by Finlay Colville, Coherent, Inc.
July 9 2009 – Next-generation c-Si cell production tooling is finally poised to make an impact within the industry.
Production equipment for c-Si cell manufacturing is slowly adjusting to changes required for next-generation cell types. Three different c-Si cell concepts are forecast, each with unique tooling to enable efficiency- or cost-prioritized manufacturing. Championed during the 1980s and 1990s, high-efficiency concepts using non-contact laser-based tooling are seeing renewed focus, as market demand hastens their long-awaited entry.
Gloom and despair was clearly in evidence down through the supply-chain for c-Si cell manufacturing equipment earlier this year . With the transition from healthy equipment order backlogs to one of near-empty production floors indeed being a wake-up call for most concerned. The impact however was not simply related to short-term bookings forecasts, but heralded an inflection point that looks set to redefine production equipment technologies once cell capex levels return to their halcyon days of the mid-2000s.
This article explains how changes in market supply-demand dynamics , coupled with less favorable financing conditions brought on by the current credit crunch climate, are together redefining the type of c-Si production equipment required by cell manufacturers over the next few years. The adoption of laser-based process tools — long considered a luxury by c-Si cell manufacturers — is used to illustrate the changes afoot, and the trends predicted for representative technologies now considered essential within specific tracks of proposed cell roadmaps.
Supplier’s market hinders progress
Historically, production technology utilized by cell manufacturers has been dictated by the supply-demand dynamics prevalent at the time. To understand this better, first let’s define relevant time periods. Throughout this article, comparison will be made directly to industry phases and assigned nomenclature, as borrowed from Stephen O’Rourke’s market-leading commentaries : initial growth (2003-2008); shake-out (2008-2012); and build-out (2012-2020). The final category of maturity (beyond 2020) is omitted here, partly on account of the rate-of-change in production technologies anticipated during shake-out and build-out phases. Rather, an additional phase is introduced prior to initial growth, here labeled ‘R&D/Technology,’ to help understand the equipment utilized extensively prior to 2003 — and how this embryonic phase largely set the scene for production technology that would be applied widely during initial growth.
So, what about the implementation of laser-based tools within c-Si cell production lines during these time periods? It’s a mute point that, had search engines been around prior to the mid-80s, a likely candidate for most-hits when looking for matches between ‘lasers’ and ‘solar’ may well have been Ray Davies’ mid-60s chart-topping hit ‘Sunny Afternoon’  when he calls out for “lazing on a sunny afternoon.” Certainly not as essential components for efficiency-enhancement above the 17% level. For the R&D/Technology phase was more about possibilities than a concerted effort to introduce non-contact production-line equipment offered by laser tools.
Prior to 2003, laser-assisted cell enhancement stages were confined mainly to the research activity at the University of New South Wales  (UNSW), Sandia Labs , European-funded projects such as LOWTHERMCELLS , and within the Fraunhofer-ISE . With the focus then on R&D achievements (the strength of the academic community) and proof-of-concept as an indication of technical prowess, there was no fundamental driver to take these processes from lab to production. Nor was there an established equipment supply-chain willing to invest the time and resource to qualify laser-tools for production use. To compound matters, suitable industrial-grade laser-based tools of the late 1980s and 1990s were not aligned with cell manufacturing needs.
Ultimately, the lack of any appreciable market-pull (demand) stood as the underlying barrier to laser-tool adoption as standardized equipment. Interestingly, high-profile was afforded to the flagship c-Si cell of the 1990s, the laser grooved buried contact (LGBC) concept . Originating at UNSW  and licensed by BP Solar, their Saturn lines were pioneering in every aspect : efficiencies greater than 17% from standard c-Si cells with laser-prepared grooves, double-diffusion for selective emitter formation, and electroless plating. Just two things really were out of sync — the time period and the lack of a tangible market demand.
As initial growth kicked in, the order of the day was ramp-up of capacity to meet the demand originating from subsidies in Japan, Germany, Spain . Equipment selection was dominated by tooling then qualified and readily available, and when configured as a complete production line, could output standard c-Si cells with efficiencies at the 13%-15% level, well below the benchmark set by research labs (UNSW PERL cell at 25% ) or the Saturn lines (over 20% demonstrated ). So, moderate-efficiency standard cell types became the mainstay of c-Si output during this time period, and as demand outstripped supply, the goal seemed simple enough — how quickly could additional lines be commissioned? For even modules based upon these cells at the $5/W mark were readily sold . Thin-film competition from the likes of First Solar  had not fully matured to the dominant level it is today, and alternative thin-film technologies based upon a:Si seemed to be more biased towards announcements of fab orders than fully-commissioned capacity levels .
What did however emerge was the idea of a high-efficiency (HE) c-Si cell concept , but not based upon low-cost p-type silicon. Rather, the technologies of Sunpower  (interdigitated back-junction, or IBJ) and Sanyo  (heterojunction with intrinsic thin-film, or HIT) used high-grade n-type silicon and production-line equipment not ideally matched to the lowest cost means of cell production. Differentiation between these HE cells and standard c-Si cells is certainly influenced by the priority afforded to cell-making revenues, when this stage is isolated within the value-chain; the comparison most evident between pure-play cell makers and vertically-integrated companies more focused on selling energy further downstream. (Note: The back-contact cells of Advent Solar 19], based upon the Emitter Wrap-Through (EWT) cell concept, can also be included within this definition of high-efficiency.) However, a clear divide was established between cells manufactured either for the lowest cost (LC) or the highest efficiency (HE). Similarly, production line equipment availability and the technologies used therein also fell into these two categories.
Laser-based production line equipment certainly had a hard task getting into standard cell production lines during this time period. The HE cells — especially LGBC production and initial EWT lines  — were using laser-tooling as an enabling technology , but accounted for a very small percentage of production output worldwide. Standard c-Si cells did come to rely upon laser tools to perform the junction isolation step , but in the mid-2000s, plasma-etching very much led the way here on account of low capex and (low-tech) alignment with manual wafer handling (especially in Asia). Understanding however that junction isolation is really a loss-preventative step, and not an efficiency-enhancement enabler, puts some perspective on what many regarded then as the ‘big’ opportunity for laser-based tools within c-Si lines .
Shake-out to the rescue
Something had to change the landscape. Enter the events of the past 12 months which have been well documented within the press recently, with the label shake-out applied mainly to consolidation of cell or panel producers expected to emerge as winners over the next few years. Shake-out also relates to a lesser degree to the different cell technologies (c-Si, thin-film types, or wildcard entrants). But shake-out also applies to the production equipment used for cell manufacturing. And with process steps for c-Si cells far more standardized compared to many of the thin-film technologies promoted today, the impact of shake-out here will be more pronounced and immediate.
Figure 1. Bottom-up analysis of historical and projected c-Si capacity added per annum. Forecasting of c-Si cells with efficiencies exceeding 17% is borrowed from the categorization proposed in Ref. #16. Actual production outputs can be estimated by combining the data above with recent analyses showing fab-utilization rates  and capex . (E) estimate; (F) forecast.
Projected cost analyses of the various cell technologies at the levelized cost of energy (LCoE) point are differentiating again two discrete tracks specific to c-Si cell production , necessary to remain competitive with thin-film approaches in the medium-term. While other factors clearly come into play here (no more so than raw material costs ), the two routes for c-Si cell production (based upon HE and LC cell types) each have their own roadmap for competitiveness . In fact, there are really three tracks opening up for c-Si cell production defined as:
- Ultra-HE cell types based upon high-grade silicon (typically n-type), characteristic of the IBJ and HIT cell types, with efficiencies exceeding 20%.
- A HE version of the standard c-Si cell, ideally based upon multicrystalline-Si (mc-Si), and providing efficiencies in the 17-20% level.
- A LC version of the standard c-Si cell, with comparable efficiencies to current cell outputs at the 14-16% level, but where tool capex and operating costs are significantly lower than used in production today.
It is therefore this differentiation, somewhat courtesy of the current shake-out phase of the industry, which forms the basis of the new production-line equipment and processes being forecast now for the next few years. Figure 1 (above) captures the projected trends out to 2013 for each of these c-Si cell types, with respective capacities added each year. The growth of high efficiency cells over this time period is illustrated in Figure 2 (below), which compares the cumulative installed capacity at year-end 2008 and 2013.
Supply chain falls into line
Fortunately, the equipment supply-chain has been primed for this change; a glance at the patent filings and license agreements over the past decade from some of the research labs flagged up earlier in this article backs this up. Further, recent EU-funded projects such as CrystalClear  and SOLASYS  are full of next-generation cell concepts awaiting industrial technology-transfer (much in the same way that the UK DTi-funded project Alpinism  between Exitech, Ltd. and BP Solar hastened industrial-grade laser tooling for the Saturn lines). It’s largely through these consortia that new laser-based processes have been illustrated, set to form part of new cell production stages to enhance the efficiency of c-Si cells.
Figure 2. With the introduction of high-efficiency c-Si cells from 2008, the proportion of installed c-Si capacity by year-end 2013 comprised of standard cells will decrease from ~90% to ~60%.
Opportunities certainly exist for new technology (including laser-based tools) to assist in next-generation cells based upon the ultra-HE cell types, but the fundamental driver there may be reduced capex and lower operating costs, rather than an increase of a few percent in cell efficiency. And scope for radically new approaches for LC cell production, possibly from reduced silicon consumption (e.g., sub-100μm thick cells ), may well see the introduction of other new tooling there. But the main driver for new equipment tooling and processes in the short-term is clearly track 2 above — the HE version of the standard c-Si cell concept . Here, immediate incremental improvements can be made via upgrading existing lines or during new capacity expansions. The goal for now being efficiency-enhancement with minimal change to production capex. But at last, a clear route-in for laser-based process tools at key enabling production stages, with a well-defined long term market pull.
The various laser-based steps for high-efficiency cells have been summarized recently in a number of review articles [20, 27, 28]. Without getting too much into technicalities, laser-based tools can be categorized  as:
- Assisting in mask writing  (for subsequent metallization, secondary diffusion in selective emitters, or surface etching to texture);
- Localized secondary diffusion  (laser doping via phosphorous- or boron-contained precursor layers);
- Contact preparation (finger grooves, through-silicon-vias , interdigitated structuring );
- Contact forming (sintering  or firing ).
When looking at the various schemes being considered today, it is indeed a testimony to the endeavors of the UNSW researchers and BP-Solar technologists that many of the efficiency-enhancement steps pursued within the current crop of HE cell candidates share so many of the concepts inherent to the original LGBC cell . Only now, production technology has matured somewhat, there exists a buoyant equipment supply-chain eager to participate, and the timing for higher finesse variants being introduced to the market is just right!
Having waited in the wings for the better part of twenty years, and having patiently watched cell production line capex grow with CAGRs in excess of 40%, next-generation c-Si cell production tooling is finally poised to make an impact within the industry — thanks mainly to the market dynamics of the new shake-out phase. As the leading cell manufacturers start to announce average c-Si cell efficiencies on mc-Si in excess of 17%, others will be forced to follow with similar high efficiency claims or choose to pursue a lower cost approach to cell manufacturing. With the implementation of high-efficiency c-Si cells directly linked to advanced laser processing within new production lines, the next few years will see increasing reliance on laser tools as they account for a greater fraction of new cell capex releases. (A follow-on feature by the author will address additional factors required for successful laser tool adoption and the timing of the various new process steps into mainstream production.)
Finlay Colville received his BSc in physics at the U. of Glasgow in 1990 and PhD in laser physics at the U. of St. Andrews in 1995, and is director of marketing for solar at Coherent Inc., 5100 Patrick Henry Drive, Santa Clara, CA 95054 USA; ph +44-7802-238-775; email [email protected]; www.Coherent.com/Solar.
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