Effects Of In-situ Heating On The Evolution Of Electron-hole Spin Configuration In Organic Semiconductor Light-emitting Devices

Organic electroluminescent devices are also called organic light emitting diodes,which emit light by carrier injection and recombination.In 1987,Tang and Van Slyke of Kodak Company in the United States prepared a sandwich-type organic electroluminescent device by thermal evaporation,which opened up a new era of organic light-emitting diode(OLED).After more than 30 years of development,OLED device manufacturing technology has been very mature,and the device performance has been greatly improved.OLEDs are widely used in lighting,display and other fields because of their high luminous efficiency,short response time,good low temperature characteristics,simple preparation process,flexibility and thinness.The light-emitting process of OLEDs is mainly divided into four steps.First,electrons and holes are injected from the cathode and the anode,respectively.Then the electrons and holes approach the light emitting layer under the action of the electric field.Then the electrons and holes reach the light-emitting layer and meet under the action of Coulomb force to form an electron-hole pair,which in turn forms an exciton.Finally,in the excited state exciton deexcitation radiation,photons are emitted.The light emission process of the device is very complicated,while the electron and hole spin mixing process and exciton evolution process inside the device are more complicated and changeable.The micro-mechanisms inside the device include hyperfine interaction,triplet-charge annihilation,triplet-triplet annihilation,singlet exciton fission,trap,etc.These processes directly affect the optoelectronic performance of the device,and it is difficult to directly observe and study.Fortunately,these microscopic processes inside the OLED often respond to the external magnetic field,so that the electroluminescence and current of the device become a function of the external magnetic field,which we call the organic magnetic field effect.The phenomenon that electroluminescence changes with a magnetic field is called a magneto-luminescence effect(MEL),and the phenomenon that a current changes with a magnetic field is called a magneto-conductive effect(MC).Different electron and hole spin mixing processes and exciton evolution processes often have their own unique MEL and MC characteristic curves.Taking advantage of this feature of the organic magnetic field effect,it can be used as a real-time efficient,non-destructive research tool to study the complex and changeable microscopic processes and interactions inside the OLEDs.At present,the research on OLEDs is mainly focused on room temperature or low temperature environment,and related research under high temperature environment still belongs to a relatively blank field.When the device is in the working state,the ambient temperature and the heat generated by the device may cause the local temperature of the organic thin film to rise,and induce the organic layer to generate traps or crystals.Structural changes may affect carrier and exciton interactions,accelerate device degradation,and even lead to catastrophic failures.Based on the above factors,no matter from the theoretical level or practical application level,the magnetic field effect is used as an efficient,sensitive,non-destructive and non-contact detection method.It is of great significance to study the effect of temperature rise on the carrier transport and exciton evolution of OLEDs.Based on the above factors,it is of great practical significance to use the magnetic field effect as an efficient,sensitive,non-destructive,non-contact detection method to study the effects of in-situ heating on the evolution of electron-hole spin configuration in organic semiconductor light-emitting devicesIn this thesis,Alq₃-based OLED and rubrene-based OLED were prepared,and the magnetic field effect characteristic curves of the devices under different currents and different temperatures were measured.We analyzed the surface morphology,electroluminescence spectrum,luminescence-current characteristics,and current efficiency,and fitted the magnetic field effect curve of the device by superposing the Lorentz and non-Lorentz functions.Eventually,the effects of temperature rise on carrier transport and exciton evolution in OLEDs were discovered,and the understanding of the internal micro-mechanisms of OLEDs under high-temperature environments was deepened.The specific research content is as follows:The first chapter mainly introduces the relevant knowledge of organic electroluminescent devices,including its definition,development history,research status.Secondly,it also briefly introduces the structure of the OLED device,the role of each organic functional layer,and the principle of light emission.Then it focuses on several models of organic magnetic field effects and their characteristic“fingerprint”line types.Finally,the basic properties of Alq₃ and Rubrene are depicted.The second chapter mainly introduces the relevant content of the preparation and measurement of OLED device,including substrate cleaning,ozone treatment,molecular beam epitaxial deposition and vacuum thermal evaporation.It also explains the measurement methods of the optical-electrical-magnetic properties of OLEDs and the means of characterizing the surface topography.The third chapter mainly introduces the effects of high temperature environment on the exciton evolution and device structure of Alq₃-based OLED by using the magneto-luminescence effect.The MEL changed little over the range 300–420 K,and then increased significantly at 440 K.Optical microscopy,atomic force microscopy,and current-density vs.voltage curves indicated that the sharp increase was caused by numerous traps generated in the emissive layer.These traps limited the diffusion of polaron pairs and excitons and ultimately promoted intersystem crossing.This work has deepened the understanding of the electron-hole interaction mechanism and exciton evolution process of OLEDs in high temperature environments,and provides a non-destructive and non-contact detection method for real-time monitoring of the structure of OLEDs.The fourth chapter mainly introduces the use of magnetic field effects to study the dependence of the light,electricity,and magnetic properties of Rubrene-based OLED on temperature,and the effects of high temperature environments on the dynamic behavior of device carriers and excitons.It was discovered for the first time that when the Rubrene OLED was heated to 380 K,the MEL characteristic line type of the device was changed from singlet exciton fission(STT)to ntersystem crossing(ISC).Analysis of the surface morphology,electroluminescence spectrum,and current–voltage curve of the device suggested that the STT-to-ISC conversion was caused by a large number of exciton traps generated in the organic layers of the device.These traps blocked the movement of excitons,which suppressed STT.By contrast,ISC was promoted by traps and became the main process.By analyzing the spectrum,the current-voltage characteristic curve and the empirical formula fitting,the trap energy was calculated quantitatively.This work has deepened our understanding of OLED exciton dynamics in high temperature environments,and has a good guiding role and practical value for studying the operating temperature range of Rubrene-based OLEDs.Based on the research in the fourth chapter,the fifth chapter mainly introduces the dependence of the light,electricity,and magnetic properties of Rubrene-based OLEDs doped with different concentrations of Ir(ppy)₃ on temperature,and analyzes the complex micromechanics inside.It was found that the luminescence of the device came from Rubrene,and Ir(ppy)₃ did not emit light.The higher the concentration of doped Ir(ppy)₃,the stronger the light emission of the device and the weaker the STT process.Heavy metal atoms caused spin orbit coupling,and the energy of Ir(ppy)₃ was transferred to Rubrene.The high temperature caused traps inside the device,promoted the mutual conversion between polaron pairs,suppressed the interaction between excitons,and eventually caused the magnetic field effect of the device to become the characteristic curve of ISC.The sixth chapter summarizes and prospects this thesis.