A team of researchers from the University of New South Wales and the Chinese solar module manufacturer LONGi found that the rear side of tunnel oxide passivated contact (TOPCon) cells, particularly the silicon nitride (SiN_x) layer, is prone to chemical degradation when exposed to sodium-based salts, resulting in a significant loss of open-circuit voltage.

Open-circuit voltage is the difference in electrical potential between two terminals of a device disconnected from the circuit without any external load and current flow.

The research, titled “Mitigating contaminant-induced surface degradation in TOPCon solar cells: Mechanisms, impacts, and mitigation,” was published in Science Direct.

The researchers utilized a 10 nm aluminum oxide (AlOx) barrier layer, deposited through atomic layer deposition (ALD), to effectively mitigate these degradation pathways.

The report said that accelerated damp heat (DH) testing demonstrates that the ALOx barrier improves the TOPCon solar cells’ long-term stability, reducing degradation and maintaining performance over extended periods.

The research was conducted on industrial n-type TOPCon solar cells with double-sided pyramid texturing and an average power conversion efficiency of approximately 23%. The rear side was structured with a tunneling silicon oxide layer and a phosphorus-doped polycrystalline silicon layer to form the n-type TOPCon configuration. It was further capped with an additional SiN_x ARC layer.

The solar cells’ electrical performance was assessed using a light output and normal analysis system from photovoltaic tools equipped with a five-rail pin contact frame to measure current-voltage characteristics.

Applying sodium acetate to the front of the TOPCon solar cells resulted in an approximate 0.87% degradation in their power conversion efficiency (PCE). This degradation was primarily due to a minor decline in the fill factor associated with a roughly 35% relative rise in the cell’s series resistance during the 20-hour DH85 (hydro-moisture meter) test.

The fill factor is a measure to gauge a solar cell’s efficiency. Solar cells with a higher fill factor are more efficient.

The researchers found that applying sodium acetate to the rear of the solar cells resulted in a 16% PCE decline. This decline was caused by a 450% relative increase in the cell’s series resistance, reducing the fill factor by 9.4%.

The report indicated that open-circuit voltage losses can reach up to 5.8%, resulting in an approximate 16% decrease in overall cell efficiency.

One of its key findings was the 10 nm AlOx barrier’s protective role on the cells. The layer was deposited through an industrial-scale ALD process. It reduced performance degradation, demonstrating only a minor PCE decrease and maintaining open-circuit voltage stability after 20 hours of accelerated DH testing.

The AlOx layer also mitigated corrosion and recombination defects induced by sodium-related salts.

The report claims that these findings confirm the AlOx layer’s potential as a viable protective solution to enhance the TOPCon solar cells’ long-term reliability under harsh environmental conditions.

Recently, China-based solar module manufacturer JinkoSolar Holding claimed that its N-type TOPCon-based perovskite tandem solar cell achieved a conversion efficiency of 33.84%, surpassing its previous record of 33.24%.

In December 2024, another solar cell and module manufacturer, Qcells, announced achieving a 28.6% tandem solar cell efficiency on a full-area M10-sized cell that can be scaled for mass manufacturing.