This is why researchers have joined forces to develop perovskite solar cells in order to distribute them commercially. However, this compound can exist in two slightly different phases: a phase that leads to excellent photovoltaic performance and another that has very little energy production.
“A big problem with FAPbI3 is that the desired phase is only stable at temperatures above 150 degrees Celsius,” said Tiarnan Doherty of the Cavendish Laboratory in Cambridge, first author of the paper. “At room temperature, it goes into another phase, which is really bad for photovoltaic energy.”
The latest solutions and alternatives to keep this material in its desired phase, which is a lower temperatures, have involved the addition of different positive and negative ions to the compound.
“That has been successful and has led to a record of photovoltaic devices, but there are still local power losses,” said Doherty, who is also affiliated with the Department of Chemical Engineering and Biotechnology. “You end up with local regions in the movie that are not in the correct phase.”
Perovskite a possible substitute for silicon
About the process of the reason why the sum of these ions improved the stability in general little was known, indeed, neither the resulting structure of perovskite was known at that time.
“There was a common consensus that when people stabilize these materials, they are an ideal cubic structure,” said Doherty. But what we have shown is that, by adding all these other things, they are not cubic at all, they are very slightly distorted. There is a very subtle structural distortion that gives some inherent stability at room temperature, ”he commented.
A distortion so small that researchers had not previously been able to detect until Doherty and his colleagues used more sensitive structural measurement techniques that had never been used extensively with materials like perovskite. Specifically, scanning electron diffraction and nuclear magnetic resonance were used to see for the first time what this stable phase looked like.
“Once we discovered that it was the slight structural distortion that gave this stability, we looked for ways to achieve this in film preparation without adding anything else to the mix.”
A study with promising results
Satyawan Nagane, research co-author, used an organic molecule called ethylenediaminetetraacetic acid (EDTA) as an additive in the perovskite precursor solution that acted as a templating agent that guides the perovskite to the desired phase, as it was being formed. EDTA connects to the mineral surface, to give a structure-directing effect, without being directly incorporated into it.
“With this method, we can achieve that desired band gap because we are not adding anything extra to the material, it is just a template to guide the formation of a film with the distorted structure, and the resulting film is extremely stable,” Nagane said.
“These findings change our optimization strategy and manufacturing guidelines for these materials,” said study lead author Dr Sam Stranks, from Cambridge’s Department of Chemical Engineering and Biotechnology. “Even small pockets that are not slightly distorted will lead to performance losses, so manufacturing lines will need to have very precise control of how and where different ‘distorting’ components and additives are deposited. This will ensure that the small distortion is uniform throughout, with no exceptions. “
Researchers hope that this study helps improve the stability and performance of perovskite. The next steps in the research will focus on integrating this approach into prototype devices in order to explore how this technique can help to obtain perovskite photovoltaic cells that help solar panels see their cost reduced in the future.