Oct 21, 2019 in Technologies
Transparent Conductive Oxide Materials

Transparent conductive oxide (TCO) materials are known for the high conductivity of electrical charge. These materials have low absorption of electromagnetic waves due to thin-film transitions that allow them to be used in heaters/defrosters, flat-panel displays, touch-panel displays, solar cells, antistatic coatings, and transparent electrodes. This paper researches TCOs and examines the carrier mobility and carrier transport properties of devices made from these materials.

Transparent Conducting Oxides (TCOs) are considered to belong to a unique class of materials. The function of these materials is to exhibit both electronic conductivity and transparency simultaneously. The use of these materials is widely applied in photovoltaics, displays, flexible electronics and low-E windows. TCOs are the base of optoelectronic devices. TCO materials are used in all these devices as they have better transmittance of incident light among all materials. They marked their lower cost and long term stability. The field of Transparent Conductors (TC’s) includes not only conventional TCOs but also graphene and polymer-based TC materials, carbon and metal nano-composites. The main feature of these materials is the existence of unique properties because of a new amorphous TCOs’ emergence, the qualities of which are as good as their crystalline counterparts.

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The advantages of transparent conducting materials are flexible electronics on polymer substrates and low-temperature processing. Having low absorption of electromagnetic waves, and light they have the visible region of the spectrum. Their electrical conductivity is up to 103 to 104 Scm-1. With the help of the development of transparent conducting oxides different optoelectronic devices were invented, such as touch panels, Si-based solar cells and flat panel displays (FPDs). TCOs are the key materials among the other materials, such as organic copper indium gallium diselenide (CIGS) solar cells, light-emitting diodes (LEDs), and blue GaN-based LEDs. They have easy fabrication and excellent conducting. The performance of the devices is slightly improved by the main properties of these materials. Its high transmittance in a suitable work function and the infrared region improve the efficiency of organic LEDs and solar cells. Using TCO increases blue LEDs’ external quantum efficiency. They are prepared with the help of thin-film technologies. The glass fibers are lossless conductors of light despite the compound and silicon semiconductors and electrical insulators that are wavelength-dependent optical resistors. Their power occupies a wide range of saving technological applications and optoelectrical circuitries. Their transparency is over 85%. The optical band gap is greater than 3 eV. The resistivity of these materials is about 10-4 ohm-cm and the controllable electrical conductivity is at least 1020 cm-3.

Focusing on general characteristics of these materials, the main attention is paid to its good physical, electrical and optical properties. Their optical and electrical transparency depends upon the presence of intrinsic or intentionally introduced defects, the number, nature, the resident morphology, and on atomic arrangements of metal cations in amorphous or crystalline oxide structures. Most of transparent conducting oxides materials are n-type (compound) semiconductors. The nonmetal part is oxygen. Most of these materials are ternary or binary compounds. They contain one or two metallic elements such as ZnO: N, ZnO: Mg, IZO, NiO: Li, NiO, CuGaO2, Cu2SrO2, CuAlO2, and thin films. The content of these metallic elements is only a few percents. In combination with all these different metal combinations or metals, materials increase a great number of semiconductors with different optoelectrical characteristics that can be changed by doping with nonmetals, metalloids, or metals.

Transparent Conducting Oxide materials have high carrier mobility. The key property of this material is a conduction band that is composed of overlapping metal s-like orbitals. Its main characteristic is to lead to a broadly dispersed conduction band. High mobility is a part of the process of semiconductors. A low temperature that is processed on plastic substrates, increased deposition rate, and uniformity improvement identifies its uniqueness among all materials. The unique features include high mobility and transparency. They are the main components to enhance solar cell efficiency. High mobility is achieved due to the existence of different semiconductors. The spectral response is increased over a wide wavelength range. The mobility of these materials improves the crystalline structure with the help of choice of deposition technique, heat treatment, and a choice of substrate. With free carrier concentration, the carrier mobility decreases. The crystallite grain sizes decrease as a function of increasing doping content. The films that have smaller grain sizes present increased grain surface that has the ability to frustrate the charge mobility between the grains. The film resident grain structure can be modified by altering the deposition parameters of a film. Because of the drop in the temperature and pressure deposition in the sputter chamber, the morphology of the film can be altered.

The existence of n-type doped conducting oxides allows having visible transparency. In this case, the conductivity is proportional to carrier mobility and free career density. If the carrier density increases, the mobility will suffer. The limit of mobility can be achieved by overcoming new approaches. When extending the wavelength, what should be considered are the materials that are conducted by an alternative mechanism that should involve charge transport within the valence band. Good conductivity and transparency in the infrared region of the spectrum can be reached with the help of mixed transition metal spinel oxides.

The conductivity magnitude is influenced by activation energy that is under charge inhomogeneity, cation site disorder and caution charge. With decreasing carrier mobility, carrier density increases. Substitution of transitions metals can lead to blue energy’s shifting and higher visible transparency of light. The spinels that contain 4d and have higher metal cations will show increased transparency at shorter wavelengths according to large separation between conduction band minimum and valence band maximum. The mobility of the materials is comparatively high due to the phase. The nearest neighbor distance is nearly identical in crystalline films and amorphous. The conduction band’s bottom is a highly ionic oxide semiconductor. Composing a spherically symmetric principal quantum number, the direct ns-orbital overlap between neighboring metal irons is allowed. The high carrier mobility is allowed in the elimination of the ns-overlap effect on oxygen-metal - oxygen bond distortions in the amorphous state. The carrier mobility is determined with the help of two methods: highly resistive density material of low carrier and conventional Hall Effect measurement, and a field that has a great effect on mobility due to TFT structures usage. In the case of in-based oxides, the measuring of the carrier mobility is not essential. The mobility can occur in the result of increased point defects due to poor quality of crystalline.

Discussing the carrier transport properties of devices made from TCO material, the main attention should be paid to carrier doping and carrier mobility taking into consideration electronic conductivity. The fundamental transport properties determine the increased importance of metal oxide channel materials. According to band theory, in reciprocal space, electronic structures are represented. The main role in crystals is played by the basic carrier transport properties. The effective masses of the hole and the electron are determined by the curvatures at the condition band minimum and valence band maximum. Carrier transport properties of devices arise from such fundamental sources as doping ions, oxygen vacancies, and interstitial metal ion impurities. Interstitial metal ion impurities and oxygen vacancies act as electron donors. When the oxygen is presented in the lattice, it acts as a doubly charged electron donor. The same way it occurs in other semiconductor crystals that have dopant ionization. The electrons jump from the valence band to the acceptor level. This process is characterized by populating the valence band with holes. From an ionized impurity scattering carrier, scattering in these oxides arises. With the scattering increasing, the mean-free path of the carriers in the oxide decreases. It leads to high resistivity and poor device performance. The thermodynamic stability of electrons and band holes are determined by the magnitude of electron affinity (EA) the ionization potential (IP). When IP decreases, the hole becomes stable. With the increasing EA, the electron becomes stable. The measurements of Hall are affected by the concentration of the hole and the conductivity’s temperature dependence.

Carrier transport properties can be changed due to degenerate conduction. When TCO materials become amorphous, the main attention is paid to structural randomness that appears at a bond angle distribution. The magnitude of the overlap is varied by the metal cations’ choice. The magnitude should be intensive when the special spread of the orbital is larger than the inter-cation distance. When the orbital is isotropic in shape, the magnitude should be insensitive to the bond angle. However, ionic amorphous materials have high mobility in comparison with the corresponding crystalline form. TCO development with higher conductivity does not depend on higher dopant concentration that is able to provide higher carrier density. Conduction electrons’ spatial separation of their parent impurity atoms (ions) should significantly increase their mobility and reduce the scattering of the carriers.

At the front side of the solar cell, the TCO layer operates as an electrical contact. It allows light to absorb the layer. At the back of the device, TCO performs as an interfacial layer. The main function of the material for this application is to improve the contact resistance between the metallic reflector and the absorber. Also, the slight improvement of TCO can be observed in the refractive index matching. TCO materials are mostly used in solar cells. High photo-conversion efficiency in solar cells requires minimum electrical and optical losses in comparison with other materials. High mobility of TCO materials is the most effective where a wide spectral range (400–1300 nm) and high transparency of >80% are required. High mobility of TCO layers can be in the superstate configuration where there is the opportunity to use the highest possible process temperature to optimize transparency and resistivity. The reason for low efficiency can lie in the high cells’ serious resistance. High mobility of TCO materials combines law resistivity with the existence of highly visible and near infra-red optical transmission.

Summing up, TCO materials are widely used nowadays. Their main characteristics of having electromagnetic waves’ low absorption in the visible region of the spectrum are identified by their uniqueness. Their glass fibers are lossless conductors of light. Being prepared with the help of thin-film technologies they are produced for optoelectrical apparatus such as optoelectrical interfaces, solar cells, circuitries, and displays. According to their main features, they are compound semiconductors. The general characteristics of TCO materials have a great influence on carrier mobility and carrier transport properties of devices due to the high conductivity of electrical charge.

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