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Brent Nelson is unlikely to underestimate the limitations of science. “I’m always quick to not say, ‘something can’t be done.’ Because people are clever and will always figure something out….Never say nothing will ever happen.” As manager of NREL’s Process Development and Integration Laboratory (PDIL), one of the planet’s leading photovoltaic research facilities, he gets a steady dose of innovation in crystalline and thin-film silicon, CIGS, CdTe, organics, measurement technologies, and variations therein to support his optimistic outlook.
But the long-time solar advocate and proficient quipster is no wide-eyed dreamer, noting that renewable energy is “basically at the Model T stage in terms of displacing conventional fuels.” When solar and the rest of the clean energy crew finally start to push the carbon cartel aside, a more than piddling amount of credit will go to collaborative R&D centers like PDIL.
As Nelson rightly observes, “It’s very difficult to support an industry that doesn’t have any process or material handling standards.” The PV industry has “sheets of glass from one square meter up to Applied’s one Yao Ming by one point two Yao Ming. They have plastic foils, metal foils, and even in the wafer silicon area they have round wafers and square wafers and rectangular wafers.
“So if you’re an equipment manufacturer and say, ‘I want to get into PV, I want to build a tool for a PV fab,’ it’s not only what technology, it might even be what company. So it’s very hard to get those kinds of cost reductions by making multiple tools for multiple companies that the semiconductor tool suppliers can.”
For PDIL to maximize the capabilities of its coterie of vacuum and atmospheric process cluster equipment and measurement/characterization tools, Nelson and his team knew they had to come up with clever yet relatively inexpensive ways to integrate the various pieces of lab gear.
One key innovation has helped standardize how the various substrates are handled as they go from deposition chamber to measurement station and back again in the 10,000 sq ft lab—a 7 × 7-inch (157 × 157mm) rigid platen that fits snugly into each system. “Within that frame, you can put a wafer, a piece of glass, a foil. I don’t care what you do in that frame, as long as you can handle that 7-inch frame. In terms of material handling, this really drives our design for these tools,” Nelson explained.
As clever as the platen handling scheme is, the samples they carry still need to be kept in ultraclean, high-vacuum environments much of the time, so their material qualities will not be compromised by any molecular or particulate contamination. A robotic interbay automation system would blow PDIL’s relatively modest budget in a flash-test moment, so a method of moving the precious research cargo between the process and measurement equipment had to be developed. The lab’s solution comes in the form of a small fleet of mobile vacuum transport pods.
“We have a bunch of cluster tools around each of these technologies, and we also have some standalone tools,” Nelson pointed out. “We can stop at any step in growing one of these devices, move it into a vacuum chamber that’s mobile, connect it up to our million-dollar x-ray photoemission spectroscopy tool or other high-end analytical tool, do science at any step along the process, looking at interface, looking at material property and composition, and then return it for processing and see how that material and interface property correlates to the device performance.”
“This mobile pod has allowed us to actually reduce the cost of this laboratory a lot. People say, ‘well, isn’t that expensive, you have a high-vacuum chamber with a battery-powered ion pump.’ Yeah, but compared with the cost of putting a million-dollar XPS on every cluster tool, it’s nothing. We didn’t have to build a cleanroom, which hugely reduced our facility costs, so these things have paid for themselves by at least a figure of 10.”
“This idea has been kicked around for a long time, but I think we’re the first ones to really implement it. There are a few places that have done it on a small scale, but to do it with full wafer size and with this collection of equipment, it’s never been scaled up like this.”
Nelson showed me the next-generation pod—the third version—being built and recounted the evolution of the transport system.
“Our first one was pretty good, we had a smaller foot, we came up with these rubber wheels that weren’t very consistent,” he said. “But then metal wheels took over, and we could align everything off the metal wheels, because we knew the floor wasn’t going to be perfect across the lab, and we had to be able to dock any one of these pods at any one of the tools, so they all had to be the same.”
“It turns out that the floor was even worse than we anticipated,” Nelson (pictured below right) chuckled. “Now we’ve gone to a big foot, so we bring it up close and then hydraulics take over and lift it up…. Then we have this big alignment tool. What we do is we park that, and we align all of our docks to that tool, and then we align the pod to one of them, and in that way, any tool can fit anywhere and…so it makes a consistent docking mechanism.”
We strolled over to an impressive collection of analytical systems. “In this bay we have most of our characterization tools that aren’t integrated directly onto our process and deposition tools,” he said. “Some of these tools [have been] developed in house, such as the reactively coupled photoconductive decay techniques, where you take a microwave antenna, and couple [it] to the sample, and pulse it with light, watch for the k signal, [and then] correlate that back to defects and lifetimes of the material.”
Although the upfront consequence of original innovation is sometimes technical isolation, at PDIL, it is often followed by technology propagation. “Because we developed the techniques, there’s no one in industry that we can go to and say, ‘OK, give us your tool but build this to our standards,’ so we’re doing all the integration on these tools. Eventually the goal is that someone would then license this technology.”
Which is precisely what’s happened. “A lot of companies are licensing our technology,” he noted. “Of our active patents in PV, we have something close to two-thirds that are licensed, which is pretty good. Part of our role is to maintain a knowledge base for the industry, so if we’re licensing technology, we’re publishing, we’re keeping that knowledge base active and vital.”
“We do thousands and thousands of measurements for industry, with people who understand what that measurement means in the context of the solar cell,” he said later in the tour. “We work hand in hand with many companies to help them understand their material interface properties, to feed that information back into their process.”
But not all of the intriguing technologies resident inside PDIL have caught the fancy of the PV manufacturing community. Take HWCVD, for example.
“One of the things we’ve worked on here for a long time that hasn’t really made it into industry is hot-wire chemical vapor deposition,” Nelson said. “We like hot-wire for a couple of reasons. In the amorphous silicon area, we’ve been able to grow at very high deposition rates. We’ve made working solar cells at 130Å per second, whereas the industry is at about 3Å per second. And we can get very high gas utilization rates, up to 80% of the silane” compared to the norm of 10-20%.
Another area of keen focus—and the basis of a CRADA with Ampulse and the NREL team’s fellow national lab rats at Oak Ridge--is the development of low-cost, high-quality epitaxial silicon growth techniques that would facilitate a radical reconfiguration of the crystalline production flow.
“The goal is if you can go from sand to gas, since the purest silicon is in the gas phase, and skip all these high-temperature steps--like growing wafers or even recrystallizing into single crystal, then you waste half of it to a sawblade, and you [end up with] a wafer that’s 10 times thicker than it needs to be--can you skip all those steps and grow high-quality epitaxial silicon on a low-cost seed? Now it becomes an experiment on how do you get a low-cost, high-quality seed,” a layer usually made of some kind of nickel oxide, about a micron thick, that gives some lattice matching with silicon, according to Nelson.
That emerging polycrystalline thin-film contender, copper indium gallium (di)selenide, garners a fair amount of attention at PDIL, with a dedicated cluster tool coming online.
“We’re scaling up our coevaporation of CIGS, which so far we’ve done on small area, to the 6-inch wafer size,” he explained. “We can grow moly (molybdenum), cad sulfide, zinc oxide, and the CIGS here. Right now, we’re in the process of validating all of our layers. We actually haven’t stuck them together in a device yet.”
“We also have an Auger spectrometer that’s hooked up directly” to the CIGS platform, Nelson continued. “The analytical system is “one of our tools that we call ‘moveable.’ Everything to run it is resident [on the tool.]. So we disconnect a few power and gas lines, and we can put that on a silicon tool or a cad telluride tool, because everything is a 10-in. flange, 1.1 meter off the ground.”
One enabling technology that PDIL does not have and puts high on its wish list is the capability to perform proper laser scribing and ultimately produce minimodules.
Nelson admitted that “monolithic integration is not a trivial step. That’s been one of our roadblocks actually. Different materials need different frequency lasers. There’s a lot more subject matter expertise that’s needed there. Until we can get to that point where we can actually develop that core competency, we’ve been reluctant to just go buy a laser, because we don’t want to have a dinosaur just sitting there.”
In the meantime, “we’re working on getting a cad telluride tool in here, as well as the epitaxial silicon tool” at the full 7 × 7 form factor, since both efforts have been limited to the smaller coupon-size samples so far.
PDIL’s point man gets a little defensive when some people wonder out loud, ‘How come it’s taken so long to get this equipment in?’ “The reality is, I’ve got six guys trying to install $25 million of equipment. It hasn’t come in any faster than we can work on it. We’re working really hard.”
“One thing I always get asked is, which technology is going to win,” Nelson shared. “I’ve been in this business since before I had grey hair, and every year, there are more technologies than there were the year before, so it’s diverging rather than converging.”
Spoken like a true believing yet worldly wise technologist, who sees the great diversity of photovoltaics pathways on a daily basis.
PHOTOS COURTESY OF NREL
