Elsevier Editorial System(tm) for Solar Energy Materials and Solar Cells Manuscript Draft

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1 Elsevier Editorial System(tm) for Solar Energy Materials and Solar Cells Manuscript Draft Manuscript Number: SOLMAT-D Title: Effect of nanowires lengths on photovoltaic properties of silicon nanowires based solar cells Article Type: Regular Manuscript Keywords: Silicon nanowires; Electroless method; Nanowire length; Photovoltaic properties Corresponding Author: Dr Mostafa Zahedifar, PhD Corresponding Author's Institution: University of Kashan First Author: Mostafa Farangi, MSc Order of Authors: Mostafa Farangi, MSc; Mostafa Zahedifar, PhD; Mohammad Hamed Pakzamir, MSc student Abstract: The fabrication of silicon nanowire- based solar cells on silicon wafer is described. Silicon nanowires (SiNWs) were synthesized by electroless etching of FZ-Si (100) wafer using HF/AgNO3 solution. Dependence of SiNWs morphology on etching duration and temperature was studied by FESEM images. Optical properties of this nanostructure were investigated by low-temperature photoluminescence at 4.2 K. The synthesized SiNWs layer was employed as active region in solar cell surface and photovoltaic properties of the fabricated cells were studied for different NWs lengths. Measurements were accomplished in 1 sun condition under AM1.5 illumination by solar simulator. Decrease in efficiency was observed with increasing SiNWs lengths which can be attributed to enhance of dangling states on nanowires surfaces. Suggested Reviewers: Nima Taghavinia PhD Researcher, Physics Department, Sharif University of technology taghavinia@sharif.edu He is an expert in nanostructured solar cells Opposed Reviewers:

2 Cover Letter Manuscript title: Effect of nanowires lengths on photovoltaic properties of silicon nanowires based solar cells Authors:M.Farangi, M. Zahedifar *, M.H.Pakzamir Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, I.R.Iran The manuscript has not been previously published, is not currently submitted for review to any other journal, and will not be submitted elsewhere before a decision is made by this journal. Mostafa Zahedifar (corresponding author)

3 *Highlights The effect of lengths of grown silicon nanowires on Si wafer on photovoltaic characteristics of fabricated solar cells was investigated. Results indicate that increasing the lengths of nanowires reduces the efficiency of the fabricated solar cells.

4 Responses to Technical Check Results Ref. SOLMAT-D Title: Effect of nanowires lengths on photovoltaic properties of silicon nanowires based solar cells Solar Energy Materials and Solar Cells Dear Prof. Ramesh Thanks due to constructing comments and suggestions regarding the submitted manuscript. We have modified the manuscript accordingly. The detailed corrections are listed below: 1. We did our best to improve the level of English throughout the manuscript and correct the grammatical errors and instances of badly worded/constructed sentences. 2. Pages were assigned page number consecutively. With my best regards Mostafa Zahedifar, Corresponding author

5 *Manuscript Click here to view linked References Effect of nanowires lengths on photovoltaic properties of silicon nanowires based solar cells M. Farangi 1, M. Zahedifar 1,2*, M. H. Pakzamir 1 1 Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, I.R.Iran 2 Physics Department, University of Kashan, Kashan, I.R.Iran Abstract The fabrication of silicon nanowire- based solar cells on silicon wafer is described. Silicon nanowires (SiNWs) were synthesized by electroless etching of FZ-Si (100) wafer using HF/AgNO 3 solution. Dependence of SiNWs morphology on etching duration and temperature was studied by FESEM images. Optical properties of this nanostructure were investigated by low-temperature photoluminescence at 4.2 K. The synthesized SiNWs layer was employed as active region in solar cell surface and photovoltaic properties of the fabricated cells were studied for different NWs lengths. Measurements were accomplished in 1 sun condition under AM1.5 illumination by solar simulator. Decrease in efficiency was observed with increasing SiNWs lengths which can be attributed to enhance of dangling states on nanowires surfaces. Keywords: Silicon nanowires; Electroless method; Nanowire length; Photovoltaic properties 1. Introduction Silicon nanostructures due to their unique optical and electrical characteristics have been widely investigated in cost-effective high efficiency solar cells. Since, physical properties of these nanostructures is dependent on their nanoscale dimensions, they can provide a reasonable electric application potential. Among different materials, SiNWs has received considerable attention due to their electronic and photovoltaic applications. SiNWs based solar cells provides potential advantages over planar waferbased or thin film solar cells and researchers control their electrical and optical properties by changing of nanowires dimensions [1 4]. SiNWs arrays on silicon substrate exhibit high optical absorption because of strong light trapping by multiple scattering of incident light among SiNWs and the optical antenna effect [5-8]. This effect causes very low reflectance and eliminates antireflective coating step in manufacturing process, so reduces the cost of solar cell [9]. In this work electroless method was used as a very simple technique to production of SiNWs on a large area of silicon substrate. A desirable SiNWs growth * Correspondig author address: zhdfr@kashanu.ac.ir, Tel:

6 direction and doping level can be achieved by selecting proper silicon substrate. The effect of nanowire geometry on main characteristics of the fabricated solar cells is also discussed. 2. Experimental procedure P-type monocrystalline FZ-Si (100) with resistivity around Ω cm from Siltronic was employed as raw wafer. For all measurements, the samples were cut into 1 1 cm 2. Si wafers were cleaned by immersing in acetone, ethanol and HF (5 M) and DI water for 10 min respectively. The etching process was accomplished in a sealed teflon vessel using a HF (5 M) and AgNO 3 (0.02 M) solution [10-12]. To investigate the effect of temperature, experiments were conducted from 40 C to 65 C at 6 steps for 60 min. The reaction time was also taken into account by performing several experiments for reaction times between 30 min to 70 min. FESEM (Hitachi S-4160) images were employed to study the SiNWs growth. PL measurements were carried out using liquid helium-bath cryostats with glass windows. The 405 nm line of a 180 mv solid state laser was employed as an excitation source in order to generate band-to-band optical transitions. The monochromator was a 60 cm single-grating with 600 grooves/mm. PL signals were detected by an InGaAs p-i-n detector (Hamamatsu, Japan) and signal amplification was based on lock-in technique. The common tube diffusion technique was employed to solar cells fabrication. Diffusion process was performed using POCl 3 at 900 C for 120 min in nitrogen atmosphere. Rear emitter layer was removed from the back surface of diffused wafer by HF, H 2 SO 4, HNO 3, and H 2 O solution, and then electroless procedure was carried out to synthesis SiNWs on the front surface. 100 nm ITO and 100 nm Al were deposited on silicon surfaces as front and rear electrodes using sputtering and thermal evaporation methods, respectively. Finally, the photovoltaic measurements were accomplished by solar simulator (Luzchem) and IVIUMSTAT (IVIUM) under AM1.5 condition. 3. Results and discussions Temperature is an important factor in electroless process which controls the nucleation rate of silver nanoclusters on silicon wafer, so its role was studied profoundly in this paper. In Fig. 1, FESEM images show a special trend for SiNWs formation. According to Fig. 1(a), Si nanostructures at 40 C are belt like, however in a closer view (inset images) the attached nanowires can be seen. Due to the sinking role of Ag nanoparticles on formation of SiNWs on silicon substrate [10,12], it can be deduced that silver nucleation on silicon wafer is not considerable in this temperature. Hence, the assembled pores by Ag nanoparticles in silicon wafer don t encounter each other, and this trend results in attached nanowires. Nucleation 2

7 process of silver nanoclusters is a very important step in electroless technique, which determines the etching amount of surface and number density of nanowires on substrate. By increasing of temperature, nucleation increases and single SiNWs are formed on substrate around 50 C and 55 C, but further rising of temperature reduces nucleation and the attached SiNWs are appeared once again at 65 C. Gaussian diagram of nucleation process is in agreement with this behavior, so 55 C was selected as optimum temperature for synthesis of SiNWs in succeeding experiments. Figs. 2, 3 show the FESEM images of SiNWs which have been synthesized at 55 C from 30 min to 70 min. Nucleation of silver nanoclusters strongly depends on AgNO 3 concentration and temperature. It is evident from Fig.2 that nucleation on silicon surface is not significantly changed with reaction time, and SiNWs have approximately the same morphology and density on silicon surface. FESEM cross-section images of SiNWs (Fig. 3) explain that the length of SiNWs strongly depends on etching time and there is a direct relevance between this quantity and SiNWs length (Fig. 4). These images show that SiNWs morphology is not a sensitive function of time due to the fact that nucleation process of Ag nanoclusters is not changed considerably on silicon surface, and during redox reaction silver atoms just diffuse to Ag nanoparticles in Si wafer and these nanoparticles sink in substrate more and more [10,11]. This behavior of Ag catalyst results in formation of a layer of SiNWs with a desired length on silicon surface. Ag + +e - Ag (1) Si+6F - SiF e - (2) A sample of 18 μm SiNWs on silicon wafer was selected for PL measurements. A typical PL spectrum of Si-nanocrystals on Si substrate is shown in Fig. 5. In the near-band-gap region somewhat intense narrow lines are seen clearly which are related to the intrinsic excitonic emission from Si substrate (FE TA, FE TO.) [13]. The broad emission band Si NC around energy ~ 1.17 ev is due to electron-hole recombination on Si nanocrystals. To examine the effects of SiNWs length on photovoltaic properties, five samples were fabricated with 1 cm 2 area and different SiNWs lengths of 18, 28, 37, 44 and 51 μm. I-V diagrams of these solar cells (Fig. 6), are different from the conventional curves and linear behavior of them give explanation for low fill factors for the fabricated solar cells. It is possible that poorly qualified junction between front electrode and SiNWs causes a high amount of series resistance in device configuration. However, special point about this figure is a decreasing trend of solar cell efficiency versus increasing SiNWs length (Table 1). This behavior comes from dangling states on SiNWs which act as recombination centers for light generated excitons. Increasing of NWs lengths, lead to increase in surface defects and recombination rate, and reduces I SC. Meanwhile, because of strong dependence of quasi levels on the carriers recombination under 3

8 illumination which set an upper limit on open circuit voltage, V OC declines by escalation of recombination current (Fig. 6). Consequently, growth of NWs lengths raises the recombination rate in solar cells and reduces their photovoltaic responses. 4. Conclusions In this work SiNWs growth on FZ-silicon wafer was considered by electroless technique and an optimum temperature for etching process was determined. It was found that SiNWs length is strongly depends on etching duration, while their morphology is not changed abruptly with reaction time. Low-temperature PL at 4.2 K was employed in order to reveal the optical characteristics of SiNWs on silicon wafer, which indicated band energy around 1.17 ev for SiNWs. This increase in band energy compared to the bulk Si proofs potential of Si nanostructure for photovoltaic purposes. On the other hand a decreasing trend for efficiency of solar cells with increasing of SiNWs length was observed. This behavior is due to rising of dangling states and defects on SiNWs surfaces by increasing NWs length. So, short SiNWs on silicon surface are recommended for photovoltaic applications. 5. Acknowledgements The authors gratefully acknowledge Prof. Valery F. Gremenok for technical assistance with photoluminescence measurements. This work was financially supported by the research council of the University of Kashan. 6. References [1] Tian, B., Zheng, X., Kempa, T.J., Fang,Y., Yu, N., Yu,J., Huang, J., and Lieber, C.M., Nature 449 (7176), (2007). [2] Marsen, M., Sattler, K., Phys. Rev. B, 60, 16, (1999). [3] Li, Q., Koo, S. M., Edelstein, M. D., Suehle, J. S., Richter, C. A., Nanotechnology, 18, 1-5, (2007). [4] Patolsky, F., Zheng, G., Lieber, C. M., Nature Protocols, 1, (2006). [5] Li, Z., Rajendran, B., Kamins, T. I., Li, X., Chen, Y., Williams, R. S., Appl. Phys. A, 80, (2005). [6] Peng, K., Huang, Z., Zhu, J., Adv. Mater., 16, (2004). [7] Peng, K., Xu, Y., Wu, Y., Yan, Y., Lee, S. T., Zhu, J., Small, 1, 11, (2005). 4

9 [8] Tang, Y. H., Zhang, Y. F., Wang, N., Lee, C. S., Han, X. D., Bello, I., Lee, S. T., J. Appl. Phys., 85, 11, (1999). [9] Th. Stelzner, M. Pietsch, G. Andr, F. Falk, E. Ose, S. Christiansen, Nanotechnology 19, 1-4 (2008). [10] S. D. Hutagalung, A. S. Y. Tan, R. Y. Tan, Y. Wahab, Advances in Micro/Nanotechnology, 7743, 1-7 (2010). [11] G. Kalita, S. Adhikari, H. R. Aryal, R. Afre, T. Soga, M. Sharon, W. Koichi, M. Umeno, J. Phys. D: Appl. Phys. 42, 1-5, , (2009). [12] J. Y. Jung, Z. Guo, S. W. Jee, H. D. Um, K. T. Park, J. H. Lee, Optics Express, 18, 103, (2010). [13] A.V. Mudryi, A.I. Patuk, I.A. Shakin, A.G. Ulyashin, R. Job, W.R. Fahrner, A. Fedotov, A. Mazanik, N. Drozdov, Solar Energy Materials & Solar Cells 72, (2002). Figure captions: Fig. 1: FESEM images of SiNWs produced by electroless method for etching times from 40 C to 65 C at 6 steps for 60 min. Fig. 2: FESEM images of SiNWs produced by electroless method. Etching times are from 30 min to 70 min at 55 C. Fig. 3: FESEM cross section images of SiNWs for etching times from 30 min to 70 min at 55 C. Fig. 4: Variation of SiNWs lengths versus etching time from 30 min to 70 min at 55 C. Fig. 5: Low temperature PL spectrum of SiNWs on silicon wafer at 4.2 K. Fig. 6: I-V diagrams of SiNWs solar cells for different nanowires lengths. Table caption: Table 1: Photovoltaic properties of fabricated solar cells for different nanowires lengths. 5

10 Table Table 1 NWs length(μm) V OC (V) J SC (ma/cm 2 ) FF% %η

11 Figure Figure 1 7

12 Figure Figure 2 8

13 Figure Figure 3 9

14 Figure L( ) Time (min) Figure 4 10

15 Figure Figure 5 11

16 Figure J SC (ma/cm 2 ) V(volt) Figure µm 30 µm 40 µm 50 µm 60 µm