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),Dhakal, Tara, et al. "Transmittance from visible to mid infra-red in AZO films grown by atomic layer deposition system." Solar Energy 86.5 (2012): 1306-1312. | https://doi.org/10.1016/j.solener.2012.01.022 conductive polymers, metal grids and random metallic networks, carbon nanotubesWu, Zhuangchun, et al. "Transparent, conductive carbon nanotube films." Science 305.5688 (2004): 1273-1276. (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),Conductive Oxide Thin Films Materion Technical Paper, "Transparent Conductive Oxide Thin Films" 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.
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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).Indium Price Supported by LCD Demand and New Uses for the Metal 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.
Fabrication Doped metal oxides for use as transparent conducting layers in photovoltaic devices are typically grown on a glass substrate. This glass substrate, apart from providing a support that the oxide can grow on, has the additional benefit of blocking most infrared wavelengths greater than 2 μm for most silicates, and converting it to heat in the glass layer. This in turn helps maintain a low temperature of the active region of the solar cell, which degrades in performance as it heats up. TCO films can be deposited on a substrate through various deposition methods, including metal organic chemical vapor deposition, metal organic molecular beam deposition, solution deposition, spray pyrolysis, ultrasonic nozzle sprayed graphene oxide and air sprayed Ag NanowireYoung-Hui Koa, Ju-Won Leeb, Won-Kook Choic, Sung-Ryong Kim, 2014, "Ultrasonic Sprayed Graphene Oxide and Air Sprayed Ag Nanowire for the Preparation of Flexible Transparent Conductive Films," The Chemical Society of Japan and pulsed laser deposition (PLD), however conventional fabrication techniques typically involve magnetron sputtering of the film. The sputtering process is very inefficient, with only 30% of planar target material available for deposition on the substrate. Cylindrical targets offer closer to 80% utilization. In the case of ITO recycling of unused target material is required for economic production. For AZO or ZnAl sputtering target material is sufficiently inexpensive that recovery of materials use is of no concern. There is some concern that there is a physical limit to the available indium for ITO.Indium, USGS report Growth typically is performed in a reducing environment to compensating acceptor defects within the film (e.g. metal vacancies), which degrade the carrier concentration (if n-type).
For AZO thin film deposition, the coating method of reactive magnetron sputtering is very economical and practical way of mass production. In this method, a zinc-aluminum target is sputtered in an oxygen atmosphere such that metal ions oxidize when they reach the substrates surface. By using a metal target instead of an oxide target, direct current magnetron sputtering may be used which enable much faster deposition rates.
Theory Charge carriers in these n-type oxides arise from three fundamental sources: interstitial metal ion impurities, oxygen vacancies, and doping ions. The first two sources always act as electron donors; indeed, some TCOs are fabricated solely using these two intrinsic sources as carrier generators. When an oxygen vacancy is present in the lattice it acts as a doubly charged electron donor. In ITO, for example, each oxygen vacancy causes the neighboring In3+ ion 5s orbitals to be stabilized from the 5s conduction band by the missing bonds to the oxygen ion, while two electrons are trapped at the site due to charge neutrality effects. This stabilization of the 5s orbitals causes a formation of a donor level for the oxygen ion, determined to be 0.03 eV below the conduction band. Thus these defects act as shallow donors to the bulk crystal. Common notation for this doping is Kröger–Vink notation and is written as:
O_O^\mathit{x} <=> {V_O^{\bullet\bullet}} + {1/2O2(g)} + 2e'
Here "O" in the subscripts indicates that both the initially bonded oxygen and the vacancy that is produced lie on an oxygen lattice site, while the superscripts on the oxygen and vacancy indicate charge. Thus to enhance their electrical properties, ITO films and other transparent conducting oxides are grown in reducing environments, which encourage oxygen vacancy formation.
Dopant ionization within the oxide occurs in the same way as in other semiconductor crystals. Shallow donors near the conduction band (n-type) allow electrons to be thermally excited into the conduction band, while acceptors near the valence band (p-type) allow electrons to jump from the valence band to the acceptor level, populating the valence band with holes. Carrier scattering in these oxides arises primarily from ionized impurity scattering at high dopant levels (>1 at%). Charged impurity ions and point defects have scattering cross-sections that are much greater than their neutral counterparts. Increasing the scattering decreases the mean-free path of the carriers in the oxide, which leads to low electron mobility and a high resistivity. These materials can be modeled reasonably well by the free electron model assuming a parabolic conduction band and doping levels above the Mott Criterion. This criterion states that an insulator such as an oxide can experience a composition-induced transition to a metallic state given a minimum doping concentration nc, determined by:
where H* is the mean ground state Bohr radius. For ITO, this value requires a minimum doping concentration of roughly 1019 cm−3. Above this level, the conduction type in the material switches from semiconductor to metallic.
Transparent conducting polymers