Transparent conductive oxides (TCO) are electronic materials that find utilization in a large variety of optoelectronic devices including flat panel displays, liquid crystal displays, plasma displays, electroluminescent displays, touch panels and solar cells. These materials also used as antistatic coatings and electro-magnetic interference (EMI) shielding.
General Electric Company (Niskayuna, NY) inventors Sergei Kniajanski and Aharon Yakimov have developed a versatile, flexible, low temperature, low cost wet process for fabrication of a wide variety of transparent conductive oxides (TCOs). Such a process not only allows reduced TCO cost and fabrication of flexible devices, but also TCOs with properties that may be better adjusted for particular technology needs, resulting in improved device efficiency. Numerous nano metal oxides and films for TCO applications are detailed in GE's U.S. Patent 7,652,157 issued on January 26th.
TCOs are of crucial importance for a number of emerging technologies, such as organic electroluminescent devices (both displays and lighting devices), and in photovoltaic (PV) devices, including: crystalline-Si heterojunction with intrinsic thin layer, amorphous silicon, CdTe, Culn(Ga)Se2 (CIGS), and organic photovoltaics. TCOs act as transparent conducting windows, structural templates, and diffusion barriers TCOs are also used for various optical coatings, in particular as infra-red reflecting coatings (heat mirrors) in automotive and building industries.
Although the main desirable characteristics of TCO materials are common to many technologies, including high optical transmissivity across a wide range of light spectrum and low electrical resistivity, specific TCO parameters vary from one system to another. Emerging technologies require new types of transparent conductors with properties better adjusted to their needs.
The number of compositions currently used as TCOs has been restricted to a few primary and binary systems. This is mainly because of two factors: 1) limited bulk solubility of crystalline metal oxide phases in each other, and 2) certain technical limitations of the currently used methods. If these challenges could be overcome, it has been shown that the number of suitable transparent and conductive binary, ternary and even quaternary phases may be larger. Some of them may potentially exist in thin films only since the phase separation in this case is kinetically precluded by the film thinness.
The compounds developed by General Electric are typically liquids with excellent wetting properties on substrates of interest and/or soluble in common organic solvents and may be conveniently applied by common coating methods. In addition, they are typically stable in dry air at room temperature, so they may be handled without any special precautions. When exposed to humid air at elevated temperatures, typically 50.degree. C.-450.degree. C., preferably 100.degree. C.-200.degree. C., the compounds are converted into metal oxides by hydrolysis with atmospheric moisture and/or thermal disproportionation. Byproducts are volatile low molecular weight siloxanes, which may be easily sequestrated.
Advantages of the GE process include: relatively inexpensive starting materials, relatively low process temperatures, low process cost, high quality film formation, ability to fabricate multicomponent films, applicability to a variety of substrates because of good wetting properties of the compounds, printability, exact control of component stoichiometry, and easy tunability of process parameters and material properties.