of the electrodes. It also requires balanced charge transport within the PLED device, increased output coupling and low density of defects that can act as exciton quenching sites. While most of these processes are generic and apply across the different colours of polymer devices, each factor needs to be optimised differently for different colour pixels.



Since the discovery of PLEDs in 1989, significant effort has been directed into the development of red, green and blue materials that exhibit stability under normal operating conditions to enable the integration of this technology into flat panel display applications in products such as televisions and in the exploding market for multimedia-enabled cell phones, PDAs and other mobile products.



Efforts over the years have also successfully produced higher efficiency OLED device structures by maximising the number of excitons that can emit light, whether through triplet harvesting systems, or by taking advantage of the higher singlet:triplet ratios observed in polymer devices.



A full colour display typically uses groups of three adjacent pixels emitting red, green and blue light. For a wide range of consumer electronic products, the useful time to half-luminance, the time taken for the device luminance to drop to half of its initial value, must exceed 10,000 hours. Although the green and red polymers currently available easily meet the stability specifications required for a range of consumer electronic products, a stable blue PLED has presented a greater challenge.



However, rapid progress has been achieved towards commercially viable blue PLED materials. For the past years, much work has been done to develop improved polyfluorene blue PLED materials and optimise device structure, including the use of an interlayer, spin-coated directly on the top of the PEDOT:PSS layer. With the interlayer, blue PLEDs with an external quantum efficiency above 5.0% have been measured, which is 35% higher than without the interlayer. CDT has made rapid progress in this area, achieving results many times better than those possible only two years ago.



With the new developments, it is now possible to access a new range of blue PLED materials optimised for a simple barium cathode architecture that is compatible with all colours. Previously, each colour required its own optimised cathode type. The development of this new common cathode architecture is a significant step forward for cost-effective manufacturing of solution-processable PLED displays.



This breakthrough in lifetime opens up new possibilities for applications such as digital still cameras (DSCs) and mobile phones. CDT also expects the technology to be applied in large-size TVs in the next few years.



A two-year programme which aims to understand the science that controls the spin states of polymer-based P-OLEDs has also concluded successfully this year. A project undertaken by CDT, together with industry display partners Philips, Covion and the universities of Cambridge, Bologna and Mons, it focused on establishing high efficiency materials using high singlet-ratio fluorescent polymers, but also worked on solution processable phosphorescent emitters.



The project achieved the production of a standard two-layer device structure with an external quantum efficiency (EQE) of 6%, almost twice the efficiency of previous materials. This impressive performance was achieved using red emitting polymers, which is typically the lowest efficiency colour in RGB displays.



Fostering Innovation and Commercialisation of PLED

With the significant potential of PLED technology, many companies are set to ride its wave. CDT is working closely with many industry leaders and strategic partners in Asia to extend potential applications of PLED technology across a wide range of sectors, allowing for faster time to market, reduced cost and reduced risk compared with a ‘go-it-alone’ approach. CDT’s PLED technology has already been licensed to world-class Original Equipment Manufacturers (OEMs), including Delta Optoelectronics, Osram Opto Semiconductors, Seiko–Epson, Philips, MicroEmissive Displays and Innoled among others.



The first PLED products were launched in 2002, among the first being a Philips electric shaver with an orange battery gauge display and a Delta Optoelectronics MP3 player which incorporated a PLED display. Dai Nippon Printing (DNP) also showcased PLED displays incorporated in a book cover.



Philips also applied PLED technology in its clamshell design Philips 639 mobile phone with a ‘Magic Mirror’ display. Incoming calls are indicated by the display, which doubles as a mirror when it is closed.



Future product ideas also include roll-up displays and displays that conform to particular products, such as illuminated car interiors and dashboards.

Asia – 95 per cent of major volume display manufacturing

Asia holds the largest potential market for PLED displays and the region has over 95 per cent of the world’s major volume display manufacturing facilities. Comparatively, there is currently no volume manufacturing for PLEDs in the US, and there are limited manufacturing facilities in Europe.



CDT is working closely with companies all over the world, but especially in Asia, in all aspects of the manufacturing supply chain to help licensees get to market and reach volume production in the shortest time possible.



In Japan, CDT has agreements with Tokki and Ulvac to ensure the optimisation of manufacturing tools for polymer production. Toppan Printing has also built a pilot line to investigate alternative printing processes for the manufacture of displays based on CDT’s PLED technology. This joint printing development programme potentially paves the way for further work involving flexible display devices.



CDT also works closely with Sumitomo Chemical, one of CDT’s long-standing research partners, in pushing forward the boundaries of materials capability, and ensuring supplies into the value chain.



Many of CDT’s licensees have also set up production facilities in Asia. Osram Opto Semiconductors has a manufacturing facility in Penang, Malaysia, while Innoled (part of Eastgate Technology of Singapore) is gearing up with a new manufacturing line for PLED displays. Delta has an active production scale-up programme.



Analysts have predicted that the market for OLED displays will reach $3billion by 2008.



Given the dramatic improvements in lifetime and efficiency, PLED technology is increasingly looking destined to spearhead the next generation of displays and lighting applications.