Primary Reference Cell Calibration
We calibrate 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) vs 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 Isc, Etot, 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 an ASD Spectro-Pro spectroradiometer and is extended using a comprehensive spectral model derived from Modtran. From these measurements, we calculate an average corrected calibration value, which relates the cell's Isc to Etot. The atmospheric parameters and cell temperature are also measured. Once a valid calibration value is obtained, the Isc is corrected for temperature and spectrum to the standard conditions.
C.R. Osterwald, K.A. Emery, D.R. Myers, and C.J. Riordan, "Extending the Spectral Range of Silicon-Based Direct-Beam Solar Spectral Radiometric Measurements," Proc. 20th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, September 26–30, 1988, pp. 1246–1250, IEEE, New York, 1989.
C.R. Osterwald, K.A. Emery, D.R. Myers, and R.E. Hart, "Primary Reference Cell Calibrations at SERI: History and Methods," Proc. 21st IEEE Photovoltaic Specialists Conf., Orlando, FL, May 21–25, 1990, pp. 1062–1067, IEEE, New York, 1990.
C. Osterwald and K. Emery, "Spectroradiometric Sun Photometry," Journal of Atmospheric and Oceanic Technology 17, 1171–1188 (2000).