Engineers have discovered the key to making polymers shine -- and they've used that knowledge to boost the efficiency of light emission from the materials to four times the best ever before reported, University of Rochester engineers report in today's issue of Science. The improvement in the efficiency of the optoelectronic polymers -- from 10 to 42 percent -- could open the door to widespread use of the inexpensive and versatile plastics in everyday electronics, say the investigators.
Optoelectronic materials, which turn light into electric signals and vice versa, are part of our everyday lives. Laser printers, fax machines, copiers, digital displays, computer monitors, fluorescent lighting, solar cells, flat-screen TVs -- even the human eye -- are based on optoelectronic materials. Thin films of the materials are coated inside devices and translate light or electricity into useful signals.
While polymers have been a low-cost and convenient alternative to conventional light-emitting materials, such as gallium arsenide, engineers have not been very successful at controlling the amount and color of light they emit. And the polymers are notoriously inefficient: much like a light bulb gives off unwanted heat to keep a room bright, so polymers waste a lot of energy by converting only a small fraction of one form of energy to another. When given a jolt of electricity, some polymers send out flashes of light -- but all too often, only a small amount of light comes out. Exactly why has long been a puzzle.
Now, engineers know why. They have discovered that a polymer's efficiency depends on how close together the polymer's molecular chains are to each other. They found that when a single polymer's chains are very close, for instance, within 3-5 angstroms (an angstrom is one ten-billionth of a meter), and when the polymer is "excited" by shining light on it or by applying a voltage, the chains form pairs of molecules known as excimers which exist for only a few billionths of a second. Excimers do not emit light efficiently.
The solution? Prevent excimers from forming by blending in another polymer or other molecules to keep the original polymer's chains separated. Just as headstrong siblings sometimes need some space apart to flourish, so do polymer chains. Properly spaced, the chains form not excimers but another type of material -- a short-lived molecular sandwich known as an exciplex -- which emits light very efficiently. Rochester engineers believe that conjugated polymer exciplexes (conjugated polymers are the major type of optoelectronic polymers) are a new class of materials.
"This opens up a whole new world," says Samson Jenekhe, associate professor of chemical engineering, who did the work along with former graduate student John Osaheni, who is now working for General Electric. "Now that we understand how light comes out of these materials, we can design approaches to make them more efficient."
Most of the exciplexes the pair has made thus far are three to five times more efficient than their pure counterparts in converting input energy into light. The engineers see an even bigger improvement in converting light into electrical charges: Candidates for such applications as solar energy conversion are 300 to 1,000 times more efficient than their excimer-forming counterparts.
Jenekhe, who worked with materials known as polybenzobisazoles (PBZA), believes that excimers are responsible for light emission from all conjugated polymers, and he explains why in the Science paper. He believes that by forming exciplexes and tweaking the type and spacing of the blending material, engineers will have better control over the color and efficiency of light a material emits. He and Osaheni have filed for a patent on the new class of materials, and their optoelectronic applications, through Research Corporation Technologies of Phoenix, Arizona.
Jenekhe's work on optoelectronic polymers is funded by the National Science Foundation, the Navy, and the University's Center for Photoinduced Charge Transfer. tr