Deposition of Al 2 O 3 thin films by sputtering for c-si solar cells passivation

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1 Deposition of Al 2 O 3 thin films by sputtering for c-si solar cells passivation Directors of Master s Thesis: PhD. Jorge Alberto García Valenzuela and PhD. Joan Bertomeu Balagueró. Departament de Física Aplicada i Òptica, Universitat de Barcelona Abstract Passivation of silicon solar cells is an important issue due to the importance of the photovoltaic industry. The present work focuses on a possible material, Al 2 O 3, to passivate such devices by using sputtering, a technique easily accessible to the industry. Characterization of the material and measurements of its passivating features have been carried out, leading to important conclusions. Index Terms 5. Nanostructured Materials: Alumina, Magnetron Sputtering, Photovoltaic Device, Silicon Solar Cells, Surface Passivation. S I. INTRODUCTION ILICON is the dominant material in the solar cell industry, with more than 85% of the devices made in crystalline Si wafers [1]. The efficiency of such devices is greatly reduced by means of the electronic recombination losses at the wafer surfaces, which can be reduced through a process called surface passivation. This process involves the presence of an outer-layer on the material to create a protective shell. A solar cell, or photovoltaic cell, is an electrical device that is able to transform light into electricity by means of the photovoltaic effect: when photons strike the surface of a semiconductor (like Si), electrons from valence band (VB) can be excited to conduction band (CB) if they get sufficient energy, where they can freely move within the semiconductor. The hole that the electron has left in the VB is also able to move: the absorbed photon has created a mobile electron-hole pair, thus being able to produce an electrical current. The electron in the CB is in a meta-stable state, and it will try to stabilize to a lower energy level by filling any empty VB state (and removing a hole), a process called recombination. So, in order to extract a current, a charge separation process is needed. This process requires a spatial asymmetry, like the one produced by the presence of an electrical field [2], [3]. In order to stablish an electrical field, a p-n junction is formed: an n-type layer of silicon (Si containing atoms with extra electrons, like P) above a p-type silicon layer (Si containing atoms with a lack of electrons, like B), which greatly increases the number of charge carriers. Once the layers are in contact, carriers accumulate in the interface forming a depletion region: electrons recombine with holes, leaving positive ions in the n-type Si, negative ones in the p- type and no mobile charge carriers, which produce an electrical field from the n-type to the p-type, which also opposes the exchange of carriers. This electric field grows until is able to arrest any further transfer of electrons and holes, leading to an equilibrium in the depletion region. When the light illuminates a solar cell surface the following can occur: it can be reflected; it can pass through the material (which is the case for the lower energy photons); or it can be absorbed (if the energy is equal or higher than the band gap of the material). If light is absorbed, a photon strikes an atom and an electron-hole pair can be formed, but because of the electric field the pair is unable to recombine: electrons are attracted to the n-type side, and holes to the p-type. Metal contacts are added to the layers, providing a way for them to recombine, and at the same time extracting the electrical current to be used [2], [3]. A schematic of the whole process can be seen in Figure 1. The presence of dangling bonds at the surface of the semiconductor, which are left by the interruption of the periodicity of the crystalline lattice, makes that the surface becomes a site with a high rate of recombination processes. These processes in the surroundings of the surface depletes the region of carriers, which produces that the carriers from higher concentration regions, by random motion, start flowing into this localized region of low carrier concentration, through the diffusion process. In order to reduce the number of dangling bonds, and hence the surface recombination, a layer of a material can be deposited on the top of the semiconductor surface. This reduction in the number of dangling bonds is known as surface passivation. How could be possible to know that the surface passivation is effective? As the surface is a place of high recombination rate, a useful way to determine the level of passivation of a surface is through the surface recombination velocity (SRV), which measures the rate at which carriers move towards the surface: if there is no recombination at the surface, the movement of carriers towards it is zero. In a surface with infinitely fast recombination, the speed of carrier towards the surface is limited by the maximum velocity that they can attain. A wide variety of materials have been used to passivate the surface of a Si solar device, such as thermally grown silicon - 1 -

2 Fig. 1. A schematic of the whole process in a solar device. First, (a) a photon illuminates the surface of a solar cell, (b) and a mobile electron-hole pair is created. Due to the presence of the electrical field (c) from the n-type Si to the p-type, electrons are pushed upwards, were they are collected by the metallic contacts; and the hole moves downwards. Finally, (d) both charge carriers found each other at the rear contact. oxide (SiO 2 ), which relies on chemical passivation (by reducing the number of dangling bonds by tying them with hydrogen introduced during the annealing process) and has achieved very low effective surface recombination velocities (S eff < 10 cm/s) [1]; silicon nitride (a-sinx:h), which uses field effect passivation (reduction in the density of one type of charge carrier at the surface due to the presence of positive interface charges) [4], which has been able to achieve S eff around 10 cm/s [5]; hydrogenated amorphous silicon (a-si:h), mainly used for thin film Si solar cells [6], achieving S eff as low as 3 cm/s [7]; alumina (Al 2 O 3 ), which is used to passivate the rear side of a solar cell, achieving S eff as low as 6 cm/s [8]; etc. The last mentioned passivating material, alumina, is the one under study in the present work. There are some researches in which it has been deposited through atomic layer deposition (ALD) [8], which can offer a great control in the deposition process but taking a considerable amount of time, which is not suitable for industrial applications. With this in mind, it is of our particular interest to study the effects and passivation degree of Al 2 O 3 deposited by means of a sputtering process, which is more adequate for industrial implementation. A. Deposition Technique II. EXPERIMENTAL DETAILS Radio Frequency (RF) Magnetron sputtering was the technique used to deposit the passivation material. Sputtering refers to the deposition of material by ejecting it from a solid target because of the collision of high energy species [9]. In this technique, a chamber is set to vacuum environment conditions in order to reduce as much as possible any possible contaminant, before introducing a noble gas as argon. Once the Ar pressure has been set to a certain value, the deposition can be started. Inside the chamber, a radiofrequency discharge is activated between a cathode (which is the target) and an anode (the substrate), thus producing the ionisation of the Ar atoms (Ar + ). Electrons are accelerated towards the anode and the positively charged argon ions are accelerated towards the cathode, which leads to an atmosphere consisting of ions, electrons, and neutral gas atoms, thus producing plasma as long as the pressure and electrical power are kept within an appropriate range (the range varies depending on the type of power source, RF or DC, and the target). If the ions striking the cathode have the necessary momentum, atoms from the - 2 -

3 from W and the deposition pressure was varied from mtorr. B. Characterization Techniques B.1. Thickness A Dektak 3030 mechanical surface profiler, which is equipped with a 25 µm diameter probe, has been used to determine the thickness of the samples. In the deposited film, some steps were created by means of a simple lift-off technique employing ink before deposition and isopropanol for cleaning it later. Thus, the depth of the steps could be measured with the profiler, which has a vertical resolution of 1 nm [12]. The obtained results were corroborated by confocal microscopy (which can produce a 3D profile, by scanning a surface and eliminating the out-of-focus light), thanks to a Sensofar PLµ 2300 optical imaging profiler device. A first set of deposited films was measured to calculate the deposition rate under the two studied parameters, assuming a constant deposition rate, and the results are plotted in Figure 3. The resulting values were used to select the deposition time required to obtain 50 nm thick thin films, which is a common value used to passivate Si wafers. Fig. 2. RF magnetron sputtering system as is installed in the Laboratory of Micro/Nanotechnologies of the Physics Faculty of Universitat de Barcelona. target are ejected in vapour phase. As the environment has been set at low pressures, condensation occurs under concurrent bombardment by energetic species, promoting nucleation, compound formation and film growth onto the substrate [10]. The process was significantly improved through the magnetic confinement of plasma particles near to the target: magnetron sputtering [11]. With this process, the number of collisions is increased, and more Ar ions appear, achieving higher deposition rates. In the present work, the equipment used was a commercial ATC-ORION 8 HV system from AJA International, Inc, which can be seen in Figure 2. Al 2 O 3 thin films were deposited at room temperature onto 5 5 cm 2 Corning glass (1737F) and p- type crystalline silicon wafers (high quality Float zone wafers, 10 cm in diameter, 280 µm in thickness and a resistivity between 1-5 Ω cm,) by RF magnetron sputtering. For this, an Al 2 O 3 target (3 inch diameter) of 99.9% purity was used. The base pressure inside the chamber was always Torr (1 Torr = Pa). The target to substrate distance was fixed at 15 cm to achieve a better homogeneity. The working gas was 99.99% Ar, and the depositions were performed with the substrate rotating at 50 rpm. In this work, we studied the effect of the radio frequency power and the deposition pressure on the alumina films deposited on glass substrates, with the aim of selecting the best conditions for their later use on the silicon wafers; the radio frequency power was varied B.2 Transmittance - Reflectance The percentage transmittance (%T) and percentage reflectance (%R) of the deposited 50 nm Al 2 O 3 thin films were measured by using a Perking Elmer Lambda 950 device in the nm wavelength interval. This system is equipped with an integrating sphere, which is able to distinguish between the specular, total (T), and diffused (Td) transmittance and reflectance (R) through the use of different configurations. The system is equipped with a deuterium and a halogen lamp, being able to take measures of transmitted and reflected light between 200 and 2500 nm. The measurements were carried out with the Al 2 O 3 facing the incident light. Fig. 3. Deposition rate as a function of deposition pressure and at different RF power, as measured by profilometer and confocal microscopy

4 B.3 X-ray Diffraction With the aim of finding some degree of crystallinity in the deposited material, the films were also measured by X-ray diffraction (XRD). In this technique, an X-ray beam is focused in the sample with a certain θ angle, and measures of the diffraction angles of the scattered beam are taken. As atoms cause that the beam of X-rays scatter in many directions, an intensity peak will appear due to constructive interference [13] when Bragg s law is fulfilled: 2 d sin nx (1) hkl In the Bragg s law, d hkl is the distance between the lattice planes, λ corresponds to the X-ray wavelength, n x stands for the diffraction order, and θ is the angle between the sample surface and the X-ray beam. In the present study, a PANalytical X'Pert PRO MRD diffractometer was used. B.4 X-ray Photoelectron Spectroscopy The X-ray photoelectron spectroscopy (XPS) technique is based on counting the emitted electrons, as a function of their kinetic energy, from the surface of a material when an X-ray beam is focusing on it. These emitted electrons correspond to the atoms located in the outer layers of the sample, and their kinetic energy can be converted in their corresponding binding energy. In this quantitative technique, the elemental composition (species) and stoichiometry of a given material can be determined. Furthermore, the kinetic energy of these electrons can provide energy concerning the chemical state and the bonded species, with the help of standard data sheets. More information can be found in [14]. In the present research, the study of the chemical bonds in the deposited materials through X-ray photoelectron spectroscopy was performed by using a PHI 5500 Multitechnique system (from Physical Electronics), with a monochromatic X-ray source from Al Kα line with an energy of ev, at 350 W, and calibrated using the 3 d 5/2 line of Ag. The binding energies have been considered by taking the carbon 1s peak as a primary standard, whose binding energy was taken as ev [15]. B.5 Surface Recombination Velocity In order to determine the effectiveness of the surface passivation, it is necessary to find the surface recombination velocity (SRV), which measures the rate at which carriers move towards the wafer surface, where they recombine. In the present work, a WCT120 Sinton lifetime tester has been used to determine the effective lifetime of the minority charge carriers (the average time which a carrier can spend in an excited state after electron-hole generation before it recombines). This effective lifetime can be related with the effective surface recombination velocity through the equation [1], [16]: 1 1 2S eff (2) W eff bulk with τ eff being the effective lifetime (which involves surface and bulk recombination processes), τ bulk is the lifetime in the bulk, W is the wafer thickness and S eff is the effective surface recombination velocity. Lifetime has been measured through two techniques: the first one is called photoconductance decay (PCD). In this technique, very short pulses of light are shone on the sample, which generates electron-hole pairs, thus enhancing the conductivity. As the light pulse ceases, these pairs start to recombine, which produces that the enhancement in the conductivity fading over time [17]. The second technique is the so-called quasi-steady state photoconductance (QSSPC) [18], which relies on the number of charge carriers present when a steady light has been shone on a sample. It implies that the intensity of the light changes sufficient slowly so that the charge carrier populations in the sample are always in steady state. Finally, for both techniques (PCD and QSSPC), the illumination has been varied over a range of intensities, always considering as the most important one, the equivalent to one sun. III. RESULTS AND DISCUSSION A. Al 2 O 3 Deposited on Glass Substrate A.1 Deposition Rate Figure 3 (already presented) shows that the deposition rate increases with the applied power. It is due to a higher energy introduced to the ions, which are able to eject more particles from the target through the momentum transfer. It is also possible to observe that the deposition rate is inversely proportional to the deposition pressure: when higher pressures are applied, it involves a shorter mean free path, because there is a higher number of particles in the argon plasma, which act as obstacles to the ejected target particles in their trajectory to the substrate, which explains the decrease in the deposition rate. A.2 XPS With the aim of corroborating that the deposited material corresponds to the desired alumina, XPS analyses have been carried out. The results shown in Table I correspond to the binding energies found for the Al 2p and for the O 2s for a different set of pressures and deposition power. As can be seen, the value found for the Al 2p is practically identical to TABLE I BINDING ENERGIES FOR Al 2p AND O 1s Sample Al 2p (ev) O 1s(eV) 150 W 1.0 mtorr W 5.0 mtorr W 1.0 mtorr W 5.0 mtorr W 1.0 mtorr W 5.0 mtorr

5 Fig. 4. (a) XPS spectra of alumina films deposited at 300 W and at different deposition pressures. Ar peaks are clearly visible in the film deposited at 1.0 mtorr. (b) Amplified spectra for the deposited material, around the Ar 2p peak. (c) Al 2p peak and fit in detail. (d) Detailed O 1s peak and fit. the reported values [15], whereas the O 2p is in very close agreement to the reported ones [15]. Figure 4 shows the binding energies obtained for 300 W (this power was chosen simply because it is the average) at different deposition pressures. It is possible to appreciate that for 1.0 mtorr there are argon peaks, which means that for lower pressures there are higher probabilities that Ar atoms from the plasma contaminate the films, being that the reason of choosing 5.0 mtorr as the appropriate one for the sputtering processes, along with the fact that at this pressure the deposition process is less aggressive, due to reduced mobility (velocity) of the ions. In the same Figure 4, it is possible to see the amplified spectra for the deposited material, around the Ar 2p peak, where it is clear that for lower pressures there is higher Ar incorporation. Also, Al 2p peak is showed in detail, whereas the curves have been fit through a Voigt function to see if they are formed by only one atomic species. The O 1s for 1.0 mtorr is well fitted through the Voigt function, whereas the other deposited film is slightly different. Figure 5 shows a comparison between the O 1s for the deposition at 1.0 mtorr and 5.0 mtorr. It can be seen that the origin of this deviance is due the presence of not fully coordinated oxygen (which is the cause for the second peak shown in the film Fig. 5. Differences in the fitted curves for the films deposited at (a) 1.0 mtorr and (b) 5.0 mtorr

6 Fig. 6. a) XPS spectra of Al 2O 3 films deposited at 5.0 mtorr and different deposition power. (b) Amplified spectra for the deposited material, around where the Ar 2p peak should be located. (c) Al 2p peak and fit in detail. (d) Detailed O 1s peak and fit. deposited at 5.0 mtorr), which indicates the presence of defects. Figure 6 shows the spectra of the samples deposited at 5.0 mtorr at different deposition rates, along with the details around the Ar 2p, Al 2p and O 1s peaks. It can be seen that the aluminium and oxygen peaks are in close concordance with the reported results [15], and there is no presence of argon species, only noise. It is worth to mention that in both Figure 4 and Figure 6 it is also appreciable a peak corresponding to C 1s, which is a common contaminant, incorporated to the deposited films from the atmosphere, usually as CO 2. Finally, Figure 7 shows the results in the deviation of the stoichiometry of the deposited films. Alumina ideally shows an O / Al ratio of 1.5 (2 aluminium atoms per 3 oxygen atoms), and XPS spectra provide information about the concentration of these atomic species by means of the intensity of its peaks (actually, the area below them, which is integrated). The intensity of these peaks has been approximated by means of the Multipak Spectrum: ESCA software, and the change of this ratio has been determined for a set of samples at different deposition power and pressure, which is shown in Figure 7. What this picture means is that there is an excess of oxygen in the deposited films. Fig. 7. O/Al ratio versus the deposition power for the sample deposited at different deposition rate and power

7 with the thickness of the film, another film with higher thickness was deposited (200 nm), whose diffractogram can be seen in Figure 9. In this figure it is possible to appreciate that there is also no crystallinity, which means that the amorphous state have remained independently of the film thickness. Fig. 8. XRD diffractograms of 50 nm Al 2O 3 films deposited at 5.0 mtorr and different deposition power. A.4 R & T At first sight, the deposited films are completely transparent, very similar to the glass substrates, so reflection and transmittance experiments have been carried out in order to verify the absorption spectra of the deposited films. It is important to state that all the deposited films, independently of the RF power or deposition pressure, show very similar results, as can be seen in Figure 10 (where the deposition pressure is constant at 5.0 mtorr, and the deposition power varies) and Figure 11 (where the deposition power is constant at 300 W and the deposition pressure varies. In glass, the transmittance is higher for shorter wavelengths, but as the wavelength increases, all the transmittances seem to converge. In order to quantify the change in the transmittance, the integrated transmittance, which is shown in table II, has been calculated. The interval of interest has been set from 400 nm to 1100 nm Fig. 9. XRD diffractogram of 200nm deposited Al 2O 3 film. A.3 XRD All the films were studied by means of X-ray diffraction in order to determine any possible degree of crystallinity. Figure 8 shows the spectra for a film deposited at 5.0 mtorr and at a different set of deposition power. As can be seen, there is no presence of multiple peaks and no pattern is described by the spectra, which implies that no crystallinity has been found in the deposited alumina. In an attempt to determine if the crystallinity is dependant TABLE II INTEGRATED TRANSMITTANCE Deposition power Deposition pressure Integrated transmitance (%) 1.0 mtorr W 2.5 mtorr mtorr mtorr W 2.5 mtorr mtorr mtorr W 2.5 mtorr mtorr 91.8 Corning Fig. 10. R & T spectra for the deposited samples at a constant pressure of 5.0 mtorr and different deposition power. Fig. 11. R & T spectra for the deposited samples at a constant power of 300 W and different deposition pressures

8 Fig. 12. R & T spectra for the 200 nm thickness sample compared with the glass substrate. wavelengths, which takes into account the visible and NIR spectrum, with the band gap of silicon as upper limit (which is 1.1 ev). As can be seen in table II, integrated transmittance does not show large changes between the samples, but they have a little lower transmittance in comparison with the glass substrate. It implies that the passivating material is almost transparent, allowing the pass of the incident light to the surface of the passivated surface (which in the present case will be silicon). The outcome slightly varies when the thickness of the deposited film changes: the 200 nm alumina film shows interference patterns, as can be appreciate in the Figure 12, but even when the transmittance spectra looks a little different, the integrated transmittance in the interval of interest is similar to the 50 nm films. B. Al 2 O 3 Deposited on Silicon Substrate After performing depositions on glass substrates, p type Fig. 14. XRD diffractigram for an alumina film deposited on silicon at 300 W. crystalline silicon substrates have been used to study the passivating effect of Al 2 O 3. As it has been determined previously, 5.0 mtorr has been considered the most appropriate deposition pressure, so, all the samples have been deposited at that pressure. The depositions have been performed to obtain 50 nm thicknesses, assuming the same deposition rates than the previous samples on glass substrates. The obtained samples have been analysed through the XPS technique, which does not show any significant difference compared with the alumina deposited on glass substrate, as is shown in Figure 13 (deposition power is 300 W). This means that the substrate does not produce any change in the XPS spectrum, which is not the case in the XRD diffractogram, as can be seen in Figure 14. It this figure, it is possible to see some changes in comparison with Figures 7 and 8 due to the substrate, but the relevant fact is that the passivating layer remains in an amorphous state, regardless that the Si substrate is crystalline. Fig. 13. a) XPS spectra of Al 2O 3 film deposited on p type silicon wafer at 300 W and 5.0 mtorr, and on glass substrate - 8 -

9 The most relevant measures on silicon substrates correspond to the ones involving the passivating features of the alumina. In order to determine these features, PCD and QSSPC measures have been carried out in three different samples, every one of them at different deposition power. In addition, the measurements have been carried out before and after an annealing at 350 C for 20 minutes. It has been found that the results for PCD and QSSPC are quite similar, so only QSSPC result are presented. The mentioned techniques provided the effective carrier lifetime (τ eff ). In a good quality wafer, as the ones used here, the recombination in the bulk is negligible, which implies that the bulk lifetime (τ bulk ) is very high in comparison with the surface recombination, so the 1/τ bulk term in equation 2 can be neglected, which produces: W Seff (3) 2 Figure 15 shows the measured lifetime of the samples, and Table III show the effective lifetime and effective SRV before and after the annealing process, for two different concentrations of light. For a Si solar cell device, the most important is the 1 sun concentration, but the lifetime tester device was unable to obtain it for the unpassivated Si wafer. However, it has been obtained for a 5 suns concentration, so it is possible to compare the degree of passivation achieved by the alumina layer. The results show that, for both concentrations, once the alumina is deposited, the effective SRV decreases as the deposition power increases. Once the samples have gone through an annealing process, the S eff decreases by around two orders of magnitude in comparison with the non-annealed samples, which shows the importance of such a process. Also, it is worth to note that the tendency relating the deposition power and S eff has been inverted: for the higher deposition power, S eff is higher. The lowest S eff found is 40 cm/s, which is showed for the sample deposited at 150 W. This result is one order of magnitude higher than values reported through other techniques, such as ALD eff TABLE III SURFACE RECOMBINATION VELOCITIES AND LIFETIMES AT DIFFERENT INCIDENT SUNLIGHTS AND DEPOSITION POWER 1 Sun No annealed samples Annealed samples Power (W) τ eff (µs) S eff (cm/s) τ eff (µs) S eff (cm/s) Suns Si (6cm/s) [8], but still it is a great improvement. At the time of the measurements, it has been possible to identify damages ( blisters ) on the surface of the annealed samples. Observations performed through optical microscopy, which can be seen in Figure 16, show that in the surface of the 300 W deposited film there are some circular regions of different nature than the rest of the film, which have a diameter around 22 µm. In the 450 W these blisters have also appeared and have a higher area and quantity than those of the previous Fig. 15. Effective lifetime measured before and after the annealing, for the three deposition power. Fig. 16. Damage found in the (a) 300W and (b) 450W deposited films after performing the annealing

10 case (being clearly visible at the naked eye, with a diameter around 28 µm). The origin of these damages blistering is unknown, but at the moment the available information is not enough to find an appropriate explanation of this phenomenon, which could be analysed in a future work. The film deposited at 150 W does not show any damage, which could imply that the deposition power could have some relevance in the production of the blisters. It is worth to mention that this blistering effect has also been reported in some other researches, although an explanation of the cause remains unknown [1], and we found an increase in the blisters production with the increase of power deposition. IV. CONCLUSIONS RF magnetron sputtering has been used to successfully deposit Al 2 O 3 on p type silicon substrates in an attempt to passivate their surfaces. First, properties of alumina films deposited on glass substrates have been characterized by means of XPS, to determine the chemical composition of depositions; XRD, trying to find any possible degree of crystallinity in the deposited films; R&T, to measure the transmittance of the passivating layer, because it is an important fact how much light can pass to the Si wafer. The findings of these techniques show that the appropriate deposition pressure is 5.0 mtorr, to avoid Ar incorporation from the sputtering process; the deposited alumina is amorphous; and the deposited films is almost completely transparent to the spectrum of interest. Once the material has been characterized, silicon substrates have been used to measure any possible degree of passivation capabilities in the alumina. The results show that the effective surface recombination velocity decreases when the Al 2 O 3 film is deposited, but this velocity reduces by around two orders of magnitude when the sample is annealed after deposition, achieving a S eff as low as 40 cm/s, which means that the material is a suitable alternative to passivate p sides of a silicon solar cell. However, the degree of passivation is lower compared with other techniques, as ALD, which can achieve velocities as low as 6 cm/s. Even so, the results obtained are very important because sputtering is a technique that can be scalable and is more suitable for industrial applications. Finally, the annealing process needs to be reviewed, because in some cases damages on the surface of the samples may appear, which could imply that this process should be performed in stages, rather than putting the samples from ambient conditions to high temperature in just one step. development of this research project. REFERENCES [1] G. Dingemans and W. M. M. Kessels, Status and prospects of Al 2O 3- based surface passivation schemes for silicon solar cells, J. Vac. Sci. Technol. A, vol. 30, no. 4, , July [2] M. A. Green, Solar cells: operating principles, technology, and system applications, California: Prentice-Hall, 1982, ch. 3. [3] J. Nelson, The Physics of Solar Cells, London: Imperial College Press, 2003, ch. 2 [4] M. Z. Rahman and S. I. Khan. (2012, October). Advances in surface passivation of c-si solar cells. Mater. Renew. Sustain. Energy [Online]. 1(1). Available: [5] S. Dauwe, Low-temperature rear surface passivation of crystalline silicon solar cells, Ph.D. Thesis, ISFH, University of Hanover, Germany [6] J. Schmidt et al. Progress in the surface passivation of silicon solar cells, in Proc. 23nd Eur. Photovolt. Sol. Energy Conf., Valencia, Spain, 2008, pp [7] S. Dauwe, J. Schmidt, and R. Hezel, Very low surface recombination velocities on p- and n-type silicon wafers passivated with hydrogenated amorphous silicon films, in Proc. 29th IEEE PVSEC., New Orleans, USA, 2008, pp [8] J. Schmidt et al. Surface Passivation of High-efficiency Silicon Solar Cells by Atomic-layer-deposited Al 2O 3, Prog. Photovolt: Res. Appl., vol. 16. no. 6, pp , March [9] M. Ohring, The materials science of thin films. USA: Academic Press, 1992 [10] D. W. Hess, Plasma material interactions, J. Vac. Sci. Technol. A, vol. 8, no. 3, pp , June [11] J. S. Chapin, Sputtering process and apparatus. US Patent , August 28, [12] P. Carreras, Doped and multi-compound ZnO-based transparent conducting oxides for silicon thin film solar cells, Ph.D Thesis, Dept. de Física Aplicada I Ȯptica, Universitat de Barcelona, Barcelona, Spain, [13] C. R. Brundle, C. A. Evans, and S. Wilson, Encyclopedia of materials characterization: surfaces interfaces, thin films. USA: Butterworth- Heinemann, 1992 [14] J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of X-ray photoelectron spectroscopy. Minnesota: Physical Electronics, Inc., 1992 [15] K. T. Ng and D. M. Hercules. Studies of nickel-tungsten-alumina catalysts by x-ray photoelectron spectroscopy, J. Phys. Chem., vol. 80, no. 19, pp , September [16] L. Fesquet, S. Olibet, E. Vallat-Sauvain, A. Shah, C. Ballif. High quality surface passivation and heterojunction fabrication by VHF- PECVD deposition of amorphous silicon on crystalline Si: theory and experiments, in Proc. 22nd Eur. Photovolt. Sol. Energy Conf., Milano, Italy, 2007, pp [17] D.T Stevenson and R.J Keyes, Measurement of Carrier Lifetimes in Germanium and Silicon, J. Appl. Phys. Vol. 26, no. 2, pp , February [18] R. Sinton and A. Cuevas, Contactless determination of current voltage characteristics and minority carrier lifetimes in semiconductors from quasi steady state photoconductance data, Appl. Phys. Lett. Vol. 69, no. 17, 2510, October, ACKNOWLEDGMENTS The author would like to thank to the FAO group for allowing him to carry out his Master thesis and the use of their equipments. Many thanks to Anna Belén Morales Vilches, for her help relating with the passivated Si substrates. Thanks to the directors of this Master thesis, Dr. Joan Bertomeu, and especially to Dr. Jorge Alberto García Valenzuela, for his continued encouragement and invaluable assistance in the