For devices that come to the National Renewable Energy Laboratory (NREL) for performance measurement, we typically follow a procedure that ensures quality measurement and follow-up. After logging in the device based on information from a cover letter or request form, we measure its area, which is crucial for determining its efficiency. We then obtain its spectral responsivity. For cells, we measure the spectral responsivity with one of two systems. For modules, however, spectral responsivity is generally provided to us by the manufacturer, or we measure it on a representative cell.
We use the information on spectral responsivity to calculate the spectral mismatch between the test device and a primary reference cell for the simulator that will be used for the subsequent current versus voltage (I-V) measurement. This is an important step in the process that was developed by NREL and is now followed by everyone in the photovoltaic (PV) industry. Correction of the spectral mismatch is necessary, because the intensity of the light source in a simulator system is set with a reference cell; and the correction factor for the spectral mismatch enables measurement of the sample's performance with respect to a reference spectrum.
We measure the I-V characteristics of the cell or module under simulated conditions. For modules, we also measure I-V performance under natural sunlight, which enables us to determine module response under "real" conditions. After measurement, the results are translated to standard conditions. For concentrator cells, we measure the I-V characteristics as a function of light level. We can also adjust the spectrum to a limited extent for multijunction concentrators to make the current matching as close as possible to reference conditions.
After all measurements have been made, we carefully review the results for anomalies or procedural errors. Finally, we prepare a report for the client, which can be as simple as a presentation of data tables, or as involved as a document that contains description, analysis, data, and recommendations.
For additional information contact Keith Emery, 303-384-6632.