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

Abstract

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.