J
jeremy
4 minutes
Alkaline Texturing of Monocrystalline Silicon: KOH Pyramids That Trap Light
Alkaline texturing covers a single-crystal wafer with tiny pyramids that trap light. The wafer sits in a hot bath of potassium hydroxide (KOH) or sodium hydroxide, at around 80 C. The alkali eats silicon faster along some crystal directions than others. What remains are random pyramids a few microns tall. The reaction is Si + 2KOH + H2O -> K2SiO3 + 2H2. Hydrogen gas bubbles off, and potassium silicate dissolves in the bath. Light that bounces off one pyramid face often hits another and gets a second chance to enter. Bare polished silicon reflects about a third of sunlight. Texturing drops that to roughly 10 percent, and the later blue coating lowers it further. Old baths added isopropyl alcohol to keep the pyramids even, but much of it boiled off as vapor. Newer additives cut alcohol use and air emissions. The waste is strong alkali, silicate and hydrogen. Hydrogen is diluted with plenty of air so it cannot build up and ignite. Spent bath is neutralized with acid, and the silicate settles out as sludge. This method only works on single crystals, since the crystals in cast wafers point every which way. After this Concept you can explain why a monocrystalline cell looks dark and velvety rather than shiny.