A team of researchers from the University of Cambridge, UK, have created a new way to stabilise perovskites to be used in inexpensive solar cells without compromising efficiency, according to a study published in the journal Science (1).
Perovskites are becoming increasingly popular as an alternative material to produce electronic devices, including solar cells and LEDs. There are different types of perovskites, but one that has attracted much attention is the formamidinium (FA)-based FAPbI3 crystal. This compound is stable, and its band gap —the way to measure energy output —is nearly perfect for photovoltaic applications.
The major problem is that this compound can exist in two phases: one leads to high performance, while the other results in poor energy output. “A big problem with FAPbI3 is that the phase that you want is only stable at temperatures above 150 degrees Celsius,” said Tiarnan Doherty from Cambridge’s Cavendish Laboratory and one of the authors in this study. “At room temperature, it transitions into another phase, which is really bad for photovoltaics.”
Researchers have tried to keep the perovskite in the desired phase by adding different positive and negative ions into the compound. This worked up to a point, but it wasn’t perfect. “That’s been successful and has led to record photovoltaic devices, but there are still local power losses that occur,” said Doherty. “You end up with local regions in the film that aren’t in the right phase.”
Now, a team from Cambridge used scanning electron diffraction, nano-X-ray diffraction, and nuclear magnetic resonance to see if they could find a better solution. They showed that minor distortions—previously undetected— are responsible for keeping perovskite in the stable phase. “There was this common consensus that when people stabilise these materials, they’re an ideal cubic structure. But what we’ve shown is that they’re not cubic at all; they’re very slightly distorted. There’s a very subtle structural distortion that gives some inherent stability at room temperature,” said Doherty.
Once they figured out that it was this slight structural distortion that gave it stability, it was just a matter of finding a way to achieve this distortion during preparation without adding other chemical elements. Instead, they added an organic compound called Ethylenediaminetetraacetic acid (EDTA) to the perovskite solution, which guides it to the desired phase as it forms. The EDTA can bind to the perovskite but does not incorporate it into the structure itself.
“With this method, we can achieve that desired band gap because we’re not adding anything extra into the material; it’s just a template to guide the formation of a film with the distorted structure – and the resulting film is extremely stable,” said Satyawan Nagane.
“You can create this slightly distorted structure in just the pristine FAPbI3 compound, without modifying the other electronic properties of what is essentially a near-perfect compound for perovskite photovoltaics,” added Dominik Kubicki.
The team hopes this work will help improve perovskite performance and stability. The aim now is to integrate this approach into prototype devices to find out how this technique can help produce better photovoltaic cells.
(1) Doherty T, Nagane T, Kubicki D, Jung Y, Johnstone D, Iqbal A, Guo D, Frohna K, Danaie M, Stranks S et al. (2021) Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases. Science, 374 (6575): 1598-1605, DOI: 10.1126/science.abl4890