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Solar Cell
Chapter 7: Manufacturing Silicon Solar Cells
Nji Raden Poespawati
Department of Electrical Engineering
Faculty of Engineering
University of Indonesia
Contents
7.1. Silicon Wafers and substrates
7.2. Silicon Solar Cell Fabrication Technologies
Silicon Wafers and substrates
Types of Silicon
Silicon or other semiconductor materials
used for solar cells can be:
1.
2.
3.
4.
single crystalline,
multicrystalline,
polycrystalline or
amorphous.
Silicon Wafers and substrates
Terminology for various
crystalline silicon (c-Si).
(continued)
types
of
Descriptor
Symbol Grain Size
Common Growth Techniques
Single crystal
sc-Si
>10cm
Czochralski (CZ) float zone (FZ)
Multicrystalline
mc-Si
1mm-10cm Cast, sheet, ribbon
Polycrystalline
pc-Si
1mm-1mm
Chemical-vapour deposition
Microcrystalline
mc-Si
<1mm
Plasma deposition
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 welldefined band structure.
Single crystalline silicon is usually grown as a large
cylindrical ingot producing circular or semi-square
solar cells(see Figure 2).
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.
Silicon Wafers and substrates
(continued)
Float Zone Silicon
To overcome these problems, Float
Zone (FZ) wafers may be used. (see
Figure 3)
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.
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).
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
Silicon Solar Cell FabricationTechnologies(continued)
Solar Cell Production
Figure 7 – Figure 15 show a commercial screen
printed solar cell fabrication plant.
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 :
1.
2.
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.
Silicon Solar Cell FabricationTechnologies(continued)
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:
1.
2.
3.
4.
5.
6.
7.
8.
9.
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%.
Silicon Solar Cell FabricationTechnologies(continued)
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.
Thank You
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.
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.
Figure 3. Schematic of Float Zone wafer growth.
(a)
(b)
Figure 4. (a) At the boundary between two crystal grains,
the bonds are strained, degrading the electronic properties
(b) A multicrystalline wafer.
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.
(a)
(c)
(b)
(d)
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.
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.).
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.).
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.).
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.).
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.).
Figure 12. Production line screen-printer. (photograph courtesy of
Eurosolare S.p.A.).
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.).
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.).
Figure 15. Array structure before lamination viewed from the
rear (photograph courtesy of Eurosolare S.p.A.).
Figure 16. Cross-section of Laser Grooved, Buried Contact
Solar Cell.
Figure 17. Schematic of high efficiency laboratory cell.