Monday, August 13, 2012

Researchers Shed New Light on Solar Cell Efficiency

The quest for more efficient solar cells is on. Currently the typical rate of conversion of sunlight into electricity is around 12% and in order to make solar power more competitive the industry works to increase this rate.

The scientific assumption is that there is a limit to the amount of energy that can be produced by a solar cell when it is hit with sunlight. Since the 1960s it is believed this limit is 33.5 percent. Five decades on and we have reached a top performance of 26 percent in the case of flat-plate, single junction solar cells, which absorb light waves above a specific frequency. Multi-junction cells could achieve higher rates of efficiency for having multiple layers and absorb multiple frequencies.

Researchers from the University of California, Berkeley recently presented a new theory about how solar cell efficiency could be increased and said the cells should resemble LEDs, that is, they should emit and absorb light. Their thesis will be presented at the Conference on Lasers and Electro Optics taking place between May 6 and 11 in San Diego, California.

Lead researcher Eli Yablonovitch said the more photons a solar cell emits, the higher its voltage, and consequently its efficiency, will be. They arrived at this conclusion while researching the reasons why solar cell efficiency has not reached its potential. They believe they have cracked it and the answer lies in the thermodynamic between absorption and emission.

They argue that designing solar cells to emit light naturally increases the voltage produced by the solar cell because the photons do not get “lost” within a cell.

Alta Devices, a company co-founded by Yablovitch, has created a prototype solar cell made of gallium arsenide (GaAs), which is used to make solar cells in satellites. The prototype achieved a record efficiency of 28.3 per cent thanks to a cell design that allowed light to escape from the cell. The reflectivity of the rear mirror was increased so that incoming photons were allowed out of the device.

The energy from the photons that come from the sun releases electrons from the cell, so they can flow freely. During this process, new photons are generated and if you allow them to escape from the cell as easily as possible, they can increase electricity generation.

The researchers expect their novel solar cell design to reach an efficiency rate of 30%. Their design is applicable to all types of solar cells.

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