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 (M eta), respectively. Furthermore, MEL is the sum of MC and M eta due to the linear relationship between EL and the product of external quantum efficiency (eta) and I, i.e., EL proportional to I eta. 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 M eta 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 M eta 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 M eta 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.