Solar Cell Chapter 7: Manufacturing Silicon Solar Cells

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Solar CellChapter 7 Manufacturing Silicon Solar
Cells

• Nji Raden Poespawati
• Department of Electrical Engineering
• Faculty of Engineering
• University of Indonesia

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Contents

• 7.1. Silicon Wafers and substrates
• 7.2. Silicon Solar Cell Fabrication Technologies

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Silicon Wafers and substrates

• Types of Silicon
• Silicon or other semiconductor materials used for
  solar cells can be
• single crystalline,
• multicrystalline,
• polycrystalline or
• amorphous.

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Silicon Wafers and substrates(continued)

• Terminology for various types of crystalline
  silicon (c-Si).

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Silicon Wafers and substrates(continued)

• Single Crystalline Silicon
• The majority of silicon solar cells are
  fabricated from silicon wafers, which may be
  either
• single-crystalline (better material parameters
  but are also more expensive) or
• multicrystalline.
• Figure 1 shows the regular arrangement of silicon
  atoms in single-crystalline silicon produces a
  well-defined band structure.
• Single crystalline silicon is usually grown as a
  large cylindrical ingot producing circular or
  semi-square solar cells(see Figure 2).

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Silicon Wafers and substrates(continued)

• Czochralski Silicon
• Single crystalline substrates are typically
  differentiated by the process by which they are
  made.
• Czochralski (CZ) wafers are the most commonly
  used type of silicon wafer, and are used by both
  the solar and integrated circuit industry.
• disadvantages
• a large amount of oxygen in the silicon wafer.
• It reduces the minority carrier lifetime in the
  solar cell, thus reducing the voltage, current
  and efficiency.
• In addition, the oxygen and complexes of the
  oxygen with other elements may become active at
  higher temperatures, making the wafers sensitive
  to high temperature processing.

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Silicon Wafers and substrates(continued)

• Float Zone Silicon
• To overcome these problems, Float Zone (FZ)
  wafers may be used. (see Figure 3)

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Silicon Wafers and substrates(continued)

• Multicrystalline Silicon
• more simple, and therefore cheaper.
• the material quality of multicrystalline material
  is lower than that of single crystalline material
  due to the presence of grain boundaries
  (introduce localised regions of recombination and
  reduce solar cell performance by blocking carrier
  flows and providing shunting paths for current
  flow across the p-n junction.
• Figure 4 shows a multicrystalline silicon.

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Silicon Wafers and substrates(continued)

• Amorphous silicon (a-Si)
• silicon in which some atoms in the structure
  remain unbonded, lacks long-range order (even
  more cheaply than multicrystalline silicon)(see
  Figure 5).

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Silicon Solar Cell Fabrication Technologies

• Screen-Printed Solar Cells
• Screen-printed solar cells were first developed
  in the 1970's.
• The key advantage of screen-printing is the
  relative simplicity of the process.
• Some techniques have already been introduced into
  commercial production while others are making
  progress from the labs to the production lines
• Phosphorous Diffusion
• Surface Texturing to Reduce Reflection
• Antireflection Coatings and Fire Through Contacts
• Edge Isolation (plasma etching, laser cutting, or
  masking the border to prevent a diffusion from
  occurring around the edge in the first place.
• Rear Contact
• Substrate
• Figure 6 shows Screen-Printed Solar Cells

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Silicon Solar Cell FabricationTechnologies(continu
ed)

• Solar Cell Production
• Figure 7 Figure 15 show a commercial screen
  printed solar cell fabrication plant.

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Silicon Solar Cell FabricationTechnologies
(continued)

• Buried Contact Solar Cells
• The buried contact solar cell is a high
  efficiency commercial solar cell technology based
  on a plated metal contact inside a laser-formed
  groove.
• Compared with screen printed solar cell
• Its performance up to 25 better
• on a large area device, a screen printed solar
  cell may have shading losses as high as 10 to
  15, while in a buried contact structure, the
  shading losses will only be 2 to 3. These lower
  shading losses allow low reflection and therefore
  higher short-circuit currents.
• A schematic of a buried contact solar cell is
  shown in the figure 16.

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Silicon Solar Cell FabricationTechnologies(continu
ed)

• High Efficiency Solar Cells
• Some of the techniques and design features used
  in the laboratory fabrication of silicon solar
  cells, to produce the highest possible
  efficiencies include
• lightly phosphorus diffused emitters, to minimise
  recombination losses and avoid the existence of a
  "dead layer" at the cell surface
• closely spaced metal lines, to minimise emitter
  lateral resistive power losses
• very fine metal lines, typically less than 20 µm
  wide, to minimise shading losses
• polished or lapped surfaces to allow top metal
  grid patterning via photolithography
• small area devices and good metal conductivities,
  to minimise resistive losses in the metal grid
• low metal contact areas and heavy doping at the
  surface of the silicon beneath the metal contact
  to minimise recombination
• use of elaborate metallization schemes, such as
  titanium/palladium/silver, that give very low
  contact resistances
• good rear surface passivation, to reduce
  recombination
• use of anti-reflection coatings, which can reduce
  surface reflection from 30 to well below 10.

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Silicon Solar Cell FabricationTechnologies(continu
ed)

• Two approaches that have been used by niche
  markets such as solar cars are (see Figure 17)
• the PERL cells produced at University of New
  South Wales, and
• the rear-contact cells developed at Stanford
  University and SunPower.

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Thank You
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Figure 1. The regular arrangement of silicon
atoms in single-crystalline silicon produces a
well-defined band structure. Each silicon atom
has four electrons in the outer shell. Pairs of
electrons from neighbouring atoms are shared so
each atom shares four bonds with the neighbouring
atoms.
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Figure 2. Single crystalline silicon is usually
grown as a large cylindrical ingot producing
circular or semi-square solar cells. The
semi-square cell started out circular but has had
the edges cut off so that a number of cells can
be more efficiently packed into a rectangular
module.
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Figure 3. Schematic of Float Zone wafer growth.
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Figure 4. (a) At the boundary between two crystal
grains, the bonds are strained, degrading the
electronic properties (b) A multicrystalline
wafer.
(a)
(b)
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Figure 5. Amorphous silicon has short-range order
to give it semiconductor properties. Extra bonds
are terminated on hydrogen atoms. The change in
average atomic spacing and presence of hydrogen
gives amorphous silicon different electronic
properties to crystalline silicon.
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Figure 6. (a) Close up of a screen used for
printing the front contact of a solar cell.
During printing, metal paste is forced through
the wire mesh in unmasked areas. The size of the
wire mesh determines the minimum width of the
fingers. Finger widths are typically 100 to 200
µm(b) Close up of a finished screen-printed solar
cell. The fingers have a spacing of approximately
3 mm. An extra metal contact strip is soldered to
the busbar during encapsulation to lower the cell
series resistance(c) Front view of a completed
screen-printed solar cell. As the cell is
manufactured from a multicrystalline substrate,
the different grain orientations can be clearly
seen. The square shape of a multicrystalline
substrate simplifies the packing of cells into a
module(d) Rear view of a finished screen-printed
solar cell. The cell can either have a grid from
a single print of Al/Ag paste with no BSF, or a
coverage of aluminium that gives a BSF but
requires a second print for solderable contacts.
(b)
(a)
(d)
(c)
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Figure 7. Crystallisation furnace for the
manufacture of multicrystalline ingots. Large
silicon slabs of approximately 0.5m by 0.5m and
20cm thick are routinely produced. By carefully
controlling the cooling of the liquid, silicon
material is produced with large grains and few
crystal defects (photograph courtesy of
Eurosolare S.p.A.).
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Figure 8. The large ingot produced by the
crystallisation furnace is sawn into smaller 10cm
by 10cm blocks. The smaller blocks are then
sliced to produce 10 cm by 10 cm wafers
(photograph courtesy of Eurosolare S.p.A.).
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Figure 9. View of the production line at
Eurosolare. While solar cell manufacture requires
clean conditions, the requirements are nowhere
near as strict as those for integrated circuit
(IC) manufacture. Hence it is not necessary for
staff to wear full cleanroom suits (photograph
courtesy of Eurosolare S.p.A.).
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Figure 10. Automatic loading of the diffusion
furnace with wafers already coated with
phosphorous. Note that the wafers just about to
enter the diffusion furnace on the right were cut
from the same ingot and have a similar
distribution of crystal grains (photograph
courtesy of Eurosolare S.p.A.).
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Figure 11. Automated unloading of the diffusion
furnace. The use of robotic equipment has
improved the reliability of cell manufacture and
reduced costs (photograph courtesy of Eurosolare
S.p.A.).
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Figure 12. Production line screen-printer.
(photograph courtesy of Eurosolare S.p.A.).
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Figure 13. Advanced screen-printing machine that
uses video cameras to quickly and accurately
align the metal contact print pattern (photograph
courtesy of Eurosolare S.p.A.).
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Figure 14. After measuring the efficiency of each
finished cell, they are sorted to minimise
mismatch on module interconnection. (photograph
courtesy of Eurosolare S.p.A.).
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Figure 15. Array structure before lamination
viewed from the rear (photograph courtesy of
Eurosolare S.p.A.).
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Figure 16. Cross-section of Laser Grooved, Buried
Contact Solar Cell.
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Figure 17. Schematic of high efficiency
laboratory cell.