Abstract
Lead halide perovskite nanocrystals (NCs) are receiving a lot of attention nowadays, due to their exceptionally high photoluminescence quantum yields reaching almost 100%, and easy tunability of their optical band gap over the entire visible spectral range by modifying composition or dimensionality/size.This thesis starts from a literature review on recent developments in the direct synthesis and ion exchange-based reactions of hybrid organic–inorganic (CH3NH3PbX3) and all-inorganic (CsPbX3) lead halide (X=Cl,Br, I) perovskite NCs. We consider their optical properties related to quantum confinement effects, single emission spectroscopy and lasing, and summarize recent developments on perovskite NCs employed as an active material in several applications such as light-emitting devices, solar cells and photodetectors.
The demonstration of the size-tunability of the bandgap of narrow size dispersion CH3NH3PbBr3 perovskite NCs by using temperature exert control by the ligand-assisted reprecipitation process is shown in Chapter 2. The fine control this allows, also beneficially improves their emission QY as high as 93% with the corresponding emission peaks covering the range from 475 to 520 nm. We further extended the procedure towards CH3NH3PbBr3 NCs in Chapter 3 through the variation of precursor’s and ligand’s concentration, demonstrating their bandgap tunability at room temperature. We proposed a qualitative model for the perovskite NC growth providing the in-depth detail on their formation process as well as a guidance for future synthetic improvements.
A one-step top-down synthesis of CH3NH3PbBr3 perovskite NCs is presented in Chapter 4 as well. Using mixture of ligands as coordinating solvents, multiple steps of reaction could be avoided, and the materials with improved stability could be obtained.
Post-preparative treatment of lead halide perovskite materials is an important approach to improve their stability. We present an approach towards stable solid-state perovskite-based luminophores in Chapter 5 with different emission colors via surface protection of CsPbX3 (X = Br or I) with a polyhedral oligomeric silsesquioxane (POSS). This treatment results in water resistant perovskite nanocrystal powders and prevents otherwise easy anion exchange between perovskite nanocrystals of different compositions mixed together in the solid state, which allows us to preserve their distinct emission spectra. Based on perovskite materials we obtain above, we subsequently used mixtures of green-emitting POSS–CsPbBr3 (or CH3NH3PbBr3 NC) and red-emitting POSS–CsPb(Br/I)3 NC powders (or red-emitting K2SiF6:Mn4+ phosphor) to fabricate single layer perovskite-based down conversion white LEDs.
Green light-emitting devices (LEDs) were fabricated and their performance has been optimized in Chapter 6. By introducing a thin film of perfluorinated ionomer sandwiched between the hole transporting layer and perovskite NC emissive layer, the device hole injection efficiency has been significantly enhanced. The beneficial role of the insulating material POSS as a solution additive or an additional hole-blocking layer to enhance the performance of electroluminescent green LEDs based on CsPbBr3 perovskite NCs was also demonstrated. POSS improved the surface coverage and the morphological features of the films deposited either from supernatant or suspension of perovskite NCs. In some devices, POSS acted as a hole-blocking layer between the perovskite NCs and 1,3,5- Tris(N- phenylbenzimidazol-2-yl) benzene, keeping both electrons and holes located within the active layer for an efficient recombination.
The thesis will end with a summary and a perspective for perovskite nanocrystal field.
| Date of Award | 18 Aug 2017 |
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| Original language | English |
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| Supervisor | Andrey ROGACH (Supervisor) |