Researchers lay nanoscale solar-cell foundation

Solar cell technology, long limited by costs and practical issues, has the potential to cross a new threshold of efficiency through an approach recently theorized by Stanford researchers.

Solar cell researchers Shanhui Fan, Zongfu Yu and Aaswath Raman took a significant step toward future solar power practicality with a Sept. 27 paper titled “Fundamental Limit of Nanophotonic Light Trapping in Solar Cells,” which lays the foundation for nanoscale solar cells–research indicating that for solar cells, less is more.

Above: a computer-produced model of a thin film, nanoscale solar cell. It is the subject of Shanhui Fan, Zongfu Yu, and Aaswath Raman’s research published last week. Graphic courtesy of Zongfu Yu

“We have developed essentially a rigorous theory to treat light trapping in nanophotonic structures in general,” said Fan, an associate professor of electrical engineering, pointing out that the team did not construct new kinds of solar cells but rather laid the theoretical foundation for future development.

Most solar panels in use today implement the 20th-century technology of making thick panels with rough surfaces to keep light rays inside the panel for as long as possible. However, the nanoscale cells that Fan’s team researched take a fundamentally different approach, using solar cells as thin as 500 nanometers, or one two-thousandth of a millimeter.

“Previously, the theory treated light as a straight ray,” said Yu, a postdoctoral research fellow. “But light has a lot of wave factors, and this in fact does not show up until you go into the nanoscale.”

Yu pointed to a previous theoretical limit for light absorption in larger solar cells and said that the theoretical limit that his team determined for nanoscale cells that dealt with the wave aspect of light turned out significantly higher. In addition to their increased efficiency, the thinner cells also cost much less because they use fewer resources.

And lower cost is a big deal. According to Scott Gould, senior engineer for building energy systems commissioning, solar power currently offsets well under one percent of Stanford’s yearly power consumption because of the impractical costs to install solar cells and the physical space required to house cells. The recent research theorizes cells that are both less expensive and more efficient, making them overall more practical for widespread use.

However, the team’s research will not make solar cells more practical overnight.

“What we have pointed out is really what outer limit of light absorption enhancement can be accomplished in different structures,” Fan said. “It’s very important [as a solar cell producer] to know how far you are from the absolute optimum as allowed by theory.”

The team hopes that its paper serves as a gateway to the optimization of solar cells as future research uses its theoretical computer models to make concrete and functional cells, but the work is unlikely to influence any commercial units for years.

Solar power is both a prominent Stanford hobby and a serious undertaking. The Stanford Solar Car Project, which produces a solar-powered car every two years, and the Stanford Solar and Wind Energy Project (SWEP) bring together student solar-power enthusiasts. Veteran Solar Car mechanical team member Matt Lambert ’12 said that although the solar cells used in the Solar Car Project generally use decades-old technology, companies including General Motors and Volkswagen sponsor the project with an interest in researching ways to implement solar cell technology.

Gould said that SWEP, a group that advocates for sustainable energy sources, has worked a lot with his department and that solar cells are on the rise in Stanford facilities. The Knight Management Center, under construction as the new home of the business school, is planned to contain 4,275 solar cell modules, more than tripling the school’s current number of modules. The huge cost of installing solar cells, however, remains the central obstacle to retrofitting current facilities with solar cells, necessitating research projects like this one.

“The hope is that all this research tells us what to do to make those cells absorb light as well as we want them to,” Raman said.

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