2. SOLAR CELL TESTING

3. Basic Structure of a Solar Cell

4. Basic Photovoltaic Cell Model
This model consists of
Built-in voltage
Current due to optical generation
Series resistance
Shunt Resistance

5. Key Parameters 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

6. 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.

7. Standard Test Conditions
AM 1.5G
Temperature = 25C
Important device characteristics can be obtained from the I-V measurements.

8. Sunlight Simulator Shutter control Status Lamp Air Mass filter housing
Lamp Power Supply
Calibration solar cell
Calibration sun meter
Wafer vacuum chuck -
backside electrical contact
Electrical probe – top side
electrical contact
Air Mass filter housing

9. 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

10. Tektronix 370B Programmable Curve Tracer
Power ON/OFF button
Signal Output Controls
Collector Supply Controls
Collector Supply Configuration
Collector Supply Voltage

11. 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.

12. 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)

13. 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.
From the origin, draw a line passing through the maximum theoretical power, PT. This is the load line PT
PMAX
ISC
IMP
VMP
VOC
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.

14. 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.

15. 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

16. 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)

17. 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

18. Shunt Resistance
Shunt resistance is the change in the voltage for change in the unit current and is ideally equal to infinity.

19. 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

20. 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.
The series resistance is calculated as:
RS = (V2-V1)/(ISC1 – ISC2) V1 V2
ISC1
ISC2

21. 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.