SOLARMATERIALS
Safe gas handling
Solar cell manufacturing is becoming important as an environmental
means of generating electricity and the growth of thin films of silicon
will form a key part of this. Dr. Michael Czerniak from Edwards
explains the exhaust gas management of the process.
Issue I 2009
T
he PECVD growth of microcrystalline New Challenges
silicon poses a number of significant To produce these twin cell structures, a
challenges regarding the treatment of the resultant conventional amorphous silicon layer is combined
exhaust gases, in terms of removing toxic, with a microcrystalline (mc-Si) layer; both
corrosive, global warming, and flammable gases deposited using extremely large capacity plasma
safely, at low cost, whilst also minimising the enhanced chemical vapour deposition (PECVD)
-pv-management.com environmental impact. Fortunately the means to equipment. Although the chemistry resembles that
achieve these goals now exist. used to make semiconductor chips, the gas flows
.solar
are orders of magnitude greater, as are the
www
Background process step times.
Although solar cells account for a tiny fraction of One aspect both processes have in common is the
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global power generation (<1%), they are use of high flows of silane (SiH4) which is activated
experiencing a phenomenal growth rate (28%) [1] by the plasma in the process to deposit silicon
as the price of solar generated electricity according to:
approaches parity with that of other sources; this is
predicted to occur sometime during the next
decade (2010-2020) [2].
There are a number of competing technologies for Not all the silane will react in the process chamber,
the manufacture of solar cells, the most widely in fact process chambers are designed so that this
used being silicon wafer based crystalline cells, does not happen in order to maintain acceptable
yielding up to around 22% efficiencies [2]. Of levels of film thickness uniformity (technically so
growing interest, however, is the competing that the gas stream does not become depleted of
technology of thin film silicon solar cells, where reagents). However, this means that there will be
microscopically thin layers of silicon are deposited significant flows of unreacted silane in the process
on large area substrates to produce low cost solar exhaust. Not only is silane toxic (TLV = 5ppm), it is
cells, a process similar to that employed to make also pyrophoric (combusts spontaneously with air)
the transistors in liquid crystal displays (LCDs). The [3] and, when incinerated, generates significant
Table 1: Abatement efficiency of simple devices is significantly inferior quantities of solid silica (SiO2) powder, around 3
challenges posed by to that of crystalline cells (~6-8%), but by grams per litre of SiH4. Based on the current silane
mc-Si exhaust gases combining 2 layers of differing material properties, gas flows used in TF silicon solar cell manufacture,
Cells highlighted in the resulting structure is, in effect, 2 cells in one this can result in tens of tons of SiO2 solids per
gray relate to the with efficiencies approaching 10% [2], yet can be chamber per year. TLV is the threshold limit value
deposition step produced in large area format at low cost. of a gas, expressed as parts per million.
Where these solar cell structures differ is that, in
order to grow mc-Si, where a much more ordered
crystalline structure is required, very high flows of
hydrogen gas must be introduced into the process
mixture, and H2 is therefore present in the process
exhaust in high quantities (100s of SLM). Hydrogen
has a reputation for its flammability, and the
exceptionally high flame speeds that are attainable
mean that it readily detonates under certain
conditions [4]. Even when combusted under
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