The fabrication and characterization of dye-sensitized solar cells with a branched structure of ZnO nanowires

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Fabrication and characterizationof dye-sensitized solar cells usingelectrospun TiO2 nanofibre as a solarlight harvesting layerMasood Hamadanian a b & Vahid Jabbari ba Institute of Nanoscience and Nanotechnology, University ofKashan, Kashan, Iranb Faculty of Chemistry, Department of Physical Chemistry,University of Kashan, Kashan, Iran

Available online: 28 Jun 2011

To cite this article: Masood Hamadanian & Vahid Jabbari (2011): Fabrication and characterizationof dye-sensitized solar cells using electrospun TiO2 nanofibre as a solar light harvesting layer,International Journal of Sustainable Energy, DOI:10.1080/1478646X.2011.583988

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International Journal of Sustainable EnergyiFirst, 2011, 1–13

Fabrication and characterization of dye-sensitized solarcells using electrospun TiO2 nanofibre as a solar

light harvesting layer

Masood Hamadaniana,b* and Vahid Jabbarib

aInstitute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran; bFaculty ofChemistry, Department of Physical Chemistry, University of Kashan, Kashan, Iran

(Received 24 February 2011; final version received 21 April 2011 )

In this work, we investigated the dye-sensitized solar cells (DSSCs) using photoanode, which were fabri-cated directly from electrospinning of TiO2 nanofibres onto the substrate. The electrochemical impedancespectroscopy (EIS) and current–voltage diagram were used to analyse electron transport in electrospunnanofibres and determine their applicability in DSSCs. In order to improve the short-circuit photocurrentof fabricated cells, we treated the electrospun TiO2 electrode with the TiCl4 aqueous solution. The modifi-cation of the photoelectrode significantly improves the power conversion efficiency (over 26%) of the solarcells attributed to the higher electron lifetime (τ ) and reduction in recombination processes as indicatedby the EIS of the solar cells.

Keywords: energy conversion; nanofibres; TiO2; electron lifetime; dye-sensitized solar cell

Introduction

The dye-sensitized solar cells (DSSCs) are recognized as third-generation solar cells due totheir low manufacturing cost and relatively high energy conversion efficiency (O’Regan andGrätzel 1991). The DSSC achieved the energy conversion efficiency of ∼10.7% using TiO2 aselectrode (Grätzel 2003). The energy conversion efficiency reached up to ∼15% for tandem cellscomprising DSSCs and Cu(In,Ga)Se2 cells (Liska et al. 2006). However, the solar cells that gavehigh conversion efficiencies have relatively lower cell working areas (<0.25 cm2). Considerableattentions were devoted in the past to understanding the electrode architecture for efficient electrondiffusion and transport (Uchida et al. 2002, Song et al. 2004, 2005, Law et al. 2005, Baxter andAydil 2006, Onozuka et al. 2006, Kokubo et al. 2007) as well as choice of electrolytes (Wang et al.2003, Komiya et al. 2004, Kato et al. 2006) and dye molecules (Horiuchi et al. 2004, Wang et al.2004, Schmidt-Mende et al. 2005a,b, Ito 2006). The TiO2 nanofibres (Song et al. 2004, Onozukaet al. 2006, Kokubo et al. 2007) and nanorods (Song et al. 2005) recently gained attention for

*Corresponding author. Email: [email protected]

ISSN 1478-6451 print/ISSN 1478-646X online© 2011 Taylor & FrancisDOI: 10.1080/1478646X.2011.583988http://www.informaworld.com

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fabrication of DSSCs due to the channelled electron transfer in them (Adachi et al. 2004, Kokuboet al. 2007).

Electrospun TiO2 nanofibre mat with nanofibres randomly overlaid, having diameters rangingfrom tens to hundreds of nanometres, and consisting of anatase-phased TiO2 single crystallinegrains with sizes of ∼10 nm, is expected to significantly outperform other forms, such as pow-der and film, of TiO2 for solar cell and photocatalytic applications. This is because of that theelectrospun nanofibre mat has a high specific surface area (ranging from hundreds to thousandsof square metres per each gram) and also controllable pore sizes among the nanofibres (rangingfrom tens to hundreds of nanometres); additionally, the thickness of the electrospun mat can bereadily manipulated. Unlike nanoscaled TiO2 particles/rods which are in a loose granular form,the electrospun nanofibres are well contained within the mat/felt. For the solar cell (specially theDSSC) application, the electrospun TiO2 nanofibre mat can provide a continuous transportingpathway for the photogenerated electrons and results in a high performance/efficiency. Presently,most DSSCs are made from powders consisting of TiO2 nanoparticles, and the photogeneratedelectrons move via hopping through the nanoparticles. It is well known that hopping is not anefficient pathway for the electron transportation, and the electron loss/recombination is occurhigh during hopping (Yu et al. 1995, Bach et al. 1998, Wold 1993).

Electrospinning is a technique that utilizes electric force along to drive the spinning processand to produce fibres. Unlike conventional spinning techniques such as solution spinning ormelt spinning that are capable of producing fibres with diameters in the micrometre range (5–15 μm), electrospinning is capable of producing fibres with diameters in the nanometre range(50–1000 nm). Unlike nanotubes, nanowires, or nanorods, most of which are made by bottom-upmanufacturing process and usually require further expensive purifications, electrospun nanofibresare made through a top-down nanomanufacturing process. Electrospun nanofibres are thereforeinexpensive, continuous, also relatively easy to align, assemble, and process into applications. Inthe recent decade, the technique of ‘electrospinning’ and its unique product of ‘nanofibres’ havebeen widely researched throughout the world (Hoffmann et al. 1995, Reneker and Chun 1996,Reneker et al. 2000, Shin et al. 2001).

Electrospun ceramic nanofibres are made by electrospinning spin dopes containing the precur-sors of ceramics followed by high-temperature annealing. Numerous electrospun ceramic, suchas silica (SiO2), zinc oxide (ZnO), and titania (TiO2) nanofibres, have been fabricated (Choi etal. 2003, Li and Xia 2003, Liu et al. 2008).

In this study, the DSSCs were fabricated by direct electrospun of TiO2 nanofibre (TiO2-NF)and TiO2 nanoparticles (TiO2-NPs) on the fluorine-doped tin oxide (FTO) glass and they werealso treated chemically to increase the TiO2 volume content by using dip-coat of TiO2 crystalfrom aqueous TiCl4 solution. The additional TiO2 layer modified the photocurrent generation ofDSSCs based on electrospun TiO2 nanofibres electrode.An energy conversion efficiency of around0.61 was achieved by TiO2-NPs and it was around 0.83% for TiO2-NF and 0.9 for TiCl4-treatedTiO2-NF electrode that showed an improvement of 26% after post-treatment.

Experimental

Materials

Titanium (IV) isopropoxide (TiP) (Merck), acetic acid (Merck), PVAc with a molecular weight of500,000 (Aldrich), N, N -dimethyl formamide (DMF) (Merck), 4-tert-butylpyridine (Aldrich),acetonitrile (Fluka), valeronitrile (Fluka), H2PtCl6 (Fluka), iodine (I2) (99.99%, Superpur1,Merck), lithium iodide (LiI) (Merck), ethyl celluloses (5–15 mPa s at 5% in toluene:ethanol/80:20

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at 25◦C, #46 070, Fluka), and Ru complex dye [Dyesol, cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicar-boxylato)-ruthenium(II)bis-tetra butyl ammonium (N719)] were used as-received.H2O was purified by distillation and filtration (Milli-Q). TiCl4 (Fluka) was diluted with waterto 2 M at 0◦C to make the stock solution, which was kept in a freezer and freshly diluted to40 mm with water for each treatment of the FTO-coated glass plates and surfaces of TiO2 porouselectrodes.

Preparation of TiO2 nanoparticles by sol–gel

The typical synthesis procedure of TiO2 nanoparticles was adopted from the previous work(Hamadanian et al. 2009): TiP (4.7 ml) was hydrolysed using 9.0 ml of glacial acetic acid at0◦C. To this solution, 98.8 ml of deionized water was added drop-wise under vigorous stirring for1 h, and subsequently the solution was ultrasonicated for 15 min in ice bath. Then the stirring wascontinued for further 4.5 h and again the solution was ultrasonicated for 15 min in ice bath untila clear solution was formed. The prepared solution was kept under dark condition for nucleationprocess for 24 h. It was then gelated in an oven at 70◦C for 12 h. The gel was dried at 100◦C andsubsequently the resulting material was powdered and then annealed in a muffle furnace at 500◦Cfor 2 h.

Preparation of TiO2 nanofibres by electrospinning

TiO2 fibres were electrospun directly onto a SnO2:F-coated glass substrate (FTO, 10 cm × 10 cm,TEC-15, Dyesol) from mixture containing 3 g of PVAc, 6 g of TiP, and 2.4 g of acetic acid as acatalyst for sol–gel reaction in DMF (37.5 ml). In a typical electrospinning, the precursor solutionwas loaded into a syringe connected to a high-voltage power supply (Bertan Model 230). Anelectric field of 15 kV was applied between a metal orifice and the FTO substrate at a distanceof 10 cm. The spinning rate was controlled by a syringe pump (FNM, Iran) at 60 l/min. Theannealing was carried out at 500◦C for 30 min in air (Figure 1).

In order to improve the performance of the film as a sensitized photoanode, the as-preparedelectrode was put into 0.2 mol/l TiCl4 aqueous solution. After being left overnight at room tem-perature in a closed chamber, the treated electrode was washed with doubly distilled water andthen annealed again for 30 min at 500◦C in air (Nazeeruddin et al. 1993).

Figure 1. Schematic diagram of electrospinning set-up.

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Figure 2. The suggested mechanism of the nanofibril formation in electrospun TiO2 fibres.

The mechanism of fibril formation

In this section, we propose the mechanism of the fibril formation and the sheath-and-core structureof electrospun TiO2 fibres. The fibre formation in the electrospinning process is considered asthe following factors: (a) the sol–gel transformation reaction with ambient moistures, (b) theconcentration change after evaporation of solvents, (c) the temperature lowering and condensationof moisture due to the heat of evaporation, (d) the dynamic phase separation phenomena in termsof miscibility, (e) the stability of separated phases, (f) the mechanical transformation, (g) thesolidification process, etc. It is very difficult to understand all the phenomena separately, sincethe process is carried out dynamically in a short time. The mechanism for the electrospun TiO2

fibres is suggested simply in three steps as illustrated in Figure 2.At first, the TiP precursor starts the sol–gel transformation with the reaction under ambient

moisture conditions. The spinning solution contains the TiO2 sol from the pre-reaction withacetic acids in polymer solution before spinning and the sol-state is converted rapidly to the gel-state. At this stage, the solvent is evaporated with increased surface area due to the decrease indiameter of flow stream. The miscible equilibrium state in spinning solution becomes unstablethermodynamically due to the concentration fluctuation of each component. The phase separationand solidification of the TiO2 phase occur abruptly in the immiscible system such as in the PVAcmatrix resulting in particle formation. On the other hand, in the miscible system such as PVAcsystem, the phase separation carried out slowly and the TiO2-rich, as well as PVAc-rich, domainsco-existed to allow the domain elongation in liquid phases. Also, the lowered temperature of thesurface accelerates the gel transformation due to the condensation of moisture resulting in thesheath formation of TiO2 fibres. The solidified fibre forms the sheath-and-core structure withfibrils.

Spectroscopy analysis

The X-ray diffraction (XRD) patterns were recorded on a Philips X’pert Pro MPD model X-raydiffractometer using Cu Kα radiation as the X-ray source. The diffractograms were recorded inthe 2θ range 10–80◦. The average crystallite size of anatase phase was determined accordingto the Scherer equation. The morphology and size were characterized using scanning electronmicroscope (SEM) (Philips XL-30ESM) and transmission electron microscope (TEM) (PhilipsEM208). UV spectra of the samples were recorded by a Perkin Elmer Lambda2S spectrometer.FT-IR spectra of the samples were recorded on a Nicolet Magna IR 550 spectrometer. For FT-IRanalysis, the as-electrospun TiO2 nanofibre mat and sol–gel-prepared TiO2 nanoparticle powderwere used for this analysis.

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Figure 3. Schematic diagram of DSSCs with compact TiO2 layer, porous TiO2 + dye, electrolyte with redox couple,and Pt-coated FTO counter-electrode.

Cell assembly and photovoltaic measurements

The TiO2 web electrode was immersed overnight in an ethanol solution containing 3 × 10−4 Mof ruthenium dye (N719). The electrode was rinsed and dried after its removal from the dyesolution. The liquid electrolyte we used consisted of 0.05 M iodine (I2), 0.1 M LiI, and 0.5 M4-tertbutylpyridine dissolved in acetonitrile. Pt-coated SnO2:F glass was used as the counter-electrode. The typical active area of DSSC was 0.25 cm2 (Figure 3).

The photocurrent–voltage characteristics of the cell were measured with an electrochemicalanalyser (CHI630A, Chenhua Instruments Co., Shanghai) under solar simulator illumination(CMH-250, Aodite Photoelectronic Technology Ltd., Beijing) at room temperature. A GCR-4 Nd:YAG laser (Spectra Physics) (λ = 532 nm, laser pulse width = 6 ns) was employed fortransient photovoltage and photocurrent measurements. A TDS3032 oscilloscope (Tektronix)was used for recording transient photovoltage and photocurrent generation. The entire energyconversion efficiency, η, is calculated by means of the following equations:

η = [VOCISCFF/Plight]. (1)

Here, VOC is the open-circuit voltage (V), ISC the short-circuit current (mA/cm2), and FF is thefill factor given by

FF =[VmaxImax

VOCISC

], (2)

where Vmax and Imax are, respectively, the voltage and current at the point of maximum poweroutput of cell.

Results and discussion

Characterization of as-prepared TiO2 films

Figure 4 shows the XRD patterns of TiO2 nanofibres (TiO2-NFs) before and after TiCl4 treatment.The prominent peaks of anatase phase of nanocrystalline TiO2 fibres that used for this study canbe seen at the 2θ values of 25.28, 37.80, 48.05, 53.89, 55.06, 62.69, 68.76, 70.31, and 75.03(PDF#00-021-1272). The base TiO2 web is anatase as shown in Figure 4(a) and the chemicallygrown TiO2 is confirmed from the mixture of anatase and rutile peaks in Figure 4(b) after treatment(Smestad et al. 1994, Ito et al. 2005).

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Figure 4. XRD patterns of TiO2 nanofibre before (a) and after (b) treatment by TiCl4.

The average crystal size was estimated from the Scherer equation on the anatase (2θ = 25.2,37.8, and 48.1◦) diffraction peaks (the most intense peaks for each sample):

D = Kλ

cos θ,

where D is the crystal size of the catalyst, λ the X-ray wavelength (1.54056Å), β the full-widthat half-maximum of the diffraction peak (radian), Kα is a coefficient (0.89), and θ the diffractionangle at the peak maximum. Average crystal sizes of TiO2 nanoparticles and nanofibres werecalculated to be around 10–15 and 12–16 nm, respectively.

SEM micrograph of the annealed (500◦C) TiO2 nanoparticles is shown in Figure 5. This imageshows global and uniform nanoparticles. Further observation indicates that the morphology ofnanoparticles is very rough and may be beneficial in enhancing the adsorption of reactants due totheir great surface roughness and high surface area. Figure 6 also shows the TEM image of theTiO2 nanoparticles. It can be seen that the sample consists of well-dispersed nanoparticles whoseparticle size of this TiO2 was found to be around 20 nm.

The fibrous structures and diameter distribution of the PVAc and PVAc/TiO2 electrospunnanofibres were examined using SEM. It can be seen that the electrospun nanofibres from PVAcand PVAc/TiO2 solutions form fibrous webs and the nanofibres are randomly distributed in thewebs, as presented in Figure 6. It was found that the average diameter of the electrospun PVAcnanofibre is decreased as PVAc was removed by calcination, and the diameter distribution becomesnarrower, indicating the improved uniformity of the fibres (Chuangchote et al. 2009). There isnot considerable shrinkage in diameter due to the elimination of the polymer component, whichresults in no stress in the fibre layer. Electrospun TiO2 web for DSSCs in this work shows thewell-organized porous electrode structure. Fabricated electrodes showed an average thickness of15 μm. This is a suitable range of thickness for DSSCs to obtain good efficiencies.

FT-IR spectra of TiO2-NFs and TiO2-NPs (Figure 7) show peaks corresponding to stretchingvibrations of the O–H and bending vibrations of the adsorbed water molecules around 3350–3450and 1620–1635 cm−1, respectively. The broad intense band below 1200 cm−1 is due to Ti–O–Tivibrations. There is no band centred at 1389 cm−1 due to the bending vibrations of the C–H bond in

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Figure 5. SEM and TEM images of TiO2 nanoparticles.

the catalysts. Also, there are no excess bands assigned for the alkoxy groups. Therefore, additionof acetic acid did not cause any residual impurities on TiO2 surface after calcinations.

Current–voltage measurement

Figure 8 presents the photocurrent density–voltage curves for cells based on electrospun TiO2-NFelectrodes under the standard globalAM1.5 irradiation. The photovoltaic characteristics of DSSCsbased on electrospun TiO2-NF and TiO2-NP electrodes are summarized in Table 1. JSC and h

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Figure 6. SEM images of PVAc/TiO2 nanofibre (a, b), TiO2 nanofibre (after 500◦C heat treatment of PVAc/TiO2) (c,d) and fibre diameter distribution in PVAc/TiO2 (e) and TiO2 nanofibres (f).

were found to be higher for the electrodes fabricated from nanofibres than the fibres, whereas VOC

was found to decrease slightly in the same order.JSC and VOC are related to the rate of electron injection to the photoelectrode and to the energy

difference between the Fermi level of the photoelectrode and the Nernst potential of the redoxcouple in the electrolyte, respectively. Recent studies indicate that JSC and VOC are influenced bythe number of carboxylic groups (–COOH) existing in the dye for anchoring to the photoelectrode.The N719 dye has two carboxylic acid groups resulting in strong anchoring and has been explainedin the framework of an adiabatic injection mechanism (Figure 9) (Yu et al. 1995, Brabec et al.2001). In this mechanism, the electronic state changes its localization from the dye to the TiO2

upon electron injection, resulting in a strong electronic coupling, which leads to an increased JSC.In addition to electronic coupling, the obvious reason can be the geometric differences betweenthe nanoparticles and the nanofibres. The variations in the observed values of JSC suggest that theelectronic coupling efficiency of photoexcited electrons could be different. This result supportsthe higher efficiency of DSSCs fabricated from nanofibres. The former ones are much bigger andhence can have much lower resistance that in turn can cause higher currents.

The UV–VIS absorption spectra of the N719 dye sensitized on the different TiO2 electrodes indiluted ethanol solution (10−5 mol L−1) are shown in Figure 10. The two broad visible bands at 502and 370 nm in N719 are assigned to metal-to-ligand charge-transfer origin. The bands in the UV at

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Figure 7. FT-IR spectra of TiO2 nanoparticles (a) and nanofibres (b).

Figure 8. I–V curve of TiO2 nanoparticles, nanofibres, and TiCl4-treated nanofibres.

308 nm with a shoulder at 304 nm are assigned as intra-ligand (π–π∗) charge-transfer transitions(Joo 2007). Notably, the TiCl4-post-treated TiO2 nanofibre-based DSSCs showed larger amountof dye loading than the untreated nanofibres and also TiO2 nanoparticles, contributing to theimproved dye loading and increase in JSC (as shown in Table 1). Therefore, the greater JSC andefficiency with the TiCl4-post-treated TiO2 nanofibre resulted from an increased number of dye

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Table 1. TiO2-based working electrodes tested with an active area of 0.25 cm2 underAM1.5G 100 mW cm−2.

Cell type JSC (mA cm−2)a VOC (mV)b FF (%)c η (%)d τ (s)

TiO2-NP 1.68 0.69 0.53 0.61 0.013TiO2-NF 2.52 0.62 0.54 0.83 0.014TiCl4-TiO2-NF 2.9 0.61 0.51 0.9 0.017

aJsc is the short-circuit current density.bVoc is the open-circuit voltage.cFF is the fill factor.dη is the overall conversion efficiency.

Figure 9. Schematic diagram of interaction between TiO2 surface and N719 dye.

Figure 10. UV–Vis absorbance of the dye desorbed from different TiO2 electrodes.

molecules chemically adsorbed onto the TiO2 nanofibre surface. The amount of dye loading maydepend on the surface area and surface conditions of the TiO2 nanofibre.

Several studies have previously reported that treatment of nanocrystalline TiO2 with TiCl4solution results in a significant improvement in device performance (Smestad et al. 1994, Papa-georgiou et al. 1997, Chuangchote et al. 2009]. The nanocrystalline TiO2 electrode with TiCl4treatment increased the necking between the nanoparticles of the film, thus facilitating the diffu-sion of photoinjected electron between particles and lowering the probability of recombination.After TiCl4 treatment at 25◦C for 24 h, the efficiency of TiO2-NF-based DSSCs was improved by

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Figure 11. EIS (Nyquist plots) of DSSCs based on TiO2 nanoparticles and electrospun TiO2 nanofibre electrode afterTiCl4 treatment.

about 26%. The insufficient conversion efficiencies of TiO2-NF-based DSSCs can be attributedto the poor adhesion of electrospun nanofibres to the FTO substrate due to nanofibres crackingand peeling away from the substrate after calcination.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) has proven to be a useful technique for the char-acteristics of electronic and ionic processes in DSSCs (Wang et al. 2002, O’Regan et al. 2007).Figure 11 shows Nyquist plots of DSSCs. Three typical semicircles in the Nyquist plots wereobserved, which correspond to the I−3 transport in the electrolyte (low-frequency semicircle),electron recombination at the TiO2/electrolyte interface together with the electron transport inthe TiO2 network (middle frequency semicircle), and charge transport process at the interfacebetween redox couple and counter-electrode (high-frequency semicircle) in order of increasingfrequency (Ferrere et al. 1997, Green et al. 2005, Kim et al. 2006). It is demonstrated that thesemicircle corresponding to TiO2/dye/electrolyte interface gets larger for the DSSC TiO2-NPsthan that for TiO2-NFs. At open circuit, the electrons that are injected from the adsorbed dye to theTiO2-NPs partially accumulate at the interface of the TiO2/dye/electrolyte and react with elec-trolyte, thereby decreasing the impedance of this network. Therefore, the larger semicircle of theDSSC (TiO2-NF) shows that the recombination of photogenerated electrons with the electrolyteby the backward transfer is retarded compared to TiO2-NPs.

According to the EIS model developed by Kern et al. (2002), the lifetime (τ ) of the injectedelectrons in TiO2 film can be estimated from the position of the low-frequency peak in Bode-phase plots of EIS spectra, through the expression τ = 1/2πf , where f is the frequency of thesuperimposed ac voltage. The fitted values of τ for the DSSC made for different photoelectrodes

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are shown in Table 1. The higher electron lifetime for the DSSC TiO2-NF results in an enhancementin power conversion efficiency when compared to the DSSC TiO2-NP.

Conclusion

In this study, DSSCs were fabricated using transparent and conductive FTO coated by TiO2

nanofibre film prepared by electrospinning. SEM images confirmed the prepared nanofibres,exceptionally long, and had an average diameter of around 300 nm. It was shown that an improve-ment with TiCl4 as much as 26% was achieved. These achievements were obtained due to areduction in grain boundaries, to an efficient high charge collection, and to rapid electron trans-port. DSSCs based on the nanofibrous TiO2 photoelectrodes have shown an energy conversionefficiency of 0.9% under irradiation of AM 1.5 simulated sunlight with a power density of100 mW cm−2, which shows good promise of electrospun nanofibrous TiO2 as the photoelec-trode in DSSCs. We believe that simple approaches such as the present one to develop nanofibreDSSCs would open up enormous possibilities in effective harvesting of solar energy for com-mercial applications, considering the fact that electrospinning is a cost-effective method for thelarge-scale production of nanofibres.

Acknowledgement

The authors are gratefully acknowledged partial support received from the National Research Council of Iran.

References

Adachi, M., Murata, Y., Takao, J., Jiu, J., Sakamoto, M., and Wang, F., 2004. Highly efficient dye-sensitized solar cellswith a titania thin-film electrode composed of a network structure of single-crystal-like tio2 nanowires made by the“oriented attachment” mechanism. J. Am. Chem. Soc., 126, 14943–14949.

Bach, U., Lupo, D., Comte, P., Moser, J.E., Weissorte, F., Salbeck, J., Spreitzer, H., and Grätzel, M., 1998. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature, 395,583–584.

Baxter, J.B. and Aydil, E.S., 2006. Dye-sensitized solar cells based on semiconductor morphologies with ZnO nanowires.Sol. Energy Mater. Sol. Cells, 90, 607–622.

Brabec, C.J., Sariciftci, N.S., and Hummelen, J.C., 2001. Plastic solar cells. Adv. Funct. Mater., 11, 15–26.Choi, S.S., Lee, S.G., Im, S.S., Kim, S.H., and Joo, Y.L., 2003. Silica nanofibers from electrospinning/sol-gel process. J.

Mater. Sci. Lett., 22, 891–896.Chuangchote, S., Sagawa, T., and Yoshikawa, S., 2009. Electrospinning of poly(vinyl pyrrolidone): Effects of solvents

on electrospinnability for the fabrication of poly(p-phenylene vinylene) and TiO2 nanofibers. J. Appl. Pol. Sci., 114,2777–2791.

Ferrere, S., Zaban, A., and Gregg, B.A., 1997. Dye sensitization of nanocrystalline tin oxide by perylene derivatives. J.Phys. Chem. B, 101, 4490–4493.

Grätzel, M., 2003. Dye-sensitized solar cells. J. Photochem. Photobiol. C, 4, 145–153.Green,A.N.M., Palomares, P.E., Haque, S.A., Kroon, J.M., and Durrant, J.R., 2005. Charge transport versus recombination

in dye-sensitized solar cells employing nanocrystalline TiO2 and SnO2 films. J. Phys. Chem. B, 109, 12525–12533.Hamadanian, M., Reisi-Vanani, A., and Majedi, A., 2009. Preparation and characterization of S-doped TiO2 nanoparticles,

effect of calcination temperature and evaluation of photocatalytic activity. Mater. Chem. Phys., 116, 376–382.Hoffmann, M.R., Martin, S.T., Choi, W., and Bahnemann, D.W., 1995. Environmental applications of semiconductor

photocatalysis. Chem. Rev., 95, 69–96.Horiuchi, T., Miura, H., and Uchida, S., 2004. Highly efficient metal-free organic dyes for dye-sensitized solar cells. J.

Photochem. Photobiol. A, 164, 29–32.Ito, S., 2006. High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness.

Adv. Mater., 18, 1202–1205.Ito, S., Liska, P., Charvet, R., Comte, P., Péchy, P., Nazeeruddin, M.K., Zakeeruddin, S.M., and Grätzel, M., 2005. Control

of dark current in photoelectrochemical (TiO2/I− − I−3 ) and dye-sensitized solar cells. Chem. Commun., 4351–4353.Joo, S.-W., 2007. Electric field-induced charge transfer of (Bu4N)2[Ru(dcbpyH)2-(NCS)2] on gold, silver, and cop-

per electrode surfaces investigated by means of surface-enhanced raman scattering. Bull. Korean Chem. Soc., 28,1405–1409.

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Vah

id J

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ri]

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6:05

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201

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International Journal of Sustainable Energy 13

Kato, T., Okazaki, A., and Hayase, S., 2006. Latent gel electrolyte precursors for quasi-solid dye sensitized solar cells:The comparison of nano-particle cross-linkers with polymer cross-linkers. J. Photochem. Photobiol. A, 179, 42–48.

Kern, R., Sastrawan, R., Ferber, J., Stangel, R., and Luther, J., 2002. Modeling and interpretation of electrical impedancespectra of dye solar cells operated under open-circuit conditions. Electrochim. Acta, 47, 4213–4225.

Kim, J.Y., Chung, I.J., Kim, J.K., and Yu, J.W., 2006. Solid-state photovoltaic devices based on perylene acid-sensitizednanostructural SnO2 with P(VdF-co-HFP) gel electrolyte. Curr. Appl. Phys., 6, 969–973.

Kokubo, H., Ding, B., Naka, T., Tsuchihira, H., and Shiratori, S., 2007. Multi-core cable-like TiO2 nanofibrous membranesfor dye-sensitized solar cells. Nanotechnology, 18, 165604.

Komiya, R., Han, L., Yamanaka, R., Islam, A., and Mitate, T., 2004. Highly efficient quasi-solid state dye-sensitized solarcell with ion conducting polymer electrolyte. J. Photochem. Photobiol. A, 164, 123–127.

Law, M., Greene, L.E., Johnson, J.C., Saykally, R., and Yang, P., 2005. Nanowire dye-sensitized solar cells. Nat. Mater.,4, 455–459.

Li, D. and Xia, Y.N., 2003. Fabrication of titania nanofibers by electrospinning. Nano Lett., 3, 555–560.Liska, P., Thampi, K.R., Grätzel, M., Bremaud, D., Rudmann, D., Upadhyara, H.M., and Tiwari, A.N., 2006. Nanocrys-

talline dye-sensitized solar cell/copper indium gallium selenide thin-film tandem showing greater than 15%conversion efficiency. Appl. Phys. Lett., 88, 203103.

Liu, Y., Sagi, S., Chandrasekar, R., Zhang, L., Hedin, N.E., and Fong, H., 2008. Preparation and characterization ofelectrospun SiO2 nanofibers. J. Nanosci. Nanotechnol., 8, 1528–1536.

Nazeeruddin, M.K., Kay, A., Rodicio, I., Baker, R.H., Müller, E., Liska, P., Vlachopoulos, N., and Grätzel, M., 1993.Conversion of light to electricity by cis-X2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) charge-transfer sen-sitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc., 115,6382.

Onozuka, K., Ding, B., Tsuge, Y., Naka, T., Yamazaki, M., Sugi, S., Ohno, S., Yoshikawa, M., and Shiratori, S.,2006. Electrospinning processed nanofibrous TiO2 membranes for photovoltaic applications. Nanotechnology, 17,1026–1031.

O’Regan, B. and Grätzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films.Nature, 353, 737–749.

O’Regan, B.C., Durrant, J.R., Sommeling, P.M., and Bakker, N.J., 2007. Influence of the TiCl4 treatment on nanocrystallineTiO2 films in dye-sensitized solar cells. 2. Charge density, band edge shifts, and quantification of recombination lossesat short circuit. J. Phys. Chem. C, 111, 14001–14010.

Papageorgiou, N., Maier, W.F., and Grätzel, M., 1997. An iodine/triiodide reduction electrocatalyst for aqueous andorganic media. J. Electrochem. Soc., 144, 876–881.

Reneker, D.H. and Chun, I., 1996. Nanometre diameter fibres of polymer, produced by electrospinning. Nanotechnology,7, 216–222.

Reneker, D.H., Yarin, A.L., Fong, H., and Koombhongse, S., 2000. Bending instability of electrically charged liquid jetsof polymer solutions in electrospinning. J. Appl. Phys., 87, 4531.

Schmidt-Mende, L., Bach, U., Humphry-Baker, R., Horiuchi, T., Miura, H., Ito, S., Uchida, S., and Grätzel, M., 2005a.Organic dye for highly efficient solid-state dye sensitized solar cells. Adv. Mater., 17, 813–825.

Schmidt-Mende, L., Kroeze, J.E., Durrant, J.R., Nazeeruddin, M.K., and Grätzel, M., 2005b. Effect of Hydrocarbon ChainLength of Amphiphilic Ruthenium Dyes on Solid-State Dye-Sensitized Photovoltaics. Nano Lett., 5, 1315–1320.

Shin, Y.M., Hohman, M.M., Brenner, M.P., and Rutledge, G.C., 2001. Electrospinning: a whipping fluid jet generatessubmicron polymer fibers. Appl. Phys. Lett., 78, 1149.

Smestad, G., Bignozzi, C., and Argazzi, R., 1994. Testing of dye sensitized TiO2 solar cells I: experimental photocurrentoutput and conversion efficiencies. Sol. Energy Mater. Sol. Cells, 32, 259–272.

Song, M.Y., Kim, D.K., Ihn, K.J., Jo, S.M., and Kim, D.Y., 2004. Electrospun TiO2 electrodes for dye-sensitized solarcells. Nanotechnology, 15, 1861–1865.

Song, M.Y., Ahn, Y.R., Jo, S.M., Kim, D.Y., and Ahn, J.P., 2005. Dye sensitization of nanocrystalline TiO2 by perylenederivatives. Appl. Phys. Lett., 87, 113–118.

Uchida, S., Chiba, R., Tomiha, M., Masaki, N., and Shirai, M., 2002. Application of titania nanotubes to a dye-sensitizedsolar cell. Electrochemistry, 70, 418–420.

Wang, S., Li, Y., Du, C., Shi, Z., Xiao, S., Zhua, D., Gao, E., and Cai, S., 2002. Application of titania nanotubes to adye-sensitized solar cell. Synth. Met., 128, 299–304.

Wang, P., Zakeeruddin, S.M., Moser, J.E., Nazeeruddin, M.K., Sekiguchi, T., and Grätzel, M., 2003. A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nat. Mater., 2,402–407.

Wang, P., Zakeeruddin, S.M., Moser, J.E., Humphry-Baker, R., Comte, P., Aranyos, V., Hagfeldt, A., Nazeeruddin, M.K.,and Grätzel, M., 2004. Stable new sensitizer with improved light harvesting for nanocrystalline dye-sensitized solarcells. Adv. Mater., 16, 1806–1811.

Wold, A., 1993. Photocatalytic properties of titanium dioxide (TiO2) Chem. Mater., 5, 280–283.Yu, G., Gao, J., Hummelen, J.C., Wudl, F., and Heeger, A., 1995. Polymer photovoltaic cells: enhanced efficiencies via a

network of internal donor-acceptor heterojunctions. Science, 270, 1789–1791.

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