Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 1–7, 2012
Hierarchical Structured TiO2 Photoanodes for
Dye-Sensitized Solar Cells
Yen-Chen Shih, Ann-Kuo Chu, and Wen-Yao Huang∗
Department of Photonics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
A novel approach has been developed to fabricate hills-like hierarchical structured TiO2 photoanodes for dye-sensitized solar cells (DSSCs). The appropriately aggregated TiO2 clusters in the
photoanode layer could cause stronger light scattering and higher dye loading that increases the
efficiency of photovoltaic device. For detailed light-harvesting study, different molecular weights of
polyvinyl alcohol (PVA) were used as binders for TiO2 nanoparticles (P-25 Degussa) aggregation. A
series of TiO2 films with dissimilar morphology, the reflection of TiO2 films, absorbance of attached
dye, amount of dye loading, and performance of fabricated DSSC devices, were measured and
investigated. An optimized device had energy conversion efficiency of 4.47% having a higher dye
loading and good light harvesting, achieving a 23% increase of short-circuit current Jsc in DSSCs.
Keywords: Hierarchical Structure, TiO2 Photoanode, Dye-Sensitized Solar Cell, Light
Scattering.
1. INTRODUCTION
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx
1533-4880/2012/12/001/007
doi:10.1166/jnn.2012.5840
1
RESEARCH ARTICLE
Recently, dye-sensitized solar cells (DSSCs) have attracted
much attention1–9 owing to their low production cost and
high conversion efficiency. In general, the photoanode
of a DSSC is a porous layer with singular-size, around
20 nm TiO2 particles coated on transparent conductive
oxide (TCO) glass, and a monolayer of the charge transfer
dye adheres to the nanoparticles. Under illumination, electrons from the photoexcited dye molecules inject into the
conduction band of TiO2 , diffuse through the porous layer
and are subsequently collected and transported to the external load by the TCO. In determining the performance of
DSSC, the porous TiO2 electrode plays an important role.
It is essential to have a TiO2 layer with a high surface area
for large dye adsorption and for it to be internally light
scattering to enhance light harvesting, gain short-circuit
current Jsc and improve the power conversion efficiency
of DSSC devices.10 The research results of Wang et al.11
demonstrated that the surface area of the TiO2 films is
important. The authors also demonstrated that, as the TiO2
nano particles decrease in size, the surface area increases,
and hence the amount of dye is increased; while large
particles are required to enhance absorption of red light
through internal light scattering. It is difficult to simultaneously increase surface area and light scattering because
they oppose each other. So far, some groups have indicated
that aggregating TiO2 nanoparticles to form clusters was
beneficial to improve the performance of DSSC.12 13 Some
studies showed better efficiency of electrolyte diffusion or
penetration and high dye loading in such structured TiO2
films; moreover, there seemed to be some scattering benefits in the clustered structure. However, the influence of
dye amount and the light-scattering effect of light harvesting were not particularly discussed.
As shown in Mie theory, light-scattering intensity is relevant to the size of the illuminated object. That is, the
scattering cross-section increases when a larger sized scattering object is used, and the application is practical for
light harvesting of longer wavelengths in solar cells.14–16
In this study, we developed a novel method to get a film
with a high surface area and good light scattering at the
same time. Furthermore, in order to find out the turning
point of these two factors that influence light harvesting,
we utilized polyvinyl alcohol (PVA) polymers with different molecular weights as binders to form a series of
hierarchical structured TiO2 films with different degrees of
aggregation as shown in Figure 1. A series of TiO2 films
with dissimilar morphology, the reflectance of TiO2 films,
absorbance of attached dye, amount of dye loading, and
performance of fabricated DSSC devices were measured
and investigated. As a result, an optimized device with
higher dye loading and good light harvesting was achieved
in our experiment at the same time as achieving a 23%
increase of short-circuit current Jsc in DSSC.
Hierarchical Structured TiO2 Photoanodes for Dye-Sensitized Solar Cells
(a)
(b)
Shih et al.
ruthenium(II)bis-tetrabutylammonium (D719 dye, Eversolar) in the tert-butanol and acetonitrile co-solvent, for
20 hours at 25 C. Then the dye absorbed TiO2 films were
rinsed with ethanol and dried at 85 C for 10 minutes.
Pt-coated ITO as counter electrode and dye-absorbed TiO2
film were sealed with hot-melt adhesive (30 m thickness,
Surlyn, Tripod), and a liquid electrolyte consisting of I2
(0.1 M), LiI (0.1 M), 1-propyl-2,3-dimethylimidazolium
iodide (DMPII, 0.6 M), and 4-tert-butylpyridine (TBP,
0.25 M, Aldrich) in 3-Methoxypropionitrile (MPN, Fluka)
was injected into the interspace of the cells.
For comparison, a conventional TiO2 film was prepared
without adding the PVA, and the device as a standard was
fabricated equally with the above methods.
2.3. Characterization and Measurement
Fig. 1. Schematic diagram of (a) conventional film and (b) hierarchical
structured film.
2. EXPERIMENTAL DETAILS
RESEARCH ARTICLE
2.1. Preparation of the TiO2 Particles Paste for
Hierarchical Structured Photoanodes
To prepare a hierarchical structured TiO2 paste, we used
PVA (CCP) as binders to aggregate the nanoparticles, and
attempted to control the size of aggregations by altering
the molecular weight (MW 31,000–99,000) of the polymers. First, 1 g PVA water solution was stirred at 65 C
overnight, and then dissolved in mixed solvent with 1.0 ml
H2 O and 0.8 ml ethanol. Then 0.6 g TiO2 powder (P-25,
21 nm Degussa) was added into the above solution. The
suspension was put into the ultrasonic cleaner for 30 minutes, then 0.05 ml Triton X-100 and 0.05 ml acetyl acetone
of surfactants were added and the suspension was put into
the ultrasonic cleaner for 30 minutes again. It was stirred
for 24 hours subsequently.
2.2. Fabrication of DSSCs
A dense TiO2 layer of about 45 nm was spin-coated on
indium tin oxide (ITO) glass (Merck) as a blocking layer to
avoid recombination from the I−
3 grabbing photoelectrons
on the conducting substrate surface. Thereafter a TiO2
porous layer was formed by the spin-coating process with
an active area of 1.3 × 0.8 cm2 . In order to evaporate the
solvent and decompose the PVA of TiO2 films, ensuring
electrical contact and mechanical adhesion on the glass,
the coated films were baked at 100 C for 10 minutes,
and calcined subsequently at 450 C for about 30 minutes in air. The average thickness of the TiO2 films was
controlled to be about 10 m measured by an -step
surface profiler. For dye absorbing, the calcined films were
immersed in a 5 × 10−4 M ruthenium dye solution, cisbis(isothiocyanato) bis(2, 2’-bipyridyl-4, 4’-dicarboxylato)
2
The surface morphology of the TiO2 porous films was
obtained by using a field emission gun-scanning electron microscopy (JEOL JSM-6700F). Reflectance and
absorbance spectra were measured by UV-vis spectrometer
(Perkin Elmer UV-Vis Lambda35) equipped with an integrating sphere. The photocurrent-voltage measurements of
DSSCs were performed using a Keithley 2400 analyser
under AM 1.5 simulated illuminations with an intensity
of 100 mW/cm2 . A 300 W xenon lamp (MODEL 6258)
was used as the light source with a filter for approximating one sunlight spectrum. The incident photo to current
conversion efficiency (IPCE) spectra were measured by
a self-designed IPCE system consisting of a xenon lamp
(Xe Light source 70051), a monochromator (Cornerstone
13074004), a power measurement (2935-C), a source measurement unit (M3500A) and a power detector (818-SL).
The amount of dye absorption was evaluated by the following method. The dye molecules were first desorbed
from the TiO2 layer by rinsing with 0.1 M NaOH aqueous solution. Then the absorbance at 511 nm of dye solution was measured by UV-vis spectrometer to identify the
concentration.
3. RESULTS AND DISCUSSION
3.1. Morphologies of TiO2 Films
In order to figure out the optical properties of TiO2 films
with different particle aggregation degrees, we used PVA
polymers with different molecular weights as binders. We
expected potential variation of light reflectance and light
absorbance with dissimilar morphology of these films. To
verify that those aggregations were actually formed in the
paste, first we diluted the paste and identified it by SEM,
as shown in Figure 2. In these pictures, we found that there
were really many micro-scale TiO2 nanoparticle aggregations, with different molecular weights of treated PVA. The
size of aggregation increased when the molecular weight
increased. We supposed that the interactions of PVA, TiO2
particles and surfactant were under the static equilibrium.
J. Nanosci. Nanotechnol. 12, 1–7, 2012
Shih et al.
(a)
(b)
(c)
(d)
(e)
(f)
RESEARCH ARTICLE
Fig. 2.
Hierarchical Structured TiO2 Photoanodes for Dye-Sensitized Solar Cells
TiO2 aggregations SEM images from diluted paste. (a), (b) Mw 31,000; (c), (d) Mw 40,000; (e), (f) Mw 54,000.
In this steady situation, many TiO2 aggregations with uniform size were formed in the paste, and size was proportional to the molecular weight of PVA because of stronger
interaction between TiO2 and PVA. Thus, in order to clarify our supposition on the aggregation mechanism, we kept
the same weight of molecules and tried to vary the amount
of PVA from 0.2 g to 0.3 g in the past for the fabrication
of hierarchical structured photoanodes. Indeed, the size of
aggregations almost remained unchanged as the amount of
PVA in the paste increased. This phenomenon could be
attributed to the hydroxyl groups in the PVA molecules
interacting with the hydroxyl groups on the TiO2 particles
and forming the clusters in the processing paste. Owing to
surfactant restraint the TiO2 -PVA clusters did not aggregate continuously and remained at a certain size as time
went by. When the PVA molecular weight increased, there
J. Nanosci. Nanotechnol. 12, 1–7, 2012
were more hydroxyl groups in each single PVA molecule.
It represented that the interaction between PVA molecules
and TiO2 particles would become stronger; thus bigger
aggregations were formed in the paste. However, when we
added more PVA molecules into the paste, the hydroxyl
groups in a single PVA molecule remained the same, so
adding more PVA did not enlarge the size of the aggregation but just increased the aggregation amount.
Figure 3 shows the surface SEM images of conventional and PVA treated films. As depicted in Figure 3, the
conventional film was formed by uniformly stacking the
nanoparticles, and we found that the TiO2 aggregation size
and porosity were proportional to the molecular weight of
the PVA. From film A to film D, the size of aggregation
increased from about 100 nm to 300 nm. This suggested
that, as higher molecular weights of PVA were treated,
3
Hierarchical Structured TiO2 Photoanodes for Dye-Sensitized Solar Cells
(a)
(b)
(c)
(d)
Shih et al.
RESEARCH ARTICLE
(e)
Fig. 3. Surface SEM images of (a) conventional TiO2 film, PVA treated TiO2 films with (b) Mw 31,000, (c) Mw 40,000, (d) Mw 54,000, and
(e) Mw 99,000.
larger TiO2 -PVA aggregations formed in the paste. Bigger
TiO2 aggregations still existed after coating the substrate
and after the polymers had decomposed during the calcining process. The side views of SEM as shown in Figure 4
show a very smooth top surface on the conventional film.
However, we clearly observed that the surface of PVA
treated film was hill-like, and that these hills were probably
formed by those micro-scale TiO2 clusters. So, we called
this micro-nano aggregation a hierarchical structured TiO2
film, and expected that the light-scattering effect could be
enhanced in this structure.
3.2. Optical Properties of the TiO2 Films and
Photoanodes
To realize the light-scattering effect in each TiO2 film,
the reflectance was measured, as shown in Figure 5(a). It
4
was apparent that the reflectance of STD was lower than
the others, because those uniformly stacking nanoparticles
could not scatter incident light well. However, by PVA
treating, those nanoparticles aggregated and formed the
hierarchical structured film, obtaining higher reflectance.
Moreover, Figure 5(b) shows the normalized reflectance
spectra, in which we found that these reflectance curves
were not similar. At the region of longer wavelengths from
600 nm to 800 nm, our results showed that if aggregation
size increased, the reflectance comparatively increased.
The phenomenon corresponds to Mie scattering theory.
So far, several studies have shown that light reflectance is
related to the scattering effect of TiO2 films.15 17 However,
we could not ensure that those reflected photons were actually well scattered in such porous films. As shown in the
schematic diagram (Fig. 1), the incident light is scattered
J. Nanosci. Nanotechnol. 12, 1–7, 2012
Shih et al.
Hierarchical Structured TiO2 Photoanodes for Dye-Sensitized Solar Cells
(a)
(a)
STD
Film A
Film B
Film C
Film D
70
Reflectance (R%)
60
50
40
30
20
10
400
500
600
Wavelength (nm)
700
800
(b)
Normalized Reflectance
(b) 1.0
STD
Film A
Film B
Film C
Film D
0.8
0.6
0.4
0.2
400
3.3. Performance of DSSCs
We have studied the optical properties of TiO2 films
with different morphologies above. Here, we discuss the
J. Nanosci. Nanotechnol. 12, 1–7, 2012
800
Fig. 5. (a) Reflectance spectra of TiO2 films (b) Normalized reflectance
spectra.
devices’ performance with light-scattering effect below.
Table I summarizes the device characteristics and the lightscattering effect. The devices A to D in the table were
fabricated with the TiO2 films A to D respectively. We
noticed that when the size of aggregation increased, the
dye amount decreased, probably owing to there being less
STD
Film A
Film B
Film C
Film D
Normalized Absorbance
by those nanoparticles and subsequently reflects backward
or traverses forward through the porous film. If we only
measure the reflectance, we neglect the photons that were
scattered in the film and traverse out. Moreover, placing a
detector in the film to recognize the actual scattering intensity is also a difficult task. Here, in order to improve the
evidence of the light-scattering effect, we tried to measure
the unit of dye absorbance, so that we could understand
how much light in the film was actually absorbed by each
dye molecule and characterize the intensity of the lightscattering effect thereafter. The absorbance spectra of each
dye molecule in TiO2 films are shown in Figure 6. These
spectra were taken by deducting the absorbance of TiO2
films from photoanodes (TiO2 with dye), and then eliminating the dye amount of each film. In Figure 6, we obviously noticed that units of dye molecule harvested more
light in the PVA treated films, and that the light absorbance
of each dye increased when the degree of aggregation size
increased. It represented that the light-scattering effect was
really stronger in the higher aggregation film.
700
400
500
600
Wavelength (nm)
700
800
Fig. 6. Absorbance spectra of each dye molecule in TiO2 films.
5
RESEARCH ARTICLE
Fig. 4. Side views SEM images of (a) conventional film and (b) PVA
treated film.
500
600
Wavelength (nm)
Hierarchical Structured TiO2 Photoanodes for Dye-Sensitized Solar Cells
Table I. Peformance of DSSCs.
Adsorbed
dye (× 10−8
mol/cm2 )
Sample
A
B
C
D
6.35
6.46
5.92
5.56
5.81
Jsc
(mA/cm2 )
Voc
(V)
FF
(%)
9 50
11 65
9 41
9 49
10 63
0.58
0.59
0.59
0.57
0.58
0.69
0.65
0.65
0.68
0.66
3 79
4 47
3 64
3 71
4 08
surface area of the films, and the scattering effect increased
owing to a higher degree of size aggregation. The shortcircuit current from devices A to D did not decrease
directly. We found that there was a turning point of
the competition between dye amount and light-scattering
effect at device C. Moreover, although film D had less
dye amount 5.81 × 10−8 mol/cm2 than conventional film
6.35 × 10−8 mol/cm2 , the short-circuit and the power conversion efficiency of device D were higher than the STD
device, which was made from conventional film. This suggested that in device D the influence of the device performance, light-scattering effect was stronger than the dye
amount. We obtained the same results from the incident
photon to the current efficiency (IPCE) spectra, as shown
in Figure 7. The quantum efficiencies of STD, device A
and device D with D719 dye were all maximized around
550 nm. For device D, the peak quantum efficiency was
almost the same as with STD, but it was apparently higher
in longer wavelength regions of about 560–700 nm. The
higher efficiency could be caused by a stronger scattering
effect in longer wavelengths of bigger TiO2 particle aggregations. Since device A had the largest amount of dye
and the best scattering effect, it had the highest quantum
efficiency over the whole spectral range and the highest
power conversion efficiency of 4.47%, enhanced by a 23%
increase of Jsc compared with STD.
50
Quantum Efficiency (%)
RESEARCH ARTICLE
STD
Device
Device
Device
Device
Scattering
effect
(525 nm)
STD
Device A
Device D
40
30
20
10
0
500
600
700
Wavelength (nm)
800
Fig. 7. Incident photo to current conversion efficiency spectra of
DSSCs.
6
Shih et al.
In addition, the quantum efficiency in the shorter wavelength region of device D was lower than the other devices
in the IPCE spectra. We supposed that this was owing to
the higher porosity and bigger pores in the film D, which
was derived from a larger size of TiO2 aggregations. When
the incident light strikes on the film, longer wavelengths
of light are scattered for sure, but shorter wavelengths of
light could not be properly scattered by such a high porosity structure. However, these photons of the shorter wavelength regions could be finally reflected forward because
of enough thickness of the film; in other words, there were
enough chances to be reflected. This corresponded to why
device D had lower quantum efficiency but a reflectance
that was not lower than the other films in shorter wavelength regions.
4. CONCLUSIONS
In this work, a series of hierarchical structured TiO2 photoanodes were prepared and characterized. We found that
Jsc was increased owing to the stronger scattering effect in
larger TiO2 film aggregations. In our study, we obtained
higher Jsc with better light scattering in photoanodes,
although the amount of the absorbed dye was less. As a
result, an optimized device A, derived from PVA 31,000
treated film, had higher dye loading and good light harvesting at the same time as achieving a 23% increase of
Jsc and producing the highest power conversion efficiency
of 4.47%.
Acknowledgment: We wish to thank the National
Science Council of Taiwan, under Grant No. NSC 1002221-E-110-046, for supporting this research.
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Received: 21 December 2011. Revised/Accepted: 22 December 2011.
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