Metal halide perovskites (MHPs) have attracted the attention of researchers particularly in the photo-absorption field. These materials rival traditional semiconductors with their cost-effective ease of synthesis, tunable bandgaps, and excellent photophysical properties. Single-junction MHP solar cells have rivaled current silicon-based photovoltaic devices, boasting a 25.2% light absorption conversion. Recently, MHPs have proven to be effective in x-ray detection. In this paper an investigation of three titanium-based MHPs was conducted. The goal of this research was to characterize the MHPs and determine if they are feasible materials to incorporate in x-ray detectors. After completing the research, two MHP species, FA2TiI6 and MA3TiCl7, were able to be synthesized and characterized. Their crystal systems were determined to be tetragonal with a P4/mmm space group. However, due to equipment limitations, their feasibility in x-ray detectors could not be determined.

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It has been 10 years since the first hybrid halide perovskite photovoltaics was fabricated. Power conversion efficiency increases from the initial 3.8% to the current 25.2%. Fabrication method envolves from spin-coating to printable technology, and we deeply experience the drastic development of hybrid halide perovskite photovoltaics. Although hybrid halide photovoltaics render a variety of advantages over traditional photovoltaics, we still cannot find any practical application of these hybrid halide photovoltaics. There exist a few issues which hinder the commercialization of this type of solar cell. Among these issues, the long-term instability of hybrid halide perovskite is the main concern for the next development. This thesis expands on investigating the instability of hybrid halide perovskites from first principles. In Chapter 1, two computational methods employed in the thesis: density functional theory and Ab initio molecular dynamics are introduced. Theoretical investigations of the instability of CH3NH3PbI3 using density functional theory method are mainly conducted at 0 K. The finite temperature effect on this instability of CH3NH3PbI3 is usually neglected. In Chapter 2 of this thesis, we combined density functional calculations and additional thermodynamic data to explain the intrinsic instability of CH3NH3PbI3 under finite temperature conditions. We also analyzed the stability under humid conditions. It is shown that the aqueous solubilities of reactants play an important role in the products’ stabilities. The Born–Haber cycle of NaCl splits the enthalpy change into several components which will give a better understanding of the origin of the corresponding enthalpy change. In Chapter 3, with the extension of the Born–Haber cycle to the hybrid halide perovskites, the reaction enthalpies which govern the intrinsic instability of ionic compounds were analyzed. We proposed a criterion that helps to filter the hybrid halide perovskites with improved stability aimed for photovoltaics. Since the instability of CH3NH3PbI3 is intrinsic. The long-term instability can be settled by discovering alternative perovskite absorber. In Chapter 4, based on literature research, we propose a three-membered ring cation which has a suitable size to fit into the Pb-I framework, leading to optimal band gap for photovoltaics. Besides, the cation has a good ionization energy which will potentially render better stability. Whereas, a comprehensive study of this cyclic ring based perovskite indicates that the instability of the three-membered ring cation will make it impossible to synthesize this theoretical structure. Moisture degradation mechanisms of CH3NH3PbI3 are investigated intensively. More importantly, for practical photovoltaics, we have to imagine different situations the modules will encounter, e.g. after a couple of years, cracks appearing on the modules are inevitable, at this stage, understanding of the degradation mechanism of CH3NH3PbI3 according to liquid water becomes important. Chapter 6 elaborately describes a comprehensive degradation mechanism of CH3NH3PbI3 under liquid water. We investigate the energy barrier for the first dissolution event of CH3NH3PbI3 in water. Furthermore, thermodynamic analyses of CH3NH3PbI3 dissolution in water clearly explain the spontaneity of CH3NH3PbI3 degradation in water. Besides, different mechanisms of CH3NH3PbI3 and CsPbI3 dissolution in water are discussed. / Dissertation / Doctor of Philosophy (PhD)

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination.

Over recent years significant progress has been made in the design and development of late transition metal cationic catalysts for olefin polymerisation. Never-the-less, the activation of catalyst precursors and generation of active species still remains a challenge. In this respect, zwitterionic catalysts could offer a range of advantages over the traditional two component catalytic systems. For example, stable zwitterions are well-defined, single component catalysts which do not require Lewis acid co-catalysts for activation. Therefore, this eliminates the possibility of anions coordinating to the active site and could provide highly active catalysts. Moreover, this could reduce the production costs. In this thesis the 6-aminofulvene-2-aldiminate (AFA) ligand system has been employed to develop zwitterionic, charge-neutral complexes, analogues of Brookhart-type cationic alkene polymerisation catalyst containing 1,2-diimine ligand. Chapter 1 of the thesis provides a comprehensive literature review of the late transition metal (Group 10) α-diimine catalytic systems and the zwitterionic early and late transition metal alkene polymerisation catalysts. Chapter 2 describes the synthesis and characterisation of some novel zwitterionic complexes [(Ph2AFA)Pd(Me)(DMAP)], [(Ph2AFA)(N,N-dimethylbenzylamine-2-C,N)- Pd(II)] and [(Ph2AFA)Ni(η 3-C3H5)] and their possible application as catalyst precursors in alkene polymerisation. In principle, upon activation these complexes should exhibit higher catalytic activity. The ideal catalyst precursor for a highly active palladium based system would be a halide-bridged dimer of the form [(Ph2AFA)Pd(μ-X)]2. Chapter 2 describes several efforts towards the synthesis of such complexes using a range of R2AFA ligands. Even with the introduction of bulky N-substituents such as cyclohexyl or tert-butyl, the halidebridged dimers could not be synthesised. Instead, the reaction between the deprotonated ligand and [PdCl2(NCPh)2] provides bis-chelated complexes [(R2AFA)2Pd]. In order to introduce more steric bulk into the AFAH ligand which might lead to a halide-bridged dimer, two more ligands N,N’-bis(2,6-diisopropyl)phenyl-6-aminofulvene-2-aldimine and N,N’-di-(2,4,6-trimethyl)phenyl-6-aminofulvene-2-aldimine have been synthesised and characterised. It has been found that the presence of the 2,6-diisopropylphenyl substituents in N,N'-bis(2,6-diisopropyl)phenyl-6-aminofulvene-2-aldimine not only prevents the coordination of two ligands to the same metal, but precludes complexation all together. Chapter 2 also describes several efforts to develop a hemi-labile complex for alkene polymerisation. Chapter 3 describes the synthesis of metalloligands of aminofulvene-aldimine (AFA) and corresponding bimetallic complexes. The AFA ligand affords transition metal complexes via both η 5- as well as κ 2-coordination modes. A new synthetic methodology has been developed to synthesise metalloligands [Cp*RuII(Ph2AFA)H][BF4], [Cp*RhIII(Cy2AFA)H][BF4]2 and [Cp*RhIII(Cy2AFA)]- [BF4]. The basicity of the monocationic Rh metalloligand is found to be significantly lower than that of its Ru analogues. This is significant as it opens a potentially easy synthetic route to bimetallic complexes. The bimetallic complex [Cp*RhIII(Cy2AFAPdCl2)][BF4] has been developed for alkene polymerisation in an attempt to investigate the charge effect in alkene polymerisation catalysis. Upon activation this monocationic Rh/Pd bimetallic complex would provide a dicationic active species which would in principle be a more highly active catalyst than the Brookhart mono cationic diimine catalysts. Chapter 4 describes all the experimental procedure and polymerisation tests in this thesis.

Abstract Methylammonium lead (II) iodide has recently attracted considerable interest which may lead to substantial developments of efficient and inexpensive industrial photovoltaics. The application of this material as a light-absorbing layer in solid-state solar cells leads to impressive efficiency of over 22% in laboratory devices. However, for industrial applications, fundamental issues regarding their thermal and moisture stability need to be addressed. MAPbI3 belongs to the perovskite family of materials with the general formula ABX3 ,where is the organic cation (methylammonium) which is reported to be a major source of instability. In this work, a variety of alkyammonium lead (II) iodide materials have been synthesized by changing the organic cation, to study the relationship between the structural and physical properties of these materials. [(A)PbI3] and (A)PbI4 series were studied. Three dimensional (3D) networks (MAPbI3,MAPbBr3), two dimensional (2D) layered systems (BdAPbI4, HdAPbI4, OdAPbI4), and one dimensional (1D) columns (EAPbI3, PAPbI3, EAPb2I6) were found for the materials. [PbI6] octahedral structural units were repeated through the material network depending on the dimensionality and connectivity of the materials. Where a bulkier cation was introduced, the crystallographic unit cell increased in size which resulted in lower symmetry crystals. The connectivity of the unit cells along the material networks was found to be based on corner-sharing and face-sharing. Lower dimensionality resulted in larger bandgaps and lower photoconductivity, and hence a lower light conversion efficiency for the related solar cells. The thermal and moisture stability was greater in the 1D and 2D materials with bulkier organic cations than with methylammonium. In total, an overview is provided of the relationship between the chemical dimensionality and physical properties of the organic-inorganic lead halide materials with focus on the solar cell application. / Svenska sammandrag: Metylammoniumbly(II)jodid har under de senaste åren genererat ett stort intresse som ett möjligt material for utveckling av effektiva och på industriell skala billiga solceller. Detta material har använts som ljusabsorberande skikt i fasta solceller med imponerande omvandlingseffektiviteter på över 22% för solceller i laboratorieskala. För att denna nya typ av solceller ska bli intressanta för produktion på industriell skala, så behöver grundläggande frågeställningar kring materialens stabilitet avseende högre temperaturer och fukt klargöras. MAPbI3 har formellt perovskitstruktur med den allmänna formel ABX3, där A utgörs av den organiska katjonen (metyammoniumjonen) och som kan kopplas till materialets instabilitet. I denna avhandling har olika alkylammoniumbly(II)jodidmaterial syntetiserats där den organiska katjonen modifierats med syftet att studera växelverkan mellan struktur och fysikaliska egenskaper hos de resulterande materialen. Material av olika dimensionalitet erhölls; tredimensionella (3D) nätverk (MAPbI3, MAPbBr3), tvådimensionella (2D) skiktade strukturer (BdAPbI4, HdAPbI4, OdAPbI4), och endimensionella (1D) kedjestrukturer (EAPbI3, PAPbI3, EAPb2I6). Flera nya lågdimensionella material (2D och 1D) tillverkats och karaktäriserats för första gången. Enkristalldiffraktometri har använts för att erhålla materialens atomära struktur. Strukturen hos material tillverkade i större mängder konfirmerades genom jämförelse mellan resultat från pulverdiffraktion och enkristalldiffraktion. Den oktaedriska strukturenheten [PbI6] utgör ett återkommande tema i materialen sammankopplade till olika dimensioner. Då större organiska katjoner används karaktäriseras i regel strukturerna av större enhetsceller och lägre symmetri. De lågdimensionella materialen ger typiskt störe elektroniskt bandgap, lägre fotoinducerad ledningsförmåga och därför sämre omvandlingseffektiviteter då de används i solceller. De lågdimensionella materialen (1D och 2D) som baseras på de större organiska katjonerna uppvisar bättre stabilitet med avseende på högre tempereratur och fukt. De tvådimensionella materialens elektroniska struktur har karaktäriserats med hjälp av röntegenfotoelektronspektroskopi, liksom röntgenabsorptions- och emissionsspektroskopi. Resultat från teoretiska beräkningar stämmer väl överens med de experimentella resultaten, och de visar att materialens valensband huvudsakligen består av bidrag från atomorbitaler hos jod, medan atomorbitaler från bly främst bidrar till edningsbandet. Sammantaget erbjuder avhandlingen en översikt av sambandet mellan kemisk dimensionalitet och fysikaliska egenskaper hos ett antal organiska/oorganiska blyhalogenidmaterial med fokus på tillämpning i solceller. / <p>QC 20170123</p>

This thesis reports the synthesis and characterization of two new copper(II) halide complexes with deprotonated N-(2-hydroxyethyl)ethylenediamine behaving as a bidentate. The magnetic properties of the new copper(II) complexes were studied from room temperature to liquid nitrogen temperatures. The magnetic data show that both complexes exhibit antiferromagnetic interactions with a singlet ground state and a thermally accessible triplet excited state. Magnetic data and infrared spectra indicate the complexes are halogenbridged. Deprotonation at an amine nitrogen is based on the presence of a hydroxyl stretching band in the infrared spectra. Electronic spectra and infrared spectra indicate the complexes are square planar. Elemental analyses, infrared spectra, electronic spectra, electron spin resonance spectra, and magnetic data are reported and discussed.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibligraphy: leaves 222-228. / by Paul Joseph Lemaire. / Ph.D.

Formation of the diaryl ether moiety remains a challenging target for organic synthesis despite modern technologies, however, better understanding of older techniques often leads to improvements. The copper-catalysed Ullmann ether synthesis, whilst attractive in many ways, is frequently problematic due to the inherent irreproducibility of the reaction on scale up. Little is yet known about the mechanism of the reaction and conflicting views are rife within the scientific community. In a well-studied example, we show that the potassium iodide formed during the reaction slows catalysis. Additionally, the deprotonation of phenol is complicated by the insolubility of the inorganic base. This results in a beneficial outcome, providing a rate enhancement and reduction of by-products, which can be further exploited to provide lower stoichiometries, improved selectivity and greater functional group tolerance. The development of an improved, more reproducible procedure in combination with reaction calorimetry has allowed the mechanism to be studied in intricate detail. Excellent agreement with a mechanistic model has led to further insight into the enigmatic aryl halide activation and provides good evidence for a single electron transfer mechanism. In addition, evidence for a dynamic catalyst resting state has been observed which adds to the complexity of the mechanism. This, in turn, leads to a fine balance of concentration and electronic effects that prove vital to the rate of reaction.