Exciplex system is a charming candidate for thermally activated delayed fluorescence(TADF)due to its intrinsic small energy difference between the lowest singlet state and triplet excited state(ΔEST).However,high emission efficiency and fast radiative decay rate are still a formidable task for the exciplex emission.Herein two novel tri(triazolo)triazine-based TADF emitters,named TTT-HPh-Ac and TTTMePh-Ac,are synthesized and characterized.Using such TADF emitters as the donor molecule and(1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-dial)tris(diphenylphosphine oxide)(PO-T2T)as the acceptor molecule,the exciplex system of TTT-HPh-Ac:PO-T2T and TTT-MePh-Ac:PO-T2T are prepared,which show a tinyΔEST of 40±20 meV and fast reverse intersystem crossing rate.As a result,very high emission efficiency(97%)and a small non-radiative decay rate are detected for the exciplex TADF system.The solution processable organic light-emitting diode using the exciplex system as the emitter achieves a maximum external quantum efficiency(EQE_(max))of 17.0%.When using the exciplex as the host matrix,the TTT-MePh-Ac:PO-T2T based solution processable device shows a better performance with an EQE_(max)of 20%with a very small efficiency roll-off of 6%at 1000 cd m^(−2).This work proves that the molecule with both intramolecular hydrogen bonding and proper twisted molecular geometry in exciplex is more favorable to enhance its emission efficiency and suppress the non-radiative transition,which provides a new way to develop efficient and stable exciplex emitters.
Xiaoyi LaiJunqing WangXiaolong LiuLei HuaBin LiWeiguo ZhuJun Yeob LeeYafei Wang
