Abstract
Thermally activated delayed fluorescence (TADF) mechanism is a significant method that enables the harvesting of both triplet and singlet excitons for emission. However, up to now most efforts have been devoted to dealing with the relation between singlet-triplet splitting (ΔEST) and fluorescence efficiency, while the significance of spin-orbit coupling (SOC) is usually ignored. In this contribution, a new method is developed to realize high-efficiency TADF-based devices through simple device-structure optimizations. By inserting an ultrathin external heavy-atom (EHA) perturber layer in a desired manner, it provides useful means of accelerating the T1 → S1 reverse intersystem crossing (RISC) in TADF molecules without affecting the corresponding S1 → T1 process heavily. Furthermore, this strategy also promotes the utilization of host triplets through Förster mechanism during host → guest energy transfer (ET) processes, which helps to get rid of the solely dependence upon Dexter mechanism. Based on this strategy, we have successfully raised the external quantum efficiency (EQE) in 4CzPN-based devices by nearly 38% in comparison to control devices. These findings provide keen insights into the role of EHA played in TADF-based devices, offering valuable guidelines for utilizing certain TADF dyes which possess high radiative transition rate but relatively inefficient RISC.
Highlights
Activated delayed fluorescence (TADF) mechanism is a significant method that enables the harvesting of both triplet and singlet excitons for emission
This contradiction is detrimental to the luminescence efficiency of thermally activated delayed fluorescence (TADF) dyes even though a compromise have been made between kr and kRISC
It excludes the formation of interfacial charge transfer (CT) state between FIrpic and 4CzPN since the hole cannot be trapped on FIrpic molecules in above four cases
Summary
Activated delayed fluorescence (TADF) mechanism is a significant method that enables the harvesting of both triplet and singlet excitons for emission. A relatively high doping concentration is usually employed to ensure an efficient direct charge-trapping, which usually yields intrinsic triplet-charge annihilation (TCA) or aggregate-induced quenching in the emissive layer (EML)[21,22,23] Another method is utilizing TADF dyes as assistant dopants along with conventional fluorescence emitters[24], in which an ideal result is obtained on the basis of following premises, i.e., 1) the formation of fluorescence emitter triplet excitons should be avoided, no matter they are resulted from direct charge-trapping on conventional fluorescence emitters or from triplet-triplet Dexter ET; 2) the potential energy loss caused by triplet excitons quenching of TADF molecules should be prevented. With the aid of EHA perturber, the host triplet excitons can efficiently transfer to guest molecules via an unusual T1H → S1H Förster ET mechanism, which gives a better opportunity to simultaneously harvest both triplet and singlet excitons of host materials
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