The new solar cells actually emit light in a process call luminescence. The light comes from new photons that get created when sunlight hits the semiconductor material in the cell – when sunlight hits the gallium arsenide, the light knocks electrons loose, but at the same time it also generates some new photons. It may sound counter-intuitive, but by letting these new photons escape the cell instead of trying to keep them in, you end up increasing the voltage that the cell can produce. “What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce,” says Eli Yablonovitch, principal researcher and UC Berkeley professor of electrical engineering.
Yablonovitch had been working to understand why scientists had only been able to achieve about 26 percent efficiency for single junction solar cells, while the theoretical limit for efficiency is considered to be about 33.5%. They looked at a thermodynamic connection between absorption and emission of light and realized that letting the new photons escape the cell (as light) has a “natural effect” of increasing the voltage produced by the cell. The higher the voltage, the more electricity you can get.
Their prototype cells have already hit 28.3 percent efficiency. Their cells are made of very thin (one micron) layers of gallium arsenide (GaAs), which reduces the cost of their cells significantly. Because they’re so thin, they’re flexible and can be integrated into roofing or building materials. The company is now looking at forming some manufacturing partnerships in order to begin mass production of its new cells.
via: press release