2D halide perovskite semiconductors have shown a variety of applications in recent years, mainly in photovoltaics, attracting a plethora of scientists to this developing multidisciplinary field. The primary goal for this Thesis, is to further exploring the field, by synthesizing new 2D hybrid halide perovskites and trying to optimize certain aspects of the crystal and electronic band structure. Towards this end, the new organic diammonium spacer (2-halo, 1,3-diammino propane, DicX, X = Br, I) was introduced, in an attempt to examine the effect of the organic halide in the photophysical properties of the layered perovskites. The new spacer was employed towards the synthesis of the 2D homologous series (DicI)(CH₃NH₃)_n-1PbnI_3n+1(n = 1-3) as well as the synthesis of members of the corresponding 2D lead-free compounds based on ordered double perovskites combining Ag⁺ and Bi³⁺ double perovskites.

As it was shown from optical measurements in the visible range, new compounds exhibit stable excitons at room temperature emitting in the wavelength range λ = 500-650 nm consistent with their Multiple Quantum Well electronic structure. However, unlike other derivatives of the 2D perovskites, the binding energy of the materials appears to be significantly reduced, as a result of the organic spacer. The extra organ iodide in the spacer reduces the dielectric contrast between the organic and the inorganic components whereas the interlayer distance between the perovskite sheets is significantly reduced from the short carbon chain. As band structure calculations also support, the new materials effectively behave as bulk perovskites, resulting in a charge-transport in all three dimensions, a property that may have great impact in the application of these materials in solar cells. In an attempt to take advantage of this property, thin films of these layered 2D halide perovskites were fabricated and characterized. Future work on these materials will deal with the manufacture and characterization of full solar cells devices, where it is anticipated that we will be able to obtain a significant increase in the photo-generated current which may lead to an overall improved power-conversion efficiency.

In the broader context of efforts towards a sustainable world, great emphasis is currently being placed on emerging technologies that are not only cost-effective, but also adaptable to a wide range of applications. The investigation of dielectric properties is a topic of continuously increasing interest, as it is inextricably linked to a multitude of technological applications in the field of photonics, electrical insulating materials as well as active dielectric materials. The precise determination, as well as the potential correlation of these properties is a prerequisite for the successful design and development of innovative technological materials with predefined, desirable properties. The scope of this Master thesis was the fabrication and characterization of composite dielectric materials, which in this case was achieved through the incorporation of Barium Titanate (ΒaΤiΟ₃), a predominantly environmentally friendly dielectric material, within a commercially available polymeric material such as Silicone. Central to this concept, was the construction of flexible samples with enhanced mechanical and dielectric properties, whereas attention was also drawn to the behavior of the dielectric properties of these materials when subjected to both constant as well as cyclic mechanical loading.