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Transcript

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 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 in Clean Room UV Lamp Housing Shutter control Cooling fan must be on UV intensity meter and calibration solar cell Air Mass 1.5 filter installed UV Lamp Power Supply Not shown 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) 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, 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. Specified voltage point for leakage current measurement 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. Alternate view of I/V Plot Current plot with 1 sun illuminating the solar cell Dark current plot (no sunlight) Zero voltage, zero current point Max photocurrent Max photocurrent about 160mA Vco and Isc Isc – where V = 0 About 150mA Voc – where I = 0 About 0.5v Jsc = (Isc)/area of cell in cm2 1) Draw a horizontal line from Isc For Pmax point 2) Draw a vertical line from Voc 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 VI FF = .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