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Metal halide perovskite semiconductors offers bright future for optoelectronic applications due to their excellent optical and electrical properties and their low-cost solution-based facile fabrication. The most of the perovskite application are based on the defective polycrystalline films and they offer inadequate moisture/thermal chemical stability. Therefore, this dissertation is dedicated to find new strategies to deploy the single-crystal perovskites to photovoltaics and new methods to reduce the moisture/thermal instability of inorganic perovskite light-emitters. In first part of this dissertation, we aimed to reveal the potential of the single crystal in photovoltaics. Single-crystal semiconductors can outperforms their polycrystalline forms in terms of photovoltaic performance due to their better structural quality and less electronic traps. However, the most efficient perovskite solar cells are based on polycrystalline films. While single crystals can perform beyond the limits of polycrystalline films, their synthesis and device integration are complex. Therefore, we aimed to create new synthetic methods to unveil the potential of the single-crystal perovskites in photovoltaics. We developed new strategies leading the perovskite single crystals to go beyond 20% power conversion efficiency in Chapter 2. Also fundamental limits of the perovskite single crystals are investigated in Chapter 3 by fabricating single crystal cells with varying thicknesses, and the electron diffusion length is calculated to be 520 μm. In Chapter 4, we propose surface modification and compositional engineering techniques to bring the perovskite single crystal photovoltaic one step beyond of the previous point by reaching 21.9% and 22.8% efficiencies, respectively. In the second part of this dissertation (Chapter 5), a novel synthetic method is offered to achieve highly stable light-emitting perovskite-related materials since the fast degradation of perovskites in the presence of water and moisture is a challenge for perovskite-based technologies and hinders the material’s potential. We demonstrated that these a direct transformation of 3D CsPbBr3 films to CsPb2Br5 exhibiting excellent stability against humidity and heat while keeping the high photoluminescence quantum yield. We believe the strategies offered in this dissertation will open an avenue in photovoltaic and light emitting applications, and can be utilized in new optoelectronic applications in future.

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Compounds in the series La1-xTiO3 have been prepared in the powder and single crystalline forms. The structures of these compounds were studied with x-ray, neutron, and electron diffraction techniques. In all cases, a distortion from the ideal perovskite structure was found. Throughout the series, transitions occur in structural, magnetic, and electrical properties. The structure of La.67TiO3 was found to be of a distorted perovskite type arising from distortions in the titanium octahedra as a result of an ordering of the lanthanum positions and A-site cation vacancies. Although this phase has been reported many times, the crystal structure has not been determined. The La.67TiO3 and La.70TiO3 phases have the same structure and were assigned to the Pmmn space group. At La75.TiO3, a Ti3+ concentration of 2 8%, the symmetry of the structure changes from Pmmn to Imma, and distortion of the perovskite structure is caused by octahedral tilting within the lattice. A further structural transition occurs at a Ti3+ concentration of 64%, La8gTiO3, where the symmetry changes from Imma to Pnma. Again distortions from the ideal perovskite structure are caused by octahedral tilts within the system. Transitions were also observed in electrical and magnetic properties. These transitions occur at the same Ti3+ concentrations as the transitions observed in the structural properties, which suggests a correlation between properties based on Ti3+ concentration. The occurrence of A-site cation vacancies within the lattice is directly related to the Ti3+ concentrations within the individual materials. Changes in the Ti3+ concentration affect the size of the unit cell as well as the electrical and magnetic properties which range from insulators at low Ti3+ concentrations to metals at high Ti3+ concentrations. / Thesis / Master of Science (MSc)

Formamidinium lead iodide perovskite (FAPI) holds promise to be used in high–efficiency solar cells. A comprehensive understanding of the immediate surface properties of this material can provide insights into its interaction with other materials and guide future engineering of its interface with other constituents of the solar cell. In this thesis, the orientations of the formamidinium cations on flat FAI and PbI2 terminated (001) surfaces of the of α–FAPI were studied with classical and ab initio molecular dynamics simulations at a temperature of 300 K. The cations on the FAI terminated surface displayed a preference to lie flat on the surface, while the cations near the PbI2 terminated surface displayed a preference to be oriented in such a way that the molecular planes were perpendicular to the surface. Average near edge X–ray fine structure (NEXAFS) spectra at the carbon and nitrogen K–edges were simulated from the trajectories of the FAI terminated structure and compared with existing experimental spectra obtained from a clean FAPI surface under ultrahigh vacuum. By comparing the experimental and calculated NEXAFS spectra it was observed that the distribution of formamidinium orientations which contribute to the experimental NEXAFS spectra is different from the one seen in the outermost FAI layer from the molecular dynamics simulation. This observation can be explained by the finite probing depth of the X–ray or the fact that the surface is not perfectly FAI terminated in the experiment. The more uniform distribution of formamidinium orientations one layer below the surface resulted in simulated NEXAFS spectra which were more similar to the experimental ones.

<p>Two-dimensional (2D) halide perovskites are an attractive class of hybrid perovskites that have additional optoelectronic tunability due to their accommodation of relatively large organic ligands. Nevertheless, contemporary ligand design depends on either expensive trial-and-error testing of whether a ligand can be integrated within the lattice or conservative heuristics that unduly limit the scope of ligand chemistries.</p><p>This work begins by investigating simulation-guided ligand design. Employing molecular dynamics (MD) simulations and machine learning (ML) models, systematic ligand exploration unveils the principles governing the stability and structural relationships of these perovskites. Promising ligand candidates undergo a refinement process informed by feasibility considerations, followed by experimental synthesis and characterization to underscore the effectiveness of simulation-informed design strategies.</p><p>Delving into the realm of anionic diffusion, a pivotal factor in 2D perovskite stability, the study examines this property using both experimental methods and simulation techniques. This parallel examination underscores the alignment between simulation predictions and real-world observations, offering nuanced insights derived from molecular simulations. Importantly, simulations serve as potent tools for hypothesis validation when ligands originate from experimental synthesis, affirming conjectures stemming from empirical insightsThe exploration extends to comprehending the molecular insights of experimental observations, thereby shedding light on factors that enhance device efficiency. We explore mechanisms for mitigating phase disproportionation, optimizing ion diffusion, modulating molecular interactions between perovskite and polymeric hole-transporting materials, and uncovering the single-molecule behavior that leads to high photoluminescence quantum yields. Notably, all simulation outcomes align with experimental findings, further validating the utility of MD analysis in the context of 2D perovskite systems.</p><p>Furthermore, this work addresses a crucial aspect of MD simulations, namely the refinement of force field models. Previously developed topology automated fixed-charge force-field interactions (TAFFI) is augmented through the incorporation of polarizability using classical Drude oscillators, resulting in a novel framework termed TAFFI-Drude. This approach enhances electrostatic properties while retaining transferability and consistency from the existing TAFFI model.</p><p>The thesis concludes with a comprehensive discussion of the findings across the aforementioned areas, highlighting the impact of simulation-driven design and insights in advancing 2D perovskite research. The implications of these discoveries for optoelectronic applications and the broader field of materials science are explored, emphasizing the potential for innovation and improvement within the realm of 2D halide perovskites.</p>

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