Thermally Activated Delayed Fluorescence Materials With Multiple Charge Transfer States And Its Performance Research

At present,the development of organic light-emitting diode(OLED)display technology has achieved important application value in society.As the light-emitting layer material used in OLED,the research of thermally activated delayed fluorescence(TADF)materials is of great significance.As a third-generation luminescence material,TADF materials can realize the transfer of excitons from the lowest excited triplet state(T₁)to the lowest excited singlet state(S₁)through reverse inter-system crossing(RISC),so that it can achieve a theoretical internal quantum efficiency of 100%without the use of heavy metal atoms.In this paper,we are committed to realizing efficient and highly stable TADF materials and explore the relationship between molecular properties and molecular design strategy under the design strategy of multiple charge transfer states from the aspects of energy level regulation,the photophysical properties and application performance of OLED devices.In the second chapter of this article,based on the design strategy of multiple charge transfer states,we selected the triarylboron unit with three connection sites as the electron acceptor and used dimethyl acridine,phenoxazine and phenothiazine as the electron donor.Three molecules with multiple charge transfer states named 3DMAC-TB,3PXZ-TB,and3PTZ-TB were designed and synthesized.The density functional theory is used to calculate the orbital distribution and corresponding energy levels of these molecules.The results prove that the highest occupied molecular orbitals(HOMO)of these molecules are distributed in the donor group,and the energy levels are almost the same,while the lowest unoccupied molecular orbitals(LUMO)is mainly distributed on the acceptor group,verifying that the material has multiple channels of charge transfer state.The donor group and the acceptor group of the molecule owned a large twist angle,thus realizing the effective separation of HOMO and LUMO and reducing the energy gap between the lowest excited singlet state and the lowest excited triplet state(?E_ST).The characterization of photophysical properties showed that the designed molecules have small?E_STand typical TADF properties,and still maintained a high fluorescence radiation rate.These light-emitting molecules were doped into DPEPO as guest materials and used as light-emitting layers in OLED devices to explore the application properties.Among them,3DMAC-TB-based devices achieved a maximum external quantum efficiency(EQE)of 36.5%,thanks to the large photoluminescence quantum yield(PLQY),while the devices based on 3PXZ-TB and 3PTZ-TB achieved the highest EQEs of 21.9%and 19.4%,respectively.These three devices all showed low efficiency roll-off under high brightness conditions and good device stability.In the third chapter of this article,based on the design strategy of the multiple charge transfer states,we regulated the donor group by introducing the phenyl group to weaken the electron donating ability of the electron donor and lower the T₁energy level of the molecule.Two sky blue materials named 3DPAc-TB and 3Spiro Ac-TB were designed and synthesized.The introduction of phenyl groups enlarged the intermolecular structure,which could effectively inhibit the aggregation-caused quenching(ACQ)of molecules.Compared with3DPAc-TB,the molecular structure of 3Spiro Ac-TB locked two free phenyl groups through chemical bonds,which enhanced the rigidity of the molecule and inhibited the aggregation quenching of the molecule more effectively,so that it could achieve a higher PLQY.Compared with 3DMAC-TB,these two molecules showed blue-shift of the fluorescence emission spectrum and also maintained TADF properties.Subsequently,we applied them to solution-processed OLED devices,achieving maximum EQEs of 12.8%and 17.3%,respectively.At the same time,it also showed a sky-blue emission and narrow full width at half maxima in the electroluminescence(EL)spectrum.This paper explored the influence of the structure on its properties and discussed the further effect on the application performance by adjusting the structure of the material.At the same time,it strengthened the relationship among the molecular structure,properties and performance,and provided an effective strategy of the development of OLED and design of TADF molecules by the design idea of multiple charge transfer states.