Most solar cells today are made of refined silicon that converts sunlight into clean electricity. Unfortunately, the process of refining silicon is far from clean and requires large amounts of energy from carbon-emitting power plants.
For a more environmentally friendly alternative to silicon, the researchers focused on thin-film perovskites – inexpensive, flexible solar cells that can be manufactured with minimal energy and with practically no CO2 emissions.
While perovskite solar cells show promise, significant challenges must be addressed before they can become commonplace, not least because of their inherent instability that makes them difficult to manufacture on a large scale.
“Perovskite solar technology is at a crossroads between commercialization and flimflammery,” said Nick Rolston, a postdoctoral fellow at Stanford University. “Millions of dollars are pouring into startups. But I firmly believe that in the next three years, if there isn’t a breakthrough that extends the life of cells, the money will dry up.”
That’s why a new perovskite manufacturing process developed at Stanford is so exciting, said Rolston. In a new study published in the November 25th issue of Joule magazine, he and his colleagues demonstrate an ultra-fast way to make sturdy perovskite cells and assemble them into solar panels that can power devices, buildings, and even the electrical grid .
“This work marks a new milestone in the production of perovskite,” said lead author of the study, Reinhold Dauskardt, Professor of Ruth G. and William K. Bowes at the Stanford School of Engineering. “It solves some of the biggest modular-scale barriers that the community has dealt with for years.”
Perovskite solar cells are thin films of synthetic crystal made from cheap, abundant chemicals like iodine, carbon, and lead.
Thin-film cells are light, bendable, and can be grown in open-air laboratories at temperatures close to the boiling point of water, a long way from the 1,650 degrees Celsius furnaces needed to refine industrial silicon.
Scientists have developed perovskite cells that convert 25 percent of sunlight into electricity, a conversion efficiency comparable to silicon. However, it is unlikely that these experimental cells will be installed on rooftops anytime soon.
“Most of the work on perovskites involves really tiny areas of active, usable solar cells. They’re typically only a fraction of the size of your little fingernail,” said Rolston, who led the study with William Scheideler, a former Stanford postdoctoral fellow, now assistant professor at Dartmouth College.
Attempts to make larger cells have created defects and pinholes that significantly reduce cell efficiency. And unlike rigid silicon cells that last 20 to 30 years, thin film perovskite eventually degrades when exposed to heat and moisture.
“You can make a small demonstration device in the laboratory,” said Dauskardt. “However, conventional perovskite processing is not scalable for fast and efficient production.”
To meet the challenge of mass production, the Dauskardt team employed a recently invented patented technology known as rapid spray plasma processing.
This technology uses a robotic device with two nozzles to quickly make thin films of perovskite. One nozzle sprays a liquid solution of chemical perovskite precursors onto a sheet of glass, while the other releases a burst of highly reactive ionized gas known as plasma.
“Conventional processing requires the perovskite solution to be baked for about half an hour,” said Rolston. “Our innovation is to use a high-energy plasma source to quickly convert liquid perovskite into a thin film solar cell in a single step.”
Using high-speed spray processing, the Stanford team was able to produce 12 meters of perovskite film per minute – about four times faster than producing a silicon cell.
“We achieved the highest throughput of any solar technology,” said Rolston. “You can think of large sheets of glass placed on rollers and continuously producing layers of perovskite at an unprecedented rate.”
In addition to a record production rate, the newly minted Perovskite cells achieved a power conversion efficiency of 18 percent.
“We want to make this process as applicable and generally useful as possible,” said Rolston. “A plasma treatment system may sound fancy, but it’s something you can buy commercially at a very reasonable cost.”
The Stanford team estimated that their perovskite modules can be made for about 25 cents per square foot – far less than about $ 2.50 per square foot, which is needed to make a typical silicon module.
Silicon solar cells are usually interconnected in encapsulated modules to increase their performance and withstand harsh weather conditions. After all, perovskite manufacturers need to build stable, efficient modules to be economically viable.
To this end, the Stanford team successfully developed perovskite modules that were still in operation after five months on the shelf with an efficiency of 15.5 percent.
Conventional silicon modules generate electricity at a price of around 5 cents per kilowatt hour. To compete with silicon, perovskite modules would have to be encapsulated in a weatherproof layer that would keep moisture out for at least a decade. The research team is currently exploring new encapsulation technologies and other ways to significantly improve shelf life.
“If we can build a perovskite module that will last 30 years, we could cut electricity costs below 2 cents per kilowatt hour,” said Rolston. “At that price, we could use perovskites for utility-scale power generation. For example, a 100 megawatt solar farm.”
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