Organic sensitizers featuring a planar indeno[1,2-b]-thiophene for efficient dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201300281

An efficient organic sensitizer (JK-306) featuring a planar indeno[1,2-b]thiophene as the π-linker of a bridging unit for dye-sensitized solar cells (DSSCs) was synthesized. The sensitizer had a strong molar absorption coefficient and a red-shifted absorption band compared with JK-305, which resulted in a significant increase in the short-circuit photocurrent density. We incorporated a highly congested bulky amino group into the 2′,4′-dihexyloxybiphenyl-4-yl moiety, an electron donor, to diminish the charge recombination and to prevent aggregation of the sensitizer. Under standard AM 1.5G solar conditions, JK-306-sensitized cells in the presence of co-adsorbents chenodeoxycholic acid (CDCA) and 4-[bis(9,9-dimethyl-9H-fluoren-2- yl)amino]benzoic acid (HC-A), which afforded an overall conversion efficiency of 8.37 % and 8.52 %, respectively. Upon changing the I^-/I ₃^- electrolyte to the Co^II/Co^III redox couple, the cell gave rise to a significantly improved conversion efficiency of 10.02 % with the multifunctional HC-A, which is one of the highest values reported for DSSCs with a cobalt-based electrolyte. Furthermore, the JK-306-based solar cell with a polymer gel electrolyte revealed a high conversion efficiency of 7.61 %, which is one of the highest values for cells based on organic sensitizers. How well does a dye work? The sensitizer reported herein has a strong molar absorption coefficient and a red-shifted absorption band, which results in a significant increase in the short-circuit photocurrent density. A device based on this sensitizer gives rise to high conversion efficiencies that are among the highest values reported for dye-sensitized solar cells based on organic sensitizers. Copyright

Full text of DOI:10.1002/cssc.201300281

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DOI: 10.1002/cssc.201300281
Organic Sensitizers Featuring a Planar Indeno[1,2-b]-
thiophene for Efficient Dye-Sensitized Solar Cells
Kimin Lim,[a] Myung Jong Ju,[b] Juman Song,[a] In Taek Choi,[b] Kwangsuk Do,[a] Hyeju Choi,[a]
Kihyung Song,[c] Hwan Kyu Kim,[b] and Jaejung Ko*[a]
An efficient organic sensitizer (JK-306) featuring a planar
indeno[1,2-b]thiophene as the p-linker of a bridging unit for
dye-sensitized solar cells (DSSCs) was synthesized. The sensitiz-
er had a strong molar absorption coefficient and a red-shifted
absorption band compared with JK-305, which resulted in a sig-
nificant increase in the short-circuit photocurrent density.
We incorporated a highly congested bulky amino group into
the 2’,4’-dihexyloxybiphenyl-4-yl moiety, an electron donor, to
diminish the charge recombination and to prevent aggrega-
tion of the sensitizer. Under standard AM 1.5G solar conditions,
JK-306-sensitized cells in the presence of co-adsorbents cheno-
deoxycholic acid (CDCA) and 4-[bis(9,9-dimethyl-9H-fluoren-2-
yl)amino]benzoic acid (HC-A), which afforded an overall con-
version efficiency of 8.37% and 8.52%, respectively. Upon
À
changing the IÀ/I3 electrolyte to the CoII/CoIII redox couple,
the cell gave rise to a significantly improved conversion effi-
ciency of 10.02% with the multifunctional HC-A, which is one
of the highest values reported for DSSCs with a cobalt-based
electrolyte. Furthermore, the JK-306-based solar cell with
a polymer gel electrolyte revealed a high conversion efficiency
of 7.61%, which is one of the highest values for cells based on
organic sensitizers.
Introduction
Dye-sensitized solar cells (DSSCs) have received intensive at-
tention owing to low-cost fabrication and high power-conver-
sion efficiency.[1] They provide a technically and economically
promising alternative to traditional silicon based p–n junction
photovoltaic devices.[2] In these cells, the sensitizer is one of
the key components. DSSCs with polypyridyl ruthenium sensi-
tizers have shown the highest efficiencies (>11 %).[3,4] A recent
study has focused on metal-free organic sensitizers because of
their advantages over ruthenium sensitizers, such as strong
molar absorption coefficient, ease of molecular tailoring, and
low material cost. Nevertheless, many organic sensitizers have
still presented a low conversion efficiency and low stability.
Two major factors for the low efficiency of organic DSSCs are
the formation of dye aggregation on the TiO2 surface[5,6] and
interfacial charge recombination.[7–9] Several strategies have
been considered for addressing both these issues such as the
use of b-cyclodextrin-encapsulated molecules,[10] the structural
modification of the sensitizers,[11] and the use of additives.[12]
Our concept toward a high photovoltaic performance with or-
ganic DSSCs is based on the interface engineering of a dye-ad-
sorbed TiO2 surface controlled by the structural modification of
the sensitizer to diminish the charge recombination and to
prevent the aggregation of sensitizers. The highly efficient or-
ganic sensitizers have a push–pull structure composed of
a donor, conjugated bridge, and acceptor. In addition to the
electron donor and acceptor units in a push–pull dye, the
p-conjugated linker is of importance in improving the cell per-
formance. A successful molecular engineering for the high effi-
ciency was achieved by incorporating planar units, such as cy-
clopentadithiophene,[13–15] dithienosilole,[16,17] carbon-bridged
phenylenevinylene,[18] indacenodithiophene,[19] cyclopenta[1,2-
b:4,5-b’]dithiophene[2’,1’:4,5]thieno[2,3-d]thiophene,[20]
and
indeno[1,2-b]thiophene bridging group,[21] into the p-linker
unit. In addition, the co-adsorption of dyes with additives, such
as decylphosophonic acid,[22] dineohexyl bis(3,3-dimethyl-
butyl)phosphinic acid,[23] chenodeoxycholic acid (CDCA),[24]
and 4-[bis(9,9-dimethyl-9H-fluoren-2-yl)amino]benzoic acid
(HC-A),[25–27] is proven to be effective in dissociating p–p stack-
ing or dye aggregation. Herein, we report the usage of new or-
ganic sensitizers (JK-305 and JK-306, Figure 1) containing the
bis(2’,4’-dihexyloxybiphenyl-4-yl)amino moiety as an electron
donor and cyanoacrylic acid as an electron acceptor bridged
by a planar indeno[1,2-b]thiophene unit as well as the co-ad-
[a] K. Lim,+ J. Song, K. Do, H. Choi, Prof. J. Ko
Photovoltaic Materials, Department of Material Chemistry
Korea University 2511 Sejong-Ro, Sejong 339-700 (South Korea)
Fax: (+82)44-867-5396
E-mail: jko@korea.ac.kr
[b] Dr. M. J. Ju,+ I. T. Choi, Prof. H. K. Kim
Global GET-Future Laboratory, Department of Materials Chemistry
Korea University, 2511 Sejong-Ro, Sejong 339-700 (South Korea)
[c] Prof. K. Song
Department of Chemistry, Korea National University of Education
Cheongwon, Chungbuk 363-791 (South Korea)
À
sorbent effect in conjunction with IÀ/I3 and CoII/CoIII redox
[+] These authors contributed equally to this work.
couples.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300281.
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N-bromosuccinimide (NBS). The Suzuki coupling reaction[29] of
the bromo product 6 with 5-formyl-2-thiopheneboronic acid
(7) yielded 5-{6-[bis(2’,4’-dihexyloxybiphenyl-4-yl)amino]-8,8-di-
methyl-indeno[2,1-b]thiophen-2-yl} thiophene-2-carbaldehyde
8. The aldehyde compound 8, upon reaction with cyanoacetic
acid in the presence of piperidine, produced the sensitizer
JK-306. To further assess the merit of a planar bridging group,
we prepared its counterpart JK-305 (see the Supporting Infor-
mation) by incorporating a dithiophene bridge as a reference.
Spectroscopic studies
The absorption and emission spectra of JK-305 and JK-306 in
ethanol are shown in Figure S1. The JK-305 dye with the bi-
thiophene linker has an absorption maximum at 451 nm,
which is assigned to the p–p* transition of the conjugated
molecule. Upon using the planar indeno[1,2-b]thiophene unit
for JK-306, the absorption band is significantly red-shifted to
471 nm. Moreover, the molar absorption coefficient at the
maximum absorption band is increased from 3.19ꢁ104 to
3.46ꢁ104 mÀ1 cmÀ1 during the p-linker planarization. The bath-
ochromic and hyperchromic effects of introducing a planar
indeno[1,2-b]thiophene unit into JK-306 can be understood
from the molecular-modeling study of the dye. The ground-
state configuration of JK-306 possesses a 1.88 twist between
the indenothiophene and the adjacent thiophene unit, which
resulted in an almost planar configuration. For JK-305, the di-
hedral angle between the aniline and the thienyl moieties and
between two thienyl moieties are 24.68 and 3.28, respectively,
which results in a more twisted configuration compared to its
counterpart JK-306. Thus, a high molar extinction coefficient
and a significant red-shift of JK-306 relative to JK-305 derive
from an increased delocalization over an entire conjugated
bridging unit.
Figure 1. Structure of JK-305 and JK-306 dyes.
Results and Discussion
Synthesis of the sensitizer
The procedures to synthesize the organic sensitizer JK-306
starting from a bis(2’,4’-dihexyloxybiphenyl-4-yl)amine (3) that
is synthesized from the reaction of 2,4-bis(hexyloxy)-4’-iodobi-
phenyl (1) and 2’,4’-bis(hexyloxy)biphenyl-4-amine (2) are
shown in Scheme 1. The N-phenylation of bromo-4,4-dimethyl-
4H-indeno[1,2-b]thiophene (4) was performed under the
Buchwald reaction conditions,[28] followed by bromination with
Electrochemical properties of these two sensitizers
were evaluated by using cyclic voltammetry in aceto-
nitrile with 0.1m tetrabutylammonium hexafluoro-
phosphate. TiO2 films stained with the sensitizers
were used as working electrodes. The oxidation po-
tential of the two sensitizers adsorbed on the TiO2
film shows a quasi-reversible couple at 0.97 and
1.03 V versus standard hydrogen electrode (NHE), re-
spectively. The oxidation potential of JK-305 is less
positive than that of JK-306, which indicates an in-
creased electron-donating ability of JK-305 compared
with that of JK-306 because of the localization of the
amino moiety and the p-linker unit. The reduction
potentials of JK-305 and JK-306 calculated from the
oxidation potential and the E0–0 values determined
from the intersection of absorption and emission
spectra are listed in Table S1. The excited-state oxida-
tion potentials (Eox) of JK-305 (À1.33 V vs. NHE) and
JK-306 (À1.18 V vs. NHE) are more negative than the
conduction band of TiO2 (À0.5 V vs. NHE). Thus, the
Scheme 1. Schematic diagram for the synthesis of JK-306; i) Pd2(dba)3 (dba= dibenzal-
acetone), P(tBu)3, tBuONa, toluene, reflux; ii) NBS, CH2Cl2, room temperature; iii) Pd(PPh3)4,
K2CO3, H2O, THF, reflux; iv) 7, piperidine, cyanoacetic acid, AcCN, CHCl3, reflux. Full reac-
tion conditions are detailed in the Supporting Information.
downhill energy offset of the lowest unoccupied mo-
lecular orbital (LUMO) in both dyes ensures the ther-
modynamic driving force for electron injection.[30]
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Theoretical calculation
We performed the molecular orbital calculation for JK-305 and
JK-306 to gain insight into the photophysical properties by
using the B3LYP/6-31G(d) method. The isodensity plots of the
frontier molecular orbitals of JK-305 and JK-306 are shown in
Figure 2. The calculation illustrates that the highest occupied
Figure 2. Isodensity surface plots of the HOMO and LUMO of JK-305 and
JK-306. The molecular orbital calculation of dyes were performed by using
the B3LYP/6-31G(d) method.
molecular orbital (HOMO) of both sensitizers is delocalized
over the p-conjugated system through the triphenyl amino
group. The LUMO of both sensitizers is delocalized over the cy-
anoacrylic acid unit with a sizable population on the next thio-
phene unit. As light excitation should be associated with vec-
torial electron flow from the HOMO to the LUMO for efficient
electron transfer, the examination of HOMO–LUMO of both
sensitizers indicates that HOMO–LUMO excitation moved the
electron distribution from the triphenylamino moiety to the
cyanoacrylic acid group.
Electrochemical impedance spectroscopy
To better understand the improved performance of the JK-306-
sensitized solar cell with HC-A instead of CDCA in the CoII/CoIII
electrolyte system, electrochemical impedance spectroscopy
(EIS) measurements were performed on the same devices. The
recombination resistance (Rct) and transport resistance (Rt) as
well as the capacitance (Cm) for the JK-306-sensitized cells
based on the HC-A and CDCA co-adsorbents in the dark are
shown in Figure 3. The respective Nyquist plots are presented
in Figure S4. As can be seen, Rct is higher for HC-A than for
CDCA in the range of applied forward bias potentials; thus, it
is expected that the dark current is lower for HC-A than for
CDCA. There was no noticeable difference in Cm between
JK-306 solar cells based on HC-A and CDCA. As shown in Fig-
ure 3b, the electron lifetime (tr =CRct)[31] for HC-A was longer
than that for CDCA in the range of applied forward bias poten-
Figure 3. a) Rt, Rct, and Cm of the JK-306-sensitized cell with CDCA and HC-A
as co-adsorbents for the CoII/CoIII redox couple under dark conditions.
b) Charge collection efficiency of the same devices as a function of applied
bias potential. c) The electron lifetime and the transport time for the same
devices as a function of forward bias potential.
tials. The co-adsorption of HC-A instead of CDCA, which forms
an insulating layer, blocks the recombination of electrons with
the [Co(bpy)3]3+ (bpy=2,2’-bipyridine) ions and thus enhances
the electron lifetime.[25–27] The longer electron lifetime contrib-
uted to a higher open-circuit voltage (Voc). Furthermore, the
photovoltaic performance is reflected by the charge collection
efficiency (hcc) derived from hcc =(1+Rt/Rct)À1 or (1+tt/tr)À1.[32]
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The hcc for HC-A is higher in the range of forward bias poten-
ative shift of TiO2 conduction band edge, and the reduced
number of surface trapping sites, contributed simultaneously
to improvements in Voc and Jsc for the JK-306-sensitized solar
cell based on HC-A.
À
tials compared to that for CDCA. For the IÀ/I3 electrolyte
system, trends consistent with the aforementioned results
were obtained and are presented in Figure S5.
To further investigate the simultaneous improvements in Voc
and Jsc (short-circuit photocurrent density), the examination of
changes in the surface trapping sites were performed by using
cyclic voltammetry[33] on each JK-306-sensitized TiO2 electrode
with HC-A and CDCA immersed in 0.1m LiClO4 in acetonitrile
(Figure S6). The capacitive currents of two electrodes at the
TiO2/LiClO4 interface are shown in Figure 4, which demonstrat-
Photovoltaic performance
The spectra of monochromatic incident photon-to-current con-
version efficiencies (IPCEs) for DSSCs based on JK-305 and
JK-306 with CDCA in combination with an iodine electrolyte
are shown in Figure 5. The onset wavelength of the IPCE spec-
trum for the device based on JK-305 is 720 nm. However, the
Figure 4. The exponential distribution of DOS was deduced by plotting ca-
pacitive charging of electrons in the surface states versus applied potential.
The capacitive charging current appearing at a negative potential is attribut-
able to the filling of surface states below the conduction band edge of TiO2
for all structures.
ed gradual onsets under a forward potential. The distribution
of trap states can be estimated from the density of states
(DOS) calculation by using DOS=(dQ/dV)(NA/F),[33] in which Q
is the total number of surface trapping sites, NA Avogadro’s
number, F Faraday’s constant, and V the potential applied to
the electrode. As shown in Figure 4, the onsets of the capaci-
tive currents were around À0.183 V for CDCA whereas the TiO2
electrode covered with JK-306 and HC-A together demonstrat-
ed onsets of around À0.283 V, which indicates that the edge
of the TiO2 conduction band moved to lower values (toward
the vacuum level). In DSSCs, Voc is determined from the differ-
ence between the quasi-Fermi level and the redox potential of
redox couple in the electrolyte;[34] therefore, a higher Voc of the
JK-306-sensitized cell with HC-A should originate from the neg-
ative shift of the quasi-Fermi level of its electrode.
Figure 5. a) Photocurrent–voltage response under simulated full AM 1.5G
sunlight using JK-305- and JK-306-sensitized TiO2 films together with CDCA
or HC-A in the presence of the iodine or cobalt electrolyte. b) IPCE as a func-
tion of wavelength.
spectrum of JK-306 tails off toward 800 nm, which contributes
to the broad spectral light harvesting. The IPCE spectrum of
JK-306 is significantly red-shifted by 80 nm compared to that
of JK-305, which is consistent with their absorption spectra.
The J–V curve for the JK-305-sensitized cell gave rise to a Jsc
of 13.12 mAcmÀ2, a Voc of 0.76 V, and a fill factor (ff) of 0.73,
which afforded an overall conversion efficiency (h) of 7.29%.
Moreover, dQ/dV of the JK-306-sensitized electrode with
HC-A was smaller than that with CDCA. This indicated a reduc-
tion in the number of surface trapping sites at which photo-
induced electrons could recombine with oxidized species (e.g.,
[Co(bpy)3]3+).[33] Therefore, we concluded that the multifunc-
tional effects, such as the breakup of dye aggregates, the neg-
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Under the same conditions, the JK-306-sensitized cell gave rise
to a Jsc of 14.10 mAcmÀ2, Voc of 0.75 V, and ff of 0.70, which af-
forded h of 8.37%. A significant increase in Jsc of JK-306 stems
from the high molar extinction coefficient and the broad and
red-shifted UV spectrum of JK-306 through the delocalization
over an entire conjugated system. Upon changing the co-ad-
sorbent from CDCA to HC-A in the JK-306-sensitized cell, the
maximum IPCE improved significantly up to 35% at 360 nm
compared to that of JK-305, which contributes to the increased
photocurrent of the cell. The efficiency of 8.52% in the JK-306-
sensitized cell was slightly improved (Jsc =16.14 mAcmÀ2, Voc =
0.75 V, and ff=0.71) compared to that of the JK-306-sensitized
cell with CDCA, which should be attributed to the role of HC-A
having multifunctional effects,[25–27] such as preventing dye ag-
gregation, a light harvesting effect in the shorter wavelength
regions around 360 nm, and fast dye regeneration through the
hole conducting effect. Moreover, the JK-306 dye exhibits
highly efficient photovoltaic performances if the CoII/CoIII elec-
Table 1. Photovoltaic properties of JK-305 and JK-306.
Dye
Electrolyte Co-adsorbent Jsc
Voc
[V]
ff
h[a]
[mAcmÀ2
]
[%]
JK-305[b] iodine[c]
JK-306[b] iodine[c]
JK-306[d] iodine[c]
JK-306[b] cobalt[e]
JK-306[d] cobalt[e]
CDCA
CDCA
HC-A
CDCA
HC-A
13.12
16.10
16.14
14.67
14.83
0.76 0.73
0.72 0.70
0.75 0.71
0.87 0.73
7.29
8.37
8.52
9.40
0.95 0.71 10.02
[a] Irradiated light: AM 1.5G (100 mWcmÀ2); cell area tested with a metal
mask: 0.16 cm2. [b] Dipping solution: 0.3 mm dye solution (EtOH/THF=
2:1) with 20 mm CDCA. [c] Iodine electrolyte: 0.6m 1,2-dimethyl-3-propyl-
imidazolium iodide, 0.05m I2, 0.1m LiI, 0.8m TBP in AcCN. [d] Dipping so-
lution: 0.3 mm dye solution (EtOH/THF=2:1) with 0.3 mm HC-A.
[e] Cobalt electrolyte: 0.22m [CoII(bpy)3](B(CN)4)2, 0.05m [CoIII-
(bpy)3](B(CN)4)3, 0.1m LiClO4, and 0.8m TBP in AcCN.
À
trolyte is used instead of the IÀ/I3 electrolyte.
The JK-306-sensitized cell was fabricated with the CoII/CoIII
electrolyte containing 0.22m [CoII(bpy)3](B(CN)4)2, 0.05m [CoIII-
(bpy)3](B(CN)4)3, 0.1m LiClO4, and 0.8m 4-tertbutylpyridine
(TBP), in acetonitrile. For CDCA, a high conversion efficiency of
9.40% under simulated sunlight (100 mAcmÀ2, AM 1.5G) was
achieved at Jsc =14.67 mAcmÀ2
, Voc =0.87 V, and ff=0.71.
Upon the use of HC-A with JK-306, the cell shows a notably
improved conversion efficiency of 10.02% with a Jsc of
14.33 mAcmÀ2, Voc of 0.95 V, and ff of 0.71. It is one of the
highest values reported for DSSCs with a cobalt-based electro-
lyte.[32,35] The Voc of 0.95 V based on the cobalt electrolyte and
HC-A is 0.2 V higher than for the iodine electrolyte under the
same conditions; this results in an overall conversion efficiency
increase of approximately 17%. This improved Voc value is at-
tributed to a large energy difference between the TiO2 conduc-
tion band and the redox couple of the cobalt electrolyte and
suppression of dark current owing to the blocking effect of
HC-A. However, the device exhibits a significant decrease in Jsc
relative to the iodine congener. The slow diffusion of the
cobalt-based cell may limit Jsc (Figure S2 and Table 1).[36]
The long-term stability is a vital factor for sustained cell op-
eration. Therefore, we substituted the liquid-based electrolyte
with a quasi-solid-state one because the cells using the liquid
electrolyte have many drawbacks, such as leakage, evapora-
tion, and corrosion of the metals by the IÀ/I3 electrolyte. The
À
photovoltaic performance of JK-305- and JK-306-sensitized
cells using a polymer gel electrolyte composed of 5 wt% poly-
(vinylidenefluoride-co-hexafluoropropylene), 0.05m I2, 0.6m
1,2-dimethyl-3-propyl-imidazolium iodide, 0.1m LiI, and 0.8m
N-methyl-benzimidazole in acetonitrile is shown in Figure 6.
The JK-305-sensitized cell gave rise to a Jsc of 12.40 mAcmÀ2
,
Figure 6. a) J–V curve and b) IPCE spectra of polymer gel-based cells with
JK-305 and JK-306. Cell area tested with a metal mask=0.16 cm2.
Voc of 0.74 V, and ff of 0.76, which afforded h of 7.00%. Under
the same conditions, the JK-306-sensitized cell gave rise to h
of 7.61% with a Jsc of 14.02 mAcmÀ2, Voc of 0.73 V, and ff of
0.74. The conversion efficiency for the JK-306-sensitized cell is
approximately 0.61%, which is superior to that of the JK-305-
sensitized cell and attributable to a significant improvement in
Jsc. This result demonstrated a trend similar to that of liquid-
based electrolytes. The efficiency of 7.61% is one of the high-
est values for a cell based on organic sensitizers.[37,38]
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Conclusions
Acknowledgements
We have designed and synthesized an efficient organic sensi-
tizer (JK-306) with a planar indeno[1,2-b]thiophene as a
p-linker and a highly congested bulky amino group as an elec-
tron donor. The devices based on the JK-306 sensitizer with
CDCA and HC-A as co-adsorbents resulted in high conversion
efficiencies of 8.37% and 8.52%, respectively. The JK-306-
based solar cell fabricated with the CoII/CoIII redox couple af-
This work was supported by a World Class University program
funded by the Ministry of Education, Science, and Technology
(MEST) through the National Research Foundation of Korea
(R31-2012-000-10035-0), the Engineering Research Council (ERC)
MEST program (2012-0000591), and the Converging Research
Center Program (2012K001287). This work was also supported by
the International Science and Business Belt Program
(2012K001573) through the Ministry of Science, ICT, and Future
Planning (former Education, Science, and Technology).
forded
a significantly improved conversion efficiency of
10.02%, which was attributable to the multifunctional effect,
such as the breakup of dye aggregates, the negative shift of
TiO2 conduction band edge, and the reduced number of sur-
face trapping sites attributed to HC-A instead of CDCA. This is
one of the highest values reported for DSSCs with cobalt-
based electrolytes. Moreover, the JK-306-sensitized solar cell
with a polymer gel electrolyte resulted in a high conversion ef-
ficiency of 7.61%, which is one of the highest values reported
for a cell based on organic sensitizers. We believe that the de-
velopment of highly efficient organic sensitizers may be possi-
ble through meticulous molecular engineering of organic
sensitizers.
Keywords: cobalt · electrolytes · impedance · sensitizers · solar
cells
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Experimental Section
The fluorine-doped tin oxide (FTO) glass plates (Pilkington TEC
Glass–TEC 8, Solar 2.3 mm thick) were cleaned in a detergent solu-
tion by using an ultrasonic bath for 30 min and rinsed with H2O
and EtOH. The FTO glass plates were immersed in TiCl4 (40 mm) at
708C for 30 min and washed with H2O and EtOH. A transparent
nanocrystalline layer on the FTO glass plates were prepared
through screen printing with the TiO2 paste (Dyesol, 18NR-T) and
then dried at 1208C. The TiO2 electrodes were gradually heated
under an air flow at 3258C for 5 min, at 3758C for 5 min, at 4508C
for 15 min, and at 5008C for 15 min. A paste for the scattering
layer containing 400 nm sized anatase particles (CCIC, PST-400C)
was deposited through screen printing and then dried for 1 h at
1208C. The TiO2 electrodes were gradually heated under an air
flow at 3258C for 5 min, at 3758C for 5 min, at 4508C for 15 min,
and at 5008C for 15 min. The resulting layer was composed of
a 6 mm thick transparent layer and a 4 mm thick scattering layer.
The TiO2 electrodes were again treated with TiCl4 at 708C for
30 min and sintered at 5008C for 30 min. The TiO2 electrodes were
immersed in the dye solution (0.3 mm in THF/EtOH=1:2 contain-
ing 20 mm CDCA or 0.3 mm HC-A) and kept at room temperature
for 12 h. Then, the Pt on FTO plates as counter electrodes was
cleaned in an ultrasonic bath in H2O, acetone, and HCl (2m, aque-
ous). The counter electrodes were prepared by coating with a drop
of the H2PtCl6 solution (2 mg of Pt in 1 mL of EtOH) on an FTO
plate and heating at 4008C for 15 min. The dye adsorbed on the
TiO2 electrode and the Pt-counter electrode were assembled into
a sealed sandwich-type cell by heating at 808C with a hot-melt
film (60 mm thick Surlyn) as a spacer between the electrodes.
A drop of the electrolyte solution was placed on a drilled hole in
the counter electrode of the assembled cell and was driven into
the cell through vacuum backfilling. Finally, the hole was sealed by
using additional Surlyn and a cover glass (0.1 mm thick).
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Received: March 30, 2013
Published online on June 20, 2013
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ChemSusChem 2013, 6, 1425 – 1431 1431