Novel dyes of branched chain polymeric metal complexes based on cyclopentadithiophene derivatives: synthesis, characterization and photovoltaic performance for dye-sensitized solar cells

Article abstract of DOI:10.1007/s11164-016-2452-8

Abstract: In this work, four novel D–A polymeric metal complexes (P1–P4) were designed and synthesized as dye sensitizers for high-performance dye-sensitized solar cells (DSSCs), which use cyclopentadithiophene or fluorene derivatives as donor (D) and the

Full text of DOI:10.1007/s11164-016-2452-8

Res Chem Intermed (2016) 42:6163–6179
DOI 10.1007/s11164-016-2452-8
Novel dyes of branched chain polymeric metal
complexes based on cyclopentadithiophene derivatives:
synthesis, characterization and photovoltaic
performance for dye-sensitized solar cells
Ye Liu1 Qiufang Xie1 Yanlong Liao1
•
•
•
Chunxiao Zhu1 Xu Chen1 Tianqi Chen1
Chaofan Zhong1
•
•
•
Received: 30 July 2015 / Accepted: 19 January 2016 / Published online: 27 January 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract In this work, four novel D–A polymeric metal complexes (P1–P4) were
designed and synthesized as dye sensitizers for high-performance dye-sensitized solar
cells (DSSCs), which use cyclopentadithiophene or fluorene derivatives as donor (D) and
the 2-(20-pyridyl)benzimidazole derivative as acceptor (A). All polymers exhibited good
thermal stability for their application in DSSCs. DSSCs based on P1 exhibited the best
power conversion efficiency of 2.49 % with the cyclopentadithiophene derivative as
donor and Cd(II) as coordination ion, which can be attributed to their stronger absorption
in the UV–visible region and lower band gap. This study will pave a new path to design
novel conjugated organic polymers as dyes for DSSCs.
&
Chaofan Zhong
zhongcf798@aliyun.com
1
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of
Education, Department of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China
123

6164
Y. Liu et al.
Graphical Abstract
Keywords Polymeric metal complexes Á Dyes Á Cyclopentadithiophene Á Structure
of donor–p–acceptor (D–p–A) Á Dye-sensitized solar cells
Introduction
With the consumption of traditional energy (such as coal, oil, and natural gas) which
causes serious environmental problems, we have to seek for other clean and
renewable new energy (wind, rain, geothermal, and solar energy). Solar energy is
recognized as one of the promising sources of energy in the twenty-first century, due
to its universality, non-pollution, and security. Dye-sensitized solar cells (DSSCs)
[1–5] are considered as one of the most promising next-generation photovoltaic
cells, which can perform efficient conversion of solar energy directly into
electricity. They have been extensively studied because of their low production
¨
cost, and flexible structure since the first report by Gratzel and co-workers in 1991
[6]. The sensitizer is one of the most critical components of DSSCs, which plays an
important role by capturing sunlight and transferring electrons. Therefore, to
develop new types of dye sensitizers with inexpensive materials for DSSCs is very
important, and it has been a challenging topic in the world.
123

Novel dyes of branched chain polymeric metal complexes…
6165
Recently, various types of sensitizers have been studied, and most of them are D–A
structures, which consist of the electron-donating (D) group and the electron-
accepting (A) group. The sensitizers based on D–A structure exhibit broad and intense
spectral absorption, which is considered to be one of the most promising category of
dyes. For most D–A sensitizers, using triphenylamine [7–9], indolines [10], carbazole
[11–13], phenothiazine [14], a diphenylamine or dialkylamine moiety as electron
donor, and a carboxylic acid or cyanoacrylic acid moiety acts as electron acceptor.
This structure has many advantages, as follows: firstly, sensitizers based on D–A have
better conjugated structure with the introduction of an electron-donating group and an
electron-accepting group, which not only can enhance light absorption in the visible
region to make the absorption spectrum redshift, but also enhance the transmission
performance of the electron. Secondly, the p–p* transitions of electron and
intramolecular charge-transformation (ICT) of molecules from an electron-donating
group to an electron-accepting group can enhance the light–harvesting ability of
sensitizers in the UV–visible region. Meanwhile, it has efficient electron injection
from the excited dye to the conduction band (CB) of TiO2 electrode with the acceptor
unit. Last but not least, it can suppress the charge recombination between the radical
cation dye and the injected electrons.
Sensitizers can be divided into pure organic and metal complexes. In the dye-
sensitized solar cell materials, bipyridyl ruthenium [15–17] is most widely used. It
has a wider absorption of the ultraviolet to near-infrared light, and an appropriate
level to achieve charge separation and regeneration of the dye compared with the
titanium dioxide photoanode and I-/I3- electrolyte. In addition, the higher stability
of the molecule structure is in line with the high performance characteristics of the
sensitizer and the energy conversion efficiency of DSSCs, which has been reached
at 12 % for ruthenium sensitizers [18, 19]. However, the absorption coefficient of
ruthenium dye is relatively low (10.000–20.000 M-1 cm-1), coupled with the high
cost of ruthenium, so that other new sensitizers have to be studied. Pure organic
dyes (metal-free organic dyes) have been thoroughly studied because of their high
molar extinction coefficients (50,000–200,000 M-1 cm-1), a variety of synthetic
methods, and changeable molecular structures. The energy conversion efficiency of
DSSCs based on pure organic dye has been 11 % [20]. However, a relatively narrow
absorption band and unstable molecule structure restrict the development of pure
organic dye sensitizers; they still need further improvement.
Accordingly, we have made significant effort in the synthesis of polymeric metal
complexes. It combines pure organic and metal complexes to form a metal-polymer,
which exhibits as many advantages of both parent materials as possible. For
example, the d–p feedback bond and the coordinate bond between the metal and the
ligand will improve the ability of electronic transmission, meanwhile, with the
flexible structure, we can obtain the target molecule by modification, and it has the
performance of higher thermal stability as metal complexes, and better dissolution
properties as pure organics.
It is worth noting that cyclopentadithiophene or fluorene derivatives are rigid flat
structures with larger bandwidth and lower energy gap, which are good electron
donors; 2-(1H-benzo[d]imidazol-2-yl)phenol has a large planar rigid structure, which
can coordinate with many metal ions, so it is a good acceptor; the carboxyl groups of
123

6166
Y. Liu et al.
3,4-diamino benzoic acid can load on the surface of titanium dioxide steadily, which
is a good coreceptor. Based on the above advantages, four novel D–A polymeric
metal complexes (P1–P4) have been designed and synthesized using cyclopenta-
dithiophene or a fluorene derivative as donor and a 2-(20-pyridyl)benzimidazole
derivative as acceptor by the Yamamoto coupling reaction for application in DSSCs
as a sensitizer. Moreover, the optical properties, thermal properties, and photovoltaic
properties of these polymeric metal complexes are also investigated in this paper.
Experimental
Instruments and measurements
Fourier transform infrared (FT-IR) spectra were measured by KBr powder and the
range of FT-IR was 450–4000 cm-1. 1H-NMR spectra were measured with a Bruker
ARX400 (400 MHz) Germany, using DMSO-d6 or CDCl3 as the solvent, and
tetramethylsilane (TMS) as the internal standard (0.00 ppm). Elemental analysis
was measured with a Perkin-Elmer 2400 II instrument under nitrogen atmosphere; it
was to test C, H, S and N of the molecular components. UV–visible spectra were
tested with a PE-Lambda 25 spectrophotometer. Samples were dissolved in N,N-
dimethylformamide (DMF), and then diluted it to
a concentration of
10-5–10-4 mol L-1. Differential scanning calorimetry (DSC) was measured with
a Perkin-Elmer DSC-7 thermal analyzer, and heating it from 25 to 250 °C at a rate
of 20 °C min-1. Thermal gravimetric analyses (TGA) were measured with a
Shimadzu TGA-7 Instrument at a heating rate of 25 °C min-1 from 25 to 600 °C in
a nitrogen atmosphere. Gel permeation chromatography (GPC) analyses were
measured with a Waters-1515 model at room temperature, using Waters styragel
3
4
5
˚
columns (10 , 10 , 10 A) as a separation column, DMF as mobile phase, and
polystyrene as calibration. Cyclic voltammetry was measured with a CHI630C
Electrochemical Workstation, using a platinum wire electrode as the auxiliary
electrode, a glassy carbon electrode as the working electrode, and a saturated
calomel electrode (SCE) as reference electrode in a 0.1 mol L-1 [Bu4N]BF4 of
DMF solution at a scan rate of 100 mV s-1
.
General procedures for fabrication of the DSSCs devices
Titania paste was prepared as follows: immersing the fluorine-doped SnO2
conducting glass (FTO) that was cleaned in 40 mmol L-1 TiCl4 solution for
30 min at 70 °C, and then washing with water and ethanol. The 20-30 nm-particles-
sized TiO2 colloid was coated on FTO glass that had been prepared by the sliding
glass rod method, and then sinter it for 30 min at 450 °C. This process was done
three times to obtain a 15 lm-thickness TiO2 film. After cooling to 100 °C, the
TiO2 film was soaked in 0.5 mmol L-1 the dye-sensitized samples in DMF and kept
for 24 h in the dark. Then the films were cleaned with anhydrous ethanol. After
drying, electrolyte that contains 0.05 mol L-1 I2, 0.5 mol L-1 LiI and 0.5 mol L-1
123

Novel dyes of branched chain polymeric metal complexes…
6167
4-tert-butylpyridine dissolved in a mixed solution of acetonitrile/3-methoxypropi-
onitrile (v/v, 7:3) was dripped on the surface of TiO2 electrodes. A Pt foil used as
counter electrode was clipped onto the top of the TiO2 as working electrode. The
dye-coated semiconductor film was illuminated through a conducting glass support
without mask. Photoelectrochemical performance of the dye-sensitized solar cell
was tested with a Keithley 2602 Source meter controlled by a computer. The
parameters of the dye-sensitized solar cell were obtained with an intensity of
100 mW cm-2 of the incident light, which was generated by a 500 W Xe lamp with
the AM 1.5 G filter with an effective area of 0.2 cm2.
Materials
All starting materials were obtained from Shanghai Chemical Reagent Co. Ltd.
(Shanghai China) and used without further purification. All solvents used were
analytical grade. 2,6-Dibromo-4,4-bis(2-ethylhexyl)-4H-thieno[30,20:4,5]cyclopenta
[1,2-b]thiophene was purchased from the company of Rui Xun Optoelectronic
Materials. 3,5-dibromo-2-hydroxybenzaldehyde, [21] 2-(1H-benzo[d]imidazol-2-yl)-
4,6-dibromophenol [22] were synthesized according to the literature methods. N,N-
dimethylformamide (DMF) was dried by distillation with CaH2. All other reagents and
solvents were commercially purchased and were used as received.
Synthesis of metal complex C1
2-(1H-benzo[d]imidazol-2-yl)-4,6-dibromophenol 0.0923 g (0.25 mmol) and 3,4-
diaminobenzoic acid 0.0389 g (0.25 mmol) were dissolved in 15 mL absolute
methanol in the 100 mL three-necked round-bottomed flask, then a methanol
solution (10 mL) of CdCl2Á2.5H2O 0.0688 g (0.3 mmol) was dropped slowly into
the three-necked round-bottomed flask under reflux condition. Then the mixture
was adjusted carefully to slightly acidic (pH = 5–6) with NaOH (0.1 mol L-1),
and reacting for 12 h. Stopping the reaction, and then cooled it to room
temperature, a light gray solid was precipitated. It was filtered, and the solid was
washed repeatedly with anhydrous methanol until the filtrate was colorless. After
dried in vacuum at 60 °C, a pale gray solid (0.1373 g) was obtained, yield 78 %.
FT-IR (KBr): 3427 (NH), 3070 (=C–H), 1592 (C=C), 1137 (C=N–Cd), 511
(N–Cd). Anal. Calcd for [C20H16Br2N4O3 CdCl2]: C 34.14, H 2.28, N 7.96. Found:
C 34.52, H 2.11, N 7.66.
Synthesis of metal complex C2
The synthetic method was the same as metal complex C1 that has been described
above, the difference was that C2 was coordinated with ZnCl2, a yellow solid
(0.1248 g) was obtained, yield 76 %. FT-IR (KBr): 3383 (NH), 1566 (C=C), 1104
(C=N–Zn), 508 (N–Zn). Anal. Calcd for [C20H16Br2N4O3 ZnCl2]: C 36.59, H 2.44,
N 8.54. Found: C 36.87, H 2.13, N 8.16.
123

6168
Y. Liu et al.
Synthesis of polymeric metal complex P1
The polymer P1 was synthesized according to the Yamamoto reaction. [23] 2,6-
Dibromo-4,4-bis(2-ethylhexyl)-4H-thieno[30,20:4,5]cyclopenta[1,2-b]thiophene
0.1120 g (0.2 mmol), metal complex C1 0.1406 g (0.2 mmol), zinc 0.0650 g
(1 mmol), bis(triphenylphosphine) nickel(II) chloride 0.1312 g (0.2 mmol),
triphenylphosphine 0.1048 g (0.4 mmol) and bipyridine 0.0031 g (0.02 mmol)
were dissolved in DMF (12 mL) in the single-necked round bottom flask, then the
mixture was heated to 90 °C slowly under nitrogen for 48 h. Cooling to room
temperature, the mixture was poured into a beaker, then addied a large amount of
ethanol, and the solid was precipitated after 5 h, and filtered. The crude product
was washed with methanol, distilled water and THF, respectively, and then dried
under vacuum for 24 h, a dark gray solid (0.0819 g) was obtained, yield 43 %. FT-
IR (KBr): 3420 (NH), 3064 (=C–H), 2961 (vinylic C–H), 1579 (C=C), 1113
(C=N–Cd), 495 (N–Cd). Anal. Calcd for [C45H56N4O3 S2CdCl2] C 57.02, H 5.91,
N 5.91, S 6.76. Found: C 56.71, H 5.38, N 6.33, S 6.54. Mn = 8.58 kg mol-1
PDI = 1.68.
,
Synthesis of polymeric metal complex P2
The synthetic method was the same as metal complex P1 that has been described
above, the difference was that P2 reacted with C2, a dark yellow solid (0.1005 g)
was obtained, yield 56 %. FT-IR (KBr): 3407 (NH), 2969 (vinylic C–H),1527
(C=C), 1085 (C=N–Zn), 497 (N–Zn). Anal. Calcd for [C45H56N4O3 S2ZnCl2] C
60.00, H 6.22, N 6.22, S 7.11. Found: C 60.57, H 6.08, N 6.43, S 7.25.
Mn = 10.83 kg mol-1, PDI = 1.59.
Synthesis of polymeric metal complex P3
The synthetic method was the same as metal complex P1 that has been described
above, the difference was that P3 reacted with alkyl fluorene, a gray solid (0.0914 g)
was obtained, yield 49 %. FT-IR (KBr): 3425 (NH), 3066 (=C–H), 2937 (vinylic
C–H), 1585 (C=C), 1122 (C=N–Cd), 502 (N–Cd). Anal. Calcd for [C49H60N4O3 Cd
Cl2] C 62.89, H 6.42, N 5.99. Found: C 62.53, H 6.21, N 6.38. Mn = 7.54 kg mol-1
PDI = 1.71.
,
Synthesis of polymeric metal complex P4
The synthetic method was the same as metal complex P1 that has been described
above, the difference was that P4 reacted with C2 and alkyl fluorene, a yellow solid
(0.0909 g) was obtained, yield 51 %. FT-IR (KBr): 3398 (NH), 2966 (vinylic C–H),
1533 (C=C), 1092 (C=N–Zn), 504 (N–Zn). Anal. Calcd for [C49H60N4O3 ZnCl2] C
66.22, H 6.76, N 6.31. Found: C 66.68, H 6.42, N 6.14. Mn = 8.97 kg mol-1
PDI = 1.64.
,
123

Novel dyes of branched chain polymeric metal complexes…
6169
Result and discussion
Synthesis and characterization
The detailed synthetic routes of the monomers and the four polymeric metal
complexes are depicted in Scheme 1. The target four polymers were obtained by the
Scheme 1 Synthesis of the monomers and polymeric metal complexes P1–P4
123

6170
Y. Liu et al.
Yamamoto coupling reaction with metal complexes C1, C2 as acceptor, and alkyl
cyclopentadithiophene or alkyl fluorene derivatives as donor.
1
The H NMR spectrum of the intermediate compound 2-(1H-benzo[d]imidazol-
2-yl)-4,6-dibromophenol is shown in Fig. 1, which was tested in DMSO-d6. The
signal of 13.86 ppm is due to the hydrogen protons of NH in the benzimidazole, and
the signals of 7.70 ppm, 7.33 ppm are attributed to the hydrogen protons of benzene
in the benzimidazole. The hydrogen proton of benzene is observed at 8.32 and
7.91 ppm.
Figure 2a, b show the FT-IR spectra of the polymers P1–P4 and their
corresponding metal complexes C1, C2. The figures show that, there is a broad
absorption band at 3427 and 3383 cm-1, which can be attributed to the stretching
vibration of N–H in the molecule, so it is not easy to see the O–H stretching
vibrations of the water molecules and -COOH. The absorption peaks at 1592 and
1566 cm-1 are due to the stretching vibration of C=C, and the absorption peaks at
1137, 1104 cm-1 can be attributed to the stretching vibration of C=N–M in the C1,
C2. Meanwhile, there are absorption peaks at 511 and 508 cm-1, respectively,
which are due to the stretching vibration of N–M in the C1 and C2. All these prove
that C1 and C2 have been successfully obtained. Compared with C1 and C2, the
corresponding absorption peaks (C=C, C=N–M, N–M) of polymers P1–P4 appeared
red shifted to some extent, which is due to the extension of the conjugation chain
after polymerization. Polymers P1–P4 have similar absorption at about 2966 cm-1
,
Fig. 1 1H NMR spectrum of 2-(1H-benzo[d]imidazol-2-yl)-4,6-dibromophenol in DMSO-d6 solution
123

Novel dyes of branched chain polymeric metal complexes…
6171
Fig. 2 a FT-IR spectra of C1, P1, P3. b FT-IR spectra of C2, P2, P4
due to the stretching vibration of alkyl chain, which illuminates that alkyl
cyclopentadithiophene or alkyl fluorene have been embedded in the molecular chain
of metal complexes successfully.
The data of gel permeation chromatography (GPC) is shown in Table 1. The
results show that the degree of polymerization of the polymer P1–P4 is not too high,
123

6172
Y. Liu et al.
Table 1 Molecular weights and thermal properties of the polymeric metal complexes
Mna (9103)
Mwa (9103)
N
PDI
Tg (°C)b
Td (°C)c
P1
P2
P3
P4
a
8.58
10.83
7.54
14.41
17.22
12.89
14.71
9
1.68
1.59
1.71
1.64
147
139
158
167
293
282
332
356
12
8
8.97
10
Determined by gel permeation chromatography using polystyrene as standard in DMF eluent
Determined by DSC with a heating rate of 25 °C min-1 under nitrogen
The temperature at 5 % weight loss under nitrogen
b
c
it is due to the fact that there is no soluble group in the metal complexes, which
result in their weak solubility in the solvent. Coupled with the larger molecular
structure of acceptor and donor, this results in a larger steric hindrance, which may
decrease the collision in the reaction, and finally they are not efficient for
polymerization.
All above results demonstrated that the target product P1–P4 has been
synthesized successfully.
Thermal properties
Thermal stability of the dye molecules determines the working life of the
photovoltaic devices. The thermal stability of four polymeric metal complexes (P1–
P4) were studied by thermal gravimetric analysis (TGA) and the differential
scanning calorimetry test (DSC). The results of TGA and DSC are shown in
Table 1, and the TGA curve of polymer P1–P4 is shown in Fig. 3. The Td (the
temperature at which they lose 5 % of their own weight) of the polymer P1–P4 are
293, 282, 332, and 356 °C, respectively. These data are all higher than 280 °C,
which is in line with the thermal performance requirements of a dye-sensitizer.
Their glass transition temperatures (Tg) are 147, 139, 158, and 167 °C, respectively.
These data suggest that the thermal performance of P3, P4 are better than that of P1
and P2, which illustrates that the rigidity structure of alkyl benzene is better than
alkyl thiophene. In conclusion, these four polymeric metal complexes (P1–P4) show
a better thermal stability, which is very favorable to extend the working life of the
device.
Optical properties
UV–Vis absorption spectra reflect the sunlight capturing capability of dye
molecules in the UV–visible region, which is an important aspect to assess the
dye sensitizer.
The UV–Vis absorption spectra of polymers P1–P4 and their corresponding
metal complexes C1, C2 are shown in Fig. 4, which is measured in DMF solution,
and the corresponding optical data are shown in Table 2. The maximum absorption
123

Novel dyes of branched chain polymeric metal complexes…
6173
Fig. 3 TGA curves of P1–P4 with a heating rate of 25 °C min-1 under nitrogen atmosphere
Fig. 4 UV–vis absorption spectra of C1, C2 and polymeric metal complexes P1–P4 in DMF solution
wavelengths of the metal complexes C1, C2 were at 386, and 377 nm, which is due
to the p–p* electronic transitions of molecule [24]. After polymerization, the
conjugate structure of the molecules is extended to some extent, due to the
combination of donor and acceptor. The maximum absorption wavelengths of P1–
P4 are red-shifted to 484, 475, 435, and 404 nm, respectively, since the charge
transfer from donor to acceptor is in the entire molecule. Meanwhile, we can also
see that polymeric metal complexes P1, P2 have relatively weak absorption peaks at
123

6174
Y. Liu et al.
Table 2 Optical and electrochemical properties of the polymeric metal complexes
Metal complex, polymer k, max (nm)a Ered (V)b Eox (V)b HOMO (eV) LUMO (eV) Eg (eV)c
C1
C2
P1
P2
P3
P4
a
386
377
484
475
435
404
-0.92
-0.97
-1.07
-1.13
1.10
1.12
1.18
1.20
-5.50
-5.52
-5.58
-5.60
-3.48
-3.43
-3.37
-3.27
2.02
2.09
2.21
2.33
The maxima absorption from the UV–Vis spectra in DMF solution
b
c
Values determined by cyclic voltammetry
Electrochemical band gap estimated from HOMO and LUMO
525 and 519 nm, although the absorption intensity is not too high, the photon flux of
this area is very large, which is very advantageous to improve the efficiency of solar
cells. Obviously, the polymer with a Cd(II) and cyclopentadithiophene derivative
has better absorption than that with a Zn(II) and fluorene derivative in the UV–Vis
area, which can be attributed to the bigger radius of Cd(II) than of Zn(II), and a
stronger electron-donating ability of cyclopentadithiophene derivative than fluorene
derivative. In summary, the introduction of cyclopentadithiophene derivative in the
donor and the introduction of Cd(II) in the acceptor can contribute to the
ultraviolet–visible absorption of dye molecules.
Electrochemical properties
The electrochemical properties are an important aspect to evaluate the performance
of a sensitizer, which can be tested by cyclic voltammetry (CV) to estimate the
lowest unoccupied molecular orbital (LUMO) energy levels and the highest
occupied molecular orbital (HOMO) energy levels of the polymer. It is an important
basis for determining whether the dye can be applied in the dye-sensitized solar
cells. On the one hand, the LUMO energy level of the dye should be higher than the
conduction band energy level (ECB) of TiO2, to achieve the charge separation; on
the other hand, the HOMO energy level of dye must be lower than the redox
potential of the electrolyte (e.g., I-/I-3 ), to achieve recycling of the dye. LUMO and
HOMO energy levels can be calculated from the onset reduction potentials (Ered
and the onset oxidation potentials (Eox) of the polymers according to equations: [25,
)
26]
HOMO ¼ ÀeðEox þ 4:40ÞðeVÞ
LUMO ¼ ÀeðEred þ 4:40ÞðeVÞ :
Eg ¼ eðEox À EredÞðeVÞ
The cyclic voltammograms of polymer P1–P4 are shown in Fig. 5, the
corresponding energy levels are shown in Fig. 6, and the corresponding data are
123

Novel dyes of branched chain polymeric metal complexes…
6175
Fig. 5 Cyclic voltammograms of P1–P4 in DMF/0.1 M [Bu4N]BF4 at 100 mV s-1
Fig. 6 The HOMO and LUMO energy levels of the polymers P1–P4
shown in Table 2. The Eox values of P1, P2, P3 and P4 are 1.10, 1.12, 1.18, and
1.20 V, respectively; The Ered values of P1–P4 are -0.92, -0.97, -1.07 and
-1.13 V, respectively. According to equations above, the corresponding HOMO
level are -5.50, -5.52, -5.58 and -5.60 eV, respectively; LUMO energy levels
are -3.48, -3.43, -3.37 and -3.27 eV, respectively. These results show that the
HOMO levels of the four polymers (P1–P4) are lower than the redox potential of the
electrolyte (-4.83 eV vs NHE), and LUMO energy levels of the four polymers (P1–
123

6176
Y. Liu et al.
P4) are higher than the conduction band energy level of titanium dioxide (-4.26 eV
vs NHE). In other words, electrons can be transferred successfully among the dye
molecules, TiO2 conduction band, and the electrolyte, to achieve charge separation
and recycling of the dye molecules. The energy gap (Eg) of polymer P1–P4 are 2.02,
2.09, 2.21, and 2.33 eV, respectively. Compared with P3 and P4, the Eg of P1 and
P2 is smaller, this is due to the stronger electron-donating of cyclopentadithiophene
derivative, which also implies that polymers with cyclopentadithiophene derivative
are more suitable to be applied as sensitizer in the DSSCs.
Photovoltaic properties
The photocurrent–voltage (J–V) curves of polymer P1–P4 as the sensitizers of dye-
sensitized solar cell are shown in Fig. 7, and corresponding data of short-circuit
current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power
conversion efficiency (g) are shown in Table 3. Seen from the data in the table,
Voc and Jsc of P1 are at maximum, which corresponds to the highest power
conversion efficiency (2.49 %). The Jsc follow the order of P1 (4.98 mA cm-2) [
P2 (4.82 mA cm-2) [ P3 (4.51 mA cm-2) [ P4 (4.36 mA cm-2). Obviously, the
Jsc of dyes do not exceed 5 mA cm-2, which is probably due to the narrow and
short absorption spectra that limited the use of long wavelengths energy. Compared
with P3 and P4, the Jsc of P1 and P2 are a little larger, which may be due to the
stronger electron-donating ability of cyclopentadithiophene derivative than the
fluorene derivative, which can produce relatively more excited state electrons.
Meanwhile, the Jsc of polymers with Cd(II) are higher than the corresponding
polymers with Zn(II), this may be due to the larger radius of Cd(II), which makes it
have a relatively strong complexing ability, and the transmission ability of electrons
Fig. 7 J–V curves of DSSCs based on dyes P1–P4 under the illumination of AM 1.5, 100 mW cm-2
123

Novel dyes of branched chain polymeric metal complexes…
6177
Table 3 Photovoltaic parameters of devices with sensitizers P1–P4 in DSSCs at full sunlight (AM 1.5 G,
100 mW cm-2
)
Polymer
Solvent
Jsc (mA cm-2
)
Voc (V)
ff (%)
g (%)
P1
P2
P3
P4
DMF
DMF
DMF
DMF
4.98
4.82
4.51
4.36
0.73
0.69
0.68
0.64
68.8
69.5
66.3
64.1
2.49
2.34
2.05
1.80
in the molecule is also relatively strong. It is not difficult to find that the Voc is
gradually decreased along with the order of P1 (0.73 V), P2 (0.69 V), P3 (0.68 V)
and P4 (0.64 V), which is attributed to the fact that their HUMO–LUMO band gaps
were broaden gradually, and the excitation of sensitizers is relatively difficult [27,
28].
The IPCE curves of polymer P1–P4 as the sensitizers of a dye-sensitized solar
cell are shown in Fig. 8. From the graph, it is evident that the four polymer
molecules all have a spectral response at 400–600 nm. Wherein, P1 has the highest
external quantum efficiency value. Although the maximum value of IPCE of dye
molecules has a certain degree of improvement, it is still relatively low compared
with the ruthenium dye, and it also shows the reason for the lower Jsc of dye
molecules. Therefore, in our future work of designing molecule structure for the
dye, we must improve the molecular structure, and try to make it redshift to expand
the absorption range of the dye, and ultimately improving the efficiency of DSSCs.
Fig. 8 Spectra of incident photon-to-current conversion efficiencies (IPCE) for DSSCs based on dyes
(P1–P4) on the ITO glass
123

6178
Y. Liu et al.
Conclusions
Four novel D–A polymeric metal complexes dyes (P1–P4) containing alkyl
cyclopentadithiophene or an alkyl fluorene derivative as electron donor (D),
benzimidazole metal complex as the electron acceptor (A) and carboxyl group
(-COOH) as an anchor group were designed, synthesized, and applied for DSSCs.
All polymers exhibited good thermal stability, which is in line with the thermal
performance requirements of the photovoltaic device. Moreover, the optical
properties and photovoltaic properties of these polymeric metal complexes are also
investigated. The results show that the polymer with cyclopentadithiophene
derivatives and Cd(II) ion complexes exhibits better efficiency because of the
stronger electron-donating ability of the cyclopentadithiophene derivative and the
stronger complexing ability of the Cd(II) ion. Although the conversion efficiency of
DSSCs has a certain degree of improvement, compared with the traditional
ruthenium dyes, there is still a long way to go. Therefore, in our future work of
designing molecule structure for the dye, we must improve the molecular structure,
and try to make it redshift to expand the absorption range of the dye, and ultimately
improving the efficiency of DSSCs.
Acknowledgments This work was financially supported by the Open Project Program of Key
Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, China
(No. 09HJYH10).
References
1. J.N. Schrauben, Y.X. Zhao, C. Mercado, P.I. Dron, J.L. Ryerson, J. Michl, K. Zhu, J.C. Johnson,
ACS Appl. Mater. Interfaces 7, 2286 (2015)
2. S. Karthikeyan, J.Y. Lee, J. Phys. Chem. A 117, 10973 (2013)
3. A.R. Pascoe, F. Huang, N.W. Duffy, Y.B. Cheng, J. Phys. Chem. C 118, 15154 (2014)
¨
4. J. Warnan, J. Gardner, L.L. Pleux, J. Petersson, Y. Pellegrin, E. Blart, L. Hammarstrom, F. Odobel, J.
Phys. Chem. C 118, 103 (2014)
5. M. Ince, J.H. Yum, Y. Kim, S. Mathew, M. Gratzel, T. Torres, M.K. Nazeeruddin, J. Phys. Chem. C
¨
118, 17166 (2014)
6. B. O’Regan, M. Gratzel, Nature 353, 737 (1991)
7. Y.L. Tan, M. Liang, Z.Y. Lu, Y.Q. Zheng, X.N. Tong, Z. Sun, S. Xue, Org. Lett. 16, 3978 (2014)
8. G.H. Wu, F.T. Kong, Y.H. Zhang, X.X. Zhang, J.Z. Li, W.C. Chen, W.Q. Liu, Y. Ding, C.N. Zhang,
B. Zhang, J.X. Yao, S.Y. Dai, J. Phys. Chem. C 118, 8756 (2014)
9. S.G. Esteban, P. Cruz, A. Aljarilla, L.M. Arellano, F. Langa, Org. Lett. 13, 5362 (2011)
10. F. Alberto, M.P. Laurence, W. Hongxia, M. Hidetoshi, M. Frank, J. Phys. Chem. C 114, 11822
(2010)
11. Y. Uemura, T.N. Murakami, N. Koumura, J. Phys. Chem. C 118, 16749 (2014)
12. P. Thongkasee, A. Thangthong, N. Janthasing, T. Sudyoadsuk, S. Namuangruk, T. Keawin, S.
Jungsuttiwong, V. Promarak, Appl. Mater. Interfaces 6, 8212 (2014)
13. T. Sudyoadsuk, S. Pansay, S. Morada, R. Rattanawan, S. Namuangruk, T. Kaewin, S. Jungsuttiwong,
V. Promarak, Eur. J. Org. Chem. 23, 5051 (2013)
14. Z. Iqbal, W.Q. Wu, D.B. Kuang, L.Y. Wang, H. Meier, D.R. Cao, Dyes Pigm. 96, 722 (2013)
15. C.Y. Chen, N. Pootrakulchote, T.H. Hung, C.J. Tan, H.H. Tsai, S.M. Zakeeruddin, C.G. Wu, M.
Gratzel, J. Phys. Chem. C 115, 20043 (2011)
16. R.M. O’Donnell, P.G. Johansson, M. Abrahamsson, G.J. Meyer, Inorg. Chem. 52, 6839 (2013)
17. W.B. Heuer, H.L. Xia, W. Ward, Z. Zhou, W.H. Pearson, M.A. Siegler, A.A.N. Sarjeant, M.
Abrahamsson, G.J. Meyer, Inorg. Chem. 51, 3981 (2012)
123

Novel dyes of branched chain polymeric metal complexes…
6179
18. Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao, P. Wang, J. Phys. Chem. C 113, 6290 (2009)
19. M. Gratzel, Acc. Chem. Res. 42, 1788 (2009)
20. M. Green, K. Emery, Y. Hishikawa, W. Warta, E. Dunlop, Prog. Photovoltaics Res. Appl. 22, 1
(2014)
21. R. Cordoba, N.S. Tormo, A.F. Medarde, J. Plumet, Bioorg. Med. Chem. 15, 5300 (2007)
´
22. D. Secci, A. Bolasco, M. D’Ascenzio, F.D. Sala, M. Yan˜ez, S. Carradoria, J. Heterocycl. Chem. 49,
1187 (2012)
23. B.K. An, Y.H. Kim, D.C. Shin, Macromolecules 34, 3993 (2001)
24. J.H. Jia, Y. Zhang, P.C. Xue, P. Zhang, X. Zhao, B.J. Liu, R. Lu, Dyes Pigm. 96, 407 (2013)
25. Y. Li, Y. Cao, J. Gao, D. Wang, G. Yu, A.J. Heeger, Synth. Met. 99, 243 (1999)
26. N.S. Cho, D.H. Wang, B.J. Jung, E. Lim, J. Lee, H.K. Shim, Macromolecules 37, 5265 (2004)
27. T. Marinado, K. Nonomura, J. Nissfolk et al., Langmuir 26, 2592 (2009)
28. C. Teng, X. Yang, S. Li et al., Chem. Eur. J. 16, 13127 (2010)
123