Aligned Single‐Crystalline Si Nanowire Arrays for Photovoltaic Applications (2005)

Silicon nanowires (SiNWs) with desirable axial crystallographic orientations can be readily prepared by a novel chemical-etching technique (see SEM image). The as-synthesized SiNW arrays significantly suppress reflection over the visible-light spectral range and are therefore promising candidates for antireflection coatings of photovoltaic cells despite exhibiting a lower efficiency than non-nanowire-based devices at this stage of development. Since the successful synthesis of carbon nanotubes (CNTs),1 great research interest has been stimulated in various one-dimensional nanostructures for their novel size- and dimensionality-dependent physical properties and their potential applications in nanoscale functional electronics and photonics.2 Silicon is the most important semiconducting material with its microelectronic technology being one of the greatest successes of the 20th century. Bulk silicon does not emit visible light since it is an indirect-bandgap material. Low-dimensional Si nanostructures can emit visible light as a result of the quantum-confinement effect. Therefore, a great deal of effort has been made to prepare various silicon nanostructures because of their potential applications in fabricating Si-based optoelectronic devices. In recent years, one-dimensional (1D) silicon nanowires (SiNWs) have been intensively investigated and various functional nanoscale devices based on SiNWs have been successfully demonstrated.3 Nanotechnology is expected to become a driver of sustainable energy development. The possible photovoltaic application of various nanostructures including carbon nanotubes, C60, and CdSe nanorods has been intensively investigated.4 As the most important material in today’s photovoltaic (PV) electricity production, Si materials have been extensively investigated.5 However, due to the limitation of present synthetic techniques for large-area aligned SiNWs, the photovoltaic investigation of SiNWs has not been reported up to now. Recently, much effort has been made to prepare SiNWs and aligned SiNW arrays by a host of approaches,6 such as chemical vapor deposition (CVD), laser ablation, thermal evaporation/decomposition, and supercritical fluid–liquid–solid (SFLS) synthesis. All these methods are quite accessible and well controlled. However, most of these methods need either high synthesis temperatures or intolerably long synthesis times due to the limitation of the growth mechanisms. Recently, we developed a facile and rapid chemical-etching technique to prepare large-area aligned SiNW arrays on single-crystal silicon wafers based on a galvanic displacement reaction.7 In a galvanic displacement process, the reduction of oxidizing metal ions to metallic species such as particles and films, and the dissolution of silicon substrates occur spontaneously on a silicon surface;8 the Si atoms act as a reducing agent for metal ions in an aqueous HF solution. In this Communication, we report that high-quality single-crystal Si nanowires with desirable crystallographic orientation and doping characteristics can be readily prepared on single-crystal Si substrates with identical crystallographic orientation and doping characteristics. Accurate control over the crystallographic axial orientations of SiNWs is of great importance to related fundamental studies and possible nanoscale electronic applications based on theoretical calculations.9 According to the remarkable optical antireflection ability of large-area SiNW arrays, a prototype SiNW-based photovoltaic cell is proposed and successfully demonstrated herein. The production of SiNW arrays is quite simple and similar to our previous report.7 The fabrication process mainly comprises two steps: 1) cleaning of the original silicon wafers, and 2) immersion of the cleaned silicon wafers into a HF-based aqueous solution containing silver nitrate in sealed vessels and treatment for the desired time. After the treatment, the as-synthesized samples were rinsed copiously in deionized water and dried at room temperature. The length of SiNWs could be effectively controlled through tuning the treatment time. The size of the film could be made as large as required and are ultimately limited only by the dimensions of vessels. Field-emission scanning electron microscope (SEM JEOL 6301F) images of as-synthesized samples show that a large-area SiNW array film could be created on an etched silicon surface (Figure 1 a1). The sample is prepared by immersing cleaned p-type (111)-oriented silicon substrates into an aqueous HF/AgNO3 solution for 20 min at 50 °C (the concentrations of HF and AgNO3 here are chosen to be 4.6 mol L−1 and 0.02 mol L−1, respectively). TEM characterizations confirm that the obtained SiNWs are singlecrystalline silicon. Figure 1 b1 shows a TEM image of a single nanowire. Figure 1 c1 is a high-resolution transmission electron microscopy (HRTEM) image of an individual SiNW; the inset shows the electron diffraction (ED) pattern recorded from the nanowire, which could be indexed as the zone axis of single-crystal Si. Analysis of ED and HRTEM indicates that the axial crystallographic orientation of the nanowire is the direction, which is identical with the orientation of the initially used silicon wafer. a) SEM cross-section image of silicon nanowire arrays. b) TEM image of an individual SiNW prepared from a p-type (111)-oriented silicon substrate. c) HRTEM image of the nanowire in Figure 1 b1 (the inset is the ED pattern recorded along the axis). d) HRTEM image of a nanowire synthesized from a p-type (100)-oriented silicon substrate (the inset is its ED pattern recorded along the axis). e, f) SEM images of SiNW arrays prepared from a polycrystalline Si substrate. SiNWs on silicon substrates with other crystallographic orientations were also investigated. Figure 1 d1 shows the HRTEM image of a nanowire prepared from a (100)-oriented silicon substrate; the inset ED pattern indicates that the axis of the nanowire is along the crystallographic direction. Our detailed TEM investigation has confirmed that the orientations of obtained silicon nanowires are always identical with the orientation of initial Si substrates. These results indicate that SiNWs with desirable axial crystallographic orientations could be fully controlled by selecting initial Si substrates with identical orientations. Unlike other etching methods, the present etching technique shows little dependence upon the crystallographic orientation and doping characteristics of Si substrates. Therefore, this etching method can be readily extended to polycrystalline Si substrates. Large-area freestanding SiNWs arrays have also been successfully prepared on polycrystalline Si substrates. Figure 1 e1 and f show SEM images of large-area SiNW arrays prepared on polycrystalline Si substrates. It can be seen that the etched polycrystalline Si surface is built up of many structural domains composed of bundles of SiNWs; these domains are separated by obvious boundaries. The typical size of these domains was equal to the size of a single Si crystallite. Evidently, these domain boundaries are the boundaries of the Si crystallites. The reduction of optical loss is one of the important factors in obtaining high-efficiency Si solar cells. To achieve this goal, the top surface of the solar cells is generally texturized10 or covered with an antireflection coating (ARC).11 Anisotropic alkaline texturization is a standard process for monocrystalline Si and is widely applied in present solar cell production. However, for polycrystalline Si, this method is not effective because only a fraction of the grains have the crystallographic orientation. It is well known that porous silicon (PSi) can reduce the reflectance to ≈5.8 % in the 400–1000 nm wavelength range and therefore can replace other surface-textured microstructure and antireflection coatings.12 The present efficiency of solar cells with a porous structure as the ARC is about 7–12 %. The black color of our as-synthesized samples implies their possible excellent optical antireflection property. Therefore, reflectance behavior of the large-area SiNW arrays was studied in view of its potential photovoltaic application as an antireflection surface. Figure 22 shows the hemispherical reflectance measurements of as-synthesized SiNW arrays on monocrystalline and polycrystalline Si substrates, PSi, and polished crystal Si. The SiNW arrays could intensively suppress the reflection drastically over a wide spectral bandwidth ranging from 300 to 1000 nm. The reflectance is about 1.4 % or much less over the range of 300–600 nm for the arrays on monocrystalline Si substrates. An average of 2 % reflectance was obtained over the same wavelength range for SiNW arrays prepared on polycrystalline Si substrates. This remarkable low reflectance could be accounted for by considering the three important properties of the SiNW arrays, namely: 1) an ultrahigh surface area due to a high density of SiNWs; 2) the subwavelength-structured (SWS) surface of the SiNW arrays, which could suppress the reflection over a wide spectral bandwidth;13 3) a possible porosity gradient throughout the SiNW arrays, which also implies a change in refractive index with depth; therefore, the behavior of the SiNW arrays more closely resembles a multi-antireflection coating.14 Hemispherical reflectance measurements of SiNW arrays, porous silicon (PSi), and polished crystalline Si. The PSi was prepared by conventional stain etching in HF/HNO3 solution. According to above results, we know that the large-area SiNW arrays show remarkable antireflection ability and have a great advantage over other texturized Si and reported ARC structures. TEM characterization has confirmed that present etching techniques show a weak dependence upon the crystal orientation and therefore can be extended to polycrystalline Si substrates. Thus, the etching technique and the produced SiNW arrays give promising results for possible applications in high-efficiency solar-cell manufacture. Based on these considerations, we have had a first try of SiNWs in the field of photovoltaic application and a prototype SiNW-based solar cell has been demonstrated. We have studied possible photovoltaic applications of SiNW arrays prepared on p-type (100)-oriented Si and polycrystalline Si substrates, respectively. Figure 33 illustrates the fabrication process of a SiNW-based photovoltaic cell using conventional Si solar cell technology. The processing sequence consists of the following steps: 1) Formation of oriented SiNW arrays on p-type silicon substrates (the thickness of the SiNW films can be efficiently controlled by adjusting the etching time); 2) chemical cleaning to remove the remaining Ag and other contaminations adhering to the SiNW arrays; 3) p–n junction preparation by POCl3 diffusion, which was carried out in a quartz tube furnace at 930 °C; 4) Vacuum evaporation of metallic aluminum onto the rear surface and sintering at 800 °C to remove the parasitic p–n junction at the backside; 5) Vacuum evaporation of metallic silver onto the rear surface and mask evaporating a Ti/Pd/Ag front grid onto the surface of the SiNW arrays, which functions as both emitter and antireflection coating. Schematic illustration of the fabrication process for preparing a SiNW-based photovoltaic cell: a) Initial p-type silicon wafer. b) Synthesis of an oriented SiNW film on the wafer by selective chemical etching in an aqueous HF/AgNO3 solution. c) Conversion of the p-type SiNW film to n-type SiNW emitters using POCl3 diffusion. d) PV cell fabrication via conventional silicon solar-cell technology, including fabrication of the front Ti/Pd/Ag grid fingers and an Al rear surface. SiNW-based solar cells with an area of 1×1 cm2 were cut out by the laser scriber to perform the test and characterization of the as-fabricated solar cells. The illuminated current–voltage (I/V) characteristic curves of the SiNW solar cells were measured under standard measuring conditions (AM1.5 spectra, 1000 W m−2, 25 °C). The dark I/V characteristic curves were measured in a dark room. Figure 4 a4 and b shows the I/V characteristics of a photovoltaic device based on SiNW arrays prepared on monocrystalline and polycrystalline Si substrates, respectively. The devices exhibited clear diode characteristics in darkness. Upon illumination of the front surface, a clear photocurrent could be detected. When the photovoltaic cell was tested in simulated sunlight, a power conversion efficiency of 9.31 %, open circuit voltage of 548.5 mV, photovoltage at maximum power of 422.0 mV, and fill factor of 0.6512 were obtained for SiNW-based photovoltaic cell on a monocrystalline Si substrate. The efficiency of the SiNW-based photovoltaic cell on a polycrystalline Si substrate is about 4.73 %. The photovoltaic conversion efficiency is not as high as expected from excellent antireflecting properties. We suggest that the low conversion efficiency may be a result of low current-collection efficiency of the front grid electrodes. Another cause may be ascribed to the enhanced surface electron–hole recombination velocity due to the ultrahigh surface area of the SiNW arrays, which would lead to the possible existence of numerous surface defects and dangling bonds. Figure 4 c4 and d shows SEM images of the front Ti/Pd/Ag grid electrode. It is clearly seen that the Ti/Pd/Ag electrode on the surface of the SiNW arrays is not a compact film, which may lead to a large longitudinal surface resistance. Therefore, it is clear that the design of the front grid electrodes should be improved. a) Current–voltage (I/V) characteristics of a photovoltaic device based on SiNW arrays prepared on a monocrystalline Si substrate. b) I/V characteristics of a photovoltaic cell based on SiNW arrays prepared on a polycrystalline Si substrate. c, d) SEM images of the front Ti/Pd/Ag grid electrode. The minority-carrier lifetime measurements confirm that the surface recombination velocity of minorities generated in SiNW arrays is much faster than that of bulk crystalline Si. The minority carrier lifetime was measured using the quasi-steady-state photoconductance decay method. As shown in Figure S1 (see Supporting Information), minority carrier lifetimes of SiNW samples decrease remarkably by about ten times compared with the original silicon wafers. The minority carrier lifetimes of SiNWs with different lengths have also been investigated. We found that the minority carrier lifetime decreases with increasing nanowire length. We suggest that the short carrier lifetime may be ascribed to the ultrahigh surface area of the SiNWs with numerous surface defects and dangling bonds, which facilitates the fast surface recombination of photogenerated minority carriers and contributes to the short carrier diffusion length. Figure S2 (see Supporting Information) shows a comparison of the spectral response characteristics of the SiNW cell with a reference cell produced on a monocrystalline Si wafer. The spectral response of the SiNW cell has a peak near 920 nm. That is to say, the SiNW cell has good absorption properties at long wavelengths. However, the spectral response of the SiNW cell is lower than that of the reference cell within the visible light region from 400 to 750 nm. Therefore, the SiNW cells have a lower fill factor and open circuit voltage compared with conventional Si solar cells on monocrystalline Si wafers. Hence, passivation layers such as indium-tin oxide or silicon nitride films should be coated onto the surface of SiNW arrays to reduce the surface recombination velocity. To improve the main photovoltaic parameters such as short-circuit current and open-circuit voltage, some technological regimes including phosphorus diffusion and contacts fabrication should be optimized. Our work is now focusing on the improvement of these technological regimes. In conclusion, we have demonstrated that single-crystal SiNWs with desirable axial crystallographic orientations could be readily prepared by a simple chemical-etching technique. The most interesting feature of the as-synthesized SiNW arrays is the significant suppression of reflection over the visible-light spectral range. Such a chemical etching technique could be used for surface texturization of mono- and polycrystalline Si solar cells due to its weak dependence upon the crystallographic orientation and doping characteristics of Si substrates. We also demonstrated that the SiNW arrays can be used as an efficient antireflection coatings for a silicon solar cell incorporating this array. However, under the present conditions, the nanowire-based photovoltaic cells still show a lower efficiency than those without nanowires; improvements that would potentially provide higher efficiencies are sought. Material preparation and characterization: The facile production of SiNW arrays is similar to that reported previously.7 The fabrication process comprises two main steps: 1) The original silicon wafers were cleaned with acetone (5 min) and ethanol (5 min), rinsed with deionized water 2–3 times, then immersed in a 3:1 mixture of H2SO4 (97 %) and H2O2 (30 %) for 10 min, thoroughly rinsed with deionized water for 10 min, and then dipped in HF solution for 1 min; 2) the cleaned silicon wafers were then immersed into an aqueous HF solution containing silver nitrate in sealed vessels and treated for the desired time at 50 °C. The concentrations of HF and AgNO3 used here were 4.6 M and 0.02 M, respectively. After treatment, the as-synthesized samples were rinsed copiously in deionized water and dried at room temperature. The as-synthesized SiNW arrays were characterized using a scanning electron microscope (SEM JEOL 6301F) and a transmission electron microscope (TEM JEOL 2010F) equipped with an energy-dispersive X-ray spectrometer and a Gatan GIF 678 system. To prepare a TEM specimen, the samples were scraped using a knife, and the scraping was collected and suspended in ethanol; then a drop was placed on a carbon copper grid and examined in a JEOL 2010F microscope. Hemispherical reflectance measurements were performed with a Hitachi U-4001 UV-IVS spectrophotometer. The properties of the SiNW-based photovoltaic cells were measured using standard photovoltaic measurement equipment at the Beijing Institute of Solar Energy Research. Photovoltaic cell processing: The detailed processing sequence for the SiNW-based photovoltaic cells consists of the following steps: 1) An oriented SiNW film on p-type silicon wafers was formed by chemical etching in an aqueous HF/AgNO3 solution; 2) the film was chemically cleaned to remove the remaining Ag and other contaminations on or inside the SiNW film in a 3:1 mixture of H2SO4 (97 %) and H2O2 (30 %), a 3:1 mixture of HCl and HNO3, and an 8:2:1 mixture of H2O, HCl, and H2O2, and then dipped in a dilute HF solution for 1 min; 3) a p–n junction was prepared by POCl3 diffusion at 930 °C for 30 min; 4) after removing the phosphorosilicate glass layer, a metallic aluminum film was thermally vacuum evaporated onto the rear surface and then treated at 800 °C to remove the parasitic p–n junction at the backside; 5) metallic silver was vacuum evaporated onto the rear surface to form a rear electrode and a Ti/Pd/Ag front grid (which functions as both an emitter and an antireflection coating) on the surface of SiNW arrays was prepared by mask evaporation; 6) the circumferential p–n junction was removed to yield a simple solar cell. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2421/2005/z500137_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.