Download SOLAR CELL TESTING

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 Density, Jsc (mA/cm2)
Max Power Voltage, Vmp
Max Power Current, Imp
Fill Factor, FF
Efficiency, 
Diode Ideality Factor, n
Shunt Resistance
Series Resistance
Reverse Saturation Current or Leakage Current
Solar cell – 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
Shutter control
Status Lamp
Air Mass filter housing
Calibration solar cell
Calibration sun meter
Wafer vacuum chuck backside electrical contact
Electrical probe – top side
electrical contact
Lamp Power Supply
Procedure for Sunlight Simulator
• 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 Tektronix 370B
curve tracer.
• Obtain IV curve and measure different parameters from
the IV characteristics
Tektronix 370B
Programmable Curve Tracer
Collector Supply
Controls
Collector Supply
Configuration
Signal Output Controls
Collector Supply
Voltage
Power ON/OFF button
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
• 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, VMP 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
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)
For AM 1.5, PIN = 1000 (W/m2)
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.
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.