Joshua Pearce is a professor of material sciences and electrical engineering at Michigan Technical University. While on sabbatical at Aalto University in Finland recently, he became involved in passivated emitter rear cell (PERC) research being conducted by Chiara Modanese, Hannu Laine, Toni Pasanen, and Hele Savin.
Silicon for photovoltaic cells comes in two grades. Pure silicon has the highest conversion rate of sunlight to electricity but is expensive. Multicrystalline silicon has a lower conversion rate but costs less. Conventional solar cells appear blue because the etching used in manufacturing allows some sunlight in the blue portion of the spectrum to be reflected back from the surface.
The Finnish team was experimenting with a dry etching process that captures more of the sunlight that hits it, which makes the cells appear black. When Pearce returned to Michigan, he continued to collaborate with the researchers in Finland. They now say the manufacturing cost of their black solar cells is 10% less than for conventional cells — a significant drop that is expected to have a significant impact on the world of solar power.
“Improving cost per unit power at the cell level can have massive effects downstream,” says Pearce. “Solar energy is already comparable to conventional forms of electricity and is the fastest growing energy source. This 10 percent drop should push solar to the forefront even faster,” he says.
What’s The Secret?
Dry etching takes an ordinary flat silicon surface and “etches it into a forest of nanoscale needles,” Pearce says. “Those needles grab the light and don’t let it get away. It’s like looking into the eyes of Darth Vader.”
Under ordinary circumstances, so much surface area would lead to reduced electrical performance, but the researchers at Aalto University found that treating the dry etched surface with an atomic layer deposition coating prevented that decrease. While the ALD process adds up to 25% to the cost of solar cells, the cells produced have a higher efficiency using the less expensive multicrystalline silicon. Combining the greater efficiency with the reduced cost of silicon led to an overall reduction in production costs of 10.8%.
Another Black Eye For Coal
“For the people that think coal technology is going to be able to compete with solar, they should know solar costs are still coming down. Most coal companies are already, or near, bankrupt now. There’s no way coal’s going to be able to compete with solar in the future,” Pearce says. “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.”
The new techniques could have political and policy implications for many countries around the world. “To make this relatively small (manufacturing) change is pretty trivial,” Pearce says. If China were to adopt this new manufacturing process, it could cement its lead in the field of PV and solar power. The European Union should “look carefully at scaling up deep reactive ion etching and ALD tools to meet the needs of the rapidly expanding PV market,” says Pearce. He also suggests the US could use this new process to gain a competitive edge over other international manufacturers and recapture its position as a leader in the field — a position it used to enjoy but let slip away.
“I don’t know which technology will end up being the one to dominate the solar field,” Pearce says. However, “the study shows the clear economic impetus to move in the direction of dry-etched black silicon PERC that wasn’t there before.”