Engineering of highly efficient tetrahydroquinoline sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2011.11.004

Four novel tetrahydroquinoline dyes by inserting isophorone and/or thiophene moieties as π bridge between the electron donating unit of substituted tetrahydroquinoline and the electron withdrawing unit of cyano carboxylic acid have been synthesized and successfully applied to dye-sensitized solar cells. Among them, DSCs sensitized by HYTIC, which shows the simplest molecular structure, exhibit improved efficiency of 7.0%. This by now is the highest efficiency for the reported tetrahydroquinoline sensitizers and comparable to the performance of N719-sensitized solar cells under the conditions employed here.

Full text of DOI:10.1016/j.tet.2011.11.004

Tetrahedron 68 (2012) 552e558
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Engineering of highly efficient tetrahydroquinoline sensitizers for dye-sensitized
solar cells
,
a
c
a,b,
*
Yan Hao a, Xichuan Yang a , Jiayan Cong , Anders Hagfeldt , Licheng Sun
*
a State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 2# Linggong Rd.,
116024 Dalian, China
b School of Chemical Science and Engineering, Center of Molecular Devices, Physical Chemistry & Organic Chemistry, Royal Institute of Technology (KTH), Teknikringen 30,
10044 Stockholm, Sweden
c Department of Physical and Analytical Chemistry, Uppsala University, PO Box 259, SE-751 05 Uppsala, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel tetrahydroquinoline dyes by inserting isophorone and/or thiophene moieties as
p bridge
Received 5 September 2011
Received in revised form 24 October 2011
Accepted 3 November 2011
Available online 10 November 2011
between the electron donating unit of substituted tetrahydroquinoline and the electron withdrawing
unit of cyano carboxylic acid have been synthesized and successfully applied to dye-sensitized solar
cells. Among them, DSCs sensitized by HYTIC, which shows the simplest molecular structure, exhibit
improved efficiency of 7.0%. This by now is the highest efficiency for the reported tetrahydroquinoline
sensitizers and comparable to the performance of N719-sensitized solar cells under the conditions
employed here.
Keywords:
Tetrahydroquinoline dyes
Dye-sensitized solar cells
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
resource limitation. Many research groups have studied organic
dyes based on coumarin,3 merocyanine,4 cyanine,5 indoline,6 hem-
The serious crisis of the conventional energy and the increasing
renewable energy demand have led to a great focus on research of
using solar energy during the recent years. As an effective way of
solar energy utilization, dye-sensitized solar cells (DSCs) have
attracted considerable attention owing to their high performance
icyanine,7 perylene,8 oligoene,9 xanthene,10 triphenylamine,9,11
phenothiazine,12 tetrahydroquinoline,13 and carbazole14 in DSCs,
and the power conversion efficiency has been improved smoothly to
approach the efficiency level for ruthenium polypyridine complexes
through the way of molecular designing and synthesis.4b,6a,11f,g,15e19
The record highest solar energy conversion efficiency has acquired
9.7e10.3%.20e22 In the past 20 years, the improved high performance
has opened up a vast range of prospects for making use of metal-free
organic dyes in DSCs.
Among the above-mentioned metal-free organic sensitizers,
tetrahydroquinoline-based dyes are a kind of promising sensitizer
for TiO2-based DSCs because of their strong ability of giving
electron, good photoresponse in the visible region, and appro-
priate energy levels matching the conduction band of TiO2, but the
in converting solar energy to electricity and potential low cost since
1
€
the original paper by O’Regan and Gratzel in 1991. With the de-
velopment of dye-sensitized solar cells, until now, record efficien-
cies of up to 12% for small cells based on Ru(II)epolypyridyl
complexes have been accompolished.2 As one of the crucial parts in
dye-sensitized solar cells, many different dye sensitizers, which
function as a light absorber have been designed and applied to DSCs
in the past decades. To date, although the highest power conversion
efficiency obtained for DSCs is based on metal complexes, the
metal-free organic dyes are expected to replace the metal com-
plexes because of lower cost, easier structural modification, and
stronger light-harvesting ability with higher molar extinction
coefficient.
solar-to-electrical energy conversion efficiency (
low compared with other organic dyes.13 On the basis of the
concept of donor- conjugated bridge-acceptor structure, we
designed and synthesized four new tetrahydroquinoline dyes
(Fig. 1) by inserting isophorone and/or thiophene moieties as
h) values remain
p
Recently, more and more attention has been directed to the ap-
plication of metal-free organic dyes in DSCs because of noble metal
p
bridge between the electron donating unit of substituted tetra-
hydroquinoline and the electron withdrawing unit of cyano car-
boxylic acid. Among them, DSCs sensitized by HYTIC, which shows
the simplest molecular structure, exhibit improved efficiency of
7.0%. This by now is the highest efficiency for the reported tetra-
hydroquinoline sensitizers.
* Corresponding authors. Tel.: þ86 411 88993886; fax: þ86 411 83702185 (X.Y.);
e-mail address: yangxc@dlut.edu.cn (X. Yang).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.11.004

Y. Hao et al. / Tetrahedron 68 (2012) 552e558
553
R
N
R
R
(b)
(a)
(c)
(a')
R
NH2
N
H
R
N
H
R
R=methyl
R=hexyl
5
R=methyl
R=hexyl
3
4
R=methyl 1
R=hexyl
6
2
R
N
CHO
(d)
R
R=methyl D0
R=hexyl D1
(e)
+
NC
COOEt
CN
O
COOEt
9
Scheme 1. Synthetic routes of mediates D0, D1, and 9. (a) Acetone, p-TsOH, cyclo-
hexane, 80e90 ꢀC, 8e10 h; (a0) 2-octanone, p-TsOH, cyclohexane, 80e90 ꢀC, 8e10 h;
(b) Raney-Ni, H2, 1 MPa, 120 ꢀC; (c) (CH3)2SO4, benzene, reflux 2 h; (d) POCl3, DMF,
55 ꢀC, 5 h; (e) CH3COONH4, CH3COOH.
Fig. 1. The molecular structures of HYTIC, HYLTIC, HYLTTC, and HYLTTIC.
2. Results and discussions
2.1. Synthesis
Tetrahydroquinoline derivatives 1e6 as the electron donor units
and the acceptor group 9 were synthesized (Scheme 1). Com-
pounds 1, 3, 5, D0 and 9 as intermediate were synthesized
according to the already reported procedures.13,23 The palladium-
catalyzed Suzuki coupling reaction and Sonogashira coupling
reaction were used to synthesize aldehydes D0, D1, D2. Finally,
condensation of the aldehydes D0, D1, D2 with compound 9, cya-
noacetic acid, respectively, by the Knoevenagel reaction gave the
dyes HYTIC, HYLTIC, HYLTTC, and HYLTTIC (Scheme 2). The
detailed synthetic procedures are described in Supplementary data.
2.2. Photophysical properties
Scheme 2. Synthetic routes of the dyes HYTIC, HYLTIC, HYLTTC, and HYLTTIC.
(a) Piperidine, CH3CN; (b) LiOH, CH3CH2OH; (c) NBS, CCl4, 1 h; (d) 2-thiophenylboric
acid, Pd(PPh3)4, THF, reflux, overnight; (e) POCl3, DMF, CH2Cl2, reflux, overnight;
(f) cyanoacetic acid, piperidine, CH3CN, reflux, 3 h.
It is well known that light-harvesting ability of sensitizer in the
visible region is one of the key factors for determining the photo-
current in actual DSCs, which can be detected by the absorption
spectra of sensitizers. The UVevis spectra of the dyes HYTIC,
HYLTIC, HYLTTC, and HYLTTIC in CH2Cl2 solutions were measured
(Fig. 2) and the characteristic data are collected in Table 1. Four
compounds HYTIC, HYLTIC, HYLTTC, and HYLTTIC all exhibit an
absorption maximum in a longer wavelength in the visible region,
located at 514 nm, 518 nm, 473 nm, and 535 nm, respectively.
This is ascribed to the intramolecular charge-transfer character.
Compared with the dye HYLTTC, the absorption maximum of other
three dyes was red shifted 41 nm, 45 nm, and 62 nm, which is the
result of the introduction of the ethylene substituted isophorone
unit with the longer conjugation than thiophene unit. Moreover,
Fig. 2 shows that the dyes HYTIC, HYLTIC, and HYLTTIC have
broader spectral response in the visible region, even from 400 nm

554
Y. Hao et al. / Tetrahedron 68 (2012) 552e558
0.6
0.6
0.4
0.2
0.0
HYLTTC
HYTIC
HYLTTC
HYTIC
On TiO
In CH Cl
2
2
2
HYLTIC
HYLTTIC
HYLTIC
HYLTTIC
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 3. The UVevis spectra of dyes HYLTTC, HYTIC, HYLTIC, and HYLTTIC on TiO2 film.
Fig. 2. The UVevis spectra of dyes HYTIC, HYLTIC, HYLTTC, and HYLTTIC in CH2Cl2
(2ꢂ10ꢁ5 M).
Table 1
Absorption, emission, and electrochemical properties of HYTIC, HYLTIC, HYLTTC, and HYLTTIC
Dye
Absorption
Emissiona
E0e0 (eV)
HOMO (V vs NHE)c
LUMO (V vs NHE)
at lmax (Mꢁ1 cmꢁ1
)
lmax onb TiO2 (nm)
lmax (nm)
(abs/em)d
a
lmax (nm)
3
HYTIC
514
518
473
535
18,620
10,752
26,665
25,176
536
487
495
510
667
669
640
775
2.029
2.031
2.136
1.872
0.823
0.874
0.934
0.974
ꢁ1.206
ꢁ1.157
ꢁ1.202
ꢁ0.898
HYLTIC
HYLTTC
HYLTTIC
a
Absorption, emission spectra were measured in CH2Cl2 solution (2ꢂ10ꢁ5 M) at room temperature.
b
c
Absorption spectra on TiO2 were obtained through measuring the dye adsorbed on TiO2 film in CH2Cl2.
The oxidation potential of the dyes was measured in CH3CN with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: glassy
carbon; reference electrode: Ag/Agþ; calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an internal reference and converted to NHE by addition of 630 mV,11a counter
electrode: Pt).
d
E0e0 was estimated from onset point of absorption spectra.
to 700 nm for the dye HYLTTIC, than that of the dye HYLTTC.
However, although the longer absorption maximum and the better
spectral response in the visible region were observed for the dyes
except HYLTTC, the lower absorbance of these dyes, which can be
described by molar extinction coefficient, limited the light-
absorption ability of these dyes in the visible region, especially
HYLTIC. It has been speculated that it may be caused by the de-
crease of coplanarity between the electron donor and the electron
acceptor in the ground state as well as in the excited state of
HYLTIC.24
The absorption spectra adsorbed on TiO2 for the dyes HYLTTC,
HYTIC, HYLTIC, and HYLTTIC are shown in Fig. 3. In comparison
with HYTIC in CH2Cl2 solution, the spectrum of HYTIC on film red
shifted 22 nm. On the contrary, the absorption spectrum of HYLTIC
on TiO2 blue shifted 31 nm than that of HYLTIC in solution and has
absorption maximum at 487 nm on the film, which may be the
reason of achieving worse photocurrent with HYLTIC sensitized
DSC than that of HYTIC. The reason for this phenomenon may be
attributed to the different aggregation formation on film for the
dyes HYTIC and HYLTIC. Obviously, both the spectral response
ranges for HYTIC and HYLTTIC on TiO2 compared to HYLTIC and
HYLTTC (400 nm-600 nm) on film are broader, from 400 nm to
700 nm, which is an advantageous spectral property for light har-
vesting in the visible region. The result indicates that the extension
2.3. Electrochemical properties
Cyclic voltammetry measurements were carried out to de-
termine the ground state oxidation potentials (Eox) correspond-
ing to the HOMO levels of the dyes. The excited state oxidation
potential (Eox) corresponding to the LUMO levels can be obtained
*
by subtracting the 0e0 transition energy (E0e0) from HOMO,
whereas E0e0 is derived from the intersection of the absorption
and emission spectra. As shown in Table 1, the calculated excited
state oxidation potentials of all dyes are much more negative
than the conduction band edge of TiO2 (ca. ꢁ0.5 V vs NHE),
suggesting a sufficiently thermodynamic driving force is pro-
vided for efficient electron injection into the TiO2 conduction
band from the excited dyes and simultaneously oxidized dyes are
formed. The ground state oxidation potentials of all dyes are
more positive than the iodine redox potential value (ca. þ0.4 V vs
NHE), ensuring efficient regeneration of the oxidized dye from
iodine redox.25
In comparison with the thiophene-based dye HYLTTC, the ex-
tended
p-conjugation for dyes HYTIC and HYLTIC by introducing
the ethenyl substituted isophorone unit lifts up the HOMO level
dramatically, resulting in the decrease of E0e0 value. This can be
ascribed to the red shift of the absorption spectrum. However, for
dye HYLTTIC owning the longest p-conjugation system, the HOMO
of the
p
-conjugation for HYLTTIC not only bathochromically shifts
level and LUMO level were both shifted downwards compared to
HYLTIC and HYLTTC, but further narrows E0e0, which is advanta-
geous for red shift of the absorption spectrum, which can induce to
the improvement of photocurrent.
the absorption maximum but also broadens its absorption spec-
trum, which is desirable for the enhanced light-harvesting ability,
yielding high photocurrent for DSCs applications.

Y. Hao et al. / Tetrahedron 68 (2012) 552e558
555
Table 2
2.4. Molecular orbital calculations
Photovoltaic performance of DSCs based on HYTIC, HYLTIC, HYLTTC, and HYLTTIC
dyes.a
To gain insight into the molecular structure and electron distri-
bution of the organic dyes, the dyes HYTIC, HYLTIC, HYLTTC, and
HYLTTIC geometries have been optimized using DFT calculations
with Gaussian 03 program.30 The calculations were performed with
the B3LYP under 6-31þG(d) basis set, given the negligible effect of
salvation on the electronic structure. The electron distributions of
the HOMO and LUMO of the four dyes are shown in Fig. 4. Compared
with HYLTTC, the red-shifted absorption wavelengths for the dyes
HYTIC and HYLTIC can be attributed to the more efficient conjuga-
tion provided by the ethylene substituted isophorone unit instead of
thiophene unit as bridge. We also notice that the HOMOeLUMO
excitation induced by light irradiation could move the electron
distribution from the whole molecule to the anchoring moiety. If
there is a similar molecular orbital geometry when dye molecules
anchored to TiO2, the position of the LUMO close to the anchoring
group will offer a better orbital overlap with the titanium 3d orbitals
than that away from the anchoring group. This favors electron in-
jection from excited dye to TiO2, yielding high photocurrent.11a
Dyes
Solvents
Jsc (mA/cm2)
Voc (mV)
Fill factor (FF)
h (%)
HYTIC
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
14.05
11.66
11.76
13.49
15.03
660
657
671
642
682
0.75
0.74
0.74
0.74
0.73
7.0
5.7
5.8
6.4
7.5
HYLTTC
HYLTIC
HYLTTIC
N719b
CH3CH2OH
a
Irradiated light: AM 1.5G (100 mW cmꢁ2); working area: 0.159 cm2; electrolyte:
0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPII), 0.06 M LiI, 0.04 M I2,
0.4 M 4-tert-butylpyridine(TBP) in acetonitrile.
b
Electrolyte: 0.6 M DMPII, 0.053 M LiI, 0.03 M I2, 0.28 M TBP, 0.05 M GuSCN in
acetonitrile.
15
10
HYLTTC
HYTIC
HYLTIC
HYLTTIC
5
0
0.0
0.2
0.4
0.6
Voltage (V)
Fig. 5. Photovoltaic performance of DSCs based on HYTIC, HYLTIC, HYLTTC, and HY-
LTTIC dyes.
with a Jsc of 14.05 mA cmꢁ2, a Voc of 0.660 V, an FF of 0.75, and a
7.0%. Under the same conditions, Jsc of 13.49 mA cmꢁ2, Voc of
0.641 V, FF of 0.74, of 6.4% was obtained for HYLTTIC sensitized
h of
h
solar cell. However, the DSCs based on dyes HYLTTC and HYLTIC
only show Jsc of 11.66 mA cmꢁ2, 11.76 mA cmꢁ2, respectively. In
comparison with the dyes HYLTTC and HYLTIC, the enhancement of
Jsc for dyes HYTIC and HYLTTIC is a result of the broadening of the
IPCE spectra on TiO2 film with the increasing
p-conjugation. The
result also demonstrates the beneficial influence of the red-shifted
absorption. The lower Voc value observed for the DSCs based on
HYLTTIC dye may be due to decreased electron lifetime with the
extension of the linker conjugation.26 Under the same test condi-
Fig. 4. Calculated isodensity surface plots and molecular orbital energy diagram of the
HOMO and LUMO for HYTIC, HYLTIC, HYLTTC, and HYLTTIC.
2.5. Photovoltaic performance
tions, the DSC sensitized N719 achieved a
h of 7.5%, with a Jsc of
The photovoltaic properties of the solar cells constructed from
these four organic dye-sensitized TiO2 electrodes were measured
under simulated AM 1.5 irradiation (100 mW cmꢁ2). The open-
circuit photovoltage (Voc), short-circuit photocurrent density (Jsc),
15.03 mA cmꢁ2, a Voc of 0.682 V, an FF of 0.73.
The incident photon-to-current conversion efficiencies (IPCEs)
data of HYTIC, HYLTIC, HYLTTC, and HYLTTIC sensitizers plotted as
a function of wavelength exhibit strikingly high efficiencies of 96%,
91%, 95%, and 90%, respectively (Fig. 6). The IPCE spectra are con-
sistent with the absorption spectra of these dyes on the TiO2 film.
Considering the light absorption and scattering loss by the con-
ducting glass, the maximum efficiency for absorbed photon-to-
collected electron conversion efficiency (APCE) is almost unity
over a broad spectral range, suggesting a very high charge collec-
tion yield. Although the maximum of IPCE value of HYTIC is similar
to that of HYLTTC, the higher photocurrent of DSC sensitized by
HYTIC relative to the HYLTTC can be mainly attributed to the
broader spectral response in the visible region, which is in agree-
ment with the absorption spectra of the dyes on TiO2 films. The
same conclusion can be obtained by comparing the IPCE response
of DSC sensitized by HYLTTIC with that of HYLTTC.
fill factor (FF), and
h are listed in Table 2. Current densityevoltage
(JeV) characteristics and incident photon-to-current conversion
efficiencies (IPCE) of devices based on the selected dyes are shown
in Figs. 5 and 6, respectively. The optimized evaluation conditions
for these four dyes are determined to be 2ꢂ10ꢁ4 M of dye in CH2Cl2
solution, 2ꢂ10ꢁ4 M chenodeoxycholic acid (CDCA) as a coad-
sorbent, and the electrolyte is 0.6 M 1,2-dimethyl-3-n-propylimi-
dazolium iodide (DMPII)/0.06 M LiI/0.04 M I2/0.4 M 4-tert-
butylpyridine(TBP) in acetonitrile. But the optimized conditions for
N719 is 2ꢂ10ꢁ4 M of dye in CH3CH2OH solution and the electrolyte
is DMPII/0.6 M, LiI/0.053 M, I2/0.03 M, TBP/0.28 M, GuSCN/0.05 M
in acetonitrile. HYTIC-sensitized solar cell shows an unanticipated
efficiency among all the other reported tetrahydroquinoline dyes,

556
Y. Hao et al. / Tetrahedron 68 (2012) 552e558
-25
100
80
60
40
20
0
HYTIC
HYLTTC
HYTIC
HYLTIC
HYLTTIC
HYTTC
-20
HYLTIC
HYLTTIC
-15
-10
-5
0
400
500
600
700
800
10
15
20
25
30
35
40
45
50
55
Wavelength (nm)
Z' (ohm)
Fig. 7. Electrochemical impedance spectra, scanned from 10ꢁ2 Hz to 107 Hz at room
temperature, for the devices based on HYTIC, HYLTIC, HYTTC, and HYLTTIC, Nyquist
plots. The cells were measured at ꢁ0.6 V in the dark. The alternate current (AC)
amplitude was set at 10 mV.
15
10
5
HYLTTC
HYTIC
HYLTIC
HYLTTIC
together with a double bond as bridge for HYTIC to keep suitable
molecular planar configuration and broaden absorption spectra,
short-circuit photocurrent densities (Jsc) were improved to a large
extent compared with the dye HYLTTC containing thiophene as
bridge. Introduction of the long carbon chain on the donor part for
HYLTIC relative to HYTIC exhibits lower Jsc mainly was caused to
the blue-shift absorption spectrum for HYLTIC on the TiO2.
Undoubtly, DSC sensitized by the dye HYLTTIC also achieved better
Jsc, which has the longest conjugation among four efficient tetra-
hydroquinoline dyes. EIS analysis reveals that the introduction of
the longest linker conjugation for the dye HYLTTIC could increase
the charge recombination arising from electrons in TiO2 film and
the Iꢁ3 ion in electrolyte, thus decreasing the Voc of DSCs consider-
ably. Among this library of new synthesized dyes and under the
0
400
500
600
700
800
Wavelength (nm)
Fig. 6. Spectral response of the photocurrent of DSCs based on HYTIC, HYLTIC,
HYLTTC, and HYLTTIC dyes.
optimized cell test conditions, the dye HYTIC shows a prominent
h
up to 7.0% (Jsc) 14.05 mA cmꢁ2, (Voc) 660 mV, (FF) 0.75 under sim-
ulated AM 1.5 irradiation (100 mW cmꢁ2), which is the highest
efficiency for the reported tetrahydroquinoline sensitizers. Further
improvement of the dye structures based on tetrahydroquinoline
by molecular engineering aiming for even better performance of
DSCs is in progress.
Electrochemical impedance spectroscopy (EIS) analysis27 was
performed to study the interfacial charge-transfer processes in
DSCs sensitized by the dyes HYTIC, HYLTIC, and HYLTTIC. The
Nyquist plots are shown in Fig. 7. By fitting the EIS spectra, some
important parameters can be obtained. The small semicircle in the
Nyquist plot represents the charge-transfer resistances (RCE) at
electrolyte/counter electrode interface. This is a process that the
oxidized species in the electrolyte get electrons from counter
electrode to produce the reduced species. The large semicircle
represents the charge-transfer resistances (Rrec) at TiO2/electrolyte
interface, referring to the back reaction between electrons in the
TiO2 film and the oxidized species (Iꢁ3 ion) in the electrolyte. It is
obvious that the Rrec values increased in the order of HYLTTIC
4. Experimental section
4.1. Materials and methods
Solvents were dried by standard procedures. All other chemicals
were purchased from commercial sources and used without further
purification. 1H NMR spectra were recorded with a Varian INOVA
400 NMR instrument (USA) with the chemical shifts by using TMS
as standard. MS data were obtained with GCT CA156 (UK) LC/Q-TOF
MS (UK) or HP1100 LC/MSD (USA) mass spectrometer. UVevis and
emission spectra of the dyes in solution were recorded in a quartz
cell with 1 cm path length on an HP 8453 (USA) and PTI700 (USA)
spectrophotometer, respectively. Electrochemical redox potentials
were obtained by cyclic voltammetry using a three-electrode cell
and an electrochemical workstation (BAS100B, USA). The working
electrode was a glass carbon electrode, the auxiliary electrode was
a Pt wire, and Ag/Agþ was used as reference electrode. Tetrabuty-
lammonium hexaflourophosphate (TBAPF6) 0.1 M was used as
supporting electrolyte in CH3CN. Ferrocene was added to each
sample solution at the end of the experiments, and the ferroce-
nium/ferrocene (Fc/Fcþ) redox couple was used as an internal
(20 U)<HYTIC (29 U)<HYLTIC (35 U)zHYTTC (33 U), indicating
that the electron recombination with the Iꢁ3 ion in electrolyte is
suppressed by the introduction of the long carbon chain in HYLTIC
compared with HYTIC, which shows an increase (11 mV) in Voc for
DSC based on HYLTIC. Compared with the other three dyes, the DSC
sensitized by HYLTTIC, which contained thiophene and isophorone
moiety as bridge, exhibited the lowest Voc, may be due to the easier
electron recombination with the longer p-conjugation bridge.
3. Conclusion
In summary, four highly efficient metal-free organic dyes con-
taining tetrahydroquinoline as donor part were synthesized and
applied in DSCs. With the introduction of the isophorone moiety,

Y. Hao et al. / Tetrahedron 68 (2012) 552e558
557
potential reference. The potentials versus NHE were calculated by
addition of 630 mV to the potentials versus Fc/Fcþ.11a Electro-
chemical impedance spectroscopy (EIS) for DSC with forward
bias ꢁ0.7 V under dark was measured with an impedance/gain-
phase analyzer (PARSTAT 2273, USA). The spectra were scanned
in a frequency range of 10ꢁ2 Hz to 105 Hz at room temperature. The
alternate current (AC) amplitude was set at 10 mV.
(Grant no. LS2010042), the Ministry of Science and Technology
(MOST) (Grant 2001CCA02500), the Swedish Energy Agency, K&A
Wallenberg Foundation, and the State Key Laboratory of Fine
Chemicals (KF0805).
Supplementary data
Supplementary data associated with this article can be found in
online version, at doi:10.1016/j.tet.2011.11.004.
4.2. DSC fabrication
The DSCs were fabricated as reported in literature.28 A layer of
13 nm (DHS-TPP3, Heptachroma, China) paste (ca. 2
coated on the F-doped tin oxide conducting glass (TEC15,
15 /square, Pilkington, USA) by screen printing and then dried for
6 min at 125 ꢀC. This procedure was repeated for eight times (ca.
16 m) and coated by a layer of 300 nm (DHS-SLP1, Heptachroma,
China) titania paste (ca. 4 m) as scattering layer. The double-layer
TiO2 electrodes (area: 6ꢂ6 mm) were gradually heated under an air
flow at 325 ꢀC for 5 min, at 375 ꢀC for 5 min, at 450 ꢀC for 15 min,
and at 500 ꢀC for 15 min. The sintered film was further treated with
40 mM TiCl4 aqueous solution at 70 ꢀC for 30 min, then washed
with ethanol and water, and annealed at 500 ꢀC for 30 min. After
the film was cooled to 40 ꢀC, it was immersed into a 2ꢂ10ꢁ4 M dye
solution in CH2Cl2 with CDCA (2ꢂ10ꢁ4 M) and maintained under
dark for 3 h. The sensitized TiO2 electrode was then rinsed with the
solvent of dye-bath and dried. The hermetically sealed cells were
fabricated by assembling the dye-loaded film as the working elec-
trode and Pt-coated conducting glass as the counter electrode
mm) was
References and notes
U
€
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We gratefully acknowledge the financial support of this work
from China Natural Science Foundation (Grant 21076039), the Na-
tional Basic Research Program of China (Grant no. 2009CB220009),
the Program for Innovative Research Team of Liaoning Province

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