Synthesis and photovoltaic properties of polymeric metal complexes containing 8-hydroxyquinoline as dye sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1080/00958972.2012.677532

Four new donor-acceptor type polymeric metal complexes (P1, P2, P3, and P4) with the same Cd(II) complex in side chain and different conjugated backbone structures were synthesized by Yamamoto coupling and applied in dye-sensitized solar cells (DSSCs) as photosensitizers. The photophysical, electrochemical, and thermal properties were investigated in detail, showing that conjugated backbone containing fluorene improved intramolecular charge transfer and increased generation of photocurrent. The highest power conversion efficiency of 0.56% (J _sc=1.63mAcm², V _oc=0.69V, FF=0.50) was obtained with a DSSC based on P3 under simulated air mass 1.5 G solar irradiation, which shows a new strategy to design photosensitizers for DSSCs.

Full text of DOI:10.1080/00958972.2012.677532

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Synthesis and photovoltaic properties
of polymeric metal complexes
containing 8-hydroxyquinoline as dye
sensitizers for dye-sensitized solar cells
Lirong Zhang a , Gaojun Wen a , Qian Xiu a , Lihui Guo a , Jinyan
Deng a & Chaofan Zhong a
a Key Laboratory of Environmentally Friendly Chemistry and
Applications of Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan, Hunan 411105, PR China
Version of record first published: 05 Apr 2012
To cite this article: Lirong Zhang, Gaojun Wen, Qian Xiu, Lihui Guo, Jinyan Deng & Chaofan
Zhong (2012): Synthesis and photovoltaic properties of polymeric metal complexes containing
8-hydroxyquinoline as dye sensitizers for dye-sensitized solar cells, Journal of Coordination
Chemistry, 65:9, 1632-1644
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Journal of Coordination Chemistry
Vol. 65, No. 9, 10 May 2012, 1632–1644
Synthesis and photovoltaic properties of polymeric metal
complexes containing 8-hydroxyquinoline as dye sensitizers for
dye-sensitized solar cells
LIRONG ZHANG, GAOJUN WEN, QIAN XIU, LIHUI GUO,
JINYAN DENG and CHAOFAN ZHONG*
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of
Education, College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, PR China
(Received 4 September 2011; in final form 28 February 2012)
Four new donor–acceptor type polymeric metal complexes (P1, P2, P3, and P4) with the same
Cd(II) complex in side chain and different conjugated backbone structures were synthesized by
Yamamoto coupling and applied in dye-sensitized solar cells (DSSCs) as photosensitizers. The
photophysical, electrochemical, and thermal properties were investigated in detail, showing that
conjugated backbone containing fluorene improved intramolecular charge transfer and
increased generation of photocurrent. The highest power conversion efficiency of 0.56%
(Jsc ¼ 1.63 mA cmꢀ2, Voc ¼ 0.69 V, FF ¼ 0.50) was obtained with a DSSC based on P3 under
simulated air mass 1.5
G solar irradiation, which shows a new strategy to design
photosensitizers for DSSCs.
Keywords: Donor–acceptor; Dye-sensitized solar cells; Polymeric metal complexes; Fluorene
1. Introduction
Increasing energy demands and concerns about global warming have encouraged
scientists to develop cheap and easily accessible renewable energy sources. In
comparison with the traditional silicon-based solar cells, dye-sensitized solar cells
(DSSCs) are promising photovoltaic cells due to versatile, energy-saving, and
environmentally friendly nature [1]. Therefore, DSSCs have been intensively studied
since the breakthrough made by O’Reagen and Gratzel in 1991 [2]. Typical dyes are
¨
ruthenium(II)–polypyridyl photosensitizers which have achieved power conversion
efficiencies over 11% in standard air mass 1.5 G sunlight [3–6], but they are not ideal
cost-effective and environmentally friendly photovoltaic devices. Organic dyes as an
alternative to ruthenium(II) complex photosensitizers exhibit many advantages [7–9]:
(1) molecular structures of organic dyes are diverse and can be easily designed and
synthesized, (2) organic dyes are superior to the Ru complexes for cost and environment
issues, (3) in solutions or thin Elms, organic dyes often show high molar extinction
coefficients, (4) organic dyes exhibit higher efficiencies than Ru complexes in
*Corresponding author. Email: zhongcf798@yahoo.com.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.677532

Dye-sensitized solar cells
1633
p-type DSSCs. These features make organic dyes attractive for photovoltaics and great
efforts have been devoted to developing organic molecule dyes [10–12]. Some
conjugated polymers used as photosensitizers in DSSCs have been reported [13–21].
PolyFuorene (PF)-based conjugated polymers have been extensively utilized in
optoelectronics due to high fluorescence quantum yield, excellent hole-transporting
properties [22, 23], and good Elm-forming properties [24, 25]. However, poor thermal
stability, high-energy barrier, and blue shift of absorptions of these polymers may limit
their large-scale application. These problems can be solved by incorporating electron-
donor or electron-acceptor units into the main PF chain to form a donor-(ꢀ-spacer)-
acceptor (D-ꢀ-A) system. Generally, D-ꢀ-A systems are the basic structure for
designing organic sensitizers due to effective light induced intramolecular charge
transfer (ICT) [26–30], which results in fast charge separation yet extremely slow charge
recombination, holding promise for constructing molecular optoelectronic devices.
8-Hydroxyquinoline (8-HQ) is one of the most important chelators for metal ions
and has been widely used. An electron-donating group in the 5- or 7-position of
8-hydroxyquinoline causes a red shift in absorption and emission spectra. Although
reports of their application as dye sensitizers for DSSCs are rare, chemical modifica-
tions between PFs and 8-HQ may satisfy application requirements to suppress charge
recombination and improve open-circuit photovoltage (Voc) [31–33].
In this article, we have designed and synthesized four D–A dyes with the same Cd(II)
complex in the side chain as electron acceptor with different conjugated backbones
containing fluorene, alkoxy benzene, or carbazole as the electron donor. The dyes are
tuned by co-monomers in the backbone chain to change the photophysical and
electrochemical properties and thus improve the photovoltaic performance. Optical,
photovoltaic, and thermal properties of the four polymeric metal complexes were
investigated.
2. Experimental
2.1. Materials
NiCl2(PPh3)2, fluorene, 8-hydroxyquinoline, Pb/C, and N-bromosuccinimide (NBS)
were obtained from Aldrich Chemical Company and used as received. Ethanol was
dried through molecular sieves and freshly distilled prior to use. N,N-dimethylforma-
mide was dried by distillation over CaH2. The other materials were of commercial grade
and used as received. 5-Formyl-8-hydroxyquinoline [34], 5-formyl-8-tosyloxyquinoline
[35], and 2,7-dibromofluorene [36] were prepared according to literature procedure. All
chemicals used were of analytical grade. Solvents were purified with conventional
methods.
2.2. Instruments and measurements
Nuclear magnetic resonance 1H NMR spectroscopy was performed in CDCl3 and
recorded on a Bruker NMR 400 spectrometer using TMS (0.00 ppm) as the internal
reference. FT-IR spectra were obtained on a Perkin-Elmer Spectrum One Fourier
transform infrared spectrometer with KBr discs. Thermogravimetric analyses (TGA)

1634
L. Zhang et al.
were run on a Shimadzu TGA-7 Instrument in nitrogen at a heating rate of 20ꢁC minꢀ1
from 25ꢁC to 900ꢁC. Differential scanning calorimetry (DSC) was performed on
materialsꢀu1sing a Perkin-Elmer DSC-7 thermal analyzer in nitrogen at a heating rate of
20ꢁC min
from 25ꢁC to 300ꢁC. UV-Vis spectra were taken on a Lambda 25
spectrophotometer. Photoluminescence (PL) spectra were taken on a Perkin-Elmer LS
55 luminescence spectrometer with a xenon lamp as the light source. Elemental analysis
for C, H, and N was carried out using a Perkin-Elmer 2400 II instrument. Cyclic
voltammetry (CV) was conducted on a CHI chi630c Electrochemical Workstation in a
0.1 mol Lꢀ1 [Bu4N]BF4 (Bu ¼ butyl) DMF solution at a scan rate of 100 mV sꢀ1 at room
temperature. The working electrode was a glassy carbon rod, the auxiliary electrode was
a Pt wire electrode, and saturated calomel electrode (SCE) was used as reference
electrode. Gel permeation chromatography (GPC) analysis was done on a WATER
2414 system with DMF as an eluent (1.0 mL minꢀ1) at 80ꢁC.
2.3. General procedure for preparation and testing of DSSCs
The FTO glass (fluorine doped SnO2, sheet resistance 14 X/sq, transmission 490% in
the visible) was first cleaned in a detergent solution using an ultrasonic bath for 15 min
and then rinsed with water and ethanol. The washed FTO glass was immersed in
40 mmol Lꢀ1 aqueous TiCl4 solution at 70ꢁC for 30 min and then washed with water
and ethanol. Titania paste was prepared from 12 g of P25 (Degussa AG, Germany)
following a literature procedure [37] and added with 3.6 mL of 1% magnesium acetate
solution [38]. Then the paste was deposited onto the FTO glass by the sliding glass rod
method. The TiO2-coated FTO glass was sintered at 450ꢁC for 30 min in air. The
sintered film was treated with 40 mmol Lꢀ1 TiCl4 aqueous solution at 45ꢁC for 30 min
and annealed again at 450ꢁC for 30 min. After the film was cooled to 80ꢁC, the TiO2
electrodes were soaked in 0.5 mmol Lꢀ1 samples dyes P1–P4 in DMF solution and
maintained under dark overnight. 3-Methoxypropionitrile solution containing LiI
(0.5 mol Lꢀ1), I2 (0.05 mol Lꢀ1), and 4-tert-butylpyridine (TBP) (0.5 mol Lꢀ1) was used
as the electrolyte. A platinized FTO glass used as counter electrode was prepared by
sputtering 200 nm Pt film on FTO glass. The dye-coated semiconductor film was
illuminated through a conducting glass support without a mask. 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 incident
light intensity of 100 mW cmꢀ2, which was generated by a 500 W Xe lamp passing
through an AM 1.5 G filter with an effective area of 0.16 cm2.
2.4. Synthesis
The synthetic routes of the monomers and polymeric metal complexes are shown in
scheme 1.
2.4.1. Synthesis of 5-formyl-8-hydroxyquinoline (1). Compound 1 was synthesized
according to the published literature [34]. Yield: 24%. 1H NMR (CDCl3, d, ppm): 10.14
(s, 1H), 9.67–9.69 (d, 1H), 8.86–8.87 (t, 1H), 7.99–8.00 (d, 1H), 7.64–7.67 (m, 1H), 7.25–
7.29 (t, 1H).

Dye-sensitized solar cells
1635
Scheme 1. Synthesis of the co-monomers and polymeric metal complexes P1–P4.
2.4.2. Synthesis of 5-formyl-8-tosyloxyquinoline (2). Compound 2 was synthesized
according to the published literature [35]; 1 (2.61 g, 15 mmol) was dissolved in
dichloromethane (80 mL) and TsCl (3.2 g, 18 mmol) was added in one portion under the
ice-water and then stirred at ambient temperature. After 12 h, the solution was poured
into cool water. The organic layer was collected and dried over anhydrous MgSO4;
removal of solvent gave yellow solid (5.12 g, 98%). 1H NMR (CDCl3, d, ppm): 10.26 (s,
1H), 9.75 (d, 1H), 9.55–9.61 (d, 1H), 8.00–8.06 (d, 1H), 7.80–7.82 (d, 1H), 7.58–7.59 (d,
2H), 7.40–7.42 (d, 2H), 7.35–7.40 (d, 1H), 0.85–0.88 (t, 3H).

1636
L. Zhang et al.
2.4.3. Synthesis of 2,7-dibromofluorene (3). Compound 3 was synthesized according to
the literature [36]. Fluorene (8.25 g, 50 mmol) and n-bromosuccinimide (17.8 g, 0.1 mol)
were added into 100 mL acetic acid; then 3.5 mL HBr was added with stirring under
nitrogen, and the reaction was continued 24 h at 90ꢁC. Yellow solid was collected by
filtration and washed by methyl alcohol, recrystallized from ethanol giving white solid
1
(6.69 g, yield 81%); m.p.: 201–202ꢁC. H NMR (CDCl3, d, ppm) 7.55 (d, 4H), 7.44
(d, 2H), 4.15 (s, 1H).
2.4.4. Synthesis of 2,7,9-tribromofluorene (4). Compound 4 was synthesized according
to the published literature [39]. A mixture of 3 (10.02 g, 31 mmol), N-bromosuccinimide
(NBS) (6.04 g, 31 mmol), and carbon tetrachloride (300 mL) was placed in a 500 mL
round bottom flask. The mixture was refluxed and benzoyl peroxide was added as
initiator. The mixture was irradiated under IR lamp for 30 min and continued to reflux
for 4 h. The product was washed twice with dilute sodium bicarbonate, twice with
water, and once with saturated sodium chloride. The combined organic layer was dried
over anhydrous MgSO4, filtered and evaporated to get the crude product. The crude
1
product was recrystallized by ethanol to obtain white solid (7.46 g, 58%). H NMR
(400 MHz, CDCl3, ꢁ, ppm): 7.70 (s, 2H), 7.58–7.62 (d, 2H), 7.51–7.53 (d, 2H), 5.56
(s, 1H). Anal. Calcd for [C13H7Br3]: C, 38.75; H, 1.75. Found: C, 38.61; H, 1.72.
2.4.5. Synthesis of 2,7-dibromo-9-triphenylphosphoniomethylbromo-fluorene
(5).
Compound 5 was synthesized according to the literature [40]. A solution of 2 (8.11 g,
20 mmol) and triphenylphosphine (6.30 g, 24 mmol) in 150 mL of dried 1,4-xylene was
reFuxed for 12 h. The reaction system was then allowed to cool to room temperature
and the white precipitate was collected by filtration, washed with dried ether and
acetone repeatedly, followed by drying; white solid was obtained (10.44 g, 81%).
1H NMR (400 MHz, CDCl3, ꢁ, ppm): 7.87 (s, 2H), 7.65–7.72 (m, 15H), 7.55 (d, 2H),
7.47 (d, 2H), 6.62 (s, 1H). Anal. Calcd for [C31H22Br3P]: C, 55.97; H, 3.33. Found: C,
55.82; H, 3.29.
2.4.6. Synthesis of 2,7-dibromo-9-2-(8-hydroxyquinoline)vinylfluorene (DB8QF) (6).
Compound 6 was synthesized according to the literature [41]. Compounds 5 (3.32 g,
5 mmol) and 2 (1.56 g, 5 mmol) were dissolved in 200 mL of absolute alcohol. Under an
ice-water bath, NaOC2H5 (2.0 g sodium in 60 mL of absolute alcohol) was added to the
solution. After 30 min, the solution was stirred for 12 h at ambient temperature. Then
potassium carbonate (1.05 g, 7.5 mmol) was added and refluxed for 8 h. Yellow solid
was collected at the end of the condensation and washed with distilled water and icy
methanol. After drying in vacuum at 45ꢁC, the compound was obtained as light yellow
1
solid (1.42 g, 65%). H NMR (CDCl3, ꢁ, ppm): 8.91 (s, 1H), 7.90–7.92 (d, 1H), 7.70–
7.76 (m, 2H), 7.67 (s, 2H), 7.52–7.58 (m, 4H), 7.02–7.08 (t, 2H). FT-IR (KBr, cmꢀ1):
3348 (O–H), 3105, 2952 (aromatic and vinylic C–H), 2912 (aliphatic C–H), 1583
(C¼N), 1446 (C¼C), 1414 (fluorene, C–H). Anal. Calcd for [C23H12ONBr2]: C, 57.77;
H, 2.53; N, 2.93. Found: C, 57.84; H, 2.61; N, 2.98.
2.4.7. Synthesis of DB8QFCd (7). Compound 7 was synthesized according to the
published procedure [35]. An ethanol solution (10 mL) of CdCl2 ꢂ 2.5H2O (0.228 g,

Dye-sensitized solar cells
1637
1 mmol) was dropped into a mixed THF solution (20 mL) of 6 (0.479 g, 1 mmol) and
2,20-bipyridine (0.156 g, 1 mmol). The reaction mixture was neutralized carefully with
1 mol Lꢀ1 aq sodium hydroxide until neutral to slightly acidic pH, refluxed for 6 h and
then recrystallized from ethanol, filtered, washed with ethanol repeatedly and dried in
vacuum at 25ꢁC. The pale precipitate was collected (0.63 g, 81%). FT-IR (KBr, cmꢀ1):
3059, 2912, 2853 (aromatic and vinylic C–H), 1574 (C¼N), 1466 (C¼C), 1414 (C–N),
1096 (C–O–M), 485 (M–N). Anal. Calcd for [C33H20ON3ClBr2Cd]: C, 50.67; H, 2.58;
N, 5.37. Found: C, 50.72; H, 2.62; N, 5.31.
2.4.8. Synthesis of 1,4-dibromo-2-methoxy-5-(octyloxy)benzene (8). Compound 8 was
1
synthesized according to the literature [42] (yield 73%). H NMR (CDCl3, ꢁ, ppm):
4.01–4.05 (m, 2H), 3.93 (s, 2H), 3.77 (s, 3H), 1.86–1.92 (m, 2H), 1.27–1.64 (m, 10H),
0.97 (t, 3H). Anal. Calcd for [C15H22Br2O2]: C, 45.71; H, 5.63; O, 8.12. Found: C, 45.82;
H, 5.68; O, 8.18.
2.4.9. Synthesis of 1,4-dibromo-2, 5-bis(octyloxy)benzene (9). The procedure for 9 was
1
similar to that of 8 giving white solid (yield 65%). H NMR (CDCl3, ꢁ, ppm): 7.16 (s,
2H), 3.96–4.04 (m, 4H), 1.81–1.89 (m, 4H), 1.34–1.54 (m, 20H), 0.95 (t, 6H). Anal.
Calcd for [C22H36Br2O2]: C, 53.67; H, 7.37; O, 6.50. Found: C, 53.81; H, 7.44; O, 6.55.
2.4.10. Synthesis of 2,7-dibromo-9,9-dioctyl-fluorene (10). Compound 10 was synthe-
1
sized according to the literature [43] (yield 44%). H NMR (CDCl3, ꢁ, ppm): 7.52 (d,
2H), 7.45 (d, 2H), 7.43 (d, 2H), 1.92 (m, 4H), 1.24–1.10 (m, 20H), 0.84 (t, 6H), 0.57 (m,
4H). Anal. Calcd for [C29H40Br2]: C, 63.51; H, 7.35. Found: C, 63.62; H, 7.41.
2.4.11. Synthesis of 2,7-dibromo-9-octylcarbazole (11). Compound 11 was synthesized
1
according to published procedure [44] (yield 75%). H NMR (CDCl3, ꢁ, ppm): 0.97
(t, 3H), 1.28 (m, 8H), 1.33 (m, 2H), 2.01 (m, 2H), 3.90 (t, 2H), 7.28 (d, 2H), 7.56 (d, 2H).
Anal. Calcd for [C20H23Br2N]: C, 54.94; H, 5.30; N, 3.20. Found: C, 60.07; H, 5.35;
N, 3.23.
2.5. General procedure of polymeric metal complexes
Polymeric metal complex was synthesized by the Yamamoto coupling method [41].
Compound 7 (0.340 g, 0.4 mmol), bis(triphenylphosphine), nickel(II) chloride (0.26 g,
0.4 mmol), compounds 8–11 (0.4 mmol), zinc (0.13 g, 2 mmol), triphenylphosphine
(0.209 g, 0.8 mmol), and 2,20-bipyridine (0.006 g, 0.038 mmol) were dissolved in DMF
(15 mL) under nitrogen. Then the mixture was stirred at 90ꢁC for 48 h. The solid was
precipitated in a large excess of ethanol solution. The crude product was washed with
ethanol, distilled water, and THF sequentially, then dried in vacuum at 60ꢁC for 1 day
to afford pale yellow solid in a yield of 44–56%.
Polymeric metal complex P1: Yield: 53%. FT-IR (KBr, cmꢀ1): 3066, 2943 (aromatic
and vinylic C–H), 1561 (C¼N), 1443 (C¼C), 1021 (C–O–M). Anal. Calcd for
[C47H40O3N3ClCd]: C, 66.99; H, 4.78; N, 4.99. Found: C, 67.08; H, 4.86; N, 4.96.

1638
L. Zhang et al.
Polymeric metal complex P2: Yield: 48%. FT-IR (KBr, cmꢀ1): 3074, 2934, 2849
(aromatic and vinylic C–H), 1554 (C¼N), 1429 (C¼C), 1066 (C–O–M). Anal. Calcd for
[C55H56O3N3ClCd]: C, 69.18; H, 5.91; N, 4.40. Found: C, 69.22; H, 6.03; N, 4.47.
Polymeric metal complex P3: Yield: 56%. FT-IR (KBr, cmꢀ1): 3067, 2929, 2851
(aromatic and vinylic C–H), 1542 (C¼N), 1418 (C¼C), 1018 (C–O–M). Anal. Calcd for
[C62H60ON3ClCd]: C, 73.65; H, 5.98; N, 4.16. Found: C, 73.67; H, 6.10; N, 4.21.
Polymeric metal complex P4: Yield: 44%. FT-IR (KBr, cmꢀ1): 3073, 2953 (aromatic
and vinylic C–H), 1563 (C¼N), 1452 (C¼C), 1068 (C–O–M). Anal. Calcd for
[C53H43ON4ClCd]: C, 70.75; H, 4.82; N, 6.23. Found: C, 70.84; H, 4.86; N, 6.34.
3. Results and discussion
3.1. Synthesis
Scheme 1 outlines the synthetic routes to 6, which was synthesized by the Wittig
reaction and four polymeric metal complexes P1–P4 which were synthesized by
Yamamoto coupling protocol.
1H NMR spectra (Supplementary material) of DB8QF (6) shows vinyl proton peaks
at 7.03 ppm and 8.91 ppm; 7.90–7.92 ppm, 7.70–7.76 ppm, ꢃ7.05 ppm were attributed to
protons of 8-hydroxyquinoline, while protons of the fluorene ring were observed at
7.67 ppm and 7.52–7.58 ppm. There was no detectable signal for OH in CDCl3 in the
spectra [45].
The FT-IR spectra of DB8QFCd and the copolymers P1–P4 show a sharp absorption
at 1574 cmꢀ1 ascribed to C¼N stretch of 8-hydroxyquinoline. Absorptions at 1464,
1414 are due to C¼C, C–N stretches, respectively; the peak at 1096 cmꢀ1 in the IR
spectra of DB8QFCd was attributed to coordination of 8-hydroxyquinoline with Cd2þ
[46]. In DB8QFCd, a new band at 485 cmꢀ1, absent in free DB8QF, can be attributed to
metal–nitrogen bonds [47]. Sharp bands at 2912 cmꢀ1 and 2853 cmꢀ1 are associated
with CH2 asymmetric and symmetric stretching vibrations, respectively, indicating that
alkyl or alkoxy aromatic derivatives have been imbedded in the molecular chain. The
average molecular weight of the coploymers (table 1) proves that copolymerization has
taken place between the monomers and gives evidence of the success of the
polymerization when taken together with the data of the elemental analysis.
GPC shows that P1–P3 have average molecular weights of 7.89, 7.20, 9.34, and
9.83 kg molꢀ1 (9, 8, 9, and 11 repeating units on average for P1–P4, respectively) with
Table 1. Polymerization, thermal, and optical properties of P1–P4.
Mn(ꢄ103)
Mw(ꢄ103)
PDI
ꢂa,max (nm)
ꢂf,max (nm)
a
Tg (ꢁC)
b
Td (ꢁC)
Polymer
P1
P2
P3
P4
7.89
7.20
9.34
9.83
15.75
11.72
18.94
21.13
1.89
1.63
2.02
2.15
98
92
116
105
317
327
269
335
397
389
411
419
466
471
454
449
aGlass transition temperature, measured from DSC traces of the polymeric metal complexes.
bThe data were obtained from TGA of the polymeric metal complexes.

Dye-sensitized solar cells
1639
Figure 1. Normalized UV-Vis absorption spectra of the polymers in DMF (10ꢀ5 mol Lꢀ1).
a relatively narrow polydispersity index (PDI) between 1.89, 1.63, 2.02, and 2.15 for P1–
P4, respectively (shown in table 1). As expected, P1–P4 were soluble in DMF, DMSO,
and toluene, but present poor solubility in THF, DCM, and MeOH.
3.2. Optical properties
The photophysical properties of the polymers were investigated by UV-Vis and
fluorescence spectroscopy in dilute DMF solution (10ꢀ5 mol Lꢀ1). The absorption
spectra of the ligand and the polymeric metal complexes are shown in figure 1 and the
corresponding optical data of the polymeric metal complexes are summarized in table 1.
For comparison, the absorption spectrum of DB8QFCd is included. Absorption spectra
of the polymers exhibited similar curves with one maximum (ꢂa,max) at 397 nm, 389 nm,
411 nm, and 419 nm, respectively, corresponding to the ꢀ–ꢀ* transitions of the
conjugated molecules. Absorption peaks of the polymers were red-shifted about 82 nm,
74 nm, 94 nm, and 104 nm in comparison with DB8QFCd, resulting from the
lengthening of ꢀ–ꢀ conjugated main chain, boding well for their photovoltaic
properties.
Fluorescence spectroscopy of polymeric metal complexes (P1–P4), recorded upon
excitation at the maximum absorption in the UV-Vis spectra, were measured in
diluted DMF solution (10ꢀ5 mol Lꢀ1). As observed from figure 2, the maximum
(ꢂe,max) emission peaks of the polymers were 476 nm, 449 nm, 457 nm, and 469 nm,
respectively. PL intensity of P1 and P2 were higher than that of P3 and P4 as a
result of electron-donating alkoxy side chain. In contrast, the ꢂe,max of P1, P2, and
P3 were bathochromic shifted because of the effect of nitrogen of carbazole on the
emission.

1640
L. Zhang et al.
Figure 2. PL spectra of the polymers in DMF (10ꢀ5 mol Lꢀ1).
3.3. Thermal properties
Thermal properties of polymers were obtained by TGA and DSC measurements and
the results are tabulated in table 1. The TGA curves (Supplementary material) of
polymers exhibit high thermal stability and the onset temperatures with 5% weight loss
(Td) of P1, P2, P3, and P4 were 317ꢁC, 327ꢁC, 269ꢁC, and 335ꢁC under nitrogen,
respectively. Synchronously, the glass transition temperatures (Tg) of P1, P2, P3, and
P4 were 98ꢁC, 92ꢁC, 116ꢁC, and 105ꢁC and no crystallization or melting peaks were
detected, indicating that the polymers are amorphous. The high thermal stability of the
polymers could beneEt to increased stability of the DSSCs, preventing morphological
change, deformation, and degradation of the active layer by current-induced heat
during operation of the photovoltaic polymers [48].
3.4. Electrochemical properties
All CV values can be given by using a method based on adiabatic potential energy
surfaces of both the donor and acceptors. CV is considered an effective tool in
investigating electrochemical properties of conjugated compounds. The oxidation and
reduction potentials revealed in cyclic voltammograms showed the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels,
which correspond to ionization potentials and electron affinities, respectively. The
polymer films coated on a glassy carbon working electrode, supported in 0.1 mol Lꢀ1
[Bu4N]BF4 (Bu ¼ butyl) in anhydrous acetonitrile solution, were measured at a
scanning rate of 50mV sꢀ1. The corresponding electrochemical data are listed
in table 2. The cyclic voltammograms of the polymers are provided in

Dye-sensitized solar cells
Table 2. Electrochemical properties of P1–P4.
1641
ox
onset
red
Polymer
E
(V)
E
(V)
onset
HOMO (eV)
LUMO (eV)
Eg (eV)
P1
P2
P3
P4
1.35
1.38
1.34
1.40
ꢀ1.14
ꢀ1.16
ꢀ1.01
ꢀ1.20
ꢀ5.75
ꢀ5.78
ꢀ5.74
ꢀ5.80
ꢀ3.26
ꢀ3.24
ꢀ3.39
ꢀ3.20
2.49
2.54
2.35
2.60
‘‘Supplementary material.’’ When SCE electrode was used as the reference electrode,
the correlation can be expressed as equations [49, 50]:
HOMO ¼ ꢀe ðEox þ 4:40Þ ðeVÞ,
LUMO ¼ ꢀe ðEred þ 4:40Þ ðeVÞ:
The results show that the reduction and oxidation potentials of P3 are
Ered ¼ ꢀ1.01 eV and Eox ¼ 1.34 eV, respectively. The energy band gap (Eg) was
2.35 eV, the energy value of the HOMO was calculated to be ꢀ5.74 eV and the
LUMO was calculated to be ꢀ3.39 eV. For P1, P2, and P4 the HOMO were calculated
as ꢀ5.75 eV, ꢀ5.78 eV, and ꢀ5.80 eV, with an Eg of 2.49 eV, 2.54 eV, and 2.60 eV,
respectively. P2 exhibited a higher energy band gap than P1 due to the steric effect.
Incorporation of the arene groups into the main PF chain impacts the energy level of
HOMO and LUMO, leading to the distinctive Eg in the polymers, which is valuable to
make a deep study of solar cell materials.
On the basis of the CV data, the LUMO of the complexes follows the order
P3 5 P1 5 P2 5 P4, which shows that the electron accepting ability of the complexes
follows the order P4 4 P2 4 P1 4 P3 [51]. Eg of the complexes follows the order
P3 5 P1 5 P2 5 P4, so P3 is more suitable for fabrication of optoelectronic devices
than either P1, P2, or P4 because a relatively low Eg can absorb light efficiently,
important to improving power conversion efficiency [52]. Values of optical band gaps of
P1, P2, P3, and P4 are 2.42 eV, 2.51 eV, 2.30 eV, and 2.52 eV, respectively, according to
figure 1. The electrochemical band gaps are higher than the optically determined ones
for two reasons: first, this may be due to interface barrier for charge injection [53] and
second, the excited state in conjugated polymers was viewed as a bound exciton.
Formation of charge carriers required a higher energy of optical absorption, with no
easy detection by linear optical spectroscopy [54].
3.5. Photocurrent–voltage measurements
Figure 3 shows current density–voltage (J–V) characteristics of the devices fabricated
by P1–P4. The corresponding open-circuit voltage (Voc), short-circuit current density
(Jsc), Ell factor (FF), and solar energy-to-electricity conversion efEciency (ꢃ) of the
polymers for devices are listed in table 3. Jsc, Voc, and FF of the device based on P3
achieved 1.63 mA cmꢀ2, 0.69 eV, and 0.50 and power conversion efEciency (ꢃ) reached
0.56%. The DSSCs based on P1, P2, and P4 obtained ꢃ values 0.34% (Jsc ¼
1.06 mA cmꢀ2, Voc ¼ 0.56 V, FF ¼ 0.56), 0.29% (Jsc ¼ 0.69 mA cmꢀ2, Voc ¼ 0.59 V,
FF ¼ 0.61), and 0.23% (Jsc ¼ 0.62 mA cmꢀ2, Voc ¼ 0.58 V, FF ¼ 0.50), respectively.
The power conversion efficiency based on P3 reached 0.56%, higher than that of the

1642
L. Zhang et al.
Figure 3. J–V curves of DSSCs based on P1–P4 under simulated AM 1.5
G solar irradiation
(100 mW cmꢀ2).
Table 3. Photovoltaic properties of DSSCs based on P1–P4 under 1.5 G solar illumination.
Polymer
Voc (V)
Jsc (mA cmꢀ2
)
FF
G (%)
P1
P2
P3
P4
0.56
0.59
0.69
0.58
1.06
0.69
1.63
0.62
0.56
0.61
0.50
0.60
0.34
0.29
0.56
0.23
devices based on P1 (0.34%), P2 (0.29%), and P4 (0.23%). This is attributed to
significant difference of the electron donor unit, with fluorene more suitable for donor
unit than alkoxy benzene derivatives or carbazole derivatives.
The low Jsc is ascribed to weak adsorption onto the surface of TiO2, low charge
separation, the narrow absorption spectra of the polymers, and low transportation
efficiency. The results indicate that modiEcation of the dyes in terms of electron-
donating monomers is an effective method to control Jsc. Moreover, a relatively high Jsc
is very important to improve PCE. The Jsc of the DSSCs based on P3 as the electron
donor unit is the highest of the polymers and P3 is more suitable for fabrication of
optoelectronic devices than others. Film-forming and weak adsorption onto the surface
of TiO2 are two major factors that limit performance of DSSCs [55, 56]. Investigation is
underway to optimize device performance.
4. Conclusions
We report synthesis and characterization of four polymeric metals with the same Cd(II)
complex in the side chain and different conjugated backbone structures. The four

Dye-sensitized solar cells
1643
materials have good thermal stability, good open-circuit voltages and fill factors but
moderate power conversion. We have found that the conjugated backbone containing
fluorene benefits ICT increases the generation of photocurrent. The ꢃ of P1–P4 are
0.34%, 0.29%, 0.56%, and 0.23%, respectively, suggesting further optimization is
essential before application in DSSCs.
To obtain outstanding ꢃ, there are still many challenges. First, Jsc based on all the
materials is very low due to low adsorption affinities on the TiO2; anchoring groups
such as ꢀCN, ꢀCOOH, or ꢀSO3H should be introduced in the structure. Structural
optimization with broad spectral coverage and excellent charge separation and
transportation are expected to produce more efficient photosensitive dyes. Our work
in these directions is underway.
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