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Measurement Process and Helpful Resources

We describe the typical process of our Photovoltaic Device Performance group, as well as the best practices we follow to provide high-quality, unbiased, and trusted measurements. You can find FAQ answers to common questions relating to solar measurements. You can also explore our publications, which are categorized according to relevant measurements-related topics.

Measurement Process

For devices that come to 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 vs 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 simulated or natural sunlight source of light does not have the same spectral irradiance as the reference spectrum. 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 direct plus diffuse lighting 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 also adjust the spectrum of the simulators for multijunction cells to make the current matching for all subcells as close as possible to reference conditions. Typically, the spectrum is adjusted so that all subcells are within 1% of their target current.

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.

Contact us to discuss our device performance capabilities and how they might help your PV technology objectives.