Molecular Engineering
Modulating the Singlet–Triplet
Energy Splitting of Indolocarbazole-Based TADF Emitters Exhibiting
AIE Properties for Nondoped Blue OLEDs with EQE of Nearly 20%
posted on 2023-12-13, 16:37authored byJinshan Wang, Yuguang Yang, Fei Gu, Xuesong Zhai, Chuang Yao, Jianfeng Zhang, Cuifeng Jiang, Xinguo Xi
The development of efficient blue thermally activated
delayed fluorescence
(TADF) emitters with an aggregation-induced emission (AIE) nature,
for the construction of organic light-emitting diodes (OLEDs), is
still insufficient. This can be attributed to the challenges encountered
in molecular design, including the inherent trade-off between radiative
decay and reverse intersystem crossing (RISC), as well as small singlet–triplet
energy splitting (ΔEST) and the
requirement for high photoluminescence quantum yields (ΦPL). Herein, we present the design of three highly efficient
blue TADF molecules with AIE characteristics by combining π-extended
donors with different acceptors to modulate the differences in the
electron-donating and electron-withdrawing abilities. This approach
not only ensures high emission efficiency by suppressing close π–π
stacking, weakening nonradiative relaxation, and enhancing radiative
transition but also maintains the equilibrium ratio between the triplet
and singlet excitons by facilitating the process of RISC. These emitters
exhibit AIE and TADF properties, featuring quick radiative rates and
low nonradiative rates. The ΦPL of these emitters
reached an impressive 88%. Based on their excellent comprehensive
performance, nondoped PICzPMO and PICzPMO OLEDs achieved excellent
electroluminescence performance, exhibiting maximum external quantum
efficiency (EQEmax) of up to 19.5%, while the doped device
achieved a higher EQEmax of 20.8%. This work demonstrated
that by fusing π-extended large rigid donors with different
acceptors, it is possible to regulate the difference in electron-donating
and electron-withdrawing abilities, resulting in a small ΔEST, high ΦPL, and fast RISC
process, which is a highly feasible strategy for designing efficient
TADF molecules.