Direct Detection of Singlet-Triplet Interconversion in OLED Magnetoelectroluminescence with a Metal-Free Fluorescence-Phosphorescence Dual Emitter

Abstract

We demonstrate that a simple phenazine derivative can serve as a dual emitter for organic light-emitting diodes, showing simultaneous luminescence from the singlet and triplet excited states at room temperature without the need of heavy-atom substituents. Although devices made with this emitter achieve only low quantum efficiencies of < 0.2%, changes in fluorescence and phosphorescence intensity on the subpercent scale caused by an external magnetic field of up to 30 mT are clearly resolved with an ultra-low-noise optical imaging technique. The results demonstrate the concept of using simple reporter molecules, available commercially, to optically detect the spin of excited states formed in an organic light-emitting diode and thereby probe the underlying spin statistics of recombining electron-hole pairs. A clear anticorrelation of the magnetic-field dependence of singlet and triplet emission shows that it is the spin interconversion between singlet and triplet which dominates the magnetoluminescence response: the phosphorescence intensity decreases by the same amount as the fluorescence intensity increases. The concurrent detection of singlet and triplet emission as well as device resistance at cryogenic and room temperature constitute a useful tool to disentangle the effects of spin-dependent recombination from spindependent transport mechanisms.

Unraveling the Important Role of High‐Lying Triplet–Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep‐Blue AIE‐Based OLEDs

“…It has been reported that a dominant RISC process will result in negative MEL, [ 37,38 ] whereas ISC typically gives a positive MEL response. [ 39,40 ] As shown in Figure 4a, the stronger negative MEL trends at high fields reflect that the RISC conversion from T 3 to S 1 is dominant over the conversion from S 1 to T 2 . Furthermore, the MEL responses at higher temperatures (300–330 K), as shown in Figure 4b, reveal favorable conversions from T 2 to S 1 and S 1 to T 3 .…”

Metal‐Free Organic Luminophores that Exhibit Dual Fluorescence and Phosphorescence Emission at Room Temperature

“…Dual-emissive electroluminescence was reported for dibenzo[a,c]phenazine 25 b dispersed in CBP film. [55]…”

Fitting the magnetoresponses of the OLED using polaron pair model to obtain spin-pair dynamics and local hyperfine fields

“…In conclusion, we demonstrate the different functional shapes of the experimentally measured MC and MEL of the Alq 3 -based OLED within the ultra-small magnetic field range (± 500 µT) under the constant current mode. The MC is displaying a characteristic "W" shaped form (dip position at ± 250 µT) while the MEL behaves in a monotonic manner, indicating two independent optical and electrical processes under the constant current mode 39 . A simplified two hyperfine field polaron pair model has been fitted to the experimentally obtained MC and MEL of the Alq 3 -based OLED.…”

“…Figure 1 a,b show the current–voltage-luminescence characteristics of a typical device and demonstrate that it is operating in a super-linear current–voltage regime with a linear relationship between luminescence and current. These relationships indicate why constant voltage conditions are not suitable for magnetic field effect measurements on such a diode, as the magnetic field dependent device current would simultaneously affect the electroluminescence 38 , 39 . Figure 1 c,d show measured diode MC (note the MC is used to allow for fitting to the theory) and MEL under a constant, 39 µA, drive condition within a range of ± 500 μT, where the MC and MEL can be defined in Eqs.…”