Plated copper front side metallization on printed seed-layers for silicon solar cells

Hochschulschrift

Zusammenfassung: In the present work, a novel front side metallization architecture for silicon solar cells based on a fine printed silver seed-layer, plated with nickel, copper and silver, is investigated. Practical challenges with this type of metallization motivated associated research, and scientific insights into different aspects of this metallization approach and beyond were obtained. The work focuses on the printing of fine seed-layers with low silver consumption using screen-printing, the corrosion of the printed seed-layers by the interaction with electrolyte solutions and the encapsulation material on module level and on the long term stability of the cells due to copper migration.To realize fine contact structures and simultaneously low silver consumption by screen-printing a high yield stress of >400 Pa for the silver pastes were shown to be beneficial. Fine contact fingers with a width of 30 μm were reproducibly printed. Seed-layers printed with ink-jet, aerosol-jet, screen-printing and flexographic printing were used to produce industrial scale 156 × 156 mm2 solar cells. A total silver consumption for the front side metallization of less than 15 mg/cell was achieved with screen-printed seed-layers. This represents a reduction of the silver consumption of at least 80% compared to solar cells with fully screen-printed silver front side contacts.The analysis of the contact corrosion due to interaction with the electrolyte solutions revealed that a site specific dissolution of the glass layer inside the fired contacts is responsible for the observed contact adhesion loss. A model of the underlying reaction mechanism was developed which explains the experimental results and provides profound understanding of the interaction between solar cell contacts and electrolyte solutions. It was found that lead glass is dissolved within the acidic aqueous systems. Several factors were identified to shift this equilibrium in favour of pronounced glass dissolution, especially the presence of complexion or precipitation agents, low pH and a cathodic potential.The acceleration of the contact corrosion by a cathodic voltage was found to depend on the nobility of further ions in the system. If noble ions like silver or copper are present, the reduction of these species at the silver contact is preferred compared to the reduction of the dissolved lead, whereas ions similarly or less noble than lead, like nickel, enable the reduction of the dissolved species, which shifts the equilibrium of the dissolution reaction strongly towards further dissolution and leads to quick and significant loss of contact adhesion. The Abstract xii described mechanism explains the observed fast gap expansion specifically at the glass silver interface in the contact. Strategies to avoid critical adhesion loss in practical application have been derived from these findings.The results regarding contact corrosion were successfully transferred to improve the understanding of a phenomenon which occurs on module level and is often described in literature. Damp heat induced degradation is known to be caused by acetic acid, released from the module encapsulation material ethylenevinyl acetate in presence of humidity and heat. With the methods developed in this work, close analogies to electrolyte corrosion could be demonstrated. It was shown that the observed loss of electrical contact between front side metallization and emitter originates from the corrosion of the glass layer inside the contacts by acidic acid, accelerated considerably by applied negative voltages. The reduction of dissolved lead was demonstrated to be the reason behind this effect. The developed model and the combined detailed understanding of the corrosion mechanism offer the possibility of considerable lifetime extension for PV-modules irrespective if cells with seed- and plate or fully printed contacts are used.The investigations of the copper migration revealed that silicon nitride anti reflection coatings deposited by industrial inline plasma enhanced chemical vapour deposition and sputtering prevent copper diffusion into the underlying silicon wafer even after a pre-treatment with hydro-fluoric acid or the mechanical load induced by screen-printing. The effectiveness of the plated nickel diffusion barrier was evaluated with cells featuring different nickel diffusion barrier thicknesses. It was shown that 20 mg plated nickel per cell, which represents a layer thickness of at least 0.2 μm, is sufficient to avoid long term stability issues due to copper migration during a module lifetime on the tested screen-printed seed-layer

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Albert-Ludwigs-Universität Freiburg, Dissertation, 2015

Classification and Topics

• Classification: Industrielle und handwerkliche Fertigung
• Keyword: Solarzelle ; Metallisieren ; Korrosion ; Kupfer

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2015
• Creator: Kraft, Achim
• Contributor: Reinecke, Holger

Further information

• DOI: 10.6094/UNIFR/10422
• URN: urn:nbn:de:bsz:25-freidok-104222
• Rights: Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:43 PM CET