Discovery highlights performance advantages for application of LEP technology in electronics displays and general lighting applications

CAMBRIDGE, United Kingdom  – The nature of one of the fundamental processes for light generation in light emitting polymer (LEP), materials has been experimentally demonstrated by scientists from the Cavendish Laboratory, Cambridge University, UK and Sultan Qaboos University, Oman. 

The discovery, announced in an article titled “Spin-dependent exciton formation in p-conjugated compounds” published in Nature Magazine (Volume 413, page 828, issue dated 25 October), has broad implications for the commercialisation of LEP technology in electronic displays and general lighting applications.

Organic light emitting diodes (OLED), comprised of two main technology branches - LEP and small molecule (SMOLED) - generate light when an electric current is applied to a thin film section. Electrons and holes are formed, which recombine through two types of states: the singlet state, which emits light, and the triplet state, which does not. The efficiency of OLEDs is limited by the ratio of light-emitting singlet states to non-emitting triplet states.

The results announced in this article show that the singlet/triplet ratio in LEPs could be greater than one, more than doubling the previously estimated maximum theoretical efficiency.

In contrast, for SMOLEDs the formation of the triplet state has been confirmed to be three times more likely than the singlet state, placing an upper limit on device efficiency of 25%. Consequently, significant research and development efforts have been expended to improve SMOLED performance, including adding phosphors and/or utilising triplet state conversion to extend singlet emission. The results announced in Nature show that very high efficiency LEPs are achievable without the addition of phosphors.

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Richard Friend, Cavendish Professor of Physics at the University of Cambridge, said “These results show that the basic semiconductor properties of polymers are different from those of  ‘small molecule’ semiconductors, in ways which make them intrinsically more efficient as light emitters in displays.”

LEP technology saw its origins in research at Cavendish Laboratory in 1989. The discovery led to the founding of Cambridge Display Technology (CDT), the owner of the fundamental intellectual property for LEP technology and the leading researcher and commercial technology developer. The announcement made today supports CDT’s own research that indicates that the potential maximum efficiency of LEPs is much higher than that considered possible from conventional theory.

Jeremy Burroughes, Product Business Unit Director at CDT elaborated, “A singlet:triplet ratio greater than 1:1 means significantly higher device efficiencies may be possible than what has been demonstrated thus far. Based on the Cavendish Laboratory findings we can expect improvements of at least a factor of two over existing efficiencies of 20 – 26 lm/W.”

“The ability of LEP to potentially operate at such high efficiencies should encourage CDT’s licensees that the technology is capable of even lower power consumption and consequently longer lifetimes than what has been demonstrated to date. Coupled with low materials utilisation, the benefits of printing techniques like inkjet printing, and a low number of manufacturing steps, this research further strengthens the belief that LEP is the technology of choice for meeting the higher performance requirements of even the most demanding electronics display applications,” added Burroughes.

The single layer LEP devices used in the research indicate that relatively simple display designs may provide very efficient cost and performance, while multi-layer LEP structures, which have experimentally been shown to provide further increase in device performance, may offer solutions for the most demanding display applications.

LEPs are being developed for inkjet and contact printing manufacturing, which, coupled with their lower activation voltage, higher efficiency and simpler manufacturing processes than SMOLED, are expected to offer a compelling technology solution for display manufacturers.

According to research from DisplaySearch, OLED display revenue is expected to grow to $4.2 billion by 2007. The OLED display market consists of a wide range of electronics products that includes mobile phones, personal digital assistants, digital cameras, camcorders, microdisplays, and eventually personal computers and consumer products.