A Cheap Route to Robust LEDs

Chemical bonds put a new spin on quantum-dot hybrid light-emitting devices.

Organic light-emitting diodes (OLEDs) are steadily making their way into commercial devices like cell phones and flat-screen displays. They're fabricated with layers of organic polymers, which make them flexible, and they use less power and less expensive materials than liquid crystal displays.

Red light: Researchers at MIT have found a potentially inexpensive way to make more-robust hybrid LEDs. The image shows a small sample of red quantum dots layered with an electrically conducting polymer on a glass substrate made using the new fabrication technique. Credit: Sreeram Vaddiraju

The downside is that because the polymers react easily with oxygen and water, OLEDs are expensive to produce--they have to be created in high-vacuum chambers--and they need extra protective packaging layers to make sure that once they're integrated into display devices, they don't degrade when exposed to air or moisture.

MIT chemical-engineering professor Karen Gleason and MIT postdoc Sreeram Vaddiraju have developed a process that aims to solve the problems of high fabrication costs and instability for OLEDs while still maintaining their flexibility. Gleason's solution is a hybrid light-emitting diode, or HLED. The device would incorporate both organic and inorganic layers, combining the flexibility of an OLED with the stability of an inorganic light-emitting material. "The idea is to have a mixed bag and capture the qualities that allow inexpensive fabrication and stability," Gleason says.

Gleason starts with a substrate of electrically conducting organic polymer, which she creates through a chemical vapor deposition process in a low-vacuum chamber. It's the only step in the process that requires a vacuum, which should make the approach cheaper than conventional methods. For the light-emitting layer, Gleason uses quantum dots, nanocrystals of inorganic semiconductors; each quantum dot can be "tuned" to emit certain frequencies of light. Although quantum dots are inflexible themselves, they're so small--two to six nanometers across--that even arranging them side by side in a continuous film still allows for flex in the material.

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While using quantum dots in light-emitting devices isn't new, Gleason's technique is. The problem is how to get the dots to stick onto a substrate in a uniform, even layer, without moving. Vaddiraju says that they use "molecular wiring." Instead of just laying down the quantum dots on top of the polymer substrate, the scientists use linker molecules between the layers to chemically bond the quantum-dot layer and the polymer together.

This "cross-linking" molecule between the layers is "a beautiful evolution of the present structures," says Vladimir Bulovic, an associate professor of electrical engineering at MIT and the first to demonstrate practical use of quantum dots in optoelectronic devices. Bulovic's research has depended on other methods of depositing quantum dots: dropping the dots onto a fast-spinning substrate, called spin casting, and, more recently, stamping them onto a surface.