| Armed with the strengths of inorganic LED and LCD, Organic Light-Emitting Diodes (OLEDs), are widely regarded as the most promising and potential lighting display technology in the new century. The way of OLEDs in the future is turning to the electrophosphorescent devices with the feasibility of carrying out 100% of internal quantum efficiency from electrofluorescent devices. Due to having a 100% of internal quantum efficiency theoretically, comparing with conventional fluorescent electroluminescence, phosphorescent OLEDs (PHOLEDs) must be able to develop a broader territory in the flat display and solid-state lighting industry in the future.The bipolar host material is one of the most potential host materials for PHOLEDs. In this thesis, by introducing the flexible spacer, the conjugation effect between the donor and the acceptor are weakened in order to increase the triplet energy levels and film stability. A series of novel bipolar hosts is designed and synthesized by employing oxadiazole and triazine as the electron transport unit, and triphenylamine and carbazole as the hole transport unit, respectively. Through the means of structure identification, electrochemical analysis, spectral analysis and thermal analysis, the properties of these bipolar host materials are well studied.Oxadiazole and triazine, which are widely used to study the phosphorescence host materials because of their highly efficient electron injection and transport capabilities, are very promising n-type materials in organic electroluminescent field. In addition, triphenylamine(TPA) and carbazole derivatives are very popular hole transporting moieties because of their high triplet energy levels and matched HOMO levels with the hole transport layer and the anode.The main contents and results are described as follows:Recent progress of host materials in phosphorescent OLEDs is reviewed in Chapter 1. Firstly, we introduced the basic concept of OLED, including the development of OLEDs, the emission mechanism, device structures and different functional materials used in OLEDs. And then three approaches to achieve electrophosphorescence are illustrated. Finally, host materials classified by their different charge transporting properties are reviewed. The new approaches and achievements of developing highly efficient organic light-emitting host materials with novel structures are focused. The design strategies and main contents of this thesis are also outlined.In Chapter 2, two bipolar transport host materials,2-(4-(3,5-bis(9-carbazolyl)phenyl)phenyl)-4,6-diphenyl-1,3,5-triazine(mCP-TRZ),2-(4-(3-(3,5-bis(9-carbazolyl)phenyl)propyl)phenyl)-4,6-diphenyl-1,3,5-triazine(mCP-L-TRZ) are facilely synthesized through Suzuki reactions. The incorporation of triazine moiety greatly improves their morphological stability, with glass transition temperature, Tg=160.65℃ and 103.26℃, respectively. mCP-L-TRZ show less intramolecular charge transfer and higher triplet energy(2.9 eV) as compared to mCP-TRZ(2.6 eV), which are suitable for blue phosphorescent OLEDs. These series of experiments affirm the designed molecular structure of this paper and are able to achieve expectation of improving the transfer properties of the host material and the stability of the light-emitting film.In Chapter 3, two compounds are synthesized by linking the electron transporting oxadiazole and the hole transporting triphenylamine through o-, p-connection. Their structures were identified through NMR. The two compounds were analyzed by UV visible absorption spectra, fluorescence emission spectra, comparing the peak absorption of two compounds preliminary validation of the ortho connection can reduce the idea of intramolecular charge transfer.In Chapter 4, an overall conclusion for this thesis is made. The main conclusions of the experiment are made. Finally, the future work on phosphorescent host materials is prospected, based on the work of this thesis. |
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