Browsing by Author "Abbasli, Madad"

Tin‐based halide perovskite solar cells can be efficient and environmentally friendly substitutions to lead‐based halide perovskite solar cells, but they have a drawback due to oxidation from Sn 2+ to Sn 4+ . Using vacuum deposition, epitaxially aligned CsSnBr 3 ultrathin films are prepared on Au(111) and Au(100) and characterized with scanning tunneling microscopy (STM), low‐energy electron diffraction (LEED), and X‐ray photoelectron spectroscopy (XPS). By co‐evaporation of the precursor molecules CsBr and SnBr 2 , few monolayers of perovskite are obtained. On Au(111), CsSnBr 3 grows in three differently oriented domains due to the hexagonal symmetry of the substrate. On Au(100), which has square symmetry, identical to CsSnBr 3 , but with about half the lattice constant of the perovskite, a (2×2) superstructure is observed. The perovskite is terminated with the (001) facet showing a square surface structure in agreement with density functional theory (DFT) calculations. Chemical analysis is performed in ultra‐high vacuum (UHV) conditions and no indication of a tin oxidation state higher than Sn 2+ is found in the films. However, after exposure to air, rapid and severe changes in the films are observed, highlighting the importance of preparing tin perovskites in a controlled environment to maintain their stability and avoid oxidation‐related issues.

This thesis investigates the growth, structural characteristics, and stability of ultrathin films of the lead-free perovskite CsSnBr3 (cesium tin bromide) on Au(111) and Au(100) substrates under ultra-high vacuum (UHV) conditions. Motivated by the need for environmentally friendly alternatives to lead-based perovskites in optoelectronic applications, this work employs a combination of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) to explore the fundamental properties of CsSnBr3 films. The study examines the behavior of precursor molecules, CsBr and SnBr2, on both gold surfaces, revealing distinct growth patterns and the influence of substrate symmetry on adsorption and subsequent film formation. Co-deposition of these precursors leads to the formation of CsSnBr3 films, the structure of which is analyzed in detail. A tetramer phase on the (001) surface of CsSnBr3 is confirmed through atomically resolved STM imaging and supported by density functional theory (DFT) calculations. Furthermore, XPS analysis provides insights into the chemical composition of the films and elucidates the degradation mechanisms upon exposure to air, highlighting the critical role of Sn4+ formation in the degradation process. The findings demonstrate that UHV-grown CsSnBr3 films initially exhibit a favorable Sn2+ oxidation state but degrade rapidly to include Sn4+ states upon air exposure, underscoring the importance of encapsulation for maintaining stability. This research advances the understanding of lead-free perovskite film growth and stability while offering valuable insights for the development of stable, efficient, and environmentally friendly optoelectronic devices.