February 21, 2011

Full-colour quantum dot displays fabricated by transfer printing

Electroluminescence image of a four-inch full-color quantum dot display with a resolution of 320 x 240 pixels. Image credit: Tae-Ho Kim, et al. ©2011 Macmillan Publishers Limited.

Nature Photonics - Full-colour quantum dot displays fabricated by transfer printing

Light-emitting diodes with quantum dot luminophores show promise in the development of next-generation displays, because quantum dot luminophores demonstrate high quantum yields, extremely narrow emission, spectral tunability and high stability, among other beneficial characteristics. However, the inability to achieve size-selective quantum dot patterning by conventional methods hinders the realization of full-colour quantum dot displays. Here, we report the first demonstration of a large-area, full-colour quantum dot display, including in flexible form, using optimized quantum dot films, and with control of the nano-interfaces and carrier behaviour. Printed quantum dot films exhibit excellent morphology, well-ordered quantum dot structure and clearly defined interfaces. These characteristics are achieved through the solvent-free transfer of quantum dot films and the compact structure of the quantum dot networks. Significant enhancements in charge transport/balance in the quantum dot layer improve electroluminescent performance. A method using plasmonic coupling is also suggested to further enhance luminous efficiency. The results suggest routes towards creating large-scale optoelectronic devices in displays, solid-state lighting and photovoltaics.

Nature News - Connect the quantum dots for a full-colour image

The display consists of a film printed with trillions of the tiny quantum dots (an average of 3 trillion per cm2). The quantum dots emit light at a specific wavelength (color) that can be tuned by changing the size of the quantum dots.

Previous attempts to make full-color quantum dot displays have faced challenges in that image quality tended to decrease with the size of the display. To overcome this challenge, the researchers in the current study used a different method for applying the quantum dots to the film’s surface. Instead of spraying the quantum dots onto the film, the researchers created an “ink stamp” out of a patterned silicon wafer. They used the stamp to pick up strips of size-selected quantum dots, and then stamp them onto the substrate. Unlike the spraying methods, this method does not require the use of a solvent, which previously reduced color brightness.

As the results showed, the new quantum dot display has a greater density and uniformity of quantum dots, as well as a brighter picture and higher energy efficiency than previous quantum dot displays. The new display is also flexible, so applications could include roll-up portable displays or flexible lighting applications. The technology could also be used in photovoltaic devices, which would especially benefit from quantum dots’ high energy efficiency.

Ink stamps have been used to print text and pictures for centuries. Now, engineers have adapted the technique to build pixels into the first full-colour 'quantum dot' display — a feat that could eventually lead to televisions that are more energy-efficient and have sharper screen images than anything available today.

Engineers have been hoping to make improved television displays with the help of quantum dots — semiconducting crystals billionths of a metre across — for more than a decade. The dots could produce much crisper images than those in liquid-crystal displays, because quantum dots emit light at an extremely narrow, and finely tunable, range of wavelengths.

The team used a patterned silicon wafer as an 'ink stamp' to pick up strips of dots made from cadmium selenide, and press them down onto a glass substrate to create red, green and blue pixels without using a solvent.

The idea may sound simple, but getting it to work was not easy, Choi explains. "It took us three years to get the details right, such as changing the speed and the pressure of the stamp to get a 100% transfer."

The team has now produced a 10-centimetre full-colour display. The pixels ware brighter and more efficient than in quantum dot displays made by rival methods, says Choi. For example, "the maximum brightness of the red pixels is about 50% better," he says. The maximum power efficiency for the red pixels is about 70% better.

Seth Coe-Sullivan, the chief technology officer of QD Vision, a company in Watertown, Massachusetts, that produces devices with lighting based on quantum dots, notes that Choi and his team's method is cheap. "We all have our eyes on making large-screen televisions, and this fabrication technique seems to be cost-effective," he says.

But Coe-Sullivan adds that it may take some time to commercialize quantum-dot displays for big items. "I can imagine that we will have small cell-phone displays using this technology within around three years," he says. "For the rest, there may be more of a wait

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