Axial-symmetric conjugated group promoting intramolecular charge transfer performances of triphenylamine sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2019.108029

The intramolecular charge transport (ICT) process directly determines the charge generation, transport, and even injection of the dye-sensitized solar cell (DSSC) sensitizer. Herein, we constructed a new series of D-π-A system by linking 4-methoxyphenyl and triphenylamine donors with an ethynyl group having an axial-symmetric conjugated system. Since the axisymmetric conjugate group overcomes the reduction of the conjugate characteristic caused by the plane distortion, the molecular ICT performance and the electronic recombination inhibition are effectively improved. As a result, the photoelectric conversion efficiency was increased from 3.43% to 6.37% by means of the extension of the π-bridge at same time. It provides a new idea for the design and development of DSSC sensitizers in the future.

Full text of DOI:10.1016/j.dyepig.2019.108029

Journal Pre-proof
Axial-symmetric conjugated group promoting intramolecular charge transfer
performances of triphenylamine sensitizers for dye-sensitized solar cells
Shaorui Chen, Juan Pei, Zhihan Pang, Wenjun Wu, Xudong Yu, Cong Zhang
PII:
S0143-7208(19)31900-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.108029
DYPI 108029
Reference:
To appear in: Dyes and Pigments
Received Date: 13 August 2019
Revised Date: 21 October 2019
Accepted Date: 5 November 2019
Please cite this article as: Chen S, Pei J, Pang Z, Wu W, Yu X, Zhang C, Axial-symmetric conjugated
group promoting intramolecular charge transfer performances of triphenylamine sensitizers for dye-
sensitized solar cells, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108029.
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2019 Published by Elsevier Ltd.

Axial-symmetric conjugated group promoting intramolecular charge
transfer performances of triphenylamine sensitizers for dye-
sensitized solar cells
a∗
a
b
b∗
a
a
Shaorui Chen , Juan Pei , Zhihan Pang , Wenjun Wu , Xudong Yu , Cong Zhang
a
College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, Hebei, China
b
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai
00237, China
2
Abstract: The intramolecular charge transport (ICT) process directly determines the charge
generation, transport, and even injection of the dye-sensitized solar cell (DSSC) sensitizer. Herein,
we constructed a new series of D-π-A system by linking 4-methoxyphenyl and triphenylamine
donors with an ethynyl group having an axial-symmetric conjugated system. Since the
axisymmetric conjugate group overcomes the reduction of the conjugate characteristic caused by
the plane distortion, the molecular ICT performance and the electronic recombination inhibition
are effectively improved. As a result, the photoelectric conversion efficiency was increased from
3
.43% to 6.37% by means of the extension of the π-bridge at same time. It provides a new idea for
the design and development of DSSC sensitizers in the future.
Key words：Triphenylamine; Dye-sensitized Solar Cells; D-π-A; ethynyl group
1
. Introduction
As a result of the growing energy demand and the pollution of fossil fuels, people urgently
need new energy to replace fossil energy. Dye-sensitized solar cells (DSSC), as the
∗
Corresponding author:
E-mail: sjz_wgq@126.com( SR Chen); wjwu@ecust.edu.cn (WJ Wu)

third-generation photovoltaic technology, have attracted considerable and sustainable attention due
to their potential application as a renewable and clean energy source [1-5]. At present, the highest
photoelectric conversion efficiencies (PCE) of DSSC based on ruthenium-complexes and
Zn-porphyrins are 11.9% and 13%, respectively [6-7]. It is worth noting that PCE value on pure
organic sensitizers over 14% under AM 1.5G irradiation [8]. Recently, SS Soni and SC Tan have
reported that novel organic ionic conductors show high performances acting as a redox mediator
and a light absorbing material at the same time [9-10]. As a crucial component in a typical DSSC,
the dye plays an important role in the light absorption, charge separation, and electron injection
[11-13]. The metal-free organic dyes have been extensively developed owing to their high molar
extinction coefficients (ε), simple preparation and purification procedures, low-cost, and
environmental friendly [14-15]. The development of low-cost and species abundance metal-free
organic dyes to replace the expensive and source-limited ruthenium dyes is highly demanded and
certainly a state of the art research topic.
Generally, metal-free organic dyes are constituted by donor (D), π-bridge (π) and acceptor (A)
moieties, so called D-π-A character, which induces the intramolecular charge transfer (ICT) from
subunit D to A through the π-bridge when a dye absorbs light [16-19]. Among them, introducing
auxiliary groups in the skeleton of D-π-A system can exhibit a significant influence on the energy
levels, light response, dye stability as well as photovoltaic performances in organic sensitizers
[20-22]. Triphenylamine (TPA) dyes are widely used in DSSCs as sensitizers for their strong
electron-donating ability and hole-transport properties [23-25]. Theoretical and experimental
studies have demonstrated that a triphenylamine moiety can effectively suppress the dye
aggregation due to its nonplanar structure and TPA-based dyes have shown promise in the

fabrication of highly efficient photovoltaic devices [26]. However, three phenyl groups of
conventional triphenylamine are noncoplanar, which weakens the intermolecular electron
interactions, therefore hindering the maximization of electron-donating ability [27-30]. In order to
improve the cell performance, a double bond is also introduced into different positions of the
π-conjugation systems to tune the molecular structure and their configurations [31]. The triple
bond π-bridge has been utilized for conjugated donor-acceptor systems because it is a rigid and
straight for conducting electrons and at the same time reduces charge recombination [32-34]. In
addition, the introduction of triple bond could red-shift the dye’s absorption spectrum due to the
enhancement of the π-spacer [35-36]. However, it is rarely introduced to the donor applied into
DSSC sensitizers and scrutinized the relationship between the structure of the dye and the
efficiency of devices.
With this in mind, herein we develop a series of new triphenylamine dyes (T-2 and T-3) (Fig.
1
), in which triphenylamine linking 4-methoxyphenyl with an ethynyl group is used as
electron-donating moiety, cyanoacrylic acid as electron acceptor, and ethylene or 1,3-butadiene
moiety as π-conjugations to bridge the donor-acceptor (D-A) systems. Its activity as sensitizer is
also compared with typical D-π-A system of T1 and N719. By comparison, T-2 and T-3
outperform the T-1 control one in terms of absorption properties and photovoltaic performance,
indicating 4-methoxyphenyl with an ethynyl group having axisymmetric conjugate group is a
promising building block for organic dyes.

H3CO
H3CO
H
H
N
H
N
COOH
CN
COOH
CN
N
COOH
CN
T-1
T-2
T-3
H3CO
H3CO
Fig. 1 Molecular structure of T-1, T-2 and T-3.
2
. Experiment
2
.1 Materials and reagents
All chemicals and solvents were analytical grade and without further purification.
Tertbutylpyridine (TBP), lithium iodide, and 1-methyl-3-propylimidazolium iodide (PMII) were
obtained from Adamas. Transparent (18 NRT) and active opaque (18NR-AO) TiO paste were
2
obtained from Greatcellsolar Ltd (Australia). The transparent fluorine-doped tin oxide (FTO)
conducting glass (fluorine-doped SnO , transmission > 90% in the visible range, sheet resistance
2
1
5Ω/square) was from Yingkou OPV Tech New Energy Co., Ltd. It was washed with a detergent
solution, deionized water, acetone and ethanol successively under ultrasonication for 20 min
before use. Thin layer chromatography purification was carried out using silica gel purchased
from Qingdao Haiyang Co. LTD, China.
2
.2 Instrumentation
1
13
H and C NMR spectra were performed using Bruker 500 MHz instrument with
tetramethylsilane as the internal standard. UV-vis absorption spectra were recorded on a UV-vis
550 spectroscope (Shimadzu). Fluorescence spectra were collected on an Edinburgh instrument
FLS-920 spectrometer with a Xe lamp as an excitation source. Cyclic voltammetry (CV) was
2

carried out on a CHI760E electrochemical workstation in CH Cl , using tetra-n-butylammonium
2
2
-1
-1
hexafluorophosphate as support electrolyte (10 M); scan rate: 100 mV s . The electrochemical
cell was equipped with three electrodes: a working electrode (plate Pt electrode), an auxiliary
electrode (platinum wire electrode), and a reference electrode (Ag/AgCl electrode). Photovoltaic
measurements were performed by employing an AM 1.5 solar simulator equipped with a 300 W
Xe lamp (model no. 91160, Newport). The power of the simulated light was calibrated to 100 mW
-
2
cm using a Newport Oriel PV reference cell system (Model 91150V). J-V curves were obtained
by applying an external bias to the cell and measuring the generated photocurrent with a Keithley
model 2400 digital source meter. The voltage step and delay time of the photocurrent were 10 mV
and 40 ms, respectively. The incident IPCE values of all DSSCs were measured by using a
Newport-74125 system (Newport Instruments). The intensity of monochromatic light was
detected by using a Si detector (Newport-71640). The EIS measurements of all DSSCs were
performed by using a Zahner IM6e Impedance Analyzer (ZAHNER-Elektrik GmbH & CoKG,
Kronach, Germany) in a frequency range of 0.1 Hz-100 kHz with an alternative signal of 5 mV at
a constant temperature of 25 ℃ in dark conditions.
2
.3 Fabrication of DSSC
All working electrodes used in this work were prepared and modified following a reported
procedure [29]. The working electrode was composed of an 8 µm thick TiO film, including a 4
2
µm transparent layer with 18 NRT and 4 µm scattering layer with 18NR-AO. The dye solutions
were 0.3 mM in acetonitrile and the photoanodes underwent dipping for 12 h in the dark to
complete the loading with sensitizers. The counter electrode was prepared by drop casting an
H PtCl solution (0.02 M in 2-propanol solution) onto FTO glass and sintered under 400 °C for 15
2
6

min. The iodine electrolyte was a solution of 0.1 M LiI, 0.05 M I , 0.6 M
2
1
-methyl-3-propylimidazolium iodide (PMII), and 0.5 M TBP in a mixture of acetonitrile and
valeronitrile (volume ratio of 85:15). For the assemblage of DSSC, the dye-covered TiO electrode
2
and Pt counter electrode were assembled into a sandwich type cell and sealed with a hot-melt
gasket of 25 µm thickness made of the ionomer Surlyn 1702 (DuPont). And then, a drop of the
electrolyte was put on the hole in the back of the counter electrode. It was introduced into the cell
via vacuum backfilling. The hole in the counter electrode was sealed by a film of Surlyn 1702 and
a cover glass (0.1 mm thickness) using a hot press. The size of the TiO electrodes used was 0.25
2
2
cm (i.e. 5 mm × 5 mm). In this work, all test data shown were the average values of five parallel
tests.
2
.4 Synthesis of dyes
N, N-dimethylformamide (DMF) were distilled with CaH . T-1 was conveniently synthesized
2
according to the literature method [37]. Other chemicals and reagents were purchased from
commercial suppliers without further purifications. The synthetic route of T-2 and T-3 are shown
in Scheme 1.

H
3
CO
I
(a)
(b)
(c)
N
CHO
CHO
N
N
N
CHO
H
3
CO
I
1
2
3
(d)
H
H
3
CO
N
COOH
NC
T-2
3
H CO
H
3
CO
H
3
H CO
3
CO
H
N
(
e)
(f)
(e)
3
N
COOH
N
CHO
NC
4
T-3
5
3
H CO
H CO
3
H CO
3
Scheme 1. Synthesis of dyes T-2 and T-3
(
a) POCl
3
, DMF, 80 ℃ , 8 h, 86%; (b) potassium iodide, I
2
, 60 ℃ , 12h, 75%; (c) Pd(II), copper iodide, P(Ph)
3
3 4
, K PO , 110 ℃ , 17
3 3 3
h, 72%; (d) cyanoacetic acid, piperidine, acetonitrile, reflux, 8 h, 51%; (e) P(Ph) CH Br, Potassium t-Butoxide, rt, 2h, 97%; (f) POCl ,
DMF, 80 ℃ , 12 h, 81% ; (g) cyanoacetic acid, piperidine, acetonitrile, reflux, 10 h, 55%.
2
.4.1 synthesis of 4-(N,N-diphenylamino)benzaldehyde (1)
phosphorous oxychloride (POCl ) (3.29 g, 21.5 mmol) was added dropwise to the mixture of
3
triphenylamine (4.9 g, 0.02 mmol) and N, N-dimethylformamide (40 mL) at 25 ℃ . The reaction
mixture was stirred and heated at 80 ℃ for 8 h. After cooling down to room temperature, the
reaction mixture was poured into 50 mL cold water and acidized with dilute hydrochloric acid and
extracted with dichloromethane (50 mL × 3). The combined organic layer was dried with
anhydrous MgSO , filtered and evaporated under reduced pressure. The residue was purified by
4
column chromatography on silica gel and eluted with petroleum ether/ethyl acetate to afford

1
compound 1 (4.72 g, 86 %) as a pale yellow solid. m.p. 128-130 ℃ (lit. [38] 129-133 ℃ ); H NMR
(
500 MHz, CDCl ) δ (ppm): 7.03 (d, J = 8.8 Hz, 2H, Ph), 7.17-7.19 (m, 6H, Ph), 7.34 (t, J = 8.0
3
Hz, 4H, Ph), 7.69 (d, J = 8.8 Hz, 2H, Ph), 9.81 (s, H, -CHO).
2
.4.2 synthesis of 4-[N,N-di(4-iodophenyl)amino]benzaldehyde (2)
The mixture of compound 1 (1.36 g, 4.98 mmol), 1.58 g (6.22 mmol) iodine and 40 mL
ethanol were heated to 60 ℃ and stirred for 30 min, then the solution of iodic acid (0.88 g, 5
mmol) in water (5 mL) were added dropwise. The reaction was stirred at 60 ℃ in nitrogen for 12h.
The solution was allowed to cool and the solid was collected, washed with ethanol and
recrystallised with CH Cl -ethanol giving the product (1.84 g, 75 %). m.p. 147.0-148.5 ℃ (lit. [39]
2
2
1
1
48.0 -150.0 ℃ ); H NMR (500 MHz, DMSO, Fig. S1) δ (ppm): 6.95 (d, J = 8.5 Hz, 4H, Ph), 7.00
(d, J = 8.5 Hz, 2H, Ph), 7.71 (d, J = 8.5 Hz, 4H, Ph), 7.76 (d, J = 9.0 Hz, 2H, Ph), 9.82 (s, H,
-CHO).
2
.4.3 synthesis of 4-(N,N-bis(4-((4-methoxyphenyl) ethynyl)phenyl)amino) benzaldehyde (3)
Under N atmosphere, compound 2 (0.52 g, 0.99 mmol), copper (I) iodide (0.076 g, 0.04
2
mmol), dichlorobis (triphenylphosphine) palladium (II) (0.026 g, 0.04 mmol), triphenyl phosphine
(0.01 g, 0.04 mmol), potassium phosphate (1.2 g, 5.66 mmol), and 4-methoxyphenyl acetylene
(0.34 g, 2.5 mmol) were dissolved in dry DMF (15 mL). The reaction was then heated at 110 ℃
for 17 h, then was quenched with water and extracted with CH Cl . The organic layer was dried
2
2
with anhydrous MgSO . Silica gel chromatography with petroleum ether/dichloromethane gave
4
1
compound 3 with the yield of 72 % (0.37 g). m.p. 183.8 -185.7 ℃ ; H NMR (500 MHz, CDCl ,
3
Fig. S2) δ (ppm): 3.83(s, 6H, -OCH ), 6.88 (d, J = 8.5 Hz, 4H, Ph), 7.10-7.12 (m, 8H, Ph), 7.46 (d,
3
J = 7.5 Hz, 6H, Ph), 7.73 (d, J = 8.5 Hz, 2H, Ph), 9.86 (s, H, -CHO); ESI-MS (m/z): 534.3 (M +

+
H) ; Anal. Calcd. for C H NO , %: C 83.28; H 5.10; N 2.62; Found C 83.27; H 5.35; N 2.76.
3
7
27
3
2
.4.4 synthesis of 3-(4-(4-(bis(4-((4-methoxyphenyl)ethynyl)phenyl)amino)phenyl) -2- cyanoacrylic
acid (T-2)
To a stirred solution of compound 3 (0.20 g, 0.37 mmol) in 10 mL of acetonitrile, cyanoacetic
acid (0.061 g, 0.72 mmol) and piperidine (3-5 drops) were added. The mixture was heated to
reflux for 9 h. After cooling, the reaction mixture was diluted with dichloromethane (20 mL),
washed with water (20 mL) and hydrochloric acid (20%, 20 mL). The organic layer was separated,
dried with MgSO and concentrated in a rotary evaporator to get crude T-2. The crude product was
4
purified by column chromatography (methanol/dichloromethane as eluent) to afford T-2 as red
1
solid. Yield 51 %, 0.12 g; m.p.153.3 -154.7 ℃ ; H NMR (500 MHz, CDCl , Fig. S3) δ (ppm):
3
3
4
.84(s, 6H, -OCH ), 6.88 (d, J = 8.5 Hz, 4H, Ph), 7.08 (d, J =9.0 Hz, 2H, Ph), 7.12 (d, J = 8.5 Hz,
3
1
3
H, Ph), 7.46-7.49 (m, 8H, Ph), 7.91 (d, J = 8.5 Hz, 2H, Ph), 8.16 (s, H,-CH=); C NMR (125
MHz, CDCl , Fig. S4) δ (ppm): 58.13, 90.29, 92.95, 116.87, 118.03, 123.42, 123.49, 127.18,
3
+
1
28.54, 135.73, 135.87, 147.73, 154.71, 157.85, 162.52; ESI-MS (m/z): 601.3 (M + H) ; Anal.
Calcd. for C H N O , %: C 79.98; H 4.70; N 4.66; Found C 80.27; H 4.55; N 4.76.
4
0
28
2
4
2
.4.5 synthesis of 4-(N,N-bis(4-((4-methoxyphenyl) ethynyl)phenyl)amino) styrene (4)
Under N atmosphere, potassium tert-butanol (t-BuOK) (0.43g, 3.84mmol) was added into 70
2
mL THF solution. After stirring for 30 minutes, t-BuOK was completely dissolved and the mixture
was cooled to 0 ℃ . Methyl triphenylphosphonium bromide (1.20 g, 3.84 mmol) and compound 3
(0.64 g, 1.19 mmol) were added. The reaction mixture was stirred for another 2 h at room
temperature, and then poured into ice water mixture followed by extraction with dichloromethane.
The organic layer was dried over MgSO . After evaporation of solvent, the crude product 4 was
4

obtained. The crude product was purified by column chromatography petroleum ether/
dichloromethane as eluent to afford compound 4 as white solid. Yield 97 %, 0.62 g; m.p. 193.0
1
-
194.7 ℃ ; H NMR (500 MHz, CDCl , Fig. S5) δ (ppm): δ: 3.85 (s, 6H, -OCH ), 5.22 (d, J = 11.0
3
3
Hz, 1H, = CH ), 5.71 (d, J = 17.5Hz, 1H, = CH ), 6.68 (dd, J = 17.5 Hz, J = 11.0 Hz, 1H,
2
2
1
2
-CH=C), 6.86 (d, J = 8.5 Hz, 4H, Ph), 7.04 (d, J = 11.0 Hz, 6H, Ph), 7.33 (d, J = 8.5 Hz, 2H, Ph),
1
3
7
.38 (d, J = 8.5 Hz, 4H, Ph); C NMR (125 MHz, CDCl , Fig. S6) δ (ppm): 55.34, 88.05, 89.04,
3
112.98, 114.03, 115.62, 117.76, 123.60, 124.91, 127.34, 132.97, 133.24, 136.09, 146.37, 146.81,
+
1
59.53; ESI-MS (m/z): 532.3 (M + H) ; Anal. Calcd. for C H NO , %: C 85.85; H 5.50; N 2.63;
38 29 2
Found C 85.77; H 5.75; N 2.76.
2
.4.6 synthesis of 4-(N,N-bis(4-((4-methoxyphenyl) ethynyl)phenyl)amino) phenyl) acrylaldehyde
(5)
At room temperature, phosphorous oxychloride (POCl ) (1.2 mL) was added dropwise to dry
3
N, N-dimethylformamide (10 mL). The mixture stirred for 1 h and compound 4 (0.67 g, 1.26
mmol) was added. Then the reaction mixture still stirred for another 12 h and was adjusted to be
neutral using 20% NaOH aqueous solution, extracted with CH Cl . The organic layer was dried
2
2
with anhydrous MgSO . Silica gel chromatography with petroleum ether/dichloromethane gave
4
1
compound 5 with the yield of 81 % (0.56 g). m.p. 163.8-165.5 ℃ ; H NMR (500 MHz, CDCl , Fig.
3
S7) δ (ppm): 3.85 (s, 6H, -OCH ), 6.67 (d, J = 8.5 Hz, 4H, Ph), 7.11 (d, J = 8.5 Hz, 6H, Ph), 7.48
3
1
3
(
d, J = 7.5 Hz, 8H, Ph), 7.72 (d, J = 8.5 Hz, 2H, Ph), 9.85 (s, 1H, -CHO); C NMR (125 MHz,
CDCl , Fig. S8) δ (ppm): 55.35, 87.71, 89.65, 114.06, 119.30, 122.85, 124.83, 126.83, 128.18,
3
+
1
29.93, 132.78, 133.03, 149.73, 152.16, 159.66, 193.62; ESI-MS (m/z): 560.4 (M + H) ; Anal.
Calcd. for C H NO , %: C 83.70; H 5.22; N 2.50; Found C 83.77; H 5.25; N 2.66.
3
9
29
3

2
4
.3.7 synthesis of (2E,4E)-5-(4-(4-(bis(4-((4-methoxyphenyl)ethynyl)phenyl)amino)phenyl)-2-cyano-2,
-pentadienoic acid (T-3)
The mixture of compound 5 (0.20 g, 0.36 mmol) cyanoacetic acid (0.061 g, 0.72 mmol) and
piperidine (3-5 drops) in 10 mL of acetonitrile was heated to reflux for 9 h, poured into 20 mL
water, extracted with dichloromethane, dried with MgSO and concentrated in a rotary evaporator
4
to get crude T-3. The crude product was purified by column chromatography (methanol /
1
dichloromethane as eluent) to afford T-3 as red solid. Yield 55 %, 0.13 g; m.p.127.3 -128.7 ℃ ; H
NMR (500 MHz, CDCl , Fig. S9) δ (ppm): 3.86 (s, 6H, -OCH ）, 6.85 (d, J = 9.0 Hz, 4H, Ph),
3
3
7
.06-7.10 (m, 6H, Ph), 7.20 (s, 1H, -CH=C), 7.23 (s, 1H, =C-CH), 7.44-7.49 (m, 10H, Ph), 8.02 (d,
13
J = 11.0 Hz, 1H, -C=CH)
; C NMR (125 MHz, CDCl , Fig. S10) δ (ppm): 55.33, 87.66, 89.77,
3
1
1
01.73, 114.04, 115.34, 119.57, 121.13, 122.47, 125.02, 128.58, 130.30, 132.79, 133.02, 145.67,
+
49.46, 149.96, 157.07, 159.64, 166.96; ESI-MS (m/z): 627.3 (M + H) ; Anal. Calcd. for
C H N O , %: C 80.49; H 4.82; N 4.47; Found C 80.57; H 4.98; N 4.66.
4
0
30
2
2
3
. Results and discussion
3
.1. Synthesis and characterization
Two dyes T-2 and T-3 were synthesised from triphenylamine as the starting material.
Compound 1 was synthesized according to the literature [38]. 4-[N, N-di (4-iodophenyl) amino]
benzaldehyde was obtained using iodine and iodic acid in the solution of ethanol. While using
potassium iodide and potassium iodate, the structure of target compound is not correct by NMR
spectra [40]. Then the solution of Sonogashira coupling was employed to attach
4
-methoxyphenylethynyl group to the compound 2 [41]. On the one hand, the aldehyde was
converted to cyanoacetic acid derivatives (T-2) by refluxing with cyanoacetic acid in the presence

of piperidine in dry acetonitrile [42]. On the other hand, a Wittig reaction between phosphonium
salt and compound 3 gave styrene derivative 4 [43]. Using Vilsmeier-Haack reaction, compound 5
was obtained, which in turn was condensed with cyanoacetic acid to give dye T-3 in good yield.
The proposed structures of all the final compounds were confirmed using characterization
techniques such as NMR, MS and elemental analysis spectroscopies.
3
.2. Photophysical properties
-5
The UV-vis absorption for T-1, T-2 and T-3 in dilute solutions of CH Cl (1.67×10 M) are
2
2
presented in Fig. 2a and the corresponding data are summarized in Table 1. All of the dyes with a
strong absorption maximum in the visible region corresponding to the highest occupied molecular
orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) transitions are observed [44].
-1
-1
The absorption spectrum of T-1 shows an absorption maximum (λmax) at 434 nm (36200 M cm ).
Compared to T-1, the absorption spectrum of T-2 shows two absorption maxima at 358 nm (62600
-1
-1
-1
-1
M cm ) and 430 nm (41000 M cm ). Both of the width and intensity of the absorption peak for
T-2 (430 nm) are bigger than those of T-1. Introduction of 1, 3-butadiene moiety as π-bridge to
-1
-1
T-2, giving T-3, causes a further red shift to 460 nm (35400 M cm ). This result suggested that
the introduction of 4-methoxyphenyl with ethynyl group having axisymmetric conjugate group to
the donor was beneficial to red shift the charge-transfer transition. Theoretical computation shows
that the ground state structure of T-1 possesses a 50.2° twist between the diphenylamine and the
o
phenyl ring, and the dihedral angle in T-2 is 49.2 because of the introduction of 4-methoxyphenyl
with ethynyl group, giving less twist than that of T-1. Furthermore, the dihedral angle in T-3 is
4
5.3° (Table 2). Introducing 4-methoxyphenyl with ethynyl group and double bond, giving T-3,
causes the largest red shift to 460 nm because of not only enhancement of the electron-donating

ability of the TPA donor but also the best delocalization over an entire conjugated system in T-3
45]. It should be noted that the red-shift usually endows the dye a wider photoresponding range,
which is favorable for light harvesting and photocurrent generation in DSSC.
[
2 2 2
Fig. 2 Absorption spectra of dyes T-1, T-2 and T-3 measured in CH Cl (a) and adsorbed onto 4 µm transparent TiO films (b).
Table 1 Photophysical and electrochemical properties for T-1, T-2 and T-3
a
b
c
d
e
f
g
ox
h
red
I
λ
max
ε
λ
max
λ
em
λ
inset
E
0-0
E
E
Dye loading amount
Dyes
-
1
-1
-2
nm
M cm
nm
nm
521
538
595
nm
V
V
V
(mol cm )
-7
-7
-7
T-1
T-2
T-3
434
430
460
36200
41000
35400
422
428
457
463
468
502
2.68 1.18 -1.50
2.65 1.19 -1.46
2.47 0.95 -1.52
2.06×10
2.08×10
2.73×10
Note:

a
Absorption parameters were obtained in CH2Cl2.
b
2 2
The molar extinction coefficient at λmax in CH Cl solution.
c
2
Absorption parameters were obtained on 4 µm nanocrystalline TiO film.
d
2 2
Emission maximum in CH Cl solution.
e
The intersection of normalized absorption and emission spectra.
f
Obtained from the intersection between the absorption and emission spectra E0-0 = 1240/λinse (Fig.S11).
g
+
The oxidation potentials of the dyes were measured in CH
2
Cl
2
solutions with ferrocene/ferrocenium (Fc/Fc ) as an external reference and
converted to NHE.
h
The Ered was calculated according to Ered = Eox – E0-0
.
I
2
The dye loads are calculated from absorbance data of the sensitized TiO electrodes.
Table 2 Optimized Conformations Calculated with Gaussian 09 program package of the Selected Dyes
The UV-vis absorption spectra for the dye-loaded TiO film (4 µm) are shown in Fig. 2b. The
2
onset points of the absorption spectra are about 530 nm for T-1, 560 nm for T-2, 610 nm for T-3.
After being absorbed on TiO nanoparticle films, the absorption band of T-3 is broader by 50 nm
2
than that of T-2 and T-2 is broader by 30nm than that of T-1. Such bathochromic shift of the
absorption spectrum is due to the introduction of axial-symmetric conjugated group
(4-methoxyphenyl with ethynyl group) and the longer π-bridge. The absorption ranges of the three

dyes are all broadened to different degrees after anchoring on TiO nanoparticle film. The TiO2
2
surfaces play an important role in organizing and orienting the adsorbed dye molecules. The
broadening of the absorption spectrum of the three dyes on TiO might be due to dye-TiO2
2
interactions, dye-dye interactions, or both of them [46-47]. In general, strong interactions between
the adsorbed dye molecules and the TiO surface are known to lead to aggregate formation, thus
2
anchoring monolayer dyes onto the TiO2 surface and accomplishing higher solar energy
conversion efficiency of DSCC [48].
3
.3. Electrochemical properties
Cyclic voltammetry (CV) was also performed to study electrochemical properties of T-1, T-2
and T-3 as well as to estimate their feasibility of electron injection from electronically excited
dyes into the conduction band of TiO and dye regeneration by redox electrolytes. CV was carried
2
out in CH Cl solution (Fig. 3) and the relevant data was listed in Table 1. The ground and excited
2
2
states of the dyes were positioned at energy levels that have sufficient driving force for both
regeneration and electron injection as shown in Scheme 2. All redox potentials are converted on
the NHE scale. The Ered of T-1, T-2 and T-3 are -1.50, -1.46 and -1.52V vs. NHE, respectively,
which is more negative than the conducting band (CB) of TiO (- 0.5 V vs. NHE), indicating that
2
the excited state of dyes T-1, T-2 and T-3 could effectively inject electrons into the CB of TiO2
[
49]. Moreover, the ground-state oxidation potentials (E ) of T-1, T-2 and T-3 are 1.18, 1.19 and
ox
-
-
0
.95 V vs. NHE, respectively and more positive than the redox potentials of I /I (0.4 V vs. NHE),
3
ensuring ample driving force for dye regeneration [50]. The introduction of 4-methoxyphenyl with
ethynyl group to triphenylamine (T-1) promotes a little decrease in the oxidation potentials. Eox
value of T-3 is significant higher compared with that of T-1 and T-2 due to the change of π-bridge,

leading to a better regeneration of the dye after electron injection into the conduction band of TiO2.
Thus, T-3 can undergo the charge transfer easily with high coplanarity and higher Eox and this
might be part of the reason why T-3-based DSSC exhibited the best photovoltaic performances
[51-52]. Therefore T-1, T-2 and T-3 are qualified to be used in DSSC in theory.
-3
Fig. 3 Cyclic voltammograms of the dyes recorded for CH
2
Cl
2
solutions (10 M) with TBAPF
6
(0.1 M) as the electrolyte (working
+
+
electrode: plate Pt; reference electrode: Ag/Ag ; an auxiliary electrode: platinum wire; calibrated with Fc/Fc as an internal reference;
-1
scan rate: 100 mV s ).
Scheme 2 Representation of HOMO and LUMO levels of T-1, T-2 and T-3 dyes (Potential vs. NHE).
.
3
.4. Theoretical calculation
In order to further understand the structural properties of the dyes and the possibility of

charge transfer from donor to acceptor on electronic excitation, the electronic structure of the dyes
was analyzed by density functional theoretical (DFT) calculations using the Gaussian 09 program
package [53]. We optimized the molecular structure of T-1, T-2 and T-3 in the vacuo. The electron
densities of HOMO and LUMO of the dyes are shown in Table 3. It is clearly that the HOMOs are
delocalized between the donor and π-bridge. In the LUMO state, the electrons are mainly
delocalized over the sectional π-bridge and anchoring group. The well separation of electron at
HOMO and LUMO levels indicates the electron can efficiently transfer from the donor part to the
acceptor part once excited, consequently giving fast electron injection from the LUMO of dyes to
the CB of TiO . It is known that the geometry of acetylene is straight line shaped due to the sp
2
hybridization of the C atom and the electron cloud of p-orbitals distributes in a cylindrical shape.
As presented in Table 3, the axial-symmetric conjugated ethynyl group can effectively offset the
reduction of the conjugation effect caused by the stereoscopic effect and promote the effective
intramolecular charge transfer process.
We can also see from Table 1 and Scheme 2 that the introduction of 4-methoxyphenyl with
ethynyl group to triphenylamine or the longer π-bridge could change HOMO-LUMO energy gaps
of the dyes more narrowly resulting from the enhancement of molecular planar configurations
with axial-symmetric distribution of π-electron in acetylene and expansion of π-bridge.
Table 3 HOMO and LUMO of the dyes T-1, T-2 and T-3 calculated with DFT method

3
.5. Photovoltaic performance of DSSC
These molecules T-1, T-2 and T-3 have the potential to act as light harvesting dyes for DSSC
and the J-V and incident photon-to-current conversion efficiency (IPCE) plots obtained are shown
in Fig.4 and Fig.5. The measured photovoltaic parameters are summarized in Table 4.
Table 4 Photovoltaic parameters of the J-V curves for the DSSC under irradiation of AM 1.5G sunlight.
J
SC
V
OC
Dyes
FF (%)
PCE (%)
-2
mAcm
mV
T-1
T-2
T-3
7.47 (± 0.14)
8.95(± 0.21)
12.21(± 0.19)
643(± 3.23)
700(± 2.45)
732(± 2.79)
71.43(± 0.09)
72.08(± 0.05)
71.73(± 0.09)
3.43(± 0.02)
4.52(± 0.02)
6.37(± 0.03)
-2
Under standard global AM 1.5 solar simulator condition (100 mW cm ), a power conversion
2
efficiency (PCE) of 3.43% was obtained for DSSC based on control dye T-1 (JSC = 7.47 mA/cm ,
VOC = 643 mV and FF = 71.43 %). Moreover, DSSCs fabricated using T-2 and T-3 sensitized cells
2
gave JSC of 8.95, 12.21mA/cm , V of 700, 732 mV and FF of 72.08, 71.73 %, corresponding to
OC

PCE of 4.52% and 6.37%, respectively. Under the same fabrication conditions, the standard N719
dye showed an efficiency of 8.62%. As depicted in table 4, the PCE of T-2 reaches the maximum
value of 4.52%, which is 31.8% higher than that of T-1 (3.43%), demonstrating that the
photovoltaic performance is improved after introducing 4-methoxyphenyl with ethynyl group to
T-1. The PCE of T-3 (6.37%) is the highest among these three dyes because of the largest VOC and
JSC. The wide gaps between the VOC and JSC of T-3, T-2 and T-1 are also partially attributed to the
different ability to prevent dark current as proved by Fig. 4 [54]. On the other hand, with the
appearances of 4-methoxyphenyl with ethynyl group and longer of π-bridge, the dihedral angles in
T-2 and T-3 are 49.2° and 45.3°, respectively. Better molecular planar configurations,
axial-symmetric distribution of π-electron in acetylene and expansion of π-bridge of T-3 than that
of T-2 leads to broadening absorption spectrum and thus higher JSC was got for DSCC sensitized
by T-3.
Fig.5 shows the IPCE as a function of excitation wavelength. The onset of the IPCE spectrum
of the devices based on T-1, T-2 and T-3 is at 532 nm, 569 nm and 610 nm, respectively. It is
impressive that T-2 bestows very broader and higher IPCE values than T-1, while introducing
4
-methoxyphenyl with ethynyl group to T-1. The IPCE value of T-2 exceeds 70% from 360 to 510
nm, with the highest value (82%) at 410 nm. Compared with T-2, dye T-3 shows an obviously
broader response range (exceed 60% in 350-540 nm) with a maximum value of 74% at 490 nm.
Although the maximum IPCE values for T-3 was lower, its photoresponse range is much broader
than that of T-2, resulting in a much higher JSC for T-3 based DSCC. As seen in Table 1, the dye
-7
-7
-7
-2
loading amounts of T-1, T-2 and T-3 are 2.06 × 10 , 2.08 × 10 and 2.72 × 10 mol cm ,
respectively, and the adsorption amount are calculated from absorption data in Figure 2 [55]. The

measured number of the dyes loading on TiO is in the order of T-1 < T-2 < T-3, in according with
2
those of IPCE spectra and JSC values determined experimentally. The increased IPCE values of
T-2 and T-3 relative to that of T-1 are also attributed to the introduction of axial-symmetric
conjugated group to triphenylamine or the longer π-bridge, which is in agreement with the
absorption spectra of the dyes on TiO films.
2
Fig. 4 J-V characteristics of solar devices based on T-1, T-2 and T-3
Fig. 5 Photocurrent action spectra (IPCE) of solar devices based on T-1, T-2 and T-3
As known, the alternation of photovoltage VOC originates from the displacement of
quasi℃ Fermi℃ level (E ) in TiO , which intrinsically stems from the change of TiO conduction
f
2
2

band edge (E ) and/or fluctuation of electron density (charge recombination rate in DSSC) with
CB
same electrolyte [56]. In order to determine the energy levels of the electrode sensitized by
different dyes, electrochemical impedance spectroscopy (EIS) was performed on T-1, T-2 and T-3
and shown in Fig. 6. As presented in Fig. 6a, the chemical capacitance (C ) of T-2 or T-3 was
µ
increased linearly with the given bias potential, and all the curves exhibited the same slope. At
fixed potential close to their VOC (e.g. 0.70V), the order of C is T-2 > T-3 > T-1. Due to the
µ
density of occupied state (DOS) which is proportional to C increases with the downward shift of
µ
the TiO conduction band, the C is inversely proportional to the position of the conduction band
2
µ
[57]. Therefore, the position of the conduction band partially determines the size of the open
circuit voltage. C of T-2 is higher than that of T-3, ruling out the downward shift of the
µ
conduction band shift as the main reason for the decreased VOC for T-2. To take insight into the
electron recombination occurring between excited electrons in conduction band with sensitizers or
electrolyte, the electron lifetime was tested as a function of potential bias. At a given potential, the
electron lifetime in cell sensitized with T-3 was obviously the longest with iodine electrolyte (Fig.
6
b), while T-1 got the shortest one, which is exactly consistent with the sequence of V . The
OC
significant increase in electron recombination lifetime means that the charge recombination arising
-
from electrons in TiO CB with the I ion in electrolyte is suppressed by the introduction of the
2
3
4
-methoxyphenyl with ethynyl group and longer π-bridge, and the ICT process was promoted with
the help of axial-symmetric electrons in ethynyl group. Thus, we can conclude that introduction of
-methoxyphenyl with ethynyl group and the extension of the π-bridge based on T-1 as the
4
primary sensitizer, the charge recombination could be repressed effectively, and the devices could
succeed superior VOC performance from T-1 to T-3 [58].

Fig.6. Chemical capacitance (a) and electron lifetime (b) as a function of bias potential obtained through electrochemical impedance spectroscopy
carried on devices with iodine electrolyte.
The stability of T-1, T-2 and T-3 sensitized photovoltaic devices were investigated by storing at
room temperature and 40-60% humidity. As shown in Fig. 7, for T-1, T-2 and T-3, the
photoelectric conversion efficiency was approximately 67%, 78% and 86% of the initial value,
respectively, after 10 days. It is indicated that the introduction of the alkynyl group into donors for
T-2 and T-3 and the expansion of conjugated bridge for T-3 can effectively improve the
photothermal stability of the sensitizers. Especially for T-3, after a short degradation in the first
three days, then the PCE stabilized for a longer period of time. By tracking their VOC and J , it
SC
was found that JSC is the determinant factor for the degradation of the PCE due to their almost
same variation tendency. The decrease in current density maybe related to the photo degradation

of trace dye. This stability test also provides a theoretical guidance for our future structural design.
Fig. 7 Stability test showing the variation in the photovoltaic parameters (Normalized PCE, VOC, and JSC) with ageing time for devices with T-1, T-2
and T-3 and with PCBM treatment (red line) under room temperature and 40 60% humidity.
-
4
. Conclusions
In summary, two novel metal-free organic dyes with ethynyl group having an
axial-symmetric conjugated system, named T-2 and T-3 were synthesized and applied in DSCC.
Introduction of the 4-methoxyphenyl with ethynyl group and the extension of π-bridge to T-1, the
decrease of dihedral angles between the benzene and diphenylamine increase the coplanar
property between donor and π-bridge, which is in favor of the intermolecular charge transfer. The
absorption spectra, short-circuit photocurrent densities (J ) and open-circuit photovoltages (Voc)
SC
were improved to a large extent. Under AM 1.5G irradiation, the energy conversion efficiencies of
DSSCs based on T-3 and T-2 are up to 6.37% and 4.52% respectively. Electrochemical impedance
spectroscopy (EIS) analysis reveals that the introduction of the 4-methoxyphenyl with ethynyl
group or the extension of π-bridge could suppress the charge recombination arising from electrons
-
in TiO film and the I ion in electrolyte and promote the ICT process, thus improving the V of
2
3
OC

DSCC considerably. This study provides useful information for further design and synthesis of
DSSC sensitizers.
Acknowledgments
This work was financed by National Natural Science Foundation of China (21603053), the
Program of Science and Technology of Hebei (17211412) and the Project of Hebei University of
Science and Technology (1182120). Wenjun Wu thanks the financial support of National Natural
Science Foundation of China (21676087) and the Scientific Committee of Shanghai
(18160723400).
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