Quantum-dot LED may be screen of choice for future electronics

MIT researchers have combined organic materials with high-performing inorganic nanocrystals to create a hybrid optoelectronic structure - a quantum dot-organic light-emitting device (QD-OLED) that may one day replace liquid crystal displays (LCDs) as the flat-panel display for consumer electronics.

The work is a collaborative effort between Moungi G. Bawendi, professor of chemistry, and Vladimir Bulovic, assistant professor of electrical engineering and computer science.

Bulovic is also affiliated with the Research Laboratory of Electronics.

Bawendi studies the electronic and optical properties of semiconductor nanocrystal quantum dots for applications ranging from biology to optical devices.

Also called artificial atoms, quantum dots are nanometer-scale 'boxes' that selectively hold or release electrons.

Unlike traditional LCDs, which must be lit from behind, quantum dots generate their own light. Depending on their size, the dots can be 'tuned' to emit any colour in the rainbow.

And the colours of light they produce are much more saturated than that of other sources. Bulovic is pursuing the use of organic and nanostructured materials as active electronic elements. Bawendi and Bulovic, with electrical engineering and computer science graduate student Seth A. Coe and chemistry graduate student Wing-Keung Woo, teamed up through MIT's Center for Materials Science and Engineering (CMSE) to create a new, improved QD-OLED.

This latest MIT QD-OLED contains only a single layer of quantum dots sandwiched between two organic thin films. (Previous QD-OLEDs used 10-20 layers.)

The researchers have demonstrated organised assemblies over a square centimetre area and the same principle could be used to make bigger components.

The MIT team's method of combining organic and inorganic materials may pave the way for new technologies and enhance understanding of the physics of these materials.

In addition to being used for extraordinarily thin, bright flat-panel displays, the QD-OLEDs also may be used in a variety of other applications; to calibrate wavelengths for scientific purposes; generate wavelengths visible only to robot eyes or "miniaturise scientific equipment in ways we haven't yet imagined," Bawendi said.

Creating bright light

The QD-OLEDs created in the MIT study have a 25-fold improvement in luminescent power efficiency over previous QD-OLEDs.

The researchers note that in time, the devices may be made even more efficient and achieve even higher colour saturation.

"One of the goals is to demonstrate a display that is stable, simple to produce, flat, high-resolution and that uses minimal power," Bulovic said.

The researchers were inspired by advances in all-organic LED (OLED) technology. OLEDs, which can be used to create TVs or computer screens only a fraction of an inch thick with the same brightness as LCDs, have been making their way into commercial electronic devices.

The group believes that QD-OLEDs will in time become complementary to OLEDs because they can be built on the same electronic platforms with compatible manufacturing methods.

Tiny building blocks

While the future of electronics and other fields may revolve around nanotechnology, researchers and manufacturers are faced with fabricating large-scale components out of building blocks invisible to the naked eye.

Creating hybrid optoelectronic devices depends on the precise positioning of functionally distinct materials, the authors write.

"How do you efficiently transport electrical charges to an active area of a hybrid device that is only a single layer of quantum dots?" Bawendi said.

The researchers used organic molecules currently used in OLEDs as an organic semiconductor to deliver an electrical charge to the quantum dots.

They used two parallel processes, which are already widely applicable in industry, to create separate but layered structures out of nanoscale materials.