Synthesis and downconversion applications for metal-metal halides

Abstract

Perovskite nanocrystals have attracted great interest in recent years for their simple synthesis, unique optical properties, such as tunable emission wavelength and narrow emission bandwidth (FWHM lower than 20 nm), and high quantum yield, as a result they have become an important category and a promising candidate for display technology. However, the inherent toxicity of lead and its instability to moisture and heat have hindered their further commercialization. To overcome these drawbacks, the replacement of Pb was broadly studied. Among various metals, environmentally friendly copper shows promising potential because of the abundance, low price, and intrinsic functional features of the copper-based compounds, and their structural, compositional, and stoichiometric flexibility. The research is divided into three parts. In Chapter 2, a synthetic method for synthesizing hexylphosphonic acid capped CsPbX₃ (X = Br, I/Br) nanocrystals were demonstrated, which can be mixed with a liquid crystal host without aggregation. By optimizing the solvents of the inkjet printing ink and hydrophobic treatment for the substrate, the as-synthesized perovskite can be printed precisely onto the bank substrate without the coffee ring effect. Moreover, the inkjet printing substrate was successfully applied to the blue mini-LED as a downconversion layer. In Chapter 3, a synthetic method for making large, well-ordered Cs₃Cu₂X₅ (X = Cl, Br, Br/Cl) single crystals with millimeter dimensions in less than 30 min is presented. Photoluminescence quantum yields (PLQYs) for single crystals of X=Cl and Br are 100 % and 27 %, respectively, with long lifetimes and strong evidence of self-trapped excitons (STE) emission. This synthesis can be used to produce white light UV-downconversion LEDs with CIE (0.3327, 0.3342) and CRI = 90 % at a color temperature of ~5500 K. Moreover, using this synthesis method, we can achieve in-situ inkjet print precursors onto a polymer matrix to achive bright and stable patterned materials. In Chapter 4, the successful synthesis of K₂CuX₃ (X = Cl, Br) and Rb₂CuX₃ (X = Cl, Br) nano/microcrystals using an anti-solvent recrystallization method is reported. These materials have violet emissions with PL peaks located from 380 nm to 400 nm and narrow FWHM of ~50 nm with high PLQYs over 48 %. These crystals show excellent photostability, remaining over 90 % PLQY after continuous UV irradiation at their PLE maximum over 3 h. Additionally, we also found that PLQYs and purity of K₂CuX₃ (X = Cl, Br) and Rb₂CuX₃ (X = Cl, Br) crystals can be improved when using a hot injection method and ~100 % PLQY can be achieved for K₂CuCl₃. Moreover, size and shape control of K₂CuX₃ (X = Cl, Br) and Rb₂CuX₃ (X = Cl, Br) were also achieved. The good photostability and high PLQYs of these A₂CuX₃ (A = K, Rb; X = Cl, Br) crystals indicate their potential in optoelectronics applications. Overall, this research includes the synthesis of lead-based CsPbX₃ (X = Cl, Br) nanocrystals, copper-based Cs₃Cu₂X₅ (X = Cl, Br) single crystals and A₂CuX₃ (A = K, Rb; X = Cl, Br) micro- and nanocrystals. This research also investigated the application of these bright emissive perovskite compounds as downconversion materials in mini-LED displays and white light LEDs.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jonathan Eugene HALPERT (Supervisor)