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“Making CIGS is kind of like baking a cake; they all have flour, eggs, and milk. But we don’t tell you everything in the recipe,” quipped Stion CEO Chet Farris. In his company’s case, the cinnamon and nutmeg can be found in the constituent ratios, molybdenum back-contact secret ingredients, a nontraditional approach to depositing the transparent conductive oxide, and other ways of sweetening its copper-indium-gallium-sulfur-(di)selenide thin-film photovoltaic confection. The pastry analogy doesn’t end there: the upstart’s roadmap calls for a tandem-junction CIGSSe device, a veritable high-efficiency layer cake.
Stion may be the only CIGS player that has been pursuing a tandem-junction architecture from day one. As I learned during my visit to the company in early December, its first-generation product is actually the bottom device of what will ultimately be a mechanically stacked dual structure.
Farris has been pleasantly surprised that the initial product, given its low bandgap (about 1.05 eV), has performed “better than we had actually anticipated,” with the Elevation Series modules coming off the company’s 10MW pilot production line regularly hitting 12-13% conversion efficiencies.
He believes that a “rational endpoint for single-junction efficiencies from a reproducibility point of view” is about 14-15%. Not champion cells, which Farris has little use for, but “consistent, center-line, tightly distributed” +/-0. 5% module efficiencies. “We have a little different strategy: first we tighten the distribution, then work on moving up the efficiency numbers.”
Admitting that the “interconnect structure is pretty loose today, especially on the first rollout of the product,” he explained that the design “set the P1 to P3 [scribe] spacing pretty wide, and about half of that space can be recovered, which is worth about three-tenths of an [efficiency] point all by itself.”
Throw in the elimination of the interconnect card and improved doping, selenization, and sulfurization, as well as other optimization of the film stack, and the efficiency could be boosted a whole point or more. “We were being very, very conservative in that design, so we’re going back and looking for ways to recoup some of that.”
Although the Gen 1 and eventually Gen 1.5 single-junction modules represent the vanguard of Stion’s first commercial push, the key strategic piece of the longer-term game plan hinges on the development and rollout of the tandem-junction CIGSSe panels, which will push efficiency numbers into the mid- to high teens.
Farris sketched a technical overview of the company’s difference-making technology.
“It’s a mechanically stacked tandem; the device structures are built completely independent of each other. This has some advantages, like not needing tunnel junctions. It allows you to optimize the diodes independent of each other.
“The bottom circuit is built in the substrate configuration, and the top circuit is built in the superstrate configuration. This allows us to exploit and not complicate the bill of materials. We have the semiconductor layers but we don’t change any [other] materials.
“We already have two sheets of glass, so we exploit the second sheet of glass for the tandem, and the EVA acts as the dielectric between the two pieces of glass. It’s a four-terminal device that gets converted to a two-terminal device in the box, and the way you match currents is by physical cell dimensions rather than to try and suboptimize the diodes.
“The high-bandgap devices ( about 1.6eV) will be higher voltage, lower current than the low bandgap devices, which will be lower voltage, higher current, so you have a current-matched diode, and the best way to do that is simply to do it mechanically,” he said.
Work on full-size tandem modules has recently begun at Stion. Farris told me during my visit that the final tool had arrived a few weeks before. Up to that point, development efforts used 5 x 5 cm2 and 20 x 20cm2 test devices, on which efficiencies of 15.7% and 15.5%, respectively, had been achieved.
During our more recent phone conversation following up on Stion’s announcement that it will build its first volume 100MW production facility in Hattiesburg, MS, the CEO provided an update, saying that the “tool performed flawlessly, the very first [full-size] panel was good that came off of it. We did the absorber formation on that particular tool, it’s an RTP [rapid thermal processing] tool. I’m very happy with the performance and uniformity--it did what we expected.”
“We’ll continue to work on that, to make sure we perfect that recipe in 2011 and get it qualified,” Farris continued. “I would anticipate that the Gen 2 product would be available in early to mid 2012.”
Speaking of those ubiquitous recipes, the firm’s cooking process doesn’t deviate that much from the standard combination of back electrode/contact—absorber—buffer (which is called the emitter in Stionese)—top contact flow and requisite isolation and interconnect scribe steps seen on other CIGS lines, but the cake does have its differentiated flavors.
Farris took me through the company’s existing facility in San Jose, where he described the circuit process and module assembly flow. About $8 million was spent on the production and R&D gear, an indication of another factor that Farris believes makes Stion “a bit different from other thin-film companies”—they don’t design, develop, and build their own equipment.
“We use industry standard equipment, optimized for each individual process step,” he pointed out. “For virtually all the tools, there are second and third sources available. There is nothing really proprietary in the mass production line from an equipment point of view.” Capital expenditures for the volume production facility are projected to be well under a dollar a watt, he added.
The manufacturing line starts with washing/drying the incoming glass and tagging each plate with a barcode. The molybdenum back electrode (along with some proprietary additives) is then sputtered on the glass, followed by a laser-scribe isolation/patterning step.
From there, the absorber layer begins to take shape, the first step consisting of copper, indium, and gallium precursor metals being sputtered (in elemental form, not as compound semiconductors, he pointed out), followed by a reactive thermal anneal process and introduction of the hydride gases to convert the stack to a semiconductor.
A “pretty standard cadmium sulfide dip” takes place next to create the buffer or emitter layer, with the wet process followed by a mechanical interconnect scribing step.
At this point, the Stion process takes a road less traveled in the CIGS community. The company has concocted its own aluminum-zinc-oxide TCO cocktail, which in and of itself is not an eyebrow raiser, but the method it uses to deposit that film does grab one’s attention—metalorganic chemical vapor deposition.
MOCVD is cheaper than the usual sputtering processes used to deposit the top contacts, Farris claimed. “More importantly, the film quality on our TCO is unique in that it has a scattering effect to it…which improves the diffusion length of the junction slightly.”
Although Stion uses this approach because of its lower cost and improved current characteristics, there’s also what the chief exec called a “serendipitous” benefit in terms of the “superior aesthetics” of the module, since its appearance is more uniform compared to some other thin-film panels.
Eventually, because of what Farris vaguely described as “intrinsic aspects of using MOCVD for TCO,” the scheme may help with another process innovation: the elimination, not the replacement, of the buffer step altogether. “We’ve already demonstrated a completely cadmium-free process, and we’re doing reliability studies,” he said, adding that modules made with the improved process should be released to the market in the first part of 2012.
Back on the production floor, once the P3 isolation scribe is lasered in and the leads attached, the manufacturing flow continues into a pretty standard layup/lamination/edge-seal/etc. module assembly sequence, which is semiautomated on the current pilot line but will be highly automated when the volume lines are installed and qual’ed in the Mississippi facility later this year.
As we walked the line, we encountered groups of TSMC employees being trained in the Stion process. The semiconductor giant plays a major role in the young company’s life, as major investor (to the tune of 21%), licensor of the single-junction technology, joint developer, and eventual supplier of modules to Stion to help fulfill its backlog of orders, which “saved us a fair amount of capital investment in order to gain access to that capacity sooner,” according to Farris.
The partners are on similar production ramp paths, with both saying they should be getting their volume lines rolling by the second half of 2011. “I think the timeline we have is pretty much the same, and it’s dictated by tool delivery schedule and nothing else. TSMC is also buying the same tools from the same vendor(s),” he revealed.
Stion is “very happy with the relationship with TSMC,” Farris said. “Obviously, you have different cultures, different styles, that’s always a little bit of a challenge to make sure you integrate with them effectively. I think the partnership is working as planned.
“They’ve helped us get these tools released and our procedures released, and work on maintenance procedures—they’re a very good manufacturing company so we’ve exploited that resource. I think it’s been a real win-win relationship, and I’m looking forward to continuing it.”
Stion’s headquarters facility doesn’t sport a big company logo or possess any slick architectural features. It’s a nondescript industrial building in a run-of-the-mill Silicon Valley business park with a simple corporate sign posted on the door.
Despite the relatively modest outward appearances, the CIGSSe company’s low-cost, measured approach and differentiated technology, as well as its strategic alliance with that certain four-letter acronymed chip foundry, may make it a serious player in the photovoltaics cake-baking market in 2011 and beyond.
To go to the first "Eye on Stion" blog dealing with the company's decision to site its first volume production facility in Mississippi, click here.
