SOLAR CELL TESTING

1
(No Transcript)
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
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
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 I0) 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 V0) 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

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
• ? 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.

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)

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.