Fraunhofer ISE warns of risk from higher than expected UV-induced degradation in TOPCon, PERC, HJT cells
Researchers at Germany's Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) have investigated the stability against UV exposure of three types of mainstream solar cell technologies – tunnel oxide passivated contact (TOPCon), passivated emitter and rear cell (PERC) and heterojunction (HJT) – and have found that all of them may suffer from severe implied voltage degradation.
They explained that UV-induced degradation (UVID) may lead to unexpected voltage and efficiency losses in the future, especially when a larger UVID track record may be available. “One prominent example of this is light and elevated temperature-induced degradation (LeTID), which has caused unforeseen losses in PERC modules during field operation,” they stated. “Recent reports suggest that a similar scenario could repeat itself due to UVID for all three of the modern cell architectures.”
The harmful effects of UV radiation have largely been associated in solar panels with UV-transparent module encapsulants and the aging of module packaging materials, which leads to encapsulant discoloration, delamination, and backsheet cracking. In particular, UV light can contribute to forming acetic acid on the module encapsulant, which corrodes the cell's contact grid. Solar cell performance is also adversely affected by UV radiation through the generation of surface defects. Within a silicon solar cell, the UV light can cause damage to the passivation layers, to the silicon beneath, and at the interface between the two.
“Currently, UV-transparent encapsulants are the standard for the module front side,” the research's lead author, Fabian Thome, told pv magazine. “The use of UV-blocking encapsulants could certainly be a strategy to reduce UVID but this comes at the cost of a lower module efficiency. We know of some manufacturers that already use this strategy. It appears to be a good intermediate solution until UVID is resolved at the cell level.”
In the study “UV-Induced Degradation of Industrial PERC, TOPCon, and HJT Solar Cells: The Next Big Reliability Challenge?,” published in RRL Solar, the researchers explained that their analysis considered both commercial and lab-level solar cells, without revealing the names of the manufacturers. The devices were exposed to radiation from UV-340 lamps without coverage.
“To establish a connection between laboratory tests and field application, we analyzed spectrally resolved data from a test site in the Negev desert, Israel, since 2019,” they said. “In the UV test sequence, three cells per group were exposed to the UV radiation from the front and two from the rear, with the respective opposite sides covered.”
The testing showed that rear exposure led to less UVID than front exposure, with all technologies suffering from voltage losses above 5 mV after 60 kWh m−2. “After UV exposure, the additional recombination – a measure for the defect formation – was more pronounced for PERC than for TOPCon; but the voltage loss was comparable,” Thome said. “This is because TOPCon has a higher passivation quality and therefore ‘feels’ even small amounts of defects. The higher the initial efficiency, the higher the sensitivity to even small amounts of additional defects.”
The analysis also showed that the passivation layers based on aluminum oxide (AlOx) and silicon nitride (SiNy), which are commonly deposited in TOPCon cells via atomic layer deposition (ALD), may enhance the UV stability of these devices compared to layers typically utilized in PERC and HJT cells, which are deposited through plasma enhanced chemical vapor deposition (PECVD).
“Components common to all three cell technologies can also be important for UV stability. “One example would be the refractive index and thickness of silicon nitride layers which determine the effective UV dose reaching the silicon,” Thome concluded.
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