Cadmium Sulfide Buffer Layer: Chemical Bath Deposition and Its Wastewater

… more less A cadmium sulfide buffer layer is a very thin coat on top of the light-absorbing film in many CIGS cells and some CdTe cells. It is only tens of nanometres thick. The factory grows it by chemical bath deposition. The coated glass dips in a warm water bath holding a cadmium salt, thiourea as a sulfur source, and ammonia. Thiourea slowly releases sulfide ions. They meet cadmium ions and form solid CdS: Cd2+ + S2- -> CdS. Only a small share of the cadmium lands on the film. Most of it stays in the spent bath or forms loose specks. That makes the bath a cadmium wastewater stream. Plants treat it by raising the pH or adding sulfide, so the cadmium drops out as a solid. Then they filter it and recycle or dispose of the sludge. The ammonia needs treatment too. To shrink the problem, makers thin the CdS layer or switch to cadmium-free buffers such as zinc oxysulfide. Each swap can cost a little efficiency. After this Concept you can explain where cadmium wastewater comes from in a thin-film factory and how it is removed or replaced.