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Solar
Cell Manufacturing
Solar
energy is rapidly becoming a viable alternative to fossil fuels. While solar
sources currently generate only a small fraction of the world’s electricity,
that fraction is projected to grow to 10% or more by 2040. Solar cell
manufacturing typically involves a number of steps that are performed under
vacuum to maintain cleanliness or to create appropriate processing conditions.
There
are three main types of solar cells in production: silicon wafer based (also
known as crystalline Si or c-Si), compound crystalline (concentrator PV), and
thin-film solar (TFS). Crystalline silicon solar cells are by far the
predominant technology.
Crystalline silicon solar cell manufacturing
requires vacuum-based processing tools for many critical steps, from mono- to
polycrystalline silicon growth to thin-film depositions on silicon wafers.
Manufacturing efficiencies can be optimized by considering cost-of-ownership (CoO)
parameters for vacuum subsystems based on dry pumps. Careful attention to vacuum
systems can make a significant contribution to reduced costs.
In
the first steps of the c-Si manufacturing process, high purity silicon ingots
are grown either by the Czochralski method of crystal growing (monocrystalline
Si ingots) or the die-casting/furnace method (polycrystalline Si ingots).
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A
solar cell made from a
monocrystalline silicon wafer
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Mono-Crystalline Silicon
Czochralski Process typical
process steps
1.
Initial evacuation into the 10-2 -
10-3 mbar range (removal of air)
2.
Backfilling with Argon to
approximately 250 mbar
3.
2nd evacuation into the 10-2
- 10-3 mbar range (removal of air traces)
4.
Pressure-rise leak test
5.
Pulling process: pressure regulated
with Argon flow at approximately 10 - 40 mbar
6.
Cooling of hot zone
7.
Venting to atmospheric pressure
Complete process time: approximately
3 - 6 days |
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Dry pumps are suggested during this process
Solar cells fabricated on
monocrystalline wafers are more efficient than polycrystalline-based
wafers
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The
silicon manufacturing process requires vacuum pumps that work reliably in
dusty environments because a crystal grower can generate several kilograms
of silicon monoxide (SiO) particles per week. In a wet pump, the particles
mix with the lubricating oil and erode internal components. Regular
replacement of the oil and filters can mitigate these effects, but even
with diligent maintenance, wet pumps require frequent rebuilding. If a wet
pump seizes during a crystal growth cycle, oil will back-stream into the
crystal grower, resulting in costly tool downtime. There is a growing
trend among silicon manufacturers to use dry pumps on crystal pullers and
growth furnaces since they offer a significantly lower total CoO. They are
inherently clean and eliminate the risk of contaminating the crystal
grower with pump oil.
Solar
Two's thermal storage system generated electricity during cloudy weather
and at night
Dry etching requires vacuum pumps that can process extremely
corrosive gases while maintaining a high gas throughput and low pressure.
Magnetically levitated turbomolecular pumps (mag-lev TMP) that are
specifically designed for harsh environments are ideal for this
application. These mag-lev TMP typically use coated components that are
compatible with the corrosive gases. They are virtually maintenance free
because they are without mechanical bearings that require lubrication and
periodic replacement.
A layer of silicon nitride is
deposited on the front surface of the cell to reduce reflection (ARC) and
passivate the surface. Typically the layer is deposited using
plasma-enhanced chemical vapor deposition (PECVD) or physical vapor
deposition (PVD). PECVD gases and byproducts-such as SiH4, NH3,
NF3, F2, H2, and HF-are pyrophoric,
flammable, toxic, and a considerable safety risk. Large amounts of
particulates are generated as well, creating a harsh environment for
vacuum pumps. Wet pumps are generally not suitable for this application,
as the high powder loads cause rapid abrasion of the pump’s internal
components and the corrosive materials rapidly degrade the sealing oil’s
lubricating properties.
Switching
from diffusion pumps to high throughput turbomolecular pumps on PVD
chambers reduces the size of the backing pumps and the required number of
high vacuum pumps.
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Helios
UAV in solar powered flight
The next step is to fabricate the
solar cells on the wafer. The first step in this process is to texture the
wafer surface, which increases the active surface area. Next, is the
doping and diffusion process, which creates the p-n junction by forming an
n-doped (electron rich) layer on top of the p-doped wafer. A layer of
phosphorous silicate glass (PSG)
forms on top of the n-doped layer, and this is
removed by either wet- or dry-etch processes.
Magnetically
levitated turbomolecular pump shown cross-sectioned.
There is no contact between the rotor
and the stator. This effectively eliminates frictional wear and vibration,
resulting in maintenance intervals as long as five years. An additional
advantage of mag-lev is the elimination of lubricants that can be
vulnerable to oxidation in corrosive environments, which can lead to
premature failu
re.
PVD processes must be able to maintain
high vacuum conditions (<5mTorr) while accommodating the flow of
process gases. Solar PVD applications typically use either diffusion pumps
or turbomolecular pumps. Both diffusion and turbomolecular pumps need
backing pumps to provide the sub-atmospheric foreline conditions they
require to operate.
After the back reflective coating and
contact layer have been deposited, the solar cell is complete and the
wafer moves to the module production line. During this process, the wafers
are electrically connected (tabbing and stringing) and then encapsulated
to provide protection against the environment. The encapsulating material,
typically ethylene vinyl acetate (EVA), is applied at high temperature
under a vacuum. As with so many other processes, material outgassed by the
EVA degrades the lubricants used in wet pumps. The lower maintenance costs
and higher availability of dry pumps provide significant benefits in
overall cost.
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Conclusion
Vacuum pumps
are critical infrastructure components of crystalline silicon solar cell
manufacturing operations, enabling essential processes from wafer formation to
final lamination. Selecting the appropriate vacuum technology for each
step is complex, but the benefits include lower up-front costs, reduced energy
consumption, decreased maintenance requirements and increased personnel safety.
For
further information about the applications of vacuum in solar module
manufacturing, please contact Osanak in our applications support department; o.mir@vpcinc.ca
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