Design of Efficient Exciplex Emitters by Decreasing the Energy Gap Between the Local Excited Triplet (3LE) State of the Acceptor and the Charge Transfer (CT) States of the Exciplex.

Xiaofang Wei,Yanwei Liu,Pengfei Wang,Zhiyi Li,Chun-Sing Lee,Taiping Hu,Honglei Gao,Guanhao Liu,Ruifang Wang,Xiaoxiao Hu,Jianjun Liu,Ying Wang

doi:10.3389/fchem.2019.00188

Abstract

A series of thermally activated delayed fluorescence (TADF) exciplex based on the TX-TerPy were constructed. The electronic coupling between the triplet local excited states (3LE) of the donors and acceptor and the charge transfer states had a great influence on the triplet exciton harvesting and ΦPL. Herein, based on this strategy, three donor molecules TAPC, TCTA, and m-MTDATA were selected. The local triplet excited state (3LE) of the three donors are 2.93, 2.72 and 2.52 eV in pure films. And the 3LE of TX-TerPy is 2.69 eV in polystyrene film. The energy gap between the singlet charge transfer (1CT) states of TAPC:TX-TerPy (7:1), TCTA:TX-TerPy (7:1) and the 3LE of TX-TerPy are 0.30 eV and 0.20 eV. Finally, the ΦPL of TAPC:TX-TerPy (7:1) and TCTA:TX-TerPy (7:1) are 65.2 and 69.6%. When we changed the doping concentration of the exciplex from 15% to 50%, the ratio of the triplet decreased, and ΦPL decreased by half, perhaps due to the increased energy gap between 1CT and 3LE. Therefore, optimizing the 1CT, 3CT, and 3LE facilitated the efficient exciplex TADF molecules.

Highlights

Organic light-emitting devices (OLED) have been widely studied due to their promising applications in large displays and solid-state lighting (Sasabe and Kido, 2013)
TX unit is promising as an acceptor unit to construct the efficient pure thermally activated delayed fluorescence (TADF) (Wang et al, 2014; Li et al, 2016) While, TX derivatives have never been used as the acceptor molecule to construct the exciplex, we functionalized the TX unit by bridging it along with the terpyridy by Suzuki coupling reactions, an electron-withdrawing group, to obtain the molecules TXTerPy
In order to further investigate the electrochemical property of the exciplex blend film based on TAPC:TXTerPy and TCTA:TX-TerPy, OLED devices were fabricated with the following structures: indium tin oxide (ITO)/TAPC(35 nm)/1,3-Bis(carbazol-9-yl)benzene

Summary

Introduction

Organic light-emitting devices (OLED) have been widely studied due to their promising applications in large displays and solid-state lighting (Sasabe and Kido, 2013). The phosphorescence of the donors show characteristic vibrational structures with the first highest peaks at 423.6 nm for TAPC, 456.6 nm for TCTA, 491.6 nm for m-MTDATA, so the related T1 energy is 2.93, 2.72 eV and 2.52 eV, respectively.

Results

Conclusion