Microstructure Analysis of Current-Fired Contacts on TOPCon Layers

We report on the analysis of the microstructure of a current-fired contact formed by screen printing, drying, and sintering of a silver paste and subsequent laser-enhanced contact optimization (LECO) for silicon solar cells with tunnel-oxide passivated contacts (TOPCon) on the TOPCon side. The analysis is based on scanning electron microscopy and energy dispersive X-ray spectroscopy. We show that a new phase is formed during LECO at individual points of the interface between the contact finger and the polycrystalline silicon layer consisting of silver and silicon. The altered material combination is confined to the metal and the polycrystalline layer only and does not penetrate through the tunnel oxide into the silicon wafer. We conclude that the mechanism for the contact formation on the TOPCon stack is very similar to the one reported for the front contact of passivated emitter and rear cells.