By George Touloupas

This article was originally published in pv magazine – January 2022 edition. Learn more about the pv magazine test here.

Together with their test partners, pv magazine decided to add LETID testing to their set of benchmarking tests for PV modules, as the industry has improved its knowledge about the risks and mitigation of this relatively new mode of PV module degradation. George Touloupas, Senior Director of Technology and Quality at CEA, provided an update.

LETID (light and elevated temperature-induced degradation) is a mode of degradation of crystalline silicon PV cells that was identified and researched a few years ago. The initial research was spearheaded by Q Cells, which also coined the term LETID in 2015, with Australia’s University of New South Wales contributing important work on the root causes and mechanism. Compared to boronoxygen LID (light-induced degradation), that develops in a period of a few weeks of operation, LETID can take years to develop and happens faster in sunnier and warmer locations. After reaching a maximum power loss, modules affected by LETID will regenerate and regain power, but this process can be very slow and is, again, dependent on local climate conditions. Overall, there can be severe energy yield loss, even if the modules eventually fully recover after many years.

Although the initial focus of LETID research was on multicrystalline PERC cells, it was soon discovered that mono PERC cells can suffer from severe LETID. In fact, other cell types, such as Al BSF and even n-type cells, can have LETID susceptibility, but to a lesser degree than PERC, in general. It has been found that metal contamination, hydrogen content and process parameters have a strong impact on LETID. Good material and process control need to address these factors to mitigate LETID.

Batch testing

In recent years, many major manufacturers have developed mitigation processes and have managed to control LETID and avoid having the originally feared severe losses. However, the need to screen products for LETID sensitivity by using an efficient testing procedure remains relevant. CEA introduced LETID testing in 2019, as part of the product requirements definition, which is done on behalf of buyers at the procurement stage, as well as a part of CEA’s batch testing protocol, which is used to decide if a large production batch, typically between 5MW and 20MW, is susceptible to a particular failure or degradation mode.

The chart to the right (Figure 1) shows LETID batch testing results from CEA’s projects since 2019, based on more than 140 tests performed on various products from multiple suppliers. When testing started in 2019, very few suppliers were ready or willing to accept testing, so 2020 was the year that batch testing for LETID picked up volume. One can see a big spread of results in both 2020 and 2021, with average degradation being about 0.80%, but reaching as high as 2.5%, highlighting the need to control LETID of production batches.

LETID tests

Q Cells and others developed tests that need months to complete, and this is not practical for a fast-moving industry. TÜV Rheinland developed an internal 2PfG series standard for LETID testing back

in 2019, with the objective to accelerate LETID testing without compromising the detection of the maximum degradation. A new draft IEC technical specification for LETID detection testing is in progress and is expected to be published in 2023, and will be based to a large extent on TÜV Rheinland’s internal LETID testing standard.

We have therefore chosen to base our LETID testing of new product samples on the draft IEC technical specification. Figure 2 below shows the test flow, described at a high level.

Similar to the existing grades for the other indoor tests, we developed grades for LETID testing results based on general industry knowledge and our experience from batch testing data, where values range from 0% to as high as 2.4%. The LETID degradation is calculated based on power after BO preconditioning, to separate LID degradation from LETID degradation (see Table 1 below).

The reason we adopted strict criteria is that the industry keeps improving, and that 50% of CEA’s LETID batch testing results in 2021 are below 0.70% (the median value).

Three samples were tested in this first lot: LONGi’s LR5-72HBD-535M, JA Solar’s JAM72S30-540MR and Trina’s TSM-505DE18M. Among these three samples, LR5-72HBD-535M was submitted by LONGi and the other two were purchased from the market.

Both LONGi and Trina modules showed negative LETID (which is a gain), achieving a grade of 100. This can be explained by some small measurement uncertainty. Overall, the results are within expectation, as CEA has observed values close to zero or slightly negative in our batch testing results. On the other hand, the LETID of JA Solar’s product was close to CEA’s average for 2021 batch testing results.

George Touloupas is CEA’s Senior Director, Technology and Quality 

Test Cooperation

pv magazine test is a cooperative effort involving pv magazine, APsystems, CEA, and Gsolar. All testing procedures are carried out at Gsolar’s test laboratory in Xi’an, China. CEA supervises these tests and designed both the indoor and outdoor testing procedures.