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Mono PERC degradation issues

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 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 degradation is real, and needs to be appropriately handled to avoid 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.

Due to ongoing responses from Panel Manufacturers, I’ve made some updates in Part 3 of this blog. All updated points will be in blue. The issue looks to be actively investigated by the big players of panel manufacturing, and a lot of them now have adequate responses. 

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?

PERC solar cells LeTID degradation

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 [8].

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.

Big claims?

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. [4] 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” [5].

Although Kersten et al. began the categorisation, I feel excerpts from Fertig et al. ‘s academic paper [7] 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 [8]:

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 [9] 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!

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.

Q.ANTUM PERFORMANCE WARRANTY

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.

Qcells Q.ANTUM LeTID PERC solar cell supression

  • 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.

temperature comparison Brisbane vs Nicosia for LeTID

Brisbane vs Nicosia temperatures. Months modified for easy comparisons.

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.

Qcells Q.ANTUM LeTID PERC solar cell supression

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) [10]

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) [11]

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) [12]

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) [13]

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.

PL Berlin LeTID Testing

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 CSP

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 cell degradation impacts

PERC Solar Modules: Risks and Mitigation Strategies (2017) [14]

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 [15] 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.

Summary

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 [15]. 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

Searching for information on LeTID was extremely difficult before I initially posted this article. All available information was in the form of scientific documents, with only one other article discussing the topic. Now, there’s plenty of information on the degradation effect, with various responses from many different panel manufacturers. I believe a reason for this initial lack of information was because R&D departments like to keep issues/breakthroughs hush hush, as making other manufacturers aware of the fact gives them a heads up they need to work on their own solutions.

So to get the data we needed, we’ve had to sign NDA’s to access internal documents, as releasing information regarding the product / processing steps would be greatly detrimental to the work put in by those R&D departments. Information is a game in of itself, and the main players of panel manufacturing need to keep on top things. They need to have solutions for unforeseen issues, such as this LeTID one, and PID in the past. I’ve updated this blog with all my current information from panel manufacturers, it’s highlighted in blue and can be found below. While I don’t have an answer for everyone, that’s not to say they have not solved the degradation issue. If you aren’t satisfied with the response from a panel manufacturer you are looking to install, the IEC 61215-2 draft standards for LeTID will be enforced for 2020 panel approval. Any panel that doesn’t pass, will not be allowed to be installed in Australia.

—–

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 LeTID perc solar cell

Results were achieved by putting different modules through 3 rounds of 162 hours at maximum power, at a temperature of 75°C.

REC

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.

Sunpower P-19 Series

Sunpower

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.

Winaico

Alison PERC solar cell degradationWe 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” [18]. 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.

Canadian PERC solar cell

Canadian

Canadian’s response has been fantastic, they’re one of the few that know what they’re talking about. I’ve been communicating my concerns and getting immediate responses in the form of data, graphs, etc. 

Canadian test their modules for LeTID at 85°C in CID, MPP mode, for 1000 hours. Their modules exhibit less than 2% degradation. These standards are well above the upcoming IEC 61215-2 draft standards, which require no more than 5% degradation at 75°C in CID, MPP mode, over 500 hours. It’s also important to note this test was conducted by a third party – the previously mentioned Alison Ciesla of UNSW.

I am confident in Canadian’s PERC modules and their ability to handle LeTID. These are one of the modules you could install with confidence even before the official IEC testing standards are enforced in 2020.

LG Solar

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

Jinko have released a statement saying their modules showed “strong results,” in a test carried out by “Wind Power Systems Quality Test Center, IEE, and CAS.” Their strong results were attributed to the company’s sustained and large investment in R&D. I am unable to find the results of the test, but the article can be found here:

JinkoSolar Creates New Testing Standard for Anti-LeTID properties in Solar Cells

I feel until more info is gathered, awaiting the enforced IEC standards in 2020 would be a safe bet here. You can’t be sure which PERC modules exhibited LeTID properties, and what standards the testing procedure meets. I’d be holding off.

Trina Solar

Trina hasn’t yet responded to my email inquiry. I would be holding off installing Trina PERC modules until the testing is enforced by IEC in 2020.

Longi

Longi seem to be well aware of the issue. I have received a document on their understanding of LeTID, and their mechanisms and solutions for it. I have not yet cited third party testing, but that’s not to say LeTID is unmitigated in Longi modules, as they are a relatively big player in the solar world. Since the main source of LeTID is the excessive hydrogen in the silicon wafer, and Longi as Mono suppliers have wafer-cell technology processes, there is a good chance they have mitigated the issue. Mitigating LeTID is significantly more problematic for module companies who purchase their cells from third party companies. Longi’s internal testing pass the draft IEC test with degradation rates within 1%, and state their standard warranty can cover LeTID.

Longi advises they are applying for a third-party test at TUV Rheinland with their mainstream products, which is expected to be completed mid-march.  If you’re on the fence, awaiting more information, waiting for the enforced 2020 IEC standards might be a good idea. However, I will update this again in April after seeing the results from TUV Rheinland, as if they pass this will trump the IEC test, and there would be no need to wait.

Risen

Risen have provided me with nice looking, in house documentation on their modules response to LeTID, which their results indicate less than 1% degradation. However, their test only lasted 96 hours, well below the literature’s recommended 500 hours. This is also a Risen document, not a third party test. There is also a possibility I have misunderstood the results, but after seeking clarification the test was indeed for just 96 hours, I haven’t heard back after 2 months. I would definitely be waiting until the enforced IEC 2020 standards before proceeding with this module. 

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:

Hanwha Q-Cells Delivers Market Leading Anti LeTID Performance in Fraunhofer CSP Test

Fraunhofer CSP PERC LeTID Degradation results

Fraunhofer CSP Test 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 [19]. 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 [2]. 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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Ben Neville

Technical Sales Manager

MC Electrical

 

 

PERC solar cell researchCitations:

[1] DNV.GL. PV Module Reliability Scorecard Report; 2017, Contributors: Meydbray J, Dross F.

[2] Krauss K, Fertig F, Menzel D, Rein S,. Light-induced degradation of silicon solar cells with aluminium oxide passivated rear side. In: 5th International Conference on Silicon Photovoltaics, SiliconPV; 2015, p. 599-606.

[3] Herguth A, Derricks C, Sperber D. A Detailed Study on Light-Induced Degradation of Cz-Si PERC-Type Solar Cells: Evidence of Rear Surface-Related Degradation. In: IEEE Journal of Photovoltaics; 2018, p 2156-3381.

[4] Ramspeck K, Zimmerman S, Nagel H, Metz H, Gassenbauer Y, Birkman B, Seigl A. Light induced degradation of rear passivated mc-SI solar cells. In: Proc 27th EU PVSEC; 2012, p 861-865.

[5] Kersten F, Engelhart P, Ploigh HC, Stekolnikov A, Lindner T, Stenzel F, Bartzsch M, Szpeth A, Petter K, Heitmann J, Muller JW. A New mc-Si Degradation Effect called LeTID. In: IEEE; 2015.

[6] Kersten F, Engelhart P, Ploigt H, Stekolnikov A, Lindner T, Stenzel F, Bartzsch M, Szpeth A, Petter K, Heitmann J, Muller JW. Degradation of multicrystalline silicon solar cells and modules after illumination at elevated temperature. In: Solar Energy Materials & Solar Cells; 2015, p 17-21.

[7] Fertig et al. Mass production of p-type Cz silicon solar cells approaching average stable conversion efficiencies of 22%. In: 7th International Conference on Silicon Photovoltaics, SiliconPV; 2017, p 338-345.

[8] Kersten F, Heitmann J, Muller JW. Influence of AL203 and Sinx passivation layers on LeTID. In: 6th International Conference of Silicon Photovoltaics, SiliconPV; 2016, p. 828-832.

[9] Kersten F, Fertig F, Petter K, Kloter B, Herzog E, Strobel MB, Heitmann J, Muller JW. System performance loss due to LeTID. In: 7th International Conference on Silicon Photovoltaics, SiliconPV; 2017, p. 540-546.

[10] Zuschlag A, Skorka D, Hahn G. Degradation and regeneration in mc-Si after different gettering steps. In: Progress in Photovoltaics: Research and Applications; 2016.

[11] Jensen MA, Morishige AE, Hofstetter J, Needleman DB, Buonassisi T. Evolution of LeTID Defects in p-type Multicrystalline Silicon During Degradation and Regeneration. In: IEEE Journal of Photovoltaics; 2017, p 2156-3381.

[12] Wagner M, Wolny F, Hentsche M, Krause A, Sylla L, Kropfgans F, Ernst M, Zierer R, Bonisch P, Muller P, Schmidt N, Osinniy V, Hartmann HP, Mehnert R, Neugaus H. Correlation of the LeTID amplitude to the Aluminum bulk concentration and Oxygen precipitation in PERC solar cells. In: Solar Energy Materials and Solar Cells; 2018, p 176-188.

[13] Chen D, Kim M, Stefani BV, Hallam BJ, Abbott MD, Chan CE, Chen R, Payne DNR, Nampalli N, Ciesla A, Fung TH, Kim K, Wenham SR. Evidence of an identical firing-activated carrier-induced defect in monocrystalline and multicrystalline silicon. In: Solar Energy Materials and Solar Cells; 2018, p 293-300

[14] Chawla D, Tolliver T. PERC Solar Modules: Risks and Mitigation Strategies. In: ICF; 2017.

[15] Kenning T. PV ModuleTech 2018: PERC field performance and bringing modules to market with JinkoSolar. In: PV Tech; 2018.

[16] Nakayashiki K, Hofstetter J, Morishige AE, Needleman L, Jensen MA, et al. Engineering Solutions and root-cause analysis for light-induced degradation in p-type multicrystalline silicon PERC modules. In: IEEE Journal of Photovoltaics; 2016, p 860-868.

[17] Eberle R, Kwapil W, Schindler F, Schubert MC, Glunz SW. Impact of the firing temperature profile on light induced degradation of multicrystalline silicon. In: Phys. Status Solidi RRL; 2016, p 1-5.

[18] Ciesla AM, Wenham S, Chen R, Chan C, Chen D, Hallam B, Payne D, Fung T, Kim M, Liu S, Wang S, et al. Hydrogen Induced Degradation. In: 7th World Conference on Photovoltaic Energy Conversion; 2018.

[19] International Technology Roadmap for Photovoltaic (ITRPV). Ninth edition; 2018.

30 Comments on PERC Solar panels – Degradation through LeTID

Ben Neville said : administrator Report 2 weeks ago

Hi Rob, Risen has been a difficult one to confirm. All of their documents on their degradation rates from LeTID are in house, and only tested up to 76 hours. There was an alternative Powerpoint presentation showing testing for 200 hours, which is better, but still below the bare minimum of 500 hours the CEC will enforce. At least Risen are responding to LeTID, but I would be much more comfortable waiting for that third party testing. Risen are probably below mid range, so my alternative pick here would be stepping up to at least a Canadian panel.

Rob said : Guest Report 2 weeks ago

Hi Ben, i have been given a quote to install Risen MonoPerc 315w, and after reading this article i am unsure whether to proceed. Could you recommend the best medium range panels in your opinion please.

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Ben Neville said : administrator Report 3 weeks ago

Hi Kevin. Not as yet, but I'm sure there's a reasonable explanation. I have chased them up directly for a response and will let you know.

Ben Neville said : administrator Report 3 weeks ago

Hi Will, Thanks for the kind words. Like you said, Longi produces their own cells, so they do have a strong say in quality - much like REC, Qcells and Sunpower (Maxeon range). Longi was one of the few manufacturers that knew what I was talking about, and their response was reasonable. I am pretty confident Longi do not have it "solved," but have it significantly minimised much in the same way REC do. They have submitted their panel for third party testing which is a strong sign of confidence, so I imagine a press release to be out soon on their results. I will add amendments to the blog when it's out.

Kevin said : Guest Report 3 weeks ago

Any updates from the Longi third-party testing at TUV Rheinland yet, you mention that they should have completed mid-march?

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Will said : Guest Report 4 weeks ago

Hi Ben, Thank you for your very informative (and worrying!) article. I installed 22x300W PERC Longi panels at about the same time your article came out last year. Have you heard anything back from Longi on their third-party test that was supposed to be completed in mid-March? I am keeping my fingers crossed that as Longi produces their own cells (one of the reasons i choose Longi) they have the issue under control. Kind regards, Will

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Ben Neville said : administrator Report a month ago

Hi Steve, Good find, I'll have to update that, they're both listed as multi. I've found documents regarding recent research are constantly being updated. This is one of those cases. If you refer to the whitepaper linked within the blog, you'll find they've changed their table data. The Multi PERC degraded by 1.1%, while the Mono PERC degraded by 1.9%

Steve Craig said : Guest Report a month ago

Hi Ben, in the REC table there are 2 columns for the twin peak 2 with the same cell type, is one mono and the other multi? If so which ones which?

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Ben Neville said : administrator Report 2 months ago

Hi Michael, Not yet. I will update the post as soon as they do.

Michael said : Guest Report 2 months ago

Have sunpower come back about LeTID with the performance p19

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Ben Neville said : administrator Report 2 months ago

Hi Margaret, I would yes. REC were one of the first to have a reasonable understanding of LeTID and seemed to have dealt with it well.

Margaret said : Guest Report 2 months ago

Are you at the position yet where you would be recommending the REC TwinPeak 2 series which have a 20 year warranty

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Ben Neville said : administrator Report 4 months ago

Hi Robert. I would recommend you simply keep an eye out on your system performance to detect any degradation. I imagine Canadian Solar will honour their warranties should a case arise. This blog post aims to assist with pre-install decisions, but also to encourage post install monitoring in light of the new information.

Robert Dobson said : Guest Report 4 months ago

We installed 10.5kw of 325 watt Canadian mono perc modules last March. I wish I had bought one extra solar panel to hold as a control to compare to a panel on the roof. I'm now working back through the bv distributor to determine if they are aware of this issue.

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Paul said : Guest Report 4 months ago

Thanks Ben, for expanding/clarifying the test requirements for LeTID. I can now see and completely agree that the "thermal cycling" tests done by DNV GL really does not simulate the conditions needed for the LeTID test (heat and Light @ maximum power). Hopefully DNV GL have read this great article and they will add LeTID testing to their barrage of tests pronto.

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Ben Neville said : administrator Report 4 months ago

Thanks Paul! Thermal cycling from DNV GL increases the IEC's 200 cycles to 600 cycles, which is a good start. However, although the temperatures exceed the trigger for LeTID, the test more covers temperature fluctuations (-40°C - 85°C), and it does not combine all conditions (light and heat). Also, the 85°C temperature is actually too high for LeTID detection, in that at these temperatures a regeneration phase kicks in, but in the real world this wouldn't be seen until after the warranty period of the panel. The literature calls for 75°C temperature at MPP (maximum power) to test for LeTID, which doesn't exist in current testing standards. The draft standards for this test exist, are are going to be enforced in 2020.

Paul said : Guest Report 5 months ago

Thank you MC Electrical for another great (and informative) article. Do you think that the “top performing” panels from the DNV GL “thermal cycling” tests give us any indication of if that panel is likely to be LeTID resistant or not?

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Mark C said : administrator Report 5 months ago

Good point Sam. It's very difficult to prove. We monitor all of our systems online, so if you saw a significant drop in monthly production figures (compared to the previous years) you could assume it was the panels. This is assuming trees haven't grown bigger). You could also compare peak production on similar days across different years. (In the case of SolarEdge, it's even harder to prove, because we have found several installations where SolarEdge optimisers were underperforming. That's a real pain to prove!)

Sam said : Guest Report 5 months ago

Also wondering how to approve your panels are degrading faster as should be expected.

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Ben Neville said : administrator Report 5 months ago

Hi Stuart. It seems all the REC modules are up there with LeTID resistance, not just the N-type. Hanwha Q Cells and Hanwha Group is not involved in land mine production. Hanwha Q Cells is an independent, separate entity from Hanwha Corporation. It's also important to note that because Hanwha Q Cells is a NASDAQ listed company, it's bound by strict rules and regulations of U.S. Securities and Exchange Commissions. This means that even Q Cells parent company cannot control Hanwha Q Cells on a whim, and that each company has a clear boundary in business operations.

Ben Neville said : administrator Report 5 months ago

Hi Dan, thanks for the kind words. It's hard for us to keep up with the ever changing PV industry - even though we are on to this issue relatively early, we've still installed a lot of PERC modules. All we can do for now is hold off and wait for third party testing to ease our doubts on the existing systems we've installed. I hope by bringing it to everyone's attention, our customers and us will keep a much closer eye on these PERC systems, and if they're degrading within expectations. Best, and most likely case scenario, is in a few weeks we get results from Sunpower indicating strong LeTID resistance. Sunpower seem very confident with the pending results, but even worse case scenario they don't come back favorable, you can have confidence Sunpower will honour their warranty promises for any modules that don't handle LeTID.

Ben Neville said : administrator Report 5 months ago

Hi Sam. Interestingly another article shares your thoughts, in that there is every potential for an influx of warranty claims that discredits the PV industry. I imagine Panel Manufacturers will honour their linear warranty should a consumer notice degradation outside of the promised range. My thoughts are cheaper PERC manufacturers whose panels are already pushing their degradation figures will be hit hardest. Hopefully higher quality panels may have enough "warranty buffer", and even some LeTID tolerance to absorb the additional unexpected degradation. Time will tell.

Sam said : Guest Report 5 months ago

Thanks for sharing this imho urgent issues with PERC panels. What would manufactors do with the performance guarantees on their panels? This could be a serious issue for them if the have to replace many and many panels. I don't hope bankruptcies are the way for avoiding guarantee claims. Bad news for PV industry....

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stuart said : Guest Report 5 months ago

Great article and Im glad to hear that the REC TP2 are at least up there the new REC Ntype should be ok?? Hanwha Corp. is involved in the production of cluster munitions ?? Banned by UN?? is this relevant to our industry? thanks

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Ben Neville said : administrator Report 5 months ago

Hi Sam. It's relatively well documented that the IEC 61215 is NOT substantial panel testing criteria. It does its best to identify infant mortality, but panel manufacturers are encouraged to source additional testing to give consumers a better idea of panel quality. Only recently have those testing bodies recognised LeTID, and it's reported they will add the relevant testing procedures to test for LeTID in the future.

Ben Neville said : administrator Report 5 months ago

HI Gary, I will see if I can chase it up for you. We found it extremely hard to find people aware of the issue. We are often referred to engineers from overseas, who can take time to respond.

Gary said : Guest Report 5 months ago

Any Info on Risen PERC 315 Watt Panels RSM120-6-315M regarding LeTID? I have been using quite a lot of these since they became available.

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Dan Graham said : Guest Report 5 months ago

Great article. I have been very impressed with the professionalism of MC Electrical in this industry. I guess my biggest concern is that MC Electrical just installed 10 kW of SunPower P19-325 in the last week of Oct 2018. So do I now need to be concerned with these panels? Thanks Dan

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Leon said : Guest Report 5 months ago

This is an excellent article. Love Q-Cells. Particularly Q-Peak.

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Sam Denso said : Guest Report 5 months ago

so......seems the IEC standards are too week since most manufacturers of solar modules pass IEC, yet degrade when placed in the sun....

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