Hydrogen can be either beneficial to silicon solar technologies, passivating bulk and surface defects, or detrimental, contributing to light- and elevated-temperature-induced degradation (LeTID) and surface-related degradation (SRD).
To promote a better understanding of recent research results on using hydrogen with silicon (Si) solar cells and the related degradation phenomena, researchers at Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) and the University of Freiburg reviewed the literature and discussed their findings. They then published Review on hydrogen in silicon solar cells: From its origin to its detrimental effects in Solar Energy and Materials.
In recent years, we have seen intensive research on hydrogen in silicon solar cells from multiple institutes. The review paper provides an overview of these results, focusing on the introduction of hydrogen into silicon solar cells and the relation between hydrogen and degradation phenomena, researcher Benjamin Hammann told pv magazine. In general, hydrogen has always been and will continue to be an important topic for silicon solar cells due to its ability to passivate defects both in the bulk and at the surface. With n-type Si dominating solar cell production and antimony (Sb) doping emerging, new questions regarding hydrogen also arise.
The researchers considered more than 170 publications and books on the topic, although not all of them were ultimately relevant for the review, according to Hammann. In the end, the review listed 77 references, covering research investigating hydrogen use with p-type silicon, boron-doped float-zone (FZ) silicon, and gallium-doped Czochralski (Cz) grown silicon, and including passivated emitter and rear cell (PERC) and tunnel-oxide passivated contact (TOPCon) technologies.
Understanding the introduction of hydrogen into silicon solar cells, along with its negative impact, helps solar cell manufacturers to assess the result of process adaptations, explained Hammann. He further noted that TOPCon solar cells, for instance, require hydrogen for excellent surface passivation, while n-type silicon is still susceptible to light- and elevated-temperature-induced degradation (LeTID), albeit less than p-type.
Stressing that much remains unknown about hydrogen in n-type silicon, Hammann said that the review serves as a good starting point for research on hydrogen in n-type silicon, as extrapolating results from one doping type to another can provide an initial hypothesis.
One of the initial findings was that LeTID is mitigated when total hydrogen concentrations are below 5 � 1014 cm3.
The team found a similar surface degradation upper tolerance limit for an aluminium oxide/silicon nitride passivation layer stack. The research team then went on to discuss various strategies to control the hydrogen content.
One important factor is the hydrogen source, typically hydrogen-rich silicon nitride, they noted. Also highlighted was that the hydrogen diffusion process occurs mainly during the fast-firing step, including both in-diffusion at around the peak temperature and out-diffusion during subsequent cool-down.
The effects of other interlayers, such as aluminium oxide or highly-doped surface-near layers, on the diffusion process were also discussed.
When asked about the findings related to SRD, Hammann said that many passivation layers, like aluminum oxide, silicon nitride, and even TOPCon layers are affected by some form of SRD.
It is highly likely that SRD is connected to hydrogen, although there are many questions left to answer, some of which we are currently investigating, he said.
The research team is now investigating SRD of TOPCon layers, and investigating UV-induced degradation of TOPCon solar cells.
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