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The perovskite family of materials is extremely large and provides a template for designing materials for different purposes. Among them, hybrid organic-inorganic perovskites (HOIPs) are very interesting and have been recently identified as possible next generation light harvesting materials because they combine low manufacturing cost and relatively high power conversion efficiencies (PCEs). In addition, some other applications like light emitting devices are also highly studied. This thesis starts with an introduction to the solar cell technologies that could use HOIPs. In Chapter 2, previously published results on the structural, electronic, optical and mechanical properties of HOIPs are reviewed in order to understand the background and latest developments in this field. Chapter 3 discusses the computational and experimental methods used in the following chapters. Then Chapter 4 describes the discovery of several hybrid double perovskites, with the formula (MA)$_2$M$^I$M$^{III}$X$_6$ (MA = methylammonium, CH$_3$NH$_3$, M$^I$ = K, Ag and Tl, M$^{III}$ = Bi, Y and Gd, X = Cl and Br). Chapter 5 presents studies on the variable presure and temperature response of formamidinium lead halides FAPbBr$_3$ (FA = formamidinium, CH(NH$_2$)$_2$) as well as the mechanical properties of FAPbBr$_3$ and FAPbI$_3$, followed by a computational study connecting the mechanical properties of halide perovskites ABX$_3$ (A = K, Rb, Cs, Fr and MA, X = Cl, Br and I) to their electronic transport properties. Chapter 6 describes a study on the phase stability, transformation and electronic properties of low-dimensional hybrid perovskites containing the guanidinium cation Gua$_x$PbI$_{x+2}$ (x = 1, 2 and 3, Gua = guanidinium, C(NH$_2$)$_3$). The conclusions and possible future work are summarized in Chapter 7. These results provide theoreticians and experimentalists with insight into the design and synthesis of novel, highly efficient, stable and environmentally friendly materials for solar cell applications as well as for other purposes in the future.

Including guest compounds inside the pores of nanoporous crystalline hosts (e.g. zeolite) is a key strategy to post-synthetically functionalise these nanoporous materials over past half a century. It yields highly active and stable heterogeneous catalysts as well as robust materials with tuneable photoluminescence properties due to geometric/quantum confinement. More recently, metal-organic frameworks (MOFs), which are hybrid hosts assembled with metal centres and organic ligands, start to be considered for creating host-guest composites. Apart from the aforementioned confinement effects, MOFs with diverse chemistries as hosts can give rise to a variety of host-guest interactions in these composite systems. It is, however, challenging to investigate these MOF-guest systems due to small MOF pore dimensions, MOF instability, poor guest loading control and limitations in guest characterisations. The thesis explores three different MOF-guest systems covering their preparation, characterisation as well as some unusual behaviour owing to MOF-guest interactions and/or confinement effects. The first system is incorporating electrically conducting poly(3,4-ethylenedioxythiophene) (PEDOT) into a $Zn_2(1,4-ndc)_2(dabco)$ ($1,4-ndc = 1,4-naphthalenedicarboxylate$, $dabco = 1,4-diazabicyclo[2.2.2]octane$) $[PEDOT@Zn_2(1,4-ndc)_2(dabco)]$, which turned the electrically insulating MOF into a semiconducting composite. The formation of nanostructured PEDOT by removing the MOF from the $PEDOT@Zn_2(1,4-ndc)_2(dabco)$ is also demonstrated. The success in MOF-PEDOT system preparation is the motivation to expand the synthesis to forming inorganic guests (the second system). Thermodynamically favourable solid/solution phases of inorganic compounds, which relate to electrochemical potential and pH, are considered to choose reaction agents and conditions to form desired guests. The application of the concept to the synthesis of $MOF-RuO_2$ catalysts ($RuO_2@MOF-808-P$), where the confined $RuO_2@MOF-808-P$ has exceptionally high activity for CO oxidation compared to unconfined $RuO_2$ with poor activity at low temperatures ($\le$150 °C) is demonstrated. In the final system, guest-induced metamorphosis by carbonising the MOF-guest composites made of HKUST-1-type MOFs and thiomolybdates is unveiled. With implications for the transformation mechanisms, MOF- and guest-dependent morphology of the carbonised products are shown. The carbonised product based upon HKUST-1(Cu) and thiomolybdate was also demonstrated as lithium-ion battery anode.

In this thesis, we investigate the nature of charge carrier generation, relaxation and recombination in a range of lead halide perovskites. We focus on understanding whether the photophysical behaviour of these perovskite materials is like that of highly-ordered inorganic crystalline semiconductors (exhibiting ballistic charge transport) or disordered molecular semiconductors (exhibiting strong electron-phonon coupling and highly localised excited states) and how we can tune these photophysical properties with inorganic and organic additives. We find that the fundamental photophysical properties of lead halide perovskites, such as charge carrier relaxation and recombination, arise from the lead halide lattice rather than the choice of A-site cation. We show that while the choice of A-site cation does not affect these photophysical properties directly, it can have a significant impact on the structure of the lead halide lattice and therefore affect these photophysical properties indirectly. We demonstrate that lead halide perovskites fabricated from particular inorganic and organic A-site cation combinations exhibit low parasitic trap densities and enhanced carrier interactions. Furthering our understanding of how the photophysical properties of these materials can be controlled through chemical composition is extremely important for the future design of highly efficient solar cells and light emitting diodes.

The hybrid organic-inorganic perovskites (HOIPs), e.g. methylammonium and formamidinium lead halide (MA/FAPbX3, X = I, Br or Cl), are a class of materials that has recently achieved remarkable performances in photovoltaic applications. This thesis describes the synthesis, structure and properties of this class of perovskites, with particular focus on their crystal chemistry, mechanical responses and structural diversity. Understanding the unique crystal chemistry of HOIPs is crucial for device design. While MA-based perovskites have been widely studied, there are still many open questions on the crystal chemistry of FA-based perovskites. In this work, FAPbX3 (X= Br or I) was shown to undergo a cubic (Pm3 ̅m) to tetragonal (P4/mbm) transition on cooling. Studies on the high-pressure crystallography of FAPbI3 exhibited a similar trend and further illustrated band gap tuning via external stimuli. In addition, the cubic lattice of FAPbBr3 was found to be more strained than its MA counterpart. The observed intrinsic strain was modelled with anisotropic line broadening and < 100 > was found to be the least strained direction. To explore potential applications in flexible devices, crystals of single (Pb-based) and double (Bi-based) perovskites were probed by nanoindentation and their mechanical properties, such as Young’s moduli (E) (10 – 20 GPa) and hardnesses (H) (0.2 -0.5 GPa), were determined. The mechanical responses of MA- and FA-based hybrid perovskites correlated well with the chemical and structural variations in these analogues, showing a general trend of ECl > EBr > EI and EPb > EBi. By analogy with classical inorganic perovskites, the hybrid phases can crystallise in both three-dimensional (3D) and low dimensional perovskite-like forms. To improve the stability and remove the toxicity in the current prototypical hybrid perovskites, compositional engineering was applied, focusing on non-toxic bismuth (Bi) as a viable alternative to lead (Pb) in future photovoltaic materials. We report a new layered perovskite, (NH4)3Bi2I9, which exhibits a band gap of 2.0 eV, comparable to MAPbBr3 and FAPbBr3. This work contributes to the materials design goal of more stable and eco-friendly perovskite devices.

Upconversion phosphor materials are attracting considerable attention for their possible applications in solar cells with improved efficiency, nanomaterials for bio-imaging, lasers and novel display technologies. Upconversion materials, usually consisting of crystals dopedwith lanthanide ions, can convert low-energy incident radiation into higher energy emittedradiation. Several mechanisms are involved, including multiple photon absorption and energy transfers between dopants. Up to now, reported upconversion efficiencies have beenrelatively low, excitation thresholds quite high, and the investigated phosphors (generally fluorides) often presented poor chemical stability (hygroscopy), limiting their industrial applicability. In this dissertation, we investigate the upconversion luminescence characteristics of rareearth-doped RE2BaZnO5 (RE = Y, Gd) phosphors, for near-infrared to visible upconversion. Being oxides, these materials have good chemical, thermal and mechanical properties. A variety of dopants, including Yb3+, Er3+, Ho3+ and Tm3+, were embedded in the host lattice, resulting in bright red, green, blue and white light emissions under 980 nm excitation and at relatively low excitation powers. Upconversion efficiencies up to ~ 5.2%, 2.6%, 1.7% and 0.3% were respectively achieved in samples doped with Yb3+, Er3+ (green and red emission), Yb3+, Ho3+ (green emission), Yb3+, Tm3+ (blue and near-infrared emission) and Yb3+, Er3+, Tm3+ (white light emission). We believe that our green, red and white emitting systems are the most efficient upconverting samples reported to date for green, red and whitelight emission, which makes them serious candidates for many of the applications listed above. The upconversion mechanisms were determined for the first time by means of indepth steady-state and time-resolved spectroscopic investigations, including concentration and power dependence studies associated with temperature-dependent lifetime measurements.

Solid state white light emitting diode lighting devices outperform conventional light sources in terms of lifetime, durability, and lumens per watt. However, the capital contribution is still to high to encourage widespread adoption. Furthermore, the colour from today's devices is unsuitable for general room illumination and thus new phosphor materials are needed. This dissertation will examine the synthesis of inorganic nanoparticles and the possibility of using hybrid inorganic-organic frameworks in the search for new lighting phosphors. Nanoparticles of the oxide compound yttrium aluminium garnet were synthesized using an emulsion technique, though it was found that the high temperature processing needed for good optical properties was not compatible with maintaining nanosized particles. In terms of hybrid framework phosphors, several aspects of this new area have been explored. The mechanical and optical properties of a dense cerium oxalate formate hybrid framework compound have been investigated. Its strength was found to be nearly as great as some classical ceramic compounds, and clearly robust enough for device applications. While the photoluminescence of the cerium oxalate formate was not suitable for solid state lighting, the impressive mechanical properties evaluated are expected to be valid for a wide range of dense inorganic-organic frameworks. A novel approach to solid state lighting phosphors was introduced by using ligand-based photoluminescence in hybrid frameworks. Novel frameworks were prepared using 9,10-anthraquinone-2,3-dicarboxylic acid in combination with calcium, manganese, nickel, and zinc. These compounds show excellent photoluminescent emission for use in solid state lighting applications, although the luminescence is quenched at room temperature due to dynamic effects. The excitation, while reaching the blue part of the spectrum, falls just short of what is needed for use today's devices. To address these issues, a second class of novel framework compounds was prepared using 9-fluorenone-2,7-dicarboxylic acid in combination with calcium, strontium, barium, cadmium, and manganese. They are more rigid structures and show good luminescence at room temperature with a photoluminescent excitation spectrum extending further into the blue than the anthraquinones. Additionally, quantum yield in the calcium fluorenone is nearly double that of its parent ligand, suggesting that there is an enhancement in luminescent properties as a result its inclusion in a framework structure. An explanation for the differences in efficiency between seemingly similar compounds are drawn from their compositions, crystal structures, photoluminescence, and specific heat properties. Finally, some structural and chemical targets for future hybrid phosphor development are identified based on the relationships identified in this work.