摘要
Organic light-emitting diodes (OLEDs) rely on efficient charge transport layers to optimize device performance, stability, and overall efficiency. One of the critical components in OLED architecture is the hole-transporting layer (HTL), which facilitates efficient hole injection and transport while maintaining thermal and morphological stability. Traditional hole-transporting materials (HTMs), such as N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1 ' biphenyl-4,4 '-diamine (NPB), are widely used; however, their limitations, including moderate thermal stability and suboptimal energy level alignment, necessitate the development of new HTMs with improved properties. To address this challenge, we designed and synthesized three novel fluorene-based HTMs: 10-hexyl-3-(2,7-diphenylfluoren-9-ylmethylene)phenothiazine (DM258), 10-hexyl-3-[2,7-di(4-fluorophenyl)fluoren-9-ylmethylene] phenothiazine (DM259), and 10-hexyl-3-[2,7-di(9-ethylcarbazolyl-3-yl)fluoren-9-ylmethylene]phenothiazine (DM260). These compounds feature a phenothiazine-fluorene core with an aliphatic chain attached to the phenothiazine unit, designed to enhance solubility and promote the formation of stable amorphous layers for solution-processed OLEDs. During this study, we investigated how different substituents on the fluorene core (benzene, fluorobenzene, and carbazole) influence the thermal, photophysical, and charge transport properties. The synthesized materials demonstrated excellent thermal stability, with decomposition temperatures exceeding 310 degrees C and glass transition temperatures above 100 degrees C, and atomic force microscopy confirmed the formation of uniform, pinhole-free films. Their HOMO-LUMO energy levels were well-suited for hole transport, and their sufficiently high triplet energy levels effectively prevented energy back-transfer, confining excitons within the emissive layer. These characteristics make them promising candidates for achieving balanced and efficient charge transport. The performance of these materials was evaluated in OLED devices, with DM260 exhibiting significant efficiency improvements. Compared to NPB-based devices, the external quantum efficiency (EQE) increased from 6.5 % to 11.4 %, power efficacy (PE) improved from 17.3 to 26.8 lm/W, and current efficacy (CE) rose from 20.6 to 34.6 cd/A at 100 cd/m2. Additionally, DM260 achieved the lowest operating voltages across various luminance levels (100, 1000, and 10,000 cd/m2), ensuring efficient charge transport with minimal energy loss. These findings highlight the critical role of molecular design in optimizing charge injection and transport, demonstrating the potential of fluorene-based HTMs for efficient solution-processed OLEDs.