During the last few years, transition metal oxides (TMO) such as molybdenum tri-oxide (MoO3), vanadium pent-oxide (V2O5) or tungsten tri-oxide (WO3) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long-term stable p-type doping of wide band gap organic materials, charge-generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi-transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO-based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution-based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.

Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications

Transition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge-injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition-metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor-like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal-oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

Significant progress in power conversion efficiencies and stabilities of polymer solar cells has been achieved using semiconducting metal oxides as charge extraction interlayers. Both n- and p-type transition metal oxides with good transparency in the visible as well as infrared region make good Ohmic contacts to both donors and acceptors in polymer bulk heterojunction solar cells. Their compatibility with roll-to-roll processing makes them very attractive for low cost manufacturing of polymer solar cells. In this review, we will present the recent results on synthesis and characterization of these metal oxides along with the device performance of the solar cells using these metal oxides as interlayers.

Metal oxides for interface engineering in polymer solar cells

MoO3 films spin-coated from a suspension of nanoparticles, which offers energetic properties nearly identical to those of thermally evaporated MoO3 films, are reported. It is demonstrated that our solution-based MoO3 acts as a very efficient hole-injection layer for organic devices.

MoO3 Films Spin-Coated from a Nanoparticle Suspension for Efficient Hole-Injection in Organic Electronics

Thin-film metal oxides are among the key materials used in organic semiconductor devices. As there are no intrinsic charge carriers in a typical organic semiconductor, all charges in the device must be injected from electrode/organic interfaces, whose energetic structure consequentially dictates the performance of devices. The energy barrier at the interface depends critically on the work function of the electrode. For this reason, various types of thin-film metal oxides can be used as a buffer layer to modify the electrode work function. This paper provides a review on recent progress in metal oxide/organic interface energetics, oxide valence structure and work function, as well as the impact of defects and interfacial reactions on oxide work functions. This review provides a rational guide to process engineers in selecting the best suitable electrode/oxide structures for a targeted applications. Organic semiconductors offer an attractive alternative to the traditional, silicon-based components of electronic devices. Cheaper to produce and more sustainable, they can also introduce different attributes, such as flexibility, to these devices. However, as organic materials do not typically possess intrinsic charge carriers — electrons or holes — all charges in the device must originate from the electrode and pass through the electrode-organic material interface, a process hindered by an energy barrier. Mark Greiner and Zheng-Hong Lu review recent achievements in a versatile class of buffer layer — thin films of transition metal oxides — that can be positioned between the two materials to reduce the energy barrier that limits charge injection. The researchers discuss how to select the most suitable metal oxide for a specific purpose, and then tune the thin film's properties by adjusting the thickness of the metal oxide layer, the oxidation state of its cations and the concentration of its defects. Over the last decade, metal oxides have proven to be important materials for organic electronics. Oxides are often used as charge-injection and charge-selective interlayers to engineer the electrical resistance at electrode/organic interfaces in organic devices. An oxide’s behavior as an interlayer depends strongly on the oxide’s electronic properties—such as its band structure and work function. The numerous degrees of freedom in an oxide’s electronic properties allow these characteristics to be easily modified. The present review outlines the use of metal oxides in organic electronics, and discusses the factors that affect the oxide’s properties that are relevant to oxide/organic interfaces.

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Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces

Electronic structure of anode interface with molybdenum oxide buffer layer

An organic light-emitting element comprising: an anode; a cathode; banks; a functional layer between the anode and the cathode; and a hole injection layer between the anode and the functional layer. The functional layer includes one or more sublayers including a light-emitting sublayer defined by the banks and that contains an organic material. The hole injection layer comprises tungsten oxide, includes an occupied energy level that is approximately 1.8 electron volts to approximately 3.6 electron volts lower than a lowest energy level of a valence band of the hole injection layer in terms of a binding energy, has a surface facing the functional layer, and has a recessed structure such that a portion of the surface overlapping with the light-emitting sublayer is located closer to the anode than other portions. The recessed structure has a recessed portion whose inner surface is in contact with the functional layer.

Organic electroluminescence element

An organic light-emitting element comprising: an anode; a cathode; banks; a functional layer between the anode and the cathode; and a hole injection layer between the anode and the functional layer. The functional layer includes at least a light-emitting sublayer defined by the banks and that contains an organic material. The hole injection layer comprises tungsten oxide and includes a crystal of the tungsten oxide, whose particle diameter is on an order of nanometers. Tungsten atoms constituting the tungsten oxide include both tungsten atoms with a valence of six and tungsten atoms with a valence less than six. The hole injection layer has a surface facing the functional layer, and a portion of the surface overlapping with the light-emitting sublayer is located closer to the anode than other portions, thereby forming a recessed structure having a recessed portion whose inner surface is in contact with the functional layer.

Organic electroluminescence element and method of manufacturing thereof

An organic EL element includes a hole injection layer yielding excellent hole conduction efficiency, and comprises: an anode; a cathode; a functional layer disposed between the anode and the cathode, and including a light-emitting layer containing organic material; the hole injection layer disposed between the anode and the functional layer; and a bank defining an area in which the light-emitting layer is to be formed, wherein the hole injection layer includes tungsten oxide, tungsten atoms constituting the tungsten oxide include both tungsten atoms with a valence of six and tungsten atoms with a valence less than six, the hole injection layer includes a crystal of the tungsten oxide, a particle diameter of the crystal being on an order of nanometers, an inner portion of the hole injection layer is depressed to define a recess, and an upper peripheral edge of the recess is covered with a part of the bank.

Organic EL element and production method for same

An organic electroluminescence panel (10) is provided with an anode (3), a cathode (11), organic light emitting layers (8(R), 8(G), 8(B)), and a first functional layer comprising a hole injection layer (5) and a hole transport layer (7). The hole injection layer consists only of metallic oxides including tungsten oxide, has the thickness of each of the R, G, and B color portions thereof to be not less than 5 nm and not more than 40 nm, and has at least one color portion thereof have a thickness different from the other color portions. The hole transport layer has the thicknesses of the R/G/B color portions thereof to be within a range wherein the thickness is nearly equal, and the organic light emitting layers have the thicknesses of the R/G/B color layers thereof to be within a range wherein the thickness is nearly equal.

Organic electroluminescence panel, method of manufacturing organic electroluminescence panel, organic light emitting apparatus using organic electroluminescence panel, and organic display apparatus using organic electroluminescence panel

Yoshiaki Tsukamoto

Papers

Electronic structure of anode interface with molybdenum oxide buffer layer

Organic electroluminescence element

Organic electroluminescence element and method of manufacturing thereof

Organic EL element and production method for same

Organic electroluminescence panel, method of manufacturing organic electroluminescence panel, organic light emitting apparatus using organic electroluminescence panel, and organic display apparatus using organic electroluminescence panel