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McMaster plan aims for solar breakthrough

Researchers at McMaster University aim to produce solar panels based on regular silicon cells, but with a secret compound deposited using MBE to form a second junction and double their efficiency.

Materials scientists in Canada are to receive $4.1 million to develop a novel high-efficiency solar cell that combines silicon and compound semiconductors.

Solar Energy firm ARISE Technologies and the Ontario Centres of Excellence (OCE) are funding the work of John Preston and Rafael Kleiman, who head up the McMaster University team.

They are aiming to combine the ubiquity and low cost of silicon-based solar cells with the high efficiencies associated with more expensive compound semiconductor technologies.

"We are aiming to develop cells suited for one-sun applications, to create what appear to be regular silicon panels "“ but which have a much higher efficiency because of the novel materials approach taken," Kleiman told compoundsemiconductor.net.

The exact nature of that materials system is being kept a closely-guarded secret, although it is expected to double the efficiency of typical silicon cells.

However, Kleiman did reveal that the team would be using McMaster s in-house MBE facility to deposit single-crystal layers of a compound semiconductor on top of the silicon host.

The professor added that GaAs will not be the material used because, at 1.45 eV, its bandgap is not sufficiently wide to provide the best conversion efficiency in a double-junction device alongside silicon.

Kleiman and colleagues have done some theoretical modeling (see figure) to work out what the best match would be. "This plot tells us clearly that for a double-junction device with silicon as the substrate, we would like our second (upper) junction to have a bandgap of about 1.68 eV," explained Kleiman, adding that the design would have a maximum theoretical efficiency of 43.5 per cent.

While triple-junction cells designed for high-concentration photovoltaic systems have already been measured to deliver a real-world efficiency close to that theoretical mark (at a 240-sun concentration), the maximum figure for triple-junctions under unfocused sunlight is much lower, and comparable to that of the silicon/III-V hybrid.

"We are targeting a more modest 30 per cent efficiency," Kleiman said, on the assumption that it would be possible to make a cell work at three-quarters of the theoretical maximum. He believes that the approach will only add a modest incremental cost to the processes currently used to make single-junction crystalline silicon cells.

Kleiman freely admits, however, that MBE will not be the ideal deposition method for the intended focus on high-volume, low-cost applications: "The later part of the project will focus on transferring the technology to a manufacturable process, such as MOCVD."

Although the practical side of the research is only at a very early stage right now, the team will be able to draw on experience gained during the photonics boom, where Canadian researchers and companies were at the cutting edge of fiber-optic technologies.

Ambitiously, the team is hoping to transfer the technology out of the lab and into a commercial fab in just three years.

However, interfacing a compound semiconductor with silicon is notoriously difficult, as Kleiman acknowledges: "I think the central goal is a detailed microscopic understanding of the III-V-to-silicon interface," concluded the researcher. "Structurally, chemically and electronically."

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