SILICON GAS: Silane, a gas made up of silicon and hydrogen, is critical to make photovoltaics to turn sunlight into electricity. But it can also be lethal.
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In 2007, outside Bangalore, India, an explosion decapitated an industrial worker, hurling his body through a brick wall. In 2005 a routine procedure at a manufacturing plant in Taiwan caused a spontaneous explosion that killed a worker and ignited a blaze that ripped through the factory, shutting down production for three months. Both incidents shared a common cause—silane, a gas made up of silicon and hydrogen that explodes on contact with air. And both incidents occurred in the same industry—solar power.
Among other environmental black marks, the process of manufacturing photovoltaic (PV) cells from silicon relies on this dangerous pyrophoric gas. As the industry gears up to meet growing demand—6.4 gigawatts of new photovoltaic installations were built worldwide in 2009 according to the European Photovoltaic Industry Association, the bulk of it silicon solar cells—what are the human health and environmental concerns related to solar power?
Of course, silane is hardly the only environmental hazard involved in solar cell production. Others include: toxic by-products from polysilicon manufacture dumped indiscriminately in China; air pollution spewed from coal-fired power plants that provide the electricity needed to produce photovoltaics; and recovering cadmium, a known human carcinogen that is a primary ingredient in some thin-film solar cells, from mining slimes. Still, only silane (SiH4) has been linked directly to any deaths as a result of the solar industry.
"Of all the toxic or reactive molecules that [solar] industry uses, silane has been involved in 10 fatalities in the last 20 years. All the others put together, it's been zero," says retired chemical engineer Eugene Ngai, president of a specialty gas safety consultancy and a silane expert. "Where I have a concern is in the photovoltaic industry…. PV could be in a light industry zone, it could be close to residential areas."
Yet, the gas is essential—despite its dangers, silane remains the best way to deliver silicon molecules to a surface, because at high temperatures (above 400 degrees Celsius) it breaks into silicon and hydrogen. "We just burn off the hydrogen, like a gas flare," says Subhendu Guha, chairman of PV maker Uni-Solar.
That means it is ubiquitous, as well. In addition to the photovoltaic industry, makers of flat-panel displays, semiconductors, even coated glass, all employ silane and all have struggled with the safety issues surrounding the compound. But it is the rapidly growing manufacture of solar cells that is driving increased production these days; hundreds of new facilities to manufacture photovoltaic cells have opened around the world in the past five years. "Practically every silicon photovoltaic company has silane in some form or other," notes Ajeet Rohatgi, director of photovoltaics research at the Georgia Institute of Technology. "Without it, you wouldn't get high efficiency…. It's one of those things you can't live without."
That's because the gas can be used to make several of the layers in a silicon photovoltaic—from the top of the cell where it is used to deposit a layer of silicon nitride that ensures that all sunlight is absorbed, to the bottom where it can be used to deposit another layer that helps reflect back any missed photons of sunlight, boosting the efficiency of the cell at converting light into electricity. "You've found a mirror that reflects light back into the cell and gives light a second pass," Rohatgi says. "That back layer is very critical to get thinner wafers [of silicon] and better back surface reflectivity and, thus, lower cost and higher efficiency."
That critical factor means tractor–trailers traverse U.S. highways transporting tanks of silane as well as cargo ships laden with canisters of the gas cross the Pacific to factories in Asia, particularly China. It also means that a vast industrial structure in rural Washington State churns out solar-grade silicon, with silane as a by-product—plagued by shutdowns due to issues with the pyrophoric gas. A few thousand tons of SiH4 are produced annually worldwide, a number that will swell in the future as more and more microelectronics, flat panels and silicon photovoltaics are produced.
Decreasing the thickness of the silicon layers—one answer to the cost challenge in photovoltaics—is not, however, a solution for the silane problem. If anything, amorphous silicon solar cells, which rely on relatively thin layers of silicon, employ more silane as part of their process, using the gas to deposit the thin layer of semiconducting materials that manufacturers such as Sharp and Uni-Solar need.
But there is an alternative. Quebec-based manufacturer SiXtron Advanced Materials has developed a way to make a more stable gas with similar properties via gasifying polymer pellets. The idea is to hook the SiXtron "Sunbox" to existing silicon photovoltaic manufacturing lines to provide a methyl silane gaseous mixture via the same pipes that would normally deliver silane from a canister. "The methyl silane gases are not pyrophoric, they are simply a flammable gas," says Bates Marshall, SiXtron's executive vice president of sales and marketing. "But a leak does not mean an explosion."
The key to the mixture's stability is replacing some of the hydrogen in typical silane with other molecules, such as carbon, to make the resulting gas less explosive. The silicon carbon nitride that this alternative deposits may even be better at helping reflect light back into a solar cell—and SiXtron's silane-free coating has been shown to reduce the loss in efficiency endemic to solar cells when they are first introduced to sunlight.
Georgia Tech's Rohatgi has been testing SiXtron's machine versus conventional silane solar cell production and found that they produce similar quality cells. "The cells we have made for SiXtron are just as good as what are being made in industry," he says. "It is great to have a technology which gives you gas without compromising efficiency. It is taking the risk out of the product."
Such an alternative may be vital, given that silane's behavior under various conditions remains a mystery, even to scientific experts. "Its behavior when released is unpredictable," admits Ngai, who has advised SiXtron. "As a result, silane has been involved in quite a number of significant incidents."
Already, such issues have driven major companies such as General Electric to opt for competing technologies, investing in thin-film solar cells made from cadmium telluride by PrimeStar. Amorphous silicon "is attractive because of the availability of silicon but you still have to have silane in high purity," says electrical engineer Danielle Merfeld, director of solar technologies at GE Global Research. Making solar cells from cadmium telluride "is a much more robust and forgiving process and material structure."
Regardless, silicon in its many forms makes up more than 80 percent of the current mix of photovoltaic production. It remains to be seen if the SiXtron alternative thrives, even though the company has partnered with solar manufacturing equipment provider Roth & Rau. Other potential alternatives in the past, such as disilane, faltered when they proved too expensive to make. "If industry sees it gives no performance loss and, if anything, it has potential for improving performance, then I see no real reason for industry not to embrace it," Rohatgi says. "It will take time."
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35 Comments
Add CommentI'm saddened to hear there have been deaths due to silane, an am glad to hear there may be a safer and equally effective alternative. But as a former safety officer in a business that used hazardous chemicals, the first question that arose for me when reading this was, "What specifically led to the explosions, and what remedies have been put in place since then?"
Reply | Report Abuse | Link to thisI guess it was beyond the scope of the article, but hazardous chemicals are used and transported every day, mostly without incident (granted, those incidents can be pretty grim). This article seemed a bit alarmist, conjuring pictures of PV plants springing up in every neighborhood, then going up in fireballs.
Again an article that runs roughshod over the diversity of manufacturing processes. Silane may be irrelevant to some string ribbon processes, perhaps others, maybe the same ones this same author ignored last month in mistakenly reporting that 1366 was the first to skip the ingot-carving process.
Reply | Report Abuse | Link to thisDave Biello, please earn the ad-revenue dollars you get from all the ads on the right hand side of our screens by making calls to a wider diversity of the industry instead of cutting and pasting from press releases. We would never allow this paucity of journalism in print.
Agree with other posters as to somewhat sensationalist nature of this little blurb. We used silane in the semiconductor industry for years. Its a well known process gas but no more or less safe than others if handled correctly. The specifics about the poor technician's death in India were gratuitous, IMHO.
Reply | Report Abuse | Link to thisWe used silane in the semiconductor industry for epitaxy for years. Its no more or less dangerous than other process gases if used correctly. Agree with the other posters about the tone and especially the title of this article. This is a bit disappointing for SCIAM to run something like this without more oversight.
Reply | Report Abuse | Link to thisAnother great advertising plug for a new product. We see more and more of these on this site to the point that it makes me wonder who benefits economically. Is SciAm paid for these "product placements". Does the showcased company write these?
Reply | Report Abuse | Link to thisWell, I guess we better stop the production of all cereal grain food product as well. There have been untold deaths from grain silo explosions over the past hundreds or perhaps thousands of years.
Reply | Report Abuse | Link to thisI agree with most of the posts that this article seemed very alarmist. There are incidents like this in even the most everyday businesses - like the grinding of grain into flour. How many unfortunates have died in that business from explosions and fire? Just being an electronic technician puts you in the top four most dangerous jobs as rated by the insurance industry.
Reply | Report Abuse | Link to thisanother reason to "amp up" nuclear power ...
Reply | Report Abuse | Link to thisDid I get that right? One of the problems with solar power is that the factories that make solar panels need conventional power in order to make their products? There may be a place for terrestrial solar power systems, but it doesn't seem like it will be in the large scale industrial sectors, but as ways to power garden lights and remote devices that monitor and send signals...low power stuff.
Reply | Report Abuse | Link to thisFurthermore, industrial accidents happen in all largescale endeavors. If we have become so adverse to risk that we can't take a step for fear of the occasional stumble, then our species has really reached its end.
Fortunately, I don't think we have and there are still bold thinkers and doers who are focusing on the prize instead of the inconvenience.
Unfortunately magazine articles like this one and like so much of our other media today panders toward our fears, anxieties and outrage knowing it causes readers, viewers, listeners to gravitate to the source and it's the cumulative numbers of eyes and ears that are the payback for the media as that's how they compute their advertising rates.
Worst disguise of a press release rewrite...evar!
Reply | Report Abuse | Link to this@dbtinc
Reply | Report Abuse | Link to thisYou forgot the ?
Hydrogen is likewise an explosive gas. But the "Hydrogen economy" proponents are not at all phased by this. Not that that is saying much ! Methane likewise, but it's not stopping "LNG" technology. As for Cadmium Telluride, this sounds about second only to nerve gas in toxicity. Now who's being alarmist ? Oh, and that little "Uranium" thing continues it's "essential" expansion. But let's not get worked-up about it !
The fact is that my gran was a brilliant cook - by name and by nature - yet she still managed to nearly blow herself up one morning being a bit sleepy with the gas cooker !
No worries ! - computer will look after it all. Same with flight, driving - gettin outta bed in the moprning ? God alone knows. All we will need to do is drink. ( and piss, of course)
@physicist
Reply | Report Abuse | Link to thisSciAm was sold recently and it looks to me like the new owners are fiddling with the content here to help defray expenses.
Its just that its beginning to look like a Murdock publication!
Its true that the article seems alarmists and is so openly advertising a new product. But the commentaries are very good. The article plus the commentaries provide a very good idea of whats going on in the field. Without the commentaries from these valuable people the article smells a little fishy. And that of course is a point of concern.
Reply | Report Abuse | Link to thisSilicon Solar Casts a Microscopic Shadow!
Reply | Report Abuse | Link to thisArticle is shocking for its lack of perspective:
Solar = 10 deaths over 20 years
Coal = ~20,000 new black lung cases each year; 10,000 deaths in mining accidents each year.
The article is typical of the press releases turned into news stories from rival solar companies or research labs that paradoxically attack the solar industry either for price or safety so that they can proclaim their company or lab as the messiah. Yet in the course of their corporate or university lab promotion they have also provided the world with these misleading headlines and naysaying soundbytes.
The article compounds the silliness by muddling silicon and silane together and considering cadmium panels as a safer alternative. It's disappointing to see SciAm becoming a shill for whatever corporate press release crosses its desk while simultaneously trying to feed the beast of dark green despair.
A more appropriate headline might have said "Silicon Solar Panels 100 Times Safer than Coal" and compared safety of solar with coal and done the basic algebra noting that even when the solar industry grows to replace every energy source on the planet, ie 1,000X, over the next 20 years, (making Peak Oil and CO2-AGW alarmism ridiculous in the process) there would be only 10,000 deaths over 20 years instead of 10,000 deaths in mine accidents each year and 20,000 new black lung cases each year with coal alone (not counting deaths from oil, gas, and nuclear power). And even that scenario assumes that silicon solar technology would not advance during that time.
And of course, since solar panels unlike coal, et al, are not fuel but a means for capturing energy that don't have to be replenished constantly with new supply, whatever deaths happen in their manufacture are for generating energy over the next 30 years. Thus the appropriate math is the potential for 10,000 solar panel manufacturing deaths using existing technology over the 30 year lifespan of solar panels versus the worldwide coal death toll of 30,000 X 30 = ~1M.
Haha! InquiringConstructivist, I thought the very same thing, only a step further, that SiXtron provided this 'journalist' with the content, to be published in article form. 'Journalists' love it when they don't have to work as hard. Just go to Profnet.com to see what I mean. Yet another example of how ScientificAmerican has lost credibility, including their blind parroting of the anthropogenic Global Warming fraud.
Reply | Report Abuse | Link to thisThis artlcle does read suspiciously like a glorified press release for SiXTron. It needlessly plays up a danger, so as to show the clear benefit of the mitigating product.
Reply | Report Abuse | Link to thisEven just the headline is misleading. "Solar Power Industry" includes concentrated solar power. But the CSP segment does not use solar cells at all. The potential danger of silane gas is nonexistent with CSP.
A better headline might have been: "Silicon Gas Casts Shadow on Solar Cell Industry". Then the article could explain in just two or three sentences the difference between PV and CSP, thereby educating readers, while at the same time, reporting accurately.
Excuse the pedantic tone, but all of this seems really obvious to me.
Just India? Not really. Mr. Subhendu Guha has a lot to explain: Uni-solar earned zero points in each of the 4 categories at the Silicon Valley Toxics Coalition, for failure to respond:
Reply | Report Abuse | Link to thishttp://www.solarscorecard.com/comp_unisolar.html
It is understandable that Uni-solar is shy. After all, the explosion in 2007 at their plant in Greenville “caused a portion of the plant to ‘pop out’ and left a large vertical hole in the side of the plant.”
http://blog.mlive.com/grpress/2007/11/explosion_force s_evacuation_at.html
Less than two weeks later, everyone within a one-mile radius of Building 1, including hundreds of people attending Greenville Community Church’s morning worship service, were evacuated after a report of a gas leak at the plant (Unisolar reportedly uses silane and germane mixtures, prone to combustion in air).
http://www.thedailynews.cc/Main.asp?SectionID=2&Artic leID=17864
The Director of Greenville Department of Public Safety, however, opined that “Their [United Solar Ovonic’s] safety control system did exactly what it was supposed to do.” Right.
whenever I hear cadmium as a solution I get the shingles: everyone knows that Cd is a polutant
Reply | Report Abuse | Link to thisI've been using a toxic, highly flammable and explosive fuel to power my personal transportation for years now.
Reply | Report Abuse | Link to thisabsolulety right!
Reply | Report Abuse | Link to thisbostongeorge6297 - Now see, you were less profane on the third submission - perhaps you should count to ten before submitting the first comment!
Reply | Report Abuse | Link to thisEvery day, I drink large quantities of a liquid that is responsible for hundreds of drownings every year, that is irreversibly toxic above a certain daily intake, that is quite dangerous in its pure form and that can react explosively with alkalis.
Reply | Report Abuse | Link to thisDrinking water casts a shadow on diets worldwide!
I would say some of the respondents to this article are a bit harsh and that takes away from the point(s) they were making..
Reply | Report Abuse | Link to thisNow yesthe author should be aware that the article by only quoting one company does sound like a blatant promotion for SiXtron, but I assume that was due to time limitations since there are other approaches besides SiXtron's to improve on the safety of Silane . Silane is used in the semiconductor industry as part of the chemical vapor deposition (CVD) processing step and with proper handling poses no serious hazard. I was not aware however Silane was alsoby some companies in manufacture of Solar cells but it makes sense since they are doing CVD. So thank you for that bit of information.
Your points about time limitation, MrLight, I think warrants the harsh responses. Lack of time is not a valid excuse for lack of research, in my opinion. And if you are correct about the existence of other approaches, then why not include those in the article?
Reply | Report Abuse | Link to thisThis piece is a far cry from being non-biased journalism.
There are hazardous with manufacturing all sorts of vital products. The larger point is to properly site manufacturing -- well away from residential Also, to have qualified staff, proper equipment maintenance and Environmental Safety & Health staff.
Reply | Report Abuse | Link to thisAnd then there are these safety comparisons just this week alone:
Reply | Report Abuse | Link to thisApril 5, 2010
Twenty-five miners dead in Raleigh County blast
http://wvgazette.com/News/201004050545
Chinese Official Warns Mine's Bosses to Prepare for Major Probe
http://www1.voanews.com/english/news/asia/Chinese-Official-Warns-Mines-Bosses-to-Prepare-for-Major-Probe-89985902.html
And how many die from oil and gas drilling, refining, and pipelines?
And how many are killed in the wars over control of relatively scarce hydrocarbon-rich real estate?
Folks: the missing point here is that the PV industry is seeing a lot of amateurs hop on the band wagon and begin PV production lines without using the proper safety protocols (think: China). This is concerning. Sure, these gases can be managed, but at a cost. For an industry striving to drive down costs and break into the mainstream, why not eliminate a hazard that can not only potentially injure workers (even when handled according to safety rules) but also threaten production uptime? It is true that journalists do not have as much time and resources as they used to, but before we jump all over this one, ask yourself, have you heard of another company offering a silane alternative that is cost-effective while creating performance parody to silane-coated monocrystalline solar cells? Perhaps this is why there are no other companies featured.
Reply | Report Abuse | Link to thisHaving worked in the amorphous silicon PV industry for more than 25 years I am well aware that silane is dangerous when mishandled. The industry has evolved equipment and procedures to minimize the hazards, but there will always be some risk. The question about the incidents cited is whether the proper precautions were in place and whether workers used them properly. Or did someone do something stupid?
Reply | Report Abuse | Link to thisBefore we condemn the solar industries as dangerous, we might compare the accident history with that of with coal mining and petroleum.
Man, I am I glad I don't subscribe to SciAm anymore. Never mind they could never deliver my subscription month-to-month. Worst, the content!! What a rag!
Reply | Report Abuse | Link to thisThankfully other comments have hit the nail on the head: this is nothing but a PR story written by and for SiXtron, the Indian silane story is needlessly gratuitous yet the story stats prove you are safer playing with silane than driving a car, the ignorance of a core high technologies demonstrably profound, a complete train wreck in terms of basic Pareto risk analysis, lack of expository writing skills, etc.
But here's the part that takes the cake: it goes all hot-and-bothered by silane, one of the safest nominally toxic gases used in any industry, but then glosses right over CdTe. Cadmium-damn-Telluride?! Are you serious?! CdTe is the alpha and omega of a toxic and dangerous chemicals compared to poor little silane. Forest; meet trees!! Pot; meet kettle. Sorry, what tiny credibility the article had to this point is utterly gone by the end.
SciAm used to be a relevant magazine. I started reading it in the late 1960s, early 1970s as kid. Had a lot to do with my career choices. SciAm used to be an unquestioned leader in science and technology. That time has passed. For quite a while it seems.
J Gruszynski
BSEE, MBA
30 years in the semiconductor industry
Doug should examine HOW MUCH conventional energy it takes to make PV modules versus the amount the modules generate. Some smart people actually actually done this and calculated an "energy payback time." Thus amorphous silicon PV in a typical installation takes about 1.5 years to repay the energy to make it. Crystalline takes about 2 years. The working life warranty on either is at least 25 years.
Reply | Report Abuse | Link to thisBy Doug's reasoning farming is a losing proposition since one must use up grain (as seed) in order to producew grain as a crop.
Did Rupert Murdoch buy SciAm? Solar power blows people up! Film at 11:00!
Reply | Report Abuse | Link to thisAs mentioned above, Silane isn't new, isn't demonic, and is dangerous. So is Acetylene, Gasoline, Natural Gas, Propane, nuclear fuel, coal dust, and coal mines. Evidently so is Axe Body spray when someone is stupid enough to spray it all over their arms and light it on fire.
I'm not sure which is more alarming, the public's declining understanding of all things scientific, or the tabloidification of science topics to get more readers (and bloggers).
In the long run, I'll bet that the tabloid science trend will do a lot more harm to people, and science, than Silane.
David Cota - Not to be always the stodgy old naysayer, well maybe I am. This is a very interesting report you refer to. However, you seem to have leapt across several fundamental boundaries on you way to solution (to another subject, by the way).
Reply | Report Abuse | Link to thisThe biggest hurdle might just be the energy cost of the suggested conversion. You'd need someone who can do the math, but I think the production of global quantities of laser powered supercooled nanoclouds of carbon to hurl at a macroscale devices constructed from charged carbon nanowires to deconstruct atoms may take significant energy resources.
Interesting unrelated experiment, though - thanks.
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"In 2007, outside Bangalore, India, an explosion decapitated an industrial worker, hurling his body through a brick wall."
Reply | Report Abuse | Link to this"In 2007, outside Bangalore, India, an explosion decapitated an industrial worker, hurling his body through a brick wall."
Reply | Report Abuse | Link to thisI would like to understand the physics of hurling a human body through a brick wall. I don't believe it can be done using the scientific method.