Preparation Of Nondoped Deep Red TADF-OLEDs By Vapor Deposition And Its Luminescence Mechanism

Organic light emitting diodes(OLEDs)are a new type of display technology.The principle of this active light-emitting display technology is to use organic materials as the light-emitting layer and directly convert electric energy into light energy under the driving of applied voltage.Its core material is organic material,which has the advantages of more variety and low price compared with inorganic material.It has been 30 years since the development of OLEDs.Red/green/blue OLEDs have made tremendous development and progress.Nowadays,OLEDs display technology is widely used in daily routine like cellular-phone,notebook computer,television,etc.In the wake of developments in science and technology development,OLEDs in the future will surely move forward with the pace of artificial intelligence,uniting with various intelligent tools to create an intelligent society for us.However,if the wavelength of OLEDs is moved closer to Deepred OLEDs(DR OLEDs)/Near infrared OLEDs(NIR OLEDs),its importance cannot be ignored.In the display field,deep red light has higher saturation,bright colors and more realistic pictures.In the field of solid-state lighting,DR OLEDs can be used as a special light source for plant growth in agriculture,and NIR OLEDs can be used to measure blood oxygen saturation,infrared imaging,etc.in medical treatment.In addition,it can also be used in light sensing,optical communication and other fields.It can be said that the importance of DR OLEDs/NIR OLEDs is no less than that of red?green?blue OLEDs,however,its technological development is slower than that of red?green?blue OLEDs,and its performance does not meet applicable requirements.In this article,we use 4,4'-bis(9-carbazole)biphenyl(CBP)as host and the red thermally activated delayed fluorescent material 7,10-bis(4-(diphenylamino)phenyl)-2,3-Dicyanopyrazinophenanthrene(TPA-DCPP)as guest to prepare DR OLEDs with high efficiency,high luminance,and low operation voltage.optimizing the device and analyzed the light-emitting mechanism.The specific work is listed one by one:(1)DR OLEDs are prepared by full evaporation method,in which the light-emitting layer is in the form of quantum well-like structure,namely "host/guest/host",and the number of quantum wells is discussed through the preparation of devices with different periods.In order to improve the carrier injection,we adjusted the host thickness,guest thickness,host/guest thickness and introduced the exciton blocking layer PO-T2 T to optimize the performance of the basic device,and finally realized the use of gradient doping to adjust the wavelength.(2)Under the premise of optimizing the basic device,we further study the influence of the position of the recombination area on the performance of the device.First,the detection sensing layer is introduced to explore the position of the recombination area.By experiments,it is found that the recombination area is near the electron transport layer,which is harmful for improving the performance of the device.Excitons quench easily through the interaction with traps from the cathode interface and polarons from unbalanced carriers.For another,some emitted photons will be re-absorbed by TPADCPP layers on the light out way if the recombination area is too far away from the anode.Therefore,it is very important to take appropriate measures to keep the recombination area away from the cathode side.We introduced a hole confinement layer among the hole transport layer with the light-emitting layer to hinder hole injection and transportation,inhibit the recombination area from moving to the cathode side,and optimize the thickness of the hole confinement layer and the type of material,and then use insulation material Li F suppresses hole injection and reduces hole current.Finally,the thickness of the electron transport layer is optimized from the angle of electron injection.The results show that the introduction of a hole confinement layer of appropriate thickness can indeed make the position of the recombination region far away from the cathode side.The optimization of the insulating material and the thickness of the electron transport layer has further improved the device performance.