Design and Preparation of Solution Processable Hyperbranched Polymers for OLED Applications

Abstract

The latest developments in organic light-emitting diodes (OLED) technology utilising thermally activated delayed fluorescence (TADF) materials are highly remarkable. The last generation of OLEDs based on TADF properties has the capability to convert triplet excitons into singlet excitons, achieving an exciton utilisation efficiency (EUE) of 100% without the inclusion of any heavy complex metal phosphorescent materials. Despite advancements in productivity, the evaporated deposition method for fabricating OLEDs presents disadvantages for producing large-scale devices, due to its strict structure requirements and significant challenges in achieving cost-effective mass production including high energy consumption, limited scalability and challenging control. As an alternative to other manufacturing processes, the solution processing method is a more straightforward, cost-effective, and easily controllable option for facilitating massive production. Nevertheless, most of the TADF dopants that have been described are mostly composed of small molecules (organic compounds with a molecular weight of 1000 daltons or less), making them unsuitable for solution deposition processes. Hence, there is a significant commercial motivation to create organic emissive materials that can be processed as solutions and possess TADF properties. In comparison to linear polymers with one-dimensional (1D) architectures, electroluminescent hyperbranched polymers (HBPs) featuring three-dimensional (3D) architectures exhibit unique molecular shapes (dendritic and spherical architectures), branching patterns, reduced intrinsic viscosity (refers to the extent to which a solute affects the viscosity of a solution), enhanced solubility, improved processability, a more stable luminescent spectrum, and the capability to tune emission colours in the emissive layers of organic light-emitting diodes (OLEDs). In addition, the incorporation of hyperbranched polymers within the OLED emitting layer effectively disperses the TADF emitting material and inhibits aggregation-caused quenching (ACQ) and exciton annihilation effects. Various strategies for the synthesis and design of solution-processable TADF hyperbranched polymers have been proposed in this study. Carbazole (Cbz) and Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) derivatives were employed as monomers in hyperbranched copolymers with minimal electronic conjugation for use as host materials due to their substantial triplet energies and their ability to facilitate the transport of holes and electrons, respectively.