New research from N.C. State University has found a way to make stacked solar cells – already the most efficient solar cells on the market – even more efficient and less expensive.

Stacked solar cells can convert nearly 45 percent of the solar energy they absorb into electricity. This high conversion rate comes from the stack. Sunlight is comprised of different colors. Stacked solar cells consist of single cells, each one capturing a different color of the spectrum, explained Salah Bedair, professor of electrical engineering at N.C. State. Stacking the cells allows for the absorption of light from across the solar spectrum.

The efficiency of stacked solar cells increases with the concentration of solar power. At one “sun,” a measure of the solar energy received by the Earth, a stacked solar cell can reach 26 percent efficiency.

The new research is the latest in advances for NCSU and Bedair dating back some 30 years.

Some energy companies are interested in using lenses to concentrate solar energy and in turn boost the amount of electricity generated from the solar cells. More suns would mean more power. But at 700 suns or more, the connecting junctions that link the cells in the stacks start to lose voltage. At that point, intensifying the solar energy increases the voltage lost resulting in diminishing efficiencies for the solar cell stack. Energy is lost as heat.

Bedair has found a way to make a connecting junction that loses little voltage. Inserting a thin film of gallium arsenide into the connecting junctions that link the individual cells the stack eliminates nearly all voltage loss without blocking solar energy.

“When we tried that, the connecting junction, rather than handling 10 suns or 100 suns, allowed us to handle 70,000 suns,” Bedair said.

Energy companies won’t need to reach 70,000 suns. Lenses are expected to concentrate sunlight at between 4,000 and 5,000 suns. At that level, Bedair says the efficiency of the solar stack can reach 46 percent, perhaps 47 percent.

The new approach to connecting junctions should lead to solar modules that cost less. The expensive components in these modules are the solar cells, not the lenses. By using lenses to concentrate sunlight and get more power out of fewer cells, solar modules can be smaller and will cost less to manufacture and deploy.

But Bedair said that his research is still four or five years away from reaching the market. His next step is to talk with N.C. State’s Office of Technology Transfer office to determine next steps.

Bedair’s research, which was funded by the U.S. Department of Energy and the National Science Foundation, was published online in Applied Physics Letters.