New method to stabilize perovskites for inexpensive solar cells

Researchers have developed a method to stabilize a promising material known as perovskite for cheap solar cells, without compromising its near-perfect performance.

Researchers at the University of Cambridge used an organic molecule as a “template” to guide the perovskite films into the desired phase as they formed. Their results are published in the journal Science.

Perovskite materials offer a cheaper alternative to silicon for the production of optoelectronic devices such as solar cells and LEDs.

There are many different perovskites, resulting from different combinations of elements, but one of the most promising to emerge in recent years is formamidinium-based (FA) FAPbI.3 crystal.

The compound is thermally stable and its inherent “band gap” – the property most closely related to the device’s power generation – is not far from ideal for photovoltaic applications.

For these reasons, efforts have been made to develop commercially available perovskite solar cells. However, the compound can exist in two slightly different phases, one leading to excellent photovoltaic performance and the other with very little energy efficiency.

“A big problem with FAPbI3 is that the phase you want is only stable at temperatures above 150 degrees Celsius,“said co-author Tiarnan Doherty of the Cavendish Lab in Cambridge. “At room temperature, it switches to another phase, which is really bad for photovoltaics. “

Recent solutions to keep the material in its desired phase at lower temperatures have involved adding different positive and negative ions to the compound.

“It has been a success and has led to the registration of photovoltaic devices but there are still local power losses occurring,“said Doherty.”You end up with local regions in the movie that are not in the correct phase. “

Little was known about why additions of these ions improved overall stability, or even what the resulting perovskite structure looked like.

There was this common consensus that when people stabilize these materials, they form an ideal cubic structure, “ Doherty said. “But what we’ve shown is that by adding all these other things, they’re not cubic at all, they’re very slightly distorted. There is a very subtle structural distortion that gives some inherent stability to room temperature. “

The distortion is so minor that it had not been detected before, until Doherty and his colleagues used sensitive structural measurement techniques that weren’t widely used on perovskite materials.

The team used scanning electron diffraction, nano-x-ray diffraction, and nuclear magnetic resonance to see, for the first time, what this stable phase actually looked like.

Once we figured out that it was the slight structural distortion that gave this stability, we looked for ways to achieve this in the preparation of the film without adding other elements to the mix. “

Co-author Satyawan Nagane used an organic molecule called ethylenediaminetetraacetic acid (EDTA) as an additive in the perovskite precursor solution, which acts as a model agent, guiding perovskite into the desired phase as it forms. EDTA binds to FAPbI3 surface to give a structure-directing effect, but does not fit into the FAPbI3 structure itself.

“With this method, we can achieve the desired bandgap because we don’t add anything more to 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. “, Nagane said.

“This way you can create this slightly distorted structure in the pristine FAPbI3 compound, without changing the other electronic properties of what is essentially an almost perfect compound for perovskite photovoltaics,“said co-author Dominik Kubicki of the Cavendish Laboratory, which is now based at the University of Warwick.

The researchers hope that this fundamental study will help improve the stability and performance of perovskite. Their own future work will consist of incorporating this approach into device prototypes to explore how this technique can help them achieve the perfect perovskite photovoltaic cells.

These results modify our optimization strategy and our manufacturing guidelines for these materials,“said lead author Dr Sam Stranks of the Cambridge Department of Chemical Engineering and Biotechnology. “Even small pockets that are not slightly deformed will result in loss of performance, so manufacturing lines will need to have very precise control over how and where the various ‘deforming’ components and additives are deposited. This will ensure that the small distortion is uniform everywhere – no exceptions. “

The work was a collaboration with the Diamond Light Source and the electron Physical Science Imaging Center (ePSIC), Imperial College London, Yonsei University, Wageningen University and Research, and Leeds University.


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