The Design,Synthesis,Characterization And Application Of High-Performance Organic Light-Emitting Materials

Organic light-emitting diode(OLED)is a flat light emitting device with sandwich structure.Holes and electrons are injected from the planar electrodes into organic semiconductor and respectively migrate toward the opposite electrodes under external electric field.The luminescent molecules can simultaneously capture the injected holes and electrons to form both 25%short-lived singlet and 75%long-lived triplet excitons based on spin statistics.In 1987,C.W.Tang et al.reported the first efficient OLED by employing a heterojunction structure,which attracted a great deal of attention from both academic groups and commercial companies.With collective efforts from world-wide researchers,OLEDs have been gradually commercialized in display industry.However,the performances of OLEDs need to be further improved with more requirements from people.In addition to modifying device structure and production process,it is of importance to design and synthesize a series of mass-produced,cheap,efficient and stable organic functional materials.In this dissertation,with the aim to obtain good performance devices,a variety of organic functional materials applied in the emitting layer of device were designed and synthesized.Accordingly,the properties and application in OLEDs of abovementioned materials were systematically studied.The study of organic functional materials can be divided into two parts:host materials with high triplet energy and novel fluorescent materials.The first part is about host materials.In this part,we designed two boron-based host materials for highly efficient blue and white phosphorescent OLEDs with low efficiency roll-off.Boron has one fewer electron than carbon configuration so that it tends to form electron-deficient multiatom networks.Therefore,its derivatives have ability of electron-acceptor and have been widely used in different fields.However,due to the poor performance of devices used borane derivatives as hosts,it is seldom being used as host materials in OLEDs.In order to shed new light on the application of borane derivatives in the field of host materials,we designed two novel bipolar hosts named SAF-3-DMB and SAF-4-DMB.The strategies were to increase the spatial separation of the highest occupied molecular orbitals(HOMOs)and lowest unoccupied molecular orbitals(LUMOs)in the molecules,incorporate specific functional groups with desirable thermal stability and tune the connecting positions of functional groups.Based on these designs,devices with borane derivatives as host materials held outstanding performances.Especially,devices employing SAF-3-DMB as a host exhibited an external quantum efficiency(EQE)of>25%.Meanwhile,the device had quite a low efficiency roll-off with an efficiency of>20%even at a high brightness of 10000 cd/m2.Furthermore,the EQE of the three-color-based white OLED used SAF-3-DMB as a host was as high as 22.9%.At a brightness of 5000 cd/m2,there was only a 3%decrease in EQE.Therefore,borane derivatives are promising host materials by reasonable chemical modification and it is worth paying attention to the applicability of borane derivatives as hosts.The second part of the dissertation is about the novel fluorescent materials and the detailed information about them are in Chapter Three and Chapter Four,respectively.According to the different mechanism of luminous,organic electroluminescence can be divided into two categories.That is organic electroluminescence and organic electrophosphorescene.Comparing to phosphorescent materials,there is no heavy metals in fluorescent materials,which makes fluorescent materials have been playing a critical role in OLEDs.Therefore,it is quite important to design and synthesize novel fluorescent materials for improving the performance of devices.In Chapter Three,in order to overcome the disadvantage that luminophores is generally weakened or quenched upon aggregation,we designed and synthesized a series of fluorescent materials with the characteristics of aggregation-induced luminescence(AIE),named SAF-2-TriPE,SAF-3-TriPE and SAF-4-TriPE.We found that incorporation of the spiro-acridine-fluorene(SAF)group,at different substitution positions on the phenyl ring affected the conjugation lengths of these compounds.Accordingly,a device based on compound SAF-2-TriPE emitted green light with EQE value of 4.22%,whereas devices that were based on compounds SAF-3-TriPE and SAF-4-TriPE emitted blue light with EQE values of 1.71%and 1.42%,respectively.These compounds also displayed different AIE performances,that is.the fluorescence quantum yield(?F)of compounds SAF-2-TriPE and SAF-3-TriPE increased obviously with the fraction of water in the THF solutions increasing;in contrast,compound SAF-4-TriPE showed an abnormal phenomenon,in that it emitted a strong fluorescence in both pure THF solution and in the aggregated state without a significant change in ?F.Overall,this systematic study confirmed a relationship between the regioisomerism of the luminophore structure and its AIE activity and the resulting electroluminescent performance in non-doped devices.On the other hand,the research based on fluorescent molecules with donor-acceptor(D-A)structure has become a hot topic with the propose of HLCT and TADF.However,compared with rich species of electron-donor units,electron-acceptor moieties with a strong electron-withdrawing capability are far from adequate.Therefore,by combined the advantages of dibenzofuran and pyrazine units,a new electron-withdrawing moiety(BFPz)is first used as an acceptor in OLED and its corresponding core unit(2-Br-BFPz)was synthesized.By incorporation this new electron-accepting moiety with a triphenylamine(TPA)group,a novel fluorescent molecule named TPA-BFPz with a high ?F of 92%is formed.Accordingly,the EQE values of the doped and non-doped blue OLEDs employing TPA-BFPz as emitter are 4.22%and 3.68%.