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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 10539ndash10543 2011

Facile Fabrication of Si Nanowire Arrays forSolar Cell Application

Xiaocheng Lilowast and Beng Kang Taylowast

School of Electrical and Electronic Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798

Large-area Si nanowire arrays have been fabricated on phosphorus doped Si surface by a facilesilver-catalyzed chemical etching process The solar cell incorporated with Si nanowire arrays showsa power conversion efficiency of 669 with an open circuit voltage of 558 mV and a short circuitcurrent density of 2513 mAcm2 under AM 15 G illumination without using any extra antireflectionlayer and surface passivation technique The high power conversion efficiency of Si nanowiresbased-solar cell is attributed to the low reflectance loss of Si nanowire arrays for incident sunlightOptimization of electrical contact and phosphorus diffusion process will be critical to improve theperformance of Si nanowires-based solar cell in the future

Keywords Si Nanowire Solar Cell Antireflection Chemical Etching Power ConversionEfficiency Silver

1 INTRODUCTION

Among the various energy projects in progress solarphotovoltaic power generation is an almost maintenance-free and renewable clean-energy technique and consideredas the most promising candidate for the future energyresource1 Si the most important semiconductor materialin micro-electronics and Si-based photovoltaic devicesincluding single-crystalline Si multi-crystalline Si andamorphous Si still play the dominant role in todayrsquosphotovoltaic market due to their high power conversionefficiency Recently nanostructured materials provide anew approach to reduce the cost and improve efficiencyin photovoltaics Si nanowires (SiNWs)2ndash4 Si nanopillar(SiNPs)5 and Si nanotips (SiNTs)6 have been theoreticallydemonstrated their good ability for improving the chargecollection efficiency and increasing the absorption for inci-dent sunlight However the experimental report on thisarea is relatively rareDuring the past few years SiNW arrays were fabricated

by various methods such as the chemical vapor deposition(CVD)7 Molecular beam epitaxy8 and Cl2 plasma basedreactive ion etching9 However rigorous experimental con-ditions and complex instruments are often required forthese methods Recently a wet chemical etching methodhas been developed to fabricate single-crystalline SiNWarrays by immersing Si wafer into the mixture solution of

lowastAuthors to whom correspondence should be addressed

AgNO3 and HF10ndash12 This technique is actually an elec-trochemical deposition of silver plus a chemical etchingprocess In this study we reported a facile process to fab-ricate large-area SiNW arrays by using a dry deposition ofsilver film plus a chemical etching process Based on thischemical etching process SiNWs-based solar cell with aconversion efficiency of 669 is achieved We hope thisfabrication process has a potential application in Si-basedsolar cell

2 EXPERIMTNAL DETAILS

In the authorrsquos experiments a 06 mm-thick single-crystal(100) p-type Si wafers with a specified surface roughnessof Ra 5sim10 nm and resistivity of 1ndash30 middot cm were usedBefore experiment Si wafers were washed with deion-ized water and acetone sequentially and then immersedinto Piranha solution (H2SO4H2O2 = 31 vv) at 90 Cfor 1 h to entirely remove the surface contamination com-pounds Then a thin layer of silver film with thicknessof 25 nm was coated on Si wafer by using a sputteringsystem with argon flow rate of 30 sccm Next the sil-ver coated Si wafers were immersed into a Teflon ves-sel containing the mixture solution of HF and H2O2 tofabricate the SiNWs at room temperature The concentra-tion of HF and H2O2 is 044 M and 46 M respectivelyThe etching duration depends on the desired length ofSiNWs After chemical etching process the Si wafers werethoroughly rinsed by deionized water for several times

J Nanosci Nanotechnol 2011 Vol 11 No 12 1533-488020111110539005 doi101166jnn20113974 10539

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IP 1556944Tue 03 Jul 2012 110142

RESEARCH

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Facile Fabrication of Si Nanowire Arrays for Solar Cell Application Li and Tay

Scheme 1 Diagram of fabrication process for SiNWs-based solar cell (a) pre-cleaning of Si wafer (b) formation of Si p-n junction wafer by aphosphorus diffusion process (c) fabrication of SiNW arrays (d) deposition of rear and front side electrode and fabrication of SiNWs-based solar cell

The residual silver particles were removed by AMP solu-tion (NH4OHH2O2H2O = 215 vv) and HPM solution(HClH2O2H2O= 218) sequentially Finally the sampleswere rinsed by deionized water and dry in N2 atmosphere

The detailed processing sequence for SiNWs-basedsolar cell is schematically depicted in Scheme 1 In gen-eral it involves the following steps(a) fabrication of planar Si p-n junction wafer with dop-ing depth of sim07 m and doping concentration of 2times1018cm3 by a POCl3 diffusion process at 930 C for50 min(b) removal of the parasitic p-n junction on back side ofSi wafer by a CMP process(c) deposition of silver film with thickness of 25 nm onSi substrate(d) fabrication of SiNW arrays on planar Si p-n junctionwafer and removal of residual silver particles by afore-mentioned chemical solutions (e) deposition of aluminumfilm with thickness of 250 nm on rear side of Si wafer andannealing at 550 C for 50 sec via a rapid thermal processto form the rear side electrode(e) deposition of TiPdAg films (6060100 nm) onSiNWs surface via a shadow mask evaporation process toform the front side electrode

After annealing in N2 atmosphere at 200 C for 6 hand cooling down to room temperature the samples werefinally cut into 1times 1 cm2 to remove the circumferentialp-n junction and yield a standard solar cellThe morphologies and structures of samples were exam-

ined by using LEO 1550 field emission scanning elec-tron microscopy (FESEM) and Raman microspectroscopy(WITEC CRM200 confocal system with exciting wave-length of 532 nm) Optical reflectance spectra wererecorded by a PerkinElmer LAMBDA 950 UVVisNIRspectrophotometer The power conversion efficiency of theas-prepared SiNWs-based solar cells was performed by asimulated Air Mass (AM) 15 G solar illumination withintensity of 100 mWcm2

3 RESULTS AND DISCUSSION

In this study SiNW arrays are fabricated by a dry sil-ver film deposition plus a wet chemical etching processWhen immersing into the chemical etching solution thecolor of Si wafer changes from gray to straw yellow and

finally turn into the black Figure 1 shows the SEM imagesof fabricated SiNW arrays with chemical etching durationof 40 sec From the top-view SEM image as shown inFigure 1(a) it can be seen that SiNWs with diameters

Fig 1 SEM images of the as-synthesized SiNW arrays (a) top viewand (b) cross-sectional view of SiNW arrays with etching duration of40 sec (c) cross-sectional view of SiNW arrays with etching duration of80 sec

10540 J Nanosci Nanotechnol 11 10539ndash10543 2011

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

ranges from 50 nm to 200 nm are uniformly distributedover large-area indicating a facile process for fabricat-ing wafer-scale of uniform SiNW arrays with low costand high throughput This wafer-scale fabrication processthus offers a great convenience for exploring their poten-tial application in solar cell Cross-sectional SEM imageas shown in Figure 1(b) indicated that all SiNWs withlength of ca 450 nm are vertically aligned on Si waferimplying that the etching process is preferred along thedirection perpendicular to Si surface Linear etching tracesand track cavity could also be easily observed on SiNWssurface The morphology on top side of SiNWs is coarserthan that of bottom side which may be related to thelonger etching duration it suffered The silver film whichactually consists of nanoclusters plays the role of catalystduring chemical etching process The silicon under the sil-ver nanoclusters is preferentially etched away and finallygoing into chemical solution in form of H2SiF4 whilethe Si surface without silver nanoclusters covered remainunchanged With the continuous dissolution of silicon andthe simultaneous sinking of silver nanoclusters the sili-con surface without silver nanoclusters covered protrudesout from the network of silver nanocluster and conse-quently SiNW arrays forms along the direction perpendic-ular to the surface of Si substrate During the chemicaletching process some silver particles also intrude intoSiNWs and leave some cavities on the sidewall of SiNWsafter the silver catalyst was removed This maybe relatedwith the undulation of solution or the produced bubblesduring chemical etching process Figure 1(c) shows thecross-sectional SEM image of SiNWs with etching dura-tion of 80 sec It can be seen that the length of SiNWsincreases remarkably and reaches 11 m with the pro-longed chemical etching duration Some SiNW bundles orclumps could also be found on sample surface Figure 2shows the Raman spectra recorded from pristine Si wafer

Fig 2 Raman spectra recorded from the pristine Si wafer surface andSiNW arrays

and SiNW arrays The typical Raman band at 5202 cmminus1

corresponds to the first-order transverse optical phononmode of Si surface Compared with that of pristine Siwafer the first-order Raman peak of SiNWs at 5189 cmminus1

shows a dramatic asymmetric broadening and lower down-shift implying that SiNWs still remain its original crys-talline structure The asymmetry and lower downshift ismainly attributed to the phonon quantum confinementeffect and the possible strain resulted from chemical etch-ing process13 According to quantum theory when thecrystalline size decreases momentum conservation will berelaxed (q = 0) and Raman-active mode will not be limitedto being at the center of the Brillouin zone The smallerthe crystalline grain the larger the frequency shift andthe more asymmetric and the broader the peak becomesThis phenomenon has been confirmed by previousreport13

The reduction of reflectance loss is one of the mostimportant factors for high efficiency solar cell Thenanoporous structure and black color of the as-preparedsamples imply their possible antireflection properties andapplications in solar cell Figure 3 demonstrates thereflectance spectra recorded from the Si wafer coveredwith different lengths of SiNW arrays Compare with thatof pristine Si wafer the presence of SiNW arrays effec-tively reduces the optical reflection loss of Si wafer Theaverage reflectance loss of SiNW arrays with length ofsim450 nm is sim401 over the range of 200ndash1000 nmwavelength which is far lower than that of pristine Siwafer and also the Si pyramidal structure as well asthe porous Si nanostrucures14ndash16 The reflectance loss ofSi surface could be further decrease to 298 as thelength of SiNWs increase to 11 m The remarkable lowreflectance loss of SiNWs covered Si wafer may be relatedwith the ultrahigh specific surface area and the typical sub-wavelength structure of SiNW arrays

Fig 3 Reflectance spectra of Si wafer and SiNW arrays with differentlengths

J Nanosci Nanotechnol 11 10539ndash10543 2011 10541

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Facile Fabrication of Si Nanowire Arrays for Solar Cell Application Li and Tay

According to the aforementioned results the as-synthesized SiNW arrays demonstrate the excellent antire-flection property and possess several advantages overthe other surface texture structures This facial pro-cess enables us to fabricate wafer-scale SiNW arrays atroom temperature with low cost and high throughputNo rigorous experimental conditions such as high tem-perature hazardous silicon precursor or complex instru-ment are required in our experiment This will pave theway to explore the possible application of SiNWs in highefficiency solar cell With this in mind here we integratethis SiNW arrays into planar Si p-n junction wafer andexplore the possible photovoltaic applications of SiNWson conventional solar cell The main fabrication processof SiNWs-based solar cell is shown in Scheme 1 anddescribed in experimental section To prevent the possi-ble chemical contamination to diffusion furnace in thisstrategy p-n junction is generated on Si wafer prior tochemical etching process By synthetically consideringthe diffusion depth of phosphorous and the impact ofthe length of SiNWs on reflectance loss the length ofSiNWs is selected as 450 nm and corresponding chemical

Fig 4 (a) Overall-view of the SiNWs-based solar cell recorded by adigital camera and (b) cross-sectional SEM image of the front side elec-trode area

etching duration is ca 40 sec Figure 4(a) shows theoptical image of the as-prepared SiNWs-based solar cellrecorded by a digital camera The black areas consist ofSiNW arrays while the block lines stand for TiPdAgfilms front grid electrode The morphology of the blackregion is same as that in Figure 1(a) Figure 4(b) demon-strates the cross-sectional SEM image of the block linearea As can be seen that TiPdAg films tightly compacton the top of SiNWs Some parts of TiPdAg films intrudethe interspace of SiNWs and paste on the side wall ofSiNWs This is believed to be beneficial for increasingthe contact area between SiNWs and electrode to someextent and improving the electron collection efficiencyFigure 5 shows the current densityndashvoltage (JndashV charac-teristic curves of the as-prepared SiNWs-based solar cellin darkness and under AM 15 G illumination conditionThe solar cell exhibits a clear diode behavior in darknesswith turn-on voltage of sim03 V and rectification ratio onthe order of 102 at 02 V Only a negligible reverse leakagecurrent was found at reverse voltage implying the goodquality of the SiNWs-based device and good controlla-bility of our fabrication process When illuminated underAM 15 G conditions SiNWs-based solar cell (filled cir-cle) exhibits an open circuit voltage (VOC of sim558 mVand a short circuit current density (JSC of sim2351 mAcm2

without using any extra antireflection layer and surfacepassivation technique Filled factor (FF) and power con-version efficiency ( of the SiNWs-based solar cell werecalculate to be 051 and 669 respectively Comparedwith that of pristine Si wafer-based solar cell (unfilledcircle) the conversion efficiency of SiNWs-based solarcell increases remarkably which is closely related withthe excellent antireflection and trapping effect of SiNWsfor incident sunlight and thus the high short circuit cur-rent density of fabricated solar cell It should be noted

Fig 5 JndashV characteristic curves of solar cells with and without SiNWarrays covered in darkness and under 15 AM illumination

10542 J Nanosci Nanotechnol 11 10539ndash10543 2011

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

that both the VOC and FF of SiNWs-based solar cell isstill lower than that of standard single-crystalline Si-basedsolar cell indicating that the optimization of phosphorusdiffusion and contact between electrodes and SiNWs isstill needed Some surface passivation technologies suchas the indium-tin oxide and silicon nitride film should beused to annihilate the surface defects and dangling bondsresulted from chemical etching process reduce the surfacerecombination velocity and finally improve the VOC andFF of the solar cell After these treatments the high per-formance SiNWs-based solar cell could be expected in thefuture

4 CONCLUSION

In summary large-area SiNW arrays were fabricated onSi surface by a facile silver-catalyzed chemical etchingprocess The as-synthesized SiNW arrays show excellentantireflection property with average reflection loss lowerthan 401 within range of 200ndash1000 nm wavelengthThe SiNWs-based solar cell shows a high conversionefficiency of 669 with VOC of sim558 mV and JSC ofsim2351 mAcm2 under 15 G illumination The high con-version efficiency of SiNWs-based solar cell is attributedto the low reflectance loss and excellent trapping effectof SiNWs for incident sunlight The optimization of phos-phorus diffusion and contact between the electrodes andSiNWs is still needed and high power conversion effi-ciency of SiNWs-based solar cell could be expected in thefuture

Acknowledgment The authors thank the financial sup-port of the MOE Tier II project of Singapore (Grant NoARC 1308)

References and Notes

1 N S Lewis Science 315 798 (2007)2 L Hu and G Chen Nano Lett 7 3249 (2007)3 M D Kelzenberg D B Turner-Evans B M Kayes M A Filier

M C Putnam N S Lewis and H A Atwater Nano Lett 8 710(2008)

4 L Tsakalakos J Balch J Fronheiser B A Korevaar O Sulimaand J Rand Appl Phys Lett 91 233117 (2007)

5 J Li H Yu S M Wong G Zhang X Sun P G Q Lo and D LKwong Appl Phys Lett 95 033102 (2009)

6 J Zhu Z Yu G F Burkhard C-M Hsu S T Connor Y XuQ Wang M McGehee S Fan and Y Cui Nano Lett 9 279 (2008)

7 J Goldberger A I Hochbaum R Fan and P D Yang Nano Lett6 973 (2006)

8 B Fuhrmann H S Leipner H R Hoche L Schubert P Wernerand U Gosele Nano Lett 5 2524 (2005)

9 C M Hsu S T Connor M X Tang and Y Cui Appl Phys Lett93 133109 (2008)

10 K Q Peng Y J Yan S P Gao and J Zhu Adv Mater 14 1164(2002)

11 K Peng Y Wu H Fang X Zhong Y Xu and J Zhu AngewChem Int Ed 44 2737 (2005)

12 K Peng Z Huang and J Zhu Adv Mater 16 73 (2004)13 C Li G J Fang S Sheng Z Q Chen J B Wang S Ma and

X Z Zhao Physica E 30 169 (2005)14 S Lee and E Lee J Nanosci Nanotechnol 7 3713 (2007)15 G H An D H Lee S H Kim B S Kim J H Yun and Y H

Choa J Am Ceram Soc 91 4148 (2008)16 B S Kim D H Lee S H Kim G H An K J Lee N V Myung

and Y H Choa J Am Ceram Soc 92 2415 (2009)

Received 30 December 2009 RevisedAccepted 30 August 2010

J Nanosci Nanotechnol 11 10539ndash10543 2011 10543

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Facile Fabrication of Si Nanowire Arrays for Solar Cell Application Li and Tay

Scheme 1 Diagram of fabrication process for SiNWs-based solar cell (a) pre-cleaning of Si wafer (b) formation of Si p-n junction wafer by aphosphorus diffusion process (c) fabrication of SiNW arrays (d) deposition of rear and front side electrode and fabrication of SiNWs-based solar cell

The residual silver particles were removed by AMP solu-tion (NH4OHH2O2H2O = 215 vv) and HPM solution(HClH2O2H2O= 218) sequentially Finally the sampleswere rinsed by deionized water and dry in N2 atmosphere

The detailed processing sequence for SiNWs-basedsolar cell is schematically depicted in Scheme 1 In gen-eral it involves the following steps(a) fabrication of planar Si p-n junction wafer with dop-ing depth of sim07 m and doping concentration of 2times1018cm3 by a POCl3 diffusion process at 930 C for50 min(b) removal of the parasitic p-n junction on back side ofSi wafer by a CMP process(c) deposition of silver film with thickness of 25 nm onSi substrate(d) fabrication of SiNW arrays on planar Si p-n junctionwafer and removal of residual silver particles by afore-mentioned chemical solutions (e) deposition of aluminumfilm with thickness of 250 nm on rear side of Si wafer andannealing at 550 C for 50 sec via a rapid thermal processto form the rear side electrode(e) deposition of TiPdAg films (6060100 nm) onSiNWs surface via a shadow mask evaporation process toform the front side electrode

After annealing in N2 atmosphere at 200 C for 6 hand cooling down to room temperature the samples werefinally cut into 1times 1 cm2 to remove the circumferentialp-n junction and yield a standard solar cellThe morphologies and structures of samples were exam-

ined by using LEO 1550 field emission scanning elec-tron microscopy (FESEM) and Raman microspectroscopy(WITEC CRM200 confocal system with exciting wave-length of 532 nm) Optical reflectance spectra wererecorded by a PerkinElmer LAMBDA 950 UVVisNIRspectrophotometer The power conversion efficiency of theas-prepared SiNWs-based solar cells was performed by asimulated Air Mass (AM) 15 G solar illumination withintensity of 100 mWcm2

3 RESULTS AND DISCUSSION

In this study SiNW arrays are fabricated by a dry sil-ver film deposition plus a wet chemical etching processWhen immersing into the chemical etching solution thecolor of Si wafer changes from gray to straw yellow and

finally turn into the black Figure 1 shows the SEM imagesof fabricated SiNW arrays with chemical etching durationof 40 sec From the top-view SEM image as shown inFigure 1(a) it can be seen that SiNWs with diameters

Fig 1 SEM images of the as-synthesized SiNW arrays (a) top viewand (b) cross-sectional view of SiNW arrays with etching duration of40 sec (c) cross-sectional view of SiNW arrays with etching duration of80 sec

10540 J Nanosci Nanotechnol 11 10539ndash10543 2011

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

ranges from 50 nm to 200 nm are uniformly distributedover large-area indicating a facile process for fabricat-ing wafer-scale of uniform SiNW arrays with low costand high throughput This wafer-scale fabrication processthus offers a great convenience for exploring their poten-tial application in solar cell Cross-sectional SEM imageas shown in Figure 1(b) indicated that all SiNWs withlength of ca 450 nm are vertically aligned on Si waferimplying that the etching process is preferred along thedirection perpendicular to Si surface Linear etching tracesand track cavity could also be easily observed on SiNWssurface The morphology on top side of SiNWs is coarserthan that of bottom side which may be related to thelonger etching duration it suffered The silver film whichactually consists of nanoclusters plays the role of catalystduring chemical etching process The silicon under the sil-ver nanoclusters is preferentially etched away and finallygoing into chemical solution in form of H2SiF4 whilethe Si surface without silver nanoclusters covered remainunchanged With the continuous dissolution of silicon andthe simultaneous sinking of silver nanoclusters the sili-con surface without silver nanoclusters covered protrudesout from the network of silver nanocluster and conse-quently SiNW arrays forms along the direction perpendic-ular to the surface of Si substrate During the chemicaletching process some silver particles also intrude intoSiNWs and leave some cavities on the sidewall of SiNWsafter the silver catalyst was removed This maybe relatedwith the undulation of solution or the produced bubblesduring chemical etching process Figure 1(c) shows thecross-sectional SEM image of SiNWs with etching dura-tion of 80 sec It can be seen that the length of SiNWsincreases remarkably and reaches 11 m with the pro-longed chemical etching duration Some SiNW bundles orclumps could also be found on sample surface Figure 2shows the Raman spectra recorded from pristine Si wafer

Fig 2 Raman spectra recorded from the pristine Si wafer surface andSiNW arrays

and SiNW arrays The typical Raman band at 5202 cmminus1

corresponds to the first-order transverse optical phononmode of Si surface Compared with that of pristine Siwafer the first-order Raman peak of SiNWs at 5189 cmminus1

shows a dramatic asymmetric broadening and lower down-shift implying that SiNWs still remain its original crys-talline structure The asymmetry and lower downshift ismainly attributed to the phonon quantum confinementeffect and the possible strain resulted from chemical etch-ing process13 According to quantum theory when thecrystalline size decreases momentum conservation will berelaxed (q = 0) and Raman-active mode will not be limitedto being at the center of the Brillouin zone The smallerthe crystalline grain the larger the frequency shift andthe more asymmetric and the broader the peak becomesThis phenomenon has been confirmed by previousreport13

The reduction of reflectance loss is one of the mostimportant factors for high efficiency solar cell Thenanoporous structure and black color of the as-preparedsamples imply their possible antireflection properties andapplications in solar cell Figure 3 demonstrates thereflectance spectra recorded from the Si wafer coveredwith different lengths of SiNW arrays Compare with thatof pristine Si wafer the presence of SiNW arrays effec-tively reduces the optical reflection loss of Si wafer Theaverage reflectance loss of SiNW arrays with length ofsim450 nm is sim401 over the range of 200ndash1000 nmwavelength which is far lower than that of pristine Siwafer and also the Si pyramidal structure as well asthe porous Si nanostrucures14ndash16 The reflectance loss ofSi surface could be further decrease to 298 as thelength of SiNWs increase to 11 m The remarkable lowreflectance loss of SiNWs covered Si wafer may be relatedwith the ultrahigh specific surface area and the typical sub-wavelength structure of SiNW arrays

Fig 3 Reflectance spectra of Si wafer and SiNW arrays with differentlengths

J Nanosci Nanotechnol 11 10539ndash10543 2011 10541

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Facile Fabrication of Si Nanowire Arrays for Solar Cell Application Li and Tay

According to the aforementioned results the as-synthesized SiNW arrays demonstrate the excellent antire-flection property and possess several advantages overthe other surface texture structures This facial pro-cess enables us to fabricate wafer-scale SiNW arrays atroom temperature with low cost and high throughputNo rigorous experimental conditions such as high tem-perature hazardous silicon precursor or complex instru-ment are required in our experiment This will pave theway to explore the possible application of SiNWs in highefficiency solar cell With this in mind here we integratethis SiNW arrays into planar Si p-n junction wafer andexplore the possible photovoltaic applications of SiNWson conventional solar cell The main fabrication processof SiNWs-based solar cell is shown in Scheme 1 anddescribed in experimental section To prevent the possi-ble chemical contamination to diffusion furnace in thisstrategy p-n junction is generated on Si wafer prior tochemical etching process By synthetically consideringthe diffusion depth of phosphorous and the impact ofthe length of SiNWs on reflectance loss the length ofSiNWs is selected as 450 nm and corresponding chemical

Fig 4 (a) Overall-view of the SiNWs-based solar cell recorded by adigital camera and (b) cross-sectional SEM image of the front side elec-trode area

etching duration is ca 40 sec Figure 4(a) shows theoptical image of the as-prepared SiNWs-based solar cellrecorded by a digital camera The black areas consist ofSiNW arrays while the block lines stand for TiPdAgfilms front grid electrode The morphology of the blackregion is same as that in Figure 1(a) Figure 4(b) demon-strates the cross-sectional SEM image of the block linearea As can be seen that TiPdAg films tightly compacton the top of SiNWs Some parts of TiPdAg films intrudethe interspace of SiNWs and paste on the side wall ofSiNWs This is believed to be beneficial for increasingthe contact area between SiNWs and electrode to someextent and improving the electron collection efficiencyFigure 5 shows the current densityndashvoltage (JndashV charac-teristic curves of the as-prepared SiNWs-based solar cellin darkness and under AM 15 G illumination conditionThe solar cell exhibits a clear diode behavior in darknesswith turn-on voltage of sim03 V and rectification ratio onthe order of 102 at 02 V Only a negligible reverse leakagecurrent was found at reverse voltage implying the goodquality of the SiNWs-based device and good controlla-bility of our fabrication process When illuminated underAM 15 G conditions SiNWs-based solar cell (filled cir-cle) exhibits an open circuit voltage (VOC of sim558 mVand a short circuit current density (JSC of sim2351 mAcm2

without using any extra antireflection layer and surfacepassivation technique Filled factor (FF) and power con-version efficiency ( of the SiNWs-based solar cell werecalculate to be 051 and 669 respectively Comparedwith that of pristine Si wafer-based solar cell (unfilledcircle) the conversion efficiency of SiNWs-based solarcell increases remarkably which is closely related withthe excellent antireflection and trapping effect of SiNWsfor incident sunlight and thus the high short circuit cur-rent density of fabricated solar cell It should be noted

Fig 5 JndashV characteristic curves of solar cells with and without SiNWarrays covered in darkness and under 15 AM illumination

10542 J Nanosci Nanotechnol 11 10539ndash10543 2011

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

that both the VOC and FF of SiNWs-based solar cell isstill lower than that of standard single-crystalline Si-basedsolar cell indicating that the optimization of phosphorusdiffusion and contact between electrodes and SiNWs isstill needed Some surface passivation technologies suchas the indium-tin oxide and silicon nitride film should beused to annihilate the surface defects and dangling bondsresulted from chemical etching process reduce the surfacerecombination velocity and finally improve the VOC andFF of the solar cell After these treatments the high per-formance SiNWs-based solar cell could be expected in thefuture

4 CONCLUSION

In summary large-area SiNW arrays were fabricated onSi surface by a facile silver-catalyzed chemical etchingprocess The as-synthesized SiNW arrays show excellentantireflection property with average reflection loss lowerthan 401 within range of 200ndash1000 nm wavelengthThe SiNWs-based solar cell shows a high conversionefficiency of 669 with VOC of sim558 mV and JSC ofsim2351 mAcm2 under 15 G illumination The high con-version efficiency of SiNWs-based solar cell is attributedto the low reflectance loss and excellent trapping effectof SiNWs for incident sunlight The optimization of phos-phorus diffusion and contact between the electrodes andSiNWs is still needed and high power conversion effi-ciency of SiNWs-based solar cell could be expected in thefuture

Acknowledgment The authors thank the financial sup-port of the MOE Tier II project of Singapore (Grant NoARC 1308)

References and Notes

1 N S Lewis Science 315 798 (2007)2 L Hu and G Chen Nano Lett 7 3249 (2007)3 M D Kelzenberg D B Turner-Evans B M Kayes M A Filier

M C Putnam N S Lewis and H A Atwater Nano Lett 8 710(2008)

4 L Tsakalakos J Balch J Fronheiser B A Korevaar O Sulimaand J Rand Appl Phys Lett 91 233117 (2007)

5 J Li H Yu S M Wong G Zhang X Sun P G Q Lo and D LKwong Appl Phys Lett 95 033102 (2009)

6 J Zhu Z Yu G F Burkhard C-M Hsu S T Connor Y XuQ Wang M McGehee S Fan and Y Cui Nano Lett 9 279 (2008)

7 J Goldberger A I Hochbaum R Fan and P D Yang Nano Lett6 973 (2006)

8 B Fuhrmann H S Leipner H R Hoche L Schubert P Wernerand U Gosele Nano Lett 5 2524 (2005)

9 C M Hsu S T Connor M X Tang and Y Cui Appl Phys Lett93 133109 (2008)

10 K Q Peng Y J Yan S P Gao and J Zhu Adv Mater 14 1164(2002)

11 K Peng Y Wu H Fang X Zhong Y Xu and J Zhu AngewChem Int Ed 44 2737 (2005)

12 K Peng Z Huang and J Zhu Adv Mater 16 73 (2004)13 C Li G J Fang S Sheng Z Q Chen J B Wang S Ma and

X Z Zhao Physica E 30 169 (2005)14 S Lee and E Lee J Nanosci Nanotechnol 7 3713 (2007)15 G H An D H Lee S H Kim B S Kim J H Yun and Y H

Choa J Am Ceram Soc 91 4148 (2008)16 B S Kim D H Lee S H Kim G H An K J Lee N V Myung

and Y H Choa J Am Ceram Soc 92 2415 (2009)

Received 30 December 2009 RevisedAccepted 30 August 2010

J Nanosci Nanotechnol 11 10539ndash10543 2011 10543

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

ranges from 50 nm to 200 nm are uniformly distributedover large-area indicating a facile process for fabricat-ing wafer-scale of uniform SiNW arrays with low costand high throughput This wafer-scale fabrication processthus offers a great convenience for exploring their poten-tial application in solar cell Cross-sectional SEM imageas shown in Figure 1(b) indicated that all SiNWs withlength of ca 450 nm are vertically aligned on Si waferimplying that the etching process is preferred along thedirection perpendicular to Si surface Linear etching tracesand track cavity could also be easily observed on SiNWssurface The morphology on top side of SiNWs is coarserthan that of bottom side which may be related to thelonger etching duration it suffered The silver film whichactually consists of nanoclusters plays the role of catalystduring chemical etching process The silicon under the sil-ver nanoclusters is preferentially etched away and finallygoing into chemical solution in form of H2SiF4 whilethe Si surface without silver nanoclusters covered remainunchanged With the continuous dissolution of silicon andthe simultaneous sinking of silver nanoclusters the sili-con surface without silver nanoclusters covered protrudesout from the network of silver nanocluster and conse-quently SiNW arrays forms along the direction perpendic-ular to the surface of Si substrate During the chemicaletching process some silver particles also intrude intoSiNWs and leave some cavities on the sidewall of SiNWsafter the silver catalyst was removed This maybe relatedwith the undulation of solution or the produced bubblesduring chemical etching process Figure 1(c) shows thecross-sectional SEM image of SiNWs with etching dura-tion of 80 sec It can be seen that the length of SiNWsincreases remarkably and reaches 11 m with the pro-longed chemical etching duration Some SiNW bundles orclumps could also be found on sample surface Figure 2shows the Raman spectra recorded from pristine Si wafer

Fig 2 Raman spectra recorded from the pristine Si wafer surface andSiNW arrays

and SiNW arrays The typical Raman band at 5202 cmminus1

corresponds to the first-order transverse optical phononmode of Si surface Compared with that of pristine Siwafer the first-order Raman peak of SiNWs at 5189 cmminus1

shows a dramatic asymmetric broadening and lower down-shift implying that SiNWs still remain its original crys-talline structure The asymmetry and lower downshift ismainly attributed to the phonon quantum confinementeffect and the possible strain resulted from chemical etch-ing process13 According to quantum theory when thecrystalline size decreases momentum conservation will berelaxed (q = 0) and Raman-active mode will not be limitedto being at the center of the Brillouin zone The smallerthe crystalline grain the larger the frequency shift andthe more asymmetric and the broader the peak becomesThis phenomenon has been confirmed by previousreport13

The reduction of reflectance loss is one of the mostimportant factors for high efficiency solar cell Thenanoporous structure and black color of the as-preparedsamples imply their possible antireflection properties andapplications in solar cell Figure 3 demonstrates thereflectance spectra recorded from the Si wafer coveredwith different lengths of SiNW arrays Compare with thatof pristine Si wafer the presence of SiNW arrays effec-tively reduces the optical reflection loss of Si wafer Theaverage reflectance loss of SiNW arrays with length ofsim450 nm is sim401 over the range of 200ndash1000 nmwavelength which is far lower than that of pristine Siwafer and also the Si pyramidal structure as well asthe porous Si nanostrucures14ndash16 The reflectance loss ofSi surface could be further decrease to 298 as thelength of SiNWs increase to 11 m The remarkable lowreflectance loss of SiNWs covered Si wafer may be relatedwith the ultrahigh specific surface area and the typical sub-wavelength structure of SiNW arrays

Fig 3 Reflectance spectra of Si wafer and SiNW arrays with differentlengths

J Nanosci Nanotechnol 11 10539ndash10543 2011 10541

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Facile Fabrication of Si Nanowire Arrays for Solar Cell Application Li and Tay

According to the aforementioned results the as-synthesized SiNW arrays demonstrate the excellent antire-flection property and possess several advantages overthe other surface texture structures This facial pro-cess enables us to fabricate wafer-scale SiNW arrays atroom temperature with low cost and high throughputNo rigorous experimental conditions such as high tem-perature hazardous silicon precursor or complex instru-ment are required in our experiment This will pave theway to explore the possible application of SiNWs in highefficiency solar cell With this in mind here we integratethis SiNW arrays into planar Si p-n junction wafer andexplore the possible photovoltaic applications of SiNWson conventional solar cell The main fabrication processof SiNWs-based solar cell is shown in Scheme 1 anddescribed in experimental section To prevent the possi-ble chemical contamination to diffusion furnace in thisstrategy p-n junction is generated on Si wafer prior tochemical etching process By synthetically consideringthe diffusion depth of phosphorous and the impact ofthe length of SiNWs on reflectance loss the length ofSiNWs is selected as 450 nm and corresponding chemical

Fig 4 (a) Overall-view of the SiNWs-based solar cell recorded by adigital camera and (b) cross-sectional SEM image of the front side elec-trode area

etching duration is ca 40 sec Figure 4(a) shows theoptical image of the as-prepared SiNWs-based solar cellrecorded by a digital camera The black areas consist ofSiNW arrays while the block lines stand for TiPdAgfilms front grid electrode The morphology of the blackregion is same as that in Figure 1(a) Figure 4(b) demon-strates the cross-sectional SEM image of the block linearea As can be seen that TiPdAg films tightly compacton the top of SiNWs Some parts of TiPdAg films intrudethe interspace of SiNWs and paste on the side wall ofSiNWs This is believed to be beneficial for increasingthe contact area between SiNWs and electrode to someextent and improving the electron collection efficiencyFigure 5 shows the current densityndashvoltage (JndashV charac-teristic curves of the as-prepared SiNWs-based solar cellin darkness and under AM 15 G illumination conditionThe solar cell exhibits a clear diode behavior in darknesswith turn-on voltage of sim03 V and rectification ratio onthe order of 102 at 02 V Only a negligible reverse leakagecurrent was found at reverse voltage implying the goodquality of the SiNWs-based device and good controlla-bility of our fabrication process When illuminated underAM 15 G conditions SiNWs-based solar cell (filled cir-cle) exhibits an open circuit voltage (VOC of sim558 mVand a short circuit current density (JSC of sim2351 mAcm2

without using any extra antireflection layer and surfacepassivation technique Filled factor (FF) and power con-version efficiency ( of the SiNWs-based solar cell werecalculate to be 051 and 669 respectively Comparedwith that of pristine Si wafer-based solar cell (unfilledcircle) the conversion efficiency of SiNWs-based solarcell increases remarkably which is closely related withthe excellent antireflection and trapping effect of SiNWsfor incident sunlight and thus the high short circuit cur-rent density of fabricated solar cell It should be noted

Fig 5 JndashV characteristic curves of solar cells with and without SiNWarrays covered in darkness and under 15 AM illumination

10542 J Nanosci Nanotechnol 11 10539ndash10543 2011

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

that both the VOC and FF of SiNWs-based solar cell isstill lower than that of standard single-crystalline Si-basedsolar cell indicating that the optimization of phosphorusdiffusion and contact between electrodes and SiNWs isstill needed Some surface passivation technologies suchas the indium-tin oxide and silicon nitride film should beused to annihilate the surface defects and dangling bondsresulted from chemical etching process reduce the surfacerecombination velocity and finally improve the VOC andFF of the solar cell After these treatments the high per-formance SiNWs-based solar cell could be expected in thefuture

4 CONCLUSION

In summary large-area SiNW arrays were fabricated onSi surface by a facile silver-catalyzed chemical etchingprocess The as-synthesized SiNW arrays show excellentantireflection property with average reflection loss lowerthan 401 within range of 200ndash1000 nm wavelengthThe SiNWs-based solar cell shows a high conversionefficiency of 669 with VOC of sim558 mV and JSC ofsim2351 mAcm2 under 15 G illumination The high con-version efficiency of SiNWs-based solar cell is attributedto the low reflectance loss and excellent trapping effectof SiNWs for incident sunlight The optimization of phos-phorus diffusion and contact between the electrodes andSiNWs is still needed and high power conversion effi-ciency of SiNWs-based solar cell could be expected in thefuture

Acknowledgment The authors thank the financial sup-port of the MOE Tier II project of Singapore (Grant NoARC 1308)

References and Notes

1 N S Lewis Science 315 798 (2007)2 L Hu and G Chen Nano Lett 7 3249 (2007)3 M D Kelzenberg D B Turner-Evans B M Kayes M A Filier

M C Putnam N S Lewis and H A Atwater Nano Lett 8 710(2008)

4 L Tsakalakos J Balch J Fronheiser B A Korevaar O Sulimaand J Rand Appl Phys Lett 91 233117 (2007)

5 J Li H Yu S M Wong G Zhang X Sun P G Q Lo and D LKwong Appl Phys Lett 95 033102 (2009)

6 J Zhu Z Yu G F Burkhard C-M Hsu S T Connor Y XuQ Wang M McGehee S Fan and Y Cui Nano Lett 9 279 (2008)

7 J Goldberger A I Hochbaum R Fan and P D Yang Nano Lett6 973 (2006)

8 B Fuhrmann H S Leipner H R Hoche L Schubert P Wernerand U Gosele Nano Lett 5 2524 (2005)

9 C M Hsu S T Connor M X Tang and Y Cui Appl Phys Lett93 133109 (2008)

10 K Q Peng Y J Yan S P Gao and J Zhu Adv Mater 14 1164(2002)

11 K Peng Y Wu H Fang X Zhong Y Xu and J Zhu AngewChem Int Ed 44 2737 (2005)

12 K Peng Z Huang and J Zhu Adv Mater 16 73 (2004)13 C Li G J Fang S Sheng Z Q Chen J B Wang S Ma and

X Z Zhao Physica E 30 169 (2005)14 S Lee and E Lee J Nanosci Nanotechnol 7 3713 (2007)15 G H An D H Lee S H Kim B S Kim J H Yun and Y H

Choa J Am Ceram Soc 91 4148 (2008)16 B S Kim D H Lee S H Kim G H An K J Lee N V Myung

and Y H Choa J Am Ceram Soc 92 2415 (2009)

Received 30 December 2009 RevisedAccepted 30 August 2010

J Nanosci Nanotechnol 11 10539ndash10543 2011 10543

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Facile Fabrication of Si Nanowire Arrays for Solar Cell Application Li and Tay

According to the aforementioned results the as-synthesized SiNW arrays demonstrate the excellent antire-flection property and possess several advantages overthe other surface texture structures This facial pro-cess enables us to fabricate wafer-scale SiNW arrays atroom temperature with low cost and high throughputNo rigorous experimental conditions such as high tem-perature hazardous silicon precursor or complex instru-ment are required in our experiment This will pave theway to explore the possible application of SiNWs in highefficiency solar cell With this in mind here we integratethis SiNW arrays into planar Si p-n junction wafer andexplore the possible photovoltaic applications of SiNWson conventional solar cell The main fabrication processof SiNWs-based solar cell is shown in Scheme 1 anddescribed in experimental section To prevent the possi-ble chemical contamination to diffusion furnace in thisstrategy p-n junction is generated on Si wafer prior tochemical etching process By synthetically consideringthe diffusion depth of phosphorous and the impact ofthe length of SiNWs on reflectance loss the length ofSiNWs is selected as 450 nm and corresponding chemical

Fig 4 (a) Overall-view of the SiNWs-based solar cell recorded by adigital camera and (b) cross-sectional SEM image of the front side elec-trode area

etching duration is ca 40 sec Figure 4(a) shows theoptical image of the as-prepared SiNWs-based solar cellrecorded by a digital camera The black areas consist ofSiNW arrays while the block lines stand for TiPdAgfilms front grid electrode The morphology of the blackregion is same as that in Figure 1(a) Figure 4(b) demon-strates the cross-sectional SEM image of the block linearea As can be seen that TiPdAg films tightly compacton the top of SiNWs Some parts of TiPdAg films intrudethe interspace of SiNWs and paste on the side wall ofSiNWs This is believed to be beneficial for increasingthe contact area between SiNWs and electrode to someextent and improving the electron collection efficiencyFigure 5 shows the current densityndashvoltage (JndashV charac-teristic curves of the as-prepared SiNWs-based solar cellin darkness and under AM 15 G illumination conditionThe solar cell exhibits a clear diode behavior in darknesswith turn-on voltage of sim03 V and rectification ratio onthe order of 102 at 02 V Only a negligible reverse leakagecurrent was found at reverse voltage implying the goodquality of the SiNWs-based device and good controlla-bility of our fabrication process When illuminated underAM 15 G conditions SiNWs-based solar cell (filled cir-cle) exhibits an open circuit voltage (VOC of sim558 mVand a short circuit current density (JSC of sim2351 mAcm2

without using any extra antireflection layer and surfacepassivation technique Filled factor (FF) and power con-version efficiency ( of the SiNWs-based solar cell werecalculate to be 051 and 669 respectively Comparedwith that of pristine Si wafer-based solar cell (unfilledcircle) the conversion efficiency of SiNWs-based solarcell increases remarkably which is closely related withthe excellent antireflection and trapping effect of SiNWsfor incident sunlight and thus the high short circuit cur-rent density of fabricated solar cell It should be noted

Fig 5 JndashV characteristic curves of solar cells with and without SiNWarrays covered in darkness and under 15 AM illumination

10542 J Nanosci Nanotechnol 11 10539ndash10543 2011

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

that both the VOC and FF of SiNWs-based solar cell isstill lower than that of standard single-crystalline Si-basedsolar cell indicating that the optimization of phosphorusdiffusion and contact between electrodes and SiNWs isstill needed Some surface passivation technologies suchas the indium-tin oxide and silicon nitride film should beused to annihilate the surface defects and dangling bondsresulted from chemical etching process reduce the surfacerecombination velocity and finally improve the VOC andFF of the solar cell After these treatments the high per-formance SiNWs-based solar cell could be expected in thefuture

4 CONCLUSION

In summary large-area SiNW arrays were fabricated onSi surface by a facile silver-catalyzed chemical etchingprocess The as-synthesized SiNW arrays show excellentantireflection property with average reflection loss lowerthan 401 within range of 200ndash1000 nm wavelengthThe SiNWs-based solar cell shows a high conversionefficiency of 669 with VOC of sim558 mV and JSC ofsim2351 mAcm2 under 15 G illumination The high con-version efficiency of SiNWs-based solar cell is attributedto the low reflectance loss and excellent trapping effectof SiNWs for incident sunlight The optimization of phos-phorus diffusion and contact between the electrodes andSiNWs is still needed and high power conversion effi-ciency of SiNWs-based solar cell could be expected in thefuture

Acknowledgment The authors thank the financial sup-port of the MOE Tier II project of Singapore (Grant NoARC 1308)

References and Notes

1 N S Lewis Science 315 798 (2007)2 L Hu and G Chen Nano Lett 7 3249 (2007)3 M D Kelzenberg D B Turner-Evans B M Kayes M A Filier

M C Putnam N S Lewis and H A Atwater Nano Lett 8 710(2008)

4 L Tsakalakos J Balch J Fronheiser B A Korevaar O Sulimaand J Rand Appl Phys Lett 91 233117 (2007)

5 J Li H Yu S M Wong G Zhang X Sun P G Q Lo and D LKwong Appl Phys Lett 95 033102 (2009)

6 J Zhu Z Yu G F Burkhard C-M Hsu S T Connor Y XuQ Wang M McGehee S Fan and Y Cui Nano Lett 9 279 (2008)

7 J Goldberger A I Hochbaum R Fan and P D Yang Nano Lett6 973 (2006)

8 B Fuhrmann H S Leipner H R Hoche L Schubert P Wernerand U Gosele Nano Lett 5 2524 (2005)

9 C M Hsu S T Connor M X Tang and Y Cui Appl Phys Lett93 133109 (2008)

10 K Q Peng Y J Yan S P Gao and J Zhu Adv Mater 14 1164(2002)

11 K Peng Y Wu H Fang X Zhong Y Xu and J Zhu AngewChem Int Ed 44 2737 (2005)

12 K Peng Z Huang and J Zhu Adv Mater 16 73 (2004)13 C Li G J Fang S Sheng Z Q Chen J B Wang S Ma and

X Z Zhao Physica E 30 169 (2005)14 S Lee and E Lee J Nanosci Nanotechnol 7 3713 (2007)15 G H An D H Lee S H Kim B S Kim J H Yun and Y H

Choa J Am Ceram Soc 91 4148 (2008)16 B S Kim D H Lee S H Kim G H An K J Lee N V Myung

and Y H Choa J Am Ceram Soc 92 2415 (2009)

Received 30 December 2009 RevisedAccepted 30 August 2010

J Nanosci Nanotechnol 11 10539ndash10543 2011 10543

Delivered by Ingenta toNanyang Technological University

IP 1556944Tue 03 Jul 2012 110142

RESEARCH

ARTIC

LE

Li and Tay Facile Fabrication of Si Nanowire Arrays for Solar Cell Application

that both the VOC and FF of SiNWs-based solar cell isstill lower than that of standard single-crystalline Si-basedsolar cell indicating that the optimization of phosphorusdiffusion and contact between electrodes and SiNWs isstill needed Some surface passivation technologies suchas the indium-tin oxide and silicon nitride film should beused to annihilate the surface defects and dangling bondsresulted from chemical etching process reduce the surfacerecombination velocity and finally improve the VOC andFF of the solar cell After these treatments the high per-formance SiNWs-based solar cell could be expected in thefuture

4 CONCLUSION

In summary large-area SiNW arrays were fabricated onSi surface by a facile silver-catalyzed chemical etchingprocess The as-synthesized SiNW arrays show excellentantireflection property with average reflection loss lowerthan 401 within range of 200ndash1000 nm wavelengthThe SiNWs-based solar cell shows a high conversionefficiency of 669 with VOC of sim558 mV and JSC ofsim2351 mAcm2 under 15 G illumination The high con-version efficiency of SiNWs-based solar cell is attributedto the low reflectance loss and excellent trapping effectof SiNWs for incident sunlight The optimization of phos-phorus diffusion and contact between the electrodes andSiNWs is still needed and high power conversion effi-ciency of SiNWs-based solar cell could be expected in thefuture

Acknowledgment The authors thank the financial sup-port of the MOE Tier II project of Singapore (Grant NoARC 1308)

References and Notes

1 N S Lewis Science 315 798 (2007)2 L Hu and G Chen Nano Lett 7 3249 (2007)3 M D Kelzenberg D B Turner-Evans B M Kayes M A Filier

M C Putnam N S Lewis and H A Atwater Nano Lett 8 710(2008)

4 L Tsakalakos J Balch J Fronheiser B A Korevaar O Sulimaand J Rand Appl Phys Lett 91 233117 (2007)

5 J Li H Yu S M Wong G Zhang X Sun P G Q Lo and D LKwong Appl Phys Lett 95 033102 (2009)

6 J Zhu Z Yu G F Burkhard C-M Hsu S T Connor Y XuQ Wang M McGehee S Fan and Y Cui Nano Lett 9 279 (2008)

7 J Goldberger A I Hochbaum R Fan and P D Yang Nano Lett6 973 (2006)

8 B Fuhrmann H S Leipner H R Hoche L Schubert P Wernerand U Gosele Nano Lett 5 2524 (2005)

9 C M Hsu S T Connor M X Tang and Y Cui Appl Phys Lett93 133109 (2008)

10 K Q Peng Y J Yan S P Gao and J Zhu Adv Mater 14 1164(2002)

11 K Peng Y Wu H Fang X Zhong Y Xu and J Zhu AngewChem Int Ed 44 2737 (2005)

12 K Peng Z Huang and J Zhu Adv Mater 16 73 (2004)13 C Li G J Fang S Sheng Z Q Chen J B Wang S Ma and

X Z Zhao Physica E 30 169 (2005)14 S Lee and E Lee J Nanosci Nanotechnol 7 3713 (2007)15 G H An D H Lee S H Kim B S Kim J H Yun and Y H

Choa J Am Ceram Soc 91 4148 (2008)16 B S Kim D H Lee S H Kim G H An K J Lee N V Myung

and Y H Choa J Am Ceram Soc 92 2415 (2009)

Received 30 December 2009 RevisedAccepted 30 August 2010

J Nanosci Nanotechnol 11 10539ndash10543 2011 10543