A solar simulator (also artificial sun or sunlight simulator) is a device that provides illumination approximating natural sunlight. The purpose of the solar simulator is to provide a controllable indoor test facility under laboratory conditions. It can be used for the testing of any processes or materials that are photosensitive, including solar cells, sun screen, cosmetics, plastics, aerospace materials, skin cancer, bioluminescence, photosynthesis, water treatment, crude-oil degradation, and free radical formation. Solar simulators are used in a wide range of research areas including photobiology, photo-oxidation, photodegradation, photovoltaics, and photocatalysis.
Classification The standards specifying performance requirements of solar simulators used in photovoltaic testing are IEC 60904-9, ASTM E927-19, and JIS C 8912. These standards specify the following dimensions of control for light from a solar simulator:
spectral content (quantified as spectral match) spatial uniformity temporal stability Spectral Coverage (SPC) (IEC 60904-9:2020 only) Spectral Deviation (SPD) (IEC 60904-9:2020 only) A solar simulator is specified according to its performance in the first three of the above dimensions, each in one of three classes: A, B, or C. (A fourth classification, A+, was introduced by the 2020 edition of IEC 60904-9 and only applies for solar simulators evaluated in the spectral range of 300 nm to 1200 nm.) For ASTM E927-19, if a solar simulator falls outside the A, B, C criteria, it is considered Class U (unclassified). Although these standards were originally defined specifically for photovoltaic testing, the metrics they introduced have become a common way of specifying solar simulators more broadly in other applications and industries. The ASTM E927-19 specifications required for each class and dimension are defined in Table 1 below. A solar simulator meeting class A specifications in all three dimensions is referred to as a Class AAA solar simulator (referring to the first three dimensions listed above).
The ASTM E927-19 standard specifies that whenever this triple-letter format is used to describe a solar simulator, it needs to be made clear which classification applies to each solar simulator metric (e.g. a Class ABA solar simulator needs to make clear which parameter(s) are Class A vs. B). The IEC 60904-9 standard specifies that the three letters must be in order of spectral match, non-uniformity, and temporal instability.
Spectral match A solar simulator's spectral match is computed by comparing its output spectrum to the integrated irradiance in several wavelength intervals. The reference percentage of total irradiance is shown below in Table 2 for the standard terrestrial spectra of AM1.5G and AM1.5D, and the extraterrestrial spectrum, AM0. Below is a plot of these two spectra.
A solar simulator's spectral match ratio, R S M {\displaystyle R_{SM}} (i.e. ratio of spectral match), is its percentage output irradiance divided by that of the reference spectrum in that wavelength interval. For example, if a solar simulator emits 17.8 percent of its total irradiance in the 400 nm–500 nm range, it would have a R S M {\displaystyle R_{SM}} in that wavelength interval of 0.98. If a solar simulator achieves a spectral match ratio R S M {\displaystyle R_{SM}} between 0.75 and 1.25 for all wavelength intervals, it is considered to have class A spectral match.
These wavelength intervals were primarily intended for the solar simulator application of testing silicon photovoltaics, hence the spectral range over which the intervals were defined was limited mainly to the originally-developed absorption region of crystalline silicon (400 nm–1100 nm). The solar simulator standards have some requirements for where the illumination spectrum must be measured. For example, the IEC 60904-9 standard requires that the spectrum be measured at four different locations in a pattern given below.
Recent material science developments have expanded the spectral responsivity range of c-Si, multi-c-Si and CIGS solar cells to 300 nm–1200 nm. Therefore, in 2020, the IEC 60904-9 standard introduced a new table of wavelength intervals (given in Table 3 below) aimed to match solar simulator output to the present needs of a wide variety of photovoltaic devices.
While the above definition of spectral range is adequate for addressing the testing needs of many photovoltaic technologies, including thin film solar cells constructed from CdTe or CIGS, it is not sufficient for testing multi-junction solar cells using high-efficiency III-V semiconductors that have wider absorption bandwidths from 300–1800 nm. For accurate spectral data outside the above-mentioned ranges, the data tables in ASTM G173 (for AM1.5G and AM1.5D) and ASTM E490 (for AM0) can be used as reference, but the specifications of solar simulators do not yet apply to anything outside 300 nm to 1200 nm for AM1.5G, and 300 nm to 1400 nm for AM0. Many solar simulator manufacturers produce light outside these regions, but the classification of light in these external regions is not yet standardized.
Spatial non-uniformity A solar simulator's spatial non-uniformity is computed via the following equation, with the result being a percentage:
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