Localized structural and bulk effects in hybrid organic-inoraginc perovskites

• Main Author: Wong, Walter Pei De
• Other Authors: Andrew Clive Grimsdale
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Functional materials; Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/151719
• Institution: Nanyang Technological University

This thesis is a study on hybrid organic inorganic perovskites (HOIP) and systematically studies the effect of localized structural distortion and disorder in perovskites, which is indirectly observed in the 3-dimensional (3D) methylammonium lead iodide (MAPbI3), and directly observed in the reduced dimensional perovskites. The study then further aims to establish a structure-property relationship in the hybrid perovskites. Chapter 1 and 2 outlines the early developments in the field of hybrid perovskite up until recent times. These chapters lay the background for the work done prior to this thesis and shows how there are knowledge gap in the field, particularly in the understanding of the structural – property relationship. Additionally, a better understanding of the material’s structure – property relationship would be valuable to the respective material’s application, like photovoltaics or efficient lighting application which are of increasing importance towards sustainability efforts. This would then allow scientists to design various aspects of the structure to exploit the respective desired material property. Chapter 3 summarizes the various background and theoretical aspects of the experimental techniques used in this thesis, namely X-ray diffraction, solid state nuclear magnetic resonance, steady-state and transient optoelectronic characterization techniques. X-ray diffraction and solid state nuclear magnetic resonance would probe the structural configuration while the suite of optoelectronic characterization techniques would investigate the optical and electronic properties of the material at various timescale. In Chapter 4, the study focuses on using first principles Density Functional Theory (DFT) simulations and spectroscopic methods to investigate the well-studied prototypical MAPbI3. DFT simulations provided an indication that charge carriers in MAPbI3 exists as localized polarons, thus, the experimental studies seek spectroscopic evidence for the existence for these self-trapped polaronic states. Through a combination of steady state photoinduced absorption, transient absorption and transient photocurrent measurements, this chapter has confirmed the existence of theoretically predicted polaronic states in MAPbI3 which finds its origins in the ‘soft’ lead – iodide bond. Chapter 5 primarily revolves around the 2-dimensional (2D) layered <110> cut of perovskite where this class of hybrid perovskites are known for several remarkable properties, such as white light emission or sometimes also called broadband emission. This work starts off re-examining a previously studied N-(3-aminopropyl)imidazole lead bromide (APIPbBr4), which was reported to exist in the 2-dimensional layered <110> cut of perovskite. However, detailed X-ray Diffraction (XRD) structural analysis of the obtained compound was shown to exist as a polymorphic API2Pb3Br10, which shows no resemblance to the <110> cut of perovskite. Hence, another compound, 2-(1H-Imidazol-1-yl)ethanammonium lead bromide (IEAPbBr4) was synthesized and was shown to exist in the <110> cut of perovskite. Detailed cation dynamics of API and IEA was further studied through Solid State Nuclear Magnetic Resonance (SSNMR) and it was shown to be considerably different. Given the huge difference between the API2Pb3Br10 and IEAPbBr4, it is therefore unsurprising that the bulk measured photophysical properties and characteristic are also different. Chapter 6 then ensued to study the significance of the perovskite A-site cation through using an imidazolium cation. It was shown to crystallize into three different polymorphs, IMIPbBr3, IMI2PbBr4 and IMI3PbBr5 (IMI = imidazolium), by varying the ratio of imidazole to lead concentrations through the precursors. The structures were thoroughly characterized by XRD and the cation dynamics probed through SSNMR. One of the polymorphs, IMIPbBr3, undergoes an unusual phase transition while the remaining two do not show any phase transition. Remarkably, all three polymorphs show distinct cationic dynamics, even though IMI2PbBr4 and IMI3PbBr5 are structurally very similar. Furthermore, photophysical measurements of each polymorph showed different characteristics which indicates the significance of cationic disorder in determining the bulk properties of HOIP.