Santa Clara, CA--A laboratory made gallium-arsenide (GaAs) thin-film photovoltaic (PV) solar cell from Alta Devices achieves a 28.2% conversion efficiency of sunlight to electricity, beating the previous record of 26.4% for a solar cell with a single p-n junction. The efficiency value, independently confirmed by the National Renewable Energy Laboratory, was made possible using a photon recycling technique.
Photons absorbed by a PV material kick electrons into the conduction band and leave behind holes. Many of the electrons that pass out of the cell are lost in the semiconductor when they recombine with a hole to produce either waste heat or a new photon. But by carefully growing a high-quality single crystal of gallium arsenide, Alta Devices says that 99% of the recombinations result in new photons that then create a new electron-hole pair and give the electron another chance to be captured as electricity; hence the term photon recycling. The Alta team also improved the reflectivity of the metal contacts on the back of the solar cell so that any photons exiting the cell are sent back in for possible reabsorption.
Because efficiency decreases in a production process once cells are packaged, Alta CEO Christopher Norris says, "We assume we will ultimately be able to achieve modules that are around 26 percent, and that’s plenty to be competitive with fossil fuels." The theoretical maximum conversion efficiency for a solar cell with a single junction is 33.5%. "We can see a path to 30 percent with our same design right now," says Norris. Adding a second junction could also increase the energy output.
Alta says that GaAs is naturally better at converting light to electricity than the chief contenders, such as silicon and cadmium telluride (CdTe), but it tends to be more expensive. Low-cost materials, such as amorphous silicon, CdTe, and copper indium gallium selenide (CIGS), are less efficient; CdTe cells are around 12%. Alta solves this problem by using only a small amount of a high-quality material in an epitaxial liftoff process--a thin film of gallium arsenide about 1 micrometer thick. Norris says an Alta module should cost about the same as a CdTe module but produce three times the energy.
The epitaxial liftoff process was developed by Eli Yablonovitch, an engineering professor at UC Berkeley and a cofounder of Alta. Alta is working on a pilot production line to produce samples of its solar cells sometime this year and expects to have early commercial shipments by late next year, Norris says. The company raised $72 million to develop its production process.