Tranparent conductive silver thin films for thermally insulating glazing and as electrodes for OLEDs – COCOTRANS

The project deals with two technologies that involve the use of transparent conductive thin films.
The first application is that of "thermal insulation reinforced" glazing. These glazing reflect the infrared radiation to the inside of the building and thus diminish thermal losses. We call them low-emissive glazing. Silver is the most commonly used material for this application, since, showing the highest conductivity, it is the most reflecting. Films have to be very thin, in order to preserve transparency in the visible. The amount of coatings of this kind which are deposited each year is huge, around 100 km2 per year, which represents a market near one milliard euros. Saint-Gobain controls 15% of this market. To keep this part of the market, it is vital to improve the performances still.
New applications of transparent conductive thin films have recently been developed, leading to what is known as transparent electronics, used for flat displays, flat light sources (OLEDs), photovoltaic cells, and smart windows. Saint-Gobain is involved in these new markets, which are very big. Sant-Gobain proposes an alternative to the use of traditional transparent conductive oxydes (TCO), such as indium-tin oxide (ITO). The Saint-Gobain solution makes use of very thin films of silver. This should bring an important advantage, because silver is chepaer that indium and its sourcing is more durable. It is thus a real breakthrough.

Two technical aspects are to be taken into account when developing the two kinds of products discussed in this proposal : 1. the conductivity of the silver thin films is to be made as high as possible. 2. the nature - ohmic or non-ohmic - of the contact between the silver film and the oxide films on both sides has to be well controlled.
COCOTRANS is made of three parts. Two of them deal with the basic understanding of the electron transport on one side, and of the nature of the silver-oxide contact on the other side. The third part is about processes that will be used to improve the characteristics of the final products.

When the thickness of the silver layer is decreased, its resistivity increases. This is due to the diminution of the electron mean free path, which comes from the interaction (scattering) of electrons with the film faces (interfaces) and with the grain boundaries. It is difficult to distinguish between these two processes and to evaluate which is dominant. In spite of many studies available in the literature, the answer to this question is not clear. We will revisit it using a totally original approach, in which hot electrons will be used instead of thermalized ones. The hot electrons will be injected on the side of the silver layer with the help of the tip of a Scanning Tunnelling Microscope. This experiment will be complemented by optical spectroscopy of silver films of various thicknesses in a very wide range of frequencies, from which information on the texture of the silver films will be drawn. It will be compared with in situ STM observation of the silver film during its growth on the oxide substrate.

The second part of the project will deals with the electrical transport across the silver/oxide interface. The experimental approach will be that of electron spectroscopy and STM spectroscopy during the growth. Use will also be made of I(V) measurements of the interface.
In parallel, new processes of deposition of the oxide/silver/oxide stack will be tested in order to improve the quality of the silver/oxide interface in regard of several criteria : size of grains, cleanness of grain boundaries, specularity of the electron reflection at the interface, ohmic or non-ohmic nature of the interface. Various techniques will be used, such as the deposition of a very thin layer of another metal or another oxide at the silver/oxide interface or the planarization of the oxide surface before the silver deposition.