Design, Synthesis, And Optoelectronic Properties Of Charge Transporting Materials For OLEDs

This dissertation consists of four chapters. Chapter 1 summarizes the advances in organic light-emitting diodes (OLEDs), including design, application and understanding of organic semiconductor materials. Chapter 2 introduces the synthesis, characterizations and applications of oxadiazole derivatives. Chapters 3 and 4 introduce the synthesis and characterizations of the polymers containing carbazolyl or 1,3,4-oxadiazolyl groups, respectively. More experimental details are given below.Part 1. Oxadiazole derivatives: synthesis, characterization, and their application in OLEDs as electron-transporting/hole-blocking materials1. A series of novel non-branched phenylene/oxadiazole oligomers used as electron-transporting/hole-blocking materials, with 2,5-bis(diaryl)-1,3,4-oxadiazole core and end-capped with different functional groups have been developed by Suzuki coupling reaction. These compounds, showing excellent thermal stabilities as determined by thermo-gravimetric analysis (TGA) and lower LUMO and HOMO levels as determined by cyclic voltammetry (CV), can be used as electron-transporting/hole-blocking materials in OLEDs.2. Through the judicious and facile fluorine end-functionalization, a more compact packing of extended Ï€-conjugated molecules, which could be well anticipated in OXD-1 and OXD-5, would favor electron transport, and contribute to efficient electron injection from the cathode into the doped emitting layer. The incorporation of sprio-fluorene as end groups made the compound have more stable morphology.3. OXD-1, OXD-2, and OXD-3 have been used as electron-transporting/ hole-blocking materials in OLEDs with the device structure ITO/NPB (60 nm)/DCJTB:Alq₃ (0.5%, 10 nm)/ETHB(20 nm)/Alq₃ (30 nm)/LiF (1nm)/Al (100 nm). The DCJTB-doped device utilizing OXD-1 as electron transporting/hole blocking layer and Alq₃ as the host achieved pure red luminescence at 620 nm originating from DCJTB, with a narrow FWHI of 65 nm, maximal brightness of 13300 cd/m² at voltage of 20.8 V and current density of ca. 355 mA/cm². High current and power efficiencies (>3.6 cd/A, 1.0 lm/W) were retained within a widerange of current densities even at low doping level. The high performance was interpreted in terms of efficient electron injection and exciton confinement leading to more balanced charge injection. Our work alleviated/overcame the "color purity-device efficiency" trade-off in NPB/Alq₃:DCJTB/Alq₃ diodes. With regard to the lower doping concentration, color purity and ease of device fabrication, it is anticipated that efficient and stable DCJTB-based red-emitting devices could be developed by making use of appropriate ET/HB materials. Additionally, OXD-1 turned out to be a good electron transport/hole block (n-type) material and may be envisaged in other optoelectronic applications.4. OLEDs used OXD-4, OXD-5, OXD-6, and OXD-7 as the electron-transporting/ hole-blocking materials with the device structure ITO/TCTA(10 nm)/ADN(30 nm)/OXD or Alq₃ (30 nm)/Mg:Ag(250 nm) showed higher performance compared with the device using Alq₃ as the electron-transporting layer. The results indicated that OXD-4, OXD-5, OXD-6, and OXD-7 have good hole-blocking abilities.Part 2. Synthesis and characterization of polymers containing carbazolyl or 1,3)4-oxadiazolyl groups via RAFT polymerization1. The controlled/living radical polymerization of methacrylates containing carbazolyl or 1,3,4-oxadiazolyl groups via RAFT polymerization was successfully demonstrated. Functional polymers poly(CzEMA), poly(CzHMA), poly(t-Bu-OxaMA), and poly(Naph-OxaMA) with hole- or electron-transfer ability were synthesized with cumyl dithiobenzoate (CDB) as a chain transfer agent (CTA) and AIBN as an initiator in a benzene solution. The dependence of molecular weight (MW) and polydispersity index (PDI) of the resulting polymers on the molar ratio of monomer to CTA, monomer concentration, and molar ratio of CTA to initiator has also been investigated. The MW and PDI of the resulting polymers were well controlled as being revealed by GPC measurements. The controlled/living features of the RAFT polymerization were confirmed by the very low polydispersity indices, feasibility to control molecular weight by changing the initial molar ratio of monomer to CTA.2. The resulting polymers were further characterized by NMR, UV-Vis spectroscopy, thermo-gravimetric analysis (TGA), and cyclic voltammetry (CV). The polymers functionalized with carbazole group or 1,3,4-oxadiazole group exhibited goodthermal stability, with a decomposition temperature of 5% weight loss about 305 ℃ and 262℃ for poly(CzEMA) and poly(CzHMA), and about 323 and 316℃ for poly(t-Bu-OxaMA) and poly(Naph-OxaMA), respectively, as determined by TGA.3. The study on the optical, thermal, and electrochemical properties of poly(CzEMA), poly(CzHMA), poly(t-Bu-OxaMA), and poly(Naph-OxaMA) prepared by RAFT polymerization indicates that the existence of CTA terminal groups dose not affect the properties of these functional polymers.It is concluded that RAFT polymerization is a promising technique to prepare hole- and/or electron-transport polymers with controlled molecular weight, narrow molecular weight distribution, and well-defined structures.