Investigating Interfacial Phenomena in Copper-Covered, n-Type Polysilicon-Based Contacts by Electron Microscopy

Copper is a promising alternative to silver as a metallization material, but because it is a notorious contaminant for silicon, it is important to understand when and how copper can induce the degradation of photovoltaic cells. Here, we prepare intentionally copper-covered, symmetric, n-type samples with polysilicon/silicon oxide passivating contacts to study the impact of annealing at 200-500°C on passivation and contact resistivity. We image cross sections using electron microscopy to correlate changes in functional properties with interfacial phenomena. Copper diffuses into polysilicon as low as room temperature, but passivation only begins to degrade after annealing at 225°C. At 250°C, passivation is fully ruined, which corresponds copper diffusing fully through the polysilicon and forming nanometersized spikes at the wafer surface. We also introduce a diffusion barrier layer between the copper and polysilicon, which improves temperature stability to about 375°C. At 400°C, passivation degrades, which appears to correspond not to copper diffusion but instead to morphological changes at the barrier layer / polysilicon interface. This study may help elucidate the conditions under which copper is suitable for TOPCon metallization.