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 . 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 . 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.
 IEC 60904-3: Measurement principles for terrestrial with reference spectral irradiance data, 2008.
 IEC 60904-9: Solar simulator performance requirements, Edition 2.0, 2007.