Solar Simulators: How They Work, What AAA Means, and How to Choose One

Solar Simulators: How They Work, What AAA Means, and How to Choose One

Solar cell efficiencies can only be compared when they are measured under the same light. Sunlight does not meet that condition: its intensity and spectrum change with the time of day, the sun angle, humidity, and clouds that in Indonesia can come and go within minutes. A solar simulator is a laboratory light source that reproduces standard sunlight, so a result measured today can be compared with one measured next month, or in another lab.

This article explains what "standard sunlight" means, what a AAA rating tells you, the main light source types, how calibration works, and what to check before choosing a solar simulator for your lab.

Standard test conditions: 1 sun, AM1.5G, 25 °C

Solar cell efficiency is reported at Standard Test Conditions (STC):

  • Irradiance of 1000 W/m² (100 mW/cm²), often called "1 sun".
  • AM1.5G spectrum: sunlight after passing through 1.5 times the thickness of the atmosphere (sun about 48° from zenith), including diffuse skylight. The reference spectrum is defined in IEC 60904-3.
  • Cell temperature of 25 °C.

A solar simulator has to deliver irradiance and spectrum as close to these conditions as possible, evenly across the sample, and stable throughout the measurement.

What AAA means

IEC 60904-9 rates solar simulators with three letters, always in the same order:

  1. Spectral match: how closely the energy in each wavelength band matches the AM1.5G spectrum.
  2. Spatial non-uniformity: how evenly the light is spread across the test plane.
  3. Temporal instability: how much the intensity fluctuates during a measurement.

Each criterion is graded A, B or C, with A the strictest. For uniformity, for example, class A allows at most 2% non-uniformity, class B 5% and class C 10%. The 2020 edition of the standard added an even stricter A+ class and new parameters such as spectral coverage and spectral deviation.

One point is often missed: a class applies to a specified area. The Ossila LED Solar Simulator G2009B2, for instance, reaches AAA over a 15 mm diameter area and ABA over 25 mm. The useful question is not "is it AAA?" but "is it AAA over my sample size?"

Light source types

Xenon arc lamps

The xenon spectrum is naturally close to sunlight and is shaped further with an AM1.5G filter. The drawbacks: the lamp needs warm-up time, produces heat, and its spectrum drifts as it ages, so it needs regular recalibration and lamp replacement.

LED

LED solar simulators combine several LEDs of different wavelengths. Because each channel can be controlled, the spectrum can be tuned, including for indoor photovoltaics. LEDs are ready within minutes, run cooler and last long (the Ossila G2009B2 is rated for up to 10,000 hours). The limit is area: compact LED simulators are generally intended for lab-scale cells, not large modules.

Halogen and metal halide

Cheaper and able to light large areas, but with a weaker spectral match. Suitable for light-exposure tests and demonstrations, less suitable for reporting efficiency.

Steady-state or pulsed

Pulsed (flash) simulators are used in industry for large modules. For research on perovskite, organic and dye-sensitized cells, a steady-state simulator is the better choice because these devices respond slowly to changes in light and voltage. Measuring them within a very short flash can give misleading numbers.

Calibration with a reference cell

Irradiance at the sample plane depends on working distance, lamp age and temperature. It is therefore reset to 100 mW/cm² before each measurement session using a calibrated reference cell placed exactly where the sample will sit.

Ideally the reference cell has a spectral response similar to the device under test. If not, spectral mismatch occurs: the reference cell reads 1 sun while the sample receives slightly more or less usable light. Rigorous publications correct for this with a mismatch factor calculated from the lamp spectrum and the spectral responses of both cells.

The Ossila Reference Solar Cell G3100A1 is a calibrated silicon photodiode with a 0–125 mW/cm² display and a 400–1100 nm spectral flattening filter, so it reads 100 mW/cm² under 1 sun from both LED and xenon sources.

From the I-V curve to efficiency

The solar simulator only supplies light. The measurement itself is made by a source measure unit or I-V test system that sweeps voltage while measuring current. The curve gives:

  • Voc: voltage at zero current.
  • Jsc: current density at zero voltage, in mA/cm².
  • FF (fill factor): maximum power divided by Voc × Jsc.
  • PCE (power conversion efficiency) = (Jsc × Voc × FF) / Pin.

Example: a cell with Jsc 22 mA/cm², Voc 1.10 V and FF 0.78 under Pin 100 mW/cm² has PCE = 22 × 1.10 × 0.78 / 100 ≈ 18.9%. A 5% error in irradiance becomes a roughly 5% error in Jsc and efficiency, which is why calibration cannot be skipped.

The most common mistakes

  • No aperture mask. Light falling outside the active area is also collected, so Jsc and efficiency look higher than they are. Use a mask with a known opening and calculate current density from that area.
  • Hysteresis in perovskite cells. Forward and reverse sweeps can differ widely. Report both directions and the scan rate, and where needed the stabilized output at the maximum power point (MPP tracking).
  • Sample heating. Under 1 sun the cell warms up and Voc drops. For long measurements, provide cooling or record the sample temperature.
  • Sample not at the calibration position. At short working distances a few millimetres already change the irradiance.
  • Infrequent calibration. Check irradiance at the start of every session, especially with ageing xenon lamps.

How to choose a solar simulator

  1. Sample size. Match it to the area over which the instrument reaches the class you need. A 25 × 25 mm cell needs a larger illuminated field than pixels a few millimetres across.
  2. Device type. Perovskite, organic and DSSC devices need a steady-state simulator and an I-V system that supports forward/reverse sweeps and MPP tracking.
  3. Spectral needs. For indoor PV or spectral tuning, an LED source with adjustable channels gives more flexibility.
  4. Measurement system. Check that the voltage and current ranges suit your devices. The Ossila Solar Cell I-V Test System, for example, works at ±10 V with current ranges from ±10 nA to ±200 mA.
  5. Reference cell. Budget for it from the start; without one, efficiency numbers cannot be defended.
  6. Budget. For groups just starting out, a low-cost solar simulator such as the ABET 10500 can be an entry point. Confirm the class and illuminated area for the configuration you choose.

Solar simulators and solar cell test equipment at AMI Scientific

Light sources and measurement systems are listed in our Solar Simulator collection, including the Ossila LED Solar Simulator G2009B2, Ossila Reference Solar Cell G3100A1, Ossila Solar Cell I-V Test Systems (manual and automated) and the ABET 10500. For device fabrication, see also spin coaters and perovskite materials.

Send us the type and size of the cells you want to test via WhatsApp and we will help you put together a configuration and a formal quotation for your institution.

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