title: Design and Properties of Novel Host Materials for Blue TADF OLEDs
Creators: name: Rodella, Francesco
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Host Material
subject: TADF
subject: blue
subject: organic light-emitting diode
subject: OLED
subject: 540

description: Organic light-emitting diodes (OLEDs) represent cutting-edge technology that promises to improve significantly display and lighting applications. However, the blue diode still lacks materials with simultaneous efficiency and stability. The latest and most promising solution to solve this problem is based on thermally activated delayed fluorescence (TADF) emitters. In the development of this new class of molecules, a fundamental role is played by the host matrix, which, together with the emitter molecules, is essential to improve the stability and efficiency of the device. The aim of the thesis is to synthesize and characterize new host materials for blue OLEDs, establish a solid strategy for their design, and study the thermal and morphological properties of hosts and emitters. The first paper establishes a fundamental strategy for synthesizing high triplet energy host materials. Through a toolbox approach, building blocks (acceptor, donor, and linker) were chosen and combined systematically to form an array of six donor-acceptor molecules. All the molecules possess a triazine acceptor, while carbazole and acridine are used as donors. The connection between donor and acceptor is altered systematically from a para-phenyl to a meta-phenyl linker until having no linker at all. By comparing the molecules, we conclude that the highest triplet energies are obtained when the linker between donor and acceptor is avoided. Furthermore, using a less conjugated donor (acridine instead of carbazole), the highest triplet energy of 3.07 eV is reached for the host ATRZ. This host possesses high thermal and morphological stability (thermal decomposition and glass transition at 341°C and 115°C, respectively). Aside from the publication, the most promising host is tested in blue TADF OLEDs, obtaining high external quantum efficiency of 10.0% and low efficiency roll-off. Furthermore, the device has shown a good lifetime with LT50 (lifetime until half luminance) of 40 hours at 100 cd/m2, which is a reasonable value among the few lifetimes reported in the literature for blue TADF emitters. In the second paper, the knowledge gained from the toolbox approach is expanded to obtain new high triplet energy hosts and validate the design strategy. Donor and acceptor units were chosen and directly coupled to limit the conjugation and provide higher triplet energy. Additionally, the donor's choice goes on the acridine unit for its high triplet state and bulkiness. This time, the donor is kept constant throughout the series, while three different pyrimidine-based acceptors with varying conjugation lengths are adopted. Their conjugated system is changed from having two phenyl rings to one phenyl ring until having only one methyl group. We conclude that the host adopting the least conjugated methyl-substituted pyrimidine acceptor provides the highest triplet energy of 3.07 eV, along with bipolar properties and high thermal and morphological stability, i.e., glass transition at 138°C and thermal decomposition at 413°C. Again, the most promising host is tested in blue TADF OLEDs, providing high efficiency of 13.6% and a low efficiency roll-off. The third paper discusses a series of four new pyridylbenzimidazole-based TADF emitters. Due to the importance of thermal and morphological stability, the thermal properties of these emitters are analyzed. The four emitters differ from the functional group on the acceptor moiety, which increases bulkiness within the series: hydrogen (without substitution), methyl, phenyl, and tert-butyl. The molecule with the phenyl group is the most stable (thermal decomposition at 355°C) due to more π conjugation and π-π interactions. Surprisingly, the morphological stability is the highest for the molecule without substitution (glass transition at 97°C), which is attributed to hydrogen bond formation in the solid-state. In the fourth paper, a model correlating TADF emitters' orientation with the host's glass transition to increase light outcoupling and hence OLED efficiency is described. The results show that the orientation of TADF emitters doped in a host film depends mostly on three parameters. i) When the molecule is highly elongated, the emitter's intrinsic property (shape) determines the orientation. ii) When emitters are less elongated, the host governs their orientation. iii) within the same glass transition range, certain hosts can rearrange more than others, boosting the horizontal orientation of the emitter. Therefore, also the alignment of the host itself influences the emitter's orientation. To summarize, the work of this thesis tackles the deficiency of strategies on how to design high triplet energy host materials by reporting a widely applicable approach for their design. Additionally, two novel hosts with triplet energy as high as 3.07 eV were obtained, possessing good thermal and morphological properties and bipolar character. The materials were tested in OLED devices delivering good efficiency and stability. Furthermore, the thermal and morphological properties of new TADF emitters were determined, providing insights into the processability and durability of this category of emitters. Finally, the morphological properties of benchmark hosts were obtained and correlated with TADF emitters' orientation to provide a model helpful to increasing efficiency in OLEDs.