Abstract
Lead halide perovskite (ABX3) nanocrystals (NCs) are nanoparticles approximately 10nm in size which absorb and emit light in the visible spectrum. The emission wavelength can be easily tuned across the whole visible spectrum with high quantum yields. The NCs absorb and reemit light with high efficiency, show narrow peak widths and emit a very uniform colour of light. Nanocrystals can be easily synthesised using high temperature and room temperature synthesis routes with few structural defects. The nanocrystals therefore have great promise in optoelectronic applications such as photovoltaic solar cells and light emitting diodes. Unfortunately, perovskite nanocrystals still have issues with material toxicity and stability limiting their use in these applications. Furthermore, technical challenges remain relating to synthesis scalability and reproducibility. In this thesis, a flow chemistry reactor was used to explore novel synthesis routes, investigate new materials and improve nanocrystal stability.Room temperature based synthesis of perovskite NCs using alcohols is relatively understudied and has previously shown poor morphological control. In this work, methylammonium lead iodide nanocrystals synthesis factors were screened and selected for further study using Design of Experiments (DOE), a multivariate methodology for studying many factors. Eight factors were selected for a broad screening DOE, followed by a refined screening DOE using five factors. The DOE highlighted the most important factors and factor interactions, and an empirical model was produced using the data. Nanocrystal photoluminescence (PL) emission was varied from 614 nm to 737 nm and the model was successfully validated showing control of the nanocrystal PL emission. Finally, morphology control was investigated using transmission emission microscopy (TEM) gaining further insight to the relationship between synthesis conditions and particle shape.
Lead halide perovskites crystal structure stoichiometry is very tuneable, with a broad range of cations which can be substituted into the A-site. Nanocrystal research often focusses on cesium, methylammonium and formamidinium cations, a comparatively narrow range of precursors. In this work a wide range of organic cations were substituted in the A-site and a novel cation, azetidinium, proved the most promising for further study. The ligand ratio and reaction temperature was screened in a microfluidic droplet flow reactor. The most promising reaction conditions were then used to synthesise AzPbI3 nanocrystals, the first bottom-up synthesis of this material. The quantum yield was measured to be approximately 1%, twice as much as AzPbI2Cl nanocrystalsmade using a top-down grinding synthesis. AzPbI3 nanocrystals were then synthesised in a batch hot injection to further investigate the nanocrystal structure and morphology. The XRD structure showed similar peaks to AzPbI3 thin films and AzPbI2Cl nanocrystals which have been previously reported in the literature. However it has not yet been established if (a) the 3D phase of AzPbI3 was synthesised and (b) whether changes in the photoluminescence emission between different ligand ratio formulations was due to changes in the nanocrystal size or changes in the crystal structure phase. Simulating the XRD pattern expected from different phases is required to fully understand the AzPbI3 nanocrystal structure.
Perovskite nanocrystals have poor stability with respect to environmental conditions such as UV light, high temperature and moisture. One solution to increase the stability is to encapsulate the nanocrystals in an inorganic silica shell. In this work, CsPbBr3 nanocrystals were synthesised using a novel droplet flow ligand assisted reprecipitation reaction, the first reported example of LARP in flow. Nanocrystals with quantum yields up to 54.4% were synthesised. In-situ ensilication was explored using ligand assisted reprecipitation and hot injection in droplet flow, the first reported use of droplet flow for an in-situ ensilication. Technical difficulties arose in the ensilication reaction and this area would benefit from further investigation.
| Date of Award | 25 May 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Petra Cameron (Supervisor), Frank Marken (Supervisor) & Alison Walker (Supervisor) |