Synthesis Of Pyrazine-based Red Thermally Activated Delayed Fluorescence Material And Research Of Its Devices

Organic light-emitting device(OLED)based on the TADF mechanism can upconvert triplet excitons to singlet state under thermal activation and then produce delayed light emission,achieving theoretically 100%internal quantum efficiency(IQE).Consequently,its external quantum efficiency(EQE)can be comparable to that of commercially available phosphorescent OLEDs.Besides,TADF materials only contain organic molecules that are low-cost and easy be produced industrially,which makes TADF-OLED very promising for commercial development in the future.At present,significant breakthroughs have been made in blue and green TADF-OLEDs.However,the development of red TADF-OLEDs is dwarfed by that of blue and green TADF-OLEDs.Here,solutions to the incompletely unsettled problems have been proposed,and we have made the following progress:(1)Aiming at the problem of low efficiency of red TADF materials,DPXZ-BPDF and TPXZ-BPF were designed and synthesized,and the basic properties and electroluminescence characteristics of these two materials were studied.The reasonable structure design made the highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)electron clouds of DPXZ-BPDF and TPXZ-BPF less overlapping,which results in a very small?Est of 0.01 eV and 0.03 eV,respectively.A strict rigid structure in DPXZ-BPDF was adopted to suppress the energy loss caused by non-radiative transition and realize efficient red light emission.Under the optimized doping concentration,high exciton utilization rate was achieved,and the PLQY reached74.6%.The maximum EQE was as high as 15.36%,and the emission wavelength is 590nm.On the other hand,TPXZ-BPF further reduced the energy gap by increasing the number of donor units,phenoxazine,making the S₁ energy level lower and redshift the emission wavelength to 608 nm.However,the increased number of donors weakened the overall molecular rigidity,leading to increased energy loss,and the maximum efficiency of the device dropped to 12.52%.At a luminance of 1000 cd·m^-2,the EQE of the devices based on DPXZ-BPDF and TPXZ-BPF can still reach 11.41%and 9.73%,demonstrating a smaller efficiency roll-off.(2)Aiming at the slow progress of red light TADF materials for solution-processed OLEDs,we designed and synthesized new orange-red light TADF molecules DPXZ-BPDPA and DPXZ-BPTPA suitable for solution process,and studied the basic properties and electroluminescent characteristics based on these two materials.The reasonable structure design made the HOMO and LUMO electron clouds of DPXZ-BPDPA and DPXZ-BPTPA less overlapping,which results in a very small?Est of 0.03 eV and 0.02eV,respectively.A tert-butyl solubilizing group was introduced to the donor part of DPXZ-BPDPA to achieve good solubility and thermal stability.The optimized doped solution-processed OLED device based on DPXZ-BPDPA achieved 585 nm orange-red light emission with maximum EQE of 2.56%.On the other hand,the donor part of DPXZ-BPTPA,triphenylamine,possessed a large degree of rotation,which endowed DPXZ-BPTPA decreased molecular rigidity and increased solubility.The optimized doped solution-processed OLED based on DPXZ-BPTPA achieved 595 nm orange-red light emission with a maximum EQE of 6.45%,which brokes the theoretical limit of traditional fluorescence efficiency of 5%,indicating the TADF characteristics of DPXZ-BPTPA.