The new equipment choices for c-Si cell producers

andarchan

In the past, the choice for c-Si cell producers was more polarized. New entrants often sided with a quick route to market, based upon standard turnkey lines. While not pushing the boundaries in terms of cell efficiencies, such an approach offered a low-risk, quick ramp-up strategy. The other advantage here was the availability of an extensive network of module integration suppliers, where connecting individual cells with standard front and back contacts was routine and guaranteed. Selling c-Si cells therefore had the widest possible module-supplier outlet for pure-play cell makers, or simply made equipment selection for downstream-integrated players (both cell and module builders) easier.

For those manufacturers pursuing non-standard cell types, full ownership of cell production technology and manufacturing tooling was a perquisite to success. And viewing the case-studies on offer here is perhaps one of the most telling observations ahead of widespread high-efficiency cell adoption throughout the industry. In comparing the relative capacities and productions of the three non-standard cell types shown in Fig. 2, there is a strong correlation. Back-junction and HIT cells were enabled by downstream-integrated players, with full-ownership on equipment (and module) tooling: conversely, wrap-through cells have struggled to gain adoption, with manufacturers often critically reliant on module tool suppliers providing customized tooling for all-back-contact interconnection.

Today, equipment tooling for c-Si fabs is governed by several factors:

The different approaches taken by established c-Si manufacturers in high-volume production vs. new entrants looking to ramp-up quickly;
The scope of full turnkey lines offered to the market, and how much they are split into front- or back-end-of-line specialist tooling; and
The range of qualified high-efficiency processes available in the market, and whether these are adopted as complete lines from single (or 'partnered') tool suppliers or as stand-alone customized inline tools.

The segregation between established players and new-entrants is likely to continue to be the main factor driving the adoption of turnkey lines; it remains unlikely that established cell producers are going to 'standardize' production, or lose control of in-house differentiation within a competitive market landscape. However, new entrants will now be able to fast-track technology by adopting one of the high-efficiency turnkey lines primed for full market release during 2010. Partial turnkey lines may emerge, offering value-added features either at the front- or back-end, depending on where the high-efficiency gains are sought from cell makers. Indeed, it may be that many of the current next-generation turnkey lines morph into optimized front- or back-end solutions. Possibly, the most likely route for new equipment adoption throughout the industry will come from customized tooling across the various process stages within the complete cell manufacturing line. In fact, for those seeking the largest efficiency enhancement, multiple new-tool/technology adoption is required -- something that further decreases the pull on turnkey lines. Customized equipment adoption is not new to the leading cell makers -- only now the choice is more varied with different high-efficiency cell concepts to pursue. The wildcard here however is the level to which established market-leaders for specific parts of the standard cell production flows adapt their tooling to modify for high-efficiency or integrate directly up/downstream with new equipment and process steps needed for the cell concepts being proposed.

While each high-efficiency cell concept has specialized process steps and associated tooling necessary to enable these, ultimately the selection of equipment from the market may well owe more to safeguarding and differentiating patents licensed, with intellectual property being protected. Arguments are already being given as to the validity of one high-efficiency concept over another from a technical standpoint, but in truth, technology exclusivity may form the underlying basis for subsequent sales and marketing strategies. While potentially confusing those starting out in high-efficiency cell decision making, this may divert the attention of cell producers to taking more ownership in customized line improvements, or simply performing customized retrofits by strategic partnering further down the equipment supply-chain. Customized retrofitting plays far more into the hands -- and comfort zone -- of established cell manufacturers; those whose initial capacity expansions were in fact differentiated in the first place.

Selective emitters to the fore -- not for the first time

There are several approaches to improving the efficiency of the standard c-Si cell concepts:

Increased light trapping by improving surface texturing;
Optimizing passivation layers (both front and rear surfaces) for reduced recombination losses;
Redistributing the phosphorous diffusion levels below and between the front surface contact grid; and
Changing metallization techniques for improved aspect ratios and increased current collection.

While each is subject to its own body of research, the advanced cell concepts based upon the standard cell type typically draw on a combination of the above efficiency-enhancement steps. No more so is this evident in what's known as the selective emitter cell types, which form the basis of the third route highlighted above. Selective emitter is rapidly becoming a buzzword within the industry, and down through the equipment supply-chain, for almost the first time. Selective emitters provide probably the most immediate route to increased cell efficiency within c-Si production today. Typically however, redesigning the front finger process comes hand-in-hand with addressing improvements to front metallization techniques compared to the historical wide (and high) lines deposited by screen-printers.

When reviewing the range of selective emitter schemes to choose from, even here there are a number of options:

Etch-back;
Screen-printed phosphorus-containing paste;
Buried-contacts;
Diffusion-masking; and
Laser-doping.

Within each also, there are different approaches. For example, laser-doping can be introduced via diffusion of phosphorous within the PSG layer prior to removal, or by the introduction of a phosphorous-containing layer after or during the SiNx deposition stage. Variants exist combining some of the steps above.

Selective emitters are not new to the industry. The research community has been acutely aware for some time that c-Si cells based upon selective emitter formation were fundamental to roadmap evolution. And efforts to commercialize them go back over ten years, before many within the existing equipment supply chain were actively involved in the solar industry. Indeed, with the research labs having had a number of years head-start compared to in-house company-located R&D efforts, intellectual property and licensing becomes a key issue. The full ramifications of this will only be played out in time.

Of more interest though are historical perspectives on the selective emitter concept: in particular, the research activity at the University of New South Wales (UNSW) during the mid-1980's coupled with the licensing and commercial implementation by BP-Solar shortly after, within their Saturn lines. It wasn't branded selective emitter then, but more appropriately high-efficiency. The concept in question was the well-known laser-grooved buried-contact (or LGBC) cell. Revolutionary in its day out of UNSW, herculean in proportions the success of BP-Solar to produce over 150MW of these cells, LGBC technology laid the foundations for many of the selective emitter schemes today, some of which can simply be regarded as high-finesse next-generation versions of the LGBC cell concept. What's different now, compared to the time period when BP-Solar were implementing these cells in production, is the market size and the participation of an equipment supply-chain co-developing the necessary production tooling to fast-track industrial qualification. For now, the equipment supply-chain is actively seeking to drive the adoption of selective emitter cell types: albeit implemented by necessity and no longer viewing them with curiosity as an esoteric, technology-led exercise.

Other factors affecting adoption timelines

While equipment manufacturers are hard at work finalizing tool supply for the new c-Si cell concepts, the timing is of course dictated by a number of external factors, including:

Silicon material cost and supply, and the overall competiveness of c-Si vs. thin-film panels at the levelized cost of energy stage;
Average wafer thickness decreases for c-Si cells, and their impact on handling equipment used within production lines;
The timing for rear-passivation layers (stacks) being implemented to increase efficiency there;
Increased thin-film utilization rates, and how quickly this adds to the overall competitive landscape relative to the demand available; and
Everything next-generation related, including radically new approaches to c-Si design, organic PV, and other potential wildcard entrants.

So, who knows, maybe reviewing the equipment landscape after the trade-show season at the end of 2010 will feature more twists and turns. Tracking market trends with so many diverse approaches to technology on offer could simply not be more important in deciding strategy and supply of equipment within the solar industry today.