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Printed Electronics World
Posted on August 7, 2008 by  & 

OLEDs to be rescued by inorganic chemistry?

OLEDs are a tough business to be in and the leaders are investing huge amounts of money to get on top of the fragile chemistry. Although versions sandwiched in glass are selling well, profits are a long way off. The advent of mass produced, flexible, low cost versions, particularly wide area types with long life, keeps slipping further into the future. Yet these are the largest potential market because they enable many exciting new product concepts to be realised rather than replacing existing displays in familiar devices. Although over 200 organisations started to develop OLEDs we now have attrition with several leaving the business in frustration every year.
To tackle the problems of instability and reactivity and to improve electronic and optical performance, an ever widening choice of elements is being employed by those in for the long haul. Fine chemical companies concerned with inorganic materials are among those coming to assist the device makers.

OLED developers look further afield

In research and development, OLEDs variously employ such materials as B, Al and Ti oxides and nitrides as barrier layers against water and oxygen, Al, Cu, Ag and indium tin oxide as conductors, Ca or Mg cathodes and CoFe nanodots and Ir and Eu in light emitting layers. For instance, the OLLA project concluded that printed organic "conductors" are too resistive to distribute potential evenly over wide area OLEDs and copper should be used.
Here are some other examples. In 2007, Jian Shen of the Oak Ridge National Laboratory in the USA and his colleagues used magnetic nano particles to dope the structure of a polymer-based OLED in research aimed at printing of these devices. The technique not only opens up a way to get more light out of an OLED, but also allows the OLED intensity to be controlled by an external magnetic field.
A typical polymer-based OLED structure contains three layers: a thin light-emitting layer held between a hole-transport layer and an electron-transport layer. The emissive layer should be thin enough to allow the electrons and holes from the transport layers to meet and recombine.
Shen and colleagues fabricated their device by using an ultrasound method to mix cobalt ferrite (CoFe) nanodots into chloroform solutions of polymers. The researchers spin-cast the CoFe-doped polymers onto a conducting glass substrate to form the OLED. They then measured the electroluminescence intensity of the doped OLED and compared it with that of a non-doped OLED.

Phosphorescent triplet OLEDs

A fairly recent development by materials supplier Merck and others is phosphorescent (triplet) technology for OLEDs. This offers the prospect of exceptionally high efficiency (a factor of 3-4 times over fluorescents) while, except for a difficult blue challenge, maintaining color purity and long lifetime for red and green. This promising triplet approach has been shown to be capable of being adapted to either vacuum or, for printing, solution processes.
Phosphorescent OLEDs, such as those employing iridium based dyes, have been developed by Pacific Northwest National Laboratory in the USA. It has used organic phosphine oxides as electron transport materials. These materials address the critical issue of achieving high quantum efficiency (photons generated per electron injected into an OLED device) at low voltages.
Devices built at PNNL using the new materials have produced external quantum efficiencies at a brightness of 800 cd/m2 as high as 11% at only 6.3 V without using conductivity doping. One class of new OLED materials developed at PNNL are based on organic phosphine oxide compounds while another is based on organic phosphine sulfides. In addition to OLEDS, these materials have the potential to be used in other devices, including photovoltaic cells and thin-film transistors.
Source: PNNL

Barrier layers

Consider barrier layers for flexible OLEDs. They need to be better than those used for any other device. We mean 10-6 grams per square meter per day of water and 10-5 cc per square meter per day of oxygen. Few currently believe that the requirements of life, flexibility, large area, low cost and volume manufacture have been met, to the extent that wide area, long life, flexible OLED lighting, signage and displays can be produced.
Developers such as Vitex, Appliflex and 3M in the USA and IMRE in Singapore use alternating metal oxide or nitride and polymer layers, examples of the inorganic layers being the oxides of boron, aluminium and titanium and there is interest in binding up the undesirables, not just preventing them from getting through. Not one of the developers of barrier layers is able to use printing as yet and samples are very hard to come by, according to various interested parties that IDTechEx has interviewed in the preparation of this article. Will those who can print tightly rollable and wide area, high integrity oxides and nitrides at low cost please step up and assist.

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Posted on: August 7, 2008

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