Light-Induced Degradation

Light-Induced Degradation (LID) is a phenomenon that depletes efficiency or causes a power drop in the solar modules when exposed to sunlight. This effect is irrespective of the type of module and the degradation is more pronounced in mono-crystalline solar cells as compared to multi-crystalline solar cells/ module technology and more on solar cells made with p-type wafers.

In mono-crystalline solar cells, the LID is caused mainly by traces of oxygen which gets combined with the molten silicon during the Czochralski process used in growing mono silicon crystals. The Boron used for doping it as p-type silicon forms a complex with oxygen and acts as a trap. It traps electron-hole pairs which otherwise would have taken part in the current generation. As a result, the efficiency and power capability of the modules reduces.

There are fundamental differences in the solidification process of mono-crystalline and multi-crystalline technology wafers. The mono-crystalline wafers are prepared by the Czochralski process and multi-crystalline wafers are obtained by the casting process. Although there is sufficient oxygen introduced during this casting process to cause the LID effect in multi-crystalline,  it will be less as compared to mono-Si as the process duration for both processes are different. Apart from this, grain boundaries and dislocations caused in multi-Si can also create traps causing LID

There are other reasons too for the occurrence of LID. It can be caused due to the surface defects causing traps to trap electron-hole pairs. There are studies that indicate that even Cu, Fe impurities in Si can cause the traps leading to a reduction in power and efficiency. 

When sunlight falls on these panels, the energy equivalent to Si bandgap is absorbed which leads to electron-hole pair generation. These generated electron-hole pairs get trapped by the traps created by the Boron-Oxygen complex before they could contribute to the total photovoltaic current. After a particular time, all these traps are completely filled with electron-hole pairs and no more absorption is possible and the generated carriers are finally available for conduction.  And therefore after a certain period of time post-exposure of modules to outdoors, the LID effect saturates. 

This effect will be more pronounced as we shift to different technologies like mono-PERC.  We will discuss this in my next article.