Study of photophysical processes in organic light-emitting diodes based on light-emission profile reconstruction

Organic light-emitting diodes (OLEDs) are emerging as a promising option for energy-efficient, flexible light sources. A key factor that needs to be measured and controlled is the shape of the emission profile, i.e. the spatial distribution of the emitting excitons across the active layer thickness. Being able to accurately measure the emission profile makes it possible to understand the fundamental (photo)physical processes involved in the device operation, providing a basis for further improving the efficiency. In order to investigate state-of-the-art devices, containing 10-20 nm thick emitting layers, emission profile measurements should provide nanometer-scale resolution. In this thesis, a method is presented and applied to reconstruct the light-emission profile, with nanometer resolution, from the measurement of wave-length, angle and polarization-dependent electroluminescence spectra. The method is introduced in chapter 2 and it is used to investigate the photophysics of OLEDs. It uses a "fit-profile" approach within which the shape of the profile is constrained by making use of our understanding of the recombination process, while still allowing more freedom than in previous studies. The method is first applied to blue-emitting and orange/red-emitting single-layer polymer-based devices. We show that a 5 nm shift of the emission profile within the emissive layer from the cathode-side to the anode-side by increasing the applied voltage can be resolved, and provide a formalism within which the resolution limits can be analyzed. Subsequently, in chapter 3 the resolution is compared to that of a more standard inverse-problem solving approach and analyzed for single-layer, double-layer and multilayer OLEDs. In all cases, the resolution is found to be in the range 1-10 nm. As a next step, in chapter 4 this method is used to determine the singlet exciton fraction in OLEDs. From standard statistical physics considerations a value of 25% is expected. Since in fluorescent materials only singlet excitons can decay radiatively, this fraction limits the maximum achievable efficiency of fluorescent OLEDs. In recent years several studies have indicated that deviations from this value may occur, in particular for polymers. The development of an accurate method for determining the singlet exciton fraction has thus become a topic of intensive discussion and great interest in the literature. We have extended a method presented by Segal et al. (2003) by exploiting the possibility to reconstruct the light-emission profile in OLEDs, and show that for the specific case of intensively studied polyfluorene-based copolymers and for a polyphenylene-vinylene-based polymer the singlet fraction is only 8-25%. The light-emission profiles obtained for the polyfluorene-based copolymers were further used to investigate the validity of charge transport and recombination models. This investigation indicates that the mobility is strongly anisotropic. A study of the emission profile in more complex double-layer small-molecule-based fluorescent OLEDs (chapter 5) is shown to provide novel insights in the photophysical processes near organic-organic interfaces. Increasing the thickness of one of the two layers is found to give rise to an emission profile shift from one side of the interface to the other, and to the occurrence of charge-transfer exciton emission. A delicate balance is shown to govern the exciton emission at both sides of the interface and the charge-transfer exciton emission from the interface itself. Since the corresponding three emission spectra are different, controlling the light-emission profile by varying one of the layer thicknesses results in the possibility to tune the emission color. As a final step, in chapter 6 an analysis is presented of the emission profile in a multilayer white-emitting OLED, investigated within the framework of the European project AEVIOM. The light-emission profile obtained provides a measure of the balance between the generation of excitons in the three emissive layers (red, green and blue). It furthermore enables making an accurate assessment of the validity of the charge transport and recombination models developed in the project, and it is shown to provide deeper insight in the role of exciton transfer processes within and between the emitting layers. The method presented in this thesis is expected to be a fundamental tool for the experimental validation of device models and for designing OLEDs with increased efficiency. The possibility to apply the method as a tool to validate state-of-the-art charge-transport and recombination models (both one-dimensional drift-diffusion models and three-dimensional Monte Carlo models) is discussed in chapter 4 and in the outlook of the thesis (chapter 7). The results presented are not only important for OLEDs, but may also be applied to other organic optoelectronic devices containing disordered organic semiconductors, such as light-emitting electrochemical cells and light-emitting field-effect transistors, as discussed in chapter 7

Spatial resolution of methods for measuring the light-emission profile in organic light-emitting diodes

An analysis is presented of the resolution limits of two alternative methods for deducing the light-emission profile in organic light-emitting diodes (OLEDs) from the angular and polarization dependent emission spectra. The comparison includes the "fit-profile" (FP) method, within which the known physics of the recombination process is employed to describe the shape of the profile using a strongly reduced number of degrees of freedom, and the Tikhonov method, which provides a more general solution. First, the cases of a delta-function shaped emission profile and a broad single-peak emission profile are investigated. It is demonstrated that for these cases a ~1¿nm resolution of the peak position may be obtained, provided that the peak is positioned optimally in the OLED microcavity. Subsequently, an analysis is given for a double-peak emission profile and for a rectangular profile, as may be obtained in multilayer OLEDs, revealing a resolution of ~10¿nm for the cases studied. It is suggested that, in general, an optimal analysis should be based on a combined Tikhonov-FP approach

Liquid hot isostatic pressing of QE22A magnesium alloy: a preliminary test

A preliminary experimental comparison of the behaviour of aluminium and magnesium alloys subjected to Liquid Hot Isostatic Pressing (LHIP) is proposed. The two metals melt at approximately the same temperature.However, as a consequence of a larger deformability of magnesium at elevated temperatures, the choice of LHIP parameters – and especially the temperature at which the pressure is applied – in the present exploratory case was constrained to values far smaller than those one would like to select in order to improve the ultimate tensile stress and the elongation to fracture

Effect of botulinum neurotoxins from Mendoza of clostridium botulinum strains on cytoskeletal proteins of mammary tumor cells

The botulinum neurotoxin serotype A (BoNT A) produced by Clostridium botulinum, which causes botulism, is used for the treatment of multiple neurological diseases and its therapeutic action against cancer is currently being evaluated. In previous studies, we have shown that BoNT A from autochthonous soil strains (Su) have different properties than the reference A Hall strain. Among these, its molecular structure, its enzymatic activity against brain SNARE proteins and its greater specific toxic activity (AE) stand out. In cells from human mammary carcinoma (MCF-7) treated with BoNTs for 45 min, we found a marked effect on the expression of cytoskeletal proteins. Therefore, in this work, we delve into the study of the action of autochthonous BoNTs A and prototype A Hall on the distribution of actin and tubulin in these cells. Native forms of autochthonous BoNT (Su strains 1935 and 1891, Tupungato) and prototype A Hall were purified by saline precipitation. Their AE values (LD50 / mg protein) were established and their electrophoretic characteristics were evaluated under non-denaturing conditions. 250 LD50 of the BoNTs were incubated to MCF-7 cell cultures for 10 or 25 min. Later, the cells were fixed and processed for indirect immunofluorescence with the use of specific antibodies that recognize tubulin or actin. The samples were visualized by fluorescence microscopy. At the two times evaluated, the three types of BoNTs produced a marked redistribution of the actin cytoskeleton, patch form, on areas coinciding with the plasma membrane. Tubulin was redistributed to multiple areas with high signal density at 10 min of incubation only in the presence of BoNT 1891. At 25 min of incubation, the cells treated with BoNTs 1891 and 1935 showed this effect, while in those incubated with A Hall, the distribution of these proteins was not modified. The notable alterations in the distribution of components of the tumor cell cytoskeleton by BoNT from native strains of Mendoza soils open new perspectives for therapy against solid tumors.Fil: Chapana, Agostina Lucía. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Guarniolo, D.. Universidad Nacional de Cuyo. Facultad de Cs.médicas. Departamento de Patología. Area de Microbiología; ArgentinaFil: Carvelli, Flavia Lorena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto de Histología y Embriología de Mendoza Dr. Mario H. Burgos. Universidad Nacional de Cuyo. Facultad de Ciencias Médicas. Instituto de Histología y Embriología de Mendoza Dr. Mario H. Burgos; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Sosa, E.. Universidad Nacional de Cuyo. Facultad de Cs.médicas. Departamento de Patología. Area de Microbiología; ArgentinaFil: Fernández, R. A.. Universidad Nacional de Cuyo. Facultad de Cs.médicas. Departamento de Patología. Area de Microbiología; ArgentinaFil: Sosa, M. A.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto de Histología y Embriología de Mendoza Dr. Mario H. Burgos. Universidad Nacional de Cuyo. Facultad de Ciencias Médicas. Instituto de Histología y Embriología de Mendoza Dr. Mario H. Burgos; ArgentinaFil: Caballero, P. A.. Universidad Nacional de Cuyo. Facultad de Cs.médicas. Departamento de Patología. Area de Microbiología; ArgentinaIV Reunión Conjunta de Sociedades de Biología de la República ArgentinaArgentinaSociedad de Biología de CuyoSociedad Argentina de BiologíaSociedad de Biología de RosarioSociedad Chilena de Reproducción y DesarrolloAsociación de Biología de TucumánSociedad de Biología de Córdob

tetraxial textiles assessment of mesoscale mechanical modelling by experimental measurements

Tetraxial technical textiles were recently manufactured by a new loom developed to weave yarns in four directions. The textile has warp, weft, and two diagonal yarns oriented at symmetrical angles (typically ±45°) with respect to the warp direction. The peculiar yarns distribution could enhance the mechanical response of the textile in multiple directions aiming to almost isotropic in-plane behaviour. For the prediction of the mechanical performance of such tetraxial textiles, a reliable and accurate predictive model is of relevant importance. The present investigation aims to adopt the finite element numerical approach at the mesoscopic scale to predict the mechanical response for any in-plane loading of tetraxial textiles. An accurate modelling of the constitutive behaviour of the fibrous yarns was adopted considering a hyperelastic model. The modelling of the tetraxial unit cell allowed to have the mechanical behaviour for uniaxial and biaxial tensile and for shear loading conditions. The assessment of the accuracy of the numerical model was performed considering a huge experimental campaign dedicated to several hybrid tetraxial textiles. The comparison highlights the accuracy of the numerical model to predict the nonlinear behavior of the fabric for any loading condition and to provide the proper mechanical model for further optimization of tetraxial textiles supposed for different industrial applications

Predictive modeling of the current density and radiative recombination in blue polymer-based light-emitting diodes

The results of a combined experimental and modeling study of charge transport, recombination and light emission in blue organic light-emitting diodes (OLEDs) based on a polyfluorene derivative are presented. It is shown that the measured temperature-dependent current-voltage curves and the voltage-dependent current efficiency are accurately described using an OLED device model that is based on the separately determined unipolar electron and hole mobility functions. The recombination rate is calculated using the Langevin formula, including recombination of holes with free as well as trapped electrons. The light emission is obtained from the exciton formation profile using independently determined values of the exciton radiative decay probability, the average dipole orientation, and assuming a fraction of singlet excitons ¿S¿¿ = ¿(22±3)%, close to the quantum-statistical value. No additional free parameter is used. This shows that predictive one-dimensional device modeling of OLEDs is feasible

The Lantern Vol. 52, No. 1, Fall 1985

• Nudes • Orion • Fragments of an Epic • Sunrise • The Planting Season • Nursing Home • Hope Chest • Childhood Swing • Relationships • Elroy, Leopold, and Max • Urban Dragon • The Farmer\u27s Wife • A Ballad of Two Lovers • Betrayal • Choices • Letting Go • Emergence of a Butterfly • Poem for Every Man • Friction • Genesis • All\u27s Well • The Willow Tree • White Wasteland • Moe\u27s Happy Christmas • Rare Bird • Carnivalhttps://digitalcommons.ursinus.edu/lantern/1127/thumbnail.jp