PVI Paper

Cleaning for high-efficiency solar cell processes

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By Eckard Wefringhaus, Director of the Quality Management Department, ISC Konstanz; Florian Buchholz, Quality Management Department, ISC Konstanz

This paper discusses the role of wafer cleaning in solar cell processing, and addresses its increasing importance with the introduction of new process steps for manufacturing high-efficiency solar cells. The requirements for
cleaning before several process steps, in relationship to the solar cell production sequence, are discussed: frontend- of-the-line (FEOL) cleaning needs to reduce metal surface concentrations by several orders of magnitude (residues from wafer sawing), while back-end-of-the-line (BEOL) cleaning needs to reduce mostly process induced contamination, which tends to be much lower. A ten-step roadmap for process integration and
optimization of new cleaning processes from lab to fab is suggested, which is based on process analytics and simple bath-lifetime simulations. A number of advanced cleaning steps are identified and their suitability for
solar cell mass production is examined. The influence of the different input variables is demonstrated, with a focus on feed and bleed settings. Finally, the need for constant monitoring of cleaning baths is highlighted, and
a device developed by Metrohm for cost-effective on-site monitoring of metallic contamination is discussed.

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The period of ‘profitless prosperity’ in the PV industry is finally at an end. Throughout 2013, despite continued economic woes, the PV industry has continued to expand and finally become a global industry. Market forecasts indicating that the sector could reach its next 100GW milestone in just the next two years suggest the industry is on the cusp of another period of strong growth. All the signs confirm this is the case, with utilization rates at their highest level since 2010, companies reporting full order books well into next year and the first tentative announcements of factory capacity expansions making the headlines.

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