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A solar cell is basically a semiconductor, which converts sunlight into electricity, sandwiched between metal contacts that carry the electrical current.
But this widely used design has a flaw: The shiny metal on top of the cell actually reflects sunlight away from the semiconductor where electricity is produced, reducing the cell's efficiency.
Now, Stanford University scientists have discovered how to hide the reflective upper contact and funnel light directly to the semiconductor below. Their findings, published in the journal ACS Nano, could lead to a new paradigm in the design and fabrication of solar cells.
"Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light," said study lead author Vijay Narasimhan, who conducted the work as a graduate student at Stanford. "Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells."
Mirror-like metal
In most solar cells, the upper contact consists of a metal wire grid that carries electricity to or from the device. But these wires also prevent sunlight from reaching the semiconductor, which is usually made of silicon.
"The more metal you have on the surface, the more light you block," said study co-author Yi Cui, an associate professor of materials science and engineering. "That light is then lost and cannot be converted to electricity."
Metal contacts, therefore, "face a seemingly irreconcilable tradeoff between electrical conductivity and optical transparency," Narasimhan added. "But the nanostructure we created eliminates that tradeoff."
For the study, the Stanford team placed a 16-nanometer-thick film of gold on a flat sheet of silicon. The gold film was riddled with an array of nanosized square holes, but to the eye, the surface looked like a shiny, gold mirror.
Optical analysis revealed that the perforated gold film covered 65 percent of the silicon surface and reflected, on average, 50 percent of the incoming light. The scientists reasoned that if they could somehow hide the reflective gold film, more light would reach the silicon semiconductor below.
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