Reference Cell Calibration

The National Renewable Energy Laboratory (NREL) calibrates primary reference cells for in-house use and for use by other national laboratories. We also do so to provide our clients and partners with a path for traceability to standards. Our laboratory is one of only four facilities in the world certified to calibrate reference cells in accordance with the world photovoltaic scale, and these measurements are accredited to International Organization for Standardization (ISO) 17025 standards. We are the only laboratory certified to calibrate primary reference cells, secondary reference cells, cells, and modules.

Most of the cells we select for calibration are obtained from organizations that have established reputations for making cells for reference, and whose cells have a history of high quality and stability. When we make a reference cell for calibration, we carefully choose materials and structures. We then make the cell in accordance with stringent procedures that ensure quality and stability. Once we make or obtain a reference cell, we subject it to carefully devised calibration procedures that minimize errors due to measurement errors from spectral correction.

As part of the reference cell certification process, it is necessary to determine that the reference detector responds linearly with total irradiance. We determine the linearity of the short-circuit current (Isc) versus the total irradiance (Etot) by illuminating a prospective reference cell with two lamps. The irradiance range is about 0.01-sun to several suns. A device is linear, if the current measured with both lamps illuminating the cell is the same as the sum of the currents with each lamp illuminating the cell. The two-lamp method is insensitive to the light spectra or spatial nonuniformity changing with irradiance.

To calibrate the cells, we concurrently measure short-circuit current, total irradiance, and spectral irradiance outdoors with the same field of view (5.0°). Total irradiance is measured with an Eppley HF primary absolute cavity radiometer. Spectral irradiance is measured with a LICOR LI-1800 spectroradiometer. From these measurements, we calculate an average uncorrected calibration value, which relates the cell's short-circuit current to total irradiance. The atmospheric parameters and cell temperature are also measured. Once a valid calibration value is obtained, the short-circuit current is corrected for temperature and spectrum to the standard conditions.

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