Research On Organic Small Molecule Luminescence And OLED Devices

Organic light-emitting diode（OLED）is an electroluminescent device,which has the advantages of low driving voltage,low power consumption,simple preparation process,high luminous efficiency and full color display.OLED has been widely used in the fields of flat-panel displays and solid-state lighting.In the past ten years,OLED has received considerable attention both in the academic and industrial communities.The main characteristic of OLED comes from the light-emitting layer,which is usually consistent of host material and guest material.Among them,guest materials can be divided into three categories according to different luminescence mechanisms:fluorescent materials,phosphorescent materials and thermally activated delayed fluorescence（TADF）materials.However,there are problems of insufficient performance enhancement and difficulty in precise control both for the host materials and the guest materials.In-depth studying the internal regulation mechanism of molecular structure,packing mode and performance is an inevitable requirement for precise control of materials.Based on this,this paper has carried out a series of works around the guest TADF material and host material in the OLED light-emitting layer.The specific contents are as follows:1.First,pure organic TADF material OCF₃-A based on a newβ-diketone boron difluoride was achieved via one-pot synthesis.Polymorphs of OCF₃-A molecule were obtained by temperature-controlled self-assembly method,and polymorph-dependent fluorescence was reported.Combining steady-state spectroscopic characterization and femtosecond transient absorption,we confirmed that the two polymorphs belong to TADF and excimer fluorescence,respectively.And we realized polymorph-based TADF and excimer fluorescence switch,and the stokes shift of excimer even reaches280 nm.Combining a series of photophysical characterizations and theoretical calculation,it can be confirmed that the two aggregates belong to J-type packing and dimer packing,respectively.And the J-type packing is in favor of TADF fluorescence emission in crystal and the photoluminescence quantum yield reaches 17.6%,which is close to low ratio doped PS films.In contrast,the dimer packing totally terminates the TADF emission and only presents the excimer emission with short lifetime.Therefore,this study provides a way to design high efficiency TADF emitters and will do some helps on the application of pure TADF emitters.2.Furthermore,we synthesized a high triplet energy（E_T）bipolar host material3,3’-6,6’-tetrabutyl-9,9’-diazole（TBC）by Suzuki coupling reaction.The photo-physical,thermal,morphological,electrochemical,charge-transporting and electroluminescence properties were comprehensively investigated.These results confirmed that TBC can serve as an ideal host material for Ph OLED in theory.In order to verify the application of TBC in Ph OLED,we fabricated green and yellow Ph OLEDs.Consequently,green-emitting TBC/Ir（ppy）₃ Ph OLED exhibits maximum external quantum,power and current efficiencies of 15.5%,40.0 lm W^-1 and 56.1 cd A^-1,respectively,with a low turn-on voltage of 2.4 V.While yellow-emitting TBC/PO-01 Ph OLED shows maximum external quantum,power and current efficiencies of11.0%,35.6 lm W^-1,35.9 cd A^-1,with a low turn-on voltage of 3.3 V.The excellent device performances illustrate that TBC is a promising candidate for the future exploration of host materials in Ph OLED.