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Indium-Free OLEDs

AMES, Iowa, Dec. 4, 2012 — A limited supply and increased demand for indium tin oxide — the chemical compound used in screen displays in computers, TVs and cell phones — has scientists searching for a viable substitute to create indium-free organic LEDs. A well-known conducting polymer just might be the solution, researchers at Ames Laboratory say.

Although indium tin oxide (ITO) has been the standard transparent conductor used as the anode for flat-screen displays, the US Department of Energy has deemed indium “near critical” in its assessment of materials vital to clean energy technology. Scientists currently are looking for a cost-effective, energy-efficient alternative.

“There are not many materials that are both transparent and electrically conductive,” said Joseph Shinar, an Ames Laboratory senior scientist. “One hundred percent of commercial display devices in the world use ITO as the transparent conducting electrode. There’s been a big push for many years to find alternatives.”

Many labs have sought replacements in zinc oxide, said Min Cai, a postdoctoral research scientist at Ames and in the Department of Physics and Astronomy at Iowa State University. “Here we are working toward something different, developing ways to use a conducting polymer.”


Ames Laboratory researcher Min Cai prepares a metal-oxide OLED. Courtesy of Ames Laboratory/US DoE.

The polymer — a mouthful — is poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate), or PEDOT:PSS. The material has been around for about 15 years but, until recently, was not transparent enough or sufficiently conductive to be a viable ITO substitute, Shinar said. However, by using a multilayer technique and special treatments, Cai and colleagues fabricated PEDOT:PSS organic LEDs (OLEDs) with vastly improved properties.

The enhanced performance is largely an effect of the difference in the optical properties between the polymer and ITO-based devices, according to computer simulations done by the scientists. PEDOT:PSS also demonstrated significant flexibility — a property ITO does not have.

“OLEDs can be made on a flexible substrate, which is one of their principal advantages over LEDs,” said Ruth Shinar, a senior scientist at the university’s Microelectronics Research Center. “But ITO is ceramic in nature; it is brittle rather than flexible.”

The research builds on the team’s continuing work to find more affordable, efficient manufacturing materials and processes for OLED manufacturing. The ongoing investigations pave the way to more cost-effective manufacturing and more-widely available OLEDs for consumers. OLED TVs have already hit the market for high-end consumers, but Joseph Shinar is confident that prices will drop as major manufacturers perfect the production process.

“We are getting there with OLED televisions,” he said. “Consumers will see them getting more affordable and more widely available in the very near future.”

The technology also is being used in lighting, in applications where diffuse light is preferred instead of point source lighting, and in art and architectural design.

The findings were reported in Advanced Materials (doi: 10.1002/adma.201202035)  

For more information, visit: www.ameslab.gov

A year ago, a University of Cincinnati professor was seeking a patent for a possible polymer ITO replacement. For more on this story, see: Professor Seeks Patent on Possible ITO Replacement


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