title: Gettering effect of polysilicon based passivating contact and its Impact and Application in silicon solar cell
Creators: name: Yang, Zhongshu
Publisher: name: The Australian National University
Year: 2024

description: Passivated contacts based on polycrystalline silicon (poly-Si) capping a silicon oxide (SiOx) interlayer have attracted significant interest in recent years. The inherent gettering effects during the fabrication process can relax the requirement for wafers and cleanrooms to some extent. This thesis focuses on understanding the gettering effects of the poly-Si SiOx structure and the impact of iron (Fe) contamination and poly-Si SiOx gettering on the performance of poly-Si SiOx based silicon solar cells. A one-dimensional (1D) diffusion-limited segregation gettering model was applied to separate the contribution of each component of the poly-Si/SiOx structure. The main gettering region is found to be the heavily doped poly-Si layer, capturing the majority of the gettered metal impurities, while the SiOx interlayer acts as a diffusion barrier slowing down the overall gettering kinetics. The 1D gettering model was validated by a 3D finite-element model, in terms of the potentially non-uniform Fe distribution due to the pinholes in the SiOx. The gettering model was further simplified with an exponential decay approximation to describe the gettering kinetics. After the poly-Si/SiOx fabrication, the blocking effect of the SiOx interlayer was studied by fitting the experimental gettering curve to the 1D diffusion-limited segregation gettering model, where a dimensionless parameter can be extracted to quantify the SiOx blocking effects. It was found that the Fe atoms go through the SiOx interlayer via a combined effect of direct diffusion and diffusion through pinholes. At low areal pinhole fractions, the Fe flux through the SiOx interlayer is dominated by direct diffusion, which is related to the SiOx stoichiometry and doping concentration in the poly-Si layer. As the areal pinhole fraction increases, more Fe flux goes through the SiOx interlayer via diffusion through pinholes, which significantly enhances the Fe transport in the SiOx interlayer. The pinhole density is found to increase with increasing activation temperatures and doping concentrations. Besides the SiOx blocking effects, the gettering strengths of the poly-Si layers deposited by different techniques were studied by fitting the experimental segregation coefficients at different temperatures with the Arrhenius equation, where the activation energy and effective gettering sites can be extracted. The gettering mechanisms of different poly-Si layers were compared and discussed, based on the results from electrochemical capacitance-voltage and secondary ion mass spectrometry, which reveal the electrically active and the total dopant profile, respectively. The phosphorus doped poly-Si layers present similar gettering strengths, despite the various underlying gettering mechanisms. The gettering by the boron doped poly-Si layers all result from inactive boron species, while there is a large variation of the gettering strengths. As the poly-Si/SiOx structure captures interstitial Fe from the silicon bulk, the impact of Fe contamination and poly-Si/SiOx gettering on the solar cell performance was investigated, by using the silicon wafers with different Fe implantation doses. The poly-Si/SiOx structure was found to be resilient to the gettered Fe contamination, in terms of passivation quality and firing stability. However, in an n-type front junction solar cell with front boron doped emitter and phosphorus doped poly- Si/SiOx on the rear, there is an obvious decreasing trend of the open-circuit voltage and short-circuit current with increasing initial bulk Fe concentrations, resulting from the different remaining bulk Fe concentrations and the increased emitter saturation current densities due to the accumulation of gettered Fe contamination in the p-type emitter. In addition, the Fe contamination was found to affect the fill factor and temperature sensitivity of the c-Si solar cells, which is suspected to be caused by an increased recombination rate in the space charge region.