Study On Microscopic Processes Of Triplet Excitons In Rubrene-based Organic Light Emitting Diodes By Utilizing Organic Magnetic Field Effects

Organic light-emitting diode(OLED)has been developing rapidly in the past 10 years due to its excellent characteristics such as low drive voltage,high luminescent intensity and efficiency,wide viewing angle,fast response speed,large area and capability of resisting bending,which make it become the most compelling technology in the field of display and lighting.In-depth study and development of new organic materials has become an important way to improve the performance of OLED,and Rubrene has attracted much interest among many organic materials because of its special photoelectric properties.Rubrene is widely used in organic devices such as organic field-effect transistor(OFET),OLED and organic solar cell(OSC)owning to its characteristics of high mobility and ES?2ET.In fact,although some researches and applications on Rubrene have been done,there are still some problems are not clear such as how does the triplet excitons react with charges or how does the interaction between excitons be influenced by Zeeman splitting in Rubrene-based OLED,etc.Effective solution to these problems can not only help us to better understand the spindependent micro-processes of OLED but also might guide us to optimize the organic applications.In this work,a series of Rubrene-based OLEDs were fabricated to solve the problems above,and the organic magnetic field effects(OMFEs)had been used as special tools to analyze the interactions between the microscopic particles.By measuring the magnetic effect curves of electroluminescence and current at different bias voltages and temperatures,and fitting the curves by an empirical formula consisted of a Lorentzian component and two non-Lorentzian components,it is found that:(1)The triplet excitons in Rubrene-based OLED are dissociated by holes while electrons in Rubrene-based OLED are scattered by triplet excitons,which produce opposite magneto-conductance(MC).(2)The magneto-electroluminescence(MEL)of device has a sign reversal at ultra-small |B|<2 mT.And the sign reversal shows a “W” shape at room temperature while present an “M” shape at low temperatures.The former is confirmed to be the resulted from hyperfine interaction and Zeeman splitting while the latter can be attributed to the zero-field splitting between triplet excitons and Zeeman splitting.The main contents of this dissertation are listed as follows:In the first introductory chapter was dealt with the basic knowledge of organic electronics,especially introduced the history of OLED development,working principle and the excited states in the device.Then,this article focused on the OMFE models of the interaction between excited states.For example: the hyperfine interaction(HFI),triplet-triplet annihilation(TTA),singlet fission(STT)and triplet-charge interaction(TQI).And ultra-small magnetic field effects(USMFEs)and the application of experimental curve fitting ware also introduced.Last but not least,detailed photoelectric properties about of Rubrene were introduced,which was the point research object of our work.In the second chapter,the fabrication and measurement of OLED in laboratory ware represented detailed,including the vacuum evaporation method of organic molecules and metal electrodes,spin-coating process of polymers and measuring methods spectra and OMFEs.In the third chapter,conventional Rubrene-based OLEDs with the structures of ITO/NPB(60 nm)/Rubrene(30 nm)/BCP(50 nm)/LiF(1 nm)/Al(120 nm),TO/NPB(60 nm)/Rubrene(30 nm)/Alq3(50 nm)/Li F(1 nm)/Al(120 nm),ITO/m-MTDATA(60 nm)/Rubrene(30 nm)/BCP(50 nm)/LiF(1 nm)/Al(120 nm)and ITO/NPB(60 nm)/Rubrene(30 nm)/BCP(50 nm)/Al(120 nm).Among them,the use of different hole and electron injection layers can change the hole and electron injection barriers and the cathode modified material of Li F layer lower the efficiency of electrons,which proved an effective way to regulate the interaction between triplet excitons and charges.The MC curves of the devices were measured at different bias voltages and fitted with an empirical formula combined of one Lorentzian and two non-Lorentzian.The results show that the TQI process occurs in the Rubrene devices and can directly and obviously affect the MC.The process includes dissociation and scattering channels which have different participant charges.The dissociation channel means that the holes dissociate by the triplet excitons which produces a negative MC;in the scattering channel the electrons scattering by the triplet excitons which leads to a positive MC.The final MC curves of the devices is the superposition of them,which is confirmed by the MC curves at low temperature.In order to make a further study of reactions of triplet excitons in Rubrene-based devices,we fabricated the OLED with structures of ITO/PEDOT: PSS/m-MTDATA(60 nm)/Rubrene(30 nm)/ BCP(50 nm)/LiF(1 nm)/Al(120 nm)and measured MEL curves at different bias voltages and temperatures.It is found that the MEL curves show a particular STT line at room temperature while present a typical TTA line at low temperature in the range of 0?|B|? 500 mT.For the low magnetic field region(|B| < 2 mT),the MEL curves at room temperature show a “W” type sign reversal,but when the temperature drop to 75 K the MEL curves change to “M” type sign reversal.Further analysis showed that the “W” type is the result of the hyperfine interaction caused by the hydrogen nucleus and the Zeeman splitting caused by the external magnetic field.And the “M” type is closely related to the TTA process,which can be attributed to combined effect of the zero-field splitting of triplet pair state and the Zeeman splitting.