Liquid Crystals that Light Up
A material containing electroluminescent liquid crystals could be used to make new kinds of OLED and LCD displays.
Liquid crystals are normally used in displays to polarize the light from a white backlight. But research by Stephen Kelly, head of chemistry at the University of Hull, in the UK, and physicist Mary O’Neill, also at the University of Hull, has shown that it is possible to make liquid crystals that emit light when electrically stimulated.
Kelly made this discovery in 2000. He and O’Neill have since worked to refine the technology. Now a company called Polar OLED, based in Leeds, UK, has been spun out to work with display companies to commercialize it. Polar OLED’s material can be used to make novel light-emitting diodes for OLED displays, as well as simple but higher-quality backlights for traditional LCDs, says Kelly.
Liquid crystals have long been known to be capable of photoluminescence–the ability to emit light when exposed to photons. But to make liquid crystals emit light when electrically stimulated, it was necessary to improve the transport of charge through the material.
Kelly’s approach achieves this by using liquid crystals containing organic compounds called aromatics. “The more aromatic rings you have, the more luminescence you get,” says Kelly. By exposing solutions of these materials to ultraviolet light, the compounds form into fixed polymerized networks that link up the liquid crystals. Depending upon the precise chemistry employed, the resulting hybrid material can be made to emit different wavelengths of light, corresponding to different colors, when a current is applied.
Red, green, and blue light-emitting liquid crystals can then be used to create individual subpixels for an OLED display. They can also be stacked on top of each other to produce white light for use in an LCD backlight, says Kelly.
High resolution OLED displays, made up of individual pixels of light-emitting organic materials, have already started to appear on the market. They offer superior brightness and viewing angles compared to LCDs, but they tend to be expensive because of the high-temperature evaporation processes used to make them.
Cambridge Display Technologies, also based in the UK, is developing a cheaper solution-based approach for printing OLED displays. But Kelly says it is hard to deposit layers of different polymers close together, or on top of one another, without destabilizing them. “The second layer will dissolve the first and the third dissolves the first two,” says Kelly.
This can be a problem, says Henning Sirringhaus, a physicist at Cambridge University, and co-founder of another company, Plastic Logic, that develops plastic electronics. “Most polymer light-emitter solvents have quite common properties,” so it can be difficult to find ones which won’t influence each other, he says.
With Polar OLED’s approach, the polymer layers are inherently stable, so stacking them or printing them close together is feasible, says Kelly. This makes it possible to print high-resolution OLEDs, he adds.
Sirringhaus says that Polar OLED’s solution is interesting, but notes that it can be difficult for a new display material to break into a mature market. “The bar is high in terms of performance,” he says.
The company’s technology could, however, also be used to make LCD backlights more cheaply. One of the big advantages of the company’s material is that it produces polarized light, says Kelly. LCDs normally require two polarizing layers in order to work. “It would make it a lot cheaper because a large part of the cost of LCDs is the polarizers,” says Kelly.
This would be a genuine advantage, says Michael Edelman, CEO of Nanoco Technologies in Manchester, UK. Nanoco is developing quantum-dot-based white backlights for displays. Backlights usually consist of white light-emitting diodes shone in at the sides of the display through waveguides, but the quality of white light produced by these LEDs has its limits.
“We will certainly keep an eye on these guys,” says Edelman. But he says it remains to be seen how well the company’s new material performs. “People have been talking about OLEDs for 15 years, about how they are going to knock LCD off its perch and take over the world,” he says. “And yet they have tended to overpromise and underdeliver.”
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