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

Abstract

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.

Work Function Tunability of Graphene with Thermally Evaporated Rhenium Heptoxide for Transparent Electrode Applications

“…This is important as the vapor composition during deposition might vary from pure Re 2 O 7 up to mixtures of Re 2 O 7 /ReO 3 , Re 2 O 7 /Re, or Re 2 O 7 /ReO 3 /Re. [52] Even though the temperature of the effusion cell was controlled to form only Re 2 O 7 , we cannot ultimately exclude the presence of other oxide mixtures. Figure 1 shows the deconvoluted XPS spectra of the Re 4f core-level region for graphene covered with rhenium oxide layers of different thicknesses.…”

“…[45][46][47][48][49]51] However, MoO 3 has a very high evaporating temperature (795 C), which might be incompatible with the evaporating of low-melting-point organic materials in the case of OPVs with inverted structures. This incompatibility turns our attention toward rhenium trioxide (ReO 3 ) with melting point of about 340 C, [44] and rhenium heptoxide (Re 2 O 7 ), which start to vaporize (melt) at about 225 C. [52] Particularly, Re 2 O 7 seems to be very promising as its melting temperature is much lower than commonly used MoO 3 and WO 3 materials. Motivated by the success of MoO 3 in the modification of graphene work function, we decided to use the cheap and nearly colorless doping material of Re 2 O 7 , which is a TMO with the highest valence state of rhenium and seems to be an effective p-dopant for OLEDs exhibiting high ambient air stability.…”

New Insights into Tunable Volatility of Ionic Materials through Counter‐Ion Control

“…Metals such as aluminum or silver and covalentnetwork solids like metal oxides, [ 20,21 ] are always evaporated at high temperatures with cleavage of metal and covalent bonds, respectively. At fi rst, sublimable materials are vaporized by heat in the vacuum chamber and then deposit on the substrate, forming fi lms.…”

“…At fi rst, sublimable materials are vaporized by heat in the vacuum chamber and then deposit on the substrate, forming fi lms. Metals such as aluminum or silver and covalentnetwork solids like metal oxides, [ 20,21 ] are always evaporated at high temperatures with cleavage of metal and covalent bonds, respectively. In comparison, organic neutral small molecules are much easier to evaporate, owing to their weak intermolecular interaction, in most cases van der Waals force.…”

X‐ray induced reduction of rhenium salts and supported oxide catalysts

“…Rhenium itself, despite its rarity and consequential cost, is widely used in high‐performance alloys, such as those used in turbines or jet engines where corrosion resistance and low thermal deformation are required, and also finds application in smart materials such as shape‐memory alloys and organic light‐emitting diode devices …”