One principal advantage of thin-film solar cells is that they use far less (maybe 1/100th the amount) of the semiconductor materials that are required to fabricate first-generation solar cells. The disadvantage is that compared with typical efficiencies of 15% for conventional solar cells, a practical efficiency of around 8% is more normal for thin-film cells.
Thin-film technologies offer the further prospect that the cells can be printed onto various flexible materials using a kind of ink-jet method, which offers numerous prospects for photo-voltaic devices in the future, beyond the simple scheme of roof-based solar panels. They are also more resistant to ionising radiation and should serve better in satellites which are in orbit above the Earth's atmosphere and hence more exposed to damage by cosmic radiation during their working lifetime of perhaps a few decades.
However, the solar-energy company MiaSolé recently reported that an efficiency of 13.8% had been achieved from thin-film panels of practical dimensions (1 m^2), rather than a much smaller lab-scale test (1). This unprecedented value has been corroborated by the DoE National Energy Laboratory (NREL).
At nearly 14%, the efficiency of the thin-film panels which are made from copper, indium, gallium and selenium (CIGS), is close to that of silicon, albeit being much cheaper to produce.
The world player in thin-film solar technologies is First Solar which makes its panels from cadmium and tellurium (Cad-Tel). While there have been steady gains in the efficiency of these, there is a practical limit of not much more than 10%, though 11.2% was reported recently (1).
While improvements in the CIGS panel efficiencies can be expected, there is the matter of accessing the materials themselves, from which they are made, most immediately indium. There are no significant ores of indium which is principally a by-product of zinc production, although roughly it is three-times as abundant (0.25 ppm) in the Earth's crust as silver (0.075 ppm). Indium is leached from slag and dust of zinc production and the metal further purified by electrolysis.
It has been estimated on the basis of the amount of indium in zinc ore stocks, there is a world reserve base of 6,000 tonnes. Given current consumption of the metal, this is sufficient for only 13 years and less than that if CIGS technology takes-off. It has been concluded therefore that in the future less than 1% of solar pv will be in the form of CIGS thin-film cells.
This prognosis is challenged by the Indium Corporation, who are the world's greatest producer of Indium, and assert that though the adaptation of more efficient recovery methods and from other ores, including tin, copper and other polymetallic deposits in hand with an expansion of mining operations, the supply of indium will prove sustainable (2).
Recycling of indium along with other rare metals (3) will also prove pivotal. It is nonetheless to be expected that there may be a supply-demand gap for indium, with a substantial escalation in its price, at least over the immediate term which is likely to impact on the inauguration of the thin-film CIGS cell industry. A rise in the price of indium is forecast as emerging CIGS and more established and steady LED and LCD markets compete for it, leading to a plan to stockpile the metal in the expectation of realising greater profits from it in the future (4).
By. Professor Chris Rhodes
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Professor Chris Rhodes is a writer and researcher. He studied chemistry at Sussex University, earning both a B.Sc and a Doctoral degree (D.Phil.); rising to become the youngest professor of physical chemistry in the U.K. at the age of 34.
A prolific author, Chris has published more than 400 research and popular science articles (some in national newspapers: The Independent and The Daily Telegraph)
He has recently published his first novel, "University Shambles" was published in April 2009 (Melrose Books). http://universityshambles.com
