Antireflective coatings add efficiency to solar panels, but the ones that are currently in use have a major drawback in the sense that they lose efficiency after the first 24 to 48 hours of use in sunlight. Even the smallest drop in efficiency can have disastrous consequences on the market, knowing that every percent counts for the user, cumulated over the solar cell’s lifetime.
Up to now, the most used antireflective coating method has been the vapor deposition of a silicon nitride film by using a highly-flamable silane gas, which can ignite when exposed to air. Transporting, storing, ventilating silane gas and other safety-keeping operation make the process very expensive.
“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.
Along with other fellow researchers from Georgia Tech, Rohatgi studied during the last 18 months how to apply the antireflective film on the solar cells without using silane. The coating used this time is called “Silexium”, has been developed by Sixtron Advanced Materials from Montreal and reduces the process of light-induced degradation by 88 percent.
Regular crystalline silicon solar cells, usually doped with boron, also contain oxygen. When sunlight hits the silicon wafer it causes the oxygen and boron to join (oxidize), thus lowering the cell’s efficiency by 3 to 5 percent. Rohatgi and his team found out that when they added Silexium film, some of the carbon in the coating ended up diffusing into the bulk of the silicon wafer, competing with the boron to make a bond with the oxygen. Because there’s less oxygen for the boron to bond with, light-induced degradation is largely avoided.
The next step that the researchers will make will be studying the oxygen content in the solar wafers after they are removed from the firing furnace, and expect it to be lower to prove their theory.
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.
It may not seem much, but it’s an advance that the industry will take if they would use Sixtron’s coatings, or others alike. Anything that could lower the price of photovoltaics is welcomed by the market.