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more environmentally sound processes may be acceptable
for PV manufacturing even though they don't meet the
stricter requirements of IC manufacturing.
It is not possible to review here in detail all the steps
involved in the manufacturing of silicon PV modules. We
will simply highlight some areas where we think
opportunities exist to make the module manufacturing
process more environmentally benign.
2. INDUSTRY INITIATIVES AND RESOURCES
Three recent initiatives by the semiconductor industry
in the environmentally benign manufacturing area could
produce results that also benefit the PV industry: (1) The
National Science Foundation and the Semiconductor
Research Corporation jointly established the NSF-SRC
Engineering Research Center for Environmentally Benign
Semiconductor Manufacturing on April 15, 1996 . SRC
is a consortium of 65 corporations and government
agencies that plans, directs, and funds the semiconductor
industry's pre-competitive, long-term research . (2) In
April 1997, the international trade association for the
semiconductor industry, Semiconductor Equipment and
Materials International (SEMI), created a new
Environmental Health and Safety Division that will explore
worldwide environmental priorities for the industry .
(3) In October 1997, the Electric Power Research Institute
(EPRI) and SEMATECH Corp. formed a new center, the
EPRI Center for Electronics Manufacturing, to address
productivity, environmental, and energy issues in the
electronics industry . SEMATECH is a non-profit R&D
consortium of U.S. semiconductor manufacturers.
The NSF-SRC Center carries out research in six areas
of semiconductor manufacturing: water conservation,
plasma processes, wet chemicals, chemical-mechanical
polishing,. emission of organics, and risk-assessment
studies. Some of these research results are presented in a
weekly teleconference seminar series hosted by the four
participating universities: University of Arizona,
Massachusetts Institute of Technology, Stanford
University, and University of California-Berkeley . A
good source of information on the Environmental, Safety,
and Health (ES&H) goals of the semiconductor industry is
the ES&H Section of the United States National Roadmap
for Semiconductors . In addition to these semiconductor
industry organizations, the United States National
Photovoltaic Environmental, Health and Safety
Information Center  regularly publishes information on
PV ES&H-related issues .
3. POLYSILICON PRODUCTION
For the feedstock material used in crystal growth, the
silicon PV industry has been relying on rejected materials
from the IC industry. These rejected materials, about 2,100
metric tons in 1997, amount to about 10% of the
semiconductor-grade polysilicon used by the IC industry.
This arrangement worked well until 1995 when a shortage
of polysilicon feedstock began to drive up the cost and
limit the growth of the silicon PV industry. If the PV
industry continues to grow at the present rate, which in
recent years has been higher than the growth rate of the IC
industry, and if crystalline silicon continues to be the
dominant technology of the PV industry, then we must
develop new sources of solar-grade polysilicon. There are
two possibilities: (1) build new factories dedicated to the
production of low-cost (< US$10/kg), solar-grade
polysilicon, and (2) find new ways to use the rejected
silicon that is not currently being used, for example,
purifying the about 30% of silicon lost from wafer-cutting
operations (kerf loss) of semiconductor-grade polysilicon
into solar-grade polysilicon. The purity requirements for
solar-grade polysilicon, according to the Solar-Grade
Silicon Stakeholders Group, are the following: it is
preferred that polysilicon have either B or P doping, with
no compensation; resistivity at 25*C should be greater than
1 ohm-cm; oxygen and carbon should not exceed the
saturation limits in the melt; and the total non-dopant
impurity concentration should be less than 1 ppma .
More than 98% of semiconductor-grade polysilicon is
produced by the trichlorosilane (SiHCl3) distillation and
reduction method [11,12]. The trichlorosilane is
manufactured by fluidizing a bed of fine pulverized
metallurgical-grade silicon (MG-Si), which is more than
98% silicon, with hydrogen chloride in the presence of a
copper-containing catalyst. The MG-Si, which costs about
US$1/kg, is produced by the reduction of natural quartzite
(silica) with coke (carbon) in an electric arc furnace. This
method of polysilicon production is very energy intensive
, and it produces large amounts of wastes, including a
mix of environmentally damaging chlorinated compounds.
About 80% of the initial metallurgical-grade silicon
material is wasted during the process. In addition, the
semiconductor-grade polysilicon material produced by this
method far exceeds the purity requirement of the PV
industry, and the cost (over US$50/kg, with most of it
attributable to the SIHC3 processes) is several times
higher than what the PV industry can afford . Every
watt of crystalline silicon PV module generating capacity
requires roughly 20 g of polysilicon. Thus, if the cost of
solar-grade polysilicon is US$20/kg, the cost of polysilicon
per watt of a crystalline-silicon PV module is US$0.40. It
is obvious that less complicated, less energy intensive,
more efficient, and more environmentally benign methods
need to be developed to meet the cost and quality
requirements of the PV industry. New methods of
producing solar-grade polysilicon should either be chlorine
free or recycle chlorine internally to reduce cost and avoid
damage to the environment.
3.1 Low-Temperature, Chlorine-free Processes for
Polysilicon Feedstock Production
The National Renewable Energy Laboratory (NREL)
and Sandia National Laboratories (SNL), with funding
from the Initiative for Proliferation Prevention (IPP)
Program, has initiated a joint research program with the
Intersolarcenter to study new chlorine-free methods of
producing solar-grade polysilicon. So far, the most
promising method developed by this project is one that
uses MG-Si and absolute alcohol as the starting materials.
This new process requires only 15 to 30 kWh of energy per
kg of polysilicon produced vs. about 250 kWh/kg of the
trichlorosilane method. The silicon yield (polysilicon and
the main by-product, silica sol) is in the 80% to 95% range
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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/m1/19/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.