LEDs and Beyond: Seeking a Perfect but Efficient White Light

  • Graduate student Lauren Powell shines a blue LED through vials of Quantum Dots, manufactured by Pittsburgh-based Crystalplex. Each vial contains trillions of nanoparticles. Photo: Ashley Murray

  • At an electronics expo near Pittsburgh, Sylvania Sales Representative Michael Reeves holds a blue LED fixture with a yellow phosphor cover. When the light turns on, the blue LED appears white. Photo: Ashley Murray

  • Robert Davis, professor in Materials Science and Engineering at Carnegie Mellon University, holds an LED. His research focuses on making the green LED. Photo: Ashley Murray

June 12, 2014

Frank Jones, a lighting distributor for Tri-State Supply, has become something of an expert on light emitting diodes, better known as LEDs.  He says that's because each of his customers wants something different.

"I have one individual customer that has an administration building.  The hallway wants one color.  The conference room, they want nice, warm 2700K, and then Gary the accountant wants nice, white bright 5000," Jones says.

First, let’s clarify those numbers. They describe the LEDs’ color, which is measured in something called Kelvin, or K.  A cool blue is 5000, while 2700 indicates a warm yellow.  Most people want a classic white, so that their rooms look the same as what they’re used to with incandescent bulbs.

Manufacturers currently make white by covering a blue LED with a yellow chemical compound called a phosphor.  But layering color this way steals about 15 percent of the LEDs’ efficiency.

"There’s no free lunch here. When you reduce that color, to the Edison color that we’ve always enjoyed, it’s a less efficient LED," says Robert Davis, a researcher at Carnegie Mellon University's Department of Materials Science and Engineering.

He and other researchers are looking  at different ways to create a soft white LED, without sacrificing efficiency. Davis says to make white light, researchers need to combine red, blue and green LEDs. While scientists can easily make red and blue, Davis says they’ve been stymied by green for years.

"The green light, still the efficiency is not high enough to combine with the red and blue at this time to produce white light," Davis says.

His graduate student Lauren Powell is trying a different approach—something called quantum dot technology. Amazon is already using it in the new Kindle Fire. Instead of trying to create a green LED, Powell is trying to apply quantum dot technology to LED lighting.

"Quantum dots are much simpler to make.  All you require are beakers, a bench top, temperature control and just the right precursors," Powell says.

By precursors, basically she means the ingredients used to make quantum dots. She displays a series of glass vials, like test tubes, which each holds trillions of tiny nano-particles are suspended in liquid. The dots are one-hundredth the width of a human hair. When she shines a blue light through them, they become excited and create energy waves that we perceive as color. Quantum dots can create a rainbow of color options, including that sought-after white.

This sidesteps the issue of needing a green LED. It also prevents loss of efficiency that with painting a blue LED with phosphor.

"If we just replace that phosphor layer with the quantum dots, we’ll get a much more pleasing color white that we’ll want to use in our homes," Powell says.

Powell says an issue with quantum dots is that researchers don’t know how long they will last in a bulb. She recently completed her disseration on their lifespan.  Pittsburgh-based quantum dot manufacturer Crystalplex supplied the dots for her work.

Davis is excited that his lab is among those pushing what could be the next frontier in lighting.

"I am absolutely after the most efficient light-emitting source.  The more efficient your light emitting source, the less electricity you’re going to need," Davis says.

And for lighting salesmen like Frank Jones, he’ll will be able to offer the most efficient lighting with the color his customers want.