Transcript Solar Cell Testing using the Keithley 4200 revised 10-11-2013

BASIC SOLAR CELL TESTING
Basic Structure of a Solar Cell
Basic Photovoltaic Cell Model
This model consists of
• Built-in voltage
• Current due to optical generation
• Series resistance
• Shunt Resistance
Key Parameters
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Open Circuit Voltage, Voc (V)
Short Circuit Current Isc (A)
Short Circuit Current Density, Jsc (mA/cm2)
Max Power Voltage, Vmp
Max Power Current, Imp
Max photo current
Fill Factor, FF
Efficiency, 
Diode Ideality Factor
Shunt Resistance
Series Resistance
Reverse Saturation Current or Leakage Current
A Solar cell is a diode
A solar cell is a diode and hence an IV curve of a solar cell under dark
conditions will look similar to that of a diode.
When illuminated, the photons interact with the material to generate electron
hole pairs, which are then driven in opposite directions by the built-in potential.
Standard Test Conditions
• AM 1.5G
• Temperature = 25C
• Important device characteristics can be obtained from the I-V
measurements.
Sunlight Simulator in Clean Room
UV Lamp Housing
Shutter control
Cooling fan
must be on
UV intensity meter
and calibration
solar cell
Air Mass 1.5 filter installed
UV Lamp Power
Supply
Not shown
Procedure for Sunlight Simulator
• Verify the stage is connected to GND
• Verify top contact probe R1 is connected to
SMU1
Procedure for Sunlight Simulator
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Make sure all fans are working
Turn ON the lamp and wait for a few minutes for it to stabilize.
Open shutter (Remember to wear safety goggles)
Using the calibration cell and the sun meter, adjust the power
supply at about 970W or 1 Sun on the sun meter.
• Replace the calibration cell with the test sample
• Make top and bottom connections to the Keithley 4200 SCS.
Keithley 4200 SCS tutorial
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Start KITE.
Select vfd in diode section. (1)
Set Anode to SMU1 and Cathode to GNDU. (2)
Click on Force/Measure button to change
sweep parameter. (3)
• Click Run Test/Plan button to start sweep. (4)
• Click Append button to do another sweep and
append the data to the previous sweep. (5)
Keithley 4200 SCS tutorial
Keithley 4200 SCS tutorial
• To save measured data, select the data tab (6)
and click save as button on the right (7).
• To view the graph, select the graph tab (8).
• To save the graph, right click on the graph and
select save as.
Keithley 4200 SCS tutorial
Open Circuit Voltage, Voc (V)
• In an ideal solar cell, Voc is independent of the illumination
intensity.
• The open circuit voltage (Voc) occurs when there is no current
passing through the cell.
V (at I=0) = Voc
• To read the open circuit voltage from the graph, locate the
point on the voltage axis where the current is zero.
Short Circuit Current Density, Jsc
• The short circuit current Isc corresponds to the short circuit
condition when the impedance is low and is calculated when
the voltage equals 0.
I (at V=0) = Isc
• To read the short circuit current from the graph, locate the
point on the current axis where the voltage is zero.
• Divide this current by the area of the solar cell under test, to
obtain the current density, Jsc (mA/cm2)
Max Power Point
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Draw a rectangle with the
origin, VOC and ISC as the 3
corners. The 4th corner will give
the maximum theoretical
power, PT.
VMP
IMP
ISC
VOC
PMAX
PT
Load Line
• From the origin, draw a line
passing through the maximum
theoretical power, PT. This is
the load line
The point where the load line
crosses the I-V curve is the
maximum power point, PMAX
for the solar cell, for a given
load, with maximum current
and maximum voltage.
Max Power Point
• The voltage at the maximum power point of the cell is the
maximum voltage, VMP.
• The current at the maximum power point of the cell is the
maximum current, IMP
• From the maximum power point, PMAX , draw a line
perpendicular to and meet the voltage axis. The maximum
power voltage, VMP is given by the value on the voltage axis.
The maximum power current, IMP is given by the value on the
current axis.
Fill Factor
• Fill Factor is the measure of the quality of the solar cell. It is the
ratio of the maximum power, Pmax to the theoretical power, PT.
FF = PMAX/PT
FF = IMP . VMP/ Isc . Voc
• Fill Factor is a number between 0.0 and 1.0. The higher the
number, the better the solar cell
Efficiency
• Efficiency is the ratio of the electrical power
output POUT, compared to the solar power input,
PIN, into the PV cell
η = POUT/PIN
POUT = PMAX (W/m2) = (Isc)(Voc)
For Standard Test Conditions
PIN = 1000 (W/m2)= 100(mW/cm2)
Reverse Saturation Current
• The saturation current I0, is the current that flows in the
reverse direction when the diode is reverse biased. It is also
called as the leakage current.
Specified voltage point for leakage
current measurement
Saturation
current, Io
Shunt Resistance
• Shunt resistance is the change in the voltage for change in the unit current
and is ideally equal to infinity.
Series Resistance
• Series resistance is due to
– Resistance of the metal contacts
– Ohmic losses in the front surface of the cell
– Impurity concentrations
– Junction depth
• Series resistance reduces both short circuit current and
maximum power output of the cell
Series Resistance
• For the measurement of internal series resistance, 2 I-V
curves of different irradiance but of the same spectrum and at
the same temperature are necessary.
V1
ISC1
ISC2
The series resistance is calculated as:
RS = (V2-V1)/(ISC1 – ISC2)
V2
Diode Ideality Factor
• The diode ideality factor n, is an indicator of the behavioral
proximity of the device under test, to an ideal diode.
•n is between 1 and 2, ideally equal to 1.
Alternate view of I/V Plot
Current plot with 1 sun
illuminating the solar cell
Dark current plot
(no sunlight)
Zero voltage,
zero current
point
Max photocurrent
Max photocurrent
about 160mA
Vco and Isc
Isc – where V = 0
About 150mA
Voc – where I = 0
About 0.5v
Jsc = (Isc)/area of cell in cm2
1) Draw a
horizontal line
from Isc
For Pmax point
2) Draw a vertical
line from Voc
3) Draw a line from
origin to where they
intersect
For Pmax point
Pmax - Max Power point
Imax = 75mA
Vmax = .35v
Fill Factor
• Fill Factor is a number between 0.0 and 1.0, the higher the
number, the better the solar cell
• Fill Factor is the ratio of the maximum power, Pmax to the
theoretical power, PT.
FF = PMAX/PT
FF = IMP . VMP/ Isc . Voc
• For the previous sample:
FF = (.075A)(.35V)/(.150A)(.50V)
FF = .02625 VI/.0750 VI
FF = .35 (unit less)
Efficiency
• Assume for previous example, area of cell = 6 cm2
η = POUT/PIN
POUT = PMAX (W/m2) = (Isc)(Voc)
PMAX = (.150A)(.0.5V) = .075W = 75mW for 6cm2
PMAX = 12.5mW/cm2
For Standard Test Conditions
PIN = 1000 (W/m2)= 100(mW/cm2)
n = 12.5 mW/cm2/100mW/cm2
n = 12.5%
Assignment on I/V plots due next
class
There will be a test on solar
cell I/V plot analysis
Alternate view of I/V Plot