CIGS cell makers battle it out for efficiency crown

Solar cells made from copper indium gallium selenide, or CIGS, are relatively new, but there's already a nice rivalry brewing in the industry.

Tucson's Global Solar has begun to produce flexible CIGS solar cells in volume that can convert a little over 10 percent of the sunlight that strikes them into electricity, which it says is a record.

Scientists at the National Renewable Energy Labs (NREL) have achieved higher results in the lab, admits Global CTO Jeff Britt. NREL has built a CIGS cell on glass that is 19.5 percent efficient and a CIGS cell on a flexible metal substrate that can convert 17.5 percent of sunlight into power.

But mass production is an entirely different story. Many of the CIGS start-ups have had trouble producing high-efficiency cells on a commercial scale. Last year, Miasole delayed releasing its solar cells because the products coming off the line were generally exhibiting 4 to 6 percent efficiency, below the company's 8 to 10 percent target.

"At the start of 2007, we were at a little over 9 percent and now we are over 10 percent," said Britt. The company also said it plans to raise efficiency to 13 to 14 percent in 2008.

Not so fast, says Martin Roscheisen, CEO of Nanosolar, who says the company's flexible CIGS cells, which started coming out last month, are at the same level.

Credit: Global Solar

A flexible solar cell made from CIGS
by Tucson, Ariz.-based Global Solar.

"And we achieve them with a process and a materials stack that's a factor less expensive," Roscheisen added.

Only a few companies have been able to get CIGS cells into production. Silicon Valley's Nanosolar started shipping CIGS cells on flexible substrates in December. Global has been producing CIGS cells for the last three years, but in limited quantities. DayStar Technologies has produced a limited number of CIGS cells, but the company uses a glass substrate.

The CIGS solar cells from these companies all rely on a similar chemistry to convert sunlight to electricity. Where they differ is in manufacturing. Nanosolar uses printing techniques to make its cells while Miasole, among others, sputters the chemicals onto a substrate. (Solopower, meanwhile, binds the CIGS chemicals to substrates through electroplating.)

Global Solar uses evaporation: the active chemicals are immersed in a solution, which then gets removed. Critics, and Roscheisen is lead among them, say evaporation can't cut it.

"Co-evaporation is very slow and very expensive," he said.

Bah, says Britt. Evaporation leads to the highest efficiency cells. It's one of the reasons NREL and other scientists have used evaporation to produce their own devices.

"You get greater control with evaporation," Britt said. "The reason they (other CIGS companies) don't do it is because they don't know how to do it. Efficiency is the key driver to success and you cannot be a success at six percent."

Evaporation is also fast. The company can coat a 1,000-foot roll of stainless steel with CIGS cells in a day, he said.

The next step for both companies will be to move from mass manufacturing to large scale mass manufacturing. Global currently has a facility that can crank out 4 megawatts worth of solar cells a year. In the first half of 2008 it will open up facilities in Tucson, Ariz. and Berlin. By the end of the year, Global will have enough capacity to produce 64 megawatts worth of solar cells a year.

Nanosolar, meanwhile, is building out a factory in Silicon Valley that will be ultimately of producing 430 megawatts worth of cells a year.

Meanwhile, HelioVolt will commence mass production in 2009, and Miasole will jump into the market soon after.

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