Highly Efficient New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine and Phenoxazine Derivatives

New hole injection layer (HIL) materials for organic light-emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical propeties of the synthesized materials were examined by UV-vis and photoluminescence spectroscopy, and by cyclic voltammetry. 10,10'-bis(4-tert-butylphenyl)-N7,N7'-di(naphthalen-1-yl)-N7,N7'-diphenyl-10H,10'H-3,3'-biphenoxazine-7,7'-diamine (1-PNA-BPBPOX) showed glass transition temperatures (T-g) of 161 degrees C, which was higher than that (110 degrees C) of Tris(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2-TNATA), a commercial HIL material. The HOMO levels of the synthesized materials were 4.9-4.8 eV, indicating a good match between the HOMO of indium tin oxide (ITO) (4.8 eV) and the HOMO of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPB) (5.4 eV), a common hole transfer layer (HTL) material. Because the synthesized materials showed minimal absorption at wavelengths shorter than 450 nm, they have good potential for use as effective HIL materials. The synthesized materials were used as the HIL in OLED devices, yielding power efficiencies of 2.8 lm/W (1-PNA-BPBPOX) and 2.1 lm/W (2-TNATA). These results indicate that 1-PNA-BPBPOX yields higher power efficiency, by a factor of 33%, than the 2-TNATA commercial HIL material. Also, 1-PNA-BPBPOX exhibited a longer device lifetime than 2-TNATA.