Transparent electrodes, i.e. films which can conduct electricity and at the same time transmit light, are of crucial importance for many optical devices, such as photovoltaic cells [Claes G. Granqvist “Transparent conductors as solar energy materials: A panoramic review” Solar Energy Materials & Solar Cells 91 (2007) 1529-1598], organic light emitting diodes [Ullrich Mitschke and Peter Bäuerle, “The electroluminescence of organic materials” J. Mater. Chem., 2000, 10, 1471], integrated electro-optic modulators [CM Lee et al., “Minimizing DC drift in LiNbO3 waveguide devices”, Applied Physics Lett. 47, 211 (1985)], laser displays [C. A. Smith “A review of liquid crystal display technologies, electronic interconnection and failure analysis Circuit” World Volume 34•Number 1•2008•35-41], photo-detectors, etc. [Yu-Zung Chiou and Jing-Jou TANG “GaN Photodetectors with Transparent Indium Tin Oxide Electrodes” Japanese Journal of Applied Physics Vol. 43, No. 7A, 2004, pp. 4146-4149]. From an application point of view, besides large optical transparency in the wavelength range of interest and adequate electrical conductivity, transparent electrodes should possess other key features, such as easy processing (e.g. possibility for large scale deposition), compatibility with other materials that form the same device (e.g. active layers), stability against temperature, mechanical and chemical stress, and low cost.
So far, transparent electrodes have been mainly fabricated using Transparent Conductive Oxides (TCOs), i.e. wide band gap semiconductors with heavy doping. Among them, Indium Tin Oxide (ITO) is the most widely used. Despite possessing large electrical conductivity and optical transparency from the visible to the infrared, TCOs present several drawbacks such as the requirement of high temperature (several hundreds of ° C.) post deposition treatments to improve mainly their electrical properties, their strong electrical and optical dependence on the doping control and their multicomponent structure that can lead to incompatibilities with some active materials. In addition they are not transparent in the UV range as it is shown in FIG. 1, which might be relevant for several applications. Often, such as in the case of ITO, they are made of elements (In) which are not easily available in large quantities and thus expensive.
Accordingly, providing a different type of transparent electrode that overcomes the aforementioned drawbacks is being investigated.
For instance R. B. Pode, et. al. (“Transparent conducting metal electrode for top emission organic light-emitting devices: Ca—Ag double layer”, Appl. Phys. Lett. 84, 4614 (2004), DOI:10.1063/1.1756674) propose a composite ultra thin metal electrode made of calcium and silver. Since the Ca is extremely sensitive to atmospheric moisture and oxygen, the metal was then protected by a layer of Ag. In fact in that reference it is said that also a double layer structure of Ca—Al has been tried for the purpose of reaching stability but the Al layer seems to be unable to protect the Ca layer from oxidation.
Annealing treatment has also been performed on Pt polycrystalline metal films to induce a structural change and promote a (111) texture [Sabrina Conoci, Salvatore Petralia, Paolo Samorì, Françisco M. Raymo, Santo Di Bella, and Salvatore Sortino “Optically Transparent, Ultrathin Pt Films as Versatile Metal Substrates for Molecular Optoelectronics” Advanced Functional Materials Volume 16, Issue 11, Pages 1425-1432]. As a consequence the metal films increase their electrical conductivity while the optical transparency does not change significantly. No oxide formation is reported, probably due to the noble nature of the metal.
Oxidation of Ru and Ir thin metal layers has been carried out at high temperatures to produce stable thin films of ruthenium oxide (RuO2) and iridium oxide (IrO2) [Jong Kyu Kim and Jong-Lam Lee “GaN MSM Ultraviolet Photodetectors with Transparent and Thermally Stable RuO2 and IrO2 Schottky Contacts” Journal of The Electrochemical Society, 151 (3) G190-G195 (2004)]. The resulting conducting metal oxides with rutile structure are attractive transparent electrodes for photodetectors and overcome the limitations of metal electrodes for Schottky junctions.
A possible alternative are Ultra thin Metal Films (UTMFs) [S. Giurgola, P. Vergani, V. Pruneri “Ultra thin metal films as an alternative to TCOs for optoelectronic applications”, Nuovo Cimento B 121, 887-897 (2006); S. Giurgola, A. Rodríguez, L. Martínez, P. Vergani, F. Lucchi, S. Benchabane, V. Pruneri, “Ultra thin nickel transparent electrodes” J. Mater. Sci: Mater. Electron. (2007) [Online publication],], i.e. metal films with a thickness in the range of 2-20 nm. However, given their metallic nature, UTMFs can easily degrade through oxidation, thus changing their electrical and optical properties. In particular this is the case for not noble metallic layers, such as Cr, Ni, Ti and Al.
To avoid oxidation and stability issues ultra thin noble metals have been used as transparent electrodes, such as gold, platinum and palladium but this is an expensive alternative. In addition for some applications one has to search for the metal that provides optimum parameters, e.g. work function or adhesion to specific substrates. It is then in some cases mandatory to use not noble metals.
Thus there still exists the need to provide an alternative method to prepare electrodes with large optical transparency in the wavelength range of interest and adequate electrical conductivity, and showing stability against temperature, mechanical and chemical stress, thus overcoming some of the drawbacks above mentioned.
The solution provided by the present invention is a method that takes actually advantage of the oxidation process to make UTMFs stable against environmental stress. The method consists in a thermal treatment in ambient atmosphere, optionally in combination with an O2 treatment to create a protecting oxide layer on the surface of the UTMFs with a controlled thickness. The method leads to an increase of the electrical resistivity and optical transparency and prevents further oxidation of the metal film underneath the protective oxide layer.