Fabrication and characterization of silicon wire solar cells having ZnO nanorod antireflection coating on Al-doped ZnO seed layer
© Baek et al; licensee Springer. 2012
Received: 5 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
In this study, we have fabricated and characterized the silicon [Si] wire solar cells with conformal ZnO nanorod antireflection coating [ARC] grown on a Al-doped ZnO [AZO] seed layer. Vertically aligned Si wire arrays were fabricated by electrochemical etching and, the p-n junction was prepared by spin-on dopant diffusion method. Hydrothermal growth of the ZnO nanorods was followed by AZO film deposition on high aspect ratio Si microwire arrays by atomic layer deposition [ALD]. The introduction of an ALD-deposited AZO film on Si wire arrays not only helps to create the ZnO nanorod arrays, but also has a strong impact on the reduction of surface recombination. The reflectance spectra show that ZnO nanorods were used as an efficient ARC to enhance light absorption by multiple scattering. Also, from the current-voltage results, we found that the combination of the AZO film and ZnO nanorods on Si wire solar cells leads to an increased power conversion efficiency by more than 27% compared to the cells without it.
Keywordssilicon microwire solar cells ZnO nanorods antireflection coating Al-doped ZnO atomic layer deposition
In recent decades, most commercial solar cells are based on crystalline silicon [c-Si], but there is increasing efforts on thin film solar cells (second generation) as well as third generation solar cells which require the use of nano/microstructures for high efficiency and low cost . Three-dimensional Si has been attracting much attention for future applications in photovoltaic devices due to their superior properties [2–9]. Si wire-based solar cells have two major advantages relative to commercial crystalline and thin-film Si absorbers. First, p-n junctions in the radial direction enable minority carriers to drift only short distances to the junction region for efficient carrier collection. That means low grade Si raw materials can be utilized, and manufacturing cost will be lowered . In addition, the enhanced light absorption by an ordered wire is attributed to the light-trapping effect to the incident light [3, 4]. Moreover, a wire array transfer technique has been studied, which not only yields c-Si wires on a flexible substrate for photovoltaic applications, but also allows the c-Si wafer to be reused for further production of aligned wire arrays [7, 8].
For the fabrication of Si nano/microstructures, a number of bottom-up methods have been developed, such as vapor-liquid-solid [VLS] growth [5–8], chemical vapor deposition [CVD] , and molecular beam epitaxy . However, these growth processes have some disadvantages as they generally need high temperature and high vacuum or discharge toxic precursors. As an alternative top-down route, a few lithographic procedures, such as electron beam lithography , and reactive ion etching [RIE]  are widely used in Si-based fabrication processes, but they are expensive, time-consuming, and not suited for mass production of ordered nanostructures on a large scale. In contrast, electrochemical etching, together with pre-patterning in a lithographic step is one of the most successful approaches in fabricating a large number of wires with a low cost and simple process. Unlike the growth techniques, vertically well-aligned Si wire arrays are reproduced by electrochemical etching with uniform periodicity . Also, the formed Si wires have smooth surfaces, unlike those formed by using deep RIE where surfaces are damaged and wavy.
Nevertheless, Si wire solar cells still face critical challenges such as relatively low cell efficiency and surface recombination losses. Here, we investigated two key factors for the Si wire solar cells in order to improve the cell performances: One is to use ZnO nanorods to increase power conversion efficiency by suppressing light reflection and increasing light scattering to the Si wire solar cells. The other is to use an Al-doped ZnO [AZO] layer to passivate the Si surface and to facilitate the nucleation of ZnO nanorods.
Recently, ZnO nanorods are regarded as an efficient antireflection coating [ARC] to take advantage of its good transparency, appropriate refractive index (n = 2), and ability to form textured coating via anisotropic growth [14, 15]. Several methods have been developed to grow ZnO nanorods, such as VLS process , CVD , and a hydrothermal method . Among them, the hydrothermal method has been regarded as a low-temperature process with a large area growth and high growth rate. ZnO nanorods with high crystal quality can be grown perpendicularly on any surface of the substrates using hydrothermal synthesis. In addition, the seed layer is also important for the growth of high-quality ZnO nanorods. Prior to ZnO nanorod growth, AZO thin films were grown on high aspect ratio Si microwire [SiMW] arrays as a seed layer by atomic layer deposition [ALD] system. We introduced the AZO thin film as a buffer layer facilitating the nucleation and alignment of ZnO nanorods because AZO thin films are attractive due to their good conductivity, high transparency, and relatively low cost [19–22]. Moreover, the AZO film was deposited on the surface of Si wire arrays to suppress the surface recombination and increase the carrier collection efficiency. The ALD technique is the best choice for constructing composite thin films extremely conformal to the structure of a high aspect ratio.
In this study, we report the fabrication of highly ordered SiMW solar cells with conformal ZnO nanorod ARC grown on an AZO seed layer. The wire arrays of a c-Si were fabricated by means of electrochemical etching combining photolithography for site-selective etching. To evaluate the cell performances, the p-n junction was prepared by a spin-on dopant [SOD] method. AZO films were prepared by the ALD process, and ZnO nanorods grown on the as-prepared AZO seed layer were synthesized using hydrothermal growth methods. The morphological, optical, and photovoltaic properties of the SiMW solar cells having ZnO ARC were also characterized.
Formation of the SiMW arrays
SiMW arrays were prepared in p-type < 100 > Si wafers with a resistivity of 1 to approximately 10 Ω cm (a boron doping density of 1015 to approximately 1016 cm-3) by electrochemical etching method. In order to make vertical arrays of Si wire with a high aspect ratio, we prepared the wafer pieces as follows: (1) The lithographical pattern was prepared on a silicon oxide layer as a mask to obtain an ordered array of a 2-μm square pattern spaced at a distance of 2 μm. (2) Then, the samples were dipped in a potassium hydroxide etchant to make inverse-shaped pyramidal notches, which would act as the regions for concentrating an electrical bias. (3) Electrochemical etching was performed with a mixed solution of hydrofluoric [HF] acid, dimethyl sulfoxide, and deionized water [DIW] (HF: (CH3)2SO: H2O = 2:5:15, v/v), respectively. (4) A thin aluminum [Al] layer with a thickness of 150 nm was deposited on the backside of the wafer to produce an ohmic contact between the Si wafer and working electrode by direct current magnetron sputtering method. After that, the electrochemical etching system was operated under a constant current density mode of different biases in a Teflon bath. A platinum wire was used as a counter electrode, and the Si wafer with Al coating on the backside was placed on a Teflon bath as a working electrode. The sample area exposed to the electrolyte solution was approximately 2 cm2.
AZO seed layer and ZnO nanorods growth
AZO thin films have been prepared by ALD technique using trimethylaluminum [TMAl] and diethylzinc [DEZn] which were used as metal precursors for Zn and Al, respectively. Metal precursors and H2O were introduced into the growth chamber separately. A high-purity N2 purge was also introduced after each metal precursor to remove the residues and by-products. Optimized AZO properties were achieved with a DEZn/TMAl cycle ratio of 19:1 . Under optimal reaction conditions, the growth rate of the ZnO films and that of the Al2O3 films were 1.5 to approximately 1.6 Å/cycle and about 0.9 Å/cycle in the substrate temperature range of 200°C, respectively. Then, ZnO ARC was synthesized using two-step methods corresponding to the formation of as-prepared AZO seed layers and the growth of nanorods. The precursors used for ZnO synthesis are zinc nitrate (99.99% purity; Sigma-Aldrich Company, St. Louis, MO, USA) and hexamethylenetetramine [HMT] (C6H12N4). The substrates were placed in a heated solution (25 mM) of zinc nitrate and HMT held for 3 h at 85°C. At the end of the growth period, the sample was removed from the solution and immediately rinsed with DIW to remove residuals from the surface.
Solar cell fabrication
A p-n junction was prepared by a solution processable SOD technique. To produce an n-type region, the phosphorus-doped SOD solution (P509; Filmtronics Inc., Butler, PA, USA) was spin-coated onto a dummy wafer, and the sample was loaded in a conventional quartz-tube furnace at 1,050°C for 5 min, while the target samples were kept at a closely spaced distance. Phosphorosilica film was removed simply by immersing the prepared specimens in buffer oxide etchant for 10 min. The active area of all devices was defined as 1 cm2. Indium/gallium eutectic metal (Ga (75%) In (25%) by weight; the melting point, approximately 15.5°C) was used to form an electrical contact on both sides. Notably, the front contact was made using eutectic liquid metals with a gold probe tip on top of a wire array.
The morphological properties of all the samples were characterized by scanning electron microscopy [SEM] (Hitachi S-4800; Hitachi, Ltd., Chiyoda, Tokyo, Japan), and secondary electron [SE] imaging of the cleaved SiMW arrays were prepared with a focused ion beam [FIB] (Seiko SMI-3050SE; Seiko Instruments Inc., Chiba, Japan). X-ray diffraction [XRD] (Rigaku D/MAX 2200H; Rigaku Corporation, Tokyo, Japan) was used to obtain crystallographic structures. The optical properties of the fabricated wire arrays were measured from the ultraviolet to the infrared region using a spectrophotometer (Cary 500; Varian Inc., Cary, NC, USA). Current-voltage measurements were carried out with a source meter (model 2400; Keithley Instruments Inc., Cleveland, OH, USA) and with a Newport 91192 solar simulator system (Newport Corporation, Irvine, CA, USA) (equipped with 1 kW Xenon arc lamp from Oriel). The light intensity was adjusted to simulated air mass [AM] 1.5 radiation at 100 mW/cm2 with a radiant power energy meter (model 70260; Oriel Instruments, Irvine, CA).
Results and discussion
The photovoltaic performances of sample A, sample B, and sample C
We have fabricated the SiMW solar cells having ZnO nanorod ARC grown on the AZO seed layer and characterized their optical and photovoltaic properties. The morphological results showed that the AZO seed layer and ZnO nanorods were conformally grown on electrochemically prepared SiMW arrays. The combination of the AZO film and ZnO nanorods on SiMW solar cells exhibits the best optical and photovoltaic performances. The photovoltaic efficiency of sample C was enhanced more than 27%, and Jsc was improved by over 31% compared to sample A. It is strongly attributed to the incorporation of the AZO thin film and ZnO nanorod ARC by suppressing light reflectance and surface recombination. The hybrid structures, i.e., SiMW solar cells with transparent conducting oxides, are a promising alternative for efficient energy-harvesting devices.
atomic layer deposition
chemical vapor deposition
focused ion beam
- J sc :
short circuit current
reactive ion etching
scanning electron microscopy
open circuit voltage
This work was financially supported by the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0001649) and by a basic research program (11-EN-03) through the Daegu-Gyeongbuk Institute of Science and Technology (DGIST) funded by the Ministry of Education, Science and Technology (MEST).
- Green MA: Third generation photovoltaics: ultra-high conversion efficiency at low cost. Prog Photovolt: Res Appl 2001, 9: 123–135. 10.1002/pip.360View ArticleGoogle Scholar
- Kayes BM, Atwater HA, Lewis NS: Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells. J Appl Phys 2005, 97: 114302. 10.1063/1.1901835View ArticleGoogle Scholar
- Peng K, Xu Y, Wu Y, Yan Y, Lee ST, Zhu J: Aligned single-crystalline Si nanowire arrays for photovoltaic applications. Small 2005, 1: 1062–1067. 10.1002/smll.200500137View ArticleGoogle Scholar
- Garnett EC, Yang P: Silicon nanowire radial p-n junction solar cells. J Am Chem Soc 2008, 130: 9224–9225. 10.1021/ja8032907View ArticleGoogle Scholar
- Tian B, Zheng X, Kempa TJ, Fang Y, Yu N, Yu G, Huang J, Lieber CM: Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 2007, 449: 885–889. 10.1038/nature06181View ArticleGoogle Scholar
- Stelzner T, Pietsch M, Andra G, Falk F, Ose E, Christiansen S: Silicon nanowire-based solar cells. Nanotechnology 2008, 19: 295203. 10.1088/0957-4484/19/29/295203View ArticleGoogle Scholar
- Putnam MC, Boettcher SW, Kelzenberg MD, Turner-Evans DB, Spurgeon JM, Warren EL, Briggs RM, Lewis NS, Atwater HA: Si microwire-array solar cells. Energy Environ Sci 2011, 8: 1037–1041.Google Scholar
- Kayes BM, Filler MA, Putnam MC, Kelzenberg MD, Lewis NS, Atwater HA: Growth of vertically aligned Si wire arrays over large areas with Au and Cu catalysts. Appl Phys Lett 2007, 91: 103110. 10.1063/1.2779236View ArticleGoogle Scholar
- Tsakalakos L, Balch J, Fronheiser J, Korevaar BA, Sulima O, Rand J: Silicon nanowire solar cells. Appl Phys Lett 2007, 91: 233117. 10.1063/1.2821113View ArticleGoogle Scholar
- Fuhrmann B, Leipner HS, Höche H, Schubert L, Werner P, Gösele U: Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy. Nano Lett 2005, 5: 2524–2527. 10.1021/nl051856aView ArticleGoogle Scholar
- Ng H, Han J, Yamada T, Nguyen P, Chen Y, Meyyappan M: Single crystal nanowire vertical surround-gate field-effect transistor. Nano Lett 2004, 4: 1247–1252. 10.1021/nl049461zView ArticleGoogle Scholar
- Huang M, Yang C, Chiou Y, Lee R: Fabrication of nanoporous antireflection surfaces on silicon. Sol Energy Mater Sol Cells 2008, 92: 1352–1357. 10.1016/j.solmat.2008.05.014View ArticleGoogle Scholar
- Meerakker JEAM, Elfrink RJG, Weeda WM, Roozeboom F: Anodic silicon etching; the formation of uniform arrays of macropores or nanowires. Phys Stat Sol A 2003, 197: 57–60. 10.1002/pssa.200306468View ArticleGoogle Scholar
- Lee YJ, Ruby DS, Peters DW, McKenzie BB, Hsu JW: ZnO nanostructures as efficient antireflection layers in solar cells. Nano Lett 2008, 8: 1501–1505. 10.1021/nl080659jView ArticleGoogle Scholar
- Chao YC, Chen CY, Lin CA, Dai YA, He JH: Antireflection effect of ZnO nanorod arrays. J Mater Chem 2010, 20: 8134–8138. 10.1039/c0jm00516aView ArticleGoogle Scholar
- Chik H, Liang J, Cloutier SG, Kouklin N, Xu JM: Periodic array of uniform ZnO nanorods by second-order self-assembly. Appl Phys Lett 2004, 84: 3376–3378. 10.1063/1.1728298View ArticleGoogle Scholar
- Wu JJ, Liu SC: Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition. Adv Mater 2002, 14: 215–218. 10.1002/1521-4095(20020205)14:3<215::AID-ADMA215>3.0.CO;2-JView ArticleGoogle Scholar
- Kwon YT, Song KY: Controlled growth of well-aligned ZnO nanorod array using a novel solution method. J Phys Chem B 2005, 109: 19263–19269. 10.1021/jp0538767View ArticleGoogle Scholar
- Song JJ, Lim S: Effect of seed layer on the growth of ZnO nanorods. J Phys Chem C 2007, 111: 596–600. 10.1021/jp0655017View ArticleGoogle Scholar
- Chung TF, Luo LB, He ZB, Leung YH, Shafiq I, Yao ZQ, Lee ST: Selective growth of catalyst-free ZnO nanowire arrays on Al: ZnO for device application. Appl Phys Lett 2007, 91: 233112. 10.1063/1.2811717View ArticleGoogle Scholar
- Shi JH, Huang SM, Chu JB, Zhu HB, Wang ZA, Li XD, Zhang DW, Sun Z, Cheng WJ, Huang FQ, Yin XY: Effect of ZnO buffer layer on AZO film properties and photovoltaic applications. J Mater Sci 2010, 21: 1005–1013.Google Scholar
- Zhang J, Que W: Preparation and characterization of sol-gel Al-doped ZnO thin films and ZnO nanowire arrays grown on Al-doped ZnO seed layer by hydrothermal method. Sol Energy Mater Sol Cells 2010, 94: 2181–2186. 10.1016/j.solmat.2010.07.009View ArticleGoogle Scholar
- Kong BH, Choi MK, Cho HK, Kim JH, Baek SH, Lee JH: Conformal coating of conductive ZnO:Al films as transparent electrodes on high aspect ratio Si microrods. Electrochem Solid-State Lett 2010, 13: K12-K14. 10.1149/1.3267051View ArticleGoogle Scholar
- Lehmann V, Föll : Formation mechanism and properties of electrochemically etched trenches in n-type silicon. J Electrochem Soc 1990, 137: 653–659. 10.1149/1.2086525View ArticleGoogle Scholar
- Kim JH, Kim KP, Lyu HK, Woo SH, Seo HS, Lee JH: Three-dimensional macropore arrays in p-type silicon fabricated by electrochemical etching. J Korean Phys Soc 2009, 1: 5–9.Google Scholar
- Jang HS, Oh BY, Choi HJ, Baek SH, Kim SB, Kim JH: Optimization of wire array formation in p-type silicon for solar cell application. Curr Appl Phys 2011, 11: S34-S38. 10.1016/j.cap.2010.11.037View ArticleGoogle Scholar
- Dasgupta NP, Neubert S, Lee W, Trejo O, Lee JR, Prinz FB: Atomic layer deposition of Al-doped ZnO films: effect of grain orientation on conductivity. Chem Mater 2010, 22: 4769–4775. 10.1021/cm101227hView ArticleGoogle Scholar
- Lee KE, Wang MS, Kim EJ, Hahn SH: Structural, electrical and optical properties of sol-gel AZO thin films. Curr Appl Phys 2009, 9: 683–687. 10.1016/j.cap.2008.06.006View ArticleGoogle Scholar
- Oh BY, Kim JH, Han JW, Seo DS, Jang HS, Choi HJ, Baek SH, Kim JH, Heo GS, Kim TW, Kim KY: Transparent conductive ZnO:Al films grown by atomic layer deposition for Si-wire-based solar cells. Curr Appl Phys 2012, 12: 273–279. 10.1016/j.cap.2011.06.017View ArticleGoogle Scholar
- Berginski M, Hüpkes J, Schulte M, Schöpe G, Stiebig H, Rech B: The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells. J Appl Phys 2007, 101: 074903. 10.1063/1.2715554View ArticleGoogle Scholar
- Baek SH, Jang HS, Kim JH: Characterization of optical absorption and photovoltaic properties of silicon wire solar cells with different aspect ratio. Curr Appl Phys 2011, 11: S30-S33.View ArticleGoogle Scholar
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