Silicon Nanowires for Photovoltaic Solar Energy Conversion

Transcription

1 Silicon Nanowires for Photovoltaic Solar Energy Conversion Kui-Qing Peng * and Shuit-Tong Lee * Semiconductor nanowires are attracting intense interest as a promising material for solar energy conversion for the new-generation photovoltaic (PV) technology. In particular, silicon nanowires (SiNWs) are under active investigation for PV applications because they offer novel approaches for solar-to-electric energy conversion leading to high-efficiency devices via simple manufacturing. This article reviews the recent developments in the utilization of SiNWs for PV applications, the relationship between SiNW-based PV device structure and performance, and the challenges to obtaining high-performance cost-effective solar cells. 1. Introduction As natural fossil fuel sources are becoming increasingly less available and more expensive, alternative renewable clean energy sources are in high demand for the coming decades, especially in light of the problematic long-term consequences of fossil fuel usages. [ 1 8 ] Among various energy sources including hydroelectricity, biomass, wind, and geothermal energy, sunlight is the most abundant natural energy resource. While the energy from sunlight striking the earth in one hour can meet our annual global energy consumption, unfortunately most of the sunlight energy is lost. In principle, photovoltaics (PVs), or the direct generation of electric power from sunlight, alone hold great promise to provide enough renewable clean energy to accommodate the ever-increasing energy demands of mankind. [ 9 12 ] In the past decades, great efforts have been undertaken to develop various inorganic and organic PV devices. [ ] The single-crystalline Si solar cell, invented some 50 years ago, [ 13 ] remains the basis of the current PV industry due to the abundance of Si materials and the high reliability and high efficiency of Si PVs. However, the high cost of Si PVs, due mainly to the high cost of solar-grade Si wafers, has greatly hindered the mass deployment of Si PVs. To reduce the cost, extensive efforts Prof. K.-Q. Peng Department of Physics Beijing Normal University Beijing, , P. R. China [email protected] Prof. S.-T. Lee Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science City University of Hong Kong SAR, Hong Kong [email protected] DOI: /adma have focused on the development of less expensive thin-film solar cells, [ ] such as amorphous Si (a-si), polycrystalline Si, cadmium telluride, and copper indium gallium selenide (CIGS), and polymerbased PVs. While these PVs have achieved high enough efficiency for practical usage, they are still not cost-competitive against traditional fossil fuels. Consequently, great efforts have been directed to develop new designs for solar cells via unconventional approaches, aiming at cost reduction and performance improvement. [ ] In this quest, a number of new strategies that may substantially improve the performance and lower the cost of PVs have emerged. Of particular interest are the PV designs based on nanostructured materials, such as nanocrystals (or quantum dots, nanoparticles), [ ] nanotubes, [ ] nanorods (or nanopillars), [ ] and nanowires, [ ] which provide unique advantages in terms of strong light absorption and efficient charge separation owing to their large surface areas. In particular, one-dimensional (1D) semiconductor nanowires, with direct wire paths for charge transport and high surface area for light harvest, are emerging as a promising candidate for building PV devices. For example, Huynh et al. reported that the use of CdSe nanorods to fabricate readily processed and efficient hybrid solar cells with polymers. [ 78 ] Similar structures have also been adopted in dye-sensitized solar cells (DSSCs) using TiO 2 or ZnO nanowires. Among 1D nanostructures, Si nanowires (SiNWs) are widely considered as an important class of nanoscale building blocks for high-performance devices [ ] due to their unique structural, electrical, optical, and thermoelectric properties in addition to their compatibility with current Si-based microelectronics. Presently, SiNWs are under intense investigation for PV applications since they may enable novel solar-to-electric energy conversion approaches for both high device efficiency and simple, low-cost manufacturing. [ ] In this article, we review the recent advances of SiNW-based solar cells as well as the associated scientific and technological challenges for their commercialization. 2. Fabrication of Silicon Nanowires SiNWs are promising nanoscale building blocks for highperformance devices such as field-effect transistors (FETs), chemical and biological sensors, and energy conversion devices. Fabrication of SiNWs with controlled diameter, length, and electronic properties are essential to these applications. Significant progress has been made in the development of 198 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

2 facile and controlled methods for SiNW fabrication in recent years. [ 133 ] In principle, there are two basic approaches for preparing SiNWs, i.e., the bottom-up and top-down approaches. The bottom-up approach is an assembly process joining Si atoms to form SiNWs. This approach includes vapor-liquidsolid (VLS) growth, [ ] oxide-assisted growth (OAG), [ ] supercritical-fluid-based and solution-based growth, [ ] and related methods. [ ] The preparation of SiNWs via the classical gold-catalyzed VLS growth process involves (1) decomposition of a Si precursor on a metal catalyst, (2) formation of the Si-metal liquid alloy, (3) Si diffusion into the Si metal alloy droplet, and (4) Si precipitation to form SiNWs from the droplet upon Si supersaturation. The bottom-up approach can readily grow SiNWs with diameters ranging from 5 nm to several hundreds of nanometers and lengths from 100 nm to tens of micrometers. On the other hand, the top-down approach prepares SiNWs via dimensional reduction of bulk Si by lithography and etching. [ ] Electron beam lithography, reaction ion etching (RIE), and the newly developed metal-catalyzed electroless etching (MCEE) of Si are widely used to fabricate SiNWs with dimensions ranging from tens to hundreds of nanometers. In general, the top-down methods have difficulty in the production of SiNWs with diameters below 10 nm Bottom-Up Fabrication: Chemical Vapor Deposition Chemical vapor deposition (CVD) via the VLS process is a popular method capable of fabricating dense, high aspect ratio, and vertically aligned SiNWs. In CVD-VLS growth of SiNWs, gold or a suitable metal serves as catalyst while a gaseous Si precursor, such as silane (SiH 4 ) or Si tetrachloride (SiCl 4 ), provides the gaseous Si reactants. The benefits of CVD VLS growth include the epitaxial SiNWs with controlled growth rates; the possibility of in situ controlled doping; SiNWs of controlled diameter, density, and length; and site-selective wire growth. Wu et al. fabricated uniform SiNWs with diameters down to 3 nm using SiH 4 as the precursor and H 2 as the carrier gas [ 138 ] and found that hydrogen mitigated radial growth through suppression of either reactant adsorption by terminating the Si surface or silane dissociation. Moreover, H 2 as the carrier gas also passivated the NW surface and reduced surface roughness. Using SiCl 4 as the precursor in VLS-CVD, Hochbaum et al. prepared vertically aligned SiNWs [ 139 ] and suggested the versatility of SiNW growth stemming from the use of SiCl 4 as the precursor. As a by-product of SiCl 4 decomposition, HCl etched the surface oxide yielding clean Si surfaces for Si precipitation from the binary liquid droplet. Epitaxial Si deposition at the interface induced growth alignment of nanowires with Si wafer surface. Without HCl, other precursors, such as SiH 4, cannot offer Si substrate cleaning or vertical SiNW alignment. The electrical properties of SiNWs can be readily tuned by introducing doping precursors, such as PH 3 and B 2 H 6, in the CVD- VLS process. [ ] Via alternate switching of the doping precursors, modulated or gradient doping profiles along the wire axial direction can be produced. Gold is the most popular metallic catalyst used for growing SiNWs via the VLS mechanism. However, gold contamination in SiNWs is unavoidable in such CVD-VLS growth. [ 173 ] Kui-Qing Peng earned a Ph.D. in materials physics at Tsinghua University (China) in 2004 and then continued as a postdoctoral researcher in Prof. S.-T. Lee s group at the Center of Super-Diamond and Advanced Films (COSDAF) at City University of Hong Kong. He is currently an Associate Professor of Materials Physics and Chemistry of the Department of Physics at Beijing Normal University. His current research is focused on the fabrication of silicon micro/nanostructures by metal-catalyzed anisotropic electroless etching (MCEE) of silicon and the application of silicon micro/nanostructures in photovoltaics, photocatalysis, lithium-ion batteries, and gas sensors. Shuit-Tong Lee is a Member of the Chinese Academy of Sciences (CAS), a Fellow of the Academy of Sciences for the Developing World, a Chair Professor of Materials Science and Director of the COSDAF at City University of Hong Kong, and Director of Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University, Suzhou. His research interests include nanomaterials and nanotechnology, organic electronics, diamond and superhard coatings, and surface and materials science. The presence of metallic contaminations frequently induces deep-level electronic states in the Si bandgap and degrades the minority-carrier lifetime and diffusion length in SiNWs, properties which are highly undesirable for photovoltaic devices. Moreover, gold or metallic catalysts are undesirable for complementary metal oxide semiconductors (CMOSs). Therefore, alternative metals, which are less detrimental to the minority-carrier lifetime and more compatible with CMOSs, such as copper and aluminum, are preferred. Several groups have grown SiNWs using alternative catalysts, such as Cu, Al, and Pt. [ 136, 137, ] Using copper as a catalyst and SiCl 4 as a Si precursor, [ 136 ] Kayes et al. achieved good control of the size, position, and uniformity of vertical, epitaxially aligned, large-area SiNW arrays on Si wafers by using a patterned oxide buffer layer to prevent catalyst migration on the surface during annealing and wire growth. Since copper is less detrimental to the minority-carrier lifetime of Si, Cu-catalyzed VLS-grown Si wires indeed showed a long effective electron diffusion length of 10 μm. [ 179 ] Aluminum can also be a suitable catalyst for VLS growth since the Al Si binary phase diagram is similar to the Au Si diagram. Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 199

3 aligned Si nanopillars on Si wafers, albeit SiNWs of a highaspect ratio are difficult to achieve via etch-mask erosion. In contrast to CVD-VLS growth, ordered arrays of <100 > - oriented SiNWs of controlled size, density, and electronic properties could be easily produced on Si wafers by the combination of RIE and lithography. For example, Hsu and coauthors fabricated uniform Si nanopillars over an entire 4 inch wafer by combining Langmuir Blodgett assembly and RIE. [ 160 ] Control over the pillar diameter and separation between the nanopillars also was demonstrated. Garnett et al. reported the fabrication of ordered SiNWs on a Si wafer through the combination of deep reactive ion etching (DRIE) and silica bead assembly. [ 162 ] Metal-Catalyzed Electroless Etching of Silicon Figure 1. a) Schematic setup of high-temperature CVD. Reproduced with permission. [ 133 ] Copyright 2010, American Chemical Society. b) Scanning electron microscopy (SEM) cross-sectional image of a SiNW array grown from Au colloids. Reproduced with permission. [ 139 ] Copyright 2005, American Chemical Society. c) Tilted SEM view of a Cu-catalyzed Si wire array over a large ( >1 cm 2 ) area. The scale bar in the inset is 10 μ m. Reproduced with permission. [ 136 ] Copyright 2007, American Institute of Physics. d) SEM cross-sectional image of Al-catalyzed SiNW array on Si(111) grown at 430 C. Reproduced with permission. [ 176 ] Copyright 2006, Nature Publishing Group. Wang et al. reported that SiNWs could be epitaxially grown on Si (111) substrates using Al catalyst at C (Figure 1d ), [ 176 ] which is considerably lower than the eutectic temperature in the Al Si binary phase diagram (577.8 C). Al can also be a p-type dopant producing a shallow acceptor in Si [ 180 ] and reduce fabrication costs and deep level impurities, thus an Al catalyst is preferred in the VLS growth of p-type SiNWs for PV applications. However, the high sensitivity of Al to oxidation necessitates a base pressure lower than 10 9 mbar to achieve highquality Al-catalyzed SiNWs. Metal-catalyzed VLS growth yields dominantly <111 >-oriented SiNWs, [ 138 ] whereas OAG predominantly produces <112 > - and <110 >-oriented SiNWs. [ ] The <100 > -oriented SiNWs are rare even for growth on [ 100 ] oriented Si substrates, but may be grown via an anodic aluminum oxide (AAO) template. [ 181 ] While conventional lithography techniques offer desirable flexibility and precision in device processing, fabrication of SiNWs with a high aspect ratio over large areas for practical applications, such as PVs, remains a challenge. In 2002, Peng et al. [ ] reported that a wafer-scale SiNW array could be readily produced via electroless etching at room temperature by simply immersing Si wafers into HF AgNO 3 solution. [ 163 ] Significantly, highly oriented SiNW arrays and Si nanostructures including porous Si and Si nanoholes can be produced by metal-catalyzed (commonly silver or gold) electroless etching (MCEE) of Si wafers in an aqueous HF solution containing oxidizing agents, such as Fe(NO 3 ) 3 or H 2 O 2. [ ] They proposed a microscopic mechanism for the MCEE method based on metal-induced local oxidation and anisotropic dissolution of Si substrates in aqueous oxidizing HF acid solution. They investigated the motility behavior of metal particles in Si [ 167 ] and showed the metal movement was preferentially along the Si [ 100 ] crystallographic orientation, thus leading to anisotropic Si(100) etching ( Figure 2 ). Compared to CVD-VLS and RIE, MCEE is a simple, 2.2. Top-Down Fabrication Reactive Ion Etching Utilizing standard Si micro-/nanofabrication technology, largearea highly uniform horizontal and vertical SiNWs can readily be fabricated on Si wafers. Electron beam lithography is commonly used for sub-ten-nanometer pattern generation, but is time consuming and expensive. [ ] Standard RIE is another popular Si micro-/nanofabrication technology that is aided by advanced lithography techniques, [ ] such as photolithography, nanosphere lithography, and nanoimprint lithography. It is widely used for producing large-area ordered vertically Figure 2. a) Schematics of collective and extended tunneling motion of Ag (or other metal) particles in a Si matrix leading to formation of Si nanostructures. b) Cross-sectional SEM image of aligned SiNW arrays by Ag-catalyzed electroless etching of p-type Si(100) in aqueous HF/ H 2 O 2 solution. c) SEM image of ordered arrays of SiNWs with controlled diameter and density by combination of MCEE and nanosphere lithography techniques. Reproduced with permission. [ 167 ] Copyright 2008, Wiley-VCH. 200 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

4 low-temperature, scalable, and speedy process. Furthermore, MCEE can readily produce vertical SiNWs with nm diameters that have desirable electrical properties and orientations from mother Si wafers. As a potential low-cost alternative to conventional bottomup fabrication, the MCEE process has recently attracted great interest for its versatility in producing wafer-scale, high aspect ratio, vertically aligned SiNW arrays. Furthermore, SiNWs produced from MCEE are uniform in terms of doping profile, crystal orientation, density, size, and shape, which are essential for applications. For example, SiNWs with controlled diameter, density, and length have been achieved via the combination of MCEE and nanosphere lithography. [ 166, 168 ] Based on the arrangement of metal catalysts via interference lithography, periodic SiNW arrays with controlled sizes and spacing have been prepared. [ 169 ] Using conventional photolithography, vertically periodic Si microwire arrays were also prepared. [ 182 ] Since the MCEE-prepared SiNWs are electrically conductive and optically active, they are particularly attractive for applications in solar cells, [ 117, 120 ] chemical sensors, [ 183 ] and thermoelectric devices. [ 132 ] 3. Properties of Silicon Nanowires for Photovoltaic Applications Silicon is the most popular material in commercial solar cell modules, accounting for 90% of the PV market. The success of Si PVs is due to several beneficial features of Si: (1) Si is plentiful and ubiquitous, being the second most abundant element on Earth; (2) Si is generally stable and nontoxic; (3) the 1.12 ev band-gap of Si is almost ideally matched to the terrestrial solar spectrum; and (4) Si PVs are readily compatible with the modern Si-based microelectronics industry. Conventional Si solar cells are made of planar junctions of p-type and n-type Si. Such PVs require relatively large amounts of Si to fully absorb the incident sunlight because Si has an indirect gap and low absorption in the visible and near-infrared spectrum. In addition, since charge carriers are collected along the light path, Si material must be of high-purity solar grade and have long minoritycarrier diffusion lengths for minimal carrier recombination and enhanced carrier collection. The requirements for large amounts of high-purity Si materials translate into the high cost of Si PVs. In recent years, the increasing demand for Si PVs has further inflated the price of raw Si materials. Consequently, Si PVs remain non-competitive in cost compared to conventional energy sources and, thus, are not yet mass deployed. Cost and efficiency are the two major challenges facing the PV industry. There are three approaches to address the cost issue of Si PV technology: (1) increase cell efficiency via novel PV designs; (2) lower the cost of solar-grade Si materials via inexpensive Si production methods; and (3) develop PV structures requiring Si materials of lower quality and less quantity. For low-cost solar energy conversion, SiNWs have emerged as an extremely attractive candidate due to their unique geometrical features. [ 85, , 162, ] First, SiNW arrays have strong optical absorption in the solar spectrum, i.e., less than 1% equivalent of Si materials in SiNW arrays can achieve the same amount of absorption as traditional planar wafer-based PV devices. Second, SiNW solar cells with radial p-n junctions offer a short collection length for charge carriers, thus allowing the use of lower-quality Si materials. Third, SiNWs can be produced with excellent electrical characteristics. These advantages may substantially reduce the production cost of SiNW-based solar cells while retaining competitive efficiencies, as discussed below Electrical Transport Properties of Silicon Nanowires Electrical transport properties of SiNWs, such as n- or p-type conducting, are important for fabricating functional electronic devices, such as FETs and solar cells. SiNWs prepared via the top-down or bottom-up approach have different electrical properties. In the top-down process, SiNWs inherit the electrical characteristics of the mother Si wafers and do not need further doping processing for conductivity. In contrast, SiNWs produced via the bottom-up approach are generally intrinsic or insulating and thus require doping processing to achieve conductivity. Since postgrowth doping of SiNWs, e.g., by ion implantation, is a difficult process, dopant incorporation is commonly performed during growth by introducing dopant precursors, such as diborane and phosphine for p- and n-type doping, respectively. Dopant or surface modification of the electronic properties of SiNWs has been studied by numerous theoretical and experimental investigations. [ , ] Cui et al. fabricated n- and p-type SiNWs by introducing boron or phosphorus dopants, respectively, during SiNW growth, [ 171 ] but the dopant concentrations were not measured. Gate-dependent transport measurements indicated reduced mobility in smallerdiameter SiNWs and the possibility of high dopant concentrations. Temperature-dependent measurements revealed the high-degree structural and doping uniformity in heavily doped SiNWs. As better device performance depends on higher carrier mobility, considerable effort has focused on improving the carrier mobility in SiNWs. The surface properties of SiNWs are expected to significantly influence the electrical characteristics owing to large surface-to-volume ratios. Indeed, Cui et al. reported remarkably enhanced carrier mobility in SiNWs after thermal annealing and passivating surface defects via chemical modification. An average mobility of cm 2 V 1 s 1 with a peak value of 1350 cm 2 V 1 s 1 [ 191 ] was achieved after surface treatment Optical Properties of Silicon Nanowires Arrays for Photovoltaic Applications The optical absorption properties of solar cells in the solar spectrum are important in determining cell efficiency. In this regard, SiNW arrays possess several unique and distinctive optical properties for PV applications owing to their high surface areas and diameters smaller than visible wavelengths Light Trapping and Enhanced Optical Absorption in Silicon Nanowires Arrays Peng et al. reported that SiNW arrays prepared by the MCEE method exhibit excellent antireflection properties. [ 117, 165 ] Figure 3a shows the hemispherical reflectance of electrolessetched SiNW arrays on single-crystal and polycrystalline Si Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 201

5 Figure 3. a) Hemispherical reflectance measurements of SiNW arrays, porous Si (PSi), and polished crystalline Si. The PSi was prepared by conventional stain etching in HF/HNO 3 solution. Reproduced with permission. [ 117 ] Copyright 2005, Wiley-VCH. b) Optical transmission ( T ), reflectance ( R ), and absorption ( A = 1 T R) of SiNWs prepared by etching of 2.7- μm-thick mc-p +nn + -Si layers on glass. Reproduced with permission. [ 185 ] Copyright 2009, American Chemical Society. c) Transmission spectra of thin Si window structures before (red) and after etching to form 2 μ m (green) and 5 μ m (black) nanowires. The spectrum from an optical model for a 7.5- μ m-thin Si window is in blue and matches very well with the planar control measurement. The insets are backlit color images of the membranes before and after etching. Clearly there is a large intensity reduction and red shift in the transmitted light after nanowires formation, suggesting strong light trapping. Reproduced with permission. [ 162 ] Copyright 2010, American Chemical Society. wafers, porous Si, and polished single-crystal Si wafers. It clearly shows that SiNW arrays drastically suppress light reflection over a wide spectral bandwidth. The reflectance is less than 1.4% over the range of nm for SiNWs fabricated on single-crystal Si wafers. This remarkably low reflectance of the SiNW array is attributed to several distinct advantages associated with the wire geometry: (1) the ultrahigh surface areas of high-density SiNWs; (2) the subwavelength light-trapping effects of SiNW arrays; and (3) the collective light scattering interactions among SiNWs, which trap light and make it travel many turns over distances much longer than the array thickness. Consequently, SiNW solar cells can have extremely low reflectance without any antireflection coatings. The optical absorption properties of SiNW array films fabricated on glass substrates by MCEE have been measured by Sivakov et al. [ 185 ] and Tsakalakos et al. [ 186 ] The SiNW films showed very low reflectance ( < 10% at nm), strong broadband optical absorption ( > 90% at 500 nm), and an optical absorption much higher than Si films of the equivalent thickness (Figure 3b ). The enhanced broadband absorption is attributed to the strong resonance among the aligned SiNW arrays, while the observed absorption is partly due to the high-density surface states in the SiNW film. Using optical transmission and photocurrent measurements (Figure 3c ), Garnet and Yang showed that the path length of incident solar radiation of SiNW array films in the AM1.5G spectrum increased by 73 times. [ 162 ] The remarkable enhancement factor in the light-trapping path length is above the theoretical limit for a randomizing scheme and superior to any traditional light-trapping method. The enhancement is ascribable to the photonic crystal enhancement effects in the devices. [ 162 ] Researchers from Caltech have investigated the sunlight trapping capability of VLS-grown SiNW arrays embedded in transparent polymer polydimethlysiloxane (PDMS). [ 121 ] The optical properties of SiNW arrays with different wire lengths, diameters, spacings, and tiling patterns were measured. They found that the well-aligned periodic wire array led to higher overall optical absorption than wires obtained by quasi-periodic or random motifs because the periodic Si wires showed a strongly anisotropic angular absorption profile, producing lowabsorbing dead spots in a photovoltaic device. These authors demonstrated various light-trapping techniques to achieve maximal absorption over the relevant wavelengths and incidence angles of solar illumination ( Figure 4 ). A remarkable near-complete absorption of sunlight above the Si bandgap has been achieved through a clever design. Specifically, the Siwire arrays were conformably coated with a SiN x antireflection coating before being embedded in PDMS, and then the optically transparent Al 2 O 3 nanoparticles were incorporated into the PDMS to induce light scattering. Both light-trapping measures increased the peak normal-incidence absorption to 0.92 and it further increased to 0.96 with the use of an Ag back-reflector. Particularly, the absorption of the Si-wire array exceeded the planar light-trapping limit for infrared wavelengths ( >800 nm), thus offering great potential to overcome the inefficiency of traditional planar Si absorbers in the infrared region. The substantially enhanced absorption properties of Si wire arrays offer great potential for creating radial p-n junction Si-wire-array PV devices with near-unity quantum efficiencies. This has been 202 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

6 optical characteristics of SiNWs has been performed by several groups aiming to design and optimize SiNW-based solar cells. [ 87, 184, 198, 199 ] Using the transfer matrix method (TMM), Hu and Chen reported the numerical analysis of the effects of nanowire diameter, length, and filling ratio on the optical absorption of periodic arrays of lightly doped SiNWs with diameters of nm. [ 184 ] The periodic SiNW array under study is schematically shown in Figure 5a. Calculations showed the electromagnetic interaction between SiNWs cannot be neglected. Figure 5b shows the reflectance and transmittance for a nanowire structure and a thin-film, revealing that SiNW array structures have much lower reflectance across a wide spectral range than Si thin-films due to the reduced density of the nanowire structure. In the high-frequency regime, the nanowire arrays have higher optical absorption than their thinfilm counterparts owing to the small reflectance of SiNWs, while similar less-optimal absorption cannot be achieved in the low-frequency regime due to the higher transmittance of the nanowire structure. However, the insufficient absorption of lowenergy photons can be overcome by employing light trapping to increase the optical path by means such as using longer SiNWs or reflecting mirrors. The absorption and reflectance of SiNW arrays with different filling ratios were also studied, revealing that SiNW arrays with smaller filling ratios yielded lower reflection and higher absorption in the high-frequency regime, while SiNW arrays with larger filling ratios show higher absorption in the low-frequency regime for the same nanowire length Device Physics of Silicon Nanowire Solar Cells Figure 4. A) Schematic and measured absorption of a Si wire array with an antireflective coating and embedded light-scatterers measured on a Ag back-reflector. B) Measured absorption ( A WA, red) of the Si-wire array from Figure 4A (which had an equivalent planar Si thickness of 2.8 μm) at normal (solid) and 50 (dashed) incidence versus the calculated normalincidence absorption of a 2.8- μ m-thick planar Si absorber with an ideal back-reflector assuming: bare, non-textured surfaces ( A Si, black) and ideally light-trapping, randomly textured surfaces ( A LT, blue). C) Illustration of the normal-incidence, spectrally weighted absorption of the AM 1.5D reference spectra corresponding to each of the three absorption cases plotted above. Reproduced with permission. [ 121 ] Copyright 2010, Nature Publishing Group. demonstrated through the photoelectrochemical (PEC) characterization of Si-wire arrays in the transparent electrolyte, which formed a conformal radial semiconductor/liquid rectifying junction. This is an important achievement since the Si wire/ polymer solar cell design achieves all these with only 1% of the equivalent Si materials used in conventional planar designs. In short, the near-complete absorption of sunlight above the Si bandgap due to excellent light-trapping effects of SiNW arrays suggests high quantum efficiency is possible for SiNW-based solar cells Optical Simulation of Silicon Nanowires Arrays The optical properties of SiNW arrays are strikingly different from those of Si bulk and thin-films, as has been shown experimentally and theoretically. Recently, detailed simulation of the There are several approaches to address the cost-efficiency challenge. One approach is to invoke orthogonalization of the light Figure 5. a) Schematic drawing of the periodic SiNW structure. The parameters are the length L, the period α, and the diameter d. In the fi gure, θ and ϕ are the zenith and azimuthal angles, respectively. b) Reflectance of nanowires and the thin fi lm. c) Absorptance of nanowire structures with various fi lling ratios obtained by TMM and the Maxwell Garnet approximation. d) Reflectance of nanowires with diameter of 50, 65, and 80 nm. Reproduced with permission. [ 184 ] Copyright 2007, American Chemical Society. Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 203

7 absorption direction to charge carrier collection, which is a key factor in determining the overall efficiency of a solar cell. Si solar cells based on nanowires with conformal radial p-n junctions are of particular interest as an optimal way to decouple [ 85, 119, 120 ] light absorption from minority-carrier collection. Kayes et al. [ 85 ] proposed a device geometry consisting of vertically aligned arrays of radial p-n junction SiNW solar cells ( Figure 6 ). Each nanowire in the array has a shallow p-n junction acting as a tiny solar cell, in which photoexcited minority carriers have to travel only a short pathway to reach the chargeseparating junction. Such a wire geometry device allows high carrier-collection efficiency even using low-quality Si, leading to lower material cost in Si PV cells. Moreover, SiNW arrays have excellent optical absorption properties owing to the subwavelength scale of SiNWs. Kayes et al. suggested that the radial p-n junction nanorod array solar cells have the potential to produce significant efficiency improvements compared to cells made from materials with diffusion lengths smaller than their optical thickness and low depletion region recombination. Optimal cells have a radius approximately equal to the minority-electron diffusion length in the p-type core and a doping level high enough so that a rod of such radius is not fully depleted. Extremely large efficiency gains (from 1.5% to 11%) are possible in Si with very low diffusion lengths ( L n = 100 nm) by exploiting the radial p-n junction nanorod geometry, provided that the trap density in the depletion region is held fixed at a relatively low level. The analytic results have motivated the new design of nanowire solar cells that allow the use of materials of much smaller quantity and lower quality while retaining reasonable efficiencies. The simulations predicted the optimal wire diameter in radial p-n junction solar cells is on the order of the minority-carrier diffusion length, which is several micrometers in low-purity Si. Therefore, microwire arrays also are very efficient not only in the absorption of sunlight but also in the collection of charge carriers. Axial p-n junction SiNW solar cells feature greater absorption of incident light. [ 185 ] Similar to radial p-n junction solar cells, minority carriers tend to recombine at surface sites rather than sweep across the axial junction in SiNWs owing to excessive surface recombination at the ultrahigh surface of SiNWs. Particularly, in contrast to the short carrier collection distance in radial p-n junction SiNW solar cells, minority carriers in a planar axial p-n junction SiNW solar cell have to travel a long distance along the wire before collection by the electrodes. Therefore, other than the advantage of light trapping, axial p-n junction SiNWs solar cells essentially function similarly to conventional planar p-n junction cells but with smaller p-n junction areas. 4. Photovoltaic Devices Based on Silicon Nanowires 4.1. Silicon Nanowire Array as an Efficient Antireflection Layer for Photovoltaic Devices Reduction of optical loss is an important consideration for high efficiency in solar cells. To achieve this, the top surface of the solar cell is typically textured or covered with an antireflection layer. Alkaline texturization is a standard process for singlecrystal Si and is widely applied in current Si-based PV industry. Porous silicon (PSi) can reduce the reflectance to 5.8% in the nm wavelength range and thus is an excellent candidate for antireflection layers. In 2005, Peng et al. reported the excellent broadband optical antireflection and absorption properties of MCEE SiNW arrays [ 165 ] and fabricated SiNW radial p-n junction solar cells by conventional phosphorous dopant diffusion. Figure 7 illustrates the processing sequence of SiNWbased photovoltaic cells: (1) fabrication of vertically aligned SiNWs on p-type Si wafers; (2) RCA (the Radio Corporation of American) cleaning to eliminate the residual metal and organic species followed by removal of Si oxide in a buffered HF solution; (3) formation of a continuous thin n + layer on SiNWs via phosphorous diffusion at 930 C; (4) evaporation of an Al layer onto the rear Si surface and removal of the rear parasitic p-n junction; and (5) deposition of Ti/Pd/Ag grid contacts onto the front sides of SiNW arrays. Those initial efforts did not produce radial p-n junction solar cells because MCEE p-type SiNWs had Figure 6. a) Schematic cross-section of the radial p-n junction nanorod cell. Light is incident on the top surface. The light grey area is n-type and the dark grey area is p-type. b) Schematic of a single rod from the radial p-n junction nanorod cell and its corresponding energy-band diagram. Reproduced with permission. [ 85 ] Copyright 2005, American Institute of Physics. Figure 7. Schematic illustration of the fabrication of functional p-n junctions for SiNW solar cells. a) Fabrication of an aligned SiNW array fi lm prepared in an aqueous HF/AgNO 3 solution. b) A radial p-n junction SiNW array solar cell via phosphorous dopant diffusion, with the n-si shell shown in red. c) A subsurface p-n junction SiNW array solar cell via phosphorous dopant diffusion. Reproduced with permission. [ 117 ] Copyright 2005, Wiley-VCH. 204 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

8 a diameter of 100 nm, which was too small for p-diffusion, so that SiNWs were completely converted to n-type throughout. Consequently, the as-prepared SiNW solar cells were similar to a conventional planar p-n junction solar cell with the SiNWs acting as an antireflection coating for enhanced optical absorption. [ 117 ] Such a cell on a single-crystal Si substrate exhibited an open circuit voltage ( V oc ) of mv and a fill factor of 0.65, giving an energy conversion efficiency ( η ) of 9.31%, which was lower than that of a planar PV similarly produced but without SiNWs. The low conversion efficiency is mainly due to the surface or interfacial recombination, high series resistance, and low-current-collection efficiency of the front-grid electrodes. Later on, Fang et al. fabricated solar cells based on slantingly aligned SiNWs on a Si(111) wafer, which yielded a much improved power conversion efficiency of 11.37%. [ 200 ] They attributed the improved device performance to the combined effects of excellent antireflection properties and better electrical contacts. However, the high surface recombination of minority carriers in SiNWs arrays remained the main obstacle to cell performance Radial p-n Junction Silicon Nanowire Photovoltaic Devices To reduce the overall cost of solar cells, efforts have focused on developing inexpensive Si-thin-film solar cells containing less Si. However, the efficiencies of 1 to 2- μm-thick thin-film solar cells are relatively low at 8 10%. The exciting theoretical results on radial p-n junction (or called coaxial core/shell nanowires) structures have motivated scientists to develop SiNW-based PV devices with conformal radial p-n junctions (or coaxial core/shell nanowires). [ , 162, 201 ] In the radial p-n junction geometry, each nanowire consists of an n-type Si layer wrapping around a p-type Si wire core to form a photovoltaic cell. An advantage of such a radial junction geometry is the short travel distances of photoexcited minority carriers to the collection electrodes, leading to enhanced carrier-collection efficiency and minimum bulk recombination. Another advantage of the radial geometry is the high tolerance of PVs for material defects, permitting the use of lower-quality Si with shorter minority-carrier diffusion lengths. Therefore, the radial p-n junction PV has the potential to meet the cost and efficiency challenge faced by planar single-crystal Si solar cells Photovoltaic Devices Based on a Single Radial p-n Junction Si Nanowire High-efficiency single-nanowire PV devices may be integrated into nanoelectronic, photonic, and sensing devices to supply energy for ultralow-power applications. In 2007, Tian et al. [ 118, 202 ] reported the experimental realization of single p-type/intrinsic/ n-type (p-i-n) coaxial SiNW solar cells, consisting of a p-type nanowire encased in intrinsic polycrystalline Si with a further layer of n-type polycrystalline Si. The p-type SiNW cores were prepared by gold-catalyzed VLS growth, using SiH 4 as the Si precursor and diborane (B 2 H 6 ) as the p-type dopant. Then, the Si i-shell and n-shell were deposited sequentially using phosphine (PH 3 ) as the n-type dopant. The p-i-n coaxial SiNWs were coated with 30 to 60-nm-thick SiO 2 by means of plasmaenhanced chemical vapor deposition (PECVD). Figure 8a Figure 8. Fabrication and characterization of the p-i-n silicon nanowire photovoltaic device. a) Schematics of the device fabrication. Left: pink, yellow, cyan, and green layers correspond to the p-core, i-shell, n-shell, and PECVD-coated SiO 2, respectively. Middle: selective etching to expose the p-core. Right: metal contacts deposited on the p-core and n-shell. b) SEM images corresponding to the schematics in (a). Scale bars are 100 nm (left), 200 nm (middle), and 1.5 mm (right). c) Dark and light current voltage ( I V ) curves. d) Light I V curves for two different n-shell contact locations. Inset is an optical microscopy image of the device. Scale bar is 5 mm. e) Real-time detection of the voltage drop across an aminopropyltriethoxysilane-modifi ed SiNW at different ph values. The SiNW ph sensor is powered by a SiNW photovoltaic device operating under 8-sun illumination ( V oc = V, I sc = na). Inset shows the circuit schematics. f) Light I V curves (1 sun, AM1.5G) of two SiNW photovoltaic devices (PV 1 and PV 2) individually and connected in series and in parallel. Reproduced with permission. [ 118 ] Copyright 2007, Nature Publishing Group. illustrates the fabrication process of the p-i-n coaxial SiNW solar cell and the SEM images corresponding to the schematics are shown in Figure 8b. Under air mass 1.5 global (AM 1.5G) illumination, the p-i-n coaxial SiNW solar cell yielded a maximum power output of 200 pw per nanowire device, a large shortcircuit current density ( J sc ) of 23.9 ma cm 2 (upper bound), and an overall power conversion efficiency of 3.4% (upper bound). J sc was linearly scaleable with wire length while the open-circuit voltage ( V oc ) was essentially independent of length. The results indicated that photoexcited carriers were collected uniformly along the length of the radial p-n junction structures and that light scattering by metal contacts did not make a significant Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 205

9 contribution to the observed photocurrent. Tian et al. showed that single core/shell SiNW PVs can serve as robust power sources to drive a SiNW ph sensor and a nanowire-based AND logic gate and larger loads were achieved by interconnecting the SiNW PVs in series and in parallel (Figure 8e,f ). These are significant developments in using individual photovoltaic power elements in nanoelectronics Photovoltaic Devices Based on Radial p-n Junction SiNW Arrays SiNW arrays consisting of radial p-n junction nanowires have a number of unique advantages for PV applications owing to the nanowire array geometry. [ 119, 120, 162, 201 ] First, the geometry of a SiNW array possesses superior antireflection characteristics, which can absorb nearly all sunlight with energy above the Si band gap. Second, the radial p-n junction nanowire geometry offers efficient carrier separation and minimal bulk recombination. Third, the wire geometry lowers the Si quality and quantity needed in Si solar cells. Finally, scalable methods, such as CVD- VLS and MCEE, can be utilized to prepare vertically aligned SiNWs over large areas on various substrates. These advantages should make solar cells from arrays of SiNW radial p-n junctions highly promising for mass deployment. Photovoltaic Devices Based on CVD-VLS-Grown SiNW Arrays : To address the cost issue of Si solar cells, researchers at the General Electric Global Research Center proposed the use of low-quality Si materials. [ 119 ] They fabricated p-n junction SiNW-array solar cells by first depositing p-type SiNW arrays via CVD-VLS on stainless steel foils precoated with a Ta 2 N film diffusion barrier and electrical contact, followed by a conformal coating of n-type amorphous Si film via a PECVD process ( Figure 9 ). The optical Figure 9. a) Schematic cross-sectional view of the SiNW solar cell architecture. The nanowire array is coated with a conformal α-si:h thin-fi lm layer. b) SEM image of a typical SiNW solar cell on a stainless steel foil, including a-si and indium tin oxide (ITO) layers with insets showing a cross-sectional view of the device and a higher magnifi cation image of an individual SiNW coated with a-si and ITO. c) Specular reflectance (log scale) for a nanowire cell (green) compared to a thin-fi lm p-i-n a-si solar cell (blue) showing a signifi cantly reduced reflectance (1%) for the nanowire cell. d) Dark and light (under simulated AM1.5 conditions) I V characteristics of a typical SiNW solar cell. Reproduced with permission. [ 119 ] Copyright 2007, American Institute of Physics. reflectance of the SiNWs-based solar cells was significantly lower than that of conventional planar solar cells by one to two orders of magnitude in the wavelength range of nm. Under AM 1.5G illumination, the best SiNWs-based PV device yielded a V oc of 130 mv and a conversion efficiency of 0.1%; the latter is far smaller than the calculated 15 18% for ideal SiNWbased solar cells. The authors suggested the low conversion efficiency may be improved by optimizing nanowire diameter, improving radial p-n junction, reducing contact resistance, and minimizing shunts. [ 119 ] Later on, Gunawan et al. [ 201 ] fabricated solar cells from Aucatalyzed CVD-VLS-grown SiNWs on a Si(100) substrate, with each SiNW consisting of an n-type core CVD coated with a conformal layer of highly conducting p-type polycrystalline Si. They found the beneficial effects of SiNWs were predominantly associated with better light trapping properties, while the gold residual impurity adversely affected the minority carrier lifetime in Si leading to enhanced carrier recombination. The severe recombination could be effectively mitigated by a conformal ALD-grown Al 2 O 3 passivation layer that improved the solar conversion efficiency from 1% to 1.8%. Solid-state solar cells fabricated from VLS-grown SiNW arrays invariably suffered from many difficulties and yielded a low V oc and small conversion efficiency (mostly <2%). The results have collectively revealed that several key issues, such as quality of radial p-n junction, local shunting, and especially gold residual impurity, are obstacles to obtaining high-performance nanowire solar cells. To achieve high-performance SiNW solar cells, it is necessary to use more friendly catalysts for VLS growth, such as Cu and Al, to reduce deep-level traps in Si, and to use a crystalline or single-crystalline outer shell to achieve a high-quality p-n junction. Photovoltaic Devices Based on MCEE and RIE SiNW Arrays: Compared to the VLS process, MCEE is a relatively simple, low-temperature, readily scalable method for producing wafersize SiNW arrays. As the MCEE process uses silver as the catalyst at room temperature, the MCEE-prepared SiNWs are free from contamination arising from high-temperature diffusion of metal impurities, which are detrimental to SiNW solar cell performance. Thus, MCEE SiNWs are potentially an excellent Si material for solar cell applications. Garnett and Yang fabricated coaxial p-type a-si/n-sinw solar cells via the combination of MCEE, CVD, and rapid thermal annealing (RTP) crystallization. [ 120 ] Figure 10 shows the structure and electrical characteristics of the SiNW solar cells. The single-crystal MCEE n-sinws with diameters of nm were coated with a layer of 150-nm-thick polycrystalline p-si shell. After RTP at 1000 C for 10 s, the rear and front side contact electrodes were formed by sputtering Ti/Ag onto the n-si and Ti/Pd onto the p-si. Under AM 1.5G illumination, the devices yielded overall efficiencies of about 0.46%. The low efficiency was attributed to high series resistance in the polycrystalline shell, reduced local shunting resistance, and the enhanced surface and junction recombination. Later, they improved the SiNW array solar cells [ 162 ] by reducing the surface roughness of the SiNWs and controlling the nanowire diameter and density via a combination of RIE and silica bead assembly. The radial p-n junction at a depth of 160 nm was formed by boron diffusion into the 206 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

10 Figure 10. SiNW solar cell structure. a) Schematic cell design with the single crystalline n-sinw core in brown, the polycrystalline p-si shell in blue, and the back contact in black. b) Cross-sectional SEM image of a completed device demonstrating excellent vertical alignment and dense wire packing. c) Transmission electron microscopy (TEM) image showing the single crystalline n-si core and polycrystalline p-si shell. The inset is the selected area electron diffraction pattern. d) I V behavior in the dark and under AM1.5 simulated sunlight irradiation. Reproduced with permission. [ 120 ] Copyright 2008, American Chemical Society. as-prepared SiNWs. The vertically ordered SiNWs array lengthened the light path by a factor of 73, leading to improved J sc over planar controls, despite the high interfacial recombination associated with the high surface area of SiNWs. The conversion efficiency of such SiNW-based solar cells became 5 6%, which is substantively higher than the radial-junction solar cells made from VLS-grown SiNWs Axial Homojunction Silicon Nanowire Photovoltaic Devices (Figure 11b ). [ 203 ] The compositional and structural variations along the axial direction were achieved during VLS growth via appropriate selection of the catalyst diameter, reactant gases, pressure, temperature, and time. The output characteristics of the axial p-i-n SiNW devices with different i-segment lengths (i = 0, 2, 4 μ m) were characterized under 1-sun AM 1.5G illumination (Figure 14c ). A systematic improvement in PV performance with increasing i-segment length was observed and the increase in both V oc and I sc is remarkable going from the p-n to the p-i-n structural motif. This is because the increased length of the undoped intrinsic region could substantially reduce the leakage current. An optimized p-i-n SiNW device with i = 4 μ m exhibited a V oc of 0.29 V, a J sc of 3.5 ma cm 2 and a fill factor (FF) of 0.51 for an overall efficiency of 0.5%. Under illumination of 4.5 suns, the maximum output power increases to 31 pw compared to 4.6 pw at 1 sun. The increase in output power under concentrated ( > 1 sun) illumination is an attractive feature, suggesting the promise of stand-alone nanowire PV devices. Lieber et al. also fabricated a SiNW tandem solar cell via integration of two photovoltaic elements with an overall p-i-n +-p + -i-n structure (Figure 11d ). Under AM 1.5G illumination and for i = 2 μ m the tandem SiNW devices exhibited a V oc of 0.36 V, which is 57% larger than that of an i = 2 μm single p-i-n device; the power output of the tandem cell increased 39% from 2.3 pw of the single cell to 3.2 pw. Ideally, the tandem solar cell should have a 100% increase in V oc and power with respect to a single cell. The difference is attributed to parasitic series resistance at the non-ideal tunneling interface between the n + and p + segments. The results indicated that the voltage and output power of the tandem solar cell can be scaled via integration of multiple single SiNW p-i-n diodes. The fabrication of p-i-n junction PV devices is costly and time-consuming, involving serial electron beam lithography processing and thus the scalability of these PV devices may be Two nanowire structural geometries may be used to fabricate PV devices, i.e., the radial and axial p-n junction nanowires. The device physics of both the axial and radial p-n junction nanowire PV devices is identical. In the axial p-n junction nanowire configuration, photoexcited electron hole pair separation takes place within the depletion region due to the built-in field established in the axial direction. Compared to the shorter carriercollection distance in the radial p-n junction nanowire structure, the carriers in the axial p-n junction nanowire have to travel a longer distance along the wire direction before collection by the electrodes Photovoltaic Devices Based on Single Axial p-i-n Junction Silicon Nanowire To elucidate the factors limiting nanowire PV devices, Lieber and co-workers investigated the properties of axial-modulation-doped p-type/intrinsic/n-type (p-i-n) ( Figure 11a ) and tandem p-i-n +-p + -i-n SiNW PV elements Figure 11. a) Schematic and SEM image of an axially modulated p-i-n SiNWs; scale bar is 1 μ m. b) Light I V characteristics for the i-length = 0, 2, and 4 μ m devices under AM 1.5G illumination. c) Schematic and SEM image of a tandem p-i-n +-p + -i-n SiNW; scale bar is 1 μm. d) I V responses recorded on p-i (2 μ m)-n (red) and p-i-n +-p + -i-n (i = 2 μ m, blue) SiNW devices under AM 1.5G illumination. Reproduced with permission. [ 203 ] Copyright 2008, American Chemical Society. Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 207

11 an issue. Kim et al. developed a method to directly grow SiNWbased photovoltaic elements in parallel and in series. [ 205 ] Briefly, SiNWs grow epitaxially from the left electrode by the Aucatalyzed VLS growth and end up in the right electrode. During growth, the reactant gases can be changed to modulate the axial composition of the SiNWs and finally patterned electrodes can be connected to the SiNWs to fabricate axial p-i-n SiNW PV devices. Under 1-sun AM 1.5G global illumination, such axial p-i-n SiNW-based PV devices yielded a J sc of 33.4 pa, a V oc of V, a FF of 0.418, and a maximum power output of 2.7 pw. The output characteristics are comparable to the above single p-i-n SiNW PV devices defined by electron beam lithography. In particular, these p-i-n SiNW PV devices may be conveniently scaled up by integrating PV elements in parallel and in series Photovoltaic Devices Based on a Vertically Aligned Axial p-n Junction SiNW Array Controlled doping is a critical step in the construction of vertical SiNW-based PV devices. There are different ways to dope SiNWs consisting of axial modulation of composition. One approach is to supply an in situ dopant source during wire growth. For example, during CVD-VLS growth p-n junction SiNWs can be produced along wire axis by changing the gaseous dopant precursor, [ 126 ] such as diborane and phosphine. Nonetheless, arrays of vertically aligned SiNWs with axial p-n junctions via the VLS method are not available. Compared to doping of SiNWs via bottom-up methods, top-down techniques such as RIE and MCEE are more straightforward to obtain SiNWs from bulk Si wafers with desirable electrical properties. Peng et al. demonstrated the first large-area vertically aligned axial p-n junction SiNWs on a Si wafer via MCEE using a HF and AgNO 3 solution. [ 204 ] The axial p-n junction nanowire showed a nonlinear and rectifying behavior. For conventional planar Si solar cells, the active Si layer has to be sufficiently thick (about 300 μ m) in order to harvest a large amount of photons. Experiment and theory have shown that vertically aligned SiNWs enable efficient broadband absorption of sunlight, so that in the form of nanowires, less than 1% of the Si material would have the same absorption efficiency as in conventional Si-wafer-based devices. The reduced need for Si materials in SiNW solar cells would decrease the production cost, since the Si material is a major cost for Si PV cells. Recently, Sivakov et al. demonstrated a solar cell from SiNWs with axial p +-n-n + junctions on glass substrates, as shown in Figure 12. [ 185 ] They first fabricated a large-grained 2.5 to 3- μ m-thick multicrystalline (mc) p +-n-n + Si layer stack on a glass substrate by electron beam evaporation (EBE) deposition and laser crystallization. Vertical SiNWs with axial p +-n-n + junctions were then fabricated from the mc-si layer by MCEE process in HF AgNO 3 solution. The arrays of SiNWs on glass exhibited the anticipated low reflectance ( < 10%, at nm) and strong broadband absorption ( > 90% at 500 nm). Under AM 1.5G illumination, the samples exhibited a maximum V oc of 450 mv and a J sc of 40 ma cm 2 for an overall efficiency of 4.4%. The authors suggested that severe shunting and large series resistance are responsible for the fair device performance. Nevertheless, such solar cells with SiNWs as the absorber fabricated on cheap glass substrates may have the potential to impact future PV technologies Photovoltaic Devices Based on n-silicon Nanowires Grown on a p-si Substrate Solar cells have also been fabricated from n-sinws grown on a p-si substrate. [ 206, 207 ] Such cells showed clear photovoltaic behavior owing to the p-n junction formed between the n-type SiNWs and the p-type Si substrate. This cell design shows inferior ability in carrier collection than the radial p-n junction design, although the SiNW array still functions as efficient sunlight absorber. In 2008, Stelzner et al. fabricated solar cells based on VLS-grown n-sinws on p-si wafers. [ 206 ] Under AM 1.5 illumination, the simple device had open-circuit voltages in the range of mv, a short-circuit current density of 2 ma cm 2, and a FF of 0.2 for an overall conversion efficiency of 0.1%. Later, Perraud et al. improved the cell design by embedding SiNWs in a spin-on-glass (SOG) matrix and subsequent chemical-mechanical polishing (CMP) of the front surface, [ 207 ] Figure 12. a) Cross-sectional SEM image of the AgNO3/HF-etched mc-p +nn + -Si layer on glass. b) Schematic representation of the I V curve measurements of SiNW based p-n junctions. c) Non-illuminated and illuminated (AM1.5) I V curves of SiNWs etched into a mc-p +nn +-Si layer on glass. SiNWs are irradiated through the glass substrate (superstate confi guration) and contacted by metal tips at four different sample positions. The right-hand scale gives real measured current values and the left-hand scale shows current density values. Reproduced with permission. [ 185 ] Copyright 2009, American Chemical Society. 208 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

12 which substantially reduced the parasitic series resistance via the formation of high-quality front electrical contact on top of the SiNW array. Under illumination at 100 mw cm 2, the cell yielded a V oc of 250 mv, the J sc of 17 ma cm 2, and a FF of 0.4 for an overall power conversion efficiency of 1.9%. The authors identified that the main limiting factor of device performance was due to the contamination by the gold catalyst or lack of passivation of surface electronic defects, yielding a high p-n junction reverse current Photovoltaic Device Based on a Silicon Nanowire Schottky Metal/Semiconductor Junction Researchers at Caltech [ 208 ] have recently fabricated a singlejunction SiNW solar cell device relying on metal/semiconductor rectifying contacts ( Figure 13a ), which were selectively introduced beneath the Al contacts to the Au-catalyzed wires by joule heating a segment of the wire. The single-wire solar cell devices were used as a platform to investigate the PV properties of SiNWs, such as the resistivity, diffusion length, and bulk and surface recombination. The dark I V characteristics indicated that the ideality factors of the single-junction SiNW PV devices ranged from 2 3.5, which were consistent with the values reported for Al-Si PV Schottky junctions. The authors suggested that the rectification was likely due to the formation of a Schottky barrier contact, an Al/Si p-n alloy junction, or a combination thereof. Under AM 1.5G illumination, the single-junction SiNW PV devices exhibited clear photovoltaic behavior with a V oc of 190 mv, a J sc of 5.0 ma cm 2 (upper bound), and a FF of 0.40 for an overall conversion efficiency of 0.46% (Figure 13b ). To study the minority carrier diffusion and recombination processes, scanning photocurrent microscopy (SPCM) experiments were performed on the single-wire solar cell devices using a confocal microscope and a near-field scanning optical microscope (NSOM) tip. They found that the photocurrent decayed exponentially away from the junction at both reverse and forward bias and that the exponential decay rate of the photocurrent did not vary with the applied bias, confirming the minority-carrier diffusion-limited transport within the wires. Minority-carrier diffusion lengths of 2 4 μ m in gold-catalyzed, VLS-grown SiNWs were observed using SPCM measurements. Large-area SiNWs-embedded Schottky solar cells were recently reported by Kim et al. [ 209 ] Briefly, multiple goldcatalyzed SiNWs were positioned between two Al and Pt electrodes using the dielectrophoretic (DEP) method and Schottky junctions were formed between SiNWs and metal electrodes. Carriers had a barrier to transport resulting in a rectifying flow. Under 1-sun illumination (100 mw cm 2 ), the device exhibited a J sc of na and a V oc of 167 mv, which is comparable to the V oc values reported for single coaxial SiNW solar cells and single Schottky-type metal/sinw junctions prepared by electrical heating. [ 118, 208 ] This work suggested a possible simple route to fabricate large-area SiNW-based solar cells without complex fabrication processes. Figure 13. a) Dark I V measurement of a single, 900-nm-diameter Si nanowire diode device. Inset: 45 view SEM image. The scale bar is 10 μ m. The rectifying junction was formed by sourcing current between the adjacent upper contacts (upward-facing arrows) until the enclosed wire segment was destroyed. The I V data were then measured between the two inner contacts (downward-facing arrows). b) Dark and light J V curves of a single Si nanowire solar cell with a 900-nm- diameter. Reproduced with permission. [ 208 ] Copyright 2008, American Chemical Society. 5. Silicon Nanowire Photoelectrochemical Solar Cells Solid-state radial p-n junction SiNW array solar cells, though theoretically and experimentally shown to be an attractive lowcost alternative to traditional planar Si-based solar cells, have to date exhibited poor performance primarily due to excessive interfacial recombination losses and large shunting. As an alternative to solid-state radial p-n junction nanowire array solar cells, the radial liquid-junction nanowire PEC solar cells have attracted intense attention owing to their potential low cost and performance advantages over their solid-state junction counterparts. [ ] In contrast to the expensive and energy-intensive high-temperature metallurgical or CVD processes needed for the solid-state junction nanowire devices, the transparent electrolyte conformally forms a rectifying junction to the top and side of the nanowires. Since the liquid junction is analogous to the radial p-n junction, the device physics principles, such as charge-carrier transport and recombination, of nanowire PEC solar cells are similar to their solid state radial p-n junction counterparts PEC Solar Cells Based on CVD-VLS-Grown Silicon Microwire Arrays Using the CVD-VLS technique and SiCl 4 as precursor, the researchers at Caltech [ 210 ] deposited arrays of vertically aligned, Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 209

13 single-crystalline n-type Si microwires with diameters of a few micrometers and lengths of tens of micrometers on highlydoped n-si wafers ( Figure 14a ). The Si microwire arrays were characterized using a PEC cell in an organic electrolyte consisting of 200 m M of dimethylferrocene (Me 2 Fc), 0.5 m M of Me 2 FcBF4, and 1 M of LiClO 4 in methanol. The liquid electrolyte acted as the p + layer on the n-sinws surface and a highly passivated surface. The vertically aligned Si microwire arrays were photoactive, yielding significantly higher photocurrents and photovoltages in the PEC cells than the control planar Si samples due to enhanced absorption and carrier collection in Si microwires (Figure 14b ) PEC Solar Cells Based on MCEE SiNW Arrays Compared to the VLS and OAG growth, the MCEE fabrication of SiNWs offers several unique advantages in addition to simple, lowcost, low-temperature, and wafer-scale production. [ ] First, the MCEE SiNWs inherit electrical properties from the mother Si Figure 14. a) A cross-section SEM image of Si wire array; scale bar is 15 μ m. b) Current density versus voltage curves for a Si wire array (solid) and control samples (dashed). The electrode potential was measured versus a Pt reference poised at the Nernstian potential of the 0.2 M Me 2 Fc/0.5 m M Me2FcBF 4 /1.0 M LiClO 4-CH3 OH cell. Reproduced with permission. [ 210 ] Copyright 2007, American Chemical Society. substrate and thus require no further doping processing for conductivity. Second, the MCEE SiNWs are an integral part of the Si substrate and are thus mechanically robust. Third, the MCEE SiNWs have rough surfaces making them highly antireflective due to strong light scattering and absorption. Fourth, combined with lithography techniques, MCEE allows the patterned and controlled fabrication of vertical SiNWs on Si wafers with desired orientations. These salient attributes make MCEE SiNW arrays a highly attractive PEC photoactive material. Peng et al. fabricated MCEE SiNW arrays and used them in PEC cells containing a mixture of an aqueous electrolyte of 40% hydrobromic acid and 3% bromine solutions, which provided a convenient and conformal wetting of all SiNWs. [ 212, 213 ] In the PEC solar cells the n-sinw array served as the sample photoelectrode while a Pt mesh served as the counter electrode. The generation of photovoltage and photocurrent in the presence of the Br 2 /Br redox couple was recorded using a Princeton electrochemical workstation. Under AM 1.5G illumination (100 mw cm 2 using an ORIEL solar simulator), the PEC measurements showed that the MCEE SiNWs are remarkably photoactive and effective at enhancing photovoltaic properties including the photocurrent and photovoltage, thereby revealing SiNWs as a promising material for PEC cells. However, the fabricated SiNWs PEC solar cells exhibited mediocre performance with a low energy conversion efficiency ( < 1%) due to serious junction recombination and poor carrier collection associated with the rough surfaces and poor junction areas of SiNWs. In light of the enhancement effects of metallic nanoparticles decoration on carbon nanotubes and nanowires, [ ] metallic nanoparticles (MNPs) were coated on n-sinws to enhance solar energy conversion ( Figure 15 ). [ 213 ] Besides the short carrier-collection distance and near-complete absorption of sunlight above the Si bandgap, the MNP-decorated SiNW electrode geometry provided additional advantages for solar cell applications. First, the conductive SiNWs provide direct 1D pathway for efficient charge-carrier transport to the Si wafer substrate. Second, MNPs serve as catalysts for electrochemical reactions on SiNW electrodes. Third, MNPs scatter impinging light among the SiNWs, which leads to increased light absorption. The photovoltaic properties of a MNP-decorated n-sinw array were characterized in an improved PEC cell using an ORIEL solar simulator at a constant light intensity of 100 mw cm 2. [ 213 ] Among the MNPs, the PtNP-decorated SiNW array yielded the highest enhancement in solar energy conversion. Figure 15c shows the output characteristics of PEC solar cells consisting of different n-si electrodes under illumination. Evidently, the SiNWs array significantly enhanced the short-circuit photocurrent density ( I sc ) to from 1.74 ma cm 2 for planar Si, while PtNP-decorated SiNWs further increased the photocurrent to 17.2 ma cm 2. The low reflectance and large surface area of SiNWs were responsible for the enhanced photocurrent. Significantly, decoration with PtNPs substantively increased the FF of the device and FF stability during measurement. The PtNP-Si nanoscale junctions and the excellent catalytic activity of PtNPs for the electrode processes are considered responsible for the remarkable enhancement in PEC performance. Furthermore, the conversion efficiency of SiNW PEC devices was roughly proportional to the PtNP amount, yielding a maximum efficiency 210 wileyonlinelibrary.com 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23,

14 Figure 15. a) Schematic illustration of a PtNP-decorated SiNW array for enhancing solar energy harvesting. b) High-resolution TEM image of a PtNP-decorated SiNW, revealing PtNP diameters of 5 nm. c) I V curves under simulated solar illumination for a PtNP-decorated n-sinw-array electrode, a PtNP-decorated planar n-si electrode, a naked n-sinw-array electrode, and a naked planar n-si electrode. d) The effects of PtNP loading or deposition time on the energy conversion effi ciency of a PtNP-decorated SiNW PEC solar cell. Reproduced with permission. [ 213 ] Copyright 2009, American Chemical Society. of 8.14% for optimal PtNP deposition (Figure 15d ). Prolonged Pt deposition on SiNWs resulted in FF degradation and lower photocurrent and thus poor overall efficiency owing to formation of a porous Si layer with large resistance. These results indicated the high potential of the PtNP-decorated SiNW system for efficient solar energy conversion. It is anticipated that better than 10% efficiency may be achieved after optimization of surface passivation, PtNP deposition, and the electrolyte solution. matrix, [ 78, 104, ] where a heterojunction is formed between inorganic nanowires and organic molecules. Such an organic inorganic hybrid nano structure offers several advantages that may lead to solar cells of 10% conversion efficiency. First, inorganic nanowires with high carrier mobility provide an electrically continuous 1D pathway for efficient carrier transport. Second, the organic/inorganic nanowire hybrid solar cell can be more flexible, less expensive, and lighter weight than conventional inorganic solar cells. Third, the hybrid nanostructure can enhance sunlight absorption and carrier collection due to largearea interfaces (for exciton dissociation) and the short heterojunction. Huang et al. fabricated blend hybrid solar cells of SiNWs and poly(3-hexylthiophene) (P3HT):[6, 6]-phenyl-C61-butyric acid methyleste (PCBM) on a glass substrate ( Figure 16 ). [ 225 ] They prepared SiNWs from a Si wafer using the MCEE method in a HF AgNO 3 solution and then transferred the SiNWs onto the P3HT:PCBM film on the ITO glass substrate. Under AM 1.5 illumination the solar cell with SiNWs showed a J sc of ma, a V oc of 425 mv, and a FF of 0.39 for a conversion efficiency of 1.93, while the cell without SiNWs exhibited a J sc of 7.17 ma, a V oc of 414 mv, and a FF of for a conversion efficiency of 1.21 (Figure 16e ). More recently, Shiu et al. reported a SiNW/ PEDOT (Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) 6. Flexible Silicon Nanowire/ Polymer Hybrid Solar Cell Current commercial solar cells are mostly based on Si wafers, but they are heavy, brittle, and unsuitable for portable applications. Organic solar cells (the thirdgeneration solar cells) have attracted considerable attention due to their flexibility, light weight, high optical absorption, and low-cost production. [ ] The major weaknesses of organic solar cells are low efficiency (currently ca. 7%) and poor stability. One promising alternative is the class of organic inorganic hybrid or composite solar cells made of inorganic nanowires embedded in polymer Figure 16. a) A schematic of the hybrid solar cell using SiNWs and a P3HT:PCBM blend. b) The setup for transferring SiNWs onto P3HT:PCBM blend. c) A schematic of the home-made machine with lateral force used to separate the Si wafer from the P3HT:PCBM blend. d) SEM image of the SiNWs transferred onto the P3HT:PCBM blend. e) Current density voltage characteristics for the solar cells with and without SiNWs under simulated AM1.5 illumination. Reproduced with permission. [ 225 ] Copyright 2008, Elsevier. Adv. Mater. 2011, 23, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 211

15 heterojunction solar cells with an improved power conversion efficiency of 5.09%, while the efficiency of planar cells was only 0.08%. [ 226 ] The results indicate that the combination of the SiNWs and the polymer blend may provide a promising approach to enhance organic PV cell performance. The Caltech group recently reported an innovative type of flexible Si solar cells consisting of VLS-grown Si wire arrays embedded in a polymer substrate. [ 121 ] They showed enhanced light absorption and carrier collection in the Si wire arrays, although the photovoltaic characteristics of the flexible hybrid solar cells were not investigated. Nevertheless, this suggests a potential route to produce flexible, lightweight, low-cost, and efficient solar cells. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No , ), National 973 project of the Major State Research Development Program of China (Grant No 2006CB933000), National Excellent Doctoral Dissertations of China (Grant No ), Beijing Natural Science Foundation (Grant No ), NCET , Beijing Nova Program (2008B24), Fok Ying Tung Education Foundation (121047), and Research Grants Council of Hong Kong SAR (Grant No. CityU5/CRF/08 and CityU101909). Received: July 6, 2010 Published online: October 7, Concluding Remarks and Outlook SiNWs have been shown both experimentally and theoretically to be an important and promising class of nanoscale building blocks for cost-effective and high-performance solar cells, primarily because of their superior properties for carrier transport, charge separation, and light absorption. On the one hand, individual and interconnected SiNW PV elements may serve as robust power nanosources to drive nanoscale electronic devices. [ 118 ] On the other hand, radial p-n junction SiNW arrays may offer a viable approach to cost-effective Si-based solar cells with performance competitive against conventional planar Si solar cells. Indeed, PEC solar cells with conversion efficiencies exceeding 8% have been demonstrated using MCEE-SiNW arrays. [ 213 ] Furthermore, Peng et al. have recently demonstrated Si-nanohole-array-based solar cells exhibiting a solar energy conversion efficiency of 9.51%, which is much higher than the reported values for SiNWs radial p-n junction solar cells. [ 227 ] Additionally, such a nanohole structure offers better mechanical robustness and higher light absorption than the free-standing SiNW array. Also, flexible solar cells based on SiNW-polymer hybrid nanostructures were possible, offering potential applications ranging from portable solar cells to roof-top and window installations. Despite the promising potential of SiNW-based solar cells, many challenges remain ahead. In particular, the excessive recombination at the high surface and interface areas of SiNWs must be reduced and kept comparable to that of the bulk in order to capitalize on the advantages of SiNWs in light absorption and charge collection. Besides optimal fabrication of high-quality p-n junctions, effective surface passivation is needed to improve device performance. Another major constraint on device performance is excessive shunting due to poor contacts or impurities at the junctions. In this regard, contacts and Si-friendly metal catalysts for SiNW growth should be optimized. 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