Yi Cui, associate professor of materials science and engineering at Stanford University, said: "The more metal you have on the surface, the more light you block. That light is then lost and cannot be converted to electricity."
Vijay Narasimhan, study co-author, explained: "Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light. Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells."
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.
Optical analysis revealed that the perforated gold film covered 65% of the silicon surface and reflected, around 50% of the incoming light. The scientists reasoned that if they could hide the reflective gold film, more light would reach the silicon semiconductor below.
The team created nanosized pillars of silicon that "tower" above the gold film and redirect the sunlight before it hits the metallic surface.
Creating silicon nanopillars turned out to be a one-step chemical process.
"We immersed the silicon and the perforated gold film together in a solution of hydrofluoric acid and hydrogen peroxide," said 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."
The silicon pillars grew to a height of 330nm, transforming the shiny gold surface to a dark red. This colour change was a clear indication to the team 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 explained. “The nanopillars act as funnels that capture light and guide it into the silicon substrate through the holes in the metal grid."
It is said that this 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.
