Photophysical Investigation of the Thermally Activated Delayed Emission from Films of m‐MTDATA:PBD Exciplex

Abstract

Understanding the delayed fl uorescence mechanism behind the creation of emissive singlets from the non-emissive triplets in exciplexes is vital for the fabrication of highly effi cient blue fl uorescent emitters, and subsequent white light applications. In this article we report the spectroscopic investigation of the exciplex formed between 4,4′,4′′-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (PBD) in a 50:50 blended fi lm. The mechanism behind extra singlet production in the blend is of E-type nature, that is, "thermally activated" delayed fl uorescence. The exciplex singlet-triplet energy splitting is estimated to be around 5 meV, smaller than previously estimated at ≈50 meV. The absence of a well defi ned separation between prompt emission and emission components with very long lifetimes, >100 ns, is indicative of such a small exchange energy, and arises through multiple cycling between the resonant singlet and triplet manifolds before eventually being emitted from a singlet state. An observed redshift of the exciplex emission with time and increasing temperature is attributed to different exciplex species being formed between the m-MTDATA and PBD molecules.

The role of exciplex states in phosphorescent OLEDs with poly(vinylcarbazole) (PVK) host

“…In both cases the key process is the enhanced triplet formation on the exciplex species. We also note that excimers of the donor also compete with other emissive species in such mixed emitter layers, [29] so it is not surprising that PVK triplet dimers also act as emissive traps as seen here.…”

Efficient Triplet Application in Exciplex Delayed-Fluorescence OLEDs Using a Reverse Intersystem Crossing Mechanism Based on a ΔES–T of around Zero

“…The argument is further confirmed by the positive MEL and MC results in relative devices. 29 To figure out the limiting factors of the EQE and the origin of the efficiency roll-off, the transient EL characteristics of the devices with various D:A molar ratios and TPBi device systems have been studied. Figure 2c indicates the EL decay curves of devices A, C, and D under electrical excitation with a pulse width of 500 ns, a pulse intensity of 14 V, and a duty of 1 kHz.…”

Thermally activated delayed fluorescence from 3nπ* to 1nπ* up-conversion and its application to organic light-emitting diodes

“…21 Current trends in the development of TADF emitters are focusing on intramolecular or intermolecular donor-acceptor (D-A)-type molecules, because chargetransfer (CT) transitions always have small singlet-triplet splitting owing to the small exchange integral between the involved wave functions. [22][23][24] Alternatively, more localized np* transitions are also overlap-forbidden and tend to have small singlet-triplet splitting. Although TADF has been observed from several aromatic ketones, these compounds emit weakly because of very slow radiative decay from the 1 np* state.…”