EE580 – Solar Cells
Todd J. Kaiser
• Lecture 08
• Solar Cell Characterization
Montana State University: Solar Cells
Lecture 8: Characterization
1
Solar Cell Operation
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
them.
Antireflection
coating
Front Contact
-
External Load
Montana State University: Solar Cells
Lecture 8: Characterization
The electron after
passing through
the load
recombines with
+ the hole
completing the
circuit
2
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
Montana State University: Solar Cells
Lecture 8: Characterization
3
Solar
Input
Circuit Diagram
Front Contact
I
Rs
Recombination
I
Current
Source
Ohmic Flow
V
Rsh
Rear Contact
Montana State University: Solar Cells
Lecture 8: Characterization
RLoad
External
Load
4
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
5
Power & IV Curve
• 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
Montana State University: Solar Cells
Lecture 8: Characterization
6
Solar Cell I-V Characteristics
Current
 qV

kT
I  I 0  e  1  I L


Dark
Current from Absorption of Photons
Voltage
Light
Twice the Light = Twice the Current
Montana State University: Solar Cells
Lecture 8: Characterization
7
Operating Point
I (mA)
V oc
0.1
I
0.2
0.3
0.4
0
I-V for a solar cell
under an illumination
of 700 W m-2
100
Slope = - 1/R
Operating point
I
I
R
(a)
0.6
V
V
I
V
0.5
Isc = Iph
200
P
(b)
Montana State University: Solar Cells
Lecture 8: Characterization
The load line for
R=3
(I-V for the load)
8
Short
Circuit
Current
IV Plot
Maximum
Power
Imp
FF 
Current
Operating
Point
I mpVmp
I scVoc
Power
Voltage (V)
Vmp
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
Montana State University: Solar Cells
Lecture 8: Characterization
Open
Circuit
Voltage
9
Resistance Effects
ISC
ISC
Current
Current
Increasing
RS
Voltage
decreasing
RP
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
Montana State University: Solar Cells
Lecture 8: Characterization
10
Loss Mechanisms in Solar Cells
Loss
Optical
Electrical
Ohmic
Reflection
Shadowing
Unabsorbed Radiation
Solar Cell Material
Base
Emitter
Contact Material
Finger
Collection Bus
Junction
Metal – Solar Cell
Montana State University: Solar Cells
Lecture 8: Characterization
Recombination
Emitter Region
Solar Cell Material
Surface
Base Region
Solar Cell Material
Surface
Space Charge Region
11
Photovoltaic Effect
Separation of holes and
electrons by Electric
Field
Absorption of
Light
Excitation
of
electrons
Voltage
Creation of
extra
electron hole
pairs (EHP)
Power = V x I
Current
Movement of charge
by Electric Field
Montana State University: Solar Cells
Lecture 8: Characterization
12
Linking Cells
• 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
– Strings – where cells are connected in series
– Blocks 2 or more strings connected together in parallel
– Joining 2 or more blocks together
Montana State University: Solar Cells
Lecture 8: Characterization
13
Parallel
connections
increase the
current output
Current
Current
Solar Cell Panels
Voltage
Voltage
Current
Series
connections
increase the
voltage output
Voltage
Montana State University: Solar Cells
Lecture 8: Characterization
Blocks
increase
both current
and voltage
output
14
Calculating Voltage and Current
• 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
15
Example: Cells Series Connected
1
Cell A
V = 0.58 V
I = 0.28 A
Cell B
V = 0.54 V
I = 0.31 A
Cell C
V = 0.61 V
I = 0.25 A
2
• 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)
Montana State University: Solar Cells
Lecture 8: Characterization
16
Example: Cells Parallel Connected
3
Cell A
V = 0.58 V
I = 0.28 A
Cell B
V = 0.54 V
I = 0.31 A
Cell C
V = 0.61 V
I = 0.25 A
4
• 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
17
Example: Block Connected
5
A
A
A
B
B
B
C
C
C
6
• 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
18
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
19