激基复合物(exciplex)和电致激基复合物(electroplex)经常同时产生于体异质结的有机发光二极管(organic light emitting diodes,OLEDs),有报道称利用这两种发射态共存的器件实现了高效率发光.器件高效率的物理机制与共存体系中自旋对态(spin-pair states)的微观过程有关,相关微观过程还有待深入研究.本文以特征的有机磁场效应(organic magnetic field effects)作为主要探测工具,通过改变器件的工作温度和给体与受体的共混比例来研究这种双发射态共存体系中自旋对态的演变过程.实验结果表明:降低发光器件的环境温度不仅会减弱从高能激基复合物到低能电致激基复合物的Dexter能量传递过程,而且因器件偏压增高还会引起电致激基复合物电荷转移(charge-transfer,CT)态的场致解离,使其数量较少,导致其CT态的反向系间窜越(reverse intersystem crossing,RISC)过程减弱.另外,增加发光层中给体的浓度会引起电荷注入更不平衡,增强过剩的空穴载流子与电致激基复合物的解离反应,这也会减弱电致激基复合物CT态的RISC过程.因此,温度降低或给体含量增多,器件的磁效率(magneto-efficiency,Mη)从原来受RISC过程主导的负磁效应转化为受系间窜越(intersystem crossing,ISC)过程主导的正磁效应.本工作有利于深入理解激基复合物和电致激基复合物共存体系中自旋对态的物理微观机制.
Regulating magneto-efficiency in coexistence systems of exciplex and electroplex by changing ambient temperature or blending ratio
As the third-generation organic light emitting diodes(OLEDs),exciplex-based OLEDs with reverse intersystem crossing(RISC)processes between charge-transfer(CT)states have potential applications for lighting and flat-panel display.Recently,researchers have been committed to improving the luminous efficiency of dual-emission OLEDs with exciplex and electroplex,but the microscopic physical processes of this dual-emission system have not been well investigated.In this article,the fingerprint organic magnetic field effects(OMFEs)are used as the main detection tools to study the spin-mixing processes of spin-pair states in the coexistence systems of exciplex and electroplex.Because the intensity of each spin-related microscopic process depends on the external magnetic field(B),alteration in applied B will lead to variation in current(I)and electroluminescence(EL)of the device.The change in I intensity caused under constant voltage(V)is referred to as magneto-conductance(MC),while the changes in EL intensity caused under constant V and I are known as magneto-electroluminescence(MEL)and magneto-efficiency(Mη),respectively.Furthermore,MEL is the sum of MC and Mη due to the linear relationship between EL and the product of external quantum efficiency(η)and I,i.e.,EL∝Iη.Previous studies have suggested that because the low-energy electroplexes are formed in exciplex-based devices where di-[4-(N,N-ditoly-amino)-pheny]cyclohexane(TAPC)and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene(TPBi)serve as donors and acceptors,respectively,the Dexter energy transfer(DET)process from the exciplex to electroplex enhances the CT-RISC process of electroplex states,resulting in the smaller value of MEL trace than that of MC trace.Thus,there is a negative value in Mη under higher bias I.It is well known that the radiation of excited states is closely related to the various evolution channels of spin-pair states in the device,and the variation in ambient temperature(T)or blending ratio can also regulate the radiation process of dual-emission systems,which will inevitably affect OMFEs from the device.This work found that the alteration in blending ratio of TAPC and TPBi in the light-emitting layer(EML)or ambient T can both cause the low-field value of MEL trace to exceed that of MC trace,i.e.,the low-field value of Mη trace undergoes a transition from negative to positive.On the one hand,the weaker DET process from the exciplex to electroplex and the electric field-induced dissociation of the CT states of electroplex result in a decrease in the number of electroplexes after T decreases.Therefore,the normalized EL spectra show that the exciplex peak increases with decreasing T,and this will weaken the CT-RISC process of electroplex states.On the other hand,an increase of donor concentration in EML leads to a more unbalanced charge injection in devices,which strengthens the dissociation of triplet CT states of weakly bound electroplexes by excess holes and then reduces the number of electroplex states.Thus,observation from normalized EL spectra manifests that the electroplex peak is impaired with the increase of donor content,and it can also attenuate the CT-RISC process of electroplex states.Both of them will convert Mη from the original negative line-shape dominated by RISC processes to positive line-shape dominated by ISC processes.This work deepens the understanding of the microscopic mechanisms of spin-pair states in dual-emission systems.
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