1. EE580 – Solar Cells Todd J. Kaiser
Lecture 08
Solar Cell Characterization
Montana State University: Solar Cells Lecture 8: Characterization

2. Montana State University: Solar Cells Lecture 8: Characterization
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
The electron after passing through the load recombines with the hole completing the circuit
Front Contact - +
External Load
Montana State University: Solar Cells Lecture 8: Characterization 2

3. 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

4. Montana State University: Solar Cells Lecture 8: Characterization
Input
Circuit Diagram
Front Contact I Rs
Recombination
Ohmic Flow
RLoad
Rsh I
Current
Source V
External
Load
Rear Contact
Montana State University: Solar Cells Lecture 8: Characterization

5. Montana State University: Solar Cells Lecture 8: Characterization
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

6. Montana State University: Solar Cells Lecture 8: Characterization
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

7. Solar Cell I-V Characteristics
Current
Dark
Current from Absorption of Photons
Voltage
Light
Twice the Light = Twice the Current
Montana State University: Solar Cells Lecture 8: Characterization

8. Montana State University: Solar Cells Lecture 8: Characterization
Operating Point
Montana State University: Solar Cells Lecture 8: Characterization

9. Montana State University: Solar Cells Lecture 8: Characterization
IV Plot
Short Circuit Current
Maximum Power
Imp
Operating Point
Current
Power
Vmp
Voltage (V)
Open Circuit Voltage
Volt (V)
0.1
0.3
0.5
0.54
0.57
Current (A)
1.3
1.2
0.75
Power (W)
0.13
0.39
0.6
0.4
Montana State University: Solar Cells Lecture 8: Characterization

10. Montana State University: Solar Cells Lecture 8: Characterization
Resistance Effects
ISC
ISC
Current
Current
Increasing RS
decreasing RP
VOC
VOC
Voltage
Voltage
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

11. Loss Mechanisms in Solar Cells
Optical
Electrical
Ohmic
Recombination
Reflection
Shadowing
Unabsorbed Radiation
Solar Cell Material
Base
Emitter
Contact Material
Finger
Collection Bus
Junction
Metal – Solar Cell
Emitter Region
Solar Cell Material
Surface
Base Region
Space Charge Region
Montana State University: Solar Cells Lecture 8: Characterization

12. Photovoltaic Effect
Separation of holes and electrons by Electric Field
Voltage
Creation of extra electron hole pairs (EHP)
Absorption of Light
Power = V x I
Excitation of electrons
Current
Movement of charge by Electric Field
Montana State University: Solar Cells Lecture 8: Characterization

13. Montana State University: Solar Cells Lecture 8: Characterization
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

14. Montana State University: Solar Cells Lecture 8: Characterization
Solar Cell Panels
Current
Current
Parallel connections increase the current output
Voltage
Voltage
Blocks increase both current and voltage output
Series connections increase the voltage output
Current
Voltage
Montana State University: Solar Cells Lecture 8: Characterization 14

15. Calculating Voltage and Current
Series connections are made by connecting one cell’s n-type contact to the p-type of the next cell
Parallel connections are made by joining each cells n-type 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

16. Example: Cells Series Connected
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 1 2
The voltage across terminals 12 is the sum of the voltages
V12 = VA + VB + VC = =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

17. Example: Cells Parallel Connected3
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 = ( )/3 = 0.58(V)
The current at the terminals 34 is the sum of the currents in each cell
I34 = (IA + IB +IC) = ( ) = 0.84(A)
Montana State University: Solar Cells Lecture 8: Characterization

18. Example: Block Connected5 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 = =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

19. Montana State University: Solar Cells Lecture 8: Characterization
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