| Organic light-emitting diode(OLED) has great potentialities in the flat-panel displays and lighting sources because of their advantages,that is,ease of fabrication,wide range of materials and low cost and so on. In the past several decades,great effort has concentrated on OLED。Although people have developed a variety of organic electroluminescence materials and devices,including the fluorescent and phosphorescent materials and devices, the industrializations are far below our expectations。It is generally recognized that fluorescent arises from singlet excitons,which account for 25% of the total excitons。So the internal quantum efficiency(IQE) of most fluorescent OLED is limited to 25%。While the IQE of phosphorescent OLEDs can approach 100% by using both singlet and triplet excitons。Although the phosphorescent OLED with iridium and platinum complexes could offer high IQE,they are rather expensive。Considering the above disadvantages in fluorescence and phosphorescence OLEDs,many research institutions and companies have been devoted to develop high-performance OLED simultaneously reducing production costs.However,the Adachi group recently reported a series of the charge transfer(CT) state-based light-emitting device to get a higher external quantum efficiency(EQE) than the fluorescent limit of 5% through reverse intersystem crossing(RISC)。For RISC mechanism,it refers to the up-conversion process from triplet to singlet excited states under thermal excitation,leading to delay fluorescence。It is obvious that a small energy gap(ΔES-T) between the singlet and triplet excited states is required。Another promising way to produce delay fluorescence is triplet-triplet annihilation process(TTA)。The TTA mechanism is considered that one extra singlet exciton will be generated by fusing two triplet excitons。Since the special intramolecular CT species have a very small ΔES-T value,the species could participate in both RISC and TTA。This leads to the coexistence of RISC and TTA that should be much better for the utilization of triplet excitons。However,determining whether the coexistence happens in fluorescent OLEDs is not an easy task。This is because the general methods can hardly distinct the delayed fluorescence is originated from which approaches 。Fortunately,under the external magnetic field,the changes of the electroluminescence of the non-magnetic OLED(magneto-electroluminescence,MEL) can be used as a novel approach to discern RISC and TTA in fluorescent OLEDs,because they are highly spin-dependent processes that can generate sizable MEL responses in OLEDs。We note that the MEL response of RISC is a low-field effect,i.e.,the MEL decreases rapid within the low-field regime(<5 m T) and then tends to be saturated.While the TTA MEL exhibits a slight increase within the range of 20 m T but a remarkable decrease in the low-field regime(>20 m T).In this work,we study the magneto-electroluminescence with the coexistence of RISC and TTA in organic light emitting diodes。Through the discussion of the spin related process among triplet excitons under the external magnetic field,a discussion was made by qualitatively analyzing the experimental phenomenon。Then,we changed the structure of device by doping concentrations,and measured the MEL under different temperatures(20 K~300 K)。We qualitatively analyzed how the doping concentration and temperature effect the MEL of the devices.The main work in this Dissertation as follows:(1) Firstly,we introduce the development history of OLED,some basic knowledge andresearch background of organic magnetic field effect。we would be focusing on several theoretical models,for example,reverse intersystem crossing,triplet-triplet annihilation,triplet-charge annihilation and so on。Then,the second charted detailedly represents the fabrication of OLED and measure methods of magnetic field effect。(2) By utilizing RISC property of fluorescen tmaterial 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran(DCJTB),the doped organic light emitting diode was fabricated。The MEL were measured under different temperatures(20 K~300 K)。The MEL curves show a sharp decrease in low field(<10 m T),followed by a slow fall in high field with increasing magnetic field,i.e。the values of the MEL are negative。The novel line shape is greatly different from the exciton-type device with intersystem crossing property。In addition,the decline of the MEL in low field and high field is modulated by current and temperature。Through the analysis of the spin related process among triplet excitons,we suggest that the MEL curves of DCJTB-doped device are caused by RISC process and triplet-triplet annihilation process。Moreover,the lifetime of triplet exciton is the main factor which influences RISC process。(3) Following such strategy,in this work,we firstly fabricated the devices by doping different concentrations of intramolecular CT states material DCJTB into phosphor host material m CP and then measured the MEL responses of these devices at room temperature。It exhibited that,as decreasing the doping concentration from 20% to 5%,the sign of low-field(<5 m T) component turns from positive to negative,and the high-field(B>20 m T) decrease of MEL was more prominent。That is quite similar to the theoretical MEL curves of RISC in low-field(<5 m T) and TTA in high-field(B>20 m T),respectively。Thus,we believe that the delayed fluorescence of DCJTB-doped devices are caused by RISC and TTA。Moreover,the RISC and TTA rates increase as the doping concentration decreasing。Subsequently,with temperature decreasing from 300 K down to 20 K,more remarkable RISC-dominanted low-field and TTA-induced high-field decrease of MEL were observed。This is attributed to the argument that because low temperature enhanced.In summary, low doping concentration and low temperature are more favorable to the RISC and TTA in the DCJTB-doped devices。This work has a certain reference value to improve the utilization rate of triplet in fluorescent OLEDs. |
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