By pass diode in PV cells
As the light fades to zero, the voltage in the cell drops. In the graph below, as the light fades (lower in the graph) the voltage also drops slightly. As the amount of light decreases the cell looks like a larger and larger load (the curves on the bottom of the graph are below the 100Ω load line). So the cell starts to dissipate energy, which is not a problem if there are a few cells in series, but if there are a lot of cells with a large amount of current, the cell can overheat or be damaged. 

To solve this problem a diode is place in series, when the voltage on the cell goes low due to lack of light, then it can bypass the cell. In the picture below, it might be good to put a protection diode at 0.4V so if the light is that low the current would go through the protection diode. 



Source: http://www.aurorasolarenergy.com/iv-curve-of-a-solar-panel/