Rational Molecular Design Strategy for Host Materials in Thermally Activated Delayed Fluorescence-OLEDs Suitable for Solution Processing

Abstract

Herein, a novel core molecule for V-shaped host molecules was synthesized, wherein two carbazoles were directly linked to cyclohexane. Cy-mCP and Cy-mCBP hosts were also successfully prepared for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF–OLEDs). The Cy-mCP and Cy-mCBP molecules contained a cyclohexane linker directly linked to two small molecular hosts (mCP and mCBP), exhibiting twice the molecular weight while maintaining the basic properties of a single host molecule with improved film-forming ability and solubility in organic solvents. These host materials showed superior thermal stability and high glass transition temperatures compared to lower molecular weight hosts. Green TADF–OLEDs were prepared using the two host materials and 2,4,5,6-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3-dicyanobenzene (t4CzIPN) emitter, achieving device efficiencies similar to that of a low-molecular-weight host. However, after the incorporation of a V-shaped host, superior characteristics were observed in terms of the thermal stability and operational stability of the device. The synthesis of V-shaped molecules by directly linking two carbazoles to a cyclohexane linker is promising for the development of different hosts for solution-processable OLEDs.

Recent endeavors and perspectives in developing solution-processable host materials for thermally activated delayed fluorescence organic light-emitting diodes

“…15–23 Solution processing is highly advantageous over vacuum processing owing to the easy processability and usage of low-cost instruments such as spin-coating, blade-casting, and inkjet printing in the former, while avoiding harsh fabrication conditions such as high temperature and pressure. Despite significant progress in the development of solution-processed OLEDs, 24–66 their efficiency and lifetimes still lag behind those of vacuum-processed devices.…”

“…Considering these challenges, small molecules, 19–31 sp 3 -cored non-conjugate linker-based dendrimers, 32–37 and polymers 38–42 with a high E T and good solubility have been demonstrated as host materials for solution-processable TADF OLEDs. Several host materials consisting of D–A, D–A–D, or D–A–D′-type architectures have been reported.…”

“…Several host materials consisting of D–A, D–A–D, or D–A–D′-type architectures have been reported. 19–42 However, these architectures can undesirably augment charge transfer between the donor and acceptor entities and extend the conjugation length, which hinders the achievement of a high E T . Although the synthesis and purification of small molecules is easier than that of polymeric hosts, it is sometimes difficult to achieve a high E T in the solid state with the former.…”

“…To address these issues, several researchers have designed and developed dendritic large molecules, as well as polymers, as host materials by incorporating cyclohexane, adamantane, silicon, or even the 9th position of acridine or fluorene as sp 3 -cored non-conjugate linkers between the donors and acceptors. 32–42 Non-conjugate linkers can break the continuous conjugation of the molecule, thereby retaining a high E T . The sp 3 -core also imparts a tetrahedral geometry and 3D orientation to the resulting molecule, which can inhibit crystallization and stabilize the morphology.…”

Theoretical Research and Photodynamic Simulation of Aggregation-Induced Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

“…Usually, it is necessary to dope TADF materials into host materials to obtain TADF emission. However, the doping method would also lead to intermolecular energy consumption. − Therefore, it is essential to design and synthesize TADF materials with high fluorescence efficiency in the solid state. , Since the concept of aggregation-induced emission (AIE) was proposed, it has attracted increasing attention for potential applications in OLEDs and biological imaging, which permits the use of fluorophore solutions at any concentration, thus overcoming the disadvantages of traditional fluorescence emitters. − Recently, it has been reported that molecular aggregation and crystal packing modes have a significant influence on TADF properties, ,, and aggregation-induced delayed fluorescence (AIDF) materials can fully combine the advantages of AIE and TADF features, thus contributing to highly efficient emission. − …”