Researchers at Rochester Institute of Technology in the US are using nanowires to capture more of the sun’s energy and transform it into usable electricity. Their work focuses on maximising how much of the solar spectrum can be taken in using tandem junction solar cells based on III-V compounds.
“III-V tandem devices are the best in the world, but because of the manufacturing and initial materials costs, they are very expensive to produce. Therefore, they are not used in consumer markets. They are used in niche applications such as space technologies. You are not going to see today’s III-V devices on large solar panels on people’s rooftops, because they are so expensive,” said Parsian Mohseni, assistant professor of microsystems engineering in RIT’s Kate Gleason College of Engineering.
He was recently awarded nearly $300,000 for an Early Concepts Grant for Exploratory Research (EAGER) from the National Science Foundation for “Two Dissimilar Materials (TDM) solar cells: bifacial III-V nanowire array on silicon tandem junction solar cells.”
EAGER grants support high-risk but potentially high-reward transformative technologies. Mohseni’s research could also open new paths toward next-generation integrated photonics and high speed transistors. These technologies are only two of the key research and development strategies in RIT’s Future Photon Initiative.
Tandem junction solar cells are groupings of multiple sub-cells each of which can absorb a particular range of a wider solar spectrum band. Mohseni’s team has been able to grow a variety of different III-V compounds for solar cells using the selective area epitaxy technique using a MOCVD system.
Mohseni’s team is developing the engineering processes on the crystal growth side to make the dissimilar materials fit together to decrease defects and gaps. By changing the architecture entirely, and using nanowires instead of thin films, this process could alleviate defects that form along conventional hetero-epitaxial bulk materials and thin films, and allow for saving nearly 90 percent of the material used to make the devices. Mohseni and his research team will use vertical nanowire structures having a diameter of approximately 100 nanometers, and lengths up to several microns, a replacement for the thin films that are currently being used for semiconductor development.
“If you are trying to absorb material in a film, you want that film to be thick enough to capture more light. If light is not absorbed, it can bounce or reflect off the film surface,” Mohseni explained. “With nanowires, if light comes in, it can still be absorbed, but if it bounces off one wire in the array, instead of going off into infinity, it can be captured by the nearby wire and be re-absorbed. This effect of multiple scattering interactions increases the light-trapping capabilities of the nanowire array. Even though we are using 90 percent less material, we can absorb light better than a thin film structure.”
The project, which will be conducted over two years, aims to expand the scope of III-V solar cells beyond niche markets, eventually incorporating the technology into homes, electrical grids or transportation systems, for example.
“It is sort of giving power back to the people – taking real energy and power-conversion technologies and putting that in the hands of the people. That’s the big picture, the long-term goal. This is one potential step toward that,” Mohseni said.