Cyclic thiourea functionalized dyes with binary π-linkers: Influence of different π-conjugation segments on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.01.022

A series of cyclic thiourea functionalized dyes containing binary π-linkers were synthesized and applied in the dye-sensitized solar cells. The only difference of these dyes is that one segment of the binary π-linkers is thiophene, n-hexylthiophene, thienothiophene, 3,4-ethylenedioxythiophene, and benzene, respectively. Among them, the dyes incorporating electron-rich thienothiophene or 3,4-ethylenedioxythiophene exhibit the broader absorption bands and higher molar extinction coefficients in the visible region, which can not only enhance the light-harvesting ability but also improve the short-circuit current density; on the other hand, the dyes bearing bulky n-hexylthiophene or 3,4-ethylenedioxythiophene can effectively suppress the intermolecular π-π aggregation and electron recombination owing to larger steric hindrance, which is beneficial to increase the open-circuit voltage.

Full text of DOI:10.1016/j.dyepig.2015.01.022

Accepted Manuscript
Cyclic thiourea functionalized dyes with binary π-linkers: Influence of different π-
conjugation segments on the performance of dye-sensitized solar cells
Shengbo Zhu, Zhongwei An, Xinbing Chen, Pei Chen, Qianfeng Liu
PII:
S0143-7208(15)00025-X
10.1016/j.dyepig.2015.01.022
DYPI 4649
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 24 October 2014
Revised Date: 24 December 2014
Accepted Date: 23 January 2015
Please cite this article as: Zhu S, An Z, Chen X, Chen P, Liu Q, Cyclic thiourea functionalized dyes with
binary π-linkers: Influence of different π-conjugation segments on the performance of dye-sensitized
solar cells, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.01.022.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Cyclic thiourea functionalized dyes with binary π-linkers: Influence of different
π-conjugation segments on the performance of dye-sensitized solar cells
Shengbo Zhua, Zhongwei Ana,b,*, Xinbing Chena, Pei Chena, Qianfeng Liuc
a Key Laboratory of Applied Surface and Colloid Chemistry, School of Materials
Science and Engineering, Shaanxi Normal University, Xi’an 710062, China.
b Xi’an Modern Chemistry Research Institute, Xi’an 710065, China.
c Xi’an Ruilian Modern Electronic Chemicals Co. Ltd., Xi’an 710077, China.
* E-mail: gmecazw@163.com
1

ACCEPTED MANUSCRIPT
Abstract
A series of cyclic thiourea functionalized dyes containing binary π-linkers were
synthesized and applied in the dye-sensitized solar cells. The only difference of these
dyes is that one segment of the binary π-linkers is thiophene, n-hexylthiophene,
thienothiophene, 3,4-ethylenedioxythiophene, and benzene, respectively. Among
them,
the
dyes
incorporating
electron-rich
thienothiophene
or
3,4-ethylenedioxythiophene exhibit the broader absorption bands and higher molar
extinction coefficients in the visible region, which can not only enhance the
light-harvesting ability but also improve the short-circuit current density; on the other
hand, the dyes bearing bulky n-hexylthiophene or 3,4-ethylenedioxythiophene can
effectively suppress the intermolecular π-π aggregation and electron recombination
owing to larger steric hindrance, which is beneficial to increase the open-circuit
voltage.
Keywords: Cyclic thiourea; Dye-sensitized solar cells; Binary π-linker; Thiophene;
Thienothiophene; 3,4-Ethylenedioxythiophene
2

ACCEPTED MANUSCRIPT
1. Introduction
With the development of global economy, energy consumption is being in the high
growth period, hence the shortage of energy sources will be a serious problem that
humanity has to face. Once a technological breakthrough is achieved, solar energy
will provide us with inexhaustible energy. Among various light-harvesting devices,
dye-sensitized solar cells (DSSCs) have attracted increasing attention due to the
features of low carbon, friendly to environment and easy fabrication [1-3]. As a key
component of DSSC, the sensitizer affects the power conversion efficiency (PCE)
because of its critical function in light harvesting and electron injection. To date,
sensitizers based on ruthenium complexes have achieved the PCE of almost 12%
[4-6]. However, the large scale application of ruthenium complexes has become a
critical problem because of resource limitation and cumbrous purification [7]. To
resolve these issues, metal-free organic dyes have been developed as alternative
sensitizers, because they exhibit advantages such as low material cost, high molar
extinction coefficients, facile modification and ease of purification [8]. In past decade,
there is a consecutive performance advancement of organic dyes as a result of
systematic molecular engineering [9]. To the best of our knowledge, DSSCs based on
single organic sensitizer has reached the PCE of 10.65% [10-13].
Most organic dyes tend to be designed with a dipolar D-π-A architecture, where
the donor (D) pushes electron into the metal oxide via the π-conjugated linker and
acceptor (A) [14-16]. Therefore, enormous efforts have been devoted toward the
design and synthesis of the three parts. So far, acceptors have been dominated by
2-cyanoacrylic acid, but various donors and linkers have been exploited. For example,
hydrazone [17-19], carbazole [20-23], coumarin [24, 25], indoline [26, 27],
phenothiazine [28, 29], phenoxazine [30], tetrahydroquinoline [31], and
3

ACCEPTED MANUSCRIPT
triphenylamine [12, 23, 32-34] are widely adopted as donors. The main advantages of
such compounds containing electron-rich nitrogen, oxygen, and sulfur heteroatoms
are good electron-donating ability and rapid charge-transporting ability [17, 29, 33,
35]. To further improve device performance, a suitable π-conjugated linker is required
between the donor and the acceptor based on the following consideration: (1) electron
is easily transferred from the donor to the acceptor [16, 29]; (2) the absorption band
could be broadened by the increase of the conjugation length [15, 36]; (3) the
intermolecular aggregation can be suppressed by the steric hindrance of π-linkers
[36-38]. Therefore, many kinds of electron-rich units have been introduced into the
π-linkers, such as thiophene [20, 39-41], thienothiophene (TT) [12, 42, 43], and
3,4-ethylenedioxythiophene (EDOT) [8, 12].
In our previous studies [44, 45], the cyclic thiourea functionalized triphenylamine
containing more nitrogen and sulfur heteroatoms was employed as an electron donor
to construct dye molecules. Compared with triphenylamine-based dyes, such dyes
exhibited better photophysical and photovoltaic performances. Moreover, the hexyl
chains attached at the N-atom sites of cyclic thiourea groups contribute to suppress
intermolecular aggregation and increase the solubility of dyes [22, 30]. Especially, the
combination of cyclic thiourea functionalized triphenylamine as the donor and
bithiophene (AZ6) or 3-hexyl-2, 2'-bithiophene (AZ261) as the π-linkers showed
broader spectral ranges and reduced intermolecular aggregation. Our results
demonstrate that the increased conjugation of π-linkers is beneficial to improve the
photocurrent density due to the broad absorption band. Therefore, a further
optimization is carried out by increase of the linker conjugation via the insertion of a
election-rich fusing ring TT or a EDOT with a small torsion angle [12]. Three new
organic dyes with different binary π-linkers were synthesized, which coded as AZ362
4

ACCEPTED MANUSCRIPT
with TT, AZ463 with EDOT, and AZ564 with benzene (Fig. 1). Through density
functional theory (DFT) calculations, we estimated these dyes have relatively small
HOMO-LUMO energy gaps (Eg), which is suitable for a wide spectral response [8].
Here, various π1 segments are inserted into the π-linkers, with a clear motivation to
further investigate the impact of π-conjugation, geometry and electron-rich segment
on photophysical, electrochemical and photovoltaic properties.
2. Experimental section
2.1. Materials and reagents
Thienothiophene
(TT),
3,4-ethylenedioxythiophene
(EDOT),
5-formyl-2-thiopheneboronic acid, N-bromosuccinimide (NBS), and Pd(PPh3)4 were
purchased from Aladdin-reagent Co.. All chromatographic separations were carried
out on silica gel (200-300 mesh). Transparent FTO conducting glass (fluorine doped
SnO2, sheet resistance 15ꢀ/square, transmission > 90% in the visible region) was
obtained from Nippon Sheet Co., Ltd.. All the chemicals and solvents used in this
work were of analytical grade and used without further purification unless otherwise
stated.
2.2. Characterizations
2.2.1. Structure confirmation
13
1H NMR and C NMR spectra were recorded on Bruker AV-300 MHz or Bruker
AV-400 MHz instruments with tetramethylsilane (TMS) as the internal standard. Gas
chromatography mass spectra (GC-MS) were acquired in the electron ionization mode
(EI) on Thermo DSQII. High resolution mass spectra (HRMS) were measured with a
Bruker maXis mass spectrometer. Elemental analysis (C H N) was carried out on a
VARIO-EL-III elemental analyzer.
5

ACCEPTED MANUSCRIPT
2.2.2. Photophysical measurements
UV-Vis absorption spectra of the dyes in MeCN-DCM (1:1, v/v, where MeCN is
acetonitrile, and DCM is dichloromethane; 1×10-5 M) solutions and on the adsorbed
TiO2 films (6.5 ꢁm) were measured with a Shimadzu UV-3600 spectrophotometer.
2.2.3. Electrochemical measurements
Cyclic voltammograms (CV) were conducted on a CHI 660D Electrochemical
Workstation at a scan rate 100 mV s-1, measured in a MeCN-DCM (3:1, v/v) solution
containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as a
supporting electrolyte (working electrode: glassy carbon; counter electrode: Pt;
reference electrode: non-aqueous Ag/Ag+ electrode) under argon atmosphere. The
redox potentials were calibrated with ferrocene/ferrocenium (Fc/Fc+) as the internal
reference.
Electrical impedance spectra (EIS) under dark with bias -0.7 V were also
measured with CHI 660D Electrochemical Workstation at frequencies of 0.05-100000
Hz. The magnitude of the alternative signal was 10 mV. Charge-transfer resistances
were determined by fitting the impedance spectra using Z-view software.
2.3. Theoretical calculations
The electronic configuration and geometry structure of the dyes were optimized
by density functional theory (DFT) calculations with the Gaussian 03 program
package at the B3LYP/6-311G(d, p) level.
2.4. Fabrication and characterization of DSSCs
TiO2 films were prepared with a screen printing method according to the
published method [45]. Then the films were immersed into a 0.4 mM dye bath in
DCM solution under dark for 24 h at room temperature. The active area of the TiO2
photoanode film was approximately 0.25 cm2. All other fabrication processes of
6

ACCEPTED MANUSCRIPT
DSSCs were also performed according to the published method [45].
Photocurrent-voltage (J-V) characteristics of the DSSCs were measured under
illumination with AM 1.5 G solar light from a 300 W xenon lamp solar simulator
(94022A, Newport Co., USA). The incident light intensity was calibrated to 100 mW
cm-2 with a standard silicon solar cell. J-V characteristics were recorded with a digital
source meter (Keithley 2400) controlled by a computer. The action spectra of
monochromatic incident photon-to-current conversion efficiency (IPCE) for solar
cells were tested on a commercial setup (QTest Station 2000 IPCE Measurement
System, Crowntech, USA).
2.5. Synthesis
The synthetic procedures of AZ6 [44], AZ261 [45], and the starting material
(4-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-yl)amino)phenyl)bo
ronic acid (1) [45] have been reported in our previous papers, and the synthesis of
intermediates 2a, 2b, and 2c can be found in the Supporting Information. The
synthetic routes of AZ362, AZ463, and AZ564 are shown in Fig. 2, and the synthetic
details are described as follows.
2.5.1. 5-(5-(4-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-
yl)amino)phenyl)thieno[3,2-b]thiophen-2-yl)thiophene-2-carbaldehyde (3a)
In a 100 mL 3-necked flask, compound 1 (700 mg, 0.90 mmol), 2a (330 mg, 1.0
mmol), tetra-n-butylammonium bromide (TBAB) (87 mg, 0.27 mmol), N, N-
dimethylformamide (DMF) (50 mL) and Pd(PPh3)4 (12 mg, 0.01 mmol) were added
in turn under a nitrogen atmosphere. Following this, K2CO3 aqueous solution (0.18
mol L-1, 10 mL) was added dropwise. The reaction mixture was stirred for 2 h at
75 °C, and then poured into ethyl acetate (EA). The organic layer was washed with
water and dried over anhydrous MgSO4. After removing the solvent, the crude
7

ACCEPTED MANUSCRIPT
product obtained was purified by column chromatography (PE/EA = 10/1, v/v; where
PE is petroleum ether, boiling range: 60-90 ºC) to give an orange semisolid (730 mg,
1
83% yield). H NMR (300 MHz, CDCl3): δ (ppm) 9.74 (s, 1H), 7.57 (d, J = 3.9 Hz,
1H), 7.42 (d, J = 8.1 Hz, 2H), 7.39 (s, 1H), 7.27 (s, 1H), 7.16 (d, J = 3.9 Hz, 1H), 7.02
(d, J = 8.4 Hz, 2H), 6.97-6.90 (m, 6H), 4.20 (t, J = 4.2 Hz, 4H), 4.10 (t, J = 4.2 Hz,
4H), 1.75-1.65 (m, 8H), 1.35-1.16 (m, 24H), 0.80 (t, J = 6.9 Hz, 6H), 0.73 (t, J = 6.0
Hz, 6H). 13C NMR (75 MHz, CDCl3): δ (ppm) 182.25, 169.50, 148.36, 147.87,
147.36, 143.03, 141.62, 140.96, 138.49, 137.32, 136.53, 133.15, 128.87, 127.72,
126.72, 124.02, 121.92, 120.44, 118.39, 114.16, 109.64, 106.05, 44.92, 44.72, 31.47,
31.40, 27.93, 27.81, 26.57, 26.44, 22.54, 22.46, 14.01, 13.99. HRMS (ESI, m/z):
[M+H]+ calcd for (C55H68N5OS5): 974.4027, found: 974.4041.
2.5.2. 5-(7-(4-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-
yl)amino)phenyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)thiophene-2-carbaldehyde
(3b).
The synthesis method resembles that of compound 3a but 2b instead of 2a, and
the crude product obtained was purified by column chromatography (PE/EA = 8/1,
v/v) to give an orange semisolid (760 mg, 87% yield). 1H NMR (400 MHz, CDCl3): δ
(ppm) 9.73 (s, 1H), 7.56-7.52 (m, 3H), 7.17 (d, J = 4.0 Hz, 1H), 7.00 (d, J = 8.0 Hz,
2H), 6.95-6.87 (m, 6H), 4.34-4.27 (m, 4H), 4.19 (t, J = 6.0 Hz, 4H), 4.09 (t, J = 5.6
Hz, 4H), 1.74-1.73 (m, 4H), 1.65-1.63 (m, 4H), 1.34-1.17 (m, 24H), 0.80 (t, J = 6.0
Hz, 6H), 0.74 (t, J = 5.6 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ (ppm) 182.35,
169.37, 147.38, 144.82, 143.15, 140.76, 140.49, 137.66, 136.95, 133.10, 128.72,
127.19, 125.83, 122.68, 121.90, 120.35, 118.20, 109.60, 108.35, 105.91, 65.10, 64.65,
44.91, 44.70, 31.47, 31.41, 27.93, 27.81, 26.57, 26.44, 22.54, 22.46, 14.03, 14.01.
HRMS (ESI, m/z): [M+H]+ calcd for (C55H70N5O3S4): 976.4361; found: 976.4349.
8

ACCEPTED MANUSCRIPT
2.5.3. 5-(4'-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-
yl)amino)-[1,1'-biphenyl]-4-yl)thiophene-2-carbaldehyde (3c).
The synthesis method resembles that of compound 3a but 2c instead of 2a, and the
crude product obtained was purified by column chromatography (PE/DCM/MeOH =
40/20/1, v/v/v; where MeOH is methanol) to give a yellow semisolid (700 mg, 85%
yield). 1H NMR (300 MHz, CDCl3): δ (ppm) 9.87 (s, 1H), 7.75-7.72 (m, 3H), 7.66 (d,
J = 6.3Hz, 2H), 7.55 (d, J = 6.3Hz, 2H), 7.45 (d, J = 3.0 Hz, 1H), 7.13-7.10 (m, 4H),
7.04-7.01 (m, 4H), 4.29 (t, J = 4.8Hz, 4H), 4.20 (t, J = 5.1 Hz, 4H), 1.86-1.81 (m, 4H),
1.76-1.73 (m, 4H), 1.45-1.25 (m, 24H), 0.89 (t, J = 5.4 Hz, 6H), 0.82 (t, J = 5.4 Hz,
13
6H). C NMR (75 MHz, CDCl3): δ (ppm) 182.62, 169.41, 153.86, 148.15, 143.31,
142.28, 141.37, 137.52, 133.22, 133.12, 131.49, 128.74, 127.66, 127, 04, 126.85,
123.95, 122.17, 120.40, 109.64, 105.97, 44.91, 44.70, 31.48, 31.41, 27.94, 27.81,
26.57, 26.43, 22.54, 22.46, 14.03, 13.99. HRMS (ESI, m/z): [M+H]+ calcd for
(C55H70N5OS3): 912.4742; found: 912.4745.
2.5.4. (E)-3-(5-(5-(4-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-
5-yl)amino)phenyl)thieno[3,2-b]thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic
acid
(AZ362)
In a 100 mL 3-necked flask, compound 3a (680 mg, 0.70 mmol), acetic acid (50
mL), cyanoacetic acid (150 mg, 1.75 mmol) and ammonium acetate (160 mg, 2.10
mmol) were added in turn under a nitrogen atmosphere. The reaction mixture was
refluxed for 5 h. After cooling to room temperature, the mixture was poured into ice
water. The precipitate was filtered, washed by distilled water, and purified by column
chromatography (DCM/MeOH = 20/1, v/v) to give a dark red solid (480 mg, 67%
1
yield). H NMR (400 MHz, CDCl3): δ (ppm) 8.40 (s, 1H), 7.66 (d, J = 4.4 Hz, 1H),
7.51-7.48 (m, 3H), 7.36 (s, 1H), 7.25 (d, J = 4.4 Hz, 1H), 7.10 (d, J = 8.0 Hz, 2H),
9

ACCEPTED MANUSCRIPT
7.06-6.99 (m, 6H), 4.27 (t, J = 7.2 Hz, 4H), 4.19 (t, J = 7.2 Hz, 4H), 1.85-1.82 (m,
4H), 1.76-1.72 (m, 4H),1.44-1.24 (m, 24H), 0.89 (t, J = 6.8 Hz, 6H), 0.82 (t, J = 6.8
Hz, 6H). 13C NMR (100 MHz, CDCl3): δ (ppm) 169.58, 167.06, 148.22, 147.60,
146.37, 142.90, 142.06, 140.87, 138.61, 136.71, 136.68, 134.89, 133.12, 128.85,
127.81, 126.65, 124.18, 121.87, 120.25, 118.16, 116.88, 114.10, 109.58, 105.98,
95.72, 44.93, 44.68, 31.48, 31.39, 27.95, 27.80, 26.58, 26.43, 22.55, 22.45, 14.03,
13.99. HRMS (ESI, m/z): [M-H]－ calcd for (C58H67N6O2S5): 1039.3929; found:
1039.3920. Anal calcd for (C58H68N6O2S5): C, 66.88; H, 6.58; N, 8.07. Found: C,
66.91; H, 6.62; N, 8.00.
2.5.5. (E)-3-(5-(7-(4-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-
yl)amino)phenyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)thiophen-2-yl)-2-cyanoacr
ylic acid (AZ463).
The synthesis method resembles that of AZ362 but 3b instead of 3a, and the crude
product obtained was purified by column chromatography (DCM/MeOH = 15/1, v/v)
to give a dark purple solid (380 mg, 52% yield). 1H NMR (400 MHz, CDCl3): δ (ppm)
8.09 (s, 1H), 7.56-7.51 (m, 3H), 7.17 (d, J = 4.4 Hz, 1H), 6.99 (d, J = 6.0 Hz, 2H),
6.92-6.88 (m, 6H), 4.38-4.28 (m, 4H), 4.18 (t, J = 5.6 Hz, 4H), 4.10 (t, J = 5.6 Hz,
4H), 1.74-1.72 (m, 4H), 1.66-1.64 (m, 4H), 1.34-1.16 (m, 24H), 0.81 (t, J = 4.8 Hz,
6H), 0.74 (t, J = 4.4 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ (ppm) 168.37, 167.55,
146.57, 146.03, 145.84, 142.00, 140.56, 138.61, 138.48, 136.77, 132.10, 127.79,
126.22, 124.48, 122.31, 120.56, 119.45, 118.70, 115.31, 108.60, 107.31, 105.05,
93.55, 64.25, 63.62, 43.90, 43.72, 30.46, 30.40, 26.92, 26.81, 25.56, 25.43, 21.52,
21.45, 13.00, 12.99. HRMS (ESI, m/z): [M-H]－ calcd for (C58H69N6O4S4): 1041.4263;
found: 1041.4252. Anal calcd for (C58H70N6O4S4): C, 66.76; H, 6.76; N, 8.05. Found:
C, 66.69; H, 6.70; N, 8.01.
10

ACCEPTED MANUSCRIPT
2.5.6.
(E)-3-(5-(4'-(bis(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-
yl)amino)-[1,1'-biphenyl]-4-yl)thiophen-2-yl)-2-cyanoacrylic acid (AZ564).
The synthesis method resembles that of AZ362 but 3c instead of 3a, and the crude
product obtained was purified by column chromatography (DCM/MeOH = 25/1, v/v)
1
to give a purple solid (480 mg, 70% yield). H NMR (400 MHz, CDCl3): δ (ppm)
8.36 (s, 1H), 7.76-7.71 (m, 3H), 7.61 (d, J = 5.6 Hz, 2H), 7.51 (d, J = 6.0Hz, 2H),
7.42 (d, J = 4.0 Hz, 1H), 7.10-7.09 (m, 4H), 7.07-6.99 (m, 4H), 4.28 (t, J = 4.8Hz,
4H), 4.18 (t, J = 4.4 Hz, 4H), 1.85-1.80 (m, 4H), 1.74-1.71 (m, 4H), 1.45-1.24 (m,
24H), 0.89 (t, J = 4.8 Hz, 6H), 0.81 (t, J = 4.8 Hz, 6H). 13C NMR (100 MHz, CDCl3):
δ (ppm) 169.40, 167.89, 154.95, 148.21, 147.55, 143.26, 141.61, 140.06, 134.84,
133.13, 133.08, 131.10, 128.77, 127.66, 127.01, 126.96, 124.31, 122.12, 120.40,
116.13, 109.64, 105.99, 97.85, 44.94, 44.72, 31.48, 31.41, 27.95, 27.82, 24.82, 26.58,
－
26.44, 22.55, 22.46, 14.02, 13.99. HRMS (ESI, m/z): [M-H]
calcd for
(C58H69N6O2S3): 977.4644; found: 977.4623. Anal calcd for (C58H70N6O2S3): C, 71.13;
H, 7.20; N, 8.58. Found: C, 71.11; H, 7.17; N, 8.59.
3. Results and discussion
3.1. Synthesis and structural characterization
The synthesis of the three dyes (AZ362, AZ463, and AZ564) is extremely
straightforward. As illustrated in Fig. 2, the target dyes were prepared by Suzuki
coupling reaction and Knoevenagel condensation reaction. Suzuki coupling reaction
of compound 1 and bromide (2a, 2b, or 2c) with Pd(PPh3)4 as the catalyst in a mixture
of K2CO3 aqueous solution and DMF afforded the corresponding intermediates (3a,
3b, or 3c). Then, the target dyes were synthesized via Knoevenagel condensation
reaction of aldehydes (3a, 3b, or 3c) with cyanoacetic acid in the presence of
11

ACCEPTED MANUSCRIPT
CH3COOH and CH3COONH4. All the intermediates and target dyes were confirmed
by standard spectroscopic methods.
3.2. Photophysical properties
The UV-Vis absorption spectra of the dyes in MeCN-DCM solutions and on TiO2
films are displayed in Fig. 3, and the corresponding photophysical data are listed in
Table 1. As shown in Fig. 3a, all of the dyes exhibit two distinct absorption bands, one
is from the local π-π* absorption in the UV region [23, 36] and the other arises from
the intramolecular charge transfer (ICT) absorption in the visible region [29, 46]. It is
noticeable that AZ564 with benzene as the π1 segment shows an overlap between the
local π-π* absorption and the ICT absorption. Compared with AZ564, the other four
dyes (AZ6, AZ261, AZ362, and AZ564) display broader absorption bands. This may
be caused by the better delocalization of electron over the whole molecule when
thiophene and its derivatives (n-hexylthiophene, TT, and EDOT) are used as the π1
segments compared to benzene [47, 48]. Compared with that of AZ6, the absorption
spectrum of AZ261 is blue-shifted by 9 nm in the maximum absorption wavelength
(λmax) together with a lower molar extinction coefficient (ε), which could be attributed
to large steric effect caused by n-hexyl chain attached to the thiophene unit. However,
the absorption bands and molar extinction coefficients of AZ362 (λmax = 488 nm, ε =
37800 M-1 cm-1) and AZ463 (λmax = 504 nm, ε = 39800 M-1 cm-1) are broader and
intenser than those of AZ6 (λmax = 475 nm, ε = 31200 M-1 cm-1), which may
demonstrate the superiority of TT and EDOT as the conjugation segments in light
harvesting. It is well known that broader absorption band and higher molar extinction
coefficient are favorable to the performance of DSSCs, as more photons can be
harvested [40]. The absorption spectra of the dyes on 6.5 ꢁm TiO2 films are shown in
Fig. 3b. The absorption spectra of all five dyes exhibit strong and broad absorption
12

ACCEPTED MANUSCRIPT
bands. Especially, the absorption intensity from 400 to 650 nm are significantly
enhanced, which could be explained by the increased delocalization of the π* orbital
of the conjugated skeleton caused by the interaction between the carboxylate group
and the Ti4+ ions, and the energy of the π* level is directly decreased [40]. This is
beneficial to harvest the visible light and thus increase the photocurrent response [46].
3.3. Electrochemical properties
To investigate the feasibility of the electron injection from the excited dye to the
conduction band of the TiO2 electrode and dye regeneration via electron donation
from the electrolyte [48], cyclic voltammetry (CV) was carried out and shown in Fig.
4. All of the dyes show oxidation/reduction processes at positive potential range and
reduction/oxidation processes at negative potential range, indicating that these dyes
have the capability to transport both electrons and holes [49]. The HOMO and LUMO
energy levels (EHOMO and ELUMO) as well as the electrochemical energy gap (Eegc ) were
calculated from the onset oxidation potentials (Eox) and the onset reduction potentials
(Ered) by assuming the energy level of ferrocene/ferrocenium (Fc/Fc+) to be −4.72 eV
below the vacuum level [50, 51]. The results are listed in Table 1. From these data, it
is observed that all of the dyes have sufficiently high ELUMO and are energetically
favorable for electron injection into the conduction band of TiO2 (ca. −4.0 eV vs.
vacuum [52]). The EHOMO also favors the regeneration of the oxidized dye by I−/I3
−
redox couple (−4.80 eV vs. vacuum [53]).
3.4. Theoretical calculations
To gain insight into the molecular structures and electron distribution, density
functional theory (DFT) calculations were conducted using the Gaussian 03 program
package at the B3LYP/6-311G(d, p) level, and the calculated results are also recorded
in Table 1. As shown in Fig. 5, the HOMOs of the five dyes are mainly distributed
13

ACCEPTED MANUSCRIPT
along the cyclic thiourea functionalized triphenylamine units and the π-linkers, and
the LUMOs are delocalized across the linkers and the acceptors. Obviously, the
HOMO and LUMO perfectly revealed that the donor-acceptor character of the cyclic
thiourea functionalized triphenylamine and the anchoring groups. The
well-overlapped HOMO and LUMO orbitals on the π-linker indirectly suggest the
well inductive or withdrawing electron tendency from the donor to the acceptor. Such
dye architecture provides an energy gradient for the excitation and facilitates the
HOMO to LUMO charge transfer transition, which is crucial to afford a favorable
energetic pathway for electron injection into the conduction band of TiO2 [43]. In
addition, it is worth noting that the Eg from the DFT calculations is bigger than the E
ec
g
from the experiments, which is a common feature of organic optoelectronic
materials and may be related with the solvent effect [42, 54]; but the relationship
between them is linear (Fig. 6).
Fig. 7 shows the optimized structure of the dyes. Dihedral angles between phenyl
and π1 segment (θ1) or π1 segment and adjacent thiophene (θ2) are listed in Table 1.
When different π1 segments are fixed to the horizontal plane, it can be seen that the
geometries of π-linkers are greatly affected. Among these dyes, AZ261 exhibits larger
steric hindrance due to the introduction of the hexyl chain, but a more twisted dihedral
angle between two thiophenes (20.33º) is found compared with that of AZ6 (15.01º).
This may weaken the ICT interactions, resulting in a blue shift of the maximum
absorption band [37], which is consistent with the experimental observation in the
absorption spectra. More interestingly, when bulky EDOT is employed to construct
AZ463, the adjacent thiophene is rotated approximately 180º. It is worth noting that
AZ463 displays excellent molecular coplanarity. Apparently, such π-linker can show
both large steric hindrance and conjugated coplanarity. Generally, large steric
14

ACCEPTED MANUSCRIPT
hindrance can effectively suppress the intermolecular aggregation and charge
recombination [37], while good molecular coplanarity can maximize the
π-conjugation pathway throughout the whole molecule and bring a remarkably red
shift in the maximum absorption band [43]. This indicates that it will probably exhibit
better photovoltaic performances.
3.5. Photovoltaic performance of DSSCs
Fig. 8 shows the IPCE spectra and J-V curves for DSSCs based on the five dyes.
As seen from Fig. 8a, the DSSCs based on AZ6, AZ261, AZ362, and AZ463 display
the maximum IPCE values of 79.65% at 490 nm, 81.76% at 480 nm, 85.33% at 500
nm, and 80.56% at 510 nm, respectively. And, the IPCE spectra of them cover in the
region from 300 nm to 700 nm. The DSSC based on AZ564, by contrast, exhibits the
smallest maximum IPCE value and narrowest IPCE spectrum. This indirectly
indicates that thiophene, n-hexylthiophene, TT, and EDOT as the π1 segments can
more efficiently convert the visible light into photocurrent. Generally, bigger integral
of the IPCE spectrum means higher electron transfer yield [55], indicating that their
cells would generate the higher conversion efficiency.
The photovoltaic performance parameters of DSSCs, including short-circuit
current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion
efficiency (PCE) are collected in Table 2, and the J-V curves are shown in Fig. 8b. As
a core dye, AZ6 yields a PCE of 7.18% (Jsc = 15.57 mA cm-2, Voc = 709.0 mV, and FF
= 0.65). At the same time, AZ261 produces a slightly higher Jsc of 15.93 mA cm-2 and
a higher Voc of 720.1 mV because of the introduction of hexyl chain, resulting in a
PCE up to 7.46%. The improved Jsc of AZ261 may be ascribed to its coplanar
structure [40], while the enhanced Voc may be attributed to the intermolecular π-π
aggregation and eletron recombination being suppressed by long hexyl chain attached
15

ACCEPTED MANUSCRIPT
at the bithiophene linker. When the π1 segment changed from thiophene to TT or
EDOT, the Jsc increases by 1.06 mA cm-2 for AZ362, 0.79 mA cm-2 for AZ463,
respectively. This is because electrons could be more easily transferred from the donor
into the TiO2 film in the presence of electron-rich atoms (sulfur and oxygen) [56]. In
addition, the broader range of light absorption for AZ362 and AZ463 may also
contribute to the higher Jsc [8]. Nevertheless, the Voc of AZ362 decreases by 40.4 mV
compared to that of AZ6, which might be due to the intermolecular π-π aggregation.
In contrast, the bulky character of the EDOT unit [8] in AZ463 may effectively
suppress the intermolecular aggregation [29, 57] and the electron recombination
between the TiO2 photoanode and the electrolyte, which can contribute to the higher
Voc and Jsc. Under the same measurement conditions, AZ564 bearing benzene without
electron-rich atoms as the π1 segment exhibits the lowest PCE of 6.25% with a Jsc as
low as 12.76 mA cm-2, even though the highest Voc is obtained.
3.6. Electrochemical impedance spectroscopy
To further elucidate the photovoltaic results and obtain more interfacial charge
transfer information, electrochemical impedance spectroscopy (EIS) was investigated.
In the Nyquist plots (Fig. 9a), there are two semicircles, and the larger semicircle
indicates charge-transfer resistance (Rrec) at the TiO2/dye/electrolyte interface; e.g., a
larger radius indicates a larger Rrec and slower electron recombination. In the Bode
phase plots (Fig. 9b), the lower-frequency peak is indicative of the charge-transfer
process of injected electrons in TiO2, and the electron lifetime (τe) can be calculated
from the peak frequency [47]. The calculated results increase in the order of AZ362
(22.1 ms) < AZ6 (25.2 ms) < AZ463 (26.5 ms) < AZ261 (29.2 ms) < AZ564 (31.3
ms). Obviously, both the Rrec and τe coincide well with Voc. The enlarged Rrec and
lengthened τe further indicate that the electron recombination between the injected
16

ACCEPTED MANUSCRIPT
electrons and the electrolyte was decreased, resulting in an increased Voc [47].
4. Conclusions
In summary, five cyclic thiourea functionalized dyes with binary π-linkers are
reported as sensitizers for DSSCs application. Our results reveal that the incorporation
of electron-rich TT and EDOT as the π1 segments can facilitate electron transfer from
the donor to the acceptor and realize the enhancement of light-harvesting ability,
resulting in an increased Jsc. On the other hand, the introduction of bulky
n-hexylthiophene and EDOT as the π1 segments can effectively suppress the
intermolecular aggregation and charge recombination, which is beneficial for the
improvement of Voc. In addition, according to the optimized structure by DFT
calculations, the combination of EDOT and thiophene as the π-linker makes AZ463
show both larger steric hindrance and better conjugated coplanarity. Therefore, DSSC
based on AZ463 exhibits the best PCE of 7.68% (Jsc = 16.36 mA cm-2, Voc = 710.0
mV, FF = 0.66). These results strongly indicate that selecting appropriate conjugation
unit as the π-linker is a promising way to improve the performance of DSSCs.
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central
Universities (No. GK201304005) and the Natural Science Foundation of China (No.
51373092). Our special thanks are also for the Fund of New Engergy Devices and
Materials provided by Mr. He Chong Ben, Hong Kong. We are grateful to Professor
Wenliang Wang at Shaanxi Normal University for theoretical calculations. We also
thank Minister Chao Gao and Miss Haimei Wu at Xi'an Modern Chemistry Research
Institute for electrochemical measurements.
17

ACCEPTED MANUSCRIPT
References
[1] O'Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal
TiO2 films. Nature 1991;353:737-40.
[2] Grätzel M. Photoelectrochemical cells. Nature 2001;414:338-44.
[3] Grätzel M. Recent advances in sensitized mesoscopic solar cells. Acc Chem Res
2009;42:1788-98.
[4] Chen C-Y, Wang M, Li J-Y, Pootrakulchote N, Alibabaei L, Ngoc-le C-h, et al. Highly efficient
light-harvesting ruthenium sensitizer for thin-film dye-sensitized solar cells. ACS Nano
2009;3:3103-9.
[5] Yum J-H, Jung I, Baik C, Ko J, Nazeeruddin MK, Grätzel M. High efficient donor-acceptor
ruthenium complex for dye-sensitized solar cell applications. Energy Environ Sci 2009;2:100-2.
[6] Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, et al. Enhance the optical absorptivity of
nanocrystalline TiO2 film with high molar extinction coefficient ruthenium sensitizers for high
performance dye-sensitized solar cells. J Am Chem Soc 2008;130:10720-8.
[7] Cheng M, Yang X, Chen C, Zhao J, Tan Q, Sun L. Effect of the acceptor on the performance of
dye-sensitized solar cells. Phys Chem Chem Phys 2013;15:17452-9.
[8] Lee MW, Kim JY, Lee DH, Ko MJ. Novel D-π-A organic dyes with
thieno[3,2-b]thiophene-3,4-ethylenedioxythiophene unit as a π-bridge for highly efficient
dye-sensitized solar cells with long-term stability. ACS Appl Mater Interfaces 2014;6:4102-8.
[9] Ito S, Miura H, Uchida S, Takata M, Sumioka K, Liska P, et al. High-conversion-efficiency
organic dye-sensitized solar cells with a novel indoline dye. Chem Commun 2008:5194-6.
[10] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-Sensitized solar cells
with an organic photosensitizer featuring orderly conjugated ethylenedioxythiophene and
dithienosilole blocks. Chem Mater 2010;22:1915-25.
[11] Bai Y, Zhang J, Zhou D, Wang Y, Zhang M, Wang P. Engineering organic sensitizers for
iodine-free dye-sensitized solar cells: red-shifted current response concomitant with attenuated
charge recombination. J Am Chem Soc 2011;133:11442-5.
[12] Zhang G, Bala H, Cheng Y, Shi D, Lv X, Yu Q, et al. High efficiency and stable
18

ACCEPTED MANUSCRIPT
dye-sensitized solar cells with an organic chromophore featuring a binary π-conjugated spacer.
Chem Commun 2009:2198-200.
[13] Yang J, Ganesan P, Teuscher J, Moehl T, Kim YJ, Yi C, et al. Influence of the donor size in
D-π-A organic dyes for dye-sensitized solar cells. J Am Chem Soc 2014;136:5722-30.
[14] Richard CA, Pan Z, Hsu HY, Cekli S, Schanze KS, Reynolds JR. Effect of isomerism and
chain length on electronic structure, photophysics, and sensitizer efficiency in quadrupolar
(donor)2-acceptor systems for application in dye-sensitized solar cells. ACS Appl Mater Interfaces
2014;6:5221-7.
[15] Liang M, Chen J. Arylamine organic dyes for dye-sensitized solar cells. Chem Soc Rev
2013;42:3453-88.
[16] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells. Chem Rev
2010;110:6595-663.
[17] Urnikaite S, Daskeviciene M, Send R, Wonneberger H, Sackus A, Bruder I, et al. Organic
dyes containing a hydrazone moiety as auxiliary donor for solid-state DSSC applications. Dyes
Pigm 2015;114:175-83.
[18] Urnikaite S, Malinauskas T, Bruder I, Send R, Gaidelis V, Sens R, et al. Organic dyes with
hydrazone moieties: A study of correlation between structure and performance in the solid-state
dye-sensitized solar cells. J Phys Chem C 2014;118:7832-43.
[19] Shen P, Liu X, Jiang S, Huang Y, Yi L, Zhao B, et al. Effects of aromatic π-conjugated
bridges on optical and photovoltaic properties of N,N-diphenylhydrazone-based metal-free
organic dyes. Org Electron 2011;12:1992-2002.
[20] Tan L-L, Xie L-J, Shen Y, Liu J-M, Xiao L-M, Kuang D-B, et al. Novel organic dyes
incorporating a carbazole or dendritic 3,6-diiodocarbazole unit for efficient dye-sensitized solar
cells. Dyes Pigm 2014;100:269-77.
[21] Wang Z-S, Koumura N, Cui Y, Takahashi M, Sekiguchi H, Mori A, et al.
Hexylthiophene-functionalized carbazole dyes for efficient molecular photovoltaics: tuning of
solar-cell performance by structural modification. Chem Mater 2008;20:3993-4003.
[22] Koumura N, Wang Z-S, Mori S, Miyashita M, Suzuki E, Hara K. Alkyl-functionalized
organic dyes for efficient molecular photovoltaics. J Am Chem Soc 2006;128:14256-7.
19

ACCEPTED MANUSCRIPT
[23] Ning Z, Zhang Q, Wu W, Pei H, Liu B, Tian H. Starburst triarylamine based dyes for efficient
dye-sensitized solar cells. J Org Chem 2008;73:3791-7.
[24] Hara K, Wang Z-S, Sato T, Furube A, Katoh R, Sugihara H, et al. Oligothiophene-containing
coumarin dyes for efficient dye-sensitized solar cells. J Phys Chem B 2005;109:15476-82.
[25] Zhang J, Li H-B, Geng Y, Wen S-Z, Zhong R-L, Wu Y, et al. Modification on C219 by
coumarin donor toward efficient sensitizer for dye sensitized solar cells: A theoretical study. Dyes
Pigm 2013;99:127-35.
[26] Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of dye-sensitized solar cells
based on metal-free indoline dyes. J Am Chem Soc 2004;126:12218-9.
[27] Wu Y, Zhang X, Li W, Wang Z-S, Tian H, Zhu W. Hexylthiophene-featured D-A-π-A
structural indoline chromophores for coadsorbent-free and panchromatic dye-sensitized solar cells.
Adv Energy Mater 2012;2:149-56.
[28] Wu W, Yang J, Hua J, Tang J, Zhang L, Long Y, et al. Efficient and stable dye-sensitized solar
cells based on phenothiazine sensitizers with thiophene units. J Mater Chem 2010;20:1772-9.
[29] Kim SH, Kim HW, Sakong C, Namgoong J, Park SW, Ko MJ, et al. Effect of five-membered
heteroaromatic linkers to the performance of phenothiazine-based dye-sensitized solar cells. Org
Lett 2011;13:5784-7.
[30] Tian H, Yang X, Chen R, Hagfeldt A, Sun L. A metal-free “black dye” for panchromatic
dye-sensitized solar cells. Energy Environ Sci 2009;2:674-7.
[31] Rakstys K, Solovjova J, Malinauskas T, Bruder I, Send R, Sackus A, et al. A structural study
of 1-phenyl-1,2,3,4-tetrahydroquinoline-based dyes for solid-state DSSC applications. Dyes Pigm
2014;104:211-9.
[32] Duan T, Fan K, Fu Y, Zhong C, Chen X, Peng T, et al. Triphenylamine-based organic dyes
containing a 1,2,3-triazole bridge for dye-sensitized solar cells via a ‘Click’ reaction. Dyes Pigm
2012;94:28-33.
[33] He J, Hua J, Hu G, Yin XJ, Gong H, Li C. Organic dyes incorporating a thiophene or furan
moiety for efficient dye-sensitized solar cells. Dyes Pigm 2014;104:75-82.
[34] Ning Z, Tian H. Triarylamine: a promising core unit for efficient photovoltaic materials.
Chem Commun 2009:5483-95.
20

ACCEPTED MANUSCRIPT
[35] Lygaitis R, Getautis V, Grazulevicius JV. Hole-transporting hydrazones. Chem Soc Rev
2008;37(4):770-88.
[36] Ning Z, Fu Y, Tian H. Improvement of dye-sensitized solar cells: what we know and what we
need to know. Energy Environ Sci 2010;3:1170-81.
[37] Lu X, Feng Q, Lan T, Zhou G, Wang Z-S. Molecular engineering of quinoxaline-based
organic sensitizers for highly efficient and stable dye-sensitized solar cells. Chem Mater
2012;24:3179-87.
[38] Ning Z, Zhang Q, Pei H, Luan J, Lu C, Cui Y, et al. Photovoltage improvement for
dye-sensitized solar cells via cone-shaped structural design. J Phys Chem C 2009;113:10307-13.
[39] Shen P, Liu Y, Huang X, Zhao B, Xiang N, Fei J, et al. Efficient triphenylamine dyes for solar
cells: Effects of alkyl-substituents and π-conjugated thiophene unit. Dyes Pigm 2009;83:187-97.
[40] Duan T, Fan K, Zhong C, Chen X, Peng T, Qin J. A new class of organic dyes containing
β-substituted 2, 2′-bithiophenene unit as a π-linker for dye-sensitized solar cells: Structural
modification for understanding relationship of structure and photovoltaic performances. J Power
Sources 2013;234:23-30.
[41] Kim S, Lee JK, Kang SO, Ko J, Yum J-H, Fantacci S, et al. Molecular engineering of organic
sensitizers for solar cell applications. J Am Chem Soc 2006;128:16701-7.
[42] Zhang G, Bai Y, Li R, Shi D, Wenger S, Zakeeruddin SM, et al. Employ a bisthienothiophene
linker to construct an organic chromophore for efficient and stable dye-sensitized solar cells.
Energy Environ Sci 2009;2:92-5.
[43] Katono M, Bessho T, Wielopolski M, Marszalek M, Moser J-E, Humphry-Baker R, et al.
Influence of the anchoring modes on the electronic and photovoltaic properties of D-π-A dyes. J
Phys Chem C 2012;116:16876-84.
[44] Wu Z, An Z, Chen X, Chen P. Cyclic thiourea/urea functionalized triphenylamine-based dyes
for high-performance dye-sensitized solar cells. Org Lett 2013;7:1456-9.
[45] Zhu S, An Z, Chen X, Chen P, Liu Q. Approach to tune short-circuit current and open-circuit
voltage of dye-sensitized solar cells: π-linker modification and photoanode selection. RSC Adv
2014;4:42252-9.
[46] Iqbal Z, Wu W-Q, Kuang D-B, Wang L, Meier H, Cao D. Phenothiazine-based dyes with
21

ACCEPTED MANUSCRIPT
bilateral extension of π-conjugation for efficient dye-sensitized solar cells. Dyes Pigm
2013;96:722-31.
[47] Wan Z, Jia C, Duan Y, Zhou L, Lin Y, Shi Y. Phenothiazine-triphenylamine based organic
dyes containing various conjugated linkers for efficient dye-sensitized solar cells. J Mater Chem
2012;22:25140-7.
[48] Wang D, Ying W, Zhang X, Hu Y, Wu W, Hua J. Near-infrared absorbing isoindigo sensitizers:
Synthesis and performance for dye-sensitized solar cells. Dyes Pigm 2015;112:327-34.
[49] Sun Q, Wang H, Yang C, Li Y. Synthesis and electroluminescence of novel copolymers
containing crown ether spacers. J Mater Chem 2003;13:800-6.
[50] Hou J, Tan Za, Yan Y, He Y, Yang C, Li Y. Synthesis and photovoltaic properties of
two-dimensional conjugated polythiophenes with bi(thienylenevinylene) side chains. J Am Chem
Soc 2006;128:4911-6.
[51] Lu Y, Xiao Z, Yuan Y, Wu H, An Z, Hou Y, et al. Fluorine substituted thiophene-quinoxaline
copolymer to reduce the HOMO level and increase the dielectric constant for high open-circuit
voltage organic solar cells. J Mater Chem C 2013;1:630-7.
[52] Kwon YS, Lim J, Song I, Song IY, Shin WS, Moon S-J, et al. Chemical compatibility
between a hole conductor and organic dye enhances the photovoltaic performance of solid-state
dye-sensitized solar cells. J Mater Chem 2012;22:8641-8.
[53] Cahen D, Hodes G. Nature of photovoltaic action in dye-sensitized solar cells. J Phys Chem
B 2000;104:2053-9.
[54] Heeney M, Zhang W, Crouch DJ, Chabinyc ML, Gordeyev S, Hamilton R, et al. Regioregular
poly(3-hexyl)selenophene: a low band gap organic hole transporting polymer. Chem Commun
2007:5061-3.
[55] Teng C, Yang X, Yang C, Tian H, Li S, Wang X, et al. Influence of triple bonds as π-spacer
units in metal-free organic dyes for dye-sensitized solar cells. J Phys Chem C 2010;114:11305-13.
[56] Li W, Liu B, Wu Y, Zhu S, Zhang Q, Zhu W. Organic sensitizers incorporating
3,4-ethylenedioxythiophene as the conjugated bridge: Joint photophysical and electrochemical
analysis of photovoltaic performance. Dyes Pigm 2013;99:176-84.
[57] Wu Y, Marszalek M, Zakeeruddin SM, Zhang Q, Tian H, Grätzel M, et al.
22

ACCEPTED MANUSCRIPT
High-conversion-efficiency organic dye-sensitized solar cells: molecular engineering on D-A-π-A
featured organic indoline dyes. Energy Environ Sci 2012;5:8261-72.
23

ACCEPTED MANUSCRIPT
Table 1
Absorption, cyclic voltammetry (CV), and density functional theory (DFT) calculations of the
dyes.
Absorption
CVc
DFTd
Dyes
λmax/nm
λmax/nm
Eox/ Ered
/
EHOMO
eV
/
ELUMO
eV
/
Eegc /
eV
EHOMO
eV
/
ELUMO
eV
/
E /
eV
g
θ1/º
θ2/º
(ε/M−1 cm−1)a
(ε/M−1 cm−1)b
V
V
475(31200)
472(54900)
464(52200)
483(56400)
511(56800)
449(55300)
0.21 -1.06
0.23 -1.08
0.23 -0.97
0.22 -0.98
0.21 -0.95
-4.93
-4.95
-4.95
-4.94
-4.93
-3.66
-3.64
-3.75
-3.74
-3.77
1.27
1.31
1.20
1.20
1.16
-5.26
-5.12
-5.24
-5.17
-5.27
-2.97
-2.67
-3.02
-2.94
-3.11
2.29 25.36 15.01
2.45 19.31 20.33
2.22 39.50 31.25
AZ6
466(21100)
488(37800)
504(39800)
419(31200)
AZ261
AZ362
AZ463
AZ564
2.23 9.33
5.96
2.16 35.18 25.58
a Measured in MeCN-DCM (1:1, v/v) solution. b Measured on 6.5 µm TiO2 films.
c Measured by cyclic voltammetry (CV) in MeCN-DCM (3:1, v/v) solution.
d Calculated by density functional theory (DFT) with the Gaussian 03 program package at the
B3LYP/6-311G (d, p) level.
1

ACCEPTED MANUSCRIPT
Table 2
Photovoltaic performance for DSSCs based on the dyes.
Dye
Jsc/mA cm-2
15.57
Voc/mV
709.0
720.1
668.6
710.0
729.2
FF
PCE/%
7.18
IPCE/% (λmax/nm)
79.65 (490)
81.76 (480)
85.33 (500)
80.56 (510)
73.26 (460)
0.65
0.65
0.66
0.66
0.67
AZ6
15.93
7.46
AZ261
AZ362
AZ463
AZ564
16.63
7.38
16.36
7.68
12.76
6.25
2

ACCEPTED MANUSCRIPT
Figure captions
Fig. 1. Chemical structures of the dyes. Herein C6H13 denotes n-hexyl.
Fig. 2. Synthetic routes of AZ362, AZ463, and AZ564. Reaction conditions: (a)
K2CO3 aqueous solution, Pd(PPh3)4, TBAB, DMF, 75 °C, 2 h; (b) cyanoacetic acid,
CH3COONH4, CH3COOH, reflux, 5 h.
Fig. 3. UV-Vis absorption spectra of the dyes in MeCN-DCM (1:1, v/v) solution (a)
and on TiO2 films (b). The absorption of a TiO2 film has been subtracted for clarity of
presentation.
Fig. 4. Cyclic voltammograms of the dyes.
Fig. 5. The frontier orbitals and energy levels of the dyes by DFT calculations.
Fig. 6. The linear fit about theoretical values Eg and their experimental values Eegc of
the dyes. R2 is the square correlation coefficient.
Fig. 7. Dihedral angles of the dyes’ π-linker by DFT calculations.
Fig. 8. (a) IPCE spectra and (b) J-V curves for DSSCs based on the dyes.
Fig. 9. EIS spectra for DSSCs based on the dyes: (a) Nyquist and (b) Bode phase
plots.
1

ACCEPTED MANUSCRIPT
Fig. 2. Chemical structures of the dyes. Herein C6H13 denotes n-hexyl.
2

ACCEPTED MANUSCRIPT
Fig. 2. Synthetic routes of AZ362, AZ463, and AZ564. Reaction conditions: (a) K2CO3
aqueous solution, Pd(PPh3)4, TBAB, DMF, 75 °C, 2 h; (b) cyanoacetic acid, CH3COONH4,
CH3COOH, reflux, 5 h.
3