2- Solar Cell

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Physics department 4th year

2-Solar cell

Aim of experiment: Studying the characteristic curves of solarcell and determination of cell parameters.

Apparatus: solar cell- power supply-ammeter-resistor box coloredfilters.

Theory of experiment:

Solar cell is a device that used to convert solar energy directly to electric

current byphotovoltaic effect.

Photovoltaic effect:

Becquerel discovered the photovoltaic effect while experimenting with anelectrolytic cell made up of two metal electrodes. Becquerel found thatcertain materials would produce small amounts of electric current whenexposed to light.

Sunlight is composed of photons, or "packets" of energy. These photonscontain various amounts of energy corresponding to the differentwavelengths of light. When photons strike a solar cell, they may bereflected or absorbed, or they may pass right through. When a photon isabsorbed, the energy of the photon is transferred to an electron in anatom of the cell (which is actually a semiconductor). With its newfoundenergy, the electron is able to escape from its normal position associatedwith that atom to become part of the current in an electrical circuit. Byleaving this position, the electron causes a hole to form.

Solar cell generations:

(1) 1st generation "mono crystalline silicon"

(2) 2nd generation "thin film cells polycrystalline "

(3) 3rd generation "polymer and dye cells"

The most common used type is 1st generation type which called "P-Njunction cell or photodiode", which will be discussed:

Construction:

It consisted of P-N junction but not made with the traditional method bycontacting two types of doped silicon (P-type) and (N-type) with eachother, but it is made by diffusion of N-type dopants into P-type crystal.Layer of P-crystal is covered by sensitive and not reflectance layer. P-layershouldn't be thick not to absorb photoelectrons which liberated when lightincident on it.
http://encyclobeamia.solarbotics.net/articles/current.htmlhttp://encyclobeamia.solarbotics.net/articles/solar_cell.htmlhttp://encyclobeamia.solarbotics.net/articles/electron.htmlhttp://encyclobeamia.solarbotics.net/articles/solar_cell.htmlhttp://encyclobeamia.solarbotics.net/articles/semiconductor.htmlhttp://encyclobeamia.solarbotics.net/articles/electron.htmlhttp://encyclobeamia.solarbotics.net/articles/current.htmlhttp://encyclobeamia.solarbotics.net/articles/circuit.htmlhttp://encyclobeamia.solarbotics.net/articles/electron.htmlhttp://encyclobeamia.solarbotics.net/articles/hole.htmlhttp://encyclobeamia.solarbotics.net/articles/current.htmlhttp://encyclobeamia.solarbotics.net/articles/solar_cell.htmlhttp://encyclobeamia.solarbotics.net/articles/electron.htmlhttp://encyclobeamia.solarbotics.net/articles/solar_cell.htmlhttp://encyclobeamia.solarbotics.net/articles/semiconductor.htmlhttp://encyclobeamia.solarbotics.net/articles/electron.htmlhttp://encyclobeamia.solarbotics.net/articles/current.htmlhttp://encyclobeamia.solarbotics.net/articles/circuit.htmlhttp://encyclobeamia.solarbotics.net/articles/electron.htmlhttp://encyclobeamia.solarbotics.net/articles/hole.html


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The photodiode is usually operated in reverse bias mode, so that thecarriers generated within the depletion region are quickly swept towardthe terminals. This is one reason why the response speed of a photodiodeis fast.

When light incident on photo cell, photons interaction with the cell hasthree probabilities:

(1) Photon would pass through the crystal without any interaction ifphoton energy is low

(2) Photon would be reflected.(3) Photon can be absorbed by silicon atoms if photon energy is higher

than band gap value.

Any material conductivity depends on the distance between "valanceband" and "conduction band" which called "band gap" and intermediate

line called "Fermi level". In case of semiconductors band gap isintermediate not high as insulators and not low as metals.

When incident photon of energy (E=h =h c/) is absorbed by siliconatoms, electrons of valance band would absorb this energy and begin toexcite to conduction band forming a hole in its location.

With time electrons would move to fill these holes producing flow of

electrons in direction opposing hole flow direction. The region whereelectrons and holes are recombined forming "depletion region". Because


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of an electric field which is created by the imbalance of chargeimmediately on either side of the junction which this diffusion creates. Theelectric field established across the p-n junction creates a diode thatpromotes charge flow, known as drift current, that opposes and eventuallybalances out the diffusion of electron and holes.

If incident photon energy is higher than the band gap, the difference inenergy is converted into heat produced by atoms vibrations whichproduces elastic waves "phonon".

There are two modes of charge carriers in solar cell:

(1) Drift carriers, driven by electrostatic field established by movedcharges as in P-N junction.

(2)Diffusion carriers, due to diffusion of carriers from highconcentration zone to low concentration zone as in polymer cellswhere there are not electrostatic field.

Characteristic solar cell equation:

Diode can be used instead of solar cell, where figure below represents theequivalent circuit:

An ideal solar cell may be modeled by a current source in parallel with adiode; in practice no solar cell is ideal, so a shunt resistance and a seriesresistance component are added to the model.

The current produced by solar cell (I), (Rs) is series resistance, (Rsh) isshunt resistance and (ID) is diode current.

(1)

The current through these elements is governed by the voltage acrossthem:
http://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Drift_currenthttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Drift_currenthttp://en.wikipedia.org/wiki/Diode


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(2)

Where

VJ = voltage across both diode and resistor RSH (volts)

V = voltage across the output terminals (volts)

I = output current (amperes)

RS = series resistance ().

By using Shockley diode equation:

(3)

Where (ID) is diode current, (I0) is reverse saturation current, (n) is idealityfactor, (K) is Boltzmann constant and (T) is temperature in Kelvin.

From ohm's low:

(4)

Substituting from eq 2, 3, 4 in 1

I= (5)

This equation doesn't have any analytical solution but has numericalsolution by using Lambert w function.

When the cell is operated at open circuit, I = 0 and the voltage across theoutput terminals is defined as the open-circuit voltage. Assuming theshunt resistance is high enough to neglect the final term of thecharacteristic equation (5), the open-circuit voltage VOC is:

-1 (6)

By taking (Ln) for both sides and put n=1;
http://en.wikipedia.org/wiki/Voltshttp://en.wikipedia.org/wiki/%C3%8E%C2%A9http://en.wikipedia.org/wiki/Open_circuithttp://en.wikipedia.org/wiki/Voltshttp://en.wikipedia.org/wiki/%C3%8E%C2%A9http://en.wikipedia.org/wiki/Open_circuit


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(7)

(8)

When the cell is operated at short circuit, (V = 0) and the current (I)through the terminals is defined as the short-circuit current. At this point,the power output of the solar cell is zero. It can be shown that for a high-quality solar cell (low RS and I0, and high RSH), short-circuit current ISC is:

(9)

As the size of cell increased, the area exposed to light is increased producing morecurrent and causing lower resistance so characteristic equation of solar cell can be

expressed by using current density term:

(10)

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Where

J = current density (amperes/cm2).

JL = photo generated current density (amperes/cm2)

Jo= reverse saturation current density (amperes/cm2)

rS = specific series resistance (-cm2)

rSH = specific shunt resistance (-cm2) .

I-V characteristic curve of solar cell:

In this graph:

The short circuit current=2A

The open circuit

voltage=0.61 V
http://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Short_circuit


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As shunt resistance decreases, the current diverted through the shuntresistor increases for a given level of junction voltage. The resultis that the voltage-controlled portion of the I-V curve begins tosag toward the origin, producing a significant decrease in theterminal current I and a slight reduction in VOC. Very low valuesofRSH will produce a significant reduction in VOC. Much as in thecase of a high series resistance.

Reverse saturation current:

If we assumed infinite shunt resistance, thecharacteristic equation can be solved forVOC:

Thus, an increase in I0 produces a reductionin VOC proportional to the inverse of thelogarithm of the increase. This explainsmathematically the reason for the reductionin VOC that accompanies increases in temperature described above.

Ideality factor:

The ideality factor (also called theemissivity factor) is a fittingparameter that describes how

closely the diode's behaviormatches that predicted by theory,which assumes the p-n junction ofthe diode is an infinite plane andno recombination occurs withinthe space-charge region. A perfectmatch to theory is indicated when n = 1.

Solar cell efficiency factors:

Energy conversion efficiency:

It is the percentage of power converted (from absorbed light to electricalenergy) and collected, when a solar cell is connected to an electricalcircuit. This term is calculated using the ratio of the maximum powerpoint, Pm, divided by the input light irradiance (E, in W/m

2) under standardtest conditions (STC) and the surface area of the solar cell (Ac in m

2).

STC specifies a temperature of25C and an irradiance of1000 W/m2 with

an air mass 1.5 (AM1.5) spectrums. Due to the difficulty in measuring these
http://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiance


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parameters directly, other parameters are measured instead:

Thermodynamic Efficiency, Quantum Efficiency, VOC ratio, and Fill Factor.

Thermodynamic efficiency limit:

Solar cells operate as quantum energy conversion devices, and aretherefore subject to the "Thermodynamic Efficiency Limit". Photonswith energy below the band gap of the absorber material cannotgenerate a hole-electron pair, and so their energy is not converted touseful output and only generates heat if absorbed. For photons withenergy above the band gap energy, only a fraction of the energy abovethe band gap can be converted to useful output. When a photon ofgreater energy is absorbed, the excess energy above the band gap isconverted to kinetic energy of the carrier combination. The excesskinetic energy is converted to heat through phonon interactions as thekinetic energy of the carriers slows to equilibrium velocity.

Solar cells with multiple band gap absorber materials are able to moreefficiently convert the solar spectrum. By using multiple band gaps, thesolar spectrum may be broken down into smaller bins where thethermodynamic efficiency limit is higher for each bin.

Quantum efficiency:

It refers to the percentage of photons that are converted to electric

current (collected carriers) when the cell is operated under short circuit

conditions. External quantum efficiency (EQE) is the fraction of incident

photons that are converted to electrical current, while internal quantum

efficiency(IQE) is the fraction ofabsorbedphotons that are converted toelectrical current. Mathematically, internal quantum efficiency is related to

external quantum efficiency by the reflectance (R) and the transmittance

(T) of the solar cell by:

IQE = EQE / (1 R T).

Maximum-power point:

A solar cell may operate over a wide range ofvoltages (V) and currents (I).By increasing the resistive load on an irradiated cell continuously fromzero (a short circuit) to a very high value (an open circuit) one can

determine the maximum-power point, it referred to as the knee of I - Vcurve.

Current (A(

Isc

Imp

Pm
http://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Current_(electricity)http://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Open_circuithttp://en.wikipedia.org/wiki/Maximum_power_theoremhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Current_(electricity)http://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Open_circuithttp://en.wikipedia.org/wiki/Maximum_power_theorem


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Fill factor (FF):

It is the ratio of the maximum power pointdivided by the open circuit

voltage (Voc) and the short circuit current(Isc):

The fill factor determines the shape of the solar cell I-V characteristics.Its value is higher than 0.7 for good cells. The series and shuntresistances account for a decrease in the fill factor. Thefill factor is auseful parameter for quality control tests.

Procedures:

Part 1: determination of filling factor.

1. Connect the circuit as shown in Figure below. Make sure that the

distance between the solar cell and light source is suitable fixed

during the experiment.

2Make the applied voltage to light source is fixed during the

experiment.

3-Change the resistance from variable resistance and record thecorresponding current.

4-Determine voltage corresponding to each resistance value.

5-Draw the relation between applied voltage and corresponding, where

the obtained graph is similar to this below:

V (volt(Vmp Voc

+

-

A

R

Photo cell

Lamp

PmIsc


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I

V

a) (b)

I

VIg1

g3I

g2Isolar cell

operatingpoint


6-Determine filling factor for this case.

7-Put colored filter in front of the light source and repeat the above steps.

8-Change filter and determine the previous variables.

9-Draw graph between corresponding wave length and maximum power

in each case.

Part 2: studying of I-V characteristic curve of solar cell as

photodiode.

1-connect the circuit as shown in

the figure below.

2-By varying the applied voltage

and determine the corresponding

current.

3-draw the relation between the applied voltage and corresponding, where

the obtained graph is similar to below:

Imp

VocVmp V (volt(


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Documents

2- Solar Cell