Ed’s Threads 071130
Musings by Ed Korczynski on November 23, 2007

PV perspective: Interview with AMAT's solar technology expert

Dr. Charles F. Gay, currently VP and GM of Applied Materials’ solar business group, is a renowned expert in PV technology and business, having been president of Arco Solar, Siemens Solar, and ASE Americas, as well as director of the US Department of Energy’s National Renewable Energy Laboratory (NREL) in Golden, CO. He found time in his busy schedule to talk with me about the incredible growth in solar business, and to explain recent changes in the photovoltaic (PV) technology landscape.

“The speed of innovation has ratcheted up quite rapidly, and there are two themes that have affected the industry over the last several years,” explained Gay. One is the scale of the industry, growing at over 40% over the last decade. This has created a dynamic where a company like Q-Cells can just show up in the market and rapidly rise to be No.2. Suntech at No.3 was virtually nonexistent three years ago.

Secondly, as the business has grown, so has the scale of manufacturing. Until recently, crystalline PV lines mainly ran old 150mm wafer equipment obsoleted from IC lines by newer 200mm tools. Less than a decade ago, a world class PV line was capable of producing fewer than 5MW/yr of cells, while today Sharp alone has over 700 MW/year of total fab capacity. Typical PV lines today are 50-100 MW, and a company wanting additional capacity builds multiple lines on site, or starts locating lines around the world depending upon customer demand.

A 100 MW/year line needs to process such a large area of material that equipment from industries other than IC manufacturing, like FPD or architectural glass, have come into mainstream use. “The process control was there, the history of making machines was there, and the expertise enabled thin-films to come onstream just when the lack of silicon had been threatening a delay in continued growth,” Gay said. Control of uniformity over large areas allows for potential cost-reduction in thin-film PV lines.

Thin-film PV panels have been able to capture an increasingly larger piece of the market. While still only ~10% of the total, it is expected to grow at a faster pace due to sheer economies of scale using large glass panels. Secondarily, thin-film lines may take extra market share while crystalline silicon line production is limited by the near-term global poly-silicon shortage.

Some crystalline solar manufacturers have responded with innovative materials engineering and supply-chain management. Using gettering, diffusion, and blanket etching of a top sacrificial layer, a PV line can essentially pull most of the impurities into a top skin that is removed. This adds fab cost, but allows for the use of less expensive "six-nines" [99.9999%] pure starting silicon that is not in short supply. “People thought maybe we can make silicon from dirty quartz using direct reduction, and maybe the silicon only needs to be six-nines pure, instead of nine-nines,” Gay said. He added that cell efficiency for single crystal is ~22% for the very best quality starting material and fab process, ~18% is a general capability for single crystal silicon, and ~16% for high-purity multicrystalline silicon.

Another example of clever materials engineering in PV is tuning the sheet resistance of the silicon using phosphorous (P) diffusion that is pattern dependent. The spacing of topside aluminum lines is determined by the sheet-resistance of the silicon after P diffusion, but P dopants interfere with the short-wavelength absorption of light. An optimization can be found by tuning the P to be higher under the lines (for reduced contact resistance) and lower between the lines (increasing conversion efficiency).

“Innovation has been happening at a faster pace due to the increased scale,” said Gay. “The size of the market is enabling additional R&D in academia, industry, and government, and also allowing for leaps in manufacturing efficiencies.”

An example of manufacturing efficiency increasing with scale is the production of “water-white” glass panels for thin-film PV. Water-white glass has low concentration of Fe₂O₃ which increases optical transmittance, and results in ~2% more light transmission, explained Gay. However, the global demand for this specialized glass was relatively small, so it was only made in relatively expensive batch furnaces. A few years ago, based on solid demand forecasts for thin-film PV panels, architectural glass companies such as Pilkington, PPG, Cardinal Glass, Asahi, etc. started retrofitting continuous float-lines for water-white production. Glass companies can sell “water-white” glass for a premium over standard green soda-lime, while still offering a cost reduction that could be cents per square foot compared to batch processing.

“All the way across the value chain, from basic science to the infrastructure for installation, there is tremendous activity in solar,” observed Gay. “It’s multiplied to the stage in Germany today there are almost as many jobs in solar as there are in automotive. Solar and wind represent for the first time in history the opportunity for job creation.”

With the global terawatt challenge remaining ahead of us, there’s lots of work to be done.—E.K.

Labels: business, energy, manufacturing, photovoltaic, solar

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