Silicon solar cell still leads the pack

Last year marked the 50th anniversary of the landmark introduction by scientists at Bell Laboratories (Murray Hill, NJ) of a silicon solar cell with 4.5% energy efficiency.

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FREMONT, CA — Last year marked the 50th anniversary of the landmark introduction by scientists at Bell Laboratories (Murray Hill, NJ) of a silicon solar cell with 4.5% energy efficiency. The anniversary provided a sobering reminder for some that solar energy technology, originally developed in the United States, is in at least one way on the verge of becoming like petroleum: an essential energy resource for which Americans are desperately dependent upon foreign suppliers.

The global solar industry grew by a factor of 10 between 1976 and 2004, with price decreases in excess of 95% from 1978 to 2004, while solar manufacturing in the United States actually declined in 2003 for the first time ever, according to a statement from the Solar Energy Industries Association (SEIA; Washington, D.C.), which issued a U.S. photovoltaic (PV) industry roadmap for 2005 and beyond at Solar Power 2004, in San Francisco last October. The SEIA plan for reinvigorating the U.S. PV industry was modeled after successful strategies in Japan, Germany, and California.

One decade ago, U.S.-based manufacturing capacity accounted for about 45% of worldwide shipments of photovoltaic modules and systems, with Europe and Japan each accounting for just over 20%. Five years later, U.S. shipments had dropped to 30% of the world total, while Japanese shipments had climbed to 40%, due in large part to an aggressively government subsidized program in Japan to integrate photovoltaic materials into building construction.1 The subsidies are already phasing out of the successful Japanese program, according to comments by Ken Zweibel, from the thin film PV partnership at the National Renewable Energy Laboratory (Golden, CO), last summer in Denver, CO, at the annual meeting of the International Society for Optical Engineering (SPIE). And notably the three largest PV manufacturers, Sharp, Sanyo and Kyocera, are also located in Japan.

Aggressive government subsidies could work in the United States also, according to Richard Swanson, president and CTO of SunPower (Sunnyvale, CA). During the coming 10 years, PV system prices are likely to fall below $3/W, he said. And a government program to stimulate the PV market during that same period by subsidizing system costs in excess of $3/W would cost about $23 billion total. Such a program would put the U.S. PV power industry on a 30% growth curve, enabling economies of scale to ultimately yield fully cost-effective, renewable and pollution-free generation of distributed or retail power (as in rooftop PV systems), along with the phasing out of subsidies. The eventual development of cost-effective bulk or wholesale power systems (to replace centralized power plants) is still at least two decades out and will require development of emerging PV technologies.

The currently established crystalline silicon PV technology might be thought of as the first generation, with thin films (based on amorphous silicon, cadmium telluride or copper indium gallium diselenide) as an emerging second generation, and organics as a distant third generation not likely to have much market impact for the next couple of decades, according to Robert Birkmire, from the University of Delaware Institute of Energy Conversion (Newark, DE), who also spoke at the SPIE meeting. Crystalline silicon solar cell technology, well developed and continuing to improve-thanks in large part to the semiconductor industry-will continue to dominate the PV landscape for at least the next 10 years.

Practical market barriers to rapid and widespread deployment of thin PV systems include a relatively long cycle time for change in energy technology and a need to demonstrate the long-term reliability that has already been demonstrated by crystalline silicon. In addition, efficiency is likely to become a major competitive factor in PV market sales. Thin film modules are approaching the 15% efficiency region of a conventional silicon solar cell, but crystalline silicon efficiencies continue to improve, and could climb to just under 25% based on currently available technology. The conventional 15% efficiency, less than half of the thermodynamic efficiency ceiling of 33%, is due largely to “implementation” losses on the order of 14%, which Swanson said he believes can be reduced to just over 4% using process improvements to boost minority carrier lifetimes in the crystalline silicon PV material above 1 ms.

New module designs and lamination materials, improved manufacturing efficiency, and thinner wafers will also contribute to reductions in PV system prices below $3/W over the next decade, Swanson said. He described wafer thickness reduction (halving every decade) as the “Moore’s Law of crystalline PV.” Another area of development is the emergence of dedicated raw material suppliers. Solar Grade Silicon (Moses Lake, WA), a former supplier of polysilicon raw material to the semiconductor industry has become the first dedicated supplier of polysilicon material to PV industry. In 2003, that company announced both the expansion of manufacturing capacity and the development of a new granular polysilicon process. Other plans to enter this area have been announced, including work at Dow Corning (Midland, MI) to refine their existing manufacturing processes for metallurgical grade silicon in order to produce silicon stock for the PV industry.

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A U.S. government program to subsidize PV-system costs in excess of $3/W could stimulate a 30% growth curve (dashed line) and provide a cost-effective distributed PV energy resource within a decade at a total cost of about $23 billion (solid line). Subsidizing less of the system cost would yield slower industry growth, take longer, and cost more in the long run (dotted line). Calculation assumes that the cost of the photovoltaic module equals the cost of the remaining system elements.
Click here to enlarge image

Like many PV business ventures in the United States, Solar Grade Silicon-a joint venture of Advanced Silicon Materials, a subsidiary of Komatsu (Japan), and Silicon Technologies, a subsidiary of Renewable Energy (Norway)-might actually be described as part of an increasingly international effort launched by and large from countries outside of the United States that are actively developing photovoltaic energy capacity. Germany has already launched an aggressive PV development program and other European countries are following suit. Rather than view the situation as a matter of international competition, however, Swanson described it, in a telephone interview last month, as an opportunity for international collaboration in which Europe, Japan, and the United States might each contribute a third.

“Manufacturing and technology expertise gravitate to where the markets are,” he said. “So since the United States represents only about 10% of the world market it makes sense that we’ve fallen a bit behind in development.”

We could wait while others develop the technology and systems and then shop around for the best deals, he added. “But that would bring about a similar situation to the one we’ve encountered with oil.”

— Hassaun A. Jones-Bey


1. P Holihan, “Technology, Manufacturing, and Market Trends in the U.S. and International Photovoltaics Industry.”

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