Friday, November 13th, 2009

Intermolecular, the company that brought combinatorial chemistry to semiconductor manufacturing R&D, has expanded its focus to look at ways to improve basic manufacturing processes for photovoltaic (PV) fabs. “I think that the PV devices of 10 years from now will look significantly different from those of today,” said Intermolecular vice president and general manager of solar and energy technologies Craig Hunter in an exclusive interview with BetaSights. Perhaps independently, Caltech and Dow will be working on low-cost thin-film PV in a four-year program that will certainly use some manner of combinatorial exploration.

The commercial PV fab industry has seen tremendous efficiency improvements over the years: crystalline silicon (c-Si) cells now provide 20-22%, multi-crystalline silicon (mc-Si) cells cost less and provide 15-17%, while thin-film PV panels are now under production at 7-11% using amorphous silicon (a-Si), mc-Si, CdTe, and CIGS absorbers. There is still room for improvement in efficiency for most PV technologies, and fab cost reduction remains essential since market demand is highly price-elastic.

Starting with high-productivity combinatorial (HPC) technology developed by Symyx Technologies—including over 650 granted and pending patents—Intermolecular has developed its own portfolio of over 90 granted or pending worldwide patents that deal with process R&D. Intermolecular has assessed the current status of R&D in PV companies as lacking the ability to thoroughly and cost-effectively explore process spaces.

To be sure, it is a long way for a new technology to go from a champion PV cell in the lab to $/kWh panels delivered into the field in volume. Combinatorial parallelism allowing for 64 experiments on a single wafer (see figure) certainly accelerates experiments, which leads to faster time-to-knowledge. The company can form 18 different cells on a single mc-Si wafer. Intermolecular’s combination of hardware parallelism integrated with an information database makes it possible to explore both faster and a greater area within the process space.

Existing fab lines need to be able to explore the edges of process spaces, and to look at complex multi-dimensional interactions. The goal is both cost reduction and line stability. For example, through exploring process interactions we might discover that the thickness of a vacuum film deposition was particularly sensitive to temperature variations across the substrate, but by using a different optimal vacuum level we can find the thickness is far less temperature dependent. Such exploration is possible with custom R&D tooling and expertise, but practically impossible on a production line.

While potassium-hydroxide plus isopropyl-alcohol (KOH + IPA) is the standard solution for Si surface texturing, Intermolecular claims to be developing a new solution that will provide at least equivalent texture with a wider processing window at significantly shorter process time. Some other PV fab technologies being explored by the company are as follows:

  • Crystalline Si: wafer texturing, materials for litho-less patterning of cells, enhanced front and backside passivation,
  • Thin-film Si: TCO composition and morphology, absorber structure for higher efficiency and improved stability,
  • CdTe: back contact and TCO for improved cost and efficiency, and
  • CIGS: higher efficiency, wider absorber deposition windows (wet, PVD), Cd-free buffer layer, improved durability TCOs.

Tony Chiang, Intermolecular CTO (founder of ALD technology company Angstrom, prior to its 2004 acquisition by Novellus), explains that PVD technology developed for IC phase-change memory (PCM) can apply to searching for an improved PV absorber. “A lot of the thin-films are similar to phase-change-memory materials,” said Chiang. “If you think about it, there are three base elements and a dopant material. It’s very analogous, and PVD is the way to address it.” Chiang proudly shows off custom built tools in the company lab with CVD, PVD, and PECVD chambers all connected to the same robotic vacuum handler, so that a single tool can also form backside contact and TCO.

For compound thin-film PV, there is a lot of potential to improve the performance of the fundamental absorber material. The two leaders today—CdTe and CIGS—rely upon relatively expensive rare-earth materials. While in principle there should be a lot of untapped potential to be found in complex combinations of common elements in the earth’s crust, such as iron (Fe), magnesium (Mg), and titanium (Ti).

Indeed, Caltech professor Nate Lewis (see figure) has been directing his researchers to experiment only with such common elements for many years now. Now his researchers will be working with Dow Chemical in a four year JDP to investigate the use of earth abundant elements to create new direct band gap PVs. “Use of earth-abundant materials can provide new technology options and could open new areas of design space,” notes Lewis. “This project will develop the science and technology base for thin-film solar-energy conversion using these widely available materials.”

Dow recently announced its first building integrated photovoltaic (BIPV) product, the POWERHOUSE Solar Shingle which uses a CIGS thin-film to provide low cost, easy installation, and a dramatically appealing aesthetic. Over a year ago, Dow announced that Global Solar would supply the CIGS technology for the first generation of shingles. –E.K.

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