Novel Magnetic Field Effects In Thermally Activated Delayed Fluorescence-Based Devices With Quantum-Well Structure

Organic light-emitting diodes(OLEDs)have advantages owning to their simple preparation process,low cost material,and ultra-thin,which are gradually being applied to TV screens,mobile phone screens,and wristbands.Usually 25% of the singlet excitons are directly transited to light-emitting and the other 75% of the triplet excitons can release their energy through the way of non-radiative transition in the OLEDs.Thus,the maximum internal quantum efficiency of the OLEDs is theoretically 25%.In addition,due to the short lifetime and low performance of OLEDs,it causes the OLEDs products not widely available for production and marketization.Therefore,it is quite necessary to improve the performance and luminous efficiency of OLEDs.Recently,Adachi et al.have synthesized thermally activated delayed fluorescence(TADF)material.The design concept of TADF material is the energy levels between singlet excitons and triplet excitons are extremely close,causing the conversion from the triplet to singlet excitons via absorbing environment heat through the reverse intersystem crossing(RISC)process.Correspondingly,the internal quantum efficiency of the TADF-based OLEDs can theoretically reach 100%,thereby increasing the electroluminescence intensity of OLEDs.Nowadays,a large number of TADF materials are constantly emerging,the device structure is constantly being optimized,and the luminous efficiency of the devices is also rising.For example,TADF-based OLEDs with the quantum-well(QW)structure are suggested.However,we have no clear understanding for the microscopic mechanism in QW-TADF-OLEDs.Therefore,it is important to study the specific physical processes in QW-TADF-OLEDs to make a better practical application in the future.In this paper,we mainly revealed a series of microscopic physical processes occurring in the QW-TADF-OLEDs.However,usually the microscopic processes in OLEDs are more complex and difficult to detect directly.Therefore,we use a highly sensitive method with non-contact and non-destructive for light-emitting devices to study many microscopic processes occurred within OLEDs.That is,organic magnetic field effects(OMFEs).This tool can effectively detect microscopic processes in OLEDs consisting of non-magnetic materials,primarily including magneto-electroluminescence(MEL)and magneto-conductance(MC).In other words,the applied magnetic field can be used to change the luminous intensity of the OLEDs and the current of the device.The physical processes occurring in OLEDs contain intersystem crossing(ISC),RISC,triplet fusion(TTA),and triplet-charge annihilation(TQA)and the like.These processes have their own fingerprint line-shape in the MFE curves.Therefore,we investigate the interactions between the carriers and excitons in QW-TADF-OLEDs by analyzing the MFE curve,device structure,and electroluminescence spectrum.The main content of this paper are listed as follows:In the first chapter,the development history of OLEDs and its application in current life are introduced,as well as the specific luminescence mechanism of OLEDs.Then the evolution history of OMFEs and its specific research contents are introduced.Finally,the several kinds of physical processes occurring in OLEDs are introduced.In the second chapter,the preparation and measurement process of QW-TADF-OLEDs are mainly introduced,including the working principle of the instruments in laboratory.In the third chapter,QW-TADF-OLEDs and non-QW reference devices based on the TADF materials used as the light-emitting layer are introduced.We measured the MEL and MC curves.Meanwhile,the M?(magneto-efficiency,M?)curves of devices are obtained.We studied the specific physical processes occurring in these kinds of OLEDs through the measured curves.The experimental results show the MEL and M? curves from QW-TADF-OLEDs present the ISC line-shapes rather than RISC,and the MC curve is positive line-shape;the MEL and M? curves from the reference device show the TTA line-shape,MC curve show the negative shape-line.The analysis shows that this difference is caused by the hyperfine interaction between the polaron pair and the charge transfer state in the quantum well and the TQA process in the built-in electric field under external magnetic field.As the temperature decreases,the line-shape of ISC from QW-TADF-OLEDs vanishes and the TTA process occurs;the TTA process from reference device disappears and the ISC line-shape occurs.Indicating the lifetime and concentration of the triplet excitons are mediated by the temperature,the competition between ISC and TTA processes would lead to the variation of MFE line-shape.In the fourth chapter,the studies on the magnetic effect of QW-TADF-OLEDs mediated by the number of QW and electron mobility are introduced.The results show when the number of QW is 7,the current-dependent MC curve within |B| < 50 mT presents a platform structure at 3 mT < |B| < 10 mT.That is,the slope of the MC curves within this specific magnetic field range is close to 0;moreover,the platform structure in the MC curve vanishes when the high electron mobility material is used,presenting a W-like line-shape.In addition,when the number of QW increases from 3 to 9,the MEL curve within |B| < 10 mT gradually shows the W-like line-shape,and the MC curve within |B| < 10 mT gradually changes line-shape from the W-like shape to the negative Lorentz shape,but the M? curve within |B| < 10 mT merely presents the V-type line-shape.Obviously,the MEL and MC curve line-shapes in the range of |B| < 10 mT are regulated by the number of QW,but the M? curve line-shape is not dependent on number of QW.We proposed these magnetic field effect behaviors occurring in QW-TADF-OLEDs are caused by the comprehensive effects of the ISC of polarons pair,the RISC of charge transfer states,and the TQA scattering and dissociation channels.This work reveals the dynamic microscopic mechanism mediated by the number of QW and electron mobility in QW-TADF-OLEDs by using the effective OMFEs tool,which provides theoretical guidance for the organic light-emitting device with the magnetic,electrical and luminescent properties.