Abnormal magnetic field effects in doped organic light-emitting devices

Abstract

Organic magnetic field effects (OMFEs), which primarily includes magneto-electroluminescence (MEL) and magneto-conductance (MC), is a non-contact, non-destructive and sensitive tool for probing the mechanisms that occur within in organic light-emitting diodes (OLEDs). Not only OMFEs can be used to detect charge transport, exciton evolution and luminescence mechanisms in OLEDs, and can also be used in magnetic devices, including magnetic switches, magnetic storage and magnetic sensors, etc. Stable magnetic devices are usually required for practical applications. There is a wealth of research that shows that doped devices exhibit improved stability when compared with non-doped devices, however, they always show weaker OMFEs responses. Therefore, it is important to achieve strong OMFEs responses from stable, doped devices. In this work, we have fabricated OLEDs using the thermally activated delayed fluorescence (TADF) material 2,3,5,6-tetrakis(3,6-diphenyl-carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) of different doping concentration as the dopant, and either tris-8-hydroxyquinoline aluminum (Alq3) and 4,4′- N , N ′-dicarbazole-biphenyl (CBP) as the host materials. The light-emitting layer in these OLEDs was Alq3: x % 4CzTPN-Ph (studied devices) or CBP:5% 4CzTPN-Ph (reference device). The MEL and MC curves from these devices were measured with different injection currents and doping concentrations at room temperature, and with varying temperatures at a constant injection current and doping concentration. Compared with the reduced OMFEs observed from the reference device, the studied devices exhibited increased amplitudes in their MEL and MC responses. For example, at an injection current of 150 μA at room temperature, the amplitude of the MEL from the studied device reached ~10% when exposed to a magnetic field of 300 mT, which was approximately 13 times larger than that observed from the reference device (~0.75%). The corresponding amplitude from the MC from the studied device was ~6% at 300 mT, which was approximately 50 times larger than that observed from the reference device (~0.12%). In addition, the MEL and MC responses from the studied devices were determined by the doping concentration. At a doping concentration of approximately 15%, the MEL and MC values from the studied devices at 300 mT reached maximum values of 13.4% and 9.3%, respectively. Both MEL and MC values from the studied device reduced with decreasing temperatures. Analysis of the energy levels and optical spectra from these studied devices indicated that the special energy level arrangement between the guest and host molecules resulted in a weak energy level trap. The external magnetic field suppressed the scattering of charges by triplet excitons, a channel for triplet-charge annihilation (TQA), which generated significant MEL and MC responses. Therefore, OMFEs that were stronger than those from the reference device were obtained. In addition, since TQA is affected by the concentration and mobility of triplet excitons and carriers, the doping concentration and working temperature can also mediate TQA by modifying the concentration and mobility of triplet excitons and carriers, which eventually affects the OMFEs. This work gives insight into the mechanisms that occur within 4CzTPN-Ph-based light-emitting diodes, and also importantly promotes practical applications of magnetic organic light-emitting diodes.