Silicon Microwire Arrays for Solar Energy-Conversion Applications

Dec 9, 2013 - Emily Warren completed her Ph.D. at the California Institute of Technology in 2012 working on the development of silicon microwires for ...
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Silicon Microwire Arrays for Solar Energy-Conversion Applications Emily L. Warren,*,† Harry A. Atwater, and Nathan S. Lewis* Division of Chemistry and Chemical Engineering, Kavli Nanoscience Institute and Beckman Institute, 210 Noyes Laboratory 127-72, California Institute of Technology, Pasadena, California 91125, United States ABSTRACT: Highly structured silicon microwire (Si MW) arrays have been synthesized and characterized as absorbers for solar energy-conversion systems. These materials are of great interest for applications in solar energy conversion, including solar electricity and solar fuels production, due to their unique materials properties, form factors, ease of fabrication, and device-processing attributes. The Si MW array geometry allows for efficient collection of photogenerated carriers from impure materials that have short minority-carrier diffusion lengths while simultaneously allowing for high optical absorption and high external quantum yields for charge-carrier collection. In addition, Si MW arrays exhibit unique mesoscale optical behavior and can be removed from the growth substrate to provide flexible, processable arrays of Si microwires ordered in a variety of organic polymers and ionomers. The unique photon-management properties of Si MW arrays, combined with their high internal surface area and controlled morphology for catalyst placement and support, allow for the use of earth-abundant electrocatalysts to produce an integrated, functional photoelectrode. These materials therefore also provide an opportunity to explore the 3-dimensional photoelectrochemical behavior of fuel-forming microstructured electrodes.

I. INTRODUCTION We describe and review herein recent progress in the development of Si microwire (Si MW) arrays for use in solar energy-conversion applications, specifically in the production of electricity and/or fuels from sunlight. We first discuss the rationale and background behind the development of energyconversion devices based on microwire arrays. We then describe the fabrication and properties of single Si microwires, as well as of arrays of Si microwires, in a variety of applications. Several design principles have recently emerged in the effort to develop an artificial photosynthetic system that would enable the direct production of fuels from sunlight. Such systems require a suitable light absorber, or combination of light absorbers, that provides the voltage required to sustain the production of solar fuels with the only chemical inputs being H2O and possibly CO2. Some existing photovoltaic materials can provide the photovoltages required to produce fuel and simultaneously oxidize water, but materials that are currently available are not optimal for such applications.1 First, the required thickness of a semiconductor is fixed by its optical absorption properties, so for example, 110 μm of Si is required to absorb 90% of the incident photons above the 1.12 eV band gap of Si. In a conventional, planar solar cell geometry, this absorber thickness defines the purity of the material required for efficient solar energy conversion, because before recombining the photogenerated carriers must reach the electrical junction where carrier separation occurs. Hence, for optimal energy-conversion efficiency, the minority-carrier collection length must equal, or exceed, the absorption depth of light. A second constraint for fuel production, such as water splitting, is that one half-reaction will liberate protons (e.g., H2O being oxidized to O2), and the other © 2013 American Chemical Society

half-reaction will consume protons (e.g., protons being reduced to H2). To preserve electroneutrality and to prevent the buildup of a proton concentration gradient, a suitably low ionic resistance path for proton (or hydroxide) conduction is required between the sites of oxidation and reduction. Finally, to ensure the safety of the system, the products must be robustly separated, which requires some type of chemical and/or physical separator having properties that are specific to the geometry as well as to the operating behavior of the entire engineered system.2 As opposed to a compact, planar structure, an array of nanowires or microwires can simultaneously address these concerns in full (Figure 1). In an array of high-aspect-ratio nanostructures or microstructures, light can be absorbed effectively along the “long” axis of the structure. Additionally, impure materials with short minority-carrier diffusion lengths can, in principle, still be used beneficially because the carriers only need to traverse the “short” distance, orthogonal to the principal direction of light absorption, to be collected and thereby produce electrical and/or chemical energy. If embedded into a suitable membrane that has the desired ionic selectivity and low gas permeability, such a structure also provides an optimal low resistance path to neutralize the proton concentration gradient that results from solar fuels production.3 In many respects, such structures resemble the architectures found in natural systems such as forests of Aspen trees, stomata, and retinal rods, in which the direction of light absorption is orthogonal to the direction of chemical species transport. The Received: June 25, 2013 Revised: November 1, 2013 Published: December 9, 2013 747

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Figure 2. Schematic of a radial p−n junction, with an inner p-type core and an outer p-type shell. Inset shows the radial band-bending diagram for a p−n junction.

versely, larger diameters produced suboptimal collection of photogenerated carriers in the radial direction. For optimized structures, the simulations indicated that a radial wire geometry should produce high energy-conversion efficiencies even for materials with low minority-carrier diffusion lengths, provided that such systems did not suffer from high rates of junction or surface recombination.6 Several different approaches have subsequently been taken to refine the optical modeling of Si nanowire/microwire array solar cells, and such methods have consistently predicted a high optical absorbance for these structured devices.7−10 Approaches that have combined optical and electrical measurements have shown that optimal geometries involve a consideration of the interplay between enhanced optical absorption and deleterious surface recombination.11−13 A. Macroporous Si. These theoretical predictions were experimentally tested shortly thereafter by etching controlled pores into high purity crystalline Si, to produce macroporous Si photoelectrodes.14 The behavior of planar crystalline n-Si photoanodes in contact with the 1,1′-dimethylferrocene+/0 ((Me2Fc)+/0) redox system in CH3OH has been thoroughly documented and shown to be in accord with the ideal Shockley diode limit on the photovoltage of such systems, providing bulk diffusion-recombination-limited photovoltages in excess of 670 mV under 1 Sun of simulated sunlight.15 These Si-liquid contacts also exhibit internal quantum yields near unity for charge-carrier collection under short-circuit conditions. Although the Si wafers used to make the macroporous Si electrodes were high purity and had a long minority-carrier collection length (in contrast with the goal of using low cost, low purity semiconductors), the experiment allowed investigation of the basic behavior of a system in which carriers were collected in a direction orthogonal to the direction of light absorption. After correction for optical losses associated with the incomplete filling fraction of the incident optical plane due to the porosity of the sample, the minority-carrier collection efficiency of the macroporous Si photoanodes was observed to be nearly unity in contact with the Me2Fc+/0−CH3OH electrolyte. Additionally, the open-circuit photovoltage (Voc) approached the Shockley diode limit, confirming the key aspects of the theoretical description of such systems. However, detailed studies of the macroporous Si system showed that the Voc systematically decreased as the junction area increased. This behavior agrees with the expectations for a

Figure 1. Schematic of a water-splitting device concept utilizing structured solar absorbers and a proton-permeable membrane for ion transport. The high-aspect-ratio structures can improve light absorption for semiconductor materials with short minority-carrier diffusion lengths, and the high surface area can enhance catalyst loading. (Image copyright 2013, E. A. Santori; used with permission.)

microwire array approach is therefore a blueprint for a system architecture that can couple the optical absorption, electrical transport, and ionic transport processes in a desirable fashion. One prototype for such a design consists of tandem micro- or nanostructured semiconductor absorbers, decorated with catalysts and embedded in a protonically selective, but product-gas impermeable, membrane (Figure 1). These considerations in turn have stimulated the design, development, and detailed exploration of the properties of nanowire and microwire arrays of semiconductors for solar energy conversion, as described in the remainder of this article.

II. FUNDAMENTAL PROPERTIES OF HIGH-ASPECT-RATIO NANO- AND MICROSTRUCTURED LIGHT ABSORBERS Several designs have historically been proposed to orthogonalize the directions of light absorption and carrier collection.4,5 Specifically, the merits of the orthogonalization concept in a radial geometry have been rigorously evaluated quantitatively using a device-physics model.6 The values of the optical and electrical parameters of conventional materials such as Si and GaAs are well-known, and device-physics simulations were used to rigorously evaluate the behavior of microwire arrays made of such materials (Figure 2). The simulations showed that exerting morphological control over semiconductors, materials with short minority-carrier diffusion lengths, which would produce very low efficiencies in a conventional planar device structure, could provide high energy-conversion efficiencies by utilization of an array of high-aspect-ratio structures. For semiconductor materials that had short minority-carrier collection lengths relative to the optical thickness of the material, optimal energyconversion efficiency was predicted when the radius of the structure was equal to the minority-carrier diffusion length in the bulk of the absorber. Smaller diameters introduced more junction area and thus more junction recombination, without a compensating increase in minority-carrier collection. Con748

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III. SI MICROWIRE ARRAYS A. Vapor−Liquid−Solid Growth of Si Microwires and Microwire Arrays. In parallel with the experimental investigations of macroporous Si and nanorod arrays of Cd(Se,Te), attention turned to implementation of the principle of orthogonalization in Si-based systems. Si is an extremely attractive material for use in photovoltaics or as a photoelectrode for electricity or fuel production, specifically as a photocathode for H2 production. Si is cathodically stable in aqueous acidic solution and with appropriate catalysts can function as an efficient absorber as part of a tandem structure for unassisted solar-driven water splitting and/or reduction of CO2 to alcohols such as methanol or butanol.17 However, no bottom-up Si deposition method had been shown to produce reasonable quality crystalline Si for use in such applications. Hence an alternative approach was pursued to fabricate the desired microwire and nanowire arrays of crystalline Si for use in photovoltaic and photoelectrochemical applications. The vapor−liquid−solid (VLS) process for growing onedimensional (1-D) semiconductor “whiskers” using a metal catalyst was reported in 1957 and then elaborated by Wagner and Ellis in 1964.18,19 During VLS growth of Si wires, a Si wafer or other growth substrate is heated to a temperature above the eutectic temperature for the metal/Si system, creating a liquid catalyst droplet. Gaseous precursors (SiCl4, SiH4, Si2H6, etc.) introduced to the reactor then decompose on the catalyst surface and supersaturate the eutectic mixture. The growth substrate provides a nucleation site for the Si to precipitate as an epitaxial crystal. The (111) crystal orientation is generally the preferred direction of growth (although other orientations have been achieved under nonstandard growth conditions20). Through use of a (111)-oriented substrate, wires of Si are formed with a growth direction that is perpendicular to the surface plane of the substrate (Figure 3).21,22

modified ideal diode equation that takes into account the internal junction area of the system relative to the projected area of the system

Voc =

κBT ⎛ Jph ⎞ ln⎜⎜ ⎟⎟ q ⎝ γJ0 ⎠

(1)

where κB is Boltzmann’s constant; T is the absolute temperature; q is the (unsigned) charge of an electron; Jph is the light-limited current density; Jo is the dark saturation current density; and γ is the ratio of the actual surface area relative to the top-down projected area (i.e., the illuminated area) of the device. In the porous morphology, the minority-carrier flux produced by the incident illumination is distributed over an internal junction area which is larger by a factor γ than the area of a planar device that has the same projected area as the sample of interest. Because the photovoltage is logarithmically proportional to the minoritycarrier flux at the semiconductor-liquid junction (eq 1), the photovoltage is reduced accordingly. This design principle, revealed by simulation and confirmed by experiment, should thus be generally applicable to any type of microstructure including nanowires, microwires, nanocrystalline particulate systems, etc., and therefore should be carefully considered in the design of optimally performing solar energy-conversion devices based on essentially any type of micro- or nanostructured semiconductor morphology. B. Cd(Se,Te) Nanorod Arrays. The experiments with macroporous Si electrodes did not directly address the behavior of semiconductors that had minority-carrier collection lengths that were shorter than the optical absorption length of the material. This issue was subsequently addressed by experiments on n-type Cd(Se,Te) photoanodes in contact with an aqueous S22−/S2− electrolyte.16 CdSe and CdTe are well-known photoanode materials that are stable in contact with such electrolytes; however, electrodeposited n-Cd(Se,Te) has minority-carrier collection lengths of only ∼1 μm, whereas the optical absorption length of Cd(Se,Te) is ∼3 μm. The orthogonalization principle was demonstrated in this system by directly comparing the behavior of a compact Cd(Se,Te) photoanode film to the behavior of a nanowire array of Cd(Se,Te). Both materials were grown by electrodeposition, with the nanowire array prepared by electrodeposition of Cd(Se,Te) into porous anodic alumina, and then dissolving the alumina template in an aqueous alkali solution to produce the desired “forest” of Cd(Se,Te) nanowires. The spectral response of the nanowire array was then compared to that of the planar compact Cd(Se,Te) film. As expected, the compact film exhibited high quantum yields for minority-carrier collection at short wavelengths, where the optical absorption depth was comparable to or less than the minority-carrier collection length. The quantum yield decreased significantly at longer excitation wavelengths, where the optical absorption depth increased for wavelengths near the band gap of the material. In contrast, the Cd(Se,Te) nanowire arrays showed a nearly constant internal quantum yield for minority-carrier collection for photon excitation energies up to and above the band gap of the Cd(Se,Te) material, regardless of the optical absorption length of the material. Hence, orthogonalization of the direction of light absorption and charge-carrier collection enabled the Cd(Se,Te) material to show efficient minoritycarrier collection under conditions for which the same material in a planar morphology showed minimal photoactivity for the same process.

Figure 3. Schematic of the patterned VLS growth process, using Cu as the VLS catalyst. The metal−Si eutectic catalyzes the decomposition of SiCl4 and H2 to produce Si and HCl. Si saturates the eutectic mixture and precipitates a Si crystal epitaxially to the crystal orientation of the substrate.

Although the initial work by Wagner and Ellis demonstrated that the VLS process was capable of producing wires with a wide range of diameters, most of the recent resurgence of interest in VLS growth has focused on the fabrication of small (99% contact yield and efficiencies of 2.8% (without light trapping) for flexible solar cells.58 2. Photocathodes for Hydrogen Evolution. In addition to the advantages of radial carrier collection, inexpensive growth techniques, and flexibility, the enhanced surface area of the microwire geometry has also attracted interest for applications in solar fuels production. A large surface area relative to the projected area allows for increased catalyst loading and decreases the required current density at each point on the surface.1 Silicon has long been considered a candidate material to serve as a photocathode for solar-driven water-splitting systems, but its surface is not catalytically active for the hydrogen-evolution reaction (HER), requiring the use of a catalyst. By changing the morphology of the material, Si MWs maintain the desirable materials properties of a Si photocathode (abundance, band gap) but should enable the use of less active, potentially less-expensive, catalysts to compete with the noble metal catalysts that are currently used for many fuel-forming reactions. The microwire geometry provides an increased ratio of surface area to projected area (γ, or roughness factor) that can range widely depending on the exact geometric details of the wires. The effect of surface-area enhancement was first investigated by a comparison of the dark catalytic properties of degenerately doped Si MW arrays with a variety of established HER catalysts. The HER performance of Ni, Pt, and Ni−Mo alloy was compared, and the catalytic HER onset improved for all of these catalysts when moving from a planar to Si MW geometry. Additionally, the Ni−Mo-loaded array performed almost as well as a Pt-loaded array, suggesting that Ni−Mo might be satisfactory under 1 Sun conditions on a morphologically structured photocathode.59 One complication to the use of Si electrodes in aqueous environments is that the band edges of Si are influenced by the pH of the solution, limiting the attainable photovoltage for unprotected Si vs the H+/H2 redox couple.60 However, introduction of a p−n junction can decouple the photovoltage of the system from the energetics of the solution/semiconductor contact. Experiments comparing p-Si MW and n+−p radial junction Si MW using the pH-independent MV2+/+(aq) redox

Figure 13. Photovoltaic energy-conversion performance of single-wire and on-substrate Si MW devices. For single wires, “light trapping” refers to the situation for a SiNx coating; for the wire array, it refers to the situation for the coating, a Ag back reflector, and with addition of scattering particles. (Adapted with permission from refs 13 and 55.)

(scattering particles, back reflector surface passivation) achieved simulated 1 Sun efficiencies of 7.9% under normal incidence illumination,55 even though again only ∼50% total absorption was achieved by the particular array and structure investigated (data points in Figure 13). The measured performance of the large-area devices was limited by incomplete absorption of the incident light at normal incidence, as well as by slightly lower photovoltages for microwire arrays than was observed for the single-wire devices. Optical and photoelectrochemical measurements of the dependence on the absorption of Si MW arrays on the angle of incidence of the illumination relative to the surface normal demonstrated that both the absorption and photocurrent increased when the sample was tilted relative to the illumination source, due to the longer path length of light through the wire array under such conditions.10,29,56 When a single-wire solar cell was measured under concentrated illumination conditions (2.3×) such that the photocurrent matched that of an optimally absorbing wire array, the resulting I−V data indicated that a large-area device of vertically oriented wires would be capable of reaching ∼17% efficiency.13 The performance of Si MW array devices can be improved by optimization of the light trapping (e.g., changing the shape of the top of the wire7 or controlled introduction of scattering particles) as well as by optimization of the surface passivation of the wires because the Voc is influenced in part by the large surface area of the wire arrays. 754

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system showed that while the measured Voc decreased with increasing pH for unprotected Si, the n+−p junctions maintained high (>500 mV) Voc values while the proton concentration was varied by over 6 pH units.61 To demonstrate that Si MWs could produce high photovoltages toward fuel-forming reactions, independent of catalyst effects, a proof-of-concept system based on radial n+−p junction Si MW arrays was investigated with Pt as the electrocatalyst.56 The Si microwires were grown and processed identically to those used for photovoltaic applications, except rather than making top electrical contacts, a thin layer of Pt was evaporated on top of the wires. Under 1 Sun of simulated sunlight, these electrodes exhibited thermodynamically defined photocathode efficiencies of 5.9% for the HER (Figure 14). Such electrodes also operated stably in acidic electrolytes for over 24 h of continuous operation at their maximum power point.

maintain high open-circuit voltages, confirming the potential for membrane-based water-splitting devices. One major challenge with these devices is the parasitic absorption by the catalyst coating. By deposition of nanoparticulate catalysts at the base of the wire array and covering the porous catalyst layer with a scattering layer (TiO2 nanoparticles), high catalytic activity, improved photocurrents, and improved stability of the electrodes in acidic conditions have been achieved simultaneously.63 3. Integrated Si MW-Based Energy-Conversion Devices. One model that has been proposed for an integrated watersplitting system is to have two semiconductor materials, a photoanode and photocathode, embedded in a membrane that can separately conduct protons and electrons, to thereby maintain charge neutrality. Modeling of this geometry as well as other proposed systems has revealed that resistive losses in the electrolyte surrounding a photoelectrochemical water-splitting device are critical to the successful performance of the system.2 The use of wire arrays directly embedded in a proton-conductive membrane can alleviate some of these issues, by minimizing the length scale of transport required through the liquid. Furthermore, studies of proton-conductive and anion-conductive membranes with, and without, embedded arrays of ordered Si MW have revealed that the addition of the wires does not drastically change the conductivities of the films nor significantly affect the rates of H2 crossover through the membrane.3 While Si alone does not have a large enough band gap to drive the water-splitting reaction, it is possible to use Si to split hydrohalic acids in a closed-loop photosynthetic system.64 Using a membrane embedded with radial p−n-junction Si MWs, unassisted splitting of HI has been achieved, by analogy to the Texas Instruments Si microspherical system for HBr splitting.64 Further studies have investigated methods to make electrical contact to Si using conductive polymers, which may be more compatible with solar fuels applications.65 4. Second-Generation Si MW Energy-Conversion Devices. Building upon the fundamental development of Si MW-based devices, recent work has focused on building more complex architectures and structures to create multicomponent systems for electricity and solar fuel production. Two approaches have been taken to grow tandem junction devices conformally on Si MW arrays. GaP is a wide band-gap material that has many complementary properties to Si. GaP has been epitaxially grown on Si MW using metal−organic chemical vapor deposition, and the resulting tandem devices have been analyzed for their energy-

Figure 14. J−E data for n+−p junction Si MW photocathodes coated with a thin Pt catalyst (red, measured in 0.5 M H2SO4), an electrodeposited Ni−Mo (blue, measured in pH = 4.5 KHP buffer) catalyst, and a polished Pt control sample (black, measured in 1.0 M H2SO4) (data from refs 56 and 62).

Although the use of Pt was suitable for demonstration purposes, a water-splitting system would preferably not rely on scarce, noble metal catalysts. Hence, Ni−Mo-coated radial n+−p junction Si MW arrays were tested as photocathodes and produced H2 from pH = 4.5 buffered electrolyte at photocathode efficiencies in excess of 2% (Figure 14).62 These devices could also be removed from the growth substrate and were shown to

Figure 15. (a) 3D schematic of a tandem junction MW array with a buried homojunction (n−p+-Si) coated with ITO and n-WO3. (b) 2D cross-section of an individual tandem junction array unit cell. Electrons and holes are collected radially in the n-WO3. Holes are collected radially in the n−p+-Si, and electrons are collected axially at the back contact. (Adapted with permission from ref 71.) 755

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conversion properties.66,67 Additionally, WO3 is a well-known semiconductor that has long been considered a good candidate for the photoanode of the water-splitting system.68,69 WO3 has accordingly been directly deposited onto Si MW arrays.70 By combining a diffused p−n junction with WO3, the overall watersplitting reaction was driven successfully under concentrated sunlight (Figure 15).71 These tandem device structures have the added advantage that each wire is capable of generating enough photovoltage to drive the reaction, so no intermediate conductive layer is needed. Such advanced designs demonstrate how a “bottom up” approach can be used to create complex structures for solar energy conversion from low-cost materials. 5. Further Developments and Impact of Si Microwire Arrays. Si microwires have demonstrated great potential in a wide range of solar energy-conversion applications. The demonstration of the orthogonalization principle has been recently extended to arrays of GaAs nanowires in contact with liquid-based redox systems,72 as well as for ZnO nanowire arrays for the photoelectrochemical oxidation of water to O2(g).73 A micro- or nanowire geometry can enable a reduction in materials utilization, and cost, for expensive semiconductor materials, while maintaining high efficiency, in accord with the recent report of 13.8% solar energy-conversion efficiency in InP-based nanowire solar cells.74 The Si microwires are also potentially useful as a structural scaffold for the formation of high-aspectratio structures of other materials, including conductors, insulators, etc. The 3-D aspects of these microwire arrays have yet to be fully exploited and may well allow independent control over otherwise correlated and interdependent properties such as reactant access, product egress, optical absorption, and catalyst activity in a variety of fuel-forming electrochemical reactions. The microwire approach is also attractive in that it allows the use of relatively impure material combinations, as well as facilitating growth of tandem structures from materials that are too strained to support lattice-mismatched growth using conventional epitaxial growth methods. Methods have also been developed recently for orientation and alignment of random dispersions of microwires in solution, allowing for the study of cohesive forces between surfaces and partially functionalized or fully functionalized microwires.75 In the foreseeable future, microwire arrays can be expected to continue to provide a platform to explore new properties of materials and to prove out new design concepts and implementations of a variety of energy-conversion systems, structures, and devices. Many of the lessons learned in the work on Si MWs ought to be transferable to micro- and nanowire systems based on other materials. The concept of orthogonalization can be applied to materials systems that have short minority-carrier diffusion lengths and to semiconductors with indirect band gaps, where the need to obtain long optical path lengths in the material must be optimized with the need to successfully collect photogenerated carriers. Although high efficiency, direct band-gap materials are not limited by light absorption or carrier transport, the light-trapping effects of the micro- and nanowire geometry can enhance absorption and therefore lower materials utilization, ultimately reducing the cost of the resulting devices. For all 3dimensionally structured materials, the benefits of adding structure must be balanced with the losses associated with larger surface areas, indicating that the precise scale and geometry must be optimized for each materials system of interest.

IV. CONCLUSIONS In less than a decade, the use of microstructured semiconductors as solar energy absorbers has gone from being a concept on paper to a highly active field of research. VLS-grown Si MW arrays have been shown to provide a platform to verify the material quality, optical properties, and performance of both photovoltaic and photoelectrochemical energy-conversion devices. This work has resulted in a robust fabrication process for Si MW arrays, as well as a path to the scalable production of freestanding polymerembedded arrays of microstructured Si. Along the way, a rigorous and detailed understanding of the materials composition, optical properties, and electrical properties of the Si MW arrays has been established. The solar energy-conversion performance of Si MW arrays has been investigated using single-wire devices, regenerative photoelectrochemistry, solid-state devices, and catalystintegrated photocathodes for solar-driven water splitting. Microstructured Si MW arrays therefore provide an interesting platform to independently optimize the materials, electrical, and chemical transport properties of energy-conversion devices. As fabrication, synthesis, and design continue to improve, these systems continue to command interest for application in the next generation of efficient solar energy-conversion devices.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: 626-395-6335. Present Address †

Dept. of Physics, 1523 Illinois St., Colorado School of Mines, Golden, CO (E.L.W.). Notes

The authors declare no competing financial interest. Biographies

Emily Warren completed her Ph.D. at the California Institute of Technology in 2012 working on the development of silicon microwires for photovoltaic and photoelectrochemical applications. She has also earned a B.S. degree in Chemical Engineering from Cornell University and an M.Phil. in Engineering for Sustainable Development from the University of Cambridge. She is currently a postdoctoral researcher at the National Renewable Energy Lab and Colorado School of Mines. 756

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Brendan Kayes, Michael Filler, James Maiolo, Joshua Spurgeon, Michael Kelzenberg, Morgan Putnam, Kate Plass, Shannon Boettcher, Daniel Turner-Evans, Hal Emmer, Adele Tamboli, Chris Chen, Elizabeth Santori, Ronald Grimm, Matthew Bierman, Heather Audesirk, Joseph Beardslee, Michael Walter, Chengxiang Xiang, Andrew Meng, Shane Ardo, Robert Coridan, Anna Beck, Ryan Briggs, Clara Cho, Leslie O’Leary, and Matthew Shaner. We acknowledge BP (support for E.L.W.), DOE DE-FG02-03-ER15483, and the Joint Center for Artificial Photosynthesis, DOE DE-SC0004993 (support for N.S.L. and H.A.A.), for financial support that allowed the preparation of this manuscript.

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Harry Atwater is the Howard Hughes Professor of Applied Physics and Materials Science at the California Institute of Technology. Professor Atwater currently serves as Director of the DOE Energy Frontier Research Center on Light-Material Interactions in Solar Energy Conversion and is also Director of the Resnick Sustainability Institute, Caltech’s largest endowed research program. Alongside his academic activities, Professor Atwater is also serving as Editor of ACS Photonics. Professor Atwater’s scientific interests have two themes: photovoltaics and solar energy as well as plasmonics and optical metamaterials. Professor Atwater received his B. S., M. S., and Ph.D. degrees from the Massachusetts Institute of Technology in 1981, 1983, and 1987, respectively. He held the IBM Postdoctoral Fellowship at Harvard University from 1987 to 1988 and has been a member of the Caltech faculty since 1988.

REFERENCES

(1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446−6473. (2) Haussener, S.; Xiang, C.; Spurgeon, J. M.; Ardo, S.; Lewis, N. S.; Weber, A. Z. Modeling, Simulation, and Design Criteria for Photoelectrochemical Water-Splitting Systems. Energy Environ. Sci. 2012, 9922−9935. (3) Spurgeon, J. M.; Walter, M. G.; Zhou, J.; Kohl, P. A.; Lewis, N. S. Electrical Conductivity, Ionic Conductivity, Optical Absorption, and Gas Separation Properties of Ionically Conductive Polymer Membranes Embedded with Si Microwire Arrays. Energy Environ. Sci. 2011, 4 (5), 1772−1780. (4) Wise, J. F. Silicon Solar Cell. U.S. Patent Application 579,801, 1966. (5) Green, M. A.; Wenham, S. R. Novel Parallel Multijunction SolarCell. Appl. Phys. Lett. 1994, 65 (23), 2907−2909. (6) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. Comparison of the Device Physics Principles of Planar and Radial P-N Junction Nanorod Solar Cells. J. Appl. Phys. 2005, 97 (11), 114302. (7) Hadiseh, A.; Ashwin, C. A.; Jennifer, A. D. Optimized Light Absorption in Si Wire Array Solar Cells. J. Opt. 2012, 14 (2), 024006. (8) Hu, L.; Chen, G. Analysis of Optical Absorption in Silicon Nanowire Arrays for Photovoltaic Applications. Nano Lett. 2007, 7 (11), 3249−3252. (9) Kosten, E. D.; Warren, E. L.; Atwater, H. A. Ray Optical Light Trapping in Silicon Microwires: Exceeding the 2n2 Intensity Limit. Opt. Express 2011, 19 (4), 3316−3331. (10) Kelzenberg, M. D.; Boettcher, S. W.; Petykiewicz, J. A.; TurnerEvans, D. B.; Putnam, M. C.; Warren, E. L.; Spurgeon, J. M.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A. Enhanced Absorption and Carrier Collection in Si Wire Arrays for Photovoltaic Applications. Nat. Mater. 2010, 9 (3), 239−244. (11) Garnett, E.; Yang, P. Light Trapping in Silicon Nanowire Solar Cells. Nano Lett. 2010, 10 (3), 1082−1087. (12) Kelzenberg, M. D.; Putnam, M. C.; Turner-Evans, D. B.; Lewis, N. S.; Atwater, H. A. Predicted Efficiency of Si Wire Array Solar Cells. Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE 2009, 001948−001953. (13) Kelzenberg, M. D.; Turner-Evans, D. B.; Putnam, M. C.; Boettcher, S. W.; Briggs, R. M.; Baek, J. Y.; Lewis, N. S.; Atwater, H. A. High-Performance Si Microwire Photovoltaics. Energy Environ. Sci. 2011, 4, 866−871. (14) Maiolo, J. R.; Atwater, H. A.; Lewis, N. S. Macroporous Silicon as a Model for Silicon Wire Array Solar Cells. J. Phys. Chem. C 2008, 112 (15), 6194−6201. (15) Gibbons, J. F.; Cogan, G. W.; Gronet, C. M.; Lewis, N. S. A 14% Efficient Nonaqueous Semiconductor/Liquid Junction Solar Cell. Appl. Phys. Lett. 1984, 45 (10), 1095−1097. (16) Spurgeon, J. M.; Atwater, H. A.; Lewis, N. S. A Comparison Between the Behavior of Nanorod Array and Planar Cd(Se, Te) Photoelectrodes. J. Phys. Chem. C 2008, 112 (15), 6186−6193. (17) Hinogami, R.; Nakamura, Y.; Yae, S.; Nakato, Y. An Approach to Ideal Semiconductor Electrodes for Efficient Photoelectrochemical

Nathan S. Lewis is the George L. Argyros Professor of Chemistry and has been on the faculty at the California Institute of Technology since 1988. He has served as the Principal Investigator of the Beckman Institute Molecular Materials Resource Center at Caltech since 1992 and is the Scientific Director of the Joint Center for Artificial Photosynthesis, the DOE’s Energy Innovation Hub in Fuels from Sunlight. Professor Lewis received his B.S. and M.S. from Caltech in 1977 and his Ph.D. from MIT in 1981. He served on the faculty at Stanford from 1981 to 1988. Professor Lewis’s primary research interests are semiconductor photoelectrochemistry and electronic noses made from arrays of vapor sensors. Professor Lewis has received numerous research and teaching awards and is currently the Editor-inChief of the Royal Society of Chemistry journal Energy & Environmental Science.

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ACKNOWLEDGMENTS The authors would like to thank many of the researchers who have contributed to the development of Si MW-based energyconversion devices and the work discussed in this article: 757

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The Journal of Physical Chemistry C

Feature Article

Reduction of Carbon Dioxide by Modification with Small Metal Particles. J. Phys. Chem. B 1998, 102 (6), 974−980. (18) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechaism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4 (5), 89−90. (19) Treuting, R. G.; Arnold, S. M. Orientation Habits of Metal Whiskers. Acta Metall. 1957, 5 (10), 598. (20) Shin, N.; Filler, M. A. Controlling Silicon Nanowire Growth Direction Via Surface Chemistry. Nano Lett. 2012, 12 (6), 2865−2870. (21) Schmidt, V.; Wittemann, J. V.; Gosele, U. Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires. Chem. Rev. 2010, 110 (1), 361−388. (22) Schmidt, V.; Wittemann, J. V.; Senz, S.; Gösele, U. Silicon Nanowires: A Review on Aspects of Their Growth and Their Electrical Properties. Adv. Mater. 2009, 21 (25−26), 2681−2702. (23) Cui, Y.; Lieber, C. M. Functional Nanoscale Electronic Devices Assembled Using Silicon Nanowire Building Blocks. Science 2001, 291 (5505), 851−853. (24) Hochbaum, A. I.; Fan, R.; He, R.; Yang, P. Controlled Growth of Si Nanowire Arrays for Device Integration. Nano Lett. 2005, 5 (3), 457− 460. (25) Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Lewis, N. S.; Atwater, H. A. Growth of Vertically Aligned Si Wire Arrays over Large Areas (> 1 cm2) with Au and Cu Catalysts. Appl. Phys. Lett. 2007, 91 (10), 103110. (26) Wang, Y. W.; Schmidt, V.; Senz, S.; Gosele, U. Epitaxial Growth of Silicon Nanowires Using an Aluminium Catalyst. Nat. Nanotechnol. 2006, 1 (3), 186−189. (27) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Crystal Growth and Its Application to Silicon. Trans. Metallurgical Society of AIME 1965, 233 (6), 1053. (28) Tamboli, A. C.; Chen, C. T.; Warren, E. L.; Turner-Evans, D. B.; Kelzenberg, M. D.; Atwater, H. A.; Lewis, N. S. Wafer-Scale Growth of Silicon Microwire Arrays for Photovoltaics. J. Photovoltaics 2012, 2 (3), 294−297. (29) Boettcher, S. W.; Spurgeon, J. M.; Putnam, M. C.; Warren, E. L.; Turner-Evans, D. B.; Kelzenberg, M. D.; Maiolo, J. R.; Atwater, H. A.; Lewis, N. S. Energy-Conversion Properties of Vapor-Liquid-SolidGrown Silicon Wire-Array Photocathodes. Science 2010, 327 (5962), 185−187. (30) Kendrick, C. E.; Redwing, J. M. The Effect of Pattern Density and Wire Diameter on the Growth Rate of Micron Diameter Silicon Wires. J. Cryst. Growth 2011, 337 (1), 1−6. (31) Perea, D. E.; Allen, J. E.; May, S. J.; Wessels, B. W.; Seidman, D. N.; Lauhon, L. J. Three-Dimensional Nanoscale Composition Mapping of Semiconductor Nanowires. Nano Lett. 2005, 6 (2), 181−185. (32) Kelzenberg, M. D.; Turner-Evans, D. B.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Lewis, N. S.; Atwater, H. A. Photovoltaic Measurements in Single-Nanowire Silicon Solar Cells. Nano Lett. 2008, 8 (2), 710−714. (33) Kelzenberg, M. D. Silicon Microwire Photovoltaics, Ph.D. thesis, California Institute of Technology, 2010. (34) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices; WileyInterscience: New York, 2006. (35) Trumbore, F. A. Solid Solubilities of Impurity Elements in Germanium and Silicon. Bell Syst. Tech. J. 1960, 39 (1), 205−233. (36) Struthers, J. D. Solubility and Diffusivity of Gold, Iron, and Copper in Silicon. J. Appl. Phys. 1956, 27 (12), 1560−1560. (37) Maiolo, J. R.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Atwater, H. A.; Lewis, N. S. High Aspect Ratio Silicon Wire Array Photoelectrochemical Cells. J. Am. Chem. Soc. 2007, 129, 12346−12347. (38) Putnam, M. C.; Filler, M. A.; Kayes, B. M.; Kelzenberg, M. D.; Guan, Y.; Lewis, N. S.; Eiler, J. M.; Atwater, H. A. Secondary Ion Mass Spectrometry of Vapor-Liquid-Solid Grown, Au-Catalyzed, Si Wires. Nano Lett. 2008, 8 (10), 3109−3113. (39) Kendrick, C.; Yoon, H.; Yuwen, Y.; Barber, G.; Shen, H.; Mallouk, T.; Dickey, E.; Mayer, T.; Redwing, J. Radial Junction Silicon Wire Array Solar Cells Fabricated by Gold-Catalyzed Vapor-Liquid-Solid Growth. Appl. Phys. Lett. 2010, 97 (14), 143108.

(40) Santori, E. A.; Maiolo, J. R., III; Bierman, M. J.; Strandwitz, N. C.; Kelzenberg, M. D.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Photoanodic Behavior of Vapor-Liquid-Solid-Grown, Lightly Doped, Crystalline Si Microwire Arrays. Energy Environ. Sci. 2012, 5, 6867− 6871. (41) Santori, E. A.; Strandwitz, N. C.; Grimm, R. L.; Brunschwig, B. S.; Lewis, N. S. Lightly Doped Si Microwires Operating Under High-Level Injection Conditions. 2013, submitted for publication. (42) Hopkins, R. H.; Rohatgi, A. Impurity Effect in Silicon for HighEfficiency Solar-Cells. J. Cryst. Growth 1986, 75 (1), 67−79. (43) Davis, J. R.; Hopkins, R. H.; Raichoudhury, P.; Blais, P. D.; Rohatgi, A.; McCormick, J. R.; Mollenkopf, H. C. Impurity Effects in Silicon Solar-Cells. J. Electrochem. Soc. 1979, 126 (3), C114−C115. (44) Istratov, A. A.; Weber, E. R. Physics of Copper in Silicon. J. Electrochem. Soc. 2002, 149 (1), G21−G30. (45) Kooij, E. S.; Despo, R. W.; Mulders, F. P. J.; Kelly, J. J. Electrochemistry of Porous and Crystalline Silicon Electrodes in Methyl Viologen Solutions. J. Electroanal. Chem. 1996, 406 (1−2), 139−146. (46) Hamann, T. W.; Lewis, N. S. Control of the Stability, ElectronTransfer Kinetics, and pH-Dependent Energetics of Si/H2O Interfaces Through Methyl Termination of Si(111) Surfaces. J. Phys. Chem. B 2006, 110 (45), 22291−22294. (47) Xiang, C.; Meng, A. C.; Lewis, N. S. Evaluation and Optimization of Mass Transport of Redox Species in Silicon Microwire-Array Photoelectrodes. Proc. Natl. Acad. Sci. U.S.A. 2012, 109 (39), 15622− 15627. (48) Putnam, M. C.; Turner-Evans, D. B.; Kelzenberg, M. D.; Boettcher, S. W.; Lewis, N. S.; Atwater, H. A. 10 μm Minority-Carrier Diffusion Lengths in Si Wires Synthesized by Cu-Catalyzed VaporLiquid-Solid Growth. Appl. Phys. Lett. 2009, 95 (16), 163116−3. (49) Allen, J. E.; Hemesath, E. R.; Perea, D. E.; Lensch-Falk, J. L.; Li, Z. Y.; Yin, F.; Gass, M. H.; Wang, P.; Bleloch, A. L.; Palmer, R. E.; Lauhon, L. J. High-Resolution Detection of Au Catalyst Atoms in Si Nanowires. Nat. Nano 2008, 3 (3), 168−173. (50) Plass, K. E.; Filler, M. A.; Spurgeon, J. M.; Kayes, B. M.; Maldonado, S.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Flexible Polymer-Embedded Si Wire Arrays. Adv. Mater. 2009, 21 (3), 325−328. (51) Spurgeon, J. M.; Plass, K. E.; Kayes, B. M.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Repeated Epitaxial Growth and Transfer of Arrays of Patterned, Vertically Aligned, Crystalline Si Wires from a Single Si(111) Substrate. Appl. Phys. Lett. 2008, 93 (3), 032112. (52) Spurgeon, J. M.; Boettcher, S. W.; Kelzenberg, M. D.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Flexible, Polymer-Supported, Si Wire Array Photoelectrodes. Adv. Mater. 2010, 22, 3277−3281. (53) Audesirk, H. A.; Warren, E. L.; Lewis, N. S. Low-Cost Fabrication Techniques for VLS Microwire Array Growth. 2013, submitted for publication. (54) Cho, C. J.; O’Leary, L.; Lewis, N. S.; Greer, J. R. In Situ Nanomechanical Measurements of Interfacial Strength in MembraneEmbedded Chemically Functionalized Si Microwires for Flexible Solar Cells. Nano Lett. 2012, 12 (6), 3296−3301. (55) Putnam, M. C.; Boettcher, S. W.; Kelzenberg, M. D.; TurnerEvans, D. B.; Spurgeon, J. M.; Warren, E. L.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A. Si Microwire-Array Solar Cells. Energy Environ. Sci. 2010, 3, 1037−1041. (56) Boettcher, S. W.; Warren, E. L.; Putnam, M. C.; Santori, E. A.; Turner-Evans, D.; Kelzenberg, M. D.; Walter, M. G.; McKone, J. R.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays. J. Am. Chem. Soc. 2011, 133 (5), 1216−1219. (57) Warren, E. L.; Turner-Evans, D. B.; Grimm, R. L.; Atwater, H. A.; Lewis, N. S. In Photoelectrochemical Characterization of Si Microwire Array Solar Cells, Proc. 38th IEEE Photovolt. Spec., 2012. (58) Turner-Evans, D. B.; Emmer, H. S.; Chen, C. T.; Atwater, H. A. Flexible, Transparent Contacts for Inorganic Nanostructures and Thin Films. Adv. Mater. 2013, in press. (59) McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.; Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Evaluation of Pt, Ni, and Ni758

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Mo Electrocatalysts for Hydrogen Evolution on Crystalline Si Electrodes. Energy Environ. Sci. 2011, 4, 3573−3583. (60) Dominey, R. N.; Lewis, N. S.; Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S. Improvement of Photoelectrochemical Hydrogen Generation by Surface Modification of P-Type Semiconductor Photocathodes. J. Am. Chem. Soc. 1982, 104 (2), 467−482. (61) Warren, E. L.; Boettcher, S. W.; Walter, M. G.; Atwater, H. A.; Lewis, N. S. pH-Independent, 520 mV Open-Circuit Voltages of Si/ Methyl Viologen2+/+ Contacts Through Use of Radial n+p-Si Junction Microwire Array Photoelectrodes. J. Phys. Chem. C 2010, 115 (2), 594− 598. (62) Warren, E. L.; McKone, J. R.; Atwater, H. A.; Gray, H. B.; Lewis, N. S. Hydrogen-Evolution Characteristics of Ni-Mo-Coated, Radial Junction, n+p-Silicon Microwire Array Photocathodes. Energy Environ. Sci. 2012, 5 (11), 9653−9661. (63) McKone, J. R. Earth-Abundant Materials for Solar Hydrogen Generation. Ph.D. Thesis, California Institute of Technology, 2013. (64) Johnson, E. L. The TI Solar Energy System Development. Electron Devices Meet., 1981 Int. 1981, 27, 2−5. (65) Yahyaie, I.; McEleney, K.; Walter, M.; Oliver, D. R.; Thomson, D. J.; Freund, M. S.; Lewis, N. S. Electrical Characterization of Si Microwires and of Si Microwire/Conducting Polymer Composite Junctions. J. Phys. Chem. Lett. 2011, 2 (6), 675−680. (66) Tamboli, A. C.; Malhotra, M.; Kimball, G. M.; Turner-Evans, D. B.; Atwater, H. A. Conformal GaP Layers on Si Wire Arrays for Solar Energy Applications. Appl. Phys. Lett. 2010, 97 (22), 221914−3. (67) Strandwitz, N. C.; Turner-Evans, D. B.; Chen, A. C. T. C. T.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Behavior of Planar and Microwire-Array Si|GaP Electrodes. Adv. Mater. 2012, 2 (9), 1109− 1116. (68) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414 (6861), 338−344. (69) Mi, Q.; Zhanaidarova, A.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S. A Quantitative Assessment of the Competition Between Water and Anion Oxidation at WO3 Photoanodes in Aqueous Electrolyte. Energy Environ. Sci. 2012, 5, 5694−5700. (70) Coridan, R. H.; Shaner, M. R.; Wiggerhorn, C.; Brunschwig, B. S.; Lewis, N. S. Electrical and Photoelectrochemical Properties of WO3/Si Tandem Photoelectodes. J. Phys. Chem. C 2013, 117 (14), 6949−6957. (71) Shaner, M. R.; Fountaine, K. T.; Ardo, S. A.; Coridan, R. H.; Atwater, H. A.; Lewis, N. S. Photoelectrochemistry of Core-Shell Tandem Junction n-p+-Si/n-WO3 Microwire Array Photoelectrodes. Energy Environ. Sci. 2013; DOI: 10.1039/C3EE43048K, in press. (72) Hu, S.; Chi, C.-Y.; Fountaine, K. T.; Yao, M.; Atwater, H. A.; Dapkus, P. D.; Lewis, N. S.; Zhou, C. Optical, Electrical, and Solar Energy-Conversion Properties of Gallium Arsenide Nanowire-Array Photoanodes. Energy Environ. Sci. 2013, 6 (6), 1879−1890. (73) Fitch, A.; Strandwitz, N. C.; Brunschwig, B. S.; Lewis, N. S. A Comparison of the Behavior of Single Crystalline and Nanowire Array ZnO Photoanodes. J. Phys. Chem. C 2013, 117 (5), 2008−2015. (74) Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Åberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgström, M. T. InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit. Science 2013, 339, 1057−1060. (75) Beardslee, J. A.; Sadtler, B.; Lewis, N. S. Magnetic Field Alignment of Randomly Oriented, High Aspect Ratio Silicon Microwires into Vertically Oriented Arrays. ACS Nano 2012, 6 (11), 10303−10310.

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