Recently there have been worldwide interests in the research of next generation solar cells by novel nanostructures and nanomaterials. Because of their near-wavelength-scale geometry, the Si nanostructure arrays have broadband low reflection and therefore strong absorption from near IR to visible region, which can be applied as an antireflection layer without further coating processes. Moreover, the high aspect ratio of nanostructure allows enhancing optical absorption while simultaneously decreasing the carrier diffusion length through the fashion of core-shell nanowires.

In this study, first we fabricated a p-type single-crystal (sc) Si nanostructure arrays as the base by statistic electroless metal deposition (SEMD) method. The morphology control of the Si nanostructures on Si substrate by four major factors was discussed in SEMD process. Next, we employed a spin-on-dopant (SOD) method to form a uniform n-type emitter on our Si nanostructures, which is suitable for the dense and high aspect ratio arrays with a super-hydrophilic surface. The thin layer of emitter facilitates to construct the core-shell structure arrays for radial p-n junction solar cell that decouples the incoming light route and carrier diffusion path into orthogonal directions. This architecture benefits to the light absorption as well as the carriers collection by allowing lateral diffusion of minority carriers to the p-n junction rather than many microns away as in Si bulk solar cells.

At last, we investigated relative reflection and photovoltaic parameters of four kinds of Si nanostructure based solar cells such as nanopores, nanowires, nanotips, and nanohills. The Si nanowires (SiNWs) based solar cell possesses the lowest total reflection (~ 1%) within the range of wavelength from 530 to 1030 nm. It also improves the conversion efficiency up to more than 25% in contrast to planar sc-Si solar cell under identical condition. In addition, the issues of length and electrode contact of SiNWs based solar cell were discussed, and several guidelines for optimizing such SiNWs based photovoltaic device were further suggested. The proposed novel design of solar cell by incorporating SiNW array revolutionizes the current architecture of solar cells, promising niche points of (1) better absorption, (2) self-antireflection, and (3) cost-effective fabricated process.

1 Dresselhaus, M. & Thomas, I. Alternative energy technologies. Nature 414, 332-337 (2001).
2 Singh, R. Why silicon is and will remain the dominant photovoltaic material. J. Nanophoton. 3, 032503, doi:10.1117/1.3196882 (2009).
3 Nelson, J. THE PHYSICS OF SOLAR CELLS. (Imperial College Press, 2003).
4 Chapin, D., Fuller, C. & Pearson, G. A New Silicon p-n junction Photocell for Converting Solar Radiation into Electrical Power. J. Appl. Phys. 25, 676-677 (1954).
5 Green, M. A. in Prog Photovoltaics Vol. 17 183-189 (2009).
6 Shockley, W. & Queisser, H. Detailed balance limit of efficiency of p-n junction solar cells. J. Appl. Phys. 32, 510-& (1961).
7 Kerr. Lifetime and efficiency limits of crystalline silicon solar cells. Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE, 438 - 441, doi:10.1109/PVSC.2002.1190553 (2002).
8 Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science 315, 798-801, doi:10.1126/science.1137014 (2007).
9 Zhu, J. et al. Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays. Nano Lett. 9, 279-282, doi:10.1021/nl802886y (2009).
10 Lin, C. & Povinelli, M. L. Optical absorption enhancement in silicon nanowire and nanohole arrays for photovoltaic application. Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS), 2010 Conference on, 1 - 2 (2010).
11 Lin, C. & Povinelli, M. L. Optical absorption enhancement in silicon nanowire arrays with a large lattice constant for photovoltaic applications. Optics Express 17, 19371-19381 (2009).
12 Tsakalakos, L. et al. Strong broadband optical absorption in silicon nanowire films. J. Nanophoton. 1, 013552 (2007).
13 Lee, C., Bae, S., Mobasser, S. & Manohara, H. A novel silicon nanotips antireflection surface for the micro sun sensor. Nano Lett. 5, 2438-2442, doi:10.1021/nl0517161 (2005).
14 Huang, Y.-F. et al. Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures. Nat Nanotechnol 2, 770-774, doi:10.1038/nnano.2007.389 (2007).
15 Shiu, S.-C., Chao, J.-J., Hung, S.-C., Yeh, C.-L. & Lin, C.-F. Morphology Dependence of Silicon Nanowire/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Heterojunction Solar Cells. Chem Mater 22, 3108-3113, doi:10.1021/cm100086x (2010).
16 Kalita, G. et al. Silicon nanowire array/polymer hybrid solar cell incorporating carbon nanotubes. J Phys D Appl Phys 42, 115104, doi:10.1088/0022-3727/42/11/115104 (2009).
17 Law, M., Greene, L., Johnson, J., Saykally, R. & Yang, P. Nanowire dye-sensitized solar cells. Nature Materials 4, 455-459, doi:10.1038/nmat1387 (2005).
18 Baxter, J. & Aydil, E. Nanowire-based dye-sensitized solar cells. Appl. Phys. Lett. 86, 053114, doi:10.1063/1.1861510 (2005).
19 Gratzel, M. Photoelectrochemical cells. Nature 414, 338-344 (2001).
20 Tian, B. et al. in Nature Vol. 449 885-U888 (2007).
21 Kelzenberg, M. D. et al. Photovoltaic measurements in single-nanowire silicon solar cells. Nano Lett. 8, 710-714, doi:10.1021/nl072622p (2008).
22 Kayes, B., Atwater, H. & Lewis, N. Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells. J. Appl. Phys. 97, 114302, doi:10.1063/1.1901835 (2005).
23 Kelzenberg, M., Putnam, M., Turner-Evans, D., Lewis, N. & Atwater, H. Predicted efficiency of Si wire array solar cells. Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE, 001948 - 001953, doi:10.1109/PVSC.2009.5411542 (2009).
24 Kayes. Radial PN junction, wire array solar cells. Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE, 1 - 5, doi:10.1109/PVSC.2008.4922460 (2008).
25 Convertino, A., Cuscuna, M. & Martelli, F. Optical reflectivity from highly disordered Si nanowire films. Nanotechnology 21, 355701, doi:10.1088/0957-4484/21/35/355701 (2010).
26 Sivakov, V. et al. in Nano Lett. Vol. 9 1549-1554 (2009).
27 Stelzner, T. et al. Silicon nanowire-based solar cells. Nanotechnology 19, 295203 (2008).
28 Garnett, E. & Yang, P. Light Trapping in Silicon Nanowire Solar Cells. Nano Lett. 10, 1082-1087, doi:10.1021/nl100161z (2010).
29 Tsakalakos, L. in Mat Sci Eng R Vol. 62 175-189 (2008).
30 Tsakalakos, L. et al. Silicon nanowire solar cells. Appl. Phys. Lett. 91, 233117 (2007).
31 Peng, K. et al. Aligned single-crystalline Si nanowire arrays for photovoltaic applications. Small 1, 1062-1067, doi:10.1002/smll.200500137 (2005).
32 Peng, K.-Q., Wang, X., Li, L., Wu, X.-L. & Lee, S.-T. High-Performance Silicon Nanohole Solar Cells. J. Am. Chem. Soc. 132, 6872-+, doi:10.1021/ja910082y (2010).
33 Zhu, J., Hsu, C.-M., Yu, Z., Fan, S. & Cui, Y. Nanodome Solar Cells with Efficient Light Management and Self-Cleaning. Nano Lett. 10, 1979-1984, doi:10.1021/nl9034237 (2010).
34 Schlosser, V. Limiting factors for the application of crystalline upgraded metallurgical grade silicon. Electron Devices, IEEE Transactions on 31, 610 - 613, doi:10.1109/T-ED.1984.21576 (1984).
35 Yoon, H. P. et al. Enhanced conversion efficiencies for pillar array solar cells fabricated from crystalline silicon with short minority carrier diffusion lengths. Appl. Phys. Lett. 96, 213503, doi:10.1063/1.3432449 (2010).
36 Kovacs, G. T. A. Micromachined Transducers Sourcebook. (McGraw-Hill Education, 2000).
37 Pavesi, L. Routes toward silicon-based lasers. Materials Today (2005).
38 Yu, D. et al. Direct evidence of quantum confinement from the size dependence of the photoluminescence of silicon quantum wires. Phys. Rev. B 59, R2498-R2501 (1999).
39 Fan, Z. et al. Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates. Nature Materials 8, 648-653, doi:10.1038/NMAT2493 (2009).
40 Kelzenberg, M. D. et al. Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications. Nature Materials 9, 239-244, doi:10.1038/NMAT2635 (2010).
41 Cui, Y., Wei, Q., Park, H. & Lieber, C. Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species. Science 293, 1289-1292 (2001).
42 Wu, Y. & Yang, P. Direct observation of vapor-liquid-solid nanowire growth. J. Am. Chem. Soc. 123, 3165-3166, doi:10.1021/ja0059084 (2001).
43 Yan, H. et al. Growth of amorphous silicon nanowires via a solid-liquid-solid mechanism. Chem Phys Lett 323, 224-228 (2000).
44 Zhang, R., Lifshitz, Y. & Lee, S. Oxide-assisted growth of semiconducting nanowires. Adv Mater 15, 635-640, doi:10.1002/adma.200301641 (2003).
45 Wang, Y., Schmidt, V., Senz, S. & Goesele, U. Epitaxial growth of silicon nanowires using an aluminium catalyst. Nat Nanotechnol 1, 186-189, doi:10.1038/nnano.2006.133 (2006).
46 Holmes, J., Johnston, K., Doty, R. & Korgel, B. Control of thickness and orientation of solution-grown silicon nanowires. Science (2000).
47 Ge, S. et al. Orientation-controlled growth of single-crystal silicon-nanowire arrays. Adv Mater 17, 56-+, doi:10.1002/adma.200400474 (2005).
48 Yu, D. et al. Synthesis of nano-scale silicon wires by excimer laser ablation at high temperature. Solid State Commun 105, 403-407 (1998).
49 Wagner, R. S. & Ellis, W. C. Vapor-Liquid-Solid Mechanism of single crystal growth. Appl. Phys. Lett. 4, 89-& (1964).
50 Chen, C.-Y., Wu, C.-S., Chou, C.-J. & Yen, T.-J. Morphological Control of Single-Crystalline Silicon Nanowire Arrays near Room Temperature. Adv Mater 20, 3811-+, doi:10.1002/adma.200702788 (2008).
51 Peng, K., Yan, Y., Gao, S. & Zhu, J. Synthesis of large-area silicon nanowire arrays via self-assembling nanoelectrochemistry. Adv Mater 14, 1164-1167 (2002).
52 Peng, K., Yan, Y., Gao, S. & Zhu, J. Dendrite-assisted growth of silicon nanowires in electroless metal deposition. Adv Funct Mater 13, 127-132 (2003).
53 Peng, K. & Zhu, J. Simultaneous gold deposition and formation of silicon nanowire arrays. J Electroanal Chem 558, 35-39, doi:10.1016/S0022-0728(03)00374-7 (2003).
54 Peng, K., Huang, Z. & Zhu, J. Fabrication of large-area silicon nanowire p-n junction diode arrays. Adv Mater 16, 73-+, doi:10.1002/adma.200306185 (2004).
55 Peng, K. & Zhu, J. Morphological selection of electroless metal deposits on silicon in aqueous fluoride solution. Electrochim Acta 49, 2563-2568, doi:10.1016/j.electacta.2004.02.009 (2004).
56 Peng, K. et al. Fabrication of single-crystalline silicon nanowires by scratching a silicon surface with catalytic metal particles. Adv Funct Mater 16, 387-394, doi:10.1002/adfm.200500392 (2006).
57 Peng, K., Lu, A., Zhang, R. & Lee, S.-T. Motility of Metal Nanoparticles in Silicon and Induced Anisotropic Silicon Etching. Adv Funct Mater 18, 3026-3035, doi:10.1002/adfm.200800371 (2008).
58 Wan, L. et al. in Appl Surf Sci Vol. 254 4899-4907 (2008).
59 Wan, L. et al. in Appl Surf Sci Vol. 255 3752-3758 (2009).
60 Dalchiele, E. A., Martin, F., Leinen, D., Marotti, R. E. & Ramos-Barrado, J. R. Synthesis, structure and photoelectrochemical properties of single crystalline silicon nanowire arrays. Thin Solid Films 518, 1804-1808, doi:10.1016/j.tsf.2009.09.037 (2010).
61 Wang, F. et al. Mask less fabrication of large scale Si nanohole array via laser annealed metal nanoparticles catalytic etching for photovoltaic application. J. Appl. Phys. 108, 024301, doi:10.1063/1.3462397 (2010).
62 Gorostiza, P., Servat, J., Morante, J. & Sanz, F. First stages of platinum electroless deposition on silicon(100) from hydrogen fluoride solutions studied by AFM. Thin Solid Films 275, 12-17 (1996).
63 Gorostiza, P., Diaz, R., Sanz, F. & Morante, J. Different behavior in the deposition of platinum from HF solutions on n- and p-type (100) Si substrates. J Electrochem Soc 144, 4119-4122 (1997).
64 Zhao, J., Wang, A., Green, M. & Ferrazza, F. 19.8% efficient "honeycomb'' textured multicrystalline and 24.4% monocrystalline silicon solar cells. Appl. Phys. Lett. 73, 1991-1993 (1998).
65 (National Science Foundation, 2009).
66 Instruments, O. (Newport, 2008).
67 Technologies, A. in Application Note B1500A-14 15 (2007).
68 Huang, Z., Fang, H. & Zhu, J. Fabrication of silicon nanowire arrays with controlled diameter, length, and density. Adv Mater 19, 744-+, doi:10.1002/adma.200600892 (2007).
69 Posthuma, N. E., van der Heide, J., Flamand, G. & Poortmans, J. Emitter formation and contact realization by diffusion for germanium photovoltaic devices. Ieee T Electron Dev 54, 1210-1215, doi:10.1109/TED.2007.894610 (2007).
70 Ingole, S. et al. Ex situ doping of silicon nanowires with boron. J. Appl. Phys. 103, 104302, doi:10.1063/1.2924415 (2008).
71 Liu, Z., Takato, H., Togashi, C. & Sakata, I. Oxygen-atmosphere heat treatment in spin-on doping process for improving the performance of crystalline silicon solar cells. Appl. Phys. Lett. 96, 153503, doi:10.1063/1.3394005 (2010).
72 Xiong, Z., Zhao, F., Yang, J. & Hu, X. Comparison of optical absorption in Si nanowire and nanoporous Si structures for photovoltaic applications. Appl. Phys. Lett. 96, 181903, doi:10.1063/1.3427407 (2010).