| Work Detail |
Researchers at Germany’s Fraunhofer ISE explained that ultraviolet (UV)-induced degradation can cause higher than expected efficiency and voltage losses in all dominant cell technologies, including TOPCon devices. The scientists hope that silicon nitride layers could be used to improve the UV stability of TOPCon compared to the PECVD layers commonly used in PERC and heterojunction cells.
Researchers at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Germany have investigated the stability under ultraviolet (UV) exposure of three types of conventional solar cell technologies – passivated oxide tunnel contact (TOPCon), passivated emitter and rear cell (PERC), and heterojunction (HJT) – and found that all of them can suffer from severe implicit voltage degradation.
They explained that UV-induced degradation (UVID) may lead to unexpected voltage and efficiency losses in the future, especially when a longer UVID history becomes available. “A prominent example of this is light-induced and high-temperature degradation (LeTID), which has caused unexpected losses in PERC modules during field operation,” they said. “Recent reports suggest that a similar scenario due to UVID could be repeated for all three modern cell architectures.”
The harmful effects of UV radiation in solar panels have largely been associated with UV-transparent module encapsulants and aging of module packaging materials, leading to encapsulant discoloration, delamination, and backsheet cracking. In particular, UV light can contribute to the formation of acetic acid in the module encapsulant, which corrodes the cell contact grid. Solar cell performance is also negatively affected by UV radiation due to the generation of surface defects. In a silicon solar cell, UV light can cause damage to the passivation layers, the underlying silicon, and the interface between the two.
“Currently, UV-transparent encapsulants are the standard for the front side of the module,” Fabian Thome, lead author of the research, explains to pv magazine. “Using UV-blocking encapsulants could be a strategy to reduce UVID radiation, but at the cost of reducing module efficiency. We know of some manufacturers already using this strategy. It seems to be a good intermediate solution until UVID is resolved at the cellular 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 explain that their analysis took into account both commercial and laboratory-grade solar cells, without revealing the names of the manufacturers. The devices were exposed to radiation from uncovered UV-340 lamps.
“To establish a connection between laboratory testing 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 UV radiation from the front and two from the back, with the respective opposite sides covered.”
Tests showed that rear exposure led to less UVID than front exposure, with all technologies suffering voltage losses greater than 5 mV after 60 kWh m-2. “Following UV exposure, additional recombination – a measure of defect formation – was more pronounced for PERC than for TOPCon; but voltage loss was comparable,” Thome said. “This is because TOPCon has a higher passivation quality and thus ‘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 demonstrated that passivation layers based on aluminum oxide (AlOx) and silicon nitride (SiNy), which are typically deposited on TOPCon cells using atomic layer deposition (ALD), can improve the UV stability of these devices compared to layers typically used on PERC and HJT cells, which are deposited using plasma-enhanced chemical vapor deposition (PECVD).
«Components common to all three cell technologies may also be important for UV stability. «An example would be the refractive index and thickness of the silicon nitride layers, which determine the effective UV dose reaching the silicon,» concludes Thome. |
