Plextronics improves LEDs and solar cells

If you’re familiar with Sony technology, particularly the 2007 OLED digital TV, you’ve witnessed technology currently being refined by a team led by Carnegie Mellon University’s vice president for research, Richard McCullough. In addition to his duties as vice president for research — which include managing corporate research and campus-wide projects and overseeing student-inclusive research projects — McCullough manages his own spin-off company, Plextronics. Launched in 2002 by McCullough and its current CEO, Andy Hannah, Plextronics is based in Harmorville in the University of Pittsburgh Applied Research Park with a current staff of about 70 people.

The mission of the company — “Light. Power. Circuitry” — describes one of the company’s projects: the development of printable solar cells made completely out of plastic. McCullough points out the scope of this project: “Imagine plastic that can be coated on any surface that could be harnessing power outside and even in low-light conditions.” The other area of focus for Plextronics is display technology. The centerpieces of this technology are the organic light-emitting diodes (OLEDs). Screens using OLEDs have paper-thin layers where displays can be made of plastic. The colors on such screens, according to McCullough, “turn out better than plasma.” The OLED technology improves upon the contemporary liquid crystal display (LCD) technology by correcting for viewing angle problems. With LCDs, one cannot see the screen unless one is sitting directly in front of it, due partially to a series of liquid crystal pixels located behind the screen either allowing or prohibiting light produced by the device to pass through.

In OLED technology, such liquid crystal pixels are not used, allowing much better visual effects while employing little energy. Light emitting diodes have been in existence for a while — similar technology is used in red-light clock radios — but only recently have they been made with organic materials. Organic diodes require much less arduous work than do inorganic, which need to be processed in a clean room. Research has been a challenging process. Swati Varshney, a senior chemistry major and member of McCullough’s research team, has worked on synthesizing materials for OLEDs and solar cells. She worked on a polymer that could emit a stable, bright white light. She modified the molecular structure of polyfluorene, a fluorescent blue substance that, despite having a high quantum yield, decomposes in current LED applications.

“I modified the reaction to give ‘living’ conditions, meaning we could get polyfluorene molecules with uniform chain length [so] that they can ... stack properly,” said Varshney. This means that the polymers could be more easily made into thin films to make OLEDs. When other reactive monomers — simpler molecules — are added to polyfluorene, the reaction produces tunable colors. The result is a stunning visual. Another problem in the process was removing organic molecules from the gold circuit board responsible for printing the images. Junior chemistry major Rebecca Potash worked on a deposition process that would detach organic molecules while leaving the circuit itself unaltered. By creating these thin, flexible electronics, the gold patterns can be “backed” directly onto the circuit while cutting down on cost and energy.

Television only scratches the surface in applicable technology, however. OLED has the potential to improve any technology that uses high-resolution micro-displays, including computer screens and mobile phones. In the future, McCullough and Hannah will look to commercializing OLEDs, as well as their solar cells and inks, while increasing their efficiency and lowering their cost. Solar cells have a promising future. Instead of the traditional silicon, which is costly and time-consuming to use, organic molecules could be used to make solar cells effectively and cheaply. This would be, according to Varshney, especially useful for applications in developing countries. “For example, solar-powered OLEDs could provide low-cost lighting to children in sub-Saharan villages so that they could study past sunset after school or work,” Varshney said, citing a presentation from the Pople Lectures held at the Mellon Institute in September.