Solar Photovoltaic Physics lecture7

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

Solar Photovoltaic Physics lecture7

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