Canadian researchers have invented a fast, low-cost way to spray solar cells onto flexible surfaces using light-sensitive materials known as colloidal quantum dots (CQDs).

“My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof,” said Illan Kramer, a post-doctoral fellow with The Edward S. Rogers Sr. Department of Electrical & Computer Engineering at the University of Toronto and IBM Canada’s Research and Development Centre.

Solar-sensitive CQDs printed onto a flexible film could be used to coat all kinds of weirdly shaped surfaces, from patio furniture to an airplane’s wing. A surface the size of your car’s roof wrapped with CQD-coated film would produce enough energy to power three conventional 100-Watt light bulbs.

He calls his system SprayLD, a play on the manufacturing process called ALD (atomic layer deposition, in which materials are laid down on a surface one atom-thickness at a time).

Until now, it was only possible to incorporate light-sensitive CQDs onto surfaces through batch processing—an inefficient, slow and expensive assembly-line approach to chemical coating. Instead, SprayLD blasts a liquid containing CQDs directly onto flexible surfaces, such as film or plastic, like printing a newspaper by applying ink onto a roll of paper.

This roll-to-roll coating method makes incorporating solar cells into existing manufacturing processes much simpler. In two recent papers in the journals Advanced Materials and Applied Physics Letters, Kramer showed that the sprayLD method can be used on flexible materials without any major loss in solar-cell efficiency.

Kramer built his sprayLD device using parts that are readily available and rather affordable. “This is something you can build in a Junkyard Wars fashion, which is basically how we did it,” said Kramer. “We think of this as a no-compromise solution for shifting from batch processing to roll-to-roll.”

“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 (ECE), vice dean, research in the Faculty of Applied Science & Engineering at University of Toronto and Kramer’s supervisor. “We were thrilled when this attractively manufacturable spray-coating process also led to superior performance devices showing improved control and purity.”

In a third paper in the journal ACS Nano, Kramer and his colleagues used IBM’s BlueGeneQ supercomputer to model how and why the sprayed CQDs perform just as well as — and in some cases better than — their batch-processed counterparts.

This work was supported by the IBM Canada Research and Development Centre, and by King Abdullah University of Science and Technology.

UofT | SprayLD — New technique offers spray-on power

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Abstract of Efficient Spray-Coated Colloidal Quantum Dot Solar Cells

A colloidal quantum dot solar cell is fabricated by spray coating under ambient conditions. By developing a room temperature spray coating technique and implementing a fully automated process with near monolayer control—an approach termed as sprayLD—an electronic defect is eliminated resulting in solar cell performance and statistical distribution superior to prior batch-processed methods along with hero performance of 8.1%.

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Abstract of Colloidal quantum dot solar cells on curved and flexible substrates

Colloidal quantum dots (CQDs) are semiconductor nanocrystals synthesized with, processed in, and deposited from the solution phase, potentially enabling low-cost, facile manufacture of solar cells. Unfortunately, CQD solar cell reports, until now, have only explored batch-processing methods—such as spin-coating—that offer limited capacity for scaling. Spray-coating could offer a means of producing uniform colloidal quantum dot films that yield high-quality devices. Here, we explore the versatility of the spray-coating method by producing CQD solar cells in a variety of previously unexplored substrate arrangements. The potential transferability of the spray-coating method to a roll-to-roll manufacturing process was tested by spray-coating the CQD active layer onto six substrates mounted on a rapidly rotating drum, yielding devices with an average power conversion efficiency of 6.7%. We further tested the manufacturability of the process by endeavoring to spray onto flexible substrates, only to find that spraying while the substrate was flexed was crucial to achieving champion performance of 7.2% without compromise to open-circuit voltage. Having deposited onto a substrate with one axis of curvature, we then built our CQD solar cells onto a spherical lens substrate having two axes of curvature resulting in a 5% efficient device. These results show that CQDs deposited using our spraying method can be integrated to large-area manufacturing processes and can be used to make solar cells on unconventional shapes.

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Abstract of Electronically Active Impurities in Colloidal Quantum Dot Solids

Colloidal quantum dot films have seen rapid progress as active materials in photodetection, light emission, and photovoltaics. Their processing from the solution phase makes them an attractive option for these applications due to the expected cost reductions associated with liquid-phase material deposition. Colloidally stable nanoparticles capped using long, insulating aliphatic ligands are used to form semiconducting, insoluble films via a solid-state ligand exchange in which the original ligands are replaced with short bifunctional ligands. Here we show that this ligand exchange can have unintended and undesired side effects: a high molecular weight complex can form, containing both lead oleate and the shorter conductive ligand, and this poorly soluble complex can end up embedded within the colloidal quantum dot (CQD) active layer. We further show that, by adding an acidic treatment during film processing, we can break up and wash away these complexes, producing a higher quality CQD solid. The improved material leads to photovoltaic devices with reduced series resistance and enhanced fill factor relative to controls employing previously reported CQD solids.