Abnormal Magneto-electroluminescence In Exciplex- And Excimer-based Devices

Light-emitting devices are convenient for our daily life.Organic light-emitting diodes(OLEDs)are widely used in the field of flat panel displays and solid-state lighting because of their advantages such as simple preparation,flexibility and ultra-thin.Traditional OLEDs are exciton-based OLEDs.According to the theory of spin statistics,25% of excitons in exciton-based OLEDs are singlet.Based on the theory of spin conservation,singlet exciton can emit fluorescence through de-excitation radiation,while triplet exciton can only be dissipated by the non-radiative transition with the help of phonons.Therefore,the internal quantum efficiency(IQE)of exciton-based OLEDs is limited to 25%.To improve the IQE of OLEDs,academia and industry are working to find ways to convert triplet excited states into singlet excited states.At present,some research groups have used the strong spin-orbit coupling effect of precious metals such as platinum and iridium to convert the triplet excited states into singlet excited states and achieve high emission efficiency.However,the expensive price of platinum and iridium limits the mass production of these highly-efficient OLEDs.Recently,exciplex-based OLEDs have attracted much attention because they can achieve theoretically IQE of 100% without the assistance of precious metals.The high IQE is attributed to the reverse intersystem crossing(RISC)process in the device.The RISC process can be described below.Holes and electrons in the exciplex located on the donor molecule or group and the acceptor molecule or group respectively.Thus,holes and electrons are separated spatially.This spatial separation results in a small energy difference between singlet and triplet exciplexes.Therefore,a triplet exciplex can convert into a singlet exciplex by absorbing ambient heat.Finally,singlet exciplexesformed by the RISC process emits delayed fluorescence in the form of de-excitation radiation,thereby achieving a theoretical increase in IQE from 25% to 100%.Because the RISC process can effectively use triplet excited states,detecting the RISC process in exciplex-based OLEDs and studying its physical mechanisms are essential for designing highly-efficient OLEDs.Recently,magneto-electroluminescence(MEL)has been used as an effective tool to study the evolution of excited states in OLEDs.MEL,that is,the applied magnetic field affects spin-mixing processes in OLEDs to change the number of singlet and triplet excited states and the emission intensity of the device.These spin-mixing processes include intersystem crossing(ISC),RISC and triplet-triplet annihilation(TTA).And these spin-mixing processes have their own unique fingerprint MEL curves.Because the current and the temperature can change the concentration and lifetime of excited states,we can investigate the evolution of excited states in the device by analyzing MEL curves of OLEDs at different currents or temperatures.In this paper,exciplex-based OLEDs were fabricated and physical mechanisms of the RISC process and the TTA process were explored by analyzing their device structures,electroluminescence spectra,MEL curves,and current-brightness-voltage characteristic curves.This work provides not only a deeper understanding of the RISC process and the TTA process,but also new ideas for designing highly-efficient OLEDs.The main contents of this article are as follows:In the first chapter,the development process of OLEDs,the device structure,the principle of light emission and its application in life are introduced.Then the measurement of MEL from OLEDs and the characteristic MEL curves corresponding to spin-mixing processes are introduced.Finally,research value and the main content of this work are introduced.In the second chapter,the preparation work before experimenting and the experimental equipment to be used are introduced.Then the experimental process of preparing OLEDs under ultra-high vacuum is introduced.Finally,methods of measuring the photoelectric characteristics and magnetic field effect of OLEDs are introduced.In the third chapter,we firstly introduce the abnormal current dependence of MEL curves from red light-emission nuclear exciplex-based OLEDs at room temperature.The MEL amplitude of the nuclear device increases with increasing current,whereas the MEL amplitude of conventional exciplex-based OLEDs decreases with increasing current.As the temperature decreased from 300 to 20 K,we found that the MEL curveof the nuclear device gradually changed from the inverted Lorentz-type line-shape representing the ISC process to the upright Lorentz-type line-shape representing the RISC process.However,MEL traces of the conventional device exhibited the ISC line-shape at different temperatures.The above two abnormal MEL from the nuclear device are attributed to the dominant RISC process.Finally,the abnormal current dependence of MEL curves from the nuclear device at low temperature is introduced,that is,the MEL amplitude increases with increasing current.This is because the dissociation process of the triplet exciplex decreases with decreasing temperature.In the fourth chapter,excimer-based OLEDs were prepared by inserting the spacer layer of different thickness between the donor and the acceptor material.When the spacer-layer thickness is small,the emission mainly comes from the exciplex.As the spacer-layer thickness increases,the emission of the device gradually changes from the exciplex emission to the excimer emission.By measuring the MEL of devices,we found that the amplitude of the MEL curve under low magnetic field intensity gradually increases and the current dependence reverse.At the same time,MEL curves of the device under high magnetic field strength gradually changed from a slow rise to a rapid decline.The high-field decrease of the MEL curve is attributed to the inhibition of the magnetic field on the TTA process.Because triplet excited states are easily quenched by thermal noise such as phonons at room temperature,TTA processes usually only occur at low temperatures.The room-temperature TTA process in excimer-based OLEDs can occur because the excimer has the low energy level and the long lifetime at room temperature.