Synthesis and evaluation of simple molecule as a co-adsorbent dye for highly efficient co-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.04.003

One simple-structure dye and two bulky dyes have been designed for exploring a new co-sensitization system utilizing synergy between simple-structure and bulky organic dyes. This small-sized dye molecule can fill up the space defects between bulky dyes, and its strong absorption band from 350 to 500 nm can complement the absorption valleys of the bulky dyes. Additionally, the dye loading of titanium dioxide surface was investigated by Thermogravimetric analysis, which confirms that the molar loading of co-sensitized device has significantly increased. These results exhibit that the simple molecule can be used not only as a co-adsorbent to inhibit charge recombination and π-π aggregation between the bulky dyes but also as a co-sensitizer to improve light-harvesting ability.

Full text of DOI:10.1016/j.dyepig.2015.04.003

Dyes and Pigments 120 (2015) 85e92
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and evaluation of simple molecule as a co-adsorbent dye for
highly efficient co-sensitized solar cells
, b,
Shengbo Zhu a, Zhongwei An a *, Xiao Sun a, Zhisheng Wu b, Xinbing Chen a, Pei Chen a
a Key Laboratory of Applied Surface and Colloid Chemistry, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, China
b Xi'an Modern Chemistry Research Institute, Xi'an, 710065, China
a r t i c l e i n f o
a b s t r a c t
Article history:
One simple-structure dye and two bulky dyes have been designed for exploring a new co-sensitization
system utilizing synergy between simple-structure and bulky organic dyes. This small-sized dye mole-
cule can fill up the space defects between bulky dyes, and its strong absorption band from 350 to 500 nm
can complement the absorption valleys of the bulky dyes. Additionally, the dye loading of titanium di-
oxide surface was investigated by Thermogravimetric analysis, which confirms that the molar loading of
co-sensitized device has significantly increased. These results exhibit that the simple molecule can be
Received 6 February 2015
Accepted 1 April 2015
Available online 11 April 2015
Keywords:
Co-sensitized solar cells
Simple-structure dye
Co-adsorbent
Complementary absorption
Molecular size matching
Cyclic thiourea functionalization
used not only as a co-adsorbent to inhibit charge recombination and
dyes but also as a co-sensitizer to improve light-harvesting ability.
p-p aggregation between the bulky
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
into
p-linkers to extend spectral response ranges [17e21]. Main
advantages of these dyes lie in bigger conjugation system and
broader absorption spectra. However, in addition to their tedious
synthetic procedures, bulky molecular structures may decrease the
amount of dye loading on TiO2 surface. Therefore, developing
simple-structure dyes as co-adsorbents will also be equally
important. Recently, Han's group reported two simple-structure
adsorbents used for co-sensitization with black dye which led to
a superior PCE up to 11.4% [22]. Kim's group employed phenothi-
azine and phenoxazine as the donors to construct some simple dye
molecules [23,24]. So far, improved conversion efficiencies have
been successfully achieved in co-sensitized solar cells using organic
dyes [18,25,26], or metal-complex and organic dye [27e33]. To the
best of our knowledge, however, there are limited investigations
available on simple organic molecules which serve as both co-
adsorbents and complementary dyes used for co-sensitized solar
cells.
With the growing severity of energy crisis and environment
problem, scientists all over the world are beginning to turn their
eyes to the exploitation of solar energy. As a promising light-
harvesting device, dye-sensitized solar cells (DSSCs) have attrac-
ted widespread attention due to the features of friendly to envi-
ronment, low carbon and easy fabrication [1e3]. In past decade,
enormous efforts have been devoted toward improving the power
conversion efficiency (PCE). To date, DSSCs based on metal-
complex sensitizers have achieved the PCE of ~12% [4e6], and
organic sensitizers also have reached the PCE of ~10% [7,8]. In recent
years, however, organic sensitizers have received more and more
attention because of their ease of syntheses, high molar extinction
coefficients and simple preparation process in comparison with
metal-complex sensitizers [9]. Researches on organic sensitizers
mainly focus on molecular modification, especially donor and/or p-
linker modification. These often give rise to large-sized and bulky
dye molecules. Generally, additional groups are introduced into
donors to avoid charge recombination and intermolecular aggre-
gation [10e16]; electron-rich conjugation segments are inserted
In our previous studies [34e36], cyclic thiourea functionalized
triphenylamine-based dyes exhibited significant photophysical and
photovoltaic performances, including that: (i) such donor con-
taining electron-rich nitrogen and sulfur heteroatoms in the cyclic
thiourea group possesses high electron-donating ability [23]; (ii)
broader and stronger spectral absorption in comparison with the
corresponding triphenylamine-based dyes; (iii) the hexyl chains
attached at the N-atom sites of cyclic thiourea groups can suppress
the intermolecular aggregation and increase the solubility of dyes
* Corresponding author. Key Laboratory of Applied Surface and Colloid Chemis-
try, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an,
710062, China.
E-mail address: gmecazw@163.com (Z. An).
http://dx.doi.org/10.1016/j.dyepig.2015.04.003
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

86
S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
[37e39]. However, the UVeVis spectra of them are characterized by
a “camelback” type, which leads to a weak absorption at around
400 nm. Besides that, there is another problem that we have to
face: these tree-shaped dyes would leave large space between dye
molecules when adsorbed on the TiO2 surface. For these reasons, a
novel simple molecule SE has been synthesized and evaluated. The
results show that SE is not only a suitable co-adsorbent but also an
effective dye in co-sensitization with dyes AZ360 or AZ362 (see
Fig. 1 for the chemical structures), with following features: (i) this
kind of non-triphenylamine dye inherits the cyclic thiourea group's
superior bloodline but avoid the drawback of bulky molecular
structures, which could be employed as fillers to fill up the space
defects between bulky dyes; (ii) the hexyl chains introduced into SE
molecule can inhibit intermolecular aggregation; (iii) the intense
spectral response between 350 and 450 nm can complement the
camelback-shaped spectra of AZ360 and AZ362 for achieving
highly efficient light harvesting [27e30]. As expected, successful
application of the simple-structure dye SE indicates the feasibility
of this idea. Especially, the co-sensitized solar cell with AZ362 þ SE
achieved the best complementary absorption and molecular size
matching, and thus produced a PCE up to 7.65%, which increased by
9.29% in comparison with that of the DSSCs based on dye AZ362
alone.
(m, 4H), 1.81e1.74 (m, 4H), 1.42e1.25 (m, 12H), 0.89e0.88 (m, 6H).
13C NMR (75 MHz, CDCl3):
(ppm) 169.62, 133.54, 132.09, 131.29,
117.59, 109.02, 85.32, 44.87, 44.83, 31.48, 31.44, 27.89, 27.79, 26.57,
26.51, 22.65, 22.53, 14.00, 13.92. GCeMS (EI, m/z) calcd. for
(C19H29IN2S): 444.11. Found: 444.18.
d
5-(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo[d]imidazol-5-yl)
thiophene-2-carbaldehyde (3). In a 150 mL 3-necked flask, com-
pound 2 (3.0 g, 6.8 mmol), 5-formyl-2-thiopheneboronic acid
(1600 mg, 10.0 mmol), tetra-n-butylammonium bromide (TBAB)
(660 mg, 2.1 mmol), N,N-dimethylformamide (DMF) (80 mL),
Pd(PPh3)4 (115 mg, 0.1 mmol), H2O (16 mL), and NaF (850 mg,
20 mmol) were added in turn under a nitrogen atmosphere.
Following this, the reaction mixture was stirred for 5 h at 60 ꢀC and
then poured into EA. The organic layer was washed with water and
dried over anhydrous MgSO4. After removing the solvent, the crude
product obtained was purified by column chromatography (PE/
EA ¼ 5/1, v/v) to give a yellow solid (940 mg, 32% yield). 1H NMR
(300 MHz, CDCl3):
d
(ppm) 9.82 (s, 1H), 7.69 (d, J ¼ 3.0 Hz, 1H), 7.48
(d, J ¼ 6.3 Hz, 1H), 7.33e7.32 (m, 2H), 7.15 (d, J ¼ 6.3 Hz, 1H),
4.28e4.22 (m, 4H),1.81e1.72 (m, 4H),1.37e1.16 (m,12H), 0.87e0.78
(m, 6H). 13C NMR (75 MHz, CDCl3):
d (ppm) 182.61, 170.37, 154.03,
142.39, 137.41, 133.04, 132.78, 128.12, 123.98, 121.59, 109.40, 106.65,
44.99, 44.92, 31.52, 31.45, 27.89, 27.86, 26.64, 26.55, 22.53, 22.52,
14.19, 13.99. HRMS (ESI, m/z): [MþH]þ calcd. for (C24H33N2OS2):
429.2034, found: 429.2068.
2. Experimental section
(E)-2-cyano-3-(5-(1,3-dihexyl-2-thioxo-2,3-dihydro-1H-benzo
[d]imidazol-5-yl)thiophen-2-yl)acrylic acid (SE). In a 100 mL 3-
necked flask, compound 3 (940 mg, 2.2 mmol), acetic acid
(50 mL), cyanoacetic acid (560 mg, 6.7 mmol) and ammonium ac-
etate (560 mg, 7.3 mmol) were added in turn under a nitrogen at-
mosphere. The reaction mixture was refluxed for 5 h. After cooling
to room temperature, the mixture was poured into ice water. The
precipitate was filtered, washed by distilled water, and purified by
column chromatography (PE/DCM ¼ 5/1, v/v; where DCM is
dichloromethane.) to give a red solid (650 mg, 59% yield). 1H NMR
2.1. Synthesis and characterization
1H NMR and 13C NMR spectra were recorded on Bruker AV-
300 MHz or Bruker AV-400 MHz instruments with tetramethylsi-
lane (TMS) as the internal standard. Gas chromatography mass
spectra (GC-MS) were acquired in the electron ionization mode (EI)
on Thermo DSQII. High resolution mass spectra (HRMS) were
measured with a Bruker maXis mass spectrometer. Elemental
analysis (C H N) was carried out on a VARIO-EL-III elemental
analyzer.
All chemicals and solvents were purchased commercially and
used without further purification unless otherwise stated. The
synthetic details of AZ360 are described in the Supporting Infor-
mation. The synthetic details of AZ362 were reported in our pre-
vious paper [36]. The synthesis of starting material 1 was reported
in our previous paper [34], and the synthetic details of SE are
described as follows:
(300 MHz, CDCl3):
d
(ppm) 8.34 (s, 1H), 7.79 (d, J ¼ 1.8 Hz, 1H), 7.54
(d, J ¼ 4.2 Hz, 1H), 7.42 (d, J ¼ 1.8 Hz, 1H), 7.39 (s, 1H), 7.19 (d,
J ¼ 4.2 Hz, 1H), 4.32 (t, J ¼ 3.9 Hz, 2H), 4.29 (t, J ¼ 3.9 Hz, 2H),
1.86e1.79 (m, 4H), 1.46e1.31 (m, 12H), 0.89 (t, J ¼ 3.3 Hz, 6H). 13
C
NMR (75 MHz, CDCl3): d (ppm) 170.49,167.60,155.54,147.71,140.18,
134.69, 133.35, 132.84, 127.60, 124.45, 121.86, 115.77, 109.44, 106.53,
90.62, 45.00, 44.91, 31.48, 31.44, 27.88, 27.87, 26.54, 26.51, 22.59,
22.53, 14.00, 13.99. HRMS (ESI, m/z): [MꢁH]－ calcd. for
(C27H32N3O2S2): 494.1936; found: 494.1938. Anal. calcd. for
(C27H33N3O2S2): C, 65.42; H, 6.71; N, 8.48. Found: C, 65.38; H, 6.73;
N, 8.49.
1,3-dihexyl-5-iodo-1H-benzo[d]imidazole-2(3H)-thione (2). In a
100 mL 3-necked flask, compound 1 (8.6 g, 20.0 mmol), toluene
(50 mL), and Lawesson reagent (6.5 g, 16.0 mmol) were added in
turn under a nitrogen atmosphere. The reaction mixture was
refluxed for 12 h. When cooling to room temperature, a yellow
precipitate began to form and then was filtered. Following this, the
filtrate was concentrated under reduced pressure and purified by
column chromatography (PE/EA ¼ 8/1, v/v; where PE is petroleum
ether, boiling range: 60e90 ꢀC; EA is ethyl acetate.) to give a yellow
2.2. Theoretical calculations
The electronic configuration and geometry structure of the dyes
were optimized by density functional theory (DFT) calculations
with the Gaussian 03 program package at the B3LYP/6-311G(d, p)
level.
viscous liquid (6.6 g, 75% yield). 1H NMR (300 MHz, CDCl3):
7.47 (s,1H), 7.20 (d, J ¼ 2.7 Hz,1H), 6.94 (d, J ¼ 8.1 Hz,1H), 4.33e4.21
d (ppm)
Fig. 1. Chemical structures of SE, AZ360, and AZ362. Herein C6H13 denotes n-hexyl.

S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
87
Table 1
2.3. UVeVis absorption measurements
UVeVis absorption and energy levels of by DFTcalculations of SE, AZ360, and AZ362.
UVeVis absorption spectra of the dyes in MeCN-DCM (1:1, v/v,
where MeCN is acetonitrile; 1 ꢂ 10ꢁ5 M) solutions and on TiO2 films
Dyes
labs/nm (ε/Mꢁ1 cmꢁ1
)
EHOMO/eV
ELUMO/eV
Eg/eV
SE
AZ360
AZ362
315 (19,700)
332 (39,200)
347 (34,700)
419 (39,400)
465 (37,500)
488 (37,800)
ꢁ5.78
ꢁ5.40
ꢁ5.24
ꢁ3.07
ꢁ3.07
ꢁ3.02
2.71
2.33
2.22
(6.5 mm) were measured with a Shimadzu UV-3600 spectrophoto
meter.
2.4. Dye loading measurements
imaginable that these tree-shaped dyes would leave large space
between molecules when adsorbed on the TiO2 surface. For
repairing the space defects, a novel simple-structure dye SE was
designed. Compared with cyclic thiourea functionalized
triphenylamine-based dyes, SE has a simple molecular structure
and small volume. Such molecules can be employed as additional
dyes to fill up the space defects between the tree-shaped dyes; on
the other hand, hexyl chains attached at the cyclic thiourea group
could suppress the intramolecular aggregation between the bulky
dyes [22]. Furthermore, two cyclic thiourea functionalized
triphenylamine-based dyes containing thienothiophene (AZ360)
TiO2 films (0.64 cm2) were prepared with a screen printing
method according to the published method35 and then immersed
into a 0.4 mM dye bath in DCM solution under dark for 24 h at room
temperature. Dye-loading properties of TiO2 films were measured
on Thermoanalyzer systems (Q1000DSC þ LNCS þ FACS Q600SDT)
at a scanning rate of 10 ꢀC minꢁ1 in the temperature range of
20e800 ꢀC under an oxygen atmosphere. For co-sensitized TiO2
films, the experimental procedures of desorption were conducted
according to the published method [35]. The desorption solutions
were measured with High performance liquid chromatography
(HPLC, LC-20AT, Shimadzu Co., Japan); chromatography column:
and thienothiophene-thiophene (AZ362) as
p-linkers were syn-
thesized for evaluating co-sensitization effectiveness.
ODS (250 mm ꢂ 4.6 mm ꢂ 5
mobile phase: MeOH/MeCN/H2O ¼ 66.5/28.5/5.0 (containing 0.2%
of triethylamine, where MeOH is methanol, v/v), 0.5 mL minꢁ1
mm); detector: SPD-10A,
l
¼ 235 nm;
As illustrated in Scheme 1, urea group at compound 1 was
turned into thiourea group by Lawesson reagent. Suzuki coupling
reaction between intermediate 2 and 5-formyl-2-thiopheneboronic
acid was carried out using Pd(PPh3)4 as the catalyst to give com-
pound 3. Finally, the obtained compound 3 was converted to the
target molecule SE via Knoevenagel condensation reaction with
cyanoacetic acid in the presence of acetic acid and ammonium
acetate. The synthesis of AZ360 resembles the reported methods of
similar dyes [35,36], and the details are described in the Supporting
Information. The chemical structures of all target molecules were
fully characterized by 1H NMR, 13C NMR, and HRMS.
.
2.5. Device fabrication and photovoltaic measurements
The active area of the TiO2 film was approximately 0.25 cm2. All
other fabrication processes of DSSCs were also performed accord-
ing to the published method [35]. Photocurrentevoltage (JeV)
characteristics of the DSSCs were measured under illumination
with AM 1.5 G solar light from a 300 W xenon lamp solar simulator
(94022A, Newport Co., USA). The incident light intensity was cali-
brated to 100 mW cmꢁ2 with a standard silicon solar cell. JeV
characteristics were recorded with a digital source meter (Keithley
2400) controlled by a computer. The action spectra of mono-
chromatic incident photon-to-current conversion efficiency (IPCE)
for solar cells were tested on a commercial setup (QTest Station
2000 IPCE Measurement System, Crowntech, USA).
3.2. Theoretical calculations
Density functional theory (DFT) calculations were conducted to
evaluate the geometries and electron distribution of the HOMO and
LUMO energy levels of SE, AZ360, and AZ362 (see Table 1). As
shown in Fig. 2, the HOMO of SE is mainly distributed along the
cyclic thiourea functionalized benzene ring and the thiophene unit,
and the LUMO is delocalized across the thiophene unit and the
acceptor. This means that electrons can successfully transfer from
the cyclic thiourea functionalized benzene ring to the acceptor for
the HOMOeLUMO transition. For AZ360 and AZ362, the HOMOs
and LUMOs perfectly revealed that the donoreacceptor character of
the cyclic thiourea functionalized triphenylamine and the
anchoring groups. The well-overlapped HOMO and LUMO orbitals
2.6. EIS measurements
Electrical impedance spectra (EIS) under dark with bias ꢁ0.7 V
were also measured with CHI 660D Electrochemical Workstation at
frequencies of 0.05e100,000 Hz. The magnitude of the alternative
signal was 10 mV. Charge-transfer resistances were determined by
fitting the impedance spectra using Z-view software.
on the
p-linker indirectly suggest the well inductive or with-
3. Results and discussion
drawing electron tendency from the donor to the acceptor. Such
dye architecture provides an energy gradient for the excitation and
facilitates the HOMO to LUMO charge transfer transition, which is
crucial to afford a favorable energetic pathway for electron injec-
tion into the conduction band of TiO2 [40].
3.1. Design and synthesis
In our previous works [34e36], a series of cyclic thiourea
functionalized triphenylamine-based dyes were synthesized. These
dyes commonly consist of a bulky donor and a rod-like linker. It is
From a co-sensitization perspective, the superiority of simple-
structure dye SE can be understood from the molecular structure.
Scheme 1. Synthetic Route of SE. Reaction conditions: (a) Lawesson reagent, toluene, reflux, 12 h; (b) 5-formyl-2-thiopheneboronic acid, TBAB, DMF, Pd(PPh3)4, H2O, NaF, 60 ꢀC, 5 h;
(c) cyanoacetic acid, CH3COONH4, CH3COOH, reflux, 5 h.

88
S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
Fig. 2. The optimized structures and frontier orbitals of SE, AZ360, and AZ362 by DFT calculations.
Cyclic thiourea functionalized triphenylamine-based dye molecules,
such as AZ360 or AZ362, possess a bulky donor along with a rod-like
linker. In contrast, SE has a simple molecular structure and small
volume, especially on the horizontal direction as observed in Fig. 2.
absorption band of SE can complement the absorption valley of
AZ360 and AZ362, especially, the combination of SE and AZ362
seems to show more perfect complementary absorption. Hence, it
is expected that the absorption defects of AZ360 and AZ362 will be
complemented by addition of SE. Based on the above assumptions,
different molar percentages of SE (10%, 30%, and 50%) were added
into AZ360 and AZ362 for UVeVis absorption measurements. As
illustrated in Fig. 5a and c, the absorption valleys of AZ360 and
AZ362 are well filled up; however, the absorption bands of
AZ360 þ SE and AZ362 þ SE become narrow along with the
The
p-linkers’ lengths of AZ360 and AZ362 are 12.77 and 16.51 Å,
respectively. For SE, however, the length of the whole molecule is
only 12.86 Å. Apparently, SE has a right shape and size to fill up the
intermolecular space between the tree-shaped dye molecules when
adsorbed onTiO2 films. Asseen fromFig. 3, thecombination ofSE and
AZ362 seems to show a better molecular size matching in length.
3.3. Optical properties
As seen from Fig. 4a, SE presents a strong absorption band in the
range of 350e500 nm along with a minor absorption peak at
around 315 nm. The former may be ascribed to the intramolecular
charge-transfer (ICT) transition between the cyclic thiourea func-
tionalized benzene ring and the cyanoacetic acid acceptor moiety
[41]; the latter might be assigned to the localized
the cyclic thiourea functionalized benzene. The intensity of ICT
transition is higher than that of the * transition (see Table 1).
p-p* transition of
p-p
However, the absorption spectra of AZ360 and AZ362 exhibit two
distinct broad absorption bands in the ranges of 300e400 nm and
400e600 nm, just like the other reported cyclic thiourea func-
tionalized triphenylamine-based dyes [34e36]. It is obvious that
there is an absorption valley at around 400 nm, in which, the ab-
sorption intensity is relatively weak. Fortunately, the strong
Fig. 3. Sketch map for molecular size matching of the dyes by DFT calculations.

S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
89
Fig. 4. UVeVis absorption spectra of SE, AZ360, and AZ362 in MeCN-DCM (1:1, v/v) solutions (a) and on TiO2 films (b). The absorption of a TiO2 film has been subtracted for clarity
of presentation.
increasing addition amount of SE. This is easy to understand that
the spectral response is gradually dominated by SE.
are described in the Supporting Information, including Fig. S1 and
Table S1). As shown in Fig. 6, all dyes adsorbed on TiO2 surfaces
were decomposed completely at around 550 ꢀC, and they show
similar decomposition behaviors. Weight percentages (W) of TiO2
with AZ360 þ SE and AZ362 þ SE at 550 ꢀC are 86.74% and 84.71%,
respectively. After careful analysis, it is found that AZ360 þ SE
exhibits less weight losses in comparison with AZ360 (W ¼ 86.11%),
while AZ362 þ SE shows more weight losses in comparison with
AZ362 (W ¼ 85.17%). These phenomena could be explained that
there is competitive adsorption between AZ360 and SE when co-
adsorbed onto TiO2 surface, thus more AZ360 molecules were
replaced by SE molecules; but filling effect may play a leading role
between AZ362 and SE due to suitable molecular sizes. HPLC
analysis of the desorption solutions provide evidences that the dye-
loading amounts (10ꢁ7 mol cmꢁ2) of co-sensitized devices with
AZ360 þ SE and AZ362 þ SE increase significantly in comparison
with those of the devices with AZ360 and AZ362 alone (2.81 vs.
2.31 and 3.35 vs. 2.33, respectively).
Compared with the results obtained in solutions, the absorption
peaks of SE, AZ360 and AZ362 on TiO2 films (Fig. 4b) are signifi-
cantly broadened, implying that energy levels of the sensitizer
molecules have somewhat changed due to the interaction with TiO2
[42]. A closer look at the absorption spectra reveals that SE shows
stronger absorption intensity in the range of 375e450 nm than
AZ360 and AZ362. This could be attributed to the powerful ab-
sorption ability of SE in this region. As seen from Fig. 5b and d,
when co-adsorbed onto TiO2 films, the absorption spectra of
AZ360 þ SE and AZ362 þ SE are higher and broader than those of
the corresponding individual dyes. This means that the addition of
SE is favorable for improving the light-harvesting ability. Never-
theless, the absorption intensity of AZ362 þ SE changes more in the
range of 375e450 nm in comparison with that of AZ360 þ SE,
indicating that SE might be a more effective co-sensitizer for
AZ362, which can be observed from Fig. 4a.
3.4. Dye loading properties
3.5. Photovoltaic performance of DSSCs
Dye loading was measured by Thermogravimetric analysis
(TGA), and the results are listed in Table 2. (Additional data of TGA
The JeV curves of co-sensitized solar cells were measured under
simulated AM 1.5G irradiation (100 mW cmꢁ2), and the
Fig. 5. UVevis absorption spectra of different molar percentages of SE added to AZ360 or AZ362 in MeCN-DCM (1:1, v/v) solutions (a and c) and on TiO2 films (b and d).

90
S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
Table 2
Photovoltaic performance and dye-loading capacity for DSSCs based on SE, AZ360, AZ362, AZ360 þ SE, and AZ362 þ SE.
Dyes
Jsc/mA cmꢁ2
Voc/mV
FF
PCE/%
IPCE/% (lmax/nm)
Dye loading/10ꢁ7 mol cmꢁ2
SE
AZ360
AZ362
7.39
14.92
15.56
15.16
16.24
618.0
680.1
690.1
690.0
720.0
0.73
0.65
0.65
0.67
0.65
3.34
6.60
7.00
7.03
7.65
61.26 (410)
73.23 (460)
77.99 (490)
75.96 (440)
85.20 (440)
2.69
2.31
2.33
AZ360 þ SE
2.81 (1.27 þ 1.54)
AZ362 þ SE
3.35 (1.63 þ 1.72)
photovoltaic parameters are summarized in Table 2. To our sur-
prise, the simple molecule SE yields a PCE of 3.34%, with a short-
circuit current density (Jsc) of 7.39 mA cmꢁ2, an open-circuit
voltage (Voc) of 618.0 mV, and a fill factor (FF) of 0.73. The results
suggest that utilizing this type of simple molecule as a co-sensitizer
is feasible. For comparison, control groups sensitized by AZ360 or
AZ362 were fabricated, which produce PCEs of 6.60%
(Jsc ¼ 14.92 mA cmꢁ2, Voc ¼ 680.1 mV, and FF ¼ 0.65) and 7.00%
(Jsc ¼ 15.56 mA cmꢁ2, Voc ¼ 690.1 mV, and FF ¼ 0.65), respectively.
Next, different molar percentages of SE (10%, 30%, and 50%) were
added into AZ362 for optimizing co-sensitized conditions, and
could contribute to occupy the space defects between the tree-
shaped dyes, thus inhibit the charge recombination and intermo-
lecular aggregation.
To further understand effects of co-sensitization on the photo-
current characteristics, the IPCE spectra were plotted as a function
of excitation wavelength and are presented in Fig. 7b and d. The
corresponding data are listed in Table 2. DSSCs based on SE, AZ360,
and AZ362 display the maximum IPCE values of 61.26% at 410 nm,
73.23% at 460 nm, and 77.99% at 490 nm, respectively. After co-
sensitization, the characteristic peaks of SE, AZ360, and AZ362
are clearly observed in the IPCE spectra of AZ360 þ SE and
AZ362 þ SE, which suggest that two organic dyes were simulta-
neously adsorbed onto the TiO2 surfaces [26]. Within the spectral
region from 350 to 700 nm, the IPCE spectra of AZ360 þ SE and
AZ362 þ SE are significantly improved in comparison with the
corresponding individual dyes, and produce the maximum IPCE
values of 75.96% at 440 nm and 85.20% at 440 nm, respectively,
indicating that complementary absorption is an effective approach
to extend the spectral response and enhance the photocurrent.
From Fig. 7b and d, however, it can be seen that SE makes a greater
contribution to AZ362 than that to AZ360, thus resulting in a higher
maximum IPCE value. These results indirectly indicate that more SE
were stuffed into the space defects between the tree-shaped dye
AZ362 due to the better molecular size matching in length.
EIS measurements were performed to further elucidate the
interfacial charge recombination process in co-sensitized solar
cells. In the Nyquist plots (Fig. 8a), there are two semicircles, and
the larger semicircle indicates charge-transfer resistance (Rct) at the
TiO2/dye/electrolyte interface, i.e., the recombination kinetics be-
tween conduction-band electrons in TiO2 and I-3 species from the
electrolyte. A larger radius indicates a larger Rct and slower electron
recombination kinetics. Obviously, co-sensitized solar cells present
the bigger Rct and slower recombination than cells based on the
individual dyes. These observations indicate that the “exposed”
TiO2 surfaces between dyes AZ360 or AZ362 may be covered by
simple-structure dye SE, which are favorable for blocking Iꢁ3 in
contract with TiO2 and reducing the back reaction probability
[44,45]. In addition, the hexyl chains attached at SE could also
impede the diffusion of Iꢁ3 onto TiO2 surfaces [28,46]. Therefore,
corresponding devices exhibit PCEs of 7.46% (Jsc ¼ 15.91 mA cmꢁ2
,
Voc ¼ 700.0 mV), 7.65% (Jsc ¼ 16.24 mA cmꢁ2, Voc ¼ 720.0 mV), and
7.26% (Jsc ¼ 14.97 mA cmꢁ2, Voc ¼ 688.8 mV), respectively. (See
Fig. S2 and Table S2)
Obviously, the addition of SE has a large impact on the photo-
voltaic performances of the co-sensitized devices. It is worth noting
that, when 50% of SE was added, the Jsc and Voc values decline
significantly, thus resulting in a smaller PCE improvement. This
might be due to competitive adsorption between the additional
and main dyes [31,43]. As a result, the additional amount of SE
should be controlled in 30%. Under the optimal conditions, co-
sensitized solar cells based on AZ360 þ SE or AZ362 þ SE were
fabricated. As seen from Table 2, the superiority of co-sensitization
is obvious. Compared with AZ360 and AZ362, the PCEs of
AZ360 þ SE and AZ362 þ SE are improved from 6.60% to 7.03% and
from 7.00% to 7.65%, respectively. This should be caused by the
improvement of Jsc and Voc. The enhanced Jsc can be explained by
the improved light harvesting due to complementary absorption
[18], while the increased Voc may be attributed to the intermolec-
ular aggregation and charge recombination being suppressed by
the insertion of co-adsorbent dye SE [22,38]. However, a closer look
at Fig. 7a and c reveals that the addition of SE seems to be more
effective for AZ362. This may be because that the simple-structure
dye SE and tree-shaped dye AZ362 show the better complementary
absorption (see Fig. 4a) and molecular size matching in length (see
Fig. 3). The better complementary absorption is favorable for
improving light harvesting, resulting in an increased photocurrent;
at the same time, the suitable molecular size matching in length
Fig. 6. TGA curves for TiO2 adsorbed SE, AZ360, AZ362, AZ360 þ SE, and AZ362 þ SE.

S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
91
Fig. 7. JeV curves and IPCE spectra and for DSSCs based on SE, AZ360, AZ362, AZ360 þ SE, and AZ362 þ SE.
there should be higher electron density in the conduction band of
TiO2 that could cause a lift-up of the Fermi level for higher Voc [30].
In the Bode phase plots (Fig. 8b), the lower-frequency peak is
indicative of the charge-transfer process of injected electrons in
TiO2. From the peak frequency, the electron lifetime (te) can be
calculated [15]. The calculated results increase in the order of SE
(11.0 ms) < AZ360 (22.6 ms) < AZ362 (24.2 ms) < AZ360 þ SE
(24.5 ms) < AZ362 þ SE (31.7 ms). Apparently, the co-sensitized
solar cells based on AZ360 þ SE and AZ362 þ SE have longer
electron lifetimes than those of the cells sensitized by AZ360 and
AZ362 alone. This means much more effective suppression of the
back reaction between the injected electrons and the electrolyte,
as an effective donor to construct dye molecule. The results suggest
that this kind of simple-structure dye can not only complement the
absorption valleys but also stuff the space defects of the bulky dyes,
consequently resulting in improved light-harvesting abilities and
enhanced photocurrent. On the other hand, the “exposed” TiO2
surfaces between bulky dyes could be covered by co-adsorbent dye
SE, which are favorable for blocking Iꢁ3 in contract with TiO2 and
reducing the charge recombination rate. In addition, the hexyl
chains introduced into SE molecule can inhibit intermolecular ag-
gregation between bulky dyes, thus resulting in an enhanced Voc of
the co-sensitized devices. Under the optimal conditions, co-
sensitized solar cell based on AZ362 þ SE shows a PCE of 7.65%,
which increases by 9.29% in comparison with that of DSSC based on
AZ362 alone (PCE ¼ 7.00%). This would spark our inspiration to
design a series of new simple molecules used for both co-
thus resulting in the improvement of Voc
.
4. Conclusions
adsorbents and co-sensitizers, thereby form
a
new co-
sensitization system with organic dyes, which is characterized by
utilizing synergy between simple and bulky dyes to improve the
photovoltaic performances of DSSCs.
A novel simple-structure dye SE was synthesized and evaluated
for co-sensitized solar cells. From DFT calculations, it can be seen
that the cyclic thiourea functionalized benzene ring can be served
Fig. 8. EIS spectra for DSSCs based on SE, AZ360, AZ362, AZ360 þ SE, and AZ362 þ SE: Nyquist (a) and Bode phase plots (b).

92
S. Zhu et al. / Dyes and Pigments 120 (2015) 85e92
[21] Ni JS, You JH, Hung WI, Kao WS, Chou HH, Lin JT. Organic dyes incorporating
Acknowledgments
the dithieno[3',2':3,4;2',3':5,6]benzo[1,2-c]furazan moiety for dye-sensitized
solar cells. ACS Appl Mater Interfaces 2014;6:22612e21.
This work was supported by the Natural Science Foundation of
China (No. 51373092) and Fundamental Research Funds for the
Central Universities (No. GK201304005) from the Department of
Education. Our special thanks are also for the Fund of New Energy
Devices and Materials provided by Mr. He Chong Ben, Hong Kong.
We are grateful to Professor Wenliang Wang at Shaanxi Normal
University for theoretical calculations. We also thank Minister Chao
Gao and Miss Haimei Wu at Xi'an Modern Chemistry Research
Institute for electrochemical measurements.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.04.003.
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