Light induced degradation (LID) is still an issue when investors and EPCs don’t ensure that cell manufacturers use adequate mitigation strategies. Indeed, in a test performed by PI Berlin on 10 different modules, eight of those degraded by one to four percent.
This was demonstrated in its presentation (slides 8 and 9) from pv magazine’s recent webinar, “Secure your investment: Discover urgently required game changing solutions in managing LeTID”, held on March 14 with initiative partner JinkoSolar.
Eternal Sun, meanwhile, explained the status of the discussion on standardized testing in the IEC 61215-2 committee, and JinkoSolar exemplified the results of the third party testing on its modules and explained how it reduces the degradation to below one or two percent.
As the webinar participation was extremely high, there was not enough time to answer all the questions posed. Below you will find answers to the most pressing.
Questions on the effect itself:
How much LID and light and elevated temperature degradation (LeTID) does JinkoSolar expect for its modules, and what do the warranties cover?
JinkoSolar: Both the LID and LeTID levels that we measure in mass production, and which are confirmed by independent laboratory studies, are within the 1 to 2% range, so well within the power warranty terms that JinkoSolar provides with its modules. It has to be noted that LID, which typically occurs in the very initial days of exposure of modules to sunlight, is covered by the first year power warranty, while LeTID, which requires a much longer period of time to develop in the order of years, is covered by the linear power warranty.
Upcoming pv magazine webinar: New approaches for bifacial modules and updated results on yield investigations
During our pv magazine webinar on Monday 29 April at 2pm (CEST), TÜV Rheinland expert Johanna Bonilla will present yield analysis for bifacial installations and insights on the IEC 60904-1-2 (2019), which describes procedures for the measurement of the current-voltage (I-V) characteristics of bifacial modules. JinkoSolar, our webinar partner, will further explain why it now offers bifacial glass-glass backsheet bifacial modules, and DuPont will discuss the reliability advantages of glass-backsheet modules.
PI Berlin tested 10 PERC-modules for LID and LeTID. Have all modules tested shown power regeneration and if so, what was the time range?
PI Berlin: Yes, they all recovered, but slower than the degradation, i.e. regeneration started at > 1000h. In the field, this would correspond operation times of four to 10 years for cold and hot climates, respectively.
How relevant are the two mechanisms, LID and LeTID with respect to each other? Which contributes more to degradation and which should investors be more concerned about?
PI Berlin: LeTID is the stronger degradation, if no counter-measures have been applied by the manufacturer, i.e. either by changing the cell process or by fast degradation/regeneration in the cell or module line.
JinkoSolar: Indeed both phenomena can lead to significant power reduction if the PV cells are not stabilized in mass production. JinkoSolar has thoroughly evaluated how to optimize the manufacturing steps and effectively implemented it to reduce, as much as possible, either LID and LeTID. Also, we keep monitoring our cells and modules to make sure power levels are consistent and within expected/acceptable limits.
Does LeTID loss completely recover in the field?
JinkoSolar: As we learned during the webinar from the different presentations, and studies available in the literature already, LeTID development and regeneration are essentially directly related to module/cell temperature. So complete regeneration, which anyway is on average estimated to occur in a timeframe in the order of years, mostly depends on the PV field location, where hotter environments clearly accelerate it. Nonetheless, the main target for manufacturers, and what we do at JinkoSolar, is to reduce the LeTID level to the minimum, so that it has almost no impact on the estimated energy yield.
LeTID depends on the module temperature. Is there any relation to the NOCT value in the data sheet (temperature under standardized operating conditions) and light induced degradation risk of a module?
PI Berlin: The NOCT value has no direct influence on the degradation mechanisms as such, but indirectly the operating temperatures influenced by weather, material and design aspects, play a role in this degradation: The higher the module temperature, the faster the degradation and the regeneration, but not its magnitude.
JinkoSolar: The trend in the industry is anyway to always improve the module temperature coefficients and that has also a positive side effect on LeTID mitigation.
Can one be sure that there is no LID and LeTID in n-type cells?
JinkoSolar: N-type silicon technology is virtually LID-free, thanks to the different doping, which avoids one of the main causes of LID, i.e. the B-O complex recombination. Some of the causes of LeTID are still under investigation and not related to the silicon dopant, so it is plausible that some minor level of degradation might also affect n-type cells. However, our preliminary studies confirm that LeTID for n-type cells is much lower than p-type ones, which is perhaps also related to the different H-passivation treatment that plays a significant role in LeTID effect.
Will LID and LeTID have effect on Immp or Vmpp or on both?
JinkoSolar: Both current and voltage parameters are somehow affected. Either LID and LeTID are caused by different and complex factors, such as recombination effects or in general Carrier-Induced Degradation phenomena, which decreases the activity or availability of active carriers, thus ultimately reducing overall cell efficiency.
Questions on measuring, testing and risk mitigation:
How one can detect LID and/or LeTID in existing installations when the effect is significant, but close to the measurement accuracy, e.g. 3%?
PI Berlin: For power measurements on the same module before and after applying accelerated LID/LeTID cycles, the uncertainty is much lower than 1%, e.g. ±0,4% at PI. In practice, we recommend removing two modules from the existing installation and sending them to a qualified lab. Alternatively, one can disconnect modules from the inverter string and monitor them carefully in the field without load and with thermal insulation from the backside (see slides 2 and 6 of the webinar presentation). The disadvantage is that this procedure takes several months and needs monitoring equipment and expert attendance.
LeTID is an effect at the cell level. As such, why not test at this level, instead of testing in module production?
PI Berlin: Yes, please ask your cell supplier to do so.
Eternal Sun: That should indeed be done, but in practice, downstream PV companies (EPC’s, investors) also like to know how much LeTID is present in the modules they intend to use for their installations.
JinkoSolar: We test our production lots at both the cell and module level to make sure that the degradation is within tolerable limits and homogenous. Given the fact either LID and LeTID are related to the cell structure aspects, measuring at both steps is somehow redundant, but that’s clearly the advantage of having a totally integrated production, which allows us to completely control each quality aspect and confirm that results are consistent. Also, given that LeTID develops over a long period of time, it is more important to have the possibility of measuring modules after their installation, and to rely on a recognized standard to evaluate and compare the test results.
PI Berlin will be presenting LeTID Results @ pv magazine Roundtables Europe, 16 May...
Paul Grunow will present PI Berlin’s LeTID results at the networking and poster sessions during pv magazine’s Roundtables Europe, 16 May (Day 2 of Intersolar Europe), between 13:30 to 14:30 and 17:00 – 18:30, ICM Conference Center, Room 5.
Join the Quality Roundtable session at 14:30 and discuss the following topics:
• Investigating poor soldering quality in modules – does auditing save money?
• Glass-glass modules are gaining market share – are they really more reliable?
• The dispute over the direct burial of cables – what is the technical best practice?
• Service desert inverters – what expectations about manufacturer service are fair and justified?
The power loss due to the LID effect must be considered in the module's nameplate power. This was the case in the old EN50380 standard, and this is the case in the actual 61215:2016 (Gate #1) standard. Can you comment, why nevertheless LID is not reflected in the power indication?
PI Berlin: The label should reflect the power in the field as used in energy yield simulations. But the stabilization for Gate No.1 in the MQT (Module Quality Tests) 19.1 from IEC 61215-2 does not catch the LeTID effect, because this degradation develops much more slowly than the known LID. Therefore, the new edition two of the IEC 61215-2 will include new procedures under MQT 23 for LeTID. In the meantime, it is recommended to ask for disclosure of the typical degradation magnitudes and time scales at the specific supplier, and if possible, have it approved by a third party.
Eternal sun: To add to PI Berlin’s comment, LeTID testing in IEC norms might not be added to the new edition IEC 61215-2, but as a separate standard. That being said, it might be that the LeTID effect has to be stated either as a pass/fail (if the module experiences a certain threshold of LeTID effect), or either as a number, indicating exactly how much the LeTID effect a certain module is undergoing (i.e. 0.5%, 5.0%, etc.). Whatever the case, as PI Berlin recommends, asking for magnitudes and timescales of LeTID from a specific supplier, or from a third party will prepare you much better for when the standard comes out, and of course, will allow you to better forecast the energy production of your PV farm. And please note: The MQT 19 stabilization does not detect LeTID within the timescale of that test, because it is done at 50°C temperature.
How should investors consider LeTID and LID in degradation parametrization for PVsyst during simulations? Do PVSyst module .pan files include LeTID?
PI Berlin: Simply subtract the degradation magnitude, i.e. the turning point from degradation to regeneration, e.g. minus 4%, from the labeled power value, because the PAN files do not include LeTID yet. Furthermore, the maximum magnitude might occur exactly at the time when the power plant is sold on the secondary market and the system might undergo a technical due diligence.
Does the stabilization of the effect in the cell production line, e.g. during the firing process, have an impact on cell efficiency?
JinkoSolar: The stabilization process of PV cells is only one of the techniques implemented in the production line to limit the LID and LeTID phenomena. In fact, a good control on both aspects can be achieved only if the whole production chain is optimized for that purpose, starting from silicon ingot production. It requires significant expertise and R&D investment to be effectively mastered and to enable the definition of the best recipe to produce highly reliable PV cells and modules with the lowest possible degradation level.
To accelerate the LID and LeTID tests, they are performed at elevated temperatures. Would you recommend testing at temperatures above 90°C, even though normally the encapsulant materials may degrade?
PI Berlin: This was recommended for the production phase only, i.e. at lamination temperatures or cell firing temperatures. Testing is done at 75°C and 85°C, according to IEC proposals.
Eternal Sun: We are currently testing at higher temperatures such as 95 or 100 °C since it seems to accelerate the LeTID effect by a factor of four. This testing is possible since modern encapsulant materials do not degrade significantly at these levels, therefore it can reduce the LeTID test time significantly. Currently we perform research to understand if the results obtained at 75 °C/85 °C are 100% comparable to those at higher temperature.
Can you recommend literature which allows a better understanding of the phenomena, the testing and the mitigation?
Eternal Sun: We presented a brief summary on our findings on testing LeTID at EU PVSEC 2018. The presentation can be downloaded here: Eternal Sun LeTID results . It provides an overview of the effect of temperature and testing different cell architectures.
Cells are treated individually for LID/LeTID. What if one out of the 72 cells in an individual panel wasn't treated effectively and still showed a degradation of 5% after exposure in the field? How much would the power output of the module be affected, due to mismatching in the serial connection?
JinkoSolar: The efficiency reductions that we monitor constantly in mass production are very consistent and limited within narrow ranges. Normally, the same LID and LeTID values can be measured at either the cell or module level, and that’s evidence of the high-level of quality control on these phenomena that is possible to achieve even at multi-GW mass production scale.
PI Berlin: This can be detected very precisely by electroluminescence. But on the power side it is more difficult. A 5% loss for one cell out of 72 cells in the module leads to less than 0.5% on the module level. The worst case experienced at PI was a 15% peak loss on the module level together with a 5% yield loss on the inverter string level after two years in the field under semi-arid conditions.
What is PI Berlin’s recommendation for investors with regards to buying PERC panels?
PI Berlin: We recommend asking the supplier to explain its counter-measures and labeling policies to prevent LeTID losses in the field. In the optimal case, this is covered by third party testing results. Some manufacturers already provide a warranty on 90% of the power after 20 years with a linear digression in between.
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