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QD-LEDs emit over whole visible spectrum

25 Jun 2009

MIT researchers develop unique structure to make RGB-emitting devices.

Researchers in the US have shown that a universal structure can be used to make hybrid organic-quantum-dot light-emitting devices that emit over the entire visible spectrum. These QD-LEDs might find use in RGB flat-panel displays in which red, green and blue QD pixels are deposited in one simple, inexpensive step.

Employing QDs of ZnCdS and ZnCdSe allowed the group to extend QD-LED colours to the deep-blue and deep-red parts of the visible spectrum, while keeping the carrier-transporting materials the same as for green and orange devices.

Hybrid organic QD-LEDs show pure colours and are robust while having the efficiency, flexibility and low processing costs of organic light-emitting devices (OLEDs).

Polina Anikeeva and Jonathan Halpert of the Massachusetts Institute of Technology and colleagues used colloidal QDs synthesized by different routes for each part of the visible spectrum (utilizing layers of CdSe, ZnS, ZnSe and the alloyed semiconductors ZnCdS and ZnCdSe). The researchers were able to employ the same organic substrate compatible with each QD type, something that was not obvious from the start.

The QD deposition method onto the substrate is rather simple, says Anikeeva. "First, we use spin casting to make a QD monolayer on top of an elastomer stamp. Next, we print the resulting monolayer inside the device structure."

Identical device structures "Because the method allows independent processing of QDs, we can use identical device structures for all of the QD colours," she told nanotechweb. "In the future, one can imagine using this contact printing method to make RGB flat-panel displays in which red, green and blue QD pixels are deposited in one easy step."

The team also observed a four-fold efficiency increase for green and a 30% increase for orange QD-LEDs compared with previous reports. This was achieved by improving the non-radiative ("Forster") energy transfer from organic carrier-transporting materials in the substrate to the QDs. "We relied heavily on exciton formation in the organic films and consecutive exciton transfer to QDs," explained Anikeeva.

While the researchers successfully found a set of carrier-transporting materials that can donate their exciton energy to green, orange and red QDs, finding materials that will be efficient exciton donors to blue QDs remains a challenge. This might be remedied by designing and synthesizing wide-bandgap hole- and electron-transporting organic materials for improved exciton energy transfer and direct charge injection into blue QDs, they suggest. Making such materials is the key to closing the efficiency gap between red-orange-green and blue QD LEDs, says Anikeeva.

The work was published in Nano Letters.

• This article first appeared on our sister website nanotechweb.org.

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