Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. Source: Wikipedia
How does Electroluminescence (EL) work?
In simple terms, Electroluminescence (or EL as its often known) involves the injection of a reverse current into a module or string. This is usually between 3-5 amps depending on the modules. This reverse current flowing through the cells of the module causes a luminescent effect that can be seen at night (or low light levels).
Instead of absorbing light through the cells for power output, the input power shines out through the cells showing itself in a very narrow band of light in the Electroluminescence range. This light is invisible to the human eye, but can be detected by specialist EL cameras (modified DSLR’s).
Images of the luminescing modules are captured by the camera, and this method enables you to clearly see areas of power producing cells (bright white/grey areas) and non-producing parts of the cells (dark grey / black areas, cracks, defects). These dark areas show electrical isolation (grid finger separation) and will negatively effect the power output of the module. Evidence can also be captured of a range of deterioration phenomena and manufacturing quality issues (like PID or flux corrosion for example).
Below you can find a small selection of the thousands of EL images SolarTester has made over the years. Thin film, polycrystalline, monocrystalline, combinations of poly and mono, from two cells per panel to 144 cells per panel; we’ve tested and evaluated them all. Continue reading after the photos
We assess the EL images based on the MBJ criteria, the latest version of these criteria can be downloaded here. The high quality of our images allows us to detect the smallest cracks. The assessment criteria have been drawn up by the MBJ partner network in collaboration with TÜV SÜD and other bodies that are involved in testing solar panels. When assessing the cracks, we look not only at the current influence on the yield of a panel, but also at the influence on the expected yield in the future. The micro cracks are divided into 3 types, non-critical, critical and very critical. The assessed cracks are ultimately used for a classification of the panel in a class A to D. This assessment is in line with the ISO-2859-1 and AQL method for performing and accepting or rejecting a sample. Below are some examples of the different types of cracks we found in the panels we tested.
An example of a non-critical crack. The power generated on both sides of the micro crack can be dissipated to the bus bars. More examples, including critical and very critical cracks, can be found in the assessment criteria.