Low-Temperature Evaporable Re2O7: An Efficient p‑Dopant for OLEDs

• Published in: Journal of physical chemistry. C 2013-07, Vol.117 (27), p.13763-13769
• Main Authors: Jia, Yifu, Duan, Lian, Zhang, Deqiang, Qiao, Juan, Dong, Guifang, Wang, Liduo, Qiu, Yong
• Format: Article
• Language: English
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Optoelectronic devices; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp400003m

Transition-metal oxides (TMOs) are one of the most promising kinds of p-doping materials for organic semiconductors. However, to be compatible with organic materials, low-temperature evaporable TMOs are highly desirable. Rhenium(VII) oxide with a very low melting temperature of only 225 °C, which is the lowest among all TMO dopants, is first investigated as a p-dopant in N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4-diamine (NPB). Systematic studies are performed compared with ReO3, a different valence state oxide of rhenium. Hole mobility improvement from 5.38 × 10–4 to 5.88 × 10–3 cm2/(V s) at an electric field of 3 × 105 V/cm is achieved by doping Re2O7 into NPB. Lower valence states of Re species in Re2O7-doped NPB than ReO3 are observed by XPS study, indicating stronger charge transfer between Re2O7 and NPB. Temperature-dependent I–V study reveals lower hole injection barrier of Re2O7 than ReO3 in hole-only devices. Crystallinity of NPB films is found to be the same before and after doping by XRD study. Absorption spectrum study reveals higher stability of Re2O7-doped NPB than ReO3 in air. Hole current is enhanced by three orders of magnitude at 2 V when utilizing both rhenium-oxide-doped NPBs in hole-only devices. OLED devices with both rhenium-oxide-doped NPBs as hole injection layer (HIL) show a similar efficiency of 3.3 cd/A at 300 mA/cm2. Also, driving voltage is reduced from 2.6 V for pure NPB to 2.5 and 2.4 V for Re2O7 and ReO3 doped NPB, respectively.