As flat-plate single-junction solar cells go, you won’t find a more efficient version than Alta Devices’ prototype made from gallium arsenide. The new design takes advantage of an old concept implemented into solar cells for the first time. Two engineers from the University of California at Berkeley, Eli Yablonovitch and his student Owen Miller, pioneered Alta Devices’ efforts.
“There’s a thermodynamic link between absorption and emission,” explained Miller. “If you have a solar cell that is a good emitter of light, it also makes it produce a higher voltage,” he said. With higher voltages, more electrical energy is available to harvest and add to power supply grids.
The theoretical limit of conversion efficiency for solar cells has been recognized since its discovery in 1961. It is the Shockley-Queisser limit, and it rests at approximately 33.5 percent for single p-n junction cells. Multi-junction cells with layered configurations are capable of slightly higher energy outputs, but still, most of the sunlight falling on a solar cell evades capture. Some is emitted as heat, and some bounces around the cell and gets lost.
Yablonovitch and Miller wanted to know why scientists have only been able to achieve about 26 percent efficiency. They focused on the energy lost from electrons bouncing around the cell, and realized that although some energy is dissipated during the process, they could boost the power output by making sure the dissipated energy is released as external fluorescence instead of heat. In effect, they are upping the emission capability. It seems counterintuitive to increase the energy moving away from the device, but the link between luminescent emission and voltage has been known for some time. The application of the theory to solar cells has not been done, until now.
One of the key ingredients to these higher efficiency solar cells is the material they are made of, gallium arsenide. Traditional cells are made from silicon, a semiconductor abundant in Earth’s crust, but gallium arsenide has properties that allow higher electron mobility. It is currently used on orbiting satellites and space exploration vehicles in NASA’s Mars Exploration Rover Mission. Gallium arsenide “absorbs photons 10,000 times more strongly than silicon for a given thickness but is not 10,000 times more expensive,” according to Yablonovitch. Alta Devices also uses rear mirrors with higher reflectivity than typical cells, which sends incoming photons back through the device.
The Berkeley Lab boasts that the cells “not only smashed previous solar conversion efficiency records, but can be produced at well below the cost of any other solar cell technology.” The National Renewable Energy Lab confirms and records efficiency claims in several categories and subcategories depending on the makeup of solar cells. Indeed the Alta Devices cells are at the top of the list with over 28 percent efficiency.
The research will be presented at the 2012 Conference on Lasers and Electro-Optics this week. Yablonovitch hopes the new technique gets attention and implementation in other types of cells, so efficiency records can continue to rise.