Scientists from the French research institute CEA-Liten have identified hydrogen migration in doped selective layers as the main driver of UV-induced degradation in silicon heterojunction solar cells. They also found that combined light and thermal light treatments can partially restore performance and improve long-term UV stability.
Researchers from the department of new energy technologies and nanomaterials (Liten) of the French Alternative Energy and Atomic Energy Commission have investigated UV-induced degradation (UVID) pathways in heterojunction (HJT) solar cells and found that light treatments that combine thermal and light activation gradually restore the selective conductivity of the layer.
“The novelty of this work lies mainly in the identification of the mechanism underlying a UV stability problem affecting the selective front layers of silicon HJT solar cells,” the study’s corresponding author, Hugo Lajoie, told pv magazine.
Using controlled UVA and UVB exposures calibrated to replicate realistic UV doses at the module level, while accounting for the optical filtering effects of glass and encapsulants, the researchers identified hydrogenated amorphous silicon (a-Si:H) layers as the key contributors to UVID sensitivity in HJT solar cells.
“The transparent conductive oxide (TCO) on the front also contributes to this degradation by UVB transmission and its hydrogen content, but selective layer modifications exert a significantly stronger influence on the amplitude and dynamics of UVID, confirming its leading role in the degradation mechanism of the HJT stack,” Lajoie continued. “Experimental evidence shows that the phosphine flow (PH₃) during plasma-enhanced chemical vapor deposition (PECVD) of the selective layer strongly controls UVID.”
The researchers also found that HJT wafers processed with a high PH₃ current in the amorphous front n-type selective layer exhibited a relative carrier lifetime loss of 63.3% after 60 kWh/m² UVA exposure, compared to only 9.5% for undoped layers, measured at an injection level of 10¹⁵ cm⁻³.
According to the research team, these findings indicate that UVID in HJT cells cannot be explained solely by the loss of chemical passivation due to the breaking of silicon-hydrogen (Si-Hₙ) bonds at the a-Si:H/c-Si interface, which has traditionally been considered the dominant degradation pathway, as UV photons can exceed the dissociation energy of Si-Hₙ bonds.
“Instead, hydrogen migration, strongly influenced by the doping level and free carrier density, due to this cleavage emerges as a major driving force, leading to the formation of electronically inactive P–Si–H–Si complexes in the front selective layer. This mechanism was indirectly proven by a UV-induced decrease in the electrical conductivity of doped selective layers. Such metastable defect configurations severely limit carrier lifetimes due to a loss of passivation due to field effects.”
The team also found that combined thermal and light activation through light soaking (LS) can gradually restore the conductivity of the selective layer, indicating light-activated hydrogen reconfiguration processes. Their analysis showed that weakly bonded hydrogen within the selective layer can migrate between Si–H–Si sites, promoting dopant activation and partial chemical repassivation of the a-Si:H/c-Si interface.
“FTIR measurements have shown that LS appears to specifically regenerate the high strain mode Si-Hₙ bonds associated with void-rich environments indicate targeted rearrangement of hydrogen in defect-prone areas and recovery of passivation quality to near initial levels,” Lajoie pointed out.Nevertheless, our results also suggest the existence of a dose-dependent threshold above which UV damage becomes increasingly irreversible.”
The study findings are presented in “Understanding UV-induced degradation mechanisms in SHJ solar cells and their reversibility: the role of hydrogen and doping”, published in Progress in Photovoltaics.
“This work highlights the need for advanced multilayer selective layer engineering in addition to optimized UV filtering at the SHJ module level,” Lajoie concluded. “Precise control of hydrogen content, bond configurations and dopant activation provides a viable path to improving UV reliability without compromising passivation and is emerging as an important lever to improve the long-term sustainability of SHJ technology.”
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