Thin films: ready for their close-up?

New types of solar cell that can be mass-produced cheaply, and integrated into building materials, are popular with venture capitalists and market analysts. But scientists are less gung ho, reports Declan Butler.

Declan Butler

From the 1950s onwards, big chunks of crystalline silicon have dominated the world of solar cells. But the dominance of these traditional cells — which make up 90% of today's 10-gigawatt-a-year installation market — is now being challenged by 'thin-film' solar cells that are micrometres or mere nanometres thick, and frequently made of materials other than silicon. Some argue that such a change in technology is the only way that solar-cell technology can hope to maintain the 50% annual growth it has enjoyed during the past five years.

Highly-purified silicon is expensive, and the fact that its other users, silicon-chip makers, manufacture high-value products from it has helped to keep it so. That presents a problem for makers of a commodity such as solar cells, a problem exacerbated by manufacturing processes that waste a significant amount of raw material. Thin films, by comparison, can in principle be made cheaply by using reel-to-reel processes similar to those of a printing press and other mass-production techniques. They can also be applied on to flexible sheets of steel and other materials, and so be integrated directly into building materials. These advantages, though, come at a cost. Thin films are normally significantly less efficient than traditional silicon cells, which have conversion efficiencies of 15% or upwards in everyday use (see table).

Nevertheless, Nanomarkets, a consultancy based in Glen Allen, Virginia, issued a report on 21 July that predicted thin films taking as much as half of the world market for photovoltaic cells by 2015. Lux Research, a New-York based analyst firm, reports that investment in the sector climbed from US$481 million in 2006 to $1.36 billion in 2007.