Solution processable small molecule based TADF exciplex OLEDs

Abstract

In this work we report the optimisation of the solution processable TADF exciplex emitter in OLED devices formed by the small molecules 9-[2,8]-9-carbazole-[dibenzothiophene-S,S-dioxide]-carbazole (DCz-DBTO2) and 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC). This exciplex, previously reported by Jankus et al. [1], has gave vacuum deposited devices having respectively current efficiency, power efficiency and EQE of 32.3 cd/A, 26.7 lm/W and 10.3 % obtained for with DCz-DBTO2:TAPC wt% ratio of 30:70. In this work we optimised the thickness and ratio of the exciplex layer using two different solvents, chlorobenzene and chloroform. The best results were achieved when the two solvents were mixed, adding 5 vol% of chlorobenzene to chloroform. With this solvent mixture comparable results to evaporated devices were achieved, 27.5 ± 3.5 cd/A, 16.5 ± 2.0 lm/W and EQE of 8.9 ± 0.6 % at the same DCz-DBTO2:TAPC wt% ratio of 30:70, demonstrating the suitability of small molecule TADF exciplexes as solution processable emissive layer for OLEDs. Highlights  Demonstrated the suitability of TADF exciplex DCz-DBTO2:TAPC for solution processable OLED emitter.  Best results were achieved with chlorobenzene:chloroform solvent mixture with a 5:95 vol% ratio.  EQE as high as 8.9 ± 0.6% were obtained which is comparable with the results previously published for the same TADF exciplex system for vacuum deposited devices.

Vibrational Damping Reveals Vibronic Coupling in Thermally Activated Delayed Fluorescence Materials

“…Future investigations of the TADF mechanism may reveal the role of other vibrational modes by mechanically dampening them in similar ways as explored here. These alternative or lowamplitude vibrational motions may be a driving factor in non-D−A TADF emitters, such as exciplexes 56,57 and multiresonance boron−nitrogen materials, where large-amplitude modes for vibronic coupling like D−A dihedral angle rocking are not obvious. 58 The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.0c03783.…”

Dendritic Interfacial Exciplex Hosts for Solution‐Processed TADF‐OLEDs with Power Efficiency Approaching 100 lm W−1

“…[ 23–25 ] Generally, exciplex could be formed in bulk phase by mixing donor and acceptor together (denoted as bulk exciplex) or at the interface of donor and acceptor layers (named interfacial exciplex). [ 26 ] Although a series of vacuum‐deposited exciplex hosts have been developed and achieved very perfect device performance, [ 8,20–22 ] exciplex‐type hosts used for vacuum‐deposited OLEDs might not suitable for solution‐processed OLEDs, because of the very limited solubility and strong intermolecular interaction, [ 27 ] which may induce phase separation or serious intermolecular π‐π stacking. In addition, compared with vacuum‐deposited counterparts with multilayer structures, solution‐processed exciplex‐based OLEDs usually use a simple device structure, because of the dissolved problem between organic layers.…”

Insights into Charge Transport in High-Efficiency Green Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with a Single Emitting Layer

“…TADF emitters for solution-processed OLEDs are challenging to synthesize; they have a high molecular weight and their synthesis/production can be expensive. 41 Research in this area is at the beginning stage because of the lack of standardization for the fabrication protocol and performance evaluation of these devices. Unlike phosphorescent emitters-based OLEDs, generally, high efficiency in solution-processed TADF OLEDs is achieved in a complex multilayer architecture.…”

“…However, in this context, the role of some important device structural parameters, for instance, the thicknesses of the HTL and the emissive layer (EML) have been the highlight. All these specifications are the heart of the solution-processed fabrication because of the influence on electrical properties; , therefore, it should be considered as an essential component when understanding the charge carriers’ transport, which is not yet described to evaluate the effect on device stability and efficiency. TADF emitters for solution-processed OLEDs are challenging to synthesize; they have a high molecular weight and their synthesis/production can be expensive .…”

“…All these specifications are the heart of the solution-processed fabrication because of the influence on electrical properties; , therefore, it should be considered as an essential component when understanding the charge carriers’ transport, which is not yet described to evaluate the effect on device stability and efficiency. TADF emitters for solution-processed OLEDs are challenging to synthesize; they have a high molecular weight and their synthesis/production can be expensive . Research in this area is at the beginning stage because of the lack of standardization for the fabrication protocol and performance evaluation of these devices.…”