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.

Sun screen: A Sixtron technician holds a solar cell layered with the company's silicon carbide anti-reflective coating. The process is safer and can be integrated into existing manufacturing lines. Credit: Sixtron

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.

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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.