Modulating carrier confinement in inorganic metal halide perovskites for enhanced efficiencies in light-emitting diodes

• Main Author: Ng, Yan Fong
• Other Authors: Nripan Mathews
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/136972
• Institution: Nanyang Technological University

Lead halide perovskites have revolutionized the field of optoelectronic research in recent years owing to the excellent semiconductor properties offered by this class of materials. Thin films of these materials possess superior electronic qualities despite being low-cost and solution-processed, with exceptional characteristics such as defect tolerance, band gap tunability, high absorption coefficient and long-range balanced charge transport. Many of such traits are not even found in well-established silicon and gallium arsenide semiconductor technologies. The past decade has seen an unprecedented rise in perovskite photovoltaic research, with power conversion efficiencies reaching 25.2%, surpassing the best-in-class thin-film photovoltaic technologies and almost catching up with silicon. While most of the research work are focused on solar cells, these remarkable properties have made halide perovskites promising semiconductor materials for various other applications, especially light emission. Inorganic and organic light-emitting diodes (LEDs) have transcended the field of artificial lighting and display, bringing significant benefits such as high brightness and efficiencies. Despite this, the high material and processing costs along with inherent limitations have hampered their widespread use. Halide perovskites, leveraging on their extraordinary optical and electronic properties, have manifested strong and efficient electroluminescence. Extremely high colour purity, along with band gap tunability that allows emission colours across the full visible spectrum, has made perovskites strong contenders for display emitters. Within two years since their first incorporation in LEDs, research has led to high luminosities and efficiencies that organic LEDs only managed to achieve after two decades. Recently, external quantum efficiencies (EQEs) have exceeded 20% with various employed strategies such as additive engineering, passivation, and dimensional modulation. Additional waveguide and outcoupling techniques have achieved a record EQE of 28.2%. While more studies have been focused on the organic-inorganic hybrid perovskites (e.g. CH3NH3PbX3 where X represents a halide), fully inorganic compounds like CsPbBr3 are more thermally stable, although poor intrinsic properties such as low exciton binding energy and defective morphology have resulted in lower photoluminescence quantum yield and EQE. The main limitation of this material can be generalized as a carrier confinement problem, and thus the objective of this thesis is aimed at overcoming this. Strategies to enhance the confinement effect can be classified into two approaches, a geometrical approach which involves physical and spatial confinement of charge carriers, and a dimensional approach which entails electronically confining the charge carriers through an energy cascade phenomenon. The geometrical approach was explored using two different methods: first, a rapid crystallization process was executed which showed that the stronger spatial confinement imparted by smaller grain sizes is essential; and second, a sequential deposition process was carried out to significantly improve the film coverage in order to confine the charge carriers within the perovskite emitter layer and reduce leakage pathways. Both studies firmly show the importance of morphological tuning in the geometrical confinement of charge carriers, confirming that the ideal morphology required is a compact film with small grain size. In the dimensional approach, a mixture of two- and three-dimensional phases, obtained through the excess addition of a large ligand molecule, phenylethylammonium bromide, has shown outstanding photo- and electroluminescence. The multiphase existence provides a cascading energy landscape where the energy or carriers funnel from the high band gap domains to the lowest band gap state, thereby electronically confining the charge carriers and significantly augmenting the carrier density within the emitter phase. In addition, the ligand molecules are found to restrict the growth of the perovskite grains, forming a smooth and compact layer of nano-sized grains. The combination of both geometrical and electronic confinement in the case of the mixed-dimensional perovskite has achieved more than fifty-fold enhancement in LED performances as compared to the pristine CsPbBr3. Its exceptional results thus spurred the last study, where multiple mixed-dimensional systems are explored with the aim of uncovering the desired characteristics of the ligand molecule. Based on a series of rigorous characterization and comparison, a list of some general guidelines for an effective ligand molecule is established, which is likely applicable to any mixed-dimensional perovskite system. Importantly, these studies lay down the various concepts governing the confinement of charge carriers in CsPbBr3 and demonstrate their significance in attaining efficient light emission. The knowledge gained will be useful in designing the next generation of perovskite emitters for light-emitting applications, while the ease of scaling up these deposition processes make commercialization of halide perovskite LEDs highly anticipated in the near future.