Panchromatic donor-acceptor-acceptor sensitizers based on 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one as a strong acceptor for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.03.002

Three new donor-acceptor-acceptor near-infrared organic dyes based on 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one as an additional acceptor were developed and used in dye-sensitized solar cells. By virtue of the simple donor-acceptor-acceptor configuration and systematic tuning of the electron-donating ability of the donor moiety, the absorption spectrum of HIQ3 on TiO₂ extended to 900 nm. Remarkably, DSCs sensitized with HIQ3 showed broad incident monochromatic photon-to-current conversion efficiency spectra across the entire visible range and extending into the near-infrared region as far as 1100 nm.

Full text of DOI:10.1016/j.dyepig.2012.03.002

Dyes and Pigments 94 (2012) 553e560
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Panchromatic donoreacceptoreacceptor sensitizers based on 4H-cyclopenta
[2,1-b:3,4-b0]dithiophen-4-one as a strong acceptor for dye-sensitized solar cells
*
Chuanjiang Qin, Ashraful Islam, Liyuan Han
Photovoltaic Materials Unit and NIMS Saint-Gobain Center of Excellence for Advanced Materials, National Institute for Materials Science, Sengen 1-2-1, Tsukuba,
Ibaraki 305-0047, Japan
a r t i c l e i n f o
a b s t r a c t
Three new donoreacceptoreacceptor near-infrared organic dyes based on 4H-cyclopenta[2,1-b:3,4-b0]
dithiophen-4-one as an additional acceptor were developed and used in dye-sensitized solar cells. By
virtue of the simple donoreacceptoreacceptor configuration and systematic tuning of the electron-
donating ability of the donor moiety, the absorption spectrum of HIQ3 on TiO2 extended to 900 nm.
Remarkably, DSCs sensitized with HIQ3 showed broad incident monochromatic photon-to-current
conversion efficiency spectra across the entire visible range and extending into the near-infrared
region as far as 1100 nm.
Article history:
Received 25 January 2012
Received in revised form
3 March 2012
Accepted 5 March 2012
Available online 20 March 2012
Keywords:
Ketones
Ó 2012 Elsevier Ltd. All rights reserved.
Donoreacceptor system
Organic dye
Near-infrared absorption
Dye-sensitized solar cells
1. Introduction
Recently, additional acceptor chromophores have been intro-
duced between the donor and the
p-bridge, leading to a D-A-p-A
In the past decade, dye-sensitized solar cells (DSCs) have
received increasing attention owing to their potential use in low-
cost production of renewable energy and in large-area, flexible,
colorful, lightweight devices [1]. Overall solar-to-electric energy
conversion efficiencies exceeding 11% have been achieved under
standard AM 1.5 conditions with DSCs based on ruthenium sensi-
tizers because of their broad and intense metal-to-ligand charge
transfer absorption across the entire visible range and extending to
the near-infrared (NIR) [2]. Because the supply of ruthenium is
limited, metal-free organic dyes have attracted considerable
architecture that facilitates intramolecular charge transfer and
adjusts the bandgap energy for harvesting more NIR light [6].
However, the resulting absorption spectra and incident mono-
chromatic photon-to-current conversion efficiency (IPCE) spectra
are inefficient at reaching to the NIR region because the additional
acceptors are only weakly electron withdrawing. A feasible strategy
for extension of the absorption and IPCE spectra into the NIR region
would be to use a more strongly electron-withdrawing acceptor
between the donor moiety and cyanoacetic acid moiety to form
a DeAeA architecture. The 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-
4-one unit is a good candidate acceptor; owing to the presence of
the electron-withdrawing ketone group, the HOMOeLUMO energy
of this acceptor (lmax ¼ 474 nm) is narrower than bithiophene
attention in the past several years [3]. Among these, donor-
p-
bridge-acceptor (D- -A) compounds are particularly attractive
p
because of their large molar extinction coefficients, readily tuned
HOMOeLUMO levels, and low-cost, easy synthesis [4]. However,
the overall conversion efficiencies of DSCs based on metal-free
organic dyes are lower than those of DSCs based on ruthenium
dyes, owing to the narrow coverage of the solar spectrum. Thus,
there is growing interest in developing NIR sensitizers for appli-
cations in DSCs [5].
(lmax
¼
302 nm) and cyclopenta[2,1-b:3,4-b0]dithiophene
(lmax ¼ 312 nm) [7]. In this study, we first introduced this strongly
electron-withdrawing acceptor into the DeAeA configuration and
synthesized three novel sensitizers, HIQ1, HIQ2 and HIQ3 (Fig. 1),
for harvesting visible and NIR light. HIQ1 has the simplest struc-
ture, with triphenylamine as the donor and 2-cyano-2-propenoic
acid as the acceptor/anchor. Two different spacers, thiophene in
HIQ2 and ethylenedioxythiophene in HIQ3, were also introduced
into the molecular backbone between the donor and the additional
acceptor to further increase the electron-donating strength of the
* Corresponding author. Tel.: þ81 29 859 2747; fax: þ81 29 859 2301.
E-mail address: Han.Liyuan@nims.go.jp (L. Han).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.03.002

554
C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
Fig. 1. Chemical structures of HIQ1, HIQ2 and HIQ3.
donor moiety. The performance of DSCs based on these NIR organic
dyes is reported.
TiO2 surface. Photovoltaic measurements were performed in a two-
electrode sandwich-type sealed-cell configuration. The dye-coated
TiO2 film was used as the working electrode, and platinum-coated
conducting glass was used as the counter-electrode. The two
2. Experimental section
electrodes were separated by a Surlyn spacer (50 mm thick) and
sealed up by heating the polymer frame. The electrolyte was
composed of 0.6 M dimethylpropylimidazolium iodide, 0.05 M I2
and 1.0 M LiI in acetonitrile. The currentevoltage characteristics
were measured using a black metal mask with an aperture area of
0.25 cm2 under standard AM 1.5 sunlight (100 mW cmꢀ2, WXS-
155S-10: Wacom Denso Co., Japan) [10]. Monochromatic incident
photon-to-current conversion efficiency spectra were measured
with monochromatic incident light of 1 ꢁ1016 photons cmꢀ2 under
100 mW cmꢀ2 in director current mode (CEP-2000BX, Bunko-
Keiki).
2.1. Materials
All chemicals and reagents were used as received from suppliers
without further purification. THF and toluene were dried over
molecular sieves and distilled under argon from sodium benzo-
phenone ketyl immediately prior to use. 4H-Cyclopenta[2,1-b:3,4-
b0]dithiophen-4-one was synthesized according to the reported
method [8]. Column chromatography was performed using
Wakogel-C300 as a stationary phase.
2.2. Analytical instruments
2.4. Synthesis
UVeviseNIR
dimethylformamide (DMF) solution or on a TiO2 film (thick-
m) with a UVeviseNIR spectrophotometer (UV-3600,
spectra
were
measured
in
N,N-
2.4.1. 4H-Cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-2-
carbaldehyde
ness ¼ 4
m
Shimadzu). 1H (600 MHz) and 13C NMR (150 MHz) spectra were
measured with a DRX-600 spectrometer (Bruker BioSpin). Mass
spectra were measured on a Shimadzu Biotech matrix-assisted
laser desorption ionization (MALDI) mass spectrometer. Cyclic
voltammetry was performed on a CH Instruments 624D potentio-
stat/galvanostat system with a three-electrode cell consisting of
a Ag/AgCl reference electrode, a working electrode, and a platinum
wire counter-electrode. The redox potentials of the dyes absorbed
on TiO2 were measured in CH3CN containing 0.1 M tetra-n-
butylammonium hexafluorophosphate. Electrochemical measure-
To a cold solution of 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-
one (4.51 g, 23.4 mmol) and DMF (1.86 g, 25.74 mmol) in 1,2-
dichloroethane (60 mL) at 0 ꢂC was added phosphorus oxy-
chloride (3.95 g, 25.74 mmol). The reaction solution was heated to
80 ꢂC and stirred for 6 h, and then saturated aqueous sodium
acetate (100 mL) was added. The mixture was stirred at room
temperature for an additional 30 min. The crude product was
extracted into dichloromethane, and the organic layer was washed
with brine and water and dried over anhydrous magnesium sulfate.
After removal of the solvent under reduced pressure, the residue
was purified by column chromatography on silica gel (ethyl acetate/
hexane,1/10, v/v) to yield a deep red solid (4.7 g, 91% yield). 1H NMR
ments were performed at a scan rate of 50 mV sꢀ1
.
2.3. Cell fabrication and characterization
(600 MHz, CDCl3)
d
: 9.79 (s, 1H), 7.62 (s, 1H), 7.34 (d, J ¼ 4.8 Hz, 1H),
7.15 (d, J ¼ 4.8 Hz, 1H) ppm. 13C NMR (150 MHz, CDCl3)
d: 182.19,
The DSCs were fabricated as follows. A double-layer TiO2 pho-
181.17, 157.51, 147.62, 145.74, 145, 142.1, 131.34, 123.6, 122.19 ppm.
MALDI-MS calcd: 219.97, found: 219.85.
toelectrode (thickness 15
electrode. A 10 m main transparent layer with titania particles
(w20 nm) and a 5 m scattering layer with titania particles
m
m; area 0.25 cm2) was used as a working
m
m
2.4.2. 6-Bromo-4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-2-
carbaldehyde
(w400 nm) were screen-printed on the fluorine-doped tin oxide
conducting glass substrate [9]. A solution of HIQ1, HIQ2 or HIQ3
(3 ꢁ 10ꢀ4 M) in acetonitrile/tert-butyl alcohol (1/1, v/v) was used to
coat the TiO2 film with the dye. Deoxycholic acid (20 mM) was
added to the dye solution as a co-adsorbent to prevent aggregation
of the dye molecules. The electrodes were immersed in the dye
solutions and then kept at 25 ꢂC for 24 h to adsorb the dye onto the
To a solution of 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-
2-carbaldehyde (4.6 g, 20.9 mmol) in DMF (50 mL) under argon was
added N-bromosuccinimide (NBS; 4.1 g, 23 mmol) in the dark. The
resulting solution was stirred for 12 h at room temperature under
argon and then extracted with dichloromethane (200 mL ꢁ 3) and
water (50 mL). The combined organic layers were dried over

C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
555
anhydrous magnesium sulfate. After removal of the solvent under
(150 MHz, CDCl3)
d: 181.94, 181.23, 158.19, 148.42, 145.59, 145.57,
reduced pressure, the residue was purified by column chromatog-
145.46, 144.87, 144.74, 144.08, 141.23, 133.52, 133.24, 130.03, 129.4,
126.46, 126.07, 125.82, 125.07, 122.7, 121.87, 117.84, 20.86 ppm.
raphy on silica gel (ethyl acetate/hexane,1/10, v/v) to give a pale red
solid (6.0 g, 96% yield). 1H NMR (600 MHz, DMSO-d6)
d
: 9.79 (s, 1H),
7.91 (s, 1H), 7.38 (s, 1H) ppm. 13C NMR (150 MHz, DMSO-d6)
d:
2.4.6. 2-Cyano-3-{6-{2-{4-[N,N-bis(4-methylphenyl)amino]
phenyl}thiophene-5-yl}-4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-
one-2-yl}acrylic acid (HIQ2)
182.04, 180.05, 157.05, 147.27, 146.14, 143.56, 140.61, 129.44, 124.76,
118.15 ppm. MALDI-MS calcd: 297.88, found: 297.98.
HIQ2 was synthesized according to the procedure described for
HIQ1 and was obtained as a black powder (0.15 g, 60% yield). FT-IR
(KBr, vmax/cmꢀ1): 3420, 3025, 2917, 2852, 2210, 1709, 1676, 1575,
2.4.3. 6-{4-[N,N-Bis(4-methylphenyl)amino]phenyl}-4H-
cyclopenta[2,1-b:3,4-b0] dithiophen-4-one-2-carbaldehyde
A mixture of 6-bromo-4H-cyclopenta[2,1-b:3,4-b0]dithiophen-
4-one-2-carbaldehyde (0.1 g, 0.35 mmol), 4-tributylstannyl-N,N-
bis(4-methylphenyl)aniline (0.22 g, 0.4 mmol), Pd(PPh3)4 (10 mg)
and anhydrous toluene (20 mL) was refluxed for 24 h under argon.
The crude product was extracted into dichloromethane, and the
organic layer was washed with water and dried over anhydrous
magnesium sulfate. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on
silica gel (ethyl acetate/hexane, 1/8, v/v) to yield a black-red solid
1430, 1262, 931, 813, 790, 761. 1H NMR (600 MHz, DMSO-d6)
d: 8.06
(s, 1H), 7.58 (s, 1H), 7.49 (d, J ¼ 8.4 Hz, 2H), 7.39 (d, J ¼ 4.2 Hz, 1H),
7.33 (d, J ¼ 4.2 Hz, 1H), 7.29 (s, 1H), 7.13 (d, J ¼ 8.4 Hz, 4H), 6.94 (d,
J ¼ 8.4 Hz, 4H), 6.87 (d, J ¼ 8.4 Hz, 2H), 2.27 (s, 6H) ppm. 13C NMR
(150 MHz, DMSO-d6) d: 181.89, 163.91, 154.33, 148.01, 146.56, 144.7,
143.85, 143.76, 143.17, 141.3, 140.99, 140.91, 133.96, 133.32, 130.62,
127.73, 126.8, 126.6, 125.23, 124.02, 121.77, 119.79, 117.39, 79.66,
20.89 ppm. HRMS (FABþ, m/z): calcd for C37H24N2O3S3, 640.0949;
found, 640.0925.
(0.12 g, 73% yield). 1H NMR (600 MHz, CDCl3)
d: 9.76 (s, 1H), 7.58 (s,
1H), 7.36 (dd, J ¼ 6.6, 1.8 Hz, 4H), 7.16 (s, 1H), 7.11 (d, J ¼ 8.4 Hz, 4H),
2.4.7. 6-{5-{4-[N,N-Bis(4-methylphenyl)amino]phenyl}-3,4-
ethylenedioxythiophene-2-yl}-4H-cyclopenta[2,1-b:3,4-b0]
7.03 (m, 4H), 6.99 (m, 2H), 2.33 (s, 6H) ppm. 13C NMR (150 MHz,
CDCl3)
d
: 181.94, 181.76, 158.76, 152.41, 149.15, 145.9, 145.07, 144.45,
dithiophen-4-one-2-carbaldehyde
143.81, 141.03, 133.68, 130.13, 129.44, 126.29, 125.35, 125.3, 121.29,
115.98, 20.88 ppm.
A mixture of 6-bromo-4H-cyclopenta[2,1-b:3,4-b0]dithiophen-
4-one-2-carbaldehyde (0.1 g, 0.35 mmol), 4-[5-tributylstannyl-3,4-
ethylenedioxythiophene-2-yl]-N,N-bis(4-methylphenyl)aniline
(0.28 g, 0.4 mmol), Pd(PPh3)4 (10 mg) and anhydrous toluene
(20 mL) was refluxed for 24 h under argon. The crude product was
extracted into dichloromethane, and the organic layer was washed
with water and dried over anhydrous magnesium sulfate. After
removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel (ethyl acetate/
hexane, 1/5, v/v) to yield a black-red solid (0.15 g, 68% yield). 1H
2.4.4. 2-Cyano-3-{6-{4-[N,N-bis(4-methylphenyl)amino]phenyl}-
4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-2-yl}acrylic acid
(HIQ1)
To a stirred solution of 6-{4-[N,N-bis(4-methylphenyl)amino]
phenyl}-4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-2-
carbaldehyde (27 mg, 0.55 mmol) and cyanoacetic acid (94 mg,
1.1 mmol) in chloroform (5 mL) was added piperidine (188 mg,
2.2 mmol). The reaction mixture was refluxed under argon for 12 h
and then acidified with 2 M aqueous hydrochloric acid (10 mL). The
crude product was extracted into chloroform, and the organic layer
was washed with water and dried over anhydrous magnesium
sulfate. After removal of the solvent under reduced pressure, the
residue was purified by flash chromatography with chloroform and
methanol/chloroform (1/10, v/v) in turn as eluents to yield a black
powder (23 mg, 76% yield). FT-IR (KBr, vmax/cmꢀ1): 3416, 3026,
2919, 2853, 2219, 1709, 1578, 1430, 1359, 1320, 1259, 1212, 1093,
NMR (600 MHz, CDCl3)
(d, J ¼ 7.8 Hz, 4H), 7.02 (m, 6H), 4.41 (s, 2H), 4.36 (s, 2H), 2.32 (s, 6H)
ppm. 13C NMR (150 MHz, CDCl3)
: 181.84, 181.56, 159.14, 147.5,
d: 9.74 (s, 1H), 7.53 (m, 3H), 7.16 (s, 1H), 7.08
d
145.28, 144.84, 144.79, 143.23, 142.7, 141.08, 139.61, 137.22, 133.02,
129.96, 129.46, 126.93, 124.94, 124.88, 121.94, 117.69, 115.89, 107.95,
65.15, 64.62, 20.85 ppm.
2.4.8. 2-Cyano-3-{6-{2-{4-[N,N-bis(4-methylphenyl)amino]
phenyl}-3,4-ethylenedioxy thiophen-5-yl}-4H-cyclopenta[2,1-b:3,4-
b0]dithiophen-4-one-2-yl}acrylic acid (HIQ3)
816, 767. 1H NMR (600 MHz, DMSO-d6)
d: 8.15 (s, 1H), 7.65 (s, 1H),
7.51 (s, 2H), 7.37 (s, 2H), 7.14(d, J ¼ 7.8 Hz, 4H), 6.96 (d, J ¼ 7.8 Hz,
HIQ3 was synthesized according to the procedure described for
HIQ1 and was obtained as a black powder (52 mg, 55% yield). FT-IR
(KBr, vmax/cmꢀ1): 3397, 3024, 2920, 2857, 2211, 1713, 1578, 1493,
4H), 6.85(d, J ¼ 7.8 Hz, 2H), 2.27 (s, 6H) ppm. 13C NMR (150 MHz,
DMSO-d6) d: 182.12, 163.99, 155.87, 151.12, 148.49, 145.86, 144.52,
143.02, 140.77, 140.02, 133.6, 130.67, 128.94, 126.66, 125.92, 125.47,
121.25, 119.14, 116.56, 79.65, 20.91 ppm. HRMS (FABþ, m/z): calcd
for C33H22N2O3S2, 558.1072; found, 558.1061.
1437, 1360, 1280, 1082, 813, 762. 1H NMR (600 MHz, DMSO-d6)
d:
8.08 (s, 1H), 7.61 (s, 1H), 7.50 (s, 2H), 7.12 (m, 5H), 6.93 (m, 6H), 4.45
(s, 2H), 4.37 (s, 2H), 2.27 (s, 6H) ppm. 13C NMR (150 MHz, DMSO-d6)
d: 181.84, 163.6, 155.8, 147.08, 145.69, 144.79, 143.55, 142.46, 141.33,
2.4.5. 6-{5-{4-[N,N-Bis(4-methylphenyl)amino]phenyl}thiophene-
2-yl}-4H-cyclopenta
140.83, 140.2, 139.82, 138.03, 130.58, 128.52, 127.13, 125.25, 125.02,
121.97, 119.37, 115.7, 115.52, 107.81, 79.65, 65.67, 65.06, 20.88 ppm.
HRMS (FABþ, m/z): calcd for C39H26N2O5S3, 698.1004; found,
698.1011.
2.4.5.1. [2,1-b:3,4-b0]dithiophen-4-one-2-carbaldehyde. A mixture
of
6-bromo-4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-2-
carbaldehyde (0.19 g, 0.65 mmol), 4-(2-(tributylstannyl)thio-
phene-5-yl)-N,N-bis(4-methylphenyl)aniline (0.52 g, 0.8 mmol),
Pd(PPh3)4 (10 mg) and anhydrous toluene (20 mL) was refluxed for
24 h under argon. The crude product was extracted into dichloro-
methane, and the organic layer was washed with water and dried
over anhydrous magnesium sulfate. After removal of the solvent
under reduced pressure, the residue was purified by column
chromatography on silica gel (ethyl acetate/hexane, 1/8, v/v) to
yield black-red solid (0.28 g, 75% yield). 1H NMR (600 MHz, CDCl3)
3. Results and discuss
3.1. Synthesis
The general synthetic routes to HIQ1, HIQ2 and HIQ3 are depicted
in Fig. 2. 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one was formyla
ted with phosphorus oxychloride in DMF to afford 4H-cyclopenta[2,1-
b:3,4-b0]dithiophen-4-one-2-carbaldehyde in excellent yield. The carb
aldehyde was brominated with NBS to give 6-bromo-4H-cyclopenta
[2,1-b:3,4-b0]dithiophen-4-one-2-carbaldehyde in high yield.4-tribu
d: 9.77 (s,1H), 7.40 (d, J ¼ 6.6 Hz, 2H), 7.18 (d, J ¼ 3.6 Hz, 2H), 7.13 (m,
2H), 7.09 (d, J ¼ 8.4 Hz, 4H), 7.03 (m, 6H), 2.32 (s, 6H) ppm. 13C NMR

556
C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
Fig. 2. Synthesis of HIQ1, HIQ2 and HIQ3: (i) POCl3, DMF, 1,2-dichloroethane reflux, (ii) NBS, DMF, rt, (iii) Pd(PPh3)4, toluene, reflux, (iv) CNCH2COOH, piperidine, CHCl3, reflux.
tylstannyl-N,N-bis(4-methylphenyl)aniline, 4-[5-(tributylstannyl)thio
phene-2-yl]-N,N-bis(4-methylphenyl)aniline, and 4-[5-tributylstann
yl-3,4-ethylenedioxythiophene-2-yl]-N,N-bis(4-methylphenyl)anili-
ne were allowed to react separately with 6-bromo-4H-cyclopenta[2,1-
b:3,4-b0]dithiophen-4-one-2-carbaldehyde via a Pd(PPh3)4-catalyzed
Stiller coupling reaction to afford the three dye precursors in moderate
yields. Finally, HIQ1, HIQ2 and HIQ3 were obtained through Knoeve-
nagel condensation between the precursors and cyanoacetic acid. All
the compounds, including the three dyes, were characterized by NMR
and MS.
charge transfer transitions between the donor moiety and the
acceptor unit [11]. Because the 4H-cyclopenta[2,1-b:3,4-b0]dithio-
phen-4-one moiety possesses substantial quinoidal character
within a conjugated backbone, it allows for stable electron delo-
calization [7]. The absorption peaks at 398 and 597 nm of HIQ1
were slightly red-shifted to 417 nm and 600 nm in HIQ2 and to 430
and 627 nm in HIQ3, owing to the increase in electron-donating
ability imparted by the thiophene and ethylenedioxythiophene
groups. HIQ3 had moderate molar extinction coefficients ( ) of
3
3.6 ꢁ 104 and 7.3 ꢁ 103 Mꢀ1 cmꢀ1 at 430 and 627 nm, respectively.
The
3
values of the three dyes increase in the order
HIQ1 < HIQ2 < HIQ3 (Table 1). All these features make HIQ1, HIQ2
and HIQ3 attractive sensitizers for nanocrystalline TiO2 DSCs in
a range covering the entire visible region and extending into the
NIR region. When adsorbed on a transparent TiO2 electrode, all
three dyes showed broad absorption spectra, and the absorption
peaks were slightly red-shifted compared with the corresponding
peaks of their solution absorption spectra, owing to interaction of
the anchoring group with surface titanium anions (Fig. 4). More-
over, the onset of absorption of HIQ3 occurred at about 900 nm,
3.2. Optical properties
The absorption spectra of all three dyes in DMF solution
exhibited strong and broad absorption over the entire visible region
extending to the NIR region (Fig. 3). The absorption spectra of HIQ1,
HIQ2 and HIQ3 were characterized by two prominent bands: the
band between 280 and 500 nm was attributed to the
sitions of the conjugated aromatic moieties, and the band at longer
wavelength (around 600 nm) was attributed to the intramolecular
pep* tran-

C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
557
Fig. 4. UVeviseNIR absorption spectra of HIQ1, HIQ2 and HIQ3 on TiO2 nanoparticles.
Fig. 3. UVeviseNIR absorption spectra of HIQ1, HIQ2 and HIQ3 in DMF.
ꢀ0.5 V vs NHE) [15]. These results indicate that electron injection
from the excited-state of the dye to the TiO2 conduction band was
possible.
making this dye a good candidate as a NIR sensitizer for DSCs. The
D-A-A configuration, with its strongly electron-withdrawing addi-
tional acceptor, showed a spectrum with a large red-shift relative to
the spectra observed with previously reported D-
dyes [12].
p-A configuration
3.4. Theoretical calculations
3.3. Electrochemical properties
To gain insight into the molecular orbitals and molecular
geometries of the dyes, we carried out time-dependent density
functional theory calculations (Fig. 6). Molecular-orbital calcula-
tions demonstrated that the HOMO of HIQ1 was largely delocalized
on the triphenylamino unit. In HIQ2 and HIQ3, the HOMOs were
spread over the triphenylamino unit through the thiophene group
and the ethylenedioxythiophene group, respectively. These results
suggest that delocalization of the HOMO on the donor moiety may
facilitate reduction of the oxidized dye by reaction with Iꢀ, making
the dye suitable for highly efficient solar cells. The LUMOs of all
three dyes were predominantly located on the cyanoacrylic units
and extended to the 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one
moieties, facilitating electron injection from the photoexcited
sensitizer to the TiO2 semiconductor. HOMOeLUMO excitation
moves the electron from the donor moiety to the cyanoacrylic acid
moiety via the additional acceptor group. Such electron density
distributions are beneficial for efficient charge separation and
electron injection. Thus, both our electrochemical and theoretical
results indicated that introducing a strongly electron-withdrawing
To evaluate the thermodynamics of electron injection from the
excited-state of the dye to the conduction band of the TiO2 elec-
trode and dye regeneration via electron donation from the elec-
trolyte, a cyclic voltammetry was obtained in a typical three-
electrode electrochemical cell with the dye-sensitized TiO2 films
as the working electrodes and 0.1 M tetra-n-butylammonium
hexafluorophosphate as the supporting electrolyte (Table 1). The
reference electrode was Ag/AgCl calibrated with Fcþ/Fc as an
internal reference. All dyes exhibited a quasi-reversible oxidative
wave. The ground-state oxidation potential values, Sþ/0, of HIQ1,
HIQ2 and HIQ3 were measured to be 1.18, 1.10 and 0.99 V (versus
NHE), respectively, and these values were low enough for efficient
regeneration of the oxidized dyes through reaction with iodide
(0.4 V; Fig. 5) [13]. The excited-state oxidation potential values, Sþ/
,
*
of the three dyes were calculated from the oxidation potential and
the optical energy gaps, E0e0, determined from the 10% maximum
absorption intensity of the absorption spectra on transparent TiO2
films [14]. The calculated Sþ/* values of HIQ1, HIQ2 and HIQ3 were
ꢀ0.66, ꢀ0.62 and ꢀ0.57 V, respectively, and were a little more
negative than the conduction band level of TiO2 (approximately
Table 1
Absorption and electrochemical properties of HIQ1, HIQ2 and HIQ3.
Absorptiona [nm]
[103 Mꢀ1 cmꢀ1])
Sþ/0 b [V]
E0e0 [V]
Sþ/ d [V]
*
c
Dye
(
3
HIQ1
HIQ2
HIQ3
398 (14.1), 597 (3.2)
417 (29.8), 600 (6.1)
430 (35.9), 627 (7.3)
1.18
1.10
0.99
1.83
1.72
1.56
ꢀ0.66
ꢀ0.62
ꢀ0.57
a
Absorption peaks and molar extinction coefficients ( 3 ) were measured in DMF
(2 ꢁ 10ꢀ5 M).
b
Ground-state oxidation potentials, Sþ/0, were measured on TiO2 containing
0.1 M (n-C4H9)4NPF6 at a scan rate of 50 mV sꢀ1 (vs. NHE).
c
Optical energy gaps, E0e0, were estimated from the 10% maximum absorption
intensity of the absorption spectra on transparent TiO2.
d
Exciþte/0d-state oxidation potentials, Sþ/*, were calculated from the expression
Fig. 5. Comparison of the conduction band (CB) of TiO2, the Sþ/0 and Sþ/* values of the
dyes, and the redox potential of Iꢀ/I3ꢀ, as well as the optical energy gaps (E0e0).
Sþ/ ¼ S
e E0e0.
*

558
C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
Fig. 6. Frontier molecular orbital profiles of HIQ1, HIQ2 and HIQ3 at the B3LYP/6-31G* level.
additional acceptor into the D-A-A configuration was a reasonable
strategy.
which is the longest-wavelength response among all NIR dyes
reported to date.
The currentevoltage characteristics (Jsc, Voc, FF, and
h) of the
HIQ1 cell were 7.76 mA cmꢀ2, 0.34 V, 0.ꢀ624 and 1.69%, respectively,
under AM 1.5 irradiation at 100 mW cm (Fig. 8 and Table 2). The
Jsc of the DSC based on HIQ2 (8.11 mA cmꢀ2) was slightly higher
than that of the DSC based on HIQ1, owing to the higher light-
harvesting efficiency of HIQ2 compared to that of HIQ1. However,
both Voc and FF of HIQ2 were lower than those of HIQ1, resulting in
3.5. Photovoltaic performance
The photovoltaic performance of DSCs sensitized with HIQ1,
HIQ2 and HIQ3 was studied under standard AM 1.5 irradiation
(100 mW cmꢀ2). The short-circuit current density (Jsc), open-circuit
voltage (Voc), fill factors (FF) and overall solar-to-electric energy
a lower
h (1.37%) for HIQ1. Among the three sensitizers, HIQ3
conversion efficiencies
(h) for each dyeeTiO2 electrode are
showed the highest Jsc (9.21 mA cmꢀ2), this result agrees well with
the proceeding IPCE measurements and absorption spectra. Thus,
we were able to increase the photocurrent simply by introducing
a more strongly electron-donating donor into the D-A-A configu-
ration based on 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one as
summarized in Table 2. IPCEs for the solar cells, plotted as a func-
tion of excitation wavelength, were calculated from Eq. (1),
IPCEð
l
Þ ¼ 1240ðIsc=lf
Þ
(1)
where Isc is the photocurrent density at short circuit (mA cmꢀ2
)
the additional acceptor. Although the
h and IPCE values were not
under monochromatic irradiation, is the wavelength of incident
l
radiation (nm) and
f
is the incident radiative flux (W mꢀ2). DSCs
based on HIQ1, HIQ2 and HIQ3 showed moderate response in the
visible region and extending into the NIR region (Fig. 7). The IPCE
values gradually increased in the order HIQ1 < HIQ2 < HIQ3 at
wavelengths >800 nm. The IPCE at 900 nm was 1.9% for HIQ1, and
the value doubled to 4% for HIQ2 and reached 9.4% for HIQ3
(Table 2). These results indicate that we not only achieved NIR
absorption but also increased the absorption intensity in the long-
wavelength region by means of simple tuning of the electron-
donating ability of the donor moiety in the D-A-A configuration
containing a strongly electron-withdrawing additional acceptor. It
is noteworthy that the IPCE response of HIQ3 extended to 1100 nm,
Table 2
Currentevoltage characteristics of DSCs sensitized with HIQ1, HIQ2 and HIQ3.
Dye
IPCE [%] at 900 nm
Jsc (mA cmꢀ2
)
Voc [V]
FF
h (%)
HIQ1
HIQ2
HIQ3
1.9
4.0
9.4
7.76
8.11
9.21
0.34
0.30
0.29
0.64
0.56
0.49
1.69
1.37
1.31
Fig. 7. IPCE spectra of DSCs based on HIQ1, HIQ2 and HIQ3.

C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
559
Dye-sensitized solar cells: a safe bet for the future. Energy Environment
Science 2008;1:655e67.
[2] (a) Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, et al. Enhance the optical
absorptivity of nanocrystalline TiO2 film with high molar extinction coefficient
ruthenium sensitizers for high performance dye-sensitized solar cells. Journal
of the American Chemical Society 2008;130:10720e8;
(b) Cao Y, Bai Y, Yu Q, Cheng Y, Liu S, Shi D, et al. Dye-sensitized solar cells with
a high absorptivity ruthenium sensitizer featuring a 2-(hexylthio)thiophene
conjugated bipyridine. The Journal of Physical Chemistry C 2009;113:6290e7;
(c) Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han LY. Dye-sensitized
solar cells with conversion efficiency of 11.1%. Japanese Journal of Applied
Physics 2006;45:L638e40;
(d) Snaith HJ, Moule AJ, Klein C, Meerholz K, Friend RH, Grätzel M. Efficiency
enhancements in solid-state hybrid solar cells via reduced charge recombina-
tion and increased light capture. Nano Letters 2007;7:3372e6.
[3] (a) Grätzel M. Recent advances in sensitized mesoscopic solar cells. Accounts
of Chemical Research 2009;42:1788e98;
(b) Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of dye-sensitized
solar cells based on metal-free indoline dyes. Journal of the American
Chemical Society 2004;126:12218e9;
(c) Zhang M, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-
sensitized solar cells with an organic photosensitizer featuring orderly
conjugated ethylenedioxythiophene and dithienosilole blocks. Chemistry of
Materials 2010;22:1915e25.
Fig. 8. JeV curves of DSCs sensitized with HIQ1, HIQ2 and HIQ3.
[4] (a) Hagfeldt A, Boschloo G, Sun LC, Kloo L, Pettersson H. Dye-sensitized solar
cells. Chemical Reviews 2010;110:6595e663;
(b) Mishra A, Fischer MK, Bäuerle P. Metal-free organic dyes for dye-sensitized
solar cells: from structure: property relationships to design rules. Angewandte
Chemie International 2009;48:2474e99.
ideal, owing to the low excited-state oxidation potentials of the
three dyes, adopting this type of D-A-A configuration is a simple,
feasible approach to the construction of NIR dyes. Further studies
on 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one involving, for
example, optimization of the structure, tuning of the energy levels
and electron injection efficiency, and optimization of the electro-
lyte solution and the redox couple are likely to result in more-
efficient NIR dyes. Additionally, co-sensitization with other dyes
to cover a broad solar spectrum may also be a good option.
[5] (a) Geiger T, Kuster S, Yum JH, Moon SJ, Nazeeruddin MK, Grätzel M, et al.
Molecular design of unsymmetrical squaraine dyes for high efficiency
conversion of low energy photons into electrons using TiO2 nanocrystalline
films. Advanced Functional Materials 2009;19:2720e7;
(b) Shi Y, Hill RBM, Yum JH, Dualeh A, Barlow S, Grätzel M, et al. Angewandte
Chemie International 2011;50:6619e21;
(c) Campbell WM, Burrell AK, Officer DL, Jolley KW. Porphyrins as light
harvesters in the dye-sensitised TiO2 solar cell. Coordination Chemistry
Reviews 2004;248:1363e79;
(d) Bessho T, Zakeeruddin SM, Yeh CY, Diau EWG, Grätzel M. Highly efficient
mesoscopic dye-sensitized solar cells based on donoreacceptor-substituted
porphyrins. Angewandte Chemie International 2010;49:6646e9;
4. Conclusion
(e) Park JK, Chen J, Lee HR, Park SW, Shinokubo H, Osuka A, et al. Doubly b-
functionalized mesoꢀmeso directly linked porphyrin dimer sensitizers for
photovoltaics. The Journal of Physical Chemistry C 2009;113:21956e63;
(f) Lee CY, She C, Jeong NC, Hupp JT. Porphyrin sensitized solar cells: TiO2
By introducing
a strongly electron-withdrawing additional
acceptor in a D-A-A configuration, we synthesized novel NIR dye
sensitizers, HIQ1, HIQ2 and HIQ3, which have 4H-cyclopenta[2,1-
b:3,4-b0]dithiophen-4-one as the additional acceptor and cyanoacetic
acid as an acceptor/anchor group. By systematically tuning the
electron-donating ability of the donor moiety simultaneously, we
were able to achieve, with HIQ3 on TiO2, a very broad absorption
spectrum extending to 900 nm, which was suitable for a NIR DSC.
DSCssensitizedwithHIQ3 showedanoverallconversionefficiency up
to 1.31%. It is noteworthy that the IPCE spectrum showed a broad
response from the visible region extending into the NIR area up to
1100 nm. Theseresults demonstrate the greatpotential ofcompounds
containing 4H-cyclopenta[2,1-b:3,4-b0]dithiophen-4-one-based D-A-
A structures as NIR sensitizers. We are carrying out additional studies
to tune the energy levels by molecular engineering of the D-A-A
structurewiththegoalofdevelopingefficientDSCsbasedonNIRdyes.
sensitization with a
Chemical Communication 2010;46:6090e2;
p-extended porphyrin possessing two anchoring groups.
(g) Guo M, Diao P, Ren YJ, Meng F, Tian H, Cai SM. Photoelectrochemical
studies of nanocrystalline TiO2 co-sensitized by novel cyanine dyes. Solar
Energy Materials and Solar Cells 2005;88:23e35;
(h) Funabiki K, Mase H, Hibino A, Tanaka N, Mizuhata N, Sakuragi Y, et al.
Synthesis of a novel heptamethineecyanine dye for use in near-infrared active
dye-sensitized solar cells with porous zinc oxide prepared at low tempera-
ture. Energy and Environmental Science 2011;4:2186e92;
(i) Jiao C, Zu N, Huang KW, Wang P, Wu J. Perylene anhydride fused
porphyrins as near-infrared sensitizers for dye-sensitized solar cells. Organic
Letters 2011;13:3652e5;
(j) Mai CL, Huang WK, Lu HP, Lee CW, Chiu CL, Liang YR, et al. Synthesis and
characterization of diporphyrin sensitizers for dye-sensitized solar cells.
Chemical Communication 2010;46:809e11.
[6] (a) Zhu W, Wu Y, Wang S, Li W, Li X, Chen J, et al. Organic D-A-p-A solar cell
sensitizers with improved stability and spectral response. Advanced Func-
tional Materials 2011;21:756e63;
(b) Cui Y, Wu Y, Lu X, Zhang X, Zhou G, Miapeh FB, et al. Incorporating ben-
zotriazole moiety to construct D-A-p-A organic sensitizers for solar cells:
significant enhancement of open-circuit photovoltage with long alkyl group.
Chemistry of Materials 2011;23:4394e401;
(c) Wang X, Wong WY, Cheung KY, Fung MK, Djurisic AB, Chan WK. Dalton
Acknowledgments
ꢀ ꢁ
Transactions 2008;40:5484e94;
(d) Wang XZ, Wang Q, Yan L, Wong WY, Cheung KY, Ng A, et al. Macromo-
lecular Rapid Communications 2010;31:861e7;
(e) Qin C, Fu Y, Chui CH, Kan CW, Xie Z, Wang L, et al. Macromolecular Rapid
Communications 2011;32:1472e7;
This work was supported by NIMS Saint-Gobain Center of
Excellence for Advanced Materials, and Core Research for Evolu-
tional Science and Technology (CREST) of the Japan Science and
Technology Agency.
(f) Wong WY, Ho CL. Accounts of Chemical Research 2010;43:1246e56.
[7] (a) Jordens P, Rawson G, Wynberg H. Synthesis of cyclopentadithiophenones.
Journal of the Chemical Society C: Organic 1970;2:273e7;
References
(b) Coppo P, Turner ML. Cyclopentadithiophene based electroactive materials.
Journal of Materials Chemistry 2005;15:1123e33.
[8] ChenCH,HsiehCH,DuboseM,ChengYJ,HsuCS. Synthesisand characterizationof
bridged bithiophene-based conjugated polymers for photovoltaic applications:
acceptor strength and ternary blends. Macromolecules 2010;43:697e708.
[9] Nazeeruddin MK, Péchy P, Renouard T, Zakeeruddin SM, Humphry-Baker R,
Comte P, et al. Engineering of efficient panchromatic sensitizers for nano-
crystalline TiO2-based solar cells. Journal of the American Chemical Society
2001;123:1613e24.
[1] (a) O’Regan B, Grätzel M. A low cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO2 films. Nature 1991;353:737e40;
(b) Nazeeruddin MK, De Angelis F, Fantacci S, Selloni A, Viscardi G, Liska P,
et al. Combined experimental and DFT-TDDFT computational study of pho-
toelectrochemical cell ruthenium sensitizers. Journal of the American Chem-
ical Society 2005;127:16835e47;
(c) Moreira Gonҫalves L, de Zea Bermudez V, Aguilar Ribeiro H, Mendes AM.

560
C. Qin et al. / Dyes and Pigments 94 (2012) 553e560
[10] Koide N, Han L. Measuring methods of cell performance of dye-sensitized
solar cells. Review of Scientific Instruments 2004;75:2828e31.
[11] Chen KF, Hsu YC, Wu QY, Yeh MCP, Sun SS. Structurally simple dipolar organic
dyes featuring 1,3-cyclohexadiene conjugated unit for dye-sensitized solar
cells. Organic Letters 2009;11:377e80.
[12] (a) Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. A high-light-har-
vesting-efficiency coumarin dye for stable dye-sensitized solar cells.
Advanced Materials 2007;19:1138e41;
Forneli A, Sastre-Santos A, Fernandez-Lazaro F, Palomares E. Interfacial photo-
induced charge transfer reactions in perylene imide dye sensitised solar cells.
Journal of Materials Chemistry 2008;18:5802e8.
[13] Oskam G, Bergeron BV, Meyer GJ, Searson PC. Pseudohalogens for dye-
sensitized TiO2 photoelectrochemical cells. The Journal of Physical Chem-
istry B 2001;105:6867e73.
[14] Nazeeruddin MK, Grätzel M. Encyclopedia of Electronchemistry. In: Bard A,
Startmann M, Licht S, editors. Semiconductor electrodes and photo-
electronchemistry. Germany: Wiley-VCH; 2002. p. 407e31.
(b) Zhou G, Pschirer N, Schöneboom JC, Eickemeyer F, Baumgarten M,
Müllen K. Ladder-type pentaphenylene dyes for dye-sensitized solar cells.
Chemistry of Materials 2008;20:1808e15. (c) Planells M, Cespedes-Guirao FJ,
[15] Hagfeld A, Grätzel M. Light-induced redox reactions in nanocrystalline
systems. Chemical Reviews 1995;95:49e68.