Rong-Huei Yi

Publication date: 15 January 2023

Source: Chemical Engineering Journal, Volume 452, Part 1

Author(s): Jiale Li, Ling Zhou, Jiawei He, Qin Xue, Liang Xu, Guohua Xie

Publication date: 15 January 2023

Source: Chemical Engineering Journal, Volume 452, Part 1

Author(s): Caifa You, Denghui Liu, Li Wang, Weiqiong Zheng, Min Li, Pu Wang, Weiguo Zhu

Compared with hole trap-assisted type exciplex cohosts, Langevin type exciplex cohosts have intrinsic advantanges to achieve long working lifetime organic light-emitting devices by restraining exciton-polaron annihilation induced transport deteriorations.

Abstract

Exciplex-forming cohosts have gained increasing interest due to their promising performances in organic light-emitting diodes (OLEDs). Such electroluminescent stabilities of the exciplex cohosts depend on their charge-carrier recombination. But intrinsic relationships between the recombination schemes and the electrical aging process of the exciplex cohosts are yet to be revealed. In this case, exciplex cohosts with two different recombination schemes, namely trap-assisted recombination (TAR) and trimolecular Langevin recombination (LR), are compared. The LR-dominated device achieves T₉₀ lifetime (time to 90% of initial luminance) of ≈8600 h at luminance of 1000 cd m⁻², which is >2.3-time lifetime improvement compared with the hole TAR-dominated ones. The corresponding degradation analyses confirms that hole carriers in hole TAR-dominated cohosts are captured by phosphorescent guests, accelerating p-type cohost deterioration through exciton-polaron decomposition on susceptible aromatic amine groups, which reveals the electrical aging mechanism of exciplex cohost based devices. Moreover, exciton managements of the recombination region further suppress luminous decline and release undesirable exciton–exciton annihilations. The current findings offer an in situ technique to observe exciton and charge carrier aging processes with different recombination mechanisms and provide a general guidance for the development of highly efficient and long-lifespan exciplex cohost based OLEDs.

Publication date: 1 January 2023

Source: Chemical Engineering Journal, Volume 451, Part 3

Author(s): Siyang Liu, Junzi Li, Huibo Yan, Fanghao Ye, Fangfang Niu, Bin Zhang, Guoping Wang, Guijun Li, Pengju Zeng

Publication date: 1 January 2023

Source: Chemical Engineering Journal, Volume 451, Part 4

Author(s): Xing Wu, Jiajie Zeng, Xiaoluo Peng, Huijun Liu, Ben Zhong Tang, Zujin Zhao

Harnessing the heavy atom effect is the most useful way for boosting reverse intersystem crossing process for thermally activated delayed fluorescence (TADF) emitters aiming to shorten the excited state lifetime and enhance the emission efficiency. A detailed investigation on S-containing TADF molecules featuring through-space charge transfer excited state shows that the heavier and bigger atom may have adverse effect because of its multiple roles.

Abstract

The harnessing of heavy atom effect of chalcogen elements offers a way for boosting the thermally activated delayed fluorescence (TADF) of purely organic luminescent materials that can harvest triplet excitons. However, the conformational and electronic variations induced by the heavy and large atoms may also have adverse effects on the TADF properties. Herein, the design, synthesis, and structures of a new type of through-space charge transfer (TSCT) emitters containing benzothiazino[2,3,4-kl]phenothiazine (DPTZ) as the donor unit are reported. The influences of S atoms on the emission properties have been systematically investigated by means of theoretical simulations, electrochemical and spectroscopic studies. Although the presence of π-stacking interactions and calculated spin-orbit coupling (SOC) values are beneficial for TSCT-TADF properties, the triplet TSCT states are uplifted to above the locally excited (LE) state of the acceptor moieties. As a result, the new emitters display longer delayed fluorescence lifetimes (τ_DF) of 255.0–114.3 μs and lower PLQYs of 45–61 % in comparison with the O-containing congeners (τ_DF=26.9–6.8 μs; PLQYs=74–71 %). This work highlights that a full consideration of various effects is essential when making use of heavy chalcogen atoms for the design of TADF emitters.

A new alkynyl Au(I) carbene complex with tunable electronic structure shows high efficiency thermally activated fluorescence (TADF). Solution-processed OLEDs made with these complexes achieve high external quantum efficiencies (EQEs) of up to 20.4 %, manifesting the bright prospect of Au(I)-TADF emitters in OLEDs.

Abstract

Two-coordinate donor-metal-acceptor type coinage metal complexes displaying efficient thermally activated delayed fluorescence (TADF) have been unveiled to be highly appealing candidates as emitters for organic light-emitting diodes (OLEDs). Herein a series of green to yellow TADF gold(I) complexes with alkynyl ligands has been developed for the first time. The complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 0.76 in doped films (5 wt % in PMMA) at room temperature. The modifications of alkynyl ligands with electron-donating amino groups together with the use of electron-deficient carbene ligands induce ligand-to-ligand charge transfer excited states that give rise to TADF emission. Spectroscopic and density functional theory (DFT) calculations reveal the roles of electron-donating capability of the alkynyl ligand in tuning the excited-state properties. Solution-processed organic light-emitting diodes (OLEDs) using the present complexes as emitters achieve maximum external quantum efficiency (EQE) of up to 20 %.

Publication date: 1 January 2023

Source: Chemical Engineering Journal, Volume 451, Part 2

Author(s): Hongyang Zhang, Yingjie Sun, Zhao Chen, Weigao Wang, Qiwei Wang, Shuming Chen, Yuqing Xu, Wai-Yeung Wong