Christelle Habis
email: christelle.habis@univ-lorraine.fr

Jean Zaraket
email: bjean.zaraket@univ-lorraine.fr

Michel Aillerie
email: cmichel.aillerie@univ-lorraine.fr

Transparent Conductive Oxides. Part II. Specific Focus on ITO, ZnO-AZO, SnO2-FTO Families for Photovoltaics Applications

Keywords: Transparent Conductive Oxides, Indium Tin Oxide, Zinc Oxide, Aluminum Zinc Oxide, Fluorine Tin Oxide

This contribution is the second part of a presentation of transparent conductive oxides, TCO. After a general overview in the first publication of properties and applications of TCO related to the type of oxide and dopant, the growth techniques and the temperature, we focus in the current one on three families of TCO used as transparent electrodes in photovoltaic, PV, cells and modules. Indeed, new generations of solar cells need optimizing TCO with improved conductivity and transparency depending on substrate, type of PV cells, and conditions of used. Thus, ITO, ZnO-AZO, and SnO2-FTO TCO families are considered. ITO presents the best combination of electrical and optical properties but TCO based on the two other families are challenging in many photovoltaic applications for obtaining low cost and environmental friendly electrodes with comparable performances. Due to the high number of publications on TCO for PV applications, this contribution does not claim to be exhaustive but makes it possible to summarize the main information concerning these materials by approaching them in a common methodology.

Introduction

Transparent conducting oxides (TCOs) are becoming a more common component of photovoltaic (PV) devices, where they serve as electrodes, structural templates, and diffusion barriers, and their work function regulates the open-circuit device voltage. They're used in everything from organic PV polymer solar cells to crystalline-Si heterojunction with intrinsic thin layer (HIT) cells. The requirements of TCOs for PV technologies are similar to those for TCOs for flat-panel display applications, and include excellent optical transmissivity throughout a wide spectrum and low resistivity. TCOs for PV applications must also use low-cost materials, with some requiring devicetechnology specific features.

This work is a second part of a general presentation of TCO based on a literature review. The first contribution of this review on TCO is presented in a previous publication title: "Transparent Conductive Oxides. Part I. General review of structural, electrical and optical properties of TCOs related to the growth techniques, materials and dopants." It introduces a general review of structural, electrical and optical properties of TCOs related to the growth techniques, materials and dopants. Based on this first one, the current one is more specifically focus on applications of TCOs as electrodes for photovoltaic applications. As specified above, and as for the first part, with the large number of works and publications present in the literature on TCOs, half of which relate to their use as electrodes in PV applications, with in addition publications presenting some given contradictory results, this contribution does not claim to be exhaustive but makes it possible for PV applications to summarize the main information, concerning these materials by approaching them in a common methodology. Considering the two parts of this contribution on TCO as a whole, mainly oriented to photovoltaic applications, it can be considered as a useful summary tool for researchers working in this domain of electrical engineering topics.

It is to be of note that, the last few years, a new generation of nanostructured TCO is currently being the subject of many researches. These aspects of materials are not considered here, and will be the subject of a future publication work. Thus, currently, three families of TCO are considered: ITO, ZnO-AZO, and SnO2-FTO used as electrodes in PV cells and modules. Based on the presentation done in Part. I, we will consider in this contribution the possible growth methods, the specific dopants and the main electrical and optical properties of this TCO for PV applications. The properties of these TCO films and families are finally compared in the final discussion.

Indium oxide (In2O3) and Indium Tin Oxide (ITO)

Indium Tin Oxide, ITO is a mixture of indium oxide (In2O3) and tin oxide (SnO2) typically in a ratio of 90/10 wt%. [1] It is widely used in transparent heating elements of aircraft and car windows, antistatic coatings over electronic instrument display panels, heat reflecting mirrors and antireflection coatings, high temperature gas sensors. ITO is also widely integrated in other electro-optic devices as transparent electrodes for solar cells, light emitting diodes, phototransistors and lasers. In addition, ITO offers the best available FoM, i.e. best performance in terms of conductivity and transparency along with excellent environmental stability, reproducibility and good surface morphology.

General properties of Indium Tin Oxide

Indium oxide mostly has a cubic structure of bixbyite type as reported in Fig. 1. Substitutional doping with Sn replaces the In 3+ ion to get indium tin oxide and Sn atoms form an interstitial bond with oxygen atoms. Tin is present with a dual valence and exists as either SnO or SnO2 which gives a direct impact on the conductivity of ITO. For lower valence state, a drop in carrier concentration occurs caused by the creation of holes acting as traps and thus reducing conductivity. However, for the case of SnO2 state, Sn 4+ releases electrons to the conduction band, thus acting as n-type donor. Moreover, ITO films have lattice parameters close to that of In2O3 and are in the range 10.12 Å to 10.31Å. [START_REF] Fan | X-ray photoemission spectroscopy studies of Sn-doped indium-oxide films[END_REF] Tuna et al. [START_REF] Tuna | High quality ITO thin films grown by dc and RF sputtering without oxygen[END_REF] reported the effect of substrate temperature and film thickness on the electrical, optical and structural properties of ITO thin films deposited by RF and DC magnetron sputtering on glass substrates. In their study they found out that the substrate temperature mostly affects the crystallization and resistivity of the samples. The characteristic peak (222) indicating the crystallization of ITO was observed starting from the annealing temperature of 150 °C up to 300 °C. Generally, the resistivity of the ITO layer was noted to decrease as the substrate temperature increases due to the increment in carrier concentration, crystallization and grain size. Nevertheless, the resistivity of the films at 400°C is found to be slightly higher than the obtained resistivity at 350°C (Fig 2 .a). This small variation might be the result of the growing number of oxygen vacancies under increased substrate temperatures. In addition, Nisha et al. [START_REF] Nisha | Effect of substrate temperature on the growth of ITO thin films[END_REF] previously reported that the diffusion of alkali ions from the substrate into the film structure at higher temperature might have a negative effect on the mobility. The increase in mobility (Fig. 2.b) for temperatures above 250 °C is attributed to the higher degree of crystallinity resulting in less grain boundary scattering. However, the increase of carrier concentration for temperature above 350 °C limits the mobility which by consequence suddenly decreases. Therefore, to get the optimum electric properties, the preferable growth temperature for ITO films is around 350 °C. High conductivity of ITO films is assumed to be owed to high carrier concentration rather than high mobility of hole. Compared to In2O3, the low mobility of hole in ITO and its dependence on substrate temperature is attributed to scattering mechanisms by ionized impurities or grain boundaries. At higher substrate temperature, an enhance of the crystallinity contributes to the increase of mobility.

The band gap of ITO has generally a value greater than 3.7eV. It ranges from 3.5 to 4.40 eV depending on the growth method and conditions as illustrated in Fig. 3 with the dependence of the band gap with the temperature of the substrat for ITO thin films [START_REF] Ray | Properties of tin doped indium oxide thin films prepared by magnetron sputtering[END_REF]. compound. [START_REF] Gupta | Band gap narrowing and the band structure of tindoped indium oxide films[END_REF] The doping with elements as tin in indium oxide, contribute in the formation of donor states at a level below the conduction band. With increasing the doping density up to a critical density, these states merge with the conduction band as illustrated in Fig. 4. The ITO film becomes degenerate when the density of free electrons from the donor atom exceeds this critical value. By consequence, the columbic interactions shift up the valence band and down the conduction band which narrows the bandgap (Fig. 4). This shift partially compensates the Burstein Moss effect.

In an attempt to identify the effect of grain size on the optical and electrical properties of ITO thin films under different annealing temperatures, Ahmed et al, deposited ITO thin films by high frequency magnetron sputtering. After deposition, the films were annealed at 250, 350, 450 and 550°C for 1 hour. [START_REF] Ahmed | The effect of post annealing temperature on grain size of indium-tin-oxide for optical and electrical properties improvement[END_REF] Table 1. Table showing the increase in grain size as the annealing temperature increases. The study showed that as the annealing temperature increased, the microstructure of the ITO films improved and caused a decrease in the band gap of the film. This change is attributed to the possibilities of oxidation, more realignment and strong interaction between film and substrate. It results, as shown in Fig. 5 an increase of the transmission response with the increase of the annealing temperature. In addition, as showed in Table 1, authors in reference [START_REF] Ahmed | The effect of post annealing temperature on grain size of indium-tin-oxide for optical and electrical properties improvement[END_REF] observed an increase in grain size (calculated from XRD spectra using Scherrer's formula) with the increase of the annealing temperature. Finally, the main output of this study displayed that at an annealing temperature of 450°C the optimum characteristics of the film were obtained.

Finally, it should be noted that indium tin oxide (ITO) thin films are mainly used as transparent electrodes for most different types of liquid crystal displays (LCDs). ITO is not widely considered as transparent electrodes for traditional silicon photovoltaic sources or panels, but only for certain thinfilm solar cells. For the latter, substitutes for ITO have already been developed, such as Al-doped ZnO (AZO) or Fluorine-doped SnO2 (FTO). However, the properties of TCOs used for flat screens require improved optical and electronic qualities. It is for these reasons that, in applications such as flat screens, the ITO has not yet been replaced. Nonetheless, the total consumption of indium used for flat screens and alkaline batteries has increased dramatically in recent years and it is widely believed that a shortage of indium may arise in the near future. Currently, researches in various teams are intensively focused on possible alternatives to the transparent ITO electrodes used in displays, focusing on the development or optimization of materials that can be produced at low cost. Among them, we note the transparent conductive oxides (TCO) and particularly the importance of zinc oxide, mainly when it is doped with metal ions as Al or Ga (AZO or GZO) and tin dioxide mainly doped with fluorine (FTO).

Thus, for photovoltaic applications as concerned in the current contribution, it is therefore these two oxide-based materials, ZnO and SnO2, which will attract our attention in the following paragraphs.

Zinc oxide (ZnO) and Aluminum doped Zinc oxide (AZO)

ZnO doped with metal ions is often chosen as an alternative to ITO in optoelectronic devices properties as its functional properties achieved currently a good level of performance and as, in addition to being a very abundant element which makes it relatively cheap. By else, it possesses an interesting set of properties that can be individually used (or manipulated) in various kinds of applications. [START_REF] Özgür | A comprehensive review of ZnO materials and devices[END_REF] For example, the large excitonic binding energy of ZnO about 60meV at room temperature makes this material an efficient light emitter in the UV spectral region interesting for optoelectronic applications. Its high conductivity and transparency, i.e. yielding to an interesting FoM makes it important for applications, not only as transparent conducting oxides (TCO) but also as thinfilm transistors (TFT). Furthermore, crystalline transparent and highly conductive ZnO thin film production can be deposited by various techniques on large scale and at room temperature making it compatible with a lot of substrate types; rigid or flexible, plastic, glass, organic and inorganic ones. Moreover, optical, electrical, magnetic, piezoelectric, catalytic and gas sensing properties make ZnO specifically attractive for nanoelectronic, optoelectronic and piezoelectric devices.

General properties of Zinc Oxide

In zinc oxide crystal lattice, the atoms are arranged in tetrahedral geometry where each Zn atom is surrounded by four O atoms and vice versa. ZnO generally crystallizes in wurtzite form (Fig. 6), but depending on the growth conditions, other crystal forms like zinc blende and rock salt exist for ZnO. Zinc oxide is a direct band gap semiconductor with a high refractive index (~2) having Eg = 3.4eV at room temperature. The concentration and mobility of electrons in ZnO were found to be in the range 10 16 ~10 17 cm -3 and 20~400 cm²V -1 s -1 , respectively. The resistivity of ZnO extends over several orders of magnitude, with values ranging from 10 -4 Ω.cm to 10 12 Ω.cm. [START_REF] Bouderbala | Thickness dependence of structural, electrical and optical behaviour of undoped ZnO thin films[END_REF] According to the procedures used for the deposition of ZnO, a low resistive (ρ = 10 -4 Ω.cm) i.e. high conductive or high resistive (ρ = 10 3 Ω.cm) i.e. low conductive films can be developed.

The low electrical conductivity of pure ZnO, when compared to ITO, is noted as a drawback. But doping by foreign elements showed to remarkably improve the conductivity and lower the sheet resistance to values of 4.3 x 10 3 Ω/cm and 3.3 x 10 3 Ω/cm for Al and Ga doping, respectively. [START_REF] Jun | Comparative studies of Al-doped ZnO and Ga-doped ZnO transparent conducting oxide thin films[END_REF] Thus, semi-conductive oxides being also often used as gas sensor and ZnO having good physical and chemical stability, it is perfectly suited for applications such as air pollution control, fire detection, leaks of harmful gases or tests for alcohol and other substances. ZnO having a high grain density and porosity, thus makes it possible to easily manufacture very sensitive gas sensors.

Various techniques can be used for the deposition of ZnO. The choice of a given technique will be drive by some practical criteria and of course, function of the targeted applications. Thus, TCO thin films must be prepared using appropriate deposition techniques that can produce low resistivity films at high deposition rates on large surface area substrates. Table 3 summarizes some deposition techniques along with the resulting associated ZnO properties. In addition, for specialized ZnO thin film materials, it is necessary to use deposition techniques that can achieve the lowest resistivity. Table 4 shows a number of deposition techniques that are usable for ZnO based thin film growth and their features.

Doping of Zinc Oxide

As mentioned above, for applications in optoelectronic devices, especially photovoltaic cells, doping ZnO is mandatory. This is how the conductivity of zinc oxide can be increased, or even optimized for the intended application thanks to the existence of intrinsic defects (vacancies and presence of interstitial atoms) allowing the introduction of doping ions, not only on these defect sites but also possible by substitution of zinc or oxygen. The presence of such intrinsic defects leads to ntype conduction. To improve its properties as transparent conductive oxide, ZnO must be doped by group III elements (B, Al, Ga, In…) or group IV element (Pb, Sn). Aluminum, indium and fluorine are the most common doping elements for ZnO. [START_REF] Douayar | Fluorinedoped ZnO thin films deposited by spray pyrolysis technique[END_REF] The use of different dopants leads to remarkable changes in the film properties. Numerous studies have been carried out on the properties of layers doped with different elements depending on the nature of dopant, its concentration and the deposition techniques. For several decades, these studies have made it possible to improve the structural, electrical and optical behavior of thin films. The works have been carried out mainly for ZnO dopants such as Al, Ga, In and Sn. As example of the change in the ZnO properties, we report in Fig. 7.a the dependence of the resistivity with the annealing temperature of pure, Al and In doped ZnO films grown by the spray pyrolysis technique. [START_REF] Nunes | Influence of the post-treatment on the properties of ZnO thin films[END_REF] We also report in Fig 7 .b the huge influence of the resistivity with the concentration of dopant (here with Sn ions) in the film allowing large possibilities of optimizations. [START_REF] Bougrine | Effect of tin incorporation on physicochemical properties of ZnO films prepared by spray pyrolysis[END_REF] These studies have shown that the conductivity could reach a maximum value between 10 2 and 10 Ω -1 .cm -1 (Fig. 7.a and 7.b). Obviously, the optical properties are also modified by doping and growth techniques. It was reported that the transmission can varies between 70% and 95% observed for different concentrations (Fig. 7.c) thicknesses and deposition or annealing temperatures. [START_REF] Hijazi | Etude des propriétés physiques et de transport dans des films d'ITO et de ZnO (dopé ou non) pour dispositifs optoélectroniques organiques[END_REF] Originally, for ZnO applications as gas sensors, aluminum and gallium were the most studied dopants and remain the most used today even for other applications because of its availability and its great electrical conductivity. [START_REF] Hjiri | Al-doped ZnO for highly sensitive CO gas sensors[END_REF], [START_REF] Zhang | ZnO-Decorated In/Ga Oxide Nanotubes Derived from Bimetallic In/Ga MOFs for Fast Acetone Detection with High Sensitivity and Selectivity[END_REF] Moreover, the deposition method also influences the choice of dopant specifically at high doping concentration. For example, in films developed by the metallic ions used as dopants are preferably locate at Zn sites in the lattice but the result of growing doped ZnO based TCO by spray pyrolysis technique, give films having more efficient optical and electrical properties with indium and gallium than with aluminum. This can be explained by looking at the ionic radius of each element; the ionic radii of Ga and In are 0.62 Å and 0.80 Å respectively which is close to that of Zn (0.74 Å). Since the doping element replaces a Zn ion, the similarity in these values cause less lattice distortion at high concentration, when compared to the Al doping having 0.53 Å as ionic radius. Within this technique, the lowest resistivity of the doped ZnO film was obtained with In dopant at a concentration of 1 at %, but for a doping concentration above 2 wt%, a weakening of the film properties occurs. [START_REF] Nunes | Effect of different dopant elements on the properties of ZnO thin films[END_REF]

Defect and Diffusion Forum Vol. 417

Focus on Aluminum doped ZnO (AZO)

Despite the better performance of TCOs based on indium and gallium doped ZnO compared to aluminum doping, it is the latter which is the most studied and developed, thanks in particular to its abundance and its comparatively attractive cost. Aluminum doped ZnO (AZO), with its unique electrical and optical characteristics, is the potential candidate to replace the expending ITO and FTO in TCO applications. In the attempt to find a material having comparable properties to that of ITO, AZO appears to be a good candidate since it has a good balance regarding the last six properties listed in Table 5. Whereas, AZO is not poor in completing the first two criteria, ITO has a good balance in fulfilling the first five but it fails with stability, toxicity and cost. [START_REF] Rombach | The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films[END_REF] In this scope, many research work is carried on to sort out limitations of AZO considering different approaches to treat this material. Different approaches are adopted to overcome the limitations in AZO, some researchers changed the doping concentration of Al in ZnO, [START_REF] Prajapati | Effect of Al dopants on the structural, optical and gas sensing properties of spray-deposited ZnO thin films[END_REF] others used different annealing temperatures, [START_REF] Ng | Effects of annealing temperature on ZnO and AZO films prepared by sol-gel technique[END_REF] or annealing duration, [START_REF] Shu-Wen | A Study of Annealing Time Effects on the Properties of Al:ZnO[END_REF] and atmospheres, [START_REF] Lennon | Effects of annealing in N2 on sputtered Al-doped ZnO thin films[END_REF] while others focused on changing the substrate temperature and precursor concentration. [START_REF] Kim | Effect of deposition temperature on the properties of Al-doped ZnO films prepared by pulsed DC magnetron sputtering for transparent electrodes in thin-film solar cells[END_REF] As already mentioned in the first part of this presentation, each of these parameters has a specific influence on the electrical and optical properties of the material.

It was reported that the incorporation of Al dopant enhances the transparency of the deposited film compared to the undoped ZnO (Fig. 8). The transmittance observed to increase up to a certain concentration limit where the transmittance is slightly reduced due to the increase in scattering and absorption photons by the crystal defects and free charge carriers. [START_REF] Babu | Electrical and optical properties of ultrasonically sprayed Al-doped zinc oxide thin films[END_REF] Therefore, the upgrade in transmittance is attributed to the Al-induced crystal defects rather than the reduction of surface roughness. [START_REF] Muiva | Effect of doping concentration on the properties of aluminium doped zinc oxide thin films prepared by spray pyrolysis for transparent electrode applications[END_REF] Figure 8. Transmittance of as deposited AZO thin films. [START_REF] Ravichandran | Properties of Sprayed Aluminum-Doped Zinc Oxide Films-A Review[END_REF] Moreover, the choice of substrate also showed to have an influence on the optical properties of AZO films where glass and quartz substrates showed better optical transparency in the visible range when compared to FTO substrates. [START_REF] Benhaliliba | A comparative study on structural, optical, photoconductivity properties of In and Al doped ZnO thin films grown onto glass and FTO substrates grown by spray pyrolysis process[END_REF], [START_REF] Manouni | Effect of aluminium doping on zinc oxide thin films grown by spray pyrolysis[END_REF] Figure 9. The variation of the sheet resistance of ZnO thin films as a function of Al doping level for as-deposited and annealed films. [START_REF] Ravichandran | Properties of Sprayed Aluminum-Doped Zinc Oxide Films-A Review[END_REF] Besides the good transparency that is a requirement for a material to be used as TCO, conductivity is also considered a crucial property to be considered. This leads the researchers to have to study the impact of different process parameters on the electrical properties of AZO films. Concerning the annealing process of AZO films, a decrease in the resistivity value to half of that of the as deposited films was observed (Fig. 9). An explanation to this phenomenon is given by some studies where they suggest that annealing causes Al atoms to migrate from the grain boundaries to electrically active sites and this leads to an increase in the concentration of free carriers. Nonetheless a certain annealing temperature limit should be respected above which a reevaporation of the film takes place and an increase in resistivity takes place caused by the creation of zinc vacancies or antisite defects. [START_REF] Igasaki | Substrate temperature dependence of electrical properties of ZnO:Al epitaxial films on sapphire[END_REF] Tin oxide (SnO2) and Fluorine doped Tin Oxide (FTO)

General properties of Tin Oxide

There are two main forms of tin oxide: the stannic oxide (SnO2) commonly called tin oxide and the stannous oxide (SnO). The existence of these two oxides reflects the dual valence of tin, with oxidation states of 2+ and 4+. Stannous oxide is not as well characterized as the stannic form of tin oxide. For example, currently, its electronic band gap is not accurately known but lies somewhere in the range of 2.5-3 eV. Thus, SnO exhibits a smaller band gap than SnO2, for which, the band gap is commonly quoted to be 3.6 eV. Also, there are no single crystals available that would facilitate more detailed studies of stannous oxide. Stannic oxide possesses the rutile structure and stannous oxide has the less common litharge structure (tetragonal crystal system). [START_REF] Batzill | The surface and materials science of tin oxide[END_REF] Finally, stannic oxide is the more abundant form of tin oxide Tin oxide is a material having a big technological importance. Thus, SnO2 have been studied for a wide range of applications such as rechargeable lithium batteries, storage of solar energy, and it is the one of technological significance in gas sensing applications and oxidation catalysts. This variety of applications depends on the morphology, phase, crystallinity, and size of the nanocrystals of the material. But in fact, dedicated to various applications, many geometrical structures and forms of tin oxide were produced like spherical particles, thin films, networks of ribbons and sheets.

In ambient conditions and atmospheric pressure, SnO2 crystallizes in tetragonal rutile structure with space group P42/mnm (Fig. 10) where the tin ions are at the center of the octahedral site formed by the oxygen ions. When doping with fluorine, the F ion replaces the oxygen. Nanostructured SnO2-x thin films are usually prepared by chemical vapor deposition and by reactive DC-magnetron sputtering techniques. As for other materials deposited in film shapes, its crystallographic phase will be conditioned by the substrate temperature during the growth process. Thus, for SnO2 films growth by DC magnetron sputtering technique, the pure tetragonal phase at low temperatures (below 90°C) and a mixture of tetragonal and orthorhombic phases at temperatures above was found. In this temperature range, the films present a wider band gap than that of a pure tetragonal phase. [39]

Doping of Tin Oxide

As for previous presented TCO, a great variety of dopants, mainly Al, In, Li was selected for tin oxide for the improvement or specialization of the SnO2 layer properties optimized to specific applications. For TCO applications, the most commonly used dopants for tin oxide are Sb, Cl and F. In the case of Sb doping, antimony (+V) substitutes tin (+IV) ion. For high doping levels, the coexistence of Sb (+V) and Sb (+III) makes the later act as an acceptor by compensation of charge carriers liberated by Sb (+V). In addition, always for high doping levels, the ions mobility decreases attributed to formation of micelles, and, the thin film color changes to darker shades reducing the transparency and leading to a low Figure of Merit. [START_REF] Rey | Etude d'oxydes métalliques nanostructurés (ZnO, SnO2) pour applications photovoltaïques notamment oxydes transparents conducteurs et cellules solaires à colorant[END_REF] Fluorine and chlorine doping share many similarities. Cl and F ions replace oxygen by releasing a charge carrier. The incorporation of these dopants also leads to the formation of uncharged micelles. Unlike Sb doping, the micelles do not carry an electrical charge, which does not cause disturbance of the tin network. In addition, films obtained by doping with chlorine or fluorine have better transparency compared to doping with antimony. Finally, fluorine doping offers an advantage over chlorine doping because of the close values of the ionic rays for fluorine (0.133 nm) and for oxygen (0.132 nm) while the ionic radius of chlorine is 0.181 nm. The consequence is that unlike the incorporation of Cl but also of Sb, the doping F can be incorporated into the tin oxide network without disturbance, thus making it possible to ensure better conductivity.

Focus on Fluorine doped SnO2 (FTO)

Many deposition methods are adopted for the preparation of fluorine doped tin oxide such as chemical vapor deposition, [START_REF] Tavakoli | Fabrication of efficient planar perovskite solar cells using a one-step chemical vapor deposition method[END_REF] sputtering, [START_REF] Banyamin | Electrical and Optical Properties of Fluorine Doped Tin Oxide Thin Films Prepared by Magnetron Sputtering[END_REF] and most commonly spray pyrolysis [START_REF] Aouaj | Comparative study of ITO and FTO thin films grown by spray pyrolysis[END_REF] using tin chlorides as precursors. As expected, the fluorine doping concentration influences the properties of the film. Increasing the F-doping level leads to an increase in the Fermi energy until reaching the conduction band edge. Hence the optical band gap exceeds 4eV and the transparency of the film in the ultraviolet (UV) range is improved.

The effect of F/Sn atomic ratio on the structural, optical and nano-mechanical properties of FTO films grown by Ultrasonic Spray Pyrolysis on glass substrate was recently investigated by Tuyen L et al.. [START_REF] Tuyen | Nanomechanical and Material Properties of Fluorine-Doped Tin Oxide Thin Films Prepared by Ultrasonic Spray Pyrolysis: Effects of F-Doping[END_REF] The films showed tetragonal rutile structure of SnO2 with preferred orientation of (110) and an increased grain size with increasing F/Sn ratio. An important result of their study is the demonstration for the electrical and optical properties of FTO of a critical concentration of the fluorine dopant located approximately at a ratio F/Sn = 0.5 corresponding to a fluorine concentration of 5.1 at% in layer. We present in Fig. 11 the results of their studies of FTO as a function of fluoride concentration. Below the critical concentration, a sharp increase in the concentration of free charge carriers is clearly observed, leading to a decrease in resistivity. However, beyond this critical concentration, there is a decrease in the concentration of charge carriers attributed to the fact that the excess fluorine no longer being able to replace the oxygen ions, it is expelled to segregate at the grain boundaries. Moreover, FTO films are acceptable have interesting transmittance (around 90% in the visible range) below the concentration threshold, whereas it appears to be reduced above the threshold (down to 80% for F/Sn=1). Authors explained this decrease by the scattering effect due to the increase of the surface roughness. In this review, different types of TCO materials such as ITO, ZnO, AZO and SnO2, FTO were discussed and compared. Several works have been presented to show the influence of the nature of TCO materials and structures on PV performance. In addition, various research efforts on the development of TCO materials have been presented, which has many advantages in terms of PV. It was noted that no type of materials can be considered perfect. Finally, the development of TCO layers itself, and the development of PV system are by combining with different research areas such as chemistry, physics, and electronics.

Conclusion

Since their discovery, Transparent Conductive Oxides were the subject of intense studies and researches in view of the diversity of materials, their important features and the multitude of application fields mainly as detectors and as transparent electrodes in various optoelectronic devices and for photovoltaic cells and modules of new generation. After a general presentation of structural, electrical and optical properties of TCO in a first publication, by the same authors, entitled "Transparent Conductive Oxides. Part I. General review of structural, electrical and optical properties of TCOs related to the growth techniques, materials and dopants.", the current one focus on TCO for photovoltaic cells and modules. Among these TCOs, Indium doped Tin Oxide (ITO), Zinc Oxide (ZnO) with its many dopants mainly aluminum (AZO) and tin oxide mainly fluorine doped (FTO), possess excellent transparency combined with high electrical conductivity. They are widely studied and were found to be the most suited for photovoltaic purposes. Due to its advantageous set of functional properties, ITO is still dominating the TCO industry. But a shortage in Indium supply started to emerge few years ago that makes it a rare and expensive material. By else, ITO thin films deposition requires vacuum based techniques and high temperatures, which is considered as a drawback since these conditions are hard to satisfy. In addition to certain risk of device damage, some substrate types are unable to handle such parameters. On the other hand, ZnO-AZO and SnO2-FTO thin films could be deposited by simple techniques with no obligation of high temperature or vacuum chamber which is suitable for many substrates types as flexible substrates.

Figure 1 .

 1 Figure 1. Indium oxide bixbyite structure. (a) Eight oxygen atoms are situated within a compressed octahedral and have six equidistant oxygen atom neighbors. (b), Eight indium atoms are situated at the corners of a highly distorted octahedron. (c), Sn doping sites in In2O3 lattice. [2]

Figure 2 .

 2 Figure 2. (a) Variation of resistivity and band gap as function of substrate temperature for ITO films. (b) Substrate temperature dependence of carrier concentration and Hall mobility of ITO 70 nm thickness. [5]

Figure 3 .

 3 Figure 3. Variation of optical band gap with different substrate temperature for ITO thin filmsdeposited by magnetron sputtering.[START_REF] Ray | Properties of tin doped indium oxide thin films prepared by magnetron sputtering[END_REF]

Figure 4 .

 4 Figure 4. Assumed parabolic band structure of pure In2O3 and mixed In2O3-SnO2 (ITO)compound.[START_REF] Gupta | Band gap narrowing and the band structure of tindoped indium oxide films[END_REF]

Figure 5 .

 5 Figure 5. Optical transmittance spectra of ITO thin films deposited by high frequency magnetronsputtering.[START_REF] Ahmed | The effect of post annealing temperature on grain size of indium-tin-oxide for optical and electrical properties improvement[END_REF]

Figure 6 .

 6 Figure 6. Zinc oxide structure: crystallizes in wurtzite form.[START_REF] Ellmer | Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells[END_REF]

Figure 7 .

 7 Figure 7. (a) Resistivity variation of doped and pure ZnO at different annealing temperatures. [19] (b) resistivity variation of tin doped ZnO at different concentrations.[20] (c) Optical transmission of the ZnO:Sn as function Sn/Zn ratio: (a) 0 at.%, (b) 2 at.%, (c) 4 at.% and (d) 6 at.% [20]

Figure 10 .

 10 Figure 10. Crystal structure of SnO2.[START_REF] Das | SnO2: A comprehensive review on structures and gas sensors[END_REF]

Figure 11 .

 11 Figure 11. (a) F/Sn ratio-dependent resistivity (ρ, black squares), carrier concentration (n, blue triangulars), and carrier mobility (µ, red spheres) of the FTO thin films. (b) Transmission spectra of FTO thin films as a function of F/Sn ratios. The inset in (b) presents the average optical transmittance (%) of the films in the visible range of 400-760 nm. [44]

Table 2 .

 2 Basic properties of zinc oxide.[START_REF] Johari | Study of zinc oxide thin film characteristics[END_REF]

	Parameters	Value
	Bandgap	3.4eV
	Density	5.606g.cm -3
	Crystal structure	Wurtzite, rock salt, zinc blend
	Stable phase at 300K	Wurtzite
	Melting point	1975°C
	Nature of oxide	Amphoteric oxide
	Lattice constants at 300K	a0= 0.32495nm
		c0= 0.52069nm
	Relative dielectric constant	8.66
	Refractive index	2.0041
	Solubility in water	0.16mg 100mL -1
	Intrinsic carrier concentration	10 20 cm -3

Table 3 .

 3 Electric properties of ZnO for different deposition methods.

	Deposition	Temperature	ρ	nV	μ	Ref
	method	(°C)	(Ω.cm)	(cm -3 )	(cm 2 .V -1 .s -1 )
	Spray	350 450	2.70x10 3 2.40x10 -2	2.4x10 15 -	0.34 -	[14] [15]
	Sputtering	150	1.00x10 3	-	3	[15]
	PLD*	400	3	1.6x10 17	12.3	[16]
	ALD**	250	5x10 -2	3x10 19	5.5	[17]

*Pulsed Laser Deposition **Atomic Layer Deposition

Table 4 .

 4 Zinc oxide thin film growth methods and features.

	Deposition method	Large area deposition	Applications
	Magnetron sputtering	Yes	LCDs, solar cells, touch
	deposition		panels
	PLD	no	LEDs
	Molecular beam deposition	No	LEDs
	Spray pyrolysis	Yes	Solar cells
	Chemical vapor deposition	Yes	Solar cells

Table 5 .

 5 Comparative table of ITO and AZO properties required to be used as TCO.

	Property	ITO	AZO
	High conductivity		-
	High carrier mobility		-
	High transparency		
	Suitable refractive index		
	Ease of processing		
	High stability	-	
	Low toxicity	-	
	Low cost	-	

Table 6 .

 6 Tin Oxide properties.

	Property	SnO2
	Mineral name	Cassiterite
	Crystal structure	Tetragonal, rutile
	Space group	P42mnm
	Lattice constant	a=0.474
		c=0.319
	Band gap [eV]	3.6
	Common n-type dopants	Sb, F, Cl

Defect and Diffusion Forum Vol. 417


Maurice Fadel
email: fadel@laplace.univ-tlse.fr

DC microgrids and battery charging

Keywords: Battery charger, SAB converter, Least square method, Power electronics, DC microgrids

This paper presents an SAB converter for connecting a DC network to an electric vehicle battery for the purpose of rapid charging. The chosen topology provides galvanic isolation and uses a resonant circuit operating above the resonant frequency. This paper proposes a simple control law to control the battery charging current by acting on the frequency of the resonant circuit using a VCO. The non-linear behavior of the converter is identified by a least squares procedure and the control law is composed of a linear controller and a non-linearity compensation block. Various simulation tests validate the proposal for a wide range of load currents.

DC grids are multiplying on a large scale and there are more and more features. For electric vehicles, these networks can be combined in an efficient way, especially if we are interested in fast battery charging. Here we present an application that allows a low voltage battery (14v) to be recharged from a more medium voltage source (400v). Fast recharging involves very high currents, so specific converters must be used to ensure good efficiency and flexible adjustment. Many topologies are candidates for this problem but if we seek to increase the efficiency and reduce the volume the solutions based on DAB (Dual Active Bridge) is quickly imposed. Here the function sought being non-reversible we have opted for an SAB (Single Active Bridge) solution. The DAB topology was developed in 1991 [START_REF] Dedoncker | Power conversion apparatus for DC/DC conversion using dual active bridges[END_REF], [START_REF] De Doncker | A three-phase soft-switched high-power-density DC/DC converter for high-power applications[END_REF] and is based on the use of two inverters connected through a high frequency resonant circuit. This topology is characterized by a high efficiency and its reversible character. The output inverter works as a rectifier and it is possible to use only diodes while losing the reversibility to give rise to the SAB. To adapt the output voltage, the resonant circuit can be equipped with a highfrequency transformer, which results in a more efficient solution (Fig. 1). The most commonly used control method for this topology is variable frequency control [START_REF] Steigerwald | High-Frequency Resonant Transistor DC-DC Converters[END_REF], which consists of varying the impedance of the resonant circuit. Depending on the placement of the switching frequency (fs) relative to the resonant frequency (f0), several conduction modes can be obtained. Two conduction modes can be obtained. These are the continuous (CCM) and discontinuous (DCM) conduction modes. When the switching frequency is higher than the resonance frequency (fs>f0), the current does not perform any half-wave during a half period of operation. Therefore, the diodes are always conductive in the secondary and only the continuous conduction mode can be obtained. The output characteristic is quite particular since it evolves according to the operating frequency. For an operation close to the resonance frequency, it is rather like a voltage source. When we move away from the resonance frequency, it becomes more like a current source (Fig. 2). In a conversion chain, this converter can be seen as an impedance, whose value is related to the reactance of the resonant circuit. Thus for high reactance (fs >> f0), the voltage drop is important. Moreover, a possible shortcircuit current at the output of the converter can be limited by increasing the operating frequency. However, the operation with a high ratio between input and output voltages induces a degradation of the power factor in the AC link [START_REF] Marzouk | Considerations to choose an appropriate charger topology for plug-in electric vehicles[END_REF]. The evolution of the current in the resonant circuit is composed of 4 different segments related to the composition of the voltages +/-Von and +/-Vout, Fig. 5. This characteristic is also visible on the output current, Fig. 6. The amplitude of the current in the resonant circuit depends on the working frequency and the impedance of the circuit at that frequency according to a nonlinear law. When operating above the resonant frequency the average output current will decrease as a function of frequency. Thus the control of this converter can be done by acting on the switching frequency of the inverter. To act continuously on the output current, the use of a VCO is very useful.

Converter sizing

The converter is sized to provide a maximum charging current of 160 A at 14 V from a high voltage battery with a minimum voltage of 360 V. Thus the parameters are given in Table 1. )

t c yf t k v d        (1)
The parameter c k represents the sensitivity of the VCO in Hz/V and  the phase at the origin.

Output stage with current filtering

To avoid AC currents flowing through the battery it is necessary to filter the output stage with an LC filter sized to attenuate twice the operating frequency of the resonant circuit [START_REF] Turksoya | Comprehensive overview of the dc-dc converter-based battery charge balancing methods in electric vehicles[END_REF]. Thus we have the following elements: Filtering the output of the inverter is especially important to protect the battery. The frequency of the current is twice the operating frequency and by setting a maximum ripple of e.g. 20%, it is easy to determine the values of the inductance and capacitor, Table 1.

II. IDENTIFICATION OF PARAMETERS CONTROL

For a frequency sweep of u=1.1 to 3 we obtain the following curves Fig. 9, which allows us to construct the following table, Table 2. The simulation is carried out with the PLECS software. Taking into account the resonant circuit model it is easy to show that the relationship between the mean value of the current Is and the reduced frequency u is given by the following relationship: 2 . 1

s ku I u    (2)
Similarly, the time constant for the average current to build up is approximately constant and the control model can be given by the following diagram: The identification of the constant k can be done by a least squares approach and the constant  can be deduced from curve 9.

Identification of gain k

Since the unknown k acts linearly, we can use a least squares method from the measurement table (Table 2).

The evaluation of the average current can be done by a simple filter eliminating twice the operating frequency, fact we can use a low pass filter set at the frequency of 5 kHz.

Calculation of the PI controller

For the determination of the parameters several methods can be used (pole placement, symmetric optimum method, ...) [START_REF] Bacha | General nonlinear control law for DC-DC symmetric switching converters[END_REF]. Here we use a simple frequency approach consisting in imposing a The principle of this anti-windup is to inhibit the integral action when the control is saturated. This is achieved by means of a multiplier associated with a binary variable.

IV. CLOSED LOOP TESTING

The tests are conducted in simulation using the PLECS software. First, we analyze the variables of the oscillating circuit, i.e. the resonant current and the input and output voltages. Fig. 16 shows this evolution for a current reference of 160 A. The variable ss I represents the current at the secondary of the transformer. In Fig. 17 we can follow the current in the battery and its average value. Fig. 16 and Fig. 17 The transformer model here is perfect and assumed to be lossless. Experimental tests will show whether magnetic losses have a significant impact on the behaviour of the converter. For decreasing changes in the reference current the same behaviour is observed, Fig. 20 and Fig. 21. In this study, we are interested in the simple control of an SAB-type converter for battery charging from a DC microgrids. The non-linear behaviour of the converter can finally be represented by a non-linear gain associated with a quasi-constant time constant. This model can then be used in a very simple way using a model inversion procedure associated with a PI-type controller. This controller can also be computed very simply using a dedicated frequency approach to impose a closed loop bandwidth. The simulation results confirm the validity of the approach and the experimental results currently underway will hopefully confirm the validity of the approach.

Figure 1 :

 1 Figure 1: SAB Topology

Figure 2 :

 2 Figure 2 : Output voltage versus Output Current

Figure 3 :

 3 Figure 3: Equivalent Circuit The switching of the bridge rectifier is natural and depends on the sign of the current in the resonant circuit. From Fig.1 it is easy to see that the voltage Vp is represented by m.Vout where m represents the inverse of the transformation ratio of the transformer. Thus the resonant circuit is located between 2 rectangular voltages Von supplied by the voltage inverter and Vp by the bridge rectifier. The switching of the bridge rectifier is natural and depends on the sign of the current in the resonant circuit. The two source (Von and Vp) are shifted by an angle link to the frequency of solicitation. The evolution of fundamental variables such as the current in the resonant circuit and the average output current can be studied with the following equivalent diagram [5]:

Figure 4 :

 4 Figure 4: Equivalent bloc diagram

Figure 5 :

 5 Figure 5: Waveforms resonant circuit for fs=1.2*fo

Figure 6 :

 6 Figure 6: Waveforms Output Current for fs=1.2*fo

Figure 7

 7 Figure 7: VCO configuration

Figure 8 :

 8 Figure 8: Output Stage with filter

Figure 9 :

 9 Figure 9: Evolution of output current versus u

Figure 10 :

 10 Figure 10: Model for converter control

Figure 11 .

 11 Figure 11. Identification of gain k

Figure 12 :

 12 Figure 12: Analyze of time constant  III. INVERTER CONTROL LAW In order to control the average current in the battery the chosen control law is based on a simple PI regulator and for this it is necessary to linearize the operation of the converter. The linearization is done by model inversion. Indeed, the behavior can be represented by the juxtaposition of a linear block and a non-linear block, Fig. 13. It is therefore necessary to invert the non-linear block by setting up a block representing the reciprocal function 1 () fu  .

Figure

  Figure 13: Control law diagram

  bandwidth in closed loop by defining a cutoff pulse of the corrected open loop. The diagram in Fig.14 explains the reasoning behind the asymptotic gain Bode diagram. It is therefore easy to deduce the parameters of the controller according to the desired bandwidth bp  determined according to the lowest operating pulsation, i.e. the resonance pulsation 0  .

Figure 14 :

 14 Figure 14: Gain curve of the Bode asymptotic diagram For a bandwidth of 5000 Hz we obtain the following parameters: 2. .5000 rd/s bp  

Figure 15 :

 15 Figure 15: Anti-windup procedure

Figure 16 :

 16 Figure 16: Evolution of resonant circuit variables

  show a controlled operation of the inverter with the charging current installed in the battery as specified.

Figure 17 :

 17 Figure 17: Evolution of output currentWe can now look at the transient regime, for example the transition from a reference current of 90A to a reference of 160A, Fig.18and Fig.19.

Figure 18 :

 18 Figure 18: Evolution of the output current at step load from 90 A to 160 A

Figure 20 :

 20 Figure 20: Evolution of the output current at step load from 160 A to 80A

Figure 21 :

 21 Figure 21: Evolution of the reduced frequency u

TABLE 1 .

 1 Converter Parameters

	Components		Value
	Lr		25 µH
	Cr		250 nF
	Ratio of transformer	m=1/21
	Ls		0.1 µH
	Cs		960 µF
	With the parameters given in table 1 the resonance
	pulsation is	0	5

TABLE 2 .

 2 Evolution of output current versus u

	Value of u	Average Value of Is ( A)
	1.1	1185
	1.3	458
	1.5	279
	1.7	201
	1.9	158
	2.1	132
	2.3	113
	2.5	99
	2.7	88
	2.9	80
	3	76


Christelle Habis
email: christelle.habis@univ-lorraine.fr

Jean Zaraket
email: bjean.zaraket@univ-lorraine.fr

Michel Aillerie
email: cmichel.aillerie@univ-lorraine.fr

Transparent Conductive Oxides. Part I. General Review of Structural, Electrical and Optical Properties of TCOs Related to the Growth Techniques, Materials and Dopants

Keywords: Transparent Conductive Oxides, Growth techniques, Dopant, Temperature effect, Electrical and optical properties of TCO

This contribution, based on literature review, presents a general overview of properties and applications of transparent conductive oxides, TCO. The requested properties of TCO are a high conductivity associated to high transparency in a wide wavelength range. The relation between the techniques used for deposition or growing TCO and the possibility of their doping with appropriate dopant and concentration were discussed relatively to their applications and properties. Thus, we present in this part, the various possible techniques for growing TCO and discuss the effect of the temperature on the functional properties. This self-consistent presentation is also considered to introduce a better understanding of the expected requirements for TCO integrated as transparent electrodes in photovoltaic cells and modules. These developments of TCO for photovoltaic applications will be presented in a second publication.

Introduction

The original known discovery of Transparent Conductive Oxide, TCO was in 1904 when Badeker [START_REF] Badeker | Concerning the electricity conductibility and the thermoelectric energy of several heavy metal bonds[END_REF] observed an oxidation of Cadmium (Cd) metal thin films deposited in a glow discharge chamber. He observed that after oxidation, the Cd films became transparent while conserving their electrical conductivity. Ever since, the value of these films grew commercially and the list of TCO expanded to other oxide materials as ZnO, TiO2, GdInOx, SnO2, In2O3, pure or doped with elements as Fluorine (F) or Aluminum (Al), and many more. [START_REF] Minami | Transparent Conductive Oxides for Transparent Electrode Applications[END_REF] Currently, at the date of this contribution, the properties of TCO layers and their optimization remain a subject of intense research all around the word and the number of publications concerning TCO continues to grow to reach 250,000 (Google Scholar, 2021) publications in rank A journals. This enthusiasm is justified by the unique properties of these TCO layers which are directly linked to their transparency and conductivity. TCO thin films have a wide range of applications including transparent heaters, heat mirrors, smart windows, and optoelectronic devices in particular the market for transparent electrodes used in touch panels, flat panel display or lamps, and thin film solar cells which is still expending linked to the increase of new emerging solar cells technologies and devices as organic and perovskite ones. [START_REF] Lewis | Applications and Processing of Transparent Conducting Oxides[END_REF] Based on the precursor tin oxide, SnO2, applications can be classified into two dominant markets: architectural applications such as energy efficient windows which currently, employ fluorine doped tin oxide, FTO, deposited by pyrolytic process, thanks to its efficiency in preventing radiative heat loss due to tin oxide low emissivity. Such coated windows are ideal for low temperature applications as, for example, glass freezer doors or for airplane cockpit windows. The second current application domain of TCO is flat panel applications such in the case of PV modules, touch screens and plasma displays which mainly employ indium doped tin oxide, ITO as external surface transparent electrodes. [START_REF] Ginley | Transparent Conducting Oxides[END_REF] Beside ITO, zinc oxide (ZnO) is also a major material used for such applications.

These last decades, an emergent third market is linked to the development of renewable energies and more specifically to the production of electricity from photovoltaic, PV solar cells, modules or panels. The resulting increase in the need for PV panels, flat or not, rigid or more recently flexible, leads to technological new developments with the development of thin-film PV panels requiring the replacement of the surface electrodes made up of metal ribbons with transparent electrodes and therefore based on TCO layers. Among all studied materials, ITO was the most employed for TCO applications, mainly for large screens due its advantageous and unique set of electrical and optical properties. Moreover, as ITO is a non-friendly material, over the years and with the increasing demand, the conception of employed TCO started to change. The change in the basic material opens the way to overcome limitations and improve the devices performance. Indeed, the urgent need of new materials for TCOs allowing the development of large area and/or flexible photovoltaic cells and display devices is accompanied by the need to increase their conductivity and transparency, which is made possible by a better understanding of the relationship between their structure and their electrical and optical properties.

It is in this context that we are carrying out this review, more specifically focus on applications of TCOs as electrodes for photovoltaic applications. Of course, as we have specified above, with the huge number of publications and work present in the literature on TCOs, half of which relate to their use as electrodes in PV applications, with in addition publications presenting some given contradictory results, it is not possible to make an exhaustive review of the properties of TCO materials and layers. It is for this reason that this contribution based on results validated in several publications, has a first main objectives to introduce a general review of structural, electrical and optical properties of TCOs related to the growth techniques, materials and dopants. This contribution as done in the current publication is the first part of a complete presentation done in two relative publications considered as a whole. Thus, the second contribution of this review on TCO is presented in a second publication title: "Transparent Conductive Oxides. Part II. Specific focus on ITO, ZnO-AZO, SnO2-FTO for photovoltaic applications". It corresponds to the second main objective dedicated to the presentation of the three main families of transparent conductive materials, ITO, ZnO-AZO, SnO2-FTO, used as electrodes in PV cells and modules. Based on the presentation done below in the current Part. I, we will consider in the second contribution the possible growth methods, the specific dopants and the main electrical and optical properties of these TCO for PV applications. The properties of these materials are finally compared in the final discussion.

General Properties of TCO and their Relevance to some Applications

The requirements for TCO are good electrical conductivity and high transparency in the visible spectrum of light. These properties are achieved in wide band gap oxides made degenerate by the introduction of dopants. [START_REF] Lewis | Applications and Processing of Transparent Conducting Oxides[END_REF] However, a lot of parameters influence the choice of a TCO depending on the application to which it is intended to be used for. These parameters include stability (physical, chemical, thermal), etchability, conductivity, plasma wavelength, thickness, deposition temperature, uniformity, toxicity and cost. Table 1 shows the most important property of each transparent conductive material studied in various works around the word. Hence, for a given application, performance is defined by a number of electrical and optical but also by physical and chemical properties. Generally, a compromise must be found by the manufacturers and designers of systems between all these properties in order to find the best material allowing to obtain the electrode adapted to the intended application. As an example, low emissivity windows reflect infrared radiation for wavelength longer than the plasma wavelength by free electrons. For cold weather, the plasma wavelength should be fairly long in order to transmit most of the solar spectrum into heat inside the substrate or the background volume. For this application FTO is the best material because it has the convenient wavelength along with excellent durability and low cost. On contrary, for hot weather, a short plasma wavelength is better so that the near IR is reflected out of the substrate. For this application silver and titanium nitrides are suitable, since silver has poor durability, it is sealed inside double-glazed panes for protection against moisture and air.

Electrical and Optical Properties of TCO

For the transparent conductor to be effective, it should have a high electrical conductivity along with low absorption of visible light. [START_REF] Gordon | Criteria for Choosing Transparent Conductors[END_REF] The first interest to control the charge carriers concentration and mobility in the TCO film is to offer the possibility of an optimization of its electrical properties. This possibility is closely related to the intrinsic material of the TCO and the type and concentration of dopant. Indeed, the conductivity of thin film is highly dependent on the carrier concentration and mobility which are related by the following relation:

𝜎𝜎 𝑛𝑛,𝑝𝑝 = 𝑛𝑛𝑛𝑛𝑛𝑛 (1)
Where n is the density of electron (hole) in the conduction band (valence band), q is the charge of the electron (1.6x10 -19 ) and 𝑛𝑛 is the mobility of charge carriers. It can therefore be seen that to become conductive, the films must either deviate from stoichiometry or by intrinsic doping such as oxygen Defect and Diffusion Forum Vol. 417 vacancies, or more generally, be doped with appropriate external impurities to form an extrinsic doping.

According to eq. 1, the possibility of increasing the conductivity of the films consists either in improving the concentration of carriers by doping, or in improving the electronic mobility μ.

Depending on material or on type of dopant, the doping can be realized by substitution, vacations or interstitial implantations. Considering the valence of the dopants or of the type of implantation on acceptor or donor sites, the doping will induce n or p type conductivity. Thus, with the TCO presented in this contribution, the doping can be done by substitution of cation (metal ion) or anion (oxygen).

However, the increase in the dopant concentration is limited by the fact that at high concentration, the ionized dopant atoms can play the role of diffusion centers reducing the mobility of electrons but also can deteriorate the optical properties by an increase of the absorption of the film at long wavelengths (near infra-red). Thus, as far as possible, it is preferred to improve the conductivity by increasing the electronic mobility 𝑛𝑛. Grain boundary scattering and ionized impurity scattering are generally considered to be the two main interacting mechanisms in conductivity. These two phenomena will then depend directly on the quality of the layers, on their thickness and therefore also on the growth or deposition methods. In particular, Minami et al. [START_REF] Minami | New n-Type Transparent Conducting Oxides[END_REF] for ZnO films obtained by different methods, and Rey et al. for FTO films have shown that the conductivity is primarily affected by grain boundaries scattering in thin films while for thicker films the ionized impurities scattering dominates. [START_REF] Rey | Electron scattering mechanisms in fluorine-doped SnO2 thin films[END_REF] Direct bandgap and high exciton energy are basic requirements for materials to be used in optical emitting devices. Whereas for TCO to be used as electrodes in solar cells, they have to exhibit a minimum carrier concentration of the order of 10 20 cm -3 and a band gap greater than 3.2eV to avoid absorption of light over most of the solar spectra. [START_REF] Minami | Transparent conducting oxide semiconductors for transparent electrodes[END_REF] The Hall Effect measurements are used to determine the electrical properties of TCO thin films. This characterization method permits to find the resistivity, charge carrier density and mobility. In the optoelectronic field especially for photovoltaic devices, it is preferable to have low resistivity to facilitate current transport.

The second interest to the doping of TCO concerns its optical properties. The first concerned optical effect is related to the Burstein Moss effect which shifts the transparency limit of a semiconductor towards short wavelengths with the increase in energy required to produce an interband electronic transition. [START_REF] Burstein | Anomalous Optical Absorption Limit in InSb[END_REF] However, the increase in the density of carriers has the effect of increasing the Fermi energy and therefore of decreasing the transparency limit when the doping level increases. Nevertheless, corresponding to the second optical effect, the doping of TCO will strongly influence the interaction of photons and excitons. Below the higher wavelength limit of the transparency band (called plasma wavelength), but above the lower transparency limit, TCO is transparent and has a dielectric type behavior. Beyond the plasma wavelength, it behaves like a metal and absorbs or reflects light. The effect of the doping is then a shift in the plasma wavelength and therefore a reduction in the transparency of the TCO in the infrared. A compromise must then be found for the TCOs used in photovoltaic applications as a function of the spectral response of the absorber. Thus, in general, with TCOs whose plasma wavelength is typically between 1 and 2 µm, the type of oxide and its doping are optimized to have a transparency range ranging from near UV to near infrared depending on the material used and its doping.

To compare the performance of transparent conductive materials, TCM quantitatively, a pertinent figure of merit, FoM is defined as the ratio of the electrical conductivity (𝜎𝜎) on the visible absorption (𝛼𝛼) and is given by Eq.2

𝜎𝜎 𝛼𝛼 = -[𝑅𝑅 𝑠𝑠 𝑙𝑙𝑛𝑛(𝑇𝑇 + 𝑅𝑅)] -1 (2)
where 𝑅𝑅 𝑠𝑠 is the sheet resistance in ohms per square, T the total visible transmission, and R is the total visible reflectance. A larger value of FoM indicates better performance and quality of the TCM. [START_REF] Ginley | Thin films for photovoltaic and related device applications[END_REF] Another expression of FoM was proposed by Jain et al. [START_REF] Jain | Indium-Tin-Oxide transparent conducting coatings on silicon solar cells and their 'figure of merit[END_REF] but without taking into account the reflection. It will not be considered here. FTO Ultrasonic Spray Pyrolysis 0.9 [START_REF] Benhaoua | Effect of film thickness on the structural, optical and electrical properties of SnO2: F thin films prepared by spray ultrasonic for solar cells applications[END_REF] Table 2 indicates that for the case of zinc oxide and tin oxide, doping with fluorine gives better material performance compared to when doped with metallic elements. This difference could be explained by the conduction band in oxide semiconductors being derived from metal orbitals. A metal dopant is electrically active when it substitutes for the primary metal (zinc or tin). In this way, the conduction band receives a strong perturbation from each metal dopant, the scattering of conduction electrons is enhanced and thus the mobility and conductivity are decreased. In contrast, fluorine substitutes for oxygen. which confining the electronic perturbation to the filled valence band and minimizing the scattering of conduction electrons. The bulk properties of TCMs such as conductivity and absorption depend on the crystallite grain size which increases with increasing film thickness. Hence, one should note that as the electrical and optical properties depend on the film thickness, FoM also dependent on it and will increase with increasing thickness.

It is to be noted that the FoM is not the only qualification criteria occurring in the development of TCO on an industrial scale. All the production processes need to be considered from the origin, the chemical and toxicity of the constituting elements and the environmental aspects for the production and treatment at the end of life. From the industrial point of view, currently the most used material is indium tin oxide (ITO). Despite its flaws imperfections and instability of purchasing price, ITO is still dominating the transparent and conductive oxide industry. As a result, effort is made in order to find a competitive transparent material having similar or overpassing performance. Indeed, Indium is a rare element on earth, which is found in small quantities in the zinc, and lead mines. Indium is then produced in much smaller amount compared to zinc and tin. Due to its rarity and the world increasing demand from the photovoltaic panel industry, its price has incredibly risen. For this reason, a renewed interest has grown in finding replacements for ITO. With today's environmental challenges, using natural and renewable resources or abundant elements became prior for sustainable development of efficient TCO and systems. [18]

Doping of Transparent Conductors

In this part, we focus on the influence of the type of extrinsic dopants on the properties of TCO and discuss the two types of doping, i.e. n and p doping. As previously shown a double interest exists in the doping of a TCO allowing respectively improving optical and electrical properties. Electronic, chemical and physical parameters of the components such as the size of the dopant ion and growing Defect and Diffusion Forum Vol. 417 process and technique parameters such as the solid solubility of the dopant in the host material network will also influence the possibility of doping and the final quality of the film. We report in Table 4 the ionic radius of the main elements considered in TCOs either as constituent elements of pure TCMs or as dopants, it is obvious that the substitution will be facilitated by ions having similar diameters. Thus, we will see below that the size of the different ions involved in the doping process will be taken into consideration in the choice of the dopant in addition to the consideration of the electronegativity, the crystal structure and other optical and electronic parameters of host and dopants.

N-type doping

Many doping are based on cation substitutions for example, indium oxide can be doped with transition metal ions as molybdenum, titanium or tin ions giving the well-known indium tin oxide, ITO intensively used as TCO for photovoltaic cells of new generation. Zinc oxide ZnO is currently doped with metallic elements such as Al, Ga, In [START_REF] Asemi | Preparation of highly conducting Al-doped ZnO target by vacuum heat-treatment for thin film solar cell applications[END_REF]- [START_REF] Chava | Improving the photovoltaic conversion efficiency of ZnO based dye sensitized solar cells by indium doping[END_REF] and tin oxide SnO2 with elements such as Sb, Nb, or metal ions such as Cu, Fe and Co. [START_REF] Zhou | Doping effects in SnO2 transport material for high performance planar perovskite solar cells[END_REF]- [START_REF] Anaraki | Low-Temperature Nb-Doped SnO2 Electron-Selective Contact Yields over 20% Efficiency in Planar Perovskite Solar Cells[END_REF] The list of dopant candidates presented above, as example is not exhaustive and there is a great deal of works on many kinds of doping. Moreover, co-doping of TCO was already studied to enhance some specific properties and results can be found in literature, such as for the Al-Ti co-doping of ZnO. Doping TCM by substitution of the oxygen anion is rare. However, this substitution process is still applied for SnO2 with fluorine dopant substituting oxygen. All dopants mentioned above are associated with the creation of an intermediary energy level under the conduction band of the host. The increase in dopant concentration develops an energy band overlapping the conduction band which reinforce the n type nature of the host semiconductor TCOs. Thus, compared to a pure undoped TCO, a larger number of electrons will participate to the circulation of the charges yielding to an increase in conduction by doping.

P-type doping

All the TCOs mentioned above are n-type semiconductors and the interest of p-type doping for TCO applications currently remains discussed and controversial. In fact, in the literature, the difficulty of creating transparent conductive oxides of p-type has been attributed to the significant localization of the holes in the oxygen levels and to the ionicity of the metal atoms. As a result, these holes are confined and require a considerable amount of energy to overcome a large barrier height to migrate inside the crystal lattice, causing poor hole mobility and conductivity. [START_REF] Sato | Transparent conducting p-type NiO thin films prepared by magnetron sputtering[END_REF], [START_REF] Kawazoe | Transparent p-Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions[END_REF] Despite that, the first attempt to make a p-type TCO was by Sato et al [START_REF] Sato | Transparent conducting p-type NiO thin films prepared by magnetron sputtering[END_REF] in 1993. They reported a p-type semitransparent TCO (40% visible transmission) and the possibility to form acceptor levels through doping in ZnO. It was then until 1997 when Kawazoe et al [START_REF] Kawazoe | P-type electrical conduction in transparent thin films of CuAlO2[END_REF] reported the properties of an oxide material (CuAlO2) that combines p-type conductivity up to 1 S.cm-1 with good optical transparency. According to the authors, this value is not comparable to the best n-type conducting oxides, but is sufficient for some applications. In recent years, more and more experimental works have focused on thin films of p-type TCO more specifically on ZnO doping with Ga, N, F, P, Sb and As. Even though some research teams obtained ZnO:Sb having a conductivity of 0.5 S/cm, mobility of 9-20 cm 2 /Vs and a hole density of 4.10 16 cm -3 , with transmission of 85%, [START_REF] David | Electro-optical and structural properties of thin ZnO films, prepared by filtered vacuum arc deposition[END_REF] the challenge to grow p-type TCO having a resistivity in the order of 10 -3 Ω.cm or better still exists.

Development and Growth of TCOs

In this section, we highlight a variety of used methods for the deposition of oxide thin films. These different deposition conditions and methods affect significantly the optical and electrical properties of ITO thin films. We finally discuss the link between the process temperature and the stability and properties of the TCO.

Generalities of growth techniques

The first developments of TCO layers on a substrate such as glass, silicon or photovoltaic cells were low-cost techniques of liquid chemistry, such as electrochemical deposits [START_REF] Wellings | Growth and characterisation of electrodeposited ZnO thin films[END_REF] hydrothermal growth [START_REF] Sahoo | Hydrothermal growth and characterization of ZnO thin film on sapphire (0001) substrate with p-GaN buffer layer[END_REF] or even deposition by the Sol-gel process. [START_REF] Znaidi | Sol-gel-deposited ZnO thin films: A review[END_REF] In order to improve the control of the growth process and therefore the quality of the layers, the first methods of producing TCO layers on a substrate by sputtering [START_REF] Takayanagi | Effect of metal mode and oxide mode on unusual c-axis parallel oriented ZnO film growth on Al/glass substrate in a reactive magnetron sputtering of Zn target[END_REF], [START_REF] Gao | ZnO thin films produced by magnetron sputtering[END_REF] and spray pyrolysis were introduced in the 1970s. The spray pyrolysis is also called atmospheric pressure deposition or aerosol-assisted chemical vapor deposition technique. More sophisticated techniques have emerged in recent decades, such as pulsed laser assisted deposition [START_REF] Park | Structure and properties of transparent conductive doped ZnO films by pulsed laser deposition[END_REF] or molecular beam epitaxy. [37] The development of these latter methods were motivated by the large-scale development of commercial TCOs requiring better control of reproducibility while improving the crystallinity of the layers, or even obtaining monocrystalline films and allowing the integration of dopants.

Furthermore, it should be noted that the development of nanostructured layers based on nanoparticles, nanowires or nanofibers have been recently developed to respond either to an improvement in the electrical and optical performance of TCO layers and / or to new applications such as for example photovoltaics on a flexible substrate. Some of the techniques mentioned above can also be used for the growth of nanostructured layers such as the deposition technique in a chemical bath, or in the vapor phase, others are then more specific such as for example the electro-spinning technique (not discussed in this article) used, for example for the development of ZnO nanofibers.

In the following part, we describe the main deposition and growth techniques of TCO films, illustrated by various examples relating to particular TCO films, in order to highlight the experimental and growth considerations to be taken into account specifically.

Sputtering

Sputtering is a widely used technique for the deposition of TCO thin films. Sputtering includes a variety of techniques that can be used in a wide range of temperature such as radio frequency (RF) or direct current (DC) sputtering, or at only high temperature as magnetron reactive and ion beam sputtering. Out of the mentioned techniques, RF and DC were found to be interesting for the industrial field since they offer high deposition rates, reproducibility and large area sputtering systems. But, the major drawback of RF deposition at room temperature is the poor electrical and structural properties since the temperature is not enough for the crystallization of ITO. Additionally, ITO films deposited by DC magnetron sputtering at room temperature, are most of the time amorphous with high electrical resistivity as reported by Quaas et. al. [START_REF] Quaas | Investigation of diffusion and crystallization processes in thin ITO films by temperature and time resolved grazing incidence X-ray diffractometry[END_REF] and Teixeira et. al. [START_REF] Teixeira | Amorphous ITO thin films prepared by DC sputtering for electrochromic applications[END_REF] The deposition temperature of ITO films by magnetron sputtering ranges from room temperature to 500°C [START_REF] Kurdesau | Comparative study of ITO layers deposited by DC and RF magnetron sputtering at room temperature[END_REF], [START_REF] Jung | Development of indium tin oxide film texture during DC magnetron sputtering deposition[END_REF]. Nevertheless, solar cells or electronic systems on plastics are types of substrates or applications requiring a low deposition temperature because higher temperatures could directly damage them or even damage the structure of the electronic device. [START_REF] Liu | Photoelectric properties of ITO thin films deposited by DC magnetron sputtering[END_REF] Figure 1. Variation of resistivity and carrier concentration in ITO layers with temperature grown by RF magnetron sputtering . [START_REF] Chityuttakan | Influence of deposition parameters on the quality of ITO films for photovoltaic application[END_REF] As an example, we report in Fig. 1 the evolution of the resistivity and of the carrier concentration with the substrate temperature during the RF magnetron sputtering process made in discharge plasma modes. We can see at first, that resistivity and carrier concentration are inversely correlated.

Electron beam and direct thermal evaporation

Electron beam and direct thermal evaporation methods use powders of metallic Sn and In for the growing of ITO films. Indium will be favorably vaporized altering the composition of alloys caused by the higher vapor pressure of In relative to Sn. Diniz et al. [START_REF] Diniz | The effects of various annealing regimes on the microstructure and physical properties of ITO (In2O3:Sn) thin films deposited by electron beam evaporation for solar energy applications[END_REF] investigated the effect of various post deposition heat treatments on the microstructure, electrical and optical properties of In2O3:Sn (ITO) thin films deposited by electron beam evaporation. The thin films were deposited on microscope glass substrates and silicon single crystal wafer. After deposition, the samples were annealed in air for 30 minutes at temperatures of 200, 250, 300, 350, 400, 450, 550°C. They observed that at low temperature, the film was amorphous while the crystallite size remarkably increases with the temperature. It was noted that for a temperature of 300°C the average grain size was about 10 to 25 nm, whereas it increases to reach 15 to 50 nm for higher temperatures. This link between temperature and structure was also reported in literature and is attributed to the fact that for a given driving force, the migration rate increases as the temperature increases. [START_REF] Rauf | A comparative study of microstructure (in ITO films) and techniques (CTEM and STM)[END_REF], [START_REF] Diniz | The effects of postdeposition annealing on the microstructure of electron-beam evaporated indium tin oxide thin films[END_REF], [START_REF] Ahmed | The effect of post annealing temperature on grain size of indium-tin-oxide for optical and electrical properties improvement[END_REF] Figure 2. a) variation of electrical resistivity and carrier concentration as function of annealing temperature for ITO films. b) optical transmission curves for a series of ITO/glass films annealed at different temperatures.

Fig. 2.a summarizes the electrical properties (resistivity and carrier concentration) of the ITO films at various temperatures. At the annealing temperature of 300°C a sharp decrease in film resistivity as well as an increase in carrier concentration are observed. The increase in film resistivity as we go further in annealing temperature is explained by the diffusion of more oxygen into the film that contribute to the decrease in the concentration of oxygen vacancies. This reduces the carrier concentration and hence the film resistivity. It is noted that optical transmission increases with increasing annealing temperatures since one of its effect is film crystallization (Fig 2 .b). Many earlier works explained the decrease of transmission for λ<400 nm by the absorption of ITO band gap which may vary in the range of 3.5 eV < Eg < 3.8 eV. [START_REF] Hamberg | Evaporated Sn-doped In2O3 films: Basic optical properties and applications to energy-efficient windows[END_REF], [START_REF] Zhao | Indium tin oxide thin films by bias magnetron rf sputtering for heterojunction solar cells application[END_REF] Higher annealing temperatures resulted in improvement in optical transmission is a result of the transition from amorphous to microcrystalline then polycrystalline. This transition reduces the disorder level and assists the incorporation of tin atoms into the indium oxide lattice so that Eg becomes more defined.

Spray pyrolysis

In the spray pyrolysis (already mentioned above and also called atmospheric pressure deposition or aerosol-assisted chemical vapor deposition technique), a chemical reaction of gaseous species takes place with a heated substrate surface. The advantages of this technique, which has become quite common for the development of electrodes, is the relatively low cost of the deposit frame, the low temperature used for the deposit allowing the deposit to be deposited on various substrates, and the possibility of depositing over large surfaces. For the experimental point of view, an additional advantage is that it enables the production of coatings having good properties on different substrate shapes with no need of vacuum use. Moreover, it is considered to be relatively cheap with respect to the apparatus. But for ITO films the main drawback is the lack of volatile and thermally stable material source. Numerous research for ZnO thin films deposition on glass substrates by the spray pyrolysis technique was made in the purpose to investigate the change in properties of the films with varying deposition parameter. ZnO thin films deposited on glass substrates with different thickness and different deposition temperature revealed the dependence of the resistivity on the substrate temperature and spraying rate (Fig. 3). In addition, the substrate temperature during the deposition process showed an effect on the film thickness; [START_REF] Rao | Effect of thickness on structural, optical and electrical properties of nanostructured ZnO thin films by spray pyrolysis[END_REF] it was found to decrease with increasing temperature (Fig. 4). This decrease is explained by the decrease in mass transport to the substrate at higher temperature and by the increase of reevaporation rate.

Thermal Stability of Transparent Conductors

The deposition temperature is decisive during a deposit because it defines the kinetics of the various reactions between the chemical species present and the deposition regime. It follows a link between this temperature and the thermal stability of the so-grown layer.

Generally, thermal stability of TCM is tested by heating the material to temperature greater than the deposition temperature. Generally, it leads to an increase in the resistance of the TCM if heated for a long period of time. Tests were made and reported in the literature to check the thermal stability of several TCMs and the results are shown in Table 5. The tested materials remained stable with no increase in sheet resistance for temperatures slightly above the deposition temperature. As mentioned in the Introduction paragraph, following this general review of structural, electrical and optical properties of TCOs related to the growth techniques, the materials and their dopants, the three main families of transparent conductive materials, ITO, ZnO-AZO, and SnO2-FTO, used as electrodes in PV cells and modules will be presented in a second contribution.

Conclusion

Since their discovery, Transparent Conductive Oxides were the subject of intense studies and researches in view of the diversity of materials, their important features and the multitude of application fields mainly as detectors and as transparent electrodes in various optoelectronic devices and for photovoltaic cells and modules of new generation. In the current contribution, we have focused on the general link between, the structural, electrical and optical properties from one side and the various possible growing processes, the operating temperature and the effect of the various dopants in term of electronegativity and concentration on the other side. Depending on the intended application and substrate, we point out the importance of choosing a suitable TCO within its specific dopant, and closely link, the choice of a appropriate growth technique at specific temperature. Thus, the presentation is based on general considerations illustrated by examples of various TCO. This publication is the first part dedicated to general considerations of a review work on transparent conductive oxides. The second part will be more dedicated to specific families of TCO integrating photovoltaic cells and modules. It will be presented by the same authors in a second linked publication entitled "Transparent Conductive Oxides. Part II. Specific focus on ITO, ZnO-AZO, SnO2-FTO for photovoltaic applications". It will covered the ITO, ZnO-AZO and SnO2-FTO TCO families.

Defect and Diffusion Forum Vol. 417

Figure 3 .

 3 Figure 3. Dependence of the resistivity of ZnO films on the substrate temperature. [50]

Figure 4 .

 4 Figure 4. Dependence of film thickness on the growth temperature. [51]

Table 1 .

 1 Summarizing table of the most important property for different Transparent Conductive Materials (TCM).[START_REF] Gordon | Criteria for Choosing Transparent Conductors[END_REF]

	Property	Material
	Highest transparency	ZnO:F, Cd2SnO4
	Highest conductivity	In2O3:Sn
	Lowest plasma frequency	SnO2:F, ZnO:F
	Highest plasma frequency	Ag, TiN, In2O3:Sn
	Highest work function, best contact to p-Si	SnO2:F, ZnSnO3
	Lowest work function, best contact to n-Si	ZnO:F
	Best thermal stability	SnO2:F, TiN, Cd2SnO4
	Best mechanical durability	TiN, SnO2:F
	Best chemical durability	SnO2:F
	Easiest to etch	ZnO:F, TiN
	Best resistance to H plasmas	ZnO:F
	Lowest deposition temperature	In2O3:Sn, ZnO:B, Ag
	Least toxic	ZnO:F, SnO2:F
	Lowest cost	SnO2:F

Table 2 .

 2 Table 2 lists the FoM for different TCO materials. Figure of Merit for some transparent conductors. [6]

	Material	Deposition method	FoM (x10 -2 Ω -1 )	Ref
	ITO	Commercial	5.9	[13]
	ITO	Atomic Layer	1.3	[14]
		Deposition
	AZO	Pulsed DC magnetron	1.99	[15]
		sputtering

Table 3 .

 3 Abundance of Indium, Tin and Zinc in ppm.[17]

	Element	Abundance (ppm)
	Indium (In)	0.049
	Tin (Sn)	2.2
	Zinc (Zn)	75

Table 4 .

 4 Table representing the ionic radii of the elements used as TCO. [19]

	Element	Ionic radius (Å)
	Oxygen (O)	1.40
	Zinc (Zn)	0.74
	Tin (Sn)	0.69
	Fluorine (F)	1.33
	Aluminum (Al)	0.53
	Chlorine (Cl)	1.81
	Gallium (Ga)	0.62
	Antimony (Sb)	0.76
	Indium (In)	0.80

Table 5 .

 5 Thermal stability of some transparent conductors. (LPCVD and APCVD are Low and Atmospheric pressure CVD, respectively).[START_REF] Gordon | Criteria for Choosing Transparent Conductors[END_REF]

	Deposition method and	Deposition temperature	Stability temperature
	material	(°C)	(°C)
	LPCVD ZnO:B	200	<250
	APCVD ZnO:F	450	<500
	APCVD SnO2:F	650	<700

Defect and Diffusion Forum Vol. 417