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Solar Cell Operation
EE580 – Solar Cells
Todd J. Kaiser
n
Emitter
p
Base
Rear Contact
Absorption of photon creates an electron hole pair. If they are within a diffusion length of the depletion region the electric field separates
electric field separates them.
Antireflection coating
• Lecture 08
• Solar Cell Characterization
Front Contact
‐
External Load
Montana State University: Solar Cells
Lecture 8: Characterization
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Montana State University: Solar Cells
Lecture 8: Characterization
Solar
Input
Solar Cell Electrical Model
• PV is modeled as a current source because it
supplies a constant current over a wide range of
voltages
• It has p-n junction diode that supplies a potential
• It has internal resistors that impede the flow of
the electrons
The electron after passing through the load recombines with + the hole completing the circuit
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Circuit Diagram
Front Contact
I
Rs
Recombination
I
Current
Source
Ohmic Flow
V
Rsh
Rear Contact
Montana State University: Solar Cells
Lecture 8: Characterization
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External
Load
Montana State University: Solar Cells
Lecture 8: Characterization
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Power & IV Curve
Electrical Losses
• Series Resistance
(Resistance of Hole & Electron Motion)
– Bulk Resistance of Semiconductor Materials
– Bulk Resistance of Metallic Contacts and
Interconnects
– Contact Resistance
• Parallel Resistance or Shunt Resistance
(Recombination of Hole and Electron)
– PN junction Leakage
– Leakage around edge of Junction
– Foreign Impurities & Crystal Defects
Montana State University: Solar Cells
Lecture 8: Characterization
RLoad
• Power (Watts) is the rate at which energy (Joules) is
supplied by a source or consumed by a load… It is a
rate not a quantity
• The power output by a source is the product of the
current supplied and the voltage at which the current
was supplied
• Power output = Source voltage x Source current
– P=V x I (Watts = Joules/second) = (Volts)x(Amperes)
• By changing the resistance of the load different currents
and corresponding voltages can be measured and
plotted
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Montana State University: Solar Cells
Lecture 8: Characterization
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Solar Cell I-V Characteristics
I (mA)
⎞
⎛ qV
I = I 0 ⎜⎜ e kT − 1⎟⎟ − I L
⎠
⎝
Current
Operating Point
V oc
0.1
I
0.2
0.3
0.4
0
0.6
0.5
V
V′
I
I-V for a solar cell
under an illumination
of 700 W m-2
Dark
V
−100
100
Slope = - 1/R
Operating point
I′
Current from Absorption of Photons
Isc = −Iph
−200
R
I
(a)
Voltage
P
(b)
The load line for
R=3Ω
(I-V for the load)
Light
Twice the Light = Twice the Current
Montana State University: Solar Cells
Lecture 8: Characterization
Short
Circuit
Current
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IV Plot
Current
Operating
Point
I mpVmp
I scVoc
8
Resistance Effects
Maximum
Power
Imp
FF =
Montana State University: Solar Cells
Lecture 8: Characterization
ISC
ISC
Current
Current
Power
decreasing
RP
Increasing
RS
Vmp
Voltage (V)
Volt (V)
0
0.1
0.3
0.5
0.54
0.57
Current (A)
1.3
1.3
1.3
1.2
0.75
0
Power (W)
0
0.13
0.39
0.6
0.4
0
Open
Circuit
Voltage
Montana State University: Solar Cells
Lecture 8: Characterization
Voltage
VOC
Voltage
VOC
Ideal case RS = 0 and RP = ∞
Both reduce the area of the maximum power rectangle
therefore reducing the efficiency and fill factor
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Montana State University: Solar Cells
Lecture 8: Characterization
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Photovoltaic Effect
Loss Mechanisms in Solar Cells
Loss
Optical
Ohmic
Reflection
Shadowing
Unabsorbed Radiation
Separation of holes and
electrons by Electric
Field
Electrical
Solar Cell Material
Base
Emitter
Contact Material
Finger
Collection Bus
Junction
Metal – Solar Cell
Montana State University: Solar Cells
Lecture 8: Characterization
Recombination
Absorption of
Light
Excitation
of
electrons
Emitter Region
Solar Cell Material
Surface
Voltage
Creation of
extra
electron hole
pairs (EHP)
Base Region
Power = V x I
Current
Solar Cell Material
Surface
Space Charge Region
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Movement of charge
by Electric Field
Montana State University: Solar Cells
Lecture 8: Characterization
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Linking Cells
Current
Parallel
connections
i
increase
th
the
current output
– Strings – where cells are connected in series
– Blocks 2 or more strings connected together in parallel
– Joining 2 or more blocks together
Voltage
Voltage
Series
connections
increase the
voltage output
Current
Montana State University: Solar Cells
Lecture 8: Characterization
Voltage
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Calculating Voltage and Current
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Cell B
V = 0.54 V
I = 0.31 A
Cell C
V = 0.61 V
I = 0.25 A
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16
Example: Block Connected
4
17
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Montana State University: Solar Cells
Lecture 8: Characterization
Cell C
V = 0.61 V
I = 0.25 A
• The voltage across terminals 34 is the average of the
voltages
• V34 = (VA + VB + VC )/3 = (0.58 + 0.54 + 0.61)/3 = 0.58(V)
• The current at the terminals 34 is the sum of the
currents in each cell
• I34 = (IA + IB +IC) = (0.28 + 0.31 + 0.25) = 0.84(A)
Montana State University: Solar Cells
Lecture 8: Characterization
Cell A
V = 0.58 V
I = 0.28 A
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Cell B
V = 0.54 V
I = 0.31 A
Montana State University: Solar Cells
Lecture 8: Characterization
• The voltage across terminals 12 is the sum of the
voltages
• V12 = VA + VB + VC = 0.58 + 0.54 + 0.61 =1.73(V)
• The current through the cells is restricted by the smallest
current produce by any of the cells
• I12 = 0.25 (A)
Example: Cells Parallel Connected
Cell A
V = 0.58 V
I = 0.28 A
Blocks
increase
both current
and voltage
output
Example: Cells Series Connected
• Series connections are made by connecting one cell’s ntype contact to the p-type of the next cell
• Parallel connections are made by joining each cells ntype contacts together and p-type contacts together
• Series connections the voltages add
• Parallel connections the current add
• Series connections the current flow is equal to the
current from the cell generating the smallest current
(limited by poorest cell)
• Parallel connections the voltage is the average of the
cells or string in parallel
Montana State University: Solar Cells
Lecture 8: Characterization
Current
Solar Cell Panels
• Solar cells are not usually used individually because they
do not output sufficient voltage and power to meet typical
electrical demands
• The amount of voltage and current they output can be
increased by combining cells together with wires to
produce larger area solar modules
• Cells can be connected in a number of ways
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A
A
A
B
B
B
C
C
C
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• The voltage across terminals 56 given by the series
voltage already calculated:
• V56 = VA + VB + VC = 0.58 + 0.54 + 0.61 =1.73(V)
• The current at the terminals 56 is the sum of the
currents in each string already calculated
• I56 = 3(Istring) = 3(0.25) = 0.75(A)
Montana State University: Solar Cells
Lecture 8: Characterization
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Summary Linking Cells
• Linking modules or batteries is similar to
connecting PV cells
– Series Connections
• Voltages are added in series connections
• The current is restricted to the smallest current
– Parallel connections
• The currents are added in parallel connections
• The voltages are averaged from each string
• Solar Cells and Modules are Matched to
improve the power generated
Montana State University: Solar Cells
Lecture 8: Characterization
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