| Organic light-emitting diodes (OLEDs) have good performance in both planar display and solid-state lighting field. For example, they are thin, light, capable of providing surface emission without UV excitation and have wide angle of view and quick response rates. So, OLEDs are displaying vast vistas in both academic and industrial fields. Accoring to quantum mechanics, phosphorescent organic light-emitting diodes (PhOLEDs) can make full use of the excition energy, however, the traditional fluorescence OLEDs use only singlet exction energy, and the energy of triplet is wasted. In order to enhance the utilization of exction energy and prevent triplet-triplet annihilation, the heavy metal complexes are doped into suitable emitting layer materials, therefore, it is important to exploit a series of new host materials.In this thesis, we synthesized a series of bipolar host materials which are based on mCP, oxadiazole and benzimidazole via ortho-, meta- and para- linkages. We obtained a series of high performance host materials through studying the molecular structure, thermal, photophysical, electrochemical, and device properties of the new materials as host.In chapter one, we introduce the concept of OLEDs, including the development, principle and perfomance parameters. Next, we describe the categories of hole/electron transport materials, bipolar transport materials, host materials, and mechanism of PhOLEDs. Finally, the design contents of this thesis are outlined.In chapter two, three new bipolar host materials,9,9'-(2'-(1H-benzimidazol-1-y 1)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (o-mCPBI),9,9'-(3'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (m-mCPBI),9,9'-(4'-(1H-benzimida-zol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (p-mCPBI), are designed and synthesized by integrating mCP with benzimidazole moiety via ortho-, meta-, and para-position of N-phenyl. The influence of different linking modes on the thermal, photophysical, electrochemical, and charge transport properties of the compounds is studied. Through the adoption the same device structure, the device performances of the blue, green, yellow, red, white phosphorescent and blue, green thermally activated delayed fluorescence (TADF) OLEDs based on the three host materials are investigated. Among the three host materials, o-mCPBI exhibits the best device performance with external quantum efficiencies of over 20% for PhOLEDs and enhanced efficiencies for TADF devices. All these devices show relatively low efficiency roll-offs at high luminance. The versatility of the benzimidazole-based bipolar host o-mCPBI, such as an extremely high triplet energy, suitable molecular orbital energy levels, improved thermal stability and more balance of charge transport, makes it a universal optical material for highly-efficient phosphorescent and TADF OLEDs.In chapter three, we have synthesized three new host materials by integrating mCP with oxadiazole moiety via ortho-, meta-, and pora-position. Which are named as 9,9'-(2'-(4"-tert-butyl-1-yl)-1,3,4-oxadiazoles-[1,1'-biphenyl]-3,5-diyl)bis(9 H-carbazole) (o-mCP/BuOXD),9,9'-(3'-(4"-tert-butyl-1-yl)-1,3,4-oxadiazoles-[1,1'-bip henyl]-3,5-diyl)bis(9H-carbazole) (m-mCP/BuOXD), and 9,9'-(4'-(4"-tert-butyl-1-yl)-1,3,4-oxadiazoles-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (p-mCP/BuOXD). The relationship between the conjugation of compound, intermolecular charge transfer and triplet energy levels is investigated by testing the properties of the host materials.In chapter four, we facilely synthesize three new hosts by integrating mCP with oxadiazole moiety, such as 2,5-bis-4'(3",5"-bis(9H-carbazole)-1"-phenyl)-1,3, 4-oxadiazoles (mCP-p-OXD),2,5-bis-3'(3",5"-bis(9H-carbazole)-1"-phenyl)-1,3,4-oxadiazoles (mCP-m-OXD),2,5-bis-2'(3",5"-bis(9H-carbazole)-1"-phenyl)-1,3,4-oxadiazoles (mCP-o-OXD). The mCP-o-OXD has a wide energy level gap, and best performance in PhOLEDs.In chapter five, a summary for this thesis is made and an outlook for PhOLEDs is proposed. |
PDF Full Text Request