**4. ZnO thin films**

Another important oxide used in PV window and display technology applications is doped ZnO, which has been learned to have a thin-film resistivity as low as 2.4 ×10–4 Ω•cm. Although the resistivity of ZnO thin films is not yet as small as the ITO standard, it does offer the significant benefits of low cost relative to In-based systems and high chemical and thermal stability. In the undoped state, zinc oxide is highly resistive because, unlike Inbased systems, ZnO native point defects are not efficient donors. However, reasonable

reports for some other promising n-type TCOs, which could find some practical applications in the future. They are titanium oxide doped with Ta or Nb, Ga2O3 doped with Sn and 12CaO・7Al2O3 (often denoted C12A7). These new TCOs are currently not capable of competing with ITO/FTO/GZO/AZO in terms of electrical or optical properties. We should also point out that n-type transparent oxides under discussion are used on top of the p-type semiconductors and the vertical conduction between the two relies on tunneling and leakage. The ideal option would be to develop p-type TCOs which are indeed substantially

Crystalline indium oxide has the bixbyite structure consisting of an 80-atom unit cell with the Ia3 space group and a 1-nm lattice parameter in an arrangement that is based on the stacking of InO6 coordination groups. The structure is closely related to fluorite, which is a face-centered cubic array of cations with all the tetrahedral interstitial positions occupied with anions. The bixbyite structure is similar to fluorite except that the MO8 coordination units (oxygen position on the corners of a cube and M located near the center of the cube) of fluorite are replaced with units that have oxygen missing from either the body or the face diagonal. The removal of two oxygen ions from the metal-centered cube to form the InO6 coordination units of bixbyite forces the displacement of the cation from the center of the cube. In this way, indium is distributed in two nonequivalent sites with one-fourth of the indium atoms positioned at the center of a trigonally distorted oxygen octahedron (diagonally missing O). The remaining three-fourths of the indium atoms are positioned at the center of a more distorted octahedron that forms with the removal of two oxygen atoms from the face of the octahedron. These MO6 coordination units are stacked such that onefourth of the oxygen ions are missing from each {100} plane to form the complete bixbyite structure. A minimum in the thin-film resistivity is found in the ITO system when the oxygen partial pressure during deposition is optimized. This is because doping arises from two sources, four-valent tin substituting for three-valent indium in the crystal and the creation of doubly charged oxygen vacancies. This is due to an oxygen-dependent competition between substitutional Sn and Sn in the form of neutral oxide complexes that do not contribute carriers. Amorphous ITO that has been optimized with respect to oxygen content during deposition has a characteristic carrier mobility (40 cm2/V s) that is only slightly less than that of crystalline films of the same composition. This is in sharp contrast to amorphous covalent semiconductors such as Si, where carrier transport is severely limited by the disorder of the amorphous phase. In semiconducting oxides formed from heavy-metal cations with (n-1)d10ns0 (n ≤4) electronic configurations, it appears that the

Another important oxide used in PV window and display technology applications is doped ZnO, which has been learned to have a thin-film resistivity as low as 2.4 ×10–4 Ω•cm. Although the resistivity of ZnO thin films is not yet as small as the ITO standard, it does offer the significant benefits of low cost relative to In-based systems and high chemical and thermal stability. In the undoped state, zinc oxide is highly resistive because, unlike Inbased systems, ZnO native point defects are not efficient donors. However, reasonable

difficult to attain.

**3. Crystal chemistry of ITO** 

degenerate band conduction is not band-tail limited.

**4. ZnO thin films** 

impurity doping efficiencies can be achieved through substitutional doping with Al, In, or Ga. Most work to date has focused on Al - doped ZnO, but this dopant requires a high degree of control over the oxygen potential in the sputter gas because of the high reactivity of Al with oxygen. Gallium, however, is less reactive and has a higher equilibrium oxidation potential, which makes it a better choice for ZnO doping applications. Furthermore, the slightly smaller bond length of Ga–O (1.92Å) compared with Zn–O (1.97 Å) also offers the advantage of minimizing the deformation of the ZnO lattice at high substitutional gallium concentrations. The variety of ZnO thin films has been expatiated elsewhere.

#### **5. Electrical conductivity of TCO**

TCOs are wide band gap (Eg) semiconducting oxides, with conductivity in the range of 102 – 1.2106 (S). The conductivity is due to doping either by oxygen vacancies or by extrinsic dopants. In the absence of doping, these oxides become very good insulators, with the resistivity of > 1010 cm. Most of the TCOs are n-type semiconductors. The electrical conductivity of n-type TCO thin films depends on the electron density in the conduction band and on their mobility: = n *e*, where is the electron mobility, n is its density, and e is the electron charge. The mobility is given by:

$$
\mu = \mathbf{e} \,\mathrm{\,\mathrm{\,\mathrm{\,\,mol}}}\,\mathrm{\,\mathrm{\,\,mol}}\tag{1}
$$

where is the mean time between collisions, and m\* is the effective electron mass. However, as n and are negatively correlated, the magnitude of is limited. Due to the large energy gap (Eg > 3 eV) separating the valence band from the conducting band, the conduction band can not be thermally populated at room temperature (kT~0.03 eV, where k is Boltzmann's constant), hence, stoichiometric crystalline TCOs are good insulators. To explain the TCO characteristics, the various popular mechanisms and several models describing the electron mobility were proposed.

In the case of intrinsic materials, the density of conducting electrons has often been attributed to the presence of unintentionally introduced donor centers, usually identified as metallic interstitials or oxygen vacancies that produced shallow donor or impurity states located close to the conduction band. The excess donor electrons are thermally ionized at room temperature, and move into the host conduction band. However, experiments have been inconclusive as to which of the possible dopants was the predominant donor. Extrinsic dopants have an important role in populating the conduction band, and some of them have been unintentionally introduce. Thus, it has been conjectured in the case of ZnO that interstitial hydrogen, in the H+ donor state, could be responsible for the presence of carrier electrons. In the case of SnO2, the important role of interstitial Sn in populating the conducting band, in addition to that of oxygen vacancies, was conclusively supported by first-principle calculations. They showed that Sn interstitials and O vacancies, which dominated the defect structure of SnO2 due to the multivalence of Sn, explained the natural nonstoichiometry of this material and produced shallow donor levels, turning the material into an intrinsic n-type semiconductor. The electrons released by these defects were not compensated because acceptor-like intrinsic defects consisting of Sn voids and O interstitials did not form spontaneously. Furthermore, the released electrons did not make direct optical transitions in the visible range due to the large gap between the Fermi level and the energy level of the first unoccupied states. Thus, SnO2 could have a carrier density with minor effects on its transparency.

TCO-Si Based Heterojunction Photovoltaic Devices 121

Consequently, the boundary in the near-IR region also shifts to the shorter wavelength with increase of the free carrier concentration. The shift in the near-IR region is more pronounced than that in the near-UV region. Therefore, the transmission window becomes narrower as the carrier concentration increases. This means that both the conductivity and the transmittance window are interconnected since the conductivity is also related to the carrier concentration as discussed above. Thus, a compromise between material conductivity and transmittance window must be struck, the specifics of which being application dependent. While for LED applications the transparency is needed only in a narrow range around the emission wavelengths, solar cells require high transparency in the whole solar spectral range. Therefore, for photovoltaics, the carrier concentration should be as low as possible for reducing the unwanted free carrier absorption in the IR spectral range, while the carrier mobility should be as high as possible to retain a sufficiently high conductivity. Optical measurements are also commonly employed to gain insight into the film quality. For example, interference fringes found in transmittance curves indicate the highly reflective nature of surfaces and interfaces in addition to the low scattering and absorption losses in the films. The particulars of interferences are related to both the film thickness and the incident wavelength, which can be used to achieve higher transmittance for TCOs. In the case of a low quality TCO, deep level emissions occurring in photoluminescence (PL) spectra along with relatively low transmittance are attributed to the lattice defects such as oxygen vacancies, zinc vacancies, interstitial metal ions, and interstitial oxygen. Highdoping concentration-induced defects in crystal lattices causing the creation of electronic defect states in band gap similarly have an adverse effect on transparency. In GZO, as an example, at very high Ga concentrations (1020–1021 cm-3), the impurity band merges with the conduction band causing a tail-like state below the conduction band edge of intrinsic ZnO. These tail states are responsible for the low-energy part of PL emission. Therefore, the defects, mainly the oxygen-related ones, in TCOs have to be substantially reduced, if not fully eliminated, through the optimal growth conditions to attain higher transmittance.

Solar cells exploit the photovoltaic effect that is the direct conversion of incident light into electricity. Electron–hole pairs generated by solar photons are separated at a space charge region of the two materials with different conduction polarities. Solar cells represent a very promising renewable energy technology because they provide clean energy source (beyond manufacturing) which will reduce our dependence on fossil oil. The principles of operation of solar cells have been widely discussed in detail in the literature and as such will not be repeated here. Rather, the various solar cell technologies will be discussed in the context of conduction oxides. Solar cells can be categorized into bulk devices (mainly single-crystal or large-grain polycrystalline Si), thin film single- and multiple-junction devices, and newly emerged technology which include dye-sensitized cells, organic/polymer cells, highefficiency multi-junction cells based on III–V semiconductors among others. Crystalline silicon modules based on bulk wafers have been dubbed as the "first-generation" photovoltaic technology. The cost of energy generated by PV modules based on bulk-Si wafers is currently around \$3–\$4/Wp and cost reduction potential seems limited by the price of Si wafers. This cost of energy is still too high for a significant influence on energy production markets. Much of the industry is focused on the most cost efficient technologies in terms of cost per generated power. The two main strategies to bring down the cost of

**7. Application of TCO in solar cells** 

The conductivity is intrinsically limited for two reasons. First, n and cannot be independently increased for practical TCOs with relatively high carrier concentrations. At high conducting electron density, carrier transport is limited primarily by ionized impurity scattering, i.e., the Coulomb interactions between electrons and the dopants. Higher doping concentration reduces carrier mobility to a degree that the conductivity is not increased, and it decreases the optical transmission at the near-infrared edge. With increasing dopant concentration, the resistivity reaches a lower limit, and does not decrease beyond it, whereas the optical window becomes narrower. Bellingham were the first to report that the mobility and hence the resistivity of transparent conductive oxides (ITO, SnO2, ZnO) are limited by ionized impurity scattering for carrier concentrations above 1020 cm-3. Ellmer also showed that in ZnO films deposited by various methods, the resistivity and mobility were nearly independent of the deposition method and limited to about 210-4 cm and 50 cm2/Vs, respectively. In ITO films, the maximum carrier concentration was about 1.51021 cm-3, and the same conductivity and mobility limits also held. This phenomenon is a universal property of other semiconductors. Scattering by the ionized dopant atoms that are homogeneously distributed in the semiconductor is only one of the possible effects that reduce the mobility. The all recently developed TCO materials, including doped and undoped binary, ternary, and quaternary compounds, also suffer from the same limitations. Only some exceptional samples had a resistivity of 110-4 cm.

In addition to the above mentioned effects that limit the conductivity, high dopant concentration could lead to clustering of the dopant ions, which increases significantly the scattering rate, and it could also produce nonparabolicity of the conduction band, which has to be taken into account for degenerately doped semiconductors with filled conduction bands.

#### **6. Optical properties of TCO**

The transmission window of TCOs is defined by two imposed boundaries. One is in the near-UV region determined by the effective band gap Eg, which is blue shifted due to the Burstein–Moss effect. Owing to high electron concentrations involved the absorption edge is shifted to higher photon energies. The sharp absorption edge near the band edge typically corresponds to the direct transition of electrons from the valence band to the conduction band. The other is at the near infrared (NIR) region due to the increase in reflectance caused by the plasma resonance of electron gas in the conduction band. The absorption coefficient (α) is very small within the defined window and consequently transparency is very high. The positions of the two boundaries defining the transmission window are closely related to the carrier concentration. For TCOs, both boundaries defining the transmission window shift to shorter wavelength with the increase of carrier concentration. The blue-shift of the near-UV and near-IR boundaries of the transmission window of GZO as the carrier concentration increased from 2.3 × 1020 cm−3 to 10 × 1020 cm−3. The blue-shift of the onset of absorption in the near-UV region is associated with the increase in the carrier concentration blocking the lowest states (filled states) in the conduction band from absorbing the photons. The Burstein–Moss effect owing to high electron concentrations has been widely observed in transmittance spectra of GZO and AZO. A comparable or even larger blue-shift in the transmittance spectra of GZO has been reported with absorption edge at about 300 nm wavelength corresponding to a bang gap of about 4.0 eV. The plasma frequency at which the free carriers are absorbed has a negative correlation with the free carrier concentration.

The conductivity is intrinsically limited for two reasons. First, n and cannot be independently increased for practical TCOs with relatively high carrier concentrations. At high conducting electron density, carrier transport is limited primarily by ionized impurity scattering, i.e., the Coulomb interactions between electrons and the dopants. Higher doping concentration reduces carrier mobility to a degree that the conductivity is not increased, and it decreases the optical transmission at the near-infrared edge. With increasing dopant concentration, the resistivity reaches a lower limit, and does not decrease beyond it, whereas the optical window becomes narrower. Bellingham were the first to report that the mobility and hence the resistivity of transparent conductive oxides (ITO, SnO2, ZnO) are limited by ionized impurity scattering for carrier concentrations above 1020 cm-3. Ellmer also showed that in ZnO films deposited by various methods, the resistivity and mobility were nearly independent of the deposition method and limited to about 210-4 cm and 50 cm2/Vs, respectively. In ITO films, the maximum carrier concentration was about 1.51021 cm-3, and the same conductivity and mobility limits also held. This phenomenon is a universal property of other semiconductors. Scattering by the ionized dopant atoms that are homogeneously distributed in the semiconductor is only one of the possible effects that reduce the mobility. The all recently developed TCO materials, including doped and undoped binary, ternary, and quaternary compounds, also suffer from the same limitations.

In addition to the above mentioned effects that limit the conductivity, high dopant concentration could lead to clustering of the dopant ions, which increases significantly the scattering rate, and it could also produce nonparabolicity of the conduction band, which has to be taken into account for degenerately doped semiconductors with filled conduction

The transmission window of TCOs is defined by two imposed boundaries. One is in the near-UV region determined by the effective band gap Eg, which is blue shifted due to the Burstein–Moss effect. Owing to high electron concentrations involved the absorption edge is shifted to higher photon energies. The sharp absorption edge near the band edge typically corresponds to the direct transition of electrons from the valence band to the conduction band. The other is at the near infrared (NIR) region due to the increase in reflectance caused by the plasma resonance of electron gas in the conduction band. The absorption coefficient (α) is very small within the defined window and consequently transparency is very high. The positions of the two boundaries defining the transmission window are closely related to the carrier concentration. For TCOs, both boundaries defining the transmission window shift to shorter wavelength with the increase of carrier concentration. The blue-shift of the near-UV and near-IR boundaries of the transmission window of GZO as the carrier concentration increased from 2.3 × 1020 cm−3 to 10 × 1020 cm−3. The blue-shift of the onset of absorption in the near-UV region is associated with the increase in the carrier concentration blocking the lowest states (filled states) in the conduction band from absorbing the photons. The Burstein–Moss effect owing to high electron concentrations has been widely observed in transmittance spectra of GZO and AZO. A comparable or even larger blue-shift in the transmittance spectra of GZO has been reported with absorption edge at about 300 nm wavelength corresponding to a bang gap of about 4.0 eV. The plasma frequency at which the free carriers are absorbed has a negative correlation with the free carrier concentration.

Only some exceptional samples had a resistivity of 110-4 cm.

bands.

**6. Optical properties of TCO** 

Consequently, the boundary in the near-IR region also shifts to the shorter wavelength with increase of the free carrier concentration. The shift in the near-IR region is more pronounced than that in the near-UV region. Therefore, the transmission window becomes narrower as the carrier concentration increases. This means that both the conductivity and the transmittance window are interconnected since the conductivity is also related to the carrier concentration as discussed above. Thus, a compromise between material conductivity and transmittance window must be struck, the specifics of which being application dependent. While for LED applications the transparency is needed only in a narrow range around the emission wavelengths, solar cells require high transparency in the whole solar spectral range. Therefore, for photovoltaics, the carrier concentration should be as low as possible for reducing the unwanted free carrier absorption in the IR spectral range, while the carrier mobility should be as high as possible to retain a sufficiently high conductivity. Optical measurements are also commonly employed to gain insight into the film quality. For example, interference fringes found in transmittance curves indicate the highly reflective nature of surfaces and interfaces in addition to the low scattering and absorption losses in the films. The particulars of interferences are related to both the film thickness and the incident wavelength, which can be used to achieve higher transmittance for TCOs. In the case of a low quality TCO, deep level emissions occurring in photoluminescence (PL) spectra along with relatively low transmittance are attributed to the lattice defects such as oxygen vacancies, zinc vacancies, interstitial metal ions, and interstitial oxygen. Highdoping concentration-induced defects in crystal lattices causing the creation of electronic defect states in band gap similarly have an adverse effect on transparency. In GZO, as an example, at very high Ga concentrations (1020–1021 cm-3), the impurity band merges with the conduction band causing a tail-like state below the conduction band edge of intrinsic ZnO. These tail states are responsible for the low-energy part of PL emission. Therefore, the defects, mainly the oxygen-related ones, in TCOs have to be substantially reduced, if not fully eliminated, through the optimal growth conditions to attain higher transmittance.
