Quantum dots enable spray-on wide-spectrum PV cells

However, cost and functionality can still be a challenge, whether structural electronics takes the form of building integrated photovoltaics or smart skin on a vehicle. 3D shapes tend to call for stretchable electronics. The smart skin cannot simply be flexible in one direction from a roll-to-roll production line if it is to be widely applied across a vehicle.

The IDTechEx report, ‘Stretchable Electronics 2014-2024’ analyses and forecasts progress in this field. However, for now, stretchable electronics is feasible only in smaller formats such as skin patches. In short, structural energy storage and energy harvesting are being pursued by building up a load-bearing structure or applying a smart skin, however, a spray-on material is a plausible alternative. It does not matter if the photovoltaics or supercapacitor storage has poor performance by unit of area when applied in this way because huge area is usually available.

Indeed, spray-on cells hold considerable promise for reducing the manufacturing costs of solar power. Within this field, colloidal quantum dots CQD have been the focus of recent research efforts, as they have the potential to soak up a wider range of the solar spectrum. A quantum dot QD is usually a 5 to 50nm III-IV nanocrystal. That is small enough to exhibit quantum mechanical properties, its excitons being confined in all three spatial dimensions so its electronic properties are intermediate between those of bulk semiconductors and discrete molecules. Researchers have studied quantum dots in transistors, solar cells, LEDs, diode lasers, agents for medical imaging and qubits in quantum computing, the first commercial release of a product being the Sony XBR X900A series of flat panel televisions in 2013.

Scientists at the University of Toronto have recently announced a method for spraying such solar cells onto flexible surfaces. They dream of coating anything from bicycle helmets to outdoor furniture one day. Quantum dots provide versatile photovoltaic material because they have a band gap that can be tuned by altering the size of their nanoparticles, soaking up different parts of the solar spectrum. Such multi-junction solar cells would have dots of a chosen spread of sizes alongside each other to widen the cells' energy harvesting potential.

In 2014, the University of Toronto scientists developed a CQD that does not have the traditional problem of binding with oxygen atoms, causing some dots to forgo their electrons and become useless. The team recorded solar efficiency of 8%, far better than earlier sprayed photovoltaics and similar to flexible organic photovoltaic coated film, though much less than that of commercially available glass panels.

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,” said 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 300W, comparing well with today’s rigid patch on a car which offers 100W peak at best. The ‘SprayLD’ is built from inexpensive, readily available components. Comprising standard air brushes and a spray nozzle used to cool steel in a mill, Kramer’s device sprays the CQDs onto the film-like materials, without incurring any major losses in efficiency of the cells. This is in some contrast to the printing of copper indium gallium diselenide and dye sensitised solar cells photovoltaics reel to reel where up to 80% reduction in efficiency is suffered compared to conventionally deposited versions on glass.

Peter Harrop, Chairman, IDTechEx, comments: “This is all very exciting. There is a huge variety of applications for photovoltaics of modest efficiency if the cost, deployment and life are adequate. You can think of anything from an electric boat to smart packaging. Indeed, the supercapacitor electric vehicle bodywork being developed by several universities could have photovoltaics sprayed on top so you make and store the electricity with little space or weight penalty, unlike today. An extra 100 miles in electric car range will come from multiple energy harvesting over and above the extra 100 miles range from better batteries. The resulting 300 miles range for affordable pure electric cars will make sales take off in a few years from now.”

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

The two day conference, ‘Electric Vehicles: Everything Will Change’ will take place in Berlin, from 28^th to 29^th April, 2015. It will concentrate on future components and smart materials for EVs on land, water and air. The conference will form a part of the major IDTechEx event covering relevant technologies such as structural electronics, printed electronics, 3D printing, in-mould electronics, wireless sensors, IoT and energy harvesting including photovoltaics deposited in different ways. The event will offer nine parallel conferences and an exhibition of over 150 stands plus many optional masterclasses on the days before and after.