Researchers seek to develop efficient quantum-dot LEDs

‘Our work paves the way to manufacture efficient and stable QD-based LEDs with really low cost, which is very important if we want to see widespread commercial use of these LEDs in large-area, full-colour flat-panel displays or as solid-state lighting sources to replace the existing incandescent and fluorescent lights,’ said Jiangeng Xue, the research leader and an associate professor of material science and engineering. ‘Manufacturing costs will be significantly reduced for these solution-processed devices, compared with the conventional way of making semiconductor LED devices.’

According to the university, a significant part of the research carried out by Xue’s team focused on improving existing organic LEDs (OLEDs). These semiconductors are multi-layered structures made up of paper-thin organic materials, such as polymer plastics, used to light up displays in an array of 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. 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 Florida University, who delved into QDs.

These nanoparticles are tiny crystals measuring just a few nanometres in width, comprised of a combination of sulphur, zinc, selenium and cadmium atoms. When excited by electricity, QDs emit an array of coloured light. The individual colours vary depending on the size of the dots. Tuning, or ‘adjusting’, the colours 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 organic and QD-based layers.

Until recently, however, engineers at Florida University and elsewhere have been vexed by a manufacturing problem that has hindered commercial development.

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 — 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.

Florida University 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 with previously reported QD-based LED devices.

The research is published in an article in the current online issue of the journal Nature Photonics.