Understanding the impact of strain (structural deformation) is crucial to the success of halide perovskite materials used in optoelectronic devices such as solar cells, X-ray detectors, and LEDs. While halide perovskites demonstrate the potential for enhanced efficiency in these devices, research is ongoing to investigate strain and defects that still hinder device performance and stability.
In experiments to understand the structural changes that can occur during device operation, scientists used Synchrotron-based Bragg Coherent Diffraction Imaging (BCDI) - an X-ray-based imaging technique - to map nanoscale strain in halide perovskite microcrystals (MAPbBr3 [MA = CH3NH3]), including strain around defects. Published in Advanced Materials, the experiments were part of a recent study from Diamond’s I13-1 beamline (Figure 1), which reveals the dynamic migration of nanoscale extended defects in halide perovskites under continuous light illumination. These insights demonstrate the highly dynamic nature of the structure of halide perovskite materials and how they evolve under operational conditions, highlighting the close links between nanoscale structure, dislocations, and device performance and stability.
![Figure 1. Inside the I13-1 beamline at the Diamond Light Source, the UK’s national synchrotron. The X-ray beam comes through the pinhole from the back right of the images towards the sample in the centre. After hitting the sample and diffracting the X-rays travel down the flight tube to the detector. Both the flight tube and detector can be moved with robot arms. Also shown is a laser for illuminating the sample during measurement and a side-mounted microscope for positioning. Adapted from Orr, K. W. P. (2024). Strain in Halide Perovskites: Characterisation, Crystallography, Consequences [Apollo - University of Cambridge Repository]. DOI: org/10.17863/CAM.108243](/dam/jcr:194da470-899f-4e9a-b514-42c2a6f210b8/Figure%201%20Inside%20the%20I13-1%20beamline.jpg)
With primary funding from the Engineering and Physical Sciences Research Council (EPSRC) and the European Research Council (ERC), the experiments took place from October 2019 – September 2023 at the University of Cambridge (UK) and Diamond Light Source (UK), and included scientists from the University of Cambridge, University College London (UK), King Abdullah University of Science and Technology Catalysis Center (KAUST) (Saudi Arabia), Brookhaven National Laboratory (USA), and Diamond Light Source.
In the race to address energy and climate challenges, the goal is to capture and emit energy efficiently, while reducing reliance on fossil fuels. In recent years, halide perovskites have been used to make high performance solar cells which offer many advantages over conventional silicon-based cells. While the latter require high crystal purity, high temperature manufacturing, and greater thickness to absorb sufficient light, thin film halide perovskite solar cells are lighter, flexible, absorb light more strongly, and are easier to manufacture, opening doors for cheaper and less energy intensive applications.
However, strain and defects in halide perovskite materials can influence their mechanical stability and energy-conversion properties, which in turn affect overall solar cell performance. Furthermore, dislocations (a specific kind of material defect uncovered in this study) formed during manufacture and operation can lead to delamination and cracking, and even mechanical failure of the solar cells, reducing their longevity.
Darren Batey, Principal Beamline Scientist on Diamond’s Beamline I13-1, said:
The world is in need of efficient solar cells and batteries to capture and store sustainable energy but these devices rely on electronic structures that degrade over time. If we can track how strain is affecting performance and stability, we can begin to explore how to engineer these materials and optimise their performance, robustness, and reliability.
Kieran Orr, who conducted the research as a PhD student at the University of Cambridge and is now a Research Fellow at Stanford University in the USA, explained:
Another exciting use of halide perovskites is in X-ray detection. X-rays are a part of the electromagnetic spectrum just like visible light, but with a different wavelength. Therefore, Halide perovskites can also absorb X-rays to produce electricity and be used in X-ray detector technologies. There are promising results which suggest that X-ray detectors based on halide perovskites could be more sensitive than current versions, meaning they can detect lower intensities. This is particularly useful for medical imaging applications (like mammography and radiography) where it is important to limit the body’s exposure to X-rays.
To gain a full understanding of strain and its impact on performance in halide perovskites, strain has to be characterised on a local scale and in three dimensions. Furthermore, experiments must be conducted under operational conditions to understand modes of device degradation and failure.
The arrangement of atoms in a halide perovskite crystal structure is like a neat 3D grid but due to an interplay of internal and external influences, strain can disrupt and distort the crystal structure as atoms are shifted from their expected positions. Every defect has a characteristic strain field around it (structural deformation due to applied stress), and with X-ray nanoimaging techniques, scientists can discover how far and in which direction atoms have moved, making it possible to identify defects such as dislocations from their characteristic local strain fields.
BCDI is an X-ray imaging technique that is optimal for mapping strain in 3D at nanometer resolution and in thicker samples similar to those in optoelectronic devices. In their research, the scientists developed a BCDI approach to visualise rich strain fields within high quality single microcrystals of halide perovskites and monitor their evolution under continuous illumination.
The experiments on Diamond’s high energy beamline I13-1 involved shooting an X-ray beam onto a halide perovskite sample and measuring the X-rays that are diffracted by the sample to learn about its internal structure (Figure 2). Because solar cells have to have light shone on them to produce electricity, these measurements were performed both in the dark and under illumination to understand what effect light has on halide perovskite structure.
Kieran explained: "In the experiment, multiple X-ray measurements were performed with the sample both in the dark and in the light over the course of roughly an hour and then computational algorithms are applied to the X-ray diffraction data to reconstruct an image of our sample. In conventional imaging (like in a normal microscope), you rely on a careful arrangement of lenses to form an image of your sample. However, BCDI effectively replaces the lenses with some very clever algorithms which we apply to the X-ray diffraction data. Because we know the physics of how X-ray interact with our sample, we can back-calculate an image of our sample from the pattern the diffracted X-rays make on the detector.
“Coherence is essential for these experiments. Exploiting the wave nature of light allows us to reveal the atomic structure of these materials at the nanoscale,” Darren says. “The X-ray beam produced at I13-1 is very bright and very stable, producing lots of coherent light and making these experiments possible.”
By tracking the strain fields in MAPbBr3, edge dislocations (a specific kind of extended defect in crystals, illustrated in Figure 3) are identified, and their extensive migrations observed through the crystal structure under visible light illumination (Figure 4). Furthermore, by considering crystals damaged by X-ray exposure, dislocation formation was found to be associated with degradation of the halide perovskite material and changes in its optoelectronic properties. These are crucial results giving insight into the surprisingly fluxional nature of the structure of these materials.
Sam Stranks, Professor of Optoelectronics at the University of Cambridge, and senior corresponding author of the work, said:
“It is becoming increasingly evident that even for large area halide perovskite modules that we may see in future solar photovoltaic arrays, understanding and controlling the structure on the tiniest of length scales is vitally important. This work therefore gives us a crucial window into how these nanoscale defects impact performance and stability, in turn giving us important guidelines on how to tune the manufacturing to ultimately get rid of the undesired effects.”
In follow-on research also using beamline I13-1, the same scientists now focused on these halide perovskite materials incorporated into a whole solar cell, finding additional dislocations and defects.
“These included an ‘anti-phase boundary, which is where the atomic arrangement of one part of the crystal has been shifted compared to neighbouring portions of the crystal,” Kieran said. “Going forward, the aim is to study the effects of applying light and voltage – both separately and together - when we operate the solar cell and produce electricity.”
Strain mapping in halide perovskites is being continued by an ongoing collaboration, funded by the Leverhulme Trust, between Imperial College London, the University of Cambridge, and two beamlines at Diamond (I13-1 and I14). The aim here is to investigate not only how to mitigate the harmful impacts of strain in halide perovskites, but how to harness strain to make better solar panels, possibly even creating more exotic devices that explicitly use strain to their advantage.
“For hardcore crystallography like this, you absolutely require synchrotrons,” Kieran said. “One can’t do this kind of research without these huge, world-class national facilities. Having a facility like Diamond means we can continue to push scientific and technological boundaries to help solve immensely important, real-world challenges, and do so in the UK.”
The forthcoming upgrade to Diamond-II will increase the synchrotron’s brightness and brilliance, potentially improving the speed of experiments and reducing exposure times. This is good news for BCDI measurements, where only one fraction of the whole scatter is recorded, provided beam-induced changes to the samples can be kept minimal. Higher quality experiments on faster timescales mean reduced radiation loss and increased efficiency, and many more groundbreaking results.
In addition to optimal science capabilities, long-term access and a collaborative learning environment played a pivotal role in this work.
“Diamond was a natural choice for our experiments,” Kieran said. “We already knew Darren and his colleagues, and having two years of guaranteed access to the beamline gave us time to develop and perfect our techniques. I visited Diamond about five times for experiments which, as a PhD student, really helped me grow in expertise.”
“Collaboration with users is how we do things at Diamond and this research is an example of a strong collaboration that fits in well with our capabilities,” Darren explained. “The process is symbiotic: we assist research groups to learn and develop the techniques themselves, and we receive feedback which helps us create the tools to make it easier for users without specialist expertise or those exploring for the first time. Our aim is to get results and together with users build solid, long-term projects for important application areas.”
To find out more about beamline I13-1 please contact the Principal Beamline Scientist: Darren Batey, Tel: +44 (0) 1235 778128; Email: darren.batey@diamond.ac.uk
[7] K. W. P. Orr, J. Diao, M. N. Lintangpradipto, D. J. Batey, A. N. Iqbal, S. Kahmann, K. Frohna, M. Dubajic, S. J. Zelewski, A. E. Dearle, T. A. Selby, P. Li, T. A. S. Doherty, S. Hofmann, O. M. Bakr, I. K. Robinson, S. D. Stranks, Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping. Adv. Mater. 2023, 35, 2305549. https://doi.org/10.1002/adma.202305549.
[10] Kieran W. P. Orr, Jiecheng Diao, Krishanu Dey, Madsar Hameed, Miloš Dubajić, Hayley L. Gilbert, Thomas A. Selby, Szymon J. Zelewski, Yutong Han, Melissa R. Fitzsimmons, Bart Roose, Peng Li, Jiadong Fan, Huaidong Jiang, Joe Briscoe, Ian K. Robinson, and Samuel D. Stranks. ACS Energy Letters 2024 9 (6), 3001-3011. https://doi.org/10.1021/acsenergylett.4c00921.
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