a-Si:H/SiNW shell/core for SiNW solar cell applications
© Ashour et al.; licensee Springer. 2013
Received: 16 June 2013
Accepted: 29 September 2013
Published: 6 November 2013
Vertically aligned silicon nanowires have been synthesized by the chemical etching of silicon wafers. The influence of a hydrogenated amorphous silicon (a-Si:H) layer (shell) on top of a silicon nanowire (SiNW) solar cell has been investigated. The optical properties of a-Si:H/SiNWs and SiNWs are examined in terms of optical reflection and absorption properties. In the presence of the a-Si:H shell, 5.2% reflection ratio in the spectral range (250 to 1,000 nm) is achieved with a superior absorption property with an average over 87% of the incident light. In addition, the characteristics of the solar cell have been significantly improved, which exhibits higher open-circuit voltage, short-circuit current, and efficiency by more than 15%, 12%, and 37%, respectively, compared with planar SiNW solar cells. Based on the current–voltage measurements and morphology results, we show that the a-Si:H shell can passivate the defects generated by wet etching processes.
KeywordsSilicon nanowire Solar cells a-Si:H Antireflection coating Metal-assisted wet chemical etching PECVD
Silicon nanowires (SiNWs) attract significant attention because of their potential applications in many fields like sensors, transistors, lithium batteries, diodes, and photovoltaics [1–5]. Particularly, they can be applied on silicon solar cells as an antireflection coating, due to low average reflectance values [6, 7]. Several synthesis methods have been used to fabricate SiNWs including chemical vapor deposition , laser ablation , thermal evaporation, and solution methods [10–12]. Among these synthesis methods, wet chemical etching has been frequently used to prepare SiNWs. Metal-assisted wet chemical etching is advantageous for achieving SiNWs with controlled diameter, length, spacing, and density, avoiding expensive and low-throughput usual lithographic processes .
Recently, it has been shown that a silicon nanowire antireflection coating (ARC) prepared by metal-assisted wet chemical etching is a near-perfect antireflection coating . The superior antireflection property of the nanowire surface is attributed to three reasons: huge surface area of SiNWs, rough surface morphology which leads to strong light scattering as well as absorption, and graded refractive index profile between air and SiNWs that closely implies a multilayer antireflection coating [6, 14, 15]. Some other properties of SiNWs, for example, crystal ordination, good doping level, and excellent uniformity, imply appropriate utilization of SiNWs in silicon solar cells.
Despite all these features, the maximum efficiency of planar solar cells using SiNW ARC does not exceed 10%. This low efficiency is attributed to many factors. One of the most important is the surface recombination velocity which strongly increases when using SiNW ARC, due to the large surface area [16, 17]. It is necessary, therefore, to passivate the SiNW surface, minimizing the surface states .
Among all materials used to passivate planar silicon solar cells, hydrogenated amorphous silicon (a-Si:H) has attracted much attention since Sanyo's first declaration of the heterojunction with intrinsic thin layer (HIT) solar cell design . From that time, HIT solar cell efficiency exceeds 22%, and the surface passivation capability of a-Si:H was intensively studied [19, 20]. Finding that interstitial a-Si:H is the main cause of reduction of the surface state density results in high-quality passivation of the silicon surface [21, 22]. Additionally, a thin layer of a-Si:H was proved to passivate all types of silicon substrates with the entire doping levels. Being deposited at temperatures below 250°C was a merit that leads to a decrease in the thermal budget of solar cell production processes. In this respect, a-Si:H is expected to be a good passivation choice for Si nanostructure solar cells. Crozier et al.  demonstrated that in situ amorphous Si/SiNW surface recombination decayed just about 2 orders of magnitude compared with SiNWs alone. The surface passivation capability of amorphous silicon was proved by the increase of lifetime and carrier diffusion length. However, this passivation effect was not investigated on the SiNW solar cell performance. In a previous study , SiNWs were synthesized using the VLS process which was a bottom-up synthesis approach. Indeed, those SiNWs differ from SiNWs synthesized by metal-assisted wet chemical etching (top-down approach), especially in the defect type and quantity, SiNW density, as well as doping mechanism .
In this work, for the first time, the fabrication of an a-Si:H/vertically aligned SiNW (shell/core) solar cell was proposed. The SiNW arrays were fabricated by metal-assisted wet chemical etching of silicon substrates, whereas the a-Si:H shell was deposited by plasma-enhanced chemical vapor deposition (PECVD). The structural, optical, and electrical properties of the a-Si:H/SiNW solar cell were all analyzed.
The growth of aligned SiNW arrays was carried out on p-type (100) silicon (0 to 1 Ω cm) wafers. The etching was carried out in a Teflon beaker containing a HF/AgNO3 solution, varying etching parameters like concentration, temperature as well as etching time. Prior to the etching, the samples were sequentially cleaned with acetone, ethanol, and de-ionized water for 5 min each followed by cleaning with a boiling piranha solution (H2SO4/H2O2 = 3:1 by volume, for 60 min) to remove any organic containment. The samples were then rinsed thoroughly with de-ionized water followed by dipping in 10% HF solution to remove any surface oxides. The cleaned silicon wafers were then immersed in the etching solution HF/AgNO3 (5.25:0.02 M). After the etching processes, the tree-like silver pattern wrapping the silicon samples was detached using a NH3OH/H2O2 (3:1) solution. Finally, the samples were rinsed with de-ionized water and air-dried.
A conventional diffusion procedure was carried out to fabricate the SiNW solar cell. The n + emitter was formed by phosphorus diffusion using a POCl3 liquid source at 850°C for 20 min. The phosphosilicate glass (PSG) that formed during diffusion was removed by dipping the samples in 5% HF for 2 min.
Hydrogenated amorphous silicon was deposited on the surface of SiNWs by PECVD. The deposition occurred under the following conditions: a power of 100 W, a temperature of 150°C, a pressure of 1 Torr, and a SiH4 gas flow of 26 sccm.
The Al back contact with 2,000-nm thickness was formed using an electron beam evaporator (Edwards Auto 306 Turbo, Sanborn, NY, USA). In order to form the back surface field (BSF), alloying of Al and Si was carried out at 900°C. The front metal contacts were made by Ag deposition (180 nm) through a metal mask using the same e-beam evaporator followed by contact sintering in forming gas at 450°C. Finally, 1 × 1 cm2 solar cells were diced for electrical characterization.
The morphology of the samples was examined using a field emission scanning electron microscope (FESEM; Carl Zeiss Supra 55VP, Oberkochen, Germany). The structure and chemical composition of the samples were investigated by Fourier transform infrared spectroscopy (FTIR). Reflection (R) spectra were obtained using a Shimadzu UV-3600 spectrophotometer (Kyoto, Japan). The J-V characteristics of the devices were measured with Keithley 237 SMU (Cleveland, OH, USA) under illumination at 100 mW/cm2 from a solar simulator with an AM 1.5G filter.
Results and discussion
Going back to earlier works, a-Si:H thin films reflect more than 45% of the incident light . Thus, it is expected that the a-Si:H/SiNW structure will be a sufficient antireflection coating combining amorphous and crystalline silicon features.
The absorption spectrum that was extracted from the measured reflection and transmission results is shown in Figure 3. It is noticeable that a-Si:H/SiNWs show a superior absorption property with an average over 87% of the incident light. Note here that the recent simulated results predicted the absorption to be around 60% to 75%  for 1-μm thickness. Using SiNWs with 3-μm lengths in this work could be the cause of such increment. As well known, SiNWs reflect less light while increasing their thickness .
Another inspiring feature of the a-Si:H/SiNW absorption spectrum is the shifting of the absorbed edge to near-infrared wavelengths. This shifting confirms the dual absorption function of both a-Si:H and SiNWs. Basically, each of them absorbed the wavelengths of the light which match to their energies. Comparing the absorption edges of our a-Si:H/SiNWs with those of amorphous silicon nanowires, it was found that the absorption edge located on the wavelength corresponds to the a-Si bandgap .
Lastly, broadband optical absorption combined with a low reflection value is a significant advantage of a-Si:H/SiNWs compared with a-Si thin films and silicon surfaces. This suggests that a-Si:H/SiNWs can be used as effective antireflection coating for silicon solar cells.
Performances of the SiNW solar cells with and without a-Si:H shell
Referring to Figure 4 and Table 1, there is also clear improvement in the short-circuit current density (Jsc). This increasing trend could not be mainly related to the trapping effect of the a-Si:H/SiNW core/shell structure. As mentioned previously, the reflection of the a-Si:H/SiNWs is slightly higher than that of the SiNWs alone. Subsequently, the main factor that leads to such increment in electrical performance is the low recombination velocity which becomes less due to the passivation effect of the a-Si:H shell as described earlier.
The calculated fill factor (FF) of the a-Si:H-passivated SiNW solar cell improved by 8%, reaching 55%. This improvement can be attributed to the decreasing contact area between the electrode and SiNWs. However, the original FF of the nonpassivated SiNW solar cell is still low. This low magnitude is more related to the main problem facing SiNW solar cells, i.e. electrode contact resistance. Hopefully, by solving the metal contact problem, the fill factor can be improved.
Our a-Si:H-passivated SiNW solar cell exhibits an improved efficiency by 37%, an open-circuit voltage by 15%, a short-circuit current by 12%, and a fill factor by 8%, as compared to the SiNW solar cell without a-Si:H. It is anticipated that the recombination rate and surface state density are decreased when the a-Si:H shell was used. However, more optimization of the a-Si:H shell thickness is needed. Moreover, more theoretical and experimental perceptions of the a-Si:H/SiNW interface is needed to maximize the a-Si:H passivation effect on the SiNW surface.
In summary, vertically aligned Si nanowires have been synthesized and implemented to a Si nanowire/a-Si:H core/shell solar cell for photovoltaic devices. Optical studies reveal that the a-Si:H/SiNWs have low reflectivity (around 5.2%) in the entire spectral range (250 to 1,000 nm) of interest for solar cells with a superior absorption property with an average over 87% of the incident light. In investigating the passivation effect of the a-Si:H shell, we find that the combination of the a-Si:H shell and SiNW solar cell leads to enhanced power conversion efficiency, open-circuit voltage, and short-circuit current by more than 37%, 15%, and 12%, respectively, compared to the SiNW cells. This is mainly due to the suppression of the surface recombination of the large surface area of SiNWs. We expect that the a-Si:H will have a significant role in passivation of the SiNW surface with more optimization of its thickness and more theoretical understanding of its interface with SiNWs.
H: Hydrogenated amorphous silicon
Field emission scanning electron microscopy
Fourier transform infrared spectroscopy
Heterojunction with intrinsic thin layer
Power conversion efficiency
Plasma-enhanced chemical vapor deposition
Transmission electron microscopy
This work has been funded by the Ministry of Science, Technology and Innovation, Malaysia, and Solar Energy Research Institute (SERI), UKM.
- Huia S, Zhang J, Chena X, Xua H, Maa D, Liua Y, Taoa B: Study of an amperometric glucose sensor based on Pd–Ni/SiNW electrode. Sensor Actuator B Chem 2011, 155: 592–597. 10.1016/j.snb.2011.01.015View ArticleGoogle Scholar
- Zaremba-Tymieniecki M, Li C, Fobelets K, Durrani ZAK: Field-effect transistors using silicon nanowires prepared by electroless chemical etching. IEEE Electron Device Lett 2010, 31: 860–862.View ArticleGoogle Scholar
- Huang Z, Zhang X, Reiche M, Liu L, Lee W, Shimizu T, Senz S, Gösele U: Extended arrays of vertically aligned sub-10 nm diameter  Si nanowires by metal-assisted chemical etching. Nano Lett 2011, 8: 3046–3051.View ArticleGoogle Scholar
- Jung JY, Guo Z, Jee SW, Um HD, Park KT, Hyun MS: A waferscale Si wire solar cell using radial and bulk p–n junctions. Nanotechnology 2010, 21: 5303–5306.Google Scholar
- Kumar D, Srivastava SK, Singh PK, Husain M, Kumar V: Fabrication of silicon nanowire arrays based solar cell with improved performance. Sol Energy Mater Sol Cells 2011, 95: 215–218. 10.1016/j.solmat.2010.04.024View ArticleGoogle Scholar
- Peng K, Xu Y, Wu Y, Yan Y, Lee ST, Zu J: Aligned single crystalline silicon nanowire arrays for photovoltaic applications. Small 2005, 1: 1062–1067. 10.1002/smll.200500137View ArticleGoogle Scholar
- Kodambaka S, Tersoff J, Reuter CM, Ross MF: Diameter-independent kinetics in the vapor–liquid-solid growth of Si nanowires. Phys Rev Lett 2006, 96: 6105–6108.View ArticleGoogle Scholar
- Zhang YF, Tang YF, Wang N, Lee CS, Bello I, Lee ST: Silicon nanowires prepared by laser ablation at high temperature. Appl Phys Lett 1998, 72: 1835–1837. 10.1063/1.121199View ArticleGoogle Scholar
- Niu J, Sha J, Yang D: Silicon nanowires fabricated by thermal evaporation of silicon monoxide. Phys E 2004, 23: 131–134. 10.1016/j.physe.2004.01.013View ArticleGoogle Scholar
- Holmes DJ, Johnston PK, Doty CR, Korgel AB: Control of thickness and orientation of solution-grown silicon nanowires. Science 2000, 287: 1471–1473. 10.1126/science.287.5457.1471View ArticleGoogle Scholar
- Huang Z, Fang H, Zhu J: Fabrication of silicon nanowire arrays with controlled diameter, length, and density. J Adv Mater 2007, 19: 744–19748. 10.1002/adma.200600892View ArticleGoogle Scholar
- Dai AH, Chang CH, Lai YC, Lin AC, Chung JR, Lin RG, He HJ: Subwavelength Si nanowire arrays for self-cleaning antireflection coatings. J Mater Chem 2010, 20: 10924–10930. 10.1039/c0jm00524jView ArticleGoogle Scholar
- Li X, Li J, Chen T, Tay KB, Wang J, Yu H: Periodically aligned Si nanopillar arrays as efficient antireflection layers for solar cell applications. Nanoscale Res Lett 2010, 5: 1721–1762. 10.1007/s11671-010-9701-3View ArticleGoogle Scholar
- Yue H, Jia R, Chen A, Ding W, Meng Y, Wu D, Wu D, Chen W, Liu X, Jin Z, Wang W, Ye T: Antireflection properties and solar cell application of silicon nanostructures. J Vac Sci Technol B 2011, 29: 1208–1212.View ArticleGoogle Scholar
- Fang H, Li X, Song S, Xu Y, Zhu J: Fabrication of slantingly-aligned silicon nanowire arrays for solar cell applications. Nanotechnology 2008, 19: 5703–5708.Google Scholar
- Dan Y, Seo K, Takei K, Meza JH, Javey A, Crozier KB: Dramatic reduction of surface recombination by in situ surface passivation of silicon nanowires. Nano Lett 2011, 11: 2527–2532. 10.1021/nl201179nView ArticleGoogle Scholar
- Lipiski M, Ziba P, Panek P, Jonas S, Kluska S, Czternastek H, Szyszka A, Paszkiewicz B: Silicon nitride for silicon solar cells. In Proceedings of the 29th International Conference of IMAPS 2005: September 19–21 2005; Darłówko. Washington, D.C.: International Microelectronics and Packaging Society; 2005:203–206.Google Scholar
- Li H, Jia R, Chen C, Xing Z, Ding W, Meng Y, Wu D, Liu X, Ye T: Influence of nanowires length on performance of crystalline silicon solar cell. Appl Phys Lett 2011, 98: 116–118.Google Scholar
- Tanaka M, Taguchi M, Matsuyama T, Sawada T, Tsuda S, Nakano S, Hanafusa H, Yukinori Kuwano Y: Development of new a-Si/c-Si heterojunction solar cells: ACJ-HIT (artificially constructed junction-heterojunction with intrinsic thin-layer). Jap J Appl Phys 1992, 31: 3518–3522. 10.1143/JJAP.31.3518View ArticleGoogle Scholar
- Rath JK, Rubinelli FA, Van der Werf CHM, Schropp REI, Van der Weg FW: Performance of heterojunction p + microcrystalline silicon n crystalline silicon solar cells. J Appl Phys 1997, 82: 6089–6095. 10.1063/1.366479View ArticleGoogle Scholar
- Terada N, Tsuge S, Toshiaki B, Takahama T, Wakisaka K, Tsuda S, Nakano S: High-efficiency a-Si/c-Si heterojunction solar cell photovoltaic energy conversion. In Proceedings of the 24th IEEE Photovoltaic Specialists Conference: December 5–9 1994; Waikoloa. Piscataway: IEEE; 1994:1219–1226.Google Scholar
- Knights CJ, Lujan AR: Microstructure of plasma-deposited a-Si : H films. Appl Phys Lett 1979, 35: 244–246. 10.1063/1.91086View ArticleGoogle Scholar
- Bandaru RP, Pichanusakorn P: An outline of the synthesis and properties of silicon nanowires. Semicond Sci Technol 2010, 25: 1–16.View ArticleGoogle Scholar
- Mahan HA, Molenbroek CE, Gallagher CA, Nelson PB, Iwaniczko E, Xu Y: Deposition of device quality, low hydrogen content, hydrogenated amorphous silicon at high deposition rates. US Patent 6,468,885 B1, 9 Oct 2002 US Patent 6,468,885 B1, 9 Oct 2002
- Xie ML, Qi WM, Chen MJ: The nature of several intense Si-H infrared stretching peaks in the neutron-transmutation-doped Si-H system. J Phys Condens Matter 1991, 3: 8519–8523. 10.1088/0953-8984/3/44/001View ArticleGoogle Scholar
- Tsai CC, Fritzsche H: Effect of annealing on the optical properties of plasma deposited amorphous hydrogenated silicon. Sol En Mater 1979, 1: 29–42. 10.1016/0165-1633(79)90054-6View ArticleGoogle Scholar
- Descoeudres A, Barraud L, De Wolf S, Strahm B, Lachenal D, Guérin C, Holman CZ, Zicarelli F, Demaurex B, Seif J, Holovsky J, Ballif C: Improved amorphous/crystalline silicon interface passivation by hydrogen plasma treatment. Appl Phys Lett 2011, 99: 3506–3508.View ArticleGoogle Scholar
- Garnett E, Yang P: Light trapping in silicon nanowire solar cells. Nano Lett 2010, 91: 3317–3319.Google Scholar
- Xie QW, Liu FW, Oh IJ, Shen ZW: Optical absorption in c-Si/a-Si:H core/shell nanowire arrays for photovoltaic applications. Appl Phys Lett 2011, 99: 3107–3109.Google Scholar
- Pankove IJ, Carlson ED: Electrical and optical properties of hydrogenated amorphous silicon. Annu Rev Mater Sci 1980, 10: 43–63. 10.1146/annurev.ms.10.080180.000355View ArticleGoogle Scholar
- Zhu J, Yu Z, Burkhard FG, Hsu MC, Connor TS, Xu Y, Wang Q, McGehee M, Fan S, Cui Y: Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays. Nano Lett 2009, 9: 279–282. 10.1021/nl802886yView ArticleGoogle Scholar
- Smith EZ, Chu V, Shepard K, Aljishi S, Slobodin D, Kolodzey J, Wagner S, Chu LT: Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films. Appl Phys Lett 1987, 50: 1521–1523. 10.1063/1.97819View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.