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Current Versus Voltage

The National Renewable Energy Laboratory (NREL) Device Performance group uses current versus voltage (I-V) measurement systems to assess the main performance parameters for photovoltaic (PV) cells and modules. I-V measurement systems determine the output performance of devices, including: open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF), maximum power output of the device (Pmax), voltage at maximum power (Vmax), current at maximum power (Imax), and conversion efficiency of the device (η). Some I-V systems may also be used to perform dark I-V measurements to determine diode properties and series and shunt resistances.

We use three I-V systems to measure performance of individual PV cells; and four for modules — two for measurements under simulated conditions; and two for measurements under outdoor conditions. All I-V measurements are made using 4-terminal Kelvin connections. This enables the use of separate channels for voltage and current measurements, which minimizes measurement errors by eliminating voltage-drop losses that could result from resistances due to cables, connections, and wiring.

All I-V systems use data acquisition systems and custom software for accurate standardized PV measurements. This includes algorithms developed by the group over the years for calculating I-V characteristics and, for most systems, for making the spectral-mismatch corrections. The custom I-V systems and flash simulators use separate meters for measuring voltage, current, and intensity; this allows us to correct the current for intensity fluctuations. The custom systems all have multiple current ranges, allowing dark I-V on cells and modules to be routinely measured.

The combined systems capabilities allow us to accurately measure the characteristics of any conceivable photovoltaic cell or module technology. The overlapping capabilities of many of the systems allow for cross checking and understanding the various error sources of the test beds.

For additional information contact Keith Emery, 303-384-6632.