chapter 6 solar cell part 1
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CHAPTER 6
SOLARCELL
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SOLAR CELL (PART I)
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WO
RLD
EN
ERG
Y C
ON
SU
MPT
ION 2010
2014Source : http://www.ren21.net/REN21Activities/GlobalStatusReport.aspx
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SOLAR ENERGY TECHNOLOGY Solar Impulse 2 to fly around the
world only by the suns rays. 35,000-kilometer, five month journey
across the globe, via India, Myanmar, China and the U.S.
March 9, 2015 took off from AbuDhabi to Oman
Features: 8-square meter single-seater cockpit. 72-meter (236-foot) wingspan 2.5 tons plane weighs
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What is Solar Energy? Energy produced by the sun Convert light into usable energysuch as electricity
Type : Bio massHydropowerSolar radiationWind power
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World top PV installer 2014 : 1.4 mil PV system installed covers small roof top system, medium
commercial and large utility-scale solar park contribute to 35.2 terawatt-hours ( 6.9 %)
31% utilize renewable energy : wind , PV world record solar power production with 25.8 GW produced at midday
on April 20 and April 21, 2015
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One of the largest PV installer Involved 400 PV companies 2011 (2.5 GW), 2012 (5 GW), 2013 (11.3 GW) installed capacity possibly reaching as much as 10 GW by 2020 200 MW Huanghe Hydropower Golmud Solar Park
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10
PHOTOVOLTAIC CELLS
PV effect :
the conversion of light (photon) into electrical energy
Types:
Inorganic Solar cell ( Single-crystal silicon, a-Si, GaAs)
Widespread
Efficiency : 15 20%
Expensive to manufacture
Organic solar cell
ultra thin, tunable colour
Efficiency : 12%
not yet to be commercialize
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PHOTOVOLTAIC CELLS
Electrochemical Dye- solar cell
Newer, less proven
Inexpensive to manufacture
Flexible
(-) : degradation issues
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PHOTOVOLTAIC CELL CONSTRUCTION
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PHOTOVOLTAIC CELL CONSTRUCTION
Solar cell : pn junction with light shining on it
To maximize efficiency , we must:- maximize e--h+ pairs - minimize recombination of e--h+ pairs
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PHOTOVOLTAIC CELL CONSTRUCTION
ACTIVE LAYER
Consist of excess electron
Consist of positively charge hole
Depletion region avoid themovement of electron andhole across region
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PHOTOVOLTAIC CELL CONSTRUCTION
As energy of light is absorbed by semiconductor e--h+ pairswill be created
Create extra mobile electron and hole.
Electric field occur as electron flow to n-doped layer and holeflow to p-doped layer photogeneration of charge carrier More electron in n-doped layer More hole in p-doped layer
Connect to electrical appliances i.e. lamp electric flow turn on light
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One PV Cell is approx 150mm indiameter
In bright sunshine it produces 0.4V d.c.
One Module is an array of approx 30cells
Connected together in series/parallel fordesired voltage
Life span is about 25 yrs
Cost varies : 300 - 600 per m2
PHOTOVOLTAIC CELL CONSTRUCTION
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Solar panel rating :
Max voltage in single PV Cell isapprox 0.4V (D.C.)
Current depends on sun intensity(Max of 2.5A)
Average voltage from a module is20V (D.C.)
PHOTOVOLTAIC CELL CONSTRUCTION
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Various factors effect power output from
panels :
Shade or Clouds
Panel position or angle
Active panels can track the sun
Temperature and solar irradiance variations
Air gap required for cooling
Partial shading will reduce performanceand can cause damage
PHOTOVOLTAIC CELL
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SOLAR CELL (PART II)
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21
SOLAR ENERGY SPECTRUM
The intensity of radiation emitted from the sun has a spectrum that resemblesa black body radiation at a temperature of about 6,000 K
This spectrum is modified by:
Effects of the solar atmosphere
Fraunhofer absorption (absorption by hydrogen)
Temperature variations on the surface of the sun facing us
The actual intensity spectrum on Earths surface depends:
on the absorption and scattering effects of the atmosphere
on the atmospheric composition
the radiation path length through the atmosphere
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22
SOLAR ENERGY SPECTRUM
0
Black body radiation at 6000 K
AM0
AM1.5
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00
0.5
1.0
1.5
2.0
2.5
Wavelength (m)
Spectral
Intensity
W cm-2 (m)-1
The spectrum of the solar energy represented as spectralintensity (I) vs wavelength above the earth's atmosphere
(AM0 radiation) and at the earth's surface (AM1.5radiation). Black body radiation at 6000 K is shown for
comparison (After H.J. Mller, Semiconductors for Solar
Cells, Artech House Press, Boston, 1993, p.10)
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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23
Light intensity variation with wavelength is typically represented byintensity per unit wavelength, called spectral intensity I, so that I is theintensity in a small interval
Integration of I over the whole spectrum gives the integrated or totalintensity, I
The integrated intensity above Earths atmosphere gives the total powerflow through a unit area perpendicular to the direction of the sun
This quantity is called the solar constant or air-mass zero (AM0) radiationand it is approximately constant at a value of 1.353 kW/m2
SOLAR ENERGY SPECTRUM
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24
Atmospheric effects depend on the wavelength
Clouds increase the absorption and scattering of the sun light andhence substantially reduced the incident intensity
On a clear sunny day, the light intensity arriving on Earths surface is roughly 70% of the intensity above the atmosphere
Absorption and scattering effects increase with the sun beams paththrough the atmosphere
The shortest path through the atmosphere is when the sun is directly above that location and the received spectrum is called air-mass one(AM1) as shown in the figure
SOLAR ENERGY SPECTRUM
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25
All other angles of incidence ( 90 increase the optical paththrough the atmosphere, and hence the atmospheric losses
Air-mass m (AMm) is defined as the ratio of the actualradiation path h to the shortest path h0, that is m=h/h0
Since h = h0sec, AMm is AMsec
It is apparent that the spectrum has several sharp absorptionpeaks at certain wavelength which are due to thosewavelengths being absorbed by various molecules in theatmosphere, such as ozone, air and water vapor molecules
SOLAR ENERGY SPECTRUM
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26
SOLAR ENERGY SPECTRUM
Direct Diffuse
(a) Illustration of the effect of the angle of incidence on the ray path length and the
definitions of AM0, AM1 and AM(sec ). The angle between the sun beam and the horizon
is the solar latitude (b) Scattering reduces the intensity and gives rise to a diffused radiation
Atmosphere
AM0
AM1
AM(sec)
h0h
(a) (b)
Tilted PV deviceEarth
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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27
In addition, the atmospheric molecules and dust particles scatter the
sun light
Scattering not only reduces the intensity on the radiation towards
Earth but also gives rise to the suns rays arriving at random angles as
shown in the figure
Consequently the terrestrial light has a diffuse component in addition
to the direct component
The diffuse component increases with cloudiness and suns position
SOLAR ENERGY SPECTRUM
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28
Solar Energy Spectrum
Direct
Direct Radiation is solar radiation reaching the Earth's surface after without having
been scattered
Diffuse
Diffuse radiation is solar radiation reaching the Earth's surface after having
been scattered from the direct solar beam
by molecules.
The diffuse component increases with cloudiness and suns position
Direct and Diffuse solar radiation
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The scattering of light increases with decreasing wavelength so that shorter wavelengths in the original sun beam experience more scattering than longer wavelengths
On a clear day, the diffuse component can be roughly 20% of the total radiation and significantly higher on cloudy days
29
SOLAR ENERGY SPECTRUM
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Example
30
-
solution
31
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solution
32
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Photovoltaic device: Structure
Consider a pn junction with a very narrow and more heavily doped n-region
Illumination is through the thin n-side
Depletion region (W) or the space charge layer (SCL) extends primarily into the p-side
There is a built-in field E0 in this depletion layer
33
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n-type semiconductor
p-type semiconductor
+ + + + + + + + + + + + + + +- - - - - - - - - - - - - - - - - -
Physics of Photovoltaic Generation
Depletion Zone
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35
Photovoltaic device: Structure
Electrodes attached to the n-side must: allow illumination to enter the device
at the same time result in a small series resistance
They are deposited on to n-side to form an array of finger electrodeson the surface
A thin antireflection coating on the surface reduces reflections and allows more light to enter the device
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36
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37
Finger electrodes
p
n
Bus electrode
for current collection
Finger electrodes on the surface of a solar cellreduce the series resistance
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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38
Photovoltaic device: EHP
n-side is very narrow so most of the photons are absorbed within:
the depletion region (W)
the neutral p-side (lp)
(Photogeneration also occur in these regions)
EHPs photogenerated in the depletion region are immediately separated by the built-in field E0 which drifts them apart
The electron drifts and reaches the neutral n+ side whereupon it makes this region negative by an amount of charge e
Similarly the hole drifts and reaches the neutral p-side and thereby makes this side positive
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39
Photovoltaic device: EHP
Open circuit voltage develops between the terminals of the device with the p-side positive with respect to the n-side
If an external load is connected then the excess electron in the n-side can travel around the external circuit, do work, and reach the p-side to recombine with the excess hole there
Without the internal field E0 it is not possible to drift apart the photogenerated EHPs and accumulate excess electrons on the n-side and excess holes on the p-side
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40
Photovoltaic device: EHP by long wavelengths
EHPs photogenerated by long wavelength photons that are absorbed in the neutral p-side can only diffuse in this region as there is no electric field
If the recombination lifetime of the electron is e, it diffuses a mean distance Le given by
where De is its diffusion coefficient in the p-side
Electrons within a distance Le to the depletion region can readily diffuse and reach this region whereupon they become drifted by E0 to the n-side
eee DL 2
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41
Photovoltaic device: EHP by long wavelengths
Only EHPs photogenerated within the minority carrier diffusion length Le to the depletion layer can contribute to the photovoltaic effect
Again the importance of the built-in field E0 is apparent
Once an electron diffuses to the depletion region it is swept over to the n-side by E0 to give an additional negative charge there
Holes left behind in the p-side contribute a net positive charge to this region
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42
Photovoltaic device: EHP by long wavelengths
Those photogenerated EHPs further away from depletion region than Leare lost by recombination
It is therefore important to have the minority carrier diffusion length Le as long as possible
This is the reason for choosing this side of a Si pn junction to be p-type which makes electrons to be the minority carriers; the electron diffusion length in Si is longer than the hole diffusion length
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43
Photovoltaic device: EHP by short wavelengths
The same ideas also apply to EHPs photogenerated by short-wavelengthphotons absorbed in the n-side
Holes photogenerated within a diffusion length Lh can reach the depletionlayer and become swept across to the p-side
The photogeneration of EHPs that contribute to the photovoltaic effecttherefore occurs in a volume covering Lh + W + Le
If the terminals of the device are shorted, then the excess electron in then-side can flow through the external circuit to neutralize the excess holein the p-side
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44
Le
Lh W
Iph
x
EHPs
exp(x)
Photogenerated carriers within the volume Lh + W + Le give rise to a photocurrent Iph. The
variation in the photegenerated EHP concentration with distance is also shown where is theabsorption coefficient at the wavelength of interest.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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45
Photovoltaic device
Current due to flow of photogenerated carriers is photocurrent
EHPs photogenerated by energetic photons absorbed in the n-side near the surface region or outside the diffusion length Lh to the depletion layer are lost by recombination as the lifetime in the n-side is generally very short (due to heavy doping)
The n-side is therefore made very thin, typically less than 0.2 m or less
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46
Photovoltaic device
Indeed, the length ln of the n-side maybe shorter than the hole diffusion length Lh
The EHP photogenerated very near the surface of the n-side however disappear by recombination due to various surface defects acting as recombination centers
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47
Photovoltaic device
At long wavelengths, around 1-1.2 m, the absorption coefficient of Si is small and the absorption depth (1/) is typically greater than 100 m
To capture these long wavelength photons we therefore need a thick p-side and at the same time a long minority carrier diffusion length Le
Typically the p-side is 200-500 m and Le tends to be shorter than this
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48
Photovoltaic device: Losses
Worst part of the efficiency limitation comes from the high energy photons becoming absorbed near the crystal surface and being lost by recombination in the surface region
Crystal surfaces and interfaces contain a high concentration of recombination centers which facilitate the recombination of photogenerated EHP near the surface
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49
Photovoltaic device: Losses
Losses due to EHP recombinations near or at the surface can as high as 40%
These combined effects bring the efficiency down to about 45%
In addition, the antireflection coating is not perfect which reduces the total collected photons by a factor of about 0.8-0.9
When we also include the limitations of the photovoltaic action itself , the upper limit to a photovoltaic device that uses a single crystal of Si is about 24-26% at room temperature
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wavelength Photon energy
Short-wavelength infrared
1.4-3 m 0.40.9 eV
Mid-wavelength infrared 38 m 150400 meV
Long-wavelength infrared 815 m 80150 meV
50
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pn JUNCTION PHOTOVOLTAIC I-V CHARACTERISTICS
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Guess what it is ??
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Iph
R
I
V V = 0
Iph
I = Id I
ph
V
Id
Isc
= Iph
R
(a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current
I in the circuit.
Light
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
PHOTOVOLTAIC I-V CHARACTERISTICS
-
Iph
R
I
V V = 0
Iph
I = Id I
ph
V
Id
Isc
= Iph
R
(a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current
I in the circuit.
Light
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
The voltage V and current I above define the convention for the direction of positive current and voltage.
PHOTOVOLTAIC I-V CHARACTERISTICS
-
Consider an ideal pn junction photovoltaic device connected to a resistive load R.
I and V define the convention for the direction of positive current and positive voltage.
If the load is short circuit the only current in the circuit is due to photogenerated (photocurrent),Iph.
Iph
R
I
V V = 0
Iph
I = Id I
ph
V
Id
Isc
= Iph
R
(a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current
I in the circuit.
Light
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
PHOTOVOLTAIC I-V CHARACTERISTICSPHOTOVOLTAIC I-V CHARACTERISTICS
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Photovoltaic I-V Characteristics
If I is the light intensity, then the short circuit current is
The photocurrent does not depend on the voltage across the pn jucntion, because it always some internal field to drift the photogenerated EHP.
If R is not short circuit the positive voltage V appears across the pn junction as a result of the current passing through.
Iph
R
I
V V = 0
Iph
I = Id I
ph
V
Id
Isc
= Iph
R
(a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current
I in the circuit.
Light
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
KIIIphsc
K is constant that depends on particular device
PHOTOVOLTAIC I-V CHARACTERISTICS
-
Under short circuit conditions (figure b), the only current in the circuit is the photocurrent Iph generated by the incident light.
Photovoltaic I-V Characteristics
K= Ratio of nonradiative to radiative losses in a solar cell material
OrK is constant that depends on particular device
PHOTOVOLTAIC I-V CHARACTERISTICS
-
If a load R is now inserted as in the figure (c), a positive voltage V appears across the diode which forward biases the diode.
There is now minority carrier injection and the resulting diode current.
Where I0 = reverse saturation current
Photovoltaic I-V CharacteristicsPHOTOVOLTAIC I-V CHARACTERISTICS
-
Thus, the total current (solar cell current),
n is the ideality factor that depends on the semiconductor material and fabrication
Photovoltaic I-V CharacteristicsPHOTOVOLTAIC I-V CHARACTERISTICS
-
Iph
R
I
V V = 0
Iph
I = Id I
ph
V
Id
Isc
= Iph
R
(a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current
I in the circuit.
Light
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
The I-V characteristics of a typical Si
solar cell (Fig.).
Normal dark characteristics being shifted
down by photocurrent Iph (short circuit),
which depend on light intensity, I.
The open circuit voltage, Voc, is given by
the point where the I-V curve cuts the V-
axis (I = 0). V
I (mA)
Dark
Light
Twice the light
0.60.40.2
20
20
0
Iph
Voc
Typical I-V characteristics of a Si solar cell. The short circuit current is Iphand the open circuit voltage is Voc. The I-V curves for positive current
requires an external bias voltage. Photovoltaic operation is always in thenegative current region.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
PHOTOVOLTAIC I-V CHARACTERISTICS
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VI (mA)
0.60.40.2
20
0
Voc
10
Isc= Iph
V
The Load Line for R = 30
(I-V for the load)
I-V for a solar cell under an
illumination of 600 Wm-2.
Operating Point
Slope = 1/R
P
I
(a) When a solar cell drives a load R, R has the same voltage as the solar cellbut the current through it is in the opposite direction to the convention thatcurrent flows from high to low potential. (b) The current I and voltage V inthe circuit of (a) can be found from a load line construction. Point P is theoperating point (I, V). The load line is for R = 30 .
LightI
R
V
I
(a) (b)
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
PHOTOVOLTAIC I-V CHARACTERISTICS
-
Since the open circuit voltage is the maximum voltage possible, andshort circuit current Isc is the maximum current possible, theunachievable maximum power = Isc Voc. It is therefore useful tocompare the maximum power output, Im Vm.
The fill factor is a measure of how close to the maximum power is aparticular operating point.
PHOTOVOLTAIC I-V CHARACTERISTICS
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The efficiency of the solar cell
The input sun-light power is
Pinput = (Light Intensity)(Surface Area)
The power delivered to the load is
PHOTOVOLTAIC I-V CHARACTERISTICS
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Two different Vsc
Two different circuit current Isc
Voc 2 Voc1 nkBT
eln
I2I1
0.551 0.0259 ln 0.5
n is the ideality factor that depends on the semiconductor material and fabrication
Isc2 Isc1I2I1
50 mA
50 W m2
100 W m2
Photovoltaic I-V Characteristics
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A Si solar cell of area 4 cm2 is connected to drive a load R as in Figure 1 . Suppose that the load is 20 and it is used under a light intensity of 1 kW m-2(Figure 1). What are the current and voltage in the circuit? What is the power delivered to the load? What is the efficiency of the solar cell in this circuit?
What is FF?
EXAMPLE
-
Figure 1
EXAMPLE
-
Figure 2
EXAMPLE
-
Series resistance and equivalent circuit Practical devices can deviate substantially from the ideal pn
junction solar cell behavior due to a number of reasons
Consider an illuminated pn junction driving a load resistance RLand assume that photogeneration takes place in the depletion region
As illustrated in the next figure, the photogenerated electron has to traverse a surface semiconductor region to reach the nearest finger electrode
All these electron paths in the n-layer surface region to finger electrodes introduce an effective series resistance Rs into the photovotaic circuit as indicated in the figure
If the finger electrodes are thin, then the series resistance of the electrodes themselves will further increase Rs
There is also a series resistance due to the neutral p-region but this is generally small compared with the resistance of the electron paths to the finger electrodes
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Neutral
n-region
Neutral
p-region
Finger
electrode
Back
electrode
Depletion
region
RL
Rs
Rp
Series and shunt resistances and various fates of photegenerated EHPs.
1999 S .O. Kasap, Optoelectronics (Prentice Hall)
Series Resistance and equivalent circuit
-
AIph Rp RLV
IIph
Id
Solar cell Load
B
Rs
The equivalent circuit of a solar cell
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Series Resistance and equivalent circuit
-
EXAMPLE Consider the equivalent circuit of a solar cell as shown in Figure below.
Show that;
[Hint: Kirchoffs current law]I Iph Id
V
Rp Iph Io exp(
eV
nkBT) Io
V
Rp
-
Figure shows the equivalent circuit with the series resistance removed. The currents flowing into node A sum to zero (Kirchoffs current law).
Currents into a node are positive and those leaving a node are negative.
Thus,
which is the required equation
EXAMPLE
-
I (mA)
V
00
0.2 0.4 0.6
5
10
Voc
Isc
Rs = 0
Rs = 20
Rs = 50
Iph
The series resistance broadens the I-V curve and reduces the maximumavailable power and hence the overall efficiency of the solar cell. The exampleis a Si solar cell with n 1.5 and Io 310-6 mA. Illumination is such thatthe photocurrent Iph = 10 mA.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Series Resistance and equivalent circuit
-
0.60.40.20246
5
15
Voltage (V)Power (mW)
Current (mA)
20
10
1 cell
2 cells in parallel
Current vs. Voltage and Power vs. Current characteristics of one cell and twocells in parallel. The two parallel devices have Rs/2 and 2Iph.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Series Resistance and equivalent circuit
-
AIphV
Iph
Id
B
Rs
RL
I/2
Id
Iph
I
RsI/2
Two identical solar cells in parallel under the same illumination anddriving a load RL.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Series Resistance and equivalent circuit
-
Series Resistance and equivalent circuit
-
Consider two identical solar cells with the properties Io = 2510
-6 mA, n = 1.5, Rs = 20 W, subjected to the same illumination so that Iph = 10 mA. Derive the corresponding current , I and voltage V.
Example
-
6.4 Open circuit voltage A solar cell under an illumination of 100 W m-2
has a short circuit current
Isc of 50 mA and an open circuit output voltage Voc, of 0.55V. What are the short circuit current and
open circuit voltages when the light intensity is halved?
Solution
The short circuit current is the photocurrent so that at
Isc2 Isc1I2I1
50 mA
50 W m2
100 W m2
= 25 mA
Assuming n1, the new open circuit voltage is
Voc 2 Voc1 nkBT
eln
I2I1
0.551 0.0259 ln 0.5 = 0.508 V
Assuming n2, the new open circuit voltage is
Voc2 Voc1 nkBT
eln
I2I1
0.55 2 0.0259 ln 0.5 = 0.467 V
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Temperature Effects Temperature decreases Output voltage and efficiency increase.
Solar cell operate best at lower temperature.
The output voltage Voc, when Voc >> nkBT/e,
I0 is reverse saturation current and strongly depend on temperature, because it depends on square of ni.
If I is light intensity,
0
lnI
I
e
TnkV
phB
oc
lnor ln00
I
KI
Tnk
eV
I
KI
e
TnkV
B
ocB
oc
-
Temperature Effects Assuming n = 1, at two different temperatures T1 and T2 but the
same illumination level, by subtraction,
Substitute,
Thus,
Rearrange for Voc2,
TkENNnBgvci
exp2
2
2
2
1
02
01
01021
1
2
2 lnlnln- lni
i
B
oc
B
oc
n
n
I
I
I
KI
I
KI
Tk
eV
Tk
eV
121
1
2
211
TTk
E
Tk
eV
Tk
eV
B
g
B
oc
B
oc
1
2
1
2
121
T
T
e
E
T
TVV
g
ococ
-
Temperature Effects Example, Si solar cell has Voc1 = 0.55 V at 20
oC (T1 = 293 K), at 60 oC
(T2 = 333 K),
V 475.0293
3331)V 1.1(
293
333)V 55.0(
2
ocV
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Solar Cells Materials, Devices and Efficiencies For a given solar spectrum,
conversion efficiency depends on the semiconductor material properties and the device structure.
Si based solar cell efficiencies 18% for polycrystalline and 22 24% for single crystal devices.
About 25% solar energy is wasted not enough energy unable to generate EHPs.
Considering all losses, the maximum electrical output power is ~20% for a high efficiency Si solar cell.
100% Incident radiation
Insufficient photon energy
h < Eg
Excessive photon energy
Near surface EHP recombination
h > Eg
Collection efficiency of photons
Voc (0.6Eg)/(ekB)
21%
FF0.85
Overall efficiency
Accounting for various losses of energy in a high efficiency Sisolar cell. Adapted from C. Hu and R. M. White, Solar Cells(McGraw-Hill Inc, New York, 1983, Figure 3.17, p. 61).
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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