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Synthesis and characterization of silicon nanowires using tin catalyst for solar cells application Minsung Jeon ⁎ , Koichi Kamisako Department of Electronic and Information Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan abstractarticle info Article history: Received 20 November 2008 Accepted 6 January 2009 Available online 8 January 20 09 Keywords: Sn-catalyzed SiNWs Reflectance Solar cells Phosphorous diffusion VLS mechanism Hydrogen radicals Tin-catalyzed silicon nanowires were synthesized for solar cells application. Voluminous silicon nanowires were fabricated on single crystalline silicon wafer. Optical reflectance and solar cell efficiency of the synthesized silicon nanowires were explored. The reflectance of as-synthesized silicon nanowires was obtained approximately 5% in the short wavelength region (λb 500 nm). A short circuit current of 2.3 mA/cm 2 and open circuit voltage of 520 mV for 1 cm 2 SiNWs solar cell was obtained. © 2009 Elsevier B.V. All rights reserved. 1. Introduction In recent years, nanowires including nanorods based solar cells have attractive interest due to their characteristics and processing benefits. The nanowires-enabled solar cells allow for decoupling the light absorption from the direction of carrier transport, such that in materials where the diffusion length of minority carriers is much shorter than the thickness of material required for optimal light absorption, current densities can be improved. Nanostructure solar cells, such as organic–inorganic materials, compound semiconductor and hetero- or homo-junction silicon structure, have been studied by many researchers [1–5]. Law et al. has been demonstrated for dye- sensitized solar cells using ZnO nanowires and produced an efficiency of 1.5% [2]. Huynh et al. have been studied polymer matrix solar cells using CdSe nanorods, with efficiency of 1.7% [3]. These results indicate that the nanowires are attractive to enhance charge transport in nanostructures solar cells compared with conventional solar cells or other nanostructured solar cells. In particular, silicon nanowires (SiNWs) are irresistible materials in the semiconductor industry because the bulk properties of silicon are well-known. Moreover, it can easily dope impurities to fabricate n- or p-type Si semiconductor [6,7]. The SiNWs have been synthesized by using various methods and metal catalysts via well-known vapor– liquid–solid (VLS) mechanism [8,9]. Moreover, various materials, such as Au, Al, Ga, In, Pb, Sn and Zn [8–15], have been used for synthesis of silicon nanostructures. Recently, hetero-junction solar cell usin g SiNWs have been demonstrated by Tsakalakos et al [4]. They fabricated Au-catalyzed SiNWs on flexible metal foil and produced a current density of ~1.6 mA/cm 2 for 1.8 cm 2 cell. Similar structures have been demonstrated by Thony et al. They fabricated an all-inorganic SiNWs solar cell using Au thin film and colloidal nanoparticles. The short circuit current (J sc ) and open circuit voltage (V oc ) of the fabricated cells were 0.53 mA/cm 2 and 125 mV, respectively [5]. As mentioned above, Au nanoparticles are well-used metal catalyst and it easily synthesizes the SiNWs at low temperature. However, it is known to be a deep level impurity in silicon. In contrast with Au, tin (Sn) appears to be the favorable catalyst because the Sn–Si alloy has relatively low eutectic temperature of 232 °C [16] and forms with extremely low content of the elemental semiconductors. In our previous work, we successfully synthesized large quantities of SiNWs using various materials by the hydrogen radical-assisted deposition method including hydrogen radicals pretreatment [12,14,17–19]. Moreover, Sn-catalyzed SiNWs were easily controlled by introducing hydrogen flow gas ratios [20]. In this letter, we synthesized SiNWs using Sn nanoparticles on single crystalline silicon (c-Si) wafer for solar cell application and their characteristics are explored. 2. Experimental details About 7 Ω cm boron-doped Cz silicon (100) wafers were used for fabrication of solar cells. Sn metal thin film as catalyst was evaporated on these c-Si wafers. The wafers were dipped in diluted hydrofluoric (5% HF) solution to remove native oxide layer and rinsed in pure water. Then, the wafers were immediately located into the evaporation vacuum chamber. The Sn metal film was deposited in situ onto the Si wafer by a thermal evaporation source at a rate of less than 1 nm/s. Materials Letters 63 (2009) 777–779 ⁎ Corresponding author. Tel./fax: +81 42 388 7446. E-mail address: joseph@cc.tuat.ac.jp (M. Jeon). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.001 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet After depositing a Sn film of approximately 7 nm thickness onto the Si wafer, the wafer was transferred into the experimental chamber and heated for 1 h at 400 °C. Hydrogen (H 2 ) gas was introduced and then hydrogen radicals generated by 2.45 GHz microwave were irradiated on the sample surface to fabricate metal nanoparticles. For synthesis of SiNWs, silane gas as Si source was introduced into the experimental chamber and it was reacted with hydrogen radicals. SiNWs were synthesized for 20 min at 400 °C. Detailed other synthesis conditions are summarized in Table 1. The optical and morphologic al characteristics of the SiNWs synthesized on c-Si wafer were estimated by spectrophotometer and field emission scanning electron microscopy (FE-SEM). Furthermore, a solar cell using the SiNWs synthesized on c-Si wafer was fabricated after diffusion to form n + emitter. The current–voltage (I–V) properties of the fabricated SiNWs solar cell were investigated by solar simulator (AM 1.5, 100 mA/cm 2 ). 3. Results and discussion After synthesis of SiNWs on p-type c-Si wafer, phospho rous diffusion was performed on the front surface using spin-on phosphors silicate g lass (PSG) coating. The n the samples were diffused to fabricate p–n + junction in quartz tube for 30 min at 900 °C in N 2 ambient. After diffusion, the samples were dipped in 5% HF solution to remove parasitic layer. Fig.1(a) ill ustrates a schematic of the SiNWs solar cell design in this work. As shown in the Fig. 1(a), the silver (Ag) contacts with thickness of 180 nm were evaporated on the surface of synthesized SiNWs without any transparent conductive ox ide (TCO) film. To investigate the morphological property of SiNWs, a FE-SEM observation was carried out after synthesis of SiNWs. Fig. 1(b) shows the top- view (15° tilted) and cross-section (see inset of Fig. 1(b)) FE-SEM images of the SiNWs fabricated on c-Si wafer. As can be seen these figures, voluminous SiNWs were synthesized whisker-like. The diameters of SiNWs on the bottom and top were approximately 60 nm and thinner than 10 nm, respectively. Their lengths extended up to ~ 1.5 μm. Moreover, the SiNWs were tapered and catalysts remained on the top of SiNWs. It indicates that the Sn- catalyzed SiNWs are synthesized via VLS mechanism [8,9]. The detailed mechanisms were well described elsewhere [14]. After diffusion, the surface morphologies were also observed. Here, the doping level in the silicon nanowires, which is measured by secondary ion-microprobe mass spectrometer, was estimated to be 1.4×10 20 atom/cm 3 base on planar Si wafer. Fig. 1(c) shows the FE-SEM image of the phosphorous-diffused SiNWs. The shapes of SiNWs after formation of pn junction were slightly curved. As mentioned above, we expect the SiNWs solar cells to show improved optical properties compared with conventional planar Si solar cells. Fig. 2(a) shows the photograph of the fabricated SiNWs solar cell. The surface color was dark brown in appearance. To examine the optical properties of the SiNWs solar cell, a reflectance was measured by spectrophotometer with integrating sphere. The reflectance of SiNWs and typical planar c-Si wafer were compared as shown in Fig. 2(b). The reflectance of the fabricated SiNWs solar cell represented less than 5% in the short wavelength regions (λ b 500 nm). In contrast with this result, we have reported SiNWs synthesized on c-Si, which have reflectance below 0.5% at λb 700 nm [20]. The difference of the reflectance might be the effect of synthesis conditions, such as thickness of metal film and SiH 4 gas flow. The reduction of reflectance is very helpful to improve the solar cell performance. Therefore, further reduction of the reflectance will be expected from the optimization of the synthesis conditions of SiNWs. For analysis of the electrical properties, the I–V curve of the fabricated SiNWs solar cell was measured by solar simulator under AM 1.5 (100 mA/cm 2 ). Fig. 3 shows the typical dark and light I–V curve of the SiNWs solar cell. Clear rectifying behavior and power generation was distinguished in the 1 cm 2 SiNWs solar cell. The short circuit current and open circuit voltage were obtained to 2.3 mA/cm 2 and 520 mV, respectively. These values were higher than that of the hetero-junction SiNWs synthesized on metal foil [4] and that of the homo-junction SiNWs solar cell [5]. As can be seen in Fig. 3, poor I–V characteristics in dark and light conditions were observed. It is due to the high series resistance and low shunt resistance. Theses limit the efficiency of the fabricated SiNWs solar cell. Here, the high series resistance may be caused by the unconnected metal Ag contact between the SiNWs and neighboring SiNWs. Moreover, it might be the effect of Fig. 1. (a) Schematic of the SiNWs solar cells. FE-SEM images of the (b) as-synthesized SiNWs and (c) phosphorous-diffused SiNWs. All the scale bars represent 2.5 μm. Table 1 Synthesis conditions for Sn-catalyzed SiNWs fabricated on c-Si (100) wafer Hydrogen radical treatment Synthesis of SiNWs Hydrogen (H 2 ) gas flow [sccm] 100 180 Silane (SiH 4 ) gas flow [sccm] – 15 Microwave power [W]40 40 Working pressure [Torr] 0.5 0.5 Temperature [°C] 400 400 Time [min] 1 20 778 M. Jeon, K. Kamisako / Materials Letters 63 (2009) 777–779 the Schottky barrier between the evaporated Al contact and silicon wafer because of insufficient annealing conditions. For low shunt resistance, it might be the leakage current from the front metal contact, i.e., local shunting across the solar cell. In addition, the poor results were caused by the unoptimized diameter distribution and the shapes of the synthesized SiNWs. It is typically not suitable for the application of electrical and optical devices. Furthermore, unremoved metal nanoparticles caused the reduction of the carrier lifetime, i.e., increasing surface recombination velocity. Therefore, the sizes and impurity removal must be controlled for further improvement. Moreover, the improvement of the cell performance owing to the deposition of TCO films, such as indium–tin oxide and fluorine doped tin oxide, on the surface of synthe- sized SiNWs will be expected. 4. Conclusion We have demonstrated tin-catalyzed silicon nanowires solar cells fabricated by the hydrogen radical-assisted deposition method on a c- Si wafer. This SiNWs solar cell structure is a promising candidate for future photovoltaic application. In particular, it is possible to reduce the thickness of Si base substrate more thinly. For improvement of conversion efficiency of the SiNWs solar cells, the reduction of the contact resistance and optimization of the nanowire sizes will be considered and the results will be presented near future. References [1] Baxter JB, Aydil ES. Appl Phys Lett 2005;86:053114. [2] Law M, Greene LE, Johnson JC, Saykally R, Yang P. Nature Mater 2005;4:455. [3] Huynh WU, Dittmer JJ, Alivisatos AP. Science 2002;295:2425. [4] Tsakalakos L, Balch J, Fronheiser J, Korevaar BA, Sulima O, Rand J. Appl Phys Lett 2007;91:233117. [5] Thony P, Delsol R, Jaussaud C, Rondel N, Rouvelere E, Poncet S, et al. Proc. 23rd European PVSEC, Valencia, Spain; Sep. 2008, pp. 670–4. [6] Pan L, Lew KK, Redwing JM, Dickey EC. J Cryst Growth 2005;277:428. [7] Kimukin I, Islam MS, Williams RS. Nanotechnology 2006;17:S240. [8] Wagner RS, Ellis WC. Appl Phys Lett 1964;4:89. [9] Givargizov EI. J Crystal Growth 1975;31:20. [10] Wang Y, Schmidt V, Senz S, Gosele U. Nature Nanotech 2006;1:186. [11] Sunkara MK, Sharma S, Miranda R, Lian G, Dickey EC. Appl Phys Lett 2001;79:1546. [12] Jeon MS, Kamisako K. Mater Lett 2008;62:3903. [13] Zhang J, Jiang F, Yang Y, Li J. J Cryst Growth 2007;307:76. [14] Jeon MS, Uchiyama H, Kamisako K. Mater Lett 2009;63:246. [15] Yu JY, Chung SW, Heath JR. J Phys Chem B 2000;104 :11864. [16] Olesinski RW, Abbaschian GJ. Bull Alloy Phase Diagr 1984;5:273. [17] Jeon MS, Kamisako K. J Nanosci Nanotechnol 2008;8:5188. [18] Jeon MS, Tomitsuka Y, Kamisako K. J Ind Eng Chem 2008;14:836. [19] Jeon MS, Kamisako K. J Alloys Compd 2008, doi:10.1016/j.jallcom.2008.09.035. [20] Jeon MS, Uchiyama H, Tomitsuka Y, Maishigi K, Kamisako K. Proc. Renewable Energy 2008, Busan, Korea; Oct. 20 08. Fig. 3. I–V characteristics of the fabricated SiNWs solar cell under dark and light conditions. Fig. 2. (a) Photograph of the fabricated SiNWs solar cell. (b) The difference of reflectance between conventional planar c-Si solar cell and SiNWs solar cell. 779M. Jeon, K. Kamisako / Materials Letters 63 (2009) 777–779 . Synthesis and characterization of silicon nanowires using tin catalyst for solar cells application Minsung Jeon ⁎ , Koichi Kamisako Department of Electronic. demonstrated for dye- sensitized solar cells using ZnO nanowires and produced an efficiency of 1.5% [2]. Huynh et al. have been studied polymer matrix solar cells using