007) 5131–5135www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Fabrication of dye sensitized solar cell using TiO2 coated carbon nanotubes

Tae Young Lee a, P.S. Alegaonkar a,b, Ji-Beom Yoo a,⁎

a Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon, 440-746, Republic of Koreab Department of Physics, University of Pune, Pune-411 007, India

Available online 20 November 2006

Abstract

We fabricated a dye sensitized solar cells (DSCs) using TiO2 coated multi-wall carbon nanotubes (TiO2-CNTs). Carbon nanotubes (CNTs) haveexcellent electrical conductivity and good chemical stability. We introduced CNTs in DSCs to improve solar cell performance through reduction ofseries resistance. TiO2-CNTs were obtained by Sol–Gel method. Compared with a conventional TiO2 cell, the TiO2-CNTs content (0.1 wt.%) cellshowed ∼50% increase in conversion efficiency, which is attributed to the increase in short circuit current density (Jsc). The enhancement in Jscoccurs due to improvement in interconnectivity between the TiO2 particles and the TiO2-CNTs in the porous TiO2 film.© 2006 Elsevier B.V. All rights reserved.

Keywords: Dye sensitized; Carbon nanotubes; Passivation layer

1. Introduction

DSCs have been attracting considerable attention because oftheir high efficiency, simple fabrication process and lowproduction cost. Cost effectiveness is an important parameterfor producing DSCs as compared to the widely used conven-tional Si-solar cells [1]. Moreover, enhanced dye sensitized solarcell efficiency would provide enormous economical advantages[2–6]. Recently, TiO2 nanoparticles have been used as a workingelectrode for DSCs due to their higher value of efficiency thanany other metal oxide semiconductor. However, the highestconversion efficiency so far reported for this device is ∼10%under air mass (AM) 1.5 (100mWcm−2) irradiation when liquidelectrolytes containing I–/I3

– redox couples was used asconjunction [7,8]. Because, photo-generated charge recombina-tion should be prevented for enhanced efficiency, solelyenlarging the oxide electrode surface area is not sufficient.Strategies to enhance efficiency include the promotion ofelectron transfer through film electrodes and the blockage ofinterface states lying below the edge of conduction band.Interface states facilitate recombination of injected conductionband electrons with I3

− ions. The efforts have been made toimprove the conversion efficiency by modifying TiO2 film.

⁎ Corresponding author. Tel.: +82 31 290 7396; fax: +82 31 290 7410.E-mail address: jbyoo@skku.ac.kr (J.-B. Yoo).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.10.056

CNTs are remarkable materials, which are being widelystudied because of their extraordinary electronic and mechan-ical properties. Polymer composites with CNTs have recentlybeen investigated for improved electrical conducting layer,optical devices and high strength composites. A composite ofpoly(p-phenylene vinylene) with CNTs in a photovoltaic deviceshowed good quantum efficiency, owing to the formation of acomplex interpenetrating network with the polymer chains [9].CNTs also conferred electrical conductivity to metal oxidenanocomposites [10]. However, only few reports have beenfound in the literature where CNTs were used in TiO2 films ofDSCs, despite of their expected potential to enhance solar energyconversion efficiency due to favorable electrical conductivity.Thus, we introduce CNTs in DSCs to improve the electricalconductivity of TiO2 film.

In this study, we incorporated TiO2-CNTs in porous TiO2

films. As a result, the value of the Jsc of DSCs was increased. Toprevent leakage current in device, thin passivated layer wasprepared between the transparent conducting glass (FTO) sub-strate and porous TiO2 film.

2. Experimental

Multi-walled CNTs (MWNTs, supplied by ILJIN Nanotech)synthesized by the thermal chemical vapor deposition (thermalCVD) method were used in the present study. The raw powdercontains MWNTs of diameter 25 nm, amorphous carbon, and

Fig. 2. XRD spectra for (a) pristine MWNTs and (b) TiO2-CNTs annealed at∼450 °C, under atmospheric conditions.

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carbon-encapsulated metal nanoparticles. MWNTs were oxi-dized in a hydrogen peroxide (H2O2) solution under ultra-sonication condition for 24 h at the temperature 50 °C toproduce finely dispersed MWNTs terminated with carboxylicacid groups. The resulting solution was filtered by a polytetra-fluoroethylene (PTFE) membrane with pore size 1 μm. At thisstep, the carbonaceous impurities were removed from the as-grown MWNTs. Raman spectrometer and Fourier transforminfrared spectrometer (FT-IR) were used to identify the for-mation of carboxylic acid groups on MWNTs.

The Sol–Gel solution (SGS)was prepared using titanium tetra-isopropoxide Ti(OPri)4, isopropanol (IPA), nitric acid (HNO3)and distilled water (H2O). The weight ratio for the SGSpreparation is kept as 1:10:1:0.2 for Ti(OPri)4:IPA:H2O:HNO3

[11]. The solution was reflux at the temperature 80 °C for a periodof 1 h, using a magnetic stirrer. For each sample, 1 g of MWNTswere mixed with 100 ml of SGS and stirred in close vials for 3 h.The impregnated MWNTs were separated from the solution byfiltration process. To obtain TiO2-CNTs, the filtrated nanotubeswere dried in an oven at 80 °C for 1 h under atmosphericconditions followed by thermal treatment at 450 °C for 1 h.

The passivation layer was introduced between the fluorine-doped SnO2 (FTO) substrate and porous TiO2 layer. To obtain auniform and flat surface, the Sol solution (Ti(OPri)4:IPA:HNO3=1:10:0.2) was spin coated. After being dried in air, thepassivation layer was annealed for 1 h at 500 °C, under atmo-spheric conditions. Scanning electron microscope (SEM) mea-surement revealed the thickness of the passivation layer 70 nm.This solution was also reflux at 80 °C for 1 h, using a magneticstirrer before spin coating.

Porous TiO2 films were prepared by coating a passivatedtransparent conducting glass substrate (Solaronix; fluorine-doped SnO2 overlayer; sheet resistance: 17 Ω/sq) with viscousslurry of TiO2 powder and TiO2-CNTs dispersed in an aqueoussolution. Initially, TiO2-CNTs (0.1–0.3 wt.%) were added inIPA and sonicated during 1 h to obtain well dispersed solutionof TiO2-CNTs in IPA. Commercially available TiO2 powder(0.5 g, P25, Degussa) and IPA included TiO2-CNTs (1 g) wereground in a mortar with distilled water (1 g), polyethyleneglycol (0.1 g, Aldrich, MW 2000) and polyethylene oxide(0.1 g, Aldrich, MW 100,000) to break up the aggregate into a

Fig. 1. SEM micrograph for TiO2 coated MWNTs (TiO2-CNTs).

dispersed paste. Adhesive tape was placed on the edges of theconductive glass to form a guide for spreading the slurry using aglass plate. The film thickness was controlled by the amount ofwater in the slurry and by the thickness of adhesive tape. Afterbeing dried, the porous TiO2 film mixed with TiO2-CNTs wasannealed for 1 h at 500 °C, under atmospheric conditions. Thefilm thickness was 10–15 um and measured with a TencorAlpha-Step profiler.

Following this process, the resulting surface-modified TiO2

films were immersed in absolute ethanol containing 0.3 mM[RuL2(NCS)2]·2H2O (L=2,2′-bipyridine-4,4′-dicarboxylicacid; Solaronix) for 12 h at room temperature. The dye-coveredelectrodes were then rinsed with absolute ethanol and dried. Ptcounter electrodes were prepared by spreading a drop of 5 mMhexachloroplatinic acid (Fluka) in IPA on the FTO glassfollowed by heating at 400 °C for 30 min in air. The Pt electrodewas placed over the dye-coated electrode, and the edges of thecell were sealed with 0.5-mm-wide strips of 100-μm-thickSurlyn (Dupont, grade 1702). The redox electrolyte consisted of0.8 M lithium iodide (LiI), 40 mM iodine (I2) and 0.2 M 4-tert-butylpyridine (TBP) in acetonitrile was introduced into the cellthrough one of the two small holes drilled in the counterelectrode. The holes were then covered and sealed with smallsquares of microscope objective glass and Surlyn.

To analyze the crystallinity of TiO2-CNTs, X-ray diffraction(XRD) data was recorded. Investigations on film morphologywere carried out by atomic force microscope (AFM) and fieldemission scanning electron microscope (FE-SEM). Currentdensity–voltage (J–V) characteristics were recorded usingKeithley (model 2400) as a source measure unit, which wasconnected between the FTO and Pt electrodes under anillumination of a 300 W Xe lamp (ILC technology Inc.). Thevoltage was scanned from −0.2 to 0.8 V in steps of 0.05 V. Theincident light intensity (100 mW cm−2) was calibrated using aNewport 818 UV photodiode detector.

3. Results and discussions

It is known that pristine MWNTs have hydrophobic surfaceand poor dispersion stability. To avoid these problems the

Fig. 3. AFM images (tapping-mode) for (a) FTO glass and (b) TiO2 passivatedFTO glass.

Fig. 5. J–V characteristics for (a) as prepared TiO2 film, (b) 0.1 wt.% TiO2-CNTs, (c) 0.2 wt.% TiO2-CNTs, and (d) 0.3 wt.% TiO2-CNTs, on TiO2 films.

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pretreatment of MWNTs is needed for many applications.Carboxylic acid groups could be generated easily by oxidationof MWNTs, by H2O2 treatment. It is a less-destructive and mildoxidation method for removing impurities as well as formingcarboxylic acid groups on nanotubes. H2O2 solution is a mild

Fig. 4. SEM images for (a) development of cracks on surface of porous TiO2 electrodthat TiO2 particles entirely covers TiO2-CNTs (content 0.1 wt.%), (c) the thicknesspassivation layer shown in (d).

acid and easy to handle. Also, reaction gases such as CO2 andH2O are non-toxic and could be released safely during theoxidation processing [12–15].

e, (b) the details of TiO2 cluster, which is marked by close circle in (a), indicatesof the porous TiO2 film and close circle with an arrow indicates details of TiO2

Table 1Photocurrent J–V parameters for CNTs incorporated TiO2 electrodes in DSCs

Composition of TiO2-CNTs

Jsc(mA/cm2)

Voc(V)

Fillfactor

Efficiency(%)

Cell area(cm2)

0 wt.% 8.49 0.60 0.65 3.32 0.330.1 wt.% 13.5 0.63 0.59 4.97 0.360.2 wt.% 11.1 0.61 0.61 4.14 0.330.3 wt.% 9.69 0.63 0.59 3.60 0.33

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H2O2 treated MWNTs have a hydrophilic surface. Thecarboxylic acid groups on the surface of MWNTs have a polarcovalent bonding by the electronegativity difference. Thus, wecould consider that H2O2 treated MWNTs have a generallynegatively charged surface. The negatively charged surface ofMWNTs enhances the stability of dispersion. These H2O2

treated MWNTs were well dispersed in SGS (Sol–Gel solution).After reaction with SGS, morphology of TiO2 coated MWNTs,recorded by SEM, is shown in Fig. 1. The coated samples werethermally treated at 450 °C in order to crystallize anatase on thenanotubes surface without any damage to MWNTs. The re-corded XRD spectra for (a) pristine MWNTs and (b) thermallytreated TiO2-CNTs are shown in Fig. 2. The most intense peaksat (002) reflection (Profile (a)) corresponds to the MWNTs andoverlaps significantly with (101) band (Profile (b)), whichcorresponds to anatase TiO2. The other peaks present in Profile(b) were attributed to the anatase form of TiO2. It indicates thatthe surface of MWNTs was covered with anatase form of TiO2.

Normally, the passivation layer consisted of materials ofsame or lower conduction band than that of porous TiO2 film.The passivation layer made by spin coating of TiO2 sol solutionhas a non-porous thin film. Thus, the passivation layer improvesthe property of interface surface and enhances the conversionefficiency by reducing the recombination of electron-hole pairs.Also, the passivation layer increases the adhesion propertybetween the FTO glass and porous TiO2 layer, reduces theleakage current by preventing direct contact of electrolyte andFTO glass [18]. Fig. 3 shows the AFM images for (a) pristineFTO glass and (b) FTO glass passivated with TiO2 thin film. Forraw FTO glass, the surface is very rough with values of r.m.s.roughness (σ) of 32.1 nm. The passivated FTO glass shows σ∼16.1 nm, i.e. the morphology is found to be smoother than thatof un-passivated FTO glass.

Fig. 4(a) shows appearance of large amount of cracks on thesurface of TiO2 film. One can see that, these large sized islandsconsist of clusters of TiO2 nanoparticles on there. The closecircle along with an arrow in Fig. 4(a) indicates the details of thecluster morphology as shown by close circle in Fig. 4(b). Thus,it can be seen from Fig. 4(b), that cluster consists of largeamount of TiO2 nanoparticles. Moreover, Fig. 4(b) indicatesthat the clusters of TiO2 particles entirely cover the aggregatedTiO2-CNTs. As a result, no TiO2-CNTs are observed on thesurface of porous TiO2 film. However, it is noteworthy that, theamount of cracks developed on TiO2 films, prepared by com-mercially available powder (P25) only, is found to be marginal.This phenomenon suggests that TiO2-CNTs play the role ofnucleation sites for clustering TiO2 nanoparticles on the surfaceof the film. Moreover, with increase in amount of TiO2-CNTs,

the number of cracks on the surface of the films is increasedsubsequently. It is thought that, the cracks generated on thesurface could be reducing the number of adsorption sites onTiO2 film as well as causing the discrimination in the con-version efficiency of DSCs. The thickness of porous TiO2 filmis shown in Fig. 4(c) and close circle with an arrow indicate thedetails of the passivation layer as shown in Fig. 4(d). As de-scribed earlier, the thickness of porous TiO2 film (10 um) wascontrolled by an adhesive tape. Whereas, the thickness of pas-sivation layer (70 nm) was controlled by the spin coating speed.

Fig. 5 shows the J–V characteristics for (a) as prepared TiO2

films, (b) 0.1 wt.% TiO2-CNTs, (c) 0.2 wt.% TiO2-CNTs, and(d) 0.3 wt.% TiO2-CNTs, on TiO2 films and Table 1 enlists otherparameters of solar cells. The value of open circuit voltage, Voc,is increased by ∼6% from 0.60 V to 0.63 V with subsequentincrease in TiO2-CNTs from 0 to 0.1 wt.%. It has been observedthat surface treatment usually increases the values of Vocregardless of nature and characteristics of coated materials onelectrode [16,17]. Furthermore, for 0.1 wt.% TiO2-CNTs, valueof short circuit photocurrent density (Jsc) is found to beincreased by ∼60% from 8.49 to 13.5 mA cm−2, whencompared to as prepared TiO2 electrode. However, Jsc decreasesthereafter (from 13.5 to 9.69 mA cm−2) with subsequentincrease in content of TiO2-CNTs from 0.1 to 0.3 wt.%. Withincrease in TiO2-CNTs contents, the gradual decrease in Jsc, isattributed to the increase in number of cracks on the surface ofporous TiO2 electrode. Consequently, the addition of TiO2-CNTs enhances the electro-conductivity of porous TiO2

electrode, but decrease the adsorption site for ruthenium dyeto making a crack in TiO2 film. Thus, TiO2-CNTs (0.1 wt.%)contained TiO2 electrodes have higher value of conversionefficiency ∼50% higher compared with as prepared TiO2

electrode of cell (Table 1). However, subsequent increase inconcentration of TiO2-CNTs (from 0.1 to 0.3 wt.%) does nothelp to increase the value of conversion efficiency further.

4. Conclusions

The J–V characteristics were studied as a function of TiO2-CNTs content (wt.%) in the porous TiO2 film, which was usedas an electrode in DSCs. We easily obtained TiO2-CNTs bySol–Gel method. SEM analysis shows that, on porouselectrode, the TiO2-CNTs were entirely surrounded by TiO2

nanoparticles. Compared with a conventional TiO2 cell, themodified TiO2 cell (0.1 wt.% TiO2-CNTs) showed ∼50%increase in the value of conversion efficiency. The enhancementin Jsc is attributed to the improved interconnectivity between theTiO2 particles and the TiO2-CNTs in the porous TiO2 film. It isemphasized that addition of the TiO2-CNTs in the TiO2 filmprovides more efficient electron transfer through the film inDSCs.

Acknowledgements

This research was funded by the KOSEF through CNNC(Center for Nanotubes and Nano structured Composite) atSungkyunkwan University.

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One of the authors (PSA) is thankful to the KoreanGovernment for awarding BK21 and also thankful to CSIR,New Delhi, India for awarding Research Associateship.

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