Novel D-π-A dye sensitizers of polymeric metal complexes based on Cd(II) with salicylaldehyde and diaminomaleonitrile: Synthesis, characterization, and photovoltaic performance for dye-sensitized solar cells

Article abstract of DOI:10.1007/s11164-014-1895-z

Being a key component of dye-sensitized solar cells, dye sensitizer and its synthesis and application have been extensively researched. In this paper, four novel polymeric metal complexes as dye sensitizers based on Cd(II) with salicylaldehyde and diaminomaleonitrile were functionally designed and synthesized. These dyes use poly(p-phenylenevinylene) or bithiophene phenylenevinylene as electron donor, Cd(II) complex unit as electron acceptor, and ethylenic bond as π linker. They were characterized by FT-IR, ¹H NMR, gel permeation chromatography, and elemental analysis. Their thermal, photophysical, electrochemical, and photovoltaic properties were also investigated by thermogravimetric analysis, differential scanning calorimetry, C-V curves, and J-V curves. All four polymeric metal complexes exhibited some photovoltaic performances. The dyes containing bithiophene (P2, P4) exhibited higher power conversion efficiency (PCE) values than the corresponding polymers without thiophene unit (P1, P3). Dual-core cadmium polymeric metal complexes showed higher PCE than mononuclear cadmium complexes by comparing P4 with P2 as well as P3 with P1. The highest PCE of compound P4 reached 2.07 % (J _sc = 4.22 mA/cm², V _oc = 0.71 mV, and FF = 69.1 %) under simulate AM 1.5G solar irradiation.

Full text of DOI:10.1007/s11164-014-1895-z

Res Chem Intermed
DOI 10.1007/s11164-014-1895-z
Novel D–p–A dye sensitizers of polymeric metal
complexes based on Cd(II) with salicylaldehyde
and diaminomaleonitrile: synthesis, characterization,
and photovoltaic performance for dye-sensitized solar
cells
Jiaomei Hu
Qiufang Xie
•
Xueliang Jin
Ye Liu Yanlong Liao
Chaofan Zhong
•
Dahai Peng
•
•
•
•
Chunxiao Zhu
•
Received: 17 June 2014 / Accepted: 9 December 2014
Ó Springer Science+Business Media Dordrecht 2015
Abstract Being a key component of dye-sensitized solar cells, dye sensitizer and
its synthesis and application have been extensively researched. In this paper, four
novel polymeric metal complexes as dye sensitizers based on Cd(II) with salicyl-
aldehyde and diaminomaleonitrile were functionally designed and synthesized.
These dyes use poly(p-phenylenevinylene) or bithiophene phenylenevinylene as
electron donor, Cd(II) complex unit as electron acceptor, and ethylenic bond as p
1
linker. They were characterized by FT-IR, H NMR, gel permeation chromatog-
raphy, and elemental analysis. Their thermal, photophysical, electrochemical, and
photovoltaic properties were also investigated by thermogravimetric analysis, dif-
ferential scanning calorimetry, C–V curves, and J–V curves. All four polymeric
metal complexes exhibited some photovoltaic performances. The dyes containing
bithiophene (P2, P4) exhibited higher power conversion efficiency (PCE) values
than the corresponding polymers without thiophene unit (P1, P3). Dual-core cad-
mium polymeric metal complexes showed higher PCE than mononuclear cadmium
complexes by comparing P4 with P2 as well as P3 with P1. The highest PCE of
2
compound P4 reached 2.07 % (Jsc = 4.22 mA/cm , V = 0.71 mV, and FF =
oc
6
9.1 %) under simulate AM 1.5G solar irradiation.
Keywords Polymeric metal complex ꢀ Dye sensitizer ꢀ Salicylaldehyde ꢀ
Diaminomaleonitrile ꢀ Cadmium
J. Hu ꢀ X. Jin ꢀ D. Peng ꢀ Q. Xie ꢀ Y. Liu ꢀ Y. Liao ꢀ C. Zhu ꢀ C. Zhong (&)
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China
e-mail: zhongcf798@aliyun.com
123

J. Hu et al.
Introduction
Energy demands and environmental pollution resulting in global warming have led
to a greater focus on renewable energy sources over the past decades. Recently,
Michael Gr a¨ tzel and coworkers reported on high-performance perovskite-sensitized
solar cells where a power conversion efficiency of 15 % was achieved with the best
device [1]. As a promising renewable energy technology that converts sunlight into
electricity, photovoltaics have attracted considerable attention with great potential
to solve urgent energy problems [2–4]. DSSCs are the only photovoltaic devices that
use molecules to absorb photons and convert them into electric charges without the
need of intermolecular transport of electronic excitation [5]. Up to now, DSSCs
based on metal-free organic dyes [6, 7], porphyrin dyes [8, 9], metal-phthalocyanine
[
10], and natural dyes [11, 12] have been developed and reported. Among them, the
generation of donor–p-conjugation-linkage–acceptor (D–p–A) sensitizers are
impressive and encouraging because the D–p–A structure can help to extend
absorption spectra of the dyes, increase the optical absorption coefficient, adjust the
HOMO and LUMO energy levels, and the energy gap DE(DE = ELUMO - EHOMO
of the dye molecules. What’s more, excited electrons can easily transfer from the
)
donor to the acceptor with the help of p bridge, and then implant into the
semiconductor TiO , thus enhancing the transmission of the charge [13].
2
The disadvantages of small organic dye molecules are their poor charge-transport
properties due to the small p conjugated system, and their poor chemical stability,
and light stability, especially the thermal stability due to the small molecular weight.
As a result, their large-scale study and applications will be affected.
For the above reasons, to synthesize the macromolecular is particularly
important. As a kind of plurality of polymers, polymeric metal complexes have
received considerable attention for these hybrid materials that provide outstanding
physical and chemical properties of both organic and inorganic components, such as
unique process ability and easy film-forming ability of polymer, prominent
luminescence efficiency, and good thermal stability of metal [14, 15]. Thousands of
papers have reported on polymeric metal complexes but articles about their
application in dye-sensitized solar cells are quite limited [16, 17]. Therefore, it is
worth synthesizing new polymeric metal complexes and studying their photovoltaic
properties. However, the high cost and rarity of the ruthenium metal may restrain
their extensive application. So, finding cheap substitute metals seems very
important. Great efforts have been made to replace Ru-based dyes with metal-
free organic dyes [18], such as phenothiazine- [19], indoline- [20], fluorene- [21],
triarylamine- [22], thiophene- [23, 24], carbazole- [25, 26], and so on.
In this study, a series of D–p–A polymeric metal complexes, which use metal-
diaminomaleonitrile as an acceptor (A), ethylenic bond as a p-conjugation linkage,
and poly(p-phenylenevinylene) (PPV) or bithiophene phenylenevinylene as donor
group (D), have been synthesized and used as sensitizers in the dye-sensitized solar
cells. The structure of P4 is shown in Scheme 1, and the synthetic routes of the four
polymeric metal complexes (P1–P4) are shown in Scheme 2. Moreover, the
thermal, optical, and photovoltaic properties of those polymeric metal complexes
have also been investigated.
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
Scheme 1 Chemical structure of polymeric metal complex P4
Experimental
Materials
NiCl (PPh ) , salicylaldehyde, diaminomaleonitrile (DAMN) and 3-methylthio-
2
3 2
phene-2-carbaldehyde were obtained from Aldrich Chemical Co. and used as
received. N,N-dimethylformamide was dried by distillation over CaH and methanol
2
was dried over molecular sieves and freshly distilled prior to use. The other
materials were common commercial-grade materials and used as received. All
chemicals used were of an analytical grade. Solvents were purified with
conventional methods.
Instruments and measurements
1
H NMR were performed in CDCl3 and recorded on a Bruker Avance 400
spectrometer. Gel permeation chromatography (GPC) analyses were measured by a
WATERS 2414 system equipped with a set of HT , HT , and HT , l-Styragel
3
4
5
columns with THF as eluent and polystyrene as standard. The FT-IR spectra were
obtained on a Perkin-Elmer Spectrum One Fourier transform infrared spectrometer
by incorporating samples in KBr pellets. Thermogravimetric analyses (TGA),
differential scanning calorimetry (DSC), and elemental analysis were performed on
a Shimadzu TGA-7 instrument, Perkin-Elmer DSC-7 thermal analyzer, and Perkin-
Elmer 2400 II instrument, respectively. UV–visible spectra of all the polymers were
taken on a Lambda 25 spectrophotometer. Photoluminescent spectra (PL) were
taken on a Perkin-Elmer LS55 luminescence spectrometer with a xenon lamp as the
light source. Cyclic voltammetry (CV) was conducted on a CHI630C Electrochem-
ical Workstation using a three-electrode system in a [Bu N]BF (0.1 M) DMF
4
4
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.
123

J. Hu et al.
Scheme 2 Synthetic route of the monomers and polymeric metal complexes P1–P4
Fabrication of DSSCs
The DSSCs devices with a sandwich structure in this paper are based on TiO2
semiconductors. Titania paste was prepared following the procedure: fluorine-doped
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
SnO conducting glass (FTO) was cleaned and immersed in aqueous 40 mmol/l
2
TiCl solution at 70 °C for 30 min. The 20–30-nm particles TiO colloid was coated
4
2
onto the above FTO glass by the sliding glass rod method and following sintered at
4
50 °C for 30 min, which was done three times to obtain a TiO film of 10–15 lm
2
thickness. After cooling to 100 °C, the TiO films were soaked in 0.5 mmol/l dye
2
samples in DMF solution and maintained under dark for 24 h at room temperature.
The films were then cleaned by anhydrous ethanol. After drying, electrolyte
containing LiI (0.5 mol/l), I (0.05 mol/l) and 4-tert-butylpyridine (0.5 mol/l) was
2
dripped on the surface of TiO electrodes. A Pt foil was used as counter electrode
2
and clipped onto the top of the TiO . The dye-coated semiconductor film was
2
illuminated through a conducting glass support without a mask. The photoelectron
chemical performance of the solar cell was measured using a Keithley 2602 Source
meter controlled by a computer. The cell parameters were obtained under an
2
incident light with an intensity of 100 mW/cm , which was generated by a 500-W
2
Xe lamp passing through an AM 1.5 G filter with an effective area of 0.2 cm .
Synthesis
Synthesis of 5-bromo-2-hydroxybenzaldehyde (1) [27]
A carbon tetrachloride solution (20 ml) of liquid bromine (2.6 ml, 0.05 mol) was
dropped into an ethanol solution (20 ml) of salicylaldehyde (5.2 ml, 0.05 mol). The
reaction system was allowed to cool to 12–15 °C. After the dropwise addition, the
reaction was continued for 2 h at room temperature. The white precipitate was
collected by filtration, washed with dried ethanol repeatedly; a white solid was
-
1
obtained (7.04 g, yield 70 %). FT-IR (KBr, cm ): 3,218, 2,880, 1,669, 1,462, 696.
1
H NMR (400 MHz, CDCl , d, ppm): 10.934 (s, 1H), 9.847 (s, 1H), 7.678 (s, 1H),
3
7
.597 (d, 1H), 6.926 (d, 1H). Mp 105–106 °C.
Synthesis of 2-amino-3-(5-bromo-2-hydroxybenzylideneamino)maleonitrile (2) [28]
An ethanol solution (100 ml) of 5-bromo-2-hydroxybenzaldehyde (1) (0.9548 g,
4
4
.75 mmol) was dropped into an ethanol solution (20 ml) of DAMN (0.5027 g,
.65 mmol), 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
-
1
ethanol, and dried under vacuum (0.5414 g, yield 40 %). FT-IR (KBr, cm ): 3,407,
,298, 3,200, 2,249, 2,208, 1,620, 1,473, 1,278, 649. Anal. Calcd for C H BrN O
3
1
1
7
4
(
2): C, 45.39; H, 2.42; Br, 27.45; N, 19.25; O, 5.50. Found: C, 45.62; H, 2.24; N,
?
1
9.43. MS: Calcd for C H BrN O[M] 291.10; found, 292.33.
11 7 4
Synthesis of L1 [28]
5
-bromo-2-hydroxybenzaldehyde (1) (0.8463 g, 4.21 mmol) and CdCl ꢀ2.5H O
2
2
(
0.4795 g, 2.10 mmol) were dissolved together in refluxing ethanol (100 ml) to give
a clear colorless solution. DAMN (0.229 g, 2.12 mmol), dissolved in ethanol
25 ml), was added dropwise and refluxing continued for 4 h. Precipitated dark
(
123

J. Hu et al.
yellow solid, filter, washed with ethanol, and dried under vacuum (1.1047 g, yield
9
0 %). Anal. Calcd for C H Br N O Cd: C, 36.99; H, 1.38; N, 9.59. Found: C,
18 8 2 4 2
-
6.35; H, 1.33; N, 9.97; FT-IR (KBr, cm ): 3,041 (=C–H), 2,921, 2,853, 1,591
1
3
(
C=N), 2,172 (C:N), 1,051 (C–O–M), 816 (Ar–H), 490 (N–M).
Synthesis of L2 [16]
CdCl ꢀ2.5H O (0.6278 g, 2.75 mmol) was dissolved in methanol (50 ml) with
2
2
stirring and refluxing. 2-amino-3-((E)-5-bromo-2-hydroxybenzylideneamino)
maleonitrile (2) (0.8005 g, 2.75 mmol) dissolved in methanol (75 ml) was then
added and the mixture was refluxed for 11 h to give a khaki-brown precipitate,
which was filtered off, washed with methanol, and dried under vacuum. Yield:
1
.059 g (96 %). Anal. Calcd for C H Br N O Cd : C, 32.91; H, 1.26; N, 13.95.
22 10 2 8 2 2
-
1
Found: C, 32.36; H, 1.64; N, 13.46. FT-IR (KBr, cm ): 3,040 (=C–H), 2,976,
,919, 1,578 (C=N), 2,375, 2,165 (C:N), 1,120 (C–O–M), 816 (Ar–H), 492 (N–
M).
2
0
Synthesis of 2,2 -(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-2,1-diyl)bis(2-bromo-
-methylthiophene) (a) and PPV (b)
4
These monomers were synthesized according to the published literature [29]. PPV
1
yield (%): 75 %. HNMR (CDCl , d, ppm): 0.882 (t, 6H), 1.38–1.82 (m, 24H), 3.93
3
(
t, 4H), 5.35 (d, 2H), 5.72 (d, 2H), 6.98 (s, 2H), 7.00–7.07 (dd, 2H). a Yield (%):
1
7
6
0
3 %. HNMR (CDCl , d, ppm): 7.69-7.67 (d, 4H), 7.535 (s, 2H), 7.470 (d, 4H),
3
.960 (s, 2H), 4.016 (d,4H), 2.325 (s, 6H), 1.87 (m, 4H), 1.256–1.303 (m, 24H),
.889 (m, 6H).
Synthesis of polymeric metal complex P1
The polymeric metal complex P1 was synthesized by using the Heck coupling
method, according to the literature [30]. A flask was charged with a mixture of
metal complex L1 (0.1754 g, 0.3 mmol), 1,4-bis(octyloxy)-2,5-divinylbenzene
(0.116 g, 0.3 mmol), Pd(OAc)2 (0.0024 g, 0.012 mmol), triethylamine (3 ml), 3-o-
tolyl phosphine (0.0220 g, 0.072 mmol) and DMF (8 ml). The flask was then
pumped into a vacuum and purged with N . The mixture was heated at 90 °C for
2
2
4 h under N . Afterwards, it was filtered after being cooled to room temperature
2
and the filtrate was then poured into methanol. The yellow precipitate was filtered
and washed with cold methanol. The crude product was purified by dissolving in
THF and precipitating into methanol to afford a light yellow solid. (Yield: 0.1486 g,
-
1
5
1 %). FT-IR (KBr, cm ): 3,045 (=C–H), 2,928, 2,854, 1,651 (C=N), 2,326
(
C:N), 1,209 (C–O–M), 827 (Ar–H), 494 (N–M). Et al. Calcd for [C H N O
4
4 48 4 4-
Cd]: C, 65.30; H, 5.98; N, 6.92; Found: C, 64.52; H, 5.98; N, 6.92; Mn = 8.9 kg/
mol, PDI = 1.36.
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
Synthesis of polymeric metal complex P2
The copolymer was synthesized by Yamamoto coupling method according to the
literature [31, 32]. L1 (0.1169 g, 0.2 mmol), bis(triphenylphosphine) nickel(II)
chloride (0.13 g, 0.2 mmol), a (0.1474 g, 0.2 mmol), zinc (0.065 g, 1 mmol),
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 nitrogen. The mixture was
then stirred at 90 °C for 48 h. The yellow solid was precipitated into a large excess
of ethanol solution. The crude product was washed with ethanol, distilled water, and
THF sequentially, and then dried in vacuum at 65 °C for 1 day to afford pale yellow
-
1
solids (0.148 g, 56 %). FT-IR (KBr, cm ): 3,042 (=C–H), 2,921, 2,852, 1,663
(
[
C=N), 2,328 (C:N), 1,220 (C–O–M), 822 (Ar–H), 492 (N–M). Anal. Calcd for
C H N O S Cd]: C, 64.75; H, 5.64; N, 5.59; S, 6.40. Found: C, 63.96; H, 5.26;
54 56 4 4 2
N, 6.23; S, 6.93; Mn = 8.1 kg/mol, PDI = 1.57.
Synthesis of polymeric metal complex P3, P4
P3 was synthesized with a similar synthetic method as P1, P4 was synthesized with
-
1
a similar synthetic method as P2, P3: (Yield: 0.1653 g, 55 %), FT-IR (KBr, cm ):
,055 (=C–H), 2,930, 2,853, 1,649 (C=N), 2,331 (C:N), 1,170 (C–O–M), 833 (Ar–
H), 492 (N–M). Et al. Calcd for [C H N O Cd ]: C, 56.09; H, 4.90; N, 10.90;
3
4
8
50
8
4
2
Found: C, 56.62; H, 5.33; N, 11.24; Mn = 9.3 kg/mol, PDI = 1.30. P4: (Yield:
-
.1464 g, 60 %), FT-IR (KBr, cm ): 3,046 (=C–H), 2,918, 2,857, 1,580 (C=N),
1
0
2
,344 (C:N), 1,164 (C–O–M), 820 (Ar–H), 496 (N–M). Et al. Calcd for
[
N, 9.37; S, 5.73; Mn = 8.6 kg/mol, PDI = 1.48.
C H N O S Cd ]: C, 57.10; H, 4.79; N, 9.81; S, 5.26; Found: C, 57.73; H, 5.32;
58 58 8 8 2 2
Results and discussion
Synthesis and characterization
The detailed synthetic routes of the four polymeric metal complexes (P1–P4) as
well as the monomers are shown in Scheme 2, and the two polymers (P1, P3) were
synthesized by the Heck coupling [33]. The other two polymers (P2, P4) were
synthesized by Yamamoto coupling [32]. The four as-synthesized polymers were
readily dissolved in common organic solvents such as DMF and DMSO at room
temperature. However, in the other solvents, they exhibit a poor solubility, such as
in chloroform and dichloromethane.
1
0
Figure 1 shows the H NMR spectra of the 2,2 -(2,5-bis(octyloxy)-1,4-pheny-
lene)-bis(ethene-2,1-diyl)bis(2-bromo-4-methylthiophene) (a). The vinyl proton
peaks appeared at around 7.67–7.69 ppm, and the hydrogen proton of thiophene
locating at the four position is observed at 7.535 ppm. The protons of octyl appeared
1
at 0.889–4.016 ppm. The H NMR shows that monomer was synthesized
successfully.
123

J. Hu et al.
The IR spectra of the ligand monomer (2), metal complex (L1, L2), and the
target polymers (P1–P4) are shown in Figs. 2 and 3. As for the monomer 2,
-
absorption peaks at 1,628 cm are due to C=N bond stretching vibration, the
1
-
absorption peaks at 3,409, 3,298, and 3,197 cm are the characteristic absorption
1
-
1
peaks of the amino-group stretching vibration, and the peak at 648 cm is the C–Br
stretching vibration absorption peak. From Figs. 2 and 3, the peaks of 1,151 and
-
,170 cm are due to C–N-M stretching vibration of metal compound L1 and L2,
1
1
respectively [34]. Combining with the results of elemental analysis, we can
?
conclude that metal Cd2 has been successfully coordinated with ligand 2, namely
complexes L1 and L2 have been successfully synthesized. Due to the introduction
of donor unit and extension of the conjugated system, the stretching vibration
1
Fig. 1 H NMR spectra of a in CDCl
3
solution
Fig. 2 FT-IR spectra of L1, P1, and P2
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
Fig. 3 FT-IR spectra of 2, L2, P3, and P4
Table 1 Molecular weights and thermal properties of the polymeric metal complexes
a
n
a
b
g
c
Polymer
M
[9103]
M
a [9103]
N
PDI
T
[°C]
d
T [°C]
w
P1
P2
P3
P4
a
8.9
8.1
9.3
8.6
12.1
12.7
12.1
12.7
11
8
1.36
1.57
1.30
1.48
162
169
181
186
323
375
350
420
9
7
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
absorption peaks of C=N, C–N–M, and N-M bond of all the target polymers
stretching vibration absorption peaks have some degree of red-shift, which can
prove that the target products have been successfully synthesized combined with the
GPC test results (Table 1) and elemental analysis results.
GPC study results of all the target polymers are shown in Table 1. The number
average molecular weight of P1–P4 is 8.9, 8.1, 9.3, and 8.6 kg/mol, and the unit of
them is 11, 8, 9, and 7, respectively. All the PDI of polymeric metal complexes are
relatively wide (P1–P4: 1.36, 1.57, 1.30, and 1.48, respectively). The changes of the
molecular weight also proved that the copolymerization has taken place between the
monomers, which is further evidence for the synthesis of the target polymers.
Optical properties
Figures 4 and 5 give the UV–Vis and normalized PL spectra of the polymeric metal
complexes P1–P4 (10–5 M in DMF solution), respectively, and the corresponding
data are summarized in Table 2.
123

J. Hu et al.
In Fig. 4, the maximum absorption values of P1–P4 are at 402, 421, 423, and
46 nm, respectively, and these strong absorption bands result from intramolecular
4
charge transfer (ICT) between the electron acceptor metal-salicylaldehyde unit and
the electron-donating alkoxy benzene. These polymers have very weak shoulder
absorption peaks in the band 457–500 nm, which is due to the charge transition of
the DAMN derivatives and d10 metal ions in the polymer. By comparing P1 with
P2 as well as P3 with P4, we found that P2 and P4 have better UV–Vis absorption,
which indicates that adding the thienyl group in polymers can promote the
absorption of light. In the later work, we will introduce more electron–donor groups
to ensure that the kmax of dyes shift to the red or infrared region and improve the
photovoltaic properties.
The normalized photoluminescent (PL) spectra of P1–P4 in DMF solution are
shown in Fig. 5. The excitation wavelengths were set according to the maximum
absorption peak of UV–Vis spectrum and the corresponding optical data are also
listed in Table 2. It can be seen that the PL peaks of P1–P4 are at 491, 469, 445, and
4
31 nm, respectively, which can be attributed to the p-p* transition of intra-ligand.
Thermal stability
As an important part of DSSCs, the dye should have good stability, which can
increase the thermal stability of photovoltaic devices. Therefore, the thermal
stability study has important implications for DSSCs. TGA and DSC were selected
to study the thermal stability of the target product, and the TGA is used to test the
target thermal decomposition temperature (Td: 5 % weight loss temperature), while
the DSC is used to test target the glass-transition temperature (Tg). The
corresponding data are listed in Table 1. The TGA results (Fig. 6) show the Td
of the four polymeric metal complexes (P1–P4) are at temperatures of 323, 375,
Fig. 4 UV-Vis absorption spectra of L1, L2, and polymeric metal complexes (P1–P4) in DMF solution
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
Fig. 5 PL spectra of the four polymeric metal complexes (P1–P4) in DMF solution
Table 2 Optical and electrochemical properties of the polymeric metal complexes
d
, max ,
e
Polymer
k
k
a
k
p, max
,
E
g, opt
eV
/
E
ox
(V)
E
red
(V)
HOMO
(eV)
LUMO
(eV)
g C
E , E /
f
g
g
h
a, onset
eV
P1
P2
P3
P4
a
402,494
421,528
423,508
446,560
491
469
445
431
2.51
2.35
2.44
2.26
1.13
1.18
1.16
1.22
-1.08
-1.02
-1.08
-0.94
-5.53
-5.58
-5.56
-5.62
-3.32
-3.38
-3.32
-3.46
2.21
2.20
2.24
2.16
k
a,max, ka, onset: The maxima and onset absorption from the UV–Vis spectra in DMF solution
p, max: The PL maxima in DMF solution
g,opt: Optical energy band gap calculated from the formula E
Values determined by cyclic voltammetry
, E : Electrochemical band gap estimated from HOMO and LUMO
b
c
d
e
k
E
g
= 1,240/ka,onset (eV)
E
g
C
350, and 420 °C in nitrogen, respectively, which means all of them are steady [35].
From the data of Table 1, we can see that the Tg of the four polymeric metal
complexes (P1–P4) follow the order P4 (186 °C) [ P3 (181 °C) [ P2
(
169 °C) [ P1 (162 °C), which suggests that polymeric metal complex have
binuclear metal hold higher rigidity than that have mononuclear metal. There is no
fixed melting point, which is due to our synthetic method, and purification means
that we only obtained the amorphous products, and this kind of structure might not
be conducive to be used in organic solar cells. The high glass-transition temperature
of them shows that these kinds of materials have great application prospects in
improving the stability of the dye-sensitized solar cells.
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J. Hu et al.
Fig. 6 TGA curves of P1–P4 with a heating rate of 20 °C/min under nitrogen atmosphere
4 4
Fig. 7 Cyclic voltammograms for P1–P4 in DMF/0.1 M [Bu N]BF at 100 mV/s
Electrochemical properties
Electronic energy level is one of the most important properties for organic materials
used in solar cells. Figure 7 shows the cyclic voltammograms of P1–P4. The cyclic
voltammetry of polymeric metal complexes were measured in DMF containing
[
Bu N]BF (Bu = butyl) as supporting electrolyte and saturated calomel electrode
4 4
(
SCE) as reference electrode at a scan rate of 50 mV/s. The lowest unoccupied
molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy
levels of the dyes are crucial properties for materials used in DSSCs. The LUMO,
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
HOMO, and energy gap (Eg) can be obtained according to Eqs. (1), (2), and (3),
respectively [36].
HOMO ¼ ꢁeðEox þ 4:40Þ ðeVÞ
LUMO ¼ ꢁeðEred þ 4:40ÞðeVÞ
Eg ¼ HOMO ꢁ LUMO
ð1Þ
ð2Þ
ð3Þ
The values obtained are listed in Table 3. The optical band gaps of P1–P4 are
estimated to be 2.51, 2.35, 2.44, and 2.26 eV, respectively. The energy levels of
HOMO and LUMO, which are -5.53 and -3.32 eV for P1, -5.58 and -3.38 eV for
P2, -5.56 and -3.32 eV for P3, and -5.62 and -3.46 eV for P4, were calculated from
the onset oxidation and reduction potential. The electrochemical band gaps of P1–
P4 are estimated to be 2.21, 2.20, 2.24, and 2.16 eV, respectively, which is
consistent with the optical band gaps, so they are suitable for fabrication of
optoelectronic devices. P4 exhibits a slightly lower band gap compared with other
three polymeric metal complexes (P1–P3); this may be due to the stronger electron-
donating ability of bithiophene phenylenevinylene units that decreased the energy
gap.
Photovoltaic properties
DSSCs devices based on these four polymeric metal complexes were fabricated and
2
tested under the illumination of AM 1.5G, 100 mW/cm for solar cell applications.
Following our conventional practice for these polymeric metal complexes using
solution process [37, 38], the active layers were spin-coated from their DMF
solutions.
Figure 8 shows the incident photon-to-current conversion efficiencies (IPCE) of
the DSSCs devices based on four polymeric metal complexes (P1–P4). From Fig. 8,
the IPCE values of the dye P3 and P4 reach to 31.4 and 34.1 %, respectively,
around the band of 450 nm. Though dye P4 has the maximum IPCE among the four
dyes, the results are not as good as the traditional ruthenium dye, which is probably
caused by low charge collection efficiency [39].
The photocurrent density–voltage (J–V) curves are reported in Fig. 9 and the
corresponding photovoltaic performance is summarized in Table 3. We can see
from Table 3 that the Voc values of P1, P2, P3, and P4 are 0.52, 0.64, 0.58, and
Table 3 Photovoltaic parameters of devices with sensitizers P1–P4 in DSSCs at full sunlight (AM 1.5 G,
1
2
00 wM/cm )
2
sc (mA/cm )
Polymer
Solvent
J
Voc (V)
FF (%)
g (%)
P1
P2
P3
P4
DMF
DMF
DMF
DMF
3.76
4.09
3.86
4.22
0.52
0.64
0.58
0.71
69.7
63.2
68.3
69.1
1.36
1.65
1.53
2.07
123

J. Hu et al.
Fig. 8 IPCE plots for the DSSCs based on the four polymeric metal complexes (P1–P4)
2
Fig. 9 J–V curves of DSSCs based on dyes (P1–P4) under the illumination of AM 1.5, 100 mW/cm
0
.71 V, respectively. The corresponding FF values are 69.7, 63.2, 68.3, 69.1. The J
sc
2
2
values follow the order P4 (Jsc = 4.22 mA/cm ) [ P2 (J = 4.09 mA/cm ) [ P3
sc
2
2
(
Jsc = 3.86 mA/cm ) [ P1 (J = 3.76 mA/cm ). We can find that the J values of
sc
sc
the target polymers (P2, P4) containing bithiophene PPV as donor are higher than
the corresponding target polymers (P1, P3) only containing PPV. The reason for
this is mainly that introduction of more thiophene units can improve electron-
donating ability, and this is more conducive to the generation of photocurrent.
Besides, the electronic conductivity of dual-core cadmium complexes is better than
mononuclear cadmium complexes. However, Jsc values of these materials are still
low; the low Jsc is related to low electron transfer efficiency and charge separation
1
23

Synthesis, characterization, and photovoltaic performance for DSSCs
and narrow absorption spectra in the region of visible light. The J value has great
sc
influence on the power conversion efficiency. Device based on P4 reached the
highest power conversion efficiency of 2.07 % than that of the devices based on P1
(
1.36 %), P2 (1.65 %), P3 (1.53 %).
Conclusions
In this paper, four polymeric metal complexes containing salicylaldehyde and
diaminomaleonitrile units with Cd(II) were synthesized and used as sensitizers in
DSSCs. The polymeric metal complexes have higher Voc and FF values. The results
show that the introduction of a thiophene and cyano group to the molecular skeleton
is conducive to broaden the spectral absorption range of polymeric metal
complexes. Increasing the number of metal ions in the ligand also can promote
the absorption of light. All the LUMO energy level of the complexes is higher than
the LUMO level of TiO , which makes the electrons transfer to the TiO smoothly.
2
2
These results strongly indicate that the polymeric metal complexes will achieve
further applications in DSSCs.
Though P4 containing binuclear metal and more thiophenes shows better optical
and photovoltaic properties, there are also some shortcomings and improvements of
the polymeric metal complexes used as dye sensitizers for dye-sensitized solar cells:
(
1) For the low solubility, the polymeric metal complexes cannot be well dissolved
in DMF; in the PV testing, those complexes could not have more adsorption. (2) The
absorption region of the polymeric metal complexes is not wide enough and so they
have a low IPCE value. (3) To get good solubility, long alkyls should be introduced
in the structure. (4) For strong adsorption onto the surface of TiO , one or two
2
anchoring groups, such as carboxylic acid or sulfonic acid groups, should also be
introduced in the structure. In order to improve the solubility and get better
photovoltaic materials, we still have much work to do and we will report the next
investigation in the future.
Acknowledgments We appreciate the financial support of the Open Project Program of Key Laboratory
of Environmentally Friendly Chemistry and Applications of Ministry of Education, China (No.
09HJYH10).
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