Solar panels are getting bigger thanks to a technology called PERC. Just two years ago, a 250W panel was the norm. Today, just about every panel manufacturer is offering Mono PERC solar modules and Poly PERC solar modules at 300W. Some are even offering an extremely affordable 330W Mono PERC module. The aim of this post is to warn of the very real degradation problems facing PERC solar cell technology.
Part one will give a basic overview of PERC, PERC solar cell efficiency, and an explanation of the significant degradation concerns facing PERC solar cell structure.
Part two will get much more in-depth citing peer review papers. I will provide extensive evidence that the problem of the PERC solar cell structure is most likely not “a storm in a teacup,” but maybe a looming disaster.
Part three consists of the replies that we have to date from PERC panel manufacturers showing that some, but not all, are taking the issue of LeTID in PERC solar cells seriously.
Part 1 – PERC – The degradation issue
What is PERC?
PERC stands for Passivated Emitter and Rear Cell. PERC was invented right here in Australia in 1980 but has only started taking off in the solar industry in the last three years. Solar panel cells can be divided into two categories:
- N-Type Silicon – High-end panels such as LG Neon and Sunpower (E and X series) solar panels use expensive N-type silicon. This premium silicon allows for the most efficient cells, meaning the highest wattage panels on the market.
- P-Type Silicon – Most solar panel manufacturers use P-Type silicon. P-Type silicon is significantly less expensive, but P-Type cells are also less efficient, meaning they make lower wattage panels. P-Type modules currently dominate the industry
PERC is a technology that increases PERC solar cell efficiency in the more affordable P-Type silicon modules. Thanks to PERC solar cells we can now get affordable panels at higher wattage.
The looming PERC disaster
Solar panels degrade over time. Traditionally, degradation has been tested and accounted for in panel specifications. While higher quality panels degrade at a slower rate, one per cent per year is acceptable for entry-level solar panels.
However, as PERC technology has become ubiquitous in the solar industry, solar academics are uncovering a very real problem with a new type of degradation in PERC solar panels. It’s called LeTID.
What is LeTID?
LeTID stands for Light and Elevated Temperature Induced Degradation. LeTID is the far more severe (and less understood) phenomenon of panel degradation. Field testing of PERC solar panels has shown degradation rates in excess of 8-10% in just three years of field testing .
That’s a problem. And it’s a bigger problem that currently LeTID is not adequately tested for by most panel manufacturers. It seems panel manufacturers are throwing caution to the wind.
The solution to LeTID
The problem of LeTID is not insurmountable. In fact, the problem has been solved completely by one company, and I have no doubt other companies will follow. However, I believe if you are looking into purchasing PERC modules, you should strongly consider selecting those PERC solar cell manufacturers who have independent documented results on how their PERC modules are coping with LeTID.
These are big claims, and big claims deserve evidence. I’ve invested considerable time reading peer-reviewed research papers and talking with PERC solar cell manufacturers before coming to this conclusion. Get your geek on, here’s a long-winded summary of our findings:
Part 2 – PERC LeTID Degradation –
a deeper look.
LID vs LeTID
LID: Light Induced Degradation. A form of degradation experienced by a panel during its first few days in the sun.
LID is a well understood, and a well-documented form of degradation [1, 2, 3]. Manufacturers take this into account by factoring a 3% power loss during the first year on the module’s linear warranty. If LID were the only problem, there would be no problem. Instead, we are looking at LeTID:
LeTID: Light and Elevated Temperature Induced Degradation. The much more severe and relatively undocumented, unreported, untested phenomenon.
Discovering LeTID in Multi (Poly) PERC Solar Cells
The first evidence of PERC solar cell degradation at elevated temperatures was identified in Multicrystalline solar cells. Ramspeck et al.  found that PERC solar cells can exhibit significantly strong power degradation compared to non-PERC solar cells. However, the issue was first categorised (again in multicrystaline panels) as LeTID by Kersten et al. (remember that name) in 2015, in their paper titled “A New mc-SI Degradation Effect called LeTID” .
Although Kersten et al. began the categorisation, I feel excerpts from Fertig et al. ‘s academic paper  summarises the potential issues surrounding it best:
If not adequately suppressed, so-called “Light and elevated Temperature Induced Degradation” (LeTID) can be a severe issue for the long-term stability of PERC …. LeTID can occur in P-type PERC with a degradation in output power of up to >6%, which cannot be suppressed in a straightforward manner by conventional processing steps …. this severe risk of LeTID to the long-term stability in the field may remain undetected.
Discovering LeTID in Mono PERC Solar Cells
Fertig et al. paper also importantly shows that LeTID does occur in Monocrystalline PERC solar cells, not just the Multicrystalline PERC solar cells which is what was originally thought. Through testing, the authors found Mono PERC modules degraded by 6% after 400h. They also applied a commercially available BO (Boron-Oxygen) stabilisation processing step to the modules, and still found 4% degradation after 400h at 75°C. This degradation percentage of Mono PERC modules matched the degradation found in Multi PERC modules with medium LeTID. There’s additional literature which I will reference later that expands on this work to give further evidence of LeTID being an issue with Mono PERC solar cells, not just Poly (Multicrystalline) PERC solar cells
Further PERC LeTID testing
Kersten et al. went on to complete further testing in 2016, looking at the influence of Aluminum oxide passivation layers on LeTID :
The degradation mechanism called LeTID, can cause a module power degradation level of above 10% and might be a potential obstacle for the industrial realization of the PERC technology.
They concluded the journal article suggesting there is no physical explanation for this type of degradation, and further work is ongoing in order to obtain a deeper understanding of LeTID and its impacts on lifetime samples of PERC solar cells.
Prevention of LeTID
In 2017, Kersten et al. performed both accelerated laboratory tests and field tests to further better understand LeTID, and prove their technology was the solution. They did so and published “System performance loss due to LeTID” in 2017  which documented their results. It’s at this point of the post that I reveal who Kersten, Fertig and others work for. None other than our much-loved Q-Cells!
Whoa, big reveal. Unless of course, you came into this post researching a bit about Q-Cells, and their Q.ANTUM technology.
Q-Cell solves LeTID
This means Q-Cells were the first to categorise the phenomenon of LeTID, so it is understandable they were the first to solve it. Their Q.ANTUM panel range is the “secret sauce” to LeTID mitigation. It utilises a type of PERC technology, so they can produce high wattage, high PERC solar cell efficiency, with the cost-effective P-type silicon, but without the severe LeTID degradation issues. They have not released their technology secrets to the public, so at the moment they are the only ones with a solution to fully mitigate LeTID.
These engineering breakthroughs also meant they could improve their initial degradation claims and their ongoing yearly degradation levels. Their new Q.Peak Duo panel is the highlight of years of R&D, which enabled them to take home the Intersolar panel award this year, beating out the highly anticipated LG Neon R.
Marketing spiel over, back to the paper!
Results of Kersten’s experiment
The field tests performed by Kersten et al. involved installing panels for three years in Cyprus and Germany. The panels were removed once every three months to test them under standard test conditions before reinstalling them outdoors. These results were compared to accelerated degradation tests in the laboratory, which came back with similar degradation levels. The below graph shows the results of their experiment and highlights the severe degradation issues of LeTID in standard PERC modules.
- The blue filled in squares show the degradation rates of Q.ANTUM modules in the field at Cyprus.
- The orange triangles are the results of the degradation rates of PERC modules in the Germany field test.
- The black squares are the results of the degradation rates of PERC modules in the Cyprus field test.
The standard PERC modules saw an initial 2% drop as a result of LID of the modules in both Cyprus and Germany. More importantly, the PERC modules saw a further 6-8% drop as a result of LeTID over 3 years (conversely, Q-Cells Q.AUNTAM only saw a total 1% initial degradation).
Light & ELEVATED TEMPERATURE induced degradation
This additional drop is much more evident in Nicosia, Cyprus, as it experiences much higher temperatures and irradiation levels. You might argue the effects of LeTID is not much to worry about in the Germany results, but our climate in Brisbane is much closer to that of Cyprus.
You’ll notice Q-Cells had no field data beyond three years. This is because LeTID is a relatively new categorisation, so much more testing is required. However, you may also notice the lab results align relatively closely to the field results, so Q-Cells were able to project their technology in the lab to over 4000h, which corresponds to more than 15 years in a harsh field environment. This enabled Q-Cells to conclude their Q.ANTUM modules were completely free of LID and LeTID.
Other Articles on LeTID
Although Kersten et al. ‘s work has been peer-reviewed for publication (meaning it’s not reasonable to call it marketing propaganda), I still feel it’s important to investigate the greater scientific communities analysis and findings on LeTID. The timeline starts with Multicrystalline observations in 2016, moving on to Monocrystalline observations in 2017-2018.
Below are several journal articles which were written after LeTID’s classification:
Degradation and regeneration in mc-SI after different gettering steps (2016) 
This academic paper by Zuschlag et al. measured the Voc (open circuit voltage) of an industrial mc-Si PERC solar cell through accelerated testing, to conclude it was obviously LeTID sensitive, showing a degradation of approx. 12% after 200h at elevated temperatures. The paper found that the concentration and distribution of metal impurities play a major role in LeTID. The authors suggested the PERC solar cell structure processing sequence should be optimised to avoid impurities during the temperature steps.
Evolution of LeTID Defects in P-type Multicrystalline Silicon During Degradation and Regeneration (2017) 
This academic paper by Jensen et al. showed a prominent LeTID defect appears after 8 minutes of exposure to 0.4 – 0.5 suns at 70-80°C. The defect increases in concentration until the point of maximum degradation. The authors recognised the engineering progress made to suppress LeTID, and delved into deeper details regarding its root cause. They looked into and compared Hydrogen, Cobalt, Oxygen and others, but concluded further research was required to pinpoint the exact LeTID defect parameters.
Correlation of the LeTID amplitude to the Aluminium bulk concentration and Oxygen precipitation in PERC solar cells (2018) 
This academic paper by Wagner et al. found strong LeTID amplitudes in PERC solar cell structure made of Boron-doped Cz (Monocrystalline) silicon material. Through statistical analysis, they found a strong correlation to the bulk concentration of Aluminium in cells originating from wafers close to the tail. They also found LeTID was strongly enhanced whenever Oxygen precipitation occurs in the cell process. The authors concluded further research was required as the exact configuration of the defect could not be extracted from their data.
Evidence of an identical firing-activated carrier-induced defect in monocystalline and multicrystalline silicon (2018) 
This academic paper by Chen et al. expanded on Fetig et al. ‘s work to confirm LeTID in Monocrystalline PERC modules. The paper presents experimental evidence that the defect responsible for carrier-induced degradation (LeTID) found in Multicrystalline, can also be found in Monocrystalline substrates. Since the paper finds the defect in both complex crystalline structures, the results provides important insights into the possible root cause of the defect. The minimal changes in surface passivation quality of both crystalline materials suggests that the degradation arises as the result of bulk defect formation, rather than surface instability. Ongoing research is recommended and required, but these recent findings are important going forward to better understand LeTID in all PERC solar cells.
Is LeTID degradation in PERC cells another degradation crisis even worse than PID? (2018)
By the title, you can imagine this isn’t an academic paper. It’s an article written as recent as November 2018, by Dr Radovan Kopecek, who suggested that PERC degradation may be worse than most people think. Even the responsible scientists for PERC production are unaware of the severe degradation effects of PERC, in particular, LeTID.
Kopecek also cites a test carried out by Pl Berlin, who found 5% degradation of a commercial Mono PERC module within just six weeks. Much like Kersten et al., they are continuing to run accelerated tests to investigate LeTID further. I think the closing lines of Kopecek’s blog piece summarises our thoughts as well:
The awareness of this issue by the main players, i.e. cell & module manufacturers on the one hand, and system installers on the other hand, will be important to avoid a flood of claims, due to dramatically underperforming PV systems, and therefore avoid a potentially severe negative impact on the credibility of PV as a whole.
Fraunhofer institute for Solar Energy Systems list specific degradation risk on their website and analyse these through testing. Common, well-known degradation risks of PERC solar modules include PID, snail trails, UV-induced degradation, and LID. However, they have a relatively new paragraph specifically referencing LeTID:
In particular, PV modules with rear passivated cells of the so-called PERC technology can be affected by “Light and Elevated Temperature Induced Degradation” (LeTID). In contrast to light-induced degradation (LID), these modules run the risk of slow but massive power degradation, which can fully or partially regenerate over the long term.
PERC Solar Modules: Risks and Mitigation Strategies (2017) 
This article written by Chawla and Tolliver indicated equipment manufacturers are working diligently to develop commercial tools for LeTID suppression, based on the approaches recommended by the literature. However, they admitted the knowledge-based associated with the LeTID suppression in PERC solar cells is limited.
PV Tech quizzed JinkoSolar  on the effects of LeTID, asking if they knew of any new types of tests becoming essential/unavoidable in the future. JinkoSolar responded that LeTID lab tests are maturing rapidly, and even starting to be accepted by some major independent engineers. Future LeTID testing may be unavoidable in the future.
There’s plenty of information out there on LID issues with PERC solar modules, but not a great deal out there at the moment on LeTID. Most PV installers aren’t talking about it yet, or aren’t aware of it. I could continue with more citations, but I hope you get the point – LeTID is an additional degradation on top of LID, and is believed to not be adequately covered in manufacturers’ linear warranty specification promises. This is simply because adequate testing is still being developed to assess LeTID. The issue was first thought to be exclusive to Multicrystalline (Poly) PERC, but more recent articles are showing Mono PERC is just as susceptible. As a result, there is every potential in the future for severely underperforming systems which use PERC solar cells, or an influx of performance warranty claims. However, that’s assuming the homeowner notices the issue in the first place.
The root cause of the degradation caused by LeTID remains unclear. Some studies link it to the higher temperatures in processing solar cells . Other studies show evidence for Hydrogen as the detrimental force causing LeTID [16, 17, 18]. As it’s still not completely understood what causes it, ongoing research is required. I feel the best way to deal with PERC degradation, for now, is to avoid selling modules without detailed LeTID results, or to have a preventative measure for it such as Q-Cells Q.ANTUM PERC.
PART 3: Panel Manufacturer Responses
With the high demand for PERC and its quick progression, there is no doubt other manufacturers are going to solve LeTID to an acceptable extent. So, if anything in the industry changes or you have some information I am lacking, please share in the comments below and I will update this section. It’s not our intention to shun a good technology, but it’s also not our intention to sell a technology that has proven degradation issues without an adequate answer. If you were looking to install PERC modules other than Q-Cells, independent testing for LeTID to show minimal degradation on those modules would be the best outcome.
REC have indicated their Mono PERC, and Multi PERC modules are LeTID resistant in this white paper.
The adjacent table shows LeTID was more pronounced in their PERC panels compared to their older non-PERC panels. This agrees with the literature. They also suggested that the occurrence of LeTID is reduced through quality manufacturing. It’s important to note that REC, like Q-Cells, manufactures their own cells. As such, their modifications in the cell manufacturing process has minimised the impact of LeTID. While most manufacturers assemble their own modules, they still import their cells. I believe if you purchase REC PERC modules, you can have confidence their future performance will not be susceptible to severe LeTID, and that any degradation should fall within their linear warranty expectations.
We approached our friends at Sunpower about our LeTID concerns with their new Chinese made P-19 panels. Sunpower ensured us that the PERC cells they purchase are high quality, but they could not show us independent results of LeTID testing. So in early December, we made an inconvenient switch from selling Sunpower P-19 325W panels to the new Q-Cells 325W DUO panels as our mid-range panel.
But, how’s this for a positive response: Sunpower are now submitting their panels to be tested by a third party to measure the degradation effects of LeTID! Until we see the results, we will only sell the high-end Sunpower 327W E and 360W X Series, N-type silicon modules. If the results of Sunpower’s 325W panel come back showing minimal degradation from LeTID, we will have full confidence selling Sunpower’s 325W P-19 modules again. We expect the results early next year.
We approached Winaico to see if they could shed any light on the topic. They put us in contact with Alison Ciesla of UNSW. Alison has been studying LeTID for a while, looking at Hydrogen’s involvement in the degradation process. I went hunting for her article, titled “Hydrogen Induced Degradation” . The paper exposed four different Multicrystalline PERC modules to 75°C, for seven days, to analyse the degradation of LeTID. The results showed “enormous variations in degradation across modules,” but that none were immune to LeTID. The authors concluded the degradation impacts have the potential to become more severe as efficiencies increase. They, like many others, recognised the importance of developing tests to identify and quantify the degradation. While it was a good response from Winaico to investigate the issue, they could not confirm whether the cells they purchase are LeTID resistant.
We have had no response from Canadian so far. It does seem they are aware of the LeTID issue, as I found on an online marketing PDF indicating “No LeTID for CSI P4 Modules.” However, the “proof” is a direct rip from Kersten et al. ‘s paper on Q-Cells’ results! I don’t know if this is a legitimate Canadian document, or one done up by a sales company. We will await Canadian’s response.
Because we sell Sunpower, LG are traditionally our frenemies. The two best panels on the market are known for taking jabs at each other. But here I’ll break step and toot LG’s horn. LG have stopped all P-Type production, so apart from their run-out panels, LG can avoid the severe degradation issues of LeTID.
Jinko hasn’t yet responded to my email inquiry.
Trina hasn’t yet responded to my email inquiry.
Longi hasn’t yet responded to my online inquiry.
Q-Cells (Third Party Testing)
Regardless of all this information coming directly from Q-Cells, the peer-reviewed process should still allow us to consider the results accurate. However, just like alternative literature was important, so too is alternative testing. A third party – Fraunhofer CSP, shared a press release on its LeTID results as recent as December 6, 2018. They performed independent accelerated laboratory tests over 700 hours (which equates to just under three years in the field) on nine different Mono PERC and Poly PERC modules, at 75°C in MPP mode. You can follow the link below to see the results:
The results of the test were presented at the EU PVSEC conference in September titled: Benchmarking Light and Elevated Temperature Induced Degradation (LeTID). The graph to the left summarises the results. We are seeing Q.ANTUM immune to any LeTID degradation after 700 hours and all other Mono PERC and Poly PERC modules, bar one, suffering varying degrees of degradation. I would be very interested to know which other module performed just as well as Q-Cells, as they would be another PERC module consumers should look at installing. My suspicions lie with REC.
The results were from industry leading modules, and we’re still seeing severe degradation even at just 700 hours. It’s clear more research needs to be done on LeTID so the necessary steps can be undertaken to avoid it during the manufacturing process.
Conclusion and Closing Thoughts
The future degradation issues facing PERC is much of an unknown, as existing standards for panel testing do not take this form of degradation into account. Future testing standards are about to be deployed, which is vital for the industry going forward . LeTID was thought to only effect Multicrystalline modules, but recent evidence shows similar degradation levels in Monocrystalline modules [3, 7, 12, 13]. I have confidence the degradation mechanisms associated with PERC solar cells are going to be mitigated, but there’s every possibility existing modules out there are going to have severe degradation issues in the near future. Q-Cells has the engineering answer to LeTID in their Q.ANTUM technology . As such, this is the only form of PERC technology we are offering going forward. We are making the tough decision to drop the Sunpower P-19 series until we receive adequate testing back which shows strong LeTID suppression. We still recommend Sunpower E series as they use N-type silicon, so degradation concerns are significantly reduced.
Ongoing research and work is required to better understand LeTID in PERC solar cells and to find its root cause. A deeper understanding will bring about better solutions for cell manufacturers. Because LeTID arises at the cell level, it’s clear panel brands who manufacture their own cells have a strong ability to mitigate LeTID. I will add any relevant updates to Part 3 of this post.
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