Nano-Scale Solar Cell Structures

I feel like more geeky stuff today, so here goes: Stanford University reports that several of its nanotechnology gurus have published a new paper about a gimmick that may revolutionize solar cell technology.

Solar cells often employ a thin layer of light-absorbing material sandwiched between reflective plates. We've known for some time that their efficiency can be increased by certain tricks for making the light bounce around in there as long as possible, because it increases the chances that each photon will be absorbed and turned into electricity rather than spat back out unused. A common method is to scratch up the top plate so that it has lots and lots of sparkly faces oriented in all directions, which randomizes the reflective patterns. For reasons that are beyond me, but that follow from the wave equations that describe the behavior of bouncing and resonating lightwaves, there is a theoretical upper limit on how much benefit can be squeezed out of this trick. The formula that describes the efficiency veers wildly up and down for reflective film thicknesses very near zero, but quickly starts to squiggle back and forth in a narrow range that approximates the efficiency limit as the film thickness increases. The Stanford guys may have found a way to exploit that interesting behavior near the "zero."

Up to now, no one paid much attention to the eccentric behavior of the efficiency formula near zero, because a solar cell's reflective film has to be thinner than a single wavelength of typical sunlight for the off-the-charts tail-end of the formula to matter. These days, however, with the explosion of nanotechnology, it's getting possible to make films even thinner than the wavelength of visible light, which is between about 400 and 700 nanometers (one billionth of a meter). This is seriously small; a single nanometer is only 10 to 30 times longer than the radius of most atoms. It is about 1/50,000 as thick as a human hair. It's so small that many of the rules of thumb that work for structures that are many times as big as a single light wavelength start to break down.

Back to that theoretical limit: for relatively thick films, it settles down to four times the square of the refractive index of the transparent layer. As this nifty graph from the Stanford gurus' paper shows, there is an initial squiggle before the function settles down around its limit, which is shown as 4n2 on the "y" axis. In this early "off the charts" territory, where the transparent layer has a thickness of less than a single wavelength of visible light (shown as "one" on the x-axis in the graph), the efficiency can be anything from very bad to very good. In the "sweet spot" between 0.5 and 1.0 wavelength (shown on the graph by the red vertical stripe), the efficiency is at least equal to the "ideal" 4n2 and can go up as high as several times that much before dropping back to normal, where it then stays no matter how thick the reflective layer gets. The new idea is to cherry-pick the best efficiencies by building nano-thin solar cell layers with a thickness of between a half and a full light wavelength, which is to say a few hundred nanometers, or still only 1/100 of the thickness of a human hair. This approach not only promises to increase the efficiency of absorbing light but also to decrease the volume (and therefore cost) of materials needed for such thin layers. So this work may lead to much cheaper and more efficient solar panels.

In related news, the Smalley Institute for Nanoscale Science and Technology at Rice University in Houston will hold an event on Sunday, October 10, celebrating the 25th anniversary of the discovery of the "buckyball," the foundation of carbon nanotechnology. This event honors the winners of the 1996 Nobel Prize in Chemistry, Robert Curl, Sir Harold Kroto, and the late Richard Smalley, who together demonstrated in 1986 that carbon vapor could condense in the form of 60-atom symmetrical balls called "buckminsterfullerenes" or "buckyballs." The idea of nanotechnology, which is not confined to carbon, dates back at least to Nobel laureate Richard Feynman's comments on the subject in the 1959. In 1981, physicists Heinrich Rohrer and Gerd Binning opened the door to great advances in the nano world by inventing the scanning tunnel microscope, a new type of electron microscope with a magnifying power of 10 million. (To the upper right is an STM image of a carbon nanotube, a stretched-out version of a buckyball.) For this accomplishment Rohrer and Binning were awarded the Nobel Prize in 1986. Today, nanotechnology is revolutionizing not only manufacturing but medicine. A young person casting about for a career could do far worse than this field.

No comments: