[Image above] Scientists at Stanford University in California have developed a new solar cell design that uses ‘invisible’ nanowires to funnel sunlight that is reflected away and lost through traditional solar cell designs. Credit: Stanford Precourt University Institute for Energy; YouTube

We’ve been seeing more trends in solar cell efficiency solutions—but scaling up that technology is still in the experimental phases.

This summer we covered research from Eindhoven University of Technology and the Foundation for Fundamental Research on Matter in the Netherlands, where scientists have developed a prototype of a solar cell that generates fuel instead of electricity.

In September, we reported on a new solution in the works for improving the efficiency of current solar cell technology. Researchers from the University of Connecticut are working on the “development of a unique, ‘green’ antenna that could potentially double the efficiencies of certain kinds of solar cells and make them more affordable,” according to a news release from the American Chemical Society.

But the most recent development in the race to improve the efficiency of solar cells, developed by scientists at Stanford University, suggests a novel method for redirecting sunlight that is otherwise reflected away and lost in standard solar cells.

“A solar cell is basically a semiconductor, which converts sunlight into electricity, sandwiched between metal contacts that carry the electrical current,” a Stanford News article about the research explains. “But this widely used design has a flaw: The critical but shiny metal on top of the cell reflects sunlight away from the semiconductor where electricity is produced, reducing the cell’s efficiency.”

The new solar cell is designed to hide the reflective upper contact and funnel that light directly into the semiconductor for added power potential.

“Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light,” Vijay Narasimhan, lead author of the study who conducted the work as a graduate student at Stanford, says in the article. “Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells.”

Narasimhan and his team created nanosized pillars of silicon that are taller than the surface of the gold film. This design allows the sunlight to be redirected to the semiconductor before it has a chance to hit the metallic surface and be reflected away and lost.

And after trial and error, the team found that creating silicon nanopillars was a simple, one-step chemical process.

“We immersed the silicon and the perforated gold film together in a solution of hydrofluoric acid and hydrogen peroxide,” Thomas Hymel, Stanford graduate student and study co-author, says in the article. “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 330 nm in height, transforming the shiny gold surface to a dark red color—an indication that the silicon nanopillars were “funneling light around the metal grid and into the silicon substrate underneath,” Narasimhan explains.


When the silicon and perforated gold film are immersed in a solution of hydrofluoric acid and hydrogen peroxide, the gold film sinks into the silicon substrate and nanopillars pop up through the holes in the film. Credit: Stanford Precourt Institute for Energy; YouTube

Narasimhan compares the process to the way a colander works when filled with water in the sink.

“When you turn on the faucet, not all of the water makes it through the holes in the colander,” he says. “But if you were to put a tiny funnel on top of each hole, most of the water would flow straight through with no problem. That’s essentially what our structure does: The nanopillars act as funnels that capture light and guide it into the silicon substrate through the holes in the metal grid.”

The team says this new technique has the potential to improve relative efficiency of solar cells by 10%.

Narasimhan explains the process in the Stanford Precourt Institute for Energy video below.

 Credit: Stanford Precourt Institute for Energy; YouTube

The research, published in ACS Nano, is “Hybrid metal–semiconductor nanostructure for ultrahigh optical absorption and low electrical resistance at optoelectronic interfaces,” (DOI: 10.1021/acsnano.5b04034).