Hot-wire Vapor Deposition Research Articles

Formation of stoichiometric, sub-stoichiometric undoped and hydrogen doped tungsten oxide films, enabled by pulsed introduction of O2 or H2 during hot-wire vapor deposition

Tungsten oxide films with various stoichiometries, undoped and hydrogen doped were deposited by heating a W wire at 660°C and at a base pressure of 13Pa set by various gasses (O2, N2, H2) or gas mixtures (N2–O2 10% in O2, forming gas, FG) and pulsed injection of O2 or H2. Using this method and dependent on the deposition conditions four classes of hot-wire (hw) tungsten oxide films were synthesized: i) stoichiometric (hwWO3), ii) oxygen deficient (sub-stoichiometric, hwWOx with x<3), iii) stoichiometric and hydrogen doped (hwWO3:H), and iv) sub-stoichiometric and hydrogen doped (hwWOx:H). Due to the pulsed injection of O2 the W wire re-oxidizes during deposition thus creating WO3 vapors continuously, so films deposited by this method do not suffer by thickness limitations and, moreover, exhibit high porosity. The optical properties of these classes of films, studied with spectroscopic ellipsometry (SE) measurements, differ substantially between them indicating corresponding differences in their electronic structures. So, hwWO3 films were semiconducting exhibiting a band gap near 3eV. Sub-stoichiometric hwWOx deposited with up to 2 injections of O2 were semi-metallic preserving some features of the electronic structure of the pure metal, while further injection of O2 leads to stoichiometric films. Fourier transform infrared spectroscopy and SE measurements indicated that hwWOx films contain H bonded with the O ions while in hwWO3:H and hwWOx:H the H is incorporated in films by direct bonding with the W ions.

High performance organic light emitting diodes using substoichiometric tungsten oxide as efficient hole injection layer

In this work we demonstrate the unique hole injection and transport properties of a substoichiometric tungsten oxide with precise stoichiometry, in particular WO2.5, obtained after the controlled hydrogen reduction during growth of tungsten oxide, using a simple hot-wire vapor deposition technique. We present clear evidence that tungsten suboxide exhibits metallic character and that an almost zero hole injection barrier exists at the anode/polymer interface due to the formation/occupation of electronic gap states near the Fermi level after oxide’s reduction. These states greatly facilitate hole injection and charge generation/electron extraction enabling the demonstration of extremely efficient hole only devices. WO2.5 films exhibit metallic-like conductivity and, thus, can also enhance charge transport at both anode and cathode interfaces. Electroluminescent devices using WO2.5 as both, hole and electron injection layer, and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (F8BT) as the emissive layer exhibited high efficiencies up to 7cd/A and 4.5lm/W, while, stability studies revealed that these devices were extremely stable, since they were operating without encapsulation in air for more than 700h.