Research Of Organic Light-emitting Diode Based On Exciplex Emitter With Multiple Reverse Intersystem Crossing Channels

Organic light-emitting diodes?OLEDs?have become one of the most high-profile display technologies in recent years because of their advantages such as self-emitting,large viewing angle,fast response speed,high contrast,and the possibility of preparing flexible devices.Among the light-emitting materials of OLEDs,thermally activated delayed fluorescence?TADF?materials can achieve 100%of the theoretical internal quantum efficiency without using precious metal atoms,which has become a current research hotspot.Unlike ordinary TADF materials,the exciplex system can achieve the separation of frontier molecular orbitals?FMOs?more naturally due to intermolecular interactions,which allows it to obtain a very small single-triplet energy level difference(?E_ST).Thus,through the reverse intersystem crossing?RISC?process,the up-conversion of triplet excitons is achieved.Compared with single-molecule TADF materials,the exciplex system is not efficient when used as a emitter,which is caused by too small fluorescence transition rate?k_F?.In order to solve the problem of low efficiency of the exciplex emitter,a new multiple RISC channel strategy was proposed.Based on this strategy,we did the following work:?1?In the reported polymer-based exciplex system,the introduction of ternary small molecules to construct a ternary exciplex system PVK:PO-T2T:mCP,the photophysical properties and film stability were studied.And the solution-processed OLED devices based on ternary exciplex system was fabrictaed.Since PVK:PO-T2T:mCP has two RISC channels,it shows extremely high photoluminescence quantum yield?PLQY?value and higher RISC rate,and maintains the excellent film-forming ability and film stability of polymer PVK.In the end,the solution-processed OLED device based on this ternary system reached a maximum enternal quantum efficiency?EQE?of 21.9%,which was the only reported polymer-based exciplex device with an EQE of more than 20%.?2?The TADF material TX-o-DMAC used to construct the exciplex system was designed and synthesized.Unlike the general TADF molecule,in this TADF material,the molecular conjugation is weakened by the central diphenyl ether group,the intramolecular effect is weakened,and it exhibits fragment properties when interacting with the electron donor material 13AB?1,3-bis?9,9-dimethylacridin-10?9H?-yl?benzene?.In the exciplex 13AB:TX-o-DMAC,RISC channel exists not only in the TX-o-DMAC,but also exists interaction between 13AB and TX-o-DMAC.The electroluminescence properties of OLED devices fabricated with 13AB:TX-o-DMAC as the light-emitting layer were studied,reaching a maximum EQE of 21.2%,compared with the single RISC channel exciplex mCP:TX-o-DMAC device,the device efficiency is significantly improved.Our research fully proves that multiple RISC channels are an effective strategy to improve the efficiency of exciplex emitters.Through this strategy,the RISC rate of the exciplex is obvious increased,which greatly improves the exciton utilization,and the device efficiency is significantly improved compared to the ordinary single-channel exciplex.