Structural and optical characterisation of all-inorganic perovskites
AbstractSince the 1990s, halide perovskites have received increasing attention in an optoelectronic context for their intrinsic properties, finding potential applications in many different sectors and devices from efficient light harvesters to photovoltaics, photodetectors, solar fuels, lighting and displays. On the other hand, their application in commercial devices is strongly hampered by their instability. The aim of the Ph.D. project was primarily structurally and optically characterize caesium lead halide perovskites, shedding light on the role of external factors which undermine their stability. The first part of the work was focused on completing the characterization of CsPbI3. Because of the rapid phase transition, it appeared urgent a fast experimental tool to rapidly identify different polymorphs. Among inorganic lead halide perovskites, CsPbI3 is the most attractive for the scientific community due to its narrow bang gap, the absorption in a large portion of the solar spectrum and the emission of a deep red light. However, it is the most vulnerable member of lead halide perovskites under ambient conditions, because of the presence of iodine. Most of the first studies were focused on finding the most efficient and stable device, with more attention to the stabilization and less to understanding the reasons of the instability, leading sometimes to misleading assignation of the different phase structures. We proposed Raman spectroscopy to fulfil this task, providing a definitive assignation of the Raman peaks with a careful analysis of the vibrational modes for the cubic and the orthorhombic phase of ABX3 perovskites, carried out by experimental measurements on CsPbI3 samples corroborated by DFT calculations. CsPbI3 perovskite was synthesized by a solid-state reaction, and its structural and optical properties were deeply investigated using a multi-technique approach. Furthermore, the optically induced degradation process and the phase evolution as a function of the synthesis time length were studied, providing new insights on the formation mechanism of secondary phases. Looking at the applications of caesium lead halide perovskites, it is essential to solve the problem of their instability. In this perspective, many studies suggest that nanocrystals are more stable than the bulk counterpart, thanks to the contribution of surface energy. But in practice, in the architecture of the devices, it becomes particularly important also the interaction among the different components in operative conditions. With a view at the potential applications in photovoltaics and photocatalysis, we realized a heterostructure of CsPbBr3 NCs and nanoporous gold. With the aim of studying the optical properties variations due to the interaction between the structures, the heterostructure was realized directly synthesising CsPbBr3 nanocrystals on gold surface, revealing a high-efficiency charge transfer. Even if more structurally stable, nanocrystals still suffer from the instability in ambient conditions. In this perspective, different kind of encapsulation have been proposed, to deal with environmental stresses. For this purpose, a preliminary work has been done to study the stability of inorganic perovskites in a mesoporous silica matrix. The mesoporous structure provides the template to synthesize nanosized perovskites, and collapsing at high temperature, encapsulates the perovskites providing good stability and preserving their excellent optical properties. CsPbCl3, CsPbBr3 and CsPbI3 NCs have been obtained with different optical and emitting properties that permitted us to obtain a first prototype of RGB matrix for lighting applications.
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Descrizione: Structural and optical characterisation of all-inorganic perovskites
Tipologia: Tesi di dottorato
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