Known as Light and Elevated Temperature Induced Degradation (LeTID) it is a relatively recently discovered phenomenon, not yet as thoroughly studied and contrary to LID, this kind of degradation can occur years later after high operating temperatures have affected the panels.
On the spec sheets we refer to NMOT values, (Nominal Module Operating Temperature) and state the nominal operating conditions: 800W/m², 20°C, 1.5AM and with wind 1 m/s. Under these conditions, the temperature of the panels can vary depending on type i.e. 44.5°C for aleo premium modules (X63) or +1°C for aleo full black modules (X83). Recent literature depict that problems with LeTID can arise when temperatures on panels exceed 50°C or more, which is not uncommon in summer when more than 70°C have been measured.
PERC cells, and in particular multi-crystalline are quite sensitive to this degradation, because their passive back is rich in hydrogen. Hydrogen could be at the origin of this phenomenon, but relevant studies are still at such an early stage, that any hypothesis is pure speculation.
Last December PV-Tech.org estimated that 80% of the cells produced globally in 2020 would be PERC-type and that demand for this technology would reach 158GW by 2022. It is therefore legitimate to wonder what moves cell manufacturers to adopt this technology and to bet on its evolution.
There are essentially 2 reasons: Low production cost and the ability to maintain yields over time.
In the case of LeTID there is not yet an official test procedure, but there are some indications that will be adopted in the next draft of IEC 61215 -1: Ed.2.0. To pass the test, cells should not degrade more than 5%.
aleo has approached this by testing of real operating conditions simulation.
486 hours in a climatic chamber at 75°C, to a current value resulting from the difference between Isc and Impp, aleo panels sailed through this test with a maximum power loss of 1,52%.
That is confidence of a stable technology for the entire expected life of a quality system.