Large-Scale Fabrication of Silicon Nanowires for Solar Energy

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Review

Large-Scale Fabrication of Silicon Nanowires for Solar Energy Applications Bing-Chang Zhang, Jiansheng Jie, Xiujuan Zhang, Xue-Mei Ou, and Xiao-Hong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06620 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

Large-Scale Fabrication of Silicon Nanowires for Solar Energy Applications Bingchang Zhang, Jiansheng Jie*, Xiujuan Zhang, Xuemei Ou and Xiaohong Zhang*

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R China

Keywords: Silicon nanowires, large-scale fabrication, solar energy conversion, photovoltaic devices, photoelectrochemical cells.

Abstract The development of silicon (Si) materials during the past decades has boosted up the prosperity of modern semiconductor industry. In comparison with the bulk Si materials, Si nanowires (SiNWs) possess superior structural, optical and electrical properties and have attracted increasing attention in solar energy applications. To achieve the practical applications of SiNWs, both large-scale synthesis of SiNWs at low cost and rational design of energy conversion devices with high efficiency are the prerequisite. This review focuses on the recent progresses in large-scale production of SiNWs, as well as the construction of high-efficiency SiNW-based solar energy conversion devices, including photovoltaic devices and photoelectrochemical cells. 1

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Finally, the outlook and challenges in this emerging field are presented.

Introduction Silicon (Si) is the dominant semiconductor material in current energy and electronic devices due to its abundance, nontoxicity, high stability, tunable electrical property and excellent photoelectric behavior. In the past decades, Si nanostructures with unique properties such as quantum size effect,1 high surface area,2 strong light trapping,3 and large thermal conductivity,4 have been extensively studied, revealing their great potential for applications in diverse fields, such as lithium ion batteries,5 supercapacitors,6 thermoelectricities,7 catalysis,8 photodetectors,9 and solar energy conversion.10 For instance, quantum size effect induced indirect-to-direct bandgap transition was observed in small-size Si nanoparticles; the strong fluorescence of Si nanoparticles shows great potential in biological probes and imaging.11-14 High surface-to-volume ratio of Si nanostructures allows more efficient catalyst loading and molecule absorption, giving rise to excellent catalytic and sensing performance of the nanostructures.2,6,8 The one-dimensional (1D) morphology and nanoscale diameter accommodate Si nanowires (SiNWs) with large strain, facilitating their applications in lithium ion batteries and flexible electronics.5,15,16 In consideration of the limited natural fossil fuel sources and global-warming effect caused by consuming fossil fuel,17-20 it has attracted increasing attention in recent years to explore new solar energy conversion materials or systems for renewable clean energy applications. Bulk-Si-based solar cells have long been the 2


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dominant commercial products because of the proper 1.12 eV bandgap of Si for solar spectrum absorption and mature processing technology of Si wafers.18,20-22 However, thick Si wafers are necessary for efficient solar energy spectrum absorption due to the indirect bandgap of Si (e.g. thickness over 100 µm to absorb 90% solar radiation with phonon energy above the bandgap of Si).23,24 To allow efficient carrier collection, high-purity Si wafers are also needed due to the long carrier diffusion length.24,25 These factors are partially responsible for the high cost of Si wafer-based solar cells. In comparison with bulk Si, SiNWs have several important advantages that are promising for solar energy conversion applications: (i) The high aspect ratio of SiNWs orthogonalizes the light absorption direction with the carrier collection direction, i.e. the length provides sufficient thickness for light absorption, while the nano-scale diameter makes sure short carrier diffusion distance.26 This means that low-purity Si materials with relatively lower carrier diffusion length could be used in the SiNW-based solar energy conversion devices, ensuring the remarkable reduction of the device costs. (ii) SiNW arrays could greatly enhance the light absorption due to the antireflection effects of the array structures and the optical antenna effects of 1D subwavelength-diameter nanostructures.27-29 Therefore, the quantity of Si materials used in SiNW-based solar energy conversion devices could be significantly reduced, which is beneficial to lower the cost of the devices.16 (iii) The SiNWs could be transferred to flexible substrates or embedded into polymer substrates. This allows the construction of flexible, stretchable solar energy conversion devices based on SiNWs, thus offering the opportunities for the fabrication of novel light-weight, wearable 3



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energy devices.30 The fabrication of SiNWs and their solar energy applications have been summarized in several previous reviews, which generally introduced the growth mechanisms of SiNWs and different types of SiNW based solar cells.17-19,31,32 Recently, enormous efforts have been made to promote the fabrication of SiNWs and their solar energy devices to practical applications,33-35 but a review to introduce these advances is still lacked. To provide guide to future applications of SiNWs, we will summarize the application-oriented latest progresses on the fabrication and solar energy applications of SiNWs in this review. In the part of SiNW fabrication, we will focus on the recent fabrication techniques promising for large-scale and continuous production of SiNWs in low cost for practical applications, such as macroscopic galvanic cell-enhanced continuous etching,33 vapor phase MCEE,34 and vertical release and roll-to-roll transfer of SiNW arrays.35 Also, the latest progresses in vapor-liquid-solid (VLS) mechanism such as the oscillatory mass transport in VLS growth will be introduced,36 which could be applied to guide the rational control and mass production of SiNWs. In the part of solar energy devices, we will emphasize the low-cost and large-scale fabrication of solar energy devices, as well as the methods to improve their durability in practical applications, such as the photovoltaic devices based on Si-inorganic and Si-organic hybrid structures,9,37 the devices fabricated with metallurgical grade Si wafer,38 low-cost photoelectrochemical co-catalysts, 25 and the protection of SiNWs.39 Finally, we present the outlook and challenges of the SiNWs for the new-generation solar energy conversion devices. 4


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2. Large-scale fabrication of SiNWs 2.1 Reactive ion etching (RIE) Top-down techniques are important approaches for large-scale and controllable fabrication of Si nanostructures. Reactive ion etching (RIE) is a kind of commonly used top-down Si micro/nano processing technique, which could etch submicron Si structures with high precision through the reaction of electric-field activated ions with Si substrates.40,41 By combining with mature lithographic techniques such as nanoimprint lithography, photolithography and natural lithography, diverse arrays of ordered 1D Si nanostructures with controlled morphologies and structures could be obtained via the RIE method (Figure 1).42-44 For instance, Cui and co-workers used Langmuir-Blodgett (LB) method to assemble nanoparticles as the etching masks in RIE process, demonstrating the fabrication of Si nanopillar (SiNP) and Si nanocone (SiNC) arrays with precisely controlled diameters and spacings over entire 4 inch wafers (Figure 1a).42 Morton and co-workers reported wide-area (cm2) pattern of SiNWs with uniform wire widths and straight sidewalls through firstly patterning the substrate with nanoimprint lithography and then RIE etching the substrate (Figure 1b).41 By patterning the substrates with traditional photolithography technique, Zhang and co-workers fabricated both SiNW and Si nanohole (SiNH) arrays with predictable and controllable geometries via RIE (Figure 1c and 1d). Also, by taking advantage of Poisson spot effect during UV light exposure, Si nanotube (SiNT) arrays could be obtained (Figure 1e).43 As a mature technique for Si electronic industry, RIE is able to 5



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rationally produce Si nanostructure arrays in wafer-scale with highly-uniform and well-controlled morphologies. However, the disadvantages such as high cost, time-consuming process and limited etching depth also restrict its wide applications in solar energy conversion devices.

Figure 1. (a) RIE fabrication of SiNWs through silica nanoparticle-assisted natural lithography. Step 1, deposition of the silica nanoparticles by LB. Step 2, shrinking of the mask by isotropic RIE of SiO2. Step 3, anisotropic etching of Si into pillars by RIE. Step 4, removal of the residual mask by HF etching. Reprinted with permission from ref 42. Copyright 2008 AIP Publishing LLC. (b),(c),(d),(e) SEM images of SiNW, SiNP, SiNH and SiNT arrays fabricated by RIE process, respectively. Reprinted with permission from ref 41. Copyright 2008 IOP Publishing. Reprinted with permission from ref 43. Copyright 2013 AIP Publishing LLC.

2.2 Metal-catalyzed electroless etching (MCEE) 2.2.1 Mechanism of MCEE 6


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In comparison with RIE method, metal-catalyzed electroless etching (MCEE) method possesses obvious advantages such as simple, low-cost and time efficient. The discovery and explanation of the MCEE phenomenon date back to the early 1990s. When Ohmi and co-workers studied the wet-chemical cleaning process of Si surface, they found that the surface of Si wafer became rougher if peroxide-containing cleaning solution was used to remove the ultrafine metal particles.45 This could be attributed to the higher electronegativity of noble metals than Si; the existence of noble metals facilitated the oxidation and the etching of Si. Motivated by the metal-assisted corrosion phenomenon, in 2000, Li and Bohn fabricated porous Si with different morphologies through intentionally depositing a thin layer of Au, Pt or Au/Pd on the Si (100) surface and then immersing the substrate in a solution containing HF and H2O2.46 In 2002, Peng and co-workers synthesized aligned SiNW arrays in an aqueous HF solution containing silver nitrate.47 Following that, they systemically investigated the synthesis of SiNWs under different conditions and further proposed a MCEE mechanism (Figure 2a-e).48,49 In this mechanism, the SiNWs are formed according to the following steps: (i) Noble metal ions near the surface of Si wafer can capture electrons from the valence band (VB) of Si and then are deposited in the form of nanoparticles on the surface of Si wafer. (ii) As noble metals such as Ag are more electronegative than Si, electrons would transfer from Si to metal nanoparticles. Thus, oxygen species in the solution would be reduced on the surface of metal nanoparticles. Si underneath the interface of Si-metal would be oxidized, as shown in Figure 2a. In other words, catalyzed site-selective oxidation of 7



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Si occurs by oxide species in the solution. (iii) The Si oxides would be immediately etched by the HF in the solution and shallow pits are formed underneath metal nanoparticles (Figure 2b). As the oxidation and etching of Si underneath metal nanoparticles proceed, metal nanoparticles move forward, tunneling through Si wafer (Figure 2c), and aligned SiNW arrays are formed eventually (Figure 2d and 2e).

Figure 2. (a-c) Mechanism of MCEE for the fabrication of SiNW array. Reprinted with permission from ref 49. Copyright 2008 John Wiley and Sons. (d), (e) are Cross-sectional and top-view SEM images of SiNWs fabricated by MCEE, respectively. Reprinted with permission from ref 48. Copyright 2005 John Wiley and Sons. (f-k) SEM and TEM images of kinked SiNWs fabricated by MCEE. Reprinted with permission from ref 53. Copyright 2010 American Chemical Society.

Based on the MCEE mechanism, further investigations on the controlled 8


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fabrication of Si nanostructure arrays with various morphologies and structures were performed. For instance, highly ordered SiNW arrays were obtained by coating a monolayer of silica colloidal crystals or polystyrene (PS) nano-spheres as masks for deposition of metal catalysts.50,51 Both SiNW and SiNT arrays could be fabricated by combining MCEE with traditional photolithography technique.52 The dimensions and spacings of the nanostructure arrays could be controlled by changing the photomask in photolithography and tuning MCEE conditions. Zhang and co-workers demonstrated the fabrication of zigzag-shaped SiNW arrays on (111)-oriented Si wafers with desired kinking angles by controlling etching temperature and etchant concentration (Figure 2f-k).53 It was found that the perturbations at the reaction sites such as hydrogen bubbles could induce the switch of etching directions to other nearby directions (e.g. from direction to direction, Figure 2i). Higher etching temperature and higher concentration of Ag+ would help overcome the potential barrier to change the etching direction from original direction to the favorable lowest-energy direction. Consequently, the merge of etching directions along and resulted in SiNWs with 125o tuning angles (Figure 2j), while the merge of etching directions along different directions led to SiNWs with 90o tuning angles (Figure 2k).53 The zigzag SiNWs may have unique applications in nanoelectronics and biological sensors such as using the zigzag nanowire transistors as probes for intracellular recording.54

2.2.2 Macroscopic galvanic cell-enhanced continuous MCEE 9



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The MCEE technique has been demonstrated a simple and convenient way to fabricate SiNW arrays.55,56 In the MCEE process, oxidizing species such as hydrogen peroxide or silver nitrate were commonly used, but for practical production, green reaction agents are surely desirable. In 2014, Peng and co-workers designed a macroscopic galvanic cell-driven MCEE process in aerated HF solution, which used the dissolved oxygen as green oxide agent to maintain the etching process (Figure 3a).57 The noble-metal-coated Si wafer and a graphite rod acted as anode and cathode, respectively, in the galvanic cell. It was found that the existence of graphite electrode facilitated oxygen reduction and subsequently accelerated the Si corrosion reaction. The etching rate of Si substrate was proportional to the surface area ratio of graphite to Si wafer. This work provides new insight into the MCEE process and offers the possibility to utilize green oxidizing species for the fabrication of SiNW arrays. Based on this result, a graphite substrate-enhanced MCEE approach was further developed by Peng and co-workers to achieved continuous mass production of SiNW arrays, as shown in Figure 3b.33 This approach utilized a long graphite as the supporting substrate and transport equipment for Si wafers. Noble-metal-coated Si wafers were placed on the graphite substrate one by one with the movement of the substrate. When the wafers passed through the aerated HF solution, large-area SiNW arrays would be produced (Figure 3c). This approach is promising to achieve continuous, mass production of SiNW arrays for practical applications.

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Figure 3. (a) Schematic diagram of the macroscopic galvanic cell driven MCEE process for SiNW preparation. Reprinted with permission from ref 57. Copyright 2014 John Wiley and Sons. (b) Schematic overview of continuous production of SiNWs through substrate-enhanced MCEE approach. (c) An SEM image of SiNWs fabricated by substrate-enhanced MCEE approach. Reprinted with permission from ref 33. Copyright 2014 Nature Publishing Group. (d) Schematic diagram of the experimental setup for vapor phase MCEE. Reprinted with permission from ref 34. Copyright 2014 American Chemical Society. 11



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2.2.3 Vapor phase MCEE Though oxygen has been used as green oxide agent for MCEE, its solubility is limited. Also, large cathodes were needed in the macroscopic galvanic cell-enhanced continuous MCEE process.57 Motivated by the natural oxidation process of Si in air and atmosphere corrosion of metals, Hildreth et al. and Peng et al. proposed a route of vapor phase MCEE for the large-scale fabrication of SiNWs. Differing from previous MCEE process, the MCEE in the new method was carried out in HF/H2O vapor at oxygen ambient.34,58 In vapor phase MCEE, the reduction kinetics could be accelerated because of the high oxygen penetration rate. The water consumption was obviously reduced and the HF could be fully utilized. A set-up diagram, Figure 3d, for vapor phase MCEE of Si wafers was also put forward to realize convenient, environmentally friendly, and scalable fabrication of SiNWs.

2.2.4 Vertical release and roll-to-roll transfer of SiNW arrays SiNW arrays fabricated by MCEE are tightly attached to Si wafers, which may largely fade the advantages of SiNW arrays: First, the unique electrical and optical properties of SiNW arrays are overshadowed by the thick Si wafers. Second, the material cost and weight of SiNW array-based devices are not obviously cut down than those of Si wafer-based devices. Third, the potential of SiNWs with large aspect ratio for flexible device applications is hindered by the rigid nature of Si wafers.

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Figure 4. (a) Schematic and SEM images of the fabrication procedure for the cracked SiNW array; Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

(b) Schematic illustration of roll-to-roll transfer process of the

multilayer SiNW array; SEM images of the three-layer SiNW array (c) before transfer, (d) after first layer transfer, and (e) after second layer transfer. Reprinted with permission from ref 35. Copyright 2014 American Chemical Society.

To achieve the transfer of SiNWs from the original substrate, SiNWs were used to be mechanically stripped by razor blade or sonication, however, which would 13



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inevitably break down the array structure.59-61 Alternatively, methods for vertical release and multilayer etching were developed to maintain the array structure of the SiNW arrays. For instance, Shiu and co-workers realized the transfer of aligned SiNW arrays through pressing Si wafer with SiNWs on the surface into uncured polymer and then detaching the original wafer after the polymer cured.62 Vlad and co-workers fabricated flexible SiNW arrays through infiltrating polymer to the arrays and then peeling off the SiNWs/polymer composite.63 Although these polymer-assisted approaches could depart the aligned SiNWs from substrates, damages on the SiNW arrays are easy to happen due to the tightly binding of SiNWs with the original substrate. In light of this, Zheng and co-workers reported the well-controlled and uniform vertical transfer of SiNW arrays by creating a horizontal crack at the middle of SiNWs (Figure 4a).64,65 SiNWs with desired length were first achieved by timed MCEE process. Then the wafer was soaked in hot water to delaminate the Ag film. Afterwards, the wafer was dried and replaced to the etching solution. The delamination of Ag film changed the equilibrium condition of Ag+/Ag, promoting the deposition of Ag particles on the sidewalls of SiNWs. Cracks were created due to horizontally etching in the subsequent wet etching step. Finally, SiNWs were transferred by using cured polymer. Jie and co-workers further designed a low-cost and time-efficient roll-to-roll technique to perform wafer-scale layer-by-layer transfer of SiNW arrays (Figure 4b).35 First, multilayer-etched SiNW arrays with horizontal cracks were formed through alternate etching-air heating-etching steps (Figure 4c). Then, thermal released tapes were tightly adhered on the top of multilayer etched 14


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SiNW arrays by stressing the tapes and SiNW arrays between two rubber rollers of a laminating machine. Eventually, a layer of SiNWs could be separated from the underlying Si substrate by peeling off the thermal released tapes (Figure 4d and 4e). It is worth noting that these SiNWs could be further released to different receiving substrates with the aid of the thermal release tape, opening the chance for the use of SiNWs in diverse fields such as flexible electronics and photovoltaics.

2.3 Vapor-liquid-solid (VLS) method Differing from the top-down etching techniques, bottom-up methods can construct SiNWs through an assembly process of Si atoms. Among various bottom-up methods, vapor-liquid-solid (VLS) method is widely used, which refers to the phase change of Si atoms, starting from a vapor phase, then forming a liquid phase and finally ending as a solid phase.66,67 In the 1960s, the VLS method was first proposed to synthesize Si whiskers by Wagner and Ellis.68,69 In 1998, Lieber et al. and Lee et al. used the laser ablation method to produce nanometer-diameter catalyst clusters and then achieve the growth of single-crystalline SiNWs via VLS method. This progress stimulated the wide interests for the VLS growth of SiNWs from the understanding of basic mechanism to bulk preparation in the past two decades.70,71

2.3.1 Growth mechanism In general opinion, VLS process includes the transport of vapor Si precursors, the dissolution of Si atoms into a liquid phase and the precipitation of Si atoms to a solid 15



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phase.60,61 First, gas precursors such as silane (SiH4),72 disilane (Si2H6),73 or the vapors of Si tetrachloride (SiCl4),74 Si monoxide (SiO)75 are transported to the surface of catalyst droplets such as Au,76 Cu,77 In,78 and Sn,75 and then decompose to Si atoms. Afterwards, the Si atoms are dissolved into the catalyst, forming alloy droplets. Last, the dissolved Si atoms reach saturation as the continuous supply of Si source, resulting in the precipitation of Si atoms from the droplet at the liquid-solid interface to form SiNWs. Based on this mechanism, the NW growth processes in a steady state, and the NW diameter is mainly determined by the size of the catalyst droplets, which will not change during the NW growth. Although the typical VLS mechanism mentioned above has guided and explained many growth experiments of SiNWs, recently, in-situ observations of the growth process challenge the traditional comprehension of planar liquid-solid interface and steady mass transport process in VLS growth.79-82 In 2010, Oh and co-workers found the oscillatory mass transport in VLS growth of nanowires for the first time.79 They observed the periodic modulation of liquid-solid and vapor-solid interfaces at the triple phase junction location. Ross and co-workers further indicated that the periodic change of the growth interface morphology was accompanied with the variation of the catalyst supersaturation, which was a general phenomenon for the VLS growth of various nanowires, including SiNWs, GeNWs and GaP NWs.80 Furthermore, they proposed a model of jumping-catalyst dynamics to describe the growth process of both straight and kinking SiNWs.81

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Figure 5. Two common quantitative geometrical relationships of one-dimensional nanostructures summarized from their own experiments and reported results: first, the periodic spacing (L) in wire direction varies linearly with the radius (r) of the nanostructures; second, the inverse of the periodic spacing (Ln) along the wire direction follows an arithmetic sequence. Reprinted with permission from ref 36. Copyright 2015 Nature Publishing Group.

Very recently, Wang and co-workers addressed the thermodynamic mechanism of SiNW growth from a new perspective - the surface curvature oscillation (SCO) of catalyst droplets during VLS processes based on the summarization of two common quantitative geometrical relationships of one-dimensional nanostructures from their own experiments and reported results (Figure 5).36 It is noted that the conventional VLS growth of nanowires usually takes place on catalyst droplets with diameter less than ten micrometers. Droplets with such a small diameter have large surface 17



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curvature. Then it would impose an extra pressure on the droplets, which is so large that it can regulate the droplets’ chemical potential and influence the absorption and deposition processes. The two processes then alternatively change the volume of the droplets and in turn the surface curvature. Under suitable conditions, the surface curvature may oscillate around an equilibrium curvature 1/Re. Consequently, the small size effect induced SCO process drives intermittent mass transport through the vapor-liquid interface and liquid-solid interface (Figure 6), resulting in a leapfrog growth of NWs. These new insights are helpful to quantitatively understand the VLS growth process and could be applied to design experiments for controlled growth of nanostructures with custom-designed morphologies, such as periodical nanostructures and ultralong SiNWs.36

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Figure 6. (a) The surface curvature oscillation (SCO) growth model of a nanowire; (b) The growth model parameters of VLS; (c) Numerical simulation plot of µl (the chemical potential of the liquid droplet) against θ (the contact angle between the liquid droplet and the solid stem) for a nanowire with the diameter of 100nm under arbitrarily defined thermodynamic properties. Reprinted with permission from ref 36. Copyright 2015 Nature Publishing Group.

2.3.2 Bulk preparation of SiNWs Low-cost and large-scale preparation of SiNWs is the basis for their practical applications in future. Though top-down technology provides well-controlled fabrication of aligned SiNW arrays, the raw materials of high-quality Si wafers are produced through high-cost and energy-intensive smelting and purification process. In contrast, the VLS method provides a route to directly transfer Si compounds to single-crystalline SiNWs, which is more promising for low-cost, bulk preparation of SiNWs. Many papers have reported the preparation of SiNWs utilizing SiH4 and SiCl4 as gas precursors, which, however, are noxious, flammable, corrosive, and not convenient to store.72,74,77,78 As a solid phase precursor, Si monoxide (SiO) is a kind of low-cost industrial raw material and is easy to store. Moreover, the disproportionation of SiO is a spontaneous reaction that would react rapidly under proper conditions, thus greatly benefiting the fast growth of SiNWs.83,84 Lee and co-workers demonstrated the rapid and large-quantity preparation of SiNWs through thermal evaporation of pure SiO powder without the use of metal 19



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catalyst.85 They also combined the evaporation of SiO and the use of Au catalyst to fabricate high-density, oriented SiNW arrays.86,87 It is found that the use of SiO as Si source benefits high growth yield, while the Au catalyst offers better control on the NW diameter and array patterns. Instead of the high melting point catalyst, Wang and co-workers proposed a method for bulk preparation of ordered SiNWs by thermal evaporation a mixture of SiO and low melting point catalyst of Tin (Sn).87 This method avoids the pre-evaporation or sputtering of catalyst on the substrate. Although it is relatively easy to realize bulk preparation of SiNWs by using SiO powder as the precursor, the need for crystalline substrates, such as Si wafers, during NW growth still limited the scalable production. To solve this problem, Wang and co-workers further developed an efficient method to grow aligned SiNW arrays on liquid metal substrate (Figure 7a and 7b).88,89 Tin was chosen as the growth substrate due to its low melting point (232 oC), high boiling point (2260 oC), low cost, and high catalytic activity for the growth of SiNWs. The liquid Sn substrate could provide sufficient catalyst for VLS growth. In addition, the flow of the liquid metal during nanowire growth could transport away the as-prepared NWs, ensuring the sequential growth of NWs on the newly exposed surface of the liquid metal. The use of liquid Sn substrate holds the promise for large-scale, continuous production of SiNWs. Moreover, the solidification shrinkage of Sn catalyst when it cooled to room temperature caused the release of the SiNW arrays from the Sn surface, avoiding the difficulty for SiNWs separation and collection.

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2.3.3 Transfer and assembly of as-grown SiNWs For flexible device applications and to allow the manufacture of SiNWs through more convenient processes such as roll-to-roll process, it is surely desirable to transfer and integrate the as-grown SiNWs on various functional substrates. In 2009, Lewis and co-workers fabricated flexible polymer-embedded Si wire arrays through casting polydimethylsiloxane (PDMS) solution onto the Si wire arrays and then removing the polymer-supported arrays after PDMS cured (Figure 7c and 7d).90 Moreover, the Si wire arrays were also embedded into an ionically conducting polymer, Nafion. Nafion possesses excellent cation exchange property and gas separation property. Therefore, an artificial photosynthetic system could be realized by connecting two Si wire array/Nafion composite films in series with a conducting polymer.91,92 Though the flexible SiNW arrays have great potential in energy applications, mechanical peeling of the SiNW arrays from the epitaxial Si wafer substrate is difficult. In light of this, Wang and co-workers fabricated naturally separated SiNW arrays on liquid metal substrate.88,89 These SiNW arrays could be further embedded into PDMS and then conveniently peeled from the metal substrate surface (Figure 7a and 7b). In these regards, this method offers a simple approach to large-scale preparation of flexible and functional SiNW arrays.88,89

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Figure 7. (a) Schematic illustration of the VLS growth of SiNW array on the surface of the liquid Sn substrate and their transfer into flexible substrate. Reprinted with permission from ref 89. Copyright 2014 IOP Publishing. (b-d) SEM images of SiNW array (b) and Si wire array (c and d) embedded in PDMS substrate. Reprinted with permission from ref 88. Copyright 2014 IOP Publishing. Reprinted with permission from ref 91. Copyright 2010 John Wiley and Sons.

3. Solar energy applications of SiNWs Solar energy is one of the most promising eco-friendly clean energy to solve the problems of energy shortage and environment pollution. Owing to the excellent physical properties and mature production technology of Si materials, as well as the strong light-trapping effect and the capability for efficient carrier separation/collection of SiNWs with three-dimensional (3D) architecture, SiNW-based solar energy devices exhibit the great potential to transform solar energy to electric or chemical energy via a photovoltaic or photoelectrochemical process.17,19,20 Recent investigations on 22


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SiNW-based solar energy devices, both in theory and in experiment, indicated that the devices were capable of achieving higher power conversion efficiency with the use of less Si materials or lower purity Si, thus providing a feasible way for low-cost and high-efficiency solar energy applications.26

3.1 Optical properties of SiNWs for solar energy applications Optical properties of SiNWs are important in determining the solar energy conversion efficiencies of devices. Different from bulk Si materials, SiNWs possess unique optical properties due to their diameters comparative to light wavelengths.93 Cao and co-workers investigated the optical properties of individual SiNWs and demonstrated that SiNWs with different diameters could generate a wide spectrum of colors by harnessing the strong resonant light scattering properties of SiNWs under white light illumination.94 Bronstrup and co-workers performed detailed theoretical analysis of scattering and absorption of individual SiNWs.29 The results revealed a strong correlation of the optical properties of individual SiNWs to their diameters. It is also found that the scattering efficiencies and absorption efficiencies bigger than 1 could occur, which means that light is scattered or absorbed from an area bigger than the geometrical area of the SiNW. In their examples, the calculated scattering efficiencies reached values up to 901% and absorption efficiencies reached values up to 449%, indicating the potential of SiNWs to harvest and scatter light very efficiently.

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Figure 8. (a) Schematic diagram of three representative processes between incident light and SiNW arrays of different parameters. (b) Calculated ultimate efficiency of SiNW arrays as a function of the nanowire length. Reprinted with permission from ref 95. Copyright 2012 IOP Publishing. (c) Schematic and measured absorption of SiNW array with an antireflective coating and embedded light-scatterers, measured on a Ag back-reflector. Reprinted with permission from ref 98. Copyright 2010 Nature Publishing Group.

Despite the excellent optical properties of individual SiNWs, the array configuration could further trap the incident light inside arrays and reduce the 24


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reflectivity owing to high surface areas and obviously increased optical path length induced by collective light scattering interactions among SiNWs (Figure 8a).27,95 The impact of diameters, lengths, periodicities and incident angles on the optical properties of SiNW arrays was studied both experimentally and theoretically.95-97 These results provide design directions for obtaining SiNW arrays with optimized solar energy harvesting capability (Figure 8b). To further enhance the light absorption, Lewis and co-workers combined anti-reflective coatings, light scatters and back reflectors with optimized SiNW arrays (Figure 8c).98 First, a SiNx layer was conformally deposited to SiNWs as an anti-reflective coating. Then, Al2O3 nanoparticles were infilled between SiNWs to scatter the light. Finally, the SiNW array was placed on an Ag back-reflector. The resulted SiNW array could achieve up to 96% peak absorption with less than 5% areal fraction of SiNWs. These anti-reflective methods provide possible routes for SiNWs as effective light absorbers for solar energy applications.

3.2 Photovoltaic devices 3.2.1 P-n junction solar cells Photovoltaic devices can absorb solar radiation in the semiconductor materials to produce photo-generated electron-hole pairs and the electron-hole pairs can be separated under the built-in electrical field at junction interface.18 In commercial Si wafer-based solar cells, p-n junctions are formed near the wafer surface via high-temperature diffusion of dopants. Similarly, SiNW-based solar cells with axial 25



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p-n junctions could be readily constructed by combining Si wafer-based junction fabrication process with Si etching technique (Figure 9a). By etching the Si wafer-based planar p-n junction to form SiNW array, the performance of the solar cells could be significantly improved due to the remarkable antireflection effect of SiNW arrays. For instance, Yu and co-workers reported a power conversion efficiency of 9.24% for SiNW-based axial p-n junction solar cells (Figure 9b).99 In contrast, the power conversion efficiency was only 3.32% for planar device counterparts (Figure 9c). It was found that the short circuit density (Jsc) could be boosted from 18.1 mA/cm2 to 34.3 mA/cm2 by fabricating SiNW array with a diameter of 200 nm and a periodicity of 400 nm on the top of a planar Si p-n junction.100,101 In addition, there existed an optimum value for the length of SiNWs in consideration of the balance between light trapping and series resistance of the device. An optimum length of 279 nm gave rise to a device efficiency of 10.15%.102

26


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Figure 9. (a) Schematic illustration of fabrication process of SiNW based axial p-n junction solar cells. (b) J-V curves of SiNW based axial p-n junction solar cells. (c) J-V curves of planar p-n junction solar cells. Reprinted with permission from ref 99. Copyright 2010 Springer Link. Table 1. Selected representative results of solar cells based on SiNW radial p-n junction. Device structures n-Si/p-SiNWs radial p-n junction by surface doping (Al+1%Si)/p+-Si/n+-SiNWs radial p-n junction by surface doping n+-Si/p-SiNWs radial p-n junction with reduced surface doping

Jsc (mA··cm-2)

Voc (V)

FF (%)

PCE (%)

Ref.

27.1

0.582

70.5

11.1

[104]

26.4

0.59

69

10.8

[103]

36.45

0.628

79.6

18.2

[105]

27



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n+-Si /p-SiNWs radial p-n junction with selectively doping SiNWs radial p-i-n junction by epitaxial growth SiNWs radial p-i-n junction with match-head structures by epitaxial growth

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33.65

0.555

68.6

12.8

[106]

40

0.44

38

10

[100]

40.0

0.46

52

10.1

[102]

The device performance of SiNW-based solar cells with axial p-n junctions is improved mainly due to the enhanced light absorption. In comparison with the axial p-n junctions, radial p-n junctions can improve the carrier collection efficiency owing to the reduced minority carrier diffusion distance in the radial direction. To form the radial p-n junctions, one can control the doping depth in thermal diffusion or grow an extra doping layer via vapor phase epitaxy on the surface of SiNWs.103-108 By taking advantage of the radial p-n junction configuration, high-efficiency (power conversion efficiency, PCE > 10%) SiNW-based solar cells could be realized (Table 1).109-112 For example, radial p-n junction solar cells formed by combining MCEE and thermal diffusion processes achieved a high PCE of 11.1% under an optimized NW length of 1 µm.110 Yoo and co-workers also reported the epitaxial growth of SiNW radial p-i-n junction solar cells with PCE of 12.8% by using a match-head structure as a built-in light concentrator.108 Though the radial p-n junction benefits light absorption as well as carrier collection for solar cells, the enhanced photo-carrier recombination caused by the heavy surface doping concentration and large surface area remains a challenge. This makes most of the reported SiNW-based solar cells have lower power conversion 28


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efficiencies than conventional planar solar cells. Therefore, appropriate surface control is essential to the further improvement of device efficiency. Oh and co-workers demonstrated that the photo-carrier recombination in radial p-n junction solar cells could be efficiently suppressed by applying a tetramethylammonium hydroxide (TMAH) etching process on SiNWs after the formation of surface emitter.111 This etching step could reduce both surface doping concentration and surface area of the SiNWs, ensuring a high efficiency of 18.2% for the radial p-n junction solar cells.

3.2.2 SiNW-inorganic hybrid devices High-efficiency p-n junction solar cells ask for expensive and complicated preparation progresses to form high-quality junctions and devices. An alternative method is to form Schottky junctions between SiNWs and other inorganic materials.113,114 Lewis and co-workers demonstrated single SiNW/Al Schottky junction solar cells with PCE of 0.46%.115 They further investigated the charge transport and collection in the devices, and found that the single SiNW possessed a long minority carrier diffusion length (>2 µm). Cui and co-workers also fabricated SiNW array-based Schottky junction solar cells with PCE of 1.2% through evaporating 10 nm Pt onto the surface of SiNWs.116

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Figure 10. (a) Schematic illustration of the SiNW/carbon quantum dot heterojunction device. (b) are SEM images of the as-prepared SiNW arrays. (c) are TEM images of carbon quantum dots. (d-f) SEM images, TEM images and EDS analysis of the carbon quantum dots coated SiNWs. (g) Photovoltaic characteristics and (h) EQE spectra of the optimal SiNW/carbon quantum dot heterojunction device compared with controlled devices. Reprinted with permission from ref 9. Copyright 2014 American Chemical Society.

Besides metals, carbon-based nanomaterials with extraordinary electronic, optical 30


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and mechanical properties were adopted to form Schottky junctions with SiNWs (Table 2).117-119 For instance, Zhu and co-workers fabricated SiNWs/carbon nanotube Schottky junction solar cells by transforming carbon nanotube thin film onto the top surface of the SiNW array.120,121 Jie and co-workers combined graphene films with SiNW arrays to fabricate SiNWs/graphene Schottky junction solar cells.122,123 Graphene films possess good conductivity and high transparency, ensuring efficient light absorption and carrier transportation/collection in the Schottky junction solar cells. By performing surface modification, controlling the number of graphene layers, and optimizing device structures, high efficiency of nearly 10% has been achieved for this kind of solar cells.124 In above devices, the carbon nanotubes or graphene films can only contact the SiNWs at the tip positions. The small contact area limits the improvement of the device performance. Xie and co-workers fabricated hybrid solar cells by coating SiNW array with carbon quantum dots (Figure 10).9 The carbon quantum dots could be coated onto the surface of SiNWs, facilitating carrier transport and collection. Due to the good surface coverage of carbon quantum dots on SiNWs, a preliminary PCE value of 9.10% was achieved.

Table 2. Selected representative results of solar cells based on SiNW hybrid structures. Jsc Device structures

(mA··cm ) -2

Voc (V)

FF (%)

PCE (%)

Ref.

Carbon quantum dots/SiNWs

30.09

0.51

59.3

9.10

[9]

Graphene/SiNWs

28.65

0.48

63.4

8.71

[123]

Graphene/P3HT/SiNWs

35.1

0.48

59

9.94

[118] 31



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PEDOT:PSS/ spiro-OMeTAD/SiNWs

30.9

0.57

58.8

10.3

[124]

PEDOT:PSS/Cu/spiro-OMeTAD/SiNWs

31.3

0.527

58.8

9.7

[125]

PEDOT:PSS/TAPC/SiNWs

34.76

0.54

69.5

13.01

[130]

PEDOT:PSS/ SiNWs

30.9

0.523

74.5

12.0

[131]

PEDOT:PSS/ low filling ratio SiNWs

30.42

0.614

70

13.11

[122]

PEDOT:PSS/Al2O3/ SiNWs

36.03

0.54

67.8

13.2

[121]

32.1

0.56

75.2

13.63

[132]

metallurgical-grade

+

PEDOT:PSS/ SiNWs /n back surface field

3.2.3 SiNW-organic hybrid devices SiNWs, as a kind of crystalline Si materials, possess high stability, high carrier mobility and strong light trapping effect, while organic materials have the characteristics of high flexibility, tunable optical properties and low-temperature, solution processing capability.125 Therefore, the SiNW-organic hybrid devices can harness the advantages of both SiNWs and organic materials, offering the opportunities for the fabrication of high-performance, low-cost hybrid solar cells. To date, various conducting organics, such as poly (3,4-ethylenedioxythiophene): poly (stylenesulfonate) (PEDOT:PSS),37,126,127 poly(3-hexylthiophene) (P3HT),124,128 and 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD),129-131 have been utilized to fabricate Si-organic hybrid solar cells, demonstrating a large progress in this field (Table 2). PEDOT:PSS, a kind of conducting polymer, which has been widely used in organic devices such as organic photovoltaics (OPVs) and organic light-emitting 32


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diodes (OLEDs), possesses good film-forming property, high conductivity, high transparency and excellent stability. In 2010, Shiu and co-workers fabricated SiNW/PEDOT:PSS hybrid solar cells by immersing SiNW array into wet PEDOT:PSS thin film.132 It was observed that the PCE could be improved from 0.08% for reference planar wafer device to 5.09% for SiNW device, indicating the feasibility to obtain high-efficiency photovoltaic devices by combining SiNWs and PEDOT:PSS. However, in this method, due to the large surface tension and long conjugated molecular chain, PEDOT:PSS was difficult to filtrate into the bottom of SiNW array. The inferior SiNW/organic contact thus resulted in a moderate device performance.132 To address this issue, Lu and co-workers achieved good polymer coverage of SiNWs by filling ethanol diluted PEDOT:PSS solution onto SiNW arrays.133 Ethanol changed the wettability of PEDOT:PSS on SiNW surface and increased the contact area between PEDOT:PSS and SiNWs. Finally, a PCE of 6.35% was demonstrated by this method. Furthermore, Sun et al. and He et al. respectively proposed the fabrication of SiNW/PEDOT:PSS hybrid solar cells by introducing a transparent hole conducting small molecule, Spiro-OMeTAD, between the SiNWs and PEDOT:PSS. Because the small molecule could infiltrate into the bottom of SiNW arrays and conformally coat on SiNWs, a structure of an organic layer surrounding SiNWs was achieved. As a result, the PCE was further improved to 10%.129-131 The high surface recombination ratio induced by the large surface area of SiNW array

is

an

important

factor

that

restricts

the

improvement

of

device

performance.126,127 Appropriate surface treatment or surface passivation is a 33



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prerequisite to achieve high-performance SiNW/organic hybrid solar cells. Sun and co-workers fabricated SiNW/PEDOT:PSS hybrid solar cells with PCE of 10.2% by passivating the surface of SiNWs with a methyl/allyl organic monolayer.134 Pudasaini and co-workers also reported SiNW/PEDOT:PSS hybrid solar cells with PCE of 10.56% by using atom layer deposition (ALD) to deposit an ultrathin alumina layer on SiNWs.135 Furthermore, a high PCE of 13% was demonstrated by Yu et al. They fabricated SiNW/PEDOT:PSS hybrid solar cells by spin-coating an intermediate 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer at the interface of SiNWs and PEDOT:PSS (Figure 11).136 The insertion of TPAC layer greatly increased the minority carrier lifetime and effectively blocked the strong oxidation at the interface of SiNWs and PEDOT.

Figure 11. (a) Schematic illustration and SEM images of SiNW/TAPC/PEDOT 34


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heterojunction solar cells. (b) Current density-voltage characteristics of the solar cells under a simulated AM1.5G illumination condition. (c) Dark current density-voltage characteristics in semi-logarithmic plot. Reprinted with permission from ref 136. Copyright 2013 American Chemical Society.

In the SiNW/organic hybrid solar cells, the SiNWs are surrounded by the thin organic layer, forming a radial heterojunction in fact. Therefore, in principle, low-purity Si could be used in this kind of solar cells owing to the short carrier diffusion distance in the radial direction of SiNWs. Recently, Sun and co-workers demonstrated the SiNW/PEDOT:PSS hybrid solar cells with PCE as high as 12% based on metallurgical grade Si wafer.38 On the other hand, the strong light trapping effect of SiNW array offers the possible to fabricate high-performance solar cells with the use of less Si materials. Cui and co-workers constructed SiNW/PEDOT:PSS hybrid solar cells with a PCE of 13.6% on 20 µm-thick Si substrate.137,138 These achievements highlight the great potential of SiNW/organic hybrid solar cells for low-cost and high-efficiency photovoltaic devices.

3.3 Photoelectrochemical devices The photoelectrochemical progress could store solar energy in the form of fuels including hydrogen and organics, greatly facilitating the energy storage and transport.139-142 Therefore, besides the SiNW-based photovoltaic devices, SiNW-based photoelectrochemical cells have gained considerable attention due to their low cost 35



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and the capability to form conformal liquid junctions.17,143 SiNWs have the advantages in terms of enhanced light absorption, efficient carrier collection and large specific surface area.143,144 However, the practical applications of SiNWs in photoelectrochemical cells still face some challenges: (i) Catalytic activity of Si is relatively low, causing large electrochemical overpotential and limited kinetics of chemical reactions. Deposition of noble metals such as Pt could effectively enhance the catalytic activity of SiNWs for water splitting.145,146 Nevertheless, the use of noble metals will suffer from high device cost. Therefore, the development of new co-catalysts with high efficiency, low cost and good durability remains a fundamental challenge. (ii) Si will experience a photo-corrosion process and be oxidized in aqueous solution. The improvement of photo-stability of SiNW-based photoelectrodes is another important issue for their applications in photoelectrochemical cells.147,148

3.3.1 High-efficiency and low-cost co-catalysts In consideration of the high material consumption of traditional deposition processes, Yang et al. and Wang et al. respectively developed the ALD method for the deposition of a highly conformal and ultrathin Pt layer on the surface of SiNWs.145,149 The Pt layer could be controlled to be as thin as 0.5 nm, thus remarkably reducing Pt loadings while keeping its catalytic activity. Hou and co-workers also proposed a bioinspired molecular, Mo3S4, as a new type of co-catalyst on SiNWs for hydrogen evolution (Figure 12).150 The photoelectrode constructed by combing SiNWs with Mo3S4 particles yielded a photocurrent of 9 mA/cm2 at applied potential of 0 V. 36


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Furthermore, Lewis and co-workers demonstrated a Ni-Mo-coated SiNW array photoelectrode, which could achieve a short circuit photocurrent density of 9.1 mA/cm2. The efficiency of 1.9 % was comparable to the performance of Pt-coated SiNW array photoelectrode under nominally the same conditions.151 In recent investigations, other low-cost materials such as WS3, MoS2 and N-doped graphene quantum sheets were intensively studied and also proved large potential as high-efficiency and low-cost co-catalysts.152-154

Figure 12. (a) Schematic of the tandem “chemical solar cell” and cubane-like Mo3S4 cluster on Si surface. (b) Photoelectrocatalytic activity measurements on planar Si and SiNWs. (c) SEM image of SiNWs. Reprinted with permission from ref 150. 37



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Copyright 2011 Nature Publishing Group.

3.3.2 Protection and passivation of SiNWs The practical photoelectrochemical fuel production was usually carried out in aqueous solution, hence the protection and passivation of SiNWs were much demanded to avoid photocorrosion and to reduce surface recombination.25,155-157 Peng and co-workers protected SiNWs from photocorrosion and further passivated their surface by deposition of 2 nm ultrathin carbon film using microwave plasma-enhanced chemical vapor deposition (CVD).25 Then Pt nanoparticles (PtNPs) were deposited onto the surface of SiNWs to form a PtNPs/C/SiNW array photoelectrode. By these means, a high PCE of 10.86% was achieved for the cells. Besides the inorganic carbon film, Zhang and co-workers realized the protection and passivation of SiNWs by modifying their surface with various organics, including 2,9,16,23-tetraamino-phthalocyanine poly-(2,9,16,23-tetraamino-phthalocyanine

of of

zinc copper

(ZnTAPc),158

(poly-CuTAPc)159,

and

PEDOT:PSS154. They also fabricated efficient visible light photoelectrodes with Ag nanoparticles/PEDOT/SiNW configuration (Figure 13).155,160 PEDOT as a transparent conducting polymer could serve as the protective layer to improve the stability of the photoelectrode. It could also facilitate the transfer of carriers from the photoelectrode to electrolyte. In addition, Ag was a plasmonic metal, and hence the decoration of Ag nanoparticles could remarkably enhance the visible light absorption. Therefore, the resulting photoelectrode achieved a solar-to-chemical energy conversion efficiency of 38


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2.86% and a stable hydrogen evolution of 36.05 µmol/(cm2·h) in a water/methanol solution.

Figure

13.

(a)

Schematic

illustration

of

the

fabrication

of

Ag

nanoparticles/PEDOT/SiNW arrays. Step 1, electro-polymerization of PEDOT. Step 2, sol-gel

decoration of AgNPs.

(b) SEM and (c) TEM images

of Ag

nanoparticles/PEDOT/SiNW structure. (d) Short-circuit photocurrent response of different photoanodes in the dark and under illumination. (e) HRTEM images of Ag nanoparticles/PEDOT/SiNW structure. (f) Current-potential characteristics of different photoanodes in the dark and under illumination. Reprinted with permission from ref 155. Copyright 2014 American Chemical Society.

3.3.3 Dual semiconductor systems for efficient solar energy conversion Though SiNWs can efficiently absorb visible light, they are incapable to utilize ultraviolet light. To solve this issue, a feasible route is to combine other broad band 39



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semiconductors with SiNWs. The dual semiconductor systems could enhance the cell performance due to wider spectrum absorption and better carrier separation efficiency.161-164 Moreover, the stability of the photoelectrode is expected to be greatly improved in case a corrosion resistive semiconductor material was adopted as the shell layer of SiNWs.165 Wang and co-workers reported a 3D branched NW heterostructures with SiNW trucks and ZnO NW branches as photoelectrodes (Figure 14a).163,166,167 The branched photoelectrode showed broadband absorption from UV to

Figure 14. (a) SEM images and incident photon conversion efficiency (IPCE) of ZnO/Si branched nanowire heterostructures. Reprinted with permission from ref 161. Copyright 2013 American Chemical Society. (b) SEM images and photocurrent of planar and porous Si/WO3 core-shell structure. Reprinted with permission from ref 159. Copyright 2014 American Chemical Society.

40


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near IR region. By tuning the doping concentration of the p-SiNW cores, the n-ZnO/p-Si branched NW array photoelectrode could be selectively used for water oxidization or reduction. In another work, Lewis and co-workers fabricated hierarchically structured SiNW/WO3 core-shell tandem photoanodes (Figure 14b).161 Through inducing porosity in electrodeposited WO3 thin film, the photocurrent density was increased by nearly 200%. Hematite (Fe2O3), as a low-cost and wide bandgap semiconductor material, was also studied by various groups to integrate with SiNWs.165,168,169 Peng and co-workers reported facile fabrication of SiNW/Fe2O3 core/shell arrays with the decoration of plasmonic gold nanoparticles as high-efficiency photoanodes.165 A solar water splitting efficiency of 6.0% calculated through photocurrent measurement was achieved under AM 1.5G illumination.165 These heterostructured photoelectrodes that combine SiNWs and wide bandgap semiconductor materials are of high efficiency and excellent stability, thus showing promising for future solar fuel production applications.

4. Conclusion and outlook SiNW arrays, with enhanced light trapping and radial carrier collection capability, show great advantages in photovoltaic and photoelectrochemical devices. Devices based on SiNW arrays could fully absorb incident light using much less Si materials than the planar wafer, and it is allowed to construct high-efficiency devices using low-purity Si materials. For practical solar energy applications, low-cost and large-scale fabrication of SiNW arrays as well as rational design and construction of 41



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high-efficiency and durable devices are prerequisite. In this review, we focus on the recent advances that have been achieved for controllable fabrication of SiNW arrays in continuous and

scalable process, as well as their photovoltaic and

photoelectrochemical device applications. Both top-down and bottom-up methods were developed to achieve large-scale production of SiNWs. Top-down methods including RIE, macroscopic galvanic cell-enhanced continuous MCEE and vapor phase MCEE possess the merit of high controllability and uniformity, while bottom-up methods such as liquid substrate VLS growth hold the advantage of low cost without the need for high-quality Si wafers. By using these SiNW arrays, high-efficiency photovoltaic and photoelectrochemical devices were realized by the means of forming radial junctions, modulating the junction interfaces, constructing hybrid systems and so on. In spite of all these progresses, there remain many problems that need to be addressed before SiNW arrays and their solar energy devices can be employed in large-scale industrial applications. First, low-cost and green fabrication of uniform SiNW arrays is still difficult to achieve. Although the reviewed top-down methods could produce uniform SiNW arrays, the demanded Si wafer substrates lead to high cost. Besides, the heavily used HF solution is not environment-friendly. On the other hand, the precise controllability and scale of bottom-up growth method need to be further improved. Second, despite the enhanced light absorption and carrier separation/collection, the efficiencies of SiNW-based solar energy devices are still not as high as expectation due to serious carrier recombination. Third, the device cost 42


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needs to be further reduced for practical applications and new device design is desired to meet the demand of the booming portable electronics. To solve these problems, the following directions might be taken into consideration. First, multilayer transfer method should be further developed for large-scale and defect-free separation of etching SiNWs from wafer substrates. This is important to reduce the device cost and facilitate the integration of SiNWs with other functional substrates. Also, through investigating the bottom-up growth of SiNWs with advanced characterization techniques like in situ electronic microscopes, it is helpful to gain more insight into the growth mechanism of the SiNWs. The controllability and scalability of the bottom-up growth of SiNWs could be effective improved. The realization of ultra-long SiNWs (centimeter or even longer) and dense SiNW films will bring exciting opportunities in novel device applications. Second, effective methods for interface modification need to be further developed to improve the device efficiency. Surface polish on the top-down SiNWs via chemical treatment could reduce the surface defects. Also, surface passivation with inorganic film coating or molecular modification is beneficial to suppress the carrier recombination. Third, ultra-thin and flexible solar energy devices could be fabricated at low cost by using multilayer-transferred SiNWs or bottom-up SiNW assemblies. These devices will have important applications in portable and wearable electronics. With the sustained progresses in large-scale fabrication of SiNWs and rational design and construction of high-efficiency devices, it is undoubtable that the applications of SiNWs in solar energy conversion will have a bright future. 43



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) (Grant Nos. 2013CB933500, 2016YFA0202400), the Major Research Plan of the National Natural Science Foundation of China (Grant No. 91333208), the National Natural Science Foundation of China (Grant No. 51373188, 61422403, 51672180, 51622306, 21673151), Collaborative Innovation Center of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project and Qing Lan Project.

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Table of Contents (TOC)

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Large-Scale Fabrication of Silicon Nanowires for Solar Energy

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