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The challenge of density functional theory is the useful approximation of the exchange - correlation energy. This energy can be approximated with the local electron density and the gradient of the density. Many different generalized gradient approximations (GGA) have been made recently and there is controversy over the best overall functional. Recent Monte Carlo simulations give evidence that the Laplacian of the density might be a better starting place than the gradient to correct the local density approximation. We have tested several Laplacian based GGA models for exchange for small atoms. We use known constraints on the exchange energy used in current GGA’s. In many models unphysical oscillations occur in the potential when using the Laplacian, and understanding and eliminating them is part of the focus of this research. We also find that mixing gradient and Laplacian seems to give a better result than only using one or the other. / Department of Physics and Astronomy

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

"MIT-3221-19." / Bibliography: p. 78-81. / Contract AT(30-1)-3221.

Thesis (Ph. D.)--University of Texas at Austin, 2007. Thesis (Ph. D.)--Université Paul Sabatier, 2007. / Thesis completed in partial fulfillment of the requirements for a joint Ph. D. from the University of Texas at Austin and the Université Paul Sabatier. Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [106]-113).

Organic light-emitting diodes (OLEDs) have been considered as the future lighting and display system and rapidly growing since 1987. It has been already used in many commercial applications such as OLED televisions, cell phone displays, and lighting systems. The OLED has higher luminous efficiency and extremely thinner layer compare to any other lighting devices, also it has flexibility and self-emission. However, there are still some drawbacks for the device performances such as lifetime especially on blue organic films, cost of manufacturing process, and moisture that we need to work on before wide-scale commercialization like LCD or LED. This thesis has focused on developing a charge distribution such as deriving empirical equations in multi-emissive layer OLED, improving external quantum efficiency (EQE) and lowering roll-off. Key results are summarized as follows: (1)We seek to establish a quantitative method to estimate the holes and electrons ratio in the recombination zones. The result shows a trend in the charge recombination ratio depending on the hole and electron transport layer (HTL/ETL) thickness. We obtained an empirical relationship between electron/hole transport layer thicknesses and emission ratio in emissive layer (EML). In addition, the electroluminescence (EL) spectra were analyzed by fitting a Gaussian distribution for the two emissive layers to calculate the intensity ratio of the energy transitions. The arrival time of hole and electrons from each electrode was determined using the thickness and mobility of NPB as hole transport layer and TPBi as electron transport layer. From these initial results, we derived an empirical mechanism to meet with an exponential relationship that can allow us to design custom- made OLEDs. (2)We fabricated White OLEDs in which the emissive layers are chemically doped with blue and red fluorescent dopants of BUBD-1 and DCJTB. This work continues by estimating of emission ratio between red and blue emissive layers by changing the thicknesses of HTL and ETL. The recombination of charge carriers was first identified the location and then we derived an empirical equation for peak intensity ratio of EL spectra with respect to thickness of the HTL/ETL to determine how recombination zone depends on the HTL and ETL thickness. The EL spectra of WOLEDs were fitted with a Gaussian distribution for the two emissive layers using host-dopant system and intensity ratio of blue and red emission peak is 61:39 when thickness of HTL and ETL are 80nm and 20nm, respectively. Also, this intensity ratio of blue and red emission peak (61:39) has the CIE color coordinates of (0.34, 0.40). We obtained a preliminary relationship between thickness of electron/hole transport layer and ratio of two emission peaks. (3)The improved external quantum efficiency (EQE) and reduced roll-off properties of blue phosphorescent organic light-emitting diodes (PHOLEDs), were obtained with structure, ITO/NPB (40 nm)/TCTA (20 nm)/mCP:FIrpic (7%)(30 nm)/TPBi (30 nm)/Liq (2 nm)/Al (80 nm) by incorporating a TCTA inter-layer. We compared the properties of BCP and TPBi as the ETL with a typical structure of HTL/ EML/ETL in OLEDs and utilized inter-layer in the optimized structure to enhance EQE to 52% at 5.5 V, also stabilize the roll-off of 63%. The use of inter-layer in blue PHOLEDs exhibits a current efficiency of 10.04 cd/A, an EQE of 6.20% at 5.5 V and the highest luminance of 10310 cd/m2 at 9.5 V.

A spectroscopic diagnostic tool has been developed to determine the electron temperature and the neutral density in helium, hydrogen and argon plasmas from absolutely calibrated spectroscopic measurements. For each gas, a method of analysis which uses models specific to each species present in the plasma (neutral atom or singly ionized atom) has been defined. The experimental electron density is used as an input parameter to the models, and the absolutely calibrated spectroscopic data are processed beforehand to obtain the populations of the upper excited levels corresponding to the observed spectral lines. For helium plasmas, the electron temperature is inferred from the experimental helium ion excited level p = 4 population using a corona model, and then the neutral density is determined from the experimental helium neutral excited level populations using a collisional-radiative model for helium neutrals. For hydrogen plasmas, combinations of the electron temperature and the neutral density are determined from the experimental hydrogen neutral excited level populations using a collisional-radiative model specific to hydrogen atoms. For argon plasmas, the electron temperature is inferred from the experimental argon ion excited level populations using a collisional-radiative model for argon ions, and then the neutral density is determined from the experimental argon neutral excited level populations using a collisional-radiative model for argon neutrals. This diagnostic tool was applied to three experiments with different geometries and plasma conditions to test the validity of each data analysis method. The helium and hydrogen data analysis methods were tested and validated on helium and hydrogen plasmas produced in the VASIMR experiment, a plasma propulsion system concept. They gave electron temperatures and neutral densities that were consistent with other diagnostics and theory. The argon diagnostic tool was tested on argon plasmas produced in the VASIMR experiment, the Helimak experiment and the Helicon experiment. The electron temperature and neutral density obtained on both the Helimak and the Helicon experiments were consistent with other diagnostics and with theory, and validated the method of analysis. An impurity problem on the VASIMR experiment made it difficult for the data analysis to be validated.

In the exploration of chemical systems through quantum mechanics, accurate treatment of the electron wavefunction, and the related electron density, is fundamental to extracting information concerning properties of a system. This work examines challenges in achieving accurate chemical information through manipulation of the one-electron basis set.

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