Device Performance Capabilities
Our Photovoltaic Device Performance group designs, develops, and implements the hardware, software, and procedures to measure current vs voltage (I-V) characteristics as a function of temperature, spectrum, and intensity for single- and multijunction cells and modules.
We use I-V measurement systems to assess the main performance parameters for 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 average about 200 cell calibrations and 250 module measurements each month. And every year, we provide consulting to hundreds of entities—individuals and groups internal to NREL, as well as the photovoltaic (PV) community at large—in the area of PV measurements and solar simulation.
Our capabilities include the use of various solar simulators and tools to measure I-V and quantum efficiency. Measurements are on cells, as well as flat-plate and concentrator modules on sun-tracking systems. We also calibrate primary reference cells for the PV community, certify efficiencies, and support standards development.
All I-V systems use data acquisition systems and custom software for accurate standardized PV measurements. This includes algorithms developed by our 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 6½-digit multimeters or high-speed analog-to-digital (A/D) converters for measuring voltage, current, and intensity; this allows us to correct the current for intensity fluctuations. The systems all have multiple current ranges, allowing a wide variety of commercial and prototype cells and modules to be measured routinely.
The combined systems capabilities allow us to accurately measure the characteristics of any conceivable PV 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.