Enemies of photovoltaics - LeTID effect

While LID has been known for almost 50 years, its LeTID variant was only discovered in 2012 with the emergence of a new cell type called PERC.

What is LeTID?

It manifests through a loss of power in the silicon modules due to exposure to sunlight, as we discussed in our article on the LID effect. So far, both phenomena are the same; the main difference between LID and LeTID is that the latter appears at high operating temperatures, above 50ºC, while LID is noticeable at low temperatures.

Apart from its appearance with higher module operating temperatures, the further development of LeTID is similar to LID. However, it differs in one point that makes it more serious: its slow recovery. While in case of LID the module stabilizes during the first two years, regeneration in case of LeTID is so slow that it affects a large portion of the module's useful life, seriously jeopardizing economic viability.

LeTID was initially detected mainly in polycrystalline silicon PERC cells. But it did not stop there: later it was also reported in monocrystalline PERC cells and even in n-type cells.

As modules with PERC cells are set to replace the hitherto traditional Al-BSF, the emergence of LeTID is a major setback. Who would buy modules, which on one hand yield 2-3% more, but on the other hand lose up to 10% of their yield after a few years of operation? As a result, the entire photovoltaic community - manufacturers, universities, laboratories, certifiers, etc. - has been working to better understand the effect and develop reliable remedies.

This is an example of how a module is affected by LID: initial (left) and after exposure to irradiation (right).

Why does it occur?

Current research points to one main suspect: hydrogen. In the manufacture of the cell, hydrogen atoms diffuse from other layers of the cell to the bulk layer. Although the specific processes within the cell are currently being analyzed, it seems quite likely that the higher the processing temperatures, the more hydrogen diffuses through the cell.

How is LeTID detected?

LeTID is not visible without the appropriate meters. As with almost all failures, it is detected by an abnormal drop in panel performance. After having ruled out other more easily viewed origins, such as hot-spots or delamination, we would have to use special electroluminescence equipment to diagnose LeTID damage, a procedure identical to the diagnosis of LID, micro-cracks and other defects. Electroluminescence is an "X-ray of the module" that allows us to see defects invisible to the human eye. During this process, the maximum current (Isc) is injected into the module, causing it to illuminate and emit radiation at a frequency only visible with special filters and cameras. In the measurements we take, we will be able to see white, gray and black areas indicating the damage it has suffered. Areas with darker colors will be those affected by LeTID, which have no electrical activity and therefore will decrease the performance of the panel.

How do you avoid the effects of LeTID?

Laboratories and manufacturers are working intensively to identify the causes of LeTID, develop test procedures for module inspection, and adapt and optimize manufacturing processes to achieve modules with the lowest risk of LeTID. We can expect that during 2020 we will still have an IEC norm that standardizes the tests to measure the effect with certainty. TÜV Rheinland has already developed its own procedure for determining LeTID. The first measures proposed in the PERC cell manufacturing process that can reduce LeTID include:

• Use raw materials low in hydrogen.
• Lower cell treatment temperatures,
• Use thinner wafers.

The desire to overcome LeTID is so great that some manufacturers are already certifying their modules in the laboratory with procedures that have not yet been completed. At present and until we know with complete clarity the reasons for LeTID, there is little we as users can do to minimize its effects. The user must act the same as in the cases of other known risks.