Synthesis And Device's Performances Of Bipolar Optoelectronic Materials Based On Benzimidazole

Organic light-emitting devices(OLEDs)have the advantages of autonomous illumination,high brightness,high color contrast,wide color gamut,and easy mass production,and have great potential in the field of next-generation solid-state lighting and full-color flat panel display.Related industries have caused great concern.Since phosphorescent materials can use both singlet and triplet excitons at the same time,the theoretical internal quantum efficiency can reach 100%,which has become a focus of attention.However,due to the long lifetime of the triplet excited state,in order to suppress the triplet-triplet annihilation and concentration quenching of the phosphorescent materials,the light-emitting layer will appear in the form of host-guest doping.Therefore,the development of host materials is of great significance.Among them,benzimidazole group-based optoelectronic materials are favored by researchers because of their good electron transport properties,excellent thermal stability and strong modification.In this work,a new series of benzimidazoles were synthesized by using benzimidazole as the core and modified by carbazole group and phenoxazine group.The photophysical properties and electroluminescence properties were studied.The specific work is as follows:(1)In order to improve the electron mobility,a strong-pull electron group,pyridine,was introduced to design and synthesize a benzimidazole/carbazole-based host material N-BCz BI,and for comparison,a pyridine group was replaced with a benzene ring to synthesize a host material BCz BI.It has been proved by research that both compounds have good thermal stability,in which the thermal decomposition temperature is above 370 °C,and the glass transition temperature is above 130 °C.Doped green phosphorescent OLEDs based on BCz BI and N-BCz BI have good performance,among which devices based on N-BCz BI have better performance of EQEmax,power efficiency and current efficiency are 24.6%,78.5 lm/W and 88.5 cd/A,respectively and low efficiency roll-off.(2)By adjusting the hole/electron transporting group ratio,we design and synthesize two host materials PBCI and MBCI based on bisbenzimidazole/carbazole,and the effects of different connection methods between two benzimidazoles on photophysical properties,thermal stability and electrochemical properties were investigated.Both the compounds PBCI and MBCI showed good thermal stability,and the thermal decomposition temperatures were 512 °C and 514 °C,respectively,where the glass transition temperature of MBCI was as high as 176 °C.The green phosphorescent devices based on PBCI host exhibited the maximum efficiencies of5.4%,7.1 lm/W and 19.6 cd/A.(3)In order to improve the luminous efficiency,three new bipolar fluorescent materials PBOZ ? MBOZ and D-MBOZ were designed and synthesized by usingphenoxazine groups to modify benzimidazole groups.The substituents of different positions and numbers were investigated.The three compounds had high quantum efficiency and good thermal stability at 340-390 °C,and the fluorescent OLEDs prepared based on D-MBOZ had the maximum efficiencies of EQEmax,power efficiency and current efficiency are 2.6%,8.5 cd/A and 5.3 lm/W,respectively.