Solar visionary and past Right Livelihood Award winner Martin Green is keeping busy. Combining his work as director of CSG Solar (a spin out formed to commercialise silicon-on-glass solar technology); with his research work with the University of New South Wales and the ARC Photovoltaic Centre of Excellence, he is also involved in exploring R&D collaboration with companies like Dow. At one such joint event, David Hopwood asked him about some of the developments in PV technology today.
REFocus: With your illustrious career in PV I am sure you will have seen most things in terms of technology. What are the most exciting technological developments in the PV sector currently, in terms of innovations that could improve the ability of PV to produce cheaper power?
Martin Green (MG): I think the most recent one that's impressed me has been the progress made with monocast silicon. It really is quite an old technology. Back in the 1980's they were able to do something similar, but the ingots have got bigger and bigger and you've got a larger and larger fraction of the mono-crystalline material. BP was working in stealth mode on the technology a few years back, and then decided to abandon it because they published their results, and the Asian companies very quickly latched onto what was going on. There are several of them now that nominally have commercial product on the market, based on the approach – JA Solar and DEK would be two of the leading companies. And in terms of the equipment suppliers, companies like GT Solar [now GT Advanced Technologies, Ed] and AGT are on the verge of (or already) offering equipment to suppliers. Also some of the wafer suppliers are now supplying wafers based on that approach, like GCL and maybe even LDK as well.
REFocus: What are the key aspects of that approach that will bring the cost down?
MG: It doesn't require much equipment modification. You just have to put a seed layer at the bottom of your crucible when you're doing the directional solidification that most manufacturers do. You need a little bit of subtlety in the initial control of temperatures - to make sure you don't melt all your seed layer away - and then it's pretty much as in normal directional solidification; you do have a slightly more complex solidification process, but the range of cell efficiencies that result from the ingots is immediate between those multi-crystalline and mono-crystalline materials, so you upgrade the average efficiency from the crystal. I think this is quite an exciting development, in that there's plenty of potential to improve the quality of material you get as more and more manufacturers work with it and tweak the process end. Also, it's a different type of mono-crystalline silicon…which can have lower oxygen composition. So from the central ingots to the extra regions of the ingots you may be able to get better quality material. This seems to be a way of producing a much better quality wafer at pretty much the same cost as the standard multi-crystalline wafer.
REFocus: Interesting…there is always a lot of debate within the PV sector as to the relative merits of crystalline technologies versus thin film technologies. What you've just told me would suggest that you think crystalline still has a major, major part to play going forward. Could you ever see a time when thin film technologies would really start to take a major portion of the market share that crystalline has?
MG: My take on this is that the recent cost reductions of crystalline (and I guess the very clear path to further cost reduction), is making it increasingly difficult for thin film technology to enter the market. The manufacturing costs of silicon now are getting very low, and it's very difficult for the thin films to get under that cost. You need many years of building up production capacity and so on, where your product is in fact going to be more expensive than the wafer product. To get underneath it, you have to invest a lot in building up a manufacturing base, and producing a product that is …going to be less attractive to the market, and therefore you have to sell at a huge loss.
REFocus: So you mentioned wafers already. What other parts of the value chain do you think really have potential for cost reduction, over the short to medium term?
MG: I think there are possibilities in modular installation. Sticking with the wafer side, along with the cost of the polysilicon coming down rapidly (and there's still more give there), there are improvements in sawing. At the moment I think 30 wafers per centimetre of ingot is a fairly standard type of yield of wafer, but I'm expecting that to go up to, say, 50 wafers per centimetre by the end of the decade, to create a yield of wafer area from that ingot.
Refocus: Have you any opinion on how far crystalline silicon manufacturing costs could come down to - within a three-year timeframe, for example?
MG: in fact there is some data out there on the rate that the manufacturing cost is reducing - the SEMI road map International Technology Roadmap for PV (ITRPV) – that was released in 2010 (and then updated in 2011) has a cost path mentioned. This was done in relative terms, but it's possible to quantify, knowing the actual cost levels the industry is presently at. But I believe the leading manufacturers are now just getting under the dollar a Watt level for manufacturing costs…and according to this road map there's a further 60% cost reduction feasible over the coming decade, through things like better wafering yield and casting bigger ingots (all relatively predictable improvements) plus improved efficiency that I think will be another contributor. So a 60% reduction from where we are now in terms of manufacturing costs seems quite feasible [NB, editor's note: The report states that due to the historical learning curve as well as industry growth, the specific costs per Watt peak (Wp) of PV modules are expected to decrease by 8%-12% per year. This corresponds to a significant cost reduction per module. To reach this, “current technology will be optimised, but new technologies also need to be implemented in production between 2013 and 2015”].
Refocus: OK, moving away from technology, there's a lot of controversy at the moment with the bankruptcy of various solar companies in the U.S., In your opinion, what's the best way to fund PV development? At an R&D level through grants? Or at the market level to build scale?
MG: I think both are needed. The market pull approach has really been effective in reducing costs over the last three years, so without the success of the German feed-in tariff programme, we wouldn't be where we are now. You also need this pipeline of new technology to drive those ongoing cost reductions, like improved sawing approaches; improved cell designs; improved material use; usage in the module and so on, which need to be funded in the early stages to prove out their potential. So I think both are needed; it's just a matter of getting the right balance between the two.
Refocus: Which the U.S. for one hasn't quite got right at the moment…?
MG: I think providing loans to some of the companies involved was sort of playing the role of a venture capitalist, and [if you're a venture capitalist] you don't expect to get a 100% success rate. So, an expected outcome if you're going to play the role of a venture capitalist with your seeding funds!
Refocus: How can industry work smarter with the academic community to develop some of these really cutting-edge research solutions that are being worked on at a very early stage, and bring them more quickly to commercialisation?
MG: We work closely with a number of manufacturers, particularly manufacturers from the Asian region, because that's where our labs are based. But we work very closely with them, with getting improved technology established, generally on their pilot production lines, so that's the first step in our interaction with them, and we're starting to see some of the results of that technology transfer resulting in commercial product. Our model is perhaps fairly unique, giving the special circumstances and the facilities available to us, but we have very strong interaction with companies throughout Asia, and working with them to improve what they're doing on a production basis.
Refocus: if you could wave a magic wand and implement something that would result in really driving the PV industry forward, what would that thing be?
MG: I've been very impressed with the progress made with the silicon wafer-based technology, and as I said before, I think it's going to be the mainstream for quite some time. But thinking about ways in which it can be supercharged, as a research group we've got very interested in [finding] ways of stacking cells of different materials on top of silicon wafers (to take advantage of very high quality silicon wafers): [so how can we] take advantage of that to provide a substrate for some more sophisticated cell structure on top of the silicon. [Also], knowing that we have to produce these cells at one a second or faster is an additional constraint.
About Martin Green: Martin Green is currently a Federation Fellow and Scientia Professor at the University of New South Wales and Executive Research Director of the ARC Photovoltaic Centre of Excellence. He is also a Director of CSG Solar, a company formed specifically to commercialise the university's thin-film, polycrystalline-siliconon-glass solar cell. His group's contributions to PV are well known, and include the development of the world's highest efficiency silicon solar cells; and several other successful spin-off companies.He is the author of 6 books on solar cells, as well as numerous papers in the area of semiconductors, microelectronics, optoelectronics and, of course, solar cells. International awards include the 1999 Australia Prize, the 2002 Right Livelihood Award (also known as the Alternative Nobel Prize), the 2004 World Technology Award for Energy, the 2007 SolarWorld Einstein Award and the 2009 Zayed Future Energy Prize (one of two finalists).