Stanford scientists have discovered how to make the electrical wiring on top of solar cells nearly invisible to incoming light—a development that could boost solar cell efficiency.

In most solar cells, the upper contact consists of a metal wire grid that carries electricity to or from the device. These wires also act like a mirror and may 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 Yi Cui, associate professor of materials science and engineering. "That light is then lost and cannot be converted to electricity."

To study the problem, the Stanford team placed a 16-nanometer-thick film of gold conducting metal on a 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% of the silicon surface and reflected, on average, 50% of the incoming light. The scientists reasoned that if they could hide the reflective gold film, more light would reach the silicon semiconductor.

The solution was to create nanosized pillars of silicon that "tower" above the gold film and redirect the sunlight before it hits the metallic surface.

Silicon nanopillars funnel light through a gold metal contact to a sheet of silicon underneath. Image credit: Vijay Narasimhan.Creating silicon nanopillars turned out to be a one-step chemical process. The researchers immersed the silicon and the perforated gold film in a solution of hydrofluoric acid and hydrogen peroxide. The gold film began to sink into the silicon substrate, and silicon nanopillars began to emerge through holes in the film.

Within seconds, the silicon pillars grew to a height of 330 nanometers, transforming the shiny gold surface to a dark red color—an indication that the metal was no longer reflecting light.

"As soon as the silicon nanopillars began to emerge, they started funneling light around the metal grid and into the silicon substrate underneath," says graduate student Vijay Narasimhan.

He compares the nanopillar array to a colander or strainer in a kitchen 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."

In their best design, nearly two-thirds of the surface can be covered with metal with a reflection loss of around 3%. Having that much metal could increase conductivity and make the cell more efficient at converting light to electricity.

According to Narasimhan, the technology could boost the efficiency of a conventional solar cell from 20% to 22%. The research team plans to test the design on a working solar cell and assess its performance in real-world conditions.

In addition to silicon, this technology can be used with other semiconducting materials for a variety of applications, including photosensors, light-emitting diodes and displays, transparent batteries and solar cells.