Hydrogen-bond-mediated structural variation of metal guanidinium formate hybrid perovskites under pressure

The hybrid perovskites are coordination frameworks with the same topology as the inorganic perovskites, but with properties driven by different chemistry, including host-framework hydrogen bonding. Like the inorganic perovskites, these materials exhibit many different phases, including structures with potentially exploitable functionality. However, far less is known about their behaviour under pressure. We have studied the structures of of manganese and cobalt guanidinium formate under pressure using single-crystal X-ray and powder neutron diffraction. Remarkably, when pressure reduces these materials' volume, they transform to a phase isostructural to cadmium guanidinium formate, which has an larger volume. Using DFT calculations, we show that this counterintuitive behaviour depends on the hydrogen-bonded network of guanidinium ions, which act as struts protecting the metal formate framework against compression. Our results demonstrate more generally that engineering desirable crystal structures in the hybrid perovskites will depend on achieving suitable host-guest hydrogen-bonding geometries.Comment: 9 pages, 3 figure

Control of Multipolar and Orbital Order in Perovskite-like [C(NH2)(3)]CuxCd1-x(HCOO)(3) Metal-Organic Frameworks

We study the compositional dependence of molecular orientation (multipolar) and orbital (quadrupolar) order in the family of perovskite-like metal–organic frameworks [C(NH2)3]CuxCd1–x(HCOO)3. On increasing the fraction x of Jahn-Teller-active Cu2+, we observe first an orbital disorder/order transition and then a multipolar reorientation transition, each occurring at distinct critical compositions xo = 0.45(5) and xm = 0.55(5). We attribute these transitions to a combination of size, charge distribution, and percolation effects. The transitions we observe establish the accessibility in for-mate perovskites of novel structural degrees of freedom beyond the familiar dipolar terms responsible for (an-ti)ferroelectric order. We discuss the symmetry implica-tions of cooperative quadrupolar and multipolar states for the design of relaxor-like hybrid perovskites

First-principles study of the structural, electronic, magnetic and ferroelectric properties of a charge ordered Iron(II)- Iron(III) formate framework

Density functional theory calculations have been performed for the structural, electronic, magnetic and ferroelectric properties of a mixed-valence Fe(II)-Fe(III) formate framework [NH$_2$(CH$_3$)$_2$][Fe$^{\rm III}$Fe$^{\rm II}$(HCOO)$_6$] (DMAFeFe). Recent experiments report a spontaneous electric polarization and our calculations are in agreement with the reported experimental value. Furthermore, we shed light into the microscopic mechanism leading to the observed value and as well how to possibly enhanced the polarization. The interplay between charge ordering, dipolar ordering of DMA$^+$ cations and the induced structural distortions suggest new interesting directions to explore in these complex multifunctional hybrid perovskites

The contribution of NMR spectroscopy in understanding perovskite stabilization phenomena

Although it has been exploited since the late 1900s to study hybrid perovskite materials, nuclear magnetic resonance (NMR) spectroscopy has only recently received extraordinary research attention in this field. This very powerful technique allows the study of the physico-chemical and structural properties of molecules by observing the quantum mechanical magnetic properties of an atomic nucleus, in solution as well as in solid state. Its versatility makes it a promising technique either for the atomic and molecular characterization of perovskite precursors in colloidal solution or for the study of the geometry and phase transitions of the obtained perovskite crystals, commonly used as a reference material compared with thin films prepared for applications in optoelectronic devices. This review will explore beyond the current focus on the stability of perovskites (3D in bulk and nanocrystals) investigated via NMR spectroscopy, in order to highlight the chemical flexibility of perovskites and the role of interactions for thermodynamic and moisture stabilization. The exceptional potential of the vast NMR tool set in perovskite structural characterization will be discussed, aimed at choosing the most stable material for optoelectronic applications. The concept of a double-sided characterization in solution and in solid state, in which the organic and inorganic structural components provide unique interactions with each other and with the external components (solvents, additives, etc.), for material solutions processed in thin films, denotes a significant contemporary target

一次元ハロゲン架橋および三次元ギ酸架橋配位高分子における構造と物性制御に関する研究

Tohoku University山下正廣課

The synthesis and characterization of mixed-organic-cations tin halide perovskites for enhanced photovoltaic cell application

Magister Scientiae - MScIn this research, novel hybrid perovskite materials were synthesized, characterized and applied in photovoltaic cells (PVCs) to enhance the performance of PVCs. Mixed-organic-cations tin halide perovskites (MOCTPs) were successfully synthesized using sol-gel method. These MOCTPs include guanidinium dimethylammonium tin iodide ([GA][(CH3)2NH2]SnI3) and guanidinium ethylmmonium tin iodide ([GA][CH3CH2NH3]SnI3). The MOCTPs were studied in comparison to their single-organic-cation tin perovskites (SOCTPs), which include guanidinium tin iodide (GASnI3), ethylammonium tin iodide ([CH3CH2NH3]SnI3) and dimethylammonium tin iodide [(CH3)2NH2]SnI3. High Resolution Scanning Electron Microscopy (HR SEM) of the five perovskite materials showed good crystallinity and tetragonal and hexagonal cubic shapes, characteristic of perovskites. These shapes were also confirmed from High Resolution Transmission Electron Microscopy (HR TEM), and the internal structure of the perovskites gave similar zone axes (ZAs) with those obtained from X-ray Diffraction (XRD). XRD showed tetragonal lattice shape for these perovskite materials. Fourier Transform Infrared (FTIR) demonstrated similar functional groups for both the SOCTPs and MOCTPs. FTIR bands that were observed are; N-H, C-H sp3, C-H aldehyde, N-H bend, C-N sp3 and N-H wag. From the 13C Nuclear Magnetic Resonance (NMR) results, the carbon atom of guanidinium iodide precursor shifts from downfield to upfield position, e.g. from 110.57 ppm to 38.49 ppm in GASnI3 SOCTP. This confirms a shift upfield of the carbon atom in guanidinium iodide precursor as it bonded to Sn metal in the perovskite chemical structure. Similar behavior was also observed for the NMR spectra of [GA][CH3CH2NH3]SnI3 MOCTP, where C-2 and C-3 atoms of ethylammonium iodide precursor shifted upfield from 37.03 ppm to 15.69 ppm and 16.06 ppm to 14.39 ppm respectively

Improvement to the thermoelectric properties of PEDOT:PSS

Thermoelectric materials can convert waste heat to electricity without moving parts. Extensive research into improving the efficiency of inorganic thermoelectric materials has allowed some materials such as bismuth tellurides to be commercialized. These materials, however, contain materials in low abundance on earth such as tellurium therefore their use and scaled production would be limited. Organic and hybrid thermoelectric materials can meet the gap for niche markets as well as be synthesized on mass, due to utilization of earth abundant elements such as carbon, sulphur, and nitrogen. The thermoelectric generators require several n and p-type materials connected for optimal efficiency. There are many ways to create inorganic thermoelectric n and p-types. However, for the organic thermoelectric materials the efficiency lags because of their lower electrical conductivity and Seebeck coefficient as well as the lack of effective strategies in the development of n-type materials. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is one of the most successful and researched p-type organic thermoelectric material and thus, the thesis will explore effective strategies for decoupling the apparent trade-offs observed when improving either the electrical conductivity or Seebeck coefficient. The first major contribution to knowledge discussed in this study is the utilization of a reducing agent treatment on PEDOT-ionic liquid composites (reader is advised to refer to chapter 5). Another significant finding was the successful development of a novel route to an n-type single walled carbon nanotube, PEDOT:PSS composite (please refer to chapter 6). The final and most significant contribution to knowledge in this research project was the development of a set of novel single walled carbon nanotube, ionic liquid, PEDOT:PSS composites whereby after a post treatment with a guanidinium iodide in ethylene glycol solution allowed for improvement of the electrical conductivity from 3.4 S cm -1 to 3665 S cm-1 and Seebeck coefficient from 12 μV K-1 to 27 μV K-1 thereby leading to an optimised power factor of 236 μW m-1 K-1 at 140 °C (please refer to chapter 7)

Using applied field, pressure, and light to control magnetic states of materials

Due to their low energy scales, flexible architectures, and unique exchange pathways, molecule-based multiferroics host a number of unique properties and phase transitions under external stimuli. In this dissertation, we reveal the magnetic- and pressure-driven transitions in [(CH3)2NH2]Mn(HCOO)3 and (NH4)2[FeCl5(H2O)], present a detailed investigation of these materials away from standard equilibrium phases, and develop rich two- and three-dimensional phase diagrams. The first platform for exploring phase transitions is [(CH3)2NH2]Mn(HCOO)3. This type-I multiferroic contains Mn centers linked by formate ligands creating Mn-O-C-O-Mn superexchange pathways. Magnetization measurements reveal two transitions - a spin-flop and a transition to the fully polarized state - and the loss of long-range order above the Neel temperature. Extending to the high-pressure regime, we perform vibrational spectroscopy across the order-disorder transition and use a correlation group analysis to determine the high pressure space groups. The superexchange pathway plays a crucial role in triggering the structural crossover to lower symmetry. Despite having driving different space groups above/below the order-disorder temperature, compression lowers each symmetry to the polar space group P1. We develop the pressure - temperature - magnetic field phase diagram for [(CH3)2NH2]Mn(HCOO)3 and articulate the potential for enhanced polarization under compression. The type-II multiferrroic (NH4)2[FeCl5(H2O)] is different. It hosts a unique set of exchange pathways mediated by through-space hydrogen- and halogen-bonding. Magnetization displays a series of transitions, including the spin-flop, transition to the fully saturated state, and many associated reorientation transitions. Extending to high-pressure studies, we employ infrared absorption and Raman scattering under compression to reveal an increase in hydrogen bonding and changes in the FeCl5H2O polyhedron that are unique to this regime. A space group analysis uncovers a sequence of space group changes that suggests it is driven to a polar space group. We generate the complete three-dimensional phase diagram, which displays the many competing structural and magnetic interactions. Together, these findings uncover magnetically-driven quantum phase transitions and reduced symmetry under compression to likely polar space groups. This work motivates extended investigations of non-equilibrium phases under external stimuli in these and other molecule-based materials with low energy scales, flexible architectures and unique spin interactions

Improving our Understanding of Metal Halide Perovskite Photovoltaic Operation and Degradation

A steady increase in global energy consumption since the industrial revolution necessitates advances in energy production, transportation, and storage. Photovoltaics offer an attractive means to meet our energy production demands while creating minimal adverse effects on the environment. However, modern day technologies do not possess the ability to scale at sufficiently low cost per kilowatt hour to become pervasive. Perovskite photovoltaics, which entered the photovoltaic world in 2009, have many inherent advantages over these commercialized technologies such as solution processability, low formation energy, high bulk defect tolerance, and high absorption coefficients that afford them the ability to revolutionize the market through very cheap and scalable fabrication processes if they can achieve high efficiency and stability. While perovskites progress in efficiency has been truly unrivaled by any other technology, stability remains a significant hurdle. Moreover, the community currently uses countless different active layer compositions, processing routes, device architectures, and until recently even degradation protocols that can lead to drastically different degradation profiles of the devices. Due to this variability, it will be desirable to develop a mechanistic understanding of degradation, as such an understanding will allow us to predict how various modifications to a device layer should impact stability and may potentially even allow the development of accelerated degradation testing protocols in the future. This thesis aims to progress our understanding of perovskite device operation and degradation, focusing on establishing trends in stability and setting up a framework that can be used to methodically improve our understanding of perovskite devices and how they function. It is broken into four main sections. The first section outlines the fields current understanding of perovskite stability, starting at the material level and working its way up to devices. As will become apparent, while the bulks of these materials appear to be relatively robust when tuned for proper tolerance factor, surface induced degradation and interactions with other layers in the device stack can lead to rapid degradation, necessitating further optimization of interfaces within the device. Ideally, these modifications should be coupled to controlled, monitored, and standardized stability testing in order to understand how and why various modifications affect stability. In this direction, the second section of the thesis focuses on establishing equipment and analysis protocols that will allow the stability of various devices to be tested under extremely similar conditions and then analyzed to extract trends. The third section then builds on the former two, presenting several in depth case studies where selective contacts, lead-based perovskites, and tin-based perovskites have been altered to enhance stability, efficiency, or our understanding of device operation. Finally, the last section moves away from the case studies, attempting to establish some trends in stability and photoemission data as well as discuss some ideal future directions for research