Synthesis And Properties Characterization Of Organic Electroluminescent Materials Based On Sulfur-containing Electron Transport Groups

The properties of fluorescent/phosphorescent host/guest materials have played a key role on the efficiency and lifetime of OLED devices. The theoretical maximum external quantum efficiency of the traditional fluorescent materials is only5%, because conventional fluorescent devices can only use25%of singlet excitons for luminous radiation. In order to utilize singlet excitons and triplet excitons effectively, Professor Forrest of Princeton University in the United States take heavy metal complexes as phosphorescent dye, and use host/guest doping system, which makes exciton utilization rate reaching100%in theory. In recent years, professor Adachi in Japan has raised the phenomenon called Thermally Activated Delayed Fluorescence (TADF), realizing theoretical100%internal quantum efficiency of fluorescence radiative emission through molecular designing to implement the intramolecular triplet excitons transiting reverse to singlet excitons. Due to its outermost six electronic structure, S element shows the characteristics of lack of electron, in the meanwhile S element has vacant3d orbitals, so sulfur-containing groups typically have a strong electron withdrawing properties. In addition, sulfone unit formed by sulfur oxidation showes strong electron injection and transport characteristics. Based on advantages mentioned above, the introduction of S element can effectively adjust the push-pull electron properties of molecules. The thesis is based on the electronic transport groups containing S element, varying the bonding position and replacing groups to design various OLED host/guest materials. Specific content is as follows:Generally introduce the basic knowledge of organic electroluminescent devices, its development history, industry status and the sulfur-containing electron transporting groups used in organic electroluminescent materials. Finally, describes the design conception and theoretical basis of materials.1,2,4-thiadiazole as a novel electron transporting group, has been first introduced to construct bipolar phosphorescent host molecules with typical hole-transporting group carbazole incorporated. Results have shown that these materials exhibit the ability of balancing hole and electron mobilities, and their glass transition temperature are about 170℃, illustrating that the introduction of1,2,4-thiadiazole group plays a key role on improving the glass transition temperature of the molecule. When materials’ glass transition temperature is high, their film still shows good film morphology after high temperature annealing. As a result of introducing these groups, the maximum external quantum efficiency (ηEQE, max) in the green PhOLEDs reaches26.1%, and the devices show a relatively low roll-off, even at the brightness of10000cd/m2,their external quantum efficiency remains above20%.Based on the shortcoming of a relative high turn-on voltage shown in the preceding chapter, adjust the electron mobility of these materials. Due to3-bromo-5-chloro-1,2,4-thiadiazole has good molecule modification property, the electron mobility can be increased gradually by taking1,2,4-thiadiazole as the center, connecting tert-butylbenzene, carbazole and benzimidazole to3,5positions of1,2,4-thiadiazole. As a result, the electron mobility of the host is enhanced with increasing the quantity of benzimidazole group, and the turn-on voltage of the devices decreases along with increasing electron mobility. Device fabricated with the electron-type material DBzTHZ as host shows the maximum power efficiency and external quantum efficiency of95.4lm/W and23.4%, respectively, which is consistent with the molecular design.Take diphenyl sulfone as the electron transport group.Via molecular design, introduce sulfone group into spirobifluorene directly, which ensures the molecule a high triplet energy level, so that it can be used as the blue phosphorescent host material. By modifying sulfone with phosphine oxide group at different positions, build two electron-type phosphorescent host materials. Device with diphenyl sulfone derivatives as host achieves good results providing theoretical guidance for the next molecular design.Based on preliminary study on property of diphenyl sulfone, introduce diphenyl sulfone sulfur and carbazole/triphenylamine groups on both sides of spiral structure through molecular design, design and synthesize a series of bipolar blue phosphorescence host material. PhOLED with these hosts with diphenyl sulfide sulfone group as the electron-transporting group shows further improved property.In view of excellent electron injection and transporting properties of sulfone, introduce a six-membered ring thioxanthone group incorporated through carbonyl groups, which increases its electron withdrawing property. Thioxanthone incorporated with carbazole or triphenylamine shows a clear TADF effect. With these compounds as the guest, the maximum external quantum efficiency of fluorescence device achieves more than10%, breaking the theoretical external quantum efficiency limit of5%of the traditional fluorescent..