New tech for more efficient solar cells

Scientists, including one of Indian-origin, have developed a new technology that could significantly improve the efficiency and lower the cost of solar cells.
Researchers at Stanford University created silicon 'nanopillars' on solar cells to hide the reflective metal contact and funnel light directly to the semiconductor below.
"Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light," said study lead author Vijay Narasimhan, who conducted the work as a graduate student at Stanford University.
"Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells," Narasimhan.

In most solar cells, the upper contact consists of a metal wire grid that carries electricity to or from the device. However, 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," said study co-author Yi Cui, an associate professor of materials science and engineering.

Metal contacts, therefore, "face a seemingly irreconcilable tradeoff between electrical conductivity and optical transparency," Narasimhan said.
For the study, the researchers placed a 16-nanometre-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 showed 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.
They 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 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," Hymel said.
Within seconds, the silicon pillars grew to a height of 330 nanometres, 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 said.
The study was published in the journal ACS Nano.