C O M P L E X S Y S T E M S A N D B I O M E D I C A L S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

6 2 H I G H L I G H T S 2 0 2 2 I

PRINCIPAL PUBLICATION AND AUTHORS

Dynamical limits for the molecular switching in a photoexcited material revealed by X-ray diffraction, A. Volte (a,b), C. Mariette (a,b), R. Bertoni (a), M. Cammarata (a,b), X. Dong (a), E. Trzop (a), H. Cailleau (a), E. Collet (a), M. Levantino (b), M. Wulff (b), J. Kubicki (c), F.-L. Yang (d), M.-L. Boillot (d), B. Corraze (e), L. Stoleriu (f), C. Enachescu (f), M. Lorenc (a), Commun. Phys. 5, 168 (2022); https:/doi.org/10.1038/s42005-022-00940-0 (a) Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, Rennes (France) (b) ESRF (c) Faculty of Physics, Adam Mickiewicz University in Poznań, Poznań (Poland) (d) Institut de Chimie Moléculaire et des Matériaux d Orsay, Université Paris-Saclay, CNRS, UMR 8182 (France) (e) Institut des Matériaux Jean Rouxel (IMN), Université de Nantes (France) (f) Universitatea Alexandru Ioan Cuza, lasi (Romania)

REFERENCES

[1] R. Bertoni et al., Nat. Mater. 15, 606-610 (2016). [2] G.S. Pawley, J. Appl. Cryst. 14, 357-361 (1981). [3] A.A. Coelho, J. Appl. Cryst. 51, 210 (2018). [4] C. Mariette et al., Nat. Commun. 12, 1239 (2021).

Preventing PbI2 formation in metal halide perovskite films by antisolvent spraying

Metal halide perovskites are promising materials for photovoltaic applications and are commonly formed via pipetting of antisolvent during perovskite film formation. Structural characterisations using electron microscopy and wide-angle X-ray scattering reveal that spraying the antisolvent prevents formation of undesired PbI2 at the surface and bulk of perovskite layers, leading to enhanced photovoltaic performance.

Metal halide perovskites are an emerging class of semiconducting materials with enormous potential for applications in optoelectronic devices such as solar cells and light-emitting diodes [1]. While they can be deposited by a broad range of methods, their processing from solution is particularly common. Most often in

this process, an antisolvent is applied by pipetting (Figure 52a) during the deposition of the precursor solution in order to trigger the crystallisation process of the perovskite layer [1].

Unfortunately, due to the solubility of some of the perovskite precursors namely the organic halides in the antisolvent, they are removed from the film, thus irreversibly altering the stoichiometry of the perovskite layer. The outcome of this compositional change during the film formation process is evidenced by the formation of PbI2 crystallites at the surface and bulk of perovskite films. At the surface, these can be directly visualised by scanning electron microscopy (SEM), where hexagonally shaped crystalline flakes can be observed (Figure 52b).

To investigate the crystalline structure of the perovskite films, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed at beamline

transition to the stable state associated with the change in volume involves only electronic rearrangements and very small atomic shifts within the unit cell. They are both ultrafast and can therefore adiabatically follow the evolution of the local volume during the propagation of strain waves. In the case of spin crossover materials, it has been established that local switching between low- and high-spin conformations is thermally activated and involves energy barriers at the scale of the molecule. This can limit the dynamics of photo-induced phase transformation, in that the state of each molecule remains thermodynamically frozen during propagation of the volumic strain set up in the material upon laser excitation. This is how it was possible to experimentally demonstrate the occurrence of a delay between the increase in crystal volume and the macroscopic molecular state switching, as shown in Figure 51.

This work revisits some fundamental aspects of the physics of light-driven phase transitions and provides new insights into the causality and correlation between intertwined degrees of freedom driven under a non- thermal regime. The results are of general interest for material science and chemical engineering applications. In particular, they may provide hints to motivate an optimised material design, scalable with size-dependent dynamics and intrinsic energetics. The findings also fuel new ideas to construct relevant theoretical models, by identifying the key degrees of freedom and understanding how their couplings drive cooperative nonequilibrium transformations in multi-functional materials.