Light and Elevated Temperature Induced Degradation

In recent years, a new type of degradation, namely LeTID, which can lead to significant power losses in solar cells has come to focus. It’s an acronym for Light and Elevated Temperature Induced Degradation. Cells with innovative designs like PERC technology show an even stronger effect than standard Al BSF multi and mono-crystalline cells. LeTID is accelerated by high irradiance at a higher temperature after hundreds of hours of light exposure. The drop in efficiency between 5% to 10% can be seen if steps are not taken to prevent it.

LeTID is sometimes referred to as Hydrogen-Induced degradation mainly caused by the release of Hydrogen. Although the exact cause of LeTID is still a matter of debate, the main reasons observed by far are:

1. Exposure to high temperature and illumination
2. Broken bonds between Si surface and hydrogen, leading to surface defects.

Due to these surface defects, there is the availability of surface which absorbs free carriers causing a reduction in efficiency/power. Over a period of time, the process gets stabilized and the saturated value of efficiency or power is obtained.

It is important to note that LeTID is also observed in mono and multi Al-BSF solar cells but its effect is not as pronounced as in mono-PERC solar cells. The reason behind this is the high performance of the PERC structure achieved by the passivation of the rear surface by depositing di-electric.

Before continuing our discussion, it is important to recall the difference between LID (Light-Induced Degradation) and LeTID. As mentioned in the earlier blog, LID is well known and its degradation mechanism is clearly laid out (based on the formation of Boron-Oxygen complexes which act as traps for the free carrier). On the other hand, LeTID is caused by hydrogen release and the mechanism is still under debate.

Research also shows that LeTID not only affects CZ grown p-type mono-PERC but also gallium doped mono, n-type Si, flat-zoned grown Si material. Thus LeTID seems to be a universal effect impacting all kinds of crystalline Si solar modules and therefore understanding the impacts of this degradation is key to making informed decisions for IPPs.

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Originally published at https://www.linkedin.com.