Thin Solid Films 515 (2007) 5131 – 5135
www.elsevier.com/locate/tsf
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 Korea
b
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) have
excellent electrical conductivity and good chemical stability. We introduced CNTs in DSCs to improve solar cell performance through reduction of
series resistance. TiO2-CNTs were obtained by Sol–Gel method. Compared with a conventional TiO2 cell, the TiO2-CNTs content (0.1 wt.%) cell
showed ∼ 50% increase in conversion efficiency, which is attributed to the increase in short circuit current density (Jsc). The enhancement in Jsc
occurs 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 of
their high efficiency, simple fabrication process and low
production cost. Cost effectiveness is an important parameter
for producing DSCs as compared to the widely used conventional Si-solar cells [1]. Moreover, enhanced dye sensitized solar
cell efficiency would provide enormous economical advantages
[2–6]. Recently, TiO2 nanoparticles have been used as a working
electrode for DSCs due to their higher value of efficiency than
any other metal oxide semiconductor. However, the highest
conversion efficiency so far reported for this device is ∼10%
under air mass (AM) 1.5 (100 mW cm− 2) irradiation when liquid
electrolytes containing I – /I3– redox couples was used as
conjunction [7,8]. Because, photo-generated charge recombination should be prevented for enhanced efficiency, solely
enlarging the oxide electrode surface area is not sufficient.
Strategies to enhance efficiency include the promotion of
electron transfer through film electrodes and the blockage of
interface states lying below the edge of conduction band.
Interface states facilitate recombination of injected conduction
band electrons with I3− ions. The efforts have been made to
improve 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 widely
studied because of their extraordinary electronic and mechanical properties. Polymer composites with CNTs have recently
been investigated for improved electrical conducting layer,
optical devices and high strength composites. A composite of
poly(p-phenylene vinylene) with CNTs in a photovoltaic device
showed good quantum efficiency, owing to the formation of a
complex interpenetrating network with the polymer chains [9].
CNTs also conferred electrical conductivity to metal oxide
nanocomposites [10]. However, only few reports have been
found in the literature where CNTs were used in TiO2 films of
DSCs, despite of their expected potential to enhance solar energy
conversion efficiency due to favorable electrical conductivity.
Thus, we introduce CNTs in DSCs to improve the electrical
conductivity 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. To
prevent leakage current in device, thin passivated layer was
prepared between the transparent conducting glass (FTO) substrate and porous TiO2 film.
2. Experimental
Multi-walled CNTs (MWNTs, supplied by ILJIN Nanotech)
synthesized by the thermal chemical vapor deposition (thermal
CVD) method were used in the present study. The raw powder
contains MWNTs of diameter 25 nm, amorphous carbon, and
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T.Y. Lee et al. / Thin Solid Films 515 (2007) 5131–5135
carbon-encapsulated metal nanoparticles. MWNTs were oxidized in a hydrogen peroxide (H2O2) solution under ultrasonication condition for 24 h at the temperature 50 °C to
produce finely dispersed MWNTs terminated with carboxylic
acid groups. The resulting solution was filtered by a polytetrafluoroethylene (PTFE) membrane with pore size 1 μm. At this
step, the carbonaceous impurities were removed from the asgrown MWNTs. Raman spectrometer and Fourier transform
infrared spectrometer (FT-IR) were used to identify the formation of carboxylic acid groups on MWNTs.
The Sol–Gel solution (SGS) was prepared using titanium tetraisopropoxide Ti(OPri)4, isopropanol (IPA), nitric acid (HNO3)
and distilled water (H2O). The weight ratio for the SGS
preparation 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 period
of 1 h, using a magnetic stirrer. For each sample, 1 g of MWNTs
were mixed with 100 ml of SGS and stirred in close vials for 3 h.
The impregnated MWNTs were separated from the solution by
filtration process. To obtain TiO2-CNTs, the filtrated nanotubes
were dried in an oven at 80 °C for 1 h under atmospheric
conditions followed by thermal treatment at 450 °C for 1 h.
The passivation layer was introduced between the fluorinedoped SnO2 (FTO) substrate and porous TiO2 layer. To obtain a
uniform and flat surface, the Sol solution (Ti(OPri)4:IPA:
HNO3 = 1:10:0.2) was spin coated. After being dried in air, the
passivation layer was annealed for 1 h at 500 °C, under atmospheric conditions. Scanning electron microscope (SEM) measurement revealed the thickness of the passivation layer 70 nm.
This solution was also reflux at 80 °C for 1 h, using a magnetic
stirrer before spin coating.
Porous TiO2 films were prepared by coating a passivated
transparent conducting glass substrate (Solaronix; fluorinedoped SnO2 overlayer; sheet resistance: 17 Ω/sq) with viscous
slurry of TiO2 powder and TiO2-CNTs dispersed in an aqueous
solution. Initially, TiO2-CNTs (0.1–0.3 wt.%) were added in
IPA and sonicated during 1 h to obtain well dispersed solution
of TiO2-CNTs in IPA. Commercially available TiO2 powder
(0.5 g, P25, Degussa) and IPA included TiO2-CNTs (1 g) were
ground in a mortar with distilled water (1 g), polyethylene
glycol (0.1 g, Aldrich, MW 2000) and polyethylene oxide
(0.1 g, Aldrich, MW 100,000) to break up the aggregate into a
Fig. 2. XRD spectra for (a) pristine MWNTs and (b) TiO2-CNTs annealed at
∼450 °C, under atmospheric conditions.
dispersed paste. Adhesive tape was placed on the edges of the
conductive glass to form a guide for spreading the slurry using a
glass plate. The film thickness was controlled by the amount of
water in the slurry and by the thickness of adhesive tape. After
being dried, the porous TiO2 film mixed with TiO2-CNTs was
annealed for 1 h at 500 °C, under atmospheric conditions. The
film thickness was 10–15 um and measured with a Tencor
Alpha-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′-dicarboxylic
acid; Solaronix) for 12 h at room temperature. The dye-covered
electrodes were then rinsed with absolute ethanol and dried. Pt
counter electrodes were prepared by spreading a drop of 5 mM
hexachloroplatinic acid (Fluka) in IPA on the FTO glass
followed by heating at 400 °C for 30 min in air. The Pt electrode
was placed over the dye-coated electrode, and the edges of the
cell were sealed with 0.5-mm-wide strips of 100-μm-thick
Surlyn (Dupont, grade 1702). The redox electrolyte consisted of
0.8 M lithium iodide (LiI), 40 mM iodine (I2) and 0.2 M 4-tertbutylpyridine (TBP) in acetonitrile was introduced into the cell
through one of the two small holes drilled in the counter
electrode. The holes were then covered and sealed with small
squares of microscope objective glass and Surlyn.
To analyze the crystallinity of TiO2-CNTs, X-ray diffraction
(XRD) data was recorded. Investigations on film morphology
were carried out by atomic force microscope (AFM) and field
emission scanning electron microscope (FE-SEM). Current
density–voltage (J–V) characteristics were recorded using
Keithley (model 2400) as a source measure unit, which was
connected between the FTO and Pt electrodes under an
illumination of a 300 W Xe lamp (ILC technology Inc.). The
voltage was scanned from − 0.2 to 0.8 V in steps of 0.05 V. The
incident light intensity (100 mW cm− 2) was calibrated using a
Newport 818 UV photodiode detector.
3. Results and discussions
Fig. 1. SEM micrograph for TiO2 coated MWNTs (TiO2-CNTs).
It is known that pristine MWNTs have hydrophobic surface
and poor dispersion stability. To avoid these problems the
T.Y. Lee et al. / Thin Solid Films 515 (2007) 5131–5135
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Fig. 3. AFM images (tapping-mode) for (a) FTO glass and (b) TiO2 passivated
FTO glass.
pretreatment of MWNTs is needed for many applications.
Carboxylic acid groups could be generated easily by oxidation
of MWNTs, by H2O2 treatment. It is a less-destructive and mild
oxidation method for removing impurities as well as forming
carboxylic acid groups on nanotubes. H2O2 solution is a mild
Fig. 5. J–V characteristics for (a) as prepared TiO2 film, (b) 0.1 wt.% TiO2CNTs, (c) 0.2 wt.% TiO2-CNTs, and (d) 0.3 wt.% TiO2-CNTs, on TiO2 films.
acid and easy to handle. Also, reaction gases such as CO2 and
H2O are non-toxic and could be released safely during the
oxidation processing [12–15].
Fig. 4. SEM images for (a) development of cracks on surface of porous TiO2 electrode, (b) the details of TiO2 cluster, which is marked by close circle in (a), indicates
that TiO2 particles entirely covers TiO2-CNTs (content 0.1 wt.%), (c) the thickness of the porous TiO2 film and close circle with an arrow indicates details of TiO2
passivation layer shown in (d).
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T.Y. Lee et al. / Thin Solid Films 515 (2007) 5131–5135
Table 1
Photocurrent J–V parameters for CNTs incorporated TiO2 electrodes in DSCs
Composition of TiO2CNTs
Jsc
Voc
(mA/cm2) (V)
Fill
factor
Efficiency
(%)
Cell area
(cm2)
0 wt.%
0.1 wt.%
0.2 wt.%
0.3 wt.%
8.49
13.5
11.1
9.69
0.65
0.59
0.61
0.59
3.32
4.97
4.14
3.60
0.33
0.36
0.33
0.33
0.60
0.63
0.61
0.63
H2O2 treated MWNTs have a hydrophilic surface. The
carboxylic acid groups on the surface of MWNTs have a polar
covalent bonding by the electronegativity difference. Thus, we
could consider that H2O2 treated MWNTs have a generally
negatively charged surface. The negatively charged surface of
MWNTs 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 were
thermally treated at 450 °C in order to crystallize anatase on the
nanotubes surface without any damage to MWNTs. The recorded XRD spectra for (a) pristine MWNTs and (b) thermally
treated TiO2-CNTs are shown in Fig. 2. The most intense peaks
at (002) reflection (Profile (a)) corresponds to the MWNTs and
overlaps significantly with (101) band (Profile (b)), which
corresponds to anatase TiO2. The other peaks present in Profile
(b) were attributed to the anatase form of TiO2. It indicates that
the surface of MWNTs was covered with anatase form of TiO2.
Normally, the passivation layer consisted of materials of
same or lower conduction band than that of porous TiO2 film.
The passivation layer made by spin coating of TiO2 sol solution
has a non-porous thin film. Thus, the passivation layer improves
the property of interface surface and enhances the conversion
efficiency by reducing the recombination of electron-hole pairs.
Also, the passivation layer increases the adhesion property
between the FTO glass and porous TiO2 layer, reduces the
leakage current by preventing direct contact of electrolyte and
FTO glass [18]. Fig. 3 shows the AFM images for (a) pristine
FTO glass and (b) FTO glass passivated with TiO2 thin film. For
raw 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 that
of un-passivated FTO glass.
Fig. 4(a) shows appearance of large amount of cracks on the
surface of TiO2 film. One can see that, these large sized islands
consist of clusters of TiO2 nanoparticles on there. The close
circle along with an arrow in Fig. 4(a) indicates the details of the
cluster morphology as shown by close circle in Fig. 4(b). Thus,
it can be seen from Fig. 4(b), that cluster consists of large
amount of TiO2 nanoparticles. Moreover, Fig. 4(b) indicates
that the clusters of TiO2 particles entirely cover the aggregated
TiO2-CNTs. As a result, no TiO2-CNTs are observed on the
surface of porous TiO2 film. However, it is noteworthy that, the
amount of cracks developed on TiO2 films, prepared by commercially available powder (P25) only, is found to be marginal.
This phenomenon suggests that TiO2-CNTs play the role of
nucleation sites for clustering TiO2 nanoparticles on the surface
of the film. Moreover, with increase in amount of TiO2-CNTs,
the number of cracks on the surface of the films is increased
subsequently. It is thought that, the cracks generated on the
surface could be reducing the number of adsorption sites on
TiO2 film as well as causing the discrimination in the conversion efficiency of DSCs. The thickness of porous TiO2 film
is shown in Fig. 4(c) and close circle with an arrow indicate the
details of the passivation layer as shown in Fig. 4(d). As described earlier, the thickness of porous TiO2 film (10 um) was
controlled by an adhesive tape. Whereas, the thickness of passivation 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 other
parameters of solar cells. The value of open circuit voltage, Voc,
is increased by ∼ 6% from 0.60 V to 0.63 V with subsequent
increase in TiO2-CNTs from 0 to 0.1 wt.%. It has been observed
that surface treatment usually increases the values of Voc
regardless of nature and characteristics of coated materials on
electrode [16,17]. Furthermore, for 0.1 wt.% TiO2-CNTs, value
of short circuit photocurrent density (Jsc) is found to be
increased by ∼ 60% from 8.49 to 13.5 mA cm− 2, when
compared to as prepared TiO2 electrode. However, Jsc decreases
thereafter (from 13.5 to 9.69 mA cm− 2) with subsequent
increase in content of TiO2-CNTs from 0.1 to 0.3 wt.%. With
increase in TiO2-CNTs contents, the gradual decrease in Jsc, is
attributed to the increase in number of cracks on the surface of
porous TiO2 electrode. Consequently, the addition of TiO2CNTs enhances the electro-conductivity of porous TiO2
electrode, but decrease the adsorption site for ruthenium dye
to making a crack in TiO2 film. Thus, TiO2-CNTs (0.1 wt.%)
contained TiO2 electrodes have higher value of conversion
efficiency ∼ 50% higher compared with as prepared TiO2
electrode of cell (Table 1). However, subsequent increase in
concentration of TiO2-CNTs (from 0.1 to 0.3 wt.%) does not
help to increase the value of conversion efficiency further.
4. Conclusions
The J–V characteristics were studied as a function of TiO2CNTs content (wt.%) in the porous TiO2 film, which was used
as an electrode in DSCs. We easily obtained TiO2-CNTs by
Sol–Gel method. SEM analysis shows that, on porous
electrode, the TiO2-CNTs were entirely surrounded by TiO2
nanoparticles. Compared with a conventional TiO2 cell, the
modified TiO2 cell (0.1 wt.% TiO2-CNTs) showed ∼ 50%
increase in the value of conversion efficiency. The enhancement
in Jsc is attributed to the improved interconnectivity between the
TiO2 particles and the TiO2-CNTs in the porous TiO2 film. It is
emphasized that addition of the TiO2-CNTs in the TiO2 film
provides more efficient electron transfer through the film in
DSCs.
Acknowledgements
This research was funded by the KOSEF through CNNC
(Center for Nanotubes and Nano structured Composite) at
Sungkyunkwan University.
T.Y. Lee et al. / Thin Solid Films 515 (2007) 5131–5135
One of the authors (PSA) is thankful to the Korean
Government for awarding BK21 and also thankful to CSIR,
New Delhi, India for awarding Research Associateship.
References
[1] A.J. McEvoy, M.K. Nazeeruddin, G. Rothenberger, M. Gra¨tzel,
Electrochem. Soc. Proc. 10 (2001) 69.
[2] S. Chappel, S.G. Chen, A. Zaban, Langmuir 18 (2002) 3336.
[3] I. Bedja, P.V. Kamat, X. Hua, A.G. Lappin, S. Hotchandani, Langmuir 13
(1997) 2398.
[4] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo,
H. Siegbahn, J. Photochem. Photobiol., A Chem. 148 (2002) 57.
[5] S. Chappel, A. Zaban, Sol. Energy Mater. Sol. Cells 71 (2002) 141.
[6] P. Guo, M.A. Aegerter, Thin Solid Films 351 (1999) 290.
[7] B. O'Regan, M. Gra¨tzel, Nature (Lond.) 353 (1991) 737.
[8] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P.
Liska, N. Vlachopoulos, M. Gra¨tzel, J. Am. Chem. Soc. 115 (1993) 6382.
5135
[9] H. Ago, K. Petrisch, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Adv.
Mater. 11 (1999) 1281.
[10] E. Flahaut, A. Peigney, Ch. Laurent, Ch. Marlieùre, F. Chastel, A. Rousset,
Acta Mater. 48 (2000) 3803.
[11] M. Takahashi, K. Tsukigi, T. Uchino, T. Yoko, Thin Solid Films 388
(2001) 231.
[12] Y. Feng, G. Zhou, G. Wang, M. Qu, Z. Yu, Chem. Phys. Lett. 375 (2003)
645.
[13] M. Yufsdasaka, M. Zhang, S. Iijima, Chem. Phys. Lett. 374 (2003) 132.
[14] Y. Li, S. Wang, Z. Luan, J. Ding, C. Xu, D. Wu, Carbon 41 (2003) 1057.
[15] K. Hernadi, A. Siska, L. Thien-Nga, L. Forro, I. Kiricsi, Solid State Ion.
141–142 (2001) 203.
[16] A. Kay, M. Gra¨tzel, Chem. Mater. 14 (2002) 2930.
[17] Y. Diamant, S.G. Chen, O. Melamed, A. Zaban, J. Phys. Chem., B 107
(2003) 1977.
[18] J.W. Lee, H.S. Lee, J.M. Choi, J.W. Park, B.C. Shin, KR Patent No. 20050030759, 31 Mar. 2005.