Cadmium Chloride Activation of CdTe Cells: Healing Grain Boundaries

… more less Cadmium chloride activation is the step that makes a CdTe cell work well. Right after deposition, the CdTe film is made of many tiny crystals called grains. The edges between grains, called grain boundaries, trap charge carriers and waste the light. The factory coats the film with CdCl2, a cadmium salt, and bakes it at roughly 400 degrees C. Chlorine moves into the grain boundaries and heals them, and the grains grow larger. Without this step, efficiency stays very low. With it, the cell reaches commercial levels. The catch is that CdCl2 dissolves in water, unlike CdTe. That makes it the more mobile and more toxic form of cadmium. Leftover salt and rinse water must be collected, not drained. Plants treat that water by dropping the cadmium out as a solid. The sludge is then recycled or sent away as hazardous waste. Researchers have shown that magnesium chloride, MgCl2, a cheap and harmless salt, can do much of the same job. After this Concept you can explain why a toxic salt is used to finish CdTe cells and how its waste is controlled.