The stamp-sized chip with two wires protruding from it doesn’t look like much. But appearances can be deceptive. This sandwich of a conventional silicon-based solar cell and an obscure mineral called perovskite has the potential to revolutionise solar electricity.
Already these experimental cells are able to convert more than 26 per cent of the sun’s rays into electricity, which is around the theoretical upper efficiency limit of conventional silicon solar cells.
In time, scientists will probably improve on that performance. Perovskite has the potential to increase that conversion efficiency to 40 per cent, according to Kylie Catchpole, a research engineer at the Australian National University, who is leading the work on these perovskite-enhanced silicon cells.
That matters. Every hour of sunlight that hits the earth carries the equivalent of a year’s worth of global human energy consumption. Improving efficiency will allow solar cells to increasingly replace more polluting forms of power generation.
A revolution with unprepossessing roots
Perovskite, which makes up a class of compounds that have a particular crystalline structure, was first identified in Russia’s Ural Mountains in 1839. But it wasn’t until 2009 that a group of Japanese scientists—led by Tsutomu Miyasaka of Toin University of Yokohama—first used perovskite to produce a solar cell. It might have remained a mineralogical curiosity had Miyasaka’s interest not been piqued by a graduate student looking to test the mineral’s photovoltaic properties a couple of years earlier. Indeed, Miyasaka, who was investigating various materials as candidates for solar cells, had never heard of perovskite until that moment.
His early efforts were inauspicious. The first perovskite solar cell (PSC) had such a low efficiency rate—an uninspiring 3.8 per cent—that several leading science journals rejected Miyasaka’s paper presenting the results.
Even so, enough scientists were intrigued by the possibilities of perovskite to investigate further. One of the first things to excite researchers was perovskite’s ability to outperform silicon in absorbing blue light, which is the high energy part of the visible electromagnetic spectrum.
With silicon-based solar cells, says Catchpole, “you just can’t take advantage of the extra energy of blue light.”
Perovskite, thus, was “the first material that was potentially cheap but would also actually increase the efficiency of solar cells,” she adds.
Advances have accelerated since. In the past two years, PSC efficiency has really jumped. Scientists in South Korea at the Ulsan National Institute of Science and Technology made a 22.1 per cent-efficient PSC quickly followed by a cell with 23.9 per cent efficiency produced by Belgium’s IMEC. And higher-still efficiencies are coming from sandwiching perovskite with other materials, as Catchpole’s work has demonstrated.
As a result, several companies are racing to develop this fast-advancing, albeit still experimental solar technology, with the hope that PSCs will hit the mass market in the next few years.
The promise and the challenges
When they do, they will be a part of a much bigger revolution. Solar electricity has boomed in the past decade, with global production exceeding 100 gigawatts in 2017, an increase of almost 1,400 per cent since 2008. Even so, it still represents a mere 1.5 per cent of the world’s total installed electricity-generating capacity.
Currently, crystalline-silicon panels account for roughly 85 per cent of the world’s photovoltaic output. Their manufacture requires expensive, multi-step processes, conducted in clean-room facilities at high temperatures—above 1000 Celsius—and in a strong vacuum. That is not ideal for high-volume production. Plus, silicon-based cells are not particularly efficient.
In theory, perovskite could replace silicon. It’s cheap and commonly available. And it can potentially be produced with inexpensive printing or spraying processes. But so far, production is at pilot volumes, which makes it hard to calculate the ultimate manufacturing costs of PSCs.
“The important thing about this approach is that there’s a way to go further,” Catchpole explains. “There’s a way to keep improving the efficiency, which hasn’t been the case with just silicon itself.”
One significant hurdle to PSC adoption is their sensitivity to heat and moisture. Current PSCs last less than a year outdoors, compared to the 20-year guarantees routinely offered by rooftop silicon solar cell manufacturers.
Part of the durability issue arises from the fact that a water-soluble form of lead is a key component of PSCs. That means we need to find a better way of keeping the lead isolated from the environment. Or—better still given its toxicity—substitute it out altogether. But there is progress on this environmental stability issue, and scientists are hopeful hurdles can be overcome before long.
Meanwhile, making PSCs competitive depends on maintaining efficiency as cells are scaled up. For now, even relatively small PSCs struggle with material defects. In 2017, Toshiba developed a flexible, 5-centimeter PSC module with an efficiency of 10.5 per cent, which is a record for anything much bigger than a postage stamp—around a quarter of the theoretically possible levels.
Solar energy is big business
Several companies are exploring perovskite solar panels. Saule Technologies, for example, is developing an inkjet printing–manufacturing process to apply PSCs on flexible foil sheets. Olga Malinkiewicz, who founded the privately backed company, said it is focusing on solar modules for three industries: automotive, construction and outer space.
Elsewhere, Iris PV, spun out of Stanford University, seeks to develop and commercialize tandem solar cells composed of silicon and perovskite.
And there is growing investor interest. For example, perovskite specialist Oxford Photovoltaics, which was spun out from Oxford University, attracted GBP16.8 million in equity investments late in 2016.
The stakes are high in the race for low-cost and reliable solar, both in terms of economies and the environment. Solar could provide 16 per cent of global electricity by mid-century, according to the International Energy Agency. And PSCs could well make a major contribution.
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