Insights into the Degradation of Amorphous Silicon Passivation Layer for Heterojunction Solar Cells

Abstract

Heterojunction (HJ) crystalline silicon (c-Si) solar cells outstanding performance relies heavily on the excellent passivation provided by the amorphous Si (a-Si:H) layer. However, recombination at the a-Si:H/c-Si interface can vary over time and become particularly detrimental for HJ modules performance making the full understanding of the recombination mechanisms at play of paramount importance. In this work, the variation of effective lifetime for high-quality n-type FZ c-Si substrates coated with a-Si:H(i) layer after several processing steps and over a period of 28 months is tracked. The root cause for degradation is identified by experimentally evaluating the surface recombination velocity (SRV) temperature-and injection-dependence before and after degradation has occurred. By applying a model for the recombination at the a-Si:H/c-Si interface to temperature-and injection-dependent SRV data, the authors are able to assess the lifetime decay as entirely ascribed to a loss of chemical passivation. Upon re-annealing the samples, only a partial recovery of lifetime is obtained suggesting that effusion of hydrogen from the a-Si:H layer has occurred. These results indicate that the usage of a capping layer is needed whereas a thorough engineering of the a-Si:H(i) layer thickness may be necessary to avoid the loss of performance of a-Si based heterojunction structures and modules.

Challenges for the future of tandem photovoltaics on the path to terawatt levels: a technology review

“…This requires either the use of diffusion barriers or lowering the temperature during III-V growth [93] o The high temperature limits the choice of Si bottom cell. For instance, the highest efficiency Si concept based on HIT structures cannot be used in this configuration because the amorphous Si passivating layers start significantly degrading after 200 °C, leading to depassivation of the heterointerfaces and increase in surface recombination velocity [108]  CIGS and CZTS also require at least one high temperature step in their synthesis (>500 °C), which besides limiting the choice of Si bottom cell can also cause contamination of the Si bottom cell by diffusion of metallic elements, in particular Cu. Therefore, a diffusion barrier is likely needed, limiting the prospects of epitaxial growth.…”

From Femtoseconds to Gigaseconds: The SolDeg Platform for the Performance Degradation Analysis of Silicon Heterojunction Solar Cells

“…In this section, we explore the correspondence between our simulations and experiments on a-Si:H/c-Si structures. Figure a shows that for the interface defect density, a wide range of values have been reported in the literature. ,− This unusually wide range is caused by many different factors, such as the different methods and protocols employed for depositing a-Si, quality and level of cleanliness of c-Si wafer before deposition, substrate morphology, orientation of c-Si, microstructure of the a-Si film, hydrogen content, and storing conditions of samples. − However, there are not many studies available correlating the impact of such differences to long-term stability of the a-Si/c-Si interface. For completeness, in Figure a, we also summarize the corresponding SRVs from the cited papers.…”

“…The Bertoni group has studied the SRV at the a-Si/c-Si interface in HJ stacks. By applying a model for the recombination at the a-Si/c-Si interface to their temperature- and injection-dependent SRV data, they analyzed the degradation of the carrier lifetime and were able to attribute it to loss of chemical passivation. , More recently, Holovský et al investigated ultrathin layers of hydrogenated amorphous silicon (a-Si:H), passivating the surface of crystalline silicon (c-Si) . These authors applied highly sensitive attenuated total reflectance Fourier-transform infrared spectroscopy combined with carrier lifetime measurements.…”

“…Wronski speculated that these states are differentiated by their different structures: DBs and mono- and divacancies, as also advocated by Smets. Other groups also analyzed their data in terms of three distinct states. , However, they focused on the alternative picture that the defect states may be the three charge states, H+, H0, and H–, of hydrogen. In addition to these experimental studies, recent theoretical and computational papers also analyzed the defect states in a-Si, and they concluded that besides DBs, highly strained bonds also contribute to midgap states significantly. , …”

Evidence for a Light‐Induced Degradation Mechanism at Elevated Temperatures in Commercial N‐Type Silicon Heterojunction Solar Cells

“…the well-known tendency of these materials to exhibit light-induced degradation via Staebler-Wronski (SW)-type defect mechanisms. [16][17][18][19][20][21] Many of these studies were, however, conducted on research samples rather than complete cells, leaving uncertainty as to the translation of observed behaviors into complete cells, featuring additional layer deposition and thermal processes.…”