Thesis (Ph.D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008. / Committee Chair: Tolbert, Laren; Committee Member: Collard, David; Committee Member: Lyon, Andrew; Committee Member: Srinivasarao, Mohan; Committee Member: Wilkinson, Angus

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.

Fluorescence energy transfer from poly(9,9-dioctylfluorene) to polymeric energy acceptors that include head-to-tail regioregular poly(3-hexylthiophene or P3HT) and poly(2-methoxy-5(2 -ethylhexyloxy)-1,4-phenylenevinylene) or MEH-PPV and monomeric acceptor meso-tetraphenylporphyrin or TPP was studied and correlated with the underlying morphology when the donor-acceptor blends were prepared as drop-coated films or breath-figure structures. It was found that the phase-separate morphology in films and breath figures was influenced by a number of factors, including material transport dynamics, solubility of the blend components in a solvent, interaction of the solvent with the substrate, and the diffusion rate of the blend components.

Thesis (M.S.)--Florida State University, 2006. / Advisor: James R. Simpson, Florida State University, College of Engineering, Dept. of Industrial Engineering. Title and description from dissertation home page (viewed Sept. 22, 2006). Document formatted into pages; contains ix, 95 pages. Includes bibliographical references.

[Truncated abstract] Over the past decade, the group III-nitride semiconducting compounds (GaN, AlN, InN, and their alloys) have attracted tremendous research efforts due to their unique electronic and optical properties. Their low thermal carrier generation rates and large breakdown fields make them attractive for the development of robust electronic devices capable of reliable operation in extreme conditions, i.e. at high power/voltage levels, high temperatures and in radiation environments. For device applications in radiation environments, such as space electronics, GaN-based devices are expected to manifest superior radiation hardness and reliability without the need for cumber- some and expensive cooling systems and/or radiation shielding. The principle aim of this Thesis is to ascertain the level of susceptibility of current GaN-based elec- tron devices to radiation-induced degradation, by undertaking a detailed study of 60Co gamma-irradiation-induced defects and defect-related effects on the electrical characteristics of n-type GaN-based materials and devices . . . While the irradiation-induced effects on device threshold voltage could be regarded as relatively benign (taking into account that the irradiation levels employed in this study are equivalent to more than 60 years exposure at the average ionising dose rate levels present in space missions), the observed device instabilities and the degradation of gate current characteristics are deleterious effects which will have a significant impact on the performance of AlGaN/GaN HEMTs operating in radiation environments at low temperatures, a combination of conditions which are found in spaceborne electronic systems.

The organic light-emitting diode (OLED) technology is emerging to be the future technology of choice for thin, flexible and efficient display and lighting panels and is a potential competitor for the existing flat panel display technologies, like liquid crystal display (LCD) and plasma display panel (PDP). OLEDs display is already making their way from both lab and industry research to display market and the pace of development of laboratory OLED design into a commercial product is very impressive. The OLED display offers several advantages over other display technologies, such as low power consumption, easy fabrication, high brightness & resolution, light weight, compact, flexible, wide viewing angle and fast response. However, OLED display is still in amateur stage in terms of their cost and lifetime. Despite of the abovementioned advantages of OLEDs, there still several issues that need to be addressed to explore the full potential of this display technology. The development of materials with high photoluminescence quantum yield (PLQY), thermal and electrochemical stability, packaging, and light extracting technology are some of the major issues. Among the emitting materials, the achievement of robust blue emitting material with high PLQY and color purity is still a challenge due to its intrinsic wide bandgap and complex device configuration. The work presented in this thesis is devoted to the development of robust blue emitting materials based on fluoranthene derivatives. Fluoranthene unit has been chosen due to its blue emission, high photoluminescence quantum yield, thermal and electrochemical stability. The thesis is organized in six chapters, and a brief discussion on the content of individual chapters is provided below. Chapter 1 provides a short description of evolution of display technology and history of OLEDs. The generation wise development of emitting materials for white OLED is concisely illustrated. The working principle, function of individual layer and factors governing external quantum efficiency of OLED device are elaborated. Finally, the important prerequisite properties of blue emitting materials for OLED application are outlined. Chapter 2 reports the design and synthesis of symmetrically and asymmetrically functionalized fluoranthene-based materials to address the issue of PL quenching in solid state, and subsequently for application in non-doped electoluminescent devices. A detailed experimental and theoretical study has been performed to understand the effect of symmetric and asymmetric functional groups on optical, thermal and electrochemical properties. The fluoranthene derivatives reported in this chapter exibited deep blue emission with high PLQY in both solution and solid state. The vacuum deposited non- doped OLED devices were fabricated and characterized utilizing these materials as emitting layer. Chapter 3 describes the rationale design of thermally stable fluoranthene derivatives as electron transport materials for OLEDs. The two derivatives investigated in this chapter comprised of two fluoranthene units linked by diphenylsulfane and dibenzothiophene linkage. The effect of rigidity provided by ring closure in molecular structure on the physical and charge transport properties has been investigated. Such materials are urgently demanded for better performance and durability of displays. In an extension to chapter 3, fluoranthene based dual functional materials possessing blue light emission and electron transport characteristics are described in Chapter 4. The application of these materials in bilayer blue OLED device successfully demonstrated. The development of such dual functional materials is an important step to not just simplify the OLED device architecture; but also has the potential to reduce the manufacturing and processing cost significantly. Chapter 5 reports the synthesis of the star-shaped fluoranthene-triazine based blue photoluminescent materials for solution processable OLEDs. The effect of chalcogen on the photophysical and electroluminescence properties has been investigated. The main advantage of such solution processable materials over small molecules is to overcome the power consuming vacuum thermal evaporation technique for deposition. Chapter 6 describes the design and synthesis of a new blue emitting material comprising of a donor moiety and an acceptor unit to observe thermally activated delayed fluorescence (TADF). However, photophysical studies did not show any sign of delayed fluorescence in this molecule. Nevertheless, a deep blue electroluminescence is achieved using a multilayer OLED device configuration.