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7/27/2019 Solar Photovoltaic Physics lecture7
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Solar Photovoltaic Physics
Lecture - 7
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Design of Solar Cells The principles for maximising cell efficiency are:
Increasing the amount of light collected by the cell that is turned into carriers;
Increasing the collection of light-generated carriers by thep-n junction;
Minimising the forward bias dark current;
Extracting the current from the cell without resistive losses.Theoretical maximum efficiency : 29 %
The maximum efficiency measured for a silicon solar cell : 24.7%
Predictions assume that there are no unabsorbed photons and that each
photon is absorbed in a material which has a band gap equal to the
photon energy.
Temperature and resistive effects: Increasing the light intensity
proportionally increases the short-circuit current. Voc increases
logarithmically with light level. Since the maximum fill factor (FF)
increases with Voc, the maximum possible FF also increases with
concentration.
The extra Voc and FF increases with concentration which allows
concentrators to achieve higher efficiencies.
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Optical Losses
Optical losses lowers the short-circuit current, i.
e
the light is reflected from the front surface, or itis not absorbed in the solar cell.
(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 (although any light whichis absorbed more than a diffusion length away from the junction will not typicallycontribute to short-circuit current since the carriers recombine).
The optical path length in the solar cell may be increased by a combination ofsurface texturing and light trapping.
There are a number of ways to reduce the optical
losses:
Top contact coverage of the cell surface can be minimised
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Antireflection (AR) Coatings Finished PV cell is coated with a material to reduce
The amount of reflected light (just like eye glasses) Usually used on cells unsuitable for texturing
Can reduce reflection to 5%
AR Coating Materials
Silicon nitrideSilicon dioxide
Zinc oxide
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Anti Reflection Coatings
A thin layer of dielectric material whose thickness causes
destructive interference between the wave reflected from
the anti-reflection coating top surface to the wavereflected from the semiconductor surfaces.
The thickness is one quarter the wavelength of the
incoming wave.Anti-reflection coating with a refractive index n1 and light
incident on the coating with a free-space wavelength 0,
The thickness d1 which causes minimum reflection is
calculated by:
Reflection is further minimised if the refractive index of the anti -reflection coating is the
geometric mean of that of the materials on either side; that is, glass or air and the
semiconductor. This is expressed by:
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Comparison of surface reflection from a
silicon solar cell, with and without a typical
anti-reflection coating.
Four multicrystallline wafers covered with films of
silicon nitride. The difference in colour is solely due to
the thickness of the film.
Surface TexturingTextured multicrystalline silicon
surface.
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Light Trapping
If light passes from a high refractive index medium to a
low refractive index medium, there is the possibility of
total internal reflection (TIR). The angle at which this
occurs is the critical angle and is found by setting 2 in
the above equation to 0.
Lambertian Rear ReflectorsLight trapping increases
the short-circuit current
(JSC) of the solar cell -
particularly for thin
devices. The path length can be enhanced by a factor up to 4n2Optical path length of approximately 50 times
the physical devices thickness
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Current & 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. Increasing the recombination increases the forward bias
current which increases the forward bias diffusion current, which in turn reduces the open-
circuit voltage.
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Minimize Parasitic Resistive Losses
Both shunt and series resistance losses decrease fill factor and
efficiency Low shunt resistance is a processing defect rather than a design
parameter
Series resistance controlled by the top contact design and
emitter resistance needs to be carefully designedTop Contacts
Metallic top contacts are necessary to collect the current
generated by the solar cell
Bus Bars are connected directly to the external leads
Fingers are finer areas of metal that collect the current and
delivers it to the bus bars
Tradeoff between resistive losses and reflection losses
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Surface Recombination Series Resistance
Base Resistance
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Emitter ResistanceThe incremental power loss in the
section dy is given by:
where is the sheet resistivity in / ; b is the distance along the finger; and y the distance between two grid fingers.
The current also depends on y and I(y) is the lateral current flow, which is zero at the midpoint between
grating lines and increases linearly to its maximum at the grating line, under uniform illumination. The
equation for the current is:
The total power loss is :
The differential resistance is given by
At the maximum power point, the generated power is:
The fractional power loss is given by:
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Contact resistance
Finger Resistance
The power loss in the element dx is:
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Metal Grid Pattern
The optimum width of the busbar, WB, occurs when the resistive loss in the busbar equals
its shadowing loss;
A tapered busbar has lower losses than a busbar of constant width; and
the smaller the unit cell, the smaller finger width, WF , and the smaller the finger spacings,S, the lower the losses.
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Solar cell design principles
Substrate Material : Silicon)
Cell Thickness : (100-500 m)Doping of Base : (1 cm): A higher base doping leads to a higher Voc and lower
resistance, but higher levels of doping result in damage to the crystal.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 thanp-type silicon so it is placed at the front
of the cell where most of the light is absorbed.
Emitter Thickness (m)
A large fraction of light is absorbed close to the front surface. By making the f ront layer
very thin, a large fraction of the carriers generated by the incoming light are created
within a diffusion length of thep-n junction.
Doping Level of Emitter (100 / )
The front junction is doped to a level sufficient to conduct away the generated electricity
without resistive loses. 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 200 m 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.
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Module Structure
(ethyl vinyl acetate)
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Module Current Depends primarily on size of solar cell and fficiency
At AM1.5 commercial cells produce 3036
mA/cm2
Typical cells produce 34 A per cell
Not temperature dependent
But depends heavily on tilt angle
Mismatch Effects Interconnecting of cells or modules not having identical properties
Module Output determined by the cell with the lowest output
Cells usually matched to each other
Shaded cell acts like poor cell
Significantly reduces output power
Localized power dissipation and localized heating
Can cause irreversible damage to module
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Packing Density
The "zero-depth concentration effect" in modules with
sparsely packed cells and a white rear surface.
In a typical module, 36 cells are connected in
series to produce a voltage sufficient to charge a
12V battery.
N is the number of cells in series;M is the number of cells in parallel;
IT is the total current from the circuit;
VT is the total voltage from the circuit;
I0 is the saturation current from a single solar cell;
IL is the short-circuit current from a single solar
cell; n is the ideality factor of a single solar cell;
and q, k, and T are constants.
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Mismatch Effects
Open Circuit Voltage Mismatch for Cells Connected in Series
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Hot-Spot HeatingOne shaded cell in a string reduces the current
through the good cells, causing the good cells
to produce higher voltages that can oftenreverse bias the bad cell.
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Mismatch for Cells Connected in Parallel Mismatch Effects in Arrays
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Heat Generation in PV Modules
Front Surface Reflection : The maximum temperature rise of the module is equal to incident power multiplied
by the reflection. Modules with a glass top surface, the reflected light contains about 4% of the incident energy .Operating Point and Efficiency of the Module: If the solar cell is operating at short-circuit current or atopen-circuit voltage, then it is generating no electricity and hence all the power absorbed by the solar cell is converted
into heat.Absorption of Light by the PV Module :How much light is absorbed and how much is reflected isdetermined by the color and material of the rear backing layer of the module.
Absorption of Infra-red Light : Light which has an energy below that of the band gap of the solar cells cannotcontribute to electrical power, but if it is absorbed by the solar cells or by the module, this light will contribute to
heating
Packing Factor of the Solar Cells : The cells will generate significant amounts of heat, usually higher than themodule encapsulation and rear backing layer. Therefore, a higher packing factor of solar cells increases the generated
heat per unit area.
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Degradation and Failure Modes
Short-Circuited Cells
Open-Circuited Cells
Interconnect Open-Circuits
Module Open-Circuits
Module Short-CircuitsModule Glass Breakage
Hot-Spot Failures
By-Pass Diode Failure
Encapsulant Failure