Functional Polymer Architectures for Solution Processed Organic Light Emitting Diodes (2010)

Organic light emitting diodes (OLEDs) prepared from electroactive materials show great potential for multicolor display and white light source applications. Unfortunately, the commercialization of multilayer OLEDs has been slow. This can be attributed in part to the vapor deposition technique used to assemble small molecule thin films. The process is not amenable to large area displays and is relatively costly. An attractive alternative solution to this problem is to replace small molecules with organic polymers, which could be solution processed in an efficient and economical manner. Polymeric materials also offer the unique potential to achieve nanoscale self-assembled structures through their functional architecture. These materials can be solution processed and mimic multilayered small molecule devices to achieve improved performance and/or balanced color. Design and synthesis and OLED testing of functional polymers which can achieve defined multilayer or nanostructured film characteristics through simple solution processing is the focus of this thesis.\tIn Chapter 1, the history of OLED materials as well as the device mechanism and key organic electronic characteristics necessary for high performance devices is introduced. There is also a discussion on the methods for device testing and characterization.\tThe design and synthesis of novel dendronized linear polymer host materials is presented in Chapter 2. These polymers should possess a rigid linear rod like architecture which had not been investigated for its potential to order a thin film in an assembly of cylindrical structures. This host material was desired in order to optimize the interface of hole and electron transporting material to achieve improved recombination in the thin film.\tSimilarly, in Chapter 3 a diblock copolymer architecture is studied as host material in OLED devices. In this work the differences between nanoscale self assembled diblock copolymers of hole and electron transporting units and random copolymers of the same composition are studied. These materials show a high external quantum efficiency of 5.6 % for a simply prepared single layer device which is enabled by the self-assembly of the functional diblock copolymer architecture.\tIn Chapter 4, these diblock copolymers are exploited not only to create nanoscale domains of hole and electron transporting domains but also to organize the site isolation of two different colored phosphorescent emitters. Polymerizable heteroleptic iridium complexes of different color were developed and covalently incorporated into separate blocks of the diblock copolymer. Following the self assembly of thin film morphology through simple spin coating, the energy transfer from blue to red emitters was greatly reduced enabling synergistic dual emission for white electroluminescence.\tChapter 5 discusses the design and synthesis of electroactive crosslinked polymer nanoparticles with nanoscale size that can achieve the site isolation of emitters. Using different polymerizable iridium complexes, batches of different colored polymer nanoparticles can be simply prepared and mixed at the device preparation stage in any ratio to yield tunable colored devices. These nanoparticles dispersions behave as light emitting inks which can be simply solution processed with predictable and stable electroluminescent color.\tIn Chapter 6, difunctional polymerizable iridium complexes are used to achieve multilayer structures of electron blocking layers and phosphorescent emissive layers. These small molecules can be solution processed to yield thin films which can be crosslinked through simple heating step. A subsequent layer can then be deposited on top to build up all solution processed multilayered devices. A select high triplet energy crosslinkable iridium complex was shown to perform well as an electron blocker and hole transporting layer in OLEDs with improved performance over the standard water soluble hole transporting layer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).