Research Highlights: Quantum Dot LEDs

Sep 7, 2011 An outstanding development in the manufacturing of stable quantum dot based light emitting diodes has recently been reported by two research groups from the University of Florida. The research article published in the online issue of Nature Photonics describes spin coating based method for easy manufacturing of stable quantum dot LEDs with maximum luminance and power efficiency values of 4,200 cd m−2 and 0.17 lm W−1 for blue emission, 68,000 cd m−2 and 8.2 lm W−1 for green, and 31,000 cd m−2 and 3.8 lm W−1 for orange-red. It is specified that incorporation of the ZnO nanoparticles allows these devices to exhibit high environmental stability. The unencapsulated devices have operating lifetimes exceeding 250 h in low vacuum with an initial brightness of 600 cd m−2

This work has been highlighted by the R&D Magazine with the exclusive comments from the scientists.
"Our work paves the way to manufacture efficient and stable quantum dot-based LEDs with really low cost, which is very important if we want to see wide-spread commercial use of these LEDs in large-area, full-color flat-panel displays or as solid-state lighting sources to replace the existing incandescent and fluorescent lights," says Jiangeng Xue, the research leader and an associate professor of materials science and engineering "Manufacturing costs will be significantly reduced for these solution-processed devices, compared to the conventional way of making semiconductor LED devices."

A significant part of the research carried out by Xue's team focused on improving existing organic LEDs. These semiconductors are multilayered structures made up of paper thin organic materials, such as polymer plastics, used to light up display systems in computer monitors, television screens, as well as smaller devices such as MP3 players, mobile phones, watches, and other handheld electronic devices. OLEDs are also becoming more popular with manufacturers because they use less power and generate crisper, brighter images than those produced by conventional LCDs (liquid crystal displays). Ultra-thin OLED panels are also used as replacements for traditional light bulbs and may be the next big thing in 3D imaging.

Complementing Xue's team is another headed by Paul Holloway, distinguished professor of materials science and engineering at UF, which delved into quantum dots, or QDs. These nanoparticles are tiny crystals just a few nanometers wide, comprised of a combination of sulfur, zinc, selenium, and cadmium atoms. When excited by electricity, QDs emit an array of colored light. The individual colors vary depending on the size of the dots. Tuning, or "adjusting," the colors is achieved by controlling the size of the QDs during the synthetic process.

By integrating the work of both teams, researchers created a high-performance hybrid LED, comprised of both organic and QD-based layers. Until recently, however, engineers at UF and elsewhere have been vexed by a manufacturing problem that hindered commercial development. An industrial process known as vacuum deposition is the common way to put the necessary organic molecules in place to carry electricity into the QDs. However, a different manufacturing process called spin-coating, is used to create a very thin layer of QDs. Having to use two separate processes slows down production and drives up manufacturing costs.

According to the Nature Photonics article, UF researchers overcame this obstacle with a patented device structure that allows for depositing all the particles and molecules needed onto the LED entirely with spin-coating. Such a device structure also yields significantly improved device efficiency and lifetime compared to previously reported QD-based LED devices.
Spin-coating may not be the final manufacturing solution, however.

"In terms of actual product manufacturing, there are many other high through-put, continuous 'roll-to-roll' printing or coating processes that we could use to fabricate large area displays or lighting devices," Xue says. "That will remain as a future research and development topic for the university and a start-up company, NanoPhotonica, that has licensed the technology and is in the midst of a technology development program to capitalize on the manufacturing breakthrough."

Original Research article available from Nature Photonics at:
http://www.nature.com/nphoton/journal/v5/n9/full/nphoton.2011.171.html

Article analysis and interview are available from R&D Magazine:
http://www.rdmag.com/News/2011/09/Manufacturing-Manufacturing-Methods-Manufacturing-Method-Paves-Way-For-Quantum-Dot-Based-LEDs/

Author:  Alice Jones

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