What are Transparent Conductive Oxides?

Jump to: Chemistry of TCOs | Examples of TCOs | Properties of TCOs | Applications of TCOs

Chemistry of Transparent Conductive Oxides

Transparent conductive oxides (TCOs) are comprised of metal or metal-combinations bonded to oxygen. These compounds are semiconducting with different optoelectronic properties. These properties can be further tuned via doping with metals, metalloids or nonmetals such as aluminum, antimony and fluorine. Metal oxides with optical transparency include tin-, indium- and zinc oxide.

A TCO is a wide bandgap semiconductor that has a relatively high concentration of free electron in its conduction band. The free electrons are present due to defects in the material or from dopants. Dopants are either acceptors or donors which cause shifts in the TCO's Fermi level. As TCOs are typically n-type semiconductors, dopants are donors. As a result, the number of electrons is increased and the donor impurity levels are situated right under the conduction band, which causes the Fermi level to be shifted towards the conduction band. For example, doping with low metallic ion concentration generates shallow donor levels, forming a carrier population at room temperature.

Why is Indium Tin Oxide (ITO) so popular?

The most famous transparent conducting oxide is indium tin oxide (ITO) with 8% by weight Sn doping. Substituting Sn⁴⁺ for In³⁺ in indium oxide introduces free electrons into the conduction band, thereby increasing the n-type conductivity. Sn⁴⁺ acts as a one-electron donor because it has one more valence electron than In³⁺, and that extra electron becomes a conduction carrier.

ITO is popular not just for its electrical conductivity and optical transparency, but also for the ease with which it can be deposited as a thin film, as well as its chemical resistance to moisture. ITO coated substrates are widely used in applications including photovoltaics, field-effect transistors and light-emitting diodes. However, indium suffers from issues of scarcity and is expensive therefore there is a need to find a substitute.

ITO Alternatives

Beyond ITO, different metal oxides and various dopants have been trialled to attempt to solve the issues mentioned above. Popular alternatives include aluminum doped zinc oxide (AZO) which can deliver low resistivities of the range 5.46 x 10⁵ to 6.99 x 10² Ω cm. Typically, 1-5 % of zinc ions are replaced by aluminum ions which significantly enhances the electrical properties of zinc oxide. Replacing Zn²⁺ with Al³⁺ introduces one extra valence electron into the system.

Examples of Transparent Conductive Oxides

Here are some examples of TCOs ranging from binary compounds, such as zinc oxide, to doped ternary oxides, such as antimony tin oxide:

Zinc Oxide Nanoparticles

From $75

Tin (IV) Oxide Powder

From $180

Indium Oxide Powder

From $135

Antimony Tin Oxide Nanopowder

From $158

Aluminum Zinc Oxide Powder

From $135

Metal Oxides

View Range

Properties of Transparent Conductive Oxides

A transparent conductive oxide must find a careful balance between electrical conductivity and optical transmittance. A reduction of electrical resistivity involves either an increase in the carrier concentration or in the mobility of the carrier. Increasing carrier concentration leads to an increase in visible absorption and therefore decreases optical transmittance. Increasing carrier mobility however does not have the same detrimental impact.

Ideal properties of a TCO

• Electrical resistivity (ρ) should be ~10^-5 Ω cm or less
• Absorption coefficient (α) < 10³ cm^-1 in the near-UV and vis range
• Optical bandgap > 3 eV

A 100 nm thick TCO film with these properties will have an optical transmission of 90% and sheet resistance (R_sh) of 10^-4 Ω cm³. The performance of TCOs can be quantified using the figure of merit as reported by Gordon (FoM^G). It is defined as the ratio of the electrical conductivity (σ) to the optical absorption (α):

The average transmittance (T) (ideally 85-90 %) and reflectance (R), respectively are measure in the 400–800 nm range.

Most TCO materials are n-type semiconductors but there are a few examples of p-type. The development of p-type TCSs with properties comparable to those of their n-type counterparts is needed for development of the next generation of transparent electronics. The challenge of p-type TCOs is due to the localized nature of the O 2p derived valence band where holes can not move freely due to the high electronegativity of oxygen. Localization leads to large hole effective masses and difficulty introducing shallow acceptors. Chemical modulation of the valence band involves the hybridization of oxygen 2p orbitals with metal d or s orbitals.

Applications Transparent Conductive Oxides

Transparent conductive oxides are used in applications that require both electrical conductivity and high transmission of visible light. Electronic devices that require transparent electrodes often use transparent conductive oxides. Transparent electronics has needed both n- and p-type TCOs for the fabrication of wide bandgap p-n homo-junctions, which has been realised by n-ZnO and p-ZnO etc. Other applications of TCOs include:

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