A Cheap Route to Robust LEDs

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The advantage of Gleason's technique, Bulovic says, is that "you end up with a very robust structure" mechanically, chemically, and electrically. "It validates an idea of stabilizing quantum dots inside organic structures by providing covalent bonds around them." Bulovic adds that there are still hurdles to overcome, but he thinks that the research "represents another one of those advancements we were hoping for in the field."

Covalent bonding solves the degradation problem, says Vaddiraju, because the linker molecules grab hold of "free bonds" in the organic material, leaving none to react in air. That effectively seals off the organic polymer layer from outside influence.

The cross-link should also take care of scaling up. Rather than dealing with the mechanical problem of depositing millions of nanocrystals onto a substrate through spin casting or stamping, the chemical reaction itself wires the dots to the substrate in a smooth, even layer. And unlike a process like spin casting, the researchers' technique uses all the dots and all the polymer. "So from a material cost point of view, we're not losing material," says Vaddiraju.

So far, the team has succeeded in creating a red HLED, which lasted 2,200 hours at 100 °C. The researchers think that's about equivalent to their goal at room temperature: 10,000 hours, or about three years at a little under 10 hours a day, which they estimate is how long a cell phone should last.

The next step is to complete testing with green and blue dots; the researchers will need all three colors working for a complete prototype. Then they'll move on to see how the device works with rapid patterning--using the dots just like ink from an ink-jet printer. Eventually, the goal is to do bulk printing. Because the layers are so thin and flexible, roll-to-roll processing will be simple and make the process even more economical. "Roll-to-roll is the same process used to put a metallization barrier layer on potato-chip bags," says Gleason. "And if it's cheap enough for potato chips, it should be cheap enough for displays."