Electrical contacts take up 5 to 10 percent of the surface of conventional solar cells, reflecting sunlight and hampering power generation. A simple chemical reaction can cause those contacts to recede while exposing more of a cell's energy-harvesting semiconductor material to the light. The method could boost the efficiency of conventional silicon solar cells from 20 percent to 22 percent, according to researchers at Stanford University. "Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light," said former graduate student Vijay Narasimhan. "Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells." The researchers created a test cell out of a silicon substrate coated in a 16-nm-thick gold film containing nanoscale square apertures. Optical analysis revealed that the gold film covered 65 percent of the silicon surface and reflected, on average, 50 percent of the incoming light. Immersing the cell in hydrofluoric acid caused silicon pillars to grow through the holes in the thin film to a height of 330 nm. This changed the gold cell dark red, an indication that it was reflecting much less light, the researchers said. "We call them covert contacts, because the metal hides in the shadows of the silicon nanopillars," said graduate student Ruby Lai. The research team then optimized the design through a series of simulations and experiments. The best design used a silicon nitride antireflection layer to achieve broadband absorption up to 97 percent, despite the fact that the gold film covered 60 percent of the cell's surface. The research team next plans to test the design on a working solar cell and assess its performance in real-world conditions. Besides gold, the nanopillar architecture will also work with contacts made of silver, platinum, nickel and other metals. And in addition to silicon, this new technology can be used with other semiconducting materials for a variety of applications, including photosensors, LEDs and displays, transparent batteries, as well as solar cells. "With most optoelectronic devices, you typically build the semiconductor and the metal contacts separately," said professor Yi Cui. "Our results suggest a new paradigm where these components are designed and fabricated together to create a high-performance interface." Funding came from the U.S. Department of Energy's Bay Area Photovoltaic Consortium. The findings were published in ACS Nano (doi: 10.1021/acsnano.5b04034).
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