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Aicha Asma Medjahed

Structure-properties correlation in hybrid perovskites for photovoltaics

Published on 21 May 2021

Thesis presented May 21, 2021

Abstract:
The intense human activity during the last three centuries is now showing its consequences, and climate change is now an undisputable truth. Developing efficient technologies to allow the broad use of low-carbon energy sources is vital to limit our impact on the environment and working on restoring the balance. Photovoltaics constitute one promising path to achieve this goal, as the sun constitutes a very powerful and sustainable source of energy. Silicon-based modules are predominant in the market, but alternative and complementary technologies are developed in research laboratories. During the last decade, a new technology appeared, generating tremendous interest in the scientific community: the hybrid halide perovskite solar cell.
Perovskite designates a structural family, which constitutes one of the most important classes of materials in solid-state sciences, due to their very versatile properties, which can be tuned through chemical engineering. The introduction of an organic cation inside an inorganic lead-halide cage gave rise to semi-conducting perovskite materials with impressive optoelectronic properties. The first material of this family to be used in photovoltaic devices was MAPbI3 (MA = CH3NH3). Research solar cells using such hybrid perovskite materials as light absorber have seen a steep rise of their efficiency. In only ten years, they reached more than 25% conversion effiency, challengin silicon-based devices. However, the in-depth understanding of the fundamental properties of these compounds and their origin is still lacking far behind the more empirical technological advances.
It is with the aim of enhancing our understanding of the structural features of these compounds that this PhD project was constructed. Mainly using X-Ray diffraction techniques, we investigated different systems. The first part of this work is dedicated to the structural investigation of MAPbI3 thin layers. By means of in-situ experiments, we identified the different steps of the crystallization mechanism through ion exchange which occur in the presence of chlorine in the precursors, under specific conditions. These results allowed to rationalize observed performances in solar cells based on similar MAPbI3 thin films. We then focused on the MAPbI3 thin films microstructure and more particularly on the peculiarity and variability of the observed texture. This behavior is shown to be related to the ferroelastic nature of the MAPbI3 cubic-tetragonal structural phase transition.
The second part of this work is dedicated to the study of the impact of ion mixing on the perovskite structure. High efficiencies and long-term stability of solar cells have been achieved in recent years by using hybrid halide perovskite materials constituted of different organic cations and different halide anions. However, the effect of each substitution on the structure is not well understood, as most of the work is performed on compounds in which two different ions are substituted simultaneously. We therefore decided to study the structure of compounds where only one ion is substituted at a time, generating three families of solid solutions: FA1−xMAxPbI3 (FA = CH(NH2)2), MAPb(I1−xBrx)3 and FAPb(I1−xBrx)3. The room temperature properties of these compounds as well as their temperature-dependent structural behavior were studied. Finally, we used the structural behavior of the mixed organic cation mixed halide compound (FAPbI3)0.85(MAPbBr3)0.15 with a composition optimized for photovoltaic applications is discussed at the light of the results obtained with the quaternary compounds.

Keywords:
Hybrid perovskites, crystalline structure, X-ray diffraction, solar cells

On-line thesis.