23-Feb-2022: Talk by Dr. M. U. Rothmann

Mathias U. Rothmann

On the 23rd of February, Dr. Mathias Uller Rothmann, a postdoc in the Semiconductor Physics group at the University of Oxford (UK) will present his talk on:
Advanced electron microscopy studies of perovskite solar cell materials.“

Electron microscopes (EM) are commonly used to study the microscopic features of a wide range of objects at a wide range of length scales, ranging from close-ups of insect anatomy, to imaging individual viruses and taking pictures of individual atomic columns. Due to their versatility and extremely high resolution, they have been instrumental in furthering our understanding of a host of materials and devices. For example, understanding the atomic-scale crystallographic properties of photovoltaic semiconductor materials such as silicon, GaAs, and CdTe, has been essential in their development from interesting materials to large-scale energy conversion industries.
However, studying photoactive hybrid perovskites with electron microscopes in general, and transmission electron microscopes (TEM) in particular, has proved to be challenging due to the large electron energies typically employed in these studies.[1] These energetic electrons enable the high resolution of electron microscopes, but they also interact strongly with the sample, possibly changing it during the interaction. These changes can be virtually instantaneous, and thus difficult to detect, under some circumstances, and the organic compounds found in hybrid perovskites have been found to be particularly vulnerable to this effect. The very close structural relationship between a number of crystallographic orientations of the pristine perovskite and lead iodide has resulted in strong ambiguity in the interpretation of EM-derived information, severely impeding the advance of atomic resolution understanding of the materials. As such, it is important to have a good understanding of this powerful technique.
In this talk, I will first go through the fundamentals of electron microscopy, explaining how images are formed and information is recorded. I will then put this into the context of studying hybrid perovskites with electron microscopes, discussing the limitations of the techniques and how to avoid accidentally studying a damaged phase. Finally, I will go through examples of some of the exciting EM work that has been done on hybrid perovskites by myself and others, including the first atomic resolution images of a hybrid perovskite thin film, [2] and give a bit of a perspective on the future of electron microscopes and hybrid perovskites.
[1] Adv. Mater. 2018, 30, 1800629
[2] Science 370, eabb5940 (2020)

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