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  • “Egg crate” structure optimizes QD-LED performance

Photonics Spectra
Feb 2012
Ashley N. Paddock, ashley.paddock@photonics.com

CAMBRIDGE, Mass. – A new quantum-dot (QD) LED “egg crate” design turns formerly troublesome ligand molecules into a critical element of a more versatile quantum-dot LED structure for applications in lighting, lasers and displays.

Ligands – organic molecules that dangle from quantum dots – are essential to quantum dot growth but can interfere with current conduction and cause functional problems later on. Researchers at Harvard School of Engineering and Applied Sciences have discovered an alternative that uses ligands to build a more versatile single-layer, egg-cratelike structure that better controls the flow of electric current – optimizing the QD-LED’s performance.


In an early design (left), the path of least resistance was between the quantum dots, so the current bypassed the dots and produced no light. Using the atomic layer deposition (ALD) technique (right), researchers funneled current directly through the dots, creating a fully functional, single-layer QD-LED. AZO = aluminum zinc oxide. Courtesy of Edward M. Likovich, Harvard University.


QDs are grown in a solution that can be deposited onto the surface of an electrode, but because the ligands are attached, the process gets complicated, according to the researchers. The core of the QD is a perfect lattice of semiconductor material, but its exterior is a lot more complicated. The dots are coated with ligands that are necessary for precise synthesis of the dots in solution, but once the QD is deposited onto the electrode’s surface, these same ligands make the steps for typical device processing more difficult.

The ligands interfere with current conduction, and attempts at modifying them could cause the quantum dots to fuse together, destroying the properties that make them useful. Organic molecules also can degrade over time when exposed to UV rays.

“The QD technologies that have been developed so far are these big, thick, multilayer devices,” said Rafael Jaramillo, a Ziff Environmental Fellow at Harvard’s Center for the Environment. “Until now, those multiple layers have been essential for producing enough light, but they don’t allow much control over current conduction or flexibility in terms of chemical treatments. A thin, monolayer film of quantum dots is of tremendous interest in this field because it enables so many new applications.”

The new QD-LED resembles a sandwich, with a single active layer of quantum dots nestled in insulation and trapped between two ceramic electrodes. To create light, current is funneled through the quantum dots, but the quantum dots also must be kept apart from one another to function. In an early design, the path of least resistance was between the quantum dots, so the electric current bypassed them and produced no light.


Electroluminescence spectra of two devices at —8-V bias. Device A (black) shows only emission due to color centers in the oxide electrodes. Device B (red) clearly shows the peak due to quantum dot excitonic emission at the expected wavelength. Courtesy of Edward M. Likovich.


Abandoning the traditional evaporation technique they had been using to apply insulation to the device, the researchers instead used atomic layer deposition (ALD), which involves jets of water. ALD takes advantage of the water-resistant ligands on the quantum dots, so when the aluminum oxide insulation is applied to the surface, it selectively fills the gaps between the quantum dots, producing a flat surface on the top and allowing more effective control over the flow of electrical current.

By exploiting the hydrophobic ligands, the scientists insulated the interstices between the quantum dots, thus creating the egg crate structure. This design enabled the scientists to funnel current directly through the QDs – despite having only a single layer of them. They were able to apply new chemical treatments to it, moving forward.

The research was published online in Advanced Materials (doi: 10.1002/adma.201101782).

Through Harvard’s Office of Technology Development, the team has applied for a provisional patent on the device. The next step will be to work toward optimizing its light-emitting efficiency. The researchers are interested in exploring other features of their ALD-QD composite. For example, the ALD oxide could protect the QD during postdeposition chemical treatments that otherwise would cause the quantum dots to agglomerate and lose their valuable quantum properties.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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