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Silicon 'pillars' improve efficiency of solar cells

26 November 2015

A solar cell is basically a semiconductor, which converts sunlight into electricity, sandwiched between metal contacts that carry the electrical current.

Silicon pillars emerge from nanosize holes in a thin gold film. The pillars funnel 97 percent of incoming light to a silicon substrate (image: Vijay Narasimhan, Stanford University)

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," says 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."

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," says 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 thus face a seemingly irreconcilable trade-off between electrical conductivity and optical transparency. According to Narasimhan, the nanostructure his team has created eliminates that trade-off.

For the study, the Stanford team placed a 16nm-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.

They achieved this by creating nanosized pillars of silicon that 'tower' above the gold film and redirect the sunlight before it strikes the metallic surface. Creating these silicon nanopillars involved a one-step chemical process.

"We immersed the silicon and the perforated gold film together in a solution of hydrofluoric acid and hydrogen peroxide," says graduate student and study co-author Thomas Hymel. "The gold film immediately began sinking into the silicon substrate, and silicon nanopillars began popping up through the holes in the film."

Within seconds, the silicon pillars grew to a height of 330nm, transforming the shiny gold surface to a dark red. This dramatic colour change was a clear indication that the metal was no longer reflecting light.

"As soon as the silicon nanopillars began to emerge, they started funnelling light around the metal grid and into the silicon substrate underneath," Narasimhan explains. "Solar cells are typically shaded by metal wires that cover five to ten percent of the top surface. 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 technology could boost the efficiency of a conventional solar cell from 20 percent to 22 percent, which represents a significant increase.

The research team plans to test the design on a working solar cell and assess its performance in real-world conditions.


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