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Mathilde Bouchard

Halide perobskites AMX3: Synthesis, cationic substitution, and structural studies

Published on 6 February 2018
Thesis presented February 06, 2018

Metal halide perovskites AMX3 (A+ is an organic or inorganic cation: Cs+ methylammonium [CH3NH3]+ (MA); M2+ is a metallic cation such as Pb2+ and X a halide anion I-, Br- or Cl-) have remarkable properties as solar cell absorbers. In the perovskite structural framework the properties of the materials can be easily tailored by modifying their chemical composition. Changing for example the halide anion modulates their band gap. This thesis deals with the synthesis and the advanced characterization of mixed halide perovskite materials – i.e. with mixed ions on the same site A, M or X – of low dimensions such as thin films and nanocrystals.
A fabrication protocol was developed for reference solar cells with CH3NH3PbI3 and CH3NH3PbI3-xClx absorbers yielding a power conversion efficiency over 10%. The perovskite thin films could be fabricated with a controlled thickness and a high reproducibility on different TiO2 substrates (compact, mesoporous or monocrystalline). The study of these thin films by laboratory and synchrotron X-ray diffraction showed that the CH3NH3PbI3-xClx crystallites exhibit a preferential (001) orientation on any kind of TiO2 substrate. By using monocrystalline TiO2 substrates we showed for the first time that the degree of orientation and the grain size increased considerably (the surface coverage was determined to be 80%) compared to mesoporous and compact polycrystalline TiO2 substrates. The presence of chlorine at the TiO2–perovskite interface and the low surface roughness of the substrate are key factors, which promote the growth of highly oriented crystallites. In the second part of the thesis, the influence of the partial substitution of lead with non-toxic homovalent metal cations (alkaline earth, 3d transition metals) on the structural and optical properties of hybrid and inorganic perovskite nanocrystals was studied. The morphology and the size of MAPb1-xMxBr3 hybrid nanocrystals synthesized by reprecipitation is clearly affected despite the low substitution (x: maximum 6% with Mg2+). Conversely, in CsPb1-xMxX3 inorganic nanocrystals synthesized by hot injection, up to 16% of Pb2+ could be replaced by Mg2+ or Sr2+, while keeping their size, shape, structure, absorption and photoluminescence properties. With a higher substitution ratio (up to 22% was achieved), the formation of the Cs4PbX6 structure is favored.

Solar cells, Perovskites, Nanostructured semiconductors, Optical properties

On-line thesis.