Using colloidal quantum dots, a team of researchers at the University of Toronto has developed photovoltaics that can produce sun from nearly all of the sun’s spectrum. The two junction devices are able to produce electrons by absorbing both the visible spectrum and most of infrared light as well.

“It captures more spectrum than if it were a one-junction device. You don’t want to go to all the far reaches of the infrared,” said Ted Sargent, Professor of Electrical and Computer Engineering at the University of Toronto, who is also the Canada Research Chair in Nanotechnology.

The team developed one junction that could absorb the infrared radiation and another for the visible spectrum of light, he said.

“The quantum dots are elemental materials, according to Sargent. They’re a metal bound with sulfur or selenium, forming a binary semiconductor,” he said. The metals include things like tin, lead or bismuth.

The devices could deliver photovoltaic materials that are up to 42 percent efficient in the future, using inexpensive deposition methods akin to inkjet printing, Sargent said. However, at this point the highest efficiency the team of researchers has reached is 5.6 percent, a world record for colloidal quantum dot technology. But that was on a single-junction device.

“It turns out that so far the best [devices] are still based on a single junction. The potential for the tandem cells is greater. But by moving to the tandem we’ve had to change the wavelengths. They need more optimization,” Sargent said. “What we’ve proved at this point, is you can make them add up their voltages and add up their power. You can make them conspire to combine their efficiencies together without loss.”

Once the technology is improved, it can be made inexpensively, according to Sargent.

“It’s a very low-cost, large area, scalable approach. We need to get our efficiencies up to silicon then we’ll beat them on cost,” he said.

Pictured: quantum dot rendering, courtesy of the Lawrence Berkeley National Lab.