Microstructure Analysis of the Interaction between Watts-Type Nickel Electrolyte and Screen Printed Solar Cell Contacts

Abstract

In this work we present the impact of Watts-type nickel electrolytes on the contact microstructure of the screen-printed silver contacts of silicon solar cells. Both an SEM and TEM analysis were used to investigate the reasons for poor contact adhesion following nickel plating. A failing interface was identified and located exactly between the glass frit and the bulk silver inside the contact. A clearly visible gap occurs at this interface due to a chemical dissolution of the boundary layer between the glass and the silver. In TEM measurements, no distinctive features in the elemental composition were identified at the failing interface that might be responsible for the site-specific dissolution. This suggests that the dissolution starts at the very thin oxide layer between the glass and the silver. Furthermore it became clear that the electrolyte reaches the identified failing interface via pores and cavities in the screen-printed silver layer. Closing these cavities prior to nickel electrolyte exposure thus lead to significantly better contact adhesion values.

Origin of corrosion effects in solar cell contacts during electrochemical nickel deposition

“…Cu reduction will be preferred because of the higher reduction potential f. The equilibrium is not shifted towards further dissolution of lead oxide, and the corrosion does not lead to contact adhesion failure transport between the place of dissolution (glass) and the place of reduction (silver) is very fast. This leads to a fast expansion of a site-specific gap formation inside the contacts along the glass silver interface, as has been observed and analysed in [4] and shown in Fig. 2.…”

“…The exposure duration was constant at 45 min for every experiment, and the temperature of the solution was set to 50°C, which is the standard operation temperature for Watts-type nickel electrolytes. These conditions have been found suitable to show corrosion effects that also occure in plating processes within the contact structure [4]. After exposure, the contacts were rinsed with deionized water, dried and stored for one day at room temperature before soldering.…”

“…Different phases are formed in the contact during firing, as has been described in various publications [7][8][9]. Depending on the used Ag paste, different metal oxides can be part of the glass; zinc oxide, for example, is often added by paste manufacturers and was observed to be part of the glass layer [4]. However, in this study, the interaction between electrolyte solutions and silver contacts is discussed with a focus on silver/silver oxide and lead/lead oxide as dominant species being present in almost every Ag-front-side metallization.…”

“…The used glass in solar cell contacts is generally lead glass containing silver after the contact formation process in the firing furnace. The silver is evenly distributed over the glass layer [4], but the largest section of the glass consists of lead oxide.…”

“…2b detailed image). The contact microstructure is further analysed in our recent publication [4]. Adhesion loss due to plating of metals has been described before [5]; however, an explanation and a model of the mechanisms behind this corrosion effect observed after nickel plating are missing to date and will be presented in this study.…”