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

• • • • • • • • • • Open Circuit Voltage, V oc (V) Short Circuit Current Density, J sc Max Power Voltage, V mp Max Power Current, I mp Fill Factor, FF Efficiency,  Diode Ideality Factor, n Shunt Resistance (mA/cm 2 ) 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.

Shutter control Status Lamp Calibration solar cell Calibration sun meter Wafer vacuum chuck backside electrical contact

Sunlight Simulator

Air Mass filter housing 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 Collector Supply Voltage Signal Output Controls Power ON/OFF button

Open Circuit Voltage, V

oc

(V)

• In an ideal solar cell, V oc illumination intensity.

is independent of the • The open circuit voltage (V oc ) occurs when there is no current passing through the cell. V (at I=0) = V oc • 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, J

sc • The short circuit current I sc corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0.

I (at V=0) = I sc

• 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, J sc (mA/cm 2 )

Max Power Point

• Draw a rectangle with the origin, V OC and ISC as the 3 corners. The 4 th corner will give the maximum theoretical power, P T . • From the origin, draw a line passing through the maximum theoretical power, P T . This is the load line V MP V OC I MP I SC P MAX P T Load Line The point where the load line crosses the I-V curve is the maximum power point, P and maximum voltage.

MAX for the solar cell, for a given load, with maximum current

Max Power Point

• • The voltage at the maximum power point of the cell is the maximum voltage, V MP .

The current at the maximum power point of the cell is the maximum current, I MP • From the maximum power point, P perpendicular to and meet the voltage axis. The maximum power voltage, V MP MAX , draw a line is given by the value on the voltage axis. The maximum power current, V MP 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, P max to the theoretical power, P T .

FF = P MAX /P T FF = I MP . V MP / I sc . V oc

Efficiency

• Efficiency is the ratio of the electrical power output P OUT , compared to the solar power input, P IN , into the PV cell P OUT η = P OUT /P IN = P MAX For AM 1.5, P IN (W/m 2 ) = 1000 (W/m 2 )

Reverse Saturation Current

• The saturation current I 0 , 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, I o

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.

V 1 I SC1 I SC2 V 2 The series resistance is calculated as: R S = (V 2 -V 1 )/(I SC1 – I SC2 )

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.