What makes TCO conducting and transparent?
The most important factor that makes TCO conducting and transparent is the band gap, smaller band gap could enhance conductivity, while larger band gap leads to higher transparency. Performance of TCO conductivity can be described in terms of resistivity, affected by charge transport properties as electron effective mass (me*: mass of an electron appears to carry), carrier concentration (n: amount of electrons or holes present), charge carrier mobility (£g: flow of electrons or holes). The enhancement of TCO conductivity can be by increasing carrier concentration or mobility. On the other hand, optical transparency requires large band gap due to absorption of materials, and it could be improved by doping elements to increase carrier concentration or enhance the simplicity of TCO structure. 
Thin film deposited technology
TCO used for different applications are normally thin films, backed by transparent or non-transparent substrates. However, the parament process is the deposition of TCO thin films which have been discussed in numerous books and literatures, and some of most common methods are summarized as followings:
(2) Chemical vapor deposition (CVD), involves a variety of techniques, such as plasma enhanced CVD (PECVD)¡Bsol-gel deposition¡Bspray pyrolysis, etc, which use heat to decompose vapor of precursor chemical to make a thin film with a desired composition in non-vacuum condition. Spray pyrolysis is widely used on a large scale deposition that a fluid of precursor is sprayed onto a hot substrate in either discrete processes or continuous process as indicated in figure 4. 
(3) Electrochemical deposition technique, includes cathodic electro-deposition from a chemical solution and anodic conversion of a metallic surface. 
Tin oxide (SnO2) regarded as N-type semiconductor, crystallizes with a rutile (tetragonal; a=b=0.474 nm and c=0.319nm) structure, wherein the tin atoms are 6 coordinate and the oxygen atoms 3 coordinate. The structure of tin oxide (SnO2) is shown in Figure 5.  Pure tin oxide shows low conductivity (~1.5¡Ñ10-2 £[-cm), hence, it can be enhanced by doping elements, such as indium (ITO) or fluorine (FTO).
Indium tin oxide (ITO), 80-90% indium oxide with a minor amount of tin oxide, has been very popular for flat panel displays (FPD). However, indium is scarce on earth so that the cost is too high to induce some issues in recent research. Consequently FTO is becoming more popular in research, with some advantages of easily produced and processed, mechanically durable, chemically stable, heat resistance and controllable morphology, etc. Although the conductivity performance may be slightly lower than ITO, FTO is generally less expensive in materials cost and manufacturing, and avoids problems with indium diffusion into the N-type TiO2 or ZnO nanostructured film following annealing treatments.  Owning to these good properties, FTO film could be served as an important role in photovoltaics (PV) industry.
1. C. G. Granqvist., Solar Energy Materials & Solar Cells 91 (2007).
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