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Department of Chemical Engineering and Biotechnology

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Researchers from University of Cambridge Department of Chemical Engineering and Biotechnology (CEB) and the Cavendish Laboratory have developed a method to stabilise the most promising perovskite for cheap solar cells, without compromising its near-perfect performance.

Perovskite materials offer a cheaper alternative to silicon for producing 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 the formamidinium (FA)-based FAPbI3 crystal.

The compound is very thermally stable and its inherent ‘bandgap’ – the property most closely linked to the energy output of the device – is not far off ideal for photovoltaic applications.

For these reasons, FAPbI3 has been the focus of many efforts to develop commercially available perovskite solar cells. However, the compound can exist in two slightly different phases, with one phase leading to excellent photovoltaic performance, and the other resulting in very little energy output.

“A big problem with FAPbI3 is that the phase that you want is only stable at temperatures above 150 degrees Celsius,” explains Tiarnan Doherty, a PhD student at the Cavendish Laboratory and now Oppenheimer Research Fellow at CEB and Research Fellow at Murray Edwards College.            

“At room temperature, it transitions into 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 into the compound.

“That's been very successful and has led to record PV devices but there are still local power losses that occur,” says Doherty. “You end up with local regions in the film that aren’t in the right phase.”

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

“There was this common consensus that when people stabilise these materials, they’re an ideal cubic structure,” says Doherty.

“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’s a very subtle structural distortion that gives some inherent stability at room temperature.

“And the local losses we see are related to areas in the film that don’t have this slight distortion – when you’re adding other components into the film, it’s very difficult to make sure they’re spread evenly, and to achieve a completely homogenous distribution. Even though on the macroscopic big picture scale it seems like they're stable, there are small local regions that cause degradation and performance losses.”

The distortion is so minor that it had previously gone undetected, until Doherty and colleagues employed very sensitive structural measurement techniques that have not been widely used on perovskite materials.

Working with the groups of Paul Midgley in the Materials Science Department and Clare Grey in the Yusuf Hamied Department of Chemistry at Cambridge, and using national facilities at Diamond Light Source and the electron Physical Science Imaging Centre (ePSIC), the team used techniques such as scanning electron diffraction, nano-X-ray diffraction and nuclear magnetic resonance to see, for the first time, what this stable phase really looked like.

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

Satyawan Nagane, a Newton International Fellow in the group, used a multifunctional organic molecule called Ethylenediaminetetraacetic acid (EDTA) as an additive in the perovskite precursor solution, which acts as a templating agent, guiding the perovskite into its desired phase as it forms. The EDTA binds to the FAPbI3 surface to give a structure-directing effect, but does not incorporate into the FAPbI3 structure itself.

“Because of the bonding in the solution of this multi-functional organic molecule, it helps to propagate the desired phase throughout the film,” explains Nagane. “And this works with very small additions of this EDTA templating agent.

“With this method, we can achieve that desired band gap you want from your FAPbI3 compound because we’re not adding anything extra into the material that changes this bandgap, it’s just a template to guide the formation of a film with the distorted structure – and the resulting film is extremely stable.”

In this way, 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,” adds Dominik Kubicki, Marie Curie Fellow in the Cavendish Laboratory and now Assistant Professor at the University of Warwick.

The researchers hope this fundamental study will open new doors for further investigation into exploiting multi-functional organic molecules to improve perovskite stability and performance. Their own future work will involve integrating this approach into prototype devices to explore how this technique may help them achieve the perfect perovskite photovoltaic cells.  

“These are extremely exciting findings because they change our optimisation strategy and manufacturing guidelines for these materials” says Sam Stranks, University Assistant Professor and Royal Society University Research Fellow in CEB, and corresponding author of the work.

“We’ve previously been guided by the ‘Goldschmidt tolerance factor’ to use the right components in the perovskite structure to make ideal cubic FAPbI3 structures, but here we show that a small degree of distortion – though not too much – is actually just right. So, we should be turning to Goldilocks rather than Goldschmidt.”

“Even small pockets that aren’t slightly distorted will lead to performance losses, and so manufacturing lines will need to have very precise control of how and where the different components and ‘distorting’ additives are deposited. This will ensure the small distortion is uniform everywhere – with no exceptions.”

Beyond the Cambridge and ePSIC teams, the work was a collaboration with Imperial College London, Yonsei University, Wageningen University and Research, and the University of Leeds.

Read the full paper, published in Science: Tiarnan A. S. Doherty et al. ‘Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases.’ Science (2021). DOI: 10.1126/science.abl4890

Read the full paper

Tiarnan A. S. Doherty et al. ‘Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases.’ Science (2021). DOI: 10.1126/science.abl4890

Image

Artist's impression of formamidinium (FA)-based crystal

Credit: Tiarnan Doherty

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