Ultra-Efficient Photovoltaics

A solar cell more than twice as efficient as typical rooftop solar panels has been developed by Spectrolab, a Boeing subsidiary based in Sylmar, CA. It makes use of a highly customizable and virtually unexplored class of materials that could lead to further jumps in efficiency over the next decade, making solar power less expensive than grid electricity in much of the country.
The cell, which employs new “metamorphic” materials, is designed for photovoltaic systems that use lenses and mirrors to concentrate the sun’s rays onto small, high-efficiency solar cells, thereby requiring far less semiconductor material than conventional solar panels. Last month Spectrolab published in the journal Applied Physics Letters the first details on its record-setting cell, initially disclosed in December, which converts 40.7 percent of incoming light into electricity at 240-fold solar concentration–a healthy 1.4 percent increase over the company’s previous world-record cell. Other groups are developing promising cells based on the new type of materials, including researchers at the Department of Energy’s National Renewable Energy Laboratory (NREL), in Golden, CO. The NREL researchers will soon publish results in the same journal showing that their NREL’s designs are tracking Spectrolab’s, improving from 37.9 percent efficiency in early 2005 to 38.9 percent efficiency today.

Metamorphic semiconductors resemble the high-efficiency cells used in space. Like the cells that grace satellites and planetary landers, they employ three layers of semiconductors, each tuned to capture a slice of the solar spectrum (solar panels have only one active layer). These semiconductor layers are assembled, one upon the next, by altering elements fed to a crystal growing in a vacuum. To avoid growing crystals filled with energy-trapping defects, device designers have until recently employed only a limited repertoire of semiconductors, such as germanium and gallium arsenide, which form similar crystal structures.

Technology Review