Transparent conducting films

There are several different metal oxides with optical band gaps of 2.5 to 4 eV, which are useful for transparent conducting oxide (TCO) coatings. The most common of these are tin-doped indium oxide (ITO), fluorine or antimony-doped tin oxide and doped zinc oxides. The preparation and physical properties of these materials prepared by CVD will be discussed. 

Critical properties of TCO coatings are electrical resistance and transparency [3], but for solar cell applications very often texture and large haze factors, i.e., ratio of diffuse to total transmission, have similar importance. Large haze factors have been shown to influence positively the efficiency of silicon solar cells, because the reflection at the TCO-silicon interface is reduced and the scattering increases the pathway of light inside the active material. The preparation and characteristics of several TCO materials have been reviewed by Chopra et al. [92] and Dawar and Joshi [93]. The optical and electrical properties of ITO and aluminum doped zinc oxide have been studied in detail by Granqvist and coworkers [94, 95], but these films were prepared by sputtering and not by CVD. Very recently they also published an overview of transparent conductive electrodes for electrochromic devices [7]. 

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Sn02 is codeposited with indium oxide to form a compound known as indium tin oxide (ITO), widely used as a transparent conductive film. ITO is usually deposited by sputtering and little work on CVD has been reported. 

Fattakhova-Rohfing, D. Brezesinski, T. Rathousky, J. Feldhoff, A. Oekermann, T. Wark, M. Smarsly, B. 2006. Transparent conducting films of indium tin oxide with 3D mesopore architecture. Adv. Mater. 18 2980-2983. 

T. Kawashima, H. Matsui, and N. Tanabe, New transparent conductive films FTO coated ITO, Thin Solid Films, 445 241-244 (2003). 

Polythiophenes (PTs)/CNTs composites have emerged as an intriguing system for use as photovoltaic devices and field effect transistors [57]. Swager and Bao independently reported methods for the assembling of PTs/CNTs systems and showed their great potential as transparent conductive films [58]. Another interesting application arises from the possibility to functionalize the polythiophene backbone for applications as chemical sensors [134]. 

Fraser IS, Motta MS, Schmidt RK, Windle AH. Continuous production of flexible carbon nanotube-based transparent conductive films. Science and Technology of Advanced Materials. 2010 Aug ll(4). 

Solar Cells—Some nanotubes exhibit a photovoltaic effect and can replace material in solar cells acting as a transparent conductive film allowing light to pass to the active layers and generate photocurrent. It has also been shown that nanotubes can be used in conjunction with other materials to act as an ignition device that is triggered with a simple camera flash. 

The crosslinkable COCs have been suggested for the use as alternatives for a glass substrate of a liquid crystal display devices and an electroluminescence (EL) display devices. Moreover, a crosslinkable COC can be used as a polarizing film, surface protective film, retardation film, transparent conductive film, light diffusion film, film for EL display devices, transparent conductive composite material, anti-reflection film, etc. Methods how to prepare these types of films have been described in detail (47). 

Ishibashi S., Higuchi Y., Ola Y. and Nakamura K. Low resistivity indium tin oxide transparent conductive films. I. Effect of introducing H2O gas of H2 gas during direct current magnetron sputtering. J. Vac. Soc. Technol, 1989 AS 3-6. 

Zhang J, Gao L, Sun J, Liu Y, Wang Y, Wang J, Kajiura H, Li Y, Noda K (2008) Dispersion of single-walled carbon nanotubes by nafion in water/ethanol for preparing transparent conducting films. J Phys Chem C 112 16370-16376... 

Optical quality thin films of metallic polymers are useful, therefore, as transparent electrodes [68]. For example, polyaniline [69], polypyrrole [70] and PEDOT [71] have been used as transparent hole-injecting electrodes in polymer LEDs (the initial demonstration of mechanically flexible polymer LEDs utilized PANl as the anode [69]). Transparent conducting films can be used for a variety of purposes for example, as antistatic coatings on CRT screens, as electrodes in liquid crystal display cells, or for fabricating electrochromic windows. 

Although ZnO also forms transparent conducting films, it has drawn surprisingly little attention for use as the anode in OLEDs. 

It is also possible to incorporate fluoride dopant atoms into the precursor. Specially synthesized trialkynyl(fluoroorgano)tin compounds were hydrolyzed to soluble oligomers, which were thermally decomposed to fluoride-doped Sn02 for use as optically transparent conducting films 92,93... 

Electrochromism is in principle a device property, although the optical function can sometimes be caused by a single layer. The basic design of an electrochromic device, presented in Fig. 3.24, consists of several layers. The substrate (mostly glass) is covered by a transparent, conducting film in contact with a film of the electrochromic substance. These films are followed by a layer of a fast ion conductor (electrolyte), an ion storage film, and another transparent conductor. The electrochromic and ion storage layers are conductors for ions and electrons while, the ion conductor has zero conductance for electrons. 

Kny et al.(5) have reported the preparation and properties of semiconducting organotin films having conductivites between 2 X 10 1 and 1 X 10 2 (ohm cm)7 Thermal treatment of such films increased the conductivity to a maximum value of 1 X 10 (ohm cm). 1 Recently, Hyneck(6) prepared a transparent conductive film of tin(II) oxide on a substrate by glow discharge... 

Vacuum filtration is another popular method in the fabrication of transparent conductive films, which are morphologically like bucky-papers . For example, Rinzler and co-workers used filtration to... 

Yang and co-workers used the separated metallic SWNTs in a systematic evaluation on transparent conductive films, which also included nanotubes from different sources coupled with various fabrication conditions. It is worth noting that the films were on a flexible substrate (PET), for which dip coating was used. Again, those films from the separated metallic SWNTs exhibited sheet resistance down to 130 Q sq for 80% optical transmittance at 550 nm (Table 6.1). The comparison of nanotube films with ITO coatings on the... 

In another study, Maeda et al fabricated transparent conductive films by air spraying enriched metallic SWNTs (from the amine-assisted post-production separation ) on to both quartz and PET substrates. The sheet resistances in the films on PET were 690 Q sq at an optical transmittance of 81% (550 nm) and 9000 Q sq at 97% (550 nm), which represented reductions by a factor of 20 in comparison with the performance in films of non-separated SWNTs. The same group also found that the effectiveness in the separation for metallic SWNTs is determined by the use of different amines at various concentrations. The proportion of metallic SWNTs against semiconducting SWNTs in the... 

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