A transparent conducting oxide (TCO) is a wide-band-gap oxide semiconductor doped so heavily that it conducts like a poor metal while still transmitting visible light. Indium tin oxide (ITO) is the benchmark, with resistivity around 10⁻⁴ Ω·cm and visible transmittance above 80%, but indium's cost and concentrated supply drive interest in fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), and non-oxide electrodes such as silver nanowires, graphene, and PEDOT:PSS.

• Transparency comes from a band gap wide enough to pass visible photons; conductivity comes from degenerate n-type doping, with carrier concentrations on the order of 10²⁰ cm⁻³.
• ITO is typically 90 wt% In2O3 and 10 wt% SnO2, and ITO production accounts for most global indium consumption, according to USGS.
• China accounted for about 70% of world refined indium output in 2024, and the U.S. warehouse price averaged $340/kg in 2024, up 42% from 2023.
• FTO, deposited pyrolytically on hot glass, is a durable choice for low-emissivity and electrically heated windows; AZO is a leading indium-free oxide alternative; IGZO is an amorphous oxide semiconductor for display transistors, not an electrode.
• Non-oxide alternatives (silver nanowires, graphene, PEDOT:PSS) win on flexibility, not yet on all-round performance and stability.
• Computed (DFT-PBE) band gaps for TCO hosts are far below measured values, so read database gaps with care.

What makes an oxide both transparent and conductive?

Most transparent oxides are insulators. As Kawazoe and co-workers put it in Nature in 1997, optically transparent oxides "tend to be electrical insulators" because of their large band gaps (≥3.1 eV), with doped In2O3, SnO2, and ZnO as the notable exceptions. A TCO reconciles the two properties with three ingredients:

1. A wide band gap. Photons with less energy than the gap pass through. For photovoltaic electrodes, a gap above 3.2 eV avoids absorbing most of the solar spectrum.
2. Degenerate doping. Donors such as Sn⁴⁺ on In³⁺ sites, F⁻ on O²⁻ sites, or Al³⁺ on Zn²⁺ sites, plus oxygen vacancies, push the Fermi level into the conduction band. Filling the bottom of the band also widens the apparent optical gap, an effect called the Burstein–Moss shift.
3. A dispersive conduction band. The conduction band of these post-transition-metal oxides is built mainly from metal s orbitals, giving light electrons and useful mobility. In heavily doped films, ionized-impurity scattering typically limits mobility to the order of 40 cm²/(V·s), although dopants such as molybdenum in In2O3 give higher mobility than tin.

The trade-off is unavoidable. Conductivity is the product of carrier density and mobility, but free electrons also absorb and reflect infrared light, and ionized dopants scatter electrons. Push the doping too high and the film loses near-infrared transparency and mobility. That infrared reflection is a defect for solar cells that use near-infrared light, and a feature for energy-saving windows.

In practice, engineers quote sheet resistance (Ω/sq), which equals resistivity divided by film thickness. A 100 nm film with a resistivity of 10⁻⁴ Ω·cm has a sheet resistance of 10 Ω/sq.

What is indium tin oxide and why is it the standard?

ITO is a solid solution of bixbyite-structured In2O3 with tin oxide. The Stadler review describes the typical composition as 90 wt% In2O3 and 10 wt% SnO2; Sn⁴⁺ replacing In³⁺ donates electrons. ITO films are usually deposited by sputtering or electron-beam evaporation and combine resistivity near 10⁻⁴ Ω·cm with transmittance above 80%. ITO became the standard electrode in LCD, OLED, and touch-panel manufacturing.

ITO's drawbacks are well known. It is brittle and typically needs high-vacuum deposition and high-temperature processing to reach its best properties, as noted in a 2017 Scientific Reports study, which limits use on flexible plastic. And its main ingredient is indium.

Why is indium a supply risk?

Indium is a by-product metal. USGS reports that it is most commonly recovered from the zinc ore sphalerite, and that the indium content of the zinc deposits it is recovered from ranges from less than 1 to 100 parts per million, so indium supply is tied to zinc production. Key figures from the USGS Mineral Commodity Summaries 2025:

• World refinery production was an estimated 1,080 tons in 2024, with China at 760 tons, about 70% of the total.
• The United States recovered no indium from ores in 2024 and was 100% import-reliant.
• The U.S. warehouse price averaged $340/kg in 2024, 42% above 2023, and peaked at $420/kg in June.
• Indium is recovered from ITO scrap, most commonly in Japan and the Republic of Korea.

For the wider picture on concentrated mineral supply, see our overview of critical minerals and rare earth elements.

How do ITO alternatives compare?

The table compares the main options. Figures are representative reported values from the cited sources, measured under different conditions, so they indicate scale rather than a head-to-head ranking.

Fluorine-doped tin oxide (FTO)

SnO2 is a rutile-structured, naturally n-type oxide. Fluorine on oxygen sites adds electrons, and films are commonly grown by chemical vapor deposition onto hot glass. A thin SnO2:F film on glass combines high conductivity, high transmittance, and low emissivity, which is why FTO is used in energy-saving windows; SnO2-coated windows reflect infrared radiation. Its chemical and thermal toughness also make FTO-coated glass a common substrate for thin-film and dye-sensitized solar cells.

Aluminum-doped zinc oxide (AZO)

ZnO has a direct band gap of about 3.3 eV. When doped with aluminum or gallium it becomes transparent and conductive; zinc and aluminum are much cheaper and less toxic than ITO's ingredients, and front contacts for solar cells and LCDs are described as an emerging commercial application, per the same source. Minami's 2005 review names AZO and gallium-doped ZnO as promising ITO alternatives because their source materials are inexpensive and non-toxic, while noting that large-area, high-rate deposition still needed development. The Stadler review describes AZO as containing about 2 wt% aluminum.

Silver nanowires, graphene, and PEDOT:PSS (non-oxide)

These are not oxides, but they compete for the same job. Silver nanowire networks conduct through a percolating web of metal wires, so they bend without cracking. Wire-to-wire contact resistance is the main limit; a 2017 study that gently welded the junctions under ordinary sunlight reached sheet resistance below 20 Ω/sq at about 87% transmittance (550 nm). Graphene grown by roll-to-roll chemical vapor deposition reached about 125 Ω/sq at 97.4% transmittance as a doped monolayer and about 30 Ω/sq at about 90% as a doped four-layer stack in a 2010 Nature Nanotechnology report. PEDOT:PSS, a conducting polymer, is printable and flexible but less conductive. USGS lists carbon nanotubes, PEDOT, copper or silver nanowires, graphene, and antimony tin oxide among developed or explored substitutes for ITO.

What is IGZO, and is it an ITO replacement?

Indium gallium zinc oxide (IGZO) is often mentioned alongside TCOs, but it plays a different role. It is an amorphous oxide semiconductor used as the channel of thin-film transistors that switch display pixels, not as a transparent electrode. In 2004, Nomura and colleagues reported amorphous IGZO deposited at room temperature on plastic with Hall mobilities exceeding 10 cm² V⁻¹ s⁻¹, an order of magnitude above hydrogenated amorphous silicon, and transistors with saturation mobilities of 6–9 cm² V⁻¹ s⁻¹. Sharp began production of LCD panels with IGZO transistors in 2012. IGZO still contains indium, so it adds to, rather than relieves, indium demand.

Where are transparent conducting oxides used?

• Displays and touch panels: ITO electrodes in LCD, OLED, and touch-panel devices; LCDs are the most common ITO end use, per USGS.
• Photovoltaics: front contacts for thin-film, perovskite, and heterojunction silicon cells, where the band gap requirement is set by the solar spectrum.
• Low-emissivity and smart glass: FTO and ITO coatings reflect thermal infrared; ITO is also used in smart windows.
• Heaters and defrosters: transparent film heaters, including demonstrations with silver nanowire networks (Scientific Reports, 2017) and FTO-coated electrically heated windows.

What do computed data say about TCO materials?

Materials databases help screen for new TCO hosts, but their band gaps need careful reading. Gaps computed with the PBE functional of density functional theory (DFT) systematically underestimate measured values, and TCO hosts are a stark example. The Materials Project ground-state structures aggregated on LatticeGraph give PBE gaps of about 0.72 eV for wurtzite ZnO, 0.63 eV for bixbyite In2O3, and 0.65 eV for rutile SnO2, versus measured gaps of about 3.3 eV for ZnO and about 3 eV for In2O3. Use computed gaps for trends and ranking, not as predictions of transparency. Our explainer on what a band gap is covers hybrid-functional and GW corrections.

The open research targets are p-type TCOs and higher-mobility hosts. Kawazoe's CuAlO2 films showed p-type conductivity up to 1 S/cm, far below n-type TCOs, and doped BaSnO3 has drawn attention as a stable transparent perovskite with high electron mobility.

Frequently asked questions

What is a transparent conducting oxide?

It is an oxide with a band gap wide enough to transmit visible light that has been doped to carry a high density of free electrons. ITO, FTO, and AZO are the main commercial examples.

What is indium tin oxide used for?

ITO is the transparent electrode in LCD and OLED displays, touch panels, some solar cells, and smart windows. ITO production accounts for most global indium consumption, according to USGS.

What are the best ITO alternatives?

For rigid, high-temperature, or outdoor coatings, FTO is the established indium-free choice; AZO is the leading low-cost oxide alternative. For flexible devices, silver nanowire networks are the most mature non-oxide option, with graphene and PEDOT:PSS used where their trade-offs are acceptable.

Is IGZO a transparent conductor?

Not in the electrode sense. IGZO is an amorphous oxide semiconductor used in display transistors, valued for mobility about ten times that of amorphous silicon.

Why are there so few p-type transparent conductors?

Oxide valence bands are dominated by localized oxygen 2p states, which make holes heavy and hard to dope. Delafossites such as CuAlO2 are the best-known exception, but their conductivity remains far below n-type TCOs.

Exploring TCO data on LatticeGraph

Compound pages for In2O3, SnO2, ZnO, CuAlO2, and BaSnO3 show computed structures, stability, and DFT band gaps alongside literature synthesis recipes and related patents. Browse the transparent conducting oxides and wide-bandgap oxides classes to compare candidates.