Synthesis And Property Studies Of Phosphorescent Host Materialsbased On Benzimidazole Derivatives

Organic light emitting diode (OLED) have drawn many attentions for their advantages of low-cost, solution-processing and flexibility. Though huge progress has been achieved in the past decades, phosphorescent OLED devices with simple structure, high performance and high stability are still rare. Besides, the relationship between molecular structure and device performance has not been understood, and it is still a challenge to improve the ultimately device performance by tuning the molecular structure. In this thesis, according to the design principle of phosphorescent host materials, a series of small molecular phosphorescent host materials based on benzimidazole have been constructed and intensively investigated. Then these materials have been used in OLED devices, and the performance of the device were optimized and comparatively studied. The main contents and results in this thesis are as follows:1. Briefly introducing the development process of the organic light emitting diode, basic principle, the application containing benzimidazole moieties in organic electrophosphorescent devices and expounded the design philosophy of this paper;2. A series of m-terphenyl-substituted C-benzimidazole phosphorescent host materials with high Tg of178℃and high ET of3.05eV were designed and synthesized. By using the series of host materials for blue PHOLEDs, the devices showed the maximum current efficiency (ηCE,max) of31.5cd/Aand EQE of16%with a low efficiency roll-off. All of the above results suggest that controlling the linkage positions of the benzimidazole and substituent group could tune properties of host materials and the efficiencies of the PhOLEDs;3. By conjugating two substituent groups to the3,6-positions of the spirofluorene, a series of host materials concurrently possessing high triplet energies and thermal stabilityfor blue PhOLEDs were synthesized. A maximum ηCE,max of34.2cd/A and a maximum power efficiency (ηPE,max) of34.41m/W based on these materials as host were achieved, respectively. It is found that even at a practical brightness of1000cd/m2, the devices have a roll-off of only7.6%. In comparison, to the conventional2,7-disubstituted homologues,3,6-disubstituted spiro-fluorene will be more suitable for blue host materials with high ET; 4. By conjugating carbazole groups to the meta or Para-positions of N-benzimidazole, two host materials mNBICz and pNBICz for blue PhOLEDs were synthesized. mNBICz was utilized as host for FIrpic-doped blue device, and exhibited high efficiencies with ηEQE,max of26.2%, ηC. max of54.5cd/A and ηp,max of52.2lm/W. Furthermore, a two-color, all-phosphor single layer white device displayed a ηEQE,max of22.9%, ηCmax of62.5cd/A and ηp,max of60.4lm/W. All of the above results suggest that the N-benzimidazole have more advantages over C-benzimidazole for blue host materials;5. By conjugating two carbazole groups to the C, N-positions of the benzimidazole, a series of phosphorescent host materials were carefully synthesized. The thermal, photophysical and electrochemical properties of benzimidazole derivatives have been improved through the adjustment of substitution ratio between carbazole and benzimidazole. Devices employing the new compounds as the host for the green emitter of Ir(ppy)3displayed a ηEQE,max of18.7%, ηc. max of68.3cd/A and ηp,max of63.7lm/W.Through the study of this series of compounds, further understanding to therelationship between molecular structure and device performanceof benzimidazole/carbazole derivatives was obtained;6. A new series of blue host materials with3,9linking topologies between the benzimidazole and carbazole moieties were facilely prepared. PhOLEDs were fabricated with DmNBICz doping with FIrpic, a ηcmaxof29.2cd/A and ηp,max of23.1lm/W were achieved. This study shows that it’s reasonable to tune the thermal stability and HOMO/LUMO of benzimidazole derivatives without changing their Er;7. A novel series of simple electron-transport (ET) type host materials based on benzimidazole and phosphine oxide group have been designed and synthesized. When these materials were utilized as both the electron transporting and host layer, significantly reduced operating voltage is achieved and the devices exhibited a maximum efficiency of ηEQE,max of23.8%and ηp,max of85.0lm/W.These results demonstrate that utilizing benzimidazole derivatives as electron-transport type host materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device configuration and enhance the performance of OLED devices.