**2. Review of TCO thin films**

#### **2.1 Development of TCOs 2.1.1 Feature of TCO**

Most optically transparent and electrically conducting oxides (TCOs) are binary or ternary compounds, containing one or two metallic elements. Their resistivity could be as low as

dominant to the open-circuit voltage. The desirable characteristics of TCO materials that are common to all PV technologies are similar to the requirements for TCOs for flat-panel display applications and include high optical transmission across a wide spectrum and low resistivity. Additionally, TCOs for terrestrial PV applications must be used as low-cost materials, and some may be required in the device-technology specific properties. The fundamentals of TCOs and the matrix of TCO properties and processing as they apply to

As an example, the In2O3:SnO2(ITO) transparent conducting oxides thin film was successfully used for the novel ultraviolet response enhanced PV cell with silicon-based SINP configuration. The realization of ultraviolet response enhancement in PV cells through the structure of ITO/SiO2/np-Silicon frame (named as SINP), which was fabricated by the state of the art processing, have been elucidated in the chapter. The fabrication process consists of thermal diffusion of phosphorus element into p-type texturized crystal Si wafer, thermal deposition of an ultra-thin silicon dioxide layer (15-20Å) at low temperature, and subsequent deposition of thick In2O3:SnO2 (ITO) layer by RF sputtering. The structure, morphology, optical and electric properties of the ITO film were characterized by XRD,

The results showed that ITO film possesses high quality in terms of antireflection and electrode functions. The device parameters derived from current-voltage (I-V) relationship under different conditions, spectral response and responsivity of the ultraviolet photoelectric cell with SINP configuration were analyzed in detail. We found that the main feature of our PV cell is the enhanced ultraviolet response and optoelectronic conversion. The improved short-circuit current, open-circuit voltage, and filled factor indicate that the device is promising to be developed into an ultraviolet and blue enhanced photovoltaic device in the future. On the other hand, the novel ITO/AZO/SiO2/p-Si SIS heterojunction has been fabricated by low temperature thermally grown an ultrathin silicon dioxide and RF sputtering deposition ITO/AZO double films on p-Si texturized substrate. The crystalline structural, optical and electrical properties of the ITO/AZO antireflection films were characterized by XRD, UV-VIS spectrophotometer, four point probes, respectively. The results show that ITO/AZO films have good quality. The electrical junction properties were investigated by I-V measurement, which reveals that the heterojunction shows strong rectifying behavior under a dark condition. The ideality factor and the saturation current of this diode is 2.3 and 1.075×10-5A, respectively. In addition, the values of IF/IR (IF and IR stand for forward and reverse current, respectively) at 2V is found to be as high as 16.55. It shows fairly good rectifying behavior indicating formation of a diode between AZO and p-Si. High photocurrent is obtained under a reverse bias when the crystalline quality of ITO/AZO

In device physics, the tunneling effect of SIS solar cell has been investigated in our current work, depending on the thickness of the ultra-thin insulator layer, which is potential for the

Most optically transparent and electrically conducting oxides (TCOs) are binary or ternary compounds, containing one or two metallic elements. Their resistivity could be as low as

SEM, UV-VIS spectrophotometer and Hall effects measurement, respectively.

current and future PV technologies were discussed.

double films is good enough to transmit the light into p-Si.

**2. Review of TCO thin films** 

**2.1 Development of TCOs 2.1.1 Feature of TCO** 

understanding of quantum mechanics in the photovoltaic devices.

10-5 cm, and their extinction coefficient k in the visible range (VIS) could be lower than 0.0001, owing to their wide optical band gap (Eg) that could be greater than 3 eV. This remarkable combination of conductivity and transparency is usually impossible in intrinsic stoichiometric oxides; however, it is achieved by producing them with a non-stoichiometric composition or by introducing appropriate dopants. Badeker (1907) discovered that thin CdO films possess such characteristics. Later, it was recognized that thin films of ZnO, SnO2, In2O3 and their alloys were also TCOs. Doping these oxides resulted in improved electrical conductivity without degrading their optical transmission. Al doped ZnO (AZO), tin doped In2O3, (ITO) and antimony or fluorine doped SnO2 (ATO and FTO), are among the most utilized TCO thin films in modern technology. In particular, ITO is used extensively in acoustic wave device, electro-optic modulators, flat panel displays, organic light emitting diodes and photovoltaic devices.

The actual and potential applications of TCO thin films include: (1) transparent electrodes for flat panel displays (2) transparent electrodes for photovoltaic cells, (3) low emissivity windows, (4) window defrosters, (5) transparent thin films transistors, (6) light emitting diodes, and (7) semiconductor lasers. As the usefulness of TCO thin films depends on both their optical and electrical properties, both parameters should be considered together with environmental stability, abrasion resistance, electron work function, and compatibility with substrate and other components of a given device, as appropriate for the application. The availability of the raw materials and the economics of the deposition method are also significant factors in choosing the most appropriate TCO material. The selection decision is generally made by maximizing the functioning of the TCO thin film by considering all relevant parameters, and minimizing the expenses. TCO material selection only based on maximizing the conductivity and the transparency can be faulty.

Recently, the scarcity and high price of Indium needed for ITO materials, the most popular TCO, as spurred R&D aimed at finding a substitute. Its electrical resistivity (ρ) should be ~10-4 cm or less, with an absorption coefficient ( ) smaller than 104 cm-1 in the near-UV and VIS range, and with an optical band gap >3eV. A 100 nm thick film TCO film with these values for and will have optical transmission (T) 90% and a sheet resistance (RS) of < 10 /. At present, AZO and ZnO:Ga (GZO) semiconductors are promising alternatives to ITO for thin-film transparent electrode applications. The best candidates is AZO, which can have a low resistivity, e.g. on the order of 10−<sup>4</sup> cm, and its source materials are inexpensive and non-toxic. However, the development of large area, high rate deposition techniques is needed.

Another objective of the recent effort to develop novel TCO materials is to deposit p-type TCO films. Most of the TCO materials are n-type semiconductors, but p-type TCO materials are required for the development of solid lasers, as well as TFT or PV cells. Such p-type TCOs include: ZnO:Mg, ZnO:N, ZnO:In, NiO, NiO:Li, CuAlO2, Cu2SrO2, and CuGaO2 thin films. These materials have not yet found a place in actual applications owing to the stability.

Published reviews on TCOs reported exhaustively on the deposition and diagnostic techniques, on film characteristics, and expected applications. The present paper has three objectives: (1) to review the theoretical and experimental efforts to explore novel TCO materials intended to improve the TCO performance, (2) to explain the intrinsic physical limitations that affect the development of an alternative TCO with properties equivalent to those of ITO, and (3) to review the practical and industrial applications of existing TCO thin films.

TCO-Si Based Heterojunction Photovoltaic Devices 115

Much as ITO is the most widely used In2O3-based binary TCO, fluorine-doped tin oxide (FTO) is the dominant in SnO2-based binary TCOs. In comparison to ITO, FTO is less expensive and shows better thermal stability of its electrical properties as well chemical stability in dye-sensitized solar cell (DSSC). FTO is the second widely used TCO material, mainly in solar cells due to its better stability in hydrogen-containing environment and at high temperatures required for device fabrication. The typical value of FTO's average transmittance is about 80%. However, electrical conductivity of FTO is relatively low and it is more difficult to pattern via wet etching as compared to ITO. In short, more efforts are beginning to be expended for TCOs by researchers owing to their above-mentioned uses spurred by their excellent electrical and optical properties in recently popularized devices. Germanium-doped indium oxide, IGO (In2O3:Ge), and fluorine-doped indium oxide, IFO (In2O3:F), reported by Romeo et al., for example, have resistivities of about 2 × 10−4 Ω cm and optical transmittance of ≥ 85% in the wavelength range of 400–800 nm, which are comparable to their benchmark ITO. Molybdenum-doped indium oxide, IMO (In2O3:Mo), was first reported by Meng et al.. Later on, Yamada et al. reported a low resistivity of 1.5 × 10−4 Ω cm and a mobility of 94 cm2/V s, and Parthiban et al. reported a resistivity of 4 × 10−<sup>4</sup> Ω cm, an average transmittance of >83% and a mobility of 149 cm2/V s for IMO. Zn-doped indium oxide, IZO (In2O3:Zn), deposited on plastic substrates showed resistivity of 2.9 × 10−<sup>4</sup> Ω cm and optical transmittance of ≥ 85%. Suffice it to say that In2O3 doped with other impurities have comparable electrical and optical properties to the above-mentioned data as

The small variations existing among these reports could be attributed to the particulars of the deposition techniques and deposition conditions. To improve the electrical and optical properties of In2O3 and ITO, their doped varieties such as ITO:Ta and In2O3:Cd–Te have been explored as well. For example, compared with ITO, the films of ITO:Ta have improved the electrical and optical properties due to the improved crystallinity, larger grain size, and the lower surface roughness, as well as a larger band gap, which are more pronounced for ITO:Ta achieved at low substrate temperatures. The carrier concentration, mobility, and maximum optical transmittance for ITO:Ta achieved at substrate temperature 400°C are 9.16 × 1020 cm−3, 28.07 cm2/V s and 91.9% respectively, while the corresponding values for ITO are 9.12 × 1020 cm−3, 26.46 cm2/V s and 87.9%, respectively. Due to historical reasons, propelled by the above discussed attributes, ITO is the predominant TCO used in optoelectronic devices. Another reason why ITO enjoys such predominance is the ease of its processing. ITO-based transparent electrodes used in LCDs consume the largest amount of indium, about 80% of the total. As reported by Minami and Miyata (January, 2008), about 800 tons of indium was used in Japan in 2007. Because approximately 80%–90% of the indium can be recycled, the real consumption of indium in Japan in 2007 is in the range of 80–160 tons. The total amount of indium reserves in the world is estimated to be only approximately 6000 tons according to the 2007 United States Geological Survey. It is widely believed that indium shortage may occur in the very near future and indium will soon

Consequently, search for alternative TCO films comparable to or better than ITO is underway. The report published by NanoMarkets in April 2009 (Indium Tin Oxide and Alternative Transparent Conductor Markets) pointed out that up until 2009 the ITO market was not challenged since the predicted boom in demand for ITO did not happen, partially due to the financial meltdown. The price of indium slightly varied from about US700\$/kg in 2005 to US1000\$/kg in 2007 and then to US700\$/kg in 2009 which is still too expensive for

enumerated in many articles.

become a strategic resource in every country.

#### **2.1.2 Multiformity of TCOs**

The first realization of a TCO material (CdO, Badeker 1907)) occurred slightly more than a century ago when a thin film of sputter deposited cadmium (Cd) metal underwent incomplete thermal oxidation upon postdeposition heating in air. Later, CdO thin films were achieved by a variety of deposition techniques such as reactive sputtering, spray pyrolysis, activated reactive evaporation, and metal organic vapor phase epitaxy (MOVPE). CdO has a face centered cubic (FCC) crystal structure with a relatively low intrinsic band gap of 2.28 eV. Note that without doping, CdO is an n-type semiconductor. The relatively narrow band gap of CdO and the toxicity of Cd make CdO less desirable and account for receiving somewhat dismal attention in its standard form. However, its low effective carrier mass allows efficiently increasing the band gap of heavily doped samples to as high as 3.35 eV (the high carrier concentration results in a partial filling of a conduction band and consequently, in a blue-shift of the UV absorption edge, known as the Burstein–Moss effect) and gives rise to mobility as high as 607 cm2/V s in epitaxial CdO films doped with Sn. The high mobility exhibited by doped CdO films is a definite advantage in device applications. Cd-based TCOs such as CdO doped with either indium (In), tin (Sn), fluorine (F), or yttrium (Y), and its ternary compounds such as CdSnO3, Cd2SnO4, CdIn2O4 as well as its other relevant compounds all have good electrical and optical properties. The lowest reported resistivity of Cd-based TCOs is 1.4×10−<sup>4</sup> Ω cm, which is very good and competitive with other leading candidates. The typical transmittance of Cd-based TCOs in the visible range is 85%–90%. Although the Cd-based TCOs have the desired electrical and optical properties, in addition to low surface recombination velocity, which is very desirable, they face tremendous obstacles in penetrating the market except for some special applications such as CdTe/CdS thin film solar cells due to the high toxicity of Cd. It should be noted that the aforementioned solar cells are regulated and cannot be sold. To circumvent this barrier, the manufacturers lease them for solar power generation instead. Consequently, our attention in this chapter is turned away for discussing this otherwise desirable conducting oxide. Revelations dating back to about 1960s that indium tin oxide (ITO), a compound of indium oxide (In2O3) and tin oxide (SnO2), exhibits both excellent electrical and optical properties

paved the way for extensive studies on this material family. In2O3 has a bixbyite-type cubic crystal structure, while SnO2 has a rutile crystal structure. Both of them are weak n-type semiconductors. Their charge carrier concentration and thus, the electrical conductivity can be strongly increased by extrinsic dopants which is desirable. In2O3 is a semiconductor with a band gap of 2.9 eV, a figure which was originally thought to be 3.7 eV. The reported dopants for In2O3-based binary TCOs are Sn, Ge, Mo, Ti, Zr, Hf, Nb, Ta, W, Te, and F as well as Zn. The In2O3-based TCOs doped with the aforementioned impurities were found to possess very good electrical and optical properties. The smallest laboratory resistivities of Sn-doped In2O3 (ITO) are just below 10−4 Ω cm, with typical resistivities being about 1 ×10−<sup>4</sup> Ω cm. As noted above, despite the nomenclature of Sn-doped In2O3 (ITO), this material is really an In2O3-rich compound of In2O3 and SnO2. SnO2 is a semiconductor with a band gap of 3.62 eV at 298 K and is particularly interesting because of its low electrical resistance coupled with its high transparency in the UV–visible region. SnO2 grown by molecular beam epitaxy (MBE) was found to be unintentionally doped with an electron concentration for different samples in the range of (0.3–3) × 1017 cm−3 and a corresponding electron mobility in the range of 20–100 cm2/V s. Fluorine (F), antimony (Sb), niobium (Nb), and tantalum (Ta) are most commonly used to achieve high n-type conductivity while maintaining high optical transparency.

The first realization of a TCO material (CdO, Badeker 1907)) occurred slightly more than a century ago when a thin film of sputter deposited cadmium (Cd) metal underwent incomplete thermal oxidation upon postdeposition heating in air. Later, CdO thin films were achieved by a variety of deposition techniques such as reactive sputtering, spray pyrolysis, activated reactive evaporation, and metal organic vapor phase epitaxy (MOVPE). CdO has a face centered cubic (FCC) crystal structure with a relatively low intrinsic band gap of 2.28 eV. Note that without doping, CdO is an n-type semiconductor. The relatively narrow band gap of CdO and the toxicity of Cd make CdO less desirable and account for receiving somewhat dismal attention in its standard form. However, its low effective carrier mass allows efficiently increasing the band gap of heavily doped samples to as high as 3.35 eV (the high carrier concentration results in a partial filling of a conduction band and consequently, in a blue-shift of the UV absorption edge, known as the Burstein–Moss effect) and gives rise to mobility as high as 607 cm2/V s in epitaxial CdO films doped with Sn. The high mobility exhibited by doped CdO films is a definite advantage in device applications. Cd-based TCOs such as CdO doped with either indium (In), tin (Sn), fluorine (F), or yttrium (Y), and its ternary compounds such as CdSnO3, Cd2SnO4, CdIn2O4 as well as its other relevant compounds all have good electrical and optical properties. The lowest reported resistivity of Cd-based TCOs is 1.4×10−<sup>4</sup> Ω cm, which is very good and competitive with other leading candidates. The typical transmittance of Cd-based TCOs in the visible range is 85%–90%. Although the Cd-based TCOs have the desired electrical and optical properties, in addition to low surface recombination velocity, which is very desirable, they face tremendous obstacles in penetrating the market except for some special applications such as CdTe/CdS thin film solar cells due to the high toxicity of Cd. It should be noted that the aforementioned solar cells are regulated and cannot be sold. To circumvent this barrier, the manufacturers lease them for solar power generation instead. Consequently, our attention in

this chapter is turned away for discussing this otherwise desirable conducting oxide.

Revelations dating back to about 1960s that indium tin oxide (ITO), a compound of indium oxide (In2O3) and tin oxide (SnO2), exhibits both excellent electrical and optical properties paved the way for extensive studies on this material family. In2O3 has a bixbyite-type cubic crystal structure, while SnO2 has a rutile crystal structure. Both of them are weak n-type semiconductors. Their charge carrier concentration and thus, the electrical conductivity can be strongly increased by extrinsic dopants which is desirable. In2O3 is a semiconductor with a band gap of 2.9 eV, a figure which was originally thought to be 3.7 eV. The reported dopants for In2O3-based binary TCOs are Sn, Ge, Mo, Ti, Zr, Hf, Nb, Ta, W, Te, and F as well as Zn. The In2O3-based TCOs doped with the aforementioned impurities were found to possess very good electrical and optical properties. The smallest laboratory resistivities of Sn-doped In2O3 (ITO) are just below 10−4 Ω cm, with typical resistivities being about 1 ×10−<sup>4</sup> Ω cm. As noted above, despite the nomenclature of Sn-doped In2O3 (ITO), this material is really an In2O3-rich compound of In2O3 and SnO2. SnO2 is a semiconductor with a band gap of 3.62 eV at 298 K and is particularly interesting because of its low electrical resistance coupled with its high transparency in the UV–visible region. SnO2 grown by molecular beam epitaxy (MBE) was found to be unintentionally doped with an electron concentration for different samples in the range of (0.3–3) × 1017 cm−3 and a corresponding electron mobility in the range of 20–100 cm2/V s. Fluorine (F), antimony (Sb), niobium (Nb), and tantalum (Ta) are most commonly used to achieve high n-type conductivity while

**2.1.2 Multiformity of TCOs** 

maintaining high optical transparency.

Much as ITO is the most widely used In2O3-based binary TCO, fluorine-doped tin oxide (FTO) is the dominant in SnO2-based binary TCOs. In comparison to ITO, FTO is less expensive and shows better thermal stability of its electrical properties as well chemical stability in dye-sensitized solar cell (DSSC). FTO is the second widely used TCO material, mainly in solar cells due to its better stability in hydrogen-containing environment and at high temperatures required for device fabrication. The typical value of FTO's average transmittance is about 80%. However, electrical conductivity of FTO is relatively low and it is more difficult to pattern via wet etching as compared to ITO. In short, more efforts are beginning to be expended for TCOs by researchers owing to their above-mentioned uses spurred by their excellent electrical and optical properties in recently popularized devices. Germanium-doped indium oxide, IGO (In2O3:Ge), and fluorine-doped indium oxide, IFO (In2O3:F), reported by Romeo et al., for example, have resistivities of about 2 × 10−4 Ω cm and optical transmittance of ≥ 85% in the wavelength range of 400–800 nm, which are comparable to their benchmark ITO. Molybdenum-doped indium oxide, IMO (In2O3:Mo), was first reported by Meng et al.. Later on, Yamada et al. reported a low resistivity of 1.5 × 10−4 Ω cm and a mobility of 94 cm2/V s, and Parthiban et al. reported a resistivity of 4 × 10−<sup>4</sup> Ω cm, an average transmittance of >83% and a mobility of 149 cm2/V s for IMO. Zn-doped indium oxide, IZO (In2O3:Zn), deposited on plastic substrates showed resistivity of 2.9 × 10−<sup>4</sup> Ω cm and optical transmittance of ≥ 85%. Suffice it to say that In2O3 doped with other impurities have comparable electrical and optical properties to the above-mentioned data as enumerated in many articles.

The small variations existing among these reports could be attributed to the particulars of the deposition techniques and deposition conditions. To improve the electrical and optical properties of In2O3 and ITO, their doped varieties such as ITO:Ta and In2O3:Cd–Te have been explored as well. For example, compared with ITO, the films of ITO:Ta have improved the electrical and optical properties due to the improved crystallinity, larger grain size, and the lower surface roughness, as well as a larger band gap, which are more pronounced for ITO:Ta achieved at low substrate temperatures. The carrier concentration, mobility, and maximum optical transmittance for ITO:Ta achieved at substrate temperature 400°C are 9.16 × 1020 cm−3, 28.07 cm2/V s and 91.9% respectively, while the corresponding values for ITO are 9.12 × 1020 cm−3, 26.46 cm2/V s and 87.9%, respectively. Due to historical reasons, propelled by the above discussed attributes, ITO is the predominant TCO used in optoelectronic devices. Another reason why ITO enjoys such predominance is the ease of its processing. ITO-based transparent electrodes used in LCDs consume the largest amount of indium, about 80% of the total. As reported by Minami and Miyata (January, 2008), about 800 tons of indium was used in Japan in 2007. Because approximately 80%–90% of the indium can be recycled, the real consumption of indium in Japan in 2007 is in the range of 80–160 tons. The total amount of indium reserves in the world is estimated to be only approximately 6000 tons according to the 2007 United States Geological Survey. It is widely believed that indium shortage may occur in the very near future and indium will soon become a strategic resource in every country.

Consequently, search for alternative TCO films comparable to or better than ITO is underway. The report published by NanoMarkets in April 2009 (Indium Tin Oxide and Alternative Transparent Conductor Markets) pointed out that up until 2009 the ITO market was not challenged since the predicted boom in demand for ITO did not happen, partially due to the financial meltdown. The price of indium slightly varied from about US700\$/kg in 2005 to US1000\$/kg in 2007 and then to US700\$/kg in 2009 which is still too expensive for

TCO-Si Based Heterojunction Photovoltaic Devices 117

all the dopants for ZnO-based binary TCOs, Ga and Al are thought to be the best candidates so far. It is also worth nothing that Zn1−xMgxO alloy films doped with a donor impurity can also serve as transparent conducting layers in optoelectronic devices. As well known the band gap of wurtzite phase of Zn1−xMgxO alloy films could be tuned from 3.37 to 4.05 eV, making conducting Zn1−xMgxO films more suitable for ultraviolet (UV) devices. The larger band gap of these conducting layers with high carrier concentration is also desired in the modulation-doped heterostructures designed to increase electron mobility. In this vein, Zn1−xMgxO doped with Al has been reported in Refs. The above-mentioned ZnO-based TCOs have relatively large refractive indices as well, in the range of 1.9–2.2, which are comparable to those of ITO and FTO. For comparison, the refractive indices of commercial ITO/glass decrease from 1.9 at wavelength of 400 nm to 1.5 at a wavelength of 800 nm, respectively. The high refractive indices reduce internal reflections and allow employment of textured structures in LEDs to enhance light extraction beyond that made feasible by enhanced transparency alone. The dispersion in published values of the refractive index is attributed to variations in properties of the films prepared by different deposition techniques. For example, amorphous ITO has lower refractive index than textured ITO. It is interesting to note that nanostructures such as nanorods and nanotips as well as controllable surface roughness could enhance light extraction/absorption in LEDs and solar cells, thus improving device performance. Fortunately, such nanostructures can be easily achieved in ZnO by choosing and controlling the growth conditions. One disadvantage of ZnO-based TCOs is that they degrade much faster than ITO and FTO when exposed to damp and hot (DH) environment. The stability of AZO used in thin film CuInGaSe2 (CIGS) solar cells, along with Al-doped Zn1−xMgxO alloy, ITO and FTO, by direct exposure to damp heat (DH) at 85°C and 85% relative humidity. The results showed that the DH-induced degradation rates followed the order of AZO and Zn1−xMgxO ≫ ITO > FTO. The degradation rates of AZO were slower for films of larger thickness which were deposited at higher substrate temperatures during sputter deposition, and underwent dry-out intervals. From the point of view of the initiation and propagation of degrading patterns and regions, the degradation behavior appears similar for all TCOs despite the obvious differences in the degradation rates. The degradation is explained by both hydrolysis of the oxides at some sporadic weak spots followed by swelling and popping of the hydrolyzed spots which are followed by

segregation of hydrolyzed regions, and hydrolysis of the oxide–glass interfaces.

In addition to those above-mentioned binary TCOs based on In2O3, SnO2 and ZnO, ternary compounds such as Zn2SnO4, ZnSnO3, Zn2In2O5, Zn3In2O6, In4Sn3O12, and multicomponent oxides including (ZnO)1−x(In2O3)x, (In2O3)x(SnO2)1−x, (ZnO)1−x(SnO2)x are also the subject of investigation. However, it is relatively difficult to deposit those TCOs with desirable optical and electrical properties due to the complexity of their compositions. Nowadays ITO, FTO and GZO/AZO described in more details above are preferred in practical applications due to the relative ease by which they can be formed. Although it is not within the scope of this article, it has to be pointed out for the sake of completeness that CdO along with In2O3 and SnO2 forms an analogous In2O3–SnO2–CdO alloy system. The averaged resistivity of ITO by different techniques is ~1 × 10−4Ω•cm, which is much higher than that of FTO. For FTO, the typically employed technique is spray pyrolysis which can produce the lowest resistivity of ~3.8 × 10−<sup>4</sup> Ω•cm. For AZO/GZO, the resistivities listed here are comparable to or slightly higher than ITO but their transmittance is slightly higher than that of ITO. Obviously, AZO and GZO as well as other ZnO-based TCOs are promising to replace ITO for transparent electrode applications in terms of their electrical and optical properties.There are also few

mass production. On the other hand, the market research firm iSupply forecasted in 2008 that the worldwide market for all touch screens employing ITO layers would nearly double, from \$3.4 billion to \$6.4 billion by 2013. Therefore, ITO as the industrial standard TCO is expected to lose its share of the applicable markets rather slowly even when alternatives become available. The report by NanoMarkets is a good guide for both users and manufacturers of TCOs.

In addition to ZnO-based TCOs, it also remarks on other possible solutions such as conductive polymers and/or the so-called and overused concept of nano-engineered materials such as poly (3, 4-ethylenedioxythiophene) well known as PEDOT by both H.C. Starck and Agfa, and carbon nanotube (CNT) coatings, which have the potentials to replace ITO at least in some applications since they can overcome the limitations of TCOs. Turning our attention now to the up and coming alternatives to ITO, ZnO with an electron affinity of 4.35 eV and a direct band gap energy of 3.30 eV is typically an n-type semiconductor material with the residual electron concentration of~1017 cm−3. However, the doped ZnO films have been realized with very attractive electrical and optical properties for electrode applications. The dopants that have been used for the ZnO-based binary TCOs are Ga, Al, B, In, Y, Sc, V, Si, Ge, Ti, Zr, Hf, and F. Among the advantages of the ZnO-based TCOs are low cost, abundant material resources, and non-toxicity. At present, ZnO heavily doped with Ga and Al (dubbed GZO and AZO) has been demonstrated to have low resistivity and high transparency in the visible spectral range and, in some cases, even outperform ITO and FTO. The dopant concentration in GZO or AZO is more often in the range of 1020–1021 cm−3 and although we obtained mobilities near 95 cm2/V s in our laboratory in GZO typical reported mobility is near or slightly below 50 cm2/V s. Ionization energies of Al and Ga donors (in the dilute limit which decreases with increased doping) are 53 and 55 meV, respectively, which are slightly lower than that of In (63 meV). Our report of a very low resistivity of~8.5×10−5 Ω cm for AZO, and Park et al. reported a resistivity of ~8.1 × 10−5 Ω cm for GZO, both of which are similar to the lowest reported resistivity of~7.7×10−5 Ω cm for ITO. The typical transmittance of AZO and GZO is easily 90% or higher, which is comparable to the best value reported for ITO when optimized for transparency alone and far exceeds that of the traditional semi-transparent and thin Ni/Au metal electrodes with transmittance below 70% in the visible range. The high transparency of AZO and GZO originates from the wide band gap nature of ZnO. Low growth temperature of AZO or GZO also intrigued researchers with respect to transparent electrode applications in solar cells. As compared to ITO, ZnO-based TCOs show better thermal stability of resistivity and better chemical stability at higher temperatures, both of which bode well for the optoelectronic devices in which this material would be used. In short, AZO and GZO are the TCOs attracting more attention, if not the most, for replacing ITO. From the cost and availability and environmental points of view, AZO appears to be the best candidate. This conclusion is also bolstered by batch process availability for large-area and large-scale production of AZO. To a lesser extent, other ZnO-based binary TCOs have also been explored. For

readers'convenience, some references are discussed at a glance below. B-doped ZnO has been reported to exhibit a lateral laser-induced photovoltage (LPV), which is expected to make it a candidate for position sensitive photo-detectors. In-doped ZnO prepared by pulsed laser deposition and spray pyrolysis is discussed, respectively. Y-doped ZnO deposited by sol–gel method on silica glass has been reported. The structural, optical and electrical properties of F-doped ZnO formed by the sol–gel process and also listed almost all the relevant activities in the field. For drawing the contrast, we should reiterate that among

mass production. On the other hand, the market research firm iSupply forecasted in 2008 that the worldwide market for all touch screens employing ITO layers would nearly double, from \$3.4 billion to \$6.4 billion by 2013. Therefore, ITO as the industrial standard TCO is expected to lose its share of the applicable markets rather slowly even when alternatives become available. The report by NanoMarkets is a good guide for both users and

In addition to ZnO-based TCOs, it also remarks on other possible solutions such as conductive polymers and/or the so-called and overused concept of nano-engineered materials such as poly (3, 4-ethylenedioxythiophene) well known as PEDOT by both H.C. Starck and Agfa, and carbon nanotube (CNT) coatings, which have the potentials to replace ITO at least in some applications since they can overcome the limitations of TCOs. Turning our attention now to the up and coming alternatives to ITO, ZnO with an electron affinity of 4.35 eV and a direct band gap energy of 3.30 eV is typically an n-type semiconductor material with the residual electron concentration of~1017 cm−3. However, the doped ZnO films have been realized with very attractive electrical and optical properties for electrode applications. The dopants that have been used for the ZnO-based binary TCOs are Ga, Al, B, In, Y, Sc, V, Si, Ge, Ti, Zr, Hf, and F. Among the advantages of the ZnO-based TCOs are low cost, abundant material resources, and non-toxicity. At present, ZnO heavily doped with Ga and Al (dubbed GZO and AZO) has been demonstrated to have low resistivity and high transparency in the visible spectral range and, in some cases, even outperform ITO and FTO. The dopant concentration in GZO or AZO is more often in the range of 1020–1021 cm−3 and although we obtained mobilities near 95 cm2/V s in our laboratory in GZO typical reported mobility is near or slightly below 50 cm2/V s. Ionization energies of Al and Ga donors (in the dilute limit which decreases with increased doping) are 53 and 55 meV, respectively, which are slightly lower than that of In (63 meV). Our report of a very low resistivity of~8.5×10−5 Ω cm for AZO, and Park et al. reported a resistivity of ~8.1 × 10−5 Ω cm for GZO, both of which are similar to the lowest reported resistivity of~7.7×10−5 Ω cm for ITO. The typical transmittance of AZO and GZO is easily 90% or higher, which is comparable to the best value reported for ITO when optimized for transparency alone and far exceeds that of the traditional semi-transparent and thin Ni/Au metal electrodes with transmittance below 70% in the visible range. The high transparency of AZO and GZO originates from the wide band gap nature of ZnO. Low growth temperature of AZO or GZO also intrigued researchers with respect to transparent electrode applications in solar cells. As compared to ITO, ZnO-based TCOs show better thermal stability of resistivity and better chemical stability at higher temperatures, both of which bode well for the optoelectronic devices in which this material would be used. In short, AZO and GZO are the TCOs attracting more attention, if not the most, for replacing ITO. From the cost and availability and environmental points of view, AZO appears to be the best candidate. This conclusion is also bolstered by batch process availability for large-area and large-scale production of AZO. To a lesser extent, other ZnO-based binary TCOs have also been explored. For readers'convenience, some references are discussed at a glance below. B-doped ZnO has been reported to exhibit a lateral laser-induced photovoltage (LPV), which is expected to make it a candidate for position sensitive photo-detectors. In-doped ZnO prepared by pulsed laser deposition and spray pyrolysis is discussed, respectively. Y-doped ZnO deposited by sol–gel method on silica glass has been reported. The structural, optical and electrical properties of F-doped ZnO formed by the sol–gel process and also listed almost all the relevant activities in the field. For drawing the contrast, we should reiterate that among

manufacturers of TCOs.

all the dopants for ZnO-based binary TCOs, Ga and Al are thought to be the best candidates so far. It is also worth nothing that Zn1−xMgxO alloy films doped with a donor impurity can also serve as transparent conducting layers in optoelectronic devices. As well known the band gap of wurtzite phase of Zn1−xMgxO alloy films could be tuned from 3.37 to 4.05 eV, making conducting Zn1−xMgxO films more suitable for ultraviolet (UV) devices. The larger band gap of these conducting layers with high carrier concentration is also desired in the modulation-doped heterostructures designed to increase electron mobility. In this vein, Zn1−xMgxO doped with Al has been reported in Refs. The above-mentioned ZnO-based TCOs have relatively large refractive indices as well, in the range of 1.9–2.2, which are comparable to those of ITO and FTO. For comparison, the refractive indices of commercial ITO/glass decrease from 1.9 at wavelength of 400 nm to 1.5 at a wavelength of 800 nm, respectively. The high refractive indices reduce internal reflections and allow employment of textured structures in LEDs to enhance light extraction beyond that made feasible by enhanced transparency alone. The dispersion in published values of the refractive index is attributed to variations in properties of the films prepared by different deposition techniques. For example, amorphous ITO has lower refractive index than textured ITO. It is interesting to note that nanostructures such as nanorods and nanotips as well as controllable surface roughness could enhance light extraction/absorption in LEDs and solar cells, thus improving device performance. Fortunately, such nanostructures can be easily achieved in ZnO by choosing and controlling the growth conditions. One disadvantage of ZnO-based TCOs is that they degrade much faster than ITO and FTO when exposed to damp and hot (DH) environment. The stability of AZO used in thin film CuInGaSe2 (CIGS) solar cells, along with Al-doped Zn1−xMgxO alloy, ITO and FTO, by direct exposure to damp heat (DH) at 85°C and 85% relative humidity. The results showed that the DH-induced degradation rates followed the order of AZO and Zn1−xMgxO ≫ ITO > FTO. The degradation rates of AZO were slower for films of larger thickness which were deposited at higher substrate temperatures during sputter deposition, and underwent dry-out intervals. From the point of view of the initiation and propagation of degrading patterns and regions, the degradation behavior appears similar for all TCOs despite the obvious differences in the degradation rates. The degradation is explained by both hydrolysis of the oxides at some sporadic weak spots followed by swelling and popping of the hydrolyzed spots which are followed by segregation of hydrolyzed regions, and hydrolysis of the oxide–glass interfaces.

In addition to those above-mentioned binary TCOs based on In2O3, SnO2 and ZnO, ternary compounds such as Zn2SnO4, ZnSnO3, Zn2In2O5, Zn3In2O6, In4Sn3O12, and multicomponent oxides including (ZnO)1−x(In2O3)x, (In2O3)x(SnO2)1−x, (ZnO)1−x(SnO2)x are also the subject of investigation. However, it is relatively difficult to deposit those TCOs with desirable optical and electrical properties due to the complexity of their compositions. Nowadays ITO, FTO and GZO/AZO described in more details above are preferred in practical applications due to the relative ease by which they can be formed. Although it is not within the scope of this article, it has to be pointed out for the sake of completeness that CdO along with In2O3 and SnO2 forms an analogous In2O3–SnO2–CdO alloy system. The averaged resistivity of ITO by different techniques is ~1 × 10−4Ω•cm, which is much higher than that of FTO. For FTO, the typically employed technique is spray pyrolysis which can produce the lowest resistivity of ~3.8 × 10−<sup>4</sup> Ω•cm. For AZO/GZO, the resistivities listed here are comparable to or slightly higher than ITO but their transmittance is slightly higher than that of ITO. Obviously, AZO and GZO as well as other ZnO-based TCOs are promising to replace ITO for transparent electrode applications in terms of their electrical and optical properties.There are also few

TCO-Si Based Heterojunction Photovoltaic Devices 119

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

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

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

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

= e / m\* (1)

concentrations. The variety of ZnO thin films has been expatiated elsewhere.

**5. Electrical conductivity of TCO** 

is the electron charge. The mobility is given by:

mobility were proposed.

effects on its transparency.

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 difficult to attain.
