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Frequently Asked Questions

Q1: How can I do business with NREL's Device Performance group?

See Working with Us.

Q2: What are the standard reporting or test conditions?

The International Electrotechnical Commission (IEC) uses the term STC to denote standard test conditions defined by a reference spectrum, with reference total irradiance measured at the reference temperature. For photovoltaics, this is ASTM G173 or IEC 60904-3 edition 3 for the global and direct reference spectra. ASTM E490 defines the currently accepted air-mass 0 (AM0) reference spectrum and solar constant for space testing in ISO/DIS standard 15387. Some groups and standards also use the term Standard Reference or Reporting Conditions to mean STC. Many researchers abbreviate the description of the reference spectrum and irradiance to "AM 1.5." The term "AM 1.5" is a geometrical quantity that refers to an optical air mass of 1.5 or zenith angle of 48.19 degrees and does not define a spectrum because the cloud cover, aerosols, water vapor, and other atmospheric absorbing and scattering parameters are unspecified.

Q3: What is the spectral error?

From IEC standard 61836, the spectral error is the "error introduced in the testing of a photovoltaic device caused by the interaction of the mismatch between the spectral responses of the test specimen and the reference device, and the mismatch between the test spectrum and the reference spectrum." This error can be applied as a correction factor before the measurements if the spectral responsivity of the test and reference device and the spectrum of the light source are known following ASTM E973 or IEC 60904-7 and first derived in the literature by Osterwald (1986). Recently, Osterwald (2015) demonstrated that a temperature-dependent spectral correction factor relaxes an incorrect assumption—that the photo-current temperature coefficient is independent of the spectrum it is measured or used under. The spectral error can be minimized by using a reference cell or module whose normalized spectral responsivity matches the normalized spectral responsivity of the device under test. Thermal detectors such as pyranometers are used for outdoor PV calibrations for historical reasons and are a poor spectral match to the responsivity of PV; see Meydbray (Part 1 and Part 2).

Q4: Where can I find more information on how to perform accurate testing?

Several books have entire chapters devoted to performance testing at the cell and module level. Several tutorials on the subject have been made into technical reports and are available.

Q5: Where can I find definitions of PV-related terminology?

Terminology for solar energy applications can be found in standards IEC standard 61836 and ASTM standard G113, ASTM E772, and ASTM E1328.

Q6: What is the uncertainty in measurements?

ISO has established a Guide to the Measurement of Uncertainty (GUM) that is the international standard method. The ISO GUM is required for ISO 17025 accreditation as a calibration lab.

Q7: What is the sample area?

Publications containing PV area definitions:

  • American Society for Testing and Materials (ASTM)
    • E948: Cells Using Reference Cells Under Simulated Sunlight
    • E1036: Standard Test Methods for Electrical Performance of Nonconcentrator Terrestrial Photovoltaic Modules and Arrays using Reference Cells
    • E1328: Terminology Relating to Photovoltaic Solar Energy Conversion
  • Institute of Electrical and Electronics Engineers (IEEE)
    • ANSI/IEEE 928: IEEE Recommended Criteria for Terrestrial Photovoltaic Power Systems
  • International Electrotechnical Commission (IEC)
    • 60904-1: Measurement of Photovoltaic Current-Voltage Characteristics
    • 60904-3: Measurement Principles for Terrestrial Photovoltaic (PV) Solar Devices with Reference Spectral Irradiance Data
  • ERDA/NASA
    • TM 73702: Terrestrial Photovoltaic Measurement Procedures, ERDA / NASA / 1022-77 / 16, June 1977.

K.A. Emery, "Solar Simulators and I-V Measurement Methods," Solar Cells 18, 251260 (1986).

M.A. Green and K.A. Emery, "Solar Cell Efficiency Tables," Progress in Photovoltaics Research and Applications 1, 2530 (1993).

Total area. This is the preferred area for reporting of results and equals the total projected area of the cell or module (the area that would be measured by taking a photograph of the device against a white background and measuring the area of the background shaded by the device). For the case of a cell attached to glass, the total area would be the area of the glass sheet.

Aperture area. In this measurement, the device under test is masked so that the illuminated area is smaller than the total cell or module area, but all essential components of the device, such as busbars, fingers and interconnects, lie within the masked area. (Masking is not required if it can be demonstrated that areas outside the aperture area are not responsive to light or are not steering light onto active areas.)

Designated illumination area. In this case, the cell or module is masked to an area smaller than the total device area, but major cell or module components lie outside the masked area. For example, for a concentrator cell, the cell busbars would lie outside of the area designated for illumination and this area classification would be the most appropriate. (Again, masking is not required if it can be demonstrated that areas outside the designated illumination area are not responding to light.)