The Role of Electron–Hole Separation in Thermally Activated Delayed Fluorescence in Donor–Acceptor Blends

Abstract

Thermally activated delayed fluorescence (TADF) is becoming an increasingly important OLED technology that extracts light from nonemissive triplet states via reverse intersystem crossing (RISC) to the bright singlet state. Here we present the rather surprising finding that in TADF materials that contain a mixture of donor and acceptor molecules the electron–hole separation fluctuates as a function of time. By performing time-resolved photoluminescence experiments, both with and without a magnetic field, we observe that at short times the TADF dynamics are insensitive to magnetic field, but a large magnetic field effect (MFE) occurs at longer times. We explain these observations by constructing a quantum mechanical rate model in which the electron and hole cycle between a near-neighbor exciplex state that shows no MFE and a separated polaron-pair state that is not emissive but does show magnetic field dependent dynamics. Interestingly, the model suggests that only a portion of TADF in these blends comes from direct RISC from triplet to singlet exciplex. A substantial contribution comes from an indirect path, where the electron and hole separate, undergo RISC from hyperfine interactions, and then recombine to form a bright singlet exciplex. These observations have a significant impact on the design rules for TADF materials, as they imply a separate set of electronic parameters that can influence efficiency.

The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

“…This mechanism arises from interactions between an electron′s spin and the magnetic nuclei of its molecule. It is therefore completely local and not quenched by significant electron‐hole separation experienced in CT states like SOC . However, the hyperfine coupling constants are very small, usually in the range of 10 −4 meV, and it therefore also appears highly unlikely that such coupling accounts for these recent observations.…”

Shorter Exciton Lifetimes via an External Heavy‐Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light‐Emitting Diodes

“…The direct intersystem crossing between the charge transfer states 1 CT and 3 CT is assumed to be very inefficient due to the vanishing spin-orbit coupling between these states (Figure 1b). [10,11] Efficient reverse intersystem crossing could be explained by hyperfine interactions. [12,13] Moreover, a mediated spin-orbit coupling process involving a higher (or lower) lying local exciton 3 LE has emerged as alternative explanation.…”

A review on organic spintronic materials and devices: I. Magnetic field effect on organic light emitting diodes

“…The model has been considered as a main mechanism for the giant OMAR effect on TADF-based OLEDs [69]. However, recent MFE study on transient photoluminescence of the same TADF blend shows compelling evidence that PP mechanism is still a dominant mechanism in such compound [113]. Since the Dg model is the newest one among OMAR models, more experiments need to be done to verify its accuracy.…”