A step by step guide to selecting the “right” Solar Simulator for your solar cell testing application
Solar Simulator or Xenon Light Source
There are many companies in the market place that claim to sell solar simulators, yet what they are really selling is a xenon light source. What are the differences between these two, even though they both use xenon short arc lamps? A solar simulator is a xenon light source; however a xenon light source is not a solar simulator. By definition of International standards (listed below in the footnote) for Solar Simulators, a solar simulator is the equipment used to simulate the solar irradiance and spectrum and it must fall into one of the three classes defined below.
If we take a xenon light source, then through the use of reflectors, filters and optics shape the light beam so that the three main characteristics of light beam meet the specified international standards, then it becomes a solar simulator. These international standards defme three classes of solar simulators; Class A, Class B and Class C and the acceptable tolerance for the three main characteristics, namely spectral match to sunlight, non-uniformity of the light beam and stability of the light beam over time. If a manufacturer does not clearly specify the class and tolerances for these characteristics, then chances are they are selling a xenon light source.
So the first thing you need to decide is what is needed for your application, a xenon light source or a solar simulator, keeping in mind that, in general, xenon light sources are cheaper than solar simulators
Type of Solar Simulator
There are two types of solar simulators available in the market for cell testing. First is 'Steady State*' (SS) system and the second is the 'Pulsed Simulator' (PS) system. Pulsed simulators can be single-pulse or multi-pulse type. Following are some of the things to consider in choosing between these two types of solar simulators.
- PS systems have lamp life that is typically between 40,000 to 1 million flashes, whereas the SS systems have a typical lamp life of 1,000 hours of continuous operation. If we take things at the face value; then 1 million flashes is better than 1,000 hours. Let us now consider the facts about cell testing. With a PS system,between 40.000 to 1 million cells can be tested before there is a need to replace the lamp. Cell testing typically takes less than a second. However when we add the time for loading and unloading of cell into the equation typical throughput is 1.000 to 1,200 cells per hour. With the SS system we will be able to test 1 million to 1.2 million cells before there is a need to replace the lamp. It is worth noting that the total number of cells that can be tested with a SS system is limited by robotics, i.e. if the cell loading and unloading can be speeded up, more than 1 million cells can be tested with SS System before the need to replace the lamp. In addition, depending on the design efficiency of a SS system, it is possible to get much longer life than the typical 1,000 hours. For example, if one design is such that the lamp runs close to full power when new, then lamp life is likely to be close to the specified 1,000 hours. If, however, another design is such that the lamp runs at lower power when new then the lamp life is likely to exceed the specified 1,000 hours. Hence, solar simulators that use mirrors are inherently less efficient design since irradiance loss from each mirror is typically 5-10%.
- Most PS systems typically have a flash duration of 2 ms to 10 ms; this means that the cell testing, i.e. I.V. measurement system must be able to perform the test within this short duration. Testing in this short duration is not a problem but generally means fewer data points for the I.V curve. It also means that the cell to be tested must be able to respond to the light in this short time duration. As a result it is important to know the response time of the device to be tested.
- For PS systems, there is a claimed advantage that since the flash is for a short duration, the cell to be tested has no appreciable increase in its temperature. Whereas the general thinking is that using a SS system one must be concerned about the increase in the cell temperature during testing. Tests' have shown that for single crystal or multi-crystalline cells, regardless of the thickness of the cell there was no significant influence on the rate of the observed heat build-up effect when cell was under the light for up to 500ms. A typical test takes about 120 ms. Thus this perceived disadvantage of a SS system is not real.
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