Introduction. Conference paper

Transcription

1 DOI /pc Pure Appl. Chem. 214; op Conference pper Fei Xiu, Ho Lin, Ming Fng, Guof Dong, Senpo Yip nd Johnny C. Ho* Fbriction nd enhnced light-trpping properties of three-dimensionl silicon nnostructures for photovoltic pplictions Abstrct: In order to mke photovoltics n economiclly vible energy solution, next-genertion solr cells with higher energy conversion efficiencies nd lower costs re urgently desired. Among mny possible solutions, three-dimensionl (3D) silicon nnostructures with excellent light-trpping properties re one of the promising cndidtes nd hve recently ttrcted considerble ttention for cost-effective photovoltic pplictions. This is becuse their enhnced light-trpping chrcteristics nd high crrier collection efficiencies cn enble the use of cheper nd thinner silicon mterils. In this review, recent developments in the controllble fbriction of 3D silicon nnostructures re summrized, followed by the investigtion of opticl properties on number of different nnostructures, including nnowires, nnopillrs, nnocones, nnopencils, nd nnopyrmids, etc. Even though nnostructures with rdil p-n junction demonstrte excellent photon mngement properties nd enhnced photo-crrier collection efficiencies, the photovoltic performnce of nnostructurebsed solr cells is still significntly limited due to the high surfce recombintion effect, which is induced by high-density surfce defects s well s the lrge surfce re in high-spect-rtio nnostructures. In this regrd, vrious pproches in reducing the surfce recombintion re discussed nd n overll geometricl considertion of both light-trpping nd recombintion effects to yield the best photovoltic properties re emphsized. Keywords: nnostructures, NMS-IX, photovoltics, silicon. *Corresponding uthor: Johnny C. Ho, Deprtment of Physics nd Mterils Science, City University of Hong Kong, 83 Tt Chee Avenue, Kowloon, Hong Kong SAR, Chin, e-mil: johnnyho@cityu.edu.hk Fei Xiu nd Senpo Yip: Deprtment of Physics nd Mterils Science, City University of Hong Kong, 83 Tt Chee Avenue, Kowloon, Hong Kong SAR, Chin; nd Shenzhen Reserch Institute, City University of Hong Kong, Shenzhen, Chin Ho Lin, Ming Fng nd Guof Dong: Deprtment of Physics nd Mterils Science, City University of Hong Kong, 83 Tt Chee Avenue, Kowloon, Hong Kong SAR, Chin Johnny C. Ho: Shenzhen Reserch Institute, City University of Hong Kong, Shenzhen, Chin; nd Centre for Functionl Photonics (CFP), City University of Hong Kong, 83 Tt Chee Avenue, Kowloon, Hong Kong SAR, Chin Article note: A collection of invited ppers bsed on presenttions t the 9 th Interntionl Conference on Novel Mterils nd their Synthesis (NMS-IX) nd the 23 rd Interntionl Symposium on Fine Chemistry nd Functionl Polymers (FCFP-XXIII), Shnghi, Chin, October 213. Introduction Over the pst decdes, highly efficient solr cells hve become one of the extensive reserch topics in vrious science nd engineering disciplines [1 6] since solr energy is by fr the most bundnt renewble clen-energy resource. For exmple, mong mny renewble lterntives, solr energy cn provide more energy in 1 h to the erth thn ll of the energy consumed by humns in 1 yer [7]. Until now, crystlline silicon (Si) remins the mjor workhorse of photovoltic (PV) mterils becuse of its well-developed infrstructure in electronic nd semiconductor industries. However, the lrge-scle implementtion of crystlline Si solr cells is still not tht economiclly ttrctive s result of the high processing nd mterils costs, coming from the high-temperture fbriction 214 IUPAC & De Gruyter

2 2 F. Xiu et l.: 3D silicon nnostructures for photovoltics steps involved nd the required thickness of silicon substrtes for the effective bsorption of solr spectrum due to its indirect bndgp of 1.1 ev. In order to mximize the development of Si PVs nd mke it n economiclly vible energy solution, it is necessry to further improve the energy conversion efficiency s well s to lower the mteril nd mnufcturing costs. In this regrd, new genertion of highly efficient, low-cost PV mterils nd novel device structures hs been recently developed to ddress the bove issue [8 12]. In prticulr, n ssortment of three-dimensionl (3D) Si nnostructures hs been fbricted with vrious processing techniques. These nnostructures re of the gret interest for PV pplictions owing to the enhnced light trpping properties plus the high crrier collection efficiency by seprting the pth for light bsorption nd crrier collection, utilizing the rdil core-shell p-n junction structure to increse the junction re [8, 13]. It hs been reported tht by controlling light t the nnometer scle using these nnostructures, development of solr cells with efficiencies up to the rnge of 5 7 % cn be reched theoreticlly, breking the Shockley-Queisser limit [14]. At the sme time, the mteril cost of these nnostructured PV devices cn be significntly reduced due to the enhnced light bsorption nd thus employing less or inexpensive silicon substrtes, without dding much to the processing cost [15]. While most studies show tht the PV performnce of nnostructured solr cells is gretly limited by the high crrier recombintion rte cused by the high density surfce or interfce defects in nnostructures with the lrge surfce re, in spite of the strong opticl enhncement [9, 16 18]. To suppress this crrier recombintion, it would be criticl to tke proper surfce tretments, such s depositing pproprite surfce pssivtion lyers (e.g., silicon oxide nd hydrogented silicon nitride) [19 23]. Recently, Wng et l. [18] hve found chemicl polishing etching (CPE) tretment s nother effective wy to reduce the surfce recombintion in high-spect-rtio nnostructures. By decresing the surfce defects nd controlling the finl morphologies of the s-mde structures with CPE, the solr cells fbricted with hierrchicl structures exhibit the impressively high efficiency of % [18]. Therefore, ll these nnostructures demonstrte the technologicl potency of being promising cndidtes for the next genertion solr cells with higher energy conversion efficiencies nd lower costs. In this report, we im to provide comprehensive review of recent progress on severl mjor ctegories of Si nnostructures with excellent photon mngement properties for PV pplictions. These nnostructures include nnowires (NWs), nnopillrs (NPs), nnorods (NRs), nnopencils (NPLs), nnopyrmids (NPMs), nnocones (NCs) nd nnodomes (NDMs), etc. We will begin nd emphsize with brief survey of recent developments in the fbriction techniques for vrious Si nnostructure formtions. Then, opticl properties of the forementioned nnostructures will be investigted in detils. Specil ttention will be pid to the different light trpping chrcteristics of these nnostructures, suggesting the pproprite geometricl nd morphologicl design guidelines for the efficient light hrvesting. Moreover, nnostructure-bsed PV pplictions will be discussed thoroughly followed by the novel pproches to reduce the surfce recombintion of crriers in the high-spect-rtio nnostructures. All these would deliver n overll geometricl considertion of both light trpping nd surfce recombintion effects in order to yield the best PV properties. Fbriction pproches for the formtion of three-dimensionl silicon nnostructures Controllble fbriction of 3D Si nnostructures is crucil for their light hrvesting properties nd corresponding PV pplictions. To control vrious geometricl nd morphologicl prmeters of the nnostructures, numerous methods including both top-down or bottom-up pproches hve been developed, such s vporliquid-solid (VLS) growth, rective ion etching (RIE), metl-ssisted chemicl etching (McEtch) nd so on. Vpor-liquid-solid (VLS) growth One of the most effective wys to chieve the bottom-up fbriction of Si nnowires cn be done by the metl-ctlyzed vpor-liquid-solid (VLS) growth technique [24]. In this pproch, s shown in Fig. 1, metl

3 F. Xiu et l.: 3D silicon nnostructures for photovoltics 3 Vpor b c Metl ctlysts Alloy liquid Nnowire Msk Film to be etched (Si) I II III Etching Temperture ( C) L Alloying I Nucletion II Growth III 36 C Polymer deposition Etching Nnosphere monolyer formtion Sphere etching nd metl mesh deposition 2 Au Ge Weight % Ge Metl mesh removl to obtin verticl NPL rrys Nnosphere removl nd metl ssisted ctlytic etching Fig. 1 Schemtic illustrtion of () the VLS NW growth mechnism (top) nd the conventionl Au-Ge binry phse digrm to demonstrte the composition nd phse evolution during the NW growth process (bottom), (b) individul steps in the deep rective ion etching (DRIE) process nd (c) the nnosphere lithogrphy followed by the metl-ssisted chemicl etching (McEtch) processes. Reproduced from Refs. [25, 53, 59] with permission from The Americn Chemicl Society, The Americn Institute of Physics nd The Royl Society of Chemistry. nnoprticles (e.g., Au) cn ctlyze the growth of nnowires by forming liquid metl droplet t n elevted temperture through which the growth toms re trnsported to the crystllizing interfce from the gs phse with silicon contining species [25]. This is known s the chemicl vpor deposition (CVD) [26 28]. Specificlly, silne (SiH 4 ) or tetrchlorosilne (SiCl 4 ) is usully used s the gs precursor providing Si constituents. Once the vporized growth species rech the substrte surfce tht is covered with metl nnoprticles nd the substrte temperture is held bove the eutectic temperture of the Au-Si lloy, the liquid lloy droplet cn be supersturted with silicon toms [29]. Then, Si NWs cn be grown by the precipittion of Si from the ctlytic lloys. Among severl methods for relizing forest of Si nnostructure, VLS technique is considered ttrctive owing to its process simplicity nd potentil sclbility suitble for lrge-re device fbriction [3]. There hve been severl in-depth studies to fbricte rdil p-n junction solr cells bsed on these VLS-grown NWs nd their PV properties re investigted ccordingly [16, 31, 32]. Menwhile, for the VLS grown NWs, it is found tht even the smllest concentrtion of ctlytic Au tom cn still reside s dopnts or deep-level energy trps in Si, which would hve detriment influence on the properties of Si NWs [33]. The corresponding overll energy conversion efficiency of fbricted solr cells is then seriously limited by the crrier recombintion within the depletion region or t the surfce of VLS grown NWs due to this Au contmintion. For instnce, Gunwn et l. reported the fbriction nd chrcteriztion of core-shell rdil p-n junction Si NW solr cells, with the NWs synthesized by the VLS method using SiH 4 s source gs while PH 3 nd B 2 H 6 s the gs-phse dopnt precursors [16]. The short-circuit current density (Jsc) is improved which is ttributed to the enhnced opticl effects of low surfce reflectnce nd light trpping in NWs within the device. However, the excessive Au residul trpped in the NWs s well s the excess surfce recombintion due to the incresed NW surfce re ultimtely limit the minority crrier lifetime nd cell performnces in spite of the strong opticl enhncement, indicting tht the mount of metl ctlyst incorported in the VLS process hs to be minimized. In this regrd, NWs produced by the non-ctlytic growth mechnism seem to suit better for the PV pplictions s there is no involvement of metl impurities [33, 34]. Much progress hs been mde in the ctlystfree growth of semiconductor NWs chieved by vrious methods, including lser bltion [35], physicl vpor deposition [36], chemicl vpor deposition [37] nd vpor phse epitxy, etc. [38, 39]. Two min steps re typiclly reported in this non-ctlytic growth [34]: (1) the nucletion of seed nnoprticles controlled by the

4 4 F. Xiu et l.: 3D silicon nnostructures for photovoltics thermodynmic size limit nd (2) the subsequent nisotropic NW growth without the ctlyst. For exmple, the non-ctlytic growth of Si NWs hs been dominted by the oxide-ssisted growth (OAG) pproch in which evportion nd deposition of oxide vpor cn form crystlline NWs with the morphous oxide shells [4 42]. In this method, oxides, insted of metls, ply n importnt role in inducing the nucletion nd growth of NWs [43 45], thus llowing the lrge-quntity production of Si NWs free of metl impurities [41]. However, this technique requires the growth t high tempertures ( > 85 C), which my restrict the prcticl implementtion for scle-up processes. Furthermore, the control of structurl defects, NW physicl dimensions nd in situ doping my not be s vible s in the metl-ctlyzed CVD method [46]. In ny cse, Kim et l. introduced non-ctlytic CVD growth of single-crystlline Si NWs by nucletion of nnocrystlline seeds on the rective oxide surfce nd subsequently chieved the nisotropic growth using the low-pressure CVD process t lower temperture ( < 65 C) [46]. The NW dimeter nd doping level could be well controlled by djusting the growth conditions, such s the growth temperture nd rtio of precursor prtil pressures. Deep rective ion etching Another pproch for relizing nnoscle verticl Si structures relies on the top-down mnufcturing, nmely high-spect-rtio nisotropic etching of Si substrtes. Deep rective ion etching (DIRE) is regrded s powerful method to fbricte verticl Si nnostructures mong vrious top-down methods [47 5]. At present, the nisotropic DRIE of Si hs lredy been mture process technology which is used for creting 3D Si structures for vrious pplictions [51, 52]. As depicted in Fig. 1b, typicl DRIE process involves the use of high-density plsm nd comprises sequence of lternting steps (no more thn few seconds long) of Si etching nd polymer deposition to protect the lredy-crved fetures from further lterl etching [53]. It is observed tht s the cycles go on, the micromchining proceeds vi series of bites into the silicon nd sclloping (or ripple ) morphology is esily to be formed on the sidewlls [51, 52]. When the feture size nd scllop size become comprble, sclloping cn become serious problem for the nnostructure [49, 5]. In order to ensure smooth sidewlls t the nnoscle, s demonstrted by Fu et l., the process conditions of DRIE hve to be well controlled to minimize the ripple effect nd this cn be chieved by optimizing the source power, bis power, gs flow rte, flow cycle time, substrte temperture, nd chmber pressure [54]. They lso illustrted n optimized DRIE process for the successful fbriction of lrge-re Si nnostructures, including nnopillrs, nnowires nd nnowlls with smooth sidewlls, emphsizing the gret potentil of DRIE for ultrsmll, lrge-re nd 3D nnofbriction. Besides the ripple effect, RIE-relted surfce contmintion nd substrte displcement dmge re lso involved: (1) surfce residues such s hlocrbon films; (2) impurity implnttion or penetrtion such s hydrogen diffusion; (3) lttice dmge due to energetic ions or rdition while het tretments cn be utilized to nnel out this dmge; (4) hevy-metl contmintion including the rector wll constituents diffusing redily into Si; (5) mobile ion contmintion such s sodium from Teflon electrodes [55]. All these could subsequently leds to the limittion in PV pplictions due to the defect-rich surfce nd residul contmintion on the fbricted structures. Wet metl-ssisted chemicl etching Recently, metl-ssisted chemicl etching (McEtch) [15, 56 59], wet but directionl etching method, hs ttrcted incresing ttention s simple nd low-cost technique for fbricting vrious Si nnostructures. This technique cn come with the bility to control vrious importnt prmeters for NW bsed PV devices, including the NW dimeter, length, orienttion, cross-sectionl shpe, doping type nd doping level, etc. Compred with dry etching processes, McEtch is free of surfce dmge becuse there re no high-energy ions involved. McEtch lso does not induce metl contmintion s hppened in the VLS method since it tkes plce t room temperture nd the metl ctlyst cnnot be incorported in Si [15]. In ddition, it is more flexible technique nd cn be used to mke higher surfce-to-volume rtio structures [47, 6], while

5 F. Xiu et l.: 3D silicon nnostructures for photovoltics 5 VLS-bsed method cn only be used to grow wires with circulr cross-sections. Furthermore, McEtch is sclble to the wfer scle esily with the lterl resolution s high s 1 nm through vrious metl ptterning schemes such s the superionic solid stte stmping [61], colloidl lithogrphy [62, 63], electron bem lithogrphy [64], or soft lithogrphy [65]. For exmple, s given in Fig. 1c, we hve ttined the Si nnopillr (NP) rrys utilizing the colloidl or nnosphere lithogrphy followed by the McEtch process [59]. In brief, monodispersed polystyrene (PS) nnospheres were ssembled into close-pck monolyer on Si (1) substrtes employing the Lngmuir-Blodgett (LB) method. The substrtes were then pre-treted with controlled oxygen plsm to result hydrophilic surfces for the enbling of uniform nnosphere coting. The dimension nd distnce pitch of the spheres could be mnipulted by the subsequent oxygen plsm etching. These obtined spheres were used s the msk nd 1.5/2 nm thick Ti/Au metl mesh ws thermlly evported. As shown in Fig. 2 nd e, Si NPs with the controllble dimeter nd periodicity could be relized vi the McEtch in HF/H 2 O 2 solution. More importntly, exploiting these NP templtes, we hve developed n nisotropic wet-chemistry only fbriction scheme for the formtion of lrge-scle, low-cost, highly regulr, single-crystlline nd high spect rtio Si nnostructures with different geometricl morphologies, rnging from nnopillr, nnorod, nnopencil nd nnocone rrys for the efficient light trpping (Fig. 2b d nd f h) [59]. In this work, once the NP templtes were obtined, they were treted with mixture of AgNO 3, HF, nd HNO 3 or H 2 O 2 for two functions. This chemicl mixture would first led to the selective deposition of Ag clusters t the NP tips nd simultneously perform Si etching. The subsequent removl of Ag clusters by HNO 3 wshing gve truncted NP tips nd this deposition/removl processes were repeted with multiple times to provide different morphologicl NP rrys with well-regulted chemicl conditions. Notbly, since Ag nnoclusters were observed to be preferentilly formed round the rim of the tip, this indicted tht Si toms in the rim region re more rective thn those in the core region. This difference in the rectivity is minly ttributed to the different crystllinity of Si in different regions, where this rim edge hs s well been observed in other mteril systems [66]. Therefore, the site-specific rectivity of Si is the key for this nisotropic etching. In ddition, the etching kinetics could be tilored by vrying the concentrtion nd the components of the [AgNO 3 + HF + HNO 3 ] mixture to chieve different morphologies. Specificlly, nnocones could be obtined by the ddition of extr AgNO 3 to HF, in which the extr AgNO 3 speeded up the formtion of Ag nnoclusters [59]. It is lso noted tht the use of the [AgNO 3 + HF + H 2 O 2 ] system yielded nnorods with the dome hed terminl, which ws ccredited to the existence of Ag(nnocluster)-ctlyzed decomposition of H 2 O 2 tht chnges the oxidtion kinetics of H 2 O 2. The detiled mechnistic processes re summrized in Fig. 3 for the fbriction of nnorods, nnopencils nd nnocones, respectively. Fig. 2 SEM imges of the fbricted high-spect (, e) nnopillr, (b, f) nnorod, (c, g) nnopencil, (d, h) nnocone rrys for the pitch of.6 nd 1.27 mm, respectively, vi the McEtch processes. Reproduced from Ref. [59] with permission from The Royl Society of Chemistry.

6 6 F. Xiu et l.: 3D silicon nnostructures for photovoltics Rim region: low crystllinity Core region: high crystllinity b 1 c 1 Wet chemicl etchnt HNO 3 wsh Nnopillrs Anisotropic Ag deposition Ag nnocluster removl Nnorods Etchnt=AgNO 3 +HF+H 2 O 2 c 2 b 2 2nd Ag deposition Nnopencils Etchnt=AgNO 3 +HF+HNO 3...b n, c n 2 nd HNO 3 wsh Nnocones Etchnt=incresed AgNO 3 +HF Fig. 3 Schemtic illustrtion of the formtion mechnism for vrious Si nnorrys with different geometricl morphologies. Reproduced from Ref. [59] with permission from The Royl Society of Chemistry. Light-trpping properties of vrious silicon nnostructures Nnowires nd nnopillrs The nti-reflection properties of silicon nnowire rrys (NWAs) hve been extensively studied theoreticlly s well s experimentlly [8, 58, 67]. As efficient bsorber mterils, NWAs cn prolong the opticl pth length of the incident light by incresing the frequency of the reflected wves bouncing between wires, reducing the opportunity for the light to escpe from the surfce. By designing NWAs with different geometries (i.e., dimeter, length nd pitch), Grnett et l. hve demonstrted the tunble light trpping properties with their NWAs [67]. Figure 4 gives the tilted cross-sectionl SEM view of ordered silicon NWAs fbricted by the DRIE process [67]. The pitch nd dimeter of the nnowires re controlled by the dimeter of msk (i.e., silic beds), while the length is determined by the etching time. Opticl mesurements re then performed on the Si substrtes before nd fter the NW fbriction, which illustrte tht NWAs yield the lower reflection property over the entire spectrl rnge from 6 to 11 nm s compred with the plnr Si. Also, there is red shift regrding the frequency of the trnsmitted light tht is expected for strong light-trpping effect. Obviously, tunble nti-reflection cn be controlled by the length of NWs nd longer NWAs exhibit the superior nti-reflection property due to n increse of the effective pth length (Fig. 4b). On the other hnd, very low reflectivity ( < 2 %) hs been relized in the 3 6 nm wvelength rnge in the cse of 12 μm long Si wires, s demonstrted by Srivstv [59]. Si nnostructures with the very low reflectivity nd correspondingly high bsorption of visible (nd infrred) light is nmed s blck silicon. However, remrkble defect concentrtions re existing on the surfce of such Si NWs due to the fbriction processes. Combining with their reltively lrge surfce re, the high surfce recombintion is resulted, which ultimtely limits the corresponding energy conversion efficiency of nnotextured blck Si solr cells [68, 69].

7 F. Xiu et l.: 3D silicon nnostructures for photovoltics 7 Trnsmission (%) µm nnowires 2 µm nnowires Plnr Opticl model b Wvelength (nm) Fig. 4 () Tilted cross-sectionl SEM imge of n ordered silicon nnowire rdil p-n junction rry solr cell. (b) Trnsmission spectr of thin silicon window structures with 2 μm (green) nd 5 μm (blck) long nnowire nd with plnr surfce (red), with the blue curve corresponding to n opticl model for 7.5 μm thin silicon window. Reproduced from Ref. [67] with permission from The Americn Chemicl Society. Compred with NWAs, Si NPs re becoming more ttrctive becuse of their unique opticl properties, especilly for their less surfce recombintion due to the reltively smller surfce re, which mke them potentil cndidte for the PV pplictions [7, 71]. Yi et l. reported the fbriction of NP-bsed Si solr cells with tunble pillr dimeters nd height [72]. They demonstrted tht the lower reflectnce of these solr cells is obtined which cn be ttributed to the enhnced light trpping nd effective bsorption of the pillr structure, in contrst to the conventionl pyrmid surfce texturized solr cell structures. Opticl nd PV chrcteristics of these cells such s the reflectivity, photovoltic conversion efficiency (PCE), externl quntum efficiency (EQE) nd so on re gretly influenced by the verge dimeter nd height of nnopillr rrys. Notbly, the NPs with smll dimeters nd lrge height cn suppress the reflection, but less dvntgeous for PV performnces since they inherently come with the lrge surfce re for the significnt surfce recombintion nd greter number of lttice defects. In their work, the lowest reflectivity ws chieved for Si NPs of 2 nm verge dimeter nd 1.5 μm height, where the reflectivity ws mesured below 5 % for the wvelength rnge of 4 1 nm, but the best performed solr cell mde with Si NPs of 6 nm verge dimeter nd 1.5 μm height (PCE %) [72]. More importntly, lthough ll these Si NWAs nd NPs hve presented the impressive light trpping chrcteristics, the lrge-scle deployment of these structures is needed for the prcticl implementtion of solr cells. In this regrd, Pudsini et l. reported tht highly ordered wfer-scle Si NP rrys were successfully fbricted by the McEtch process in combintion with the nnosphere lithogrphy [9]. The fbricted NPs were then incorported into the Si/orgnic polymer hybrid solr cell devices, with the im to utilize these semiconductor nnostructures s n efficient electron ccepting phse for the low-cost hybrid PVs. Figure 5 demonstrtes SEM imges of the verticlly ligned Si NP rrys with the hexgonl order [9]. The pitch of NP rrys is 65 nm nd Si NPs with different heights re fbricted by vrying the Si etching time. Reflectivity spectr presented in Fig. 5b show tht the stronger nti-reflection property could be observed s the height of NP increses. Further investigtion in the effect of NP height on the corresponding PV performnces ws crried out nd found tht the mesured externl quntum efficiency (EQE) of the Si NP/PEDOT:PSS solr cells improves with the increse in the NP height up to.4 μm due to the light trpping effects. Beyond.4 μm height, the EQE decreses drsticlly with the increse in the NP height, despite of their effective light trpping especilly in the wvelength rnge of 4 8 nm. This observed phenomenon of the pillr height dependent light trpping cn be relted to the increse in crrier recombintion with the further increse of Si NP height. It lso worth to note tht s mjority of the short-wvelength photons re bsorbed in first 1 nm of Si or so, the most significnt drop in EQE is witnessed t those short wvelengths for cells with the tller pillrs, while there is no substntil chnge for the wvelengths >8 nm, since most of the long-wvelength photons re bsorbed in the bulk of Si. All these indicte n effective surfce pssivtion, especilly the junction pssivtion, being inevitble to further dvnce the electricl performnce of these NP-bsed hybrid solr cells.

8 8 F. Xiu et l.: 3D silicon nnostructures for photovoltics b Reflectivity (%) Reflectivity SiNP:PEDOT:PSS H=.2 µm SiNP:PEDOT:PSS H=.4 µm SiNP:PEDOT:PSS H=.8 µm SiNP:PEDOT:PSS H=1.2 µm EQE Externl quntum efficiency (%) Wvelength (nm) Fig. 5 () Top view SEM imge of Si NP rry textured silicon surfce; the inset shows the higher mgnifiction imge of the sme smple, where it is possible to identify the hexgonl order of the NP rry. (b) Reflectivity spectr nd mesured externl quntum efficiency of the fbricted Si NP/PEDOT:PSS hybrid solr cells with different Si NP height. Reproduced from Ref. [9] with permission from The Americn Chemicl Society. Tpered nnostructures: nnopyrmids, nnocones, nd nnodomes Along with NWAs nd NPs, fine-designed nnostructures with the tpered geometry nd shpe, such s nnocones, nnopyrmids nd nndomes, etc, hve illustrted superior ntireflection properties due to their grded trnsition of the effective refrctive index between the nnostructures nd ir [73 77]. Among these tpered nnostructures, nnocones (NCNs) hve been considered s the optiml structure for light hrvesting since their opticl reflection cn be significntly reduced over brod rnge of wvelength through their smller tip s compred with NWAs nd NPs [78 8]. Wng et l. performed systemtic simultion studies on the opticl properties of silicon NCNs nd contrsted them with those of NWAs [8]. NCN rrys re found to hve the significntly enhnced solr bsorption nd efficiencies over NW rrys due to the reduced reflection from their smller tip nd reduced trnsmission from their lrger circulr bse. Furthermore, these nti-reflection properties re found to be insensitive to the tip dimeter, fcilitting their fbriction for prcticl pplictions. This wy, the opticl bsorption enhncement in morphous Si NCs ws experimentlly investigted by Yi et l. [81]. The fbriction process of NCs is depicted in Fig. 6 d. A monolyer of silic nnoprticles is used s n etch msk during chlorinebsed RIE process nd either NWs or NCs cn be obtined through controlling the process conditions becuse the etching rte of silic is much lower thn tht of morphous Si. It could be found tht in contrst with the morphous film nd NWs, grded trnsition of the effective refrctive index is observed for NCs due to the grdul shrinking of the dimeter from the root to the top (Fig. 6e g). Providing the perfect impednce mtching between morphous Si nd ir through the grdul reduction of the effective refrctive index, NCs disply the stronger bsorption over lrge rnge of wvelengths nd ngles of incidence (Fig. 6h i). These excellent nd omnidirectionl light bsorption chrcteristics cn provide higher integrted power genertion over conventionl plnr structures; s result, these morphous Si NCs cn offer promising PV structure s both bsorber nd ntireflection lyers for the efficient solr cells. Nnopyrmid (NPM) is nother kind of tpered nnostructure with the squre bse. This NPM structure cn be obtined by the most common surfce texturiztion technique through the nisotropic chemicl etching of silicon in the lkline solution [82, 83]. Mvrokeflos et l. demonstrted n inverted nnopyrmid light-trpping scheme fbricted t wfer-scle vi low-cost wet etching process [84]. Commercil Si-on-insultor (SOI) substrtes were selected with the desired Si nd SiO 2 thickness. Then, hole rrys were ptterned in negtive photoresist on the top Si surfce followed by the subsequent wet etching in the queous KOH solution to yield the inverted pyrmid structure. KOH wet chemistry ws utilized s it etched

9 F. Xiu et l.: 3D silicon nnostructures for photovoltics 9 b c d SiO 2 -Si:H ITO Refrctive index e f g Thickness (nm) Thickness (nm) Thickness (nm) Absorption h.9.9 i Thin film.5.4 Nnowire Thin film.4 Nnocone Nnowire.3.3 Nnocone Wvelength (nm) Angle of incidence ( ) Absorption Fig. 6 ( d) Schemtic illustrtion of 1 μm thick morphous Si deposited on the ITO-coted glss substrte, monolyer of silic nnoprticles on top of the morphous thin film, NW rrys nd NC rrys. The effective refrctive index profiles of the interfces between ir nd (e) morphous Si thin film, (f) 6 nm NW rrys, nd (g) 6 nm NC rrys. (h) Mesured bsorption of smples with morphous thin film, NW rrys nd NC rrys s the top lyer over lrge rnge of wvelength t the norml incidence. (i) Mesured ngulr dependent bsorption of three smples t the wvelength of λ = 488 nm. Reproduced from Ref. [81] with permission from The Americn Chemicl Society. crystlline Si nisotropiclly long < 1 > crystl direction. The fbricted NPM structures re presented in Fig. 7. Smples of vrying ridge size yielded bsorptnce fluctutions tht re most pronounced t short wvelengths [84]. Mgnified SEM imges disply the seprtion between successive pyrmids being bout 1 nm with the fluctution of 1 nm (Fig. 7 inset). These inverted pyrmids exhibit n obvious opticl bsorption enhncement throughout the entire solr spectrum s compred with tht of the flt film (Fig. 7b). Notbly, the bsorptnce of this 1 mm thick Si lyer with the inverted NPM structure is found to pproch the Yblonovitch limit with the miniml ngle dependence [85], confirming the effectiveness of trpping photons in NPMs. Similrly, nnodomes (NDMs) combine mny nnophotonic effects in effectively reducing the opticl reflection nd enhnced bsorption over brod spectrl rnge. Zhu et l. hve illustrted nnodome morphous Si solr cells with the efficient light mngement fbricted on NCN substrtes fter the deposition of solr cell lyers, s shown in Fig. 8 c [86]. These NDM devices consist of only 28 nm thick hydrogented morphous silicon bsorber lyer, which cn provide the much higher bsorption thn flt film devices over the entire spectrum nd lrge rnge of incident ngles (Fig. 8d f). This nti-reflection effect minly comes from the tpered shpe of NDM structures with the better effective reflective index mtching with ir, prticulrly coupling light into the morphous lyer with suppressed reflection s well s scttering light long the in-plne dimension. This scttering cn increse the light trveling pth by trpping photons. Notbly, there is significnt interference oscilltions ppered in the flt film devices

10 1 F. Xiu et l.: 3D silicon nnostructures for photovoltics b Absorptnce Absorption in Ag.3 Flt film.2 Theory Experiment.1 Yblonovitch Wvelength (µm) Fig. 7 () Top view SEM imges of the 1 μm thick crystlline Si film illustrting the ptterned inverted nnopyrmids on 7 nm period. A 9 nm SiN x lyer is deposited on the inverted pyrmids nd SiO 2 (1 μm) nd silver (2 nm) lyer re t the bckside of the c-si film. The top left inset is the close-up cross-sectionl SEM picture of the sme device. (b) Comprison of the theoreticl nd experimentl bsorptnce spectr, the Yblonovitch limit corresponding to the c-si thickness nd the clculted bsorptnce in the Ag lyer for the top Si film thicknesses of 1 μm. Reproduced from Ref. [84] with permission from The Americn Chemicl Society. for the long wvelength bove 55 nm, while NDM devices still show reltively flt brodbnd dsorption for the long wvelength region, indicting the NDM structure is fesible for thin film solr cells with only submicrometer thick bsorber lyers, ttributed to the reduced reflection nd voiding interference oscilltions for longer wvelengths. Also, the NDM structure is in principle not restricted to ny prticulr mteril system nd its fbriction is comptible with most post-solr mnufcturing. Therefore, ll these cn initite mny exciting opportunities for n ssortment of PV devices to further improve efficiency nd minimize the mterils usge. b c C 8 nm TCO 28 nm p-i-n--si 8 nm TCO 1 nm Ag Nnocone substrte d e f Absorption (%) Nno Flt Nno with ITO Flt with ITO Wvelength (nm) Absorption (%) Nno 2 Flt Nno with ITO Flt with ITO Wvelength (nm) Absorption (%) Nno Flt 2 Nno with ITO Flt with ITO Wvelength (nm) Fig. 8 SEM imges with 45 tilt ngle of () nnocone qurtz substrtes nd (b) morphous Si nnodome solr cells fter the deposition of multilyers of mterils on nnocones. (c) Schemtics of the cross-sectionl structure of nnodome solr cells. (d f) Light bsorption mesurement of nnodomes nd flt substrtes with nd without ITO. (d) Integrting sphere mesurement results of bsorption under the norml incidence, (e) 3 ngle of incidence, (f) 6 ngle of incidence. Reproduced from Ref. [86] with permission from The Americn Chemicl Society.

11 F. Xiu et l.: 3D silicon nnostructures for photovoltics 11 Nnoholes nd nnowells Besides verticlly ligned positive nnostructures discussed bove, negtive nnostructures such s nnoholes (NHs) nd nnowells (NWs) lso demonstrte the impressive light mngement property. Hn et l. investigted silicon NH rrys s light bsorbing structures for photovoltics vi the simultion nd compred them to tht of NR rrys [87]. Figure 9 b show the schemtic illustrtions of NH nd NR rrys. The bsorption spectr for NH nd NR rrys with the structure thickness of mm nd 2.33 μm were clculted nd shown in Fig. 9c. Results show tht the bsorption of NH rry is higher thn tht of NR rrys t the wvelength < 75 nm in regrdless of the structure thickness. While t the long wvelength region (75 nm < λ < 1 μm), the thin NH rry (1.193 μm thickness) cn bsorb more strongly thn the NR rry, indicting tht the light trpping in the smll volume is more efficient for the NH rry. Experimentlly, McEtch processes hve lso been employed to fbricte these negtive nnostructures s nti-reflection lyers. For exmple, nnopore-type blck Si hs been chieved by one-step Ag-ssisted chemicl etching involving HF nd H 2 O 2 solution [88]. Lower reflectivity cn be compromised between the sufficient Ag ctlyst to crete lrge numbers of nnopores ( 1 nm in dimeters) on Si surfce nd the excessive Ag which would induce the deeply etched chnnels creting potentil short-circuit in the subsequent solr cell fbriction. By controlling the etching condition, the lowest reltive effective reflectivity of.17 % over wide wvelength rnge of 3 1 nm is relized for the nnopore Si mde of [Ag + ] = 5 mm with 2 min etching nd HF: H 2 O 2 : H 2 O rtio of 1:5:2. The simplicity nd effectiveness of this structure cn mke such processing esily sclble for industril pplictions. Applictions in solr cells For photovoltic pplictions, 3D nnostructures cnnot only led to improved photon hrvesting properties utilizing the light trpping mechnism, but lso enhnce the photo-crrier collection efficiency due to the shorted diffusion length in p-n junctions [89 91]. Generlly, there re two min configurtions of p n junction, i.e., rdil (core-shell) junctions nd xil junctions. The rdil junction cn enhnce the crrier collection efficiency s compred to the xil junctions, s long s the rdii of the nnostructures re much smller thn the minority diffusion length [89, 92]. In rdil p n structures, the seprtion nd collection of photo-generted crriers tke plce in the rdil direction nd such geometry provides short trvel distnces of photo-excited minority crriers to the collection electrodes, leding to the improved crrier-collection efficiency [18, 32, 93]. Furthermore, the rdil geometry of p n junction in 3D nnostructures permits the use of lower-qulity silicon with shorter minority-crrier diffusion length. For exmple, rrys of verticl rods with c 1..8 b Absorptnce.6.4 Hole, d=1.193 mm Rod, d=1.193 mm.2 Hole, d=2.33 µm Rod, d=2.33 µm Wvelength (µm) Fig. 9 () Schemtic illustrtions of the nnohole nd (b) nnorod rrys. (c) Clculted bsorptnce spectr for the nnohole nd the nnorod rry structures when the thickness d is 2.33 μm nd mm. Reproduced from Ref. [87] with permission from The Americn Chemicl Society.

12 12 F. Xiu et l.: 3D silicon nnostructures for photovoltics core-shell p n junction structure hve been theoreticlly shown to be cpble of improving photo-generted crrier collection for poor-qulity mteril with short minority crrier diffusion lengths, s reported by B. M. Kyes et l. [15], indicting high tolernce of mteril defects for PVs [91]. Regrdless of the bove-mentioned potentil dvntges inspired by the rdil p-n junction with excellent light trpping properties, most studies show tht the PV performnce of nnostructured solr cells is not stisfying in spite of the strong opticl enhncement [9, 16 18]. The degrdtion of device performnce is mostly ttributed to the high recombintion effect cused by high-density surfce defects s well s the lrge surfce re in these high-spect-rtio nnostructures [67, 93, 94]. A competition between the improved bsorption nd incresed surfce recombintion ws observed nd investigted by Grnett et l., by chnging the silicon film thickness nd nnowire length [67]. Figure 1 nd b show the photovoltic performnce of 5 μm tll NW s well s plnr control solr cells under AM 1.5G illumintion while devices with different silicon bsorber thicknesses were discussed. It could be seen tht NW cell fbricted from n 8 μm thin silicon bsorber gve 4 % higher Jsc versus the plnr control cell, indicting tht the light-trpping effects dominte for very thin bsorbing lyers. While for NW cell using the 2 μm silicon bsorbing lyer, it hd 14 % lower Jsc thn the plnr control solr cell, becuse the recombintion effect is more importnt for thicker cells tht lredy bsorb lrge frction of the incident light. In order to further explore the opticl trpping dvntges nd recombintion disdvntges of nnowire rrys, Fig. 1c depicts the photovoltic chrcteristics for 8 μm cells fbricted with vrious nnowire lengths, leding to different roughness fctors. The roughness fctor (RF) is defined s the ctul surfce re of the structure divided by the geometric re (e.g., RF of plnr cell is 1) [67]. The trends of Voc, FF, nd Jsc show tht the incresed surfce nd junction res led to the enhncement of both recombintion (i.e., lower Voc nd FF) nd light trpping effect (i.e., higher Jsc). It lso ppers tht the light-trpping effect domintes over the recombintion effect for this Si bsorber thickness, s the Jsc continues to increse with higher RF. To suppress the recombintion effect, it would be criticl to tke proper surfce tretments or design improved nnorry geometries tht llow for lower roughness fctors without scrificing the light trpping. Deposition of surfce pssivtion lyers cn effectively reduce the surfce recombintion through the reduction of interfce trps nd the implementtion of field-effect pssivtion [95]. Chen et l. demonstrted highly efficient nnotextured blck silicon solr cell with n + emitter/p bse structure pssivted by the Al 2 O 3 lyer through the tomic lyer deposition (ALD) (shown in Fig. 11) [19]. The nnotextured blck silicon wfer covered with the 11 nm thick Al 2 O 3 lyer exhibited very low totl reflectnce of 1.5 % in brod spectrum from 4 to 8 nm, s presented in Fig. 11b. Figure 11c gives the illuminted current-voltge (I V) curves of the cells with nd without the Al 2 O 3 pssivtion lyer. By compring cell 1 (without pssivtion lyer) nd cell 2 (with Al 2 O 3 pssivtion lyer), one could find tht Jsc nd η were enhnced 5.5 % nd 7.3 % by the s-deposited Al 2 O 3 surfce pssivtion lyer. Thus the efficiency enhncement cn be minly ttributed to the increse of Jsc due to the suppression of surfce recombintion by the s-deposited pssivtion lyer. Current density (ma/cm 2 ) µm nnowires Plnr Voltge (V) b Current density (ma/cm 2 ).7 V 5 µm nnowires oc J sc Normlized J 19 FF Plnr sc Voltge (V) Roughness fctor (RF) Fig. 1 () Photovoltic performnce of 5 μm nnowire nd plnr control solr cells fbricted from () 8 μm nd (b) 2 μm thin silicon bsorbers. (c) Photovoltic response s function of roughness fctor. Reproduced from Ref. [67] with permission from The Americn Chemicl Society. c V oc, V or FF J sc, ma/cm 2

13 F. Xiu et l.: 3D silicon nnostructures for photovoltics 13 n + -Si p-si P + (BSF) Al Ni/Ag b c 4.5 Nnotextured blck Si Al 4. Nnotextured blck Si+5 nm Al O O 3 Nnotextured blck Si+11 nm Al 2 O Reflectnce (%) Wvelength (nm) Voltge (V) Current density (ma/cm 2 ) -5 Cell-1 Cell Fig. 11 () Schemtic digrm of the nnotextured blck silicon solr cell with n n+ emitter/p bse structure. (b) Totl reflectnce spectr of the nnotextured blck silicon wfer with nd without the Al 2 O 3 lyers. (c) Illuminted I V chrcteristics of the cells with (cell 2) nd without (cell 1) 11 nm Al 2 O 3 pssivtion lyer. Reproduced from Ref. [19] with permission from The Americn Chemicl Society. In ddition to Al 2 O 3, severl mterils hve demonstrted with the excellent surfce pssivtion effect, such s silicon oxide nd hydrogented silicon nitride lyer mnufctured by plsm enhnced chemicl vpor deposition (PECVD) [2, 21]. Also, the combintion of PECVD bsed silicon oxide nd silicon nitride hs been proved to be very promising pproch for surfce pssivtion, which hs been successfully integrted in mny cell structures [22, 23]. Specificlly, the surfce recombintion cn be reduced by the deposition of surfce pssivtion lyer becuse of the following resons: (1) the decrese in interfcil stte density, i.e., the so-clled chemicl pssivtion, nd (2) the reduction in minority crrier concentrtion ner the interfce by the built-in electric field, which is referred to the field-effect pssivtion [96]. The built-in electric field estblished by the pproprite fixed chrge in the surfce pssivtion lyer, cn give rise to repelling of minority crriers wy from the interfce, leding to lower surfce recombintion rte. Insted of nnostructured silicon solr cells, other semiconductor solr cells lso get benefits from this surfce pssivtion. Mrini et l. reported tht in-situ pssivtion tretment could drmticlly improve the externl quntum efficiency nd totl power conversion efficiency of GAs-bsed nnopillr rry solr cells [97]. Holm et l. [98] lso demonstrted tht the surfce pssivtion plys crucil role in the GAsP single nnowire solr cells, s summrized in Tble 1. Besides, chemicl polishing etching (CPE) tretment hs been found s nother effective wy to reduce the surfce recombintion in high-spect-rtio nnostructures by decresing the surfce defects nd control the finl morphologies of the s-mde structures, s reported by Wng et l. [18] In their work, lrge-re uniform hierrchicl structures combining NPMs nd NWs were obtined by the metl-ssisted chemicl etching. A CPE solution which is the mixture of nitric cid (HNO 3 ) nd hydrofluoric cid (HF) ws pplied to the hierrchicl structures leding to n tomiclly smooth nd contmintion-free surfce (Fig. 12 c). Reflectnce spectr of the hierrchicl structures with different CPE durtions nd J-V chrcteristics of Si Tble 1 Summry of the current sttus of solr cell efficiency bsed on three-dimensionl silicon, compound semiconductor nd hybrid nnostructures. Group Solr cell type PCE ( %) Miin-Jng Chen et l. [19] Nnowire-bsed silicon solr cells 18.2 Yi Cui et l. [86] Nnodome-bsed silicon solr cells 5.9 Futing Yi et l. [72] Nnopillr-bsed silicon solr cells Jr-Hu He et l. [18] Hierrchicl structure-bsed silicon solr cells (pyrmids + nnowires) Pushp Rj Pudsini et l. [9] Nnopillr-bsed hybrid Si/PEDOT:PSS solr cells 9.65 Yi Cui et l. [79] Nnocone-bsed hybrid Si/PEDOT:PSS solr cells 11.1 Huffker DL. et l. [97] GAs nnopillr-rry solr cells 6.63 Jeppe V. Holm et l. [98] GAsP single-nnowire solr cells 1.2

14 14 F. Xiu et l.: 3D silicon nnostructures for photovoltics b c Anisotropic etching with KOH/IPA solution d Metl-ssisted chemicl etching on pyrmid surfces e 4 Pyrmid Pyrmid+NW Pyrmid+NW+CPE (3s) Pyrmid+NW+CPE (6s) Pyrmid+NW+CPE (9s) R (%) Wvelength (nm) 1 CPE for removing defects nd controlling morphologies f 3 J (ma/cm2) 2 Pyrmid Pyrmid+NW Pyrmid+NW+CPE (3s) Pyrmid+NW+CPE (6s) Pyrmid+NW+CPE (9s) Fig. 12 ( c) Schemtics of the experimentl procedures. (d) The corresponding solr cell structure. (e) Totl reflectnce spectr of hierrchicl structures with different CPE durtions over the wvelength regions of 3 11 nm. (f) J V chrcteristics the solr cells with different structures under AM 1.5G illumintion. Reproduced from Ref. [18] with permission from The Americn Chemicl Society. heterojunction solr cells (Fig. 12d) re shown in Fig. 12e nd f, respectively. The lowest reflectnce could be observed in the pyrmid-nw hierrchicl structure without CPE tretment due to its excellent light trpping property, while poor PV performnce is demonstrted which is ttributed to high-density crrier trpping centers. After treted by CPE, ll the structures exhibit higher photovoltic conversion efficiencies even though there is decrese in the reflectnce due to the formtion of reltively polished surfces. Especilly, Jsc of the solr cell with hierrchicl structures is incresed from to ma/cm2 fter 6 s CPE, owing to the smoother surfce nd the decrese in junction re. Nevertheless, Jsc decreses s the CPE time prolongs further, demonstrting n overll geometricl considertion of both light trpping nd recombintion effect should be emphsized to yield the best photovoltic properties for the hierrchicl structures bsed solr cell. More importntly, this unique cell structure lso illustrtes the omnidirectionl PV properties, exhibiting the dily generted power enhncement of 44.2 % s compred to conventionl micropyrmid control cells nd demonstrting the technologicl potency of 3D hierrchicl structures for the next-genertion solr cells [18]. Furthermore, thin-film solr cells re presently considered s nother promising route to lower the mteril nd processing cost of photovoltics [99]. However, the PV performnce of thin-film solr cells is significntly ffected by thinning down the ctive lyer nd the corresponding light trpping becomes n importnt limiting fctor. By combining low-cost thin-film solr cells with nnostructured surfce texturiztion, the light hrvesting properties could be improved. Nnocone structures with excellent light trpping properties were employed in the hybrid Si thin film solr cells, s reported by Jeong et l. [79]. With very thin ctive mteril nd inexpensive processing steps, hybrid Si nnocone/polymer solr cells could demonstrte high power conversion efficiency bove 11 %, demonstrting n economiclly vible lterntive energy solution (Tble 1). In ddition to the surfce texturiztion, recent reserch hs lso shown tht integrting plsmonic nnostructures is n lterntive fesible pproch to enhnce the light trpping properties for thin-film solr cells by using metllic nnoprticles to couple light into the underlying opticl modes of the semiconductor [1]. In order to enhnce the opticl bsorption in the solr spectrum, tuning the surfce plsmon resonnce cn be utilized. Specificlly, the excittion of surfce plsmons cn be chrcterized by the strong scttering s well s the enhncement of electric field round the surrounding re of nnoprticles [11]. Photocurrent enhncements

15 F. Xiu et l.: 3D silicon nnostructures for photovoltics 15 hve been reported from both inorgnic, orgnic solr cells nd hybrid inorgnic-orgnic devices such s dye sensitized solr cells [12 14]. For exmple, Mtheu et l. [14] hve described n increse in the power conversion efficiency of 2.8 % nd 8.8 % from Si-bsed devices using 1 nm Au colloidl prticles nd 15 nm silic prticles, respectively. The similr pproch hs lso been pplied to GAs thin-film solr cells with n 8 % increse in the short-circuit current density utilizing 11 nm silver prticles [15]. Bsed on ll these results, the potentil of incorporting plsmonics into solr cell structures looks very encourging. In ny cse, specil ttention must be pid to the density of nnoprticles deposited on the top cell surfce in order to void the shdowing effect, which would degrde the corresponding light bsorption if not properly optimized [99]. Conclusion In this review, the controllble fbriction of three-dimensionl silicon nnostructures hs been summrized. Among vrious fbriction techniques, McEtch is more flexible nd promising method to fbricte high-spect-rtio nnostructures for photovoltic pplictions becuse there is no surfce dmge nd metl contmintion cused in the fbriction process. Menwhile, the light trpping properties of severl mjor ctegories of silicon nnostructures re systemticlly compred nd it is reveled tht nnostructures with diverse configurtions hve demonstrted excellent but unique photon mngement properties. Prticulrly, nnocone (NCN) structure hs been considered s one of the optiml structures for light hrvesting due to their reduced reflection over brod rnge of wvelengths through grded effective refrctive index, nd their smll tips lso contribute to reduced reflectnce compred with other nnostructures. Even though these nnostructures with rdil p n junction cn demonstrte improved photon hrvesting properties nd enhnced photo-crrier collection efficiencies, the photovoltic performnce of nnostructure-bsed devices is still significntly limited by the high recombintion effect coming from high-density surfce defects in nnostructures with the lrge surfce re. In this regrd, vrious pproches hve been discussed to reduce the surfce recombintion in these high-spect-rtio nnostructures, such s depositing pssivtion lyers s well s chemicl polishing etching tretment. All in ll, concurrent improvement in both electricl nd opticl chrcteristics of the rdil junction Si nnostructures is the most importnt issue for putting highefficiency nnostructured bsed solr cells into prctice. Acknowledgments: This reserch ws finncilly supported by the City University of Hong Kong (Project no ). References [1] D. Ginley, M. Green, R. Collins. MRS Bull. 33, (28). [2] A. Hochbum, P. Yng. Chem. Rev. 11, (21). [3] M. Lw, J. Goldberger, P. Yng. Annu. Rev. Mter. Res. 34, (24). [4] B. Tin, T. J. Kemp, C. M. Lieber. Chem. Soc. Rev. 38, (29). [5] Z. Y. Fn, D. J. Ruebusch, A. A. Rthore, R. Kpdi, O. Ergen, P. W. Lue, A. Jvey. Nno Res. 2, (29). [6] K. Yu, J. Chen. Nnoscle Res. Lett. 4, 1 1 (28). [7] N. S. Lewis, D. G. Nocer. Proc. Ntl. Acd. Sci. USA 13, (26). [8] L. Dupré, D. Buttrd, P. Gentile, N. Puc, Amit Solnki. Energy Procedi 1, (211). [9] P. R. Pudsini, F. Ruiz-Zeped, M. Shrm, D. Elm, A. Ponce, A. A. Ayon. ACS Appl. Mter. Interfces 5, (213). [1] R. Yu, Q. Lin, S. F. Leung, Z. Fn. Nno Energy 1, (212). [11] Y. Long, M. Yu, B. Sun, C. Gu nd Z. Fn. Chem. Soc. Rev. 41, (212). [12] Z. Fn, A. Jvey. Nt. Mter. 7, (28). [13] M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evns, M. C. Putnm, E. L. Wrren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis, H. A. Atwter. Nt. Mter (21). [14] W. R. Wei, M. L. Tsi, S. T. Ho, S. H. Ti, C. R. Ho, S. H. Tsi, C. W. Liu, R. J. Chung, J. H. He. Nno Lett. 13, (213). [15] B. M. Kyes, H. A. Atwter, N. S. Lewis. J. Appl. Phys. 97, (25). [16] G. Oki, G. Suprtik. Sol. Energy Mter. Sol. Cells 93, (29).

16 16 F. Xiu et l.: 3D silicon nnostructures for photovoltics [17] S. K. Srivstv, D. Kumr, P. K. Singh, M. Kr, V. Kumr, M. Husin. Sol. Energy Mter. Sol. Cells 94, (21). [18] H. P. Wng, T. Y. Lin, C.W. Hsu, M. L. Tsi, C. H. Hung, W. R. Wei, M. Y. Hung, Y. J. Chien, P. C. Yng, C. W. Liu, L. J. Chou, J. H. He. ACS Nno 7, (213). [19] W. C. Wng, C. W. Lin, H. J. Chen, C. W. Chng, J. J. Hung, M. J. Yng, B. Tjhjono, J.J. Hung, W. C. Hsu, M. J. Chen. ACS Appl. Mter. Interfces 5, (213). [2] J. M. Kopfer, S. Keipert-Colberg, D. Borchert. Thin Solid Films 519, (211). [21] G. Dingemns, W. M. M. Kessels. J. Vc. Sci. Technol. A 3, 482 (212). [22] J. Schmidt, M. Kerr, A. Cuevs. Semicond. Sci. Technol. 16, (21). [23] O. Schultz, M. Hofmnn, S. W. Glunz, G. P. Willeke. Proceedings of the 31st IEEE Photovoltic Specilists Conference, (25). [24] R. S. Wgner, W. E. Ellis. Appl. Phys. Lett. 4, 89 (1964). [25] Y. Wu, P. Yng. J. Am. Chem. Soc. 123, (21). [26] Y. Wng, V. Schmidt, S. Senz U. Gösele. Nt. Nnotechnol. 1, (26). [27] J. J. Hou, N. Hn, F. Y. Wng, F. Xiu, S. P. Yip, A. T. Hui, T. F. Hung, J. C. Ho. ACS Nno 6, (212). [28] N. Hn, F. Y. Wng, A. T. Hui, J. J. Hou, G. C. Shn, F. Xiu, T. F. Hung, J. C. Ho. Nnotechnology 22, (211). [29] T. Hnrth, B. A. Korgel. J. Am. Chem. Soc. 124, (22). [3] W. I. Prk, G. Zheng, X. Jing, B. Tin, C. M. Lieber. Nno Lett. 8, (28). [31] A. Mrtí, N. López, E. Antolín, E. Cánovs, C. Stnley, C. Frmer, L. Cudr, Solid Films 511, (26). [32] L. Tsklkos, J. Blch, J. Fronheiser, B. A. Korevr, O. Sulim, J. Rnd. Appl. Phys. Lett. 91, (27). [33] Z. W. Pn, Z. R. Di, L. Xu, S. T. Lee, Z. L. Wng. J. Phys. Chem. B 15, (21). [34] S. J. Kwon. J. Phys. Chem. B 11, (26). [35] W. S. Shi, Y. F. Z. N. Wng, C. S. Lee, S. T. Lee. Appl. Phys. Lett. 78, (21). [36] Y. Zhng, H. Ji, D. J. Yu. Phys. D: Appl. Phys. 37, (24). [37] M. J. Biermn, Y. K. A. Lu, A. V. Kvit, A. L. Schmitt, S. Jin. Science 32, (28). [38] P. J. Poole, L. J. efebvre, J. Frser. Appl. Phys. Lett. 83, (23). [39] J. Noborisk, J. Motohis, T. Fukui. Appl. Phys. Lett. 86, (25). [4] N. Wng, Y. H. Tng, Y. F. Zhng, C. S. Lee, I. Bello, S. T. Lee. Chem. Phys. Lett. 299, (1999). [41] R. Q. Zhng, Y. Lifshitz, S. T. Lee. Adv. Mter. 15, (23). [42] C. P. Li, C. S. Lee, X. L. M, N. Wng, R. Q. Zhng, S. T. Lee. Adv. Mter. 15, (23). [43] Y. F. Zhng, Y. H. Tng, N. Wng, D. P. Yu, C. S. Lee, I. Bello, S. T. Lee. Appl. Phys. Lett. 72, 1835 (1998). [44] N. Wng, Y. F. Zhng, Y. H. Tng, C. S. Lee, S. T. Lee. Phys. Rev. B 58, R16 24 (1998). [45] S. T. Lee, Y. F. Zhng, N. Wng, Y. H. Tng, I. Bello, C. S. Lee, Y. W. Chung. J. Mter. Res. 14, 453 (1999). [46] B. S. Kim, T. W. Koo, J. H. Lee, D. S. Kim, Y. C. Jung, S. W. Hwng, B. L. Choi, E. K. Lee, J. M. Kim, D. Whng. Nno Lett. 9, (29). [47] Y. Hung, X. Hun, C. M. Lieber. Smll 1, 142 (25). [48] S. H. G. Teo, A. Q. Liu, G. L. Si, C. Lu. Int. J. Nnosci. 4, (25). [49] L. Siniemi, H. Keskinen, M. Arom, L. Luosujrvi, K. Grigors, T. Kotiho, J. M. Mkel, S. Frnssil. Nnotechnology 18, (27). [5] K. J. Morton, G. Nieberg, S. Bi, S. Y. Chou. Nnotechnology 19, (28). [51] F. Lärmer, A. Schilp. U.S. Ptent (1996). [52] W. Lng. Mter. Sci. Eng. 1, R. 17 (1996). [53] H. Rhee, H. Kwon, C. K. Kim, H. J. Kim, J. Yoo, Y. W. Kim. J. Vc. Sci. Technol. B 26, 576 (28). [54] Y. Q. Fu, A. Colli, A. Fsoli, J. K. Luo, A. J. Flewitt, A. C. Ferrri, W. I. Milne. J. Vc. Sci. Technol. B 2, (29). [55] G. S. Oehrlein, S. M. Rossngel, (Ed.), Noyes, Prk Ridge, NJ, pp. 196 (199). [56] X. L. Li. Curr. Opin. Solid Stte Mter. Sci. 16, (212). [57] X. L. Li, P. W. Bohn. Appl. Phys. Lett. 77, (2). [58] Z. Hung, N. Geyer, P.Werner, J. de Boor, U. Gosele. Adv. Mter. 23, (211). [59] H. Lin, H. Y. Cheung, F. Xiu, F. Y. Wng, S. P. Yip, N. Hn, T. F. Hung, J. Zhou, J. C. Ho, C. Y. Wong. J. Mter. Chem. A 1, (213). [6] L. Schubert, P. Werner, N. D. Zkhrov, G. Gerth, F. M. Kolb, L. Long, U. Gosele, T. Y. Tn. Appl. Phys. Lett. 84, 4968 (24). [61] W. Chern, K. Hsu, I. S. Chun, B. P. D. Azeredo, N. Ahmed, K. H. Kim, J. M. Zuo, N. Fng, P. Ferreir, X. Li. Nno Lett. 1, (21). [62] X. H. Li, R. Song, Y. K. Ee, P. Kumnorkew, J. F. Gilchrist, N. Tnsu. IEEE Photon. J. 3, (211). [63] Y. K. Ee, P. Kumnorkew, R. A. Arif, H. Tong, H. Zho, J. F. Gilchrist, N. Tnsu. IEEE J. Sel. Topics Quntum Electron. 4, (29). [64] I. S. Chun, E. K. Chow, X. Li. Appl. Phys. Lett. 92, (28). [65] J. A. Rogers, R. G. Nuzzo. Mter. Tody 8, 5 56 (25). [66] X. H. Yng, F. G. Zeng, X. J. Li. Mter. Sci. Semicond. Process. 16, 1 (213). [67] E. Grnett, P. Yng. Nno Lett. 1, (21).

17 F. Xiu et l.: 3D silicon nnostructures for photovoltics 17 [68] B. Tin, X. Zheng, T.J. Kemp, Y. Fng, N. Yu, G. Yu, J. Hung, C.M. Lieber. Nture 449, (27). [69] S.W. Boettcher, J. M. Spurgeon, M. C. Putnm, E. L. Wrren, D. B. Turner-Evns, M. D. Kelzenberg, J. R. Miolo, H. A. Atwter, N. S. Lewis. Science 327, 185 (21). [7] T. J. Kemp, B. Tin, D. R. Kim, J. Hu, X. Zheng, C. M. Lieber. Nno Lett. 8, (28). [71] O. Gunwn, K. Wng, B. Fllhzd, Y. Zhng, E. Tutuc, S. Guh. Prog. Photovolt: Res. Appl. 19, (211). [72] J. Liu, M. Ashmkhn, X. Zhng, G. Dong, Y. Lio, B. Wng, T. Zhng, F. Yi. Energy Technol. 1, (213). [73] Y. F. Hung, S. Chttopdhyy, Y.J. Jen, C. Y. Peng, T. A. Liu, Y. K. Hsu, C. L. Pn, H. C. Lo, C. H. Hsu, Y. H. Chng, C. S. Lee, K. H. Chen, L. C. Chen. Nt. Nnotechnol. 2, (27). [74] Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, J. W. P. Hsu. Nno Lett. 8, (28). [75] T. Lohmuller, M. Helgert, M. Sundermnn, R. Brunner, J. P. Sptz. Nno Lett. 8, (28). [76] J. Zhu, Z. Yu, G. F. Burkhrd, C. M. Hsu, S. T. Connor, Y. Xu, Q. Wng, M. McGehee, S. Fn, Y. Cui. Nno Lett. 9, (29). [77] Z. Yu, H. Go, H. Wu, H. Ge, S. Y. Chou. J. Vc. Sci. Technol. B 21, (23). [78] K. X. Wng, Z. Yu, V. Liu, Y. Cui, S. Fn. Nno Lett. 12, (212). [79] S. Jeong, E. C. Grnett, S. Wng, Z. Yu, S. Fn, M. L. Brongersm, M. D. McGehee, Y. Cui. Nno Lett. 12, (212). [8] B. M. Wng, P. W. Leu. Nnotechnology 23, 1943 (212). [81] Y. Cui. Nno Lett. 91, (29). [82] N. Mrrero, B. Gonzlez-Diz, R. Guerrero-Lemus. Sol. Energy Mter. Sol. Cells 91, (27). [83] X. S. Hu, Y. J. Zhng, H. W. Wng. Sol. Energy Mter. Sol. Cells 94, (21). [84] A. Mvrokeflos, S. E. Hn, S. Yerci, M. S. Brnhm, G. Chen. Nno Lett. 12, (212). [85] E. J. Yblonovitch. Opt. Soc. Am. 72, (1987). [86] J. Zhu, C. M. Hsu, Z. F. Yu, S. H. Fn, Y. Cui. Nno Lett. 1, (21). [87] S. E. Hn, G. Chen. Nno Lett. 1, (21). [88] Y. T. Lu, A. R. Brron. PCCP 15, 9862 (213). [89] S. H. Tsi, H. C. Chng, H. H. Wng, S. Y. Chen, C. A. Lin, S. A. Chen, Y. L. Chueh, J. H. He. ACS Nno 5, (211). [9] H. C. Chng, K. Y. Li, Y. A. Di, H. H. Wng, C. A. Lin, J. H. He. Energy Environ. Sci. 4, (211). [91] K. Q. Peng, S. T. Lee. Adv. Mter. 23, (211). [92] E. C. Grnett, M. L. Brongersm, Y. Cui, M. D. McGehee. Annu. Rev. Mter. Res. 41, (211). [93] E. C. Grnett, P. Yng. J. Am. Chem. Soc. 13, (28). [94] M. M. Adchi, M. P. Anntrm, K. S. Krim. Sci. Rep 3, 1 6 (213). [95] A.G. Aberle. Prog. Photovolt Res. Appl. 8, 473 (2). [96] B. Hoex, J. J. H. Gielis, M. C. M. vn de Snden, W. M. M. Kessels. J. Appl. Phys. 14, (28). [97] G. Mrini, A. C. Scofield, C. H. Hung, D. L. Huffker. Nt. Commun. 4, 1497 (213). [98] J. V. Holm, H. I. Jørgensen, P. Krogstrup, J. Nygr d, H. Liu, M. Agesen. Nt. Commun. 4, 1498 (213). [99] V. E. Ferry, J. N. Mundy, H. A. Atwter. Adv. Mter. 22, (21). [1] H. A. Atwter, A. Polmn. Nt. Mter. 9, 25 (21). [11] S. Pilli, M.A. Green. Solr Energy Mterils & Solr Cells 94, (21). [12] H. R. Sturt, D. G. Hll. Appl. Phys. Lett. 73, 3815 (1998). [13] V. E. Ferry, M. A. Verschuuren, H. B. T. Li, R. E. I. Schropp, H. A. Atwter, A. Polmn. Appl. Phys. Lett. 95, (29). [14] P. Mtheu, S. H. Lim, D. Derkcs, C. McPheeters, E. T. Yu. Appl. Phys. Lett. 93, (28). [15] K. Nkym, K. Tnbe, H. A. Atwter. Appl. Phys. Lett. 93, (28).