Research Article Electrodeposition and Low-Temperature Post-Treatment of Nanocrystalline SnO 2 Films for Flexible Dye-Sensitized Solar Cells

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1 Nnomterils Volume 2012, Artile ID , 6 pges oi: /2012/ Reserh Artile Eletroeposition n Low-Temperture Post-Tretment of Nnorystlline SnO 2 Films for Flexile Dye-Sensitize Solr Cells Shengjun Li, Yongjun Li, Zeng Chen, Junhui Liu, n Weifeng Zhng Key Lortory of Photovolti Mterils of Henn Provine n Shool of Physis n Eletronis, Henn University, Kifeng , Chin Corresponene shoul e resse to Weifeng Zhng, wfzhng@126.om Reeive 1 Novemer 2011; Aepte 28 Deemer 2011 Aemi Eitor: Anukorn Phurungrt Copyright 2012 Shengjun Li et l. This is n open ess rtile istriute uner the Cretive Commons Attriution Liense, whih permits unrestrite use, istriution, n reproution in ny meium, provie the originl work is properly ite. SnO 2 porous films were eletroeposite on Ti sustrte using pulse-potentil tehnique. These eposite films were elt with ifferent post-tretments. The effets of the post tretments were investigte through X-ry iffrtion, FE-SEM, n Rmn spetr. These investigtions showe tht rystl nnoporous SnO 2 films were otine through low-temperture post-tretment proess. The photoeletri onversion effiieny of the SnO 2 film trete with oth the wter vpour tretment n ultrviolet illumintion ws improve to e 0.35% whih ws omprle with the onversion effiieny of the film sintere t 550 C. 1. Introution Dye-sensitize solr ells (DSCs) hve ttrte gret sientifi n tehnologil interests s photovolti evies for their utiliztion of nontoxi mterils, simple preprtion proess, n high photoeletri onversion effiieny up to 11% [1, 2]. Conventionlly, the working eletroe ws prepre se on the sustrte of trnsprent onutive glss whih nnot e ene n tens to e rshe. As sustitute, flexile sustrte, suh s poly(ethylene terephthlte) ote with inium-ope tin oxie (ITO/PET), hs een opte in the DSCs [3, 4]. The PET sustrte is lightweight n flexile, ut it nnot withstn the temperture ove 150 C. To hieve high-qulity nnoporous photoeletroe t low temperture, vrious methos hve een evelope, suh s hyrotherml synthesis, hemil vpor eposition, physil vpor eposition, n eletroeposition [5 7]. In ontrst with other tehniques, the eletroeposition metho shows mny vntges, for exmple, it is usully rrie out elow 100 C n n give rigi ontrol on film thikness n omposition. The eletroeposite semionutor oxies show high porosity n fine sustrte hesion. Vrious metl oxies, suh s TiO 2,ZnO,In 2 O 3,n ZrO 2, hve een suessfully eletroeposite [7 10]. During the eletroeposition proess, the eletrons keep flowing through the nnoporous film. So the eletroeposite film is expete to possess fine eletron trnsport properties even without high-temperture sintering [8, 11]. SnO 2 hs een extensively use in gs sensors n lithium ion tteries for its trnsprent property n high eletroni moility [12 15]. The pplition of SnO 2 in DSCs hs lso een reporte [16 19]. Generlly, the SnO 2 films show low photoeletri onversion effiieny euse of the severe hrge reomintion n low onution n ege [20, 21]. But these rwks oul e overome y the surfe oting of other oxies, suh s Al 2 O 3,ZnO,nMgO [22, 23]. More importntly, highly rystllize SnO 2 oul e otine t low temperture, so SnO 2 shoul e kin of suitle photonoe mteril for flexile DSCs. In this pper, we eletroeposite SnO 2 film n posttrete the film though vrious proesses. Ti foil ws selete to e the sustrte of nnorystlline SnO 2 films for the omprison of vrious post-tretment proesses. After the wter vpour tretment with HNO 3 n UV light irrition, the photoeletri properties were omprle with tht of the hightemperture-trete eletroes.

2 2 Nnomterils 2. Experimentl 2.1. Preprtion n Chrteriztion of SnO 2 Films. The preprtion of nnorystlline SnO 2 films ws rrie out with simple three-eletroe system. Ti sheet (thikness of 0.1 mm) n Pt sheet were use s working n ounter eletroe, respetively. The istne etween the working eletroe n the ounter eletroe ws ontrolle to e out 3.5 m. Ag/AgCl eletroe with sturte potssium hlorie queous solution worke s the referene eletroe. The eletrolyte onsiste of 0.05 M SnCl 2 2H 2 O n 0.1 M HNO 3. The eposition temperture ws fixe t 75 Cynoil th. The Ti sheet whih ws use s flexile metl sustrte for nnorystlline SnO 2 film ws ultrsonilly lene in etergent solution, etone, n eionize wter, sequentilly. The expose surfe re of the working eletroe ws out 1.5 m 2. Eletrohemil eposition experiments were rrie out with CHI660C eletrohemil worksttion (Shnghi Chen-Hu Instrument Co, Chin). Pulsepotentil tehnique ws opte in the eposition proess with high potentil of 0.1 V (versus Ag/AgCl) n low potentil of 0.6 V (versus Ag/AgCl). The pulse time ws 10 s n eposition time ws 2 h. The s-grown SnO 2 films were yellow. The films were rie in ir n trete s the following proess: (1) wter vpour tretment in n utolve with 25 ml eionize wter for 12 hours t 150 C (pure wter vpour tretment), (2) wter vpour tretment in n utolve with 25 ml 0.3 M HNO 3 queous solution for 12 h t 150 C (HNO 3 tretment), (3) UV light irrition tretment t 258 nm for 20 min fter the wter vpour tretment with HNO 3 solution (UV-tretment), n (4) sintering t 550 C for 1 h in ir. After the post tretment, the eletroeposite SnO 2 films turne to e white. The rystlline phse of the smples ws hrterize y X-PertPro X-ry iffrtometer with monohromtize Cu Kα irrition (λ = nm) n morphologies were etermine using HithiS-4300F fiel emission snning eletron mirosope (FE-SEM). The Rmn spetr were mesure y lser Rmn spetrometer (Renishw-1000) t n output power of nm soli-stte lser Frition n Chrteriztion of Dye-Sensitize Solr Cells. For DSCs frition, the smples were immerse in mol L 1 ethnol solution of N3 ye for 24 h. A trnsprent Pt ounter eletroe ws prepre y spreing 5mmolL 1 H 2 PtCl 6 queous solution on n FTO glss sustrte n pyrolyze t 390 C for 15 min. The mixture of 0.6 M imethylpropylimizolium ioie, 0.1 M ioine, 0.5 M 4-tertutylpyriine, n 0.1 M lithium ioie in methoxy etonitrile ws selete to e the eletrolyte of DSCs. The photoeletrohemil hrteristis of DSCs were mesure y the photourrent-voltge urve (I V urves) mesurement with CHI660C Eletrohemil Worksttion uner the simulte solr light. A 500 W xenon lmp ws use s the light soure. The inient light intensity ws 100 mw m 2 mesure y Rition Meter (FZ-A, Beijing Norml University, Chin) n the tive ell re ws Intensity (.u.) θ (eg) Figure 1: XRD imges of the eletroeposite SnO 2 films otine t ifferent post tretment onitions () untrete, () pure wter vpour tretment, () HNO 3 tretment, () UV-tretment, n (e) line t 550 C m 2. The eletrohemil impene spetr (EIS) were lso rrie out with CHI660C. The impene mesurement of DSC ws reore t the open iruit voltge uner light illumintion over frequeny rnge of M Hz with mplitue of 10 mv. 3. Results n Disussion The rystllinity of the SnO 2 film efore n fter post tretment ws investigte y X-ry iffrtion whih is shown in Figure 1. The eletroeposite SnO 2 film without ny post tretment lrey shows the property of rystlline (shown in Figure 1 urve ), ut the reflet peks re wek n ro. So, it is neessry to el with the eposite films with post tretments. Here, four ifferent post tretment proesses were opte: wter vpour tretment with pure wter t 150 C (urve ), wter vpour tretment with 0.3 M HNO 3 solution t 150 C (urve ), wter vpour tretment with 0.3 M HNO 3 solution t 150 C, n UV-tretment (258 nm) for 20 min (urve ), sintering t 550 C for 1 h (urve e). After wter vpour tretment, the iffrtion pek intensity of SnO 2 eme stronger whih inite the enhnement of rystllinity egree. In ition, the iffrtion pek intensity of eletroeposite SnO 2 films trete with HNO 3 tretment n UV-tretment ws further improve. Figure 2 shows FE-SEM imges of the SnO 2 films otine through ifferent post-tretment proesses. After pure wter vpour tretment, rystl nnoporous SnO 2 film ws otine. But the SnO 2 prtiles exist in n gglomertion stte. The prtile size is s lrge s out nm whih is isvntgeous for the sorption of ye. With the ition of HNO 3 in the wter vpour tretment proess, more miropores were forme whih filitte the infiltrtion eletrolyte. At the sme time, the prtile size of SnO 2 ws erese for the sorption of more ye moleulr. Compring the FE-SEM imges of the other three post-tretments, there ws no ovious ifferene e

3 Nnomterils 3 Tle 1: Performne hrteristis of DSCs se on the nnorystlline SnO2 film uner ifferent post tretment proesses. Js (ma m 2 ) Post tretment onitions Pure wter vpour tretment HNO3 tretment UV-tretment Cline t 550 C Vo (mv) FF () () () () η (%) Figure 2: FE-SEM of the eletroeposite SnO2 films otine t ifferent onitions, () pure wter vpour tretment, () HNO3 tretment, () UV-tretment, n () line t 550 C. In orer to give more insight into the struturl hnge of the SnO2 films otine through ifferent post tretments, Rmn spetr re presente in Figure 3. The sttering pek t out 557 m 1 of the low-temperture-trete films isppers in the spetrum of the film sintere t 550 C whih might e use y the smller prtile size of the films without high-temperture rystlliztion proess [24]. However, the SnO2 film trete with pure wter vpour shows lrger prtile size in SEM spetr thn tht of the high-temperture sintere films. This phenomenon further suggeste tht the SnO2 existe s n gglomertion stte though smll SnO2 prtiles were eletroeposite from the eletrolyte. The pek t 1600 m 1 orrespone to the virtion of the OH units sore t the surfe of SnO2 nnoprtiles whih filitte the sorption of ye [24]. In ition, n extr pek ws oserve t the frequeny of 1045 m 1 fter the HNO3 wter vpour tretment. This new sttering pek shoul e ssoite with the intertion of the HNO3 n SnO2 nnoprtiles. The ition of HNO3 h me some signifint hnges for the SnO2 films. The SnO2 films otine uner ifferent post tretment were sensitize with N3 ye n use to ssemle DSCs. Figure 4 shows the urrent-voltge hrteristis of the ells uner simulte solr illumintion (t 100 mw m 2 ) n Tle 1 gives the hrteristi prmeters, tht is, open iruit voltge (Vo ), short iruit urrent (Js ), fill ftor (FF), n overll light to eletriity onversion effiieny (η). The SnO2 films trete with pure wter vpour show low Js (0.60 ma m 2 ) n fill ftor (0.48). With the ition of HNO3, the Js n fill ftor were oth enhne to e 1.02 ma m 2 n The improvement of the short iruit might e use y the inrese of surfe roughness

4 4 Nnomterils 5 Intensity (.u.) e Current ensity (ma m 2 ) Rmn shift (m 1 ) Figure 3: Rmn spetr of the eletroeposite SnO 2 films otine t ifferent onitions, () untrete, () pure wter vpour tretment, () HNO 3 tretment, () UV-tretment, n (e) line t 550 C Voltge (V) Figure 4: Photourrent-voltge hrteristi for DSCs se on the eletroeposite SnO 2 films otine t ifferent onitions, () pure wter vpour tretment, () HNO 3 tretment, () UVtretment, n () line t 550 C. (shown in Figure 3) whih permitte more single moleulr sorption on the SnO 2 film. The reson ws tht some SnO 2 nnoprtiles might e ethe wy uring the HNO 3 tretment. The enhnement of the fill ftor shoul e relte with the etter rystlliztion n esorption of the surfesore wter. After the UV-tretment, the J s ws signifintly inrese to e 1.65 ma m 2. The onversion Effiieny ws improve from 0.09% of the pure wter trete SnO 2 film to 0.35% of the film fter UV-tretment whih ws ontriute to the enhnement of the rystllinity (shown in Figure 1). This onversion effiieny ws expete to e further inrese y the optimiztion of the wter vpour n UV-tretment onitions, suh s the wter vpour pressure, the onentrtion of HNO 3, n UV-tretment time. It n e seen tht the photoeletri properties of the SnO 2 films is possile to give omprle performne with the films sintere t 550 C (0.56%) [25]. To further explore the effet of the post tretments on the eletron trnsport performne, eletrohemil impene of these SnO 2 films ws investigte. Impene spetr of these DSCs re shown in Figure 5. Generlly, the impene spetrum of testing ells onsiste of three min omponents, whih were ttriute to hrge trnsfer t Pt/ eletrolyte (ω 1 ), TiO 2 /eletrolyte (ω 2 ), n iffusion impene of I 3 in eletrolyte (ω 3 )[26, 27]. The hrteristi frequeny of (ω 2 ) oul reflet the reomintion lifetime (τ e ) of the injete eletrons [28]. The hrteristi frequenies ( f mx ) re 25.7, 3.7, 3.1, n 2.1 for the wter vpour tretment, wter vpour tretment with HNO 3, UV-tretment n high-temperture sintering, respetively. After the wter vpour tretment of SnO 2 film in pure wter, the eletron lifetime shoul e muh shorter thn the high-temperture sintere films (oring to τ e = 1/2πf mx ). But the ition of HNO 3 uring the wter vpour tretment proess improve the eletron lifetime signifintly. The results might e use y the intertion of the NO 3 n SnO 2 nnoprtiles exept for the etter rystllintion n esorption of Phse (eg) m 100 m k 10 k 100 k Frequeny (Hz) Figure 5: Impene spetr of the eletroeposite SnO 2 films otine t ifferent onitions, () pure wter vpour tretment, () HNO 3 tretment, () UV-tretment, n () line t 550 C. the surfe-sore wter. For the surfe lening effet of UV-tretment, the eletron lifetime ws further improve to e lmost the sme s tht of the high-temperture sintere film. 4. Conlusions SnO 2 porous films were frite y eletroeposition t 75 C through pulse-potentil tehnique. These eposite films proeee with ifferent post tretments. The effets of the post tretments on the SnO 2 film were investigte through X-ry iffrtion, FTIR-sorption spetrosopy, FE-SEM, n Rmn spetr. The wter vpour tretment n UV-tretment re helpful to improve the rystllinity

5 Nnomterils 5 n to relese the surfe-sore wter moleules of the eletroeposite SnO 2 film. With the ition of HNO 3 in the wter vpour tretment proess, the prtile size ws erese n new sttering pek ws foun in the Rmn spetr. The SnO 2 film fter the omintion tretment of the wter vpour tretment with HNO 3 solution n UV-tretment gives photoeletri onversion effiieny of out 0.35 with open iruit voltge 340 mv, shirt iruit urrent 1.65 ma m 2, n fill ftor 0.63 whih re ll omprle with the films sintere t 550 C. Aknowlegments This pper ws supporte y the Bsi n Frontier Tehnology Reserh Progrms of the Deprtment of Siene n Tehnology of Henn Provine (no ), the Key Tehnologies R&D Progrm of Henn Provine (no ), the Ntion Nturl Siene Fountion of Chin (no ), n the Reserh Fun of Henn University (no. 2010ZRZD07). Referenes [1] B. O Regn n M. Grätzel, A low-ost, high-effiieny solr ell se on ye-sensitize olloil TiO 2 films, Nture, vol. 353, no. 6346, pp , [2] A. Hgfelt, G. Boshloo, L. Sun, L. Kloo, n H. 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