Scientists claim 10% saving for black-Si cell production with dry-etching

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The common perception is that black-Si solar cells – known for their low reflectivity and a higher absorption of visible and infrared light – are too expensive compared to traditional crystalline cells, due to the higher costs determined by their production process at a commercial scale, and especially when using dry-etching and atomic layer deposition (ALD).

A new study in Energies, conducted by scientists from Finland’s Alto University and the U.S.-based Michigan Technological University, however, intends to show production costs for black-Si PERC cells may be reduced by around 10% even using dry-etching, a process for making solar cells more efficient at capturing light.

According to the research team, dry-etching is not only crucial for improving the cell’s light absorption, but also because it enables diamond saw wafering, enhances metal gettering, and may prevent power conversion efficiency degradation under light exposure. The importance of diamond wire sawing for the resurgence of black-Si technology was highlighted by pv magazine editor Mark Hutchins, in an article in the November 2017 print edition of the magazine.

To calculate costs for each manufacturing process of black-Si PERC cells, the researchers based their analysis on a theoretical 1 GW factory and calculated costs per unit of power based on a 22% cell efficiency. The current efficiency record for a black-Si cell produced at industrial scale – set in the third quarter of last year – is 20.78%.

Dry-etching cost rise offers overall saving

“This introduces a source of uncertainty in the step costs calculations,” the research team admit, adding: “The results show an increase in the overall cell processing costs [of] between 15.8% and 25.1%, due to the combination of black-Si etching and passivation by double-sided atomic layer deposition. Despite this increase, the cost per unit [of] power of the overall PERC cell drops by 10.8%.”

The scientists also claim the adoption of dry-etching in the process determines a cost increase of only a factor of 2.62 in the step cost, according to the best-case scenario in the research. The overall cell processing costs, however, are lower for black-Si cells even in the worst-case scenario. The research team explains the surface passivation step also offers the potential for cost saving, as black-Si PERC cells can “effectively be passivated with a double-side ALD step, prior to capping the rear side by SiN [silicon nitride].”

The cost of passivation in the process may be 50% lower than the cost for the corresponding steps using PECVD (plasma-enhanced chemical vapor despoition) and CVD for the standard Czochralski (Cz) process used in monocrystalline cells, the researchers claim.

The scientists specified the economic viability of the production of dry-etched black-Si cells has already been verified in a pilot production line, but did not name the manufacturer.

They point out, however, only metal assisted catalyzed etched (MACE) black-Si cells have so far been deployed by manufacturers to industrial production. One such manufacturer is Chinese module maker Wuxi Suntech Power Co. Ltd, which announced the start of a 500 MW production line in January.