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Paint it Black: One-Step Etch Cuts Solar Cell Costs

NREL's technique provides the solar cell manufacturing industry with a quicker, cheaper way to produce large volumes of high-performance PV devices.

Photo of a group of six people (one woman and five men) standing together in front of various laboratory machinery. Enlarge image

The scientists responsible for developing NREL's game-changing black silicon. In the front row, left to right, are Anna Duda, Howard Branz, and Vern Yost. In the back row, left to right, are Matt Page, Scott Ward, and Hao-Chih Yuan.

Cost is a major obstacle for the photovoltaic (PV) industry on its path to utility-scale deployment because generally, when it comes to solar cells, higher efficiencies mean higher price tags. But a team of NREL scientists has defied this expectation with their "black silicon" nanocatalytic wet-chemical etch. Its name may be a mouthful, but this one-step process creates high-efficiency solar cells based on an innovative antireflection approach that promises to significantly reduce manufacturing costs, thereby helping propel PV toward cost-competitiveness.

A New Approach to Antireflection

Any light reflected from a solar cell surface is wasted, so PV manufacturers constantly seek ways to increase the amount of light absorbed by their solar cells. But standard antireflection methods, including antireflective coatings and etching large pyramids into the cell surface, add considerable cost to a solar cell—and they succeed only to a point: cells using both techniques still absorb only between 93% and 97% of the sunlight they receive.

NREL has long been a leader in PV research and development, and led by Principal Scientist Howard Branz of NREL's Silicon Materials and Devices group, a team from the National Center for Photovoltaics had an idea. Using an untried antireflection principle, the team invented a simple process that submerges silicon wafers (the most common solar cell material) in a chemical bath. There, catalytic reactions rapidly etch millions of tiny holes in the silicon. This turns the wafers black, allowing them to absorb more than 98% of the sunlight they receive. At a cost of just a few cents per watt, the one-step technique has produced a black silicon cell with a validated 16.8% conversion efficiency.

A Cheaper Method, A Better Material

As a replacement for conventional antireflection methods, NREL's black silicon etch could reduce the cost of energy delivered over the life of a silicon PV array by 2% or 3%—significant progress toward the nation's solar energy goals. Plus, the NREL method actually simplifies solar cell processing, speeds throughput, uses less costly machinery, and produces fewer harmful byproducts than conventional antireflection techniques, making NREL's technology particularly appealing to PV manufacturers.

NREL won an R&D 100 Award in 2010 for the cutting-edge process, but in the lab's spirit of innovation, Branz's team hasn't stopped there. They are working to produce cells with efficiencies greater than 19% by reducing reflection and improving the conversion efficiency for blue light in the solar spectrum.

Learn more about black silicon.

Photo credit: Dennis Schroeder

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