The future of solar power looks bright. Stanford scientists have developed a nanostructure that makes the electrical wiring on top of solar cells almost invisible to incoming light. This could potentially increase the efficiency of conventional cells from 20 to 22 percent, which sounds small, but is significant according to the researchers.
The new design uses silicon nanopillars to reflect light away from the wires and into the solar cells, which are made of semiconductor materials sandwiched between two metal contacts that carry the electricity generated by sunlight hitting the semiconductor. The metal contact on top reduces the efficiency of the solar cell because it blocks and reflects light away.
"Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light," lead author of the study, Vijay Narasimhan, said in a statement. "Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells."
In the study, the team coated a silicon wafer with a 16-nanometer-thick film of gold. To let light through, the metal layer was perforated and it ended up covering 60 percent of the silicon. This set up allowed for 50 percent of the light to reach the solar cell. To improve efficiency, the team constructed nanopillars (which, as they sound like, are really small pillar-like structures) on the gold film to increase the amount of light reaching the silicon.
"We immersed the silicon and the perforated gold film together in a solution of hydrofluoric acid and hydrogen peroxide," said Thomas Hymel, coauthor of the study published in ACS Nano. "The gold film immediately began sinking into the silicon substrate, and silicon nanopillars began popping up through the holes in the film."
The pillars grow to a height of 330 nanometers and act like funnels for light approaching the solar cell. The new configuration allowed for 97 percent of the light to reach the silicon wafer.
"Solar cells are typically shaded by metal wires that cover five to 10 percent of the top surface," Narasimhan said. "In our best design, nearly two-thirds of the surface can be covered with metal, yet the reflection loss is only three percent. Having that much metal could increase conductivity and make the cell far more efficient at converting light to electricity."
The team is now planning to test this design to establish the performance of this technology in real-world conditions.
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