Photophysical processes in ternary metal halides and sulfides for optoelectronic applications

Abstract

Over the past decade, perovskites have garnered widespread attention due to their excellent optoelectronic properties and simple synthesis routes. Considering the stability of the material and potential environmental pollution, a series of novel semiconductor materials have been developed as strong competitors. These ternary metal halides and sulfides exhibit excellent optical and electrical properties, such as tunable bandgaps, high color purity, high photoluminescence quantum yields, high stability, and low toxicity. All of the properties make these materials highly desirable for high-performance optoelectronic applications. In fact, all of these exceptional properties are supported by deep photophysical processes. By exploring and optimizing these processes, the potential of these materials can be further developed, and their device performance can be improved. With the development of transient spectroscopy, more details in these photophysical processes have become traceable. Based on the investigation of the photophysical processes, this thesis studies the unique properties of different ternary metal halides and sulfides and explores their potential for a range of optoelectronic applications. In the first project, we discuss the feasibility of the photovoltaic devices based on the singlet fission effect and fabricate AgBiS₂/Pentacene singlet fission solar cells. The process of triplet generation in pentacene and carrier transfer in heterojunction are meticulously traced by transient absorption spectroscopy. Subsequently, an additional annealing treatment is adopted, leading to enhanced singlet fission and more efficient carrier transfer, and eventually the singlet fission solar cells exhibit near-unity internal quantum efficiency. In the second project, a new low temperature injection method is developed for the synthesis of Cs₃Cu₂Br₅ nanorods. The separation of the nucleation and the growth stages results in highly controllable aspect ratio of the products. The polarized emission of the Cs₃Cu₂Br₅ nanorods is investigated by a single particle spectroscopy, and the highest recorded degree of polarization of the single nanorod is 0.88, showing great potential of this materials for fabricating polarized emission devices after appropriate alignment treatment. The third project focuses on the strongly confined CsPbBr₃ quantum dots. By simply changing the precursor temperature, the quantum dots exhibit tunable emission ranging from violet-blue (433 nm) to pure-green (515 nm). The fabrication of deep-blue light emitting diodes is further tried with these strongly confined quantum dots. In the last project, we explore the energy funneling process in quasi-2D perovskite and summarize the various effects of the metal dopants in the devices. Overall, this research includes the synthesis of numerous ternary metal halides and sulfides, the characterization of ultrafast photophysical processes in different systems, and the fabrication of some optoelectronic devices. By integrating the intrinsic mechanisms with the extrinsic properties, we aim to explore superior and novel materials, as well as higher-performance optoelectronic devices.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jonathan Eugene HALPERT (Supervisor) & Liang Guo (Supervisor)