Layered copper oxychalcogenides for next generation p-type transparent conductors

Vision: To develop an understanding of how the structure and composition of a class of mixed oxygen-sulfur and oxygen-selenium compounds, known as layered oxychalcogenides, controls their optoelectronic properties and use this to develop a 'p-type' transparent conductor with commercially viable transparency and conductivity.

Transparent conductors, as the name implies, are materials that are transparent while also having high electrical conductivity. They are an indispensable class of material for modern electronics, being found in the screens of smart phones and tablet computers, in solar cell panels and in coatings for thermally efficient glazing. The global market for transparent conductors in 2018 was in excess of $7.1 billion. However, this market is dominated by just two materials, indium tin oxide (ITO) and fluorine doped tin oxide (FTO), and crucially both of these are 'n-type' where the conduction originates through movement of negatively charged particles - electrons. In 'p-type' materials, the conduction is in electron deficient layers and is considered as movement of 'holes' carrying a formal positive charge. In non-transparent semiconductor electronics both n-type and p-type materials are used, and their combination allows formation of diodes and transistors which are the fundamental building blocks of modern electronic devices.

In contrast, for transparent conductors there is no stable p-type conductor with sufficient transparency or conductivity to rival n-type ITO or FTO. The discovery of such a p-type transparent conductor would be transformative in enabling new technology. In combination with existing n-type transparent conductors, the availability of p-type transparent coatings would allow for formation of transparent p-n junctions that could be used to create transparent transistors, and hence transparent electronics. This would open up the possibility of integrated electronics on windows, mirrors and car windscreens - for example, providing a head-up-display for cars or in the next generation of smart glasses. Additional applications of transparent electronics would include integrated photovoltaics on windows, invisible RFIDs for security, and more efficient, brighter and lighter LEDs. A p-type transparent conductor would also reduce materials supply risk by providing alternative options for existing transparent conductor applications.

Our proposal is to investigate the layered oxychalcogenides, a materials class that has already produced at least eight known examples with p-type conductivity, if not optical transparency. From the literature, at least six different structure classes of layered oxychalcogenides are known; considering all of these and applying a series of design rules allows us to generate a target phase space of 950 oxysulfides and oxyselenides in which to identify transparent conductive materials. This is too large a phase space to investigate by exhaustive synthesis, so we will instead use a selective approach to create a compound library and use this to gain an understanding of the effect of composition on optoelectronic properties. Assessment of conductivity for this compound library will allow us to improve our design strategy and effectively target high mobility materials. We will also take advantage of computational materials modelling in collaboration with the research group of Prof David Scanlon at UCL, to more rapidly assess the stability and properties of target compositions.

By the end of the project we will have synthesized and characterised between 20 and 40 new layered oxychalcogenides and identified from within this set the most promising candidates for transparent p-type conductor applications. We will also generate a new understanding of the structure-property relations in this important class of solid-state material, and use this to identify and optimise at least one new p-type transparent conductor with conductivity equal to commercial materials.

Transparent conducting oxides (TCOs) are worth $7.1 billion a year and represent a growing market, with films of TCOs being essential components in television and display screens, and the touch screens of portable electronic devices. Less obvious but significant applications of TCO films include defrost coatings for vehicle windows, and passive thermal insulation coatings on glazing, For all these applications the materials used are n-type TCOs, chosen from just a handful of compounds. These TCOs represent only a fraction of the potential of transparent conductor technology as the contribution possible from p-types is missing. In this project, we will address this with the identification of one or more stable, high mobility p-type transparent conductors (pTCs).

An effective p-type transparent conductor would open up entirely new areas of technology through pairing with existing n-type TCOs to create transparent pn junctions and transistors, and hence transparent electronics [1]. This could enable new technology by allowing display systems on transparent surfaces, for example smart glasses, or head-up displays for car windscreens, which would improve safety by allowing the driver access to information while keeping their eyes on the road. Transparent conductor pn junctions could also be used for a transparent photovoltaic coating on 'smart' windows, allowing the energy in sunlight outside of the visible range to be converted to electricity, and making better use of the available 'real estate' for energy collection in the urban environment - helping to address energy security and the smart cities agenda. A further use for transparent electronics would be in active matrix LED screens where the select transistor is embedded with each pixel; if this were transparent, it would produce more efficient, brighter screens, which would allow for significantly increased battery life for mobile devices, as up to 60% of battery usage is consumed by screen illumination.

The availability of a pTC would also expand the choice of device architecture in both LEDs and in photovoltaic cells, reinvigorating research in these areas, and furthering development of energy efficient and sustainable energy technology. Both types of device rely on at least one transparent charge carrier layer, to allow transmission of light, which currently has to be the n-side of the junction. The availability of a p-type layer would give greater design flexibility by allowing the transparent layer to be on either the p or n doped side of the junction.

The discovery of new transparent conductors would also provide significant benefit by reducing the supply risk associated with current TCOs, and improve sustainability. For example, at the moment all high value TCO applications make use of indium tin oxide, and this presents a risk as indium has been recognised as a critical element by the UN, US and EU, with current and projected demand rapidly depleting known reserves - in less than 10 years in some estimates [2]. Our new transparent conductors will alleviate this risk through diversification, and may in fact be more suitable for transparent conductor application due to economic or process advantages.

Our project is at an early stage of technological development, and these represent the long-term impacts. More short term impact will arise though the development and training of people. The PDRA we recruit will gain new experience of materials chemistry, and they will mentor and train project students assigned to the work. This will have an important impact in providing skilled individuals for UK industry. The PDRA will also take advantage of the opportunities for public speaking and outreach to improve their communication skills, and to help inspire the next generation of researchers, ensuring the continuation of the talent pipeline.

[1] Ohta et al, Materials Today, 7(6), 2004, p1369.
[2] Chapter1, Element Recovery and Sustainability, edited by AJ Hunt