Creating nano-textures on slicon with dry etching makes black silicon (black-Si) that is more efficient at capturing light than standard etching treatments. It has no colour because the dry etching process takes a normally flat silicon surface and etches it into a forest of nanoscale needles.
Normally such a high surface area with many surface defects would hurt electrical performance, but researchers at Aalto University found that when the silicon is also treated with an appropriate atomic layer deposition (ALD) coating, the effects of surface defects are mitigated.
“Improving cost per unit power at the cell level can have massive effects downstream,” said Joshua Pearce, professor of material sciences and electrical engineering at Michigan Tech who was working at Aalto on sabatical. The costs of solar energy are comparable to conventional forms of electricity and this 10 percent drop should push solar to the forefront even faster.
Typical thinking has been that the cost of black-Si cells from dry etching and ALD are too expensive for practical use. “Margins are extremely tight. Everyone’s trying to push costs as low as possible,” he said.
The project found that production of individual black-Si passive emitter rear cells (PERC) were between 15.8 and 25.1 percent more expensive than making conventional cells, but the efficiency gains and the ability to go to the less-expensive multicrystalline silicon starting material far outweighed those extra costs: overall the cost per unit power dropped by 10.8 percent.
“This study points to where the future is going to go in PV manufacturing and what countries might want to do to give themselves a competitive advantage,” he added.
Researchers were also able to get information on manufacturing costs from companies, which is not public, but were allowed to use for this study, along with published literature on solar cells. While the spot price for solar cells may change day by day – or even by hour – the results still hold. “That’s 10 percent decline between cell types from whatever the number is that day,” he said. This is because the comparisons were made on relative costs, not absolute costs. That’s also why tariffs were not factored into the calculations.
While the production process can still be optimised to pull out a few more percentage points of efficiency, the next step for this study is to be used by policy makers to accelerate PV manufacturing. Pearce belives the EU should “look carefully at scaling up deep reactive ion etching and ALD tools to meet the needs of the rapidly expanding PV market”, while the US could use this data at a policy level to leap frog international manufacturers and invest in producing the new machines to manufacture these types of solar cells.
“I don’t know which technology will end up being the one to dominate the solar field,” he said, however “The study shows the clear economic impetus to move in the direction of dry-etched black silicon PERC that wasn’t there before.”
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