Crystallization | EPFL Graph Search

Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposition directly from a gas. Attributes of the resulting crystal depend largely on factors such as temperature, air pressure, and in the case of liquid crystals, time of fluid evaporation. Crystallization occurs in two major steps. The first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent. The second step is known as crystal growth, which is the increase in the size of particles and leads to a crystal state. An important feature of this step is that loose particles form layers at the crystal's surface and lodge themselves into open inconsistencies such as pores, cracks, etc. The majority of minerals and organic molecules crystallize easily, and the resulting crystals are generally of go

Spatial and Morphological Control of Cyanine Dye Thin Films

Nicolas Alexandre Serge Leclaire

Cyanine dyes are organic semiconductor compounds with light absorption and emission properties useful for emerging technologies such as solar cells and light-emitting devices. The characteristics of these materials in the solid state depend on their organization of the constituting building blocks. This thesis focuses on controlling the morphology of cyanine dye thin films at different length scales and clarifying the resulting properties. When microstructures present features whose size matches visible light wavelengths, new properties may arise from light-matter interactions. Here the properties resulting from the light-matter interactions of cyanine droplet films cast from solution are studied. Based on experimental evidence, it is shown that dye droplet ensembles scatter light with different efficiencies and wavelength ranges depending on their dimensions. FDTD simulations are used to show that this effect results fromscattering enhancement at the absorption edge of the dye where the refractive index varies considerably. Simulations also provide a better understanding of individual dropletsâ interaction with light. While earlier work had hypothesized that the observed scattering phenomenon were due to crystalline clusters within the droplets, this work highlights the contribution of the dye filmmorphology. Cyanines also form single crystals whose fabrication induces molecular-scale order in the material. Previous work demonstrated that thin single crystals could be grown by solvent vapor annealing of dye droplets. Here it is shown that in uncontrolled conditions, cyanine single crystals destabilize to formdendritic crystals. In-situ microscopy observations highlight the solute reservoir role of the droplet distribution surrounding a growing crystal; when the distance between droplets and the crystal front is large, the solute supply is diffusion-limited. Moreover variations in local pressure equilibrium between the droplets and crystal front lead to advection fluxes which perturb the crystal growth. These observations help design configurations to either prevent crystal destabilization or take advantage of the dendritic growth in a controlled manner. In addition, the patterning of crystals on a substrate is relevant for their application in devices. A practical challenge is to induce single crystal growth at specific locations. Here, surfaces patterned with SAMs of hydrophilic and hydrophobic thiols are used to create dye droplet arrays from which crystals can be grown. This method is shown to yield local crystallization of the dye and to prevent crystal destabilization through better control of the droplet distribution. By varying the dye solution concentration, partial control over crystal density is achieved, however it proved difficult to control the number of nuclei per droplet. A more controlled evaporation and solvent vapor annealing system might be necessary to master the nucleation process. Finally the structure and optical properties of cyanine single crystals are addressed. The crystal structure was determined by X-ray diffraction. Structural aspects are shown to lead to excitonic couplings, which are evidenced by orientation-dependent spectroscopic measurements of single crystals. Although further investigation of the absorption band structure is necessary, the results are promising for photovoltaic devices as they might improve exciton transport compared to amorphous layers.

EPFL2018

Investigation in Crystal Growth/Morphology and Interface Engineering of Perovskite Solar Cells by Different Deposition Methods

Nadja Isabelle Desiree Klipfel

A new and promising technology has emerged to compete with traditional silicon photovoltaic semiconductors. The low material and processing costs and high-power conversion efficiencies of organic-inorganic metal halide perovskites make these a promising technology for the future. Despite record efficiencies for small-area devices, stability remains challenging. Due to rapid progress in the field, avenues for large-area commercial production are being explored. One approach is via thermal evaporation, which is presented in this work. The focus is on the vacuum-deposition method itself, the interface engineering of perovskite layers, and the study of a family of low-cost hole-transporting materials (HTM). To improve reproducibility among research groups in vacuum-deposition processes, a better understanding and control of the crystal formation is essential. Of the experimental parameters, two were investigated that were not yet considered in-depth or combined: deposition speed and underlayer material. It was shown that perovskite growth rate changes affect preferred crystal orientation. In addition, it was found that the final perovskite composition is influenced by its underlayer chemistry. Multi-source vacuum deposition of perovskite layers is complex due to the laborious calibration process necessary for proper stoichiometry. Single-source evaporation of pre-synthesized perovskite powders can reduce effort and time. The feasibility of obtaining pure phase films was demonstrated and confirmed by multiple analysis methods.Interfaces in contact with the perovskite can be a source of defects with a deteriorating effect on stability and efficiency. The charge transport processes at the interface of electron transport material (ETM) and perovskite were investigated. The results showed a synergetic effect between cTiO2 and C60 and the importance of C60 as a charge extraction layer. The perovskite hole transporting material (HTM) interface was also systematically studied. Monitoring of dopants showed, for the first time in literature, TFSI anion migration through grain boundaries to the perovskite surface. It was further shown that non-radiative recombination pathways are reduced, indi-cating self-passivating of crystal defects during migration processes.The state-of-the-art HTM 2,2',7,7'-tetrakis (N,N-di-p-methoxyphenylamine)-9,9'spirofluorene (spiro-OMeTAD) is widely employed in spin-coated PSCs. Devel-oping a less costly manufacturing alternative to this material that retains similar properties is a key challenge. A new class of Zn (II) and Cu (II)-based phthalocyanine HTMs functionalized with butyl- and ethylhexyl- groups in the periphery was investigated. Devices fabricated with the Zn (II)-based HTM (n-butyl functionalization) demonstrated the best performance. The device architecture with the highest stability maintained 80% of its initial power conversion efficiency (PCE) over 20h of illumination. At 20.2%, the perovskite architecture with the highest PCE also was the least stable, indicating a trade-off between these two characteristics for this HTM class.

EPFL2022

Investigation of Sulfate Attack by Experimental and Thermodynamic Means

Wolfgang Kunther

This work investigates sulfate attack in complex sulfate environments by exposing different binder types to various sulfate solutions and comparing predicted phase and volume changes with experimental data. The most important aspects of this work can be grouped in three topics: The comparison of the predicted volume increase with the experimentally observed length changes. This part of the work shows that volume increase cannot be linked directly to the observed expansions. Additionally, the volume increase model does not provide an explanation for the driving force for the space filling to overcome mechanical constraints. A more plausible explanation of expansion lies in the theory of crystallization pressure, in which crystals forming from a supersaturated solution seem to be a more applicable explanation for the deterioration differences. However, this is difficult to verify directly as it is impossible to measure the solution concentration or individual crystals exert pressure on their surroundings. It is observed that expansion occurs in systems where thermodynamic modelling predicts the co-existence of ettringite with gypsum. In such a case, if monosulfate and gypsum are both present locally, the solution can be highly supersaturated with respect to ettringite within the exposed materials. An increase of sulfate contents in the C-S-H phase has been observed for these cases, indicating increased sulfate concentrations in the pore solution while providing the necessary confinement for the crystals in intimate phase mixtures to exceed pressure. The presence of bicarbonate ions in the sulfate solution reduces the expansion significantly for the CEM I and CEM III/B binders, and reduced the sulfate uptake in the phase assemblage. The CEM III/B mortars showed a highly leached zone at the surface in which also calcite was observed, this is attributed to the destabilisation of ettringite in the presence of high concentrations of bicarbonate ions. The microstructural characterization combined with the information from thermodynamic modelling suggests that conditions of high supersaturation with respect to ettringite are unlikely to occur in samples exposed to solutions containing bicarbonate ions. Degradation of mortars exposed to complex magnesium containing sulfate solutions showed that the presence of sodium, potassium and calcium in a magnesium solution reduce the deterioration symptoms significantly for the CEM I and CEM III/B mortars. The experimental observations suggest that sulfate attack in natural environments is not only less severe, due to reduced sulfate concentrations in the field, but are likely to be affected by the presence of bicarbonate anions and the common occurrence of different cations. Both of these aspects reduce the deterioration significantly and can be assumed to occur in most natural waters.

EPFL2012