Physics department

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Solar cell -2
Aim of experiment: Apparatus:
.filters Studying the characteristic curves of solar .cell and determination of cell parameters

solar cell- power supply-ammeter-resistor box colored

:Theory of experiment
Solar cell is a device that used to convert solar energy directly to electric .current by photovoltaic effect :Photovoltaic effect Becquerel discovered the photovoltaic effect while experimenting with an electrolytic cell made up of two metal electrodes. Becquerel found that certain materials would produce small amounts of electric current when exposed to light. Sunlight is composed of photons, or "packets" of energy. These photons contain various amounts of energy corresponding to the different wavelengths of light. When photons strike a solar cell, they may be reflected or absorbed, or they may pass right through. When a photon is absorbed, the energy of the photon is transferred to an electron in an atom of the cell (which is actually a semiconductor). With its newfound energy, the electron is able to escape from its normal position associated with that atom to become part of the current in an electrical circuit. By leaving 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-N junction cell or photodiode", which will be discussed: Construction: It consisted of P-N junction but not made with the traditional method by contacting two types of doped silicon (P-type) and (N-type) with each other, 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-layer shouldn't be thick not to absorb photoelectrons which liberated when light incident on it.

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The photodiode is usually operated in reverse bias mode, so that the carriers generated within the depletion region are quickly swept toward the terminals. This is one reason why the response speed of a photodiode is fast. When light incident on photo cell, photons interaction with the cell has three probabilities: (1) Photon would pass through the crystal without any interaction if photon 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 "valance band" and "conduction band" which called "band gap" and intermediate line called "Fermi level". In case of semiconductors band gap is intermediate not high as insulators and not low as metals. When incident photon of energy (E=h =h c/) is absorbed by silicon atoms, electrons of valance band would absorb this energy and begin to excite 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 where electrons and holes are recombined forming "depletion region". Because

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of an electric field which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates. The electric field established across the p-n junction creates a diode that promotes charge flow, known as drift current, that opposes and eventually balances out the diffusion of electron and holes. If incident photon energy is higher than the band gap, the difference in energy is converted into heat produced by atoms vibrations which produces elastic waves "phonon".

There are two modes of charge carriers in solar cell: (1) Drift carriers, driven by electrostatic field established by moved charges as in P-N junction. (2) Diffusion carriers, due to diffusion of carriers from high concentration zone to low concentration zone as in polymer cells where there are not electrostatic field. Characteristic solar cell equation: Diode can be used instead of solar cell, where figure below represents the equivalent circuit:

An ideal solar cell may be modeled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The current produced by solar cell (I), (Rs) is series resistance, (Rsh) is shunt resistance and (ID) is diode current.

(1)
The current through these elements is governed by the voltage across them:

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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 ideality factor, (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 numerical solution by using Lambert w function. When the cell is operated at open circuit, I = 0 and the voltage across the output terminals is defined as the open-circuit voltage. Assuming the shunt resistance is high enough to neglect the final term of the characteristic equation (5), the open-circuit voltage VOC is:

-1
By taking (Ln) for both sides and put n=1;

(6)

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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 highquality 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 more current and causing lower resistance so characteristic equation of solar cell can be expressed by using current density term:

(10) (11)
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

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Effectors on I-V characteristic curve of solar cell: (1) Temperature: Temperature affects the characteristic equation. While increasing T reduces the magnitude of the exponent in the characteristic equation, the value of I0 increases in proportion to exp (T). The net effect is to reduce VOC (the open-circuit voltage) linearly with increasing temperature. The magnitude of this reduction is inversely proportional to VOC; that is, cells with higher values of VOC suffer smaller reductions in voltage with increasing temperature.

(2)Series resistance: As series resistance increases, the voltage drop between the junction voltage and the terminal voltage becomes greater for the same flow of current. The result is that the current-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal voltage V and a slight reduction in ISC, the shortcircuit current. Very high values of RS will also produce a significant reduction in ISC; in these regimes, series resistance dominates and the behavior of the solar cell resembles that of a resistor. Shunt resistance:

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As shunt resistance decreases, the current diverted through the shunt resistor increases for a given level of junction voltage. The result is that the voltage-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal current I and a slight reduction in VOC. Very low values of RSH will produce a significant reduction in VOC. Much as in the case of a high series resistance. Reverse saturation current: If we assumed infinite shunt resistance, the characteristic equation can be solved for VOC:

Thus, an increase in I0 produces a reduction in VOC proportional to the inverse of the logarithm of the increase. This explains mathematically the reason for the reduction in VOC that accompanies increases in temperature described above. Ideality factor: The ideality factor (also called the emissivity factor) is a fitting parameter that describes how closely the diode's behavior matches that predicted by theory, which assumes the p-n junction of the diode is an infinite plane and no recombination occurs within the space-charge region. A perfect match 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 electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance (E, in W/m2) under standard test conditions (STC) and the surface area of the solar cell (Ac in m2).

STC specifies a temperature of 25C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrums. Due to the difficulty in measuring these

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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 are therefore subject to the "Thermodynamic Efficiency Limit". Photons with energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity. Solar cells with multiple band gap absorber materials are able to more efficiently convert the solar spectrum. By using multiple band gaps, the solar spectrum may be broken down into smaller bins where the thermodynamic 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 of absorbed photons that are converted to electrical 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).

Isc Imp

Pm

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Fill factor (FF):

Vmp

Voc

(V (volt

It is the ratio of the maximum power point divided 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 shunt resistances account for a decrease in the fill factor. The fill factor is a useful 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.
Photo cell

A R

+ -

Lamp

Make the applied voltage to light source is fixed during the 2 .experiment Change the resistance from variable resistance and record the -3 .corresponding current .Determine voltage corresponding to each resistance value -4 Draw the relation between applied voltage and corresponding, where -5 :the obtained graph is similar to this below

Isc

Pm

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Imp

4th year

Vmp
.Determine filling factor for this case -6

Voc

(V (volt

.Put colored filter in front of the light source and repeat the above steps -7 .Change filter and determine the previous variables -8 Draw graph between corresponding wave length and maximum power -9 .in each case Part 2: studying of I-V characteristic curve of solar cell as .photodiode connect the circuit as shown in-1 .the figure below By varying the applied voltage -2 and determine the corresponding .current draw the relation between the applied voltage and corresponding, where-3 :the obtained graph is similar to below

Ig1 Ig2 I g3

solar cell
operating point

(a)

(b)

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4th year