Review on hydrogen in silicon solar cells: From its origin to its detrimental effects

In this work, we present an overview of the current understanding of hydrogen in modern silicon solar cells. The ambivalent nature of hydrogen poses a significant challenge for solar cells: While hydrogen is highly beneficial due to the passivation of bulk and surface defects, it is also detrimental to long-term stability, being associated with two degradation phenomena. Specifically, we examine the relation between hydrogen and light- and elevated-temperature-induced degradation (LeTID) and surface-related degradation (SRD). Our findings indicate that LeTID is mitigated when total hydrogen concentrations are below 5 × 1014 cm−3. For the surface degradation of an aluminium oxide/silicon nitride (Al2O3/SiNx:H) passivation layer stack, our data indicate the existence of a similar upper tolerance limit. Thus, managing hydrogen content is key to reducing these degradation phenomena. Therefore, we discuss various strategies to control the hydrogen content. One important factor is the hydrogen source, typically hydrogen-rich silicon nitride. Furthermore, the hydrogen diffusion process is discussed that occurs mainly during the fast-firing step, including both in-diffusion at around the peak temperature and out-diffusion during subsequent cool-down. Additionally, we consider the effects of other interlayers, such as Al2O3 or highly-doped surface-near layers, on the diffusion process. Thus, depending on the cell process, the most suitable adjustments can be employed to achieve optimum hydrogen management.