Thinner, larger solar cells challenge current module assembly tech Current module assembly methodologies based on solder processes, while effective for today’s cell dimensions, are challenged by the move to thinner, larger solar cells.
The much discussed and highly sought after grid parity for solar technology is arguably the primary driver for widespread, mainstream adoption of solar electricity solutions. While significant advance has been made over the last decade, the higher cost as compared to the price tag of traditional grid power has left consumers a bit lukewarm when it comes to photovoltaics (PV). But new advances may help change this dynamic and push the industry ever closer to heretofore elusive grid parity.
|Figure 1. Soldered interconnection; silicon pull-out from the cell is observed .|
Reigning crystalline silicon (c-Si) cell technology, which currently accounts for approximately 80% of the PV market, today has an efficiency of roughly 15% and an average module end unit price per Watt peak (Wp) of US $2.00 – $2.50 (or, a production cost in the range of US $1.50 – $1.80 per module). Indeed, the performance of c-Si cells is what has led to their market dominance and, while thin-film modules are a lower cost solution from a production point of view, their current status as the less efficient of the two technologies has limited implementation – at least for now.
Figure 2. Adhesive shows good adhesion to the tab and bus bar: a cohesive failure mode is observed .
Improving solar conversion efficiency in tandem with reducing raw materials and high volume production costs is imperative for the growth of PV technology. It has been estimated that a turnkey system price should be in the range of US $2.50 – $3.00 per Wp and that full production (taking into account other factors than just the system alone) costs averaging $1.25 per Wp are the target needed to reach grid parity .
In this work, we focus on c-Si module assembly and how new developments in materials and processing techniques can help lower overall manufacturing costs, thus enabling further adoption of solar systems.
Challenges with current c-Si module assembly
While there are subtle variations on the assembly methodology used for c-Si modules, generally speaking, the production process is as follows: the metallized c-Si cells are connected into a string and then different strings are joined to form the module. Pre-applied solder-metallized copper ribbon is used to achieve the electrical connection between the different cells and strings. String formation (cell interconnection) is accomplished by attaching the ribbon from the top of one cell at the bus bar parallels to the bottom side of the adjoining cell at the silver or silver-aluminum backside firing paste. To date, the dominant interconnect materials used to make these copper ribbon connections have been tin-lead (SnPb) eutectic solders.
But market factors are now challenging conventional SnPb solder’s future role as the most viable interconnect solution for crystalline photovoltaic modules. First, legislative measures that have been implemented at the board assembly level dictate the elimination of lead from solder materials and it’s likely this will come to pass in the PV market as well. If so, it means that higher melt point (250° to 260°C) lead-free solders would be required and, for thinner and more temperature-sensitive solar substrates, these processing requirements may be insurmountable. Second, the push to reduce costs and extend efficiency by incorporating thinner and larger c-Si cells is problematic for solder processes as cells may crack or break during soldering [2, 3]. In addition, the rigid nature of the interconnect and the coefficient of thermal expansion (CTE) mismatch between the silicon and copper tabs may also result in damage to the silicon while in service, thus decreasing module efficiency or inducing complete failure. The solder itself can also crack, which causes lower module efficiency through greater electrical resistance . These issues have therefore forced module assemblers to seek alternative interconnect methods for modern, thinner c-Si cells.
Electrically conductive adhesives provide possible solution
Current c-Si cell thicknesses are generally in the 180 micron range and, at this dimension, solder connection processes are still relatively robust and arguably the most cost-effective solution. But solder’s dominance is being upended as the PV industry pushes toward cells as thin as 160µms over the next 24 months, and below 120µms soon thereafter. Not only are the thicknesses of the cells being reduced, but the viable live area is also being increased. Current 6″ x 6″, 180µm cells will give way to 8″x 8″, 160µm cells in the not too distant future.
|Figure 3. New ECA shows stable contact resistance at 85°C and 85% relative humidity (RH).|
Because these dimensions will be required to achieve grid parity (thinner silicon reduces materials cost), alternative materials will be needed for cell connection as soldering processes will induce stresses that these more fragile cells cannot withstand. At the same time, these new interconnect materials must be as effective a conductor as solder and provide equal or greater reliability.
Figure 4. Next-generation ECA also exhibits contact resistance stability when subjected to thermal cycling.
Recently, there have been advances in electrically conductive adhesive (ECA) technology that are proving to deliver the conductivity, throughput, limited stress and flexibility needed to make thinner, larger solar cell modules a high volume production reality.
These newer-generation ECAs provide significant advantages over their solder counterparts, including lower temperature processing, fast curing and unmatched flexibility, to name a few. Solder attachment processes for traditional tin-lead solders require reflow temperatures of around 220°C, while emerging lead-free soldering dictates temperatures in the 250°C to 260°C range. These higher temperature processes are not conducive to assembly of thinner, larger cells, as the CTE mismatch between the silicon and the copper tabs will likely cause cell cracking or breakage.
Newer-generation ECAs, however, resolve this issue by enabling a very low cure temperature of 150°C. In addition, the cure time of these latest ECAs has been reduced to a mere five seconds, which is consistent with that of solder processes. The greater flexibility of the adhesives, along with the lower temperatures required for curing induce much lower stresses than that of solder, as can be seen in Figs. 1 and 2 .
Figure 5. Peel strength of the novel ECA is equal to or better than solder. Solder peel strength is about 3N/mm.
Not only are solder processing temperatures a concern for modern, thinner cell assembly, but the inherent rigidness of solder connections – even low melt solders— also introduces stress onto the cell as it exists in the module. This can result in cracking or breaking either during assembly or in the field, conditions that both contribute to lower yields and higher costs. So, the flexibility afforded by ECAs is another key advantage. This flexibility allows for a better matching of CTE and also provides the ability to use thicker tabbing ribbons, which reduces shadowing and improves solar cell efficiency. Thicker, narrower ribbons can only be used if there is a connection method that can withstand the stress. ECAs deliver the flexibility required for use of thicker ribbons while solder arguably cannot.
Aside from all of these compelling advantages, though, the fact remains that ECAs must deliver equal or better performance to that of solder in order to be considered a viable alternative. Testing of a newer-generation ECA confirms such a performance, with good adhesion to both Ag and SnPbAg coated tabs, very stable contact resistance on Ag firing paste of c-Si cells in both damp heat and thermal cycling (Figs. 3 and 4) and a peel strength equal to or better than solder (Fig. 5)
In addition to the advantages delivered for assembly of c-Si solar cell modules, some of the more recently formulated ECAs can also be used for thin-film solar cell processes in situations where assembly firms require fast cure times.
Future developments: back contact panel assembly and ECAs
As solar cell technology progresses, more innovative cell and module design, assembly methods and materials are being considered to facilitate higher efficiency and lower cost. One such development is a module assembly process using back (or rear) contact solar cells. In the case of back contact solar cells, which encompass both metallization wrap through (MWT) and emitter wrap through (EWT) designs, the negative and positive poles may be contacted at the back of the cell. The interconnect of these cells is achieved either through the use of ribbon technology or by a conductive pattern built into the module back foil .
With back contact panel assembly, ECAs are also proving their advantages as the most flexible, low stress, low temperature method of module assembly. Already the ultra-fast cure times, flexibility and equality to solder performance have been confirmed. What’s more, some ECAs can be cured alongside the ethylene vinyl acetate (EVA) lamination cure profile, which further reduces process steps and, therefore, lowers cost. Future developments of ECAs for back contact modules will focus on long-term reliability and compatibility with lower-cost back contact foil metals. For today’s standard copper back contact foils, an ECA solution is already available. Compatibility with aluminum is currently in development, with a viable, commercial ECA expected to hit the market in 2011/2012.
The cost of solar energy has been reduced significantly over the last five years, but further improvements are required to reach grid parity. Current module assembly methodologies based on solder processes, while effective for today’s cell dimensions, are challenged by the move to thinner, larger solar cells. New electrically conductive adhesives that deliver fast cure times, low temperature processing and superior flexibility are providing the robust interconnect required for higher yield, higher efficiency and, therefore, lower cost assembly of modern PV modules.
For the solar industry to push toward mainstream adoption, significant changes to older manufacturing methods must occur and electrically conductive adhesives are leading the way – not just for current c-Si front to back contact designs, but for c-Si back contact structures and thin-film solar modules as well. ECAs may well be the technology that leads the future of solar cell production.
The authors would like to thank ECN for providing the images of the peel strength evaluation.
1. W. Sinke, “Stringing it Out; Innovative solar module assembly technology,” Renewable Energy World, March/April 2008.
2. A. Henckens, H. Goossens, et al., “Short-circuiting Corrosion: Overcoming problems when bonding electronic components without solder,” European Coatings Journal, May 2010.
3. I.J. Bennett, et al., “Low-stress interconnection of solar cells,” 22nd European Photovoltaic Solar Energy Conference, Sept. 2007.
4. ECN Experimental Report on Electrically Conductive Adhesives, not published (internal report).
Tom Adcock received his BS in chemical engineering from Lehigh U. and is a Marketing Manager at Henkel Corp., Assembly Electronics Group, 14000 Jamboree Rd., Irvine, CA 92606 USA; ph.: 949-789-2500; [email protected]
Anja Henckens received her PhD in polymer/organic chemistry from the U. of Hasselt, Belgium and is a Research Chemist at Henkel Corp.