When a cell is shaded it produces less current and then impacted all the others connected in serie. This results in a loss of power delivered by the module but also a risk of hotspot due to the fact that the polarity of the shaded cell is reversed making it function as a receptor instead of a generator. For these reasons, the manufacturers assemble by-pass diode in parallel to the cells connected in serie which are short-circuited the sub-layout of cells impacted by shading. The power loss and hoiptspot risk is then reduced. 

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