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I-V characterization of solar cells and panels using a source-measure unit.

The focus for researchers and manufacturers of solar or photovoltaic cells and panels is finding ways to improve cell efficiency and maximize energy extraction. To help achieve these goals, electrical characterization of the cell as well as photovoltaic materials is an important part of research and development as well as during the manufacturing process.

Current-voltage (I-V) characterization of a cell provides insight into important parameters about a cell's performance, including its maximum current ([I.sub.max]) and voltage ([V.sub.max]), open-circuit voltage ([V.sub.OC]), short-circuit current ([I.sub.SC]), and efficiency ([eta]). These I-V characteristics can easily be generated using a source-measure unit (SMU), a test instrument that can source and measure both current and voltage. Because SMUs have four-quadrant source capability, they can sink the cell current as a function of an applied voltage.

Figure 1 shows the use of an SMU instrument for I-V characterization of an illuminated solar cell. Since current only starts to flow when a load is connected to the output of an illuminated solar cell, the SMU is serving as the load in this configuration. As a result, the measured current is negative.

The solar cell

A solar cell may be represented by the equivalent circuit model shown in Figure 2, which consists of a light-induced current source ([I.sub.L]), a diode that generates a saturation current, series resistance ([r.sub.s]), and shunt resistance ([r.sub.sh]). The series resistance is due to the resistance of the metal contacts, ohmic losses in the front surface of the cell, impurity concentrations, and junction depth. The series resistance is an important parameter because it reduces both the cell's short-circuit current and its maximum power output. Ideally, the series resistance should be 0 ([r.sub.s] = 0). The shunt resistance represents the loss due to surface leakage along the edge of the cell or to crystal defects. Ideally, the shunt resistance should be infinite ([r.sub.sh] = [infinity]).

If a load resistor ([R.sub.L]) is connected to an illuminated solar cell, then the total current becomes

I = [I.sub.s] ([e.sub.qV]/kT])-1) - [I.sub.L]

where

[I.sub.s] = current due to diode saturation

[I.sub.L] = current due to optical generation

As noted previously, several parameters are used to characterize the efficiency of the solar cell, including the maximum power point ([P.sub.max]), the energy conversion efficiency ([eta]), and the fill factor (FF).

These points are shown in Figure 3, which shows a typical forward bias I-V curve of an illuminated solar cell. The maximum power point ([P.sub.max]) is the product of the maximum cell current ([I.sub.max]) and the voltage ([V.sub.max]) where the power output of the cell is greatest. This point is located at the "knee" of the curve.

The fill factor (FF) is a measure of how far the I-V characteristics of an actual solar cell differ from those of an ideal cell. The fill factor is defined as

FF = [I.sub.max] [V.sub.max]/[I.sub.SC] [V.sub.OC]

As defined, the fill factor is the ratio of the maximum power ([P.sub.max] = [I.sub.max] [V.sub.max]) to the product of the short circuit current ([I.sub.SC]) and the open circuit voltage ([V.sub.OC]). The ideal solar cell has a fill factor equal to one but losses from series and shunt resistance decrease efficiency.

Another important parameter is the conversion efficiency, which is defined as the ratio of the maximum power output to the power input to the cell:

[eta] = [P.sub.max]/[P.sub.in]

where

[P.sub.max] = the maximum power output (W)

[P.sub.in] = the power input to the cell defined as the total radiant energy on the surface of the cell (W)

Forward-biased I-V measurements

Forward-biased I-V measurements of the solar cell are made under controlled illumination. The SMU is set up to output a voltage sweep and measure the resulting current. For this measurement, the voltage source is swept from [V.sub.1] = 0 to [V.sub.2] = [V.sub.OC] - When the voltage source is 0 ([V.sub.1] = 0), the current is equal to the short-circuit current ([I.sub.1] = [I.sub.SC]). When the voltage source is an open circuit ([V.sub.2] = [V.sub.OC])/ then the current is equal to zero ([I.sub.2] = 0). The parameters [V.sub.OC] and [I.sub.SC] can in turn be derived from the sweep data. Figure 4 shows the I-V curve of an illuminated PV panel generated by an SMU.

Conversion efficiency ([eta]) can be calculated if the power input to the cell is known and current density (J) can be derived based on the area of the cell.

Reverse-biased I-V measurements

The leakage current and shunt resistance can be derived from the reverse-biased I-V data. Typically, the test is performed in the dark. The voltage is sourced from 0 V to a voltage level where the device begins to break down. The resulting current is measured as a function of the voltage.

Depending on the size of the cell, the leakage current can be as small as picoamps. When making these very sensitive low-current measurements (nanoamps or less), you may need to use low-noise cables and place the device in a shielded enclosure to shield it electrostatically. One method for determining the shunt resistance of the PV cell is from the slope of the reverse-biased I-V curve, as shown in Figure 5.

A solar cell or panel is connected to an SMU as shown in Figure 6. A four-wire connection is made to eliminate the effects of the lead resistance. When connecting the leads to the solar cell, the Force LO and Sense LO connections are made to the cathode terminal. The Force HI and Sense HI connections are made to the anode. The connections should be made as close as possible to the cell to prevent the resistance of the solar cell's terminals from affecting measurement accuracy.

Automating I-V measurements

Many SMUs can be remotely controlled using test scripts through a LAN, USB, or GPIB interface. In this example, automated I-V characteristics testing was performed on a polycrystalline silicon solar panel. The SMU was programmed to sweep voltage from 0 V to 20 V in 115 steps and to measure the resulting current in a four-wire configuration. The results of graphing the I-V characteristics of this cell are shown in Figure 7. Notice the test on the solar panel was executed with light (Light ON) and in the dark (Light OFF). As previously discussed, the measured current in the "Light ON" graph is negative because the SMU is sinking current. If desired, the curve can easily be inverted in the spreadsheet.

In addition to automating I-V measurements, some SMUs feature a user-customizable interface to display the derived maximum power, short-circuit current, open-circuit voltage, or other user-derived calculations on a single screen (Figure 8) for faster and easier analysis.

Conclusion

Measuring the electrical characteristics of a solar cell is critical for determining the device's output performance and efficiency. Key measurements include maximum current and voltage, opencircuit voltage, short-circuit current, and efficiency. SMUs can simplify solar cell testing by automating the I-V measurements and pulling together results for easy display and analysis.

Mary Anne Tupta is a senior applications engineer in the Keithley Instruments Product Line at Tektronix in Cleveland. She has a M.S. degree in physics and a B.S. degree physics/electronic engineering from John Carroll University. Tupta has been an applications engineer at Keithley since 1988. mtupta@keithley.com

By Mary Anne Tupta, Keithley Instruments Product Line, Tektronix

Caption: Figure 1. Circuit diagram showing an SMU connected to a solar cell for I-V characterization

Caption: Figure 2. Equivalent circuit of a solar or photovoltaic cell

Caption: Figure 3. Typical forward bias I-V characteristics of solar cell

Caption: Figure 4. Example solar panel I-V sweep generated on the graph screen of a Keithley Model 2460 SMU

Caption: Figure 5. Typical reverse-biased characteristics of a solar cell

Caption: Figure 6. Four-wire connections from an SMU to the solar cell or panel

Caption: Figure 7. Solar cell I-V sweeps generated by an SMU

Caption: Figure 8. SMU showing key solar cell measurements on a single screen, including maximum power, short-circuit current, and open-circuit voltage
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Title Annotation:SOLAR CELL TEST
Author:Tupta, Mary Anne
Publication:EE-Evaluation Engineering
Article Type:Report
Geographic Code:1USA
Date:Mar 1, 2017
Words:1393
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