Safer Salts For Solar-Cell Production

Substituting magnesium chloride for toxic cadmium chloride could free cadmium-telluride solar-cell manufacturers from a costly and hazardous process without sacrificing efficiency, according to a new study.

At the heart of a standard CdTe solar cell is a photovoltaic junction formed by the interface between neighboring thin layers of CdS and CdTe. Without processing the layers after deposition, a cell converts incident solar power into electric power with less than 5% efficiency. Manufacturers bump this up to between 10 and 20% using a junction activation step.

For more than two decades, activation has relied on CdCl₂, usually deposited as a thin coating on the cell’s CdTe layer. When the cell is heated, chloride diffuses through its stacked structure and reforms the physical and electronic characteristics of the cadmium-containing layers, leaving a more efficient photovoltaic junction.

Jon D. Major and his colleagues at the University of Liverpool, in En­gland, have now succeeded in activating cells using MgCl₂ instead of CdCl₂ without any loss of efficiency (Nature 2014, DOI: 10.1038/nature13435).

The replacement salt is “completely benign and much lower cost” than CdCl₂, Major said at a press conference. On an industrial scale, MgCl₂ is available for less than 1% of the price of CdCl₂ and requires no specialized waste disposal. “Why it hasn’t been used before, we genuinely don’t know,” he said.

Some aren’t convinced a replacement salt will result in much savings. For example, in a statement released by First Solar, the company that holds the record for CdTe solar-cell efficiency, Chief Technology Officer Raffi Garabedian said that CdCl₂ activation “is not a major cost driver in our manufacturing process.”

But smaller start-ups could reap benefits from the research, says Alvin Compaan, professor emeritus of physics at the University of Toledo and cofounder of the thin-film photovoltaic developer Lucintech. Major and coworkers “got some impressive results,” Compaan says. “I think it’s a very important advance.”