Influence of the substrate material on the surface morphology of electrochemically

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1 pss-heder will e provided y the pulisher Review copy not for distriution 0 Influence of the sustrte mteril on the surfce morphology of electrochemiclly deposited ZnO lyers (pss-logo will e inserted here y the pulisher) Dorin Dimov-Mlinovsk *,, Konstntin Lovchinov, Mxim Gnchev, Orlin Angelov, Jochim Selnd Grff nd Alexnder Ulyshin Centrl Lortory of Solr Energy nd New Energy Sources, Bulg.Acd.Sci., ulv. Tzrigrdsko chussee, Sofi, Bulgri SINTEF, Forskningsveien, P.O. Box, Blindern, 0, Oslo, Norwy Received August 0, revised Jnury 0, ccepted ZZZ Pulished online ZZZ Keywords ZnO, electrochemicl deposition. nnorods, nnowlls, nnotues, SEM * Corresponding uthor: e-mil pss@wiley-vch.de, Phone: + 0, Fx: + 0, We: Introduction ZnO thin films nd cotings hve ttrcted gret interest due to their suitle properties for ppliction in optoelectronic devices, sensors, displys nd different thin film solr cells [-.] Recently, development of cost-effective methods for friction of one dimensionl ZnO nnostructures, such s nnowires, nnorods nd nnotues hve ttrcted extensive ttention ecuse of their potentil pplictions for the dvnced solr cell structures [,]. The nnometer size ZnO sed nnostructures hve very lrge surfce res per unit volume, which offers possiility to improve light hrvesting properties of solr cell structures in cse if such nnostructures re used s ntireflection cotings [, ]. ZnO sed lyers nd nnostructures with different morphologies cn e deposited y different techniques such s chemicl nd In this pper we report results of friction of ordered ZnO nnorods (NRs) or nnowlls (NWs) electrochemiclly deposited on different highly conductive sustrtes. The following types of conductive sustrtes hve een used: (i) highly doped p-type multi-crystlline Si, nd (ii) Copper thin pltes. The influence of seeding Ag lyer deposited on multi-si sustrtes on the growth of ZnO NRs hs een studied s well. It is oserved tht morphology of the deposited ZnO structures depends strongly on the type of the sustrte used. It is found tht ZnO structures with different shpes, such s nnorods, nnotues or nnowlls cn e grown on top of highly conductive sustrtes used in this work. It is oserved tht opticl reflection of the deposited lyers depends on the sustrte used s well s on the time of the electrochemicl deposition of ZnO lyers. It ws found tht: (i) for ZnO NWs/Cu structures, diffused reflection exhiit strong enhncement compred to pure Cu sustrte; (ii) in cse of ZnO NRs/Si structures reflection is decresed compred to tht for pure Si sustrte. Possile pplictions of ZnO NRs or NWs sed structures for the processing of dvnced Si sed solr cells re discussed. electrochemicl deposition, sol-gel processing, spry pyrolysis, Vpor Liquid Solid (VLS), rc dischrge, pulsed lser deposition etc. [-]. Electrochemicl deposition is n ttrctive method for friction of ZnO nnowires ecuse of its simplicity. Additionlly it is low cost method, which cn e used on industril scle. This pper reports results of friction of one dimentionl nnostructures (nnorods/nnowlls), otined y electrochemicl deposition of ZnO sed lyers on different conductive sustrtes (Si, Cu). According to our knowledge, highly doped p+ Si sustrte is used for first time s sustrte for electrochemicl deposition of ZnO nnostructured films. p+-si cn e considered s: (i) n emitter for the n-type Si sed solr cells nd (ii) s supporting highly doped low-cost sustrtes for thin film - Si:H sed solr cells processed in sustrte configur-

S. Bhrs et l.: Mnuscript preprtion guidelines 0 tion. Such solr cells cn e deposited on top of nnostructured ZnO/Si structures, which exhiit enhnced light hrvesting properties.

Mesurements of the diffused reflection spectr of ZnO sed nnostructures deposited on Si nd Cu sustrtes hve een performed s well.

0 t temperture of 0 C using threeelectrode electrochemicl cell nd sturted clomel electrode (SCE) s reference electrode. Since the potentil of Zn in the electrolyte is.0 V vs.

V. ZnO lyer deposition time vries etween nd min. The thickness of the grown ZnO films estimted from cross section SEM imges ws in the rnge of 0..0 µm.

2 S. Bhrs et l.: Mnuscript preprtion guidelines 0 tion. Such solr cells cn e deposited on top of nnostructured ZnO/Si structures, which exhiit enhnced light hrvesting properties. An nlysis of the surfce morphology of such lyers is performed y mens of Scnning Electron Microscope (SEM). Mesurements of the diffused reflection spectr of ZnO sed nnostructures deposited on Si nd Cu sustrtes hve een performed s well.. Experimentl ZnO nnostructured films hve een deposited y mens of n electrochemicl process from cid queous solutions of ZnCl (. 0 - M) nd KCl (0. M) with ph.0 t temperture of 0 C using threeelectrode electrochemicl cell nd sturted clomel electrode (SCE) s reference electrode. Since the potentil of Zn in the electrolyte is.0 V vs. SCE the deposition process of ZnO is crried out t.0 V vs. SCE preventing metl Zn deposition. Good qulity ZnO films hve een otined t redox potentil within the rnge etween +0. nd +0. V vs. SCE. Zinc peroxide (ZnO ) ws formed on the smples with d dhesion to the sustrte t redox potentil higher thn +0. V. ZnO lyer deposition time vries etween nd min. The thickness of the grown ZnO films estimted from cross section SEM imges ws in the rnge of 0..0 µm. ZnO lyers hve een deposited on different types of conductive sustrtes, such s high conductive p-type multi-crystlline Si (multi-si) sustrtes with resistivity ~ 00 mω/. As-cut, s well s etched in Ar + r.f plsm Si sustrtes nd Cu thin pltes hve een used. The influence of the presence of seeding Ag lyer deposited on the plsm etched multi-si sustrte hs een studied s well. Multi-Si sustrtes with thickness out 0µm hve een fricted using wire swing of Si lock otined y csting method from metllurgicl grde Si feedstock. Si sustrtes hve een hevily doped y Boron upon the Si ingot growth. Si sustrtes hve een clened in cetone, isopropnol nd deionized wter followed y tretment in H SO +H O solution nd wshing in deionized wter efore ZnO deposition. Another set of smples with electrodeposited ZnO lyers hs een prepred on Ag uffer film ( or nm) deposited y mgnetron sputtering on polished multi-si sustrte. In cse of nm thick Ag lyer n nneling t C for min efore the ZnO growth hs een pplied in order to form Ag nnodots, which hve een used s nno-seeds upon the growth of the ZnO sed nno-structures. The surfce morphology of the deposited ZnO sed structures hs een studied y mens of Scnning Electron Microscope (SEM) Philips nd FEI Qunt 0F, nd the thickness of the coting is estimted from the cross section imges. Direct nd diffuse reflection spectr hve een mesured y spectrophotometer Shimdzu UV-00 in the rnge of 0 00 nm employing n integrting sphere for the diffuse reflection. Results nd discussion. ZnO on multi-si Si sustrtes. Fig. shows SEM surfce imges of the s-cut (Fig. ) nd plsm etched (Fig. ) multi-si sustrtes. The s-cut multi-si sustrtes hve rough surfce with lrge, reltively flt grins, which hve size out 0 µm. Moreover, some smller grins nd prticles, which hve size out µm cn e lso seen (Fig.). After Ar plsm etching Si surfce of the multi-si smple is rther smooth nd consists of only lrge grins, which hve size of out 0 µm. Fig. shows SEM microgrphs of ZnO lyer Figure SEM imges of the surfce of s-cut () nd plsm etched () multi-si sustrtes. Mrkers correspond to 0 µm. Figure SEM imges of the ZnO lyer grown on s-cut multi-si sustrte t 0 C nd 000 mv for 0 min: surfce view () nd cross section view (). Mrkers correspond to µm.

pss-heder will e provided y the pulisher 0 c e Figure.

(c) nd (e) nd cross section view (), (d) nd (f) deposited t 000 mv nd 0 C for

sustrte t 0 C, 000 mv for 0 SEM imges of ZnO lyers grown on plsm etched multi-si

it cn e min: () surfce view nd () cross section; mrkers correspond to µm.

sustrte is given in (c) mrker corresponds to 0 nm.

The grown lyer consists of nno tues, which hve hexgonl shpe nd different dimeters

The nnotues hve different orienttion ngles, which re cused y the high surfce

The surfce morphology of the ZnO coting on micron scle follows the morphology of

SEM imges of ZnO lyers grown on plsm etched multi-si it cn e seen tht the

3 pss-heder will e provided y the pulisher 0 c e Figure. SEM imges of the ZnO lyer grown on plsm etched multi-si sustrte: surfce view (), (c) nd (e) nd cross section view (), (d) nd (f) deposited t 000 mv nd 0 C for different time: min () nd (); 0 min (c) nd (d); min (e) nd (f). Mrkers correspond to µm. Figure. SEM imges of ZnO grown on nm seeding Ag lyer deposited on polished multi-si sustrte t 0 C, 000 mv for 0 SEM imges of ZnO lyers grown on plsm etched multi-si sustrte re shown in Fig.. From Fig. it cn e min: () surfce view nd () cross section; mrkers correspond to µm. For comprison the surfce view of ZnO grown on nm Ag lyer deposited on c-si sustrte is given in (c) mrker corresponds to 0 nm. c d f grown on s-cut multi-si surfce. The grown lyer consists of nno tues, which hve hexgonl shpe nd different dimeters of out 0 00 nm (Fig. ). The verged thickness of the lyer is out 0 nm. The nnotues hve different orienttion ngles, which re cused y the high surfce roughness of the multi-si sustrte. The surfce morphology of the ZnO coting on micron scle follows the morphology of the s-cut multi-si sustrte (Fig. ). SEM imges of ZnO lyers grown on plsm etched multi-si sustrte re shown in Fig.. From Fig. it cn e seen tht the morphology of such lyers is very different from tht oserved for ZnO lyers grown on s-cut multi- Si sustrtes. Perfect hexgonl-like plne rods with n verge dimeter of out 0 nm nd height of nm cn e seen from Fig. in this cse. It hs to e noted tht n increse of the ZnO lyer deposition time leds to the increse of the nno-rods height. Moreover, some ending of nno-rods with lrger height cn e seen s well. Figure. SEM imges of the surfce of ZnO nnorods nd nnotues () grown on multi-si sustrte covered y uffer nm Ag film nneled t 0 C for min (). Mrkers correspond to µm ().

Duffuse Reflection, (%) S. Bhrs et l.: Mnuscript preprtion guidelines 0. ZnO on poly-si sustrte with Ag uffer lyer Surfce morphology of the ZnO nnorods grown on.

, it cn e seen tht hexgonl nnorods hve very inhomogeneous distriution nd different ngles of growth on the surfce of the Si sustrte Dimeters of ZnO NRs re out etween 0 nd 00 nm.

4 Duffuse Reflection, (%) S. Bhrs et l.: Mnuscript preprtion guidelines 0. ZnO on poly-si sustrte with Ag uffer lyer Surfce morphology of the ZnO nnorods grown on. nm Ag uffer lyer deposited y mgnetron sputtering on plsm etched multi-si sustrte is demonstrted in Fig. () nd (). From Figs., it cn e seen tht hexgonl nnorods hve very inhomogeneous distriution nd different ngles of growth on the surfce of the Si sustrte Dimeters of ZnO NRs re out etween 0 nd 00 nm. Similr morphology hs ZnO sed structure, which is grown on nm Ag uffer lyer deposited on c-si sustrte, s it is shown in Fig. c. However in this cse the verge size of the hexgonl NRs is out 0 nm, which cn e explined y the different growth kinetics of ZnO NRs on thin Ag lyer compred to tht for the cse of Si sustrtes. Deposition of ZnO film on top of nm Ag uffer lyers nneled t C for min results in totlly different morphology of the finl ZnO sed structure. In this cse unches of nnorods nd nnotues, 00 0 nm thick re formed s cn e seen in Fig.. As shown in Fig., these structures strt to grow from the Ag seeding nnodots, which hve dimeters of out 00 nm. These nnodots re formed fter nneling of the Ag film.. ZnO on Cu thin plte The surfce morphology of ZnO structures grown electrochemiclly on Cu thin pltes for different times is demonstrted in Fig.. From Fig. it cn e seen tht ZnO structures deposited for 0 min consist of: (i) nnorods with dimeters of out 0 nm; c Figure. SEM imges of the ZnO deposited y electrochemicl grown on Cu thin pltes for different time () 0 min, () 0 min, (c) min t 0 C nd 000 mv. Mrkers correspond to µm. (ii) wlls, which look like whiskers nd situted etween NRs. These wlls hve different lengths from to µm nd width (dimeter) of out 0 nm. An increse of the deposition time up to min leds to the disppernce of the ZnO nnorods nd to formtion of nno-wlls (Fig.,c). From Fig. nd c it cn e seen tht the density nd the height of the nnowlls re higher for the longer deposition times. It cn e ssumed tht upon the longer ZnO deposition time the only growth of nnowlls occurs. It seems tht the previously grown NRs (0 min growth) re trnsformed into nnowlls upon the longer electrodeposition process... Diffuse reflection of ZnO grown on poly-si susstrte Diffuse reflection spectr of the deposited ZnO structures on multi-si sustrtes re shown in Fig.. Reflection spectr hve een recorded in the opticl rnge of 00 nm. For comprison, reflection spectr of ZnO structures deposited on the s-cut (Fig., curve ) nd Ar + plsm etched multi-si sustrtes (Fig., curve ) re shown s well. It is seen tht the plsm etching leds to n increse of the diffuse reflectnce from the pure multi- Si sustrte. However, ZnO lyers grown on etched multi- Si exhiit lower vlues of the reflectnce in the spectrl region under investigtion (Fig., curves,, nd ). The only one exception hs to e noted - in cse of ZnO grown on plsm etched multi-si sustrte covered y nm Ag seeding lyer (Fig., curve ), slightly higher reflectnce vlues compred to those for pure plsm etched , (nm) Figure. Spectr of diffuse reflection of: s-cut multi-si sustrte, plsm etched multi-si sustrte, ZnO deposited on plsm etched multi-si with nm Ag seeding lyer for 0 min, ZnO deposited on s-cut multi-si for 0 min, ZnO deposited on plsm etched multi-si for min, ZnO deposited on plsm etched multi-si for 0 min, ZnO deposited on plsm etched multi-si for min.

5 Direct Reflection, (%) Diffuse Reflection, (%) pss-heder will e provided y the pulisher 0 Si sustrtes hs een oserved (Fig., curve ). It cn e ssumed in this cse tht Ag lyer contriutes in the dditionl reflection from this structure EDS nlysis shows tht the deposited films consist of ZnO. More detiled nlysis of the composition of the ZnO sed lyer (including depth resolved dependencies) is required. Such nlysis is suject for further studies of the ZnO sed structures presented in this work... Diffuse reflection of ZnO grown on Cu thin plte Diffuse reflectnce spectr of ZnO films grown on top of the Cu thin plte sustrtes re shown in Fig.. All spectr hve een recorded in the spectrl rnge of 00 nm. The corresponding spectrum of the pure Cu plte is shown, s well, for comprison. Fig. shows direct reflection spectr of the sme smples. From Fig it cn e seen tht in the whole spectrl region under investigtion the diffuse reflection of the pure Cu plte is much lower thn the direct reflection of the sme Cu plte. Asolute vlues of the diffused reflection of the electrochemiclly deposited ZnO lyers re close to vlues of the direct reflection for the sme ZnO/Cu structures. From Fig. it cn e seen tht n increse of the electrochemicl deposition time of the ZnO lyers results in n increse of the dif Cu sustrte min 0 0 C 0 min 0 0 C 0 min 0 0 C , (nm) Cu sustrte min 0 0 C 0 min 0 0 C 0 min 0 0 C , (nm) Figure. Spectr of diffuse () nd direct reflection () of ZnO thin lyers deposited on Cu plte for different times: 0, 0 nd min. The corresponding spectr for pure Cu plte re presented s well. fused reflectnce vlues due to the chnges of the surfce morphology. Indeed, formtion of higher nd thicker nnowlls cn e seen from the SEM imges, tken for ZnO/Cu structures processed for 0 nd min. It is necessry to note tht the solute vlue of the diffused reflection depends on numer of prmeters of the ZnO sed structures grown on Cu sustrtes. The following prmeters of the ZnO nno-structures cn e mentioned in this regrd - the size nd the shpe of the nnowires nd nnowlls, the surfce roughness of the deposited films, the refrctive index, etc. Further detiled studies re required to explin the oserved experimentl results, which show tht ZnO sed structures deposited on Cu sustrtes exhiit rther strong reflection creted y ZnO/Cu mirror, which reflects light in the wide rnge of the spectrum. It hs to e noted tht such reflection cn e potentilly utilized for the thin Si solr cell fricted on top of such mirror-like ZnO/Cu sed structures. Conclusion It is demonstrted tht ZnO lyers grown y electrochemicl deposition on different highly conductive sustrtes (Si, Cu) hve different morphologies. It is oserved tht ZnO nnorods with hexgonl shpe cn e formed on multi-si sustrte etched y n Ar + plsm. It is shown tht in some cses ( minutes deposition on etched Si sustrte) ZnO sed NRs deposited on Si sustrtes provide formtion of n ntireflection coting (ARC, reflection < 0%), which cn e used for Si sed solr cells. Although further optimiztion of such ZnO NRs sed structures re required, the otined result cn e considered s promising technologicl development towrds low-temperture nd cost effective non-vcuum technology for the processing of ARCs for Si sed s well s other types of solr cells. It is shown tht deposition of ZnO sed structures on Cu thin plte sustrtes leds to the formtion of ZnO sed nnowlls. An increse of the diffused reflection with the increse of the deposition time is oserved. It cn e ssumed tht tht such mirror-like ZnO/Cu sed structures with n enhnced diffuse reflection cn e utilized for the thin Si solr cell fricted on top of such structures. Further investigtions trgeting processing of solr cells (like thin film -Si:H sed) on top of ZnO/Cu sustrtes fricted using the proposed pproch re required to clrify this point. Acknowledgements The work hs een supported y the FP - project NnoPV. References

6 S. Bhrs et l.: Mnuscript preprtion guidelines 0 [] E. Fortunto, L. Rniero, L. Silv, A. Gonclves, A. Pimentel, P. Brquinh, H. Agus, L. Pere, G. Gonclves, I. Ferreir, E. Elngovn, nd R. Mrtins, Sol. Energy Mter. Sol. Cells, 0 (0). [] J. Poortmnns nd V. Arhipov, Thin Film Solr Cells (John Wiley nd Sons, Ltd., Chichester, 0). [] D. Dimov-Mlinovsk, H. Nichev, O. Angelov, V. Grigorov, nd M. Kmenov, Superlttices Microstruct., (0). [] Y. Co, L. Mio, S. Tnemur, M. Tnemur, Y. Kuno, Y. Hyshi, nd Y. Mori, Jpn. J. Appl. Phys., (0). [] R. Kopenkmp, K. Boedecker, M. C. Lux-Steiner, M. Poschenrieder, F.Zeni, C.Levy-Clement, nd S. Wgner Appl.Phys. Lett., (00). [] J. Y. Chen, K. W. Sun, Solr Energy Mt&Solr Cells, (0)]. [] N. Koprinrov, M. Mrinov, D. Dimov-Mlinovsk, I. Strov, M. Konstntinov, H. Nichev, nd Tz.Tzchev J. Optoelectronics&Advnced Mterils, (0). [] T. Stelzner T, J. Blch, B.A. Fronheisner, B. A. Korevr, F.Sulim nd J. Rnd, Appl. Phys. Lett., (0). [] X. Kong, Y. Ding nd Z.J.Wng, Phys. Chem. B 0, 0 (0). [0] Y. Di, Y. Zhng, Q. K. Li nd C. W. Nn, Chem. Phys. Lett., (0). [] Y. S. Lim, J. W. Prk, S. T. Hong nd J. Kim, Mt. Sci. Engineering B, 00 (0). [] D. Dimov-Mlinovsk, P. Andreev P, M. Sendov- Vssilev, H. Nichev nd V. Mikli, Proc.th EPVSEC (Hmourg, Germny - Sept. 0), pp. -. [] T. Dedov, I.O. Acik, M. Krunk, V. Mikli, O. Voroujev, nd A. Mere, Thin Solid Films, ().

Structural and Optical Properties of ZnO Nanostructured Layers Deposited by Electrochemical Method on Conductive Multi-Crystalline Si Substrates

Bulg. J. Phys. 40 (2013) 229 236 Structural and Optical Properties of ZnO Nanostructured Layers Deposited by Electrochemical Method on Conductive Multi-Crystalline Si Substrates M. Petrov 1, K. Lovchinov