I had a nice chat Sunday with Ted Sargent, a professor of electrical and computer engineering at the University of Toronto who is also Canada Research Chair in nanotechnology. Sargent and his team have for the past six years led research on the development of colloidal quantum-dot (CQD) solar cells. The basic concept, which I first wrote about back in May 2005, has to do with the ability to design nanoparticles that are tuned to absorb a specific part of the electromagnetic spectrum of sunlight — that is, the ability to create quantum dots that can tap into both the visible and invisible spectrum of light. The potential, of course, is the ability to place these particles in some kind of liquid that becomes part of a high-volume spray-on process for manufacturing solar PV cells. Sargent’s goal is to produce a solar cell that’s both extremely low cost to make but without sacrificing efficiency, which is generally the case today.
Over the years Sargent’s group has steadily improved the technology, including extracting electrons from the solar cells more efficiently. From efficiencies of less than 1 per cent six years ago they’re now up above 5 per cent. Another way of improving efficiency is to design tandem and triple-junction solar cells, where each layer is specifically and optimally tuned to the slice of spectrum it wants to capture. “You can change the colours of light they absorb very readily by just changing the size of the nanoparticles,” Sargent explained to me. But if you want two layers — one for invisible spectrum and one for visible — it means you have to stack them in a way where you don’t compromise the efficiency of one or both layers. Sargent, whose group just released its latest study in the journal Nature Photonics, says they have cracked that nut. “The bottom line is, we’ve figured out how to stack these two solar cells on top of each other and make a totally smooth path for electrons to flow between these two cells.” It removes a major barrier to progress.
Achieving this flow isn’t so easy. Sargent says there’s a high wall for these electrons to pass, so the trick is to lower the wall separating the front and back layers (both made of titanium dioxide boosted with light absorbers) or make the wall easier to surmount. The solution was what Sargent’s team labelled the Graded Recombination Layer, which is composed of a series of conductive metal oxides — starting with molybdenum, then indium-tin, then zinc and finally titanium –that help transport the electrons between layers. “It’s a staircase, a little escalator, that brings electrons from the front cell to the back cell without having the losses,” says Sargent, who also likes to make comparisons to an electron ladder. “We ended up building a ladder with step heights that make it easier for the electrons to surmount.”
The device they built is only a couple of millimetres square, but that’s all you need to show the approach works. It achieved 4.2 per cent efficiency, with about two-thirds of that coming from visible spectrum and the remaining third from infrared spectrum. This is less than the single-junction efficiency record, but Sargent says for this experiment the objective wasn’t to improve efficiency but to establish an efficient flow between the two cells. The next step will be to up the efficiency, and even look at triple- or quadruple-junction cells.
As far as technology commercialization goes, Sargent’s goal is to achieve 10 per cent efficiency, which with the potential for super low-cost manufacturing would allow for the manufacture of flexible solar PV modules for about 50 cents per watt peak solar — a highly competitive price-point. After that, the group will shoot for 18 per cent efficiency. The goal is to achieve the highest efficiencies at the lowest possible cost. “We don’t want to sacrifice either,” he says. Asked about competition from other solar technologies, such as organic solar cells or thin-film cells based on CIGS or cadmium telluride, he’s optimistic about the superiority of the quantum dot approach. “We have solid reasons to believe we can be superior on cost and have a roadmap to be potentially better in terms of efficiency.”