Silicon solar cells have enjoyed incremental efficiency increases over the past few years, with many now able to convert up to 25% of sunlight into electricity. However, manufacturing silicon wafers, which need to be around 300 microns thick to absorb all the sunlight that reaches them, requires temperatures of around 1,200 degrees Celsius, a procedure that brings significant costs. Telluride, a cheaper alternative to silicon, is however highly toxic and known to cause cancer.
Such conditions prompted Goutam Dalapati and other colleagues from Singapore’s A*STAR institute of Materials Research & Engineering to investigate a copper-zinc-tin-sulfide (CZTS) compound, which offers the optical and electrical properties required in solar cells, and is made from non-toxic, widely available materials that are cheaper than silicon to process.
“CZTS is a semiconducting compound with a higher absorption coefficient than silicon,” says Dalapati, “so it’s able to absorb more visible light and produce more electricity than silicon, and can be used for very large-scale applications, like roofs and solar farms.”
Solar cells made from CZTS have the potential for up to 30% efficiency, he claims. However, they require high-quality, thin film of CZTS with no impurities, and a suitable material for the interface layer that sits underneath the CZTS, helping to collect electrical charge.
The thin films of CZTS were grown using a technique called quaternary sputtering, where a single target made from CZTS was used as a source for depositing the film.
There are several advantages offered by this method over deposition methods. These include excellent uniformity over large areas and reduced reliance on toxic precursors.
“The composition and structural properties of the CZTS layer depend on the deposition process and the sulfurization,” explains Dalapati. “By using a single-step sputtering target we were able to produce a thin film with a uniform composition and smooth surface, which limits the formation of defects, and is a highly reproducible process.”