Thesis presented February 06, 2018

Abstract:
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 CH₃NH₃PbI_3-xCl_x 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 CH₃NH₃PbI_3-xCl_x 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 MAPb_1-xM_xBr₃ hybrid nanocrystals synthesized by reprecipitation is clearly affected despite the low substitution (x: maximum 6% with Mg²⁺). Conversely, in CsPb_1-xM_xX₃ 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.

Keywords:
Solar cells, Perovskites, Nanostructured semiconductors, Optical properties

On-line thesis.