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• 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 diodeA 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 RoomAir Mass 1.5 filter installedShutter control

UV Lamp Housing

Cooling fan must be on UV Lamp Power

SupplyNot shown

UV intensity meter and

calibration solar cell

Procedure for Sunlight Simulator

• Verify the stage is connected to GND• Verify top contact probe R1 is connected to

SMU1

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 Keithley 4200 SCS.

Keithley 4200 SCS tutorial

• 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)

PT

PMAX

ISC

IMP

VMP VOC

Load Line

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

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.

Saturation current, Io

Specified voltage point for leakage current measurement

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.

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

V1

V2ISC1

ISC2

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

Dark current plot (no sunlight)

Current plot with 1 sun illuminating the solar cell

Zero voltage, zero current

point

Max photocurrent

Max photocurrent about 160mA

Vco and Isc

Voc – where I = 0About 0.5v

Isc – where V = 0 About 150mA

Jsc = (Isc)/area of cell in cm2

For Pmax point

2) Draw a vertical line from Voc

1) Draw a horizontal line from Isc

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 VIFF = .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