Introduction Transparent conducting oxides (TCOs) are a class of materials with numerous applications. What is a transparent conducting

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1 AZO The Replacement for ITO? With applications in electronic screens and displays, LEDs and solar cells, transparent conducting oxides (TCOs) are considered key materials for a range of sectors. These extensive applications take advantage of the unique characteristics of TCOs, which include high electrical conductivity, transparency in the visible range and good chemical and thermal stability. Tin-doped indium oxide (ITO) is currently the most popular TCO, but concerns surrounding the cost and supply of indium have resulted in an increasing effort to find alternatives. The most promising candidate for the replacement of ITO is aluminium-doped zinc oxide (AZO). Fig. 1 Schematic representation from nanostructured AZO powder to solar cells Introduction Transparent conducting oxides (TCOs) are a class of materials with numerous applications. Exhibiting transparency and electrical conductivity even at very narrow thicknesses, TCO thin films are frequently used in the production of optoelectronic devices from conducting glass and touchscreen panels to solar cells. There are significant concerns surrounding the availability and cost of the most frequently used material for the generation Keywords transparent conducting oxides, aluminiumdoped zinc oxide, tin-doped indium oxide of TCO thin films tin-doped indium oxide (ITO). Aluminium-doped zinc oxide (AZO) offers an effective TCO alternative that has a considerably lower price tag with equivalent, and often enhanced properties. Innovnano have developed a range of superior nanostructured AZO sputtering targets that can provide researchers with highly optimised and cost-effective tools for the development of next generation TCO thin films. In addition to enhanced properties, these targets have lower associated production costs, leading to decreased cost of the end products a major advantage. What is a transparent conducting oxide (TCO)? TCOs are based on simple oxides CdO, In 2, SnO 2, Ga 2 and ZnO which are intrinsically or extrinsically doped in order to provide electrical properties similar to metals, whilst also maintaining transparency. Nuno Neves INNOVNANO Antanhol Portugal 62 ceramicapplications 3 (2015) [1]

2 TECHNOLOGY INSIGHTS Fig. 2 Typical point defects observed in: intrinsically doped ZnO a) oxygen vacancy (Vo) and b) Zn 2+ in an interstitial position; and extrinsically doped ZnO, c) Al 3+ occupying a Zn 2+ site in the ZnO lattice The doping procedure is a process whereby impurities are intentionally introduced, in order to modulate the electrical properties and thus establish electrical conductivity as a result of free carriers, in this case electrons. The process of extrinsic doping involves the addition of metal ions with different valences to the crystal lattice structures of the simple oxides in order to form these free carriers. For intrinsically doped TCOs, the electrons come from intrinsic defects, such as oxygen vacancies or interstitial metal cations. In both cases, it is the structural imperfections that give rise to the increased carrier concentrations and consequently electrical conductiv ity [1]. Doped TCOs are deposited as thin films for a wide variety of applications demanding electrical conductivity and optical transparency. A range of deposition techniques can be used for this with sputtering being the most common method. Properties of TCOs TCOs have such varied applications as a result of their extensive array of properties, which can be summarised as: High electrical conductivity (low electrical resistivity) Transparency in the visible range (high levels of transmittance) Ease of formation and deposition Good chemical and thermal stability. One of the key characteristics that define a TCO is high electrical conductivity. TCOs show low electrical resistivity in the range of 10 4 Ω cm (and sheet resistances of approximately 20 Ω sq 1 ), high carrier concentrations of cm 3 and also high carrier mobilities ranging from cm 2 V 1 s 1 [4, 5]. In addition to their good electrical conductivity, TCOs are highly transparent, exhibiting optical absorption coefficients less than 0,1 and with greater than 80 % transparency Tab. 1 Overview of the typical values of INNOVNANO AZO targets, compared with AZO and ITO targets available on the market [4, 8] Powder characterization throughout the visible spectrum when deposited as thin films [6]. As well as these two defining properties, TCOs benefit from good chemical and thermal stability, in Commercial AZO ITO INNOVNANO AZO Particle size [nm] >500 >500 <100 Specific surface area [m 2 /g] Homogeneity Good Good Very good AZO targets production Conformation HP, HIP HP, HIP UP, HP Sintering cycle [ C] Targets properties Density [%] >95 >95 >95 Grain size [µm] <10 <10 <3 Grain size distribution Very good Very good Very good Secondary phase distribution Good Very good Very good Electrical resistivity [Ω cm] < < < Thin films properties Electrical conductivity [Ω cm] Transmittance [%] >80 >80 >80 Resistivity spatial distribution Good Very good Very good ceramicapplications 3 (2015) [1] 63

3 Fig. 3 SEM image of the homogeneous microstructure and reduced porosity of an INNOVNANO AZO target Fig. 4 AZ ceramic sputtering target add ition to ease of formation and deposition. It should also be noted that the electrical and optical properties of sputtered thin films are dependent on the deposition parameters and also the characteristics of the sputtering targets. Grain size, density, oxygen content and homogeneity play particularly important roles in determining the quality of the sputtering target. The past, present and future of TCOs The utility of TCOs is based on their optical transparency in the visible range and high electrical conductivity. It was in 1907 that cadmium oxide (CdO), the first TCO, was produced [2], although it took a further 40 years for TCOs with superior properties to be developed. These materials included SnO 2, In 2 and ZnO and experienced their first application during the Second World War, when SnO 2 was used as a defroster at high altitudes in warplanes [3]. In the following years these early TCOs continued to be developed and used in a range of applications, often replacing metals such as gold and silver. But it was the severe increase in oil prices leading to the search for alternative energy sources, as well as the widespread success of touchscreens and other electronic displays, which indirectly resulted in the extensive and ongoing progress in TCO development. With high optical transparency and electrical conductivity, TCO thin films are ideal for applications in photovoltaics as a component of solar cells, and are also used in a very broad range of products and applications including [3]: Energy-efficient windows Optical microscopes Abrasion- and corrosion-resistant coatings Satellites Gas sensors LEDs Flat panel displays Electrochromic windows Thermoelectric materials. Commonly used TCOs Currently the most extensively used TCO in industry is ITO (In 2 :Sn) as it has exceptional electrical and optical properties with an electrical resistivity of 10 4 and a transmittance greater than 85 %. However, there are considerable issues surrounding the availability and cost of indium, which is a scarce resource in nature that has experienced increased consumption and thus commands a high price on the market. Additionally, the best quality ITO films with the lowest resistivity are usually deposited at high temperatures, which can cause serious problems for applications in temperature sensitive technology such as organic solar cells and LEDs [4]. SnO 2 doped with fluorine or cadmium (SnO 2 :F and SnO 2 :Cd) is also widely used and accounts for the largest quantity (by area) of deposited TCOs. SnO 2 is inexpensive in terms of raw materials and processing, however the conductivity is not as good as for ITO, and fluorine and cadmium are also highly toxic, so have major environmental drawbacks. As with ITO, the best SnO 2 -based materials are also deposited at high temperatures, limiting their use in temperature sensitive technologies [7]. AZO an alternative for ITO The challenge for the future of TCOs is to find low cost and abundant materials with optimised properties that can replace the scarce and expensive option of ITO, which is currently the most popular TCO in the market. The disadvantages and limitations surrounding the most commonly used TCOs have prompted investigations into alternative materials with enhanced properties. The ITO substitute making the most ground and holding the most potential is aluminium doped ZnO (AZO), which is of special interest to the photovoltaic industry with widespread applications in solar cells. A study by P. J. M. Isherwood et al., at the Centre for Renewable Energy Systems Technology (CREST) at Loughborough University found that AZO thin films deposited between room temperature and 300 C have sheet resistances equivalent to or better than ITO thin films at a given transmission value [4]. The excellent sheet resistances and transmission values were also evident across a wide 64 ceramicapplications 3 (2015) [1]

4 TECHNOLOGY INSIGHTS range of deposition temperatures, including room temperature, demonstrating the usability of AZO for temperature-sensitive technologies. The study tested thin films deposited by a process of radio frequency (RF) sputtering using AZO ceramic sputtering targets developed by Innovnano. These targets are unique in the fact that they use an AZO nanostructured powder, synthesised by the proprietary process of emulsion detonation (EDS). In addition to equivalent, and sometimes improved, electrical and optical properties, AZO has further advantages over ITO and other common TCOs, including lower cost, greater abundance, high thermal stability, layer flexibility and lack of toxicity [8]. AZO sputtering targets are therefore ideally poised to replace ITO targets as the TCO of choice in a number of applications, especially those where temperature sensitivity is a consideration. The importance of nanostructure The characteristics of ceramic sputtering targets directly affect the properties of sputtered thin films and so their performance is defined by the quality of the starting materials the ceramic powders. An important consideration when trying to yield ceramic powders of the highest quality, with enhanced properties to be translated to the production of superior thin films, is the manufacturing process. At Innovnano, the proprietary EDS method is used for the synthesis of nanostructured ceramic powders, providing superior powder characteristics. This process uses high temperatures, high pressures and ultrafast quenching for the detonation of an emulsion containing the zinc and aluminium precursors, producing a structure composed of individual and agglomerated nano particles. As nanoparticles, with a very small crystallite size (19 nm [4]), there is a high degree of amorphization and the specific surface area is high, improving the reactivity and sinterability of the powders. A significant and unique aspect of the synthesis process is that it allows for rapid and simple alteration of the powder com pos ition, with the Al-dopant being uniformly incorporated into the ZnO structure to create a homogeneous powder and consequently sputtering targets with a fine and homogeneous microstructure. Fig. 5 AZO material deposited as a back contact in an a-si thin film solar cell The result is that sputtering targets can be manu factured with different dopant levels and therefore tailored towards a wide range of uses and requirements [4]. Quality AZO sputtering targets As well as the characteristics of the nanostructured powders, there is a high correlation between the quality of the AZO sputtering targets themselves and the quality and performance of TCO thin films and the end products. It is therefore critical that the sputtering targets possess the desired structural, mechanical, thermal and electrical properties. At INNOVNANO, AZO sputtering targets are prepared by hot-pressing, with a sintering cycle that involves lower temperatures ( C) up to 250 C lower than normal [8]. These lower temperatures are possible due to the high sinterability of the nanostructured AZO powder, which also permits the formation of highly dense targets (>98 % of theoretical density) [4]. Lower sintering temperatures also min im ise grain growth to maintain a homo geneous microstructure and optimum, smaller grain sizes. These characteristics are demonstrated by TEM and SEM images of AZO targets, which show a compos ition of small, regular crystals of between 0,5 µm and 1 µm. In addition to enhanced properties, the possibility of sintering at lower temperatures leads to lower production costs of the sputtering targets and consequently decreased costs for the end products. Importantly, AZO sputtering targets demonstrate electrical conductivity that is comparable to ITO targets. The study at CREST found that AZO targets produced from the sintering of INNOVNANO s AZO nanostructured powders had excellent electrical properties, with an electrical resistivity of Ω cm, a carrier concentration of cm 3 and a mobility of 39 cm 2 V 1 s 1 [4]. AZO thin films and deposition The enhanced mechanical properties of Innovnano s nanostructured AZO sputtering targets are achieved through homogeneous microstructure, small grain size and reduced porosity. When combined with optimal electrical properties and decreased manufacturing costs, these high quality AZO sputtering targets produce high performance thin films. As well as sputtering target characteristics, the deposition conditions and parameters also influence the properties of AZO thin films, with deposition temperature and film thickness having especially important effects on the electrical properties of the sputtered thin film. Differences in these parameters affect the sheet resistance and result in large variances, ranging from 3 to 35 Ω/sq. Film thickness also has an effect on the optical properties of the film, with thicker films showing decreased transmission across the visible and infrared ranges [4]. Overall, the AZO films deposited from the sputtering targets in the CREST study [4] at temperatures up to 300 C demonstrate equivalent (and often lower) sheet resistances to those seen for ITO. This presents AZO as an ideal TCO substitute for ITO. Case study: AZO in the photovoltaic sector One of the most important TCO applications is in the solar power industry, for ceramicapplications 3 (2015) [1] 65

5 the manufacture of photovoltaic cells. Although c-si (crystalline silicon) solar cells currently hold the largest share of the market in this sector, thin film photovoltaics are becoming increasingly more important, including CIGS (copper indium gallium (di)selenide), CdTe (cadmium telluride) and a-si (amorphous silicon) thin film solar cells. The thin films production market has grown from around 4 % of the global solar photovoltaics market in 2003 to approximately 30 % in 2013 [9] and it is predicted that in the long-term thin film photovoltaic technology will surpass conventional c-si technology [10]. Increases in efficiency and reductions in the manufacturing costs of CIGS and CdTe solar cells especially, have led to them be- coming important players in the photovoltaic market. Thin film solar cells have TCO layers, which are used as front and back contacts, as shown in Fig. 5. AZO provides the necessary properties for these applications high transparency in the visible range of the solar spectrum, high conductivity, lack of toxicity, high carrier mobility, high stability against atomic hydrogen and an appropriate refractive index. Due to their significantly lower costs, AZO thin films offer a highly attractive material for this TCO layer, especially when compared to the more commonly used and expensive option of ITO. In fact, AZO thin films are already used in some CIGS solar cells as a front contact and also as a back contact in a-si thin film solar cells (Fig. 5). Summary AZO provides the best alternative to ITO for a range of applications, but most importantly in the photovoltaic sector, where temperature sensitivity during production can be an important consideration. Nanostructured AZO ceramic powders and sputtering targets developed by Innov - nano provide superior mechanical and electrical properties for the next generation of high performance thin films, with uniform thickness and excellent optical, morphological and electrical characteristics. Importantly, these sputtering targets also offer improved target lifetimes and higher sputtering efficiency, resulting in lower production costs for thin film manufacturers. References [1] Norton, D.P.; et al.: ZnO: growth, doping and processing. Materials Today (2004) [2] Badeker, K.: Concerning the electrical conductibility and the thermoelectric energy of several heavy metal bonds. Ann. Phys. (Leipzig) 22 (1907) [4] [3] Gordon, R.: Criteria for choosing transparent conductors. MRS Bulletin 25 (2000) [4] Isherwood, P.J.M.; et al.: High quality aluminium doped zinc oxide target synthesis from nanoparticulate powder and characterisation of sputtered thin films. Thin Solid Films 566 (2014) [5] Wager, J.; et al.: Transparent electronics. New York 2008 [6] Ingram, B.J.; et al.: Chemical and structural factors governing transparent conductivity in oxides. J. of Electroceramics 13 (2004) [7] Fortunato, E.; et al.: Transparent conducting oxides for photovoltaics. MRS Bulletin 32 (2007) [8] Neves, N.; et al.: Aluminium doped zinc oxide sputtering targets obtained from nano structured powders: Processing and application. J. of the Europ. Cer am. Soc. 32 (2012) [9] Growth opportunities in global thin film solar PV module market : Trend, forecast and opportunity analysis. Published 2012 by Research and Markets [10] Thin-film photovoltaic (PV) cells market analysis to 2020 CIGS (copper indium gallium diselenide) to emerge as the major technology by Published 2010 by GBI Research CERAMICAPPLICATIONS Components f or high performance Issue 2/2015 Circulation at: Hybrid Expo; Stuttgart/DE; MATERIALICA; Munich/DE; Ceramitec 2015; Munich/DE; Aquatech; Amsterdam/NL; EMA 2016; Orlando/US; ICACC; Daytona Beach/US; Editorial Deadline: Contact: Karin Scharrer (k.scharrer@goeller-verlag.de) Editorial Topics: Energy and environmental technology Mechanical and chemical process engineering Bioceramic components and medical devices Electronic and magnetic components Wear and corrosion protection High temperature application Coatings Advertising Deadline: Corinna Zepter (c.zepter@goeller-verlag.de) Isabelle Wilk (i.wilk@goeller-verlag.de) 66 ceramicapplications 3 (2015) [1]