Cyanopyrimidine–Carbazole Hybrid Host Materials for High-Efficiency and Low-Efficiency Roll-Off TADF OLEDs

Abstract

Two isomeric host materials (Sy and Asy) comprising carbazole (donor) and CN-substituted pyrimidine (acceptor) were synthesized, characterized, and utilized as host materials for green and blue thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Both molecules have high triplet energy and small energy difference between singlet and triplet states, leading to feasible TADF. The different linking topologies of carbazole and CN groups on the pyrimidine core provide distinct photophysical properties and molecular packing manners, which further influence the efficiency as they served as hosts in TADF OLEDs. As compared to Asy-based cases, the Sy-hosted TADF OLED device gave higher maximum external quantum efficiencies (EQE) of 24.0% (vs 22.5%) for green (4CzIPN as a dopant) and 20.4% (vs 15.0%) for blue (2CzTPN as a dopant) and low efficiency roll-off. The high horizontal dipole ratio (Θ ≈ 88%) for both emitters dispersed in Sy and Asy hosts accounts for the high device efficiency. A clear molecular structure-physical property-device performance relationship has been established to highlight the importance of symmetrical structure in TADF host material design.

New exciplex systems composed of triazatruxene donors and N-heteroarene-cored acceptors

“…In considering the subtle balance between the limited HOMO/LUMO overlap and S 1 to S 0 transition probability, OLED devices employing TADF emitters that can achieve an external quantum efficiency (EQE) of over 36% have been reported. [5][6][7][8][9][10] These exciting results verify the promising potential of organic TADF materials for giving highefficiency OLEDs. Beyond the intramolecular charge transfer (ICT) approach, TADF can also be realized by the recombination of holes and electrons accumulated at the interface of a hole-transporting material (HTM) and an electron-transporting material (ETM) due to the large energy level off-sets.…”

Suppressing dimer formation by increasing conformational freedom in multi-carbazole thermally activated delayed fluorescence emitters

“…Consequently, the optimization of TADFactive structures has stimulated significant research interest in the past few years. 4,[10][11][12][13][14] However, applying the TADF mechanism towards stable and efficient blue OLEDs has not been straightforward. Current state-of-the-art materials still struggle in terms of roll-off efficiency and device operational lifetime.…”

P‐168: High Efficiency (EQE>30%) TADF‐OLED with Lightly‐doped Emitter (0.5%) by using TADF‐Host

“…On the other hand, heavy doping concentration may lead the issue about concentration quenching of excitons to degrade the device efficiency. To mitigate this issue, the exciplex host or TADF host were employed to lower the dopant concentration by supporting the efficient energy transfer between host and dopant [6][7][8][9].…”