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NREL Sows the Seeds for a Thin-Film Solar Cell Bounty

From the 2007 Research Review.

A recent crop of thin-film solar cell companies is growing in influence in the U.S. solar cell industry, thanks in part to years of care and nurturing by NREL.

Solar cells work by capturing photons of sunlight in a photovoltaic material, which is generally a semiconductor with at least one loosely-bound electron that can be freed from its atom by sunlight within a narrow range of wavelengths. Most of today's solar cells are made from wafers of crystalline silicon, which is also used in computer chips. But NREL has long been involved in the development of alternative solar cells made from thin films of semiconductors such as amorphous silicon, cadmium telluride, and alloys of copper, indium, gallium, and selenium known as "CIGS."

These thin-film solar cells have a number of advantages over traditional crystalline silicon solar cells: they use less semiconductor material, so they are generally less expensive, and they can be deposited rapidly onto a variety of substrates, such as glass, stainless steel, or even plastic. And while crystalline silicon is used to form individual solar cells, which are then assembled into large rectangular modules, thin-film manufacturers actually deposit the entire module at one time, avoiding some of the labor-intensive assembly that individual solar cells require. This provides potential cost savings while allowing for a wide range of possibilities for the form of the final product.

A perfect example of this wide range of possibilities can be found in the Uni-Solar brand of solar shingles, which consist of amorphous silicon deposited onto flexible stainless steel. The solar module is encapsulated in a clear plastic to yield a product that looks and functions like a shingle but also produces electricity.

United Solar Ovonic, a subsidiary of Energy Conversion Devices, Inc., first introduced the Uni-Solar shingles in the mid-1990s. The company then evolved the product into "peel and stick" laminates that can be directly bonded to the roofs of buildings. The solar laminates are one of the most popular examples of building-integrated photovoltaic devices, that is, solar cells that also serve a structural purpose for the building.

Building an Industry

United Solar Ovonic is also an example of the thin-film solar revolution that is now taking place. NREL has long supported the research efforts of a number of companies that explored each of the thin-film technologies in what could be described as a friendly competition. At the same time, in-house teams of NREL scientists and engineers also tried to perfect the technologies. The story of United Solar Ovonic (often referred to by its Uni-Solar brand name) is one of those cases in which industry pulls ahead of a national laboratory while building on the laboratory's breakthroughs.

"In the thin-film area, we had national teams for each technology, so there was an amorphous silicon team, and Uni-Solar was the prime industrial member in that team," says Bolko von Roedern, the NREL technical monitor for the amorphous silicon work. "Historically, for amorphous silicon, the industry group always had the highest efficiencies, whereas for CIGS and cadmium telluride, our in-house groups were the world-record holders. In the U.S. amorphous silicon program, Uni-Solar was the champion cell maker."

A key to Uni-Solar's success was its ability to lay down three separate layers of amorphous silicon, each tuned to a different frequency of sunlight. This created a cell with three active semiconductor layers, called a "triple-junction" cell. The top active layer in a triple-junction cell captures a portion of the solar spectrum, and the remaining sunlight penetrates to the second layer, which captures another part of the solar spectrum, and likewise for the third and final layer. As a result, the cell converts more of the solar spectrum into electricity, achieving a higher conversion efficiency.

But despite its technical leadership, Uni-Solar still learned from NREL's research, particularly when it came to material characterization and the techniques for depositing the amorphous silicon onto a substrate. For instance, NREL pioneered hot-wire deposition, in which a material such as silicon is vaporized by a hot wire in a vacuum chamber, and the vapor then condenses onto the cooler surface of the substrate. Uni-Solar helped NREL to make high-efficiency solar cells using this technique.

"NREL developed the hot-wire deposition technology and other deposition technologies, but Uni-Solar often collaborated with NREL to achieve better results," says von Roedern.

In 1997, Uni-Solar started up its amorphous silicon pilot line, which had the capacity to produce enough solar modules in one year to generate 5 megawatts of power. In the solar cell industry, this is referred to as a 5-megawatt production line, big enough to provide 2-kilowatt solar power systems to 2,500 homes each year. This Uni-Solar pilot line was the first significant thin-film manufacturing facility in the United States.

The success of Uni-Solar's pilot line led the company to launch its first full-scale manufacturing plant in 2002. The facility, located in Auburn Hills, Michigan, produces the amorphous silicon solar modules using a roll-to-roll process similar to the printing presses used by newspapers. As a result, the Michigan production plant had a 25-megawatt production capacity, and back then that was big enough to boost U.S. solar cell production capacity by 20%. The process handles 9 miles of stainless steel at one time, advancing at a rate of about 2 feet per minute through the machine, to deposit triple-junction cells only1 micron thick.

Uni-Solar eventually increased its production speed at the Auburn Hills plant, boosting its annual capacity to about 28 megawatts. In 2007, Uni-Solar added a second production line and doubled the plant's production capacity. Currently, more than 50% of the company's products are used in building-integrated applications such as their "peel and stick" roofing laminates.

Uni-Solar produced 48 megawatts of solar cells in 2007, and the company is now building four 30-megawatt production lines at a new facility in Greenfield, Michigan. When the first line begins production in 2008, the company will have an annual production capacity of 88 megawatts, and that will quickly grow to 178 megawatts. For comparison, U.S. manufacturing plants produced about 266 megawatts of solar cells in 2007, so amorphous silicon is having a significant impact on the domestic front.

"It has been a dream for a long time," says von Roedern, "and now a dream that has been realized."

New Competition Arises

Uni-Solar and other manufacturers have achieved significant results for amorphous silicon solar cells. At the same time, NREL's work with the other thin-film technologies, CIGS and cadmium telluride, has triggered new industries that are growing even faster.

Their current growth is particularly impressive when you consider the humble beginnings of today's leading companies. For CIGS, for instance, Global Solar Energy is currently the largest manufacturer depositing CIGS thin films onto a flexible substrate. But when NREL started working with Global Solar back in 1996, the company had a much lower profile.

"Global Solar was a spin-off from ITN Energy Systems, of Littleton, Colorado, which was also supported by NREL," says Harin Ullal, the lead project manager for NREL's work with the CIGS and cadmium telluride industries. "They basically had a building with four walls when we gave them the contract. We helped them get started developing the technology for CIGS."

NREL developed a laboratory procedure to produce CIGS solar cells with 19.9% conversion efficiencies using glass as a substrate. Global Solar licensed the technology from NREL and is using the same procedure, but is instead depositing the coating on flexible stainless steel in a roll-to-roll production process.

Global Solar started in Tucson, Arizona, with a 4-megawatt pilot line. In early 2008, the company opened its first full-scale manufacturing line, which has a capacity of 40 megawatts. The company is also building a 35-megawatt manufacturing plant in Berlin, Germany, and it plans to establish a new 100-megawatt manufacturing line in Tucson by 2010.

"NREL has been a steadfast partner to Global Solar throughout the changes that have occurred in our company and industry," says Jeff Britt, chief technology officer for Global Solar. "The technical support from NREL has led us in new directions and provided capabilities we could not find anywhere else in the world. In particular, their work on high-efficiency CIGS solar cells and the transfer of that know-how has directly benefited our production line."

"Firsts" from First Solar

But NREL's biggest thin-film success to date is its work with First Solar, Inc. This manufacturer of cadmium telluride solar modules started out as a small company called Solar Cells, Inc.

"We started working with Solar Cells, Inc., way back in 1991 when they were about a six-person operation, almost a garage-type operation, on the campus of the University of Toledo in Ohio," says Ullal.

NREL was attracted to the company because it employed some experts from the glass industry as well as an expert in cadmium telluride technology. Solar Cells, Inc. was focused on depositing cadmium telluride on glass, forming rigid solar modules similar in appearance to solar modules made from crystalline silicon solar cells.

"We supported Solar Cells, Inc. in their early years through joint research projects, and after a couple of years or so, they produced their first prototype module, which was rated at an efficiency of 6% or so," says Ullal.

Solar Cells, Inc. kept making steady progress and eventually started a pilot line with a capacity of about 2 to 3 megawatts. In 1999, private investors formed the First Solar company as a joint venture with Solar Cells, Inc., and later bought out Solar Cells' stake in the company. In its early years, First Solar developed a range of novel technologies to make it easier to manufacture their thin-film-on-glass solar modules. Along the way, the company continued to collaborate with NREL.

"We've worked with them quite closely," says Ullal. "For example, we've done a lot of measurements and characterizations of their materials, and we've helped them with NREL's technical knowledge. We've also helped them through testing their modules and systems at our outdoor test facilities."

First Solar launched a plant in Perrysburg, Ohio, that started with one production line and soon expanded to three production lines, each designed to produce 25 megawatts of solar modules per year. Today, each of those lines has been expanded to 45 megawatts, for a total of 135 megawatts of U.S. production capacity. The company has also improved the conversion efficiency of its solar modules to 10.6% while cutting its manufacturing costs from $2.96 per watt in 2003 to $1.14 per watt today.

"NREL expertise played a key role in helping First Solar get established in its early days," says David Eaglesham, vice president of technology at First Solar. "NREL plays a very crucial role for the photovoltaic community in the United States and provides invaluable expertise and learning for companies getting started in this business."

First Solar now operates a manufacturing plant in Germany with four lines, for a total of 180 megawatts, making it the largest U.S.-based manufacturer of solar cells and the world leader for thin-film solar modules. The company is now the world's fifth-largest manufacturer of solar cells, producing 206 megawatts of solar cells in 2007.

First Solar is also constructing four new manufacturing plants in Malaysia, each one featuring four 45-megawatt production lines. Once those plants begin operating, the company's total production capacity will exceed 1,000 megawatts per year. For comparison, about 3,700 megawatts of solar cells were produced globally in 2007.

"It's an amazing story," says Ullal. "NREL and DOE can really be proud of those accomplishments."

The company's manufacturing growth is indeed impressive, but it was its financial performance that drew the world's attention in 2007. The company launched its initial public offering in late 2006 and garnered more than $400 million in investments. Then in 2007, its stock went up by a factor of 8, making it the top one-year performer on the Wall Street Journal's Shareholder Scoreboard.

While NREL can't take credit for the company's financial success, that success is built on the cadmium telluride technology that NREL helped develop. With a number of companies now successfully manufacturing solar modules using the three main thin-film technologies, NREL has undoubtedly helped to bring the nascent thin-film solar industry to life. But rather than resting on its laurels, NREL continues to work with new companies that are bringing fresh ideas to the thin-film solar cell industry.

The current CIGS manufacturing roster includes HelioVolt, which plans to build a 20-megawatt plant in Austin, Texas, in 2008; Ascent Solar, which plans to construct a 25-megawatt production line near Denver, Colorado, by 2009; and SoloPower, Inc., which is using electrodeposition to build CIGS solar modules on flexible thin-film substrates. SoloPower is in the process of building a 20-megawatt manufacturing plant in San Jose, California.

For cadmium telluride, the roster includes AVA Solar, which plans to begin pilot production in northern Colorado in 2008, and PrimeStar Solar, which has entered into a cooperative research agreement with NREL. It's worth noting that General Electric Company has also made a significant investment in PrimeStar Solar.

The future of thin-film solar cell manufacturing looks promising indeed, as former start-ups are becoming manufacturing giants, while a new crop of start-ups hope to inject added competition into the rapidly growing field.