Solar Energy Materials & Solar Cells

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1 Solar Energy Materials & Solar Cells 102 (2012) Contents lists available at SciVerse ScienceDirect Solar Energy Materials & Solar Cells journal homepage: Facile metallization of dielectric coatings for plasmonic solar cells S. Bastide a,n, T. Nychyporuk b, Z. Zhou b, A. Fave b, M. Lemiti b a Institut Chimie et Matériaux Paris-Est (ICMPE), CNRS-UMR-7182, UPE, 2-8 rue Henri Dunant, Thiais, France b Université de Lyon, Institute for Nanotechnologies of Lyon (INL), INL-UMR5270, CNRS, INSA de Lyon, 7 avenue Jean Capelle, Bat. Blaise Pascal, Villeurbanne, F-69621, France. article info Available online 3 December 2011 Key words: Metal nanoparticles Hydrogenated silicon nitride Silicon oxide Silicon nanoparticles Electroless metal deposition Plasmonic solar cell abstract Electroless deposition of Ag nanoparticles on PECVD SiN x :H dielectric layers is a new straightforward chemical bath process well adapted to the realization of plasmonic solar cells. Depending on the stoichiometry, SiN x :H contains a certain number of Si nanoclusters embedded in the silicon nitride matrix, that act as reducing agents for Ag ions in solution and thus allows the deposition of Ag nanoparticles directly on the surface. We investigate the influence of the layer stoichiometry and deposition parameters on the nucleation and growth of Ag nanoparticles, and show how they can be adjusted to control the nanoparticle size and density. We also demonstrate that this facile metallization process can be extended to PECVD SiO x layers, another dielectric coating of interest in photovoltaics. & 2011 Elsevier B.V. All rights reserved. 1. Introduction In the past few years, plasmonic nano-structures have appeared as a promising tool to enhance the optical properties of photovoltaic devices [1 3]. For thin film solar cells, scattering of incident light by metal (e.g. Ag and Au) nanoparticles (NPs) on the front or rear side, can strongly improve light trapping and hence absorption [4,5] due to their localized surface plasmon resonance (SPR) properties. Fig. 1 shows the architecture of a plasmonic solar cell using light scattering from metal NPs deposited at the front side. Thanks to the angular spread, not only light has an increased optical path length in the substrate, but the fraction scattered at an angle above the critical angle for reflection at the Si/dielectric rear interface is totally reflected. A metal reflector at the rear allows to reflect the rest of the light (below the critical angle) towards the front interface where it is again scattered by the metal NPs [3]. Recent studies have reported on problems with locating the metal NPs at the front such as absorption losses at wavelength below the surface plasmon resonance due to destructive interference between scattered and incident light [4,6,7]. Putting the scattering NPs on the rear side of the solar cell is currently seen as the best way to solve this problem since light trapping is mostly required for those long wavelength photons that reach the rear side. It also allows to optimize separately the front antireflection coating and the back scattering layer, i.e. the characteristics of the metal NP (size, shape and coverage) and of the underlying dielectric layer (refractive index and thickness) [8,9]. n Corresponding author. address: bastide@glvt-cnrs.fr (S. Bastide). The metal nanoparticles can be deposited by different techniques, the most commonly used being thermal evaporation of metal layers with a subsequent annealing step [10], deposition from colloids and through porous templates. However, these methods are cost and time expensive and cannot be applied for large scale device production. We have recently reported on a new and facile technique for producing Ag nanoparticles on SiN x :H dielectric films used in photovoltaic devices [11]. Ag can be deposited by dipping the SiN x :H coatings in a solution of HF and a Ag salt. The extent of Ag deposition depends on the stoichiometry of the films. We have determined that the driving force for this process is the presence of Si nanoclusters that play the role of sacrificial anode in presence of HF for the reduction of the Ag salt. If we consider the dissolution regime where 2 electrons per Si atom are involved [12], the overall electroless reaction can be summarized by: Siþ6HFþ2Ag þ -H 2 SiF 6 þ2agþ2h þ þh 2 This metal-assisted etching of silicon is known as a deposition method of noble metals on Si and has been studied extensively for etching Si nanostructures, especially Si nanowires [13,14]. In this work, we go further in the investigation of the electroless deposition of Ag by examining the influence of the SiN x :H stoichiometry and chemical conditions on the NP features. Based on our previous findings, it was also logical to envision the possibility of depositing Ag with the same electroless process on other dielectrics containing a fraction of a Si phase. Here, we show for the first time that PECVD SiO x, a standard dielectric material used in solar cells, can indeed be metallized using the same electroless plating method /$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi: /j.solmat

2 S. Bastide et al. / Solar Energy Materials & Solar Cells 102 (2012) Materials and methods P-type (100) oriented c-si wafers, with resistivity of 1 10 O cm were used as substrates for the growth of 300 nm thick SiN x :H and SiO x layers by low frequency plasma enhanced chemical vapor deposition (LF-PECVD) in a semi industrial system operating at 440 khz [15]. ForSiN x :Hagasmixturecomposedofpuresilane (SiH 4 ) and ammonia (NH 3 ) was used. For SiO x layers, NH 3 was replaced by N 2 O. The deposition temperature was less than 400 1C. The gas flow ratios R¼NH 3 /SiH 4 (or NO 2 /SiH 4 ) was adjusted between 0.1 and 30 in order to control the stoichiometry of the layers. In our experimental PECVD conditions, there is much less hydrogen in oxide films than in nitride ones, we therefore use the following notations SiO x and SiN x :H for the materials. Electroless metallization was performed by putting in contact the dielectric layers with an aqueous solution of HF containing AgNO 3 at 0.14 M and 0.5 mm, respectively. The Si substrate was isolated from the solution by an O ring seal. After metallization, the samples were rinsed thoroughly in deionized water and dried with nitrogen. Fig. 1. Example of architecture of a thin film Si solar cell with light trapping based on the surface plasmon properties of metal nanoparticles. The metal nanostructures were observed with a LEO 1530 scanning electron microscope (SEM) and by atomic force microscopy (AFM) in the tapping mode for a scanned area of 1 mm 2. Quantitative analysis of the NP dimensions and surface coverage was performed by computer processing of recorded images using ImageJ and SPIP software for SEM and AFM images, respectively. 3. Results and discussion 3.1. Deposition of Ag NPs on SiN x :H Ag NPs formed on SiN x :H coatings by HF-AgNO 3 dips exhibit features (density and size) that depend on the layer stoichiometry [11]. Under equal chemical conditions (0.14 M HF, room temperature, 5 min dip), Ag deposition is more pronounced as x tends to 0 (Si rich layer), i.e. inversely as R the PECVD gas ratio NH 3 /SiH 4 (because of its more convenient range, R (0 30) rather than x (0 1.33) is used to characterize the layer stoichiometry). Conversely, for a fixed stoichiometry, the NP features may change with the chemical conditions. To go further in the description of the system, we have first examined the influence of the treatment time. A set of experiments lasting from 1 sec to 60 min was conducted on SiN x :H layer made at R¼5, typical for Si antireflection coatings (n¼2.0; fraction of Si10 vol.%, from XPS analysis). AFM observations of the surface are shown in Fig. 2 and the corresponding evolution of the NP density and size as a function of time in Fig. 3. During the first 15 sec the number of Ag NPs rapidly increases (nucleation period), reaches a maximum value at 475 mm 2 after 2 min and then decay exponentially to 70 mm 2 after 60 min. The decay of the NP density must result from a merging process and indeed a large proportion of NPs exhibits a strong shape anisotropy (circularity 51) corresponding to several spherical NPs stuck together (especially after 5 min). More or less the same Fig. 2. AFM images (tapping mode) of SiN x :H surfaces after Ag deposition in HF-AgNO 3 for 1 sec to 60 min.

3 28 S. Bastide et al. / Solar Energy Materials & Solar Cells 102 (2012) Fig. 3. Size and density of Au NPs as a function of deposition time. Fig. 5. Total volume of Ag deposited by mm 2 as a function of the temperature (HF 0.14 M, 5 min). deposition. Measurements of the etch rate of SiN x :H in 0.14 M HF at 15, 25 and 35 1C give 0.9, 1.7 and 2.8 nm/min, respectively. Changing the bath temperature from 15 to 35 1C leads to an increase in etch rate by a factor of 3.1. The volume ratio of deposited Ag found experimentally is 2.7, in relatively good agreement with the latter Electroless deposition of Ag NPs on SiO x layers Fig. 4. Maximum NP density as a function of R, taken from plots of the NP density with time as shown in the inset for R¼1, 3, 7 and 30. evolution is followed for samples with other stoichiometries (see R¼1, 3, 7 and 30 in the inset of Fig. 4), with a maximum density reached after 2 min. As shown in Fig. 4, the maximum density changes by almost three orders of magnitude depending on R. The NP size increases almost linearly with time (Fig. 3, right axis) to reach a mean value of 100 nm after 60 min. Note that this size represents the sphere equivalent diameter calculated from the NP volume (taking into account the coverage, the NP mean height and density). If we consider the total volume of deposited Ag as a function of time, it increases linearly with time as well (not shown here). This is rather surprising if we suppose that Ag deposition relies on the Si nanoclusters in contact with the electrolyte, i.e. those at the very surface of the layer. However, it is well known that HF dissolves SiN x :H [16] and as a consequence the Si nanoclusters embedded in the layer are progressively uncovered ( iceberg effect ) [11]. When brought in contact with the electrolyte, new Ag nanoclusters are formed that aggregate with already existing NPs. Hence, the process can be viewed as a constant supply of Ag accompanied with a merging of NPs, both factors contributing to an increase in NP size with time. Results from the metallization of SiN x :H layers (R¼5; 5 min) at different bath temperatures support this interpretation. As shown in Fig. 5, the total volume of deposited Ag per mm 2 clearly increases with the temperature (5 min treatment). Obviously changing the temperature must affect the dissolution rate of SiN x :H and therefore the volume of Si made available for Ag SiO x is another Si based dielectric commonly used in photovoltaics as an antireflection coatings (often in double or triple stacks with SiN x ) [17] and for surface passivation [18,19]. The PECVD process is based on the use of SiH 4 and N 2 O as precursor gases and the stoichiometry can also be adjusted by controlling the gas ratio R to form Si rich SiO x (low R) to stoichiometric SiO 2 (R¼30). A Si phase is also found in the form of c-si nanoclusters, and its fraction is inversely proportional to the value of R [20]. Because of these similarities with SiN x :H, the electroless plating described here should work for SiO x as well. Ag deposition was carried out in the same chemical conditions, i.e M HF, 0.5 mm AgNO 3 and 5 min dip at room temperature. Fig. 6 shows topographical SEM views of the Ag nanostructures obtained on SiO x as a function of the stoichiometric ratio R. The general trends are similar to those for SiN x :H: (i) well defined individual Ag nanoparticles are obtained; (ii) the surface coverage decreases inversely as R, i.e. when the layer becomes Si poor. However, there are also significant differences. Up to R¼10, the surface coverage seems to be identical for all stoichiometries and a bimodal distribution of size is observed with maxima at 40 and 10 nm. At R¼30, the surface coverage is much lower and only small NPs are observed, with a mean diameter of 20 nm. Hence, for SiO x layers the stoichiometry induced tuning of Ag NP size and density occurs in a much narrower range of R. What we observe indirectly via Ag deposition should correspond to a strong variation in stoichiometry in this range but the reason for this is not clear yet and a detailed investigation of the electroless plating of SiO x for R values between 10 and 30 has been undertaken Application in a solar cell process Integration of the electroless Ag deposition in a solar cell process can be foreseen practically. Taking advantage of the PECVD step, the SiN x :H antireflection coating/passivating layer could be stacked with a top sacrificial layer serving to form Ag NPs during a subsequent HF-AgNO 3 dip. The NP density and size

4 S. Bastide et al. / Solar Energy Materials & Solar Cells 102 (2012) Fig. 6. SEM views of SiO x layers with different stoichiometries (R) after metallization in HF-AgNO 3 for 5 min at 25 1C. will be controlled by the layer stoichiometry and thickness in accordance with their required plasmonic properties for light scattering. With this respect, it is important to note the large range in NP density obtainable by adjusting the layer stoichiometry (cf. Fig. 4), although the maximum spread (after 2 min) is obtained for a NP size limit of 40 nm in the present conditions. This must be compared to thermal evaporation and dewetting of a metal film, the most commonly used technique at present. With this method, the NP size is well controlled through the initial film thickness, but the density cannot be modified since dewetting depends mostly on the physical properties of the metal and underlying material. For wafer based Si cells, wet treatments can be problematic if contacts of the solution with the non-treated side must be avoided (e.g. corrosion in HF). There are however available industrial equipment today that allow single side treatment of wafers and hence could be adapted to one side electroless plating. The treatment time is another important industrial constraint. To obtain large NP (e.g. 100 nm) in the conditions of this study, 60 min are needed, which is not really suitable for a typical wafer process flow. However, a higher HF concentration and/or bath temperature can significantly reduce this duration. For instance, PECVD SiN x :H (R3 5) is etched in concentrated HF (14.6 M) at 70 nm/min instead of 1.7 nm/min in diluted HF (0.14 M), i.e. the treatment time can be reduced from 60 to 1.5 min for the same amount of deposited Ag. Again, this compares favorably to thermal evaporation of a metal film which requires a subsequent dewetting for min at C [6,8], although there is probably room for improvements in this case either. 4. Conclusion Electroless deposition of Ag nanoparticles in HF-AgNO 3 solutions on PECVD SiN x :H dielectric layers used in Si solar cells is a straightforward method that allows good control of the nanoparticle density and size. The density depends strongly on the stoichiometry and the size increases almost linearly with the treatment time, hence a good level of control can be achieved. We assume that the mechanism relies on the release of Si nanoclusters due to the dissolution of SiN x :H by HF. Si plays the role of a reducing agent for Ag ions in solution and, as the dissolution goes on, the surface is under constant supply of Ag. As a result, the Ag nanoparticles grow in size and eventually merge. To support this rationale we gave evidences based on the temperature dependence of the SiN x :H dissolution rate and by demonstrating that Ag can be deposited with the same electroless technique on PECVD SiO x, another dielectrics that also incorporates a Si phase. Ag deposition on SiO x is found to follow approximately the same trends than on SiN x :H. This facile metallization of dielectric coatings is therefore an promising tool to deposit Ag nanoparticles with possibly a unique control of their plasmonic properties. Its implementation in a solar cell process can take advantage of already existing fabrication steps and equipment and could be useful for the realization of various plasmonic solar cell architectures involving dielectrics like SiN x :H or SiO x. References [1] D. Derkacs, S.H. Lim, P. Matheu, W. Mar, E.T. Yu, Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles, Applied Physics Letters 89 (1 3) (2006) [2] S. Pillai, K.R. Catchpole, T. Trupke, M.A. Green, Surface plasmon enhanced silicon solar cells, Journal of Applied Physics 101 (1 8) (2007) [3] H.A. Atwater, A. Polman, Plasmonics for improved photovoltaic devices, Nature Materials 9 (2010) [4] K.R. Catchpole, S. Pillai, Absorption enhancement due to scattering by dipoles into silicon waveguides, Journal of Applied Physics 100 (1 8) (2006) [5] L. Hu, X. Chen, G. Chen, Surface-plasmon enhanced near-bandgap light absorption in silicon photovoltaics, Journal of Computational and Theoretical Nanoscience 5 (2008) [6] F.J. Beck, A. Polman, K.R. Catchpole, Tunable light trapping for solar cells using localized surface plasmons, Journal of Applied Physics 105 (1 7) (2009) [7] S.H. Lim, W. Mar, P. Matheu, D. Derkacs, E.T. Yu, Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles, Journal of Applied Physics 101 (1 7) (2007) [8] S. Pillai, F.J. Beck, K.R. Catchpole, Z. Ouyang, M.A. Green, The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions, Journal of Applied Physics 109 (1 8) (2011)

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