Efficient Small Molecule Organic Dyes Containing Different Bridges in Donor Moieties for Dye-Sensitized Solar Cells

Article abstract of DOI:10.1016/j.electacta.2017.08.089

Small molecule (molecular weight is around 500 Da) organic dyes were synthesized and used as sensitizers for the dye-sensitized solar cells (DSSCs). The optical and electrochemical properties, theoretical studies and photovoltaic parameters of DSSCs baseed on these dyes were systematically investigated. In order to extend the conjugation extent, two extra phenyl units were attached to triphenylamine (TPA) units bridged with single bonds, double bonds or triple bonds, respectively. The DSSCs based on FWDS exhibited well performances: the short-circuit current density (J_sc) of 10.04, 10.91 and 9.12 mA cm^?2, respectively; the open circuit voltage (V_oc) of 0.77, 0.75 and 0.78 V, respectively; the power conversion efficiency (η) of 5.65%, 6.04% and 5.19%, respectively. Two extra phenyl units attached via double or triple bonds increase the planar nature of these molecules, increasing the loading amount, which ultimately can correlate into the received efficiency. The results indicated that it is an effective way to get efficient dyes by introducing extra phenyl rings into donor moiety especially adopting double bonds as the bridge. Finally, efficient DSSCs were achieved by using small molecule TPA-type dyes FWD1, FWD2 and FWD3 as the sensitizers.

Full text of DOI:10.1016/j.electacta.2017.08.089

Accepted Manuscript
Title: Efficient Small Molecule Organic Dyes Containing
Different Bridges in Donor Moieties for Dye-Sensitized Solar
Cells
Authors: Haobin Wang, BenZhou Bao, Xiangyu Hu, Jing-Kun
Fang
PII:
S0013-4686(17)31730-9
DOI:
Reference:
http://dx.doi.org/10.1016/j.electacta.2017.08.089
EA 30093
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Revised date:
Accepted date:
16-7-2017
13-8-2017
14-8-2017
Please cite this article as: Haobin Wang, BenZhou Bao, Xiangyu Hu,
Jing-Kun Fang, Efficient Small Molecule Organic Dyes Containing Different
Bridges in Donor Moieties for Dye-Sensitized Solar Cells, Electrochimica
Actahttp://dx.doi.org/10.1016/j.electacta.2017.08.089
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Efficient Small Molecule Organic Dyes Containing Different Bridges
in Donor Moieties for Dye-Sensitized Solar Cells
Haobin Wang, BenZhou Bao, Xiangyu Hu, Jing-Kun Fang*
Department of Chemistry, School of Chemical Engineering, Nanjing University of Science and
Technology, Xiaolingwei Street No. 200, Nanjing 210094, China
E-mail of corresponding author Jing-Kun Fang: fjk@njust.edu.cn
Graphical abstract
1

Abstract
Small molecule (molecular weight is around 500 Da) organic dyes were synthesized and used as
sensitizers for the dye-sensitized solar cells (DSSCs). The optical and electrochemical properties,
theoretical studies and photovoltaic parameters of DSSCs baseed on these dyes were
systematically investigated. In order to extend the conjugation extent, two extra phenyl units were
attached to triphenylamine (TPA) units bridged with single bonds, double bonds or triple bonds,
respectively. The DSSCs based on FWDS exhibited well performances: the short-circuit current
-2
density (Jsc) of 10.04, 10.91 and 9.12 mA cm , respectively; the open circuit voltage (Voc) of 0.77,
0.75 and 0.78 V, respectively; the power conversion efficiency (η) of 5.65%, 6.04% and 5.19%,
respectively. Two extra phenyl units attached via double or triple bonds increase the planar nature
of these molecules, increasing the loading amount, which ultimately can correlate into the received
efficiency. The results indicated that it is an effective way to get efficient dyes by introducing extra
phenyl rings into donor moiety especially adopting double bonds as the bridge. Finally, efficient
DSSCs were achieved by using small molecule TPA-type dyes FWD1, FWD2 and FWD3 as the
sensitizers.
Keywords: Dye-sensitized solar cells; Organic dyes; Small molecule dyes; Triphenylamine;
Photovoltaic performances
2

1 Introduction
Photosensitizers for Dye-Sensitized Solar Cells (DSSCs) have been widely developed since DSSC
was firstly reported by Grätzel [1]. Even though the Ruthenium-based complex are regarded as the
most efficient and successful sensitizers [2-4], issues such as containing heavy metal and high cost
limit their application. Organic dyes have been extensively investigated for DSSCs for the sake of
their environmental safety and low cost [5-9]. Triphenylamine (TPA) unit is widely applied in
DSSCs as an excellent building block for its electron donating property and pyramidal structure
[10-13]. Wielopolski and coworkers reported that the extended conjugation of TPA as a donor
would make the LUMO+1 shift to lower energy levels and give a lower energy gap between
LUMO and LUMO+1 resulting in wider absorption spectra [14]. Thus, many extended conjugation
TPA-type dyes were developed with well light harvesting capabilities and high power conversion
efficiencies [15-17]. Both single bonds and multiple bonds could be options for bridging the
different conjugation units [18-20]. Due to the lack of coplanarity of the connecting parts, dye
aggregation could be suppressed by adopting single-bond-bridge, which could improve the device
performance efficiently [12, 21]. However, the lack of coplanarity would meanwhile limit the
absorption spectra range and hence restrict the utilization of the solar energy [22, 23]. This problem
can be avoided by adopting multiple-bond-bridge (double or triple bond) leading to an extended
conjugation range [24, 25].
Scheme 1. Molecular structures of TPA-type dyes
3

2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid (TC), as the simplest TPA-type dye, has
been reported by Teng and coworkers [26]. Obviously, the scant conjugation extent of TC would
restrict its light harvesting capability and hence low power conversion efficiency was observed
(2.6%). In this work, based on dye TC, three conjugation extended TPA-type dyes (FWD1, FWD2
and FWD3) were designed and synthesized by introducing two extra phenyl units to TPA units
bridged with single bonds, double bonds or triple bonds, respectively (Scheme 1). The extra phenyl
units were introduced into donor moiety aiming to reduce to energy gap, and we want to investigate
the different effects caused by adopting different kind of bridges to the donor part. The structures
of FWDs are shown in Scheme 1. The photophysical and electrochemical properties of FWDs as
well as their applications in DSSCs are reported.
2 Results and discussion
2.1 Designand synthesis of dyes
Scheme 2. Synthetic routes of dyes FWDs.
4

Scheme 2 depicts the synthetic routes for FWDs. Intermediate 1~3 could be easily synthesized
from 4-(bis(4-iodophenyl)amino)benzaldehyde as the starting material by a Suzuki reaction with
phenylboronic acid, a Heck reaction with styrene or a Sonogashira reaction with phenylacetylene,
respectively. Then the aldehyde 1~3 reacted with cyanoacetic acid would give the desired dyes
FWD1, FWD2 and FWD3 by Knoevenagel reaction. All of the procedure gave the desired
1
products in good yields. Dyes FWD1, FWD2 and FWD3 were fully characterized with H NMR,
13C NMR and HRMS, etc.
2.2 UV-Vis absorption spectra
Fig. 1. UV-Vis absorption spectra of FWDs in EtOH solutions (a) and adsorbed on TiO2 films (b).
The absorption spectra of FWDs were measured to study their light harvesting capabilities. The
UV-vis absorption spectra of FWDs in EtOH solutions at a concentration of 5×10-5 M and
adsorbed on 2 μm transparent TiO2 films are shown in Fig. 1. The corresponding data are listed in
Table 1.
Table 1
Optical and electrochemical data of FWDs.
λabs/nma
λabs/nm
Dye
Eox/Vb
E0-0/Vc
Ered/Vd
μ/Debye Band gap e
-1
(ε/104 M-1 cm ) (TiO2)
410 (2.30)
414
FWD1
FWD2
FWD3
1.30
1.09
1.35
2.46
2.34
2.48
-1.16
-1.25
-1.13
8.38
8.31
8.06
3.06
2.83
2.90
332 (1.69)
424 (3.12)
375 (4.11)
424
410 (3.06)
355 (4.16)
413
a Measured in EtOH solution.
b First oxidation peak potentials (Eox) (vs NHE) were calibrated with ferrocene (0.63 V vs NHE).
c E0-0 transition energy measured at the onset of absorption spectra.
d Ered = Eox - E0-0
e DFT/B3LYP calculated values.
.
5

Two major strong absorption bands of FWDs in the visible region were observed, which can
be ascribed to the localized aromatic π-π* transition of the TPA moieties and the intramolecular
charge transfer (ICT) from the donor (TPA) to the acceptor (cyanoacetic acid) with some
delocalized π-π* transition character [27]. The absorption maximum (λabs) of FWD2 (424 and 375
nm), adopting double bonds as the bridge between the phenyl and triphenylamine units, exhibited
obvious bathochromic shifts compared with that of FWD1 (410 and 332 nm) adopting single
bonds as the bridge and FWD3 (410 and 355 nm) adopting triple bonds as the bridge, indicates
that double bond is an efficient unit to enlarge the adsorption range. FWD2 and FWD3 showed a
significantly higher molar extinction coefficient (ε) compared to that of FWD1, indicating
improved light harvesting capabilities for FWD2 and FWD3 due to their better coplanarity, which
always results in a wider adsorption range and higher molar extinction coefficient [28].
In general, hypsochromic shift would be observed after attaching dyes on TiO2 films due to
the H-aggregation (plane-to-plane π stacking) of the dyes on semiconductor surface [29]. The
absorption maxima for FWDs on 2 μm TiO2 transparent films were 414, 424 and 413 nm,
respectively. The data were almost same with that in EtOH and indicated that no H-aggregations
for FWDs on TiO2 films were observed. The reason for this result should be the pyramidal
structure of the triphenylamine moiety that avoid the issue of plane-to-plane π-stacking.
2.3 Electrochemical properties
The Cyclic voltammetry was performed to investigate the electrochemical properties of FWDs
and also the possibilities of electron injections from the excited dyes into TiO2 conduction band
and dye regenerations by redox electrolytes [26]. The cyclic voltammograms are represented in
Fig. 2 and the relevant data are summarized in Table 1.
Fig. 2. CV plots of FWDs.
Fig. 3. The energy band diagram of FWDs.
6

The first oxidation peak potentials (Eox) (refers to the HOMO level) of FWDs were 1.30, 1.09
and 1.35 V, respectively. The results are summarized in energy band diagrams in Fig. 3. The values
were more positive than the I-3/I- redox potential value (0.4 V vs NHE), which guarantees efficient
dye regeneration of the oxidized dyes by the electrolyte [30]. Eox value of FWD2 is significant
lower compared with that of FWD1 and FWD3, which could be attributed to its double bonds
moiety. It suggests that FWD2 can be easily excited to its higher energy level. Thus, FWD2 can
undergo the charge transfer easily and this might be part of the reason why FWD2-based DSSC
exhibited the best photovoltaic performances.
The band gap energies (E0-0) of FWDs were 2.46, 2.34 and 2.48 V, respectively, which were
estimated from the onset wavelengths in the absorption spectra of the dyes in EtOH [31]. Ered
(refers to the LUMO level) can be calculated from Eox - E0-0. The Ered of FWDs (-1.16, -1.25 and
-1.13 V, respectively) were sufficiently more negative than the Fermi level of TiO2 (-0.5 V vs
NHE), providing the thermodynamic feasibility of electron injection from the excited dye
molecules into the conduction band of the TiO2 semiconductor [30]. These results clearly
demonstrated that FWDs could be potentially efficient sensitizers to be used in DSSCs.
2.4 Theoretical calculation
To investigate the geometrical structures and electron distributions of FWDs, the geometries and
energies of the dyes were optimized by density functional theory (DFT) calculations with the
B3LYP exchange correlation functional under the 6-31G (d) basis set implemented in the Gaussian
09 program. The optimized structure and electron distribution of FWDs are shown in Fig. 4, with
the isodensity surface values fixed at 0.02.
Fig. 4. The optimized structure and electron distribution of the HOMO and LUMO of FWDs.
According to the corresponding molecular orbital profiles, the HOMOs of these dyes
uniformly distributed over the whole molecules, while the LUMOs delocalized over the electron
7

accepter parts. It means that the electrons can easily transfer from the electron donor part to the
acceptor part and hence an efficient electron injection from the excited dyes into the acceptor
group-connected TiO2. In the optimized structures of FWD1, the two benzene rings of the biphenyl
part could not be coplanar, which restricted the conjugation extent. For FWD2 and FWD3, the
two benzene rings of the diphenylethene and diphenylethyne parts were parallel, which would
exhibit an extended conjugation range and hence the higher molar extinction coefficients. The
smallest calculated band gap (LUMO-HOMO) of FWD2 is well coincidence with the λabs of
FWD2 (Table 1). FWD3 showed the smallest dipole moment (μ = 8.06 Debye) compared with
FWD1 (8.38 Debye) and FWD2 (8.31 Debye). The small dipole moment might be adverse to
intramolecular charge transfer from donor to acceptor part resulting in the lowest power conversion
efficiency for FWD3-based DSSC [27].
2.5 Photovoltaic properties
Fig. 5. J-V curves (a) and IPCE plots (b) of DSSCs based FWDs.
Table 2
Photovoltaic performances parameters and simulated EIS data of DSSCs based on FWDs.
-2
-2
-2
-2
-2
Dye
Jsc/mA cm
Voc/V
0.77
FF
η/%
5.65
6.04
5.19
2.60
Γ/mol cm τe/ms
Rs/Ω cm RCE/Ω cm Rrec/Ω cm
FWD1
FWD2
FWD3
TCa
10.04
10.91
9.12
5.26
0.73
0.74
0.73
0.78
1.74×10-7 96.8
2.17×10-7 96.8
1.95×10-7 179.7
7.1
11.7
8.9
14.2
-
362.6
382.0
501.9
-
0.75
0.78
0.64
15.6
10.7
-
-
-
a See ref. [26].
Fig. 5a shows the current density-voltage (J-V) curves of the DSSCs sensitized with FWDs under
AM 1.5G irradiation (1 sun, 100 mW cm-2), the cells were measured without mask in a 0.16 cm2
working area. Their corresponding photoelectrochemical properties, namely short-circuit current
8

density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (η) data
are listed in Table 2. FWD2-based DSSC exhibited the highest short-circuit photocurrent density
-2
(Jsc) of 10.91 mA cm while the Jsc values for FWD1-based and FWD3-based DSSCs were 10.04
-2
and 9.12 mA cm , respectively. High open circuit voltage (Voc) values were observed for these
DSSCs (0.77 V for FWD1, 0.75 V for FWD2 and 0.78 V for FWD3). As a result, excellent power
conversion efficiencies (η) were achieved by FWDs-based DSSCs (5.65% for FWD1, 6.04% for
FWD2 and 5.19% for FWD3). The best power conversion efficiency for FWD2-based DSSC was
mainly because of its highest short-circuit photocurrent density, which was attributed to the widest
absorption spectrum for FWD2 and also probably the highest dye loading amount (Γ) of FWD2
on TiO2 [32]. All of the DSSCs based on FWDs showed significant improved performances
compared with that of TC. The results indicated that it is an effective way to get efficient dyes by
introducing extra phenyl rings into donor moiety, especially adopting double bonds as the bridge.
The IPCE (Incident Photon-to-current Conversion Efficiency) spectra of the DSSCs exhibited
response in the region of 300-600 nm and showed plateau nearly 80% in the range from 400 to
510 nm (Fig. 5b). The broadest IPCE response range of FWD2-based DSSC explained the highest
Jsc value of it from the J-V measurement, which is consistent with the widest absorption spectrum
of FWD2.
2.6 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) of the DSSCs in dark under an applied dc voltage
equivalent to Voc of the device were measured to further determine photovoltaic properties of
FWDs. Nyquist plots and Bode phase plots of DSSCs based on FWDs and the equivalent circuit
are shown in Fig. 6 and the relevant data are listed in Table 2.
Fig. 6. Nyquist plots (a) and Bode plots (b) of the DSSCs based on FWDs and the equivalent circuit (c).
9

-2
-2
The series resistance (Rs) values of these three cells (7.1 Ω cm for FWD1, 15.6 Ω cm for
-2
FWD2 and 10.7 Ω cm for FWD3) are similar because of the same electrolyte and electrode in
both materials and surface area [33]. The small charge values of transfer resistances at the interface
-2
-2
-2
of Pt/electrolyte (RCE) (11.7 Ω cm for FWD1, 8.9 Ωcm for FWD2 and 14.2 Ω cm for FWD3)
represented the rapid reduction of I-3 ions on Pt counter electrode [34]. The larger semicircle at
lower frequencies indicates charge recombination resistance at the TiO2/dye/electrolyte (Rrec),
which give information about the charge transfer at TiO2/Dye/Electrolyte interface. Large Rrec
-2
-2
-2
values of these three cells (362.6 Ω cm for FWD1, 382.0 Ω cm for FWD2 and 501.9 Ω cm for
FWD3) guaranteed high open circuit voltage [34]. The electron lifetime (τe) can be estimated
according to τe = 1/2πf [35], while f is the peak frequency at lower frequency region in the Bode
phase plots (1.6 Hz for FWD1, 1.6 Hz for FWD2 and 0.9 Hz for FWD3) corresponds to the
TiO2/dye/electrolyte interface. Typically, higher τe value would lead to a higher Voc [36]. FWD3
showed the highest τe (179.7 ms). As a result, the highest Voc was observed for FWD3-based
DSSC.
3 Experimental
3.1 Reagents and materials
All reagents obtained from commercial sources were used as received, unless otherwise noted.
Toluene and diisopropylamine were distilled from CaH2 before used. FTO conductive glasses
(sheet resistance 7 Ω/sq, 2.2 mm thick) were used for DSSCs fabrication. Electrolyte for DSSCs
fabrication: LiI (0.1 M), I2 (0.1 M), 4-tert-butylpyridine (1.0 M) and 1,2-dimethyl-3-
propylimidazolium iodide (0.6 M) in acetonitrile.
3.2 Instruments and characterization
10

1
Melting points were measured on an X-4A apparatus. H NMR and 13C NMR spectra were
recorded at room temperature on Bruker DRX300 or Bruker AVANCE III 500 instruments and
calibrated with tetramethylsilane (TMS) as an internal reference. HRMS were recorded by Waters
Q-TOF MicroTM. UV-Vis absorption spectra were recorded by THERMO FISHER
EVOLUTION220 at room temperature. Cyclic voltammetry measurements of dyes were carried
out with a Versa STAT3 electrochemical workstation in tetrahydrofuran (THF) (1.0×10−3 M)
containing 0.1 M Bu4NPF6 as the supporting electrolyte, and a three-electrode system (glassy
carbon as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as reference
electrode). Ferrocene was used as the external standard. The scan rate was 50 mV/s. The IPCE
were measured by PECCELL PEC-S20 action spectrum measurement with monochromatic
-2
incident light under 100 mW cm with bias light in DC mode (150 W xenon lamp). Current
density-voltage measurements were carried out using simulated 1.5 AM sunlight with an output
-2
power of 100 mW cm (PECELL-L15). Electrochemical impedance spectroscopy (EIS) were
measured by Zahner Zennium Impedance Analyzer in the frequency range of 10-1-105 Hz under 1
sun bias illumination under an open-circuit condition. The amount of dye load was measured by
desorbing the dye from the films with 0.1 M tetrabutylammonium hydroxide in THF/H2O (1:1)
and measuring the corresponding UV-vis spectrum.
3.3 Synthesis of sensitizers
All reactions were carried out under an atmosphere of nitrogen unless otherwise noted. 4-(bis(4-
iodophenyl)amino)benzaldehyde was synthesized according to the literature [27].
3.3.1 4-(di([1,1'-biphenyl]-4-yl)amino)benzaldehyde (1)
To a round-bottom flask were added 4-(bis(4-iodophenyl)amino)benzaldehyde (1470 mg, 2.80
mmol), Pd(PPh3)4 (325 mg, 0.28 mmol), K2CO3 (3100 mg, 22.4 mmol) and phenylboronic acid
11

(821 mg, 6.72 mmol), 1,4-dioxane (32 mL) and water (8 mL), and the mixture was stirred at 80°C
overnight. After cooling, the reaction mixture was extracted with dichloromethane. The organic
layer was dried over anhydrous Na2SO4. After filtration, the solvents were removed by rotary
evaporation. The crude product was chromatographed on silica gel eluting with petroleum ether
and dichloromethane (1:1) to afford 1010 mg (84.8%) of 1 as a yellow solid.
M.p.: 101.7-102.7°C. 1H NMR (300 MHz, CDCl3) δ: 7.14 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.7 Hz,
4H), 7.35 (t, J = 7.5 Hz, 2H), 7.45 (t, J = 7.4 Hz, 4H), 7.57-7.61 (m, 8H), 7.73 (d, J = 8.7 Hz, 2H),
9.84(s, 1H).
3.3.2 4-(bis(4-styrylphenyl)amino)benzaldehyde (2)
To a round-bottom flask were added 4-(bis(4-iodophenyl)amino)benzaldehyde (1470 mg, 2.80
mmol), Pd(PPh3)4 (325 mg, 0.28 mmol), K2CO3 (3100mg, 22.4 mmol) and styrene (1170 mg, 6.72
mmol), 1,4-dioxane (32 mL) and water (8 mL), and the mixture was stirred at 90°C overnight.
After cooling, the reaction mixture was extracted with dichloromethane. The organic layer was
dried over anhydrous Na2SO4. After filtration, the solvents were removed by rotary evaporation.
The crude product was chromatographed on silica gel eluting with petroleum ether and
dichloromethane (1:1) to afford 1220 mg (91.2%) of 2 as a yellow solid.
M.p.: 114.4-115.2°C. 1H NMR (500 MHz, CDCl3) δ: 7.08 (s, 2H), 7.09 (s, 2H), 7.12 (d, J = 8.5
Hz, 2H), 7.16 (d, J = 8.5 Hz, 4H), 7.28 (d, J = 8.5 Hz, 2H), 7.37 (t, J = 7.5 Hz, 4H), 7.48-7.52 (m,
8H), 7.72 (d, J = 8.5 Hz, 2H), 9.84(s, 1H).
3.3.3 4-(bis(4-(phenylethynyl)phenyl)amino)benzaldehyde (3)
To a round-bottom flask were added 4-(bis(4-iodophenyl)amino)benzaldehyde (1470 mg, 2.80
mmol), Pd(PPh3)2Cl2 (196 mg, 0.28 mmol), CuI (53 mg, 0.10 mmol) and phenylacetylene (600 mg,
6.16 mmol), toluene (40 mL) and diisopropylamine (5 mL), and the mixture was stirred at room
12

temperature overnight. Then the reaction mixture was extracted with dichloromethane. The
organic layer was dried over anhydrous Na2SO4. After filtration, the solvents were removed by
rotary evaporation. The crude product was chromatographed on silica gel eluting with petroleum
ether and dichloromethane (1:1) to afford 1250 mg (94.0%) of 3 as a yellow solid.
1
M.p.: 77.9-78.8°C. H NMR (300 MHz, CDCl3) δ: 7.11-7.15(m, 6H), 7.34-7.36 (m, 6H), 7.48-
7.54 (m, 8H), 7.74 (d, J = 9.0 Hz, 2H), 9.86(s, 1H).
3.3.4 2-cyano-3-(4-(di([1,1'-biphenyl]-4-yl)amino)phenyl)acrylic acid (FWD1)
To a round-bottom flask were added 1 (766 mg, 1.80 mmol), CH3COONH4 (333 mg, 4.32 mmol),
cyanoacetic acid (291 mg, 3.42 mmol) and acetic acid (40 mL) was heated to reflux for 5 h. After
cooling, the mixture was poured into 200 mL of water. After filtration, the residue was
recrystallized with CH2Cl2 to afford 476 mg (53.8%) of FWD1 as a red solid.
M.p.: 152.0-152.9°C. 1H NMR (300 MHz, DMSO) δ: 7.03 (d, J = 8.7 Hz, 2H), 7.31 (d, J = 8.4 Hz,
4H), 7.36 (d, J = 7.2 Hz, 2H), 7.47 (t, J = 7.7 Hz, 4H), 7.67-7.75 (m, 8H), 7.97 (d, J = 8.7 Hz, 2H),
8.17(s, 1H). 13C NMR (125 MHz, DMSO) δ: 99.42, 117.28, 119.55, 123.93, 126.36, 126.51,
127.47, 128.21, 128.02, 132.82, 136.03, 139.33, 144.70, 151.22, 152.85, 164.46. HRMS (ESI-)
-
m/z: calcd for (M-H)- C34H23N2O2 :491.1765, found: 491.1768.
3.3.5 3-(4-(bis(4-styrylphenyl)amino)phenyl)-2-cyanoacrylic acid (FWD2)
To a round-bottom flask were added 2 (860 mg, 1.80 mmol), CH3COONH4 (333 mg, 4.32 mmol),
cyanoacetic acid (291 mg, 3.42 mmol) and acetic acid (40 mL) was heated to reflux for 5 h. After
cooling, the mixture was poured into 200 mL of water. After filtration, the residue was
recrystallized with CH2Cl2 to afford 411 mg (42.0%) of FWD2 as a red solid.
M.p.: 154.0-155.1°C. 1H NMR(300 MHz, DMSO) δ: 7.01 (d, J = 8.7 Hz, 2H), 7.18-7.31(m, 10H),
7.38 (t, J = 7.5 Hz, 4H), 7.59-7.67 (m, 8H), 7.96 (d, J = 9.0 Hz, 2H), 8.17 (s, 1H) 13.62 (br, 1H).
13

13C NMR (125 MHz, DMSO) δ: 98.57, 117.07, 119.70, 123.85, 126.04, 126.54, 127.63, 128.06,
128.39, 128.78, 132.93, 134.18, 137.09, 144.53, 151.22, 153.26, 164.18. HRMS (ESI-) m/z: calcd
-
for (M-H)- C38H27N2O2 :543.2078, found: 543.2084.
3.3.6 3-(4-(bis(4-(phenylethynyl)phenyl)amino)phenyl)-2-cyanoacrylic acid (FWD3)
To a round-bottom flask were added 3 (1040 mg, 2.20 mmol), CH3COONH4 (407 mg, 5.28 mmol),
cyanoacetic acid (356 mg, 4.18 mmol) and acetic acid (48 mL) was heated to reflux for 5 h. After
cooling, the mixture was poured into 200 mL of water. After filtration, the residue
chromatographed on silica gel eluting with dichloromethane and methanol (30:1 to 10:1) to afford
610 mg (51.3%) of FWD3 as a red solid.
M.p.: 152.3-153.0°C. 1H NMR(500 MHz, DMSO) δ: 7.10-7.14 (m, 6H), 7.42-7.43 (m, 6H), 7.54-
7.55 (m, 8H), 7.89 (d, J = 8.5 Hz, 2H), 7.98 (s, 1H). 13C NMR(125 MHz, DMSO) δ: 89.13, 89.43,
118.12, 122.22, 122.40, 125.04, 126.50, 128.78, 131.33, 132.00, 133.05, 145.83, 149.38, 150.04,
164.00. HRMS (ESI-) m/z: calcd for (M-H)- C38H23N2O2-:539.1765, found: 539.1772.
3.4 Fabrication of DSSCs
Working electrodes (4*4 mm, 0.16 cm2) with a 15 μm thick transparent TiO2 (20 nm) layer and a
5 μm thick TiO2 (200 and 400 nm) scattering layer were heated at 80 °C for 30 min before using.
The electrodes were immersed in the dye solutions (0.3 mM in CHCl3) for 12 h. Then the
electrodes were flushed with EtOH and dried in air for use. Liquid electrolytes, which consists of
LiI (0.1 M), I2 (0.1 M), 4-tert-butylpyridine (1.0 M) and 1,2-dimethyl-3-propylimidazolium iodide
(0.6 M) in acetonitrile, was added dropwise onto the electrodes. The counter electrodes and
working electrodes were sandwiched to be dye-sensitized solar cells without mask for the
measurements.
4 Conclusions
14

In summary, three small molecule (molecular weight is around 500 Da) organic dyes (FWD1,
FWD2 and FWD3) were synthesized by introducing two extra phenyl units to TPA units of TC
bridged with single bonds, double bonds or triple bonds, respectively. The optical and
electrochemical properties, theoretical studies and photovoltaic parameters of DSSCs baseed on
these dyes were systematically investigated. The DSSCs based on FWDs exhibited well
-2
performances: the short-circuit current density (Jsc) of 10.04, 10.91 and 9.12 mA cm ,
respectively; the open circuit voltage (Voc) of 0.77, 0.75 and 0.78 V, respectively; the power
conversion efficiency (η) of 5.65%, 6.04% and 5.19%, respectively. Two extra phenyl units
attached via double or triple bonds increase the planar nature of these molecules, increasing the
loading amount, which ultimately can correlate into the received efficiency. The results indicated
that it is an effective way to get efficient dyes by introducing extra phenyl rings into donor moiety,
especially adopting double bonds as the bridge. Finally, efficient DSSCs were achieved by using
small molecule TPA-type dyes FWD1, FWD2 and FWD3 as the sensitizers.
Acknowledgment
This work was supported by the Natural Science Foundation of Jiangsu Province (BK20140780).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://XXXXXXXXXXXXX.
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Figure captions
Fig. 1. UV-Vis absorption spectra of FWDs in EtOH solutions (a) and adsorbed on TiO2 films
(b).
Fig. 2. CV plots of FWDs.
Fig. 3. The energy band diagram of FWDs.
Fig. 4. The optimized structure and electron distribution of the HOMO and LUMO of FWDs.
Fig. 5. J-V curves (a) and IPCE plots (b) of DSSCs based FWDs.
Fig. 6. Nyquist plots (a) and Bode plots (b) of the DSSCs based on FWDs and the equivalent
circuit (c).
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Fig 1
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Fig 2
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Fig 3
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Fig 4
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Fig 5
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Fig 6
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