How UV can lead to degradation in TOPCon modules and the latest on identifying potential problems
The rapid adoption of the next generation of PV cells and modules has brought the issue of UV induced degradation (UVID) back as a topic of discussion. Manufacturers and laboratories are working to develop a better understanding of the phenonium and ways to ensure the latest generation of TOPCon modules go the distance.
There is no doubt that TOPCon technology has won the race, at least in the short term, as the commercial solar cell technology of the day. An acronym of Tunnel Oxide Passivated Contact, TOPCon is displacing mono-crystalline PERC from the solar market in a remarkably short period of time.
Its other n-type rival, heterojunction (HJT) solar cells, very much has its place – the REC Alpha series an excellent example – but in terms of production volumes, TOPCon has surged past the rest.
While a simplification, the primary reasons for TOPCon’s supremacy – in the short term at least – is that it is more of a progression than step change for manufacturers. While HJT represents a simplified production process when compared with TOPCon, it is also more of a technological leap. And HJT uses a lot of silver, in its metallization – a major cost hurdle for producers.
A look at the latest ITRPV Roadmap shows that international experts expect TOPCon to take about 49% market share in 2024 – leaving about 40% to PERC and 10% for HJT. And, according to the experts who contribute to the ITPRV, PERC will likely be squeezed from the market next year.
Degradation challenges
The upside of TOPCon is pretty huge. Its theoretical maximum cell efficiency is 28.7% – compared to 24.5% in PERC. And manufacturers have made rapid progress towards that target. Trina has achieved 23.2% module in its Vertex range – with cell efficiencies considerably higher.
However, there have been concerns about TOPCon’s durability since early 2023. And testing results, like the Kiwa PVEL scorecard, have shown a wide range of outcomes for TOPCon modules – some good, others quite worrying.
It is certainly true that a common story within the development of the PV industry has been initial growing pains as the industry transitions to a newer, higher efficiency technologies.
The experts from DNV make this case in a short paper they have prepared about degradation in TOPCon modules. DNV’s Henry Hieslmair provides a good summary of the pattern from the past: “With PERC, the industry was caught by surprise with light-induced degradation (LID) and light at elevated temperature-induced degradation (LeTID). With transformerless inverters, we learned about potential-induced degradation (PID) – the hard way.”
UVID re-emerges
Turning to TOPCon in 2024, one particular concern with the technology has been UV induced degradation (UVID), which the DNV experts address in their paper. While it may seem counterintuitive that PV modules react badly to sunlight, exposure to harsh UV rays does appear to be cause the power output from some PV cells to drop off. It can additionally cause degradation to materials like certain encapsulants, or EVA.
It's an issue that should be monitored closely by the Australian PV industry, with plentiful sunshine both a blessing and a curse.
UVID has been turning up in testing. In its latest scorecard results, PVEL reports that one module it tested showed 5% power loss after 60 kWh/m2 of UV testing and more than 12% following 120 kWh/m2. It notes that a checkerboard pattern is evident under electroluminescence (EL) imaging – which is also what is seen when LeTID is picked up.
The PVEL team add that both TOPCon and HJT modules are currently being supplied with extended warranties, given that negatively charged (n-type) solar wafers, which do not exhibit Light Induced Degradation (LID). If UV is causing degradation after a relatively short period of time, these warranty claims might not stack up.
PVEL says that 120 kWh/m2 is the equivalent of 6-24 months of outdoor exposure. It would be safe to say Australian conditions lie in the lower end of that range.
Quality assurance provider Kiwa PI Berlin confirms that it has started to see TOPCon modules in the field exhibiting this patter. Kiwa acquired PVEL and PI Berlin in 2021 and 2022 respectively.
“We are starting to see these patterns – similar to LeTID and PID – a checkerboard pattern in TOPCon modules in southern Europe that have been operation for six months to one year,” says PI Berlin’s Steven Xureb. “Now we’re doing these sensitivity tests afterwards on spare modules, to see whether they are really UV sensitive.”
Xureb says there are many unknowns about UVID and its long-term impact, but he says the encapsulant material appears to be crucial as to how UV light impacts a module’s performance.
“It's clearly associated with the encapsulant,” says Xureb. “Encapsulants have UV blockers in them and it's obviously a balance of not cutting out too much of the light spectrum that reduces the efficiency of the cell versus the effect of the UV negatively on the degradation. I think that's a balance that some of the module manufacturers and the encapsulant manufacturers are dealing with.”
New tests
PI Berlin is currently working on a project to develop a methodology to identify the UVID early, to give module buyers more confidence in the long term performance of TOPCon products. The testing PVEL does is effective in picking up UVID, in part because it goes well beyond the 15 kWh/m2 of testing specified in the standard (IEC 61215). But for quality assurance, far quicker testing will be required.
“We’re trying to identify an early marker of UV degredation,” says Xureb. “What we're trying to do is see statistically if there's something that we could hang our hat on. If at 120 [kWh/m2], you're seeing 15% degradation, at 15 [kWh/m2], and at 60 [kWh/m2], can you already see a trend that's happening or is it something that really happens only in the last dosage?”
To conduct the testing, PI Berlin will be purchasing modules off the market. It will be testing modules that use both EVA and polyolefin (POE) as encapsulant material.
The PI Berlin hope to have some results as soon as October.
The technological progression of PV is undoubtedly a good thing – delivering more power for lower cost. But each change in technology brings with it risks. The emergence of UVID in TOPCon modules appears to be a continuation of that trend, but one that the PV industry is proving relatively quick to address.