Core to any analysis of a solar cell or photovoltaic module is the assessment of its energy conversion efficiency. In a laboratory, this is typically done by using a solar simulator in combination with high-precision electronics. The solar simulator should be as close to the standardised solar spectrum as possible. This solar cell spectrum is defined in standards such as the IEC AM1.5G [1]. Also, the solar simulator should also have a sufficiently short and long-term stability and uniformity. The requirements are summarised in Table I and II. Solar simulators are quantified in Classes A to C, where A is the best and C the worst in terms of the spectral match, uniformity, and temporal stability.
The solar cell is placed on a temperature-controlled chuck, typically maintaining the temperature at 25 oC. The front of the solar cell is contacted using bars with various current-voltage pins, while a full area copper chuck typically contacts the rear of the solar cell with one voltage probe. The contacts are connected to a four-quadrant current voltage source meter which allows for the measurement of the current-voltage characteristics of the solar cell.
Table I: Global solar irradiance distribution in the 400 – 1100 nm spectrum according to the IEC [1, 2].
| Wavelength range (nm) | Percentage of total irradiance in the 400 – 1100 wavelength range | |
| 1 | 400 − 500 | 18.4 % |
| 2 | 500 − 600 | 19.9 % |
| 3 | 600 − 700 | 18.4 % |
| 4 | 700 − 800 | 14.9 % |
| 5 | 800 − 900 | 12.5 % |
| 6 | 900 − 1100 | 15.9 % |
Table II: Definition of solar simulator classifications according to the IEC [2]. The short-term temporal stability refers to the stability of the light source during the measurement of one data set (e.g., the irradiance, current, and voltage) while the long-term instability refers it is related to the time required to take one entire I-V measurement.
| Classification | Spectral match to intervals specified in Table I | Non-uniformity of irradiance | Temporal instability | |
| Short-term instability | Long-term instability of irradiance | |||
| A | 0.75 – 1.25 | 2 % | 0.5 % | 2 % |
| B | 0.6 – 1.4 | 5 % | 2 % | 5 % |
| C | 0.4 – 2.0 | 10 % | 10 % | 10 % |
The solar simulator shown in the video below, supplied by the company Wavelabs from Germany, is one of the first that is employing a set of 20+ different colours light emitting diodes (LEDs) to replicate the solar spectrum. This offers some unique advantages compared to single light source simulators.
- The spectrum of the solar simulator can be changed by changing the relative intensities of the 20+ LEDs and can achieve an unprecedented match to AM1.5G and other spectral standards.
- Pulse length can be changed to allow for the measurement of high-capacitance solar cells.
- Fast quantum efficiency measurements can be made for 20+ wavelengths, including light-biasing required for multi-junction solar cells.
- LEDs have typically very long lifetimes, hence, reduced operating costs (and increased up-time)
- In-situ measurement of spectral intensity ensuring excellent repeatability.
[1] IEC 60904-3: Measurement principles for terrestrial with reference spectral irradiance data, 2008.
[2] IEC 60904-9: Solar simulator performance requirements, Edition 2.0, 2007.