Record efficiency for solar cell.

Record efficiency for solar cell 

A two-layer solar cell developed at Sandia National Laboratories in Albuquerque, N.M., demonstrated a 31 percent sunlight-to-electricity conversion efficiency -- the highest ever recorded, according to an announcement by its designers last week. The device is a stacked multijunction cell, which means it has multiple photosensitive layers, each optimized for different wavelengths of light.

The upper, gallium-arsenide layer in the Sandia device -- sensitive to wavelengths from the ultraviolet through the visible portion of the spectrum--converted into electricity 27.2 percent of the light striking it. Unabsorbed light passed through to an underlying silicon-crystal layer, which is sensitive to light into the near-infrared. Even though the silicon layer is sensitive to a broader spectrum of frequencies, it's less efficient than the gallium arsenide in tapping the energy of the shorter wavelenghts, points out Dan Arvizu, supervisor of Sandia's solar-cell work. That's why it was placed on the bottom, he explains. Neither the specific gallium-arsenide layer nor the silicon layer used in this multifunction photovoltaic cell is ideal for such a device, he adds; they're just the "two most mature" options available at this time.

Peak efficiencies were achieved at intensities between 35 and 50 watts per square centimeter, a 350- to 500-folf concentration of natural sunlight. In fact, these crystalline multijunction photvoltaic devices are designed for use with solar concentrators. Though the best commercially available cells for use with concentrators have efficiencies of just 18 to 20 percent, Arvizu expects it won't be long before future two-layer multijunction cells achieve solar-conversion efficiencies near 35 percent.