863/articles/Photovoltaics-World/volume-20100/issue-5/features/reducing-cost
-improving-efficiency-in-solar-modules.html
Reducing cost/improving efficiency in solar modules Tom Adcock, Anja
Henckens, Henkel Corporation, Irvine, CA USACurrent 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 [4].Reigning
crystalline silicon (c-Si) cell technology, which currently accounts for
approximately 80% of the photovoltaic (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 [4].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
[1].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 assemblyWhile 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 [1]. These issues have therefore forced module
assemblers to seek alternative interconnect methods for modern, thinner c-Si
cells.Electrically conductive adhesives provide possible solutionCurrent
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 [4]. 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 ECAsAs 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 [1].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.ConclusionThe
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.AcknowledgementsThe authors would like to thank ECN for providing
the images of the peel strength evaluation.References1. 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
tom.adcock@us.henkel.comAnja Henckens received her PhD in polymer/organic
chemistry from the U. of Hasselt, Belgium and is a Research Chemist at
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