| Organic light-emitting diodes(OLEDs)have been widely used in the commercial field as a new generation of display technology,but there is still the problem of efficiency roll-off at high brightness level,which hinders the further development and breakthrough of OLEDs.The red,green,and blue primary color devices are the basic conditions for OLED full-color display.However,the development of blue light devices is still lagging behind.Due to the high cost,poor stability and short life of phosphorescent devices,people have turned their attention to fluorescent devices with easy-to-synthesize materials and stable luminescence.Therefore,this article gets an insight into the efficiency roll-off phenomenon in blue fluorescent organic light-emitting diodes.The efficiency roll-off is the result of the combined effects of many physical processes.The exciton-charge interaction has been proved to be the main mechanism experimentally,so this paper studies and analyzes the physical process of efficiency roll-off in OLED from the two angles of flow charge quenching and static charge quenching.The research content in this article mainly includes the following parts:(1)Chapter 1 mainly describes the development of organic light-emitting diodes,existing problems and related basic theories,and finally expounds the idea and research significance of this paper.The OLED displays have been successfully expanded to the global market,and lighting products are also developing rapidly.However,there are still problems in the efficiency and stability of blue organic light-emitting devices.This paper studies the physical mechanism of the efficiency roll-off of the organic light-emitting diodes.(2)Chapter 2 mainly introduces the process of sample preparation and measurement.It includes the selection of organic materials and electrode materials,the preparation of experiments,the preparation and packaging of devices,and the steady state measurement and transient measurement of samples.(3)Chapter 3 mainly studies the changes of the photoluminescence response of unipolar devices under the action of electron current and hole current,which is equivalent to simulating the interaction between excitons and flowing charges.Firstly,the effects of exciton-exciton annihilation and exciton dissociation caused by electric field on this experiment are excluded.It is proved that exciton-charge interaction is the main mechanism of efficiency roll-down in fluorescence OLEDs.Then the structure design of the unipolar device is explained:In order to effectively distinguish the quenching effect of electrons and holes on excitons,different materials are selected as functional layers to design the structure for energy level matching.Electron-only device and hole-only device are prepared in this way.Exciton-polaron interaction is observed by steady-state and transient-state measurement.Experimental results showed that the quenching effect of the holes with lower carrier density was more significant.Through the analysis of the energy level structure and carrier mobility of the light-emitting layer,it is found that the trapping of holes by the guest molecules adds additional carrier density,shortens the exciton-charge distance and promote the exciton-hole interaction.It is inferred that excitons are mainly quenched by static charges rather than moving charges.(4)Chapter 4 focuses on the quenching of excitons by stored charges.The PMMA insulation layer is spin-coated on ITO substrate in the experiment,thus forming a capacitor structure.When positive(negative)bias is applied,holes or electrons can accumulate on one side of the emitting layer.The quenching effect of stored charges on excitons in devices is studied by photoluminescence.It can be further proved that excitons are mainly quenched by stored charges,and the quenching effect of stored holes on excitons is higher than that of stored electrons.On the one hand,the physical process of interaction between static charge and exciton is analyzed:the interfacial charge will accumulate on the hole injection side or electron injection side of the luminescent layer,forming an overlap region with the luminescence region.The exciton-charge interaction will occur in this region.On the other hand,the bound charge affects the carrier balance and the effective radiation efficiency of excitons in the light-emitting layer through the trap-assisted non-radiative recombination method,resulting in light-emitting loss and reducing the light-emitting efficiency of the device.This is because the bound charges will recombine non-radiatively with the free charges,causing the carrier transport in the light-emitting layer to lose balance,and some carriers cannot continue to form excitons.It also leads to a narrow exciton recombination region,which reduces the distance between excitons and polarons.In addition,the non-radiation recombination caused by trap state will compete with radiation recombination,thus reducing the effective radiation efficiency of the device.This will lead to an increase in the driving voltage of the device and loss of light emission,which directly affects the overall performance of the device.(5)Chapter 5 focuses on the preparation of highly efficient organic light-emitting diode devices based on mCBP:DPAVBi doping systems.The doping concentration is an important factor that affects the generation and deactivation of excitons,and the efficiency of energy transfer between the host and the guest.A slight fluctuation in doping concentration may cause significant changes in device performance.In this regard,we prepared organic light-emitting diode devices with different doping concentrations to explore the effect of doping concentration on exciton-charge interaction.It is found that the doping of the guest will have a significant impact on the energy disorder of the light-emitting layer.Finally,the blue organic fluorescent device with good comprehensive performance is obtained,which doping concentration is 10wt.%,and its luminescence intensity is 58900 cd/m~2,and the maximum current efficiency is 11.75 cd/A.And some optimization suggestions are put forward for adjusting the light-emitting area or carrier trapping.(6)Chapter 6 summarizes the main work described in this paper,and carries on the corresponding prospect.In this paper,the internal mechanism of efficiency roll-down is further improved,and the quenching effect of charge on excitons is discussed.This provides a useful reference for slowing down the efficiency roll-off of blue fluorescent organic light-emitting diodes.And some optimization suggestions are put forward for adjusting the light-emitting area or carrier trapping,which will help to improve the efficiency roll-off phenomenon in fluorescent organic light-emitting diodes in the future. |
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