Dissertations / Theses on the topic 'Perovskite'

Orientador: Pascoal José Giglio Pagliuso
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Há muitas décadas os óxidos de metais de transição são tema de grande interesse científico devido à grande variedade de propriedades físicas interessantes que apresentam, com suas possíveis aplicações tecnológicas. Mais recentemente, por exemplo, os óxidos de metais de transição com propriedades multiferróicas ganharam destaque na comunidade científica como potenciais dispositivos magneto-eletrônicos. Muitos óxidos de metais de transição se formam na estrutura cristalina chamada perovskita simples, com simetria cúbica ou distorcida. Muitos outros óxidos podem se cristalizar numa variante da perovskita simples, a chamada perovskita dupla ordenada (PDO), que possui fórmula geral A2B¿B¿¿O6, onde o íon A ocupa os vértices do cubo enquanto os cátions B¿ e B¿¿ se alternam nos centros dos octaedros de oxigênio. Dois compostos com estrutura PDO bastante estudados são o Sr2FeReO6 e Sr2FeMoO6 devido ao fato de apresentarem, entre outras propriedades interessantes, comportamento meio-metálico (halfmetal), magnetrorresistência por tunelamento à temperatura ambiente, ferrimagnetismo com TC acima de 400K em ambos os compostos. As propriedades estruturais, eletrônicas e magnéticas dessas PDOs estão altamente conectadas e são fortemente dependentes do grau de hibridização dos orbitais d dos cátions B¿¿. Assim, se fazem importantes os estudos de novos compostos PDO a fim de se investigar as idéias correntes propostas em literatura e, nesse contexto, reportamos aqui os resultados da síntese e caracterização das séries inéditas Ca2-xLaxFeIrO6 e Sr2-xLaxFeIrO6, onde o Ir, assim como o Re e Mo, é metal de transição, no caso com caráter 5d, e pode assumir diferentes estados de valência. As medidas de magnetização indicaram que estes sistemas tendem a evoluir de antiferromagnéticos nas extremidades das séries, x = 0 e x = 2, para ferrimagnéticos em regiões intermediárias da série. Medidas realizadas no composto de maior magnetização da série de Sr, o Sr1.2La0.8FeIrO6, indicaram que este composto se ordena ferrimagneticamente em torno de 700 K, sendo esta a mais elevada TC já reportada para perovskitas duplas. Medidas de resistividade em função da temperatura indicaram que os compostos apresentam comportamento isolante e praticamente nenhum efeito magneto-resistivo. No composto antiferromagnético Sr2FeIrO6 foi estudada a resistividade sob efeito de pressão e, embora não tenha ocorrido nenhuma transição metal-isolante, ocorre uma diminuição sistemática da resistência do material e da inclinação da curva à medida que a pressão aumenta, indicando um comportamento do tipo isolante de Mott nesse composto. Neste trabalho são apresentados também resultados dos estudos realizados na perovskita TbMnO3. Realizamos neste óxido medidas de susceptibilidade magnética, calor específico, Ressonância Paramagnética Eletrônica (EPR) e absorção de microondas para várias temperaturas. A susceptibilidade magnética e o calor específico confirmaram para a amostra estudada as temperaturas de transição de fase magnética (TN = 41 K) e ferroelétrica (Tlock) já reportadas em literatura. Os espectros de EPR mostraram para todo o intervalo de temperatura uma única linha consistente com uma forma de linha Lorentziana e um valor de g independente da temperatura g = 1.96(3) consistente com Mn3+ em um meio isolante. A largura de linha sofreu um alargamento com a temperatura seguindo uma lei do tipo C/T. Esse alargamento impediu a observação dos espectros de ressonância em torno das regiões de temperaturas das transições de fase magnética e ferroelétrica. Devido à forte dependência da constante dielétrica com a freqüência, as medidas realizadas com a cavidade de campo elétrico não permitiram a observação de qualquer anomalia em torno das temperaturas de transições
Abstract: For many decades the transition metal oxides are subject of great scientific interest because of the wide variety of interesting physical properties and their potential technological applications. More recently, for example, oxides of transition metals with multiferroic properties have been considered as potential magneto-electronic devices. Many transition metal oxides form in the perovskite crystalline structure, with cubic or distorted symmetry. Many other oxides can crystallize in a variant of the simple perovskite, called the ordered double perovskite (ODP), which has the general formula A2B'B''O6, where the A ion occupies the vertices of the cube while the cations B 'and B'' alternate in the centers of the oxygen octahedra. Sr2FeReO6 and Sr2FeMoO6 are two compounds with the ODP structure which were extensively studied due to their interesting properties such as half-metal behavior, tunneling magnetoresistance at room temperature and ferrimagnetic order (TC above 400 K). The structural, electronic and magnetic properties of these ODPs are highly correlated and are strongly dependent on the strong d orbitals hybridization of the of the B'' cations. Therefore, studies of new ODP compounds are important in order to investigate the current ideas proposed in the literature and improve the understanding of their physical properties. We report here our results of synthesis and characterization of the unpublished series Ca2-xLaxFeIrO6 and Sr2-xLaxFeIrO6, where the Ir such as Re and Mo are transition metal, with d character that can assume different valence states. The magnetic measurements indicated that those systems tend to evolve from antiferromagnetics at the ends of the series, x = 0 and x = 2, to ferrimagnetic for intermediate regions of the series. Measurements performed in the compound of higher magnetization in the Sr serie, Sr1.2La0.8FeIrO6 indicated that this compound orders ferrimagnetic around 700 K, which is the highest TC ever reported for double perovskites. Resistivity measurements as a function of temperature indicated that these compounds also exhibit insulating behavior and virtually no magneto-resistive effect. In the antiferromagnetic compound Sr2FeIrO6, the effect of pressure on the resistivity was investigated, and although no metal-insulator transition was seen, there is a systematic decrease of the resistance and the slope of the curve as the pressure increases, indicating a Mott insulator-like behavior in this compound. This work also presents results on the TbMnO3 perovskite. We have performed magnetic susceptibility, specific heat, Electron Paramagnetic Resonance (EPR) and microwave absorption measurements at various temperatures. Magnetic susceptibility and specific heat data confirmed the ocurrence of a magnetic (TN = 41 K) and ferroelectric (Tlock) phase transition. The EPR spectra showed, for the entire temperature range measured, a single Lorentzian line shape and T independent g-value = 1.96 (3), consistent with the resonance of Mn3+ in an insulating environment. The width line broadens with the decreasing temperature following a C/T law. This broadening prevented the observation of the resonance spectra near the magnetic and ferroelectric phase transitions. Because of the strong frequency dependence of the dielectric constant, the measurements performed with the electric field cavity also did not allow observation of any anomaly around the ferroelectric transition
Doutorado
Física da Matéria Condensada
Doutor em Ciências

Uma tecnologia importante para reduzir a quantidade de poluentes atmosféricos lançados na atmosfera, é a utilização de catalisadores, que convertem os gases altamente poluentes como o NO, para outros gases inofensivos ao meio ambiente. Neste trabalho, os óxidos do tipo perovskitas La2CuO4, LaNiO3, LaMnO3, La1,4Sr0,6CuO4, La0,7Sr0,3NiO3 e La0,7Sr0,3MnO3 foram preparados através do método da co-precipitação, caracterizados por difração de raios-X, redução a temperatura programada, fisissorção de nitrogênio e análise química, e, posteriormente avaliados frente as reações de redução de NO com CO, e decomposição direta de NO. Ambas as reações foram testadas nas temperaturas de 400oC e 500oC e o tempo de reação foi de 7 à 10 horas. Através dos ensaios catalíticos verificou-se que o catalisador La2CuO4 apresentou a maior atividade para a reação de redução, e quando substituiu-se parcialmente o lantânio pelo estrôncio, houve uma melhora significativa para todos os catalisadores. As análises de DRX indicaram que, mesmo após os ensaios catalíticos, a estrutura cristalina dos catalisadores foi preservada.
A important technology to reduce the atmospheric pollution is the use of catalysts, to transform high pollutant as NO in other inoffensive gases to the environment. In this work, the perovskite oxides La2CuO4, LaNiO3, LaMnO3, La1,4Sr0,6CuO4, La0,7Sr0,3NiO3 e La0,7Sr0,3MnO3 were prepared through co-precipitation method and characterized by X-ray diffraction and temperature programmed reduction, nitrogen physsisorption and subsequent valued on the reduction of NO by CO and the direct decomposition of NO. These reaction were tested at 400oC and 500oC temperatures and times of reaction between 7 and 10 hours. Through the catalytic tests the La2CuO4 catalyst shown the best activity to the reduce reaction, and when the La is partially substituted by strontium all the catalyst showed a better significant for all the catalysts. The XRD analysis shown that the catalytic structure of the catalysts were preserved after the catalytic test yet.

The synthesis, crystal structures and dielectric properties of perovskite oxynitrides of the type LnTiO2N (Ln = La, Nd and mixtures) and ATaO2N (A = Ca, Sr, Ba and mixtures), have been investigated. The end-member oxynitrides and their associated LaxNd1-xTiO2N, CaxSr1-xTaO2N and BaxSr1-xTaO2N solid solutions were successfully prepared by ammonolysis of the appropriate precursor oxides at temperatures in the range 900-1200ºC. The complete range of LaxNd1-xTiO2N solid solution are orthorhombic perovskites, which show a small increase in unit cell parameters with increasing La content across the range. Compositions in the CaxSr1-xTaO2N series are tetragonal for x < 0.6 and orthorhombic for x > 0.6. BaxSr1-xTaO2N solid solutions are tetragonal for x < 0.5 and cubic for x > 0.5. It was impossible to form any perovskite solid solution in the BaTaO2N-CaTaO2N series. Attempts were made to densify these oxynitrides by hot-pressing and spark plasma sintering, but a characteristic of all these oxynitride structures is that they are stable up to relatively low temperatures (1250oC) with decomposition occurring before any sintering can be achieved. A reducing environment is necessary to prevent oxidation, but this must not be too reducing to promote conversion of the transition metal into a lower oxidation state. As a result, fully dense samples were not obtained and dielectric property measurements could not be made on these samples. As an alternative, pure single phase LaTiO2N, NdTiO2N and LaxNd1-xTiO2N solid solutions were prepared as » 3 μm thick surface layers on dense pellets of the corresponding La2Ti2O7, Nd2Ti2O7 and (LaxNd1-x)2Ti2O7) oxides by ammonolysis. The bulk dielectric properties of these VI coated samples were then measured by LCR bridge techniques. The presence of an oxynitride layer significantly increased the measured dielectric constant of all samples, compared with the pure oxides, but a significantly higher dielectric loss was also observed. This lossy behaviour is believed to be due to the presence of a more conductive region of reduced La2Ti2O7 (of typical composition 2 2 7 La TiIV TiIIIO -x x -z ) situated immediately below the oxynitride layer, produced during synthesis by the presence of hydrogen in the nitriding ammonia atmosphere. The dielectric constant of all these oxynitrides was also measured in particulate form by impedance analysis of slurries and the dielectric constant calculated by a method of mixtures. Compared with the parent oxides, the dielectric constants were noticeably larger, ranging from 100-1500. These values are in reasonable agreement with the limited amount of data available in the literature, and show that this group of materials merits further exploration, providing easier synthesis routes can be developed, which also result in low loss final materials being obtained.

Perovskite solar cells (PSC) performance has risen rapidly the last few years with the current record having power conversion efficiency (PCE) of 22.1 %. This has attracted a lot of attention towards this alternative solar cell that can be manufactured with less energy and toxic material than traditional silicon solar cells. The purpose of this thesis is to reproduce high performance PSC from known recipe by Zhang et al. with potential of PCE reaching above 18 %. The thesis covers the theory regarding how a PSC operates, how they are measured and which parameters are important for a high performance PSC. The thesis includes a detailed manuscript on how to manufacture high performance PSC layer by layer and how to characterize the performance of the cells by IV-measurements. Furthermore, it includes scanning electron microscopy (SEM), by which the cells surface layers and cross-section could be evaluated. The result shows that it is possible to reproduce the PSC from literature and achieve a PCE of 18.8 %. However, the cells PCE decrease by 15 % during 2 hours of constant illumination, due to lack of stability. The manufactured PSC was used to power two catalysts that splits water into O2 and H2 and managed to reach a solar to hydrogen conversion efficiency (STHCE) of 13 %.

Perowskite und Perowskit-artige Materialien sind von großem Interesse, da sie eine Vielzahl von strukturellen und physikalischen Eigenschaften haben, welche die Möglichkeit bieten, sie für unterschiedliche Anwendungen einzusetzen. Die Methode der Liquid-Delivery Metal Organic Chemical Vapour Deposition (LD-MOCVD) wurde gewählt, da sie eine gute Kontrolle über die Zusammensetzung ternärer Oxide und eine hohe Homogenität der Filme ermöglicht. Darüber hinaus können mit dieser Methode Filme hergestellt werden, die aus Elementen bestehen, für welche nur feste Precursor oder welche mit niedrigem Dampfdruck zur Verfügung stehen. Ziel dieser Arbeit war es, mit Hilfe der LD-MOCVD Filme aus SrRuO3, Bi4Ti3O12 und (Na,Bi)4Ti3O12 abzuscheiden und den Einfluss der Wachstumsbedingungen auf die Eigenschaften der Filme zu untersuchen. Zusätzlich wurde die Wirkung der Verspannung, die durch die Gitterfehlanpassung zwischen Substrat und Film entsteht, auf die physikalischen Eigenschaften der Schichten untersucht. SrRuO3 Filme wurden auf gestuften SrTiO3(001), NdGaO3(110) und DyScO3(110) Substraten gewachsen, deren Oberflächenterminierung durch oberflächensensitive Proton-induzierte Auger-Elektronen-Spektroskopie (AES) bestimmt wurde. Die Substrate wurden unter verschiedenen Bedingungen durch Änderung der Temperdauer und -atmosphäre präpariert. Die systematische Untersuchung der Beziehung zwischen Verspannung und Curie-Temperatur von dünnen SrRuO3(100) Filmen erfolgte unter Verwendung von Substraten mit unterschiedlichen Gitterkonstanten. Die beobachtete Curie-Temperatur sank mit erhöhter kompressiver Verspannung und nahm mit erhöhter tensiler Verspannung zu. Um stöchiometrische und epitaktische Bi4Ti3O12(001) Filme zu wachsen, war aufgrund der Flüchtigkeit des Bismuts ein Bi Überschuss in der Precursor-Lösung notwendig. Die Substitution von Bi durch Na in Bi4Ti3O12 wurde zum ersten Mal in LD-MOCVD-Filmen erreicht.
Perovskites and perovskite-like materials are actually of great interest since they offer a wide range of structural and physical properties giving the opportunity to employ these materials for different applications. Liquid-Delivery Metal Organic Chemical Vapour deposition (LD-MOCVD) was chosen due to the easy composition control for ternary oxides, high uniformity and good conformal step coverage. Additionally, it allows growing the films, containing elements, for which only solid or low vapour pressure precursors, having mainly thermal stability problems over long heating periods, are available. The purpose of this work was to grow SrRuO3, Bi4Ti3O12 and (Na, Bi)4Ti3O12 films by LD-MOCVD and to investigate the influence of the deposition conditions on the properties of the films. Additionally, the effect of the strain due to the lattice mismatch between substrates and films on the physical properties of the films was also investigated. SrRuO3 films were grown on stepped SrTiO3(001), NdGaO3(110) and DyScO3(110) substrates, which were prepared under different conditions by changing the annealing time and atmosphere. The termination of the substrates was measured by surface sensitive proton-induced Auger Electron Spectroscopy (p-AES) technique. Another systematic study of the relation between epitaxial strain and Curie temperature of thin SrRuO3(100) films was performed by using substrates with different lattice constants. The observed Curie temperature decreased with compressive and increased with tensile strain. Thin films of Bi4Ti3O12 as well as (Na, Bi)4Ti3O12 were successfully deposited. In order to grow stoichiometric and epitaxial Bi4Ti3O12(001) films, Bi excess in the precursor solution was necessary, due to the volatility of Bi. Substitution of Bi with Na in Bi4Ti3O12 was achieved for the first time for the films deposited by LD-MOCVD.

Perovskite solar cells are a very promising photovoltaic technology which was first reported in 2009 and developed very rapidly. The crystallisation within perovskite films is highly dependent on processing environments, such as temperature, humidity, atmosphere, even light, which makes the fabrication of perovskite solar cells rather lab-dependent and poorly reproducible. One strategy to overcome this problem is to develop a controlled synthesis of perovskite nanocrystals which can then be ordered into films in a separatestep. In this thesis, optimisation of planar perovskite solar cells is carried out by the engineering of perovskite film fabrication methods. Different deposition methods along with different process factors such as solvents, temperature and precursor recipes are compared. One step spin-coating method with the recipe of MAI:PbCl2=3:1 gives the best PCE of 12.1 ± 0.7 % in air with controlled humidity of < 35%, showing high reliability and reproducibility. Doping of TiO2 layers with Zn2+, Sn4+ and Nb5+ ions are carried out to investigate the impacts of doping ions in different valence states on the electron-transporting properties of TiO2 ETLs. The different doping ions shift the flat band potential differently. Zn2+ largely negatively shifts the flat band potential, whereas Nb5+ positively shifts and Sn4+ barely changes the flat band potential of TiO2. the Zn-doping of the TiO2 ETL decreases the performance of the cells. However, when a thin layer of Zn-doped TiO2 is deposited on top of the pristine TiO2 layer as interlayer, the cell efficiency is slightly improved. Following the cell optimisation, to achieve better control over the crystallisation process, a facile flow reactor is developed for the synthesis of MAPbX3 perovskite nanocrystals at low temperature, which are further used for perovskite solar cells. The nanocrystals show narrow size distribution, good emissive properties and high stability. The bandgap of the nanocrystals was easily tuned between 485-745 nm by changing the halide composition. The photoluminescence of the MAPbI3 NCs in the first supernatant can also be tuned by changing the process parameters such as temperature, residence time and ligand concentration. However the impacts are more complex in the second supernatant in toluene with the appearance of multiple peaks in the PL spectra. It could be resultedfrom the formation of smaller NCs due to the reprecipitation of the incompletely removed reactants when added into toluene, or the fragmentation of the NCs upon dispersion into toluene, but better understanding is still needed. In the last part of the thesis, the synthesised MAPbI3 nanocrystals are investigated in perovskite solar cell applications. They have been applied as interlayers at the perovskite HTM interface, where they improved the stability of the devices towards moisture. The nanocrystals and their bulk by-products are also used as active light-absorbing layers for perovskite solar cells, delivering the best PCEs of 0.51% and 1.2% respectively, and notably showing outstanding water resistance. Further improvements in the cell performance could potentially be achieved by the removal of the insulating long chain ligands using effective ligand exchange treatments.

Les cellules photovoltaïques et les diodes électroluminescentes pérovskites requièrent des couches d'injection/extraction de charges, qui sont cruciales pour plusieurs processus importants régissant les performances et la durée de vie. Bien que des études intensives aient été consacrées au développement de couches interfaciales innovantes de type p, des matériaux présentant des propriétés hautement ajustables et une stabilité photochimique élevée restent très recherchés. Cette thèse explore l’utilisation d’oxydes pérovskites comme couches d’interface pour des applications optoélectroniques en raison de leurs propriétés ajustables et de leur stabilité en conditions ambiantes. Des couches minces d’oxydes de pérovskites de type SrTi0.7Fe0.3O3-δ (STFO) sont utilisées comme couches d'extraction ou d'injection de charge pour les cellules solaires et les diodes électroluminescentes à base de pérovskites halogénées. En utilisant la technique de dépôt par laser pulsé (PLD), des couches minces STFO hautement cristallines ont été déposées sur des substrats en verre/FTO à des températures relativement modérées (<400 °C) par rapport aux techniques de dépôt traditionnelles. Des post-traitements thermiques, soit par traitement thermique rapide (RTP), soit par recuit thermique conventionnel (TA), ont été utilisés dans le but d’améliorer la plus grand cristalline des films polycristallins et pour ajuster leurs propriétés optiques et électroniques. Lorsqu'ils sont déposés sur la pérovskite d’oxyde, les couches d'halogénure de pérovskite de type FA0.85Cs0.15Pb(I0.85Br0.15)3 (adaptées à la conversion d'énergie photovoltaïque PV) présentent des tailles de grains plus grandes et un meilleur ordre cristallin par rapport à des films similaires déposés sur une couche interfaciale de type p de référence telle que le PEDOT:PSS commercial. De plus, la présence de l'oxyde a entraîné une nette réduction de la phase de pérovskite halogénée optiquement inactive, illustrant l’impact positif de la couche d’oxyde pérovskite sous-jacente. Les couche de STFO recuites (RTP et le TA) induisent par ailleurs une plus grande durée de vie de l’exciton dans la couche active par rapport au PEDOT :PSS. De façon similaire, la cristallisation d’une pérovskite halogénée quasi-2D de type (PEA)2(MA)PbBr4 (adaptée à la fabrication de LED) sur des couches de STFO a été réalisée avec succès, conduisant à des propriétés similaires que pour des couches références déposées sur du PEDOT:PSS. Dans ce cas, la pérovskite quasi-2D déposée sur STFO a montré une durée de vie de l’exciton relativement longue. Bien que l'intégration de couches minces de STFO dans les deux types de dispositifs ait donné lieu à des performances limitées, ce travail démontre le fort potentiel de la classe des oxydes de pérovskites pour la création de dispositifs tout-pérovskite efficaces et stables
Halide organic-inorganic photovoltaics and light-emitting diodes require suitable charge injection/extraction layers, which are crucial for several important processes governing performance and lifetime. While intensive research has been devoted to developing innovative p-type interfacial layers, materials with highly tunable properties and high photochemical stability remain in demand. This thesis explores oxide perovskites as interlayers for optoelectronic applications due to their stable physical properties under ambient conditions. SrTi0.7Fe0.3O3-δ (STFO) oxide perovskite thin film is utilized as charge extraction/injection layers for planar halide perovskite solar cells and light-emitting diodes. Using pulsed laser deposition (PLD), highly crystalline STFO thin layers on glass/FTO substrates have been successfully processed at relatively moderate temperatures (<400 °C) as compared to traditional deposition techniques. Additional thermal treatments, either by rapid thermal processing (RTP) or conventional thermal annealing, have been applied to the oxide thin films to further improve the larger crystal of the polycrystalline layers, and to tune their optical and electronic properties. When deposited on top of the oxide perovskite, FA0.85Cs0.15Pb(I0.85Br0.15)3 halide perovskite layer (suitable for photovoltaic PV energy conversion) show larger grain sizes and better crystalline order than compared to similar films deposited on top of reference p-type interlayer such as commercial PEDOT:PSS. Furthermore, the presence of the oxide resulted in a clear reduction of the fraction of optically inactive halide perovskite phase. This observation suggests that the perovskite interlayer positively impacts the growth mechanism of the halide perovskite active layer. Finally, annealed STFO layers induce longer exciton lifetime in the halide perovskite active layer, compared PEDOT:PSS. Similarly, the crystallization of the (PEA)2(MA)PbBr4 quasi-2D perovskite (suitable for light-emitting LED applications) on STFO layers was found to be of high quality, leading to comparable properties of layers deposited on top of classical PEDOT:PSS. Moreover, quasi-2D perovskite on STFO showed quite a long exciton lifetime. Although STFO thin films integrated into both halide perovskite PV and LED devices have conducted to limited performance, this work demonstrates the high potential of oxide perovskites towards efficient and stable all-perovskite devices

En aquesta tesi es planteja fabricar una cel·la solar nano-estructurada de perovskita utilitzant alúmina nano-porosa anoditzada (NAA en les seves sigles en anglès) com a suport. Es va escollir la perovskita perquè les cel·les solars d'aquest material han assolit una eficiència molt similar a les cel·les existents de silici. A més a més, són barates i fàcils de preparar. El fet que la cel·la estigui nano-estructurada aportarà estabilitat davant la radiació, la temperatura i la humitat, sent aquest el principal problema d'aquests dispositius. Els nano-porus de la NAA tenen una forma cilíndrica molt ben definida on la grandària es pot controlar fàcilment sent tots els nano-porus iguals, el qual permetrà un major control sobre l'homogeneïtat del material infiltrat. Per tant l'objectiu de la tesi és aplicar la tecnologia de NAA a les cel·les solars de perovskita (CSP). Per això, primer va tenir lloc un procés de familiarització amb la fabricació i caracterització de NAA, així com dels CPSs d'alta eficiència, mitjançant mètodes estàndards coneguts. Un cop es va aconseguir la fabricació de NAA amb diferents mides de porus, la capa barrera d'alúmina que existeix entre l'alumini i el fons del porus va haver de ser eliminada, per poder aprofitar l'alumini (base de la NAA) com a contacte elèctric. Pel qual es va investigar i desenvolupar un nou mètode, ja que els mètodes existents no són adequats per eliminar la capa de barrera de gruixos superiors als 200 nm. Finalment es va estudiar la infiltració dels materials que formen una CSP en els nano-porus, mitjançant mètodes simples de deposició. Es va obtenir una cel·la solar nano-estructurada de perovskita utilitzant com a suport NAA, dels quals resultats d'eficiència són humils, pel fet que l'estructura plantejada en aquest treball és totalment innovadora. Fet que obre un ampli camí per futurs treballs.
En esta tesis se plantea fabricar una celda solar nano-estructurada de perovskita utilizando alúmina nano-porosa anodizada (NAA de sus siglas en inglés) como soporte. Se eligió la perovskita ya que las celdas solares de este material han alcanzado una eficiencia muy similar a las celdas existentes de silicio. Además, son baratas y fáciles de preparar. El hecho de que la celda este nano-estructurada aportará estabilidad frente a la radiación, temperatura y humedad, siendo este el principal problema de estos dispositivos. Los nano-poros de la NAA tienen una forma cilíndrica muy bien definida cuyo tamaño se puede controlar fácilmente siendo todos los nano-poros iguales, lo cual permitirá un mayor control sobre la homogeneidad del material infiltrado. Por lo que el objetivo de esta tesis es aplicar la tecnología de NAA a las celdas solares de perovskita (CSP). Para ello primero tuvo lugar el proceso de familiarización con la fabricación y caracterización de NAA, así como de CSPs de alta eficiencia, mediante métodos estándar conocidos. Una vez se consiguió la fabricación de NAA con diferentes tamaños de poro, la capa barrera de alúmina que existe entre el aluminio y el fondo del poro tuvo que ser eliminada, para poder aprovechar el aluminio (base de la NAA) como contacto eléctrico. Para lo cual se investigó y desarrolló un nuevo método, ya que los métodos existentes no son adecuados para eliminar capa de barrera de espesores superiores a los 200 nm. Finalmente se estudió la infiltración de los materiales que forman una CSP en los nano-poros, mediante métodos simples de deposición. Se obtuvo una celda solar nano-estructurada de perovskita utilizando como soporte NAA, cuyos resultados de eficiencia son humildes, debido a que la estructura planteada en este trabajo es totalmente novedosa. Lo cual abre un amplio camino para futuros trabajos.
In this thesis, the nanostructured perovskite solar cell manufacture using nanoporous anodic alumina (NAA) as a scaffold is proposed. The perovskite was chosen since the solar cells made with this material have achieved very similar efficiency to silicon cells. Also, they are cheap and easy to prepare. The fact that the cell will be nanostructured will provide stability against radiation, temperature and humidity, this being the main problem of these devices. The NAA nanopores have a very well defined cylindrical shape, whose size can be easily controlled, all nanopores being ident, which will allow greater control over the homogeneity of the infiltrated material. Therefore, this thesis aims to apply NAA technology to perovskite solar cells (PSCs). First, the familiarization process with the manufacture and characterization of NAA, as well as of high-efficiency PSCs, through known standard methods were carried out. Once the manufacture of NAA with different pore sizes was achieved, the alumina barrier layer that exists between the aluminium and the bottom of the nanopores had to be removed, to take advantage of the aluminium (base of the NAA) as an electrical contact. For which a new method was investigated and developed since existing methods are not suitable for removing barrier layer thicknesses greater than 200 nm. Finally, the infiltration of the materials that form a PSC within the nanopores was studied, utilizing simple deposition methods. A full working nanostructured perovskite solar cell was obtained using NAA as a scaffold, whose efficiency results are modest because the structure proposed in this work is novel. Which opens a wide path for future work.

Perovskites and perovskite related materials are materials that are candidates for applications such as oxygen permeable membranes, cathodes for SOFC and high-temperature oxygen sensors. This arises from the potential high ionic conductivity and the chemical stability even at low partial pressures of oxygen. From an application point of view, it is important to have knowledge about the oxygen transport properties in these materials. Oxygen transport in mixed conducting oxides involves two inherently different processes; oxygen exchange between bulk gas and surface and solid state diffusion. The objective of this work has been to obtain fundamental understanding of these transport properties in mixed ionic and electronic conductors. For that purpose two materials systems with significant differences in electronic conductivty and oxygen vacancy concentration were chosen as objectives for the investigation, viz.: Sr-substituted LaCoO₃ and Al-substituted SrTiO₃.

All transport properties (diffusion and surface exchange) have been assessed by electrical conductivity relaxation, and the work also evaluate the pros and cons using this specific method to obtain transport data for the materials in question.

In the first two papers (Paper 1 and Paper 2) transport properties are derived for La_1-xSr_xCoO_3-δ (x=0 (LC), 0.2 (LSC-02) and 0.5 (LSC-05)). In Paper 1 “chemical transport coefficients”, D_chem and k_chem, are reported. More fundamental transport coefficients, such as oxygen component diffusion coefficient (D_O) and vacancy diffusion coefficients (D_V), are also deduced and discussed. Activation energies for D_O and D_V, were determined. The activation energies for D_O varies from 279 kJ/mol for LC to 174-222 kJ/mol for LSC-02 and 90-105 kJ/mol for LSC-05, decreasing with increasing Srcontent. The activation energies for the vacancy diffusion coefficient, DV, are smaller than for the component diffusion coefficients and typical values are 77 kJ/mol for LC, 85 kJ/mol for LSC-02 and 75 kJ/mol for LSC-05, that is, almost independent of Sr-content. The enthalpies of vacancy formation decreases with increasing Sr content. The values are 206 kJ/mol for LC, 75 kJ/mol for LSC-02 and 15 kJ/mol for LSC-05, which agrees well with values reported in the literature. However, the vacancy diffusion coefficient showed an unexpected increase at high concentrations of oxygen vacancies, corresponding to δ=0.27-0.30. The phenomena with a PO₂ dependent D_V is discussed.

In Paper 2, the oxygen surface exchange coefficient, k₀, is derived from “chemical values” reported in Paper 1, and used as a basis to deduce probable reaction mechanisms associated with surface exchange. The temperature dependency plots showed that for the composition with x = 0.5, the k0 made a shift in activation energy from ~120 kJ/mol to ~15 kJ/mol above 950 °C. It is suggested that this significant shift in activation energy might be due to an oxygen adsorption/desorption mechanism on the surface becoming rate controlling at high temperatures. The composition with x=0.2 did not show this shift in activation energy. Relations between possible rate controlling reactions and reaction rates (k₀) were established, and formed the basis for discussions on probable rate controlling processes. There are reasons to assume that for oxidation prosesses a rate controlling reaction involving a direct “installation” of an oxygen molecule into two vacancies is dominating, while a dissociation of an oxygen molecule generally gives a better description for a reduction process.

In Paper 3 the oxygen transport properties in SrTi_1-xAl_xO₃ (x=0 (ST), 0.02 (STA-02) and 0.05 (STA-05)) were determined in O2/N2 mixtures. In this contribution the electrical conductivity is also presented in a large P_O2- interval (O₂/N₂- and CO/CO₂-mixtures). Electrical conductivity for pure SrTiO₃ (ST) in terms of P_O2 applied well with defect chemistry reported in the literature. For the two Al-substituted compositions the electrical conductivity followed predicted behaviour at high and low P_O2’s. However, in the medium P_O2 range we were not able to describe the conductivity behaviour in terms of classical defect chemistry. Reasons for the discrepancy is discussed.

D_chem for ST and STA-02 are reported and are, along with their corresponding activation energies, 187 and 104-180 kJ/mol, respectively, in good accordance with values from literature. Furthermore, values for the component diffusion coefficient, D_O, and the vacancy diffusion coefficient, D_V, are reported for ST at 950 °C, the only composition where oxygen vacancy concentrations are available in the literature. Values for kchem in STA-02 and STA-05 are also reported, and show pronounced P_O2 dependencies. For STA-05 the activation energy for k_chem is found to vary between 90 and 105 kJ/mol. Due to a high uncertainty, activation energies are not reported for STA-02.

Reported D_chem and k_chem values for related materials in literature indicate increasing numeric values with decreasing concentration of oxygen vacancies. It is reasoned that this is due to an ever increasing thermodynamic factor with decreasing population of vacancies. The implications for the component diffusion coefficient is discussed.

In Paper 4 the oxygen transport properties in SrTiO₃ pure and with Al were investigated in mixtures of CO/CO₂. D_chem are reported for ST and STA-02 while k_chem are reported for ST, STA-02 and STA-05. The D_chem showed a P_O2-dependency, which can be explained by the variation in the thermodynamic factor. The introduction of Al in the sample increases the value of D_chem, probably due to the introduction of more oxygen vacancies. STA-02 showed a discrete increase in D_chem in the CO-rich atmospheres, this may be due to phase transition or phase separation at P_O2 ~10^-17 atm. The k_chem showed a maximum at P_CO/P_CO2 = 1 for STA-02 and STA-05. This behaviour corresponds well with a rate controlling reaction involving a charged and adsorbed CO₂ molecule. The same maximum is also reported in the literature for BaTiO₃ wihtout and with 1.8 % Al and for La_0.9Sr_0.1FeO₃.

This work has examined chemical diffusion and surface exchange coefficients with electrical conductivity relaxation in two material systems with distinct differences in electrical conductivity and oxygen vacancy concentrations. The main focus has been to elucidate properties of the transport coefficients based on own measurements, but also include transport coefficients from other material systems from literature as references. The vacancy diffusion coefficients have been examined, showing that they increase with increasing concentration of oxygen vacancies in materials where the concentration of vacancies is high. No obvious reason for this behaviour has been found, however, it may be related to a change in activation energy. It is rather well established in the literature that for materials where the concentration of vacancies may be characterized as dilute, we should expect a D_V independent of the population of vacancies. Finally, based on own results and data reported in the literature it appears that with respect to the oxygen surface flux the oxygen vacancy concentration seems to be the property of most importance. That is, for oxidation processes the oxygen exchange flux will increase with vacancy population.

The perovskite family has many novel features including, in certain compounds, either high temperature superconductivity or colossal magnetoresistance (CMR). The electrical and magnetic properties of grain boundaries in these materials also have their own unique properties. These distinct characteristics can be exploited to make devices with a range of potential applications. In this thesis, two types of devices are presented. Superconducting grain boundaries have been studied extensively and can form Jospehson junctions. The grain boundaries have a reduced supercurrent but a substantially enhanced sensitivity to magnetic fields relative to the bulk material. As current in nearby control loops can generate magnetic flux in the grain boundary, Josephson vortex flow transistors can be made. The properties of one type of this device are presented as a function its shape and relative dimensions. Tapering the junction width is shown to produce a substantial current gain over a significant field range. These devices can be used as current based logic gates. In 1995, reports of the low magnetic field response in polycrystalline films of the CMR manganites were published. These low-field effects were attributed to the grain boundaries present in the granular materials. To isolate and maximise this response, a Wheatstone bridge structure has been patterned into various bicrystal CMR films. The properties of the grain boundaries are reported as a function of temperature, the magnitude and orientation of the magnetic field, and also as a function of the grain boundary angle. CMR grain boundaries may replace the giant magnetoresistive read heads which are the current state of the art in magnetic read head technology. The possibility of a common transport mechanism in these two types of materials has been explored and several similarities have been found to exist.

Organo-metallic halide perovskite solar cells (PSCs) are considered as a promising alternative photovoltaic technology due to the advantages of low-cost solution fabrication capability and high power conversion efficiency (PCE). PSCs can be made using a conventional (n-i-p) structure and an inverted (p-i-n) configuration. PCE of the conventional p-i-n type PSCs is slightly higher than that of the inverted n-i-p type PSCs. However, the TiO2 electron transporting layer adopted in the conventional PSCs is formed at a high sintering temperature of >450 °C. The TiO2 electron transporting layer limits the application of conventional PSCs using flexible substrates that are not compatible with the high processing temperature. The hole extraction layer (HEL) in the inverted p-i-n type PSCs can be prepared by low-temperature solution fabrication processes, which can be adopted for achieving high performance large area flexible solar cells at a low cost. Inverted PSCs with a PCE range from 10 to 20% have been reported over the past few years. In comparison with the progresses of other photovoltaic technologies, the rapid enhancement in PCE of the PSCs offers an attractive option for commercial viability. The aim of this PhD project is to study the origin of the improvement in the performance of solution-processable inverted PSCs. The surface morphological and electronic properties of the HEL are crucial for the growth of the perovskite active layer and hence the performance of the inverted PSCs. Enhancement in short circuit current density (Jsc), reduced loss in open circuit voltage (Voc), improvement in cha Organo-metallic halide perovskite solar cells (PSCs) are considered as a promising alternative photovoltaic technology due to the advantages of low-cost solution fabrication capability and high power conversion efficiency (PCE). PSCs can be made using a conventional (n-i-p) structure and an inverted (p-i-n) configuration. PCE of the conventional p-i-n type PSCs is slightly higher than that of the inverted n-i-p type PSCs. However, the TiO2 electron transporting layer adopted in the conventional PSCs is formed at a high sintering temperature of >450 °C. The TiO2 electron transporting layer limits the application of conventional PSCs using flexible substrates that are not compatible with the high processing temperature. The hole extraction layer (HEL) in the inverted p-i-n type PSCs can be prepared by low-temperature solution fabrication processes, which can be adopted for achieving high performance large area flexible solar cells at a low cost. Inverted PSCs with a PCE range from 10 to 20% have been reported over the past few years. In comparison with the progresses of other photovoltaic technologies, the rapid enhancement in PCE of the PSCs offers an attractive option for commercial viability. The aim of this PhD project is to study the origin of the improvement in the performance of solution-processable inverted PSCs. The surface morphological and electronic properties of the HEL are crucial for the growth of the perovskite active layer and hence the performance of the inverted PSCs. Enhancement in short circuit current density (Jsc), reduced loss in open circuit voltage (Voc), improvement in charge collection efficiency (ηcc) through suppression of charge recombination were investigated systematically via controlled growth of the perovskite active layer in solution-processed inverted PSCs. Poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT:PSS) is one of the widely used solution processable conductive materials for hole transporting in different optoelectronic devices. PEDOT:PSS HEL also is a perfect electron blocking layer due to its high LUMO level. However, it has been reported that PEDOT:PSS HEL is related to the deterioration in the stability of PSCs due to its acidic and hygroscopic nature. Modification of PEDOT:PSS using solvent additives or incorporating metallic oxide nanoparticles for improving the processability and the performance of the inverted PSCs were reported. This work has been focused primary on realizing the controlled growth of perovskite active layer via HEL/perovskite interfacial modification using sodium citrate-treated PEDOT:PSS HEL and WO3-PEDOT:PSS composite HEL. Apart from investigating the properties of the modified PEDOT:PSS HELs, the purpose of the work is to improve the understanding of the effect of modified HEL on the growth of the perovskite layer, revealing the charge recombination processes under different operation conditions, analyzing change extraction probability, and thereby improving the overall performance of the PSCs. PCE of >11.30% was achieved for PSCs with a sodium citrate-modified PEDOT:PSS HEL, which is >20% higher than that of a structurally identical control device having a pristine PEDOT:PSS HEL (9.16%). The incident photon to current efficiency (IPCE) and light intensity-dependent J-V measurements reveal that the use of the sodium citrate-modified PEDOT:PSS HEL helps to boost the performance of the inverted PSCs in two ways: (1) it improves the processability of perovskite active layer on HEL, and (2) it enables to enhance the charge extraction efficiency at the HEL/perovskite interface. The suppression of charge recombination in the PSCs with a modified HEL also was examined using photocurrent-effective voltage (Jph-Veff) and transient photocurrent (TPC) measurements. Morphological and structural properties of the perovskite layers were investigated using the scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements. The results reveal that high quality perovskite active layer on the modified HEL was attained forming complete perovskite phase. The surface electronic properties of the modified PEDOT:PSS and pristine PEDOT:PSS layers were studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements. XPS results reveal that treatment of sodium citrate partially removes the PSS unit in the PEDOT:PSS, resulting in an increase in the ratio of PEDOT to PSS from 0.197 for a treated PEDOT:PSS HEL to that of 0.108 for the pristine PEDOT:PSS HEL. UPS measurements also show that there is an observable reduction in the work function of the modified HEL, implying that sodium citrate-modified PEDOT:PSS HEL possesses an improved electron blocking capability, which is beneficial for efficient operation of the inverted PSCs.;The performance enhancement in MAPbI3-based PSCs with a tungsten oxide (WO3)-PEDOT:PSS composite HEL also was analyzed. The uniform composite WO3-PEDOT:PSS HEL was formed on indium tin oxide (ITO) surface by solution fabrication process. The morphological and surface electronic properties of WO3-PEDOT:PSS composite film were examined using AFM, XPS, UPS and Raman Spectroscopy. SEM images reveal that the perovskite films grown on the composite HEL had a full coverage without observable pin holes. XRD results show clearly that no residual of lead iodide phase was observed, suggesting a complete perovskite phase was obtained for the perovskite active layer grown on the composite HEL. The volume ratio of WO3 to PEDOT:PSS of 1:0.25 was optimized for achieving enhanced current density and Voc in the PSCs. It is demonstrated clearly that the use of the WO3-PEDOT:PSS composite HEL helps to improve the charge collection probability through suppression of the charge recombination at the MAPbI3/composite HEL interface. The charge extraction efficiency at the perovskite/PEDOT:PSS and perovskite/composite HEL interfaces were investigated by analyzing the PL quenching efficiency of the MAPbI3 active layer. It is shown that the PL efficiency quenching at the MAPbI3/composite HEL samples is one order of magnitude higher than that measured for the perovskite/pristine PEDOT:PSS sample, suggesting an enhanced hole extraction probability at the MAPbI3/composite HEL interface. The combined effects of improved perovskite crystal growth and enhanced charge extraction capabilities result in the inverted PSCs with a PCE of 12.65%, which is 22% higher than that of a structurally identical control device (10.39%). The use of the WO3-PEDOT:PSS composite HEL also benefits the efficient operation of the PSCs, demonstrated in the stability test, as compared to that of the control cell under the same aging conditions. With the progresses made in improving the performance of MAPbI3-based PSCs, the research was extended to study the performance of efficient PSCs with mixed halide of MA0.7FA0.3Pb (I0.9Br0.1)3. The effect of the annealing temperature on the growth of the mixed MA0.7FA0.3Pb (I0.9Br0.1)3 perovskite active layer was analyzed. It was found that the optimal growth of the mixed perovskite active layer occurred at an annealing temperature of 100°C. UPS results reveal that the ionization potential of 5.76 eV measured for the mixed cation perovskite is lower than that of MAPbI3-based single cation perovskite layer (5.85 eV), while the corresponding electron affinity of the mixed perovskite was 4.28 eV and that for the MAPbI3 layer was 4.18 eV, respectively. The changes in the bandgap and the energy levels of the MA0.7FA0.3Pb (I0.9Br0.1)3 and MAPbI3 active layers were examined using UV-vis absorption spectroscopy and UPS measurements. Compared to the MAPbI3-based control cell, a 23% increase in Jsc, a 15% increase in Voc and an overall 25% increase in PCE for the MA0.7FA0.3Pb (I0.9Br0.1)3 were achieved as compared to that of the MAPbI3-based PSCs. An obvious improvement in charge collection efficiency in MA0.7FA0.3Pb (I0.9Br0.1)3-based PSCs operated at different Veff was clearly manifested by the light intensity dependent J-V characteristic measurements. PL quenching efficiency also shows the charge transfer between MA0.7FA0.3Pb (I0.9Br0.1)3 and PEDOT:PSS HEL is one order of magnitude higher as compare to that in the MAPbI3-based PSCs, suggesting the formation of improved interfacial properties at the MA0.7FA0.3Pb (I0.9Br0.1)3/HEL interface. The impact of incorporating mixed MA0.7FA0.3Pb (I0.9Br0.1)3 perovskite active layer on PCE and the stability of the PSCs was further studied using a combination of TPC measurement and aging test. The stability of MA0.7FA0.3Pb (I0.9Br0.1)3- and MAPbI3-based PSCs with respect to the aging time was monitored for a period of >2 months. The MA0.7FA0.3Pb (I0.9Br0.1)3-based PSCs are more stable compared to the MAPbI3-based PSCs aged under the same conditions. The aging test supports the findings made with the TPC and light intensity dependent J-V measurements. It shows that the improved interfacial quality at the perovskite/HEL and the enhanced charge extraction capability are favorable for efficient and stable operation of MA0.7FA0.3Pb (I0.9Br0.1)3-based PSCs.

A range of electrical characterization techniques previously used for DSSC have been transferred to the study of planar perovskite devices. These include impedance spectroscopy (EIS), intensity modulated photovoltage spectroscopy (IMVS) and open-circuit voltage decay measurements (OCVD). An investigation into the observed response from these measurements has been carried out in order to gain a deeper understanding of device operation. Multiple processes with time constants on the microsecond, millisecond and second timescale were observed. The complimentary frequency and time domain techniques have been employed, showing excellent agreement between the two types of measurement. The high frequency (microsecond) process was found to be purely electronic in nature, which was linked to recombination. The geometric capacitance was shown to dominate this response, with accumulation of charge in the planar perovskite layer not observed. The lower frequency (millisecond and second timescale) processes were found to be linked to the coupling between recombination and the movement of ions. The low frequency EIS and IMVS measurements revealed that the recombination resistance was frequency dependent. The rate of change of the recombination resistance was found to be linked to the diffusion of ionic species. Activation energies for these processes were obtained (EA=0.55-0.66 eV) and shown to be in good agreement to computationally calculated values from literature for iodide vacancy migration. The same slow processes were also studied in the time domain using open-circuit photovoltage rise and decay measurements from well-defined equilibrium conditions. Comparable activation energies were also found using these techniques. The vacancy defect concentration was calculated to be 3x1019 cm-3, which is high enough for ionic double layers at the contacts to completely screen the built-in voltage across the perovskite at equilibrium in the dark. The slow dynamic processes observed under illumination or applied bias are therefore due to the rearrangement of ions in response to a changing electric field. As this rearrangement occurs, the rate of recombination is altered.

This project focuses on simulation performance of perovskite solar cells using two models. One is a simplified model developed for perovskite absorber layer of PSCs by using matlab program to investigate the effect of density of state, relative dielectric permittivity and band gap energy of the perovskite material on the device performance. The other model is based on SCAPS to investigate the influence of hole mobility and band gap offset of different hole transport materials on device performance.

Depuis 5 ans, les pérovskites hybrides organiques-inorganiques sont apparues comme une nouvelle classe de semiconducteurs possédant des propriétés optoélectroniques très intéressantes pour les dispositifs photovoltaïques et émetteurs de lumière. Cette thèse porte sur une étude expérimentale de spectroscopie optique, qui s’inscrit dans le champ d’exploration des propriétés optiques et des effets excitoniques des pérovskites hybrides CH3NH3PbX3 avec X = I ou Br. Nous avons étudié les propriétés optiques de couches minces déposées par spin-coating et de monocristaux élaborés en solution. Les couches minces présentent une structure granulaire et une densité élevée de défauts qui induisent une grande variabilité des propriétés optiques. L’étude des monocristaux nous a permis de mettre en évidence les propriétés intrinsèques du matériau : émission d’excitons libres, couplage électron-phonon, dynamique de recombinaison des porteurs de charge. De plus, nous avons exploré l’impact de la transition de phase orthorhombique-tétragonale sur les propriétés optiques de CH3NH3PbI3. Enfin, nous avons quantifié l’effet de la réabsorption sur les propriétés d’émission des pérovskites hybrides. L’estimation précise de cet effet est particulièrement importante pour l’interprétation des propriétés optiques des pérovskites hybrides et explique la grande hétérogénéité des résultats dans la littérature
In the last five years, hybrid organic-inorganic perovskites have emerged as a novel class of semiconductors owing to their interesting electronic and optical properties for photovoltaic and light-emitting devices. This thesis reports an experimental study using optical spectroscopy to explore the optical properties and excitonic effects of hybrid perovskites such as CH3NH3PbX3 with X = I or Br.We studied the optical properties of spin-coated thin films and solution processed single crystals. Thin films present a granular structure and a high density of defects which induce a great variability of the optical properties. The study of single crystals allows us to highlight the intrinsic properties of material: free exciton emission, electron-phonon coupling and charge carriers recombination dynamics. Besides, we have investigated the impact of the orthorhombic-tetragonal phase transition on the optical properties of CH3NH3PbI3. Finally, we have quantified the effect of reabsorption on the emission properties of hybrid perovskites. The accurate estimate of this effect is particularly important for the interpretation of the optical properties of hybrid perovskites and explains the great heterogeneity of the results in the literature

Les perovskita orgàniques/inorgàniques híbrides han atret molta atenció des que es van introduir per primera vegada en un dispositiu fotovoltaic fa deu anys, obtenint una eficiència de el 3%. Des de llavors, l’eficiència de les cel·les solars de perovskita ha augmentat fins a gairebé igualar l’eficiència de les cel·les fotovoltaiques comercials de silici. A més, permet la fabricació de dispositius flexibles a un preu reduït. A causa de les seves propietats optoelectròniques excepcionalment bones, també s’està duent a terme una intensa recerca per trobar diferents aplicacions d’aquest tipus de materials, ja sigui com a sensors, làsers o díodes emissors de llum. No obstant això, encara s’han d’abordar alguns problemes que presenten, com la inestabilitat química o estructural en condicions ambientals. Per tal de comprendre millor l’estabilitat estructural i el paper exercit per la interacció entre el catió orgànic i la xarxa inorgànica, estudiem les propietats estructurals i optoelectròniques de les perovskita de la família FAxMA1-xPbI3 a diferents pressions (fins a 15 GPa) i temperatures (de 10 a 385 K). La nostra investigació d’aquest material és realitzada per mitjà d’espectroscòpia òptica no invasiva, com fotoluminiscència (PL), Raman i elipsometría. En els articles aquí recopilats es mostra el primer diagrama de fase complet de perovskita de iodur de plom de cations mixtes (formamidinio i metilamonio), en funció de la temperatura i la composició. Aquest diagrama pot servir per avaluar la concentració relativa òptima dels cations orgànics per estabilitzar la fase cúbica pel que fa als canvis de temperatura. Aquests materials també presenten una dependència atípica del bandgap amb temperatura, que en la literatura s’atribueix exclusivament a la renormalització del gap causa de la interacció electró-fonó. No obstant això, és aquest treball mostrem que els efectes d’expansió tèrmica també juguen un paper decisiu en la dependència del gap amb temperatura. De totes les combinacions de la família de perovskita d’halur organometàl·lic, MAPbI3 és probablement el més estudiat a causa de les seves excel·lents propietats. Se sap que la interacció entre el moviment dels cations orgànics i la gàbia inorgànica rígida té un paper decisiu en l’estructura cristal·lina d’aquest material. Per exemple, a causa de el desordre dinàmic de les molècules de metilamonio, la perovskita MAPbI3 adopta una fase cúbica altament simètrica a altes temperatures. Quan es refreda, tant la contracció de la xarxa com la reducció de la simetria a causa d’una transició a una fase ortorrómbica bloquegen les molècules de MA en els buits de la xarxa inorgànica. En els articles aquí compilats observem per primera vegada un efecte similar però a temperatura ambient, aplicant pressió hidrostàtica a el material. En ambdós casos, el bloqueig dels cations de MA es pot observar indirectament a través d’una reducció dràstica dels amples de línia de fonons en els mesuraments Raman. Demostrant, d’aquesta manera, que és possible alterar les propietats vibratòries del material aplicant una pressió controlada. Finalment, la modificació del valor del bandgap i la variació de l’estructura de bandes de sistema mixt FAxMA1-xPbI3 s’avalua en funció de la composició de FA amb elipsometría i fotoluminiscència.
Las perovskitas orgánicas/inorgánicas híbridas han atraído mucha atención desde que se introdujeron por primera vez en un dispositivo fotovoltaico hace diez años, obteniendo una eficiencia de alrededor del 3%. Desde entonces, la eficiencia de las celdas solares de perovskita ha aumentado hasta casi igualar la eficiencia de las celdas fotovoltaicas comerciales de silicio. Además, permite la fabricación de dispositivos flexibles a un precio reducido. Debido a sus propiedades optoelectrónicas excepcionalmente buenas, también se está llevando a cabo una intensa investigación para encontrar diferentes aplicaciones de este tipo de materiales, ya sea como sensores, láseres o diodos emisores de luz. Sin embargo, todavía deben abordarse algunos problemas que presentan, como la inestabilidad química o estructural en condiciones ambientales. Con el fin de comprender mejor la estabilidad estructural y el papel desempeñado por la interacción entre el catión orgánico y la red inorgánica, estudiamos las propiedades estructurales y optoelectrónicas de las perovskitas de la familia FAxMA1-xPbI3 a diferentes presiones (hasta 15 GPa) y temperaturas (de 10 a 385 K). Nuestra investigación de este material es realizada por medio de espectroscopía óptica no invasiva, como fotoluminiscencia (PL), Raman y elipsometría. En los artículos aquí recopilados se muestra el primer diagrama de fase completo de perovskitas de yoduro de plomo de cationes mixtos (formamidinio y metilamonio), en función de la temperatura y la composición. Este diagrama puede servir para evaluar la concentración relativa óptima de los cationes orgánicos para estabilizar la fase cúbica con respecto a los cambios de temperatura. Estos materiales también presentan una dependencia atípica del bandgap con temperatura, que en la literatura se atribuye exclusivamente a la renormalización del gap debido a la interacción electrón-fonón. Sin embargo, es éste trabajo mostramos que los efectos de expansión térmica también juegan un papel decisivo en la dependencia del gap con temperatura. De todas las combinaciones de la familia de perovskitas de haluro organometálico, MAPbI3 es probablemente el más estudiado debido a sus excelentes propiedades. Se sabe que la interacción entre el movimiento de los cationes orgánicos y la jaula inorgánica rígida tiene un papel decisivo en la estructura cristalina de este material. Por ejemplo, debido al desorden dinámico de las moléculas de metilamonio, la perovskita MAPbI3 adopta una fase cúbica altamente simétrica a altas temperaturas. Cuando se enfría, tanto la contracción de la red como la reducción de la simetría debido a una transición a una fase ortorrómbica bloquean las moléculas de MA en los huecos de la red inorgánica. En los artículos aquí compilados observamos por primera vez un efecto similar pero a temperatura ambiente, aplicando presión hidrostática al material. En ambos casos, el bloqueo de los cationes de MA se puede observar indirectamente a través de una reducción drástica de los anchos de línea de fonones en las mediciones Raman. Demostrando, de ésta manera, que es posible alterar las propiedades vibratorias del material aplicando una presión controlada. Finalmente, la modificación del valor del bandgap y la variación de la estructura de bandas del sistema mixto FAxMA1-xPbI3 se evalúa en función de la composición de FA con elipsometría y fotoluminiscencia.
Hybrid organic/inorganic perovskites have attracted a lot of attention since they were first introduced in a working photovoltaic device ten years ago, yielding an efficiency of around 3%. Since then, the efficiency of the perovskite solar cells has risen to almost stand toe to toe with that of commercial silicon photovoltaics. Besides, it allows the fabrication of flexible devices at an inexpensive cost. Due to its exceptionally good optoelectronic properties, there is also an intense research for different applications of this type of materials, such as sensors, lasers or light-emitting diodes. However, they still present some issues that need to be addressed, such as chemical or structural instabilities under ambient conditions. In order to better understand the structural stability, and the role played by the interaction between the organic cation and the inorganic framework, we studied the structural and optoelectronic properties of perovskites of the family FAxMA1-xPbI3 at different pressures (up to 15 GPa) and temperatures (10 to 385 K). We investigated this material by noninvasive optical spectroscopy means, such as photoluminescence (PL), Raman and ellipsometry. In the articles here compiled, the first complete phase diagram of mixed cation (formamidinium and methylammonium) lead iodide perovskites is provided as a function of temperature and composition. This serves to assess the best relative concentration of the organic cations to stabilize the cubic phase with respect to temperature changes. These materials also present an atypical dependence of the bandgap with temperature, which in the literature is ascribed exclusively to a huge electron-phonon renormalization. However, here we show that thermal expansion effects also play a decisive role in the temperature behavior of the fundamental gap. From all the combinations in the family of organometal halide perovskites, MAPbI3 is probably the most studied due to its outstanding optoelectronic properties. It is known that the interplay between the movement of the organic cations and the rigid inorganic cage has a decisive role in the crystalline structure of this material. For instance, due to the dynamic disorder of the methylammonia, MAPbI3 adopts a highly symmetric cubic phase at high temperatures. When cooling down, both the contraction of the lattice and the reduction of symmetry due to a transition to an orthorhombic phase lock the MA molecules in the cage voids. We are able to observe for the first time a similar effect but at room temperature, by applying hydrostatic pressure to the material. In both cases, the locking of the MA cations can be indirectly observed through a drastic reduction of the phonon linewidths in Raman experiments. We have shown, in this way, that it is possible to alter the vibrational properties of the material by applying a controlled hydrostatic pressure. Finally, the tuning of the bandgap and the variation of the band structure of the mixed-system FAxMA1-xPbI3 is evaluated as a function of FA composition with ellipsometry and photoluminescence.

Orientador: Pascoal Jose Giglio Pagliuso
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Um grande número de óxidos de metais de transição se forma em estrutura perovskita simples ou em uma de suas variantes, e muitos deles apresentam propriedades físicas interessantes, como supercondutividade, magneto-resistência colossal e ferroeletricidade. Uma das variantes da estrutura perovskita simples, a perovskita dupla ordenada (PDO), pode se cristalizar em simetria cúbica ou em variantes distorcidas, e possui fórmula geral A2B¿B" O6, em que o íon A ocupa os vértices do cubo enquanto os cátions B¿ e B¿¿ se alternam nos centros dos octaedros de oxigênio. Dentre os compostos com estrutura perovskita dupla já reportados na literatura, Sr2FeRe O6e Sr2FeMoO6são particularmente interessantes devido à suas interessantes propriedades físicas e à sua potencialidade como dispositivos magneto-eletrônicos. As propriedades estruturais, de transporte e magnéticas desses materiais estão altamente conectadas, e acredita-se que essas propriedades das PDO sejam fortemente dependentes do grau de hibridização dos orbitais d dos cátions B" (ex.: Re e Mo). Portanto, para se comprovar a potencialidade dos compostos PDO como dispositivos magnetoeletrônicos, além entender os mecanismos microscópicos responsáveis por suas propriedades magnéticas e eletrônicas, é essencial que se investigue em detalhes outros exemplos de PDOs que possam confirmar as idéias correntes propostas na literatura. Nesse trabalho descrevemos o processo de síntese e as propriedades físicas da série inédita Ca2-xL axFeIrO6, onde o Ir, assim como o Re e o Mo, é metal de transição, no caso com caráter 5d, e pode possuir diferentes estados de valência. Espectros de difração de raios-x e Refinamento Rietveld mostraram que os compostos da série se cristalizam em uma estrutura monoclínica, com grupo espacial P21/n, com uma grande presença de desordem catiônica nos sítios Fe/Ir. Medidas de magnetização indicaram que, aparentemente, os compostos tendem a evoluir de antiferromagnéticos nas extremidades da série, x = 0 e x = 2, para ferromagnéticos em regiões intermediárias da série. Medidas de espectroscopia Mössbauer mostraram que a valência do ferro é +3 ao longo da toda série, de modo que, possivelmente, a mudança na natureza das interações magnéticas estaria sendo causada pela alteração da valência do Ir devido a dopagem com La. Medidas de calor específico revelaram uma anomalia característica de uma transição magnética somente para a mostra de x = 0. Foram feitas medidas de resistividade em função da temperatura, em que se observou que os materiais apresentam comportamento isolante e praticamente nenhum efeito magneto-resistivo. Para as amostras no centro da série (em torno de x = 1,0) a presença de loops de histerese nas curvas MxH e um comportamento irreversível nas curvas de MxT sugerem uma competição entre fases ferrimagnéticas e antiferromagnética para esta região de concentração. Qualitativamente, nossos resultados podem ser interpretados considerando-se a mudança de valência do Ir, as regras de Goodenough-Kanamori-Anderson e a presença de desordem catiônica
Abstract: Perovskite structure and its variants host a great number of transition metals oxides that present a variety of interesting physical properties such as superconductivity, ferroelectricity and colossal magneto-resistance. A variant of the simple perovskite structure, the ordered double perovskite (ODP), also can grow in cubic (or lower) symmetry, with a general formula A2B¿B" O6, where the cation A occupies the vertices of the cube while B¿ and B" sits alternately at the center of the oxygen-octahedron. Among the compounds with ordered double-perovskite structure, Sr2FeReO6 and Sr2FeMoO6 are particularly interesting due to their interesting physical properties and their potentiality as magneto-electronic devices, having highly connected structural, transport and magnetic properties. It has been proposed that the magnetic and conducting ground states of ODP systems are strongly dependent on the delocalization level of the cation B" 5d electrons (ex: Re, Mo). However in order to prove the potentiality of composites ODP as magnetoeletronics devices, and to understand the microscopical mechanisms responsible for its magnetic and electronic properties, it is necessary to further investigate other examples of ODP that can confirm the current ideas proposed in literature. In this work we have synthesized and studied the Ca2-xLaxFeIrO6 series, where Ir, as well as Re and Mo, is transition metal with a 5d character which can possess different valence states. X-rays diffraction spectra and Rietveld Refinement analyses have shown that the Ca2-xLaxFeIrO6 compounds series crystallized in a monoclinic structure, space group P21/n, with an unavoidable cationic Fe/Ir site disorder. Measurements of temperature/field dependent magnetization have indicated that the magnetic interactions in these compounds evolves from antiferromagnetic in the extremities of the series, x = 0 and x = 2, for ferromagnetics in intermediate regions of the series. Mössbauer spectroscopy measurements revealed that the valence of the Fe is +3 in the whole series, in a way that the change in the nature of the magnetic interactions can be possibly caused by the variation of Ir valence due the La doping. Specific heat measurements have revealed an anomaly associated to a magnetic phase transition only for the x = 0 compound while measurements of electrical resistivity as a function of the temperature have shown insulating behavior and absence of magneto resistance for all studied samples. For the samples with x ~ 1.0, the presence of ferromagnetic loops and ZFC and FC hysteresys in the MxT curves indicates the competition between ferrimagnetic and antiferromagnetic phases in a disordered system. Qualitatively, all our results can be understood in terms of a valence changes in the Ir ions, the Goodenough-Kanamori-Anderson rules and the role of cationic disorder
Mestrado
Física da Matéria Condensada
Mestre em Física

O mundo moderno é extremamente dependente de combustíveis fósseis como fonte de energia primária e essa forte dependência leva a problemas políticos, econômicos e ambientais. Como possível solução a esses problemas tem-se as células combustíveis, pois são dispositivos que geram energia elétrica limpa diretamente de reações eletroquímicas produzindo, além da energia elétrica, apenas calor e água. Logo, percebe-se que essas células são fontes de energia confiáveis, renováveis e não poluentes, que contribuem para o desenvolvimento sustentável. Devido a isso, este trabalho teve como objetivo principal a síntese (por um método inédito) e a caracterização de materiais porosos a base de cromita de lantânio, LaxSr1-xCryFe1-y(Mn1-y)O3-?, para possível implementação como material de anodo e catodo de célula a combustível de óxido sólido (SOFC). Particularmente, estudos com anodos nos quais o transporte eletrônico é feito por materiais cerâmicos ao invés de metais são a área mais promissora na pesquisa recente. Além disso, materiais a base de manganita de lantânio dopadas com estrôncio são na atualidade os materiais mais usados na construção do catodo da SOFC. Nesta tese os materiais foram sintetizados pelo método sol-gel com agentes direcionador e dilatador de estrutura, resultando em materiais porosos em forma de esponja e com a estrutura perovskita, porém com fases espúrias. Foi estudada a influência do processamento de calcinação e de dopagem sobre as estruturas cristalográficas e porosas dos materiais. A maior temperatura de calcinação favoreceu a formação da estrutura perovskita com a retenção da fase romboédrica e reduziu a presença das fases espúrias, porém reduziu a porosidade, principalmente dos menores mesoporos, e a área superficial dos materiais. Por outro lado, ao dopar o sítio B os materiais com 75 %mol de La e calcinação a 1000 °C, observou-se a formação de um maior volume de mesoporos, ao mesmo tempo, que produziu uma maior quantidade de mesoporos menores e favoreceu a retenção da fase romboédrica da estrutura perovskita. Quanto ao comportamento eletrocatalítico, as células com eletrodos confeccionados a partir de La0,33Sr0,66Cr0,33Mn0,33O3-? apresentaram os melhores resultados tanto para anodo como para catodo entre as amostras avaliadas na tese. Além do mais, foram obtidos dois materiais, um cerâmico (La0,33Sr0,66Cr0,33Mn0,33O3-?) e um compósito cerâmico (La0,33Sr0,66Cr0,33Mn0,33O3-? + ZrO2 8%mol Y2O3) bons candidatos a catodo da SOFC. Esses materiais possuem uma composição química não encontrada na literatura para tal finalidade, ou seja, são inéditos.
The modern world is extremely dependent on fossil combustibles as primary source of energy and, this dependence brings political, economic and ambient problems. As a possible solution to these problems are the fuel cells, because they are devices that generate clean electric energy directly from electrochemical reactions, producing besides electric energy, heat and water. Therefore, these cells are reliable, renewable and non-pollutant sources, that contribute to the sustainable development. Related to it, this work had the main goal the synthesis (by a new method) and characterization of porous materials based on lanthanum chromite, LaxSr1-xCryFe1-y(Mn1-y)O3-?, for possible use as anode and cathode material of Solid Oxide Fuel Cell (SOFC). In particular, studies of anodes in which the electronic transport is performed by ceramic materials instead of metals are the most promising recent research area. Moreover, materials based on lanthanum manganite doped with strontium are now a days the more used materials for SOFC cathodes. In this thesis, the materials were synthesized by the sol-gel method with directing and swelling structure agents, resulting in porous sponge materials with perovskite structure, but having spurious phases. The influence of the calcination and doping of the materials upon the crystallographic and porous structures were studied. Higher calcination temperature favored the formation of the perovskite structure and reduced the presence of spurious phases, but reduced the porosity, mainly of smaller mesopores and the surface area. On the other hand, doping the B site in materials with 75 %mol of La and the calcination at 1000 oC produced a higher mesopore volume, a higher amount of small mesopores and favored the retention of the rhombohedral perovskite structure. Regarding the catalytic behavior, the cells with electrodes of La0,33Sr0,66Cr0,33Mn0,33O3-? presented the best results as anode and cathode among the evaluate samples. Moreover, two materials were obtained, a ceramic one, (La0,33Sr0,66Cr0,33Mn0,33O3-?) and a ceramic composite, (La0,33Sr0,66Cr0,33Mn0,33O3-? + ZrO2 8%mol Y2O3), good candidates as SOFC cathodes. These materials have a chemical composition, which were not reported in the literature for this application, being unique.

In the present thesis the reported results let us to conclude that the developed aqueous sol–gel procedure could be successfully used for the low-temperature synthesis of monophasic perovskite gadolinium aluminate ceramics. It was also demonstrated that the selection of raw materials for the sol–gel processing should be done with care. Monophasic perovskite structure gadolinium aluminate GdAlO3 has been synthesized by sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material and 1.2 ethanediol, natural tartaric acid, citric acid or mixture of citric acid and ethanolamine as complexing agent. According to the XRD results, the Gd2O3 instead of Gd(NO3)36H2O preferably should be used as starting material for the preparation of GdAlO3. Monophasic perovskite structure strontium substituted gadolinium aluminate Gd0.90Sr0.10AlO3– was synthesized by the same sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material, strontium nitrate as strontium ion source and EDTA as complexing agent. It was proved that better gadolinium ion source is gadolinium oxide than gadolinium nitate hexahydrate, better complexing agent – EDTA than natural tartaric acid in the synthesis of GdAlO3 and strontium substituted Gd1–xSrxAlO3– (x = 0.10, 0.25, 0.50, 0.75) samples. According to the XRD results, increasing amount of strontium reduces amount of peaks of perovskite structure gadolinium aluminate. Besides, 800 ºC temperature is too low for the... [to full text]
Šiame darbe vienfazis perovskito struktūros gadolinio aliuminatas GdAlO3 zolių-gelių metodu buvo gautas 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, kompleksus sudarančiu reagentu – 1,2 etandiolį, natūralią vyno rūgštį, citrinų rūgštį arba citrinų rūgšties ir etanolamino mišinį. Sintetinant GdAlO3, gautų tyrimų rezultatai parodė, kaip yra svarbu zolių gelių procese pasirinkti tinkamas pradines medžiagas bei kompleksus su metalais sudarančius reagentus: gadolinio jonų šaltiniu naudojant Gd2O3 buvo gauti geresni rezultatai nei naudojant Gd(NO3)3•6H2O, vienfazis GdAlO3 gali būti sėkmingai susintetintas kompleksus sudarančiu reagentu naudojant tiek natūralią vyno rūgštį, tiek 1,2 etandiolį, tiek citrinų rūgštį, tiek citrinų rūgšties ir etanolamino mišinį. Nustatyta, kad sintetinant stronciu pakeistus gadolinio aliuminatus Gd1–xSrxAlO3– (x = 0,10, 0,25, 0,50, 0,75), tinkamesnis gadolinio jonų šaltinis yra gadolinio oksidas nei gadolinio nitratas heksahidratas, kompleksus sudarantis reagentas – EDTA nei natūrali vyno rūgštis. Vienfazis perovskito struktūros stronciu pakeistas gadolinio aliuminatas Gd0,90Sr0,10AlO3– buvo susintetintas zolių-gelių metodu 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, stroncio jonų šaltiniu – Sr(NO3)2, kompleksus sudarančiu reagentu – EDTA. Be to, 800 ºC iškaitinimo temperatūra yra per maža šiems junginiams sintetinti. Pirmą kartą pasiūlytas vandeninis zolių gelių metodas lantano ir gadolinio kobaltatams... [toliau žr. visą tekstą]

Notre etude experimentale de la perovskite kznf#3 montre que les valeurs de la conductivite electrique a haute temperature ne sont pas caracteristiques d'un electrolyte solide. Nous attribuons le transport ionique a la migration des lacunes de fluor creees par dissolution de l'oxygene dans le reseau. L'energie de migration de la lacune de fluor est d'environ 0,9 ev. Par une etude en dynamique moleculaire de la perovskite mgsio#3 nous mettons en evidence lors du chauffage a la pression constante de 310 kb, une transition de phase orthorhombique-quadratique, qui s'amorce a 2600 k. Cette transition de phase s'accompagne d'un mode acoustique de phonons caracterise par un mouvement de precession des octaedres sio#6. Dans la phase quadratique, sans introduire de defauts dans le systeme, nous mettons en evidence une diffusion rapide de l'oxygene, comparable a celle d'un electrolyte solide. Cette forte mobilite anionique semble etre associee au mode acoustique observe a la transition de phase. Les proprietes elastiques et thermodynamiques sont fortement modifiees a la transition de phase; le module de cisaillement et les vitesses sismiques chutent, la chaleur specifique et la dilatation thermique augmentent. La temperature de la transition de phase croit avec la pression; a haute temperature dans la phase orthorhombique, une baisse de la pression accentue les distorsions de la structure; aux basses pressions, la phase perovskite se destabilise

This thesis focuses on the study of hybrid perovskites properties for the purposes of photovoltaic applications. During the almost four years PhD project that has lead to this thesis the record photovoltaic efficiency for this technology has in- creased from 10.9% to 22.1%. Such a significant pace of development can be com- pared with few other materials. It is for this reason that hybrid perovsites have at- tracted impressive research efforts. We approached the study of such unique ma- terials using computational ab-initio techniques, and in particular Density Func- tional Theory. We considered different materials, but most of the attention was concentrated on MAPI (CH 3 NH 3 PbI 3 ). The results are divided in three chapters, each exploring a different material prop- erty. The first chapter reports the electronic structure of the material bulk, sur- faces, and other electronic-related properties such as the rotation barrier for the organic component and the Berry phase polarization. The second chapter focuses on the vibrational properties primary employing the harmonic approximation but also extends to the quasi-harmonic approximation. The outcome of these calculations permitted us to calculate theoretical IR and Ra- man spectra which are in good agreement with different experimental measure- ments. The quasi-harmonic approximation was used to calculate temperature dependent properties, such as the Grüneisen parameter, the thermal dependence of heat capacity and the thermal volumetric expansion. The third and last chapter reviews the thermodynamic properties of binary halide compounds. The cobination of ab-initio calculations with the generalised quasi- chemical approximation has allowed to study the stability of mixed composition perovskites. The results certified a set of stable structures that could stand at the base of observed phenomena of photo-degradation of hybrid perovskite based devices. All three chapters have been written to understand the chemical and physical behaviour of hybrid perovskites and to extended and contribute to the under- standing of experimental work.

Sr\(_2\)Co\(_2\)O\(_5\) with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters \(a\) = 5.4639(3) Å, \(b\) = 15.6486(8) Å and \(c\) = 5.5667(3) Å based on refinement of NPD data collected at 4K. Electron microscopy revealed L R L R intralayer ordering of chain orientations which requires a doubling of the unit cell along the \(c\) parameter, consistent with the assignment of the space group \(Pcmb\). However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group \(Imma\) appears more appropriate. Low-temperature fluorination of this material and the related brownmillerite phases Sr\(_2\)CoFeO\(_5\) and Sr\(_2\)Fe\(_2\)O\(_5\) has been found to give different products depending on the starting material. Sr\(_2\)Co\(_2\)O\(_5\) and Sr\(_2\)CoFeO\(_5\) are amenable to fluorine insertion reactions with corresponding oxidation of transition metal cations, forming cubic perovskite phases of the nominal compositions SrCoO\(_{2.5}\)F\(_{0.5}\) (\(a\) = 3.8574(3) Å) and SrCo\(_{0.5}\)Fe\(_{0.5}\)O\(_{2.5}\)F\(_{0.5}\) (\(a\) = 3.872(2) Å). Fluorination of Sr\(_2\)Fe\(_2\)O\(_5\), however, results in a 1:1 mixture of two phases that were assigned to SrFeO\(_2\)F (\(a\) = 3.9481(3) Å) and SrFeO\(_{3-δ}\) (\(a\) = 3.8775(3) Å). Fluorine insertion into the oxygen defect superstructure manganite Sr\(_2\)MnO\(_{3.5+x}\) has been shown by TEM to result in two levels of fluorination. In the higher fluorine content sections, the fluorine anions displace oxygen anions from their apical positions into the equatorial vacancies, thus destroying the superstructure and giving a K\(_2\)NiF\(_4\) type structure (\(a\) = 3.8210(1) Å and \(c\) = 12.686(1) Å). Ce\(_2\)MnN\(_3\)F\(_{2-δ}\) with tetragonal symmetry (\(P4/nmm\) \(a\) = 3.8554(4) Å and \(c\) = 13.088(4) Å based on neutron powder diffraction) and a structure related to LaSrMnO\(_4\)F has been synthesised via low-temperature fluorination of the ternary nitride Ce\(_2\)MnN\(_3\). Two F\(^-\) anions are inserted but no F\(^-\)/N3\(^-\) substitution takes place, instead a structural rearrangement whereby one F\(^-\) expands the manganese coordination from four to six occurs and the second F\(^-\) inserts in alternate layers of interstitial sites, in a staged fashion. The effective magnetic moment in Ce\(_2\)MnN\(_3\)F\(_{2-δ}\), μ\(_{eff}\) = 5.38 μ\(_B\), is consistent with an intermediate value between that of Mn\(^{3+}\) (4.9 μ\(_B\)) and Mn\(^{2+}\) (5.9 μ\(_B\)) supporting the proposed stoichiometry, Ce\(_2\)MnN\(_3\)F\(_{2-δ}\).

This thesis reports on the chemical vapour deposition (CVD) growth of carbon nanostructures (mainly carbon nanotubes (CNTs)) on perovskite oxide surfaces with the aid of various catalysts. Two types of perovskite oxide, single crystal SrTiO3 (001) and polycrystalline BaSrTiO₃, have been used as catalyst supports (in metal-catalyst-involved CVD routes) or as catalysts (via metal-catalyst-free CVD routes) for the growth of carbon nanostructures. In metal-catalyst-involved cases, SrTiO₃ (001) single crystal has been proven, for the first time, to serve as a substrate for the growth of CNTs. Fe and Ni catalysts can be tailored in a controllable manner on SrTiO3 (001) surfaces prior to the CNT synthesis, forming truncated pyramid shaped nanocrystals with uniform size distributions. The growth of vertically aligned CNT carpets was realised with the aid of Fe on SrTiO₃ (001) surfaces, and it was further found that the CNTs grow via a base growth model. Furthermore, it is possible to grow helical carbon nanostructures on BaSrTiO3 substrates by introducing a Sn catalyst into the system. The synthesised helical carbon nanostructures follow a tip growth mode, where the structural and chemical aspects of catalyst particles gave rise to a wide range of carbon morphologies. CNTs were also grown on single crystal SrTiO₃ (001) and polycrystalline BaSrTiO3 substrates via metal-catalyst-free routes. The surface-roughness-tailored growth of CNTs was surprisingly achieved on a series of engineered SrTiO₃ (001) surfaces, where a correlation between the surface roughness/morphology of the substrates and the relevant catalytic activity was revealed. The growth of CNTs arises because the catalyst fabrication methods lead to the formation of SrTiO₃ asperities with nanoscale curvatures, over which the CNTs are generated throughout a lift-off process. Facet-selective growth of CNTs was observed on polycrystalline BaSrTiO₃ surfaces, where BaSrTiO₃ (110) facets lead to the growth of CNTs on them, whereas the (001) facets result in no growth at all. This observation was further analysed in the content of the adsorption and diffusion of carbon species on distinct BaSrTiO₃ facets, before reaching the conclusion that the formation of CNTs occurs through a metal-free, stack-up process driven by the assembly of the carbon fragments.

This thesis explores a range of photoactive metal halide perovskite materials for use in photovoltaic applications. These materials are of huge interest due to their outstanding optoelectronic properties which result in high photovoltaic power conversion efficiencies. In particular, this thesis discusses perovskites with stoichiometry ABX₃ where A is a singly charged cation, for example methylammonium (MA), B is predominantly lead (Pb²⁺), and X is iodide (I-) and/or bromide (Br^-). At present the commercial applications of these materials are limited by the chemical instability of the A-site cation. In this thesis, the effect of chemical substitution of the A-site is investigated as a way to increase the stability of the perovskite material. Full replacement with the inorganic cation caesium (Cs⁺) is shown to significantly improve the chemical stability. However, the inorganic lead halide perovskites with ideal bandgaps for photovoltaic applications exhibit structural instability. Routes to achieve both chemical and structural stability for these perovskites are discussed. Consequently, this thesis represents pioneering work in the field of inorganic halide perovskites and will greatly assist the development of stable inorganic perovskite materials for optoelectronic applications such as tandem photovoltaics and LEDs. Chapters 1 and 2 of this thesis present the motivation for perovskite materials to be used in solar cells, along with relevant background information about these materials and solar cell operation in general. Chapter 3 details the methods utilised in the experimental results chapters which follow. The first experimental results chapter, Chapter 4, shows how incorporation of Br^- in place of I^- in CsPbI₃ leads to increased ambient stability of the perovskite structure, and the first solar cells with CsPbI₂Br as the absorbing photovoltaic material are reported. Chapter 5 remedies the deficit of information about the optoelectronic properties of the CsPbI_3-xBr_x (0 ≤ x ≤ 3) perovskites through magneto-optical measurements on thin-films. These measurements raise questions about the room temperature perovskite structure of the CsPbI_3-xBrx compositions with small x, previously thought to be cubic perovskite, which is shown in Chapter 6 to be an orthorhombic perovskite polymorph. This finding motivates preliminary work presented in Chapter 7 aimed at chemical stabilisation of this orthorhombic perovskite polymorph. Finally, Chapter 8 summarises the work presented in this thesis, and recommends further research for the development of stable perovskite materials for photovoltaics.

Magister Scientiae - MSc (Biotechnology)
Organometallic halide perovskite solar cells have developed as one of the most promising contenders for the production of solar cells. The generic perovskite (PS) structure ABX3 allows manufacturing a broad range of PS materials by simple modification of building blocks A, B, and X (A = organic group, B = metal, and X = halide). The preparation of a series of solution-processed solar cells based on methylammonium (MA) lead halide derivatives, CH3NH3PbX3 (X=Cl, Br, and I) has been reported. These solar cells showed tunable optical properties depending on the nature and ratio of the halides employed. The photovoltaic performance of perovskite solar cells is dependent on the absorption of the light spectrum. The photovoltaic performance can be improved by widening the light absorption towards the near infrared spectrum. A panchromatric material that absorbs all the light from the ultra-visible to near infrared of the solar spectrum is an ideal photoactive layer. The optimal bandgap for single junction solar cells is known to be 1.1 and 1.4 eV. However, the bandgaps of methylammonium lead trihalide perovskites are beyond this range, and this inspired the realization of the lower bandgap perovskites.
2021-08-31

Global energy consumption has gradually increased in the last few decades. Limited fossil fuels sources call for research on sustainable and renewable energy resources to meet future demand. The direct conversion of sunlight into electricity is one of the most promising ways to meet future energy demands with minimal environmental impacts. Solar technology provides an ecofriendly and renewable energy route to convert photon energy into electricity. Since 2009, metal halide perovskite solar cells have achieved the most notable progress in the field of photovoltaics with a certified power conversion efficiency of 25.7%, outperforming many proven commercial photovoltaic technologies, e.g., multicrystalline Si and CdTe-based solar cells. However, long-term stability and the toxicity of lead are two major concerns that hinder further development of perovskite solar cells. To tackle the toxicity issue, several lead-free alternatives have been proposed, with bismuth considered one of the most promising substitutes for lead due to their similar electronic configurations. This thesis aims to improve the efficiency of bismuth-based solar cells using film fabrication, doping and compositional tuning, and interface engineering. Formation of the heterojunction structure by introducing proper electron donor/acceptor with Bi-based light absorbers is also explored for several bismuth-based solar cells. Bismuth triiodide is the most fundamental material in the bismuth family and possesses interesting optical properties. Chapter 2 reports on a facile low-temperature solution processed bismuth triiodide/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) binary quasi-bulk heterojunction solar cell. The employment of antisolvent and solvent vapor annealing were found effective to improve film quality and enlarge the grain size of bismuth triiodide nanoparticles. To promote the separation of the excitons, the bismuth triiodide and PCBM layers were innovatively utilized as the electron donor and acceptor. The effective separation of excitons at the bismuth triiodide/PCBM binary quasi-bulk heterojunction was experimentally confirmed by Kelvin probe force microscopy. After optimization, a champion power conversion efficiency (PCE) of 1.5% with record short-circuit current density was achieved in an inverted solar cell. This work provides a new means to enhance the performance of bismuth-based solar cells. An all inorganic bismuth-based CsBi3I10 solar cell was recently proposed as a light absorber due to its appropriate bandgap, long carrier lifetime, and long stability. In Chapter 3, an air-processed, antisolvent-free, and low-temperature method is proposed for synthesizing high-quality CsBi3I10 films. The structural and electronic properties of CsBi3I10 were studied by density functional theory (DFT) calculations. For film fabrication, nitrogen gas was applied to accelerate the evaporation of the solvent and hasten oversaturation of the solution during spin-coating. In addition, the subtle introduction of PCBM in the precursor solution facilitated the excitation separation, which enhanced performance. The solar cell based on CsBi3I10/PCBM film yielded a high PCE of 1.2% with great light-soaking stability. In Chapter 4, an effective doping strategy is proposed to improve the crystallization and optoelectronic property of CsBi3I10 thin films. The retarded crystallization during thermal annealing was observed for antimony (Sb)-incorporated CsBi3I10 films. The Sb-doped CsB3I10 films’ morphologies changed dramatically as evidenced by surface scanning electron microscope (SEM). In addition, reduced bandgaps were observed for Sb-doped CsB3I10 films, which could contribute to better light absorption. In addition, suitable channels for constructing heterojunction structures were created via solvent engineering to facilitate the charge transfer. The solar cell based on optimized film produced a PCE of 1.54% with excellent reproductivity. This work provides insight into manipulating the crystallization of Bi-based perovskite films via alloying and solvent engineering. Halide perovskite solar cells have demonstrated high power conversion efficiency. Compositional engineering and surface passivation technologies have drawn great attention to enhance their energy conversion efficiency and moisture resistance. In Chapter 5, the density functional theory method is employed to understand: 1) the effects of compositional engineering at the A site of perovskites; 2) the 3-butenoic acidbased passivation layer on the structural, electronic, and optical properties of halide perovskites. It was found that the electronic and optical properties of CsPbI3 can be tuned by the mixing of caesium (Cs) and formamidinium (FA) cations. Moreover, the calculation of adsorption energies on mixed-cation Cs1−xFAxPbI3 surfaces revealed much stronger adsorption strength of 3-butenoic acid compared to that of CsPbI3, which will facilitate blocking of the interaction between perovskite surfaces and water molecules. The calculated results indicated that adopting such an organic molecule as a passivation layer does not compromise the excellent electronic and optical properties of Cs1−xFAxPbI3 perovskites. The theoretical understanding of A cation engineering and organic molecule-based surface passivation in this work will help improve the overall performance of perovskite solar cells. This thesis makes significant contributions to the development of Bi-based solar cells and the passivation layer of perovskite through: (i) designing heterojunction structure by introducing proper electron donor/acceptor to facilitate exciton separation in Bi-based materials for high-efficiency solar cells; (ii) controlling film crystallization via suitable solvent engineering, such as antisolvent and solvent vapor annealing, and applying gas blowing during spin-coating to manipulate growth and crystallization so as to improve the morphology and quality of Bi-based films; (iii) utilizing doping strategy to retard fast crystallization as well as tuning the optoelectronic properties of Bi-based films; (iv) proposing an air-prepared method for low-temperature film fabrication; and (v) theoretically studying the compositional engineering and surface passivation technologies of perovskite solar cells.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

L'energia fotovoltaica s'ha convertit en una de les alternatives més populars com a font d'energia renovable. Es basa en la transformació directa de radiació solar en electricitat. Es troba disponible a escala global i a més no necessita de cap transformador per convertir l'energia mecànica en energia elèctrica, el que fa que sigui fàcil d'implementar. Avui en dia, el material més utilitzat per a aplicacions fotovoltaiques segueix sent el silici. En canvi, el desenvolupament de noves tecnologies, més barates, fàcils de processar i que a més poden utilitzar-se en substrats flexibles, ha sorgit com a alternativa al silici. De totes elles, les perovskita basades en halurs de plom s'han convertit en una de les millors opcions per a la comunitat científica a causa de les excel·lents propietats fotovoltaiques que presenta. Tot i que les eficiències dels dispositius preparats amb perovskita han arribat al 25%, un valor que es troba molt proper al seu màxim teòric, els processos que tenen lloc en aquests dispositius encara no són del tot coneguts. En aquesta tesi es tracta d'obtenir informació sobre els processos dels transportadors de càrrega, des de com es generen fins a la recombinació, tant en les interfícies com a l'interior del propi material. Per això, s'han utilitzat diferents tècniques de caracterització avançades com el fotovoltatge transitori (TPV), la fotocorrent transitòria (TPC), l'extracció de càrrega (CE) i l’espectroscòpia d'absorció transitòria en l'escala del femtosegon (FSTA), obtenint importants conclusions sobre pèrdues i processos que afecten la recombinació de transportadors de càrrega que porten a pitjors eficiències
La energía fotovoltaica se ha convertido en una de las alternativas más populares como fuente de energía renovable. Se basa en la transformación directa de radiación solar en electricidad. Se encuentra disponible a escala global y además no precisa de ningún transformador para convertir la energía mecánica en energía eléctrica, lo que hace que sea fácil de implementar. Hoy en día, el material más utilizado para aplicaciones fotovoltaicas sigue siendo el silicio. En cambio, el desarrollo de nuevas tecnologías, más baratas, fáciles de procesar y que además pueden utilizarse en sustratos flexibles, ha surgido como alternativa al silicio. De todas ellas, las perovskitas basadas en haluros de plomo se han convertido en una de las mejores opciones para la comunidad científica debido a las excelentes propiedades fotovoltaicas que presenta. Aunque las eficiencias de los dispositivos preparados con perovskitas han alcanzado el 25%, un valor que se encuentra muy cercano a su máximo teórico, los procesos que tienen lugar en estos dispositivos aún no son del todo conocidos. En esta tesis se trata de obtener información acerca de los procesos de los transportadores de carga, desde cómo se generan hasta la recombinación, tanto en las interfaces como en el interior del propio material. Para ello, se han utilizado distintas técnicas de caracterización avanzadas como el fotovoltaje transitorio (TPV), fotocorriente transitoria (TPC), la extracción de carga (CE) y la espectrocopía de absorción transitoria en la escala del femtosegundo (fsTA), obteniendo importantes conclusiones sobre pérdidas
Photovoltaics have become one of the most popular renewable source of energy. Photovoltaic technologies transform sunlight into electricity, and they are also available worldwide, and they do not depend on the conversion of motive power, making this technology quite easy to implement. Nowadays, silicon is still the most used material for photovoltaics. Anyway, new photovoltaic technologies have emerged as alternatives to silicon, as they are cheaper, easier to process, and, they are possible to use on flexible substrates. Among them, lead halide perovskites have become one of the most popular choice in the scientific community, due to the great properties that this material presents. While efficiencies have risen above 25%, which is close to their maximum theoretical limit, there is still debate about the processes happening in the device. In this thesis, we try to gain insight into charge carrier processes from their generation to their recombination at both perovskite interfaces, and also in the bulk of the material. Using advanced characterization techniques, such as transient photovoltage (TPV), transient photocurrent (TPC), charge extraction (CE), and femtosecond transient absorption spectroscopy (fsTA) we obtained important findings about charge carrier losses, and artifacts affecting charge carrier recombination in functional devices that lead to lower power conversion efficiencies.

Lithium Air (Li/O2) batteries are energy conversion devices that produce electricity from the oxidation of lithium metal at the anode and the reduction of molecular oxygen at the cathode. These batteries are considered as promising rechargeable cells for high power applications due to their high power density ranging from 1000 to 2000 Wh/kg. However, one of the most significant challenges is the need to separate the metallic lithium anode from any oxygen or water-containing environment while at the same time allowing fast and efficient lithium ion transport through the electrolyte. Therefore, lithium ion conducting materials that are water and CO2 resistant are a prerequisite. Common materials used as anode protective films and/or Li+ conducting electrolytes for lithium air batteries are perovskite-type oxides (formula: ABO3). Perovskites are good candidates for this application because of their versatility, particularly in regards to ionic conductivity. In the present work, a low cost perovskite family such as SFO (SmFeO3) is developed as a lithium ion conducting material by the introduction of Li+ into its lattice. The perovskites have been synthesized using a solid-state reaction method (SSR) and characterized using different techniques such as powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-ray Spectroscopy (EDS) and electrochemical impedance spectroscopy (EIS). The synthesized perovskites are based on samarium lithium ferrite and divided into two groups depending on the formal presence of vacancies in the stoichiometric formula. The first group (SLFO) with no formal vacancies has the stoichiometric formula of SmxLi1-xFeO2+x (where x = 0.1, 0.2, 0.3, 0.5 and 0.7). While the second group (SLFO*) was generated with less metal atoms than specified in the perovskite structure, thereby generating a structure with intrinsic vacancies and with the formula, Sm(x)Li([1-x] – [0.1] or [0.2]) FeO3-δ (where x = 0.3, 0.4, 0.5 and 0.6). Finally, the effect of varying Li and Sm concentrations in both groups and vacancies created in the lattice for the second group, on the ionic conductivity is explored.

Perovskite oxide compounds have generated a great amount of interest over the past twenty years, as they exhibit exotic magnetic, electric, and magnetoelectric properties, which, apart from their intrinsic interest, would have a wide range of applications in industry, with special utilization in the engineering of data storage devices.Starting with a general definition of the perovskite structure of ABO3 materials, the magnetic and transport properties of cobalt and manganese based compounds are reviewed. The objective of this thesis is focused on the investigation of the structures, and the magnetic and transport properties of some typical cobalt or manganese based three-dimensional perovskite compounds doped with various elements.Polycrystalline samples of RE1-xSrxCoO3 (RE = Gd, Nd, Ho, Y, and Dy), REMnxCo1-xO3 (RE = Gd and Ho), and La0.33Sr0.66Fe1-xCrxO3 were fabricated and characterized in detail using X-ray diffraction, Rietveld structural refinement, magnetic and transport measurements, X-ray absorption near-edge (XANES) spectroscopy, and far-infrared (FIR) spectroscopy.Perovskite GdCo1-xMnxO3 (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8) compounds were prepared by solid-state reaction. Structures were characterized using X-ray diffraction and the Rietveld refinement method. The compounds crystallized in the orthorhombic structure with Plmn space group. Crystal lattice parameters constants decreased with increasing Mn doping level. DC magnetization and ac susceptibility were studied over a wide temperature range and different frequencies. A typical spin glass state was observed in all samples around the same temperature of 122 K, where the compounds changed from the paramagnetic to the ferromagnetic state. For x = 0.5, secondary transitions were observed both in dc and in ac measurements, which were independent of the frequency, but depended on the doping level. The valences of Co and Mn were estimated with possible spin states.The structures, magnetism, giant magnetoresistance, and spin and valence states of Co ions were investigated in perovskite Gd1-xSrxCoO3 (x = 0-0.5) compounds synthesized by a solid state reaction method. Results showed that all the samples are ferromagnetic around 150 K, and the magnetization and coercive field increase with the Sr doping level. A spin glass state was observed. Furthermore, the resistivity drops systematically with x. Giant magnetoresistance of over 60% was observed for the Gd0.5Sr0.5CoO3 sample. Spin state assessment indicates that the Co3+ and Co4+ ions are present in intermediate and high spin states.Polycrystalline perovskites compounds HoMn1-xCoxO3 (x = 0-0.8) were prepared by conventional solid-state reaction. Rietveld refinement indicated that, instead of a hexagonal structure with P63cm symmetry for HoMnO3, the compounds doped with Co crystallized in the GdFeO3-type orthorhombic structure containing distorted (Co/Mn)O6 octahedral. The dc magnetization was measured in fields up to 5 T over a wide temperature range from 350 K down to 5 K. It was found that the system gradually changed from paramagnetic for x = 0 to ferromagnetic with a Curie temperature, TC, of 90 K for x = 0.5, then to paramagnetic again for x = 0.8. X-ray absorption near-edge spectra (XANES) were collected to determine the valences of both Co and Mn ions. The results showed that Co and Mn were present mainly as mixed valences of Co2+ + Co3+, Mn4+ + Mn3+, Co3+ +Co4+, and Mn4+, depending on the value of x. When the XANES results were combined with Curie-Weiss fitting, it was found that both Mn4+ and Co3+ were present as high spin states for x > 0.2, whereas, Co4+ appeared in either intermediate or high spin states, closely corresponding to the lattice variations along the (010) direction. HoMn1-xCoxO3 (x = 0-0.8) compounds were also studied by far-infrared (FIR) spectroscopy in order to gain information on the infrared active phonon modes. The data indicated that phonon modes significantly changed with increasing cobalt doping level. Four main bands were assigned as external, torsional, bending, and stretching bands. The external vibration energy remained the same at 1190 cm-1 for Co doping with x < 0.5 and shifted to higher energy for x > 0 5. The torsional and bending bands exhibited splitting. The stretching band is at 600 cm-1 for all samples, but the bandwidth is reduced as Co doping is increased. The transmission spectrum of HoMn4/5Co1/5O3 was analyzed to obtain the spectrum of the optical density. The minimum number of oscillators to obtain a reliable fit is 5, using a sum of non-interacting harmonic oscillators.Cr doping effects on the crystal structures and magnetic properties for La1/3Sr2/3Fe1-xCrxO3 were studied in this work. It was observed that single phase samples with the cubic crystal structure can only form within a narrow solubility range. Magnetic measurements showed that the resulting compounds are magnetic insulators.

Neste trabalho foram avaliadas as atividades de catalisadores do tipo perovskita LaNi1-xCoxO3 frente à reação de reforma a vapor de etanol. Devido à baixa área superficial, característica de óxidos do tipo perovskita, esses foram suportados em SiO2, Al2O3 e ZrO2, a fim de verificar o efeito do suporte na atividade catalítica.
Os catalisadores foram preparados pelo método da co-precipitação e caracterizados por espectrometria de emissão atômica por plasma induzido, difração de raios X pelo método do pó, adsorção de nitrogênio pelo método B.E.T, redução a temperatura programada e espectroscopia de absorção de raios X.
Para estudar o processo de redução e a possibilidade de oxidação durante a reação catalítica, foram realizados estudos in situ da reação de reforma a vapor, através da espectroscopia de absorção de raios X. Estes dados foram comparados com os resultados de aplicação das técnicas de RTP e DRX às amostras parcial e totalmente reduzidas e foi proposto um mecanismo de redução do óxido do tipo perovskita durante o processo de ativação do catalisador.
Todos os catalisadores mostraram-se ativos nas reações de reforma a vapor de etanol e a seletividade dos produtos foi dependente do tipo do catalisador avaliado. De acordo com os resultados obtidos, destaca-se o catalisador não suportado LaNiO3, com conversão de etanol de 99% e seletividade para H2, CO e CO2 de 4,8; 1,1 e 1,3, respectivamente. Além disso, o ensaio com dois catalisadores simultâneos (LaNiO3 + LaCoO3) foi o que apresentou melhor estabilidade na reação, com 100% de conversão de etanol e seletividade semelhante à obtida pelo catalisador LaNiO3.
In this work, the catalytic activity of perovskite oxides, LaNi1-xCoxO3, was evaluated in the ethanol steam reforming. Due to the low surface area, characteristic of perovskite oxides, these catalysts were supported on SiO2, AI2O3 and ZrO2 and the effect of the support was evaluated. The catalysts were prepared by the co-precipitation method and characterized by Atomic Induced Plasma Spectroscopy, X-Ray Powder Diffraction (XRD), Nitrogen adsorption by B.E.T. method, Temperature Programmed Reduction (TPR) and X-ray Absorption Near Edge Structure (XANES).
The catalytic process was accompanied by XANES in situ to verify changes in the oxidation state of the active phase during the activation process with H2 and also during the process of steam reforming of ethanol. By relation of these results with TPR and XRD, for samples partially and fully reduced, it was proposed a mechanism for the reduction of the perovskite oxides during the conditions of activation.
All catalysts showed activity for the ethanol steam reforming with the selectivity dependent of the catalyst evaluated. Summarizing, the results showed that the unsupported catalyst LaNiO3 presented the better performance, with the ethanol conversion of 99% and selectivity for H2, CO and CO2 of 4.8; 1.1 and 1.3, respectively. Moreover, the test using two simultaneous catalysts (LaNiO3 + LaCoO3), showed better stability in the reaction, presenting ethanol conversion of 100% and selectivity to H2, CO and CO2 similar to the LaNiO3 catalyst.

Orientador: Alexandre Mesquita
Resumo: O titanato de cálcio CaTiO3 é um material com estrutura de tipo perovskita que tem sido aplicado como luminóforo. É bem estabelecido que a inserção de íons de terra rara no sítio ocupado pelo Ca2+ provoca mudanças significativas nas suas propriedades estruturais e produz efeitos luminescentes na faixa do visível. Entretanto não existem trabalhos reportando a inserção do La3+ no sítio do Ca2+ no que se refere às características estruturais e luminescentes desses materiais. Portanto, o presente trabalho estuda as propriedades fotoluminescentes apresentadas pela matriz à base de titanato de cálcio dopada com praseodímio, lantânio e alumínio. O material foi sintetizado a partir do método Pechini, que se mostrou um método eficaz na preparação de acordo com a caracterização estrutural realizada. A técnica de difração de raio X permitiu identificar que todas as amostras de CaTiO3 apresentam-se na fase cristalina com simetria ortorrômbica. Através da espectroscopia de absorção de raio X na borda K do Ti4+ constatamos a ocorrência de maior simetria do Ti4+ em relação aos átomos ao seu redor quando aumentamos a temperatura de calcinação e que a substituição de íons no sítio do Ca2+ não altera essa simetria. Em razão do aumento da concentração dos dopantes, tanto no sítio do Ca2+ quanto no sitio do Ti4+, foram observados alterações nos modos de vibração dos espectros Raman. Constatou-se o aumento da torção entre os octaedros de TiO6 em relação ao aumento da concentração dos dopantes. No... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The calcium titanate CaTiO3 is a perovskite structure material which has been used as phosphor. It is well established that the incorporation of rare earth ions in the place occupied by Ca2+ cause significant changes in their structural properties and produce luminescent effects in the visible range. However, there are no studies reporting the insertion of La3+ on the Ca2+ site with respect to structural and luminescent characteristics of these materials. So, this work studies the photoluminescent properties presented by the matrix based on calcium titanate doped with praseodymium, lanthanum and aluminum. The material was synthesized by the Pechini method, which proved an effective method of preparation according to the structural characterization performed. The technique of X-ray diffraction showed that all samples are organized and exhibit orthorhombic symmetry. Using X-ray absorption spectroscopy at the K edge of Ti4+, we found that the occurrence of major symmetry of Ti4+ with respect to the neighbor atoms when the calcination temperature was increased and that the substitution of ions Ca2+ by Pr3+ does not change this symmetry. With the concentration of dopants increasing at the Ca2+ and Ti4+ sites, changes of the vibration modes in the Raman spectra were observed. It was found an increased torque between the TiO6 octahedra as a function of the increase in concentration of dopants. In the luminescence spectra, increasing the structural defect density enhanced the light e... (Complete abstract click electronic access below)
Mestre

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

Aquesta tesis inclou el treball fet en ICIQ sobre fabricació, caracterització, i modelització de cel·les solars de perovskita híbrida. Agafades des de la recerca en altres tipus de cel·les solars, les eines de anàlisi, les metodologies i, més important, la seva interpretació han sigut analitzades i adaptades a aquest nou tipus de dispositiu. Llavors, aquestes tècniques han sigut utilitzades per analitzar i entendre la influència de quatre diferents i nous transportadors de forats electrònics sobre el voltatge de cel·les de perovskita. Un altre estudi ha investigat la acumulació dels electrons en les cel·les per mig de petits canvis en el gruix de cada capa i analitzant les mostres per mig de les mateixes tècniques. Des de la meva estada internacional en els grups de Dr. Piers Barnes i Prof. Jenny Nelson en Imperial College London un altre estudi ha sigut complert sobre la complexa interpretació dels resultats de espectroscòpia de impedància en presència de ions mobles en les cel·les de perovskita. També està exposat tot el programari lliure que ha sigut desenvolupat per la adquisició i processament de dades i per la modelització deriva-difusió de cel·les solars de perovskita. Una versió actualitzada de aquesta tesis es pot trobar en https://github.com/ilario/documents_in_latex-PhD_thesis/
Esta tesis incluye el trabajo hecho en ICIQ sobre fabricación, caracterización, y modelización de celdas solares de perovskita hibrida. Provenientes desde la investigación en otros tipos de celdas solares, las herramientas de análisis, las metodologías, y, aún más importante, su interpretación han sido analizadas y adaptadas a este nuevo tipo de dispositivo. Entonces, estas técnicas han sido utilizadas para analizar y entender la influencia de cuatros diferentes y novedosos transportadores de huecos electrónicos sobre el voltaje de celdas de perovskita. Otro estudio ha investigado la acumulación de electrones en las celdas utilizando pequeños cambios en el grosor de cada capa y analizando las muestras con las mismas técnicas. Desde mi estancia internacional en los grupos del Dr. Piers Barnes y de la Prof. Jenny Nelson en Imperial College London otro estudio ha sido llevado al cabo sobre la complexa interpretación de los resultados de espectroscopia de impedancia en presencia de iones móviles en las celdas de perovskita. Además, se expone todos los programas libres que han sido desarrollados para la adquisición y procesamiento de datos y para la modelización deriva-difusión de celdas solares de perovskita. Una versión actualizada de esta tesis se puede encontrar en https://github.com/ilario/documents_in_latex-PhD_thesis/
This thesis includes the work done in ICIQ about fabrication, characterization, and modelling of hybrid perovskite solar cells. Coming from other kind of solar cells, the analysis tools, the methods, and, most importantly, their interpretation have been analysed and adapted to this new kind of device. Then, these techniques has been employed for analysing and understanding the influence of four different and novel hole transport materials on perovskite solar cells voltage. Another study focussed on the electrons accumulation in devices employing small variations in each stacked layer thickness and analysing the samples using the same techniques. From by international stay in Dr. Piers Barnes and Prof. Jenny Nelson groups in Imperial College London another study was originated exploring the complex interpretation of impedance spectroscopy results when applied on perovskite solar cells with mobile ions. Finally, all the free software that has been developed for data acquisition and processing and for drift-diffusion modelling of perovskite solar cells have been exposed. An updated version of this thesis can be found on https://github.com/ilario/documents_in_latex-PhD_thesis/