Endowing TADF luminophors with AIE properties through adjusting flexible dendrons for highly efficient solution-processed nondoped OLEDs.

Dan Liu,Ben Zhong Tang,Wei Jiang,Wen Wen Tian,Jing Yi Wei,Zheng Zhao,Yue Ming Sun

doi:10.1039/d0sc02194f

Abstract

The amalgamation of thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) properties, termed AIE-TADF, is a promising strategy to design novel robust luminescent materials. Herein, we transform 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN) from an ACQ molecule into an AIEgen by simply decorating the 5CzBN core with alkyl chain-linked spirobifluorene dendrons. By increasing the number of flexible dendrons, these materials can not only show obvious AIE-TADF characteristics and uniform film morphology, but can also exhibit better resistance to isopropyl alcohol, which are beneficial to fully solution-processed OLEDs. Notably, 5CzBN-PSP shows great device efficiency with an external quantum efficiency (EQE), current efficiency and power efficiency of 20.1%, 58.7 cd A-1 and 46.2 lm W-1, respectively and achieved record-breaking efficiency in solution-processed nondoped OLEDs based on AIE emitters. This work demonstrates a general approach to explore new efficient emitters by the marriage of AIE and TADF which could potentially improve their performance in various areas.

Highlights

The reduction of the manufacturing cost of organic light-emitting diodes (OLEDs) remains one of the key challenges in the commercialization of OLED technology.[1]
Long-lived triplet excitons in thermally activated delayed fluorescence (TADF) molecules can be up-converted into radiable singlet excitons through the reverse intersystem crossing (RISC) process, the lower RISC rate inevitably results in quenching of many triplet excitons in the aggregated state by triplet–triplet annihilation (TTA), singlet– triplet annihilation (STA) and triplet-polaron annihilation (TPA).[18,19]
Tang and co-workers have reported a series of new luminogens with aggregation-induced emission (AIE) properties since 2001.20–22 These AIE compounds effectively overcome the drawbacks of aggregation-caused quenching (ACQ) and achieve efficient solid-state luminescence

Summary

Introduction

Complexes.[7,8,9] In addition, several small-molecule TADF emitters have been employed in solution-processed OLEDs and achieved relatively high device performance.[10,11,12] It is vital to emphasize that the majority of TADF emitters are doped in appropriate host matrices to weaken the intermolecular interactions and exciton quenching.[13,14] As is known, strong luminescence of conventional organic uorophores in dilute solution is normally weakened or quenched in their aggregated states, and there is no exception for most TADF materials.[15,16,17] long-lived triplet excitons in TADF molecules can be up-converted into radiable singlet excitons through the reverse intersystem crossing (RISC) process, the lower RISC rate (kRISC) inevitably results in quenching of many triplet excitons in the aggregated state by triplet–triplet annihilation (TTA), singlet– triplet annihilation (STA) and triplet-polaron annihilation (TPA).[18,19] The aggregation-caused quenching (ACQ) effect seriously limits their application and reduces the device performance. There is room for further development to extend the structural diversity of solution-processable AIE-TADF materials and enhance their device performances In this contribution, a novel type of AIE-TADF molecule was developed by constructing a core–dendron structure, which has not been reported in previous research. With increasing the number of exible branches, the compounds showed better solubility, a more smooth surface morphology, more obvious AIE features and higher photoluminescence quantum yields (PLQYs). By employing these AIE-TADF materials (5CzBN-SSP, 5CzBN-DSP, and 5CzBNPSP) as emitters, fully solution-processed nondoped OLEDs achieved high external quantum efficiencies (EQE) of 7.3%, 13.9% and 20.1%, which far exceed those of the fully solution processed OLEDs based on 5CzBN. The glass transition temperatures (Tg) of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-

Results and discussion

Findings

Conclusions