Orientador: Pascoal Jose Giglio Pagliuso / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-10T19:13:39Z (GMT). No. of bitstreams: 1 Bufaical_LeandroFelixdeSousa_M.pdf: 1501359 bytes, checksum: 7c304c6377511ffd74495c952b919ec0 (MD5) Previous issue date: 2006 / 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

Submitted by Erika Demachki (erikademachki@gmail.com) on 2015-03-04T21:01:51Z No. of bitstreams: 2 Dissertação - Lidianne Dias de Alvarenga - 2013.pdf: 2431358 bytes, checksum: ad3757d9260c06bc86b5d120f613787e (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Erika Demachki (erikademachki@gmail.com) on 2015-03-04T21:02:03Z (GMT) No. of bitstreams: 2 Dissertação - Lidianne Dias de Alvarenga - 2013.pdf: 2431358 bytes, checksum: ad3757d9260c06bc86b5d120f613787e (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-03-04T21:02:03Z (GMT). No. of bitstreams: 2 Dissertação - Lidianne Dias de Alvarenga - 2013.pdf: 2431358 bytes, checksum: ad3757d9260c06bc86b5d120f613787e (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2013-07-16 / Crystalline samples of CaTiO3: X with X = Tb or/and Yb were stoichiometrically prepared by the polymeric precursor method. The structure of the doped sample was analyzed by X-ray diffraction measurements. The diffraction pattern showed a single phase perovskite-related type orthorhombic. The X-ray diffraction was used to evaluate order-disorder materials calcined at different temperatures, and to calculate the lattice parameters and crystallite size of the materials. XANES spectra were used to evaluate the local order around the Ti atoms and correlating with pre presence of clusters TiO5-TiO6. The photoluminescent property of the obtained materials was investigated for the powders calcined at different temperatures and observed a greater issue for photoluminescent material heat treated at 500 ° C. / Amostras estruturalmente ordenadas e desordenadas de CaTiO3:X com X= Tb e/ou Yb foram estequiometricamente preparadas pelo método dos precursores poliméricos. A estrutura cristalina da amostra dopada foi analisada pelas medidas de difração de raios X. O padrão de difração obtido para as amostras estruturalmente ordenadas revelou uma única fase relacionada a fase perovsquita do tipo ortorrômbica do material. A difração de raios X também foi utilizada para avaliar a ordem-desordem estrutural dos materiais calcinados a diferentes temperaturas, bem como para calcular os parâmetros de rede e tamanho do cristalito dos materiais. Espectros de XANES foram utilizados para avaliar a ordem local ao redor dos átomos de Ti e correlacionar a altura da pré-borda com a presença de clusters de TiO5-TiO6. A propriedade fotoluminescente dos materiais obtidos foi investigada para os materiais calcinados em diferentes temperaturas, observando uma maior emissão fotoluminescente para o material tratado termicamente a 500ºC.

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.<br /> 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.<br /> 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.<br /> 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).<br /> 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.<br /> 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.

T* based La₀.₉Ln₀.₉Sr₀.₂CuO₄ (Ln = Sm & Gd) has been investigated as cathode material for intermediate temperature solid oxide fuel cell using Ce₀.₉Gd₀.₁O₁.₉₅ (GDC) and La₀.₉Sr₀.₁Ga₀.₈Mg₀.₂O₃-δ (LSGM-9182) as the electrolyte material. Both oxides crystallize in tetragonal P4/nmm symmetry. The structural and phase stability has been confirmed up to 800 °C by High temperature XRD studies. The coefficient of thermal expansion (CTE) and oxygen content decrease with decreasing size of the Ln³+ ions from Ln = Sm to Gd. While the decrease in CTE is due to the increasing co-valence of the Ln–O bond, the decrease in electrical conductivity at high temperature is due to the increasing oxide ion vacancies and a bending of the O–Cu–O bonds. The highest value of DC conductivity has been observed for the LSSCu, which showed a metal like temperature dependence. LGSCu showed a semiconductor to metallic temperature dependence of conductivity with a maximum of 25 Scm-¹. From the microstructural characterization and the polarisation resistance measurement of the symmetric cells at temperature ranges from 700 - 800 °C, 900 °C has been chosen as the most suitable sintering temperature and LGSCu has shown the minimum polarization resistance of 0.35 Ωcm² and 0.09 Ωcm² at 800 °C using GDC and LSGM-9182 electrolytes respectively under OCV condition. To improve the ASR of LGSCu, the composite of LGSCu and GDC with varying wt. % of GDC has been optimised and it shows the ASR of 0.12 Ωcm² using GDC as the electrolyte because it enhance the triple phase boundary region. The maximum power density of single-cell SOFCs fabricated with the La₀.₉Ln₀.₉Sr₀.₂CuO₄ (Ln= Sm & Gd) cathodes, La₀.₉Sr₀.₁Ga₀.₈Mg₀.₂O₃-δ (LSGM-9182) electrolyte, and Ni–Ce₀.₉Gd₀.₁O₁.₉₅ cermet anode exhibit 720 and 824 mWcm-² at 800 °C respectively. The phase pure T* Nd₁.₃₂Ce₀.27Sr₀.₄₁CuO₄-δ (NCSCu) has been synthesized by combustion method and its crystal chemistry, thermal and electrochemical properties, and catalytic activity in SOFC were evaluated using LSGM-9182 as the electrolyte. It shows promising performance and can be used as potential cathode materials for IT-SOFC. The effect of B-site Ni and Co substitution for Cu on the structural and electrochemical properties of the T* La₀.₉Gd₀.₉Sr₀.₂CuO₄ has been investigated as cathode materials for intermediate temperature solid oxide fuel cells using LSGM-9182 as the electrolyte. At a given temperature, the electrical conductivity gradually increases with increasing Ni content and the CTE gradually decreases. Ni doping has also improved the electrochemical performance. Sr doped A /A //B₂O₅+δ (A / = Rare Earth, A // = Ba or Sr and B = Transition Metals) layered perovskites improves the electrochemical performance due to the increase in electrical conductivity and smaller size difference between Ln+³ and Sr+². However these layered perovskites suffer from high thermal expansion coefficient (20-23 x 10-6 K-1) which does not match with the state of the art electrolyte materials. B-site transition metal doped layered perovskites of compositions SmBa₀.₅Sr₀.₅Co₂-ₓO₅+δ (M = Cu, Ni, Fe) have been investigated as cathode material for intermediate temperature solid oxide fuel cell using LSGM-9182 as the electrolyte material. Phase purity has been confirmed by XRD technique. The crystal cell parameters have been found out using Rietveld refinement by FULLPROF software. The substitution of Cu, Ni and Fe for Co lowers the CTE of Co-based materials by suppression of the spin state transition of Co³+ which will be highly advantageous for long term SOFC application. The introduction of transition metals exhibit inferior electrochemical performance to pristine cathode using LSGM-9182 as the electrolyte but still shows reasonable power density with advantage of lower CTE value thereby can be explored as promising cathode material for IT-SOFCs.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16396号 / 工博第3477号 / 新制||工||1525(附属図書館) / 29027 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 乾 晴行, 准教授 宇田 哲也 / 学位規則第4条第1項該当

Abstract Nd-modified lead-zirconate-titanate (PNZT) thin films were deposited on MgO(100), Si(100) and Al2O3(1102) single-crystal substrates using the pulsed-laser-ablation technique with a XeCl excimer laser. The post-annealing heat-treatment technique was used for the crystallisation of the films. The structural characterization, microstructure and the chemical composition of the thin films and of the ceramic Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets after ablation were studied using x-ray diffraction, scanning electron microscopy and energy-dispersive x-ray spectroscopy, respectively. The formation of the particulates on the target surface during the ablation process and the effect of the particulates on the quality of the thin films were studied. Typically, the ferroelectric PNZT thin films for the capacitor structures were deposited at the laser-beam fluence of around 1.0 J/cm2 and annealed at the temperatures from 600 to 700 °C. The dielectric and, especially, the polarization properties and the residual macroscopic stress state of the PNZT thin films were studied. The relationship between the electrical properties of the films and the nature of the stress state was also investigated. The average growth rate of the PNZT films increased linearly with increasing laser-beam fluence above the threshold value of around 0.4 J/cm2. The composition of the PNZT films varied strongly with the deposition laser-beam fluence and annealing temperature. The phase structure of PNZT films ablated from Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets could be adjusted between tetragonal and rhombohedral structures by changing the incident laser-beam fluence on the target surface. The surface of the target after ablation was covered by the laser-cone structure and the topmost layer of the target was amorphous having TiO2 and ZrO2 structures with separate segregated lead droplets. On MgO substrates, values of the relative dielectric constant er from 430 to 560 and of the remanent polarization εr of the order of 18 μC/cm2 were achieved in PNZT films which were under a compressive stress of the order of 300 MPa. On silicon substrates, εr was around 100 and the polarization properties of the films were modest due to a strong tensile stress of the order of 400 MPa. The Poole-Frenkel conduction mechanism with the activation energy of around 0.2 eV was found responsible for the leakage conductivity in the capacitor structures with PNZT films.

Les travaux de thèse ont porté sur la substitution cationique et anionique du composé LaFeO3, élaboré en couche mince par pulvérisation cathodique magnétron réactive (deux cibles métalliques et mélange gazeux Ar+O2). La première partie de la thèse est consacrée à la substitution du lanthane par d'autres terres rares plus petites : Praséodyme, Néodyme et Samarium. Les quatre pérovskites LaFeO3, PrFeO3, NdFeO3 et SmFeO3 ont donc été synthétisées et caractérisées (caractérisations optiques, électriques et structurales), et l'influence de la substitution sur les propriétés physiques est discutée. Les travaux ont montré que la substitution du lanthane par d’autres terres rares plus petites entraîne une modification des propriétés, que nous pouvons directement relier à la distorsion de la maille. Dans une deuxième partie, le dopage à l’azote du composé LaFeO3 a été réalisé, en utilisant de l'azote en tant que gaz réactif (deux cibles + mélange gazeux Ar+O2+N2). Les conditions de dépôt ont été étudiées dans le but d'optimiser la synthèse. A partir de mesures in situ des paramètres de la cible en fonction des débits de gaz, associées aux mesures post dépôt (composition, épaisseur, conductivité électrique), des conditions particulières de synthèse ont été retenues, afin d’optimiser le dopage à l’azote. L’utilisation de ces conditions de synthèses spécifiques ont permis la réalisation de plusieurs dépôts LaFeO3-xNx qui ont ensuite été caractérisés (caractérisations chimiques, optiques, électriques et structurales). Il en résulte que le dopage à l’azote du composé LaFeO3 entraîne une modification du gap de la pérovskite, ainsi que des propriétés électrique, et optique, malgré une faible teneur en azote. / The work done trough this PhD deals with cationic and anionic substitution of LaFeO3 thin films deposited by reactive magnetron sputtering (co-sputtering of two targets in Ar+O2 mixture). The first part of the word is devoted to substitution of lanthanum by smaller rare earth, Praseodymium, Neodymium, and Samarium. The four perovskites LaFeO3, PrFeO3, NdFeO3 and SmFeO3 have been synthesized and characterized (optical, electrical and structural properties), and this cationic substitution is discussed. It results in a shift of the properties which can be directly linked with lattice distortion due to smaller rare earth. In a second part, nitrogen doping of LaFeO3 has been studied, by using nitrogen as reactive gas (co-sputtering of two targets in Ar+O2+N2 mixture). Deposition conditions have been studied in order to optimize the perovskite synthesis. From in situ measurements of target voltage variation with flow rate associated to post-deposition measurement (thickness, composition, electrical conductivity), specified conditions have been found for nitrogen doping optimization. Different LaFeO3-xNx thin films have been deposited following these specified conditions, and characterized (chemical, optical, electrical and structural analysis). Nitrogen doping of LaFeO3 leads to bandgap decrease, associated to modification of optical and electrical properties, even if a small amount of nitrogen can be added to the structure.

This work focused on the study of three independent projects involving perovskite oxide materials and their applications as solid oxide fuel cell (SOFC) anodes and electrochemical oxygen sensors. The underlying theme is the versatility and tune-ability of the perovskite structure. Reactivity and conductivity (ionic as well as electronic) are modified to optimize performance in a specific application. The effect of Ce doping on the structure and the conductivity of BaFeO3 perovskite materials is investigated and the resulting materials are applied as oxygen sensors. The new perovskite family, Ba1-xCexFeO3-δ (x=0, 0.01, 0.03, and 0.05), was prepared via a sol-gel method. Powder XRD indicates a hexagonal structure for BaFeO3 with a change to a cubic perovskite upon Cerium doping at the A site. The solubility limit of Ce at the A site was experimentally determined to be between 5-7 mol %. Bulk, electronic and ionic conductivities of BaFeO3-δ and Ba0.95Ce0.05FeO3-δ were measured in air at temperatures up to 1000˚C. Cerium doping increases the conductivity throughout the entire temperature range with a more pronounced effect at higher temperatures. At 800˚C the conductivity of Ba0.95Ce.05FeO3-δ reaches 3.3 S/cm. Pellets of Ba0.95Ce.05FeO3-δ were tested as gas sensors at 500 and 700˚C and show a linear, reproducible response to O2. Promising perovskite anodes have been tested in high sulfur fuel feeds. A series of perovskite solid oxide fuel cell (SOFC) anode materials: Sm0.95Ce0.05FeO3-δ, Sm0.95Ce0.05Fe0.97Ni0.03O3-δ and Sm0.95Ce0.05Fe0.97Co0.03O3-δ have been tested for sulfur tolerance at 500°C. The introduction of the extreme 5% H2S enhances the performance of these anodes, verified by EIS and CA experiments. Post mortem analyses indicate that the performance XII enhancement arises from the partial sulfidation of the anode, leading to the formation of FeS2, Sm3S4 and S on the perovskite surface. Testing in lower concentrations of sulfur, more common in sour fuels, 0.5% H2S, also enhances the performance of these materials. The SCF-Co anode shows promising stability and an increase in exchange current density, io, from 13.72 to 127.02 mA/cm2 when switching from H2 to 0.5% H2S/99.5% H2 fuel composition. Recovery tests performed on the SCF-Co anode conclude that the open cell voltage (OCV) and power density of these cells recover within 4 hours of H2S removal. We conclude that the formation of metal sulfide species is only partially reversible, yielding an anode material with an overall lower Rct upon switching back to pure H2. Combining their performance in sulfur containing fuels with their previously reported coke tolerance makes these perovskites especially attractive as low temperature SOFC anodes in sour fuels. A new perovskite family Ba1-xYxMoO3 (x=0-0.05) has been investigated in regards to electrical conductivity and performance as IT-SOFC anode materials for the oxidation of H2. Refinement of p-XRD spectra as well as SEM imaging conclude that the solubility limit of Y doping at the A site is 5 mol%, beyond which Y2O3 segregation occurs. The undoped BaMoO3 sample has a colossal room temperature conductivity of 2500 S/cm in dry H2. All materials maintain metallic conductivity in the temperature range of 25-1000°C with resistance increasing with Y doping. The Ba1-xYxMoO3 (x=0, 0.05) materials exhibit good performance as SOFC anode materials between 500-800°C, with Rct values at 500°C in dry H2 of 3.15 and 6.33 ohm*cm2 respectively. The catalytic performance of these perovskite anodes is directly related to electronic conductivity, as concluded from composite anode performance.

Defect passivation and surface modification of perovskite semiconductors play a key role in achieving highly efficient and stable perovskite solar cells (PSCs) and light-emitting diodes (LEDs). This dissertation describes three novel strategies for such defect passivation and surface modification. In the first strategy, we demonstrate a facile approach using inorganic perovskite quantum dots (QDs) to supply bulk- and surface-passivation agents to combine high power conversion efficiency (PCE) with high stability in CH3NH3PbI3 (MAPbI3) inverted PSCs. This strategy utilizes inorganic perovskite QDs to distribute elemental dopants uniformly across the MAPbI3 film and attach ligands to the film’s surface. Compared with pristine MAPbI3 films, MAPbI3 films processed with QDs show a reduction in tail states, smaller trap-state density, and an increase in carrier recombination lifetime. The strategy results in reduced voltage losses and an improvement in PCE from 18.3% to 21.5%, which is among the highest efficiencies for MAPbI3 devices. The devices maintain 80% of their initial PCE under 1-sun continuous illumination for 500 h and show improved thermal stability. In the second strategy, we reduce the efficiency gap between the inverted PSCs and regular PSCs using a trace amount of surface-anchoring, long-chain alkylamine ligands (AALs) as grain and interface modifiers. We show that long-chain AALs suppress nonradiative carrier recombination and improve the optoelectronic properties of mixed-cation mixed-halide perovskite films. These translate into a certified stabilized PCE of 22.3% (23.0% PCE for lab-measured champion devices). The devices operate for over 1000 hours at the maximum power point (MPP), under simulated AM1.5 illumination, without loss of efficiency. Finally, we report a strategy to passivate Cl vacancies in mixed halide perovskite (MHP) QDs using non-polar-solvent-soluble organic pseudohalide (n-dodecylammonium thiocyanate (DAT)), enabling blue MHP LEDs with enhanced efficiency. Density-function-theory calculations reveal that the thiocyanate (SCN-) groups fill in the Cl vacancies and remove deep electron traps within the bandgap. DAT-treated CsPb(BrxCl1-x)3 QDs exhibit near unity (~100%) photoluminescence quantum yields; and their blue (~470 nm) LEDs are spectrally stable with an external quantum efficiency (EQE) of 6.3% – a record for perovskite LEDs emitting at the 460-480 nm range relevant to Rec. 2020 display standards.

Lead Halide Perovskites (LHPs) have emerged as an outstanding optical material, chiefly as attractive options for studies of light emission, due to their high quantum efficiencies, broad wavelength tuneability via chemical substitution, and facile growth conditions. LHPs have also been increasingly considered as an ideal candidate for exploring applications of exciton-polariton condensation, with a recent explosion of research in this area. The physical properties of LHPs are distinct from traditional materials often used to study exciton-polaritons, leading to debates over photo-physical mechanisms of stimulated emission, and interpretation of experimental results. This thesis addresses these debates in two parts, discussing (1) how the relatively low exciton-binding energy and phonon-bottleneck effects often leads to exciton dissociation prior to the laser powers needed to observe stimulated emission, and (2) how the optical birefringence associated with bulk CsPbBr3 at cryogenic temperatures will produce novel optical potentials which amount to a synthetic spin-orbit coupling of exciton-polaritons within a perovskite microcavity. These conclusions are reached by a combination of static and time-resolved spectroscopies, along with polarization-resolved Fourier-imaging optical techniques.