In 1907 German physicist Gustav Mie realized that tiny metal particles in stained glass scattered light in ways that produced beautiful colors. Now, a related interplay between light and matter explains why incredibly thin nanowires made of semiconductors like germanium may prove to be effective components for solar cells (see "Nanowires scatter light very efficiently" and "Complementary zinc oxide nanowires may yield cheaper LEDs and solar cells"). Combining Mie's work with more recent theory, a Stanford University (Stanford, CA) team has discerned how to tune and improve the light absorption efficiency of the wires.
The Stanford research appears in the July 5 online edition of the journal Nature Materials. "For many solar cells if you can get just a couple of percent improvement in energy conversion efficiency, people are very happy," said Mark Brongersma, an associate professor of materials science and engineering and a senior author on the paper. "Here we show that we can boost the light absorption by a factor of 10 for some wires at some wavelengths of light. Researchers around the world are already investigating the use of nanowires in solar cells and there is a potential to get significant boosts in their efficiencies."
Because the work explains how to improve light absorption with germanium, a material compatible with computer chips, it may also have applications in improving data communications by bridging optical and electronic signals, even in the smallest spaces on densely packed chips.
In the work by Brongersma's team, the germanium nanowires were as thin as 10 nm in radius. They were grown by Joon-Shik Park, a visiting researcher from the Korea Electronics Technology Institute in the group of materials science and engineering professor Bruce Clemens. Brongersma doctoral student Linyou Cao, the lead author on the paper and main driver of the research, hooked the germanium wires up electrically and showed that in the presence of visible and infrared light they act like wispy antennas, capturing particularly resonant wavelengths of the light and bouncing them around inside the wire. Because the resonant light bounces around for a while, it has a larger chance to be absorbed in the semiconducting material. When it is absorbed, it excites electrical charges in the material that can be measured as an electrical current from the wire. In a solar panel, that current is the electricity the panel generates. The crux of the team's discovery is that different sizes of wire will absorb different frequencies of light more efficiently.
Brongersma said the team's next goal is to build prototype solar cells that make use of the tuning guidelines established in the Nature Materials paper.
For more information, go to news.stanford.edu.
