A facile method to prepare SnO
2
nanotubes for use in efficient SnO
2
–TiO
2
core–shell dye-sensitized solar cells†
Caitian Gao,
*
a
Xiaodong Li,
a
Bingan Lu,
b
Lulu Chen,
a
Youqing Wang,
a
Feng Teng,
a
Jiangtao Wang,
a
Zhenxing Zhang,
a
Xiaojun Pan
a
and Erqing Xie
*
a
Received 14th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2nr30349c
A high-efficiency photoelectrode for dye-sensitized solar cells (DSSCs) should combine the
advantageous features of fast electron transport, slow interfacial electron recombination and large
specific surface area. However, these three requirements usually cannot be achieved simultaneously in
the present state-of-the-art research. Here we report a simple procedure to combine the three conflicting
requirements by using porous SnO
2
nanotube–TiO
2
(SnO
2
NT–TiO
2
) core–shell structured
photoanodes for DSSCs. The SnO
2
nanotubes are prepared by electrospinning of polyvinyl
pyrrolidone (PVP)/tin dichloride dihydrate (SnCl
2
$2H
2
O) solution followed by direct sintering of the
as-spun nanofibers. A possible evolution mechanism is proposed. The power conversion efficiency
(PCE) value of the SnO
2
NT–TiO
2
core–shell structured DSSCs (5.11%) is above five times higher
than that of SnO
2
nanotube (SnO
2
NT) DSSCs (0.99%). This PCE value is also higher than that of
TiO
2
nanoparticles (P25) DSSCs (4.82%), even though the amount of dye molecules adsorbed to the
SnO
2
NT–TiO
2
photoanode is less than half of that in the P25 film. This simple procedure provides
a new approach to achieve the three conflicting requirements simultaneously, which has been
demonstrated as a promising strategy to obtain high-efficiency DSSCs.
Introduction
Dye-sensitized solar cells (DSSCs) have attracted a great deal of
attention as a promising candidate for future green energy due to
their facile, low-cost, and environmentally-friendly fabrication
process.
1–3
Recently, Yella et al.
4
reported a new record power
conversion efficiency (PCE) of >12% by using a porphyrin-
sensitized nanocrystalline TiO
2
photoanode together with
cobalt(
II/III)-based redox electrolyte. However, the nanocrystal-
line TiO
2
photoanode is still a limiting component that needs to
be further improved before the technology can be commercial-
ized. The overall sunlight-to-electric power conversion process of
a DSSC can be summarized as the combination of photo-
generation, charge-carrier transport, and collection.
1,5
In other
words, a high-efficiency photoanode for DSSCs should combine
the advantageous features of fast electron transport, slow inter-
facial electron recombination, and high specific surface area.
Considerable attention has been focused on 1D core–shell
structures for the purpose of speeding up electron transport
and slowing recombination to achieve a high charge collection
efficiency.
6–11
However, in the present state-of-the-art research,
these 1D core–shell structures usually increase the charge
collection efficiency at the expense of specific surface area,
resulting in a low PCE.
One promising approach to achieving the three conflicting
requirements simultaneously is to use electrospun SnO
2
nano-
tubes in SnO
2
nanotube–TiO
2
(SnO
2
NT–TiO
2
) core–shell
structured photoanodes for dye-sensitized solar cells. Firstly, we
fabricated SnO
2
nanofibers and nanotubes as photoanodes
for DSSCs by electrospinning followed by direct sintering the
as-spun nanofibers. Electrospinning generates 1D polycrystalline
SnO
2
nanomaterials with extremely high aspect ratios and
specific surface areas, and thus, is different from most other
methods used to prepare SnO
2
nanomaterials, such as template
methods,
12
and the hydro-thermal method.
13
Therefore, the
electrospun 1D structures, such as nanofibers and nanotubes,
can provide a direct pathway for electron transport and large
surface area for dye loading. On the other hand, SnO
2
is an
excellent metal oxide semiconductor with higher electron
mobility (100 to 200 cm
2
V
1
S
1
) and larger band gap (3.6 eV)
than TiO
2
, indicating a faster transport of photogenerated elec-
trons and more excellent long-term stability compared to TiO
2
for DSSCs applications.
7,14,15
However, SnO
2
-based DSSCs were
developed with less success due to at least two weak points: (1)
a
School of Physical Science and Technology, Lanzhou University, Lanzhou
730000, Gansu, People’s Republic of China. E-mail: xieeq@lzu.edu.cn;
caitiangao10@163.com
b
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of
Education, State Key Laboratory for Chemo/Biosensing and
Chemometrics, Hunan University, Hunan, People’s Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nr30349c
This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 3475–3481 | 3475
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