Transparent conducting films (TCFs) are thin films of optically transparent and electrically conductive material. They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives include wider-spectrum transparent conductive oxides (TCOs), conductive polymers, metal grids and random metallic networks, carbon nanotubes (CNT), graphene, nanowire meshes and ultra thin metal films. TCFs for photovoltaic applications have been fabricated from both inorganic and organic materials. Inorganic films typically are made up of a layer of transparent conducting oxide (TCO), most commonly indium tin oxide (ITO), fluorine doped tin oxide (FTO), niobium doped anatase TiO2 (NTO) or doped zinc oxide. Organic films are being developed using carbon nanotube networks and graphene, which can be fabricated to be highly transparent to infrared light, along with networks of polymers such as poly(3,4-ethylenedioxythiophene) and its derivatives. Transparent conducting films are typically used as electrodes when a situation calls for low resistance electrical contacts without blocking light (e.g. LEDs, photovoltaics). Transparent materials possess wide bandgaps whose energy value is greater than those of visible light. As such, photons with energies below the bandgap value are not absorbed by these materials and visible light passes through. Some applications, such as solar cells, often require a wider range of transparency beyond visible light to make efficient use of the full solar spectrum.
Transparent conducting oxides
Overview Transparent conductive oxides (TCO) are doped metal oxides used in optoelectronic devices such as flat panel displays and photovoltaics (including inorganic devices, organic devices, and dye-sensitized solar cells). Most of these films are fabricated with polycrystalline or amorphous microstructures. Typically, these applications use electrode materials that have greater than 80% transmittance of incident light as well as electrical conductivities higher than 103 S/cm for efficient carrier transport. In general, TCOs for use as thin-film electrodes in solar cells should have a minimum carrier concentration on the order of 1020 cm−3 for low resistivity and a bandgap greater than 3.2 eV to avoid absorption of light over most of the solar spectra. Mobility in these films is typically limited by ionized impurity scattering due to the large amount of ionized dopant atoms and is on the order of 40 cm2/(V·s) for the best performing TCOs. Current transparent conducting oxides used in industry are primarily n-type conductors, meaning their primary conduction is as donors of electrons. This is because electron mobilities are typically higher than hole mobilities, making it difficult to find shallow acceptors in wide band gap oxides to create a large hole population. Suitable p-type transparent conducting oxides are still being researched, though the best of them are still orders of magnitude behind n-type TCOs. An early p-type TCO was the delafossite oxide CuAlO2; in 1997, Kawazoe et al. reported p-type electrical conduction in transparent CuAlO2 thin films, helping establish p-type transparent oxide semiconductor research. The lower carriers' concentration of TCOs with respect to metals shift their plasmonic resonance into the NIR and SWIR range. To date, the industry standard in TCOs is ITO, or indium tin oxide. This material boasts a low resistivity of ~10−4 Ω·cm and a transmittance of greater than 80%. ITO has the drawback of being expensive. Indium, the film's primary metal, is rare (6000 metric tons worldwide in 2006), and its price fluctuates due to market demand (over $800 per kg in 2006). For this reason, doped binary compounds such as aluminum-doped zinc oxide (AZO) and indium-doped cadmium oxide have been proposed as alternative materials. AZO is composed of aluminum and zinc, two common and inexpensive materials, while indium-doped cadmium oxide only uses indium in low concentrations. Several transition metal dopants in indium oxide, particularly molybdenum, give much higher electron mobility and conductivity than obtained with tin and Ta is a promising alternative dopant for tin oxide. Other novel transparent conducting oxides include barium stannate and the correlated metal oxides strontium vanadate and calcium vanadate. Binary compounds of metal oxides without any intentional impurity doping have also been developed for use as TCOs. These systems are typically n-type with a carrier concentration on the order of 1020 cm−3, provided by interstitial metal ions and oxygen vacancies which both act as donors. However, these simple TCOs have not found practical use due to the high dependence of their electrical properties on temperature and oxygen partial pressure. In current research, labs are looking to optimize the electrical and optical characteristics of certain TCOs. Researchers deposit TCO onto the sample by using a sputtering machine. The targets have been changed and researchers are looking at materials such as IZO (Indium Zinc Oxide), ITO (Indium Tin Oxide) and AZO (Aluminum Zinc Oxide), and they are optimizing these materials by changing parameters within the sputtering deposition machine. When researchers vary parameters such as concentration of the gases within the sputtering, the pressure within the sputtering machine, power of the sputtering, and pressure, they are able to achieve different carrier concentrations and sheet resistivities within the machine. Carrier concentrations affect the short circuit current of the sample, and a change in sheet resistivity affects the fill factor of the sample. Researchers have varied parameters enough and found combinations that will optimize the short circuit current as well as the fill factor for TCOs such as indium tin oxide. Alternatives to ITO have been used such as graphene, or silver nanowire meshes.
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