Silanol dyes for solar cells: Higher efficiency and significant durability

Article abstract of DOI:10.1002/aoc.1612

Azobenzene-containing silanol dyes were synthesized, and their applicability to dye-sensitized solar cells was investigated. Silanol dyes showed better effectiveness when compared with conventional carboxy-substituted azobenzene dyes. Moreover, silanol dy

Full text of DOI:10.1002/aoc.1612

Full Paper
Received: 25 August 2009
Revised: 13 November 2009
Accepted: 24 November 2009
Published online in Wiley Interscience
(www.interscience.com) DOI 10.1002/aoc.1612
Silanol dyes for solar cells: higher efficiency
and significant durability
Masafumi Unnoa,b∗, Kenji Kakiagea, Masaki Yamamuraa, Takehiro Kogurea,
Toru Kyomena,b and Minoru Hanayaa,b
Azobenzene-containing silanol dyes were synthesized, and their applicability to dye-sensitized solar cells was investigated.
Silanol dyes showed better effectiveness when compared with conventional carboxy-substituted azobenzene dyes. Moreover,
silanol dyes showed better durability than carboxyl dyes; ∼90% of silanol dyes remained intact on the TiO2 electrode of the
solar cell after being immersed in water for 96 h, whereas in the case of carboxy dyes this figure was less than 20%. Copyright
c
ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: silanol; dye-sensitized solar cell; higher efficiency; better durability
Introduction
The energy conversion efficiencies of dye-sensitized solar cells
(DSSCs) are approaching 12% after Gra¨tzel’s development of
the nanocrystalline cell.[1,2] However, in the last decade, the
improvement in the energy conversion efficiency has been less
than1%, indicating thatnewmaterialsarenecessarytorealize15%
efficiency for application. In recent applications of DSSCs, another
problem has been encountered, namely durability. The existing
sensitizing dyes contain sulfonic, phosphonic or carboxy groups
boundtoTiO2;[3,4] thesebondsaresusceptibletohydrolysis, which
affects the lifespan of the dye. This is a serious problem because
a shorter lifespan has a direct impact on the total cost. Moreover,
tight sealing is required for preventing water from penetrating
the device. In order to find a solution to these problems, we
examined the use of silanols for dye-sensitized solar cells. In earlier
studies, we had reported the versatility of silanols as starting
compounds for cage siloxanes[5–8] and laddersiloxanes,[8,9–12] or
ashostmolecules.[13–15] Wehadalsoprovedthatsilanolsformvery
strong bonds with most metal oxides such as alumina, titania or
silica.[16] Further, addition of silicon atoms to a π-system enhances
electron delocalization – a bathochromic shift and hyperchromic
effect are observed in most cases.[17] These two strong points
prompted us to develop silanol dyes for solar cells. In this paper,
we describe the introduction of silicon atoms to dyes and the
impact on the effectiveness and durability of solar cells.
Scheme 1. Bonding schemes of carboxylic acid.
that the starting materials were consumed without any problem.
Therefore, we hypothesize that the target compounds could
not be obtained because of the instability of silyl esters of
carboxylic acids. During the procedure of purification (using
column chromatography or distillation), silyl esters underwent
decomposition, which indicates that they were unstable in the
presence of moisture. These results clearly indicate that the
bond between carboxyl acid and silanol is not stable. Since the
nature of the bonding between carboxylic acid and silanol or
titanol is similar, the chemical bonding of carboxylic acid with
titanium oxide is also expected to be weak and liable to hydrolysis
(Scheme 1). In order to establish the durability of solar cells, anchor
point substituents must be carefully chosen; conventional carboxy
groups may not serve the purpose.
Since the method of modification of carboxylic dyes did not
work, we then considered the introduction of silyl groups into
the dyes. With this modification, electron delocalization brings
Results and Discussions
∗
Synthesis of Silanol Dyes
Correspondence to: Masafumi Unno, Gunma University, Department of
Chemistry and Chemical Biology, Graduate School of Engineering,
1-5-1Tenjin-cho, Kiryu, Gunma376-8515, Japan. E-mail:unno@gunma-u.ac.jp
Our first approach involved the transformation of the carboxy
group to silyl substituents. Through this approach, a substantial
number of dyes with carboxyl groups can be utilized as
starting compounds. We used benzoic acid or salicylic acid as
model compounds and examined their reaction with various
chlorosilanes. Unfortunately, we could not obtain the target
compounds in the reaction conditions that we used. We used
gas chromatography (GC) to trace the reaction, and we observed
a
Department of Chemistry and Chemical Biology, Graduate School of
Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515,
Japan
b
International Education and Research Center for Silicon Science, Graduate
School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma
376-8515, Japan
c
Appl. Organometal. Chem. 2010, 24, 247–250
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

M. Unno et al.
Scheme 2. Preparation of azobenzene-substituted silanols.
Table 1. Summary of UV-vis spectra in acetonitrile solutions
Compounds
λmax, nm (ε)
Azobenzene
440 (530), 316 (20500), 227 (13000)
445 (1480), 323 (46900), 229 (24900)
444 (650), 322 (22900), 228 (10500)
2
3
quantitative comparison of sensitizing properties between the
dyes containing silanol and carboxy groups. Figure 2 shows the
absorption spectra in visible-light region of 2 and 3 adsorbed
on the TiO2 electrodes, which were used in the cells for I–V
measurements. The spectra were obtained by subtracting the
absorption due to the TiO2 electrode from those of dye-adsorbed
TiO2 electrodes. From these spectra, the relative values of
difference absorbance at the absorption maxima were estimated
to be 0.125 for 3 and 0.153 for 2. In the wavelength region the
ε value of 2 in solution is about 2.3 times larger than that of 3,
thus we could estimate the relative amount of the molecules of 3
adsorbed on TiO2 electrode to that of 2 to be 1.9 : 1. As compound
2 has two azobenzene moieties per molecule, the total number of
azobenzene moieties on the electrode surfaces is considered to
be fairly close between the cases of 2 and 3. Thus, we can safely
assume that the number of azobenzene moieties on the device is
the same for 2 and 3, and that direct comparison of I–V properties
is possible. The result of the I–V measurement is shown in Fig. 3.
The cells containing 2 exhibited higher short-circuit current (Jsc)
values than the cells containing 3 or the cells containing no dye.
As a result, the light-to-electric energy conversion efficiency (η)
of the cells with 2 is higher than that of the cells with 3 by a
factor of 2.7 under the present test conditions. The results of the
measurement are summarized in Table 2. These results reveal that
2 is effective as a sensitizing dye for DSSCs. They also prove that
electron transfer from the light-excited dye to the TiO2 electrode
of 2 through the Si–O–Ti bonds is more efficient than that of 3
through the ester-like bond.
Figure 1. UV–vis absorption spectra of 2, 3 and azobenzene in acetonitrile
solutions: the solid line represents the result for 2, the broken line that for
3 and the dotted line azobenzene.
about bathochromic shift and hyperchromic effect. Both these are
favorable for solar cell dyes.
Admittedly, azobenzene is not the best sensitizer for a solar
cell; however, easy access to azobenzene-substituted silanol is
advantageous for the comparison of electronic properties and
stability of silanol and carboxylic acid dyes. Moreover, the fact that
azobenzene has a simple formula makes the orbital calculations
easier. Therefore, we then tried to synthesize silylazobenzene.
All attempts at the preparation of 4-phenylazosilanetriol (1)
failed, probably because of the decomposition of triols by
condensation. On the other hand, the synthesis of bis(4-
phenylazophenyl)silanediol (2) proceeded well and we obtained
the product as an orange-yellow solid that was stable in both
air and moisture (Scheme 2). The UV–vis spectra of azobenzene
derivatives are summarized in Fig. 1. While the absorption of 4-
carboxyazobenzene 3 is similar to that of azobenzene, the effect
of silyl substituents of 2 is noticeable. Notably, the absorption
coefficient of 2 is 2.3 times higher than that of 3 in the visible
light region of λ = 445 nm. Thus, silanol dye 2 is clearly useful for
DSSCs. The results of the UV–vis spectra of the silyl derivatives in
solution are summarized in Table 1.
Durability of Silanols Dyes
Effectiveness of Silanol Dyes as a Sensitizer for Solar Cells
In the case of solar cells with conventional carboxy dyes, instability
of the dye-adsorbed electrodes to water is a major problem. In
order to prevent ingress of moisture, tight sealing is necessary,
and even after this, effectiveness decreases after some time. This
instability is based on the weakness of the ester-like bond formed
The silanol dye 2 was observed to work as a sensitizer in a dye-
sensitized solar cell (DSSC), as reported shortly in our previous
paper.[18] In this work, we measured the I–V properties of the
cells using 2 and 3 as sensitizing dyes to obtain a precise and
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 247–250

Silanol dyes for solar cell
Figure 4. Changes of the absorptions due to the dyes 2 and 3 adsorbed
on the TiO2 electrodes with the soaking time in water at 25 ◦C: open circles
represent the results for 2 and solid circles that for 3. The absorption ratios
were evaluated by the normalization of absorbance at λ = 414 nm with
respect to those before soaking in water.
dye 3 lost more than 80% of its volume after 96 h, while silanol dye
2 retained about 90% of its volume. This property of silanol dyes of
bonding tightly to the TiO2 electrode could be a great advantage
in the utilization of silanol dyes as sensitizing dyes for DSSCs.
These results clearly indicate that silanol dyes are promising
from the viewpoints of both higher effectiveness of electron
transfer as well as durability.
Figure 2. UV–vis absorption spectra of 2 and 3 adsorbed on TiO2
electrodes: the solid line represents the result for 2 and the broken
line 3.
Conclusion
The results of our investigation of alternative compounds
for practical application in dye-sensitized solar cells can be
summarized as follows: (1) introduction of silyl groups into dyes
resulted in bathochromic shift and higher absorption coefficient;
(2) transformationofknowndyeswithcarboxylicgroupstosilanols
or alkoxysilanes was unsuccessful; (3) silanol dyes showed better
effectiveness and durability than carboxyl dyes.
We are currently investigating more effective variants of silanol
dyes as well as the quantitative comparison of the durability of
dyes for solar cells.
Figure 3. I–V properties of the cells using 2 and 3 as sensitizing dyes
and that without dye under a simulated sunlight irradiation: the solid line
represents the result for 2, the broken line that for 3 and the dotted line
without dye.
Experimental
The reactions were carried out under either an argon or
a nitrogen atmosphere. Fourier transform nuclear magnetic
resonance spectra were obtained using a Jeol model AL-300
Table 2. I–V properties of the cells using 2 and 3 as sensitizing dyes
and with no dye
(1H at 300.00 MHz) and Jeol model α-500 (1H at 500.00 MHz, 13
C
125.65 MHz, 29Si at 99.25 MHz). Chemical shifts were reported as
δ units (ppm) relative to SiMe4, and residual solvent peaks were
used for standards. For 29Si NMR, SiMe4 was used as an external
standard. Electron ionization mass spectrometry was performed
with a Jeol JMS-DX302 and JMS-700. Electrospray ionization mass
spectra (ESI-MS) were recorded on an Applied Biosystems/MDS-
Sciex API-100 spectrometer. Infrared spectra were measured with
a Shimadzu FTIR-8700. UV–vis spectra were recorded on a Hitachi
U-3010 spectrometer, and an integrating sphere was equipped to
the spectrometer for the measurements of the dyes adsorbed on
the TiO2 electrodes.
Dyes
Jsc (mA cm−2
)
Voc (V)
FF
η (%)
3
0.715
1.674
0.307
0.492
0.513
0.408
0.433
0.485
0.411
0.152
0.416
2
With no dye
0.0515
between the dye and the TiO2 electrode. In the case of silanol
dyes, on the other hand, the chemical adsorption of the dye to the
TiO2 electrode is performed by the formation of a Si–O–Ti bond
between the silanol moiety of the dye and the TiO2 electrode, and
the formed Si–O–Ti bond is expected to be much more stable
than the ester-like bond. To check the durability, we soaked the
dyes on TiO2 in water at 25 ◦C, and measured the change of the
absorption at λ = 414 nm (see Fig. 2). As shown in Fig. 4, carboxy
Synthesis of bis(4-Phenylazophenyl)silanediol (2)
Butyl lithium (0.70 mmol, hexane solution) was added to a
solution of 200 mg (0.649 mmol) of 4-iodoazobenzene[19] in
c
Appl. Organometal. Chem. 2010, 24, 247–250
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
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M. Unno et al.
20 ml of Et2O under stirring at −70 to −20 ◦C. After 1 h,
72 mg (0.47 mmol) of tetrachlorosilane was added at the same
temperature and stirred for 17 h. The resulting solution was
washed three times with 20 ml of brine and evaporated. The
residuewasrecrystallizedfromhexane–ethertoyield56 mg(20%)
ofbis(4-phen◦ylazophenyl)silanediolasanorange-redpowder.M.p.
154.2–157.9 C. 1H NMR (500 MHz, CDCl3) δ 7.20 (m, 1H), 7.45 (m,
3H), 7.70–7.95 (m, 5H). 13C NMR (125.65 MHz, CDCl3) δ 22.3, 122.4,
123.1, 129.2, 131.3, 136.2, 152.7, 154.0 ppm. 29Si NMR (99.25 MHz,
CDCl3) δ−21.8 ppm. IR (KBr, cm−1) ν 323, 3065, 1593, 1385, 1120,
1074, 835, 766, 685, 598, 554. MS m/z (%) 423 (100), 259 (50). Anal.
calcd for C24H20N4O2Si: C, 67.96; H, 4.75; N, 13.21. Found: C, 68.66;
H, 4.49; N, 13.32.
Acknowledgment
This research was partially supported by the Ministry of Education,
Science, Sports and Culture, Grant-in-Aid for Scientific Research
(B), 21350070. We acknowledge the generous gift of silicon
compounds by Shin-Etsu Chemical Co. Ltd and Momentive
Performance Materials Inc.
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The photovoltaic measurements were performed for the elec-
trochemical cell consisting of a dye-adsorbed TiO2 electrode, a
counter electrode, a polyethylene film spacer (100 µm thick), and
an organic electrolyte. A Pt-sputtered FTO-coated glass plate was
used as the counter electrode, and a solution of 0.3 M LiI and 0.015
M I2 in acetonitrile–ethylene carbonate (2 : 8 in volume) was used
as the electrolyte. The photovoltaic performance of the solar cells
was assessed from the I–V properties of the cells measured with
a solar simulator of OTENTO-SUN III (Bunkoh-Keiki) and a source
meter of R6240A (Advantest). The aperture area of the cells was
maintained at 1.0 cm2 using a shading mask and the I–V proper-
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condition (100 mW cm−2) at 25 2 ◦C.
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 247–250