Synthesis and characterisation of perovskite and chalcogenide semiconductor materials for opto-electrical applications

Colloidal synthesis strategies hold significant promise for a wide range of applications, specifically in optoelectronics as absorbers and phosphors. Colloidal nanocrystals (NCs) are now a fundamental building block in nanoscience due to the surfactant-assisted precision synthesis that provides an acutely narrow size distribution, highly regular morphologies, controllable surface chemistry and enhanced optical properties. Careful design is required to produce nanocrystals that are suitable for a wide range of applications. Building on the knowledge gained from research of metal chalcogenide quantum dots, metal-halide perovskite materials have become a dominant research area in recent years due to several factors, including exceptionally high optical absorption, long carrier lifetimes and diffusion lengths, and high defect tolerance. An example of a popular perovskite nanomaterial is CsPbBr3, which can be manipulated by varying synthesis parameters such as precursors and surfactants, to achieve different morphologies and properties. Herein, CsPbBr3 and a range of other materials are synthesised by solution-based protocols, both colloidally and otherwise. These materials are then systematically characterised structurally and optically to understand the effects of the synthesis parameters. First, the manipulation of the surfactant system of colloidal CsPbBr3 perovskite nanocrystals is explored in Chapter 4. The typical oleic acid and oleylamine ligand combination employed in the synthesis leads to optically bright NCs with a narrow size distribution. However, they are easily displaced, leading to insufficient stability for real-world application. This study introduces phosphonic acids of various lengths into the system leading to small, near monodisperse NCs and a precise control parameter for particle size and hence material bandgap. Further, the substitution of the primary ligand oleylamine with trioctylphosphine oxide leads to increased reaction yield and demonstrates the potential of phosphorous-based ligands in the perovskite NC synthesis. Next, Chapter 5 focuses on the substitution of the B cation away from the toxicity of Pb towards Sn and Ge. Moving up the group 14 column leads to more covalent bonds and a more stable perovskite structure which counteracts the fear that the Ge(II) will oxidise to Ge(IV). However, Ge(II) does readily oxidise during processing making the material difficult to form using organic and colloidal synthesis strategies. Therefore, this system utilises an aqueous solvent and a reducing agent, which creates a stable perovskite. An array of Ge perovskites are formed successfully by incorporating Sn into the lattice up to 25%. Chapter 6 continues the theme of alloying elements using organic colloidal synthesis. Indium chalcogenide materials are synthesised using a hot injection synthesis rather than the typical mechanical exfoliation route. The system requires high temperatures to dissolve elemental chalcogenide precursors in oleylamine however, it is a relatively unexplored phosphine free system that produces variations of In2(S1-xSex)3, In2(S1-xTex)3 and In2(Se1-xTex)3. Finally, Chapter 7 explores attempts to form Ge based perovskites via a colloidal synthesis strategy, resulting instead in the preferential formation of alkali halides. The investigation then provides a detailed overview of the surfactant effect on CsGeBr3 microcrystals. Additive types based on amines, phosphorous, polymers, sulphur and silicon are all known to manipulate perovskite structures in various ways; thus, they are explored systematically and their effects are discussed. Amines and triblock copolymers show the most robust control over the perovskite morphology.