Hong Lin, Ning Wang, Luozheng Zhang, Chunfu Lin and Jianbao Li
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AZojomo (ISSN 1833-122X) Volume 3 December 2007
Results and Discussion
The dye-sensitized solar cell (DSC) is highlighted by its low cost in recent
years. In the present study, TiO2 nanotubes (TNTs) with high surface
area were prepared, which can be used in the anodic electrode of the
dye-sensitized solar cell (DSC). The preparation methods, heat-treatment effects
and photovoltaic properties of the electrodes were discussed. Firstly, titanate
nanotubes were synthesized by hydrothermal method with commercial antase-type
TiO2 powder as the raw material. TNTs can be obtained by
heat-treating the as-prepared titanate nanotubes at 450°C. Secondly, the
electrode with 30% TiO2 nanotubes for the DSC was designed and
prepared successfully. By using the nano-electrode as the photoanode of the DSC,
the light-to-energy conversion efficiency of 5.42% was obtained.
Dye-sensitized solar cell (DSC), electrode, TiO2, nanotube,
With the decrease of energy resources and the improvement of the
environmentalism, sustainable energy becomes more and more necessary and
important. Experts estimated that the market demand for solar cells would keep
on with an increasing rate of 25-30% annually. The DSC comes into the world from
the last decades , with the advantages of low cost and simple processing. The
schematic diagram of the DSC is shown in Figure 1. It is composed of transparent
conductive glass (TCG), nanostructured TiO2 electrode with absorbed
dye, an electrolyte including redox of I-/I3-,
and counter electrode with evaporated Pt on TCG.
Figure 1. Schematic diagram and the
working mechanism of DSC
It has been calculated that the theoretical conversion efficiency of the DSC
is about 33%. The current highest efficiency, however, is only about 11% .
This is usually ascribed to the low transfer rate of the photo-induced electrons
in TiO2 (see arrow 4 in Figure 1), which is about
102~100 s-1, much lower than the recombination
rate of the electrons and holes.
On the other hand, the TiO2 electrode is generally made up of
nanocrystalline particles with a porous structure, which provides a rather large
surface area. As shown in Figure 2, the surface area of the TiO2
electrode decides the adsorption amount of the dye, which in turn directly
affects the absorption of incident light.
Figure 2. Illustrated diagram of
TiO2 electrode with adsorbed dye.
In the present study, TiO2 nanotubes (TNTs) were utilized in the
electrode. Comparing with the nanocrystalline particles, TNTs may offer larger
surface area and may increase charge transfer rate by the increase of diffusion
length. The preparation methods, heat-treatment effects and photovoltaic
properties of the nano-electrodes are now described and discussed.
All chemicals were analytical grade and used without further purification.
Commercial anatase titania powder (3.75 g, diameter: ~100nm) was dispersed in an
aqueous solution of NaOH (10 M, 30 ml) and moved into a Teflon-lined autoclave
. The autoclave was heated at 130°C for 20 h. After hydrothermal treatment,
the precipitate was repeatedly centrifugated and washed
with distilled water until the pH value was near to 7~8. Subsequently, the above
solution was filtrated and air-dried at 80°C to get an as-prepared sample.
Finally, the as-prepared sample was annealed at 450°, 550° and 650°C for 1 h.
An aqueous paste was obtained by mixing 30% the above as-prepared sample with
70% nanocrystalline TiO2 (P25, Degussa AG, Germany) powder.
TiO2 electrode was obtained by depositing a film (thickness: ~5µm)
from the paste on a TCG (ITO), and heat-treating at 450°C for 1 h. A DSC was
assembled using the above electrode and other traditional materials . The
electrodes were immersed for 8 h in a 3×10-4 M solution of the
sensitizer dye, RuL2(SCN)3 (Solaronix, L = 4,
4’-dicarboxy-2, 2’-bipyridine) in pure ethanol. Pt sputtered ITO glass was used
as a counter electrode. In the photochemical cell configuration,
RuL2(SCN)3 /TiO2 films on the ITO glass were
employed in a sandwich-type cell incorporating Pt sputtered ITO glass and a
non-aqueous electrolyte consisting of 0.04 M LiI, 0.02 M iodine in acetonitrile.
Tertbutylpyridine (TBP) was used or not used in the electrolyte. The cell, whose
active area was 0.123 cm2, was tested under 30 mW irradiation with a
500 W xenon lamp. The photovoltaic properties of the DSC were measured using a
Source Meter (Keithley-4200, Keithley Co. Ltd., USA).
Phase identification of the samples was carried out by X-ray diffraction
analysis (XRD, RIGAKU, D/Max-RB, Japan). The morphology of the samples were
observed by a transmission electron microscope (TEM, JEOL JEM-200CX, 200 kV,
Japan). The crystallographic characteristics of the products were evaluated by
selected-area electron diffraction (SAED), an accessory of TEM. Nitrogen
adsorption-desorption measurements were carried out at 77 K using a
Micromeritics ASAP 2010 to determine the Brunauer-Emmett-Teller (BET) surface
Results and Discussion
The XRD result of the samples annealed
at different temperature is shown in Figure 3. The as-prepared sample was mainly
composed of monoclinic H2Ti3O7. At 450ºC, the
sample was mainly composed of anatase TiO2, with a trace of
monoclinic H2Ti3O7 and triclinic
Ti5O9. When the temperature was increased, more anatase
and triclinic Ti5O9 were obtained in increasing amounts.
With the increase of the heat-treating temperature to 650ºC, well-crystallized
triclinic Ti5O9 and anatase TiO2 formed without
Figure 3: XRD patterns of the
as-prepared sample (a) and samples heat-treated at 450°C (b), 550°C (c) and
650°C (d). (M: H2Ti3O7; A: anatase
TiO2; T: Ti5O9).
Figure 4 shows TEM images of the as-prepared sample and the samples
heat-treated at 450°, 550° and 650°C for 1 h. From Figure 4 (a) and its bottom
right inset (HRTEM image), it can be clearly seen that all the raw
TiO2 particles changed to nanotubes at the present experimental
condition, and all the nanotubes are open-ended with multiwall. Their inner
diameter and outer diameter are approximately 3-5 nm and 8-12 nm, respectively.
From Figure 3 we know that these as-prepared nanotubes are not TiO2
but H2Ti3O7 (titanate). After heat-treated at
450°C for 1 h, the titanate became to TiO2, and the outer diameter of
the nanotubes clearly increased (Figure 4 (b)) to about 20 nm. When the
temperature was increased to 550° and 650°C, tubular crystals were hardly found
in the sample and a great number of rod-shaped and granular crystals formed, and
the size of these rod-shaped and granular crystals increased with the increase
of the temperature.
Figure 4. TEM images of the
as-prepared sample (a) and samples heat-treated at 450°C (b), 550°C (c) and
The BET surface area of the as-prepared sample was up to 375.6
m2·g-1. When the sample was heat-treated, its BET surface
area decreased. The higher the temperature, the smaller the BET surface area of
the sample. The BET surface area was 182.5 m2·g-1 after
heat-treated at 450°C, which is the highest BET surface area among the annealed
samples. The surface area was reduced to 71.2 m2·g-1 after
heat-treated at 650°C.
As shown in Figure 3, the as-prepared sample is titanate
(H2Ti3O7) but not TiO2, and the
TiO2 can be obtained by heat-treating the as-prepared sample at more
than 450°C. Moreover, the sample heat-treated at 450°C has the largest surface
area. Therefore, the electrode of our DSC was obtained by using 30% as-prepared
samples, and heat-treating at 450°C for 1 h. This means that the electrode we
used contains 30% TNTs with high surface area.
Figure 5 shows the typical current-voltage curves of the DSC with the TNTs
electrode with and without the TBP modification. It can be found that
the photovoltage increases from 0.49 V to 0.63 V after TBP modification, which
can be explained from Eqs. (1):
Where Voc, q,
and ER/R- are the open circuit voltage, the
electric charge transferred in a redox cycle, the quasi-Fermi energy level of
TiO2 and the Nernst potential of the redox couple (R/R-),
respectively. TBP and TNTs react to form a new complex, which increases
and then Voc also increased.
Figure 5. Current-voltage curves of
the DSC with the TNTs electrode with and without the TBP modification
(30 mW irradiation).
It is also found that the photocurrent density is decreased from 5.8
mA·cm-2 to 4.5 mA·cm-2 after TBP modification, which can
be ascribed to that TBP molecules is less conductive.
The light-to-energy conversion efficiency for the DSC with 30% TNTs as the
electrode is up to 5.42% with TBP modification. In fact, when 100% P25 (without
TNTs) was used as the electrode with TBP modification, the photocurrent density,
photovoltage and conversion efficiency were 5.5 mA·cm-2, 0.66 V and
7.68%. The conversion efficiency of DSC with 30% TNTs was lower than that
without TNTs, which is ascribed to that the inner surface of the TNTs may not be
fully used to adsorb the dye. Therefore, further investigation would focus on
the adsorption process of the dye on TiO2.
In this paper, the preparation method and heat-treatment effect of TNTs has
been demonstrated. TNTs with relative high surface area were synthesized by a
hydrothermal method at 130°C and then by heat-treatment at 450°C. The surface
area and the outer diameters of the TNTs (the sample heat-treated at 450°C) are
182.5 m2·g-1 and 20 nm, respectively. The TNTs are
open-ended with multiwall. The light-to-energy conversion efficiency for the DSC
with 30% TNTs is 5.42%.
The authors would like to express their gratitude to the support from The
Project-sponsored by SRF for ROCS, SEM and the support from Tsinghua Basic
Research Foundation (JCpy2005055).
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Hong Lin, Ning Wang, Luozheng Zhang, Chunfu
Lin and Jianbao Li
State Key Lab of New Ceramics and Fine
Department of Material Science and Engineering
This paper was also published in “Advances in Technology of Materials and
Materials Processing Journal, 9 (2007) 5-8”.