Preparation of transparent conductive AZO thin films for solar cells

A study of the effect of technology parameters (sputtering power, substrate temperature and post-deposition annealing) on structural, electrical and optical properties of aluminium-doped zinc oxide (AZO) thin films was carried out. The optimal technology parameters of preparation were found to get necessary properties of AZO thin films for application in solar cells - the high figure of merit (F ges 4 %/Omega), low electrical sheet resistance (Rs les 10 Omega/square) and high optical transmittance (T ges 82%, including the glass substrate).


Introduction
Transparent conducting oxides (TCOs) based on ZnO are promising for application in thinfilm solar photovoltaic cells (PVCs) and various optoelectronic devices (Minami, 2005). Desired parameters of ZnO and doped ZnO:Al (AZO) thin films are given by their role in superstrate configuration of tandem Si solar cell (Zeman, 2007): the light enters the cell through the glass substrate where two pin absorber thin-film structures are placed between two TCO layers with back metal contact. The upper front contact AZO layer should fulfill several important requirements: high transparency in VIS/near IR solar spectrum; high electrical conductivity; suitable surface texture in order to enhance light scattering and absorption inside the cell; high chemical stability and adhesion to silicon. Moreover, bottom ZnO interlayer between Si and metal (usually Ag) contact is acting as barier and adhesion layer as well as optical matching layer to Ag back contact to improve its reflection of radiation, particularly in near IR region (Dadamseh et al., 2008). Optimization of the front contact TCO has proven to be crucial for getting the high cell efficiency (Berginski et al., 2008). RF sputtering is owning several advantages in comparison with the other physical and chemical deposition methods: a low-temperature ion-assisted deposition of metals, semiconductors, insulators, the before/post deposition modification of substrate/thin -film surface by ions on the micro-/nano-level; change of deposition rate in wide range (0,1 to 10 nm/s); to control further parameters which are important for thin film growth (substrate temperature, plasma density, composition of working gas, ion bombardment of film during deposition). In addition there is a significant contribution of secondary electron bombardment to the atomic scale heating of the film when it is prepared by the RF diode sputtering. Therefore RF sputtering of AZO films from ceramic target is often used to get the best their electrical and optical properties. An influence of different technological parameters was investigated: partial pressure of oxygen (Tsui & Hirohashi, 2000), substrate temperature (Fu & Zhuang, 2004), (Ali, 2006), (Berginski et al., 2008), substrate bias voltage (Ma & Hao, 2002), (Lim & Kim, 2006), post-deposition annealing (Fang at al., 2002), (Oh et al., 2007), (Berginski Fig. 1. Crystalline structure zone model of sputtered ZnO thin films: Zone 1 -porous structure of tapered amorphous or crystalline nanograins separated by voids, Zone Tdense polycrystalline structure of fibrous and nanocrystalline grains, Zone 2 -columnar grain structure, Zone 3 -single-crystal micrograin structure, Zone NT -nanostructures and nanoelements. The aim of present work has been to find correlations among the technological parameters (power density, substrate temperature and post-deposition annealing) and structural / electrical / optical properties of AZO thin films. In the beginning to accelerate our investigation of desirable thin film properties we used the RF diode sputtering where one can get continual changes of thin film thickness (of composition also) in one deposition run.

Modelling and simulation
Computer simulations have proved to be an indispensable tool for obtaining a better understanding of solar photovoltaic cells (PVC) performance and for determining trends for optimizing material parameters and solar cell structures. We focused on the simulations of both the parasitic effect in real bulk PVCs and progressive thin film solar PVCs, based on amorphous silicon and transparent conductive layers of ZnO, ZnO:Al. Sputtering is an important technique for deposition of both multicomponent thin films for solar applications as well as multilayer coatings with only few nanometers thin layers (socalled superlattices) which exhibit superior hardness, high wear, corrosion resistance and thermal stability (Panjan, 2007). Sputter deposition is attractive particularly in industrial applications due to the need of high deposition rates and uniform coverage over large areas. Therefore it is desirable to know what influence has the sputter system arrangement on spatial distribution of sputtered particles on the top of substrate (so-called deposition profile), i.e. on homogeneity of growing film properties.

Electric properties of PVC
The most important electric parameters, which are used to characterize the quality of PVC, are defined: the short-circuit current I SC (the current through the solar cell when the voltage www.intechopen.com

Semiconductor Technologies 274
across the solar cell is zero), the open-circuit voltage V OC (the maximum voltage available from a solar cell, at zero current), the fill factor FF (indicating how far the product I SC V OC is from the power delivered by the PVC) and the conversion efficiency (η). The conversion efficiency is defined as the ratio of the photovoltaically generated electric output of the cell to the radiation power falling on it P in : where FF is the fill factor of PVC I m V m /I SC V OC (or area ratio A / B in Fig. 2), E is value of irradiance and J SC is the short-circuit current density I SC /A. The values of V m and I m are the co-ordinates for maximal power point (he designates the optimal operating point of PVC), and can be estimated from the open circuit voltage and short circuit current: V m ~ (0.75-0.9)×V OC , I m ~ (0.85-0.95)×I SC (Goetzberger & Hoffmann, 2005). Efficiency is measured under standard test conditions (temperature of PVC 25°C, irradiance 1000 Wm -2 , air mass 1.5). By means of the two-diode model we achieved the better description of PVC. Diode D 1 is representing the carrier injection current I INJ and diode D 2 the recombination current I R . The values of saturation current densities and ideality factors for this diodes are different: J s1 = 1e-12 Acm -2 , J s2 = 1e-8 Acm -2 , m 1 = 1 (ideal diode), m 2 = 2. We define the size of PVC area A = 100 cm 2 . The other components are of resistive nature, a parallel (or shunt) resistance R P and the series resistance R S . For obtaining high efficiency of PVC, the parallel parasitic resistance R P (described loss currents at the edges of the solar cell and surface inhomogeneities) should be as high as possible and the series resistance R S (the resistance through the wafer, the resistance of the back surface contact and the contact grid on the front surface) as a low as possible, ideally it's a deal: R S = 0, R P → ∞. The values of parasitic resistances depend on PVC size, consequently also from area. In PSpice is the value of the short circuit current I SC assigned to a voltage-controlled current source (G-device, Fig.2) and is given by: We considered that the value of short-circuit current density J SC is given at standard test conditions. The effect of parasitic resistances R S and R P on C-V characteristic is shown in the Fig.4. As can be seen, for parasitic serial resistance R S (Fig.4a) the values of the short-circuit current and of the fill factor (it follows too efficiency) can be expressively reduced. At the high values of R S occur the big reduction of the short-circuit current value I SC . The opencircuit voltage is independent of the series resistance. The product R S .I SC should never have been greater than 25 mV in praxis (outside temperature 25 °C). The parallel resistance also degrades the performance of the PVC, Fig.4b Small values of the parallel resistance heavily degrade the fill factor (i.e. efficiency). Also are the value of open-circuit voltage reduced, the short-circuit current is independent of the parallel resistance.
Selected parameters of illuminated PVC with the parasitic resistances R S and R P is shown in the Tab.1. The parameters I SC and V OC are assigned from the graph, parameters FF and η are calculated by (eq. 1).

ASA model of thin film solar cell the 2nd generation
Progressive solar PVC, 2 nd and 3 rd generation with higher efficiency of 20÷40%, are formed in thin film structures, predominantly based on amorphous silicon (a-Si:H, p-i-n junction) as the absorber material and transparent conducting oxide (TCO) semiconductors for transparent electrodes, e.g. single junction p-i-n a-Si PVC structure "glass/TCO/a-Si:H (p-i-n)/TCO/Ag or Al (reflective back contact)" or tandem solar cell structure "glass/TCO/a-Si:H (p-i-n)/μc-Si:H (p-i-n)/ TCO/Ag or Al (reflective back contact)" (Zeman, 2007). For the simulation of the thin film PVC we have used the ASA program, developed at Delft University of Technology (Zeman et al., 2005), which is designed for the simulation of multilayered heterojunction device structures.
We focused for "superstrate" configuration of thin-film solar PVC: Glass/ZnO:Al/a-Si:H (pi-n)/ZnO/Al (reflective back contact). Schematic structure of a single junction a-Si:H PVC is shown in Fig. 5. The active device consists of three layers: a p-type a-Si:H layer, an intrinsic a-Si:H layer and an n type a-Si:H layer. This layers form a p-i-n single junction. The doped layers set up an internal electric field across the intrinsic a-Si:H layer and establish low loss ohmic electrical contacts between the a-Si:H part of the PVC and the external electrodes (Zeman, 2007). The thickness of the i-region should be optimized for maximum current generation. In practice is limit the i-region thickness to around 0.5 μm (Nelson, 2003). Transparent conducting oxides based on ZnO are promising for application in thin-film solar photovoltaic cells. The upper front contact Zno:Al layer should fulfil several important requirements: high transparency in VIS/near IR solar spectrum; high electrical conductivity; suitable surface texture in order to enhance light scattering and absorption inside the cell; high chemical stability and adhesion to silicon. Moreover, bottom ZnO interlayer between Si and metal (usually Ag) contact is acting as barier and adhesion layer as well as optical matching layer to Ag back contact to improve its reflection of radiation, particularly in near IR region (Dagamseh et al., 2008). Optimization of the front contact TCO has proven to be crucial for the high cell efficienty (Berginski et al., 2008). Computer simulations for single junction a-Si:H PVC structure (Fig. 4) we compile in ASA software. The thicknesses of particular layers are show in Fig. 4. All important electric properties are set direct in the C-V characteristic for illuminated p-i-n PVC structure (Fig.6). Also in this case are the parameters I SC and V OC assigned from the graph, parameters FF and η are calculated by (eq. 1). Reflection in the near IR region has a tendency to increase with the decrease of thickness of front ZnO layer. From dependences of quantum efficiency on wavelength result the fact, that the simple TF PVC structure shows lower effectiveness in the near IR region.

Sputtering
Computer simulations of magnetron sputtering corresponding to multi-source and multifold substrate rotation facilities have been progressively improved , [Jehn, 1999]. These calculations consider over-cosine distribution of sputter-deposited fluxes on the substrate area A s , dJ/dA s ~ cos n Θ, 1 < n ≤ 2, which reflects the kinetic energy losses of sputtered particles due to their collisions with working gas atoms.
We have applied the model and simulations of the spatial distribution of sputtered particles (Tvarozek et al., 1982) -deposition profile -in our diode system consisting of the planparallel arrangement of target and substrate in the distance of D ( Fig. 9) with these idealized assumptions: (a) Material of the target is emitted uniformly from the target area A t ; (b) Angular distribution of the intensity of particles rejected from the target (sputtered particle flux J in the direction given by an angle Θ to the normal of surface) conforms with the Knudsen cosine law (Kaminsky, 1965) or with its slight modifications, J 0 is the sputtered flux perpendicular to the target surface; (c) Scattering of particles on the way to substrate is neglected; (d) Accommodation coefficient of particles condensed on the substrate is equal 1 (resputtering is not included).

Semiconductor Technologies 280
Then the flux of material sputtered from the target surface element dA t to the substrate area element dA s in the point P is given where Θ is an angle between dA t -dA s path and the normal of target/substrate planes (dA t = dA s cos Θ ). By integrating of Eq. (4) over the target area A t we can get the deposition profile G(P) in the arbitrary point P on the substrate It is useful to normalize the deposition profile G N (P) towards the maximal value of G(0) which is usually the beginning of the co-ordinal system across the substrate For computing and simulations of deposition profiles we used Agilent Visual Engineering Environment (Agilent VEE Pro, 2009) -graphical language programming environment optimized for the use with electronic instruments, that provides a quick path to measurement and analysis. . In this case, simulations and experiments confirmed that an intensity of particles emitted from target approximately conforms to the Knudsen cosine law (Fig. 10 a). Changes of thin film thicknesses were less than 5% in the central substrate region of diameter ≤ ½ of target diameter. In lateral distances larger than target radius, the thin film thickness decreased down to 1/4 of the maximal value.
The detail comparison of various theoretical and experimental deposition profiles has shown that the sputtered fluxes from target follow slightly over-cosine angular distribution during the diode sputtering at pressures below 1 Pa (Fig. 10 b). Sputtered particle flux J in the direction given by an angle Θ towards normal of target surface can be described by equation where the coefficients were estimated from experimental deposition profiles (approximately c 2 ≈ 0.1 c 1 ).
Using the RF diode sputtering in the low-pressure region (p ≤ 1.3 Pa), the mean free path of sputtered particles (~ 10 -2 m) is comparable with the distance of target -substrate and therefore we can assume "collision-less" regime, particularly for high energetic particles passed through RF discharge. Neutral sputtered particles and energetic species (ions neutralized at the target and reflected from it) sputtered from the target conform to the Knudsen cosine law slightly modified by over-cosine dependence and their spatial distribution on the substrate (deposition profile) is described by power cosine dependence (Eq. 5). This "directional" sputtering can be exploited for preparation of strong textured thin films, e.g. c-axis inclined ZnO (Link, 2006) applicable in solar cells and in various optoelectronic devices, but also in an engineering of the nanoscale morphology, e.g. for preparation of a new class of optical nanomaterials -sculptured thin films -consisting of shaped, parallel, identical nanowires generally grown by physical vapor deposition techniques (Lakhtakia, 2008), (Jen, 2009).

Technology
We prepared ZnO:Al thin films in a planar RF sputtering diode system Perkin Elmer 2400/8L, using a ceramic target (ZnO+2 wt.% Al 2 O 3 ) in Ar working gas at constant pressure of 1.3 Pa. Films with thickness from 560 nm to 800 nm, depended on the sputtering power density and deposition time, were deposited on Corning glass 7059 substrates. Three technology parameters (chosen to tailor the physical properties of the AZO films) were investigated: the sputtering RF power density E Ф (1.1 ÷ 4.4 x 10 4 W/m 2 ), the substrate temperature T s (RT, 100 ÷ 300C) and the post-deposition annealing at temperatures T a (200 ÷ 400C) in the forming gas (80% N 2 + 20% H 2 ) for 1 hour. The real structure characterization (crystallite size and their crystallographic orientation, micro-strains and biaxial lattice stresses) of the films was investigated by X-ray diffraction (XRD) analysis. The XRD patterns were carried out using an automatic powder X-ray diffractometer AXS Bruker D8 equipped with a position sensitive area detector Histar. Cobalt K radiation (λ = 0.179 nm) was used as an X-rays source. Six strongest diffraction lines in the range of 2 -diffraction angle from 30 o to 80 o of a ZnO powder are used as reference patterns. The position, height, integrated intensity and FWHM are the main four parameters that characterize the diffraction lines. The broadening of a diffraction line is a result of a real material structure, where the size of the crystallites and the micro-strains are the most important contributors to the broadening of the line. In this study we used a procedure utilizing an integral breadth of a diffraction line (Langford, 1978), (Delhez et al., 1982). Equation 8 characterizes the integral breadth β that includes two parameters namely the height and the integrated intensity: and Gaussian parts of the integral breadth of the line represent the size of the crystallites and the micro-strains, respectively. The average crystallite size and micro-strains are determined using Equations 9 and 10, respectively (Delhez et al., 1982): where <D> is the average crystallite size in the direction perpendicular to the diffracting lattice planes,  is the X-ray wavelength used and  is the Bragg's angle; www.intechopen.com

Preparation of transparent conductive AZO thin ilms for solar cells 283
where <ε> is the average micro-strain in the diffracting volume. Biaxial lattice stresses were calculated from a shift of the most intensive diffraction line (002) according to the equation (Sutta et al., 1982) o where d 0 is the strain free reference lattice spacing, d is the lattice spacing obtained from the experiment, E is Young's modulus and μ is Poisson's ratio. Preferred orientation of crystallites (texture) in a certain direction can be approximately characterized by the Harris texture index (Okolo et al., 2005) where n is the number of investigated diffraction lines, I i is the observed intensity and R i is the corresponding intensity of the sample with randomly oriented crystallites (this value can be calculated from the theory if the structure of material is known or it can be picked up from the standard diffraction data files). In our case the R i values were calculated from the theory. The maximal value that the Harris texture index can reach is equal to the number of lines taken into the calculation. For samples with completely randomly oriented crystallites the Harris texture index is equal to one. The grain size was observed by transmission electron microscopy (TEM) by JOEL Electron Microscope. The thickness of AZO thin films was measured by DEKTAK 150, their electrical resistivity was obtained by Van der Pauw method and optical transmittance was determined by spectrometer Specord 210 and Avantes AVASPEC Fiber Optic Spectrometer. The optical transmittance in the UV spectrum region (blue-shift of the absorption edge) gives the information about the width of optical band-gap. The absorption edge for direct inter-band transitions is given by where A is a constant for a direct transition, a is the optical absorption coefficient, which is given from dividing absorbance by film thickness, and h is the photon energy. The direct band-gap of materials was obtained by plotting and extrapolation of (αh) 2 vs. h.

Deposition profile and properties of sputtered AZO thin films
AZO thin films were sputtered in the planar diode sputtering system (Fig. 9) at power density 4 times higher than the value of minimal energy flux density (~ 1 x 10 4 W/m 2 ). Corning glass substrates with dimension of 10x10 mm were placed on different positions under the target, ZnO+2 wt. % Al 2 O 3 , with diameter of 152 mm. We have applied the model of spatial distribution of sputtered particles in our diode system (see Section 2.4) to get continual changes of thickness of thin films in one deposition run from 700 nm in the center to 300 nm at the border of substrate holder (Fig. 10 a). All sputtered AZO films were polycrystalline with the columnar structure and a very strong texture in the [001] direction perpendicular to the surface, what was confirmed by XRD, TEM and SEM analyses (Fig. 11,  Fig.12). Properties of AZO thin films have been changed in lateral direction across the substrate holder. In the central region we can expect the highest power density (corresponding to deposition profile) what caused also an increase of mean grain size of 50 nm in comparison with value of 20 nm in peripheral position (Fig. 11 a). Evaluation of XRD patterns also showed the tendency mentioned previously (Fig. 11 b): the average size of crystallites <D> (regions of coherent X-ray scattering) changed from 54 nm to 136 nm (in central region) and compressive lattice biaxial stresses σ 1 +σ 2 increased from -3.8 GPa to -6.4 GPa (in peripheral position). Resistivity of AZO films placed in middle substrate region was in the range of 10 -2 Ωcm and gradually towards the side of holder, has increased up to 3 Ωcm (Fig. 11 c). Remarkable increase of resistivity with position may be explained by the variation of particular fluxes of sputtered particles, dominantly by change of mutual ratios of sputtered ZnO, Al and O fluxes. Particularly, the bombardment of growing film by negative O ions generates large amount of oxygen intersticials that act as trap for free electrons, thereby increasing the resistivity. Transmission of films sputtered in the central region was about 89 % and it increased to 91 % at the edge of substrate holder. The course of the optical band-gap dependence on substrate position is copying the transmittance dependence (Fig. 11 d).
For the explanation of previous mentioned phenomena, both effects the negative oxygen ion bombardment, or resputtering respectively, of growing film and the local atomic-scale heating of film surface by energetic secondary electrons, should be taken in the consideration. We assume, that it is a difference of the spatial distribution between energetic negative oxygen ions and secondary electrons, coming from the target and accelerating at it. Even the both fluxes have obtained the same kinetic energy (in order of 10 -100 eV), the mean velocity of electrons and ions differs of 10 3 -times because another masses. Therefore in the case of RF diode sputtering, the distribution of negative oxygen ions sputtered from the target will be more uniform across its area (in comparison with sputtered neutral particles) since the flux of negative oxygen ions is collimated by the induced electric field of target. The distribution of secondary electrons bombarding the growing film will follow the deposition profile because the induced target voltage has no significant influence on their angular emission due to small electron mass and high velocity. Then we can expect a higher local atomic-scale heating of the growing film in the central region.
We can conclude, that our arrangement provides sputtering of AZO thin films with relatively homogenous properties in the central region of diameter of 50 mm.

Influence of technological parameters on structural properties
Based on previous results we used the central region for sputtering and the effect of three technological parameters on structure of AZO thin films has been investigated: power density, substrate temperature and post-deposition annealing. An increase of both sputtering power density up to 4,4.10 4 W/m2 and substrate temperature to 200°C as well as post-deposition annealing provided highly textured films with a preferential c-axis [001] orientation (Fig. 13, 15, 17). Typical columnar structure of AZO films and their surface morphology are shown in Fig. 12. Fig. 12. The cross-section of typical polycrystalline AZO thin film with columnar structure in [001] direction perpendicular to the substrate and a hillock surface morphology Significant asymmetry of (002) diffraction line when room substrate temperature was used indicates that there is a region with heterogeneous structure at the substrate -film interface (Fig. 13 a). Further energy delivered to the growing film by heating the substrate from RT to 200°C improved the crystallinity of the films (Fig. 15).The asymmetry of (002) line as well as heterogeneous regions completely diminished at higher substrate temperatures and small shifts from the reference line position were observed, which indicates only unimportant lattice strains (Fig. 15 b). The widths of (002) lines became narrower and dimensions of crystallites growing were from 60 to 120 nm for films deposited at RT and to more than 200 nm for those deposited at higher substrate temperatures. The widths of azimuthal line profiles also decreased from 15 to 3.5° with increasing energy delivered to the growing film during the deposition. This indicates lower declination of individual crystallites from the normal to the substrate (stronger texture). At the highest substrate temperature (300°C) also the other diffraction lines appeared, which is caused by more randomly oriented crystallites of the film. Other authors report the same changes with the temperature (Fu et al., 2003). In this case the film loosed its anisotropy. These phenomena induced by power density and temperature during deposition indirectly confirmed by changes of the surface morphology of films (Fig. 14, Fig. 16).  The shift up of the 2 with increasing RF power, as well as substrate and annealing temperatures, is a result of the increase of Al 3+ substituents (Al 3+ that substitute for Zn 2+ in the ZnO lattice) and a reduction of the interplanar distance, which changes the lattice distortion in AZO films from compressive to tensile lattice stresses. An increase of the power density and temperature during growth are resulting in the larger grains (growth from 60 to more than 200 nm) and better crystalline structure (no line asymmetry Post-deposition annealing in the forming gas at 400 °C diminished an asymmetry of (002) diffraction line (Fig. 17). www.intechopen.com

Semiconductor Technologies 288
The average size of crystallites was  100 nm at T A = 200°C and it increased to  400 nm by the growth of annealing temperature up to T A = 400 °C (Fig. 17 b). Post-deposition annealing reduced biaxial lattice stresses due to elimination of grain boundary defects. Both effects can be explained by recrystallization and phase transformation.

Influence of technological parameters on electrical and optical properties
The substrate temperature increase during sputtering caused the decrease of sheet resistance from 300 Ω/square to 50 Ω/square with a minimum 36 Ω/square at T s = 200°C (Fig. 18 a). Sputter power density has changed considerable R s from 1137 Ω/square at E Φ = 1.1.10 4 W/m 2 to 300 Ω/square at E Φ = 4.4.10 4 W/m 2 (Fig. 18 b). After annealing (over the T A > 200°C) the decrease of the sheet resistance to 10.4 Ω/square was observed (Fig. 18  c). The lowering of the sheet resistance by temperature (during or after deposition) was caused by both the improvement of crystalline structure and the increase of free electron concentration particularly, what has been supported by optical measurements.  (c) annealing temperature on the sheet resistance AZO thin films sputtered at a higher substrate temperature showed the "blue shift" in optical spectrum (Fig. 19), so-called the Burstein -Moss effect, which is dependent on the dopant concentration and the effective mass state density (Sernelius et al. 1988). The widening of the optical band-gap with the substrate temperature is originated by the increase of the electron concentration caused by Al doping (Liu et al. 2006), (Moon et al. 2006). No significant influence of the substrate temperature on the transmittance was observed (Fig. 20).
www.intechopen.com  The dependence of transmittance on the power density showed a minimum at a middle powers and the highest value of 93 % was reached at power density of E Φ = 4.4.10 4 W/m 2 (Fig. 21). The post-deposition annealing had a positive effect on the increase of transmittance accompanied with the decrease of the optical band-gap width (Fig. 22)

Summary
An influence of substrate temperature, power density, post-deposition annealing temperature on sheet resistance and transparency of sputtered AZO thin films are summarized in Table 1.
There is an inherent trade-off between how high transparent and low resistive TCO layer is, which is represented by the figure of merit: F = T opt /R s , where T opt (%) is the transmittance for AZO layer and R s (Ω/square) is the sheet resistance. The higher figure of merit (F > 1% / Ω) there is the better the performance of the TCO thin film. The substrate temperature increase during sputtering caused an improvement of the figure of merit to the value of F = 2.6 %/Ω at 200 ºC (Fig. 23 a). It showed a low value of 8 x 10 -2 %/Ω at the initial power density E Φ = 1.1 x 10 4 W/m 2 which became greater with growth of power density (Fig. 23 b). The greatest effect on the figure of merit exhibited the post-deposition annealing what increased of it up to value of F =8.9 %/Ω at T A = 400 ºC (Fig. 23 c).

Conclusion
AZO thin films sputtered at power density of 4.4 x 10 4 W/m 2 , substrate temperature of 200ºC and annealed in the forming gas at 400ºC showed a highly (002) oriented crystalline structure with the larger grains. Optimal substrate temperature could not surpass 200ºC and on the other side, an annealing temperature should be higher than 200ºC. Desired properties of AZO thin films for solar cell application -the high figure of merit (F ≥ 8 %/Ω), low sheet resistance (R s ≤ 10 /square) and the highest transmittance (T opt ≥ 86% ) were obtained in highly oriented AZO films prepared at high values of power density, optimal substrate and annealing temperatures. The next experiments are focusing on the incorporation of AZO films into the real thin film solar cell, on the evaluation of their behaviour and on the (a) (b) (c)

www.intechopen.com
Semiconductor Technologies 292 enhancement of light scattering by AZO film using chemical or ion etching modification of their surface.