G

E

Ao

BWa

b

a

ARRAA

KIFNF

1

hteflmgafspbattimpd

(

0d

ARTICLE IN PRESS ModelA-17883; No. of Pages 6Electrochimica Acta xxx (2011) xxx– xxx

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

n inverted fabrication method towards a flexible dye sensitized solar cell basedn a free-standing TiO2 nanowires membrane

o-Lei Chena,1, Hao Hua,1, Qi-Dong Taia, Nan-Gang Zhanga, Feng Guoa, Bobby Seboa,ei Liua, Ji-Kang Yuanb, Jian-Bo Wanga,∗, Xing-Zhong Zhaoa,∗

School of Physics and Technology, Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, Wuhan University, Wuhan 430072, ChinaDepartment of Applied Physics, The Hong Kong Polytechnic University, 999077, Hong Kong, China

r t i c l e i n f o

rticle history:eceived 28 August 2011eceived in revised form 4 November 2011ccepted 5 November 2011

a b s t r a c t

This paper reports an inverted fabrication process for the photoanode of a flexible dye sensitized solar cell(DSSC). This procedure involves assembling a free-standing TiO2 nanowires/nanoparticles hybrid mem-brane, via high temperature annealing, sputtering an indium tin oxide (ITO) layer onto this membrane,and transferring these onto a polydimethylsiloxane (PDMS) substrate. The inverted procedure prevents

vailable online xxx

eywords:nverted fabrication processree-standinganowires/nanoparticles hybrid membranelexible dye sensitized solar cell

thermal decomposition of polymer substrate, whilst enabling effective thermal treatment of the func-tionalized titanium oxide. The flexible DSSC fabricated in this way has an efficiency of 2.7%, which iscomparable with rigid device constructed using similar materials.

© 2011 Elsevier Ltd. All rights reserved.

. Introduction

In the past two decades, dye sensitized solar cells (DSSCs)ave attracted expanding attentions due to their several advan-ages over conventional photovoltaic cells, such as low-cost,asy-fabrication, and environmental friendliness [1]. Wherein, theexible version: employing metal or conductive transparent poly-er substrates, rather than rigid fluorine-doped tin oxide (FTO)

lass, offers the promise of enabling practical applications in therea of portable solar cells [2–4]. However, the metal substrates,or example titanium and stainless steel, whilst they can with-tand high temperature treatment, are opaque. In contrast to, theolymer substrates, which cannot withstand post heating process,ut are transparent. Firstly, the rigid devices can only be oper-ted under rear-illumination, where intensity loss occur when lightraverses the counter electrode and electrolyte. Moreover, the elec-ron path length in TiO2 nanostructures is longer for rear-siderradiation [5]. Secondly, poor crystallinity in the absence of ther-

Please cite this article in press as: B.-L. Chen, et al., An inverted fabricatfree-standing TiO2 nanowires membrane, Electrochim. Acta (2011), doi:10

al annealing leads to larger grain boundary resistance, thus lesshoto-generated electrons may be collected. Both of these effectsiminish the performance of the DSSCs as they are currently fabri-

∗ Corresponding authors. Tel.: +86 27 8764 2784; fax: +86 27 6875 2569.E-mail addresses: wang@whu.edu.cn (J.-B. Wang), xzzhao@whu.edu.cn

X.-Z. Zhao).1 These authors contributed equally to the work.

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.11.036

cated. Although several low-temperature sintering methods havebeen used to overcome the problem of thermal decomposition oftransparent flexible conductive polymer substrate, whilst enablingeffective thermal treatment of TiO2 porous film [4,6,7], the poorinter-particle connection remains a fundamental drawback dimin-ishing the practical light-to-electrical energy conversion yield [8].Recently, some film transfer techniques have been developed toovercome those problems [8–10] and although high efficiency wasobtained, the methods need multiple steps in their implementa-tion. A major challenge here is to find a facile way to fabricate aporous TiO2 nanostructure via high temperature sintering on theflexible polymer substrate.

Accordingly, we present an inverted fabrication method for thephotoanode of a flexible photoanode of dye sensitized solar cell,based on a free-standing TiO2 nanowires membrane. Wherein, thekey point is the design of suitable TiO2 membrane for inverted fabri-cation of photoanode. This requires a material composed of anatasephase TiO2, combining the features of flexibility and large surfacearea. A free-standing TiO2 nanowires membrane, which exhibitsgood bending properties (e.g. not cracking or delaminating), evenafter high temperature treatment, is beneficial. Additionally, itis known that surface area of TiO2 nanowires membrane canbe increased by adding TiO2 nanoparticles, which improves the

ion method towards a flexible dye sensitized solar cell based on a.1016/j.electacta.2011.11.036

dye-loading in DSSCs [11,12]. To this end, we have developeda free-standing TiO2 nanowires/nanoparticles hybrid membranevia high temperature annealing as a backbone material in thismethod.

dx.doi.org/10.1016/j.electacta.2011.11.036dx.doi.org/10.1016/j.electacta.2011.11.036http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactamailto:wang@whu.edu.cnmailto:xzzhao@whu.edu.cndx.doi.org/10.1016/j.electacta.2011.11.036

ING ModelE2 imica

2

2

btNlrawo

2

2a

2

s[sw

2

nsasdacof

2

aBEob(TJ

2

ea(s00tiw

ARTICLEA-17883; No. of Pages 6B.-L. Chen et al. / Electroch

. Experimental

.1. Nanowires synthesis

The precursor to TiO2 nanowires membrane was synthesizedy a typical hydrothermal method. An amount of 6.4 g commercialitanium oxide nanopowder (P25) was mixed with 80 mL of 10 MaOH solution. The suspension was then transferred into a Teflon-

ined stainless steel autoclave and heated at 170 ◦C for 4 days. Theesulting solid was then re-suspended in 500 mL of deionized waternd stirred for several hours, then collected, filtered, and washedith 0.1 M hydrochloric acid solution to remove remaining trances

f Na ion. Then, the white product was dried at 80 ◦C for 8 h.

.2. Nanowires/nanoparticles suspension

The as-prepared nanowires were dispersed in ethanol with0 wt.% methylcellulose and 30 wt.% P25. The mixture was stirredt room temperature for 6 h.

.3. Nanowires membrane fabrication

The nanowires membrane was produced by printing the suspen-ion on a hydrophobic Teflon substrate using doctor-blade method13]. Subsequently, the membrane was peeled off from the sub-trate and annealed in an oven at 500 ◦C. The sintered membraneas treated with 40 mM TiCl4 aqueous solution at 70 ◦C for 30 min.

.4. Photoanode assembly

A thin ITO layer was deposited on the as-prepared TiO2anowires/nanoparticles hybrid membrane using D.C. magnetronputtering. The sputtering method was according to previous liter-ture [18]. The as-treated membrane was sensitized by soaking inolution of N3 dye (cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-icarboxylato)-ruthenium(II)) at 60 ◦C for 12 h. Subsequently, anluminum foil was paste on the edge of sensitized membrane usingonductive epoxy adhesive. Finally, PDMS pre-polymer (in the ratiof A/B = 5/1) was cast on the hybrid membrane and cured at 85 ◦Cor 30 min.

.5. Materials characterizations

Power X-ray diffraction (XRD) measurements was carried outt a speed of 0.05◦ s−1 by a Bruker diffractometer (D8 Advance,ruker, Germany). Scanning electron microscope (SEM) images andnergy-dispersive X-ray spectroscopy (EDX) profile were obtainedn JEOL 6700F (JEOL, Japan). UV–vis spectrum were measuredy Lambda 650S (Perkin Elmer, UK). Atomic force microscopyAFM) images were obtained on SPM-9500J3 (Shimadzu, Japan).ransmission electron microscopy (TEM) images were obtained onEM2010FEF (JEOL, Japan).

.6. Photovoltaic properties measurement

The sensitized photoanode was assembled with Pt counterlectrode by filling electrolyte between the two electrodes. Thell-flexible demonstration was fabricated with bendable Pt/Ti foilPECCELL Technologies, Inc., Yokohama, Japan). The electrolyteolution was 1 M PMII (1-methyl 3-propyl imidazolium iodide),.04 M LiI, 0.03 M I2, 0.1 M GuSCN (guanidinium thiocyanate),

Please cite this article in press as: B.-L. Chen, et al., An inverted fabricatfree-standing TiO2 nanowires membrane, Electrochim. Acta (2011), doi:10

.5 M TBP (4-tert-butylpyridine) in component solvent of acetoni-rile and propylene carbonate (v:v = 1:1). J–V and electrochemicalmpedance spectroscopy (EIS) measurement of the membrane and

ere carried out under AM 1.5 illumination of 78.8 mW cm−2

PRESS Acta xxx (2011) xxx– xxx

(active area 0.25 cm2) using Newport 91192 light simulated solarlight source with 0.25 cm2 active area.

3. Results and discussion

The inverted fabrication procedure for flexible photoanodeof dye sensitized solar cell is summarized in Fig. 1. The TiO2nanowires/nanoparticles hybrid membrane was prepared bythe doctor-blade method [13], using an ethanol based TiO2nanowires/nanoparticles suspension, on a Teflon substrate placedin an oven. As the solvent evaporating, a free-standing TiO2nanowires/nanoparticles hybrid membrane was peeled off eas-ily from the hydrophobic substrate. Moreover, TiCl4 treatmentwas used to improve connectivity between the nanowires’ aggre-gates and nanoparticles [14]. Subsequently, a thin ITO layer wasdeposited on the membrane to form a transparent current collector.After that, the whole membrane was soaked into dye N3 acetoni-trile solution. Finally, a PDMS stander was casted on the membrane.Herein, it is notable that this strategy is quite different from others,in which the TiO2 film was fabricated by compressing on commer-cial conductive polymer substrates (e.g. ITO/PEN). The developedinverted fabrication method is low-cost, efficient, and easy to oper-ate. Furthermore, lower consumption was introduced in the wholeprocess by the absence of commercial conductive polymer sub-strates. Aside from the inverted fabrication method employed here,the sintered hybrid membrane has good stability, which is the guar-antee for kinds of post treatment to the material. The free-standinghybrid membrane maintains its flexibility even after 500 ◦C postheating process.

The fabrication of TiO2 nanowires/nanoparticles hybrid mem-brane is based on free-standing nanowires membrane. In ourexperiment, TiO2 nanowires were synthesized using hydrother-mal method which is available in previous literature [15,16]. Aas-prepared free-standing nanowires membrane is shown in Fig. 2aand b. The membrane exhibits uniform surface morphology andgood flexibility even after high temperature post heating process(Fig. 2c). The cross-section image (the insert figure of Fig. 2a)indicates a three-dimensional porous nano-structure of this TiO2nanowires network. A SEM image (Fig. 2b) shows the surfaceview of TiO2 nanowires/nanoparticles hybrid membrane. Nanopar-ticles distribute equably in the membrane. Additionally, the hybridmembrane with no delamination and crack, displays acceptableflexibility after thermal treatment at 500 ◦C (Fig. 2d).

XRD pattern shows that the nanowires after being calcined at500 ◦C transformed from titanate to anatase phase (Fig. 3a). How-ever, the reflection peak at 2� ∼ 28◦ and 43◦ can be attributed tothe remnant TiO2(B) nanowires, which can also be observed inTEM image (Fig. 4). The lattice spacings are (2̄ 0 1)TiO2(B) 50 nm and(1 1 0)TiO2(B) 36 nm, respectively [16]. The TiO2(B) is a metastablephase, which form a negatively charged layered structure. Thehydrogen ion and trace sodium ion, which is the remnant of ionexchange process, sit in between the adjacent layers, resulting invariable interlayer distances, which would explain the flexibility ofthe nanowires membrane [15].

Fig. 5a is the SEM cross-section image of TiO2nanowires/nanoparticles hybrid membrane, which has beendeposited a thin layer ITO, and Fig. 5b is the elements (Ti and In)linear scan from EDX profile. Both of these information confirmthat a thin layer ITO was sputtered on the surface of the hybridmembrane, which is not compact, but coats the nanowires andnanoparticles in the first micron of our membrane. Furthermore,no significant diffusion of indium was observed in our analyses,

ion method towards a flexible dye sensitized solar cell based on a.1016/j.electacta.2011.11.036

which is similar with the TiO2 films prepared by low temperaturemethod [17,18].

To investigate the impact on optical properties of this flexi-ble DSSCs photoanode, fabricated by using inverted method, three

dx.doi.org/10.1016/j.electacta.2011.11.036

Please cite this article in press as: B.-L. Chen, et al., An inverted fabrication method towards a flexible dye sensitized solar cell based on afree-standing TiO2 nanowires membrane, Electrochim. Acta (2011), doi:10.1016/j.electacta.2011.11.036

ARTICLE IN PRESSG ModelEA-17883; No. of Pages 6B.-L. Chen et al. / Electrochimica Acta xxx (2011) xxx– xxx 3

Fig. 1. Inverted fabrication procedure for flexible photoanode of dye sensitized solar cell. (a) Assembly of TiO2 nanowires/nanoparticles hybrid membrane. (b) TiCl4 treatmentof TiO2 nanowires/nanoparticles hybrid membrane. (c) Deposition a thin ITO layer on TiO2 nanowires/nanoparticles hybrid membrane using magnetron sputtering. (d)Sensitization of TiO2 nanowires/nanoparticles hybrid membrane using dye N3. (e) Assembly of Al foil electrode on ITO layer using conductive epoxy adhesive. (f) Casting atransparent PDMS stander on the membrane.

Fig. 2. Characterization of 500 ◦C post heated TiO2 nanowires membrane and TiO2 nanowires/nanoparticles hybrid membrane. (a) SEM image of 500 ◦C post heatedTiO2 nanowires membrane. The inset shows a cross-section SEM image of 500 ◦C post heated TiO2 nanowires membrane. (b) SEM image of 500 ◦C post heated TiO2nanowires/nanoparticles hybrid membrane. (c) Flexibility display of 500 ◦C post heated TiO2 nanowires membrane. (d) Flexibility display of 500 ◦C post heated TiO2nanowires/nanoparticles hybrid membrane.

dx.doi.org/10.1016/j.electacta.2011.11.036

ARTICLE IN PRESSG ModelEA-17883; No. of Pages 64 B.-L. Chen et al. / Electrochimica Acta xxx (2011) xxx– xxx

F(

U(iwl[bAstitws

ig. 3. XRD spectra of TiO2 nanowires. (a) Nanowires after hydrothermal reaction.b) Nanowires after heating at 500 ◦C.

V–vis absorbance measurements were carried out respectivelyshown in Fig. 6a). It was found that the thin ITO layer resultedn a significant increase of the absorbance, especially in short

avelength area. The increase is due to the absorbance of ITOayer, which is in good accordance with former reported work19]. After casting PDMS polymer stander on the hybrid mem-rane, the material’s absorbance decrease in full wavelength range.nd the decrease in long wavelength area is a little larger thanhort wavelength area. On the other hand, Fig. 6b indicated thathe PDMS stander lead to significant decrease of reflection, whichs corresponding to the result of absorbance. It can be supposed

Please cite this article in press as: B.-L. Chen, et al., An inverted fabricatfree-standing TiO2 nanowires membrane, Electrochim. Acta (2011), doi:10

hat the PDMS casting smoothes the surface of hybrid membrane,hich causes those changes in UV–vis spectrum. Furthermore, the

moother surface weakens the scattering effect of TiO2 nanowires,

Fig. 4. TEM micrographs of a TiO2 nanowire after heating at 500 ◦C.

Fig. 5. Cross-section information of TiO2 nanowires/nanoparticles hybrid

membrane with a thin layer ITO. (a) SEM cross-section image of TiO2nanowires/nanoparticles hybrid membrane with a thin layer ITO. (b) EDXprofiles of the hybrid membrane.

which leads to larger diminution for absorbance in long wavelengthrange.

Fig. 7 presents three-dimensional AFM images of hybrid mem-brane and the membrane with PDMS stander. It can be seenthat surface topography is quite different for both membranes.As expected, the casting of PDMS on hybrid membrane decreasesthe roughness of the membrane (shown in Table 1), which meansthis casting process smooth the surface of hybrid membrane. Thischange is consistent with the results in UV–vis spectrum.

Fig. 8 shows the J–V performance for flexiblenanowires/nanoparticles hybrid membrane photoanode aswell as rigid one which has been assembled by doctor-blademethod [13] on FTO conductive glass substrate, using same hybrid

−2 −2

ion method towards a flexible dye sensitized solar cell based on a.1016/j.electacta.2011.11.036

materials. The photocurrents are 6.2 mA cm and 6.4 mA cmfor half-flexible and rigid solar cells, respectively. Additionally,the half-flexible photoanode presents poorer fill factor and photo-voltage (FF = 0.55, Voc = 633 mV) compared to rigid one (FF = 0.67,

Table 1Surface roughness parameters.a

Ra (nm) Rq (nm)

Hybrid membrane 70.6 87.2Hybrid membrane/PDMS 39.5 49.4

a Ra, average roughness (2 �m scan size); Rq, root-mean-square roughness (2 �mscan size).

dx.doi.org/10.1016/j.electacta.2011.11.036

ARTICLE IN PRESSG ModelEA-17883; No. of Pages 6B.-L. Chen et al. / Electrochimica Acta xxx (2011) xxx– xxx 5

Fig. 6. UV–vis spectrum of various membranes. (a) UV–vis absorption spec-trum for sensitized nanowires/nanoparticles hybrid membrane, sensitizednanowires/nanoparticles hybrid membrane with ITO layer, and sensitizednanowires/nanoparticles hybrid membrane with ITO layer and PDMS stander. (b)UbI

VboT

doc

tpDrt

TCms

Fig. 7. Three-dimensional AFM images of various membranes. (a) Sensitizednanowires/nanoparticles hybrid membrane. (b) Sensitized nanowires/nanoparticleshybrid membrane hybrid membrane with PDMS stander.

V–vis reflection spectrum for sensitized nanowires/nanoparticles hybrid mem-rane with ITO layer, sensitized nanowires/nanoparticles hybrid membrane with

TO layer and PDMS stander, and pure PDMS membrane.

oc = 709 mV). Finally, the half-flexible photoanode fabricatedy inverted method gives an overall power conversion efficiencyf 2.7%, which is comparable with 3.8% for rigid one (shown inable 2).

Fig. 8 also displays the performance of an all-flexible DSSCemonstration, using this photoanode. Efficiency of 2.5% wasbtained. The Jsc and Voc are comparable with that of half-flexibleell.

The photovoltage and fill factor of flexible photoanode is poorerhan rigid device. It can be assumed that these losses are caused by

Please cite this article in press as: B.-L. Chen, et al., An inverted fabrication method towards a flexible dye sensitized solar cell based on afree-standing TiO2 nanowires membrane, Electrochim. Acta (2011), doi:10.1016/j.electacta.2011.11.036

oor connection between ITO layer and TiO2 membrane in flexibleSSC. Wherein, the ITO layer was directly deposited on porous and

ough hybrid membrane which may lead to poorer conductivityhan compact and continuous ITO thin film on glass substrates. This

able 2ell performance of full-flexible, half-flexible and rigid dye sensitized solar cells,easured under illumination with AM 1.5 simulated sunlight, 78.8 mW cm−2. Jsc:

hort-circuit current density; Voc: open-circuit voltage; FF: fill factor; �: efficiency.

Jsc (mA cm−2) Voc (mV) FF � (%)

Half-flexible 6.2 633 0.55 2.7All-flexible 6.0 600 0.55 2.5Rigid 6.4 709 0.67 3.8

Fig. 8. J–V curves of half-flexible dye sensitized solar cell and all-flexible dye sensi-tized solar cell fabricated by inverted method and rigid dye sensitized solar cell insame system as reference.

dx.doi.org/10.1016/j.electacta.2011.11.036

ARTICLE ING ModelEA-17883; No. of Pages 66 B.-L. Chen et al. / Electrochimica

Fig. 9. Nyquist plots of half-flexible and rigid DSSCs obtained at open circuit con-d0a

hflc

iea[idttlcii[h

4

flshtps

[

[

[[

[

[

[

[

[

[[20] J. Halme, P. Vahermaa, K. Miettunen, P. Lund, Advanced Materials 22 (2010)

itions under 78.8 mW cm−2, AM 1.5 simulated irradiation with active area of.25 cm2. The symbols are experimental data, and the solid lines are fitted resultsccording to the inset equivalent circuit.

igher resistance may cause photovoltage and fill factor losses inexible device. Furthermore, PDMS casting on hybrid membraneould also lead to increase of resistance.

Electrical impedance spectroscopy (EIS) is shown in Fig. 9. Thempedance of DSSC can be studied effectively by the insertedquivalent circuit model. The equivalent circuit is constructed byssigning different cell components and their interfaces resistances20]. For this study, two typical semicircles are observed as shownn our previous work [21]. Semicircle at middle range of frequenciesescribes the charge transfer resistance at the interface betweenhe electrolyte and TiO2 hybrid membrane, which is at the focus ofhis work. By fitting the experimental data with the inset equiva-ent circuit, the charge transfer resistance (Rct) are 39 � and 52 �,orresponding to rigid cell and half-flexible cell respectively. Thencrease in charge transfer resistance at the electrolyte/photoanodenterface was caused by the decrease in surface area in photoanode22], which is according to ITO sputtering and PDMS casting on theybrid membrane. This result is consistent with Jsc data in Fig. 8.

. Conclusions

In summary, this inverted method has been used to fabricateexible photoanode of dye sensitized solar cell based on free-tanding TiO2 nanowires/nanoparticles hybrid membrane with

Please cite this article in press as: B.-L. Chen, et al., An inverted fabricatfree-standing TiO2 nanowires membrane, Electrochim. Acta (2011), doi:10

igh temperature thermal treatment. In conjunction with thereatment of the hybrid membrane by TiCl4 solution, the flexiblehotoanode shows comparable performance with rigid device inimilar system. A photocurrent density of 6.2 mA cm−2 for flexible

[[

PRESS Acta xxx (2011) xxx– xxx

DSSC is close to 6.4 mA cm−2 for rigid one in same system. Theconversion efficiency of flexible DSSC is 2.7%, comparable to 3.8%of rigid cell. Therefore, this inverted method proves a new way toresolve the mismatch problem regarding thermal decompositionof polymer substrate and thermal annealing of TiO2. Moreover, thismethod is also facile, convenient, and easy to be scaled-up. Andit is speculated that the overall energy efficiency could be furtherimproved by optimizing the composition of hybrid membrane,and the stability study is ongoing.

Acknowledgments

The financial support of this work by the National Basic ResearchProgram (No. 2011CB933300) of China, and the National ScienceFund for Talent Training in Basic Science (Grant No. J0830310) aregratefully acknowledged.

References

[1] B. O’Regan, M. Gratzel, Nature 353 (1991) 737.[2] M.G. Kang, N.-G. Park, K.S. Ryu, S.H. Chang, K.-J. Kim, Solar Energy Materials

and Solar Cells 90 (2006) 574.[3] D. Kuang, J.R.M. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S.M.

Zakeeruddin, M. Gratzel, ACS Nano 2 (2008) 1113.[4] F. Pichot, J.R. Pitts, B.A. Gregg, Langmuir 16 (2000) 5626.[5] S. Ito, S.M. Zakeeruddin, P. Comte, P. Liska, D. Kuang, M. Gratzel, Nature Pho-

tonics 2 (2008) 693.[6] T.N. Murakami, Y. Kijitori, N. Kawashima, T. Miyasaka, Journal of Photochem-

istry and Photobiology A: Chemistry 164 (2004) 187.[7] D. Zhang, T. Yoshida, T. Oekermann, K. Furuta, H. Minoura, Advanced Functional

Materials 16 (2006) 1228.[8] Q. Miao, L. Wu, J. Cui, M. Huang, T. Ma, Advanced Materials 23 (2011) 2764.[9] M. Durr, A. Schmid, M. Obermaier, S. Rosselli, A. Yasuda, G. Nelles, Nature

Materials 4 (2005) 607.10] L. Yang, L. Wu, M. Wu, G. Xin, H. Lin, T. Ma, Electrochemistry Communications

12 (2010) 1000.11] P. Joshi, L. Zhang, D. Davoux, Z. Zhu, D. Galipeau, H. Fong, Q. Qiao, Energy and

Environmental Science 3 (2010) 1507.12] B. Tan, Y. Wu, The Journal of Physical Chemistry B 110 (2006) 15932.13] M.G. Kang, K.M. Kim, K.S. Ryu, S.H. Chang, N.-G. Park, K.-J. Kim, Journal of

Electrochemical Society 151 (2004) E257.14] N. Teı̌treault, E. Horvaı̌th, T. Moehl, J.R.M. Brillet, R. Smajda, S.P. Bungener, N.

Cai, P. Wang, S.M. Zakeeruddin, L.S. Forroı̌, A. Magrez, M. Gratzel, ACS Nano 4(2010) 7644.

15] W. Dong, A. Cogbill, T. Zhang, S. Ghosh, Z.R. Tian, The Journal of Physical Chem-istry B 110 (2006) 16819.

16] R. Yoshida, Y. Suzuki, S. Yoshikawa, Journal of Solid State Chemistry 178 (2005)2179.

17] H.C. Weerasinghe, P.M. Sirimanne, G.V. Franks, G.P. Simon, Y.B. Cheng, Journalof Photochemistry and Photobiology A: Chemistry 213 (2010) 30.

18] S.I. Boyadzhiev, M.M. Rassovska, Y. Matsumura, Y. Kotani, R.S. Yordanov, I.R.Yordanova. Characterization of reactive sputtered ITO and TiO2/ITO thin filmsfor applications at elevated temperatures, 33rdInt. Spring Seminar on Electron-ics Technology (2010) 65–70.

19] S. Bhagwat, R.P. Howson, Surface and Coatings Technology 111 (1999) 163.

ion method towards a flexible dye sensitized solar cell based on a.1016/j.electacta.2011.11.036

E210.21] Q. Tai, B. Chen, F. Guo, S. Xu, H. Hu, B. Sebo, X. Zhao, ACS Nano 5 (2011) 3795.22] J. Chen, C. Chen, C. Wu, C. Lin, Y. Lai, C. Wang, H. Chen, R. Vittal, K. Ho, Journal

of Materials Chemistry 20 (2010) 7201.

dx.doi.org/10.1016/j.electacta.2011.11.036

An inverted fabrication method towards a flexible dye sensitized solar cell based on a free-standing TiO2 nanowires membrane1 Introduction2 Experimental2.1 Nanowires synthesis2.2 Nanowires/nanoparticles suspension2.3 Nanowires membrane fabrication2.4 Photoanode assembly2.5 Materials characterizations2.6 Photovoltaic properties measurement

3 Results and discussion4 ConclusionsAcknowledgmentsReferences