Investigation Of The Mechanism Of Enhancement Performance In Rubrene-Based Fluorescent OLEDs

Organic light-emitting diodes(OLEDs)have been in development for more than two decades,and they have quickly become popular with researchers due to their advantages of cheap supplies,fast response,wide viewing angle,and good flexibility.In recent years,in order to make it better put on the market,experts are committed to study the stability and luminous efficiency of OLED.In the family of organic materials,rubrene has become a hot material among organic materials with its unique photophysical properties,which makes it play different functional materials in organic semiconductors such as OLEDs,organic field effect transistors(OFETs)and organic solar cell(OSCs).In order to study the physical mechanism of rubrene in these devices and improve their performance in various aspects,doping the original rubrene-OLED device with a light-emitting layer,introducing new materials,and changing the structure of the device are all conventional methods for researchers to prepare devices.In the measurement,the organic magneto-electroluminescence(MEL)occupies a great advantage in many measurement methods due to its non-contact,zero damage,and zero interference.It has been used in the measurement of rubrene OLED devices for many times,and has made a lot of remarkable achievements in its optical-electrical-magnetic properties.However,due to the complexity of its internal physical mechanism,there are still many problems that need to be solved urgently.In this paper,rubrene acts as different functional layers in different OLEDs.The relationship between excitons and polaron pairs of these devices under different concentrations,different injection currents,and different operating temperatures is studied through the relationship between MEL and luminous intensity.Thus,its internal physical interaction law is more clearly understood,which lays a certain foundation for improving the photoelectric conversion rate of the device.This paper mainly includes the following four parts:(1)The first chapter introduces the definition,development history,device structure,classification of spin-pair state and electron-hole injection ? transport ? composite luminescence mechanism of OLED.In addition,the basic knowledge of some commonly used organic magnetic response fingerprint curve is also introduced.Finally,the basic knowledge of the rubrene material.(2)The second chapter mainly introduces the specific preparation methods and main steps of the OLED devices involved in this article,including cleaning substrates,spin-coated polymer materials,and molecular beam epitaxy.In addition,it also briefly introduces the measurement requirements and principles.(3)In chapter 3,we used rubrene as a sensitizer and the conventional fluorescent material DCJTB as a common guest to be incorporated into the wide-gap host material m CP to prepare a rubrene-assisted light-emitting OLED.After measuring and analyzing its MEL response and current-luminescence(I-B)curve,it was found that the electroluminescence(EL)efficiency of this device is significantly higher than that of conventional DCJTB doped devices(m CP: y% DCJTB).This is because in the target device,besides the direct charge injection of DCJTB dopant also present in the reference device,there is a reverse intersystem crossing(RISC)of triplet excitons via strong triplet fusion(TF)of rubrene and multiple energy transfer(ET)channels sensitized by rubrene.Importantly,the EL originating from rubrene sensitization is stronger than that from the DCJTB guest itself.In addition,both the EL efficiency and the contribution of RISC via TF of rubrene were increased at high sensitizer concentration,large bias current and low operating temperature.Clearly,these current and temperature dependences are in sharp contrast to the RISC features of traditional thermally activated delayed fluorescence-sensitized OLEDs.These findings demonstrate that traditional fluorescent rubrene can be used as an effective sensitizer and reveal a feasible approach to realize high-performance OLEDs.(4)The fourth chapter introduces the mixing of different proportions of phosphorescent materials Ir(ppy)3 with strong spin-orbit coupling(SOC)characteristics in rubrene films to prepare hybrid devices of different proportions.By measuring and studying their MEL and I-B curves at different injection currents and temperatures,it was found that the singlet fission(SF)process of different proportions of hybrid devices at room temperature shows the characteristics of increasing first and then decreasing with the increase of Ir(ppy)3.However,the luminous efficiency of OLEDs shows a monotonically increasing characteristic.Obviously,this is not consistent with the results of traditional rubrene doped devices(m CP: y%rubrene)with increasing doping concentration,enhanced SF but reduced luminous efficiency.The reason for this phenomenon was found after research and discussion: the molecular space of rubrene,intersystem crossing(ISC)caused by the strong SOC of Ir(ppy)3 and ET process between triplet exciton of Ir(ppy)3 and singlet exciton of rubrene.The combined action of these three micro-mechanisms leads to the abnormal relationship between MEL and luminescence of the device,and the current density and the working temperature of the device also have a good regulatory effect on them.Clearly,this work provides a good reference value for understanding the internal mechanism of enhanced EL efficiency and micro-processes of rubrene devices.