Spray-on cells hold considerable promise for reducing the manufacturing costs of solar power. Within this field, colloidal quantum dots (CQD) have also been the focus of a number if research efforts, as they have the potential to soak up a wider range of the solar spectrum. Scientists at the University of Toronto have been aboard the quantum dot train for some time now and their latest breakthrough involves a new method for spraying solar cells onto flexible surfaces, a development that could one day see them coat anything from bicycle helmets to outdoor furniture.

In such spray on solar cells, quantum dots would act as the absorbing photovoltaic material. Because they have a band gap that can be tuned by altering the size of their nanoparticles, they can be made to soak up different parts of the solar spectrum. This could prove particularly valuable if they were to be used in multi-junction solar cells, where dots small and large could sit alongside each other to widen the cells' energy harvesting potential.

Earlier this year, University of Toronto scientists developed a new kind of CQD that doesn't bind with oxygen atoms, a problem that causes some dots to forgo their electrons and become useless. The researchers recorded a solar efficiency of eight percent, much less than that of commercially available panels, but were optimistic about the dot's potential to power new kinds of devices.

The scientists say that until now, integrating CQDs into materials has only been possible through batch processing, a procedure that is inefficient and expensive. But Illan Kramer, one of the university's post-doctoral fellows, has developed an instrument where the dots can be sprayed onto flexible surfaces such as film or plastic, an advance that could make the process a lot simpler.

"My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof,” says Kramer.

Printing light-sensitive CQDs onto thin and bendable materials could theoretically see them applied to just about any irregular surface, as well making cells cheaper and easier to manufacture. According to the researchers, covering the roof of a car in CQD-laden film would generate power enough for three 100 W light bulbs.

Kramer's system is dubbed SprayLD and is built from inexpensive, readily available components. Comprising standard air brushes and a spray nozzle used to cool steel in a mill, Kramer's contraption sprays the CQDs onto the film-like materials, and reportedly does so without incurring major losses in performance of the cells. The researchers tell us they achieved an efficiency of 7.2 percent.

“As quantum dot solar technology advances rapidly in performance, it’s important to determine how to scale them and make this new class of solar technologies manufacturable,” said Professor Ted Sargent, Kramer's supervisor and vice dean, research at the University's Faculty of Applied Science & Engineering. “We were thrilled when this attractively manufacturable spray-coating process also led to superior performance devices showing improved control and purity.”

Kramer's research has been published across a series of journal articles, initially in Applied Physics Letters, then in Advanced Materials and most recently in ACS Nano.

You can see a video demonstration of the SprayLD system below.

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