Organic dyes with imidazole derivatives as auxiliary donors for dye-sensitized solar cells: Experimental and theoretical investigation

Article abstract of DOI:10.1016/j.dyepig.2013.12.024

In order to study the influence of different imidazole derivatives in triphenylamine-based organic dyes, two different imidazole derivatives are introduced into the phenyl ring of the triphenylamine core, coded as CD-4 and CD-6, respectively. Their photophysical, electrochemical properties and the performances of the corresponding dye-sensitized solar cells (DSSCs) are further investigated. Due to the better molar extinction coefficient, quantum efficiency (QE) and longer lifetime of excited electron, the DSSC based on CD-4 has the higher overall conversion efficiencies as 4.11% than that of CD-6 as 1.51% under full sunlight (AM 1.5G, 100 mW cm^-2) irradiation. Density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations were carried out to study the ground state structures, the electronic structures and the optical properties of the two dyes. The simulated UV-vis absorption spectra for the two dyes are in excellent agreement with the experimental results.

Full text of DOI:10.1016/j.dyepig.2013.12.024

Dyes and Pigments 104 (2014) 48e56
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Organic dyes with imidazole derivatives as auxiliary donors for
dye-sensitized solar cells: Experimental and theoretical investigation
,
a
b
Ximing Chen a, Chunyang Jia a , Zhongquan Wan , Xiaojun Yao
*
a State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science
and Technology of China, Chengdu 610054, PR China
b State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to study the influence of different imidazole derivatives in triphenylamine-based organic dyes,
two different imidazole derivatives are introduced into the phenyl ring of the triphenylamine core, coded
as CD-4 and CD-6, respectively. Their photophysical, electrochemical properties and the performances of
the corresponding dye-sensitized solar cells (DSSCs) are further investigated. Due to the better molar
extinction coefficient, quantum efficiency (QE) and longer lifetime of excited electron, the DSSC based on
CD-4 has the higher overall conversion efficiencies as 4.11% than that of CD-6 as 1.51% under full sunlight
(AM 1.5G, 100 mW cmꢀ2) irradiation. Density functional theory (DFT) and time dependent density
functional theory (TD-DFT) calculations were carried out to study the ground state structures, the
electronic structures and the optical properties of the two dyes. The simulated UVevis absorption spectra
for the two dyes are in excellent agreement with the experimental results.
Received 26 September 2013
Received in revised form
20 December 2013
Accepted 24 December 2013
Available online 3 January 2014
Keywords:
Dye-sensitized solar cells
Organic dyes
Triphenylamine
Ó 2014 Elsevier Ltd. All rights reserved.
Imidazole derivatives
Cyanoacetic acid
Density functional theory
1. Introduction
ruthenium dyes need noble metal Ru which is scarce and disad-
vantage to environmental protection.
Since the seminal work of dye-sensitized solar cells (DSSCs) was
reported in 1991 by Grätzel and O’Regan, it has attracted the
considerable attention of many research groups owing to their high
efficiencies and low costs [1e3]. DSSCs typically contain four
components: a mesoporous semiconductor metal oxide film, a dye,
an electrolyte/hole transporter, and a counter electrode [4]. As a
critical component in DSSCs, the dye sensitizers play a vital role in
providing electron injection into conduction band (CB) of TiO2 upon
light irradiation. Nowadays, DSSCs based on ruthenium dyes and
porphyrin dyes have shown very impressive solar to electric power
conversion efficiencies. The DSSC based on black dye with donore
acceptor type coadsorbent has reached an overall solar energy
For the above reasons, lots of efforts have been dedicated to the
development of metal-free organic dyes due to their high molecular
extinction coefficients, simple preparations, low costs, and envi-
ronment friendly [7,8]. Up to now, many different types of organic
dyes have been developed as the sensitizer for DSSCs. The DSSCs
based on merocyanine [9,10], coumarin [11e13], indoline [14e16],
squaraine [17,18], hemicyanine [19,20], phenothiazine [21e24],
triphenylamine [25e27], fluorene [16,28,29], carbazole [30e32],
tetrahydroquinoline [33e35] and benzo[1,2-b:4,5-b0]difuran (BDF)
[36] have been developed and shown good performances. In
particular, it encourages to note that a promising
h
up to 10.3% has
been demonstrated by Wang et al. [37].
conversion efficiency (
h) of 11.4% [5], and a new record efficiency of
Most organic dyes are consisted of electron donor, p spacer and
12.3% has been obtained by co-sensitization of the porphyrin dye
YD2-o-C8 and the organic dye Y123 [6]. Although ruthenium and
porphyrin dyes have high efficiencies, the large-scale application of
them is limited due to many practical issues. For example, the
synthesis and purification of ruthenium and porphyrin dyes are
very complicated and the yields of them are very low, especially the
acceptor. However, the rod-like molecules are elongated, which
may facilitate the recombination of electrons with the triiodide and
the formation of aggregates between the neighbor dye molecules
[38]. Therefore, organic dyes with a starburst conformation were
designed and synthesized by introducing additional electron donor
groups into the DepeA molecule to form the starburst 2DepeA
structure [39e41], which avoids the charge recombination process
of injected electrons with the triiodide in the electrolyte and the
formation of aggregates between the neighbor dye molecules.
Among the organic dyes, triphenylamine and its derivatives are a
* Corresponding author. Tel.: þ86 28 83201991; fax: þ86 28 83202569.
E-mail address: cyjia@uestc.edu.cn (C. Jia).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.12.024

X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
49
starburst conformation, which is very helpful to avoid the forma-
tion of aggregates between neighbor dye molecules. Besides,
imidazole derivatives could be used for the good auxiliary electron
donor in the organic dyes based on the following reasons: 1) it has
the conjugated chain which is advantage to the charge-transfer
transition from the auxiliary electron donor to first electron
donor; 2) charge recombination after electron injection maybe
retarded because of decreasing positive charge density at the donor
by electronic delocalization of the two substituents at positions 4
and 5 of the imidazolyl ring [42]. Based on these reasons, we
introduce the different imidazole derivatives to the structure of
triphenylamine, and the 2-cyanoacetic acid acts as acceptor group
and ethanol. A thin film of TiO2 was prepared on the FTO substrate
with the compact TiO2 layer through blade coating with glass rod.
After drying the nanocrystalline TiO2 layer at 80 ꢂC, the TiO2 thin
film with more layers was achieved by repeating the blade coating
above process two times. The resulting working electrode was
composed of a 14
mm thick transparent TiO2 nanoparticle layer.
Finally TiO2 electrodes were treated at 450 ꢂC for 30 min. After
cooling to room temperature, the electrodes were immersed in
40 mM TiCl4 aq. at 70 ꢂC for 30 min, and washed with water and
ethanol again, then recalcined at 450 ꢂC for 30 min. After the sin-
tering, when the TiO2 electrodes cooled to 80 ꢂC, the electrodes
were immersed in a dye bath containing 0.2 mM CD-4, CD-6 or
N719 in ethanol and left overnight. The films were then rinsed with
ethanol to remove excess dye. In our experiment, open cells were
fabricated in air by clamping the different dye electrode with
platinized counter electrode. The electrolyte used here is composed
to constitute two new 2DepeA dyes coded as CD-4 and CD-6. Their
photophysical, electrochemical properties and the performances of
the corresponding DSSCs are further investigated. At the same time,
N719 acts as the purpose of comparison dye, which was success-
fully applied in DSSCs. The corresponding molecular structures of
the two dyes and N719 are shown in Scheme 1.
of 0.6
M 1,2-dimethyl-3-propylimidazolium iodide (DMPII),
0.0653 M LiI, 0.03 M I2, 0.28 M 4-tertbutylpyridine (TBP) and 0.05 M
guanidium thiocyanate (GuSCN) in acetonitrile.
2. Experimental section
2.3. Fabrication of the samples for measuring the dye adsorbed
amount and absorption spectra on TiO2
2.1. Materials and characterization
The measurement of the dye adsorbed amount was according to
the literature [44] and the adopted TiO2 films were same as the
fabrication of DSSCs. The measurement procedure as follows: the
All solvents and other chemicals were reagent grade and used
without further purification. 4,40-dimethoxybenzil, 2,20-thenil, 2-
cyanoacetic acid and triphenylamine were purchased from Asta-
tech. HRMS data were obtained with a micrOTOF-Q II instrument.
1H NMR spectra were measured on Varian Mercury Plus 400 MHz
and Bruke 500 MHz NMR instruments. Mp data were obtained on
X4 melting point detector (FUKA, Beijing, China). INSPECT F Scan-
ning electron microscopy (SEM) (FEI, Netherlands) is used to
measure the thickness of the TiO2 film. Absorption spectra were
measured with SHIMADZU (model UV2550) UVevis spectropho-
tometer. Cyclic voltammetry experiments were performed on a CH
Instruments 660C electrochemical workstation with a scanning
rate of 50 mV/s in dimethylformamide (DMF) (5.0 ꢁ 10ꢀ4 M) con-
taining 0.1 M n-Bu4NPF6 as the supporting electrolyte, platinum as
counter and work electrodes and Ag/AgCl as reference electrode.
14
12 h in a dye bath and further employed for the measurement of the
dye adsorbed amount. The 5 m thickness TiO2 films were sensi-
mm thickness (area: 7 ꢁ 12 mm) TiO2 films were sensitized for
m
tized for 12 h in a dye bath, which were adopted for absorption
spectra measurement of the dyes on TiO2 surface.
2.4. Photovoltaic characterization
The irradiation source for the photocurrent action spectrum
measurement is
a
simulated AM 1.5G solar irradiation
(100 mV cmꢀ2) (SAN-EI ELECTRIC, XES-301S). The currentevoltage
characteristics of photovoltaic devices were measured by using a
KEITHLEY 2400 semiconductor parameter analyzer. The tested so-
lar cells were masked to a working area of 0.16 cm2. The action
spectra of quantum efficiency (QE) for solar cell were performed by
using QEX10 quantum efficiency measurement system. Electro-
chemical impedance spectroscopy (EIS) data were obtained in the
dark under forward bias 0.7 V, scanning from 10ꢀ2 to 105 Hz with ac
amplitude of 10 mV by using CH Instruments 660C electrochemical
workstation.
2.2. Fabrication of DSSCs
TiO2 colloid was prepared according to the literature [43]. The
washed FTO glass substrates were immersed in 40 mM TiCl4 aq. at
70 ꢂC for 30 min to form a compact layer of TiO2, which plays an
important role in suppressing the charge recombination of DSSCs at
the interface between FTO and electrolyte, then washed with water
2.5. Synthesis
O
Compound 2 [45] was prepared according to the reported
literature (as shown in Scheme 2).
H
N
N
OTBA
O
N
2.5.1. Synthesis of compound 3
OH
O
acetic acid (70 mL) solution of 4,40-dimethoxybenzil
COOH
CN
A
N
O
(189.00 mg, 0.70 mmol), compound 2 (315.00 mg, 1.05 mmol) and
ammonium acetate (1080.00 mg, 14.00 mmol) were charged
sequentially in a three-necked flask and heated under reflux for
12 h. The reaction mixture was poured into ice-cold water. The
resulting precipitate was filtered, washed with water, and dried.
Then the residue was purified by silica gel column chromatography
with petroleum ether/ethyl acetate (4:1, v:v) as eluent to afford
compound 3 as a yellow solid (126.00 mg, yield 32.67%). Mp:
N
N
NCS
NCS
CD-4
CD-6
Ru
N
HO
H
N
S
S
O
N
N
ABTO
O
N719
COOH
CN
210 ꢂC. 1H NMR (500 MHz, CDCl3)
d: 9.84 (s, 1H), 7.85e7.83 (d,
J ¼ 8.5 Hz, 2H), 7.72e7.71 (d, J ¼ 8.5 Hz, 2H), 7.60e7.58 (d, J ¼ 8.5 Hz,
Scheme 1. Molecular structures of CD-4, CD-6 and N719.
2H), 7.40e7.35 (m, 5H), 7.23e7.18 (m, 4H), 7.10e7.09 (d, J ¼ 9.0 Hz,

50
X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
O
COOH
CN
O
H
O
N
N
O
N
CD-4
CD-6
(d)
)
b
(
O
O
3
O
O
N
N
)
a
(
S
O
S
O
COOH
CN
1
2
O
H
S
S
N
N
N
(d)
)
c
(
4
O
Scheme 2. Synthetic routes of the dyes CD-4 and CD-6. (a) POCl3, DMF, reflux, 72 h (b) 4,40-dimethoxybenzil, ammonium acetate, acetic acid, reflux, yield 32.67% (c) 2,20-thenil,
ammonium acetate, acetic acid, reflux, yield 18.46% (d) 2-cyanoacetic acid, piperidine, acetonitrile, reflux (CD-4, yield 72.36%; CD-6, yield 77.49%).
2H), 6.94e6.93 (d, J ¼ 8.5 Hz, 2H), 6.87e6.85 (d, J ¼ 9.0 Hz, 2H), 3.85
(s, 3H), 3.82 (s, 3H). HRMS (ESI, m/z): calcd for C36H29N3O3:
551.2209, found 552.2426 [M þ H]þ.
(m, 2H), 8.00e7.97 (d, J ¼ 8.8 Hz, 2H), 7.70e7.66 (d, J ¼ 16.8 Hz, 2H),
7.49e7.45 (t, J ¼ 8.0 Hz, 4H), 7.37e7.28 (m, 3H), 7.25e7.24 (d,
J ¼ 7.6 Hz, 2H), 7.18e7.13 (m, 2H), 7.05e7.02 (d, J ¼ 8.8 Hz, 2H).
HRMS (ESI, m/z): calcd for C33H22N4O2S2: 570.1184, found 571.1266
[M þ H]þ.
2.5.2. Synthesis of compound 4
A acetic acid (50 mL) solution of 2,20-thenil (154.00 mg,
0.70 mmol), compound 2 (210.00 mg, 0.70 mmol) and ammonium
acetate (1080.00 mg, 14.00 mmol) were charged sequentially in a
three-necked flask and heated under reflux for 15 h. The reaction
mixture was poured into ice-cold water. The resulting precipitate
was filtered, washed with water, and dried. Then the residue was
purified by silica gel column chromatography with petroleum
ether/ethyl acetate (3:1, v:v) as eluent to afford compound 4 as a
yellow solid (65.00 mg, yield 18.46%). Mp: 198 ꢂC. 1H NMR
2.6. Theoretical calculation methods
The dyes CD-4 and CD-6 before and after binding to (TiO2)9 [46]
in vacuum are calculated at density functional B3LYP [47] level
using the 6-31G* for C, H, O, N, S atoms and effective core potential
(ECP) LANL2DZ and its accompanying basis set for Ti atom for both
geometry optimizations and frequency calculations. None of the
frequency calculations generated imaginary frequencies, indicating
that the optimized geometries are true energy minima. Electronic
populations of the HOMO and LUMO are calculated to show the
position of the localization of electron populations along with the
calculated molecular orbital energy diagram. As for the absorption,
coulomb-attenuating method CAM-B3LYP [48] functional was
chosen to calculate the vertical excitation energies and the oscil-
lator strengths [49] within the framework of TD-DFT. The solvent
effect of ethanol (the solvent used to record the experimental
spectra) on the absorption spectra, was considered using non-
equilibrium implementation of the conductor-like polarizable
continuum model (CPCM), which returns valid solvent effects when
there are no specific interactions between the solute and the sol-
vent molecules [50]. All calculations have been performed with the
Gaussian 09 packages [51].
(500 MHz, CDCl3)
d
: 9.84 (s, 1H), 7.84e7.82 (d, J ¼ 8.5 Hz, 2H), 7.73e
7.71 (d, J ¼ 8.5 Hz, 2H), 7.43e7.42 (d, J ¼ 5 Hz, 1H), 7.38e7.35 (t,
J ¼ 7.8 Hz, 2H), 7.33e7.30 (m, 2H), 7.25e7.18 (m, 6H), 7.14e7.13 (t,
J ¼ 4.25 Hz, 1H), 7.10e7.09 (d, J ¼ 9.0 Hz, 2H), 7.00e6.99 (t,
J ¼ 4.25 Hz, 1H). HRMS (ESI, m/z): calcd for C30H21N3OS2: 503.1126,
found 504.1264 [M þ H]þ.
2.5.3. Synthesis of CD-4
A
CH3CN (20 mL) solution of compound 3 (68.00 mg,
0.12 mmol), 2-cyanoacetic acid (35.00 mg, 0.41 mmol) and a few
drops of piperidine were charged sequentially in a three-necked
flask and heated to reflux under a nitrogen atmosphere for 10 h.
After cooling to room temperature, the solvents were removed by
rotary evaporation, and the residue was purified by silica gel col-
umn chromatography with dichloromethane/ethanol (6:1, v:v) as
eluent to afford the dye CD-4 as a deep yellow solid (55.00 mg, yield
3. Results and discussion
72.36%). Mp: 225 ꢂC. 1H NMR (400 MHz, DMSO-d6)
d: 8.16 (s, 1H),
8.09e8.07 (d, J ¼ 8.4 Hz, 2H), 7.97e7.94 (d, J ¼ 8.8 Hz, 2H), 7.47e7.43
(m, 6H), 7.29e7.24 (m, 5H), 6.98e6.93 (t, J ¼ 9.2 Hz, 6H), 3.77 (s,
6H). HRMS (ESI, m/z): calcd for C39H30N4O4: 618.2267, found
619.2344 [M þ H]þ.
3.1. Synthesis and characterization of the new dyes
The synthesis of the two new triphenylamine-based organic
dyes CD-4 and CD-6 is outlined in Scheme 2. The two dyes CD-4
and CD-6 were synthesized by the similar stepwise synthetic
protocol. First, compound 2 was prepared according to the re-
ported literature (as shown in Scheme 2). Then 4,40-dimethox-
ybenzil and 2,20-thenil were reacted with compound 2 and
ammonium acetate in the acetic acid, respectively, which got
compounds 3, 4. Finally, compounds 3, 4 with about three fold
excess of 2-cyanoacetic acid afforded the target dyes CD-4 and CD-
6 in acetonitrile using piperidine as catalyst. The structures of all
dye molecules were characterized unambiguously with proton
nuclear magnetic resonance (1H NMR) spectroscopy and mass
analysis.
2.5.4. Synthesis of CD-6
A CH3CN (20 mL) solution of compound 4 (61.40 mg, 0.12 mmol),
2-cyanoacetic acid (31.00 mg, 0.36 mmol) and a few drops of
piperidine were charged sequentially in a three-necked flask and
heated to reflux under a nitrogen atmosphere for 10 h. After cooling
to room temperature, the solvents were removed by rotary evap-
oration, and the residue was purified by silica gel column chro-
matography with dichloromethane/ethanol (6:1, v:v) as eluent to
afford the dye CD-6 as a deep yellow solid (53 mg, yield 77.49%).
Mp: 208 ꢂC. 1H NMR (400 MHz, DMSO-d6)
d: 8.19 (s, 1H), 8.15e8.09

X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
51
1.2
CD-4
CD-6
30000
25000
20000
15000
10000
5000
0
CD-4
CD-6
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
250
300
350
400
450
500
550
600
Wavelength / nm
Wavelength / nm
Fig. 2. Absorption spectra of TiO2 films sensitized by CD-4 and CD-6.
Fig. 1. Absorption spectra of CD-4 and CD-6 in ethanol.
10
3.2. Optical properties
CD-4
CD-6
The UVevis absorption spectra of the dyes CD-4 and CD-6 are
showninFig.1, and thecorrespondingdata are summarized inTable 1.
In ethanol solution, CD-4 and CD-6 exhibit two major bands
appearing at 339e405 nm and 338e411 nm, respectively. The former
8
6
4
2
0
is ascribed to a localized aromatic pep* transition and the later is of
charge-transfer character between the triphenylamine donating unit
and the anchoring moiety. Noticeably, the dye CD-4 has stronger
absorption than that of CD-6 over the entire absorption range. This
result indicates that the imidazole derivative in CD-4 has stronger
electron donating ability than that of the imidazole derivative in CD-
6. The molar extinction coefficients of the maximum absorption
wavelengths (lmax) for the two dyes are clearly high, which indicates
good light harvesting abilities (see Fig. 1).
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
The absorption spectra of CD-4 and CD-6 on the surface of 5 mm
Photoenergy hv / eV
thick TiO2 are shown in Fig. 2. When the two dye molecules are
adsorbed on the TiO2 films, the absorption peaks broaden, and are
more or less red-shifted compared to those in ethanol solution,
indicating strong interactions between the dyes and the TiO2 sur-
face, which should favor the light harvesting of the solar cells and
thus increase the photocurrent response region. The red-shifted
spectra of CD-4 and CD-6 may result from J-type aggregation [52]
of the dyes on TiO2 surface. Additionally, the lmax of CD-6 on TiO2
film is red-shifted 30 nm in comparison to that in ethanol solution
while CD-4 shows a red shift of 25 nm, indicating that CD-6 has a
more tendency to form J-type aggregation on TiO2 surface. Thus,
the introduction of imidazole derivative bearing thiophene group is
Fig. 3. Plot of (ahv)2 vs. hv.
advantage to form the J-type aggregation for the corresponding
triphenylamine dyes on TiO2 surface.
Based on the Tauc relation, the energy gap (Eg) can be obtained
by plotting (ahv)2 vs. hv and extrapolating the linear portion of
Table 1
UVevis and electrochemical data.
Dye
lmaxa/nm(εb/Mꢀ1 cmꢀ1
)
lmaxc/nm
Eoxd/V
Ege/eV
Eredf/V
(vs. NHE)
(vs. NHE)
CD-4
CD-6
405 (28,979)
411 (24,873)
430
441
1.08
1.15
2.55
2.46
ꢀ1.47
ꢀ1.31
CD-6
CD-4
a
Absorption is measured in ethanol solutions (1.0
ꢁ
10ꢀ5 M) at room
temperature.
b
c
The molar extinction coefficient at lmax of the absorption spectra.
Absorption spectra of the dyes adsorbed on TiO2 electrodes.
Eox was measured in DMF with 0.1 M n-Bu4NPF6 as electrolyte (scanning rate:
d
50 mV/s, working electrode and counter electrode: Pt wires, and reference elec-
trode: Ag/AgCl), potentials measured vs. Ag/AgCl were converted to normal
hydrogen electrode (NHE) by addition of þ0.2 V.
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
e
E / V vs. Ag/AgCl
Eg was estimated from the absorption spectra of TiO2 electrodes sensitized by
the dyes.
f
Ered was calculated from Eox ꢀ Eg.
Fig. 4. Cyclic voltammograms of CD-4 and CD-6 dissolved in DMF.

52
X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
(DMF) solution using 0.1 M n-Bu4NPF6 as supporting electrolyte
-2.0
-1.5
-1.0
-0.5
0.0
(Fig. 4), and the corresponding data are summarized in Table 1.
HOMO levels of CD-4 and CD-6 are determined to be 1.08 and 1.15 V
vs. NHE, respectively (summarized in Table 1), which are more
positive than the Iꢀ/I3ꢀ redox potential (0.4 V vs. NHE). It implies
that the oxidized dyes formed from respective electron injection
into the CB of TiO2 will favorably accept electrons from Iꢀ ions
thermodynamically. On the other hand, the excited-state oxidation
potentials (Ered), which correspond to the LUMO level, can be ob-
tained by the Eox and Eg of the dyes, namely, Eox ꢀ Eg. LUMO levels of
the two dyes (CD-4: ꢀ1.47 V; CD-6: ꢀ1.31 V vs. NHE) are more
negative than the CB of TiO2 (at approximately ꢀ0.5 V vs. NHE) [53].
Generally, a minimal driving force of 0.2 V is sufficient to ensure fast
excited-state injection and regeneration of the oxidized dye [54].
Thus, the excited electrons injection of two dyes are guaranteed to
be efficient as well as the regeneration of the oxidized dyes. In
comparison with the LUMO level of CD-6, CD-4 has a higher LUMO
level, indicating that excited-state dye CD-4 has a stronger electron
driving force, which is advantage to promote the speed of injecting
electron and improve the performance of the corresponding DSSC.
The schematic energy levels of CD-4 and CD-6 based on absorption
and electrochemical data are shown in Fig. 5. According to Fig. 5,
the two dyes are considered to have proper electronic energy levels
as promising sensitizers in DSSCs.
-1.47
2.55
-1.31
2.46
-0.5
CB
0.4
I-/I3-
0.5
1.08
CD-4
1.15
CD-6
1.0
1.5
Fig. 5. Schematic energy levels of CD-4 and CD-6 based on absorption and electro-
chemical data.
(ahv)2 to zero as shown in Fig. 3 [45]. The Eg of CD-4 and CD-6 are
estimated to be 2.55 eV and 2.46 eV, respectively. The bigger red
shift of CD-6 adsorbed on TiO2 film can be evidenced by the lower
energy band-gap as compared with that of CD-4.
3.3. Electrochemical properties
Cyclic voltammograms were performed to evaluate the possi-
bility of electron transfer from the excited dye molecule to the CB of
TiO2 and the regeneration of oxidized sensitizers. Cyclic voltam-
metry measurements were performed in dimethylformamide
3.4. Theoretical calculations
The ground state geometries and molecular orbital spatial dis-
tributions from HOMO ꢀ 1 to LUMO for CD-4 and CD-6 before and
Fig. 6. The optimized ground state geometries for CD-4 and CD-6 before and after binding to (TiO2)9 in vacuum.

X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
53
after binding to (TiO2)9 in vacuum are shown in Figs. 6 and 7,
CD-4
CD-6
100000
80000
60000
40000
20000
0
respectively. As shown in Fig 6, the imidazole ring and phenyl ring
of the triphenylamine core is coplanar, expressing strong conju-
gation degree, which is advantage to electron transfer from auxil-
iary electron donor to triphenylamine core. The dihedral angel
between two substituents at positions 4 and 5 of the imidazolyl
ring in CD-4 and CD-6 are 69.0ꢂ and 42.6ꢂ, respectively. So, CD-4
shows a better steric hindrance, indicating that it has the better
ability of anti-aggregation on the TiO2 surface, which is very
important to improve the performances of DSSCs. When the two
dyes are bonded to (TiO2)9 cluster, the TieO bonds of CD-4e(TiO2)9
Experiment results
ꢀ
and CD-6e(TiO2)9 are about 2.04 A, suggesting that the two dyes
could adsorb on the TiO2 anatase (101) surface firmly. As shown
in Fig 7, it is found that HOMOs of CD-4 and CD-6 are mainly
localized on the auxiliary donor and triphenylamine area, whereas
LUMOs are distributed at the 2-cyanoacetic acid units and neighbor
benzene ring, which indicates that there are good electron-
separated states between HOMOs and LUMOs. The LUMOs of the
two dyes are mainly distributed at the anchoring group (eCOOH),
which could lead to a strong electronic coupling with TiO2 surface
and boost the electron injection efficiency. Thus, the
HOMO / LUMO excitation induced by light irradiation could move
the excited electron from the auxiliary donor and triphenylamine
unit to the 2-cyanoacetic acid, and injected into the CB of TiO2
through the anchoring group. In order to prove the conclusion, the
HOMO ꢀ 1, HOMO and LUMO of dyese(TiO2)9 complex were also
studied. As shown in Fig. 7, there is the electron distribution mostly
delocalized on the dye molecule in the HOMO and HOMO ꢀ 1,
whereas the LUMO shows injected electron delocalized dominantly
on the (TiO2)9 cluster. The result indicates that efficient electron
250
300
350
400
450
500
550
Wavelength / nm
Fig. 8. The simulated UVevis absorption spectra for free dyes in ethanol.
injection from LUMOs of dyes to the CB of TiO2 can be performed
through a carboxylic acid acceptor group and the intermolecular
charge-transfer (CT) transition takes place in the dyee(TiO2)9
complex when the dye sensitizers were adsorbed on TiO2 surface.
The simulated UVevis absorption spectra by TD-DFT calcula-
tions are shown in Fig. 8, and the major electron excitations and
corresponding electron transition are listed in Table 2. It can be
found that the broad band in 270e500 nm region for CD-4 is
composed of three electron excitations calculated at 396.7, 314.8
and 283.0 nm while CD-6 is at 393.3 and 304.5 nm. The results
Fig. 7. Electron distributions in HOMO-1, HOMO and LUMO orbitals of CD-4 and CD-6 before and after binding to (TiO2)9 in vacuum.

54
X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
Table 2
CD-4
Electronic transition configurations, computed excitation energies and oscillator
strengths (f) for the main optical transitions of the absorption bands in visible and
near-UV region for CD-4 and CD-6 in ethanol solution (H ꢀ 1 ¼ HOMO ꢀ 1,
H ¼ HOMO, L ¼ LUMO, etc.).
CD-6
16
14
12
10
8
CD-4+5 mM CDCA
CD-6+5 mM CDCA
N719
Dye Wavelength E
f
Composition
(nm)
(eV)
CD-4 396.7
314.8
283.0
CD-6 393.3
304.5
3.13 1.3802 H ꢀ 0 / L þ 0(þ49%) H ꢀ 1 / L þ 0(þ41%)
3.94 0.6197 H ꢀ 0 / L þ 1(þ66%) H ꢀ 1 / L þ 0(17%)
4.38 0.5874 H ꢀ 0 / L þ 2(þ67%) H ꢀ 1 / L þ 1(þ11%)
3.15 1.3818 H ꢀ 0 / L þ 0(þ57%) H ꢀ 1 / L þ 0(þ33%)
4.07 0.7778 H ꢀ 0 / L þ 2(þ68%) H ꢀ 1 / L þ 0(þ10%)
6
4
2
correspond well with the experimental results that CD-4 is at 404,
340 and 291 nm and CD-6 is at 408 and 340 nm. The simulated
absorption spectra have the similar absorption bands to the
experimental spectra, which also shows the accuracy of this theo-
retical calculation. From the view of Table 2, the lmax of CD-4
correspond to H-0 / L þ 0(þ49%) and H ꢀ 1 / L þ 0(þ41%), while
CD-6 is H ꢀ 0 / L þ 0(þ57%) and H ꢀ 1 / L þ 0(þ33%). By this
token, not only lmax of the two dyes come from the transition from
the HOMO to LUMO, but also come from the HOMO ꢀ 1 to LUMO.
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage / V
Fig. 10. Current densityevoltage curves of DSSCs based on CD-4, CD-6 and N719.
photovoltaic performances of DSSCs. In comparison with CD-6,
both the Jsc and Voc values of CD-4 are improved by introducing the
p-methoxyphenyl group rather than thiophene group to imidazole
part. The higher Jsc of CD-4 can be deduced from the higher molar
extinction coefficient and QE than that of CD-6.
3.5. Photovoltaic properties
To further investigate why dyes CD-4 and CD-6-sensitized solar
cells have such a different performance, the adsorbed amount of
dyes CD-4 and CD-6 on TiO2 surface were measured and are shown
in Table 3. By desorbing the dye in a basic solution, the dye
adsorbed amount was estimated by measuring the absorption
spectra of the resultant solution. The concentrations of CD-4 and
CD-6 on TiO2 surface were determined as 9.73 ꢁ 10ꢀ8 and
2.24 ꢁ 10ꢀ7 M cmꢀ2, respectively, which indicates that the Jsc value
of CD-6-sensitized DSSC should be higher than that of CD-4.
However, the measurement result is completely different as the Jsc
value of CD-6-sensitized DSSC is lower than that of CD-4. This
result maybe due to CD-6 having a better planarity on the imidazole
The quantum efficiency (QE) and the photocurrent densitye
photovoltage (JeV) curves of the corresponding DSSCs are shown in
Figs. 9 and 10. The detailed photovoltaic parameters are summa-
rized in Table 3.
As shown in Fig. 9, the DSSC based on CD-4 shows a higher QE
value in the spectra range of 300e600 nm than that of CD-6, pro-
ducing a maximum QE of 46.57% at 476 nm. The QE spectra
changing tendencies of the dyes are in excellent agreement with
their UVevis absorption spectra. The JeV curves of the DSSCs based
on CD-4, CD-6 and N719 performed under simulated AM 1.5 solar
irradiation (100 mW cmꢀ2) are shown in Fig. 10, where N719 is
included for comparison. According to the data in Table 3, the
h
unit than that of CD-4, which leads to the formation of
p-stacked
values of 4.11% and 1.51% were obtained by the DSSCs based on CD-
4 and CD-6, respectively. Under the same measurement conditions,
the DSSC based on N719 generated an efficiency of 5.71%
(Jsc ¼ 15.24 mA cmꢀ2, Voc ¼ 0.65 V, and ff ¼ 0.57). It’s clear that the
Jsc values are in order of CD-4 (Jsc ¼ 8.60 mA cmꢀ2) > CD-6
(Jsc ¼ 5.33 mA cmꢀ2), which are in good agreement with the QE
results (Fig. 9) of the two dyes. According to Fig. 10 and Table 3, it is
clear that introducing different imidazole derivatives into the
formwork of triphenylamine core will give a big difference to the
aggregation and further causes CD-6 residing in the adsorbed
system rather than attach to the TiO2 surface. When chenodeox-
ycholic acid (CDCA) is used as coadsorbent to dissociate the
p-
stacked sensitizer aggregation, the DSSC based on CD-6 afford a
higher Jsc value (6.26 mA cmꢀ2) while that of CD-4 shows a lower Jsc
value (7.99 mA cmꢀ2). The result indicates that CD-4 has a better
ability of anti-aggregation than CD-6, which is in accordance with
above analysis for the adsorbed amount of the dyes. The best per-
formance in this work was observed for the device made with CD-4,
which exhibited a Jsc of 8.60 mA cmꢀ2, Voc of 0.63 V, ff of 0.75 and
h
of 4.11%.
50
45
40
CD-4
CD-6
3.6. Electrochemical impedance spectroscopy (EIS) analysis
35
30
25
20
15
10
5
Electrochemical impedance spectroscopy (EIS) analysis [55,56]
was performed to study the interfacial charge-transfer processes
Table 3
Photovoltaic performances of DSSCs based on CD-4, CD-6 and N719.
Dye
Jsc/mA cmꢀ2 Voc/V ff
Adsorbed amount
[10ꢀ4 mM cmꢀ2
se/ms
h%
]
CD-4
CD-6
8.60
5.33
7.99
0.63
0.51
0.64
0.75 0.973
0.55 2.24
13.5
2.0
/
4.11
1.51
3.36
0
CD-4 þ 5 mM
0.66
0.61
0.57
/
/
/
300
400
500
600
700
CDCA
CD-6 þ 5 mM
CDCA
N719
6.26
0.53
0.65
/
/
2.02
5.71
Wavelength / nm
15.24
Fig. 9. The QE spectra of the DSSCs sensitized with CD-4 and CD-6.

X. Chen et al. / Dyes and Pigments 104 (2014) 48e56
55
in DSSCs based on the different dyes. The Nyquist and Bode plots
4. Conclusions
for CD-4 and CD-6-sensitized cells are shown in Fig. 11a and b,
respectively. With the bias voltage applied, the first small semi-
circle (higher than 103 Hz) is attributed to charge-transfer at the
Pt/electrolyte interface, the second larger semicircle at lower fre-
quencies (in the 10e100 Hz region) to the charge recombination
processes at the TiO2/dye/electrolyte interface. As shown
in Fig. 11a, the radius of second larger semicircle at lower fre-
quencies is in the order CD-4 > CD-6, indicating that the recom-
bination rate is in the order CD-6 > CD-4. This is the main reason
why the Voc value of CD-4 (0.63 V) is higher than that of CD-6
(0.51 V).
As shown in the Bode phase plots (Fig. 11b), the low-
frequency peak is indicative of the charge-transfer process of
injected electrons in TiO2. The higher Voc of CD-4 can be further
explained by electron lifetime. The obtained electron lifetimes of
DSSCs based on the CD-4 and CD-6 are 13.5 ms and 2.0 ms by the
In summary, we have designed and synthesized two new
triphenylamine-based organic dyes, which constituted the 2De eA
structure and were applied in DSSCs. The effects of introducing
different imidazole derivatives into the formwork of triphenylamine
core on the optical, electrochemical and photovoltaic properties
p
were studied. The DSSC based on CD-4 has obtained the
h value of
4.11% (Jsc ¼ 8.60 mA cmꢀ2, Voc ¼ 0.63 V, ff ¼ 0.75) while that of CD-6
has obtained only 1.51%, (Jsc ¼ 5.33 mA cmꢀ2, Voc ¼ 0.51 V, ff ¼ 0.55).
The Jsc of CD-4 is higher than that of CD-6, which can be deduced
from the better molar extinction coefficient and QE. At the same
time, the EIS results are in good agreement with the different Voc
values of DSSCs based on the two dyes. The higher Voc of CD-4 can be
further explained by the longer electron lifetime. The DFT and TD-
DFT calculations are very good to explain the experimental results,
especially the simulated UVevis absorption spectra for the two dyes
are in excellent agreement with the experimental UVevis absorp-
tion spectra, which indicates that the theoretical calculation is very
important for exploring high efficiency organic dyes. Our findings
demonstrate that the corresponding imidazole derivative in CD-4 is
very promising electron donor, introducing it into the triphenyl-
amine dye can strongly improve photovoltaic performance.
Although the overall conversion efficiencies of the two dyes are not
very high, the results will still afford significant value for future
equation se ¼ 1/2
pf (f is the frequency of the low-frequency
peak). The longer electron lifetimes of CD-4 could explain the
significant enhancement in Voc for CD-4 sensitized cell in com-
parison with that of CD-6 (as shown in Table 3). Here, the cor-
responding imidazole derivative in CD-4 can increase electron
lifetime by 6.75-fold with respect to CD-6, indicating that
imidazole derivative in CD-4 is better auxiliary electron donor
than that in CD-6.
development of efficient 2DepeA sensitizers. Further structural
optimization, such as broadening the absorption spectra and tuning
the energy levels, is very likely to generate more efficient sensitizers
and this work is currently underway in our laboratory.
20.0
CD-4
CD-6
17.5
Acknowledgments
15.0
12.5
10.0
7.5
We thank the National Natural Science Foundation of China
(Grant No. 21272033), the Innovation Funds of State key Laboratory
of Electronic Thin Films and Integrated Device (Grant No.
CXJJ201104) for financial support.
5.0
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