A Safer Way to Coat Long-Lasting Solar Cells
Antireflective film helps the cells maintain their yield.
A venture spun out of two Quebec universities says it has developed a safer way of adding antireflective coatings to crystalline silicon solar cells that also boosts their lifetime energy yield.
In the solar photovoltaic market, even the smallest improvement in efficiency can have a meaningful impact on manufacturers’ bottom line, which is why antireflective coatings are so important. These thin coatings, which cause solar cells to appear blue, maximize how much sunlight is absorbed and reduce surface defects that can lower performance.
However, the most popular coating method–the vapor deposition of a silicon nitride film using silane gas–comes with major risks. Silane can ignite when exposed to air; the gas is costly to transport, and silicon cell manufacturers must invest in special storage, ventilation, and other safety measures to prevent accidents.
“The potential for damage is huge,” says Ajeet Rohatgi, director of the Photovoltaic Research Center at the Georgia Institute of Technology. Cells coated this way are also affected by a phenomenon called light-induced degradation that occurs once after the first 24 to 48 hours of sunlight exposure. “In a cell with 18 percent efficiency, you will see efficiency drop [almost immediately] to 17.7 or 17.5 percent, and you’ve lost that for the life of the cell,” he says.
Rohatgi and his team of researchers at Georgia Tech have spent the past 18 months testing a new silane-free process for applying antireflective film to solar cells, which was developed by Montreal-based Sixtron Advanced Materials. The coating–a silicon carbide nitride material carrying the trade name Silexium–reduces light-induced degradation by up to 88 percent.
Crystalline silicon wafers, which are usually doped with boron, also contain oxygen. When sunlight first hits a new cell it causes boron and oxygen to combine, resulting in a 3 percent to 5 percent degradation in cell efficiency. The researchers found that when the Silexium film is added, some of the carbon in the coating ends up diffusing into the bulk of the silicon wafer. They believe the carbon competes with the boron to make a bond with oxygen. Because there’s less oxygen for the boron to bond with, light-induced degradation is largely avoided.
Abasifreke Ebong, assistant director of Georgia Tech’s Photovoltaic Research Center, says to confirm that this is happening, the next step is to study the oxygen content of the solar wafers after they’re removed from the firing furnace. If the oxygen is lower, the theory holds. “That’s the data we’re waiting for,” he says.
According to Mike Davies, senior vice president at Sixtron, every 0.1 percentage of net efficiency spared from light-induced degradation results, on average, in a $600,000 gain in profit margin for each 60-megawatt cell production line.
Sixtron’s system eliminates the silane gas hazard, relying instead on a proprietary solid polymer material that contains silicon and carbon. Using heat and pressure, the solid is converted to a less dangerous methyl silane gas during the cell-coating process. The solid-to-gas conversion takes place inside the company’s gas-handling cabinet system, called SunBox, which has been designed to plug directly into industry-standard systems that exist on most cell-production lines.
Joshua Pearce, a professor of advanced materials at Queen’s University in Kingston, Ontario, says Sixtron may be overstating the risks of using silane in a photovoltaic cell plant. “There are standard safety procedures that make working in a photovoltaic factory very safe,” he says. Still, he adds, “anything to drop the cost of photovoltaic, even if by a small amount, is a great contribution.”
Sixtron says it is already working with the top three providers of photovoltaic cell manufacturing equipment in Germany, and has interest from several others. The company plans to rent out the system at a cost roughly the same as using a silane-based system. Importantly, it avoids the need to use other light-induced degradation reduction strategies, based on alternative manufacturing methods or the use of higher-cost wafers doped with gallium.