The Impact of Annealing on Copper-Plated Heterojunction Solar Cells

Why It Matters

The findings reveal that standard fast‑anneal steps can degrade mechanical integrity of HJT cells, forcing manufacturers to rethink process windows to maintain adhesion without compromising efficiency.

Key Takeaways

• Fast annealing raises Cu and ITO microstrain.
• Self‑annealing preserves (100) Cu texture, lowers silicon stress.
• Preferred (100) Cu orientation reduces Young’s modulus mismatch.
• Uniform plating and surface prep critical for adhesion.
• HJT cells cannot tolerate >250 °C anneal like PERC/TOPCon.

Pulse Analysis

Heterojunction (HJT) solar cells promise higher efficiencies than conventional silicon technologies, yet their low‑temperature processing limits the thermal budget for post‑plating steps. The UNSW study highlights that copper contacts, while offering excellent conductivity, introduce mechanical challenges when subjected to rapid annealing. By heating cells to 205 °C for less than a minute, the researchers observed a contraction of the copper lattice and an expansion of the ITO lattice, which together amplified microstrain and generated localized stress concentrations up to two micrometres wide in the silicon substrate. These stress hotspots can act as nucleation sites for cracks, jeopardising long‑term reliability.

In contrast, samples left to self‑anneal at ambient conditions retained a preferential (100) crystal orientation in the copper layer. This orientation aligns with a lower Young’s modulus, reducing the mismatch with the silicon wafer and thereby limiting strain transfer. Nano‑indentation confirmed that the mechanical strength of the copper remained unchanged across treatments, indicating that the observed differences stem from microstructural evolution rather than bulk material weakening. The X‑ray diffraction data further showed that fast annealing promoted grain growth in the Cu (200) direction, a less favorable texture for stress mitigation.

For manufacturers, the practical takeaway is clear: HJT production lines must balance the need for strong copper‑silicon adhesion with the risk of inducing thermal strain. Optimising electrolyte composition, plating current density, and post‑plating anneal profiles can preserve the (100) texture while still achieving reliable contact formation. As the industry scales HJT modules, integrating in‑line stress monitoring—such as Raman mapping—could pre‑empt failure modes that traditional electrical testing might miss. Continued research into low‑temperature anneal chemistries will be pivotal for unlocking the full commercial potential of copper‑based HJT cells.