Importance of blocking layers at conducting glass/TiO2 interfaces in dye-sensitized ionic-liquid solar cells

Article abstract of DOI:10.1246/cl.2006.252

Thin Nb₂O₅ film works as a potential blocking layer when deposited between fluorine-doped tin oxide and nanocrystalline TiO ₂ layer, improving V_oc and conversion efficiency of the dye-sensitized TiO₂ solar cells using ionic liquid electrolytes. Copyright

Full text of DOI:10.1246/cl.2006.252

252
Chemistry Letters Vol.35, No.3 (2006)
Importance of Blocking Layers at Conducting Glass/TiO2 Interfaces
in Dye-sensitized Ionic-liquid Solar Cells
Jiangbin Xia,1 Naruhiko Masaki,1 Kejian Jiang,1 Yuji Wada,2 and Shozo Yanagidaꢀ1
1Center for Advanced Science and Innovation, Osaka University, Suita, Osaka 565-0871
2Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
(Received November 30, 2005; CL-051484; E-mail: yanagida@mls.eng.osaka-u.ac.jp)
Thin Nb2O5 film works as a potential blocking layer when
TiO2, which can suppress back electron transfer from FTO to
electrolytes without lowering conductivity between them as
shown in Figure 1.
deposited between fluorine-doped tin oxide and nanocrystalline
TiO2 layer, improving Voc and conversion efficiency of the dye-
sensitized TiO2 solar cells using ionic liquid electrolytes.
The spray pyrolysis method12a with different precursors list-
ed in Table 1 has been applied to form the corresponding metal
oxide layers on FTO during the fabrication of the structured
FTO/metal oxide/porous TiO2 (Nanoxide-T, Solaronix, 5.5
mm) electrodes. The general procedure is as follows; each etha-
nol solution (0.02 M) of the precursor was sprayed layer-by-lay-
er appropriate repetitions onto the FTO substrate keeping over
400 ꢁC. Then, the substrates with the blocking layers were an-
nealed at 500 ꢁC for 1 h (some XRD data refer to supplementary
information). The Ru dye, Z-907 (Ru-520 DN, Solaronix) and
the ionic-liquid electrolyte composed of HMImI (1-hexyl-3-
methylimidazolium iodide) with iodine ([Iꢂ]:[I2] = 10:1) are
employed for ionic-liquid DSCs. The cells without any blocking
layer or with liquid electrolyte based on methoxyacetonitrile are
also fabricated for comparison.
In dye-sensitized TiO2 solar cells (DSCs) that compose of
a dye-adsorbed nano-TiO2 layer on fluorine-doped tin oxide
(FTO) glass cathode as the window electrode, redox electrolytes
as charge carrier and the counter anode electrode, unidirectional
charge flow with no electron leakage at the interfaces is essential
for the high energy-conversion efficiency. Recently, much atten-
tion1–3 has been paid to improve the performance of the ionic-
liquid DSCs because of their features such as high ionic conduc-
tivity, nonvolatility, electrochemical stability, and nonflamma-
bility.4–6
According to the unidirectional electron-transporting princi-
ple of DSCs, there are four important interfaces in the devices as
shown in Figure 1. These are, the interface of FTO/TiO2, TiO2/
dye, dye/electrolyte, and electrolyte/counter electrode (usually
platinized FTO electrode). Recently, many researchers pay
much attention to the interface of TiO2/dye especially through
core–shell structured electrodes by introducing Nb2O5, SrTiO3,
and Al2O37–11 for TiO2 electrode surface modification. Howev-
er, except for the establishment of model for DSCs,12–15 few
groups investigate the modification of the interface of FTO/
TiO2 and its effectiveness by employing a compact TiO2
layer.12b,15
As shown in Table 1, while the blocking MgO, ZnO, Al2O3,
Eu2O3, and SiO2 layers do not give any improvement of Voc, the
Nb2O5 blocking layer improves Voc effectively, with keeping re-
spectable fill factors (FF), giving the high conversion efficiency
as well as the TiO2 blocking layer. Figure 2 presents the J–V
curve of the novel type solar cell and that of nonblocked refer-
ence under AM 1.5 irradiation of 100 mW/cm2.
Table 1. Comparison of the parameters of the ionic-liquid
DSCs using mesoporous TiO2 electrodes with several metal
oxide blocking layers on FTO after optimization
In this letter, we first develop Nb2O5 as blocking layer
which gives an improvement in open-circuit photovoltage
(Voc) and fill factor, leading to respectable energy-conversion
efficiency in ionic-liquid DSCs. The improvement may come
from the Nb2O5 forming potential barrier between FTO and
Blocking Voc
materials /mV
Jsc
/mA
Precursors
FF ꢀ/%
—
a
none
TiO2
Nb2O5
MgO
580
641
649
590
8.30
7.85
7.43
7.38
0.57 2.8
0.65 3.3
0.67 3.2
0.64 2.8
Nb(OCH2CH3)5
Mg(OCH3)2
Sn(CH3COCHCO
CH3)4
Zn(CH3COCHCO
CH3)2
SnO2
ZnO
578
572
8.63
4.44
0.57 2.8
0.54 1.4
Al(OCH(CH3)2)3
EuCl3
Al2O3
Eu2O3
579
621
5.35
6.06
0.69 1.8
0.61 2.3
Tetramethylcyclo-
tetrasiloxane
—
SiO2
567
6.75
0.57 2.2
noneb
Nb2O5
704 10.17
710 9.54
0.68 4.9
0.72 4.8
b
Nb(OCH2CH3)5
aDiisopropoxytitanium bisacetylacetonate as precursor.
bLiquid electrolyte: 0.1 M LiI, 0.3 M 2,3-dimethyl-1-propyl-
imidazolium iodide, 0.05 M I2, 0.5 M 4-tert-butylpyridine in
methoxyacetonitrile.
Figure 1. Schematic views of interfaces in the DSC device and
the electron transfer of the new structured electrode.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
253
adding additives like TBP (4-tert-butylpyridine) to improve
the Voc of the cell, such kind of modified substrate is a good
choice for the improvements of Voc. Other facile methods for
construction of the effective blocking layer are currently being
undertaken.
In summary, we have demonstrated that among the exam-
ined metal oxides, Nb2O5 can form an effective blocking layer
at FTO/nano-TiO2 interfaces and greatly improve Voc and fill
factor, proving the importance of blocking layer at FTO and
TiO2 interfaces in ionic-liquid DSCs.
This research was supported by the New Energy and
Industrial Technology Development Organization (NEDO)
under the Ministry of Economy, Trade and Industry.
Figure 2. J–V curves of cells employing Z-907 sensitized
FTO/nano-TiO2 ( ) and FTO/Nb2O5/nano-TiO2 electrodes
(
) under AM 1.5 irradiation. (Electrolyte:HMImI:I2 = 10:1).
When the thicker Nb2O5 layer (obtained by 50-times pyrol-
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Published on the web (Advance View) January 28, 2006; DOI 10.1246/cl.2006.252