Part III – I-V Characterization of Photovoltaic Cells using PXI

This article is the third in a series of 3 tutorials on assessing the performance of photovoltaic cells through I-V characterization. This series includes an overview of PV cells, and describes the theory behind I-V characterization. The tutorials also include an example setup using National Instruments hardware and a free downloadable library of LabVIEW code for performing the I-V analysis. The other two articles in this series are:

• Part I – Photovoltaic Cell Overview
• Part II – Photovoltaic Cell I-V Characterization Theory and LabVIEW Analysis Code

Testing Photovoltaic Cells

Two simple tests can be performed in order to obtain all of the necessary data for solar cell I-V characterization.  These tests are the Forward-Bias (Illuminated) test and the Reverse-Bias (Dark) Tests, and can both be done with Source Measure Unit (SMU) that can both sink and source current and voltage when connected to a solar panel.

Forward Bias (Illuminated) Test

The forward test gives the illuminated I-V curve for a PV cell as described in the above section.  In a light test, an I-V sweep is conducted where the voltage is swept upwards starting at V=0, whilst measuring the sinking current.  The following values can be calculated using the forward-bias (illuminated) Test:

• Open Circuit Voltage (V_OC)
• Short Circuit Current (I_SC)
• Maximum Power (P_MAX), Current at P_MAX (I_MP), Voltage at P_MAX (V _MP)
• Fill Factor (FF)
• Shunt Resistance (R_SH)
• Series Resistance (R_S)
• Maximum Efficiency (η_MAX)

Reverse-Bias (Dark) Test

By blocking all light to prevent it from exciting a PV cell, the cell can be tested as a passive diode element to determine its breakdown diode properties and internal resistances.  The I-V parameters that can be obtained via the reverse-bias (dark) test are:

• Shunt Resistance (R_SH)
• Series Resistance (R_S)

 Ambient condition considerations:

Many of the I-V analysis parameters, including the maximum efficiency (η_MAX), are affected by ambient conditions such as temperature and the intensity and spectrum of the incident light.  For this reason, it is recommended to test and compare PV cells using similar lighting and temperature conditions. 

PV cells to be used in space are typically tested under Air Mass 0 (AM0) conditions, while terrestrial cells are tested using the Air Mass 1.5 (AM1.5) standard.  Also, cells are typically tested at a temperature of 25°C and with a light intensity of 1 sun (1000 W/m²).

Typical I-V Test System

Let us now look at a typical setup for performing I-V characterization.  This system is based on virtual instrumentation technologies developed by National Instruments.  NI graphical system design and virtual instrumentation technologies empower researchers to focus on innovation by simplifying device I-V performance characterization through high level software development tools such as NI LabVIEW, and tightly integrated, easy to use hardware modules such as the NI PXI-4130 Power SMU.

Figure 1 depicts an example test system for I-V characterization. 

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Figure 1 - Example of a Test System for I-V Characterization

First, a light source provides incident radiation to excite the solar panel.  Next, a Source Measure Unit (SMU) such as the PXI 4130 SMU is used to sweep the voltage and measure the current from the PV cell or module.  Additionally, several sensors are required to measure the ambient conditions in which the test is conducted.  A light intensity meter, or pyranometer, is used to measure the irradiance of the incident light and a temperature sensor (thermocouple, thermistor, or RTD) is needed to obtain the temperature at which the test is conducted.  A data acquisition system with analog input capabilities, such as NI M-Series DAQ, is then used to acquire these sensor measurements and, along with the SMU, interface with a computer.  Software such as NI LabVIEW is used to acquire, analyze and display the results of the I-V characterization tests, and assess the main performance parameters for the solar panel.

A cooling system can be added to counter balance the heating from the light source by maintaining the ambient temperature and the temperature of the solar cell.

Source Measure Unit (SMU) for I-V Characterization

A Source Measure Unit (SMU) is a precision power sourcing instrument that provides voltage and current sourcing and measurement resolution at or below 1 mV and 1 µA, respectively. In addition, SMUs feature a 4-quadrant output that can both source and sink current and voltage for testing either forward or reverse PV characteristics.  An example of an SMU is the NI PXI-4130 Power SMU. 

Figure 2 - NI PXI-4130 Power SMU

With the NI PXI-4130 Power SMU (Figure 2), you can perform sweeps of both current and voltage to determine the IV characteristics of photovoltaic cells or modules and diodes. This module can sink up to 10 W and has isolated outputs capable of sourcing upto ±20 V and 2 A.  It also has a 1 nA current measurement resolution rating, and provides software selectable output capacitance to enhance stability.  NI source measure units ship with ready-to-run LabVIEW example programs for performing IV sweeps.

It is possible to cascade multiple NI PXI-4130 Power SMU units in series to attain larger voltages when sourcing power (for reverse-bias tests).

Other test system options:

In place of the NI PXI-4130 Power SMU, other third-party SMUs with a GPIB interface may be used that meet required specifications for PV testing. Drivers for these third-party instruments specifically created for NI software can be found at the Instrument Driver Network portal. 

Summary

This section gave an example of a test system to characterize the I-V response of photovoltaic (solar) cells.  More information about photovoltaic cells and I-V characterization theory can be found in the previous two parts of this tutorial:

• Part I – Photovoltaic Cell Overview
• Part II – Photovoltaic Cell I-V Characterization Theory and LabVIEW Analysis Code

 

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