Spray on, printable and other new thin film technology looks set to provide a major boost to the global solar market.
Currently being developed by researchers and a small number of companies, the new film materials offer the potential of lighter and cheaper manufacturing.
“This field is moving so rapidly that I’m sure in a few years you will start seeing products you can actually hold in your hand,” says Dr Jao van de Lagemaat from the National Renewable Energy Laboratory in Golden, Colorado.
The most promising of the new film technologies is perovskite cells, named after the 19th century Russian mineralogist Lev Perovski.
Unlike silicon-based photovoltaic (PV) cells, perovskite cells are soluble in a variety of solvents so can be easily sprayed on to a surface, similar to inks or paints. That potentially makes the cells much cheaper to manufacture and means that the light-gathering film can be attached to flexible materials, opening up a range of new applications.
“You could, in a factory, print these solar cells using a similar process as is used for printing newspapers,” says Van de Lagemaat. “Your solar panels would come out as a roll at the end.”
What has really got people excited about perovskites though is the rapid increase in efficiency that materials scientists have achieved with them in the lab. In seven years they have gone from converting 3.8% of the light that falls on them into electricity, to more than 20%.
That figure might not sound hugely impressive but consider that traditional silicon-based cells, with their decades of research behind them, only achieve 24% or 25% efficiency in the lab and around 18% in real-world applications. The theoretical maximum is around 33% energy conversion.
Although recently touting it as a “gamechanger” for the solar market, Prof Yang Yang at the department of materials science and engineering at the University of California, Los Angeles remains cautious: “We have to face reality. To put them on the rooftop and the power plant requires a significant improvement in the material.”
One problem is connected with the material’s inherent advantage – their solubility. That, combined with heat sensitivity, means the cells are not as stable as silicon PV. Instead of lasting for 25 years or more they degrade over a period of months or a few years. That might not matter for short-lived disposable products such as cellphones, but would exclude the technology from the large-scale solar farm market, for example.
Researchers are working to improve the material’s inherent stability or come up with coatings that would encapsulate the perovskite, but that may add cost.
Another issue is disposal. Perovskites typically contain small amounts of lead – not enough to prevent their development (the lead in a single car battery is apparently enough for hundreds of square metres of perovskite solar cells), but enough to make the search for non-toxic alternatives an active line of research.
Oxford PV, a spin-off from researchers at Oxford University, announced two large investments (£8.1m and £8.7m) in late 2016 from investors including Statoil Energy Ventures and Legal General Capital. The company has also announced it is partnering with an unnamed major global solar manufacturer and intends to bring a product to market by the end of next year.
Aside from perovskites, organic PV can also be printed as a thin film on to a flexible substrate. In this case though the light-activated layer, or layers, are made up of conducting organic materials, usually polymers.
Like perovskites, organic PV has stability issues and, at around 13%, the efficiencies that scientists have achieved in the lab are not as good. But it does have other advantages. It does not contain toxic elements, for example, and can be engineered to be transparent and coloured. That means it could potentially be retrofitted to buildings as a tinted window coating.
Another new approach is so-called quantum dots, which are semiconducting particles that can be coated on to a surface. The technology is further from commercialisation but theoretical work from Van de Lagemaat’s team suggests that, in combination with perovskites, it may be possible to manufacture a panel that is 30% efficient.
Leonie Greene, head of external affairs at the UK’s Solar Trade Association, believes the industry in general is adopting a wait-and-see posture. “There are lots of areas of research and we wait to see which can make it out of the lab into commercialisation,” she says.
“We shouldn’t forget,” she adds, “that commercially available solar, where conversion efficiencies of over 20% are commonly available, are already providing power cheaper than other sources of power in many parts of the world.”