Tuesday, 22 May 2012

Luminescent 'LED-type' design breaks efficiency record

To produce the maximum amount of energy, solar cells are designed to absorb as much light from the Sun as possible. Now researchers from the University of California, Berkeley, have suggested & demonstrated a counterintuitive concept:  - solar cells should be designed to be more like LEDs, able to emit light as well as absorb it. The Berkeley team will present its findings at the Conference on Lasers and Electro Optics (CLEO: 2012), to be held May 6-11 in San Jose, Calif.

"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.

Since 1961, scientists have known that, under ideal conditions, there is a limit to the amount of electrical energy that can be harvested from sunlight hitting a typical solar cell. This absolute limit is, theoretically, about 33.5 percent. That means that at most 33.5 percent of the energy from incoming photons will be absorbed and converted into useful electrical energy.

For five decades, researchers were unable to come close to achieving this efficiency: as of 2010, the highest anyone had come was just more than 26 percent. (This is for flat-plate, "single junction" solar cells, which absorb light waves above a specific frequency. "Multi-junction" cells, which have multiple layers and absorb multiple frequencies, are able to achieve higher efficiencies.)

More recently, Yablonovitch and his colleagues were trying to understand why there has been such a large gap between the theoretical limit and the limit that researchers have been able to achieve. As they worked, a "coherent picture emerged," says Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch's group. They came across a relatively simple, if perhaps counterintuitive, solution based on a mathematical connection between absorption and emission of light.

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