The disruptive potential of thin and kerfless wafers

In late July, a thin silicon solar technology startup based in an office park on the outskirts of Silicon Valley announced that it had raised $31 million in venture capital (VC) in its fourth funding round. This is a lot of money, particularly given that VC for solar manufacturing technologies dried up in mid-2012.
Many solar technology companies have made big promises to increase efficiencies and reduce costs, and Solexel is no different. The company is targeting 20% efficient PV modules in 2015, with a cost road map to bring module costs down to $0.40 per watt in 2017. However, Solexel also has significant accomplishments to back this up, including reaching a cell efficiency of 21.2% this year with its 35 micron wafers, which are roughly 20% the thickness of standard silicon wafers and thin enough to bend. The company has attracted over $100 million in funding to date, including from solar technology leader SunPower, which participated in earlier rounds but not the D round.
Meanwhile, in another office park in a leafy suburb outside of Boston, another solar startup using an alternative approach to wafering was named a New Energy Pioneer by Bloomberg New Energy Finance (BNEF) in April. The company, 1366 Technologies, has a different approach, casting wafers with a conventional thickness from molten silicon. Like Solexel, this approach uses silicon more efficiently, and results in a more uniform wafer quality than industry standard processes.
This is not just a concept, as both companies are actively producing wafers on pilot lines. Solexel has a 1 MW line, and in the first quarter of 2013, 1366 built25 MW of Direct Wafer manufacturing capacity at its facility in Bedford, Massachusetts, which it is using to refine its process.

Replacing ingots, saws and slurry

Solexel and 1366 are the leading lights of thin and kerfless wafer technologies, which aim to replace the current polysilicon, ingot and wafer process. In this field is also Crystal Solar, which uses an epitaxial lift-off technique similar to Solexel, and GT Advanced Technologies’ Hyperion technology, which it purchased from Twin Creeks.
It is easy to see why these companies are targeting this part of the value chain. By any account, the process of forming polysilicon into ingots and sawing it into wafers is tedious, wasteful and consumes a tremendous amount of space and energy.
Sawing silicon ingots into wafers alone can take up to seven hours and wastes up to 44% of the material as silicon dust. This dust results from the kerf; the area where the wire saws pass through, which is why wafers formed directly without sawing are called “kerfless.” And while a move to diamond wire sawing reduces kerf thickness and thus the volume of silicon dust, 1366 estimates that the solar industry wastes 100,000 metric tons (mt) of silicon annually, and 100,000 kilometers of steel wire.
These companies seek to replace these processes with direct wafer formation based either on silane gas deposition on a substrate or, in the case of 1366, molten silicon. These technologies offer fewer steps, the potential for lower equipment costs, and more efficient use of silicon, which 1366 estimates can result in a total wafer cost reduction of 54%.

Breakage and efficiency

But just as these technologies offer big benefits, there are also drawbacks to this approach. Thin wafers are much more delicate than the standard 160 – 180 micron wafers, and as such require special equipment. “If you go with the thinner wafers, you have to change the whole production line, because handling thin wafers is really tricky,” notes VLSI Research Europe MD John West.
Solexel has addressed this problem by keeping its wafers adhered to a substrate during the initial phases of cell processing. While this helps with potential breakage issues, it also means that the company is re-engineering more of the production process, which makes it more expensive and complicated to replicate at commercial scale. Additionally, to make a compelling value proposition it is not enough to reduce waste or simplify production steps, but to do so with a process that results in PV cells that are at least close to the efficiencies of cells made using standard processes. “Saving silicon is great, but if this comes at the expense of cell efficiency, the technology doesn’t stand a chance,” noted solar analyst Andrew Gabor in a 2012 report by GTM Research.

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While many thin and kerfless wafer startups have failed at this, 1366 says that it is consistently producing wafers that serve as the basis for cells with 17% or greater efficiency, using standard cell processes. The back contact cells that Solexel produces with its thin silicon wafers have achieved efficiencies as high as 21.2%, with modules at 19% efficiency. This is well ahead of most conventional PV technologies.

Bad timing

However, thin and kerfless wafer technologies must reach economies of scale to be able to deliver on their promise of cost reduction. GTM Research’s Shyam Mehta says that Solexel’s latest $31 million round is not nearly enough to do this. “It is going to finance their working capital and allow them to do some R&D, but it is not nearly enough for them to build a big factory in Malaysia, which is what they want to do.” Raising money is particularly difficult at this time. Due to massive industry overcapacity along the entire PV value chain, equipment spending began declining in mid-2011. And while the first quarter of 2014 saw the industry’s first positive book-to-bill ratio in over three years, market volumes are less than 20% what they were in the first half of 2011.
“The way the industry is playing out right now is that none of the manufacturers have any money,” notes VLSI’s West. “Even buying regular equipment – it has pretty much come to a standstill. Even if they wanted to get the money, nobody would give it to them.” Another factor is the collapse in polysilicon prices. While polysilicon prices were above $50 per kilogram as recently as the first half of 2011, by the end of the year they had fallen to around $25 per kilogram, and have been in the $16 – $27 range ever since.
“The value proposition for thin and kerfless wafer technologies is tied to the cost of polysilicon,” explains Mehta. “Even though the market has recovered and pricing is up, that still is not enough of a trigger.” These difficult market circumstances may be why some companies are using thin wafer technologies for other applications. GT Advanced Technologies is exploring sapphire applications for its Hyperion ion implant technology, but says that it is not planning commercial deployment for solar applications at this time.

A superior solution?

Despite these short-term market issues, many analysts do see thin and kerfless wafer techniques as potentially viable alternatives to the current production processes for ingot and wafers. “It depends on what time frame you are talking about,” says Mehta. “My thinking is that there is definite potential for thin or even kerfless wafer technologies to make broad penetration in the market over the long run.” John West of VLSI agrees. “Kerfless technology is a definite one for the long term. There’s only so far that polysilicon can fall.” As judged by venture capital investment in Solexel, investors also see long-term potential.
“VCs have generally stayed away from large bets on solar technology firms – specifically CSP, CPV and thin film companies,” explains Mercom Capital CEO Raj Prabhu. “After investing over $2 billion in these companies since 2010, most VC investments turned sour when the price of c-Si modules crashed. Solexel has been one of the few solar technology companies raising funds consistently over the last few years.” Some companies are already making plans for commercialization.
Startup 1366 Technologies is probably the farthest along, and says it intends to start construction on a facility with an initial capacity of 250 MW annually in the second quarter of 2015. While the company has not yet made a final site selection, the project has the support of Japanese chemical maker Tokuyama, as well as a $150 million U.S. Department of Energy (DOE) loan guarantee granted in 2011.
The startup 1366 appears to have learned lessons from the many companies in this space that have failed. “I think we have a careful and methodical approach to building a company,” says 1366 CEO Frank van Mierlo. “We are cognizant of the difficulties and have mapped out our progress carefully.” For Solexel the path towards commercialization is inherently more complicated. While 1366 plans to initially be a wafer supplier, Solexel is attempting to replicate a larger part of the supply chain. This can be a dangerous proposition, and many analysts cite such an approach as a factor in the failure of “string ribbon” PV wafer and module maker Evergreen Solar.
However, the innovations that Solexel offers are not limited to reduced silicon use, and a large part of the company’s value proposition appears to hinge on the cell processing stage. “They are adding a lot of tweaks to get to that 20% module level,” says Mehta. “You aren’t going to get high efficiency by simply using thinner wafers.” In addition to the high efficiency play, Solexel is also addressing cost concerns, including reducing the use of silver. But even with all of these benefits, Mehta says Solexel will need the assistance of a strategic investor or even a buyer to be able to scale its technology.
Mehta also notes that it is still very early in the game for these technologies. “If you are asking which of these companies will be successful, they are all piloting version 1.0 of the technology that they are trying to commercialize, and since when has that ever been successful?
“Ultimately a successful process, and company, is going to emerge through multiple iterations on a concept. It’s about half a decade out until we will see any real traction on this front.”