Cyclic thiourea/urea functionalized triphenylamine-based dyes for high-performance dye-sensitized solar cells

Article abstract of DOI:10.1021/ol4001685

Six cyclic thiourea/urea functionalized triphenylamine-based dyes (AZ1-AZ6) containing 2-cyanoacrylic acid as an acceptor and various linkers (phenyl, biphenyl, and bithiophene) were synthesized. They exhibited high photovoltaic performance owing to an improved short-circuit photocurrent density (J _sc) and open-circuit voltage (V_oc). Among them, AZ6 bearing a cyclic thiourea group and bithiophene linker showed the highest power conversion efficiency (PCE) up to 7.29%, which was comparable to that of N719 (PCE = 7.36%).

Full text of DOI:10.1021/ol4001685

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Cyclic Thiourea/Urea Functionalized
Triphenylamine-Based Dyes for High-
Performance Dye-Sensitized Solar Cells
Zhisheng Wu,† Zhongwei An,*,†,‡ Xinbing Chen,† and Pei Chen†
Key Laboratory of Applied Surface and Colloid Chemistry (MOE), School of Materials
Science and Engineering, Shaanxi Normal University, 710162, P. R. China, and Xi’an
Modern Chemistry Research Institute, Xi’an 710065, P. R. China
gmecazw@163.com
Received January 20, 2013
ABSTRACT
Six cyclic thiourea/urea functionalized triphenylamine-based dyes (AZ1ꢀAZ6) containing 2-cyanoacrylic acid as an acceptor and various linkers
(phenyl, biphenyl, and bithiophene) were synthesized. They exhibited high photovoltaic performance owing to an improved short-circuit
photocurrent density (Jsc) and open-circuit voltage (Voc). Among them, AZ6 bearing a cyclic thiourea group and bithiophene linker showed the
highest power conversion efficiency (PCE) up to 7.29%, which was comparable to that of N719 (PCE = 7.36%).
Since the remarkable progress made by O0Regan and
purification procedures, organic sensitizers have received
increasing attention.5
€
Gratzel in 1991, dye-sensitized solar cells (DSSCs) have
aroused worldwide scientific research interest because of
their easy fabrication process and low cost compared with
traditional silicon-based solar cells.1 As a key component
of DSSCs, the sensitizer affects the power conversion
efficiency (PCE) due to its critical function in light harvest-
ing and electron injection. To date, several sensitizers have
been developed including metal complex and organic
compounds, where organic sensitizers exhibit a high PCE
up to 10%,2 which is comparable to that of metal complex
sensitizers, such as a ruthenium-based complex (PCE
of 11%)3 or zinc porphyrin complex (PCE of 12%).4 In
addition, due to their inexpensive raw material and easy
Organic sensitizers are commonly composed of three
units, namely the electron donor, π-conjugation linker,
and electron acceptor. This structural character is favor-
able for optimizing DSSCs’ performance by changing/
modification of the different parts in the molecules.6
Therefore, enormous efforts have been devoted toward
the design and synthesis of the three units. So far, the
electron acceptors have been dominated by 2-cyanoacrylic
acid, but various linkers and donors have been exploited.
For example, oligoene,7 oligophenylene,8 and oligothio-
phene9 are widely used as linkers due to their prominent
€
(5) (a) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int.
† Shaanxi Normal University.
Ed. 2009, 48, 2474. (b) Ooyama, Y.; Harima, Y. Eur. J. Org. Chem. 2009,
2903.
‡ Xi'an Modern Chemistry Research Institute.
€
€
(1) (a) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (b) Gratzel,
M. Nature 2001, 414, 338.
(2) Zeng, W. D.; Cao, Y. M.; Bai, Y.; Wang, Y. H.; Shi, Y. S.; Zhang,
(6) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595.
(7) (a) Hara, K.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.;
Sayama, K.; Arakawa, H. Chem. Commun. 2003, 252. (b) Kitamura,
T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T.;
Yanagida, S. Chem. Mater. 2004, 16, 1806.
M.; Wang, F. F.; Pan, C. Y.; Wang, P. Chem. Mater. 2010, 22, 1915.
(3) Gratzel, M. J. Photochem. Photobiol. A 2004, 164, 3.
(4) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
€
Nazeerudine, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddine, S. M.;
€
(8) Im, H.; Kim, S.; Park, C.; Jang, S.-H.; Kim, C.-J.; Kim, K.; Park,
N.-G.; Kim, C. Chem. Commun. 2010, 46, 1335.
Gratzel, M. Science 2011, 334, 629.
r
10.1021/ol4001685
XXXX American Chemical Society

electron-transport properties, while carbazole,10 coumarin,11
indoline,12 tetrahydroquinoline,13 and triphenylamine2,14
are adopted as donors because of their good electron-
donating ability as well as excellent stability.
Based on aforementioned ideas, we synthesized six novel
organic dyes with cyclic thiourea/urea functionalized tri-
phenylamine as the electron donor and 2-cyanoacrylic acid
as the acceptor. These donors and acceptor are directly
connected (AZ1 and AZ2) or bridged by oligophenylenes
(AZ3, AZ4, and AZ5) or bithiophene (AZ6). Their struc-
tures are shown in Figure 1, and the synthetic protocols
were described in detail in the Supporting Information.
For comparison, TC105 as a reference dye was synthesized
according to the literature.17
The UVꢀvis absorption spectra of the dyes in CH3CN
solution (10ꢀ5 M) are illustrated in Figure 2. Their photo-
physical and electrochemical data are collected in Table 1.
Normally, all of the dyes displayed two relatively broad
absorption bands in the ranges from 290 to 400 nm and
from 400 to 600 nm, respectively. The former absorption
Despite the significant progress was achieved by organic
sensitizers, much work is still desired to further improve
the photovoltaic performance, for example, by expanding
absorption bands and inhibiting intermolecule aggrega-
tion.15 Here, we try to introduce cyclic thiourea/urea
groups into a triphenylamine electron donor unit based
on the following considerations: (1) electron transfer from
the donor to the acceptor could be facilitated because
sulfur/oxygen and nitrogen heteroatoms in the cyclic
thiourea/urea groups tend to increase the electron-donat-
ing ability; (2) the intermolecule aggregation and undesired
charge recombination are expected to be suppressed due to
the long alkyl chains attached at the N-atom sites of cyclic
thiourea/urea groups;10a (3) cyclic thiourea/urea groups
may effectively delocalize the positive charges of the
oxidized dyes due to sulfur/oxygen and nitrogen heteroa-
toms with lone pair electrons.16
Figure 2. UVꢀvis absorption spectra of AZ1ꢀAZ6 and TC105
in CH3CN solution (10ꢀ5 M).
Table 1. Optical and Electrochemical Properties of the Dyes
Figure 1. Structures of the cyclic thiourea/urea functionalized
triphenylamine dyes AZ1ꢀAZ6 and TC105.
a
b,c
d
c
λmax
/
ε/
Eox
/
E0ꢀ0
V
/
Eox ꢀ E0ꢀ0
/
dye
AZ1
nm
M
ꢀ1 cmꢀ1
V
V
423
423
428
426
347
443
410
34 830
39 950
29 680
33 530
66 000
40 690
28 570
0.91
0.93
0.73
0.80
0.84
0.79
1.05
2.18
2.14
2.03
2.07
2.25
1.98
2.38
ꢀ1.27
ꢀ1.21
ꢀ1.30
ꢀ1.27
ꢀ1.41
ꢀ1.19
ꢀ1.33
(9) (a) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci,
€
AZ2
S.; De Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gratzel, M.
J. Am. Chem. Soc. 2006, 128, 16701. (b) Liang, Y.; Peng, B.; Liang, J.;
Tao, Z.; Chen, J. Org. Lett. 2010, 12, 1204. (c) Do, K.; Kim, D.; Cho, N.;
Paek, S.; Song, K.; Ko, J. Org. Lett. 2012, 14, 222.
(10) (a) Koumura, N.; Wang, Z. S.; Mori, S.; Miyashita, M.; Suzuki,
E.; Hara, K. J. Am. Chem. Soc. 2006, 128, 14256. (b) Lee, W.; Cho, N.;
Kwon, J.; Ko, J.; Hong, J.-I. Chem.;Asian J. 2012, 7, 343.
(11) (a) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo,
A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B
2003, 107, 597. (b) Liu, B.; Wang, R.; Mi, W.; Li, X.; Yu, H. J. Mater.
Chem. 2012, 22, 15379.
(12) (a) Horiuchi, T.; Miura, H.; Uchida, S. Chem. Commun. 2003,
3036. (b) Wu, Y.; Zhang, X.; Li, W.; Wang, Z.-S.; Tian, H.; Zhu, W. Adv.
Energy Mater. 2012, 2, 149.
(13) Hao, Y.; Yang, X.; Cong, J.; Hagfeldt, A.; Sun, L. Tetrahedron
2012, 68, 552.
AZ3
AZ4
AZ5
AZ6
TC105
a Measured in CH3CN solution (10ꢀ5 M) at rt. b Measured in
acetonitrile solution containing 0.1 M tetrabutylam monium hexafluor-
ophosphate (TBAPF6) as electrolyte (working electrode: glassy carbon;
counter electrode: Pt (area: 0.023 cm2); reference electrode: saturated
calomel electrode (SCE)); calibrated with ferrocene/ferrocenium
(Fc/Fcþ) as an external reference and converted to NHE by addition
of 0.07 V.18 c Values are reported versus NHE. d Estimated from onset
wavelength in absorption spectra.
(14) (a) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson,
N. A.; Ai, X.; Lian, T. Q.; Yanagida, S. Chem. Mater. 2004, 16, 1806.
(b) Thomas, K. R. J.; Hsu, Y.; Lin, J. T.; Lee, K.; Ho, K.; Lai, C.; Cheng,
Y.; Chou, P. Chem. Mater. 2008, 20, 1830. (c) Ning, Z. J.; Tian, H. Chem.
Commun. 2009, 5483.
(15) Qu, S.; Qin, C.; Islam, A.; Wu, Y.; Zhu, W.; Hua, J.; Tian, H.;
Han, L. Chem. Commun. 2012, 48, 6972.
(16) Zhang, M.-D.; Pan, H.; Ju, X.-H.; Ji, Y.-J.; Qin, L.; Zheng,
H.-G.; Zhou, X.-F. Phys. Chem. Chem. Phys. 2012, 14, 2809.
(17) Teng, C.; Yang, X. C.; Yang, C.; Tian, H. N.; Li, S. F.; Wang,
X. N.; Hagfeldt, A.; Sun, L. C. J. Phys. Chem. C 2010, 114, 11305.
B
Org. Lett., Vol. XX, No. XX, XXXX

band was attributed to the localized πꢀπ* transitions
of the conjugated systems while the latter was assigned to
the intramolecular charge transfer between donor and
acceptor.19 Generally, compared with TC105 (410 nm,
28 570 Mꢀ1 cmꢀ1), the cyclic thiourea/urea functionalized
dyes red-shifted from 10 to 40 nm with a higher molar
extinction coefficient, even the dyes with a shorter bridge
linker such as AZ1 (423 nm, 34830 Mꢀ1 cmꢀ1) and AZ2
(423 nm, 39950 Mꢀ1 cmꢀ1). It is worth pointing out that
AZ5 with biphenyl as the bridge linker did not show an
absorption peak in the visible region while AZ6 with
bithiophene as the bridge linker red-shifted to 443 nm with
a molar extinction coefficient up to 40690 Mꢀ1 cmꢀ1. The
main reason for this phenomenon may be the electrons of
bithiophene are richer and allow better delocalization than
biphenyl. The UVꢀvis absorption spectra demonstrated
that the introduction of cyclic thiourea/urea functional
groups not only broadens the absorption bands but also
increases the molar extinction, which is desirable for a
high-performance sensitizer with a higher short-circuit
photocurrent density (Jsc) for DSSCs.
Figure 3. (a) JꢀV curves for DSSCs based on the dyes under
illumination of AM 1.5 G simulated sunlight (100 mW cmꢀ2).
(b) IPCE spectra of the same DSSCs.
Table 2. Photovoltaic Performance Data of the Dyesa
JSC
/
VOC
/
FF/
%
PCE/
%
IPCE/%
dye
AZ1
mA cmꢀ2
mV
(λmax /nm)
3
Cyclic voltammetry (CV) was performed in acetonitrile
solutions to test first oxidation potentials (Eox), corre-
sponding to the HOMO levels of the dyes (Table 1,
9.7
9.9
9.3
9.9
9.0
14.8
16.2
8.3
739
770
739
780
780
749
740
709
62.7
65.0
68.9
69.0
63.5
65.9
61.4
65.6
4.51
4.94
4.73
5.33
4.44
7.29
7.36
3.87
81 (440)
81 (490)
75 (450)
86 (430)
80 (450)
81 (510)
71 (510)
85 (420)
AZ2
AZ3
AZ4
Figure S1). The LUMO levels of the dyes could be calcu-
20
AZ5
lated from Eox ꢀ E0ꢀ0
,
in which the E0ꢀ0 was estimated
AZ6
from the onset wavelength of the absorption spectra. As
shown in Table 1, the HOMO levels of these dyes were more
positive than the iodine/iodide redox potential value (0.4 V
vs NHE), so the oxidized dye molecules could be regenerated
effectively by the redox couple. The LUMO levels of these
dyes were sufficiently more negative than the conduction
band energy level of the TiO2 electrode (ꢀ0.5 V vs NHE),
which is energetically favorable for the electron injection
from the excited dye into the conduction band of TiO2.
The geometries of the dyes were optimized, and their
frontier molecular orbitals were calculated by density
functional theory (DFT) at the B3LYP/6-311G (d,p) level.
Their electron distributions of the HOMO and LUMO
are shown in Figure S2, and the energy levels are listed in
Table S1. HOMOs were expanded from triphenylamine to
the cyclic thiourea/urea functional groups, while LUMOs
were mainly delocalized at the cyanoacrylic acid acceptor.
This distribution of electrons will facilitate electron injec-
tion from the excited dye to the conduction band of
TiO2. The energy gaps (ΔELUMOꢀHOMO) of AZ3 (2.47 eV,
Table S1) and AZ4 (2.47 eV) were narrower than that of
TC105 (2.71 eV), which indicated that the introduction
of thiourea/urea functional groups would facilitate the
electron transfer from the donor to the acceptor. These
are in line with UVꢀvis results.
N719
TC105
a Measured under AM 1.5G irradiation (100 mW cmꢀ1) with
0.25 cm2 working area. The thickness of TiO2 film was ∼12 μm, and
the electrolyte was 0.6 M 1-butyl-3-methylimidazolium iodide (BMII),
0.1 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M guanidinium
thiocyanate in acetonitrile.
The photocurrentꢀvoltage (JꢀV) and incident photon to
current efficiency(IPCE) curves are shown in Figure 3, and
the corresponding photovoltaic data including Jsc, open-
circuit voltage (Voc), fill factor (FF), and PCE are listed
in Table 2. Encouragingly, the cyclic thiourea/urea func-
tionalized dyes exhibited distinctly improved PCEs
(4.44ꢀ7.29%) due to the increased Jsc accompanied by
the enhanced Voc relative to TC105 (Jsc = 8.3 mA cmꢀ2
,
Voc = 709 mV, FF = 65.6%, and PCE = 3.87%).
Specifically, the PCEs of AZ1 (Jsc = 9.7 mA cmꢀ2
Voc = 739 mV, and FF = 62.7%) and AZ2 (Jsc
,
=
9.9 mA cmꢀ2, Voc = 770 mV, and FF = 65.0%) were
4.51% and 4.94%, respectively. It is worth noting that
AZ2 displayed a higher Voc than AZ1, which may be
ascribed to alkyl chains attached at thiourea groups pos-
sibly inhibiting intermolecular aggregation as well as sup-
pressing back reactions between the injected electrons
and the electrolyte.10a And the result seems to support that
a more significant effect was obtained by the longer alkyl
chains. Furthermore, the PCE of AZ3 increased to 4.73%
(Jsc = 9.3 mA cmꢀ2, Voc = 739 mV, and FF = 68.9%),
The DSSC characteristics of dyes were evaluated
under standard AM 1.5 G simulated sunlight irradiation.
(18) Yang, J.; Guo, F.; Hua, J.; Li, X.; Wu, W.; Qu, Y.; Tian, H.
J. Mater. Chem. 2012, 22, 24356.
(19) Kim, S. H.; Kim, H. W.; Sakong, C.; Namgoong, J.; Park, S. W.;
Ko, M. J.; Lee, C. H.; Lee, W. I.; Kim, J. P. Org. Lett. 2011, 13, 5784.
€
(20) Gao, P.; Tsao, H. N.; Gratzel, M.; Nazeeruddin, M. K. Org.
and that of AZ4 increased to 5.33% (Jsc = 9.9 mA cmꢀ2
,
and FF = 69.0%) with a Voc up to 780 mV, which
increased beyond 70 mV compared with that of TC105.
Although the Voc reached 780 mV as well, a poor PCE
Lett. 2012, 14, 4330.
Org. Lett., Vol. XX, No. XX, XXXX
C

with Voc. The enlarged Rrec and lengthened electron life-
time further indicated that the electron recombination
between the injected electrons and the electrolyte was
decreased, resulting in an increased Voc.23 AZ4 showed a
slower electron recombination than AZ3 and resulted
in a larger Voc; the mechanism is unclear and still under
investigation. Compared with AZ5, AZ6 displayed a faster
electron recombination due to a better overlap between the
HOMO and the conduction band of TiO2 (Figure S2),24
which resulted in a decreased Voc.
In summary, six novel cyclic thiourea/urea functiona-
lized triphenylamine-based dyes were synthesized and
their DSSC performances were studied as well. Compared
with TC105, the cyclic thiourea/urea functionalized dyes
AZ1ꢀAZ6 exhibited a higher Jsc accompanied by a larger
Voc, therefore obtaining improved PCEs. The improved Jsc
may be ascribed to the broadened absorption bands as the
result of the introduction of cyclic thiourea/urea functional
groups, while the enhanced Voc may be attributed to the
intermolecular πꢀπ aggregation and charge recombina-
tion being suppressed by long alkyl chains attached at
cyclic thiourea/urea functional groups. Cyclic thiourea
functionalized dye AZ6, in which the bithiophene is
the linker bridge, yielded a satisfying PCE up to 7.29%
(Jsc = 14.8 mA cmꢀ2, Voc = 749 mV, and FF = 65.9%).
This result can be in competition with N719 (PCE =
7.36%). Overall, this work has provided evidence that
the cyclic thiourea/urea functionalization is a promising
approach for the further development of highly efficient
organic dyes.
Figure 4. EIS spectra of DSSCs tested at ꢀ0.7 V forward bias in
the dark: (a) Nyquist and (b) Bode phase plots.
(4.44%) was obtained by AZ5 due to its relatively lower Jsc
(9.0 mA cmꢀ2) and FF (63.5%). Thanks to the sharply
increased Jsc, AZ6 yielded an exciting PCE up to 7.29%
(Jsc = 14.8 mA cmꢀ2, Voc = 749 mV, and FF = 65.9%),
which was comparable to N719 (PCE = 7.36%, Jsc
=
16.2 mA cmꢀ2, Voc = 740 mV, and FF = 61.4%). The
corresponding triphenylamine dye (coded as dye 414b or
1P-PSS21) was reported by Chouand Chow independently
and yielded PCEs of 6.15% and 6.17%, respectively.
Compared with these results, the improved photovoltaic
performance of AZ6 may originate from the introduction
of cyclic thiourea functional groups. To some extent, the
integral of the IPCE spectra related to the values of Jsc of
DSSCs. The IPCE curve (Figure 3b) of AZ6 exhibited
a significantly larger area than other organic dyes and
resulted in the largest Jsc. The IPCE spectra of other dyes
basically showed consistent Jsc as well as PCE values.
Electrochemical impedance spectroscopy (EIS)22 was
applied to investigate the interfacial charge transfer pro-
cess in DSSCs. There were two semicircles in the Nyquist
plots (Figure 4a), and the larger semicircle in the low-
frequency region corresponded to charge-transfer resis-
tance (Rrec) at the TiO2/dye/electrolyte interface; e.g.,
a larger radius indicated a larger Rrec and slower electron
recombination. All the cyclic thiourea/urea functionalized
dyes AZ1ꢀAZ6 showed a larger Rrec than TC105. In
addition, the electron lifetime could be calculated from
the Bode phase plots (Figure 4b)23 and the calculated
results increased in the order TC105 (6.2 ms) < AZ3
(13.3 ms) < N719 (19.7 ms) < AZ1 (21.1 ms) < AZ6
(24.0 ms) < AZ2 (35.1 ms) < AZ4 (41.7 ms) < AZ5
(42.6 ms). Both the Rrec and electron lifetimecoincidedwell
Acknowledgment. Research was supported by the Pro-
gram for Changjiang Scholars and Innovative Research
Team in University (IRT1070) and the Fundamental Re-
search Funds for the Central Universities (GK200902002,
GK201002002, and GK261001001). We are grateful to
Professor Wenliang Wang at Shaanxi Normal University
for theoretical calculations and Associate Professor
Honglan Qi at Shaanxi Normal University for cyclic
voltammetry measurements. We also thank Senior Engi-
neer Jianwen Wang and Mr. Linuo Jin at Xi’an Ruilian
Modern Electronic Chemicals Co. Ltd. for DSSCs’ device
fabrication.
Supporting Information Available. Experimentaldetails,
DSSCs device fabrication, characterization details, and
calculated molecular geometries. The material is available
free of charge via the Internet at http://pubs.acs.org.
(21) Chang, Y. J.; Chow, T. J. Tetrahedron 2009, 65, 4726.
(22) (a) Fabregat-Santiago, F.; Bisquert, J.; Garcia-Belmonte, G.;
Boschloo, G.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2005, 87, 117.
(24) Maggio, E.; Martsinovich, N.; Troisi, A. Angew. Chem., Int. Ed.
2013, 52, 973.
€
(b) Wang, Q.; Moser, J. E.; Gratzel, M. J. Phys. Chem. B 2005, 109,
14945.
(23) Wan, Z.; Jia, C.; Duan, Y.; Zhou, L.; Lin, Y.; Shi, Y. J. Mater.
Chem. 2012, 22, 25140.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX