Solar Cell
Chapter 6: Design of Silicon Solar Cells
Nji Raden Poespawati
Department of Electrical Engineering
Faculty of Engineering
University of Indonesia
Contents
6.1. Optical Properties
6.2. Reducing Recombination
6.3. Top Contact Design
6.4. Solar Cell Structure
Optical Properties
Basic Solar Cell Design
Solar cell design involves specifying the
parameters of a solar cell structure in order to
maximize efficiency, given a certain set of
constraints.
Fig. 1 shows Evolution of silicon solar cell
efficiency.
Optical Properties
(continued)
In designing such single junction solar cells,
the principles for maximizing cell efficiency
are:
1. increasing the amount of light collected by
the cell that is turned into carriers;
2. increasing the collection of light-generated
carriers by the p-n junction;
3. minimizing the forward bias dark current;
4. extracting the current from the cell without
resistive losses.
Optical Properties
(continued)
Optical Losses
Optical losses chiefly effect the power from a solar cell by
lowering the short-circuit current.
Sources of optical loss in a solar cell is illustrated in Figure 2.
There are a number of ways to reduce the optical losses:
 Top contact coverage of the cell surface can be minimized
(although this may result in increased series resistance);
 Anti-reflection coatings can be used on the top surface of
the cell.
 Reflection can be reduced by surface texturing.
 The solar cell can be made thicker to increase absorption
 The optical path length in the solar cell may be increased by
a combination of surface texturing and light trapping.
Optical Properties
(continued)
Anti-Reflection Coatings
Anti-reflection coatings on solar cells are similar to those
used on other optical equipment such as camera lenses.
The minimum reflection is calculated by:
……………………………(6.1)
Where :
n1 : a refractive index of transparent material (ARC)
l0 : a free-space wavelength and
d1 : the thickness
n0 : a refractive index of glass or air
n2 : a refractive index of semiconductor
Optical Properties
(continued)
Figure 3 illustrates use of a quarter wavelength
anti-reflection coating to counter surface
reflection.
For photovoltaic applications, the refractive
index, and thickness are chosen in order to
minimize reflection for a wavelength of 0.6mm.
Comparison of surface reflection from a silicon
solar cell, with and without a typical antireflection coating is depicted in Figure 4
Optical Properties
(continued)
Surface Texturing
Surface texturing, either in combination with an anti-reflection
coating or by itself, can also be used to minimize reflection.
Surface texturing can be accomplished in a number of ways:
1.
A single crystalline substrate can be textured by etching
along the faces of the crystal planes. (random pyramid)
2.
the pyramids are etched down into the silicon surface rather
than etched pointing upwards from the surface (inverted
pyramid)
3.
using a photolithographic technique as well as mechanically
sculpting the front surface using dicing saws or lasers to cut
the surface into an appropriate shape (multicrystalline
wafers).
Figure 5 is shown the surface texturing which are used those
methods
Optical Properties
(continued)
Material Thickness
The amount of light absorbed depends on the optical path
length and the absorption coefficient.
For silicon material in excess of 10 mm thick, essentially all the
light with energy above the band gap is absorbed. The 100%
of the total current refers to the fact that at 10 mm, all the
light which can be absorbed in silicon, is absorbed.
In material of 10 microns thick, only 30% of the total available
current is absorbed. The photons which are lost are the orange
and red photons.
Optical Properties
(continued)
Light Trapping
a solar cell with no light trapping features may have an optical
path length of one device thickness, while a solar cell with good
light trapping may have an optical path length of 50, indicating
that light bounces back and forth within the cell many times.
Light trapping is usually achieved by changing the angle at which
light travels in the solar cell by having it be incident on an angled
surface.
the angle at which light enters the solar cell (the angle of refracted light)
can be calculated:
………………………………(6.2)
Optical Properties
(continued)
In a textured single crystalline solar cell, the presence of
crystallographic planes make the angle q1 equal to 36°
as shown in Figure 6.
Lambertian Rear Reflectors
A Lambertian back reflector is a special type of rear
reflector which randomizes the direction of the reflected
light.
A Lambertian rear surface is illustrated in the figure 7.
Reducing Recombination
Recombination Losses
Recombination losses effect :
1.the current collection (the short-circuit current)
2.the forward bias injection current (open-circuit voltage).
The main areas of recombination are :
1.at the surface (surface recombination)
2.the bulk of the solar cell (bulk recombination)
The depletion region is another area in which recombination
can occur (depletion region recombination).
Reducing Recombination
(continued)
Current Losses Due to Recombination
In order for the p-n junction to be able to collect all of the
light-generated
carriers, both surface and
bulk
recombination must be minimized.
In silicon solar cells, the two conditions commonly required
for such current collection are:
1.
the carrier must be generated within a diffusion length of
the junction, so that it will be able to diffuse to the
junction before recombining; and
2.
in the case of a localized high recombination site, the
carrier must be generated closer to the junction than to
the recombination site. For less severe localized
recombination sites, carriers can be generated closer to
the recombination site while still being able to diffuse to
the junction and be collected without recombining.
Reducing Recombination
(continued)
The quantum efficiency of a solar cell quantifies the effect of
recombination on the light generation current. The quantum
efficiency of a silicon solar cell is shown in Figure 8.
Figure 9 is illustrated Quantum efficiency curves for three
different types of crystalline silicon solar cells.
Voltage Losses Due to Recombination
The open-circuit voltage is the voltage at which the forward
bias diffusion current is exactly equal to the short circuit
current.
The forward bias diffusion current is dependent on the
amount recombination in a p-n junction and increasing the
recombination increases the forward bias current.
Reducing Recombination
(continued)
high recombination   the forward bias diffusion current , which in
turn reduces the open-circuit voltage.
The recombination is controlled by the number of minority carriers at
the junction edge, how fast they move away from the junction and
how quickly they recombine.
Consequently, the dark forward bias current, an hence the open-circuit
voltage is affected by the following parameters:
1.
the number of minority carriers at the junction edge. Minimizing
the
equilibrium
minority
carrier
concentration
reduces
recombination. Minimizing the equilibrium carrier concentration is
achieved by increasing the doping;
2.
the diffusion length in the material. The diffusion length depends
on the types of material. High doping reduces the diffusion length;
3.
the presence of localized recombination sources within a diffusion
length of the junction. A high recombination source close the the
junction will allow carriers to move to this recombination source
very quickly and recombine, thus dramatically increasing the
recombination current. The impact of surface recombination is
reduced by passivating the surfaces.
Reducing Recombination
(continued)
Effect of doping (ND) on diffusion length and open-circuit voltage
assuming well passivated surfaces is shown in Figure 10.
Surface Recombination
Surface recombination can have a major impact both on the short-circuit
current and on the open-circuit voltage.
Lowering the high top surface recombination is typically accomplished by
reducing the number of dangling silicon bonds at the top surface by
growing a "passivating" layer (usually silicon dioxide) on the top surface.
Techniques for reducing the impact of surface recombination is depicted
in Figure 11
Top Contact Design
Series Resistance
In addition to maximizing absorption and minimizing
recombination, is to minimize parasitic resistive losses.
Both shunt and series resistance losses decrease the fill factor
and efficiency of a solar cell.
A detrimentally low shunt resistance is a processing defect
rather than a design parameter. However, the series resistance,
controlled by the top contact design and emitter resistance,
needs to be carefully designed for each type and size of solar
cell structure in order to optimize solar cell efficiency.
The series resistance of a solar cell consists of several
components as shown in Figure 12
Top Contact Design (continued)
Base Resistance
The resistance and current of the base is assumed to be
constant.
The resistance to the current of the bulk component of the
cell, or the "bulk resistance", Rb, is defined as:
………………………………..(6.3)
taking into account the thickness of the material. Where:
L = length of conducting (resistive) path
rb = "bulk resistivity" (inverse of conductivity) of the bulk cell
material (0.5 - 5.0 W cm for a typical silicon solar cell)
A =cell area,and
w = width of bulk region of cell.
Top Contact Design (continued)
Sheet Resistivity
The "sheet resistivity", which depends on both the resistivity
and the thickness.
For a uniformly doped layer, the sheet resistance is defined as:
……………………………………..(6.4)
where
r
is
the
resistivity
t is the thickness of the layer.
of
the
layer;
and
The sheet resistivity is normally expressed as ohms/square
or W/
For non-uniformly doped n-type layers, ie., if r is nonuniform:
…………………..(6.5)
Top Contact Design(continued)
Emitter Resistance
Based on the sheet resistivity, the power loss due to the
emitter resistance can be calculated as a function of finger
spacing in the top contact.
Idealized current flow from point of generation to external
contact in a solar cell is shown in Figure 13.
Top Contact Design
(continued)
Contact Resistance
Contact resistance losses occur at the interface between the
silicon solar cell and the metal contact.
To keep top contact losses low, the top N-layer must be as
heavily doped as possible.
Figure 14 shows points of contact resistance losses at
interface between grid lines and semiconductor.
Top Contact Design
(continued)
Metal Grid Pattern
The design of the top contact involves not only the minimization of
the finger and busbar resistance, but the overall reduction of
losses associated with the top contact.
These include resistive losses in the emitter, resistive losses in the
metal top contact and shading losses.
The critical features of the top contact design which determine how
the magnitude of these losses are :
1.
the finger and busbar spacing,
2.
the metal height-to-width aspect ratio,
3.
the minimum metal line width and
4.
the resistivity of the metal.
These are shown in the figure 15.
Top Contact Design
(continued)
Design Rules
for
practical reasons most top surface metalization
patterns are relatively simple and highly symmetrical.
A symmetrical contacting scheme can be broken down into
unit cells and several broad design rules can be
determined. It can be shown (Serreze, 1978) that:
1.
the optimum width of the busbar, WB, occurs when the
resistive loss in the busbar equals its shadowing loss;
2.
a tapered busbar has lower losses than a busbar of
constant width; and
3.
the smaller the unit cell, the smaller finger width, WF ,
and the smaller the finger spacings, S, the lower the
losses.
Solar Cell Structure
Silicon Solar Cell Parameters
For silicon solar cells, the basic design constraints
on :
1.
surface reflection,
2.
carrier collection,
3.
recombination and
4.
parasitic resistances
The result in an optimum device of about 25%
theoretical efficiency. A schematic of such an
optimum device is shown in Figure 16.
Solar Cell Structure(continued)
Basic Cell Design Compromises:
Substrate Material (usually silicon)
Bulk
crystalline
silicon
dominates
the
current
photovoltaic market, in part due to the prominence of
silicon in the integrated circuit market.
Cell Thickness (100-500 µm)
An optimum silicon solar cell with light trapping and
very good surface passivation is about 100 µm thick.
Doping of Base (1 W·cm)
A higher base doping leads to a higher Voc and lower
resistance, but higher levels of doping result in damage to
the crystal.
Solar Cell Structure(continued)
Reflection Control (front surface typically textured)
The front surface is textured to increase the amount of light coupled
into the cell.
Emitter Dopant (n-type)
N-type silicon has a higher surface quality than p-type silicon so it is
placed at the front of the cell where most of the light is absorbed.
Thus the top of the cell is the negative terminal and the rear of the
cell is the positive terminal.
Emitter Thickness (<1mm)
A large fraction of light is absorbed close to the front surface. By
making the front layer very thin, a large fraction of the carriers
generated by the incoming light are created within a diffusion length
of the p-n junction.
Solar Cell Structure(continued)
Doping Level of Emitter (100 W/ )
The front junction is doped to a level sufficient to conduct away the
generated electricity without resistive looses. However, excessive
levels of doping reduces the material's quality to the extent that
carriers recombine before reaching the junction.
Grid Pattern (fingers 20 to 200mm width, placed 1 – 5 mm
apart)
The resistivity of silicon is too low to conduct away all the current
generated, so a lower resistivity metal grid is placed on the surface
to conduct away the current. The metal grid shades the cell from the
incoming light so there is a compromise between light collection and
resistance of the metal grid.
Rear Contact.
The rear contact is much less important than the front contact since
it is much further away from the junction and does not need to be
transparent. The design of the rear contact is becoming increasingly
important as overall efficiency increases and the cells become
thinner.
Thank You
Figure 1. Evolution of silicon solar cell efficiency.
Figure 2. Sources of optical loss in a solar cell.
Figure 3. Use of a quarter wavelength antireflection coating to counter surface reflection.
Figure 4. Comparison of surface reflection from a silicon solar cell,
with and without a typical anti-reflection coating.
(a)
(b)
(c)
(d)
Figure 5. (a) A square based pyramid which forms the surface of an
appropriately textured crystalline silicon solar cell.(b)Scanning electron
microscope photograph of a textured silicon surface.(c) Scanning
electron microscope photograph of a textured silicon surface. (d)
Scanning electron microscope photograph of a textured multicrystalline
silicon surface.
Figure 6. Reflection and transmission of light for a
textured silicon solar cell.
Figure 7. Light trapping using a randomized reflector on the
rear of the cell. Light less than the critical angle escapes
the cell but light greater than the critical angle is totally
internally reflected inside the cell. In actual devices, the
front surface is also textured using schemes such as the
random pyramids mentioned earlier.
Figure 8. Typical quantum efficiency in an ideal
and actual solar cell, illustrating the impact of
optical and recombination losses.
Figure 9. Quantum efficiency curves for three different types of
crystalline silicon solar cells. The buried contact and screen printed
curves are internal quantum efficiencies, while the PERL is an
external quantum efficiency. The PERL cell has the best response to
infrared light since it has a well passivated, highly reflective rear
incorporating light trapping.
Figure 10. Effect of doping (ND) on diffusion
length and open-circuit voltage assuming well
passivated surfaces.
Figure 11. Techniques for reducing the impact of
surface recombination.
Figure 12. Resistive components and current flows in a solar cell.
Figure 13. Idealised current flow from point of generation to
external contact in a solar cell. The emitter is typically
much thinner than shown in the diagram.
Figure 14. Points of contact resistance losses at interface
between grid lines and semiconductor.
Figure 15. Key features of a top surface contacting scheme.
Figure 16. Basic schematic of a silicon solar cell. The top
layer is referred to as the emitter and the bulk material is
referred to as the base.