The aim of the work presented in this thesis is to study the opto-electronic properties of semi-conducting perovskite materials when being used in unconventional solar cell device configurations. Being a young technology, perovskites as solar cell materials have seen an unparalleled rise in the research community which has driven the fastest performance inflation to power conversion efficiencies competing with the ones of long established single crystalline technologies. The ability to process perovskites inexpensively makes them the new hope in the fight against climate change. Herein device architectures were developed with a special focus on potential commercial applications. Initially the work in this thesis has been motivated by the interest in crystal growth and morphology of perovskite thin-films, which has led to the study of confined crystal growth within microstructures. Controlling the crystal domain geometry enabled the fabrication of enhanced semi-transparent devices. More efforts were directed into the improvement of specifically neutral colour semi-transparent devices, which could be improved via a simple treatment of selectively attaching shunt-blocking layers. Furthermore, a back-contacted perovskite device design was introduced, which allows not only for the fabrication of a new type of perovskite solar cell, but also represent a great material testing platform to study perovskite and electrode characteristics. This led to the discovery of charge transport distances, that exceed those of other thin-film devices. Finally, perovskite-on-silicon tandem solar cell designs were analysed through a rigorous optical model to estimate the expected real world energy yield from such systems. Important implications include the fact that two terminal tandem solar cells come close to four-terminal configurations and can overall compete, in relative terms, well with established single junction silicon cells.

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.

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Parmi les matériaux fonctionnels, les oxydes à structure pérovskite présentent une très grande variété de propriétés, de très nombreux dispositifs avancés les utilisent donc comme éléments actifs. On compte au nombre de ces propriétés l'antiferroélectricité, caractérisée à l'échelle atomique par un agencement antiparallèle de dipôles électriques. Le zirconate de plomb, PbZrO3, est considéré comme l'archétype de l'antiferroélectrique à structure pérovskite. Malgré sa découverte il y a plus de 80 ans, la physique présidant sa relation structure-Propriété reste incomprise. Cette étude de doctorat vise donc à étudier la structure de PbZrO3 sous diverses sollicitations, notamment de température, champ électrique, et pression hydrostatique, les propriétés électriques de ses différentes phases ainsi qu'à proposer des modèles permettant d'expliquer leur relation. L'accent sera mis en particulier sur la phase dite “intermédiaire” et qui présente des propriétés ferroélectriques. La compréhension acquise au cours de cette étude permet de proposer des dispositifs innovants à base de matériau antiferroélectrique. / Among functional materials, oxides with a perovskite structure exhibit a large variety of properties which make them an active material of choice in a wide range of advanced devices. Antiferroelectricity is one such property and is characterized at the atomic scale by an antiparallel pattern of electrical dipoles. Lead zirconate (PbZrO3) is considered as the archetype of antiferroelectric perovskites. Despite its discovery more than 80 years ago, the physics presiding over the relationship between its structure and its properties remain to be established. This thesis aims to study the structure of lead zirconate under various conditions (temperature, electric field, hydrostatic pressure), the electrical properties of its phases, as well as to propose models explaining their relationship. This study has a particular focus on the so-Called “intermediate” phase which exhibits ferroelectric properties. The knowledge acquired during this study enables the proposal of innovative devices based on antiferroelectrics.

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Orientador: Pascoal José Giglio Pagliuso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-15T19:42:04Z (GMT). No. of bitstreams: 1 Bufaical_LeandroFelixdeSousa_D.pdf: 2467040 bytes, checksum: 4460b75c04d570fb27584b7dfff2c5f3 (MD5) Previous issue date: 2010 / 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.

Ce travail est consacré à la recherche de nouveaux matériaux sans plomb à structure pérovskite, de formules (A⁺,Bi³⁺)BO₃ et (A⁺,Bi³+)(B,B’)O₃ où A⁺ = {Li⁺, Na⁺, K⁺, Rb⁺}. Le but visé est le remplacement des pérovskites à base de plomb actuellement utilisées dans les dispositifs électroniques. La possibilité de substitution du plomb par un pseudo-ion (A⁺,Bi³⁺) a été étudiée pour (i) les pérovskites simples PbTiO₃ et PbZrO₃ et (ii) les pérovskites complexes Pb(Mg⅟₃Nb⅔)O₃ et Pb(Zn⅟₃Nb⅔)O₃.La synthèse de la composition (Li⅟₂Bi⅟₂)TiO₃ (LBT) a conduit à un composé pyrochlore de type Bi₂Ti₂O₇. Les autres compositions ont permis d'obtenir les pérovskites (Na⅟₂Bi⅟₂)ZrO₃ (NBZ), (K⅟₂Bi⅟₂)ZrO₃ (KBZ), (Na⅟₂Bi⅟₂)(Mg⅓Nb⅔)O₃ (NBMN), (K⅟₂Bi⅟₂)(Mg⅟₃Nb⅔)O₃ (KBMN) et (K⅟₂Bi⅟₂)(Zn⅟₃Nb⅔)O₃ (KBZN). Un important travail de synthèse a été effectué, en utilisant plusieurs voies et différents précurseurs, afin d'essayer d'éliminer les phases secondaires qui accompagnent généralement ces pérovskites.Les structures cristallines ont été étudiées par diffraction de neutrons à l'aide de la méthode de Rietveld. Celles-ci ont été déterminées pour LBT, NBZ, KBZ et KBMN. Les propriétés électriques de ces matériaux ont également été étudiées. Aucun d'entre eux n'est ferroélectrique. KBZ est électrostrictif. Enfin, NBZ, NBMN et KBMN montrent d'intéressantes propriétés de relaxeurs. / The present work focused on the research of new lead-free perovskite materials with the formula (A⁺,Bi³⁺)BO₃ and(A⁺,Bi³+)(B,B’)O₃ with A⁺ = {Li⁺, Na⁺, K⁺, Rb⁺}. The aim of this study is to find potential candidates to replace the lead-based perovskite used in the electronic devices nowadays. The possibility to substitute the lead ion par a couple (A⁺,Bi³⁺) has been examined for (i) the simple perovskites PbTiO₃ and PbZrO₃ and (ii) the complex perovskites Pb(Mg⅟₃Nb⅔)O₃ and Pb(Zn⅟₃Nb⅔)O₃.The synthesis of the composition (Li⅟₂Bi⅟₂)TiO₃ (LBT) has led to a pyrochlore Bi₂Ti₂O₇-like compound. The other compositions have conducted to the perovskites compounds (Na⅟₂Bi⅟₂)ZrO₃ (NBZ), (K⅟₂Bi⅟₂)ZrO₃ (KBZ), (Na⅟₂Bi⅟₂)(Mg⅓Nb⅔)O₃ (NBMN),(K⅟₂Bi⅟₂)(Mg⅟₃Nb⅔)O₃ (KBMN) and (K⅟₂Bi⅟₂)(Zn⅟₃Nb⅔)O₃ (KBZN). A lot of work has been carried out for the syntheses, using different precursor or process, in order to suppress the secondary phases which are often obtained with these perovskites. The crystalline structures have been studied by neutron diffraction, using the Rietveld method. These have been determined for the LBT, NBZ, KBZ and KBMN compounds. The electrical properties of these materials have also been studied. None of them is ferroelectric. KBZ is électrostrictif. Finally, NBZ, NBMN and KBMN present some interesting relaxor properties.

La couche D" située à la frontière manteau-noyau est une région complexe caractérisée par une forte anisotropie à différentes échelles. Inaccessible de par sa profondeur et caractérisée par des conditions P-T extrêmes, l’étude de cette région de la Terre représente un défi majeur qui ne peut être abordé qu’au travers d’observables géophysiques et d’expériences de hautes pressions. Les causes de l’anisotropie sismique de D" sont toujours l’objet de débats. La contribution de l’orientation préférentielle des cristaux reste cependant une piste privilégiée compte-tenu de la structure très anisotrope de la post-perovskite (ppv). De plus, D" est une couche limite thermique à l’interface entre le noyau constitué d’un alliage de fer liquide et le manteau inférieur constitué de silicates visqueux. Les propriétés physiques de D" sont donc particulièrement importantes pour comprendre les transferts thermiques en provenance du noyau et leur contribution à la convection mantellique. Ce phénomène implique l’écoulement plastique de roches contrôlé par le déplacement de défauts cristallins. Cependant, pour la ppv, les informations concernant les systèmes de glissement majeurs ou les défauts sont extrêmement parcellaires. Pour les phases de hautes pressions, la modélisation numérique représente une approche de choix pour obtenir des informations sur les mécanismes de déformations élémentaires difficiles à obtenir par voie expérimentale. Le but de ce travail est d ‘étudier à l’échelle atomique les défauts majeurs de la ppv MgSiO3 ainsi que leurs mobilités afin d’évaluer la capacité de cette phase à se déformer plastiquement par glissement de dislocations dans les conditions de D". / The D’’ layer, located right above the core-mantle boundary, represents a very complex region with significant seismic anisotropy both at the global and local scale. Being a part of inaccessible deep Earth interior, characterized by extreme P-T conditions (>120 GPa, 2500 K), this region is very challenging for interpretation relying only on the direct geophysical observations and high-pressure experiments, leading often to contradictory results. Thus, the reasons of the pronounced anisotropy in D’’ are still debated. Among them, contribution of the crystal preferred orientation in anisotropic silicate post-perovskite (ppv) phase is commonly considered as substantial. Furthermore, the D’’ is a thermal boundary layer located at the interface between liquid iron alloy, constituting the outer core, and solid although viscous silicates of the lowermost mantle. As such, its physical properties are critical for our understanding of the heat transfer from the core, driving mantle convection. The latter is governed by plastic flow, controlled by the motion of defects in crystals. However, for the ppv, information about mechanical properties, easy slip systems, dislocations and their behavior under stress is still scarce. For high pressure phases, numerical modelling represents a powerful tool able to provide the intrinsic properties and the elementary deformation mechanisms, not available for direct observations during high-pressure experiments. The aim of this study is to access the structure and mobility of dislocations in MgSiO3 ppv, relying on the atomic-scale modeling, in order to infer the ability of this phase to plastically deform by dislocation glide at D’’ conditions.

Ferroelectric and antiferroelectric ultrathin films have attracted a lot of attention recently due to their remarkable properties and their potential to allow for device miniaturization in numerous applications. However, when the ferroelectric films are scaled down, it brings about an unavoidable depolarizing field. A partial surface charge compensation allows to control the residual depolarizing field and manipulate the properties of ultrathin ferroelectric films. In this dissertation we take advantage of atomistic first-principles-based simulations to expand our understanding of the role of the partial surface charge compensation in the properties of ferroelectric and antiferroelectric ultrathin films. The application of our computational methodology to study the effect of the partial surface charge compensation in ferroelectric ultrathin films led to the prediction that, depending on the quality of the surface charge compensation, ferroelectric thin films respond to an electric field in a qualitatively different manner. They can be tuned to behave like a linear dielectric, a ferroelectric or even an antiferroelectric. This effect was shown to exist in films with different mechanical boundary conditions and different crystal symmetries. There are a number of potential applications where such properties of ferroelectric thin films can be used. One of these potential applications is energy storage. We will show that, in the antiferroelectric regime, ferroelectric thin films exhibit drastic enhancement of energy storage density which is a desirable property. One of the most promising applications of ferroelectric ultrathin films that emerged only recently is the harvesting of the giant electrocaloric effect. Interestingly, despite numerous studies of the electrocaloric effect in ferroelectric thin films, it is presently unknown how a residual depolarizing field affects the electrocaloric properties of such films. Application of state-of-the-art computational methods to investigate the electrocaloric effect in ferroelectric films with partial surface charge compensation led to the prediction that the residual depolarizing field can perform a dual role in the electrocaloric effect in these films. When the depolarizing field creates competition between the monodomain and nanodomain states, we predict an enhancement of the electrocaloric effect due to the frustration that increases the entropy of the state and therefore the electrocaloric temperature change. On the other hand, when the depolarizing field leads to a formation of nanodomains, thin films either exhibit a small electrocaloric effect or lose their electrocaloric properties altogether to the irreversible nanodomain motion. When the residual depolarizing field is weak enough to permit the formation of monodomain phases, the electrocaloric effect is significantly reduced as compared to bulk. We believe that our findings could potentially reveal additional opportunities to optimize solid state cooling technology. While the electrocaloric effect has been a popular topic of interest in recent years [12], there still exists numerous gaps in the fundamental understanding of the effect. In particular, it is presently unknown whether the scaling laws, known to exist for magnetocaloric materials, can be applied to ferroelectric and antiferroelectric electrocalorics. We predict the existence of scaling laws for low-field electrocaloric temperature change in antiferroelectric and ferroelectric materials. With the help of first-principles-based simulations, we showed computationally that the scaling laws exist for antiferroelectric PbZrO3 along with ferroelectrics PbTiO3, BaTiO3 and KNbO3. Additional evidence of the scaling laws existence are provided using experimental data from the literature. Interestingly, our studies on ferroelectric films predicted the existence of antiferroelectric behavior in ultrathin films with partial surface charge compensation. One may wonder whether it is possible to stabilize the ferroelectric phase in antiferroelectric films and what role the surface charge screening would play in such a transition. Motivated to address these fundamental questions, we used computational experiments to study antiferroelectric ultrathin films with a residual depolarizing field. Our studies led to the following predictions. We found that PbZrO3 thin films exhibit the ferroelectric phase upon scaling down and under the condition of efficient surface charge compensation. We also found a strong competition between the antiferroelectric and ferroelectric phases for the thin films of the critical size associated with antiferroelectric-ferroelectric phase transition. This finding motivated us to study the electrocaloric effect in PbZrO3 thin films with antiferroelectric-ferroelectric phase competition. We found that high tunability of the phase transition by the electric field leads to a wide range of temperatures associated with a strong electrocaloric effect. In addition, we found that epitaxial strain provides further tunability to the electrocaloric properties. In summary, our studies led to a broader and deeper understanding of the abundantly many roles surface charge compensation plays in ultrathin ferroelectrics and antiferroelectrics.