Novel 4-trifluoromethylthiazole-5-carboxylic acid as acceptor in photosensitized dyes

Article abstract of DOI:10.1246/cl.2012.1560

Novel photosensitized dyes containing 4-trifluoromethyl-thiazole-5- carboxylic acid as an acceptor are synthesized. Stilbene-type dyes with this acceptor show high performance as dye-sensitized solar cells. The trifluoromethyl group is assumed to act as a suppressor of electron back-donation from the TiO₂ conduction band to the electrolyte and as an accelerator of charge separation in the photoexcited state.

Full text of DOI:10.1246/cl.2012.1560

doi:10.1246/cl.2012.1560
1560
Published on the web November 17, 2012
Novel 4-Trifluoromethylthiazole-5-carboxylic Acid as Acceptor in Photosensitized Dyes
Satoru Iwata,*1 Misa Aoyama,1 Satoshi Uchida,2 and Kiyoshi Tanaka*1
1Laboratory of Molecular Control, Faculty of Science and Technology, Seikei University, Musashino, Tokyo 180-8633
2Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo 153-8904
(Received July 6, 2012; CL-120721; E-mail: iwata@st.seikei.ac.jp, tanaka@st.seikei.ac.jp)
Novel photosensitized dyes containing 4-trifluoromethyl-
The absorption spectra of the obtained dyes are shown in
Figure 1, and the absorption and the redox potential data are
summarized in Table 1. The absorptions of the dyes with no CF3
group, MCL08 and MCL09, are slightly blue-shifted compared
to those of the CF3-containing dyes, MCL05 and MCL06. The
oxidation potentials, Eox, were measured from the peak potential
Ep (revised using the ferrocene oxidation potential). The redox
energy gaps (E0-0) were calculated from the cross points between
the abscissa and the tangent to the curve where the slope was the
thiazole-5-carboxylic acid as an acceptor are synthesized.
Stilbene-type dyes with this acceptor show high performance
as dye-sensitized solar cells. The trifluoromethyl group is
assumed to act as a suppressor of electron back-donation from
the TiO2 conduction band to the electrolyte and as an accelerator
of charge separation in the photoexcited state.
Dye-sensitized solar cells (DSCs) have reached the stage of
practical applications. High performances of DSCs have been
reported using ruthenium complexes, which exhibit © (solar
energy to electricity conversion efficiency) values above 11%.1
Recently, metal-free sensitizers have been reported to have
effective performances (© = 7-10%).2
strongest, and the Ered values were calculated from Eox ¹ E0-0.
The HOMO and LUMO energy levels of the dyes were
calculated using DFT with B3LYP6-31G* under vacuum
conditions (Figure 2). The experimental HOMO and LUMO
levels were estimated from the values of Eox and Ered
,
respectively, according to the following equation.7
The architecture of a metal-free sensitizer needs an electron
donor, a spacer (³-conjugated bridge), and an electron acceptor
linked with an anchor. Conjugated amines such as triphenyl-
amine have been widely used as donor parts,3 and stilbenes,
phenylenes, and conjugated thiophenes have been used as
spacers.4 However, the diversity of acceptors is not enough;
most effective sensitized dyes consist of cyanoacrylic acid or
rhodanine acetic acid.5 In this communication, we report novel
sensitized dyes with trifluoromethyl (CF3-) thiazolecarboxylic
acid as an acceptor. Previously we reported that azo dyes
containing trifluoromethyl-thiazolecarboxylate show long wave-
length absorption,6 and so it is expected that the introduction of
trifluoromethyl-thiazolecarboxylic acid to the sensitized dyes as
an acceptor will lead to absorption over a wide range of
wavelengths of sun light.
The synthetic procedures for the dyes are shown in
Scheme 1 (details are shown in the ESI14). Dyes with a CF3
group, i.e., MCL05 and MCL06, and dyes with no CF3 group,
i.e., MCL08 and MCL09, were synthesized via Horner-
Wadsworth-Emmons reactions using the corresponding thia-
zolemethylphosphonates 3a and 3b obtained by Michaelis-
Arbuzov reactions of (bromomethyl)thiazole 2a or 2b.
Figure 1. Absorption spectra of dyes in acetonitrile solution,
[dye] = 1.0 © 10¹5 M.
Table 1. Absorption and redox properties of dyes
-
a/nm
Eoxb/V
(vs. NHE)
abs
Dye
E0-0c/eV Eredd/V
¹1
(¾/©104 M¹1 cm
)
MCL05
MCL06
MCL08
MCL09
420 (2.4)
433 (3.3)
399 (2.7)
423 (2.3)
1.06
0.86
1.02
0.83
2.54
2.45
2.63
2.48
¹1.48
¹1.59
¹1.61
¹1.65
Y
Y
Y
N
S
N
S
N
S
a
b
Me
CH2Br
CH2PO(OEt)2
EtOOC
EtOOC
EtOOC
1a (Y = CF3)
1b (Y = H)
2a (Y = CF3) 36%
2b (Y = H) 51%
3a (Y = CF3) quant.
3b (Y = H) 88%
aAbsorption was recorded in an acetonitrile solution (1.0 ©
10¹5 M). bEox shows oxidation peak potential vs. NHE. Cyclic
voltammetry was measured in an acetonitrile solution
containing 0.1 M tetrabutylammonium perchlorate as a sup-
porting electrolyte. Working electrode: platinum, counter
Y
Y
NR2
NR2
N
N
S
c
d
S
EtOOC
HOOC
4a (Y = CF3, R = Ph) 60%
4b (Y = CF3, R = Et) 23%
MCL05 (Y = CF3, R = Ph) 39%
MCL06 (Y = CF3, R = Et) 33%
4c (Y = H, R = Ph)
4d (Y = H, R = Et)
48%
14%
MCL08 (Y = H, R = Ph)
MCL09 (Y = H, R = Et)
87%
75%
electrode: platinum wire, reference electrode: Ag/Ag+ cali-
¹1
brated with ferrocene/ferrocenium, scanning rate: 100 mV s
.
cThe E0-0 value was estimated from the cross point between the
Scheme 1. Synthesis of dyes. Reagents (a) NBS, AIBN/CCl4;
(b) P(OEt)3; (c) R2NC6H4CHO, MeONa/THF; (d) KOH/EtOH,
HCl.
abscissa and the tangent to the curve where the slope was the
d
strongest. Ered was calculated from Eox ¹ E0-0
.
Chem. Lett. 2012, 41, 1560-1562
© 2012 The Chemical Society of Japan
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1561
Table 2. Photovoltaic performances of DSSCsa,b
¹2
Dye
Voc/V
Jsc/mA cm
©/%
ff
MCL05
0.68
0.66
0.63
0.63
0.67
0.67
0.61
0.61
0.70
0.67
8.8
6.8
10.7
8.7
9.4
6.3
9.7
8.2
12.9
12.9
3.6
2.8
4.0
3.3
3.6
2.7
3.7
3.1
5.1
4.8
0.60
0.63
0.59
0.60
0.57
0.64
0.62
0.62
0.57
0.56
MCL06
MCL08
MCL09
D149c
aIrradiated with 300 W Xe lamp (100 mW cm¹2), and light >
650 nm was filtered out. The cell size: 0.25 cm2, counter
electrode: platinum-coated FTO glass. The electrolyte ingre-
dients; I2 (0.05 M), LiI (0.10 M), 1,2-dimethyl-3-propylimida-
zolium iodide (0.60 M), t-butylpyridine (0.05 M) in 3-meth-
Figure 2. Diagram of HOMO and LUMO energy levels.
b
oxypropionitrile. Values listed in boldface are derived from
c
the measurements under AM1.5G. The reference data using
chenodeoxycholic acid as a coadsorbent.
Figure 3. Photocurrent-voltage (xenon lamp) and dark cur-
rent-voltage charts of DSCs.
E ðeVÞ ¼ ꢀ½E ðV vs: NHEÞ þ 4:2ꢁ
ð1Þ
Figure 4. IPCE spectra and absorption spectra on TiO2 of the
DSCs. The absorption spectra on TiO2 were normalized12 and
absorptions <350 nm were deleted due to TiO2 self-absorbance.
The calculated values are comparable to the measured values.
With regard to the energy levels of DSCs, the following
requirements should be satisfied; the HOMO energy level is
¹
¹
below the I /I3 redox potential (¹4.6 eV or 0.4 V vs. NHE),
and the LUMO energy level is above the conduction band (CB)
edge of TiO2 (¹3.7 eV or ¹0.5 V vs. NHE).8 Moreover, it has
been reported that the energy gaps between the CB and the
LUMO should be more than 0.2-0.3 eV,9 and the HOMO energy
It has been reported that the dark current is caused by the
reverse current from the CB of TiO2 to the electrolyte (the back-
donation current) and that the back-donation current at low
voltage reflects a small open-circuit voltage (Voc).1,11 Indeed, the
order of Voc values of the DSCs is MCL09 < MCL06 <
MCL08 < MCL05 (under irradiation with a xenon lamp), and
this order is comparable to the order of the starting voltages of
their dark currents (Figure 3). Therefore, the diphenylamino and
CF3 groups seem to defend the back-donation current by their
steric hindrance effects. The © values are dependent on the short-
circuit photocurrent densities (Jsc); MCL06, having the largest
Jsc value, shows the best performance. The CF3 group in
MCL06 is assumed to play an effective role in charge separation
in the photoexcited state, leading to the large Jsc value. The
influence of the light source (AM1.5G or xenon lamp) on the
cell performance is explained by the light source spectra, i.e., the
¹
¹
level separated from the I /I3 redox potential by as little as
0.15 eV for efficient electron injection.10 All the dyes seem to
satisfy these requirements.
The DSCs were fabricated using screen-printed TiO2
(0.25 cm2) on FTO glass. The photovoltaic performances of
the DSCs were measured using a xenon lamp (light > 650 nm
filtered out) and an AM1.5G solar simulator, where the light
intensity was adjusted to 100 mW cm¹2 (the light source spectra
are shown in Figure S114). The photocurrent-voltage and the
dark current-voltage charts are shown in Figure 3, and the
photovoltaic properties are summarized in Table 2, where
D149 dye2 data are added as a reference for our fabricating
conditions. These results show that the thiazolecarboxylic
acid works as a new type of acceptor and anchor unit of the
DSC.
¹2
light intensity was adjusted to 100 mW cm in all spectrum
areas, so the narrow spectrum light (xenon lamp) has a relatively
strong intensity. This is the reason why xenon-lamp irradiation
shows good performances compared to the AM1.5G lamp.
Chem. Lett. 2012, 41, 1560-1562
© 2012 The Chemical Society of Japan
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1562
The incident photo-to-current conversion efficiency (IPCE)
Baschloo, T. Brinck, A. Hagfeldt, L. Sun, J. Org. Chem.
2007, 72, 9550. c) G. Li, K.-J. Jiang, Y.-F. Li, S.-L. Li, L.-M.
Yang, J. Phys. Chem. C 2008, 112, 11591.
spectra and the absorbance spectra on the TiO2 cell are shown in
Figure 4. The absorption spectra on the TiO2 cell are broad
compared to those in solution. The IPCE spectra show a similar
pattern to absorption on the TiO2 cell. The stilbene-type dyes
show satisfactory IPCE spectra; in particular, MCL05 recorded
86% at 430 nm. When photoabsorption loss by the cell13 is
considered, this record shows almost perfect photo-to-current
conversion.
In conclusion, novel photosensitized dyes containing 4-
trifluoromethylthiazole-5-carboxylic acid as an acceptor show
high performances, with © values of 3-4% in DSCs. The CF3
group is assumed to act as a suppressor of electron back-
donation from TiO2 to the electrolyte and as an accelerator of
charge separation in the photoexcited state. The optimizations of
the cell fabrication conditions are not enough, so superior
performances are expected.
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© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/