Novel dye sensitizers of main chain polymeric metal complexes based on complexes of diaminomaleonitrile with Cd(II), Ni(II): Synthesis, characterization, and photovoltaic performance for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jorganchem.2013.09.006

Four new donor-π-acceptor (D-π-A) dyes (P1, P2, P3, P4) were synthesized, characterized and applied in dye-sensitized solar cells (DSSCs). Poly(p-phenylenevinylene) (PPV) or Bisthien PPV was used as donor, ethilenic link or thiophene as π-bridge, and Cadmium complexes or Nickel complexes as acceptor in these D-π-A type sensitizers. The introduction of the polymerization of π system and the metal complexes has greatly increased the light absorption. Increasing the number of thienyl group in the molecule to a certain extent was effective in further lowering both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels to attain higher open-circuit voltage (V_oc). Among them, P3-based cell shows excellent photovoltaic performance and power conversion efficiency up to 2.25% under AM 1.5G irradiation (100 mW cm ^-2).

Full text of DOI:10.1016/j.jorganchem.2013.09.006

Journal of Organometallic Chemistry 749 (2014) 26e33
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Novel dye sensitizers of main chain polymeric metal complexes based
on complexes of diaminomaleonitrile with Cd(II), Ni(II): Synthesis,
characterization, and photovoltaic performance for dye-sensitized
solar cells
Wei Zhang, Xueliang Jin, Xiaoguang Yu, Jun Zhou, Guipeng Tang, Dahai Peng, Jiaomei Hu,
*
Chaofan Zhong
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan,
Hunan 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2013
Received in revised form
Four new donor-
dye-sensitized solar cells (DSSCs). Poly(p-phenylenevinylene) (PPV) or Bisthien PPV was used as donor,
ethilenic link or thiophene as -bridge, and Cadmium complexes or Nickel complexes as acceptor in
these D- -A type sensitizers. The introduction of the polymerization of system and the metal com-
plexes has greatly increased the light absorption. Increasing the number of thienyl group in the molecule
to a certain extent was effective in further lowering both the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy levels to attain higher open-circuit voltage
p-acceptor (D-p-A) dyes (P1, P2, P3, P4) were synthesized, characterized and applied in
p
27 August 2013
p
p
Accepted 5 September 2013
Keywords:
Dye-sensitized solar cells
Polymeric metal complex
Thiophene
Diaminomaleonitrile
Photovoltaic performance
(Voc). Among them, P3-based cell shows excellent photovoltaic performance and power conversion ef-
ꢀ
2
ficiency up to 2.25% under AM 1.5G irradiation (100 mW cm ).
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
the third generation solar cells is to deliver electricity at a large
scale competitive price, that is, less than 0.5 $/Wp, which can
complete with fossil energy. From the current point of research
progress, Dye-sensitized solar cells will be the most potential third-
generation solar cells.
Solar cells, or photovoltaics, were recognized as the main
methods and approaches to solve the world’s future energy prob-
lems [1]. Since 1954, Chapin, Fuller, and Pearson at Bell Laboratories
have improved the efficiency of a Si cell to 6% [2]. Especially in the
last twenty years, the research and practical applications of solar
cells have made a lot of progress. But it can not also compete with
fossil fuels on the issues of cost and practicality. At present, the
research and applications of solar cells have been to enter the third
generation [3,4]. The first generation is crystalline silicon solar cells,
whose energy conversion efficiency can reach 24%, but the cost is
more than 3 $ per peak watt (Wp); The second generation solar
cells, for example, amorphous silicon, CIGS, and CdTe, are based on
thin film technologies, whose energy conversion efficiency was up
to 19%, and the cost was down to 1 $ per peak watt (Wp) [5]. In
order to further reduce the costs and investments, scientists have
embarked on the third generation organic solar cells. The goal for
Dye-sensitized solar cells (DSSCs), also known as Grätzel cells,
were invented by Grätzel and O’Regan in 1991 [6]. There are more
advantages of dye-sensitized solar cells than the first-generation
and second-generation solar cells: relatively small weight; low
production cost; short energy payback time; high light-to-electrical
conversion efficiencies, and so forth [7e9]. The key indicator to
measure the performance of dye-sensitized solar cells is the con-
version efficiency. The theoretical conversion efficiency could be
achieved about 20%, but the maximum efficiency of the current
experimental study is just 12% [10,11]. There are a lot of factors that
affect the conversion efficiency of the solar cells, and the photo-
sensitizer is one of the most critical parts in dye-sensitized solar
cells, because it determines the light absorption properties, elec-
tron transport, and the composite performance. Therefore, many
scientists of chemistry, physics and materials take much interest in
the new high-performance dye sensitizers, and it has become the
most favorite of worldwide research and development applications.
*
Corresponding author.
E-mail addresses:
C. Zhong).
zhongcf798@aliyun.com,
zhongcf798@yahoo.com.cn
(
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.09.006

W. Zhang et al. / Journal of Organometallic Chemistry 749 (2014) 26e33
27
Thousands of dyes have been synthesized and tested in DSSCs so
far, and it can be divided into three categories. The first type of dye
sensitizer is metal complexes, especially the ruthenium (Ru(II))
complexes. Light absorption in the visible part of the solar spec-
trum is due to a metal to ligand charge transfer (MLCT) process [12].
Among them, the most prominent is N719 and N749, and their
efficiency could be reached to 10.4% and 11.1%, respectively [13]. As
we all know, rare metal ruthenium is very expensive and pollute
environment badly. Therefore, it is difficult to continue the break-
through and development.
phenylenevinylene) (PPV) or bithiophene phenylenevinylene as
donor group (D), which are shown in Scheme 1. The work of Wong
et al. offers an attractive avenue towards conjugated materials with
broad solar absorptions and demonstrates the potential of metal-
lopolyynes for both visible and NIR light power generation [37,38].
Moreover, thermal properties, optical properties, and photovoltaic
properties of polymeric metal complexes are also investigated in
this paper.
2. Experimental section
The second type of dye sensitizers is porphyrin and phthalocy-
anine. Because they exhibit intense spectral response bands in the
near-IR region and possess good chemical, optical and thermal
stability. However, the energy conversion efficiency has been
hovering around 3%e9% [14e17]. Until 2010 and 2011, also the
Grätzel team synthesized the zinc porphyrin YD2 [18], YD2-o-C8
2.1. Materials
2 3 2
NiCl (PPh ) , diaminomaleonitrile (DAMN) and 3-methyl-2-
thiophenecarboxaldehyde were obtained from Aldrich Chemical
Co. and used as received. N,N-Dimethylformamide was dried
[19], the energy conversion efficiency of 11.2% and 12.7% of the
2
by distillation over CaH , and methanol was dried over molecular
record. Owing to the poor solubility of such dyes, it is difficult for
film formation on the surface of the TiO , so its development will
2
also be restricted.
The third type of sensitizers is organic dyes. They include n-type
dyes such as coumarin dyes [20e22], triphenylamine dyes [23,24],
carbazole dyes [25e27], perylene dyes [28e30], and p-type dyes
sieves and freshly distilled prior to being used. The other materials
were common commercial grade and used as received. All chem-
icals used were of an analytical grade. Solvents were purified with
conventional methods.
2.2. Instruments and measurements
[
31,32]. Compared with the noble ruthenium complex dyes, organic
dyes have many advantages: structure diversity, easy design and
synthesis, low cost, fewer environmental pollution problems, high
extinction coefficient, and a higher conversion efficiency of 5%e10%
1H NMR were performed in CDCl
and recorded on a Bruker
3
Avance 400 spectrometer. Gel Permeation Chromatography (GPC)
analyses were measured by a WATER 2414 system equipped with a
[33]. The organic dyes have poor chemical stability, light stability,
3 4 5
set of HT , HT and HT , l-styrayel columns with THF as eluent and
especially the thermal stability due to a small molecular weight.
Therefore, it is difficult to achieve large-scale research and
application.
People have come up to a preliminary conclusion on the dyes’
synthesis from the research and development of the above dye
sensitizers in order to obtain more efficient dye sensitizers. To
design a new dye, we should bear in mind the following rules: wide
absorption band and a high absorption coefficient; favorable HOMO
and LUMO energies for efficient charge transfer; a good carrier
transport bridges to enhance the carrier transport; good solubility;
polystyrene as standard. The FT-IR spectra were obtained on a Per-
kineElmer Spectrum. One Fourier transform infrared spectrometer
by incorporating samples in KBr pellets. Differential Scanning
Calorimetry (DSC), Thermogravimetric analysis (TGA) and Elemental
analysis were performed on PerkineElmer DSC-7 thermal analyzer,
Shimadzu TGA-7 Instrument, and PerkineElmer 2400 II instrument,
respectively. UVeVisible spectra of all the polymers were taken on a
Lambda 25 spectrophotometer. Photoluminescent spectra (PL) were
taken on a PerkineElmer LS55 luminescence spectrometer with a
xenon lamp as the light source. Cyclic voltammetry (CV) was con-
ducted on a CHI630C Electrochemical Workstation using a three
2
strong anchoring on the surface of TiO ; a high light stability,
chemical stability and thermal stability for durable devices and so
on. The first three features are the most important, and mainly
depend on the dye molecules to the light absorption and electron
transfer. This can be changed by tuning its ground state, the exci-
4 4
electrode system, in a [Bu N]BF (0.1 M) DMF solution at a scan rate
of 100 mV/s. A glassy carbon rod, a Pt wire electrode and a saturated
calomel electrode (SCE) were used as working electrode, auxiliary
electrode and reference electrode, respectively.
tation state energy and the energy gap
ground state). In order to reduce the
gap of metallated polymer can extend toward the near-infrared
NIR) range of the solar spectrum [34,35]), thereby shifting the
max to longer wavelengths, the following major approaches can be
adopted: (i) enlargement of the systems, (ii) transition from ar-
D
E (
D
E ¼ E excited state ꢀ E
D
E of a given dye (Low band
2.3. Synthesis
(
l
2.3.1. Synthesis of 5-bromo-3-methyl-2-thiophenecarboxaldehyde
(1)
p
Following the preparation as described in U.S. Publication NO.
omatic to quinoidal structures, (iii) introduction of donor acceptor
substituents, (iv) polymerization.
It is well known that metal chelates of salicylaldehyde and
diaminomaleonitrile (DAMN) derivatives are the most super ma-
terials as an electron transporter, and increasing the number of
thiophene units in poly(p-phenylenevinylene) can further extend
2
2006/0199836, Br (7.34 mL, 0.14 mol) was added to a solution of 3-
methyl-2-thiophenecarboxaldehyde (17.1 mL, 0.14 mol) in chloro-
ꢁ
form (118 mL) dropwise at 0 C over a period of 20 min. The re-
action was allowed to slowly warm to room temperature and stir
for 2 h. The brown/red solution was diluted with 150 mL of CH
and washed with water, 1.5 M K HPO and brine. The organic layer
was dried over anhydrous Na SO and concentrated. The residue
2 2
Cl
2
4
the
photocurrent, as the result of the red-shifted absorption of the
sensitizer loaded TiO film [36]. Although chemists rarely report
p
-conjugation, which can also increase the short circuit
2
4
was purified by flash chromatography (silica gel, hexanes:EtOAc,
2
100:0e50:50) to afford 25.1 g (65% yield) of the title compound as a
1
their application as dye sensitizers for dye-sensitized solar cells,
there is no doubt that polymeric substance contains salicylalde-
hyde and DAMN may satisfy application requirements to make an
effective way to improve open-circuit photovoltage (Voc).
brown solid: H NMR (400 MHz, CDCl
3
) 9.94 (s, 1H), 6.74 (s, 1H),
2.58 (s, 3H).
2.3.2. Synthesis of (2-amino-3-(5-bromo-3-methylthiophen-2-yl)
methyleneamino)-fumaronitrile (TDAMN)
According to the above-mentioned points, we have designed
and synthesized four D-
acceptor (A), ethilenic bond as a
p
-A dyes possessing a metal-DAMN as an
An ethanol solution (100 mL) of 5-bromo-3-methyl-2-
thiophenecarboxaldehyde (1) (2.0326 g, 0.01 mol) was dropped
p-conjugation linkage, and poly(p-

28
W. Zhang et al. / Journal of Organometallic Chemistry 749 (2014) 26e33
Scheme 1. Synthetic route of the monomers and polymeric metal complexes P1eP4.
to an ethanol solution (20 mL) of DAMN (1.0810 g, 0.01 mol), and
the mixture was refluxed for 16 h. The yellow solution was cooled in
ice to give fine yellow needles, which were filtered off, washed with
cooled ethanol, and dried under vacuum (2.2137 g yield 75%). FT-IR
charged with a mixture of metal complex L1. (0.2315 g, 0.3 mmol),
1,4-Bis(octyloxy)-2,5-divinylbenzene (0.116 g, 0.3 mmol), Pd(OAc)
(0.0024 g, 0.012 mmol), triethylamine (3 mL), 3-o-tolyl phosphine
(0.0220 g, 0.072 mmol) and DMF (8 mL). Then the flask was
2
ꢀ1
(
KBr, cm ): 3406, 3299, 3196, 2234, 2202, 1616, 1589, 1420, 1381,
24. Anal. Calcd for C10 BrN S (2): C, 40.69; H, 2.39; N, 18.98; S,
pumped into a vacuum and purged with N
2
. The mixture was
ꢁ
8
H
7
4
heated at 90 C for 18 h under N
2
. After that, it was filtered after
1
C
0.86. Found: C, 41.35; H, 2.24; N, 19.47, S, 11.42. MS: Calcd for
10 7 4 3
H BrN S [M] 293.96; found, 294.95. H NMR (400 MHz, CDCl ):
cooled to room temperature and the filtrate was poured into
ethanol. The yellow precipitate was filtered and washed with cold
ethanol. The crude product was purified by dissolving in DMF and
precipitating into ethanol to afford a light yellow solid. (Yield:
0.2965 g, 56%). FT-IR (KBr, cm ): 2921, 2851, 1643 (C]N), 2328
(C^N), 1095 (CeOeM), 784 (AreH), 495 (NeM). Et al. Calcd for
þ
1
7.62 (s, 1H), 6.74 (s, 1H), 2.35 (s, 2H), 2.21 (s, 3H).
ꢀ
1
2
.3.3. Synthesis of L1 [39]
CdCl $2.5H O (0.0572 g, 0.25 mmol) was dissolved in methanol
50 mL) with stirring and refluxing. 2-amino-3-((E)-(5-bromo-3-
methylthiophen-2-yl)methyl-enea-mino) fumaronitrile (2)
0.1476 g, 0.5 mmol) dissolved in methanol (75 mL) was then added
2
2
(
2 8 2 2
[C46H52Cl N O S Cd]: C, 55.45; H, 5.26; N, 11.25; S, 6.44. Found: C,
56.21; H, 5.73; N, 11.32; S, 6.92. Mn ¼ 13.0 kg/mol, PDI ¼ 1.31.
(
and the mixture refluxed for 11 h to give a khaki-brown precipitate,
which was filtered off, washed with methanol, and dried under
2.3.6. Synthesis of polymeric metal complex P2
With the similar synthetic method as P1 afford a yellow
ꢀ
1
vacuum. Yield: 0.1794 g (93%). Anal. Calcd for C20
2
H12Br Cl
2
N
8
S
2
Cd:
solid (0.1807 g, 52%). FT-IR (KBr, cm ): 2928, 2849, 1657 (C]N),
2355 (C^N), 1108 (CeOeM), 762 (AreH), 496 (NeM). Et al.
C, 31.13; H,1.57; N,14.52; S, 8.31. Found: C, 31.54; H,1.62; N,13.98; S,
ꢀ
1
8
.39. FT-IR (KBr, cm ): 3040 (]CeH), 2976, 2919, 1570 (C]N),
52 10 8 2
Calcd for [C46H N O S Ni]: C, 55.48; H, 5.26; N, 14.07; S, 6.44.
2
175 (C^N), 1035 (CeOeM), 931 (AreH), 494 (NeM).
Found: C, 56.03; H, 5.75; N, 14.71; S, 6.02. Mn ¼ 11.9 kg/mol,
PDI ¼ 1.36.
2.3.4. Synthesis of L2
In the same manner as described for L1 afford a light brown
2.3.7. Synthesis of polymeric metal complex P3
solid. Yield: 91%. Anal. Calcd for C20
H
12Br
2
N
10
O
6
S
2
Ni: C, 31.16;
The copolymer was synthesized by Yamamoto coupling method
according to the lit [41,42]. L1 (0.1543 g, 0.2 mmol), bis(-
triphenylphosphine) nickel(II) chloride (0.13 g, 0.2 mmol), 2,2 -
H, 1.57; N, 18.17; S, 8.32, 14.52. Found: C, 30.82; H, 1.62; N, 18.24;
ꢀ
1
0
S, 8.40. FT-IR (KBr, cm ): 3038 (]CeH), 2973, 2931, 1642, 1569
C]N), 2427 (C^N), 1030 (CeOeM), 939 (AreH), 492 (NeM).
(
(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-2,1-diyl)bis(2-bromo-
4-methylthiophene) (0.1474 g, 0.2 mmol), zinc (0.065 g, 1 mmol),
2
.3.5. Synthesis of polymeric metal complex P1
The polymeric metal complex P1 was synthesized by using the
triphenylphosphine (0.1045 g, 0.4 mmol), and a little bipyridine
(0.003 g, 0.019 mmol) were dissolved in DMF (7.5 mL) under ni-
trogen. Then the mixture was stirred at 90 C for 48 h. The yellow
ꢁ
Heck coupling method, according to the literature [40]. A flask was

W. Zhang et al. / Journal of Organometallic Chemistry 749 (2014) 26e33
29
3. Results and discussion
3.1. Synthesis and characterization
The detailed synthetic routes of the four polymeric metal com-
plexes (P1, P2, P3, P4) as well as the monomers are shown in
Scheme 1. Meanwhile, the two polymers (P1, P2) were synthesized
by the Heck coupling [43], the other two polymers (P3, P4) were
synthesized by the Yamamoto coupling [41]. The four as-synthesized
polymers could be dissolved in common organic solvents such as
DMF and THF at room temperature. But in the other solvents they
exhibit a poor solubility, such as in acetone and chloroform.
þ
Fig. 1 shows time-of-flight mass spectrometry of (TDAMN). [M]
2
93.96; found, 294.95. It indicates that the compound has been
1
successfully synthesized (Fig. 2 H NMR spectra of TDAMN in CDCl
3
solution).
The IR spectra of the ligand monomer (2), metal complex
L1, L2) and the target polymers (P1, P2, P3, P4) are shown in
(
Fig. 1. Time-of-flight mass spectrometry of (TDAMN).
Figs. 3 and 4. As for the monomer 2, the absorption peaks at 3406,
ꢀ1
3299, 3196 cm are the characteristic absorption peaks of the
amino-group stretching vibration, absorption peaks at 2234, 2202
solid was precipitated into a large excess of ethanol solution. The
crude product was washed with ethanol, distilled water, and THF
are due to C^N bond stretching vibration, absorption peaks at
1616 cm are due to C]N bond stretching vibration, and 1590 is
the bond stretching vibration of C]C. From Figs. 3 and 4, the peaks
ꢀ1
ꢁ
sequentially, then dried in vacuum at 65 C for one day to afford
ꢀ1
ꢀ1
ꢀ1
pale yellow solids (0.1480 g, 61%). FT-IR (KBr, cm ): 3051 (]CeH),
of 1035 cm and 1030 cm are due to CeNeM stretching vibra-
tion of metal compound L1 and L2 respectively [44]. And then
integrating with the results of elemental analysis, we can come up
2
4
924, 2862, 1645 (C]N), 2210 (C^N), 1113 (CeOeM), 761 (AreH),
97 (NeM). Anal. Calcd for [C56 Cd]: C, 56.58; H, 5.09;
H
2 8 2 4
60Cl N O S
2
þ
2þ
N, 9.43; S, 10.79. Found: C, 57.12; H, 4.56; N, 9.51; S, 10.82.
Mn ¼ 10.7 kg/mol, PDI ¼ 1.67.
with that metal Cd and Ni which have been successfully co-
ordinated with ligand 2, namely, complexes L1 and L2 have been
successfully synthesized.
2
.3.8. Synthesis of polymeric metal complex P4
Gel permeation chromatography (GPC) study for all the target
polymers is shown in Table 1. The number average molecular
weight of P1, P2, P3 and P4 are 13.0, 11.9, 10.7 and 9.5 kg/mol, and
the units of them are 13, 12, 9 and 8 respectively. All the PDI of
polymeric metal complexes are relatively wide (P1, P2, P3, P4: 1.31,
1.36, 1.67 and 1.53, respectively). The changes of the molecular
weight are also proved that the copolymerization has been taken
With the similar synthetic method as P3 afford a yellow solid.
ꢀ
1
Yield (%): 63%. FT-IR (KBr, cm ): 3049 (]CeH), 2915, 2856, 1659
C]N), 2217 (C^N), 1115 (CeOeM), 719 (AreH), 494 (NeM). Anal.
Calcd for [C56 Ni]: C, 56.61; H, 5.09; N, 11.79; S, 10.80.
(
60 10 8 4
H N O S
Found: C, 57.25; H, 6.57; N, 11.68; S, 10.37. Mn ¼ 9.5 kg/mol,
PDI ¼ 1.53.
Fig. 2. 1H NMR spectra of TDAMN in CDCl
solution.
3

30
W. Zhang et al. / Journal of Organometallic Chemistry 749 (2014) 26e33
Table 1
Molecular weights and thermal properties of the polymeric metal complexes.
a
3
a
3
b
ꢁ
c
ꢁ
Polymer
M
n
[ꢂ10 ]
M
w
[ꢂ10 ]
PDI
T
g
[ C]
d
T [ C]
P1
P2
P3
P4
13.0
11.9
10.7
9.5
17.0
16.2
17.9
14.5
1.31
1.36
1.67
1.53
153
131
144
120
325
302
353
330
a
Determined by gel permeation chromatography using polystyrene as standard.
Determined by DSC with a heating rate of 20 C/min under nitrogen.
The temperature at 5% weight loss under nitrogen.
b
c
ꢁ
of light than polymers containing metal-Ni. And it also indicates
that adding thienyl group into polymers can promote the absorp-
tion of light.
The normalized photoluminescent (PL) spectra of P1, P2, P3 and
P4 in DMF solution are shown in Fig. 6, the excitation wavelengths
were set according to the absorption peak of UVevis spectrum, and
the corresponding optical data are also listed in Table 2. It can be
seen that the PL peaks of them are at 486, 497, 455 and 470 nm,
respectively, which can be attributed to the
intra-ligand.
pep* transition of
Fig. 3. FT-IR spectra of TDAMN, L1, P1, P3.
place between the monomers, which is a further evidence for the
target polymers that have been successfully synthesized.
3.3. Thermal stability
The thermal properties of the polymeric metal complexes were
3
.2. Optical properties
investigated by differential scanning calorimetric (DSC) and
thermo-gravimetric analysis (TGA), and the corresponding data are
also reported in Table 1. The TGA image which present in Fig. 7
shows that P1, P2, P3 and P4 possess good thermally stability
The UVevis and normalized PL spectra of the polymeric metal
ꢀ5
complexes P1, P2, P3 and P4 (10 M in DMF solution) are shown in
Figs. 5 and 6 respectively, and the corresponding data are sum-
marized in Table 2.
with 5% weight loss at temperatures (T
330 C in nitrogen, respectively. We could see that P1, P2, P3 and P4
d
) of 325, 302, 353, and
ꢁ
ꢁ
The ligand L shows a UVevis normalized absorption peak
have glass transition temperature (T
g
) ranged from 120 to 153
C
located at about 350 nm, corresponding to the
p
e
p
* electron
and followed the order P1 > P3 > P2 > P4, which suggests that
polymeric metal complex containing metal-Cd has hold higher ri-
gidity than that containing metal-Ni.
transitions of the conjugated molecules, which was observed in the
ICT between the electron acceptor TDAMN unit and the electron
donating PPV moiety. In Fig. 5, the maximum absorption of P1, P2,
P3 and P4 are at 420, 405, 451 and 422 nm, respectively. Those
polymers have very weak shoulder absorption peak in the band
3.4. Electrochemical properties
4
50e550 nm due to the charge transition of the DAMN derivatives
The electrochemical behaviors of the polymers were investi-
10
and d metal ions in the polymer. The one which shows a better UV
absorption by comparing P2 with P4 as well as P1 with P3, in-
dicates that polymers containing metal-Cd have better absorption
gated by cyclic voltammetry, which is one of the important prop-
erties for organic materials used in solar cells. Fig. 8 shows the
cyclic voltammetry curves of P1, P2, P3, P4. The cyclic voltammetry
4 4
was measured in DMF solution which used [Bu N]BF as supporting
electrolyte and a saturated calomel electrode (SCE) as the reference
electrode with a scan rate of 100 mV/s. The lowest unoccupied
molecular orbital (LUMO), highest occupied molecular orbital
g
(HOMO) energy levels and energy gap (E ) can be handled conve-
niently by equations as follows [42].
Table 2
Optical and electrochemical properties of the polymeric metal complexes.
a,maxa,
l
a,onset
l
p,max
b
E
g,opt/eVc
E
E
red
HOMO LUMO Eg,EC/eVe
(eV) (eV)
Polymer
l
ox
d
d
(V) (V)
P1
P2
P3
P4
420, 561
405, 528
451, 585
422, 566
486
497
455
470
2.21
2.35
2.12
2.19
1.11 ꢀ0.92 ꢀ5.51 ꢀ3.48 2.03
1.06 ꢀ1.04 ꢀ5.46 ꢀ3.36 2.10
1.17 ꢀ0.82 ꢀ5.57 ꢀ3.58 1.99
1.12 ꢀ0.89 ꢀ5.52 ꢀ3.51 2.01
a
l
a,max
, la,onset: The maxima and onset absorption from the UVevis spectra in
DMF solution.
b
l
p,max: The PL maxima in DMF solution.
g,opt: Optical energy band gap calculated from the formula E
c
E
g
¼ 1240/
l
a,onset
(
eV).
d
Values determined by cyclic voltammetry.
e
Fig. 4. FT-IR spectra of TDAMN, L2, P2, P4.
Eg,EC: Electrochemical band gap estimated from HOMO and LUMO.

W. Zhang et al. / Journal of Organometallic Chemistry 749 (2014) 26e33
31
ꢁ
Fig. 7. TGA curves of P1eP4 with a heating rate of 20 C/min under nitrogen
Fig. 5. UVevis absorption spectra of L1, L2 and polymeric metal complexes P1eP4 in
DMF solution.
atmosphere.
the four polymeric metal complexes. Measurement of DSSCs merits
such as corresponding open-circuit voltage (Voc), short-circuit
current density (Jsc), fill factor (ff), and power conversion effi-
HOMO ¼ ꢀeðEox þ 4:40Þ ðeVÞ
LUMO ¼ ꢀeðEred þ 4:40Þ ðeVÞ
(1)
(2)
(3)
ciency (h) are listed in Table 3. It can be seen from the table that the
Voc values of P1, P2, P3 and P4 are 0.65, 0.61, 0.72 and 0.71,
E
g
¼ HOMO ꢀ LUMO
respectively. The corresponding ff values could be achieved a
higher level about 70%. However, the Jsc values increased from
The units of Eox and Ered are measured through electrochemical
curve and the data obtained are listed in Table 3. The reduction and
2
2
4
.19 mA/cm for P2 to 4.75 mA/cm for P3. The power conversion
efficiency based on P3 reached 2.25%, which is the highest of the
device based on those four polymeric metal complexes.
oxidation potentials of P1 were measured to be Eox ¼ 1.11 V and
E
red ¼ ꢀ0.92 V, respectively. The energy value of the HOMO was
The low Jsc is chalked up to the long distance between donor and
acceptor, which lead to the low charge separation and trans-
portation efficiency, and it also can be seen from the input photon
to converted current efficiency (IPCE) curves as shown in Fig. 9. The
IPCE value of P1, P2, P3 and P4 are only about 27% at 425 nm, and is
very low compared with N719 which could be achieve to 80% at
calculated to be ꢀ5.512 eV, and the energy value of the LUMO was
calculated to be ꢀ3.48 eV, so the energy band gap was 2.03 eV. In
the same way, we can get the related data of other polymers. The E
g
of the complexes follows the order, P3 > P4 > P1 > P2, so P3 is
more suitable for fabrication of optoelectronic devices than P4, P1
and P2, because a relatively low E
g
can absorb light efficiently, it’s
425 nm [46]. As discussed in the UVevis absorption spectra section,
very important to improve power conversion efficiency [45].
those two groups of data are proportional development, and the
IPCE value is decided by the absorption of light.
3.5. Photovoltaic properties
4. Conclusion
Fig. 10 displays the irradiation source for the photocurrent
densityevoltage (JeV) measurement of the DSSCs devices based on
In this paper, four polymeric metal complexes containing thio-
phene and diaminomaleonitrile units with Cd(II), Ni(II) were
Fig. 6. PL spectra of the four polymeric metal complexes in DMF solution.
4 4
Fig. 8. Cyclic voltammograms for P1eP4 in DMF/0.1 M [Bu N]BF at 100 mV/s.

32
W. Zhang et al. / Journal of Organometallic Chemistry 749 (2014) 26e33
Table 3
Photovoltaic parameters of devices with sensitizers P1eP4 in DSSCs at full sunlight
dissolved in DMF, and in the UV test and PV test, those complexes
could not have more adsorption. Therefore, in order to get good
solubility, long alkyls should be introduced in the structure. The
absorption region of the polymeric metal complexes is not wide
enough, thus they have low IPCE value. For strong adsorption onto
ꢀ2
(
AM 1.5G, 100 mW cm ).
Polymer
Solvent
J
sc (mA/cm2)
V
oc (V)
ff (%)
h (%)
P1
P2
P3
P4
DMF
DMF
DMF
DMF
4.38
4.19
4.75
4.52
0.65
0.61
0.72
0.71
69.7
70.1
65.8
65.3
1.98
1.79
2.25
2.10
2
the surface of TiO , one or two anchoring groups, such as carboxylic
acid groups or sulfonic acid groups should also be introduced in the
structure [47,48]. For the purpose of improving the solubility and
getting better photovoltaic materials, we still have much work to do
and we will report the next investigation in the future.
synthesized, and as sensitizers were used for application. The four
materials have showed high stabilities, good open-circuit voltage,
fill factor, and power conversion efficiency under simulated
AM 1.5G illumination (100 mW cm ). These results strongly pre-
dict the importance of their further investigation in DSSCs.
However, there are many challenges to be overcome: Jsc and
IPCE are very low; at the same time, the electron injection effi-
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