Researchers from the SLAC National Accelerator Laboratory and the National Renewable Energy Laboratory are looking to make solar module manufacturing cheaper. And the way to do that, taking silver out of the manufacturing process.
“By far the most commercially viable option for photovoltaic energy generation, crystalline silicon, continues to dominate the industry with over 90 (percent) market share,” the researchers wrote in a study appearing today in Nature Communications. “Optimally designed silver front-contacts in the majority of (crystalline silicone) solar cells utilize narrow grid lines (approximate width of 50 µm) to minimize shading loss and achieve high current, high-fill factor, and hence, high photo-conversion efficiency.”
But silver isn’t cheap. SLAC staff scientist Mike Toney, one of the study’s authors, believes silver accounts for roughly 10 to 15 percent of the total cost to manufacture a solar cell. To find an alternative, the researchers had to first get a glimpse inside the firing process during manufacturing, a previous unknown.
“Being able to detect this is so hard mostly because it happens so fast,” Toney told R&D Magazine. Toney has been a staff scientist with SLAC for close to 13 years.
They knew that a paste comprised of silver particle, glass, and lead oxide was printed onto the solar cell’s surface. From there, the cell goes through a furnace set at 1,500 F in under a minute, and then cools. The result, a cell with electrical contacts.
But the activity inside the furnace was like a “black box,” according to National Renewable Energy Laboratory’s Maikel van Hest, who led the research.
According to Toney, scientists had been debating what occurs during the heating process for roughly 10 years.
To glimpse the process, the researchers used X-ray diffraction during the firing process to reveal the chemistry. The X-ray was fired through the window of a specially designed furnace.
“The result showed that lead oxide plays a key role in forming the contact, etching away the solar cell’s antireflective coating so silver can move through, pool and eventually harden in small pits on the silicon surface,” according to SLAC. “Once cooled, the finished contact contains solid silver blobs that have been squeezed together by heat; tiny silver particles in a layer of solid glass; and solid silver on the silicon surface. All three types of silver are needed to make the contact effective.”
Unlike before, the researchers now have a method for measurement during the contact formation process. They hope it will aid in research for finding an alternative to silver.
“It gives us some kind of pointers as to the chemical nature of the (alternative) metal that one would want to try to use,” Toney concluded.
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