Synthesis and characterization of conjugated polymers with main-chain donors and pendent acceptors for dye-sensitized solar cells

Article abstract of DOI:10.1039/c3ra42161a

Herein, we report three conjugated polymers with main-chain donors and pendent π-acceptors as photosensitizers for dye-sensitized solar cells (DSSCs). The polymer PPTZF (which contains a phenothiazine (PTZ) chromophore and has a 9,9-dioctyl-fluorene (F) group as the donor unit in the main chain and a cyanoacrylic acid acceptor in the side chain, linked by a thiophene unit) was used as a basic structure. Then 9,9-dioctyl-carbazole (CZ) and triphenylamine (TPA) were selected to replace F and PTZ, respectively, as structural modifications to get another two polymers: PPTZCZ and PTPACZ. Finally, we fabricated devices and examined the effects of different donors with flexible alkyl chains on the photovoltaic performance of DSSCs. Our studies have revealed that the PTPACZ dye, which has a degree of polymerization (DP) of ～4, exhibited excellent photovoltaic performance with a relatively high power conversion efficiency of 4.4% under standard (Global Air Mass 1.5) illumination. Charge extraction and transient photovoltage decay measurements have demonstrated that the charge recombination lifetime (τ) is a crucial influencing factor on the open-circuit voltage of these polymer dyes. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42161a

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Synthesis and characterization of conjugated polymers
with main-chain donors and pendent acceptors for dye-
sensitized solar cells
Cite this: DOI: 10.1039/c3ra42161a
Haijun Tan,a Chunyue Pan,*ac Gang Wang,a Yingying Wu,a Yiping Zhang,a
Xuzhu Chen,a Yingping Zou,a Guipeng Yu*ac and Min Zhang*b
Herein, we report three conjugated polymers with main-chain donors and pendent p-acceptors as
photosensitizers for dye-sensitized solar cells (DSSCs). The polymer PPTZF (which contains a phenothiazine
(PTZ) chromophore and has a 9,9-dioctyl-fluorene (F) group as the donor unit in the main chain and a
cyanoacrylic acid acceptor in the side chain, linked by a thiophene unit) was used as a basic structure. Then
9,9-dioctyl-carbazole (CZ) and triphenylamine (TPA) were selected to replace F and PTZ, respectively, as
structural modifications to get another two polymers: PPTZCZ and PTPACZ. Finally, we fabricated devices
and examined the effects of different donors with flexible alkyl chains on the photovoltaic performance of
DSSCs. Our studies have revealed that the PTPACZ dye, which has a degree of polymerization (DP) of y4,
exhibited excellent photovoltaic performance with a relatively high power conversion efficiency of 4.4%
under standard (Global Air Mass 1.5) illumination. Charge extraction and transient photovoltage decay
measurements have demonstrated that the charge recombination lifetime (t) is a crucial influencing factor
on the open-circuit voltage of these polymer dyes.
Received 2nd May 2013,
Accepted 1st July 2013
DOI: 10.1039/c3ra42161a
www.rsc.org/advances
It is critical to get valid absorption and electron injection as
well as transference by designing suitable polymer struc-
Introduction
tures.11,18–20 In 2011, Kirk S. Schanze et al. introduced a
terthiophene segment as the donor in the backbone and
achieved a power conversion efficiency of y3%. Another
important outcome of their work is the demonstration that the
performance of the polymers in the DSSCs varies with the
degree of polymerization (DP). In particular, a higher incident
photon-to-collected electron conversion efficiency (IPCE) and
power conversion efficiency are obtained for the polymer with
the lowest DP.21 Inspired by the design of small organic dye
molecules with donor–p (bridge)–acceptor (D–p–A) structures,
Bin Liu et al. synthesized a conjugated polymer dye containing
an electron donating backbone (triphenylamine) and an
electron accepting side chain (cyanoacetic acid) with con-
jugated thiophene units as the linkers, resulting in a power
conversion efficiency of 3.39%.22 In 2011, Jong-Beom Baek
et al. synthesized RCP-1, which comprises an electron-
donating backbone (carbazole) and an electron-accepting side
chain (cyanoacetic acid) connected through conjugated viny-
lene and terthiophene, and tested it as a photosensitizer. The
well-defined donor (D)–acceptor (A) structure of RCP-1 has
made it possible, for the first time, to reach a power
conversion efficiency of over 4% in DSSCs with an organic
polymer sensitizer.23
Dye-sensitized solar cells (DSSCs) have attracted much atten-
tion in recent years as a promising alternative to traditional
photovoltaic devices based on inorganic semiconductor
technology.1–4 Among all the components, the dye plays a
crucial role in improving the conversion efficiency.5 DSSCs
based on metal complex sensitizers have shown very out-
standing solar-to-electric power conversion efficiencies.6,7
However, they are not attractive in terms of cost and
environmental issues, so great efforts have been dedicated to
the development of metal-free organic dyes.8–11 Conducting
polymers are remarkable in that they combine the electrical
and optical properties of metals and inorganic semiconductors
with the mechanical properties and processing advantages of
organic polymers.12–15 The advantages of tunable energy
levels, large absorption coefficients and excellent structural
stability owing to thermal and solvent resistance make
conjugated polymers possible candidates as sensitizers in
DSSCs.16,17
aInstitute of Organic function macromolecule, College of chemistry and chemical
engineering, Central south university, Changsha 410083, China.
E-mail: panchunyue@sina.com; gilbertyu@csu.edu.cn; Tel: +86 137 8612 0351
bChangchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China. E-mail: min.zhang@ciac.jl.cn
cKey Laboratory of Resources Chemistry of Nonferrous Metals Ministry of Education,
Based on the research mentioned above, combining the
outstanding properties of electron-rich donor and pendent
Central South University, Changsha 410083, China
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first printed onto a fluorine-doped SnO2 (FTO) conducting
glass electrode and further coated with a 5 mm thick scattering
layer of 350–450 nm-sized titania particles. The film thickness
was measured using a benchtop Ambios XP-1 stylus profil-
ometer. The cycloidal TiO2 electrode (y0.28 cm2) was stained
by immersing it into a 150 mmol mL21 dye solution, in a mixed
solvent of THF and AN (v/v, 1 : 1) for 10 h. After being washed
with AN and dried by air flow, the sensitized titania electrode
was assembled with a thermally platinized FTO electrode. The
electrodes were separated by a 35 mm thick Bynel (DuPont) hot-
melt gasket and sealed by heating. The internal space was
filled with a liquid electrolyte using a vacuum backfilling
system. The electrolyte-injecting hole on the counter electrode
glass substrate, made with a sand-blasting drill, was sealed
with a Bynel sheet and a thin glass cover by heating. The
infiltrated electrolyte is composed of 1.0 M DMII, 0.02 M
iodine (I2), 1 M TBP, 0.1 M GNCS and 0.05 M LiI in AN.
acceptor conjugated polymers may lead to the development of
a series of promising dyes for high-performance polymer
DSSCs. Herein, we report the synthesis of polymer PPTZF
(which contains a phenothiazine (PTZ) chromophore and has
a 9,9-dioctyl-fluorene (F) group as the donor unit in the main
chain and a cyanoacrylic acid acceptor in the side chain,
linked by a thiophene unit) was used as a basic structure. Then
9,9-dioctyl-carbazole (CZ) and triphenylamine (TPA) were
selected to replace F and PTZ, respectively, as structural
modifications to get another two polymers: PPTZCZ and
PTPACZ (Fig. 1). Finally, we fabricated devices and examined
the effects of different donors with flexible alkyl chains on the
photovoltaic performance of DSSCs.
The result shows that the PTPACZ dye, which has a DP of
y4, exhibited excellent photovoltaic performance with a
relatively high power conversion efficiency of 4.4% for DSSCs
under standard (Global Air Mass 1.5) illumination. To the best
of our knowledge, this is the highest overall efficiency
achieved in polymer DSSCs. Our study suggested that the
structure–property relationship in polymers with strong
electron-donating main chains and good conjugated side
chain units, as well as electron-withdrawing groups, was
similar to the small organic dye molecules of D–p–A type.
Therefore, the design strategy of small organic dyes could be
applied in designing polymer dyes.
Photovoltaic measurements
A Keithley 2400 source meter and a Zolix Omni-l 300
monochromator equipped with a 500 W xenon lamp were
used for the measurement of photocurrent action spectra, with
a wavelength sampling interval of 10 nm and a current
sampling time of 2 s, under full computer control. The
Hamamatsu S1337 1010BQ silicon diode used for IPCE
measurements was calibrated in the National Institute of
Metrology, China. During the photocurrent action spectra
measurements, a white light-emitting diode was used to bias a
testing cell with y1.0 mA cm2 of background current. A model
LS1000 4S-AM1.5G-1000-W solar simulator (Solar Light
Company, USA) in combination with a metal mesh was
employed to give an irradiance of 100 mW cm2. The light
intensity was tested with a PMA 2144 pyranometer and a
calibrated PMA 2100 dose control system. J–V characteristics
were obtained by applying a bias potential to a testing cell and
measuring the photocurrent with a Keithley 2602 source meter
under full computer control. The measurements were fully
automated using Labview 8.0. A metal mask with an aperture
area of 0.158 cm2 covered the testing cell during all
measurements. An antireflection film (l , 380 nm, ARKTOP,
ASAHI Glass) was adhered to the DSC photoanode during IPCE
and J–V measurements. The short-circuit photocurrent den-
sities measured under this solar simulator are well consistent
with the integral of the IPCEs over the AM1.5G spectrum
(ASTMG173-03), within 5% errors.
Experimental section
Materials
Acetonitrile (AN) and tetrahydrofuran (THF) were distilled
before use. Phenoxazine was purchased from Inno-Chem
Science Technology Co., Ltd. N-bromosuccinimide (NBS),
cyanoacetic acid, piperidine, copper powder, 5-formyl-2-thio-
pheneboronic acid, 1-bromo-4-iodobenzene and 1-bromo-iso-
octane were purchased from Energy Chemical. Lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-ethyl-3-methy-
limidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI),
1,3-dimethylimidazolium iodide (DMII), guanidinium thiocya-
nate (GNCS) and 4-tert-butylpyridine (TBP) were purchased
from Sigma-Aldrich.
UV-vis, photoluminescence and voltammetric measurements
Electronic absorption spectra were measured on an Agilent
G1103A spectrometer. Emission spectra were recorded with a
PerkinElmer LS55 luminescence spectrometer. The emitted
light was detected with a Hamamatsu R928 red-sensitive
photomultiplier. A computer-controlled CHI660C electroche-
mical workstation was used for square-wave voltammetric
measurements with a three-electrode electrochemical cell. All
potentials were reported against the ferrocene/ferrocenium
(Fc/Fc+) reference.
Transient photovoltage decay and charge extraction
measurements
Transient photoelectrical experiments were measured with an
Autolab-PGSTAT302N electrochemical workstation. The steady
and perturbing lights on the photoanode side of a testing cell
were supplied with white and red light-emitting diodes,
respectively. We used the red light to generate a photovoltage
perturbation near the open-circuit photovoltage of a testing
cell under a certain white light and measured the voltage decay
process thereafter. The change in photovoltage caused by the
red pulse of a testing cell was below 5 mV. Normally, the
transient signal follows a monoexponential decay, thus the
electron lifetime can be obtained by fitting an exponential
Cell fabrication
A screen-printed double layer film of interconnected TiO2
particles was used as the mesoporous negative electrode. A 7
mm thick transparent layer of 25 nm-sized titania particles was
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hyde was catalyzed by Pd(PPh3)4 in toluene for 48 h under
alkaline conditions. Following the polymerization reaction,
the polymer dyes were synthesized via Knoevenagel condensa-
tion reactions between the aldehyde groups of the precursor
polymers and cyanoacetic acid in the presence of piperidine.
All of the monomers were synthesized according to the
traditional methods: bromination, high temperature solid-
state Ullmann and Suzuki coupling.
Synthesis of 5-(4-(diphenylamino)phenyl)thiophene-2-car-
baldehyde (2). Compound 1 (19.8 g, 61.1 mmol), Pd(PPh3)4
(0.5 g), 5-formylthiophen-2-yl boronic acid (11.7 g, 73 mmol)
and KOAc (24.8 g, 183.3 mmol) were added to 150 mL THF.
The mixture was heated to 80 uC under argon, stirred and
refluxed for 24 h. After cooling to room temperature (RT), the
solvent was removed by evaporation and the residue was
dissolved in CH2Cl2 (200 mL). The organic portion was washed
with brine and water and dried with anhydrous MgSO4. The
solvent was removed by evaporation, and the residue was
purified by column chromatography using silica gel (PE–
CH2Cl2 = 2 : 1–1 : 1, v/v) to yield 0.5 g golden crystals (60.1%).
1H NMR (CDCl3, 400 MHz, ppm): d = 9.9 (s, 1H), 7.7 (d, 1H), 7.5
(d, 2H), 7.3 (m, 5H), 7.18 (d, 4H), 7.14–7.1 (m, 4H). GC-MS: m/z
= 355.1.
Synthesis of 5-(4-(bis(4bromophenyl)amino)phenyl)thio-
phene-2-carbaldehyde (3). Compound 2 (1.54 g, 4 mmol) was
dissolved in THF (100 mL), then NBS (1.4 g, 8 mmol) was
added away from light. After stirring at 0 uC for 1 h, the
mixture was heated to RT for 12 h, and the reaction was
quenched with water. Then the solvent was removed by
evaporation, and the residue was dissolved in CH2Cl2. The
organic portion was washed with brine and water several
times, then dried with anhydrous MgSO4. The solvent was
removed by evaporation, and the residue was purified by
column chromatography using silica gel (PE–CH2Cl2 = 4 : 1–
2 : 1, v/v) to yield 1.93 g yellow powder (89.4%). 1H NMR
(CDCl3, 400 MHz, ppm): d = 9.88 (s, 1H), 7.73–7.72 (d, 1H),
7.56–7.54 (d, 2H), 7.41–7.40 (d, 4H), 7.32–7.31 (d, 1H), 7.07–
7.05 (d, 2H), 7.01–6.99 (d, 4H). GC-MS: m/z = 512.9.
Scheme 1 Synthetic routes of PTPACZ, PPTZCZ and PPTZF. i) NBS, CCl4; ii)
Pd(PPh3)4, KOAc, 5-formylthiophen-2-yl boronic acid, THF; iii) NBS, THF; iv)
4-iodobromobenzene, potassium carbonate and copper sulfate; v) Pd(PPh3)4,
toluene, K2CO3; vi) cyanoacetic acid, piperidine, CHCl3.
function to the photovoltage decay. The electron density was
estimated by the charge extraction method. A testing cell was
first kept at open-circuit under white light and subsequently,
the white light was turned off upon switching the cell from
open-circuit to short-circuit to record the resulting current
transient, and the electron density was obtained by current
integration.
Synthesis of 10-(4-bromophenyl)-10H-phenothiazine (4).
Phenothiazine (4 g, 20 mmol), 4-iodobromobenzene (6.2 g,
22 mmol), potassium carbonate (3.9 g, 30 mmol) and copper
sulfate pentahydrate (0.25 g, 4 mmol) were added to a 250 mL
three-necked round-bottomed flask. The mixture was heated
to 170 uC for 8 h. 50 mL acetic ether was added into the flask
after the temperature was cooled to 130 uC, and then the
mixture was refluxed for 30 min. After cooling to RT, the
mixture was extracted with acetic ether and washed with water.
The organic layer was dried with MgSO4. The solvent was
removed by evaporation and the residue was purified by
column chromatography using silica gel (petroleum ether) to
yield 3.80 g white crystals (53.5%). 1H NMR (CDCl3, 400 MHz):
7.699 (s, 2H), 7.26 (s, 2H), 7.0 (d, 2H), 6.89 (s, 4H), 6.23 (s, 2H).
GC-MS: m/z = 323.
Measurements of molecular weight and thermal stability
The molecular weights of the polymers were measured by gel
permeation chromatography (WATERS 2414) with three
columns in a series, from 1000–1 000 000, with polystyrene
(PS) as a standard sample. Thermogravimetric analysis (TGA)
was performed at a heating rate of 10 K min21 under N2
atmosphere using a PerkinElmer TGA7.
Results and discussion
Synthesis
The general synthetic routes to prepare PPTZF, PPTZCZ and
PTPACZ are shown in Scheme 1, and detailed synthesis and
characterization data are provided below. The precursor
polymers 9–11 with aldehyde groups were synthesized via
Suzuki cross-coupling reactions. The cross-coupling of borates
with alkyl-substituted-carbazole or fluorene, and dibromide
triphenylamine or phenothiazine with thiophene carbalde-
Synthesis of 5-(4-(10H-phenothiazin-10-yl)phenyl)thiophene-
2-carbaldehyde (5). Compound 4 (0.71 g, 2 mmol), 5-for-
mylthiophen-2-yl boronic acid (0.37 g, 2.4 mmol), Pd(PPh3)4
(0.1 g) and KOAc (0.6 g, 6 mmol) were added to 20 mL THF.
The mixture was heated to 80 uC under argon, stirred, and
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Fig. 1 Structures of the polymer dyes.
refluxed for 8 h. After cooling to RT, the solvent was removed
by evaporation, and the residue was dissolved in CH2Cl2 (50
mL). The organic portion was washed with brine and water
and dried with anhydrous MgSO4. The solvent was removed by
evaporation, and the residue was purified by column
chromatography using silica gel (PE–CH2Cl2 = 2 : 1–1 : 1, v/v)
to yield 0.5 g golden crystals (60.1%). 1H NMR (CDCl3, 400
MHz, ppm): 9.91 (s, 1H), 7.82–7.80 (d, 2H), 7.77–7.76 (d, 1H),
7.44–7.43 (d, 1H), 7.39–7.37 (d, 2H), 7.14–7.12 (d, 2H), 6.965
(m, 4H), 6.51 (d, 2H). GC-MS: m/z = 385.1.
Fig. 2 UV-vis absorption spectra of PTPACZ, PPTZCZ and PPTZF in THF.
Synthesis of 11. The synthesis procedure for 11 was the
same as that for 9. Purification afforded a black solid (62.1%).
1H NMR (CDCl3, 400 MHz, ppm): 9.943 (s, 1H), 7.9 (d, 2H), 7.8
(m, 2H), 7.7–7.6 (m, 3H), 7.6–7.2 (m, 9H), 6.5 (d, 1H), 6.3 (m,
2H), 2–0.7 (m, 34H).
Synthesis of PTPACZ. A mixture of 9 (0.37 g, 0.5 mmol in
repeat unit), cyanoacetic acid (0.128 g, 1.5 mmol), piperidine
(0.5 mL) and CHCl3 (20 mL) was refluxed for 12 h under argon.
After cooling to RT, the mixture was washed with 2 M HCl (20
mL). The organic portion was dried with anhydrous MgSO4.
The solvent was removed by evaporation to yield 0.23 g dark-
red solid (55.3%). 1H NMR (DMSO, 400 MHz, ppm): 7.9 (m,
1H), 7.82–7.16 (m, 20H). GPC: (THF, polystyrene standard), Mn
= 2.315 kg mol21, Mw = 3.384 kg mol21, PDI = 1.46.
Synthesis of 5-(4-(3,7-dibromo-10H-phenothiazin-10-yl)phe-
nyl)thiophene-2-carbaldehyde (6). Compound 5 (1.54 g, 4
mmol) was dissolved in THF (100 mL), then NBS (1.4 g, 8
mmol) was added away from light. After stirring at 0 uC for 1 h,
the mixture was heated to RT for 12 h, and the reaction was
quenched with water. Then the solvent was removed by
evaporation, and the residue was dissolved in CH2Cl2 (100
mL). The organic portion was washed with brine and water
and then dried with anhydrous MgSO4. The solvent was
removed by evaporation, and the residue was purified by
column chromatography using silica gel (PE–CH2Cl2 = 4 : 1–
2 : 1, v/v) to yield 1.93 g yellow powder (89.4%). 1H NMR
(CDCl3, 400 MHz, ppm): 9.93 (s, 1H), 7.89–7.87 (d, 2H), 7.79–
7.78 (d, 1H), 7.48–7.47 (s, 1H), 7.40–7.37 (d, 2H), 7.20–7.16 (d,
2H), 7.0–6.98 (m, 2H), 6.18–6.16 (d, 2H). GC-MS: m/z = 542.9.
Synthesis of 9. 2,7-Di(boronic acid pinacolester)-9-(1-octyl-
nonyl)carbazole (0.658 g, 1 mmol), compound 3 (0.513 g, 1
mmol) and Pd(PPh3)4 (0.1 g, 0.1 mmol) were dissolved in
degassed toluene (10 mL) in a 25 mL two-necked round-
bottom flask. A 2 M aqueous solution of K2CO3 (3 mL) was
added. Then the mixture was stirred vigorously under argon at
100 uC for 48 h. The resulting mixture was then poured into
250 mL of methanol. The precipitate was collected by
filtration, and then the solid product was extracted with
methanol and hexane for 24 h, respectively, in Soxhlet
apparatus to remove monomers. Then the final product was
extracted with CHCl3 for 24 h to yield 0.7 g yellowish-black
solid (94%). 1H NMR (CDCl3, 400 MHz, ppm): 9.85 (s, 1H), 8.20
(m, 2H), 7.80 (s, 1H), 7.67 (s, 5H), 7.60 (s, 1H), 7.48 (m, 5H),
7.35 (s, 4H), 7.13 (d, 2H), 2.33–1.13 (m, 28H), 0.80 (m, 6H).
Synthesis of 10. The synthesis procedure for 10 was the
same as that for 9. Purification afforded a dark-brown filmy
solid (63.5%). 1H NMR (CDCl3, 400 MHz, ppm): 9.9 (s, 1H), 8.1
(m, 2H), 7.9 (d, 2H), 7.8 (d, 1H), 7.6 (m, 2H), 7.5 (m, 7H), 7.4 (s,
2H), 7.3 (s, 2H), 6.5 (d, 1H), 2.3–1.9 (m, 4H), 1.17 (m, 25H), 0.8
(m, 6H).
Synthesis of PPTZCZ. The synthesis procedure for PPTZCZ
was the same as that for PTPACZ. Purification afforded a dark-
red solid (82.0%). 1H NMR (DMSO, 400 MHz, ppm): 8.48 (s,
1H), 8.2–7.2 (m, 19H). GPC: (THF, polystyrene standard), Mn =
2.363 kg mol21, Mw = 3.021 kg mol21, PDI = 1.27.
Synthesis of PPTZF. The synthesis procedure for PPTZF was
the same as that for PTPACZ. Purification afforded a yellowish-
1
red filmy solid (78.4%). H NMR (DMSO, 400 MHz, ppm): 8.1
(m, 1H), 8.0–6.25 (m, 18H), 1.6–0.8 (m, 33H). GPC: (THF,
polystyrene standard), Mn = 2.949 kg mol21, Mw = 3.249 kg
mol21, PDI = 1.34.
Absorption spectra and electrochemical properties
The light-harvesting capacities of the polymers at the
molecular level were first compared by recording the UV-
visible absorption spectra (Fig. 2) of PPTZF, PPTZCZ and
PTPACZ in THF, and the detailed parameters are listed in
Table 1. One can see from Scheme 1 that PPTZF is a basic
structure, and two different kinds of structural modification to
the backbone of PPTZF have been selected to examine their
effect on the performance of the polymer. As shown in Fig. 2,
PPTZF has two distinct absorption peaks at 330 and 372 nm,
neither of which are shifted upon replacing the 9,9-dioctyl-
9H-fluorene (F) of PPTZF with 9-(heptadecan-9-yl)-
9H-carbazole (CZ) to form PPTZCZ. But the corresponding
absorption coefficients are enhanced from 24.0, 19.3 to 42.0,
44.2 6 103 (mg mL21 21 cm21, respectively. The PPTZCZ has a
)
abs
max
maximum absorption wavelength (l ) of 370 nm, which is
red-shifted to 372 nm upon replacing the phenothiazin (PTZ)
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Table 1 Photophysical and electrochemical data for PTPACZ, PPTZCZ and PPTZF in THF
a/nm
e
a/103 (mg mL21 21
)
cm21
l
a/nm
max
E0–0b/eV
2.38
ED/D+c/V
0.45
ED*/D+d/V
21.93
22.52
22.50
abs
max
abs
max
pl
Dye
l
PTPACZ
PPTZCZ
PPTZF
372
455
329
370
330
372
75.2
23.5
42.0
44.2
24.0
19.3
561
480
478
2.77
0.25
2.76
0.26
a
abs
max
abs
max
pl
max
The maximum absorption wavelength (l ), maximum molar absorption coefficient (e ), and PL maximum wavelength (l ) were derived
b
from the static electronic absorption and emission spectra in THF solution. The zero–zero transition energy (E0–0) was estimated from the
intersection points of normalized absorption and emission spectra. The ground-state redox potential (ED/D+) was reported with Fc/Fc+ as a
c
d
reference. The excited-state redox potential ( ED*/D+) was estimated by the equation ED*/D+ = ED/D+ 2 E0–0/e without considering any entropy
change during the light excitation.
of PPTZCZ with triphenylamine (TPA) to form PTPACZ, and
PPTZF are 21.93 V, 22.52 V, 22.50 V with the Fc/Fc+ redox
the absorption coefficient is distinctly increased from 42.0 to
couple as a reference.
75.2 6 103 (mg mL21 21 cm21. Moreover, the PTPACZ has
)
Photovoltaic performance of DSSCs
another distinct absorption peak (23.5 6 103 (mg mL21 21
)
cm21, 455 nm) in the visible region.
Fig. 5 shows the photocurrent action spectra of DSSCs which
were made from 7 + 5 mm thick bilayer titania films coated
with PPTZF, PPTZCZ and PTPACZ in THF–AN (v/v, 1 : 1). The
onsets of the IPCE spectra increase in the order PPTZF ,
PPTZCZ , PTPACZ. In particular, the PTPACZ cell has a much
broader and higher IPCE peak, which is in good agreement
with the absorption spectra of these dyes. As presented in
Fig. 6A, we recorded the photocurrent density–voltage (J–V)
characteristics under simulated AM1.5 G illumination (100
mW cm22) and collected the detailed photovoltaic parameters
in Table 2. The PPTZF and PPTZCZ cells display power
conversion efficiencies (g) of 3.0 and 3.5% respectively. The
small efficiency improvement is mainly related to the short-
To fabricate an efficient DSSC, the energy-offsets of dye
molecules with respect to the titania nanocrystals and redox
electrolytes is very important. The E0–0 (zero–zero transition
energies) were estimated from the intersection points of the
normalized absorption and emission spectra (Fig. 3) of 200 mg
mL21 dye solution (THF), and were 2.38 eV, 2.77 eV and 2.76
eV for PTPACZ, PPTZCZ and PPTZF, respectively. Then we
measured the ground-state redox potential (ED/D+) of these
three dyes in THF, which can be accurately derived by
averaging the anodic and cathodic peak potentials of square-
wave voltammograms (Fig. 4). Then the excited-state redox
potential (ED*/D+) can be calculated using the equation ED*/D+
=
ED/D+ 2 E0–0/e without considering any entropy change during
light excitation. The ED*/D+ values of PTPACZ, PPTZCZ and
Fig. 4 Square-wave voltammograms of PTPACZ, PPTZCZ and PPTZF in THF with
0.1 M 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide sup-
porting electrolyte.
Fig. 3 Normalized electronic absorption and emission spectra of PTPACZ,
PPTZCZ and PPTZF in THF.
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Table 2 Photovoltaic parameters obtained at simulated AM1.5 G conditionsa
dye
Voc/mV
Jsc/mA cm22
Fill factor (FF)
g/%
PPTZF
PPTZCZ
PTPACZ
742
775
769
5.30
6.06
8.12
0.77
0.75
0.71
3.0
3.5
4.4
a
The validity of our photovoltaic data is confirmed by comparing
the calculated Jsc via wavelength integration of the product of the
standard AM1.5 emission spectrum (ASTM G173-03) and measured
IPCE spectra with the experimental Jsc, showing less than 5% error.
Also note that all the cells show a linear dependence of
photocurrent on light intensity.
Fig. 5 Photocurrent action spectra of the polymer dyes.
Assuming Jsc is proportional to photocarrier generation flux,
we further measured the current density–voltage (J–V) curves of
these three cells at various light intensities and plotted Voc as a
function of Jsc in Fig. 6B. At a given Jsc, the photovoltages
increase in the order PPTZF , PTPACZ , PPTZCZ. For a
certain redox electrolyte in DSSCs, it is well recognized that a
rise or fall of Voc stems from a shift of electron quasi-Fermi-
level (EF,n) in titania, which could be related to a variance in
the titania conduction band edge (Ec) and/or a fluctuation of
electron density.24,25 At a given photocarrier generation flux,
the electron density is controlled by the interfacial recombina-
tion of titania electrons with electron accepting species in an
electrolyte and/or dye cations. Thus, we further performed
charge extraction and transient photovoltage decay measure-
ments to understand the alterative order of Voc.
circuit photocurrent density (Jsc) and open-circuit photovoltage
(Voc) increasing from 742 mV and 5.30 mA cm22 to 775 mV and
6.06 mA cm22, respectively, because of the better light-
harvesting capacity of PPTZCZ than PPTZF due to replacement
of the F unit with CZ.
Furthermore, the PTPACZ cell yielded an efficiency of 4.4%
with a Jsc of 8.12 mA cm22 and a Voc of 769 mV, because TPA
has much better donor-capability than PTZ. This is in good
agreement with the absorption and IPCE spectra of these dyes.
As depicted in Fig. 7A, at the same Voc or potential bias the
PTPACZ and PPTZCZ cells have similar extracted electron
densities (de), but the PPTZF cell is not the same. With respect
to the PTPACZ and PPTZCZ cells, there is an y15 mV upward
shift in the PPTZF cell of the titania conduction-band edge
against the electrolyte Fermi-level. However, the relatively
higher conduction-band edge of the PPTZF cell does not lead
to an improved Voc, owing to a shortened charge recombina-
tion lifetime (t) at a given de as presented in Fig. 7B. At the
Fig. 6 (A) J–V characteristics measured under irradiation of 100 mW cm22
,
Fig. 7 (A) Extracted electron density in titania against open-circuit photovol-
tage. (B) Electron lifetime as a function of extracted electron density for cells
made with titania films coated with polymer dyes (PPTZF, PPTZCZ and PTPACZ)
in THF–AN (v/v, 1 : 1).
simulated AM1.5 sunlight. (B) Plots of open-circuit photovoltage as a function of
short-circuit photocurrent density. Cells were tested using a metal mask with an
aperture area of 0.158 cm22. An antireflection film was adhered to the testing
cell during measurements.
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Notes and references
¨
1 B. O’Regan and M. Gratzel, Nature, 1991, 335, 7377.
¨
2 M. Gratzel, J. Photochem. Photobiol., C, 2003, 4, 145.
3 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595.
¨
4 M. K. Nazeeruddin, P. Pechy and M. Gratzel, Chem.
Commun., 1997, 1705.
5 M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni,
¨
G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Gratzel, J. Am.
Chem. Soc., 2005, 127, 16835.
6 L. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi,
S. Zhang, X. Yang and M. Yanagida, Energy Environ. Sci.,
2012, 5, 6057.
7 A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M.
K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S.
Fig. 8 TGA plots of PPTZF, PPTZCZ and PTPACZ.
¨
M. Zakeeruddin and M. Gratzel, Science, 2011, 334, 629.
8 M. Ye, X. Xin, C. Lin and Z. Lin, Nano Lett., 2011, 11, 3214.
9 M. Xu, S. Wenger, H. Bala, D. Shi, R. Li, Y. Zhou, S.
same extracted electron density (de) the charge recombination
lifetime (t) of these three dyes increase in the order of PPTZF
, PTPACZ , PPTZCZ. This is in good agreement with the
alterative order of Voc. Thereby, the charge recombination
lifetime (t) is a crucial influential factor on the Voc of these
polymer dyes.
¨
M. Zakeeruddin, M. Gratzel and P. Wang, J. Phys. Chem. C,
2009, 113, 2966.
10 D. Zhou, N. Cai, H. Long, M. Zhang, Y. Wang and P. Wang,
J. Phys. Chem. C, 2011, 115, 3163.
11 W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang,
F. Wang, C. Pan and P. Wang, Chem. Mater., 2010, 22, 1915.
12 J. Kim, J. K. Koh, B. Kim, S. H. Ahn, H. Ahn, D. Y. Ryu, J.
H. Ki and E. Kim, Adv. Funct. Mater., 2011, 21, 4633.
13 V. S. Sajia, K. Zongb and M. Pyoa, J. Photochem. Photobiol.,
A, 2010, 212, 81.
14 Z. Zhang, Y. Liu, Y. Yang, K. Hou, B. Peng, G. Zhao,
M. Zhang, X. Guo, E. Kang and Y. Li, Macromolecules, 2010,
43, 9376.
Thermal properties
The thermal properties of PPTZF, PPTZCZ and PTPACZ were
determined by TGA under nitrogen atmosphere. As shown in
Fig. 8, these three polymer dyes exhibited good thermal
stability, with 5% weight-loss temperatures (Td) of 250 uC, 239
uC and 244 uC, respectively. This is desirable for the
application of polymer dyes in DSSCs.
15 C. Duan, W. Cai, F. Huang, J. Zhang, M. Wang, T. Yang,
C. Zhong, X. Gong and Y. Cao, Macromolecules, 2010, 43,
5262.
Conclusions
16 G. D. Sharma, P. Suresh, S. K. Sharma and M. S. Roy, Sol.
Energy Mater. Sol. Cells, 2008, 92, 61.
In summary, we have synthesized three conjugated polymers
(PPTZF, PPTZCZ and PTPACZ) which have number average
molecular weights of 2300–3000 (DP y4), and investigated the
influence of different donor units in the backbone on the
polymer molecule’s energy level and photovoltaic character-
istics. Then, we developed a PTPACZ cell, which has TPA and
CZ as the backbone, that displays a power conversion
efficiency of 4.4%. Up to now, it’s the highest efficiency
achieved in DSSCs with polymer dyes applied as the sensitizer.
Our work demonstrated that the engineering of small organic
molecules are also suitable for polymer dye design.
17 M. B. Frederik and C. Krebs, Sol. Energy Mater. Sol. Cells,
2006, 90, 1425.
18 F. Huang, K. Chen, H. Yip, S. K. Hau, O. Acton, Y. Zhang,
J. Luo and Alex K.-Y. Jen, J. Am. Chem. Soc., 2009, 131,
13886–13887.
19 M. Xu, R. Li, N. Pootrakulchote, D. Shi, J. Guo, Z. Yi, S.
¨
M. Zakeeruddin, M. Gratzel and P. Wang, J. Phys. Chem. C,
2008, 112, 19770.
20 Y. Liu, X. Guo, N. Xiang, B. Zhao, H. Huang, H. Li, P. Shen
and S. Tan, J. Mater. Chem., 2010, 20, 1140.
21 Z. Fang, A. A. Eshbaugh and K. S. Schanze, J. Am. Chem.
Soc., 2011, 133, 3063.
22 W. Zhang, Z. Fang, M. Su, M. Saeys and B. Liu, Macromol.
Rapid Commun., 2009, 30, 1533.
23 D. W. Chang, S. J. Ko, J. Y. Kim, S. M. Park, H. J. Lee, L. Dai
and J. B. Baek, Macromol. Rapid Commun., 2011, 32, 1809.
24 B. O’Regan and J. R. Durrant, Acc. Chem. Res., 2009, 42,
1799.
Acknowledgements
This work was supported by grants from the Natural Science
Foundation of China (NOs. 20976199, 21204103 and 51103146)
and the National 973 Program (No. 2011CBA00702).
´
25 F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Sero and
J. Bisquert, Phys. Chem. Chem. Phys., 2011, 13, 9083.
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