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Synthesis, structure and photoelectrochemical properties of single crystalline silicon nanowire arrays
Thin Solid Films 518 (2010) 1804–1808
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis, structure and photoelectrochemical properties of single crystalline silicon nanowire arrays E.A. Dalchiele a,⁎, F. Martín b, D. Leinen b, R.E. Marotti a, J.R. Ramos-Barrado b a
Instituto de Física, Facultad de Ingeniería, Herrera y Reissig 565, C.C. 30, 11000 Montevideo, Uruguay Laboratorio de Materiales y Superficie (Unidad Asociada al CSIC), Departamentos de Física Aplicada & Ingeniería Química, Universidad de Málaga, Campus de Teatinos s/n, E29071 Málaga, Spain
b
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Silicon Nanowires Photoelectrochemistry Photovoltaic solar cell
a b s t r a c t In the present work, n-type silicon nanowire (n-SiNW) arrays have been synthesized by self-assembly electroless metal deposition (EMD) nanoelectrochemistry. The synthesized n-SiNW arrays have been submitted to scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and optical studies. Initial probes of the solar device conversion properties and the photovoltaic parameters such as short-circuit current, open-circuit potential, and fill factor of the n-SiNW arrays have been explored using a liquidjunction in a photoelectrochemical (PEC) system under white light. Moreover, a direct comparison between the PEC performance of a polished n-Si(100) and the synthesized n-SiNW array photoelectrodes has been done. The PEC performance was significantly enhanced on the n-SiNWs photoelectrodes compared with that on polished n-Si(100). © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last years, much attention has appeared in the literature on the use of low-dimensional semiconductor nanostructures, especially one-dimensional (1D) semiconductor nanorods and nanowires, for the optimization of the photovoltaic energy conversion and reducing cost issues in solar cells [1–8]. The approach used to attempt such optimization is based in the orthogonalization of the directions of light absorption and charge-carrier collection [9–11]. For instance, the semiconducting absorber comprised of a vertically oriented array of high aspect ratio nanowires could in principle provide sufficient optical absorption along its axial dimension, while facilitating collection of carriers radially over a distance sufficiently short to compensate for a short minority carrier collection length (if an inexpensive low-quality material is used) [7–11]. Therefore, the requirements on the minority carrier diffusion length of the absorber material are reduced significantly in comparison to the planar geometry [7–11]. In particular, for an absorber material that is carrier collection limited and does not have an excessively high rate of depletion-region recombination, analysis of the device physics has shown that such nanorods arrays can potentially offer improved performance relative to traditional planar junctions [7,10,11]. For instance, ZnO nanowire- and nanorodbased dye-sensitized solar cells have been produced [2,12], and the
⁎ Corresponding author. Tel.: +598 2 7110905; fax: +598 2 7111630. E-mail address: dalchiel@fing.edu.uy (E.A. Dalchiele). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.037
photovoltaic applications of carbon nanotubes, CdSe and Cd(Se,Te) nanorods have been extensively investigated [9,13,14]. Silicon (Si) as a consequence of its excellent performance, is the most important semiconducting material since the Si-based devices have dominated microelectronic technology for many decades [15,16], and silicon-based solar cells play a dominant role in the photovoltaic market at present. More recently, photovoltaic cells based on silicon nanowire arrays have emerged as a promising candidate for solar energy harvesting [6,10], and promise to lower costs. In fact, silicon nanowires offer several performance and manufacturing benefits that may impact future photovoltaic applications. Silicon nanowires (SiNWs) were first obtained in 1964 by Wagner and Ellis by a vapor–liquid–solid (VLS) mechanism [17]. Since this very first synthesis, many methods have been developed for the growth of SiNWs, such as pulsed laser deposition (PLD) [18,19], gas-phase molecular beam epitaxy (GS-MBE) [20], electron beam lithography (EBL) [21], chemical vapor deposition (CVD) [22,23], and laser ablation [24]. Furthermore, a wet-chemical etching technique in an ionic silver HF solution has been used to prepare SiNW arrays [25,26]. Also, we have focused on this etching technique, denominated here as self-assembly electroless metal deposition nanoelectrochemistry (briefly as SAEMD-nanoelectrochemistry), because we see that this method has several advantages with respect to other SiNWs preparation techniques. In fact, in detail: i) since the asprepared SiNWs are an integral part of the silicon wafer substrate, they provide an uninterrupted electronic pathway, i.e.: SiNWs have a direct, high quality electrical contact with the underlying silicon substrate, are robust and do not peel off from the substrate; ii) the SiNWs have direct 1D electronic pathways allowing for efficient charge transport; iii) the
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electrical properties of the SiNWs are directly hereditary from the bulk silicon substrate, thus there is no need of further doping for conductivity control and iv) due to the rough surface of the SAEMD-nanoelectrochemistry SiNWs, they exhibit a black appearance and they are almost non-reflective due to strong light scattering and absorption. All these advantages make the EMD SiNWs an efficient and promising photoactive material. In the present work, we report on the synthesis of SiNWs and their characterization in view of their potential application in photovoltaic cells. In this way, n-type silicon nanowire (n-SiNW) arrays have been synthesized by the SAEMD-nanoelectrochemistry technique. The synthesized n-SiNW arrays have been submitted to scanning electron microscopy (SEM), transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and optical studies. Initial probes of the solar device conversion properties and the photovoltaic parameters such as short-circuit current, open-circuit potential, and fill factor of the n-SiNW arrays have been explored using a liquid-junction in a photoelectrochemical (PEC) system under white light. Moreover, a direct comparison between the PEC performance of a polished n-Si(100) and the synthesized n-SiNW array photoelectrodes has been done. 2. Experimental Single-side polished Si(100) wafer chips (TOPSIL), 500 μm thick, n-doped (9–15 Ω cm resistivity) with approximate areas of 1 cm2, were first washed in boiling acetone for 10 min and subsequently in isopropanol at room temperature, with sonication for 5 min. They were oxidized in H2O2/HCl/H2O (2:1:8) at 353 K for 15 min to remove any trace of heavy metals and organic species, rinsed copiously with deionized water, etched with 10% aqueous HF for 10 min, rinsed with water again, dried under a stream of N2 and immediately used either as a substrate in the SiNW growth process or in the preparation of the planar polished n-Si(100) photoelectrodes. The n-type silicon nanowire arrays have been synthesized by self-assembly electroless metal deposition (SAEMD) nanoelectrochemistry on the silicon wafer chips in an ionic silver HF solution through selective etching. The cleaned Si wafer chips were immersed into an aqueous HF/AgNO3 solution for 15–30 min at 50 °C (the concentration of HF and AgNO3 here are chosen to be 4.6 M and 0.02 M, respectively). The length of SiNWs could be effectively controlled through tuning the treatment time. After the treatment, the as-synthesized samples were rinsed copiously in deionized water and dried at room temperature. Then, the SiNW arrays were dipped in 30 wt.% HNO3 aqueous solution for 60 s and repeated for several times to remove all residual Ag from the nanowire surfaces. After the nitric acid bath no Ag peaks appeared in the XPS spectra of the nanowires (results not shown). Eventually, and in order to prepare the planar polished n-Si and SiNW array photoelectrodes, ohmic contact was made to the back of the polished n-Si and SiNW samples by scratching the Si surface, rubbing it with Ga–In eutectic and attaching a copper contact to it. The edges of the silicon squares and the copper were isolated with an adhesive Teflon tape leaving a defined exposed area of 0.196 cm2 facing the solution. Before the electrochemical experiments, the electrode surface was again etched for 15 min with the 10% aqueous HF solution (after this procedure an atomically smooth and hydrogen terminated surface is obtained). Scanning electron microscopy (SEM) pictures were obtained with a JEOL JSM-5410 apparatus. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) were carried out with a Philips CM-200 microscope operated at 200 kV. Specimens for TEM were prepared by removing the electrodeposited material by grating with a scalpel, collected and ultrasonically dispersing in 1 ml of ethanol. A small drop of the suspended solution was placed onto a porous carbon film supported on a TEM nickel grid, and was dried in air prior to observation.
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XPS surface analysis (X-ray source: Mg Kα (1253.6 eV), 15 kV, 300 W) and depth profiling were carried out with a PHI 5700 equipment recording survey spectra and high-resolution multiregional spectra. The pressure in the analysis chamber was about 10− 7 Pa. The sputter rate at 4 keV Ar+ and beam rastering of 3 × 3 mm2 can be assumed to be approximately 3.4 nm/min in SiO2 and 3.6 nm/min in silicon. These values are in reference to the sputter rate of 4 nm/min determined in Ta2O5 under identical sputter conditions. XPS atomic concentrations were calculated from Si 2p, O 1s, C 1s, F 1s and Ag 3d photoelectron peak areas using Shirley background subtraction and sensitivity factors supplied by the spectrometer manufacturer (PHI). Optical reflectance spectra between 400 to 900 nm were measured with an S2000 Ocean Optics spectrophotometer. Photoelectrochemical studies were carried out in a three-armed cell with a platinum counter electrode and saturated calomel electrode as reference. All potential values in this paper are reported vs. this reference electrode. This photoelectrochemical studies have been performed in a 0.5 M KNO3 + 0.1 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 solution. All solutions were prepared from analytical grade reagents and 18 MΩ cm Millipore water. The electrolyte solutions were purged with nitrogen for 30 min prior to each experimental series and then kept under flowing nitrogen during the experiments. A 200 W tungstenhalogen lamp (ORIEL) was used as the light source. The incident power at the electrode surface was determined as 40 mW/cm2 (using a previously calibrated solar cell with a Kipp & Zonen piranometer), after eliminating the light reflection and absorption effects at the Pyrex glass cell and electrolyte, respectively. The PEC performance of the n-Si and nSiNWs photoelectrodes has been assessed by using linear sweep voltammetry both in the dark and under illumination by using an Autolab Electrochemical Analyzer (model PGSTAT 30, Eco Chemie BV, The Netherlands). 3. Results and discussion 3.1. Morphological and structural characterization The SEM images of a typical SiNW array synthesized by the SAEMD-nanoelectrochemistry approach on n-type Si(100) substrate (30 min etching) are shown in Fig. 1. SEM observations reveal that large quantities of silicon nanowire arrays could be produced on the silicon wafer chip. The density of SiNW arrays (Fig. 1a) is about 109/ cm2, which shows little changes with different etching times. Fig. 1b shows the cross-section details of the SiNW array in which all SiNWs are distinguishable and most of them are vertical to the wafer surface, exhibiting a length about 30 μm and diameters in the range of 100– 160 nm. Some disorder SiNWs arise from cutting and loading of the SEM sample. According to the literature reports [25,26] and our experimental results, the SiNW length can be controlled by varying the synthesizing time (etching time), and the diameter of SiNW does not show noticeable change during synthesizing time. Furthermore, the morphology and structure of SiNWs have been characterized in detail using TEM and HRTEM analysis. Fig. 2a shows TEM image of a typical SiNW of ca. 130 nm diameter. The SiNW were single crystalline, as shown by the SAED pattern (inset of Fig. 2a) and HRTEM image of the Si lattice of a SiNW in Fig. 2b. A lattice spacing of d ∼ 0.314 nm has been determined (see Fig. 2b), which corresponds to the (111) plane of cubic silicon (0.31380 nm [27]). A further detailed analysis of HRTEM indicates that the axial crystallographic orientation of the SiNWs is the [100] direction (forming an angle of 54.7° with the [111] direction) (see Fig. 2b), which is identical with the orientation of the initially used silicon wafer. So, those HRTEM investigations confirmed that the axial crystallographic directions of as-synthesized one-dimensional silicon nanostructures are determined by the orientation of the initial silicon wafers used. More specifically, SiNW with desirable orientations could be created through selecting silicon wafers with identical orientations.
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Fig. 1. SEM images of well aligned SiNW arrays: (a) top view and (b) cross-sectional view.
Fig. 2. (a) Typical TEM image of an individual SiNW and its corresponding selected area electron diffraction (SAED) pattern (as inset). (b) An HRTEM image of this SiNW.
3.2. XPS characterization
passivation layer and the carbon contamination at the sample surface are almost eliminated, more effectively in the silicon wafer area which is flat compared to the nanowire area with its high surface roughness. Hence for the latter, even after 20 min of 4 keV Ar+ ion etching, some silicon dioxide remain due to shadow effects for the ion beam. The spectra of the original surfaces in Fig. 3, i.e. before 4 keV Ar+ ion etching, were referenced in binding energy to the C 1s peak at 285.0 eV corresponding to the adventitious surface contamination of aliphatic carbon. After sputtering, spectra were referenced keeping the position of the Si 2p peak of silicon (Si0) at its original value. In the case of the untreated silicon wafer area, spectra had not been shifted whereas for the nanowire area spectra of the original surface were shifted by −0.8 eV in binding energy. The latter is probably due to both, some surface charging since the nanowire surface is highly passivated, and the different surface morphologies (surface roughness) in that area where the nanowires have been produced. Binding energy values of the peaks and the energy difference of Si 2p peaks corresponding to Si0 and Si4+ states are indicated in Fig. 3. It is interesting that this energy difference increases from 4.0 eV in the silicon wafer area to 4.9 eV in the nanowire area. Considering the binding energy values of O 1s, C1s and Si 2p at 533.1 eV, 285.0 eV and
For the sake of comparison, two areas of the same sample (sample NWSi4) were analyzed by XPS: a) the SiNW area (NW) i.e. where the silicon nanowires have been produced by SAEMD-nanoelectrochemistry, and b) the area of the untreated silicon wafer chip (SS). Fig. 3 shows the O 1s, C 1s and Si 2p photoelectron spectra of both areas in its original state and after 10 and 20 min of 4 keV Ar+ ion etching. Table 1 shows the corresponding atomic concentrations calculated for carbon, oxygen and silicon. In the nanowire area, silver was below detection limit (<0.01 at.%) which shows the effectiveness of the procedure in eliminating the electrochemical agent. Only a small amount of fluorine (<1 at.%) was detected, falling below detection limit after Ar sputtering. Therefore, both elements which come from the electrochemical treatment, have not been included in Table 1. Comparing the atomic concentrations of the original surfaces (Table 1), we observe a very low carbon surface contamination in the nanowire area (see also Fig. 3). This is due to a better and more effective surface passivation of silicon to silicon dioxide in the nanowire area; the oxygen content is nearly double of that in the silicon wafer area albeit both areas show about the same amount of silicon. By prolonged 4 keV Ar+ ion etching the
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Fig. 3. O 1s, C 1s and Si2p photoelectron spectra for the SiNW area (NW) and the bare silicon wafer area (SS) of the original surfaces and after 10 and 20 min of 4 KeV Ar+ etching.
103.9 eV, respectively, in the nanowire area, they agree well with those of highly passivated n-type silicon [28]. The binding energy values of O 1s, C1s and Si 2p (Si0 and Si4+ states) at 532.8 eV, 285.0 eV, 103.5 eV and 99.5 eV, respectively, in the silicon wafer area, also agree well with values for n-type silicon with surface oxidation [29]. However, the shift of the Si 2p (Si0) to 99.0 eV in the nanowire area seems to indicate that the electrochemical treatment produce an even more n-type silicon material. Core level shifts to lower binding energy of about 0.5 eV have been observed for increasing amounts of arsenic doping in monocrystalline silicon [30]. Nevertheless, comparing the binding energy values of O 1s and Si 2p (Si4+ state) of the nanowire area with the wafer area, shows that some surface charging in the passivation layer of the nanowires may also contribute to the increase in energy difference of Si 2p Si0 and Si4+ states. 3.3. Optical characterization Nanowires not only offer the advantage of more efficient charge transport compared to other nanostructured materials configurations, but also present the potential for improved optical absorption characteristics [31]. In fact, the prepared SiNW array samples are black in appearance and non-reflective to the naked eye. In order to quantify this fact, the spectra of Fig. 4 compare the optical reflectance of the SiNW array with the one of a bare polished n-Si. Both spectra were normalized to the maximum reflectance of the bare silicon sample. For this reason the polished Si sample is almost flat and equal to 100%. The reflectance of the SiNW array is less than 5% smaller than
the one of the bare polished n-Si sample, in the entire measured spectra. In either case no clear structure can be found. 3.4. Photoelectrochemical study Fig. 5 shows typical steady-state j-E characteristics under 40 mW/ cm2 white illumination of polished n-Si and n-SiNW (etching time 3− 15 min) array photoelectrodes in the 0.1/0.1 M Fe(CN)4− 6 /Fe(CN)6 solution with Eo = 0.209 V vs. SCE. Here, the potential was swept from cathodic to anodic direction, and the curves were obtained immediately after etching the electrodes in a 10% HF solution for 10 min. Representative photoelectrochemical output data, i.e.: open-circuit potential, Voc, short-circuit photocurrent density, Jsc, fill factor and conversion efficiency, from these curves are given as insets in Fig. 5. It can be appreciated that the SiNW array photoelectrodes exhibited a Voc value lower than that of the polished n-Si ones. The last, is in agreement of previously reported photoelectrochemical studies in which nanorod (Cd(Se,Te)) [9] and nanowire (Si)[32] array electrodes yielded smaller Voc values than the respective planar ones. The SiNW
Table 1 Atomic concentrations (%) of the silicon nanowire (NW) and the silicon wafer surface (SS) before and after 4 keV Ar+ sputtering. Surface
C
O
Si
NW original NW 10 min Ar+ NW 20 min Ar+ SS original SS 10 min Ar+ SS 20 min Ar+
8.9 2.2 2.0 32.5 6.3 1.7
53.9 20.1 13.7 29.6 2.2 1.8
37.2 77.7 84.3 37.9 91.5 96.5
Fig. 4. Reflectance as a function of wavelength for an n-SiNW array Si wafer chip sample and a bare polished n-Si (normalized to the maximum reflectance of a bare silicon sample).
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measured under 40 mW cm− 2 white illumination conditions. Voc was lower, in average, for SiNWs array photoelectrodes, while Jsc, fill factor and overall efficiencies of the SiNWs array photoelectrodes were generally superior to those of the bare polished Si junction devices. Acknowledgments The authors are grateful to Universidad de Málaga, Málaga, Spain, for the financial support received. E.A.D. and R.E.M. also acknowledge the financial support of PEDECIBA-Física, and especially to C.S.I.C., Universidad de la República, Montevideo, Uruguay. The authors also thank A. Martínez Orellana and Gregorio Martín (Málaga, Spain) for SEM and TEM measurements. References Fig. 5. Current density-voltage characteristics for typical n-Si nanowire array and clean polished n-Si photoelectrodes in 0.5 M KNO3 + 0.1 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 solution under white illumination (40 mW/cm2). The blue and red solid lines are the behavior of the n-SiNW array and bare polished n-Si photoelectrodes, respectively. The equilibrium formal redox potential (Eo = 0.209 V vs. SCE) for the [Fe(CN)6]3−/4− redox couple corresponds to the right axis of the plot.
array electrodes exhibited, however, higher overall short-circuit current densities ( Jsc) than the polished planar n-Si electrodes (see inset Fig. 5). The last is in line with a previously published work [33]. Jsc provides information on the net quantum yield for charge separation and also serves as a measure of the yield of minority carriers that survive the cross of the solid–liquid interface [34]. This enhancement in the photocurrent of the SiNW photoelectrodes compared with the planar polished n-Si ones can be due to the excellent antireflective ability of the SiNW arrays and to a better collection of minority carriers. Moreover, the SiNW arrays exhibited better fill factors than the planar n-Si samples investigated in the present study (see inset Fig. 5). This fact can be in principle attributable to an improved ratio of charge-transfer relative to surface recombination, as a result of increasing the internal junction area [9]. The optimization of PEC efficiency required not only high Jsc values but also high values of the fill factor [34]. Furthermore, the overall efficiency of the SiNW array photoelectrodes is approximately twice that of those of planar n-Si junction devices (see inset Fig. 5). 4. Conclusions Vertically aligned arrays of high aspect single crystalline n-type silicon nanowires have been synthesized by self-assembly electroless metal deposition (EMD) nanoelectrochemistry. XPS has shown that silicon nanowires are of pure n-type silicon with an effective surface passivation. Silver, the main electrochemical agent, was not detected by XPS which shows the effectiveness of the electrochemical procedure in eliminating the electrochemical agent. Their photoelectrochemical properties were measured and compared with those of its untreated bare polished Si sample counterpart. The open-circuit photovoltage (Voc), short-circuit current density ( Jsc), fill factor, and overall energy conversion efficiency for both types of electrodes were
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis, structure and photoelectrochemical properties of single crystalline silicon nanowire arrays E.A. Dalchiele a,⁎, F. Martín b, D. Leinen b, R.E. Marotti a, J.R. Ramos-Barrado b a
Instituto de Física, Facultad de Ingeniería, Herrera y Reissig 565, C.C. 30, 11000 Montevideo, Uruguay Laboratorio de Materiales y Superficie (Unidad Asociada al CSIC), Departamentos de Física Aplicada & Ingeniería Química, Universidad de Málaga, Campus de Teatinos s/n, E29071 Málaga, Spain
b
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Silicon Nanowires Photoelectrochemistry Photovoltaic solar cell
a b s t r a c t In the present work, n-type silicon nanowire (n-SiNW) arrays have been synthesized by self-assembly electroless metal deposition (EMD) nanoelectrochemistry. The synthesized n-SiNW arrays have been submitted to scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and optical studies. Initial probes of the solar device conversion properties and the photovoltaic parameters such as short-circuit current, open-circuit potential, and fill factor of the n-SiNW arrays have been explored using a liquidjunction in a photoelectrochemical (PEC) system under white light. Moreover, a direct comparison between the PEC performance of a polished n-Si(100) and the synthesized n-SiNW array photoelectrodes has been done. The PEC performance was significantly enhanced on the n-SiNWs photoelectrodes compared with that on polished n-Si(100). © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last years, much attention has appeared in the literature on the use of low-dimensional semiconductor nanostructures, especially one-dimensional (1D) semiconductor nanorods and nanowires, for the optimization of the photovoltaic energy conversion and reducing cost issues in solar cells [1–8]. The approach used to attempt such optimization is based in the orthogonalization of the directions of light absorption and charge-carrier collection [9–11]. For instance, the semiconducting absorber comprised of a vertically oriented array of high aspect ratio nanowires could in principle provide sufficient optical absorption along its axial dimension, while facilitating collection of carriers radially over a distance sufficiently short to compensate for a short minority carrier collection length (if an inexpensive low-quality material is used) [7–11]. Therefore, the requirements on the minority carrier diffusion length of the absorber material are reduced significantly in comparison to the planar geometry [7–11]. In particular, for an absorber material that is carrier collection limited and does not have an excessively high rate of depletion-region recombination, analysis of the device physics has shown that such nanorods arrays can potentially offer improved performance relative to traditional planar junctions [7,10,11]. For instance, ZnO nanowire- and nanorodbased dye-sensitized solar cells have been produced [2,12], and the
⁎ Corresponding author. Tel.: +598 2 7110905; fax: +598 2 7111630. E-mail address: dalchiel@fing.edu.uy (E.A. Dalchiele). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.037
photovoltaic applications of carbon nanotubes, CdSe and Cd(Se,Te) nanorods have been extensively investigated [9,13,14]. Silicon (Si) as a consequence of its excellent performance, is the most important semiconducting material since the Si-based devices have dominated microelectronic technology for many decades [15,16], and silicon-based solar cells play a dominant role in the photovoltaic market at present. More recently, photovoltaic cells based on silicon nanowire arrays have emerged as a promising candidate for solar energy harvesting [6,10], and promise to lower costs. In fact, silicon nanowires offer several performance and manufacturing benefits that may impact future photovoltaic applications. Silicon nanowires (SiNWs) were first obtained in 1964 by Wagner and Ellis by a vapor–liquid–solid (VLS) mechanism [17]. Since this very first synthesis, many methods have been developed for the growth of SiNWs, such as pulsed laser deposition (PLD) [18,19], gas-phase molecular beam epitaxy (GS-MBE) [20], electron beam lithography (EBL) [21], chemical vapor deposition (CVD) [22,23], and laser ablation [24]. Furthermore, a wet-chemical etching technique in an ionic silver HF solution has been used to prepare SiNW arrays [25,26]. Also, we have focused on this etching technique, denominated here as self-assembly electroless metal deposition nanoelectrochemistry (briefly as SAEMD-nanoelectrochemistry), because we see that this method has several advantages with respect to other SiNWs preparation techniques. In fact, in detail: i) since the asprepared SiNWs are an integral part of the silicon wafer substrate, they provide an uninterrupted electronic pathway, i.e.: SiNWs have a direct, high quality electrical contact with the underlying silicon substrate, are robust and do not peel off from the substrate; ii) the SiNWs have direct 1D electronic pathways allowing for efficient charge transport; iii) the
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electrical properties of the SiNWs are directly hereditary from the bulk silicon substrate, thus there is no need of further doping for conductivity control and iv) due to the rough surface of the SAEMD-nanoelectrochemistry SiNWs, they exhibit a black appearance and they are almost non-reflective due to strong light scattering and absorption. All these advantages make the EMD SiNWs an efficient and promising photoactive material. In the present work, we report on the synthesis of SiNWs and their characterization in view of their potential application in photovoltaic cells. In this way, n-type silicon nanowire (n-SiNW) arrays have been synthesized by the SAEMD-nanoelectrochemistry technique. The synthesized n-SiNW arrays have been submitted to scanning electron microscopy (SEM), transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and optical studies. Initial probes of the solar device conversion properties and the photovoltaic parameters such as short-circuit current, open-circuit potential, and fill factor of the n-SiNW arrays have been explored using a liquid-junction in a photoelectrochemical (PEC) system under white light. Moreover, a direct comparison between the PEC performance of a polished n-Si(100) and the synthesized n-SiNW array photoelectrodes has been done. 2. Experimental Single-side polished Si(100) wafer chips (TOPSIL), 500 μm thick, n-doped (9–15 Ω cm resistivity) with approximate areas of 1 cm2, were first washed in boiling acetone for 10 min and subsequently in isopropanol at room temperature, with sonication for 5 min. They were oxidized in H2O2/HCl/H2O (2:1:8) at 353 K for 15 min to remove any trace of heavy metals and organic species, rinsed copiously with deionized water, etched with 10% aqueous HF for 10 min, rinsed with water again, dried under a stream of N2 and immediately used either as a substrate in the SiNW growth process or in the preparation of the planar polished n-Si(100) photoelectrodes. The n-type silicon nanowire arrays have been synthesized by self-assembly electroless metal deposition (SAEMD) nanoelectrochemistry on the silicon wafer chips in an ionic silver HF solution through selective etching. The cleaned Si wafer chips were immersed into an aqueous HF/AgNO3 solution for 15–30 min at 50 °C (the concentration of HF and AgNO3 here are chosen to be 4.6 M and 0.02 M, respectively). The length of SiNWs could be effectively controlled through tuning the treatment time. After the treatment, the as-synthesized samples were rinsed copiously in deionized water and dried at room temperature. Then, the SiNW arrays were dipped in 30 wt.% HNO3 aqueous solution for 60 s and repeated for several times to remove all residual Ag from the nanowire surfaces. After the nitric acid bath no Ag peaks appeared in the XPS spectra of the nanowires (results not shown). Eventually, and in order to prepare the planar polished n-Si and SiNW array photoelectrodes, ohmic contact was made to the back of the polished n-Si and SiNW samples by scratching the Si surface, rubbing it with Ga–In eutectic and attaching a copper contact to it. The edges of the silicon squares and the copper were isolated with an adhesive Teflon tape leaving a defined exposed area of 0.196 cm2 facing the solution. Before the electrochemical experiments, the electrode surface was again etched for 15 min with the 10% aqueous HF solution (after this procedure an atomically smooth and hydrogen terminated surface is obtained). Scanning electron microscopy (SEM) pictures were obtained with a JEOL JSM-5410 apparatus. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) were carried out with a Philips CM-200 microscope operated at 200 kV. Specimens for TEM were prepared by removing the electrodeposited material by grating with a scalpel, collected and ultrasonically dispersing in 1 ml of ethanol. A small drop of the suspended solution was placed onto a porous carbon film supported on a TEM nickel grid, and was dried in air prior to observation.
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XPS surface analysis (X-ray source: Mg Kα (1253.6 eV), 15 kV, 300 W) and depth profiling were carried out with a PHI 5700 equipment recording survey spectra and high-resolution multiregional spectra. The pressure in the analysis chamber was about 10− 7 Pa. The sputter rate at 4 keV Ar+ and beam rastering of 3 × 3 mm2 can be assumed to be approximately 3.4 nm/min in SiO2 and 3.6 nm/min in silicon. These values are in reference to the sputter rate of 4 nm/min determined in Ta2O5 under identical sputter conditions. XPS atomic concentrations were calculated from Si 2p, O 1s, C 1s, F 1s and Ag 3d photoelectron peak areas using Shirley background subtraction and sensitivity factors supplied by the spectrometer manufacturer (PHI). Optical reflectance spectra between 400 to 900 nm were measured with an S2000 Ocean Optics spectrophotometer. Photoelectrochemical studies were carried out in a three-armed cell with a platinum counter electrode and saturated calomel electrode as reference. All potential values in this paper are reported vs. this reference electrode. This photoelectrochemical studies have been performed in a 0.5 M KNO3 + 0.1 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 solution. All solutions were prepared from analytical grade reagents and 18 MΩ cm Millipore water. The electrolyte solutions were purged with nitrogen for 30 min prior to each experimental series and then kept under flowing nitrogen during the experiments. A 200 W tungstenhalogen lamp (ORIEL) was used as the light source. The incident power at the electrode surface was determined as 40 mW/cm2 (using a previously calibrated solar cell with a Kipp & Zonen piranometer), after eliminating the light reflection and absorption effects at the Pyrex glass cell and electrolyte, respectively. The PEC performance of the n-Si and nSiNWs photoelectrodes has been assessed by using linear sweep voltammetry both in the dark and under illumination by using an Autolab Electrochemical Analyzer (model PGSTAT 30, Eco Chemie BV, The Netherlands). 3. Results and discussion 3.1. Morphological and structural characterization The SEM images of a typical SiNW array synthesized by the SAEMD-nanoelectrochemistry approach on n-type Si(100) substrate (30 min etching) are shown in Fig. 1. SEM observations reveal that large quantities of silicon nanowire arrays could be produced on the silicon wafer chip. The density of SiNW arrays (Fig. 1a) is about 109/ cm2, which shows little changes with different etching times. Fig. 1b shows the cross-section details of the SiNW array in which all SiNWs are distinguishable and most of them are vertical to the wafer surface, exhibiting a length about 30 μm and diameters in the range of 100– 160 nm. Some disorder SiNWs arise from cutting and loading of the SEM sample. According to the literature reports [25,26] and our experimental results, the SiNW length can be controlled by varying the synthesizing time (etching time), and the diameter of SiNW does not show noticeable change during synthesizing time. Furthermore, the morphology and structure of SiNWs have been characterized in detail using TEM and HRTEM analysis. Fig. 2a shows TEM image of a typical SiNW of ca. 130 nm diameter. The SiNW were single crystalline, as shown by the SAED pattern (inset of Fig. 2a) and HRTEM image of the Si lattice of a SiNW in Fig. 2b. A lattice spacing of d ∼ 0.314 nm has been determined (see Fig. 2b), which corresponds to the (111) plane of cubic silicon (0.31380 nm [27]). A further detailed analysis of HRTEM indicates that the axial crystallographic orientation of the SiNWs is the [100] direction (forming an angle of 54.7° with the [111] direction) (see Fig. 2b), which is identical with the orientation of the initially used silicon wafer. So, those HRTEM investigations confirmed that the axial crystallographic directions of as-synthesized one-dimensional silicon nanostructures are determined by the orientation of the initial silicon wafers used. More specifically, SiNW with desirable orientations could be created through selecting silicon wafers with identical orientations.
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Fig. 1. SEM images of well aligned SiNW arrays: (a) top view and (b) cross-sectional view.
Fig. 2. (a) Typical TEM image of an individual SiNW and its corresponding selected area electron diffraction (SAED) pattern (as inset). (b) An HRTEM image of this SiNW.
3.2. XPS characterization
passivation layer and the carbon contamination at the sample surface are almost eliminated, more effectively in the silicon wafer area which is flat compared to the nanowire area with its high surface roughness. Hence for the latter, even after 20 min of 4 keV Ar+ ion etching, some silicon dioxide remain due to shadow effects for the ion beam. The spectra of the original surfaces in Fig. 3, i.e. before 4 keV Ar+ ion etching, were referenced in binding energy to the C 1s peak at 285.0 eV corresponding to the adventitious surface contamination of aliphatic carbon. After sputtering, spectra were referenced keeping the position of the Si 2p peak of silicon (Si0) at its original value. In the case of the untreated silicon wafer area, spectra had not been shifted whereas for the nanowire area spectra of the original surface were shifted by −0.8 eV in binding energy. The latter is probably due to both, some surface charging since the nanowire surface is highly passivated, and the different surface morphologies (surface roughness) in that area where the nanowires have been produced. Binding energy values of the peaks and the energy difference of Si 2p peaks corresponding to Si0 and Si4+ states are indicated in Fig. 3. It is interesting that this energy difference increases from 4.0 eV in the silicon wafer area to 4.9 eV in the nanowire area. Considering the binding energy values of O 1s, C1s and Si 2p at 533.1 eV, 285.0 eV and
For the sake of comparison, two areas of the same sample (sample NWSi4) were analyzed by XPS: a) the SiNW area (NW) i.e. where the silicon nanowires have been produced by SAEMD-nanoelectrochemistry, and b) the area of the untreated silicon wafer chip (SS). Fig. 3 shows the O 1s, C 1s and Si 2p photoelectron spectra of both areas in its original state and after 10 and 20 min of 4 keV Ar+ ion etching. Table 1 shows the corresponding atomic concentrations calculated for carbon, oxygen and silicon. In the nanowire area, silver was below detection limit (<0.01 at.%) which shows the effectiveness of the procedure in eliminating the electrochemical agent. Only a small amount of fluorine (<1 at.%) was detected, falling below detection limit after Ar sputtering. Therefore, both elements which come from the electrochemical treatment, have not been included in Table 1. Comparing the atomic concentrations of the original surfaces (Table 1), we observe a very low carbon surface contamination in the nanowire area (see also Fig. 3). This is due to a better and more effective surface passivation of silicon to silicon dioxide in the nanowire area; the oxygen content is nearly double of that in the silicon wafer area albeit both areas show about the same amount of silicon. By prolonged 4 keV Ar+ ion etching the
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Fig. 3. O 1s, C 1s and Si2p photoelectron spectra for the SiNW area (NW) and the bare silicon wafer area (SS) of the original surfaces and after 10 and 20 min of 4 KeV Ar+ etching.
103.9 eV, respectively, in the nanowire area, they agree well with those of highly passivated n-type silicon [28]. The binding energy values of O 1s, C1s and Si 2p (Si0 and Si4+ states) at 532.8 eV, 285.0 eV, 103.5 eV and 99.5 eV, respectively, in the silicon wafer area, also agree well with values for n-type silicon with surface oxidation [29]. However, the shift of the Si 2p (Si0) to 99.0 eV in the nanowire area seems to indicate that the electrochemical treatment produce an even more n-type silicon material. Core level shifts to lower binding energy of about 0.5 eV have been observed for increasing amounts of arsenic doping in monocrystalline silicon [30]. Nevertheless, comparing the binding energy values of O 1s and Si 2p (Si4+ state) of the nanowire area with the wafer area, shows that some surface charging in the passivation layer of the nanowires may also contribute to the increase in energy difference of Si 2p Si0 and Si4+ states. 3.3. Optical characterization Nanowires not only offer the advantage of more efficient charge transport compared to other nanostructured materials configurations, but also present the potential for improved optical absorption characteristics [31]. In fact, the prepared SiNW array samples are black in appearance and non-reflective to the naked eye. In order to quantify this fact, the spectra of Fig. 4 compare the optical reflectance of the SiNW array with the one of a bare polished n-Si. Both spectra were normalized to the maximum reflectance of the bare silicon sample. For this reason the polished Si sample is almost flat and equal to 100%. The reflectance of the SiNW array is less than 5% smaller than
the one of the bare polished n-Si sample, in the entire measured spectra. In either case no clear structure can be found. 3.4. Photoelectrochemical study Fig. 5 shows typical steady-state j-E characteristics under 40 mW/ cm2 white illumination of polished n-Si and n-SiNW (etching time 3− 15 min) array photoelectrodes in the 0.1/0.1 M Fe(CN)4− 6 /Fe(CN)6 solution with Eo = 0.209 V vs. SCE. Here, the potential was swept from cathodic to anodic direction, and the curves were obtained immediately after etching the electrodes in a 10% HF solution for 10 min. Representative photoelectrochemical output data, i.e.: open-circuit potential, Voc, short-circuit photocurrent density, Jsc, fill factor and conversion efficiency, from these curves are given as insets in Fig. 5. It can be appreciated that the SiNW array photoelectrodes exhibited a Voc value lower than that of the polished n-Si ones. The last, is in agreement of previously reported photoelectrochemical studies in which nanorod (Cd(Se,Te)) [9] and nanowire (Si)[32] array electrodes yielded smaller Voc values than the respective planar ones. The SiNW
Table 1 Atomic concentrations (%) of the silicon nanowire (NW) and the silicon wafer surface (SS) before and after 4 keV Ar+ sputtering. Surface
C
O
Si
NW original NW 10 min Ar+ NW 20 min Ar+ SS original SS 10 min Ar+ SS 20 min Ar+
8.9 2.2 2.0 32.5 6.3 1.7
53.9 20.1 13.7 29.6 2.2 1.8
37.2 77.7 84.3 37.9 91.5 96.5
Fig. 4. Reflectance as a function of wavelength for an n-SiNW array Si wafer chip sample and a bare polished n-Si (normalized to the maximum reflectance of a bare silicon sample).
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measured under 40 mW cm− 2 white illumination conditions. Voc was lower, in average, for SiNWs array photoelectrodes, while Jsc, fill factor and overall efficiencies of the SiNWs array photoelectrodes were generally superior to those of the bare polished Si junction devices. Acknowledgments The authors are grateful to Universidad de Málaga, Málaga, Spain, for the financial support received. E.A.D. and R.E.M. also acknowledge the financial support of PEDECIBA-Física, and especially to C.S.I.C., Universidad de la República, Montevideo, Uruguay. The authors also thank A. Martínez Orellana and Gregorio Martín (Málaga, Spain) for SEM and TEM measurements. References Fig. 5. Current density-voltage characteristics for typical n-Si nanowire array and clean polished n-Si photoelectrodes in 0.5 M KNO3 + 0.1 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 solution under white illumination (40 mW/cm2). The blue and red solid lines are the behavior of the n-SiNW array and bare polished n-Si photoelectrodes, respectively. The equilibrium formal redox potential (Eo = 0.209 V vs. SCE) for the [Fe(CN)6]3−/4− redox couple corresponds to the right axis of the plot.
array electrodes exhibited, however, higher overall short-circuit current densities ( Jsc) than the polished planar n-Si electrodes (see inset Fig. 5). The last is in line with a previously published work [33]. Jsc provides information on the net quantum yield for charge separation and also serves as a measure of the yield of minority carriers that survive the cross of the solid–liquid interface [34]. This enhancement in the photocurrent of the SiNW photoelectrodes compared with the planar polished n-Si ones can be due to the excellent antireflective ability of the SiNW arrays and to a better collection of minority carriers. Moreover, the SiNW arrays exhibited better fill factors than the planar n-Si samples investigated in the present study (see inset Fig. 5). This fact can be in principle attributable to an improved ratio of charge-transfer relative to surface recombination, as a result of increasing the internal junction area [9]. The optimization of PEC efficiency required not only high Jsc values but also high values of the fill factor [34]. Furthermore, the overall efficiency of the SiNW array photoelectrodes is approximately twice that of those of planar n-Si junction devices (see inset Fig. 5). 4. Conclusions Vertically aligned arrays of high aspect single crystalline n-type silicon nanowires have been synthesized by self-assembly electroless metal deposition (EMD) nanoelectrochemistry. XPS has shown that silicon nanowires are of pure n-type silicon with an effective surface passivation. Silver, the main electrochemical agent, was not detected by XPS which shows the effectiveness of the electrochemical procedure in eliminating the electrochemical agent. Their photoelectrochemical properties were measured and compared with those of its untreated bare polished Si sample counterpart. The open-circuit photovoltage (Voc), short-circuit current density ( Jsc), fill factor, and overall energy conversion efficiency for both types of electrodes were
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