Highly outstanding are the most recent improvements in OLED technology that make use of TADF materials. Based on TADF characteristics, the most recent generation of OLEDs can achieve 100% theoretical exciton utilisation efficiency (EUE) without the need of any heavy complex metal phosphorescent substances by converting triplet excitons into singlet excitons. Despite improvements in productivity in the deposition of active layers in OLED devices through evaporation, this method is still unsuitable for large-scale device manufacture because of the high costs involved, and control challenges. The solution processing approach is an easier, more affordable, and more manageable choice for enabling large-scale production as compared to other manufacturing procedures. Since the majority of TADF dyes that have been reported are primarily composed of small molecules, solution deposition techniques are inappropriate for their use. Therefore, the development of organic emissive materials with TADF properties and the ability to be deposited in active layers through solution techniques is a highly desirable goal.
Electroluminescent hyperbranched polymers (HBPs) with three-dimensional (3D) architectures have a number of advantages over linear polymers with one-dimensional (1D) architectures. These advantages include being able to tune emission colours in the emissive layers of organic light-emitting diodes (OLEDs), decreased intrinsic viscosity, improved solubility, enhanced processability, more stable luminescent spectrum, and the ability to display unique molecular shapes and branching layouts. Furthermore, the OLED emitting layer's integration of hyperbranched polymers prevents exciton annihilation and aggregation-caused quenching (ACQ) while also efficiently dispersing the TADF emitting material.
This work explores approaches to prepare solution-processable hyperbranched polymers for use either as hosts to TADF dyes in blends or as materials with covalently attached TADF dyes. Oxadiazole derivatives are used as repeat units in these materials due to their high triplet energies values as well as their ability to transport electrons effectively in devices. The hyperbranched polymers and copolymers are designed to have limited electronic conjugation between monomer repeat units in order to maintain as similar high triplet energy levels in the host polymers as in their oxadiazole repeat units. In Chapter 2, two hyperbranched polymers with oxadiazole repeat units are
iii described. The hyperbranched polymers were prepared through AB 2 polycondensation strategy using trans-esterification reactions with a single oxadiazole-based monomer, in the presence and absence of the core molecule 4-nitrophenyl acetate. In Chapter 3, oxadiazole-based hyperbranched polymers using A 2 and B 3 monomers were prepared through an A2 B 3 polycondensation approach to obtain ether linked copolymers.
In Chapter 4, the Suzuki polycondensation method was utilised to synthesise four hyperbranched polymers with thermally activated delayed fluorescence (TADF) properties. These polymers were based on oxadiazole units and were prepared employing a side chain approach. Through copolymerisation with varied monomer ratios, TADF units (10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10H phenoxazine (TRZ-PXZ)) were attached as part of the polymers repeat units. Mass spectrometry, elemental analysis, 1 H NMR and 13 C NMR, and other techniques were used to investigate the structure of the intermediate monomers and hyperbranched polymers prepared. Studies of the optical, electrochemical and thermal properties of all of the hyperbranched polymers were undertaken using several techniques that included UVvis spectroscopy, photoluminescence spectroscopy, cyclic voltammetry (CV), and thermal gravimetric analysis (TGA).
