When solar panels loose efficiency, how does that happen?
Potential Induced Degradation (PID) is a relatively recently discovered phenomenon which can occur in solar panels although a concise understanding of what causes it is still being finalised. Tests by the German Fraunhofer Institute showed that solar panels which are susceptible can lose more than 30% of their rated power through this effect and as much as 90% in extreme cases. What is known is that sodium enrichment occurs between the chemicals used on the surface of the solar cell and the glass in certain circumstances, causing a leakage of current between the cells and the solar panel frame. The intensity of the degradation depends on the module type, the ambient environmental conditions and the position of the solar panel in the array string. A high system voltage (eg 600V +) and high ambient humidity can lead to a film of moisture on the solar panel surface. A drift of positive ions occurs from the glass towards the solar cell in the opposite direction, which is believed to cause degradation in the solar cell.
LG panels performed very well in the PID tests performed by the world renowned Fraunhofer testing laboratory as well as TUV testing laboratory in Germany in 2012. After a prolonged PID test at the Fraunhofer testing laboratory the LG panels still had 100% output, while of 13 panels tested, 8 had lost between 20% and 90% of generating output (PID Brochure).
Light Induced Degradation (LID) is another phenomenon that occurs in solar panels. All solar cells degrade when they are first exposed to sunlight. It is generally accepted that the effect is most pronounced if boron oxygen defects are more prevalent within the silicon structure, although it can occur on the surface layers too. The result is simply described as an increase in the density of the material which makes the movement of electrons more difficult, which results in less power flow.
Thin Film solar panels for example, exhibit relatively higher levels of initial degradation (in the first 30 days) but manufacturers understand this well and over-rate their power to take into account the losses they know will occur.
In Mono Crystalline and Poly Crystalline solar panels, the effect is more subtle (in the order of around 0.7% power loss per annum). What this means for the end customer is that solar panels over the years become slightly less efficient and will over time produce slightly less electricity. Over 25 years it is around 20%. This is why it is important to start off with a high quality, solid efficiency panel.