Expanding our use of solar energy is an important part of the fight against climate change by decarbonizing the economy. The silicon-based solar cells in widespread use today are pretty good – efficient, relatively cheap, and long-lasting – but making them is extremely energy-intensive.
“A lot of energy is embodied in the device,” says Timothy Kelly, a materials chemist who studies photovoltaics at the University of Saskatchewan. “It takes a long time to produce more energy than was used in the manufacturing process.”
Over the past decade, however, a promising new class of photovoltaic material has come to the fore – perovskite crystals made of a combination of organic and inorganic materials, such as lead-halide. These can be made quickly and cheaply by mixing two beakers on a benchtop, says Kelly, then coating the mixture on large sheets of plastic or glass. They have an efficiency on par with traditional silicon cells, and are able to absorb a broader spectrum of light so in theory that efficiency could be even higher.
But while they are easy to make, they are also easy to break. A silicon-based solar panel can last for 25 years or more, but perovskite panels break down much more quickly. “They don’t have the stability yet to be used outdoors,” says So Min Park, a materials scientist at Northwestern University in Evanston, Illinois.
Park has been working on improving the stability of perovskite cells. While a postdoctoral researcher in Ted Sargent’s lab at the University of Toronto she developed a perovskite solar cell that can stand up to high temperatures for more than 1500 hours.
To improve their efficiency perovskite cells typically have a thin, two-dimensional layer of material, called a passivation layer, on top of the perovskite crystals. But these layers, often made with ammonium, don’t stand up well to heat. The passivation layer tends to mix in with and disrupt the crystal structure of the underlying perovskite layer, leading to it breaking down – a fatal flaw in a device that is intended to be exposed to bright sunlight for much of its life.
So Park and her colleagues developed a new material for the passivation layer, replacing the bulky ammonium ions with 3,4,5-trifluoroanilinium. This new material does not penetrate into the perovskite layer, making the solar cells much more stable at high temperatures.
The team tested the cells at 85oC and 50% humidity and found a T85 – the amount of time it takes for the cell’s performance to degrade to 85% of its original value – of 1,560 hours. That’s a record performance for a perovskite cell, says Park.
Kelly says that level of stability is getting close to what is needed to make perovskite cells commercially viable for large-scale solar power generation. “Hot, sunny locations tend to be ideal sites for solar farms, so that’s the kind of stability you would need to withstand those hot summer days in places like Phoenix, or the Middle East, anywhere that sees a lot of insolation,” he says.
It will still take some time, however, before highly efficient and stable perovskite solar cells are ready to be deployed, says Park. “There is still much more to be done,” she says, particularly around scaling up to match the size of traditional silicon solar panels. “This is the beginning for perovskite solar cells.”