New diketo-pyrrolo-pyrrole (DPP) sensitizer containing a furan moiety for efficient and stable dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.09.009

A new metal-free organic sensitizer containing a furan moiety as the π-spacer based on the diketo-pyrrolo-pyrrole unit was synthesized through simple synthetic routes and with low cost for the application of dye-sensitized solar cells. Two corresponding d

Full text of DOI:10.1016/j.dyepig.2011.09.009

Dyes and Pigments 92 (2012) 1384e1393
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New diketo-pyrrolo-pyrrole (DPP) sensitizer containing a furan moiety for
efficient and stable dye-sensitized solar cells
*
*
Sanyin Qu, Bing Wang, Fuling Guo, Jing Li, Wenjun Wu , Cong Kong, Yitao Long, Jianli Hua
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new metal-free organic sensitizer containing a furan moiety as the
p-spacer based on the diketo-
Received 21 June 2011
Received in revised form
8 September 2011
Accepted 12 September 2011
Available online 22 September 2011
pyrrolo-pyrrole unit was synthesized through simple synthetic routes and with low cost for the appli-
cation of dye-sensitized solar cells. Two corresponding dyes with benzene and thiophene spacers were
also synthesized for the purpose of comparison. On the basis of optimized DSSC test conditions, the
sensitizer containing the furan shows prominent solar energy conversion efficiency (h) of 5.65%
(Jsc ¼ 15.96 mA cmꢀ2, Voc ¼ 541 mV, ff ¼ 0.65) under simulated full sunlight irradiation. The dyes were
also tested in a solvent-free ionic liquid electrolyte devices and the stability of devices was performed
over 2000 h at full sunlight. The sensitizer containing the furan moiety exhibited good stability and
better photovoltaic performance of up to 4.41% power conversion efficiency.
Keywords:
Diketo-pyrrolo-pyrrole
Metal-free
Dye-sensitized solar cell
Furan
Ó 2011 Elsevier Ltd. All rights reserved.
Ionic liquid electrolyte
Stability
1. Introduction
Though many factors control the lifetime and cost of the cells,
a crucial element in DSSCs, the sensitizer exerts a significant
The ever-increasing demands for renewable energy sources
have led to intense research on a variety of light-harvesting devices.
Among these, dye-sensitized solar cells [1] have attracted much
attention as a promising, efficient, molecular photovoltaic device.
At present, DSSCs based on Ru (II)-polypyridyl complexes have an
influence on these two points. In order to improve the cells’
stability, several effective strategies, such as introducing oligo-
thiophene linkers [16], starburst carbazole antenna donors [17] and
thermally and photochemically stable fluorophores [18] into the
sensitizers, are employed in DSSCs. The holes in these dye-
sensitizers were delocalized over the large moieties instead of
a specific atom, thus avoid forming a localized radical center and
thus show relatively good stability [19]. Diketo-pyrrolo-pyrrole
(DPP) and some of its derivatives are commercialized pigments
with a large fluorophore and exceptional light, weather and heat
stability. In our previous studies [20], we have introduced DPP into
DSSCs and achieved relatively high efficiency. Since triphenylamine
and DPP are both commercially available compounds with low cost,
also with the good stability of the DPP fluorophore, making them
promising pigments for practical application, therefore it is
meaningful to further expand the DPP series of dyes.
As reported so far, most of the metal-free dye-sensitizers with
high efficiency rely on thiophene or thiophene-based heterocycles
as the
p-spacer. In particular, some of the dyes based on coumarin-
thiophene dye [21], carbazole-oligothiophene dye [19] and
benzothiadizole-thiophene dye [22] show good stability. This could
be explained by the fact that holes in such dyes were located on the
thiophene moieties. And furan, thiophene’s oxygen analog, which
has a higher oxidation potential, would be more efficient for the
overall solar energy conversion efficiency (h) approaching 12% in
small area [2] and with a module efficiency exceeding 8% [3] under
simulated AM 1.5 irradiation (100 mW cmꢀ2). On the other hand,
metal-free organic dyes have also been developed for DSSCs due to
their high molar absorption coefficient, simple synthesis process
and structure adjustability. Recently, novel organic dyes based on
cyanine [4], merocyanine [5], coumarin [6], carbazole [7], indoline
[8], hemicyanine [9], oligoene [10], xanthene [11], phenothiazine
[12] and phenoxazine [13] have been investigated as sensitizers for
DSSCs, and solar cells based on triphenylamine and dithienosilole
have been reported [14] with an energy conversion efficiency
exceeding 10%. However, at present, the photovoltaics sector is still
dominated by Si and there has been rare production of DSSCs on
a large scale. The bottleneck in the further practical application of
DSSCs is the lifetime and cost of the cells [15].
* Corresponding authors. Tel.: þ86 21 64250940; fax: þ86 21 64252758.
E-mail addresses: wjwu@ecust.edu.cn (W. Wu), jlhua@ecust.edu.cn (J. Hua).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.009

S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
1385
hole location and reinforce the stability of the dye-sensitizers
[19,23]. However, furan has received much less attention
compared to the thiophene unit. Recently, furan units have also
been incorporated into some conjugated polymers and oligomers
as an alternative to thiophenes for photovoltaics and have revealed
that the optical and charge carrier mobility can be quite comparable
to that of thiophenes [24]. Small chromophores containing a furan
unit applied in dye-sensitized solar cells have also been reported
and showed relatively high efficiency [25]. There have been reports
showing that the incorporation of a furan moiety can somewhat
improve polymer solubility substantially and gain high molecular
weight [26]. The introduction of a furan moiety in DPP-based dye-
obtained from the Geao Science and Educational Co. Ltd. of China and
cleaned by water, acetone, distilled water and ethanol in turn. TiO2
paste was purchased from Solaronix (Switzerland). Lithium iodide
was obtained from Fluka. Tetrahydrofuran (THF) was pre-dried over
4 Å molecular sieves and distilled under argon atmosphere from
sodium benzophenoneketyl immediately prior to use. Dichloro-
methaneandpyridineweredistilledundernormalpressureanddried
over calcium hydroxide. Starting material 3-[4-[4-(N,N-bis(4-
methoxyphenyl)amino)phenyl]pheny-l]-6-(4-bromophenyl)-2,5-di-
n-butyl-pyrrolo [3,4-c]pyrrole-1,4-dione (1) was synthesized
according to our previous studies [20]. DPP-III, included for
comparison, was synthesized in our earlier studies [20]. All other
chemicals were purchased from Aldrich and used as received without
further purification.
sensitizers and compared with other
p-spacer would be inter-
esting. More importantly, furan derivatives can be synthesized from
a variety of natural products; hence, the dye-sensitizers based on
furan can be considered as the renewable and sustainable synthetic
resources and allow for production of DSSCs at a large scale [26,27].
Based on the above consideration, we have designed and
synthesized a new DPP-based organic sensitizer DPP-I containing
2.3. Preparation of solar cells
A screen-printed TiO2 particle was used as photoelectrode. The
thickness of TiO2 film was controlled by the mesh size of screen
printing with different layers and measured by a surface profile
(Dektak Co., Model DAKTAK II). Sintering was carried out at 450 ꢁC
for 30 min. Before immersion in the dye solution, these films were
soaked in the 0.2 M aqueous TiCl4 solution overnight in a closed
chamber and washed with water and ethanol, which can signifi-
cantly increase the short circuit photocurrent. Then the films were
heated again at 450 ꢁC for 30 min followed by cooling to 80 ꢁC and
dipping into a 3 ꢂ 10ꢀ4 M solution of dyes in dichloromethane for
12 h at room temperature. To prepare the counter electrode, the Pt
catalyst was deposited on the cleaned FTO glass by coating with
a drop of H2PtCl6 solution (0.02 M in 2-Propanol solution) with the
heat treatment at 400 ꢁC for 15 min. A hole (0.8-mm diameter) was
drilled on the counter electrode by a Drill-press. The perforated
sheet was cleaned by ultrasound in an ethanol bath for 10 min.
About the assemblage of DSSCs, the dye-covered TiO2 electrode and
Pt-counter electrode were assembled into a sandwich type cell and
a furan moiety as a
and DPP-III with a benzene and a thiophene
p
-spacer, and the corresponding dyes DPP-II
-spacer for the
p
purpose of comparison were designed and synthesized; corre-
sponding molecular structures of the three dyes are shown in
Scheme 1. The sensitizers have been applied to sensitization of
nanocrystalline TiO2-based solar cells and the effect of
p-spacer
linkers on light absorption, energy level, photocurrent and photo-
voltage will be detailed in this paper.
2. Experimental section
2.1. Equipment
NMR spectra were obtained on Brücker AM 500 spectrometer.
The absorption spectra of the dyes in solution and adsorbed on TiO2
films were measured with a Varian Cary 500 spectrophotometer. MS
were recorded on ESI mass spectroscopy. The cyclic voltammograms
of dyes were obtained with a Versastat II electrochemical work-
station (Princeton applied research) using a normal three-electrode
cell with a Pt working electrode, a Pt wire counter electrode, and
a regular calomel reference electrode in saturated KCl solution. The
supporting electrolyte was 0.1 M TBAPF6 (tetra-n-butylammonium
hexafluorophosphate) in acetonitrile/THF(3:1, v/v) as the solvent.
sealed with a hot-melt gasket of 25
mm thickness made of the
ionomer Surlyn 1702 (Dupont). The size of the TiO2 electrodes used
was 0.28 cm2. A drop of the electrolyte was put on the hole in the
back of the counter electrode. It was introduced into the cell via
vacuum backfilling. Finally, the hole was sealed using a UV-melt
gum and a cover glass (0.1 mm thickness). The electrolyte
employed was a solution of 0.6 M PMII (1-propyl-3-methyl imi-
dazolium iodide), 0.1 M LiI, 0.05 M I2, in a mixture of acetonitrile
andmethoxypropionitrile(volume ratio, 7:3). For the solvent-free
ionic liquid electrolyte, which consists of 1-butyl-3-methyl imida-
zolium, iodine, benzimidazole and guanidine thiocyanate
(BMII:I2:BI:GNCS ¼ 40:1.67:0.67:3.33).
2.2. Materials
Optically transparent FTO conducting glass (fluorine doped SnO2,
transmission >90% in the visible, sheet resistance 15 U/square) was
2.4. Photoelectrochemical measurements
The photocurrent action spectra were measured with a Model
SR830 DSP Lock-In Amplifier, a Model SR540 Optical Chopper
(Stanford Research Corporation, USA), a 7IL/PX150 xenon lamp
with power supply and a 7ISW301 Spectrometer. The irradiation
source for the photocurrent densityevoltage (JeV) measurement is
an AM 1.5 solar simulator (91160, Newport Co., USA). The incident
light intensity was 100 mW cmꢀ2 calibrated with a standard Si solar
cell. The tested solar cells were masked to a working area of
0.159 cm2. Volt-current characteristic were performed on the
Model 2400 Sourcemeter (Keithley Instruments, Inc. USA). The
electrochemical impedance spectroscopy (EIS) measurements of all
the DSSCs were performed using a Zahner IM6e Impedance
Analyzer (ZAHNER-Elektrik GmbH &CoKG, Kronach, Germany). The
frequency range is 0.1 Hze100 kHz. The applied voltage bias
is ꢀ0.60 V. The magnitude of the alternating signal is 5 mV.
Scheme 1. Molecular structures of dyes DPP (I w III).

1386
S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
2.5. Synthesis
148.3, 148.0, 146.0, 142.4, 139.5, 137.5, 130.4, 129.4, 129.3, 129.2,
128.3, 127.4, 127.1, 125.7, 125.3, 124.9, 118.6, 115.0, 111.6, 109.4, 108.6,
55.2, 30.7, 19.2, 19.2, 13.3, 13.3. HRMS (m/z): [M-H]- calcd. for
(C54H47N4O7): 863.3445; found, 863.3445.
2.5.1. Synthesis of 5-[4-[3-[4-(4-(N, N-bis (4-methoxyphenyl) amino)
phenyl) phenyl]-2,5-di-n-butyl-pyrrolo [3,4-c]pyrrole-1,4-dione]
phenyl]furan-2-carbaldehyde (2)
Compound 1 (0.20 g, 0.25 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol),
and Na2CO3 (1.02 g, 0.01 mol) in THF (10 mL) and H2O (5 mL) were
heated to 45 ꢁC under a nitrogen atmosphere for 30 min. A solution
of 5-formylfuran-2-ylboronic acid (0.070 g, 0.50 mmol) in THF
(5 mL) was added slowly, and the mixture was heated under reflux
for further 12 h. After cooling to room temperature, the mixture
was extracted with CH2Cl2 (30 mL). The organic portion was
combined and removed by rotary evaporation. The residue was
purified by column chromatography on silica (CH2Cl2/ethyl
acetate ¼ 1/70, v/v) to give a red solid. (Yield: 78.5%). mp
115e117 ꢁC. IR (KBr): 2953, 2931, 1671, 1606, 1503, 1245, 1099, 1030,
2.5.4. Synthesis of 2-cyano-3-[5-[4-[3-[4-(4-(N,N-Bis(4-methoxy
phenyl)amino) phenyl)phenyl]-2,5-di-n-butyl-pyrrolo [3,4-c]
pyrrole-1,4-dione]phenyl]benzene-2-yl]acrylic acid (DPP-II)
Compound 3 (0.080 g, 0.10 mmol), 2-cyanoacetic acid (0.11 g,
1.29 mmol), and piperidine (0.5 mL) in THF (20 mL) were heated to
reflux under a nitrogen atmosphere for 6 h. After cooling to room
temperature, the precipitate was filtered. The residue was purified
by column chromatography on silica (CH2Cl2/ethanol ¼ 10/1, v/v) to
give a dark red solid. (Yield: 57.8%). mp 223e225 ꢁC. IR (KBr): 3445,
2962, 2930, 2221, 1650, 1592, 1510, 1380, 1243, 1090, 1041, 823. 1H
NMR (500 MHz, DMSO)
d
: 8.37 (s,1H), 8.18 (d, J ¼ 8.04, 2H), 8.00 (m,
823. 1H NMR (CDCl3, 500 MHz)
d
: (ppm) 9.69 (s, 1H), 7.95 (d,
8H), 7.84 (d, J ¼ 8.33, 2H), 7.66 (d, J ¼ 8.55, 2H), 7.11 (d, J ¼ 8.63, 4H),
6.96 (d, J ¼ 8.82, 4H), 6.84 (d, J ¼ 8.18, 2H), 3.76 (m, 10H), 1.46 (m,
4H), 1.18 (m, 4H), 0.79 (t, J ¼ 7.13, 7.13, 6H). 13C NMR (125 MHz,
J ¼ 8.8 Hz, 2H), 7.92 (d, J ¼ 9.2 Hz, 2H), 7.89 (d, J ¼ 8.4 Hz, 2H), 7.69
(d, J ¼ 8.4 Hz, 2H), 7.46 (d, J ¼ 8.8 Hz, 2H), 7.34 (d, J ¼ 4.0 Hz, 1H),
7.10 (d, J ¼ 8.4 Hz, 4H), 6.98 (d, J ¼ 8.4 Hz, 2H), 6.94 (d, J ¼ 3.6 Hz,
1H), 6.86 (d, J ¼ 8.8 Hz, 4H), 3.81 (m,10H),1.61 (m, 4H),1.29 (m, 4H),
DMSO) d: 165.8, 160.9, 160.6, 156.1, 154.9, 154.2, 152.8, 149.8, 146.8,
146.8, 146.6, 144.5, 141.7, 139.5, 136.9, 129.2, 127.5, 127.2, 125.8,
123.0, 122.6, 122.1, 118.7, 116.5, 115.1, 114.4, 111.4, 109.7, 109.2, 55.2,
30.6, 19.3, 19.2, 13.9, 13.4, 11.3. [M-H]- calcd. for (C56H49N4O6):
873.3652; found, 873.3662.
0.87 (m, J ¼ 7.2 Hz, 7.2 Hz, 6H).13C NMR (CDCl3, 125 MHz)
d: (ppm)
177.4, 171.1, 162.9, 162.6, 158.2, 156.2, 152.5, 149.0, 146.5, 143.7,
140.5, 131.1, 130.9, 129.3, 127.6, 127.0, 126.5, 125.8, 125.5,120.1,114.8,
110.6, 109.6, 109.1, 68.0, 60.4, 55.5, 42.0, 41.9, 31.6, 31.6, 25.6, 21.1,
20.0, 14.2, 13.6.
3. Results and discussion
2.5.2. Synthesis of 5-[4-[3-[4-(4-(N, N-bis (4-methoxyphenyl)
amino)phenyl) phenyl]-2,5-di-n-butyl-pyrrolo [3,4-c]pyrrole-1,4-
dione] phenyl]benzene-2-carbaldehyde(3)
3.1. Synthesis
The synthetic routes of the dyes are shown in Scheme 2. DPP-III
and3-[4-[4-(N,N-bis(4-methoxyphenyl)amino)phenyl]phenyl]-6-
(4-bromophenyl)-2,5-di-n-butyl-pyrrolo [3,4-c]pyrrole-1,4-dione
(1) were obtained as reported earlier [20]. DPP-I and DPP-II were
synthesized by Suzuki coupling reaction of compound 1 with 5-
formylfuran-2-ylboronic acid, 4-formylphenylboronic acid, fol-
lowed by Knoevenagel condensation reaction with cyanoacetic
acid. The structure of all of the key intermediates and two novel
organic DPP sensitizers have been confirmed by 1H NMR, 13C NMR
and HRMS.
Compound 1 (0.20 g, 0.25 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol),
and Na2CO3(1.02 g, 0.01 mol) in THF (10 mL) and H2O (5 mL) were
heated to 45 ꢁC under a nitrogen atmosphere for 30 min. A solution
of 4-formylphenylboronic acid (0.075 g, 0.50 mmol) in THF (5 mL)
was added slowly, and the mixture was heated under reflux for
further 12 h. After cooling to room temperature, the mixture was
extracted with CH2Cl2 (30 mL). The organic portion was combined
and removed by rotary evaporation. The residue was purified by
column chromatography on silica (CH2Cl2/ethyl acetate ¼ 1/70, v/v)
to give a red solid. (Yield: 53.4%). mp 101e103 ꢁC. IR (KBr): 2956,
2932, 1655, 1598, 1507, 1246, 1096, 1028, 827. 1H NMR (500 MHz,
3.2. Material
CDCl3)
d
: (ppm) 10.09 (s,1H), 7.98 (m, J ¼ 8.4 Hz, 8.4 Hz, 4H), 7.91 (d,
J ¼ 8.4 Hz, 2H), 7.81 (t, J ¼ 5.6 Hz, 6.0 Hz, 4H), 7.72 (d, J ¼ 8.4 Hz, 2H),
7.48 (d, J ¼ 8.8 Hz, 2H), 7.11 (d, J ¼ 8.8 Hz, 4H), 7.01 (d, J ¼ 8.8 Hz,
2H), 6.87 (d, J ¼ 8.8 Hz, 4H), 3.83 (m,10H),1.65 (m, 4H),1.33 (m, 4H),
The TiO2 paste was important part during the preparation of
dye-sensitized solar cells. In this study, the TiO2 paste was
purchased from Solaronix (Switzerland) instead of P25 (Degussa)
used in the previous studies. Commercial available P25 produced
by Degussa (Evonik) is widely used in dye-sensitized solar cells
because of its relatively high activity and low cost. However, it is
known that P25 is composed of anatase and rutile crystallites [28],
and rutile is worse than anatase for solar energy conversion [29].
Thus, P25 is not the best choice for high efficiency dye-sensitized
solar cells. The TiO2 paste purchased from Solaronix (Switzerland)
were prepared by sol-gel route and with pure anatase crystallite,
which is better for solar energy conversion. It has been reported
that those cells with highest solar conversion efficiency (near or
higher than 10%) were prepared with TiO2 paste by sol-gel route
[16,30].
0.88 (t, J ¼ 7.2 Hz, 7.6 Hz, 6H). 13C NMR (CDCl3, 125 MHz)
d: (ppm)
191.8, 162.9, 162.7, 148.8, 148.1, 147.4, 143.4, 142.1, 135.7, 133.2, 130.4,
129.4, 129.3, 128.3, 127.8, 127.8, 126.8, 126.3, 124.8, 123.4, 123.3,
110.4, 109.7, 77.4, 77.0, 76.7, 31.7, 31.6, 29.7, 20.1, 13.7.
2.5.3. Synthesis of 2-cyano-3-[5-[4-[3-[4-(4-(N,N-Bis(4-methoxyphe-
nyl)amino) phenyl)phenyl]-2,5-di-n-butyl-pyrrolo [3,4-c]pyrrole-1,4-
dione]phenyl]furan-2-yl] acrylic acid (DPP-I)
Compound 2 (0.10 g, 0.12 mmol), 2-cyanoacetic acid (0.11 g,
1.29 mmol), and piperidine (0.5 mL) in THF (20 mL) were heated to
reflux under a nitrogen atmosphere for 6 h. After cooling to room
temperature, the precipitate was filtered. The residue was purified
by column chromatography on silica (CH2Cl2/ethanol ¼ 10/1, v/v)
to give a dark red solid. (Yield: 46.3%). mp 241e243 ꢁC. IR (KBr):
3440, 2956, 2933, 2220, 1670, 1591, 1512, 1240, 1091, 1033, 825. 1H
3.3. Absorption properties in solutions and on TiO2 film
NMR (500 MHz, DMSO),
d
: 8.09 (d, J ¼ 9.45, 3H), 8.01 (d, J ¼ 8.55,
Absorption spectra of the three dyes in a diluted solution of
dichloromethane are shown in Fig. 1 and their absorption data are
listed in Table 1. In the UVeVis spectra, the dyes exhibit three major
prominent bands, appearing at 300e325 nm, 340e425 nm, and
500e550 nm, respectively. The absorption band of 300e325 nm
2H), 7.91 (d, J ¼ 8.46, 2H), 7.81 (d, J ¼ 8.52, 2H), 7.64 (d, J ¼ 8.74, 2H),
7.56 (m, 2H), 7.10 (d, J ¼ 8.90, 4H), 6.96 (d, J ¼ 8.95, 4H), 6.83 (d,
J ¼ 8.75, 2H), 3.78 (m, 10H), 1.44 (m, 4H), 1.18 (m, 4H), 0.78 (m, 6H).
13C NMR (125 MHz, DMSO)
d: 165.6, 161.6, 161.4, 157.2, 156.0, 148.7,

S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
1387
Scheme 2. The synthetic procedure of the dyes DPP (I w III).
was ascribed to a localized aromatic
triarylamine [31], and the middle band was due to the
transition of diketo-pyrrolo-pyrrole (DPP)-conjugated molecule
with the -spacer of either benzene, furan or thiephene group.
pep
* transition of typical
pep
*
molecules show about three times higher absorption coefficients.
The greater maximum absorption coefficients of the organic dyes
allow a correspondingly thinner nanocrystalline film, which could
benefit DSSCs with ionic liquid electrolyte for better conversion
efficiencies [34].
p
Compared with DPP-I and DPP-III, DPP-II. exhibits a middle peak
at 343 nm (ε ¼ 4.77 ꢂ 104) blue shifted by about 50 nm. This may be
caused by the decrease of co-planarity between diketo-pyrrolo-
pyrrole (DPP) moiety and the electron acceptor due to the intro-
duction of benzene unit [32]. The bands at around 500e550 nm can
be attributed to the intramolecular charge transfer (ICT) between
the triphenylamine donor and the acceptor, in which DPP (I w III)
show strong visible band at 525 nm, 511 nm and 524 nm owing to
the introduction of diketo-pyrrolo-pyrrole (DPP) group, which
could favor light-harvesting in DSSCs. The corresponding long
wavelength maximum extinction coefficients of the three dyes are
3.88 ꢂ 104, 3.28 ꢂ 104 and 4.21 ꢂ 104 Mꢀ1 cmꢀ1, respectively. In
comparison with conventional ruthenium complexes (for example,
1.39 ꢂ 104 Mꢀ1 cmꢀ1 at 541 nm for N3) [33] the present dye
Fig. 2 shows the absorption spectra of DPP (I w III) on TiO2 films
after 12 h adsorption. As shown in Fig. 2, the absorption peaks for
DPP (I w III) on the TiO2 film are at
l
¼ 590, 566 and 589 nm, the
absorption bands are red-shifted by 65, 55 and 65 nm compared
with the solution spectrum, respectively. The red shifts of the
absorption spectra on TiO2 of DPP (I w III) could be ascribed to the
aggregation or electronic coupling of the dyes on the TiO2 surface.
The interaction between the carboxylate group and the surface Ti4þ
ions may lead to increased delocalization of the
p
* orbital of the
conjugated framework. The energy of the
p
* level is decreased by
this delocalization, which explains the red shift for the absorption
spectra.
3.4. Electrochemical properties
1.6
To evaluate the possibility of electron transfer from the excited
DPP-I
1.4
DPP-II
Table 1
DPP-III
1.2
Optical properties and redox potential of DPP (I w III).
b
d
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Dye
lmaxa/nm
lmax
HOMOc/V
(vs.NHE)
E0e0
(eV)
LUMOe/V
(vs. NHE)
(ε ꢂ 10ꢀ4 Mꢀ1 cmꢀ1
)
(nm)
DPP-I
525 (3.88), 409(3.47),
323 (3.21)
511 (3.28), 343 (4.77),
308 (3.56)
524 (4.21), 388 (4.57),
305 (3.91)
590
566
589
0.94
0.95
1.12
1.95
2.01
1.91
ꢀ1.01
ꢀ1.06
ꢀ0.79
DPP-II
DPP-III
Absorption maximum in CH2Cl2 solution (3 ꢂ 10ꢀ5 M).
Absorption maximum on TiO2 film.
HOMO were measured in acetonitrile/THF(3:1, v/v) with 0.1 M tetra-n- buty-
a
b
c
lammoniumhexafluorophosphate (TBAPF6) as electrolyte (working electrode: Pt;
reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an
external reference. Counter electrode: Pt).
300
400
500
600
700
d
E0-0 was estimated from the absorption thresholds from UVeVis absorption
Wavelength / nm
spectra of the dyes.
e
Fig. 1. UVeVis absorption spectra of DPP (I w III) in CH2Cl2 solution (3 ꢂ 10ꢀ5 M).
LUMO is estimated by subtracting E0-0 to HOMO.

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S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
1.91 eV, respectively. The estimated excited-state potential corre-
2.5
2.0
1.5
1.0
0.5
0.0
DPP-I
sponding to the LUMO levels are ꢀ1.01, ꢀ1.06 and ꢀ0.79 V,
respectively. The examined HOMO levels and the LUMO levels are
listed in Table 1.
DPP-II
DPP-III
The HOMO levels of the three dyes are much more positive
than the iodine/iodide redox potential value (0.4 V), ensuring
that there is sufficient driving force for efficient dye regeneration
through the recapture of the injected electrons from Iꢀ by the
dye cation radical. From the LUMO values, all of the three dyes
can complete the process of electron injection from the excited
dye molecule to TiO2 conduction band, with a more negative
level than the energy level of the TiO2 electrode (ꢀ0.5 V vs. NHE).
Furthermore, the energy gaps between the LUMO of the dyes
DPP-I and DPP-II and TiO2 conduction band was larger than
DPP-III, indicating that the electron injection process from the
excited dye molecule to TiO2 conduction band is more efficient
[35].
400
500
600
700
Wavelength / nm
Fig. 2. Absorption spectra of DPP(I w III) adsorbed on TiO2 film.
3.5. Photovoltaic performance of liquid electrolyte DSSCs
Fig. 4 shows the action spectra of incident monochromatic
photon-to-current conversion efficiency (IPCE) for DSSCs based
on DPP (I w III). The dye-coated TiO2 film was used as working
electrode, platinized FTO glass as counter electrodes and 0.1 M
LiI þ 0.05 M I2 þ 0.6 M MPII in acetonitrile and methox-
ypropionitrile(volume ratio, 7:3) mixture solution as redox elec-
trolyte. The solar cells based on the three dyes exhibit high IPCE
values above 60% in the range of 500e600 nm and with the
highest value of 83.2% at 535 nm for DPP-I, 87.9% at 530 nm for
DPP-II and 73.5% at 543 nm for DPP-III, respectively. By
comparison, both of DPP-I and DPP-II show higher maximum
IPCE values and energy conversion efficiencies than the DPP-III
dye with thiophene unit. The IPCE at a specific wavelength is
expressed as
dye to the conduction band of TiO2, cyclic voltammetry was per-
formed in acetonitrile/tetrahydrofuran solution, using 0.1 M tet-
rabutylammonium hexafluorophosphate as the supporting
electrolyte, Pt as working electrode and counter electrode and
saturated calomel electrode (SCE) as reference electrode. The
examined highest occupied molecular orbital (HOMO) levels and
the lowest unoccupied molecular orbital (LUMO) levels are
collected in Table 1. The SCE reference electrode was calibrated
using a ferrocene/ferrocenium (Fc/Fcþ) redox couple as an external
standard and the E1/2 of the Fc/Fcþ redox couple was found to be
0.37 V versus the SCE reference electrode [12b]. The potentials
versus NHE were calibrated by addition of 0.63 V to the potentials
versus Fc/Fcþ [12a]. Therefore, the potentials measured vs SCE
were converted to normal hydrogen electrode (NHE) by addition
of þ0.26 V. It can be obtained from the oxidative cyclic voltam-
metry plot (shown in Fig. 3) that the first half-wave potentials of
DPP (I w III) are 0.68, 0.69 and 0.86 V (vs. SCE). Therefore, the
ground state oxidation potential corresponding to the HOMO
levels are 0.94, 0.95 and 1.12 V (vs. NHE) by addition of 0.26 V,
respectively. From Fig. 1, we know that the absorption thresholds
for dyes DPP (I w III) are 636, 616 and 649 nm in CH2Cl2 solution,
which correspond to the band gap energy (E0e0) of 1.95, 2.01 and
IPCEð
l
Þ ¼ LHEð
l
Þ
finj
$
hc ¼ LHEð
l f
Þ$ ð
l
ÞET
Where LHE(l) stands for light-harvesting efficiency of the photo-
electrode, 4inj is the quantum yield of charge injection from the
dye excited-state into TiO2, hc is the efficiency of the injected
charge collection at the back contact, and
4
(l)
ET
is defined as an
electron transfer yield, which is a product of the electron injection
yield and the charge collection efficiency. Considering similar
unity LHE(l) values for all the dyes, higher IPCE values of the
100
90
80
70
60
50
40
30
20
10
0
DPP-I
DPP-II
DPP-III
-10
400
500
Wavelength / nm
600
700
Fig. 3. Cyclic voltammetry plots of DPP(IwIII) in acetonitrile/THF(3:1, v/v).
Fig. 4. Photocurrent action spectra of the TiO2 electrodes sensitized by DPP (IwIII).

S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
1389
Table 2
16
Photovoltaic performance of DSSCs based on DPP (I w III) and N719 dye for 16
thick TiO2 film with liquid electrolyte.
m
m
DPP-I
DPP-II
DPP-III
14
12
10
8
Dye
Jsc/mA cmꢀ2
Voc/mV
ff
h
(%)
Amounta
(mol cmꢀ2
)
DPP-I
DPP-II
DPP-III
N719
12.43
14.53
10.32
16.10
597
625
567
689
0.60
0.66
0.66
0.67
4.43
6.03
3.89
7.47
9.76 ꢂ 10ꢀ8
1.41 ꢂ 10ꢀ7
1.12 ꢂ 10ꢀ7
a
Amount of the dyes adsorbed on TiO2 film.
6
4
DSSCs sensitized by the dyes with benzene and furan as
means that the DSSCs sensitized by the dyes with benzene and
furan linkers have higher electron transfer yield than the
p-spacer
2
4(l)
ET
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
dye with the thiophene linker. Therefore, the introduction of
benzene and furan rings into DPP-based dye structure has a posi-
tive effect on IPCE. With judicious molecular structure design to
introduce benzene and furan linkers to tune the HOMO-LUMO
levels appropriately, higher IPCE thus higher Jsc leading to higher
energy conversion efficiency could be obtained with these DPP-
based dye sensitized solar cells [36].
Voltage / V
Fig. 5. Currentevoltage characteristics of DSSCs based on DPP(IwIII)under irradiation
of AM 1.5G simulated solar light (100 mW cm-2) with liquid electrolyte.
The currentevoltage characteristics of the DSSCs based on 16 mm
thickness of TiO2 film was measured at 100 mW cmꢀ2 under
simulated AM 1.5G solar light conditions. The DPP-I-sensitized cell
gave a short circuit photocurrent density (Jsc) of 12.43 mA cmꢀ2, an
open circuit voltage (Voc) of 0.597 V, and a fill factor of 0.60, cor-
responding to an overall conversion efficiency of 4.43% (see
Table 2). Under the same conditions, the DPP-II- and DPP-III-
sensitized cell gave a Jsc of 14.53 and 10.32 mA cmꢀ2, Voc of 0.625
and 0.567 V, and a fill factor (ff) of 0.66 and 0.66, corresponding to
an overall conversion efficiency of 6.03% and 3.89%, respectively.
DPP-II has the highest photocurrent due to more loading, faster and
more effective electron injection efficiency. From Fig. 5 and Table 2,
we can also see that DPP-II has a little higher open circuit voltage
than the other two dyes, this could be ascribed to less aggregation
occurring for DPP-II by the decrease of co-planarity between DPP
moiety and the benzene spacer. Compared to DPP-III, DPP-I has
higher photocurrent, indicating that furan moiety may have even
better charge carrier mobility than thiophene moiety.
achieved their highest efficiency with thinner TiO2 film, this is
ascribed to higher molar extinction coefficient of the two dyes
than DPP-II.
3.6. Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) analysis was
performed to elucidate the photovoltaic findings further. Fig. 7
showed the electrochemical impedance spectra for the DSSCs
based on DPP (I w III) under a forward bias of ꢀ0.60 V in the dark.
Three semicircles were observed in the Nyquist plots (Fig. 7(a)). The
large semicircle in the Nyquist plots located in the middle is
attributed to the dark reaction impedance caused by charge
transportation at the TiO2/dye/electrolyte interface, and the other
two small semicircles located in the low- and high- frequency
regions are assigned to the charge-transfer at counter electrode and
diffusion of I3- in the electrolyte, respectively [38]. The radius of the
larger semicircle increases in the order DPP-III < DPP-I < DPP-II,
indicating that the electron recombination resistance augments
from DPP-III, DPP-I to DPP-II [39]. The electron lifetime values
derived from curve fitting are, 23.9, 53.7 and 14.8 ms for DPP-I,
DPP-II and DPP-III-sensitized solar cells, respectively. The longest
electron lifetime observed with DPP-II-sensitized cell indicated
there exists efficient suppression of the back reaction of the
injected electron with the I3ꢀ in the electrolyte, which is reflected
in the improvement of the Voc, yielding substantially enhanced
device efficiency. At the same time, DPP-I exists longer lifetime
than DPP-III-sensitized cell, indicating that the sensitizer con-
taining the furan moiety may be more efficient for suppression of
the back reaction of the injected electron with the I3ꢀ in the elec-
trolyte. The Bode phase plots shown in Fig. 7(b) likewise support
the differences in the electron lifetime for TiO2 films derived with
the three dyes.
For further optimization of the device performance, the effect
of the thickness of the TiO2 film layer on the performance was
carried out, for which Fig. 6 and Table 3 show currentevoltage
characteristics obtained with DSSCs based on DPP (I w III) with
the thickness ranging from about 8
a remarkably high photocurrent density (Jsc) of 15.96 mA cmꢀ2
open circuit voltage (Voc) of 0.541 V and a fill factor (ff) of 0.65,
corresponding to an overall power conversion efficiency ( ) of
mm to 23 mm. DPP-I yielded
,
h
5.65% based on 13 mm thickness of TiO2 film and DPP-III yielded
a high photocurrent density (Jsc) of 10.79 mA cmꢀ2, open circuit
voltage (Voc) of 0.606 V and a fill factor (ff) of 0.68, corresponding
to an overall power conversion efficiency (
h) of 4.44% based on
8
m
m thickness of TiO2 film. As can be inferred from Table 3, the
photocurrent of DPP-II-sensitized solar cell increased with the
increasing thickness of TiO2 nanocrystalline layer at first, when
the thickness reached to 16 mm, the solar cell yielded a remark-
ably high photocurrent density (Jsc) of 14.53 mA cmꢀ2 under
standard global AM 1.5G solar light conditions due to high
adsorption. The cell showed an open circuit voltage (Voc) of
0.625 V and a fill factor (ff) of 0.66, corresponding to an overall
3.7. Photovoltaic performance of solvent-free ionic liquid electrolyte
DSSCs
power conversion efficiency (
the TiO2 film increased from 16
Voc decreased, this is because the increased surface area also
enhanced the possibility for injected electrons to recombine with
the I3ꢀelectrolyte [37]. Compared to DPP-II, DPP-I and DPP-III
h
) of 6.03%. When the thickness of
m to 21 m, both of the Jsc and
To lower the cost of photovoltaic power production, a substan-
tial improvement in the DSSC efficiency by structure modification is
still necessary. Nevertheless, long-term stability is also a vital
parameter for sustained cell operation. However, the use of organic
solvents is undesirable as they are likely to be volatile, so we
m
m

1390
S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
Table 3
Currentevoltage characteristics obtained with DPP (I w III)-sensitized solar cells for
various thicknesses of the nanocrystalline TiO2 films.
Dye
Thickness/um
Jsc/mA cmꢀ2
Voc/V
FF
h/%
DPP-I
8
11
13
16
23
8
12
14
16
21
8
11.79
12.48
15.96
12.43
12.92
9.47
10.53
13.92
14.53
13.52
10.79
10.32
10.38
10.13
0.624
0.580
0.541
0.597
0.577
0.705
0.657
0.628
0.625
0.619
0.606
0.567
0.553
0.551
0.60
0.67
0.65
0.60
0.62
0.63
0.66
0.65
0.66
0.62
0.68
0.66
0.63
0.65
4.43
4.87
5.65
4.43
4.65
4.19
4.56
5.68
6.03
5.21
4.44
3.89
3.59
3.64
DPP-II
DPP-III
16
18
22
DPP-III reaches of 40.5% at 540 nm, respectively. In the spectral
range 500e600 nm, the IPCE data of DPP (I w III) exceed 50%, 60%
and 40%, respectively. On the other hand, the IPCE value of DPP-III
is higher than that for DPP-II in the range of 600e700 nm. As
a result, the IPCE performance of the ionic liquid electrolyte DSSCs
based on DPP-II and DPP-III is similar. Compared to DPP-II, the
IPCE value of DPP-I is higher while there exists the opposite result
in liquid electrolyte. This could be explained by the different
thickness of TiO2 films used in the liquid electrolyte and solvent-
a
b
Fig. 6. CurrenteVoltage characteristics obtained with DSSCs based on DPP (IwIII)with
various thicknesses of the nanocrystalline TiO2 films.
substituted the liquid electrolyte with a solvent-free ionic liquid
electrolyte [40]. DPP (I w III) were evaluated as sensitizers for the
solvent-free ionic liquid electrolyte dye-sensitized solar cell using
1-butyl-3-methyl imidazolium, iodine, benzimidazole and guani-
dine thiocyanate (BMII:I2:BI:GNCS ¼ 40:1.67:0.67:3.33) as redox
electrolyte [41]. Fig. 8(a) shows the incident monochromatic
photon-to-current conversion efficiency (IPCE) of DPP (I w III)-
sensitized solar cells with ionic liquid electrolyte based on 8 mm
thickness of TiO2 film. From Fig. 8 (a) we can see that the three dyes
can efficiently convert visible-light to photocurrent in the region
from 350 to 700 nm. DPP-I reaches its maximum of 64.2% at
540 nm, DPP-II reaches its maximum IPCE of 53.3% at 530 nm and
Fig. 7. Impedance spectra of DSSCs based on DPP (IwIII) measured at -0.60 V bias in
the dark. (a) Nyquist plots; (b) Bode phase plots.

S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
1391
photocurrent density (Jsc) of 12.68 mA cmꢀ2, an open circuit
voltage (Voc) of 0.568 V and a fill factor (ff) of 0.61, corresponding
a
to an overall conversion efficiency (h) of 4.41% and DPP-II-sensi-
tized cell gave
a short circuit photocurrent density (Jsc) of
7.78 mA cmꢀ2, an open circuit voltage (Voc) of 0.650 V and a fill
factor (ff) of 0.58, corresponding to an overall conversion efficiency
(
h
) of 2.93%. Under the same conditions, the DPP-III-sensitized cell
gave Jsc values of 8.14 mA cmꢀ2, Voc of 0.549 V and ff of 0.68,
corresponding to the of 3.03%. Obviously, DPP-I and DPP-III
show a higher efficiency than DPP-II (Table 4). The larger of
h
h
b
Fig. 8. (a) IPCE and (b) Photocurrentevoltage curves of TiO2 electrodes obtained with
solvent-free ionic liquid electrolyte solar cells sensitizedby dyes with DPP (IwIII).
free ionic liquid electrolyte DSSCs. Since the diffusion of Iꢀ and I3ꢀ
ions in ionic liquid electrolyte is slow because of their high
viscosity, a thin nanocrystalline TiO2 film is necessary to reach high
conversion efficiencies in solvent-free ionic liquid electrolyte
DSSCs. For this reason, metal-free organic dyes with high absorp-
tion are ideal for solvent-free ionic liquid DSSCs [34]. As a result,
DPP-I with a furan
DPP-II.
p-spacer achieved better IPCE performance than
Photovoltaic performances of DPP (I w III)-sensitized TiO2 film
electrodes with a solvent-free ionic liquid electrolyte under stan-
dard global AM 1.5 solar condition (100 mW cmꢀ2) are listed in
Table 4, and the corresponding photocurrent-voltage curves are
shown in Fig. 8 (b). DPP-I-sensitized cell gave a short circuit
Table 4
Performance parameters of solvent-free ionic liquid electrolyte solar cells sensitized
by DPP (I w III).a
Dye
Jsc (mA cmꢀ2
)
Voc (V)
ff
h(%)
DPP-I
DPP-II
DPP-III
N719
12.68
7.78
8.14
0.568
0.650
0.549
0.636
0.61
0.58
0.68
0.67
4.41
2.93
3.03
5.65
13.30
a
Illumination: 100 mW cmꢀ2 simulated AM 1.5 G solar light; electrolyte con-
taining: 1-butyl-3-methyl imidazolium, 1-methyl-3-trimethylsilyl imidazolium,
Fig. 9. Stability test photovoltaic parameter (Jsc, Voc, ff, and h) variations with aging
iodine,
benzimidazole
and
guanidine
thiocyanate(BMII:
time for the device based on DPP (I w III) and solvent-free ionic liquid electrolyte
I2:BI:GNCS ¼ 40:1.67:0.67:3.33).
during successive 1 sun visible-light.

1392
S. Qu et al. / Dyes and Pigments 92 (2012) 1384e1393
DPP-I and DPP-III than DPP-II in ionic liquid electrolyte cells can
be tentatively attributed to the higher molar extinction and light-
harvesting efficiency of the photoelectrode and the prevention of
dye aggregates on the semiconductor. In addition, the efficiency in
the case of DPP-III is lower than that of DPP-I, which is ascribed to
the reduction of the photocurrent due to the less energy gaps
between the LUMO of the dye DPP-III and the TiO2 conduction
band. Long-term stability measurements of devices with solvent-
free ionic liquid electrolyte were performed over 2000 h at full
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Scientific Committee of Shanghai (10520709700).

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