"Covert contacts" enable more efficient solar cell design
You've probably noticed that solar panels sitting on people's roofs appear to be broken up into grids. These grid lines are actually metal contacts and, although they're necessary for conducting the electrical current generated by the underlying semiconductor, they reduce the amount of sunlight reaching the semiconductor layer. Now researchers at Stanford University have developed a way to make these reflective metal contacts almost invisible to incoming light, thereby increasing solar panel efficiency.
Although the upper metal contacts are relatively thin, they can cover around five to 10 percent of the surface area of a solar panel, which means five to 10 percent of the sunlight that would otherwise go towards generating electricity is simply reflected away. What the Stanford researchers have essentially done is find a way to squeeze the underlying semiconductor through the metal contacts so they are almost invisible to the the incoming light.
To achieve this, the researchers placed a 16-nanometer-thick film of gold conducting metal on a flat sheet of silicon. Although the gold film appeared as a solid gold film to the naked eye, it was actually perforated with an array of nanosized square holes that meant it only covered 65 percent of the silicon surface and reflected, on average, 50 percent of the incoming light.
Immersing the gold film and silicon in a solution of hydrofluoric acid and hydrogen peroxide caused the gold film to sink into the silicon substrate as silicon nanopillars poked through the holes in the gold film. The result of this one step chemical process was what the term have dubbed "covert contacts," with the shiny gold surface turning to a dark red in a matter of seconds as the silicon pillars grew to a height of 330 nanometers.
"As soon as the silicon nanopillars began to emerge, they started funneling light around the metal grid and into the silicon substrate underneath," says study lead author Vijay Narasimhan, who likened the nanopillar array to a colander in a 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."
Through a series of simulations and experiments, the research team optimized the design so that nearly two-thirds of the surface could be covered with metal with only a reflection loss of just 3 percent. Narisimhan says building solar cells with that much metal could increase conductivity to significantly improve the solar efficiency of the panel, estimating a conventional solar cell could be boosted from 20 to 22 percent efficiency.
While the initial breakthrough was made with gold contacts, study co-author Ruby Lai says the nanopillar architecture will also work with silver, platinum, nickel and other metals. Additionally, the team says the technology can also be used with semiconducting materials other than silicon, opening up potential applications in photosensors, LEDs, displays and transparent batteries.
The team now plans to put the new design to the test in a working solar cell under real-world conditions.
Narasimhan discusses the technology in the video below.
Source: Stanford University