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recipes for cooking light

In a world roiling in new technologies, few companies have received the early stage notoriety of Pittsburgh’s own Plextronics. Spun out of CMU in 2002, the company’s technology centers on the molecular self assembly of non-metallic nanowires that, depending on their molecular structure, are able to absorb light and transform it into electrical current or, alternatively, to use electrical current to emit light. Given the technology’s revolutionary nature, I couldn’t keep myself from asking Plextronics’ CEO Andy Hannah for a brain-dump and a plant tour, which he happily granted.


During my visit, I witnessed a deceptively simple demonstration in which one of my guides placed a high-intensity lamp facedown on a light absorption panel to illuminate a light emission panel about a foot away. Both panels were coated with compositional variants of a material called regioregular polythiophene (RRPT), a material developed in 1992 by Plextronics Co-Founder and Dean of CMU’s Mellon College of Chemistry, Dr. Rick McCullough.


Although conductive polymers were first discovered in the 1970s, they experienced little promise of commercial success until Dr. McCullough figured out how to synthesize conductive molecules that self-assembled into linear structures with sulfur heads attached to carbon backbones of various atomic lengths in a single pot (reactor) process that could be performed at room temperature. What is more, as though taking on lives of their own, McCullough’s molecules also assembled themselves, head to tail, to form nanowires that conducted electricity. Since electrical conductivity is essential to both absorbing and emitting light, and the active ingredients in the invention are extremely common, this technology appeared to be a natural winner in a world looking for better ways to produce energy and more efficient ways to use it. Absorption would generate electricity from the Sun during the day. Emission would utilize that energy to generate light at night.


The key to both absorption and emission may be found at the edges of quantum theory. No… don’t stop reading… I promise to keep this simple.


It is little known that contrary to most atomic logo designs, electrons do not orbit atomic nuclei in perfect circles. Rather, they whiz around their nuclei inside cloud-like energy shells or bands that resemble oblique ellipses, flattened eggs and figure-eights more than hoola-hoops. Each band has a requisite energy level for the reception or rejection of electrons, depending on its average distance from the nucleus. The farther from the nucleus a band is, the higher the energy of its electrons, but the closer to the nucleus an electron is, the more energy it takes to bump it to the next higher band.


Meanwhile, over on the Sun, after its eight-minute journey to Earth, an inbound light particle, or photon, can strike one of McCullough’s conductive polymers and excite an electron to a higher energy level. In turn, the electron will jump to an energy band farther from its nucleus. When the electron departs its home band, it leaves behind a positively charged hole, where it once lived. So while the electron is up there in its new snooty neighborhood, known as the conduction band, acting all cool and stuff, the hole wants it back or at least it wants some other electron back. The distance between the bands is known as the band gap.


The band gap is key to Plextronics’ technology, because by designing materials with different band structures, the electrons can be coaxed into staying in the conduction band for longer or shorter periods of time (were talking about nanoseconds here). In the case of absorption from solar light, the materials are tuned to trap the electrons long enough to move along the conductive polymer nanowires, thereby generating electrical current. In the case of emission, the process is reversed. An electrical current stimulates electrons to jump and fall, back and forth, between bands, thereby emitting photons (light) as they recombine with their holes.


The result is light absorption when the Sun is shining and light emission when it’s not. Pretty good recipes in anybody’s cookbook.


This story first appeared inTom Imerito’s TEQ column, Innovation Chronicles. You can read it on the Pittsburgh Technology Council’s website.

©Copyright 2007 Thomas P. Imerito/ dba Science Communications

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