| With the advantages of low drive voltage,fast response and high luminance,organic light-emitting diodes?OLEDs?have drawn great attentions in recent years and gradually get commercialized.At present,the majority of the materials used in commercial OLEDs are phosphorescent complexes containing noble metals such as Ir and Pt.These heavy metals suffer from limited reserves,high prices and lack of intellectual properties.Therefore,developing metal-free pure organic fluorescence materials is of great significance.Small fluorescent molecules have the advantages of low cost and high purity when comparing with phosphorescent materials.But they can only utilize 25%singlet excitons according to the spin statistics,which restrict the maximum of the external quantum efficiency of these traditional fluorescence materials to be 5%in OLEDs if the out-coupling efficiency is 20%.So developing pure organic small molecule which can use the remaining 75%triplet excitons has been a hot research focus in recent years.The ways for organic small molecules to utilize triplet excitons mainly include thermally activated delayed fluorescence?TADF?,triplet-triplet annihilation?TTA?,“hot exciton”,free radicals and some other mechanisms.TADF and“hot exciton”mechanisms,using charge transfer states with weak exciton binding energy to realize reverse intersystem crossing?RISC?process from triplet to singlet state,can theoretically achieve 100%internal quantum efficiency and high device performances.TADF and“hot exciton”based small molecules used in electroluminescence are almost D-A type molecules consisting of electron donor and acceptor groups,which realize the control of emission wavelength by tuning the relative strength of donor and acceptor.Blue and red materials are poorly developed when compare with the green counterpart.The main reason is that blue materials with wide bandgaps require the constructing groups to have higher LUMO and lower HOMO energy levels,which results in the scarcity of the appropriate building groups.And as for red-emitting materials with narrow bandgaps,their lowest excited singlet states tend to be consumed by nonradiative transition process subjected to the energy gap low,leading to lower photoluminescence efficiency and worse device performances.Thus,developing blue and red organic materials with high efficiency for electroluminescence remains main research concerns for OLED.In this dissertation,we focus on designing pure organic blue/red fluorescent materials which can utilize triplet energy and fabricating their high-performance devices.Based on the electron donor 9,9-diphenyl-9,10-dihydroacridine?DPAC?,different acceptors with various electron-withdrawing ability are introduced to construct a series of high-efficiency D-A type organic small molecules.Then the relationships between structure and property are investigated.DPAC has suitable electron-donating ability with a sp3 hybridized N atom;its rigid structure is beneficial to attain high efficiency;DPAC has larger volume because of the strong steric hindrance effect since acridine is substituted by two benzene rings,which help adjusting the interactions for DPAC based D-A molecules in aggregated state.The main results of this dissertation are summarized as follow:?1?Based on the donor DPAC,weak electron acceptor diphenyl-sulfone and its derivative are introduced to get two high-efficiency deep blue or blue emission materials DPACDPS and DPACDBTDO.It is found that the change of triplet energy of the acceptor directly influences the energy levels and TADF property of the resulting compounds.Both of them possess high photoluminescence quantum efficiency?PLQY?,the PLQY of DPACDBTDO thin film is as high as 100%.The doped device using DPACDBTDO as emitting layer obtains a maximum external quantum efficiency?EQE?of 13.1%(34.2 cd A-1),with electroluminescent emission peak at 500 nm and CIE coordinates of?0.255,0.455?.?2?The strong electron acceptor,dicyanopyrazine derivative is introduced to DPAC to construct a TADF material namely DPACPhAP with red emission,and it is also introduced to DMAC to achieve DMACPhAP for comparison.Under the premise of effective separation of HOMO and LUMO,the connecting ways between donor and acceptor are adjusted to increase the photoluminescence efficiency of both molecules.The pure-red doped devices based on DPACPhAP and DMACPhAP achieve the maximum EQE of 14.1%(10.3 cd A-1)and 14.4%(12.6 cd A-1),respectively,and the EL peaks of both devices are at 640 nm.This is a good result among pure-red TADF electroluminescent devices.?3?DPAC and assistant acceptors are introduced to the both sides of classic naphthothiadizole?NZ?group to give three highly-efficient red materials named DPACNZP,DPACNZPCN and DPACNZPPI.They all have big torsion angles beyond50°between NZ group and bridging benzene which attributes to the spatial steric hindrance of NZ moiety.The NZ group seems to be“wrapped up”,which can effectively weaken the fluorescence quenching and dipole-dipole interactions of devices in aggregation states.Thus,the nondoped device based on DPACNZP reaches a high EQE of 4.6%(6.3 cd A-1)with red emission peaked at 624 nm and the CIE coordinates of?0.599,0.397?,which is a good result for red-emission nondoped devices.Further,combining the analysis of exciton utilization rate in devices and energy levels calculated by theoretical calculations,it is believed that the triplet harvesting mechanism of these NZ derivatives belongs to“hot exciton”model.The exciton utilization efficiency?EUE?of red-emission doped device based on DPACNZPPI is as high as 74%,in which the maximum EQE reaches 8.7%(11.9 cd A-1)with emission peak at 620 nm and CIE coordinates of?0.604,0.392?.This is an excellent device result in red electroluminescence without TADF characters,and is also the best result among the reported NZ-containing molecules.In summary,we have designed and synthesized a series of high-efficiency materials with emission from deep blue to red and further understand the structural characteristics and the potentials of DPAC.The intermolecular interactions are effectively weakened by means of decorating acceptor containing two active sites with large-volume DPAC and other assistant acceptors,which provide new ideas for further molecule design.Based on the effective separation of HOMO and LUMO via the big-torsion structures between DPAC and different acceptors,both TADF or“hot exciton”mechanisms using the weak exciton-binding charge transfer state to harvest triplets are realized according to the different triplet energy levels,which is meaningful for further material design. |
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