Conjugated Carbazole/Thiazole Organic Semiconductors And Their Applications In OLEDs

In recent years, with rapid development and further reseach, commercialization of organic light emitting diodes (OLEDs) has appeared (mobile phones, TVs etc.). However, most high efficient devices reported have complicated material system/device structures that render them much difficult and expensive to fabricate. Meanwhile, with mature research of main-layer materials, people give more attention to materials (such as electrodes, injecting materials etc.) whose research are scarce, and have achieved some breakthroughs. In this thesis, we built multifunctional materials based on groups of indole-carbazole and carbazole-carbazole, and studied relationships between structures and properties. Then, we designed novel hole-injection materials based on thiazole and used them to improve performance of devices with multifunctional materials.1. In chapter II, we synthesized three multifunctional compounds of DPDT-ICZ, DNDT-ICZ and DPA-PCTP-CA. NMR, mass spectrometry, elemental analysis etc. were used for characterization. Studies of crystal structures revealed molecular interactions were weak. With such properties, these materials could form uniform thin films during evaporation and have the possibility of being excellent luminescent materials, hole-transport materials and hosts. We investigated three hole-injection materials of BTDA-TCNQ, TDA-TCNNQ and TDA-TCNAQ. Their simple and cheap process of synthesis, good solubilities which are much better than common hole-injection material HATCN extend their range of application.2. In chapter Ⅲ, thermodynamic, spectroscopic and electrochemical studies of DPDT-ICZ, DNDT-ICZ and DPA-PCTP-CA showed they might be very stable and excellent deep-blue luminescent materials and hosts for phosphorescent devices from green to red. According to electrochemical and spectroscopic studies, their HOMO energy levels were about-5.0eV and LUMO energy levels were about-2.10eV which made them to have the potentiality of excellent hole-transport materials. Thermodynamic, spectroscopic and electrochemical studies of BTDA-TCNQ, TDA-TCNNQ and TDA-TCNAQ indicated they might be stable hole-injection materials with strong electron-withdrawing characteristics.3. In chapter Ⅳ, we applied DPDT-ICZ and DNDT-ICZ of indole-carbazole compounds to devices. Single-carrier devices figured out they were typical hole-transport materials and their hole-transport capabilities were weaker than NPB which was beneficial for them to achieve charge balance in devices. Their non-doped devices emitted deep-blue light and showed high brightness and stabilities, while their low efficiencies limited their application. In quinacridone’s devices which used them as hole-transport materials, high efficiencies and low roll-off of efficiencies indicated they were good hole-transport materials. In yellow and red phosphorescent devices which used them as hosts, high efficiencies and low roll-off of efficiencies proved they were good hosts. Based on facts mentioned above, we used them as both hole-transport materials and hosts to build multifunctional-material devices. Good performance of devices revealed they were excellent multifunctional materials.4. In chapter V, devices with DPA-PCTP-CA of carbazole-carbazole compounds were carried out. Hole-only devices revealed that its hole-transport capability was much lower than NPB, even lower than electron-transport ability of Alq3. Moreover, we applied it to fluorescent and phosphorescent devices as hole-transport material. Although efficiencies of these devices were lower than devices which used NPB and DPDT-ICZ as hole-transport materials, their fairly good performance indicated it was still a suitable hole-transport material. As a result, we used it as hole-transport material and host simultaneously in multifunctional-material devices. However, bad performance of devices indicated that it was not a good multifunctional material. 5. In chapter VI, single-carrier devices of NPB of bottom contact, top contact and doped contact or fluorescent devices which used BTDA-TCNQ, TDA-TCNNQ, TDA-TCNAQ, MoO3, HATCN as hole-injection materials indicated BTDA-TCNQ, TDA-TCNNQ, TDA-TCNAQ were better hole-injection materials than MoO3and HATCN. However, efficiencies of devices which used them as hole-injection materials to improve devices of multifunctional materials above were only slightly increased. This might be due to limited hole-transport abilities of emitting layers. At the same time, current density-voltage curves indicated they were still good hole-injection materials.To sum up, we designed three multifunctional materials and three hole-injection materials, and studied their physical and electroluminescent properties. Furthermore, we used our hole-injection materials to revise multifunctional-material devices. Results showed DPDT-ICZ and DNDT-ICZ were excellent multifunctional materials; BTDA-TCNQ, TDA-TCNNQ and TDA-TCNAQ were excellent hole-injection materials. These properties were consistent with properties that we hoped our materials had when we designed them step by step. The results indicate ideas that designing and modifying molecules from properties which we need are reasonable. Our studies provide some materials that are about relationships between structures and properties, and also produce some excellent materials of OLEDs.