A COMPARATIVE CHEMICAL STUDY OF THE TOP LAYERS OF TWO DIFFERENT QUALITY SILICON SOLAR CELLS

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1 A COMPARATIVE CHEMICAL STUDY OF THE TOP LAYERS OF TWO DIFFERENT QUALITY SILICON SOLAR CELLS M.J. Ariza and D.Leinen Departamento de Fisica Aplicada I, Facultad de Ciencias, Universidad de Málaga, E-2971 Malaga, Spain., Phone: , Fax: , F. Martín Departamento de Ingenieria Química, Facultad de Ciencias, Universidad de Málaga, E-2971 Malaga, Spain., Phone: , Fax: O, Abstract - Two solar cells of different quality, i.e. one yielding a high photo-current of 3 Ampere (good cell) and the other one a low photo-current of only 1.6 Ampere (bad cell), have been studied by XPS combined with Ar + depth profiling for chemical and compositional analysis of the surfaces and in depth, i.e. through the anti-reflection coating (TiO2), the passivation layer (SiO2) up into the phosphor doped silicon bulk. At the surface the elemental composition is similar for both cells, although the bad one presents slightly more carbon and oxygen, and considerably more lead and silver, than the good one. During profiling, carbon is also detected in the TiO2 and the SiO2 passivation layers and in the silicon bulk. The carbon amount versus depth is always considerably higher in the bad cell than in the good one, and in both cells is higher in the TiO2 film than in the other materials (good cell: ~ 1 % C in TiO2 and ~.5 % C in Si; bad cell: ~ 2 % C in TiO2 and ~ 1 % C in Si). Furthermore, XPS depth profiling shows that silver atoms have not diffused in the same way in both cells, since only the good cell presents silver atoms up into the silicon bulk making thus a good front contact. On the other hand, carbon atoms are impurities for the silicon bulk and can act as recombination centers for charge carriers. Hence, the reason for the differences in the photo-current is most probably due to a bad electrode contact together with a mayor amount of carbon impurities in the lower quality cell. 1. INTRODUCTION At the end of the manufacturing process of positive doped mono-crystalline silicon solar cells each device is subjected to an efficiency test in order to classify cells of different quality and to exclude bad ones, i.e. which yield a too small photo current under standardized conditions, for the assembly of the solar panel. Furthermore, the quality test permits to mount in each solar panel cells of equal power and thus to exploit to a maximum the power generated in each panel. Solar panels are classified according to its output power and a quality (output power) dependent price fixing for each panel can thus be made making the product more competitive for the market. Although techniques applied for manufacturing monocrystalline silicon solar cells are well established already since about two decades (Kelly, 1978; Partain, 1995), the reason why some cells yield a worse result than others is often unknown. A low photocurrent can be due to defects or impurities in the silicon bulk which would induce the recombination of the charge carriers before their recollection at the electrodes, as well as due to a bad electrical contact of the electrodes to the silicon bulk material. The front contact is usually made with metallic deposits of silver in form of fingers. After the serigraphic finger painting, the silver atoms have to diffuse during the final firing process at 973 degree Celsius through the TiO2 anti-reflection layer and the SiO2 passivation layer up into the phosphor doped silicon bulk in order to make a good electrical top contact to the cell. Since the charge carriers have to be produced and recollected in a range of the top 5 nm of the silicon wafer, a chemical analysis of this zone could be useful in determining the reasons why some cells yield a worse result than others. Therefore, a technique which uses X- ray photoelectron spectroscopy (XPS) together with Argon ion depth profiling could be an adequate tool to get more insight into this problem since this kind of analysis yields compositional and chemical information. In this work we compare with XPS depth profiling two silicon solar cells cut in wafers from the same p-doped monocrytalline silicon crystal and manufactured in the same manufacturing line but which are of different quality, i.e. yielding 3. Ampere (good cell) and 1.6 Ampere (bad cell) photocurrent under standardized conditions. We present a study of the chemical composition versus depth through the TiO2 anti-reflection layer, the SiO2 passivation layer up into the phosphor doped silicon bulk. This study has been done on the photovoltaic active part of the solar cell, i.e. far away from the metallic fingers, the front contact of the solar cell device.

2 bad one only 1.6 Ampere, in the following called SC1.6, under the same conditions of illumination. This means a difference in output power of approximately 5%. Figure 1: SEM pictures from SC3. (top) and SC1.6 (bottom) 2. EXPERIMENTAL 2.1 Materials Two monocrystalline silicon solar cells manufactured by ISOFOTON (Málaga, Spain) have been studied. Monocrystalline p(boron)-doped silicon grown in <111> direction by the Czochralski method and cut in slices (wafers) of.3 to.35 mm thickness have been used as source material for solar cell manufacturing. The manufacturing process can be roughly separated in the successively following fabrication steps of surface texturing, phosphor diffusion, SiO2 passivation layer formation, TiO2 anti-reflection coating and crystallization into the rutile phase by calcination, finger painting and firing of the final device for electrical contact formation (see for more information also in other contribution to this volume (Mariza, Leinen and Martin, 2)). Both solar cells under study proceed front the same monocrystal and the same manufacturing serie, but yield significantly different photocurrents; the good one of 3. Ampere, in the following called SC3., and the 2.2 XPS depth profiling XPS measurements were carried out with a PHI 57 spectrometer equipment including an Ar ion gun coupled to the spectrometer main camber. Multiregion spectra were recorded from a 38 µm diameter analysis area at 45 take-off-angle with a concentric hemispherical energy electron analyzer operating in the constant pass energy mode at ev, using Mg Ka radiation ( ev) as excitation source working at a power of 3W (15 KeV). Under these conditions, the Au 4f 7/2 line was recorded with 1.16 ev FWHM at a binding energy of 84. ev. The spectrometer energy scale was calibrated using Cu 2p 3/2, Ag 3d 5/2 and Au 4f 7/2 photoelectron lines at 932.7, and 84. ev, respectively. The pressure in the analysis chamber was lower than ~1-7 Pa and during 4 KeV Ar + depth profiling at about 1-5 Pa. The Ar ion gun was positioned at 5 with respect to the sample surface and the sputtered sample area was 4x4mm 2 in order to be sure that the analyzed area during depth profiling is sputtered uniformly and hence at equal depth at a given sputter time. The sputter rate is assumed to be 1.2 nm/min as determined for Ta 2 O 5 with a depth resolution of about 1 nm. Two types of depth profiles have been performed: 1) sequence programmed profiling, consisting of sequences of 2 min sputter time, 2 s delay time (to reduce 4 KeV Ar+ sputter induced sample charging) and thereafter recording of the multiregion spectra of elements in mayor concentration (Ti, O, Si) by only a few sweeps over each recording region (Ti 2p, O ls and Si2p) up to a total sputter time equivalent to a certain number of sputter sequences. 2) manual profiling, consisting of sputtering for a certain time (sequence) up to the desired depth, closing (after each sputter sequence) the Ar gas inlet and thus lowering the pressure to ~1-7 Pa before recording the multiregion spectra for a sufficiently long time to be able to detect elements in minor concentration. In this way, C ls, Ag 3d, Pb 4f and P 2p besides Ti 2p, Ols and Si 2p photoelectron peaks have been recorded after each sputter sequence. A detection limit of about.2 % for C and.5 % for Ag in atomic concentration has thus been achieved. PHI ACCESS ESCA-V6. F software package was used for data acquisition and analysis. The atomic concentrations were calculated from the photoelectron peak areas using Shirley background subtraction (Shirley, 1972) and sensitivity factors provided by the spectrometer manufacturer (PHI, 2). 2.3 SEM analysis

3 The morphological aspect of both solar cells have been studied with a JOEL JMS 63 electron microscope working at 1 KeV. For this, both samples have been couvered with a thin gold film in order to prevent any charging during the electron bombardment. 3. RESULTS AND DISCUSSION 3.1 SEM The SEM pictures of the surfaces of both samples are shown in Figure 1. It can be seen that the wafer surface of SC3. presents pyramids of more or less uniform size whereas the one of SC1.6 shows pyramids of varying size between 1 and 1 µm approximately. These pyramids have been formed by chemical etching of the Si(1) wafer surface. They are of quadratic base with four Si(111) faces. Figure 1 reveals that the texturing process has not been equally efficient for both surfaces although both wafers have passed the same humid line for texturing in the manufacturing process. Texturing of SC3. (good) Ti2p t (min) Ar+ SC1.6 (bad) Ti2p t (min) Ar+ Figure 2: Ti,O,Si depth profiles of SC3. (top) and SC1.6 (bottom) the p-si wafer surface increases the photovoltaic active surface in about 25-3%. (Partain, 1995). Because of this, there must be other reasons for to explain the large difference measured in efficiency (about 5%) of both solar cells. 3.2 XPS depth profiling O1s O1s Si2p Si2p In Figure 2 the relative atomic concentrations of Ti, O and Si versus 4 KeV Ar+ sputter time (type 1 of profiling, see experimental part) are shown for both solar cells. In both depth profiles, we detect for about the first 25 min of sputtering Ti and O atoms according to the TiO 2 anti-reflection layer. Then Ti concentration drops whereas the Si concentration increases and O concentration stays constant due to profiling in the SiO 2 passivation layer. Finally, the O concentration drops to zero and only Si is detected when reaching the silicon bulk of the solar cells. During 4 KeV Ar+ sputter profiling, the atomic concentrations measured not necessarily have to reproduce the stoichiometric composition of the layers. This is due to preferential sputter removal of some elements as, for instance, occurs in TiO2 where O is sputtered preferentially (Malherbe, Hofmann and Sanz, 1986). Table 1: TiO 2 and SiO 2 layer thickness in min of 4 KeV ar + sputtering (assumed sputter rate of 1.2 nm / min as determined in Ta 2 O 5 ) Sample TiO 2 SiO 2 SC3. 34 (47) 63 (64) SC (44) 65 (66) The atomic concentrations found in both depth profiles reflect the layer system of the solar cell: TiO2 anti-reflaction layer on top, SiO2 passivation layer underneath and the Si wafer as substrate, in accordance to the manufacturing process of the solar cell device (Ariza, Leinen and Martin, 2). In Table 1 we present the layer thickness (equivalent to sputter time) determined from the depth profiles of Figure 2 for both solar cells. The differences in layer thickness comparing both solar cells are smaller than the depth resolution determined from the profiles. The depth resolution has worsen considerably compared to the one determined for the Ta 2 O 5 /Ta interface when calibrating the 4 KeV Ar+ sputter rate. It is found to be about 25 min 4 KeV Ar+ sputtering for the TiO 2 /SiO 2 interface and to be about 5 min 4 KeV Ar+ sputtering for the SiO 2,Si interface due to the high roughness of the textured wafers (Hofmann, 1995). The bad depth resolution, in that case, due to the surface morphology of the samples, is the reason why no differences in the layer structure of the elements in mayor concentration can be observed in the depth profiles of SC3. and SC1.6. However, the profiles of Figure 2 permit us to determine the sputter time (depth) necessary to reach the different layers and interfaces in both solar cells and thus make possible to sputter to a certain depth of interest and record then mutiregion spectra of sufficient recording time for each photoelectron peak to be able to detect elements in minor concentration. Those

4 elements are phosphor, lead, silver and carbon. These elements have been found as traces at the photovolataic active part of the solar cell surface (see also (Ariza, Leinen and Martin, 2)). The distribution at the surface and versus depth of these elements is decisive for the efficiency of the solar cell device: phosphor is the element which makes the p-n junction in the p-doped silicon bulk; silver has to diffuse through the surface layer system into the silicon bulk but not passing the p-n junction (in that case it would make a short-circuit) in order to make a good electrical front contact; carbon has isochemical property to silicon and can diffuse thus easily into the silicon bulk and act there as recombination center for charge carriers; lead atoms can act as dispersion centers for light absorption thus reducing the light transmission into the silicon bulk for electron-hole pair generation. Table 2: Atomic concentrations at the solar cell surface %Ti %Si %O %C %P %Ag %Pb SC SC Table 2 shows the atomic concentrations determined for the surfaces of both solar cells, i.e. before 4 KeV Ar+ sputter profiling. As can be seen in Figure 3 which shows the depth profile of type 2 (see experimental part), the mayor amount of the elements in minor concentration as C, P, Ag and Pb have been found at the surfaces. In Table 2 one can observe that at the surface of SC1.6 (bad cell) a mayor amount of C, Pb and P has been found compared to SC3.. A mayor amount of P at the surface indicates a mayor redistribution of this element through the layer system during the thermal processes of manufacturing as has been shown elsewhere (Ariza, Leinen and Martin, 2). This could also affect the depth of the p-n junction in the solar cell device. On the other hand, a mayor amount of C and Pb at the surface leads to a reduction of transmitted light into the region of the p-n junction. Depth profiling has shown that Pb and P concentration decrease below XPS detection limit (about.1%) in about 8-1 min of 4 KeV Ar+ sputtering. However, C and Ag concentrations could be followed for both solar cells profiling through the layers up into the silicon bulk. The corresponding atomic concentrations of Ag and C determined from the Ag 3d and the Cls photoelectron peaks are shown versus 4 KeV Ar+ sputter time in Figure 3. In the lower part of the figure, the C concentration is presented in an expanded scale to point out the differences between both cells. In Figure 3 (top) we can see that the Ag concentration drops for SC1.6 in about 3 min below the XPS detection limit whereas for SC3. it drops too within the first minutes but stays then at detectable concentrations of about.1 to.2 atom % and this until deep into the silicon bulk. This shows that in SC1.6 silver has only be detected in the zones of TiO 2 and SiO 2 layers but not in the silicon bulk. In comparison to SC3. where silver is present up into the silicon bulk we can deduce that silver atoms have not diffused sufficiently in SC1.6 in order to make a good electrical contact. On the other hand, the carbon concentration decreases drastically in only 1 min of Ag3d t (min) Ar+ C1s SC3. SC1.6 SC3. SC t (min) Ar+ Figure 3: Silver and carbon atomic concentration versus 4 KeV Ar + sputter time sputtering, but stays at detectable levels in the whole depth range studied. As we can se in Figure 3, the carbon concentration is always higher in the TiO 2 layer than in the SiO 2 layer or the Si bulk and besides that is always higher in SC1.6 than in SC3.. This can be seen on bottom of Figure 3 showing the C concentration versus sputter time in an expanded scale. Although the elements in minor concentration are quite near the detection limit for XPS, we could discern differences in chemical composition which together with the differences found in morphology for both cells can explain the large difference in photocurrent measured. On the one hand, the mayor concentration of lead found at the surface of SC1.6 diminish the number of photons which reach the p-n junction. Furthermore, the mayor amount of carbon detected in all the studied depth of SC1.6 means a larger amount of impurities which can act as centers for charge carrier recombination. On the other hand, in SC3. exists a mayor concentration of silver in

5 all the studied depth range which helps to recollect the electrons produced by photoeffect near the p-n junction. On the contrary, it is most probable that the front contact of SC1.6 is not very good because of insufficient diffusion of the silver atoms. The XPS results have shown that each manufacturing process of the solar cell device is decisive for the final product quality since the differences found for both solar cells range from surface texturing to differences in atomic concentration which have been originated in the successive thermal treatments applied during the manufacturing of the solar cell device. 4. CONCLUSIONS The comparison of a solar cell of good quality (high photocurrent of 3. A) with one of bad quality (low photocurrent of only 1.6 A) has shown that all manufacturing processes are decisive for the final quality of the solar cell device. For the solar cell of bad quality, it was found that the surface texturing is more irregular, and the amount of impurities as carbon and lead in the uppermost atomic layers is considerably higher, whereas silver atoms have not diffused sufficiently from the top into the silicon bulk in order to make a good electrical front contact of the solar cell device. Therefore, the difference in quality (photocurrent) determined for both cells can be ascribed to a less efficient absorption of light due to dispersion centers as Pb and C in the TiO2 anti-reflection layer as well as to a bad efficiency for charge carrier recollection due to a bad front contact and a mayor amount of recombination centers (carbon impurities) in the silicon bulk of in the bad quality solar cell REFERENCES Ariza M.J., Leinen D. and Martin F. (2). A comparative chemical study of the top layers of two different quality silicon solar cells, other contribution to EUROSUN2 Hofmann S. (1995). Depth profiling in AES and XPS, pp , in Practical Surface Science Edited by D. Briggs and M.P. Seah, John Wiley & Sons, Chichester Kelly H. (1978). Photovoltaic Power Systems: A Tour through the Alternatives, Science 199,634 MalherbeJ.B., Hofmann S., Sanz J.M., (1986). Preferential sputtering of oxides: A comparison of model prediction with experimental data. Applied Surface Science 27, Moulder J.F., Stickle W.F., Sobol P.E., Bomben K.D. (1992), in Handbook of X-ray Photoelectron Spectroscopy. Ed.by J. Chastain. Published by PHI, 1992 Partain L.D. (1995). Solar cells and their applications. (Wiley Series in Microwave and Optical Engineering, Kai Chang, Series Editor, p.p.55-78, Wiley Interscience, New York PHI (2), Physical Electronics, 659 Flying Cloud Drive, Eden Prairie, Mn 55344, USA. Shirley DA. (1972) Phys. Rev. B 5, 479.