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STANFORD University scientists are using nanotechnology to increase the efficiency of solar cells, by reducing the impact the upper metal contact has on the incoming light.
Solar cells convert sunlight into electricity, but structurally are a semiconductor sandwiched between metal contacts that carry the electrical current. The upper contact usually consists of a metal wire grid, which carries electricity to or from the device.
However, the shiny metal of the contact above the cell reflects sunlight away from the semiconductor, reducing the conversion efficiency of the cell. Until now, this was considered an unavoidable issue, duye to the irreconcilable tradeoff between electrical conductivity and optical transparency.
The Stanford University students developed a novel way, based on nanotechnology, to make the metal contact nearly invisible to incoming light, reducing its impact on solar conversion.
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.
The researchers then analysed the setup. Using optical analysis, they found that the perforated gold film covered 65 percent of the silicon surface, while reflecting 50 percent of the incoming light.
To render the gold film "invisible", the researchers chose to create nano-sized pillars of silicon that tower above the metal, redirecting the sunlight before it has the chance to hit the metallic surface.
One benefit from this approach is the relatively simple technique for creating the required silicon nanopillars: by immersing the silicon and the perforated gold film together in a solution of hydrofluoric acid and hydrogen peroxide.
This caused the gold film to start sinking into the silicon substrate, with silicon nanopillars popping up through the holes in the film.
The silicon pillars grew to a height of 330nm, and began funnelling and guiding incoming light around the metal grid into the silicon substrate beneath.
Study lead author Vijay Narasimhan, who conducted the work as a graduate student at Stanford, said the technique could significantly improve the efficiency and thereby lower the cost of solar cells.
The research team then optimised the design through a series of simulations and experiments.
"Solar cells are typically shaded by metal wires that cover 5-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 3 percent. Having that much metal could increase conductivity and make the cell far more efficient at converting light to electricity."
The research team plans to test the design on a working solar cell and assess its performance in real-world conditions.
Besides gold, the nanopillar architecture will also work with contacts made of silver, platinum, nickel and other metals.
In addition to silicon, this new technology can be used with other semiconducting materials for a variety of applications, including photosensors, light-emitting diodes and displays, transparent batteries, as well as solar cells.