Solar Power World

Could there be a world where photovoltaic cells are no longer made with silicon? The answer to that question is a definite maybe — but one company is out there trying to do it.

Natcore Technology, a Red Bank, N.J.-based technology company, recently announced their breakthrough technology that has the potential to revolutionize the way thin-film PV cells are created. The company achieved the discovery under a joint-research agreement with Rice University.

A research team working under company cofounder Dr. Andrew Barron fabricated two families of multilayer quantum dot films, one with silicon and the other with germanium, both of which have demonstrated the ability to produce a photo-generated current.

The discovery could eliminate the need for a silicon wafer subcell, which will accelerate the drive toward a low-cost tandem cell.

To understand the importance of this discovery, Natcore cofounder Dr. Dennis Flood says you have to understand the basics of tandem cells and how they work. Ordinarily, solar cells don’t harvest energy equally well from across the entire spectrum, and they are limited in efficiency. Putting two solar cells tuned to different parts of the solar spectrum in optical series (i.e., in tandem) with one another allows more of the energy to be collected. “There are a couple of companies making those kinds of cells, but they can cost up to $300 per watt and are used in spacecraft,” Flood says. “It’s really not available for terrestrial use.” In the last few years, those same companies have made a concentrator version of their space cells for terrestrial use. Focusing high intensity light on the cell lets them make the cells smaller and therefore cheaper.

Flood says it would be nice if companies could achieve the same goals without the focus by using quantum dots. That this is possible was proven by Dr. Martin Green at the University of New South Wales, when he came up with the third generation solar cells. He used nanostructures to generate layers of quantum dots of silicon embedded a silicon layer. While that process is good for chip production, it’s not practical for solar cell production.

“Sometimes that can take hours, and you have little or no independent control over what you get,” Flood says. “But it showed that using quantum dots could be done.”

What Natcore can do with its technology is that they can fabricate the dots and grow the silicon dioxide separately.

“Then we pour the dots over the silicon dioxide and shake them a little bit to make sure they’re properly aligned,” Flood says. “It gives us totally independent control over every step of the process.”

Flood also says Natcore’s system allows cells to absorb light at 1.7 EV, compared with 1.1 EV for the typical solar cell.

Here’s how the new system works. Each film is comprised of silicon or germanium quantum dots embedded in a silica matrix, which is produced using a Natcore-patented Liquid Phase Deposition (LPD) silica growth technology. Unlike preceding attempts to make such layers using chemical vapor deposition (CVD) technology, Natcore’s approach decouples quantum dot formation from the silica layer growth and allows for completely independent selection of quantum dot type, size and spacing within the silica layer.

The photo-generated current measurements are the first of its kind for this sort of structure and showed that the Si and Ge quantum dots were photoactive in different spectral regions. The larger Ge quantum dots were responsive to an infrared-rich light source, and the Si quantum dots were responsive to a UV-rich light source, which was consistent with the company’s expectations.

The smaller Si quantum dots (the dots diameters were 1 nm to 2 nm) responded more readily to shorter wavelengths of light, while the larger Ge dots (5 nm and 6 nm) responded more readily to longer light wavelengths.

“We’ve created the first all quantum-dot tandem solar absorber,” Flood says. “It’s the critical first step in creating a silicon-free cell.”

Flood says the most exciting part of the discovery is that the industry could eventually get beyond the silicon subcell.

“We can get rid of the silicon wafer entirely,” Flood says. “We will have a thin-film, dual-junction solar cell that will be very close to 30% efficiency. We can even see a future where we might be able to get to an efficiency of 35%.”

He believes it’s feasible and inexpensive to produce and can be done on different types of substrates. The manufacturing process is roll-to-roll, which means that underutilized manufacturing equipment of the sort available at Kodak’s Eastman Business Park in Rochester, N.Y. -i.e., existing technology, the same technology that produces film — can be used to produce the solar cells.

“The dryers are already there, all the permits are there, all the regulatory emissions control procedures are there — it doesn’t require any new work,” Flood says. “This approach should be able to get solar cells out the door at 10% of the capex cost of a silicon solar cell fab.”

In his 30 years in the solar industry, Flood says he’s never been a part of anything more exciting than this.

“I’ve watched a bunch of developments over my career, and I’ve never been more excited than I am about this one,” Flood says. “We’re hoping to revolutionize the industry with our discovery.” SPW