Fluoranthene-Based Materials for Non-Doped Blue Organic Light-Emitting Diodes
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.