Researchers at German research institute ISC Konstanz have investigated the effects of dirrent techniques used for aluminum oxide (AlOx) layers deposition techniques the UV-induced degradation (UVID) susceptibility of TOPCon solar cells and have found that atomic layer deposition (ALD) approaches are preferable to plasma-enhanced chemical vapor deposition (PECVD) methods.
“Our work confirms previous findings that both the thickness of the AlOx passivation layer and the deposition technique, with PECVD being more susceptible to UVID than ALD, play an important role in the UV stability of the front-side passivation,” corresponding author David Bäurle told pv magazine.
“A key novel aspect of our study is the detailed comparison of different ALD approaches such as spatial, thermal, and plasma-enhanced ALD,” he went on to say. “We found that the widely used industrial spatial and thermal ALD processes, as well as a tube-based plasma-enhanced ALD approach, exhibit similar UV stability. Furthermore, to the best of our knowledge, this is the first study showing that a thermally grown interfacial SiOx layer formed prior to AlOx deposition can further improve the UV stability, even for cells featuring an ALD-based front-side passivation.”
“In addition, we investigated commercially available TOPCon solar cells from five different manufacturers,” Bäurle further explained. “All groups showed a mean relative efficiency degradations between 2% and 3% after an accumulated UV dose of 60 kWh/m². Most interestingly, one group included a single outlier with almost 6% relative degradation despite initially exhibiting a passivation quality comparable tho the other cells in its group. This findings suggest that process-related variations can also have a significant impact on a cell’s susceptibility to UV-induced degradation. One possible explanation is that local inhomogeneities, most likely in the AlOx passivation layer, made this particular cell more susceptible to UV-induced degradation.”
In the study “UV degradation in TOPCon solar cells — Impacting factors and mitigation strategies,” published in Solar Energy Materials and Solar Cells, the researchers explained they investigated how passivation technologies influence the UV stability of TOPCon solar cells by analyzing both laboratory-made samples and commercially produced devices. They fabricated test samples using n-type Czochralski silicon wafers and examined several structures, including simplified lifetime test samples, TOPCon precursor structures before metallization, and fully processed solar cells.
They deposited AlOx-based passivation layers using plasma-enhanced atomic layer deposition (PEALD) and covered with a protective silicon nitride (SiN) capping layer. These layers are critical because they reduce surface recombination and help maintain cell efficiency during UV exposure.
To determine which deposition method provides the best UV resistance, the researchers compared different AlOx layer designs, including PEALD-deposited AlO, PECVD-deposited AlO, and ALD-based AlO combined with a thin thermal silicon oxide (SiO) interlayer.
In addition to laboratory samples, the study analyzed industrial TOPCon cells from several manufacturers to compare PEALD performance with other commercially used ALD approaches, such as spatial ALD and thermal ALD. This allowed the researchers to evaluate whether laboratory findings translate to real industrial production conditions.
The analysis showed that AlO deposition technique has a major impact on UV stability, with PECVD-based layers showing significantly higher degradation than PEALD, thermal ALD, and spatial ALD passivation stacks. Increasing the AlO thickness improved UV resistance, especially for PEALD layers, likely by reducing the impact of hydrogen migration and defect formation at the silicon/passivation interface.
The scientists also found that the integration of a thin thermally grown SiO interlayer beneath PEALD AlO further enhanced UV stability by strengthening chemical passivation and limiting UV-induced interface defect generation. Further analyses showed that UV exposure increases both interface trap density and fixed charge; however, the initial gain in field-effect passivation can temporarily offset losses in chemical passivation.
Moreover, the research group ascertained that the industrial TOPCon cells exhibited comparable UV-induced efficiency losses, primarily driven by voltage degradation, although variations within production batches influenced the final stability. It also found that the laboratory-fabricated TOPCon cells with PEALD AlO achieved UV performance similar to industrial references, with thicker AlO layers and SiO interfacial layers delivering the highest stability.
“These findings suggest that, although process choices such as deposition technique and layer thickness influence the UVID susceptibility, process-related non-uniformities may in some cases have a similar strong effect,” the academics concluded.
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