Cambridge Display Technology News
Perhaps the most exciting development in the display industry in the last fifteen years has been the discovery and development of polymer light emitting diodes (PLEDs).
It all started in the Cavendish Laboratory of Cambridge University in 1989, when it was found that ‘organic’ LEDs could be made using conjugated polymers. So began the history of Cambridge Display Technology (CDT).
Green flask of polyphenylene vinylene
In particular, polyphenylene vinylene (PPV) was found to emit yellow-green light when sandwiched between a pair of electrodes. The initial device efficiencies were very low, but the researchers quickly realised the commercial potential of this discovery, especially for the manufacture of displays which emit their own light. These would offer significant advantages over the main display technology we still use today (liquid crystal display or LCD), in which a separate light source has to be filtered in several stages to produce the image we see.
PLEDs have a number of intrinsic advantages over liquid crystal devices:
* PLED is an emissive technology: it emits light as a function of its electrical operation.
* A PLED display consists of polymer material manufactured on a substrate of glass or plastic, and does not require additional elements such as backlights, filters and polarizers.
* PLED technology is very energy efficient and lends itself to the creation of ultra-thin lighting displays that will operate at lower voltages.
The resulting benefits include brighter, clearer displays with viewing angles approaching 180 degrees; simpler construction resulting in cheaper, more robust display modules, and fast response times allowing full colour video pictures even at low temperature.
How do PLEDs work?
Since the discovery of PLEDs in 1989, significant effort has been directed into the development of red, green and blue materials that exhibit high efficiency and stability under normal operating conditions, and to enable integration into flat panel display (FPD) applications.
For a wide range of consumer electronic products, the useful lifetime (time taken for the device luminance to drop to half of its initial value) must exceed 10,000 hours. Full colour displays typically use groups of three adjacent pixels emitting red, green and blue light. Although the green and red polymers currently available can meet the stability specifications required for a range of consumer electronic products, a stable blue PLED has until now presented a greater challenge.
PLEDs can be used to provide light of almost any visible hue and beyond
In the past several years, Dow Chemical, Sumitomo Chemical, CDT and Covion (a subsidiary of Merck KGaA), have worked on developing new blue PLED materials and optimised device structures. CDT has now announced the achievement of an extrapolated lifetime of over 100,000 hours from laboratory devices with brightness 100 cd/m² at room temperature.