Performance Metrics

Key Performance Metrics for Solar Simulators

Measuring solar simulator performance allows for comparisons between different systems and technologies. These performance metrics enable researchers and solar cell producers to find the simulator best suited to their needs.

Ranking Solar Simulators

Solar simulators are rated by several key metrics including their ability to match reference spectra (link to cell characteristic page), their spatial non-uniformity within the incident beam and their temporal stability. For terrestrial simulators, each rating is given a letter grade from A to C, with A indicating the best quality. A solar simulator with Class A in all three categories is called Class AAA. Space solar simulators require higher performance than terrestrial, improving on Class AAA performance and using calibrated isotype standards for spectral matching.

Spectral Match

Some solar simulators match the spectral output of the sun and overall output power while other solar simulators match performance to calibrated reference standards. Typically small-area and terrestrial, single-junction solar cells use the spectral match method, which ensures that a solar cell is detecting the correct spectral mix of sunlight. Space, multi-junction and large-area simulators use the calibrated reference standards method. This method ensures that the solar simulator is exciting the reference standards in a way that is consistent with the target spectrum and confirms that all the junctions of a multi-junction cell are producing the correct current.

Terrestrial class A spectral match denotes a simulator that is within +/- 25% of six (terrestrial) or eight (space) individual spectral intervals. Although an unofficial rating, Class A+ spectral matching is within +/- 12.5%. Space solar cells typically match current to calibrated reference standards to within +/- 1%.

Spatial Non-uniformity

Solar simulators must also have uniform illumination across the beam, known as spatial non-uniformity. This metric determines if cells positioned in different locations in the illumination beam will produce the same current. Good spatial non-uniformity is needed to ensure that all cells in a string or circuit are generating the correct amount of power. A terrestrial simulator with Class A spatial non-uniformity is within +/- 2% across multiple areas of the measurement beam. Space solar simulators typically require +/- 2% to +/- 1% spatial non-uniformity.

Temporal Stability

High-grade temporal stability is also highly valuable, as it ensures that measurements taken at different times generate similar results. Terrestrial class A temporal stability is when the simulator is within a +/-2% change over time. Space simulators typically require temporal stability between +/- 2% and +/- 1%.

Industry Standards and Specifications

ASTM Inc. provides the ASTM E927 specification, which dictates performance requirements and parameters to classify both pulsed and steady-state solar simulators operating indoors for terrestrial solar cells. The classifications are based on the simulator’s spectral match to a reference spectral irradiance, spatial irradiance non-uniformity and temporal instability of its irradiance.

The American Institute of Aeronautics and Astronautics provides two important standards for solar cell simulators for space. AIAA S-111A is a standard that describes qualifications for solar cells that are used in space. The AIAA S-112A standard details the qualification and quality requirements for electrical components that are used in space solar panels. It also defines requirements for solar panel manufacturers' quality systems, as well as how to qualify and characterize electrical components on solar panels.

Producing Solar Simulator Calibration Standards

Many applications, including space, require greater spectral match than provided by Class A+, which is the calibrated reference standard method. This need for better spectral match is why the space industry matches the current in all cells to the current of calibration standards. These calibration standards, one for each junction of a multi-junction solar cell, are flown to high altitudes with balloons and are sometimes referred to as isotypes.

The balloon flight method ensures that solar simulators are highly accurate, as high-altitude calibration is the most accurate method of generating calibration standards. NASA’s Jet Propulsion Laboratory (JPL) used very large high-altitude balloons to create calibration standards until the early 2000s.

Calibration Standard Creation Methods

Today, the methods used to create solar cell calibration standards include balloon and plane flights. There are also synthetic calibration standards that are manufactured.

According to NASA’s AM0 calibration round robin study, balloon flights provide the least measurement uncertainty and are therefore the most accurate when compared to aircraft and indoor methods.

However, large balloon flights incur high costs while smaller balloons are a more cost-effective method. Plane flights are generally priced lower than large balloons but not less than small balloons. Ultralight planes are often used, and they provide a mid-range level of accuracy. Standard jets provide even less accuracy.

Generating synthetic calibration standards produces the least accurate standards while also being expensive and time-consuming. However, the main benefit of the synthetic method is that it can be done within a lab at any time of day and in any weather conditions.

Automated, Real-time User Calibration Improves Performance

In general, solar simulators are calibrated at the factory, often rated between Class A+AA and Class CCC. Beyond factory calibrations, a fully automated calibration system can significantly improve solar simulator performance beyond industry standard specifications.

The automated system can scan calibration standards within the light beam and measure the spectral, spatial and temporal performance while conducting automated beam adjustment to further improve performance.

Automated solar simulators also enable rapid measurement of multiple circuits via an all-junction, all-cell, short-circuit current mismatch (JCM) measurement. During JCM, all isotypes are moved to every cell location to either verify or calibrate the light beam intensity both spectrally and spatially. JCM measurements performed before and after solar cell testing enable full characterization of spectral, spatial and temporal solar simulator metrics, at the specific locations relevant to the test, at the time of test. The JCM method can produce performance dramatically superior to other methods.

Current Matching to Isotypes

By updating the output beam according to performance during JCM, it is possible to generate more accurate light output. Angstrom Designs simulators, when factory calibrated, are Class A+AA with +/- 1% current match to reference standards.

At minimum performance, a Class A+AA simulator’s three 2% errors (spectral, spatial and temporal) can compound into as much as 6% total error. However, we have demonstrated that through automated user calibration with the JCM method, a solar simulator can achieve a total error that is more than an order of magnitude better — over ten times the best classification today.

An array of LED heads an on automated frame

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