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The Promise and Unanswered Questions About CZTS Solar Technology

After spending years working on commercializing copper-indium-gallium-selenide (CIGS) thin films, AQT Solar is now focusing on engineering thin films that use copper, zinc, tin and sulfur (CZTS) instead. Last week, the Silicon Valley company said it had created a prototype CZTS panel and could one day deliver cheap cells for its customers, who will then assemble the cells into panels.

The announcement came just several months after a thin film expert publicly cautioned AQT about making empty promises and “giving another technology a bad name.” It was not hard to see why the expert, Rommel Noufi at the National Renewable Energy Laboratory, was wary of any company making bold claims. AQT, along with many other venture-backed startups, has been trying to carve out a sizable slice of the market with CIGS cells and panels. But CIGS technology has yet to fulfill its destiny despites receiving billions of dollars in private investments, and some of the companies have been around for about a decade.

CIGS thin film makers such as Stion, Nansolar and SoloPower are soldiering on with plans for factory expansion and hopefully sales, too, while a few have gone bankrupt in the past year, including Solyndra and Soltecture. Global Solar Energy is looking for investors in earnest, something that its CIGS compatriots such as Nanosolar, HelioVolt and MiaSole have done in the past 12 months to survive a global market that has been marked by an oversupply of panels and falling prices.

AQT has missed some goals on CIGS cell production and sales it set a few years back, and soon after it announced an $18.7 million round in January this year, it said it had turned its attention to CZTS cells. The company also has said it would continue to make CIGS cells.

Can AQT really find success in developing and selling CZTS cells? The answer to that is what makes it interesting to follow AQT’s attempt at bringing to market the CZTS technology. CZTS technology falls in the “novel” category, and its efficiency puts it toward the low end of the solar cell technology spectrum. The company declined to disclose the efficiency or dimensions of its prototype, 60-watt CZTS panel. When he spoke at a Photon conference in San Francisco a few months back, AQT’s CEO, Michael Bartholomeusz, promised to commercialize the CZTS technology in 2013.

So what is it about the CZTS blend that is worth the effort and what are the obstacles? I caught up with Noufi and a scientist at IBM – which has lined up companies as research partners to work on CZTS thin films – to learn more.  One of IBM’s research partners is Japan-based Solar Frontier, a subsidiary of Showa Shell and a maker of CIGS cells and panels. Solar Frontier has built a 900 MW factory to leap ahead of all those venture-backed startups and is now the largest CIGS thin film maker in the world.

Solar technology is very much about material science, and an important factor to consider is how easily and cheaply will companies be able to buy the ingredients they need. It turns out that zinc and tin are more abundant and found in more diverse regions around the world than indium and gallium. That means getting a steady of supply of these two materials could involve less political hassles and lower costs. But whether the savings will be significant enough is an unanswered question because the two materials represent only part of a much longer equation for calculating production costs.

Using CZTS also makes it possible to create a much thinner cell than a silicon cell for absorbing sunlight and produce electricity, so there could be savings in using a smaller quantity of materials. The crystal structure of the CZTS layer lends itself to becoming high-performing multicrystalline film, which would be easier to make than the usually more expensive but also more efficient monocrystalline version, said David Mitzi, manager of IBM’s photovoltaic technology research.

Finally, what drew IBM to CZTS research is the pace of efficiency improvement since the mid-1990s, Mitzi said. By 2008, the cell efficiency hit 6.7 percent, and in 2010, IBM reported a record 9.7 percent cell efficiency. Last year, the company published a paper in Progress in Photovoltaics that touted a cell efficiency of 10.1 percent (selenium was used as well).

Mitzi said 15 percent efficiency, at the minimum, is a good goal to aim for if CZTS is to compete against other solar cell technologies. In various academic and company labs, CIGS cell efficiencies have reached around 20 percent while CIGS panel efficiency has hit 15.7 percent (by MiaSole). First Solar, which uses cadmium-telluride for its thin films, said it’s able to achieve 17.3 percent cell efficiency and 14.4 panel efficiency in its lab.

“We hope to get to that (15 percent efficiency) in two years or so,” Mitzi said. “There are many unknowns.”

Indeed, whether CZTS cells can be commercially viable is very much in question. Researchers haven’t spent as much time studying the prospect of using CZTS for solar cells as they have using CIGS and other compounds. They are working on answering key questions such as the best ways to deposit the CZTS materials, how materials interact and change during the production process and how to best control the electronic and optical properties of the materials, Noufi said. He, too, sees 15 percent efficiency as a good goal to reach in order to determine the market potential of the CZTS technology.

“It’s in a pure R&D stage. If people start talking about manufacturability and cost, then it’s premature,” Noufi said. “To be able to talk about costs, you’ll have to have a well defined process in place. You would have to define your capital equipment, throughput and yields to some degree, and reach a competitive efficiency in order to be able to do the cost analysis.”

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