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Halide perovskites have attracted the attention of a broad segment of the optoelectronics field, owing to their outstanding optical and electrical properties; simple low-temperature solution processing; low-cost raw materials; and tunable bandgaps. The main objective of this dissertation is engineering the materials’ properties of halide perovskites – their crystallinity, composition, and dimensionality – in order to understand the fundamental photophysical processes leading to their extraordinary behavior and to translate this understanding into optoelectronic applications. This dissertation is divided into two parts: the first focuses primarily on halide perovskites as a photonic source from an emission perspective, whereas the second is devoted to fundamental investigation of emergent photophysical concepts in halide perovskite materials including photon recycling and hot carriers. In the first part of this dissertation, we studied the synthesis and characterization of Cs-Pb-Br-based perovskite-related single crystals to elucidate the origin of the materials’ emission properties. After that, we presented perovskite nanocrystals (NCs) as a color converter in solid state lighting and visible light communication. Perovskites NCs’ converted white light (with a high color rendering index of 89 and a color correlated temperature of 3236 K) exhibits an extraordinary modulation bandwidth of 491 MHz, and data transmission rate of 2 Gbit/s. In the second part of this dissertation, we developed a facile synthesis method for perovskite microwires and demonstrate efficient photon recycling in those microwires with conclusive spectroscopic evidence. Subsequently, we investigated hot charge carriers in halide perovskites solar cells by a combination of laser spectroscopy and density functional modelling. Furthermore, we presented that hot holes were extracted at the device interface between the perovskite absorber and a hole transport layer. The findings and methodologies described in this dissertation represent a significant advance for utilizing the optical properties of halide perovskites, bring new fundamental photophysical insights to the field of halide perovskites, and provide a new powerful approach for designing the interface of perovskite solar cells to efficiently extract the hot charge carriers.

The recent discovery of a two-dimensional electron (2DEG) gas at interfaces between nonpolar SrTiO₃ (STO) with other polar perovskites has lead to an enormous amount of research. Among this 2DEGs most interesting properties are two-dimensional superconductivity and ferromagnetism, sometimes concurrent. This study provides a starting point in understanding the reconstruction of bulk perovskite t₂[subscript g] bands near a surface or polar interface. First a symmetry constrained [k arrow] · [p arrow] model is developed for an arbitrary pseudocubic bulk perovskite. This [k arrow] · [p arrow] model is applied to studies of bulk STO under external strain and to the Shubnikov - de Haas effect in lightly doped STO to high magnetic fields. Then a simplified electronic structure model is developed for surfaces and interfaces. This model includes non-linear and non-local screening effects by a single polar lattice mode. Generalization of the lattice screening model is discussed. Bonding within a single perovskite layer is then investigated further to understand Rashba interactions and their connection with microscopic material parameters. Next the optical conductivity of quantum confined t₂[subscript g] bands is investigated. Finally some possible future work based on the ideas developed in this thesis are explained. / text

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The work documented in this thesis concerns the control and modification of semiconducting perovskite thin films for their use in perovksite solar cells (PSCs). PSCs are a promising new thin-film technology, offering both high solar to electricity conversion efficiencies and cheap fabrication costs. Herein, the boundaries of perovskite solar cell research are pushed further by tackling several challenges important to the field. Initially, this work focuses on understanding why the best PSCs made so far have been mesostructured devices, with the perovskite infiltrated into a scaffold. It is shown that this can be seen as simply a fabrication aid; without the scaffold, thin films easily dewet from the substrate. By understanding the crucial parameters important in carefully controlling this dewetting, it is minimised, and it is shown that scaffold-free planar heterojunction devices with high efficiencies can be fabricated. This work leads on to the next section; the development of semi-transparent perovskite solar cells. In their present state, PSCs cannot compete with silicon as stand-alone modules. Here, the morphological control has been leveraged to realise a different embodiment – semi-transparent perovskite devices for use in building-integrated photovoltaics. Competitive efficiency and transparency are demonstrated. Moreover, a hybrid self-tinting power-generating window concept is fabricated, by combining the photovoltaic and electrochromic technologies. In the third section of the thesis, the limitations of the most studied perovskite material, methylammonium lead halide, are addressed: its overly wide bandgap and thermal instability. To address these, the chemical constituents of the perovksite are altered, and the development of more efficient and more stable materials are reported. These are likely to be important for perovskite modules to pass international certification requirements for commercialisation. Finally, an in-depth study on the effect of ambient moisture, relevant for considering scale-up and the fabrication environment needed, is carried out. It is shown that the presence of some moisture during film fabrication allows a reduction of defect states in the perovskite material, enhancing device performance and film quality.

Halide perovskites have some unique advantages as optoelectronic materials. Metal halide perovskites have been attracting enormous attention for applications in optoelectronic devices such as photodetectors, light-emitting devices and field-effect transistors. The remarkable semiconducting properties have been intensively investigated in recent years. However, the performance of optoelectronics devices based on the conventional perovskite is limited by the ion migration, the mobility of the carriers and the light absorption in the near infrared region and so on. In a decade, numerous attempts are studied to further breakthrough the limitations using both physical and chemical methods. This dissertation is devoted to overcoming the drawbacks by integrating the state-of-art perovskite with other functional materials and to further deciphering the carrier transport mechanics behind the mixed dimensional heterostructures. Field-effect transistors are the workhorse of modern microelectronics. Proof-of-concept devices have been made, utilizing solution-processed perovskite as transistors. Beyond the Field-effect transistors, photodetectors can be construct with a transistor configuration. In this dissertation, we exploited Au dimers with structural darkness to enhance the light harvesting, and utilize sorted semiconducting single-walled carbon nanotubes to enhance the conductivity of thin-film. At last, we developed a hybrid memtransistor, modulable by multiple physical inputs using hybrid perovskite and conjugated polymer heterojunction channels to realize neuromorphic computing.

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Several bismuth-containing transition metal perovskites that are of interest as potential multiferroic materials have been synthesised and studied. These materials have been structurally characterised and their physical properties have been examined at varying temperatures and pressures. The new series of substituted bismuth ferrite perovskites BixCa1-xFeO3, where x = 0.4 - 1.0, has been prepared. A disordered cubic phase (x = 0.4 - 0.67) and the coexistence of rhombohedral and cubic phases (x = 0.8 and 0.9) have been observed. The x = 0.8 sample is located at the phase boundary and shows a transformation from cubic to rhombohedral symmetry at 473 - 573 K. All samples are antiferromagnets at room temperature and have Néel temperature of 623 - 643 K. Ferroelectric order is suppressed in the disordered cubic phase. BixLa1-xMnO3 materials with x = 0.8, 0.9 and 1.0 were synthesised at 3 - 6 GPa. For x = 1.0 and 0.9 samples a highly distorted perovskite structure with monoclinic space group C2/c was adopted and ferromagnetic behaviour was observed with Curie temperatures of 101 and 94 K, respectively. Bi0.8La0.2MnO3 shows an O'-type orthorhombic Pnma structure and canted A-type antiferromagnetic ordering below 80 K. A new phase of BiNiO3 has been discovered at 4 - 5 GPa below 200 K, in which a Pb11 symmetry has been revealed with a = 5.2515(2) Å, b = 5.6012(3) Å, c = 7.6202(4) Å and β = 90.20(1) º at 4.3 GPa and 100 K. This new Phase Id is derived from the ambient Phase I Bi3+ 0.5Bi5+ 0.5Ni2+O3, where the charge disproportionated Bi3+/Bi5+ cations become disordered. The updated P-T phase diagram of BiNiO3 is presented. BiCu3Mn4O12 has been studied by neutron diffraction from 5 to 400 K. The incorporation of Mn3+ into the Cu site has been observed, showing that the true composition is BiCu2.5Mn4.5O12. The ordering of Mn and Cu moments below transition temperature 320 K is found to be ferromagnetic rather than ferrimagnetic as proposed previously.