Researchers at the Australian National University (ANU) have looked at the effects of gettering mechanisms on passivating contacts based on polysilicon and silicon monoxide (SiOx) for tunnel oxide passivated contacts (TOPCon) solar cells. The getting technique, which reduces defects in wafer manufacturing, could become more important in the future, as cell efficiencies increase and become increasingly sensitive to traces of metallic impurities.
This technique involves three steps and is used during crystal growth to remove containments and other forms of defects in wafers. Via this process, the impurities are initially released into a solid solution and then undergo diffusion through the silicon. Finally, they are trapped in an area away from the active circuit regions of the wafer.
In “Comparing the Gettering Effect of Heavily Doped Polysilicon Films and Its Implications for Tunnel Oxide-Passivated Contact Solar Cells,” which was recently published in RRL Solar, the scientists considered deposition techniques such as plasma enhanced chemical vapour deposition (PECVD) or low-pressure chemical vapor deposition (LPCVD). They also looked at different doping techniques such as in-situ doping, ion implantation, or thermal diffusion, and with different dopant types and doping concentrations.
“In our work, we surveyed a wide range of poly films, which were made by different techniques, in order to identify the factors that affect the gettering effects of poly-Si/SiOx during its formation,” researcher Zhongshu Yang told pv magazine. “Our findings are applicable to commercial TOPCon cells.”
The scientists analyzed the gettering strength of poly-Si films from four different types of P-doped poly-Si and three different types of B-doped poly-Si.
“P-doped poly are commonly used in n-type TOPCon cells, which is still the focus of many manufacturers, due to the excellent surface passivation of the P-doped poly and the absence of light induced degradation. The application of P-doped poly in n-type TOPCon cells can significantly remove impurities from the silicon bulk,” Yang said. “On the other hand, the application of the p-type TOPCon cells based on B-doped poly is limited, although the p-type TOPCon cells have their unique advantages that easy to transfer from the existing PERC cell fabrication system and less sensitive temperature coefficient. From our finding, the gettering strength by B-doped poly varies significantly depending on the film deposition and doping techniques.”
They said the gettering strength of the P-doped poly mainly depends on the doping concentration, but attributed the gettering by B-doped poly to inactive boron, which relates to the fabrication process.
“It is worth noting that the Si wafers used in our article are flat zone (FZ) wafers with implanted iron (Fe) impurities. While in industry, Czochralski (CZ) wafers are most used, which may have different impurities and defects,” Yang explained. “The contamination level from the fabrication process in industry is also difficult to be generally quantified. However, the remaining fraction of the impurity in the bulk after the fabrication process is still valid, at least for Fe impurities, the one of the most common and harmful transition metal impurities.”
The research group recently built a 27.6%-efficient monolithic perovskite-silicon n-type tandem solar cell based on TOPCon tech for the bottom cell. They built the 1 square-centimeter cell with a dense hole transport layer based on nickel(II) oxide (NiOx) with a thickness of 10 nanometers. The research group said this minimizes the likelihood of pinholes that can result in shunt paths.
The group also compared the firing resistance of p-type and n-type poly-Si/SiOx passivating contacts and found that the former have better resistance to hydrogen diffusivity and higher passivation quality after firing. The scientists concluded that the diffusivity of hydrogen in p-type poly-Si is lower than in n-type, which prevents the accumulation of excess hydrogen around the oxide which deteriorates the passivation quality.
“The results extend our understanding of the firing effect on poly-Si/SiOx structures, which may help to further improve the performance of doped poly-Si contacts,” they said.
The scientists also recently conducted a review of all gettering technologies in solar wafer manufacturing and offered a comprehensive analysis of the possible gettering sinks and routes for different solar cell architectures.
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