NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 22 of 144
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POCI3 liquid source dopant, which generates toxic P205
and C12 effluents and requires frequent cleaning of
diffusion tubes using HF solutions. Belt furnaces are more
environmentally benign because they can use water-
soluble, non-toxic, spin-on or spray-on dopants or vapor
dopants and do not require HF cleaning. Optically
enhanced doping methods, such as the solar furnace doping
proposed by NREL , are also environmentally benign
Edge trimming to remove electrical shorts between
the front and back junction can be done either by laser
cutting or plasma etching. Because of its very high
throughput, CF4 + 02 plasma etching is commonly used
by the PV industry. However, CF4 is one of the
perfluorinated compounds (PFCs) that has no known
natural destruction mechanisms, and thus, has a large
global-warming potential . The PV industry needs to
either find an alternative to the CF4 and 02 plasma etch or
improve the effluent treatment to include PFC capture and
Antireflection coatings can be deposited by vacuum
evaporation, plasma deposition, atmospheric-pressure
chemical vapor deposition, and spin-on liquids. Silane,
which is pyrophoric, is commonly used in depositing
silicon nitride antireflection coatings . A safer
alternative, chlorosilane, which is non-pyrophoric, has
been used successfully by the IC industry for silicon nitride
For metal electrodes on the solar cells, it is not
desirable to use silver-tin-lead solder baths after screen
printing to enhance the conductivity of the metal grids
because of the added lead content to the cells.
7. MODULE ASSEMBLY
7.1 No-Clean Flux
Flux, typically a derivative of pine resin, is applied to
cell interconnection strips before soldering to act as a
deoxidizer and to ensure better adhesion between the
solder and solar cells. Conventional flux leaves residues on
the cell surfaces that need to be cleaned with a
chlorofluorocarbon (CFC) compound. CFCs are known to
cause ozone depletion in the atmosphere . Recently,
water-soluble fluxes and no-clean fluxes, low-residue
fluxes that could be left on the solar cell after soldering
have become widely available . In an NREL-funded
program, Siemens Solar was able to eliminate the CFC
usage in the manufacturing facility by switching from
conventional solder paste to a "no-clean" solder paste .
The no-clean process both eliminates the environmental
damaging CFC emissions and reduces costs. Siemens Solar
also found that, by using a water-soluble flux, the CFC
usage can be reduced by about 60% over a conventional
flux. However, it appeared that water rinse of the cells
retained moisture during the lamination sequence and
caused module reliability problems.
7.2 Lead-Free Solder
Lead is a well-known hazard to human health. When
disposed of in landfills, it can leach into soils and pollute
ground water. It is important for the PV industry to remove
or minimize the use of lead in modules so that proper
disposal at the end of module life will not become a
problem . For example, some European countries have
proposed a ban on the landfill disposal of electronic
products containing lead. There are two sources of lead in
a crystalline-silicon PV module: solder-dipped electrodes
and solder-coated copper ribbons. The practice of dip-
coating solar-cell contact electrodes is no longer necessary
with modern screen-printed electrodes, but it is still used
by a significant number of module manufacturers. The
lead-tin solder that coats the surfaces of copper ribbons for
tabbing strips is needed to prevent the oxidation of the
copper and to improve the solderability of the ribbons.
However, lead-free alternatives to lead-tin solder have
been investigated extensively by the printed-circuit-board
industry . For example, the National Center for
Manufacturing Sciences in Ann Arbor, Michigan, has
recently completed a US$10-million project that evaluated
79 lead-free solder alloys and found seven promising
replacement candidates . The International Tin
Research Institute (ITRI) in Middlesex, England, has also
done extensive studies on lead-free solder alloys .
A very promising alternative for the tin-36% lead-2%
silver ribbon coatings commonly used by the PV industry
is the tin-3.5% silver alloy. It is identified as a promising
alternative for the standard tin-37% lead alloy by both
NCMS and ITRI. The silver in the alloy is needed to
increase the pull strength of the ribbon. The 2210C melting
temperature, although higher than the 183*C for the
standard lead-tin alloy, is acceptable. Because it is a binary
alloy, it should have excellent stability. Its resistance to
high-temperature fatigue is also good. The present cost of
the alloy, at US$0.10 per cubic centimeter, is about twice
the standard lead-tin alloy, which is the cheapest of the tin
alloys. The cost difference between the silver-tin alloy and
the lead-tin-silver alloy is small.
As the silicon PV industry continues to rapidly
expand, the environmental impact of its manufacturing
processes and products will receive increasing attention. It
is particularly important for a renewable energy technology
to address its environmental impact during manufacturing
because one of the primary benefits of renewable energy
generation is its low environmental impact. We have
discussed several alternative approaches in this paper that
are both cost effective and environmentally benign.
However, the manufacturability and reliability of most of
these alternative approaches need further investigation. We
propose that the silicon PV industry form an association of
government laboratories, equipment suppliers, and cell and
module manufacturers to promote more environmentally
benign manufacturing approaches. This association can
also coordinate the PV industry's interactions with the
environmental associations of the integrated-circuits and
printed-circuit-board industries mentioned in this paper.
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc707815/m1/22/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.