Reduction of Tungsten Oxide: A Path Towards Dual Functionality Utilization for Efficient Anode and Cathode Interfacial Layers in Organic Light‐Emitting Diodes

Abstract

Here, we report on the dual functionality of tungsten oxide for application as an efficient electron and hole injection/transport layer in organic light‐emitting diodes (OLEDs). We demonstrate hybrid polymer light‐emitting diodes (Hy‐PLEDs), based on a polyfluorene copolymer, by inserting a very thin layer of a partially reduced tungsten oxide, WO2.5, at the polymer/Al cathode interface to serve as an electron injection and transport layer. Significantly improved current densities, luminances, and luminous efficiencies were achieved, primarily as a result of improved electron injection at the interface with Al and transport to the lowest unoccupied molecular orbital (LUMO) of the polymer, with a corresponding lowering of the device driving voltage. Using a combination of optical absorption, ultraviolet spectoscopy, X‐ray photoelectron spectroscopy, and photovoltaic open circuit voltage measurements, we demonstrate that partial reduction of the WO3 to WO2.5 results in the appearance of new gap states just below the conduction band edge in the previously forbidden gap. The new gap states are proposed to act as a reservoir of donor electrons for enhanced injection and transport to the polymer LUMO and decrease the effective cathode workfunction. Moreover, when a thin tungsten oxide film in its fully oxidized state (WO3) is inserted at the ITO anode/polymer interface, further improvement in device characteristics was achieved. Since both fully oxidized and partially reduced tungsten oxide layers can be deposited in the same chamber with well controlled morphology, this work paves the way for the facile fabrication of efficient and stable Hy‐OLEDs with excellent reproducibility.

Transition metal oxides as hole-selective contacts in silicon heterojunctions solar cells

“…Nonetheless, the absence of the W +5 oxidation state on the air-exposed surface could be caused by tungsten reoxidation by air, not excluding oxygen vacancies from the material bulk. The characteristic defect states that lie within the band gap of these non-stoichiometric TMOs were also identified near the Fermi level [28,29,30], although in a minor degree for WO x (Fig. 2(d)).…”

“…These results limit the oxide thickness on the final device to only a few nanometers and justifies the need for an ITO collecting electrode. Ultimately, actual conductivities will depend on the morphology of thermally-evaporated TMOs, which has been reported as completely amorphous for MoO x [20] and nano-crystalline for WO x [30]. Further study of preparation methods and post-deposition treatments could promote oxide crystallinity in order to enhance film conductivities.…”

“…The oxygen deficiency in MoO 3-x was calculated at x ~0.5, with a 0.5% relative content of Mo +5 cations in the total composition. Thirdly, the W4f core level of WO 3 was also decomposed into two doublets centered at 36.8 and 34.8 eV (Δ BE =2.2 eV, 4:3 area ratio), characteristic of W +6 and W +5 cations [30].…”

Omnidirectional antireflective properties of porous tungsten oxide films with in-depth variation of void fraction and stoichiometry

“…The evolution of the oxide reduction was monitored by measuring the intensity of the W 4f core levels and calculating the ratio between the W 6 þ component versus the W components with lower oxidation states (e.g., 5þ, 4þ). By depositing the first layers of the film (e.g., the first 7 nm) under H 2 environment it was found that they were sub-stoichiometric WO x (i.e., metallic [14]) with x¼2.5 and their refractive index equal to 3. Then the deposition ambient was switched to N 2 resulting in the deposition of a ca.…”

“…Due to their high porosity these films exhibited refractive index depending on thickness (i.e., deposition time) varying between 2 for thinner films and 1.2 for thicker ones, within the visible range [12]. By switching the deposition ambient from N 2 to H 2 , sub-stoichiometric films, described as hwWO x with xo3, were also obtained, with their degree of stoichiometry (x) and refractive index depending on the exact H 2 partial pressure [13][14][15].…”