Jon Excell

Nano-technique could make solar cells less “shiny” and more efficient

News

Researchers at Stanford University in the US have figured out a way of boosting the efficiency of solar cells by reducing the degree to which they reflect light.

Solar cells consist of a semiconductor sandwiched between metal contacts that carry the electrical current. However, the shiny metal on the top of the cell hampers its efficiency by reflecting sunlight away from the semiconductor.

The Stanford team, which was led by graduate student Vijat Narasimhan, has used nanotechnology to make this upper contact invisible to incoming light.

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.

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 per cent of the silicon surface and reflected, on average, 50 per cent 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.

Silicon pillars emerge from nanosize holes in a thin gold film. The pillars funnel 97 percent of incoming light to a silicon substrate, a technology that could significantly boost the performance of conventional solar cells.
Silicon pillars emerge from nanosize holes in a thin gold film. The pillars funnel 97 percent of incoming light to a silicon substrate, a technology that could significantly boost the performance of conventional solar cells.

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.

These pillars were produced using a one-step chemical process which involved immersing the silicon and perforated gold film in a solution of hydrofluoric acid and hydrogen peroxide “The gold film immediately began sinking into the silicon substrate, and silicon nanopillars began popping up through the holes in the film,” said study co-author Thomas Hymel.

Explaining how the pillars work, Narashiman compared the nanopillar array to a colander in your kitchen sink. “When you turn on the faucet, not all of the water makes it through the holes in the colander. 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 group claims that the technology could boost the efficiency of a conventional solar cell from 20 to 22 per cent, and now plans to test the design on a working solar cell.  “Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells,” said Narasimhan.