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Controlling the microstructure of the thin films of halogenated hybrid perovskites used in solar cells is essential to optimize their performance. Irig researchers are interested in the deformation state of perovskite films from diffraction (Diffraction is the behavior of waves when they encounter an obstacle that is not theirs …) X-rays and study theorientation (In the literal sense, orientation designates or materializes the direction of the East (dawn …) from Net (A computer network is a set of equipment connected to each other to exchange information …) crystal clear in these films of microscopy (Microscopy is the observation of a sample (placed in a microscopic preparation …) X-rays of diffraction. The stability of the different domain orientations is studied by DFT and illustrates the determining role of theenvironment (The environment is everything that surrounds us. They are all natural elements and …) chemistinterface (An interface is an area, real or virtual, that separates two elements. The interface …) substrate-film.

In the context (The context of an event includes the circumstances and conditions surrounding it; the …) the development of photovoltaic technologies, based on solar cells halogenated hybrid perovskites (PHH) do theobject (In general, the word object (from the Latin objectum, 1361) designates an entity defined in …) intense studies for ten years. These cells now achieve energy conversion efficiency values ​​close to those of solar cells in silicon (Silicon is a chemical element of the family of crystallogens, symbol Si …), of the order of 25.5%. However, many challenges remain to be addressed to match the performance of silicon, particularly with regards to performance stability in the time (Time is a concept developed by humans to learn the …)stability that requires, among other things, the mastery (The master’s degree is a university degree or diploma corresponding to the degree or title of …) structural properties of thin layers by PHH. However, questions remain, for example, about the stress state of the crystal lattice that constitutes these thin PHH layers and about its orientation.

It is well known that the state of deformation of the perovskite layer affects its stability, just as the orientation of the crystal lattice potentially affects the optoelectronic properties of the perovskite. In order to check the microstructure thin layers of PHH, Irig researchers used laboratory X-ray diffraction (XRD) and X-ray diffraction microscopy on synchrotron (The term synchrotron refers to a large electromagnetic instrument intended …) identify and understand the mechanisms governing the deformation state and crystalline orientation of MAPbI thin films3 one of materials (A material is a material of natural or artificial origin that man shapes to …) Reference PHH.

Initially, Irig’s researchers were interested in the deformation state of perovskite films. For this, they took XRD measurements on site which allowed them to challenge the commonly accepted assumption that PHH layers synthesized at 100 ° C exhibit deformation at temperature (Temperature is a physical quantity measured using a thermometer and …) environment due to the difference between the values ​​of the coefficient of expansion (Expansion coefficients measure the relative increase in volume of a system when …) thermal (Thermal is the science that deals with the production of energy, the use of …) (CDT) of the PHH and the substrate on which the thin layers are deposited. Their measurements have in fact highlighted the possibility of obtaining thin layers of MAPbI3 completely (Completion or fully automatic, or by Anglicism completion or …) relaxed (without deformation) thus demonstrating the absence of direct correlation between the difference in CTE and the deformation of thin films.

In a second step, the researchers investigated the orientation of the crystal lattice in these thin layers using X-ray diffraction microscopy at the European Synchrotron. Radiation (Radiation is a transfer of energy in the form of waves or particles, which can be …) Facility (ESRF in Grenoble), a technique that allows you to visualize all the crystalline domains that have the same orientation. Their results made it possible to attribute the origin of the double texture sometimes observed in XRD measurements to the presence of Twins ferroelastic (Image) self forming (In pitch, fundamental frequency changes are perceived as variations of …) during the cubic-tetragonal structural transition undergone by MAPbI3 around 57 ° C.


A twin is an association of several identical crystals whose orientations are not random but connected by operations of symmetry (Generally the term symmetry refers to existence, in a …) characteristics of the crystal structure of Material (A material is a material of natural or artificial origin that man shapes …) in question.

Finally, these experimental studies were supplemented by calculations of theory (The word theory comes from the Greek theorein, which means “to contemplate, observe, …) of the functional (In mathematics, the term functional refers to certain functions ….) of the density (The density or relative density of a body is the ratio of its density to the …) (DFT) which showed that the chemical environment at the interface with the substrate (“first layer” of the MAPbI network3 Type MAI or PbI2) influences the orientation of the perovskite.

These various results constitute an important advance in the understanding of the microstructure of the layers of halogenated hybrid perovskites, paving the way for the control of their stress state and their texture, with the aim of improving their optoelectronic properties, as well as the performance and stability of the device.

The halogenated hybrid perovskites Having a together (In set theory, a set intuitively designates a collection …) favorable properties for photovoltaics, rarely present simultaneously in other types of materials – a large coefficient (In mathematics a coefficient is a multiplying factor that depends on a certain …) fromabsorption (In optics, absorption refers to the process by which the energy of a photon is absorbed …)weak excitonic effects, power (In common sense, energy means anything that allows you to do work, produce energy, etc.) of forbidden band allowing a good collection of the photons (In particle physics, the photon is the elementary particle mediating the interaction …)good properties of transport (Transport is the act of carrying something, or someone, from one place to another, the most …), high fault tolerance. Furthermore, these physical properties can be modulated by engineering (Engineering refers to all functions ranging from design and studies to …) chemistry, which allows you to optimize the perovskites to adapt them to various photovoltaic challenges such as stability or the production of multi-junction cells.

The plot of a thin layer characterizes the orientation of the crystal lattice in the direction perpendicular (In plane geometry, two lines are said to be perpendicular when they intersect in …) in the layer plane.
The microstructure of a crystalline thin film is defined by the state of stress, the texture and the length (The length of an object is the distance between its two furthest ends …) coherence of the crystal lattice.

References:
Medjahed AA, Zhou T, Alvarez Quiceno JC, Dally P, Pochet P, Schülli TU, Djurado D, Reiss P and Pouget S
Microstructure of methylammonium lead iodide perovskite thin films: a comprehensive study of deformation and texture.
Advanced energy materials2022.

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