NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 20 of 144
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vs. 6% to 20% for the trichlorosilane method. The eventual
cost goal is US$10 per kg of solar-grade polysilicon.
The basic processing stages of this chlorine-free
polysilicon production process are the following:
1. The reaction of metallurgical-grade silicon with
alcohol proceeds at 2800C in the presence of a
Si + 3 C2H50H catalyst Si(0C2H5)3H + H2 (1)
2. The disproportion (i.e., simultaneous oxidation and
reduction) of triethoxysilane in the presence of a
catalyst will lead to the production of silane and
4 Si(OC2H5)3H catalyst S44 + 3 Si(0C2H5)4 .(2)
3. Dry ethanol and such secondary products as high-
purity SiO2 or silica sol can be extracted by
hydrolysis of tetraethoxysilane. The alcohol will be
returned to Stage 1.
Si(OC2H5)4 + 2 H/O - SiO2 + 4 C2H50H. (3)
4. Silane is decomposed pyrolytically to pure silicon and
hydrogen at a temperature of about 9000C:
SiH4 850*- 900'C Si + 2 H2. (4)
The purity requirements for solar-grade silicon are not as
high as those for electronic applications. Thus, the silane
will undergo a simplified cycle of purification, and at
Stage 4 the less expensive and less energy-consuming
process of a fluidized bed reactor can be used, instead of
the well-known Siemens Process .
3.2 Purification of Metallurgical Silicon
NREL and ENEA (National Agency for New
Technologies Energy & Environment) have proposed a
novel method of producing solar-grade polysilicon by
directly purifying MG-Si pellets. The process uses the very
large surface areas, produced by porous silicon etch on the
surfaces of the silicon wafer, as sites for gettering
impurities in the subsequent high-temperature annealing.
The details of this process will be presented seperately at
this conference .
3.3 New Sources of Silicon Waste from the Electronic
When wafers are sliced from silicon ingots using a
multiple-wire saw, a layer of silicon about 250
micrometers thick is lost per wafer. This kerf loss is higher
for inner-diameter (ID) saws. Depending on the wafer
thickness, this kerf loss represents from 25% to 50% of the
ingot material, several times the quantity of the material
that is presently used by the PV industry. Presently, the
solar industry uses mainly Cz ingot top and tails, pot scrap,
and rejected wafers from the IC industry . If a method
can be developed to produce solar-grade polysilicon by
purifying the kerf remains of semiconductor-grade ingots,
enough polysilicon would be generated for over 300
MW/year of crystalline-silicon solar cells, i.e., more than
two times the size of the current silicon solar-cell
4. CRYSTAL GROWTH
There are four types of crystalline-silicon solar cells:
single-crystal, polycrystalline, ribbon, and silicon film
deposited on low-cost substrates. In 1997, market share of
the worldwide PV cell and module shipment for the four
types of crystalline-silicon solar cells were 49.6% for
single-crystal, 34.0% for polycrystalline, 3.2% for ribbon,
and 0.4% for silicon film . Crystal growth from a silicon
melt generates relatively little waste. The main concern is
the energy required and the amount of argon gas used
during crystal growth. Electricity and argon needed for Cz
growth are the highest among the four types of silicon
materials . Recently, however, the world's largest
manufacturer of Cz silicon solar cells, Siemens Solar,
Industries, announced a joint project with the Northwest
Energy Efficiency Alliance to cut the amount of electricity
used to grow crystals and yield savings of 40% to 50%
5. WAFER SLICING AND CLEANING
5.1 Wafer Slicing
In the last six years or so, the PV industry has made
the transition from using ID saws for wafer slicing to using
multiple-wire saws. Multiple-wire saws can improve wafer
yield per unit length of ingots by over 50% because of
lower kerf loss and thinner wafers. However, wafer slicing
is still one of the most expensive processes in silicon solar-
cell manufacturing because of the large quantities of
consumables (stainless-steel wire and abrasive slurry) and
the kerf loss. During wafer slicing, ingots are bonded to a
ceramic submount with hot-melt adhesive and sliced into
wafers using multiple steel wires to which an abrasive
slurry is fed. The slurry is composed of silicon carbide
(SiC) and mineral-oil-based or glycol-based slurry vehicle.
Oil-based slurry is commonly used by the PV industry.
Compared to the water-soluble, glycol-based slurries more
commonly used by the IC industry, oil-based slurries
produce more environmentally damaging wastes and
require more extensive wafer cleaning. The added cost and
the process changes needed for the PV industry to switch
over to glycol-based slurry need to be investigated.
Methods of proper disposal or recycling of the stainless-
steel cutting wire also need be studied, as does the effective
recovery of the SiC in the slurry. The development of
water-base slurries will also help reduce cost and
5.2 Wafer Cleaning and Etching
The cost of chemical waste disposal is high. It is
important for the PV industry to find ways to reduce
chemical consumption and waste generation through
source reduction, recovery, recycle, reuse, and
substitution. Because wafer cleanliness for PV is not as
critical as for IC manufacturing, a safe choice, in terms of
making sure the highest quality and most extensive
cleaning procedures are used, is not necessarily the right
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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/m1/20/ocr/: accessed May 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.