| Organic light-emitting diode（OLED）is a new generation of solid-state lighting and flat panel display technology with many advantages,such as low energy consumption,fast response speed,and being self-luminous.As the third-generation luminescent material,the thermally activated delayed fluorescence（TADF）materials are characterized by high internal quantum efficiency（IQE）like phosphorescent materials and low cost like the traditional fluorescence materials,and have been one of the research hotspots in both industry and scientific communities.At present,there are still some challenging problems in developing novel TADF materials and TADF-OLEDs.For example,it is difficult for traditional TADF molecules to achieve a large radiation transition rate（kr）and a small singlet-triplet energy splitting（ΔEST）simultaneously.In addition,the TADF-OLEDs usually suffer from severe efficiency roll-off at high driving region due to long triplet lifetimes of TADF emitters in the order of microseconds or even milliseconds.Accordingly,this dissertation is devoted to develop novel TADF materials.The molecular design and performance optimization strategies are focused on intramolecular hydrogen bonding,site isolation via barrier groups,and dual-channel charge transfer.The detailed research contents are as follows:（1）A group of blue TADF materials SON-Cz,SON-t Bu Cz and SON-Ph Cz were prepared by regulating the exciton dynamics parameters through intramolecular hydrogen bonds.The hydrogen bond between the pyridine N atom and the carbazole 1-H atom effectively improves the molecular rigidity,reduces the non-radiative transition of excitons（knr）,and increases the oscillator strength,and kr.In spite of the flattening of molecular conformation caused by the hydrogen bonds that is not favorable for spatial HOMO/LUMO separation,the pyridine enhances the acceptor strength and increased the molecular transition dipole moment as well.As a result,TADF feature was strongly enhanced with reducedΔEST values,and the PLQYs were simultaneously increased as compared with hydrogen bond-free control molecule.The external quantum efficiency（EQE）of the SON-t Bu Cz and SON-Ph Cz TADF-OLEDs reached up to 29.59%and 28.22%,respectively,which are among the best values for sky-blue TADF-OLEDs so far.In addition,the all-TADF white OLED with SON-t Bu Cz as blue host emitter also achieved an EQE of 23.51%which is one of the highest efficiencies of all-TADF single layer WOLEDs so far.（2）Four TADF molecules PT-BZ-DMAC,PS-BZ-DMAC,PT-BZ-PXZ and PS-BZ-PXZ were designed and synthesized by attaching thioether or sulfone on the benzophenone acceptor.The doped TADF-OLEDs of these emitters realized EQEs of 17.71%(49.55 cd A-1),20.55%(64.55 cd A-1),18.08%(57.99 cd A-1),and 8.89%(25.47 cd A-1).The single crystal analysis and photophysical study revealed that the electron withdrawing sulphone enhanced the acceptor strength and caused red shift of the corresponding emitters,while the thioether only acted as barrier group to reduce the intermolecular interaction.PT-BZ-DMAC exhibited an EQE of 17.34%in the non-doped OLED,which was comparable to that（17.71%）of the doped device.This was attributed to the thioether and the methyl groups on DMAC moieties to reduce the unwanted intermolecular interactions.（3）Using D-σ-A motif as the molecular framework,dual CT-channel TADF molecules,Pmd-S-DMAC,Pmd-S-TPA,Pmd-S-PXZ were constructed with pyrimidine as the acceptor and thioether as theσspacer between donor and acceptor.Through-bond charge transfer（TBCT）was blocked up by the un-conjugated thioether.However,the intramolecular through space charge transfer（TSCT）was verified by the photophysical study.While the single crystal analysis revealed the extra close interaction between the donor and the acceptor units of the neighboring molecule,suggestion the existence of intermolecular TSCT effect.As a result,these TADF emitters are characterized by dual-CT-channel feature.The doped TADF-OLEDs of these emitters realized sky-blue to green emission with EQEs of 17.88%(39.51 cd A-1),16.18%(44.47 cd A-1)and 20.74%(62.14 cd A-1),respectively.（4）Triazine,with stronger electron-withdrawing ability than pyrimidine,was selected as acceptor to construct D-σ-A type dual-CT-channel TADF emitters Trz-S-DMAC,Trz-S-TPA and Trz-S-t Bu TPA.The doped TADF-OLEDs of these emitters realized EQEs of 18.73%(52.43 cd A-1),23.17%(66.12 cd A-1),and 26.60%(76.95 cd A-1),respectively.A“head-to-tail”arrangement style was detected in the single crystal of Trz-S-TPA,in which the donor of one molecule was stacked with the donor of the neighboring molecule in an edge-to-face style,leading to a short donor-acceptor distance of 2.69（?）and the most effective intermolecular TSCT.The efficiency of the Trz-S-TPA device does not decrease significantly with increasing the doping concentration.The maximum EQE of its non-doped device reaches 20.60%(60.90cd A-1),and remains 19.56%and 13.45%at the brightness of 100 and 1000 cd m-2,which are higher than those of most of the green non-doped TADF-OLEDs reported so far. |
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