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Near stoichiometric compositions of Ba3+3xB1+yNb20g (B=Co or Zn) perovskites were studied by microstructure analysis and optical techniques. Materials considered in the present research belong to the family of perovskites exhibiting disorder-1:2 order phase transitions that are important for microwave applications. It was found that deviation from stoichiometry involving cation deficiencies on Ba-or B-positions facilitates formation of an ordered structure for small values of cation deficiencies. Excessive deviation from the nominal values as well as introduction of extra cations destabilizes the perovskite structure leading to the precipitation of secondary phases. Formation of a Ba-deficient Bs6BNb9030 (B = Co or Zn) phase influences the grain growth rate through reduction in the surface energy of grains. In combination with large strain in precursor materials caused by applied pressure during fabrication and high sintering temperature this results in increased porosity and lower density. Appearance of Raman active modes in the considered Ba3+3xBl+yNbz0g materials was attributed to the formation of a 1:2 cation ordered structure. It was shown that microwave losses are influenced by the degree of 1:2 cation ordering that depends on the formation of secondary phases as well as a densification process. The appearance of an "extra" peak in Raman spectra was attributed to the formation of 1:1 cation order described based on the "space-charge" model. Changes in the position of the mode, attributed to "breathing-type" vibrations of oxygen anions from materials having "partially" ordered 1:1 structure to those having 1:2 ordered structure, indicate formation of more rigid oxygen octahedra associated with lower microwave losses. Structural distortion caused by 1:2 cation ordering results in changes in the mutual orientation of transition metal-ligand molecular orbitals and the appearance of two emission bands signifying formation of two different Nb06 octahedra. The first octahedron, present in the 1:2 ordered structure, gives origin to the lower energy photoluminescence band, while the second one, forming a disordered cubic structure, produces an emission peak at higher energies with the variation in the position of the maximum depending on the type of cation on the B-site. Changes in the maximum position of the high-energy peak were attributed to different structure distortions caused by off-center motion of Nb^5+ and stabilization by neighboring B06 octahedra. The stabilization power of B06 octahedra depends on the covalency of B-0 bonds and is larger for cobalt containing perovskites. / Thesis / Doctor of Philosophy (PhD)

Topotactic routes have been used to produce new materials with tunable electronic and magnetic properties. The host materials were the single and triple layered Ruddlesden-Popper compounds, NaLnTiO4 (Ln = La and Gd) and A2La2Ti3O10 (A = Li and K). These compounds consist of [LnTiO4]- and [La2Ti3O10]2-, respectively, perovskite layers interleaved with two alkali metal ion strata. The topotactic routes used in this research were ion exchange and intercalation. Ion exchange was used to replace the cations in the interlayer space with a cationic unit: vanadyl or the transition metal ion, nickel. This ion exchange route opens the structure to further chemistry because each alkali metal ion is replaced by a divalent ion and a vacancy. In these vacancies other atoms can be inserted. Reductive intercalation with alkali metals is of special interest due to their propensity for forming mixed valence compounds. Mixed valency is usually correlated with semiconductive, metallic or superconductive behaviors, and unusual magnetic properties (CMR). Na0.1(VO) Na0.1(VO) 0.45LaTiO4, Na0.05(VO)0.48GdTiO4 and Li0.3Ni0.85La2Ti3O10 have been obtained by ion exchange reactions. Also, the reactivity of Na0.05(VO)0.48GdTiO4, (VO) La2Ti3O10 and Li0.3Ni0.85La2Ti3O10 is probed by an intercalation reaction with n-BuLi. Lithium insertion between the perovskite blocks reduces the titanium in the perovskite sheets and produces new mixed valence titanates. Lithium intercalation in the vanadyl compounds, Na0.05(VO)0.48GdTiO4 , as well as (VO)La2Ti3O10 gives rise to new magnetic properties. The crystal structures, thermal behavior, electronic and magnetic properties of these new compounds will be discussed.

Halide perovskite solar cells, with rapid efficiency improvements from ~10% to ~23% in 6 years, have attracted significant attention due to their remarkable performance, low processing cost and their potential to become a strong alternative candidate to silicon solar cells. Significant development has also been achieved in halide perovskite-based LEDs with EQE improved from below 1% to ~20% in less than 4 years. This remarkable progress can mainly be attributed to the optimisation of halide perovskite properties. This dissertation focuses on the correlation between optical and electrical properties of halide perovskites and their remarkable performance. Bandgap tunabilities of halide perovskites in blue to green regions through mixing Br-and Cl-and in near infra-red region by substituting Pb2+ with Sn2+ are demonstrated. The absorption and PL spectra are consistent with each other supporting the bandgap tunability. Corresponding EL spectra, which are consistent with their PL spectra, are also demonstrated for blue to green regions. Terahertz measurements coupled with PLQE and transient PL results reveal that the high carrier mobilities are the main reason behind the high efficiency of tin-rich samples. A novel perovskite-polymer-bulk heterostructure is introduced and studied comprehensively. Correlations between their optoelectronic properties and remarkable performance on timescales ranging from femtosecond to microsecond are presented. Transient optical spectroscopy reveals the energy transfer from 2D regions to 3D regions happens in 1 ps. The 20% EQE of the LEDs based in this structure is consistent with conventional thin-film optical models giving internal quantum efficiency of ~100%. This in agreement with near-unity PLQE value of the pristine emissive layer material and the dominant bimolecular recombination process observed in nanosecond-scale transient PL measurements. Two typical interfacial engineering methods to improve the quality of halide perovskite and device performance are then presented. Optimised NiOx is adopted to improve the anode interface. From transient photovoltaic measurements, we find the charge collection ability of NiOx is superior to that of PEDOT:PSS. This is also the main reason behind their better photovoltaic device performance. A unique anti-solvent treatment with additive modifies both the bulk and surfaces of halide perovskites and improves the device performance significantly. Transient PL and PLQE measurements demonstrate that non-radiative recombination pathways are significantly reduced.

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.

Perovskite photovoltaics is one of the fastest growing opto-electronic technologies with device efficiencies currently exceeding 23%. The opportunity to deposit these abundant materials with large area solution processing techniques could make perovskites viable for low-cost production. However, since perovskite materials are prone to degradation, their lifetime needs to be improved to that of silicon solar cells before these devices can be commercialized. Moreover, unlike most semiconductors, trap densities in polycrystalline perovskite films in high-performing devices have been determined to be relatively large, suggesting a remarkable defect tolerance in perovskite films that needs to be understood in the context of the nature of the trap states and any residual non- radiative losses. These non-radiative losses are observed as photoluminescence heterogeneity within perovskite films, even for high-performing perovskite systems. In this work, we explore the degradation kinetics of perovskite devices under stress conditions and find that further stability improvements should focus on the mitigation of trap generation during ageing. Furthermore, we fabricate perovskite solar cells with a novel back-contact structure, in which electron- and hole-selective electrodes are co-positioned on the back side of the cell and spaced by 100 μm. By utilising grazing-incidence X-ray diffraction, we show that even in the earliest stages of conversion of precursors to perovskite we achieve remarkably high open-circuit voltages, suggesting that the defect tolerance of perovskites appears at an early stage in the conversion process. Moreover, we employ scanning X-ray diffraction with nanofocused beam and obtain detailed information, revealing overlapping grains located at different depths within perovskite films. We find that the critical grain size is the longer-range structural super-grains rather than the grains viewed with conventional microscopy techniques. These findings further highlight the presence of structural defects in perovskite materials and provide important insights towards improving the optoelectronic behaviour of these materials.

El nitrógeno presenta propiedades electrónicas como polarizabilidad y electronegatividad, así como características cristaloquímicas -radios iónicos, índices de coordinación- muy próximas a las del oxígeno. Ello permite que ambos aniones puedan sustituirse mutuamente en las mismas posiciones cristalográficas, formando disoluciones sólidas. En este sentido nos podemos plantear diseñar nuevos edificios cristalinos de fases nitruradas de metales de transición que presenten estructuras similares a las de los óxidos con aplicaciones por sus propiedades magnéticas, catalíticas, conductividad electrónica o iónica, superconductividad, como materiales para baterías, como dieléctricos, ferroeléctricos, etc.El trabajo experimental realizado ha permitido aislar con éxito dos términos de una nueva familia de oxinitruros de Ruddlesden-Popper de estroncio y niobio. Ésta se formuló como un intercrecimiento de n capas de tipo perovsquita que contienen un nitrógeno por capa [SrNbO2N] y una capa de tipo cloruro sódico [SrO], es decir [SrO][SrNbO2N]n. Dicha formulación inicial se realizó por analogía con los oxinitruros conocidos SrNbO2N y Sr2TaO3N, si bien se esperaba poder modificar el estado de oxidación formal del niobio desde +5 hacia valores inferiores mediante dopaje con metales electropositivos en la posición del estroncio, o por variación de la relación N/O, incluyendo la posible existencia de vacantes aniónicas. Se han obtenido como fases puras los términos n= 1 y n= 2 mediante la reacción:n+1 SrCO3 + n/2 Nb2O5 + n NH3 _ Srn+1NbnO2n+1Nn + 3n/2 H2O + (n+1) CO2Además de los términos n= 1 y n= 2 se han observado miembros de la familia con n= 3 y 4 como defectos extensos en imágenes de microscopía electrónica de alta resolución. Asimismo se ha podido preparar el nuevo compuesto Sr3Ta2O5N2 mediante un procedimiento análogo al de las fases de niobio, lo que teniendo en cuenta la existencia del término n= 1 (Sr2TaO3N), permite formular la familia homóloga de tántalo [SrO][SrTaO2N]n.El estudio de estas fases mediante difracción de electrones conduce al grupo espacial I4/mmm, que es el de más alta simetría de entre los observados en compuestos de tipo Ruddlesden-Popper.Hasta el momento, los oxinitruros preparados no presentan ninguna transición superconductora entre 300 y 4 K, ni carácter metálico. ¿Por qué los óxidos con valencia mixta d0-d1 suelen ser metálicos y los oxinitruros preparados no presentan este comportamiento? A priori parece que ambos tipos de compuestos deberían mostrar las mismas propiedades físicas ya que en los dos casos tenemos una ocupación parcial de las bandas del bloque t2g del metal y estas bandas deben presentar una buena deslocalización. Para entender mejor las propiedades de transporte en los oxinitruros preparados, y debido a que por otro lado, en los óxidos laminares con configuración electrónica d0 se plantea a menudo la posibilidad del dopaje para variar el estado de oxidación formal del metal de transición, se ha desarrollado un modelo cualitativo que permite la comprensión de las propiedades de transporte de sistemas laminares relacionados con la estructura perovsquita para contages electrónicos entre d0 y d1. Inicialmente se analiza la estructura electrónica correspondiente al caso hipotético de un sistema construido a partir de octaedros ideales y posteriormente se analizan distintos casos reales de estructuras con dos, tres y cuatro capas de octaedros en los que se estudia el efecto que tienen distintos factores cristaloquímicos sobre la naturaleza y la dispersión de las bandas.Este estudio nos aporta una herramienta muy útil para la predicción de la naturaleza de los niveles parcialmente ocupados que determinan las propiedades de transporte de este tipo de compuestos, conociendo su estructura y número de electrones. Asimismo, a partir de los resultados de este análisis podemos entender porque los oxinitruros preparados por el momento no presentan comportamiento metálico. / Nitrogen presents some properties like polarizability, electronegativity, ionic radii and coordination numbers quite similar to oxygen. This allows that both anions can be exchanged in the same crystallographic position, creating solid disolutions. Following this approach we can design the formation of new transition metal nitride compounds with analogous structures to their parent oxides. The substitution of oxygen by nitrogen will promote the modification of their physical properties (magnetic, catalytic, electronic or ionic conductivity, superconductivity, applications as dielectric materials, electrodes for batteries, ferroelectrics,...).The experimental work has allowed us to synthesize two members of the new family of Ruddlesden-Popper strontium niobium oxynitrides. This compounds consist of an intergrowth of n perovskite layers with one nitrogen per layer [SrNbO2N] and a layer of [SrO] with NaCl structure. So we have the general formula [SrO][SrNbO2N]n. This materials were prepared following the approach used for the already known SrNbO2N and Sr2TaO3N oxynitrides. With the aim of modifying the niobium oxidation state from +5 to lower values, we carried out some experiments in order to substitute the strontium for lanthanum and to change the N/O ratio. We have obtained as pure phases the members n= 1 and n= 2 following the reaction:n+1 SrCO3 + n/2 Nb2O5 + n NH3 _ Srn+1NbnO2n+1Nn + 3n/2 H2O + (n+1) CO2Apart from the n= 1 and n= 2 members we have observed the n= 3 and n= 4 phases as extended intergrowths in high resolution transmission electron microscopy images. We have also been able to synthesize the new compound Sr3Ta2O5N2 using the ammonolysis system. Since the n= 1 phase, Sr2TaO3N, had already been prepared, we could establish the new family [SrO][SrTaO2N]n.The space group I4/mmm was determined by electron diffraction. This group has the highest symmetry among the observed for the Ruddlesden-Popper phases.Up to now, the prepared oxynitrides are not metallic and magnetic measurements do not show any superconducting transition down to 4 K. Why the mixed valence oxides are often metallic and the prepared oxynitrides do not have this behaviour? It looks like both compounds would present the same physical properties since they have partial occupation of the t2g bands, and this bands would have a good delocalization. In order to understand the transport properties of the prepared oxynitrides and due to the current interest in the intercalation of alkali-metal cations in the layered oxide systems, we have developed a qualitative model that is useful to understand the transport properties of laminar perovskite related systems with electronic configurations d0-d1. In a first stage the electronic structure of an idealized system (regular octahedrons) is constructed. Then, several compounds with n= 2, n= 3 and n= 4 are analyzed. We have studied the effect of different factors on the nature and dispersion of the bands.With this study we have developed a really useful tool to predict the nature of the partially occupied bands that determine the transport properties of this type of compounds. We just need to know the crystal structure and the electron count. It has also been useful to understand the properties of the prepared oxynitrides.

La tesi constitueix un estudi de diverses perovskites mixtes de coure i titani, entre les quals cal citar La2CuTiO6 (estructura tridimensional amb els ions Cu(II) i Ti (IV) desordenats) i Ln2Ba2Cu2Ti2O11 (estructura laminar amb Cu(II) i Ti(IV) ordenats). El treball aprofondeix en les relacions entre mètodes de síntesis, estructura i possibilitat de presentar propietats superconductores en els òxids del sistema Ln-A-Cu-Ti-O (A=Ba, Sr). Ambdós tipus d'òxid han estat sintetitzats tant pel mètode ceràmic com via sol-gel i s'ha estudiat el camí de reacció en tots els casos. Com a resultats més significatius cal destacar la inducció de bidimensionalitat i ordre en sistemes originalment desordenats, que ha donat lloc a l'aïllament d'un nou tipus de perovskites laminars (Ln2Ba2Cu2Ti2O11). Cal destacar també els diferents intents d'obtenció d'òxids de fórmula Ln1+xA2-xCu2TiO8-&#948; (Ln= lantànid, A=alcal·linoterri) que han donat lloc a la síntesi d'un òxid superconductor de la mateixa família (YSr2Cu2.7Ti0.3O7.06) amb una temperatura crítica de 26K.Hom duu a terme una caracterització en profunditat dels òxids esmentats (anàlisi de l'estequiometria catiònica i el contingut en oxigen, caracterització estructural mitjançant difracció de raigs X i neutrons sobre pols, difracció d'electrons i microscòpia electrònica de transmissió i mesura de les propietats magnètiques i de transport). Es realitza també un estudi de les possibilitats de dopatge de Ln2Ba2Cu2Ti2O11, preparant-ne un gran nombre de derivats amb substitució catiònica. L'oxigenació d'aquests derivats substituïts ha estat assajada per diferents mètodes entre els quals cal esmentar l'electroquímica i l'alta pressió d'oxigen.La caracterització acurada dels diferents òxids obtinguts permet d'establir correlacions estructurals significatives que ajuden a la comprensió dels factors que determinen la superconductivitat en aquest tipus d'òxid. Es duu a terme també una discussió dels factors que afecten l'ordre (tant de cations com de vacants d'oxigen) i la dimensionalitat estructural en perovskites mixtes. / La tesis constituye un estudio de diversas perovsquitas mixtas de cobre y titanio, de entre las cuales cabe destacar La2CuTiO6 (estructura tridimensional que presenta los iones Cu(II) y Ti (IV) desordenados) y Ln2Ba2Cu2Ti2O11 (estructura laminar con Cu(II) y Ti(IV) ordenados). El trabajo profundiza en las relaciones entre métodos de síntesis, estructura y posibilidad de presentar propiedades superconductoras de los óxidos del sistema Ln-A-Cu-Ti-O (A=Ba, Sr). Ambos tipos de óxidos se han sintetizado tanto por el método cerámico como vía sol-gel y en todos los casos se ha estudiado el camino de reacción. Como resultados más significativos cabe citar la inducción de bidimensionalidad y orden en sistemas originalmente desordenados, que ha dado lugar al aislamiento de un nuevo tipo de perovsquitas laminares (Ln2Ba2Cu2Ti2O11). Es necesario destacar también los distintos intentos de obtención de óxidos de fórmula Ln1+xA2-xCu2TiO8-&#948; (Ln= lantánido, A=alcalinotérreo) que han dado lugar a la síntesis de un óxido superconductor de la misma familia (YSr2Cu2.7Ti0.3O7.06) con una temperatura crítica de 26K.Se ha llevado a cabo una caracterización en profundidad de los citados óxidos (análisis de la estequiometría catiónica y el contenido de oxígeno, caracterización estructural mediante difracción de rayos X y neutrones sobre polvo, difracción de electrones y microscopía electrónica de transmisión y medida de las propiedades magnéticas y de transporte). Se ha realizado también un estudio de las posibilidades de dopaje de Ln2Ba2Cu2Ti2O11, preparando un gran número de derivados de estas fases con sustituciones catiónicas. La oxigenación de dichos derivados se ha ensayado también utilizando diferentes métodos entre los que cabe destacar los electroquímicos y el tratamiento bajo alta presión de oxígeno. La caracterización cuidadosa de los compuestos obtenidos permite establecer correlaciones estructurales significativas que ayudan a la comprensión de los factores que determinan la superconductividad en este tipo de óxidos. Se discuten también los factores que afectan el orden (tanto de cationes como de vacantes de oxígeno) y la dimensionalidad estructural en perovsquitas mixtas. / The thesis is a study of different mixed copper-titanium perovskites, like La2CuTiO6 (three-dimensional structure with Cu (II) and Ti (IV) disordered in the octahedral positions) and Ln2Ba2Cu2Ti2O11 (layered structure with ordering of Cu(II) and Ti(IV)). The work gives an insight in the relationship among the synthetic methods, the structure and the possibility of becoming superconductor, in the system Ln-A-Cu-Ti-O (A=Ba, Sr). Both types of oxides have been synthesized by the ceramic and sol-gel methods and a study of the reaction path has been made as well. The most significant results are: the induction of bidimensionality and order in systems originally disordered (which has led to the isolation of the new type of layered perovskites Ln2Ba2Cu2Ti2O11), and also the several attempts made to synthesize oxides with the general formula Ln1+xA2-xCu2TiO8-&#948; (Ln= lanthanide, A=alkaline earth) which have led to the obtaining of a superconducting oxide with a critical temperature of 26 K (YSr2Cu2.7Ti0.3O7.06).A detailed characterization of the above oxides is made: cationic stoichiometry and oxygen content analices, structural characterization by means of x-ray and neutron powder diffraction, electron diffraction and electron microscopy, and magnetic and transport properties. A study of the doping possibilities in Ln2Ba2Cu2Ti2O11 has also been made, and several derivatives of these oxides with cationic substitution have been prepared. The oxigenation of these substituted derivatives has been attempted by different methods such as electrochemistry and high oxygen pressure.The study of the structure and properties of the oxides synthesized yields significant structural correlations which help to understand the factors that determine superconductivity in these oxides. A discussion is also made concerning the factors that affect cationic and/or oxygen vacancy ordering and bidimensionality in mixed perovskites.

This dissertation computationally and analytically investigates ways to model solar cells when the lateral motion of charge carriers and light are relevant. We focus on back-contact perovskite solar cells, and assessing the experimental technique of scanning photocurrent microscopy as a means to investigate them. Solar cells are three-dimensional objects frequently modelled as being one-dimensional. However, for more complex designs of solar cell or if the cell is only point-illuminated, one-dimensional modelling is insufficient. In the first study, some conditions for reducing the complexity of two-dimensional drift-diffusion simulations are investigated for a back-contact perovskite cell. Analytic expressions for the relationship in both the low extraction velocity and high extraction velocity regimes are demonstrated, and the conditions where these approximations break down are investigated. These findings are then applied a point-excited film with an extended electrode, a problem encountered during scanning photocurrent microscopy. We show the current recorded in this case should decay exponentially with the distance between excitation and electrode, with a decay constant that can be related to device parameters. The characteristic equilibration time for the system to reach this current is demonstrated to increase linearly with distance. Between this gradient and the exponent, information about the diffusion and recombination mechanics can be extracted from a variety of systems. Photon recycling is the process in whereby photogenerated carriers recombine to generate light that is absorbed again within the solar cell. In the second section, we apply the findings of the first section to show that experimental results published elsewhere are best explained by photon recycling in methylammonium lead iodide perovskite back-contact solar cells. However we do not have an established theoretical model for long-ranged lateral optical transport in these solar cells. Three models are developed: a bimolecular model for unscattered, coherent transport; a photon diffusion model for frequently scattered, noncoherent light; and a monomolecular, assisted-diffusion model. The modal nature of coherent optical transport is considered and modifications to previous one-dimensional theories are made. The nature of the photon diffusion model is discussed, as are theoretical shortcomings. All three models are then solved numerically and compared to experimental results. The low-scattering photon diffusion models correspond well to the experiment. The third investigation involves the performance of different architectures of back-contact perovskite cells. These cells potentially offer increased current due to less shadowing by front electrodes. We compare them to each other and to traditional vertical structures. It is found that, in terms of internal transport, the back-contact solar cells give less efficient performance than the vertical design. The best of the back-contact cells investigated is a flat interdigitated design. The increase in efficiency from optical factors would have to exceed 10% for the overall efficiency of back-contact cells to be higher than vertical devices. We also develop a model of photon recycling appropriate for short-ranged, bulk 2D transport and demonstrate that in perovskites, it produces little change in power conversion efficiency (and small changes in short-circuit voltage) when compared with the standard drift-diffusion equations with the second-order recombination constant is adjusted.