Organic dyes incorporating the cyclopentadithiophene moiety for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.09.019

Three triarylamine organic dyes (XS28-30) containing a cyclopentadithiophene unit as the conjugated bridge have been designed and synthesized for a potential application in dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and

Full text of DOI:10.1016/j.dyepig.2011.09.019

Dyes and Pigments 92 (2012) 1292e1299
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Organic dyes incorporating the cyclopentadithiophene moiety for efficient
dye-sensitized solar cells
*
*
Xiaobing Cheng, Siyuan Sun, Mao Liang , Yongbo Shi, Zhe Sun, Song Xue
Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three triarylamine organic dyes (XS28e30) containing a cyclopentadithiophene unit as the conjugated
bridge have been designed and synthesized for a potential application in dye-sensitized solar cells
(DSSCs). Their absorption spectra, electrochemical and photovoltaic properties have been investigated.
The incorporation of ethyl-substituted cyclopentadithiophene is highly beneficial to light-harvesting and
Received 28 April 2011
Received in revised form
15 August 2011
Accepted 22 September 2011
Available online 1 October 2011
preventing close
pep aggregation, thus favorably generating high efficiency. For a typical device, a solar
energy conversion efficiency (
h
) of 5.8% based on XS29 was achieved under simulated AM 1.5 solar
irradiation (100 mW cmꢀ2) with a short-circuit photocurrent density (JSC) of 14.4 mA cmꢀ2, an open-
circuit voltage (VOC) of 601 mV, and a fill factor (ff) of 0.68. These results suggest that the functional-
ized cyclopentadithiophene unit is a promising candidate for DSSCs.
Keywords:
Dye-sensitized solar cell
Organic dye
Ó 2011 Elsevier Ltd. All rights reserved.
Triarylamine
Cyclopentadithiophene
Conjugated bridge
Photovoltaic
1. Introduction
produces an effective intramolecular charge transfer from D to
A during photoexcitation. Aside from the electron donor and elec-
Recently, dye-sentsitized solar cells (DSSCs), considered as
a credible alternative to conventional inorganic silicon-based
solar cells, have attracted much attention relevant to global
environmental issues [1]. It has been found that the nature of
photosensitizers, such as redox potential, structure and photo-
physical properties, etc., play an important role in determining
the overall cell efficiencies. To date, two kinds of photosensi-
tizers, ruthenium dyes [2,3] and metal-free organic dyes, were
developed for DSSCs. Metal-free organic dyes were regarded as
an alternative to ruthenium dyes owing to their high molar
absorption coefficient, simple synthesis procedure, and low cost.
Various kinds of metal-free organic dyes such as coumarin-
[4e6], indoline- [7e10], triphenylamine- [11e19], phenothia-
zine- [20], dimethylfluorene- [21], truxene- [22e25], mer-
ocyanine- [26], hemicyanine- [27] and carbazole- [28] have been
investigated as sensitizers for DSSCs.
tron acceptor in a typical pushepull dye, the conjugated bridging
segment is widely recognized of its significance for performance
control of DSSCs [29]. Organic dyes based on thiophene,
oligothiophene moiety [28], 3,4-ethylenedioxythiophene (EDOT)
[12], thieno[3,2-b]thiophene (TT) [30] and dithieno[3,2-b:20,30-d]
thiophene (DTT) [31] as efficient bridging segments have been re-
ported, showing high values of the overall conversion efficiencies.
Cyclopentadithiophene, emerged as
a attractive building
block for the development of conducting materials [32], has
attracted our interest for design of photosenzitizers due to its
special properties of rigid conjugation structure and facile
introduction of alkyl chains. Organic dyes using the alkyl-
functionalized cyclopentadithiophene as the conjugated bridge
segment are expected to improve optical properties, suppress
dye aggregation, and retard charge recombination. In this study,
we report on the synthesis, characterization, and photovoltaic
properties of three new organic dyes (XS28e30, Scheme 1),
which contain functionalized 4,4-diethyl-cyclopenta(2,1-b:3,4-
b0)dithiophenes moiety as the conjugated bridge. Moreover, we
perform density functional theory (DFT) and time-depended
density functional theory (TD-DFT) calculations to provide
a detailed characterization of the structural and optical proper-
ties of the three sensitizers.
By far the most common investigated organic dyes are usually
composed by an electron donor (D), a conjugated bridging segment
(p), and an electron acceptor (A). This DepeA dipolar architecture
* Corresponding authors. Tel.: þ86 22 60214250; fax: þ86 22 60214252.
E-mail addresses: liangmao717@126.com (M. Liang), xuesong@ustc.edu.cn
(S. Xue).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.019

X. Cheng et al. / Dyes and Pigments 92 (2012) 1292e1299
1293
Scheme 1. Structure of organic dyes (XS28e30).
2. Experimental sections
Cyclic voltammetry measurements were performed at room
temperature on a computer controlled LK2005A electrochemical
workstation with Pt-wires as working electrode and counter elec-
trode, Ag/AgCl electrode as reference electrode with a scan rate of
100 mV sꢀ1. Tetrabutylammonium perchlorate (TBAP, 0.1 mol/L)
and acetonitrile were used as supporting electrolyte and solvent,
respectively. The measurements were calibrated using ferrocene as
standard. The redox potential of ferrocene internal reference is
taken as 0.63 V vs NHE.
2.1. Materials and instruments
The synthetic routes of the XS28e30 dyes are shown in
Scheme 2. N,N-Dimethylformamide was dried over and distilled
from CaH2 under an atmosphere of nitrogen. Phosphorus oxy-
chloride was freshly distilled before use. Titanium (IV) isoprop-
oxide, tertbutylpyridine and lithium iodide were purchased from
Aldrich. All other solvents and chemicals used in this work were
analytical grade and used without further purification.
2.3. Fabrication of DSSCs
1H NMR and 13C NMR spectra were recorded on a Bruker AM-
400 spectrometer. The reported chemical shifts were against TMS.
Mass spectra were recorded on a LCQ AD (Thermofinnigan, USA)
mass spectrometer. The melting point was taken on a RY-1 ther-
mometer and temperatures were uncorrected.
The TiO2 paste used for the preparation of the nanocrystalline
films consists of 18 wt.% TiO2, 9 wt.% ethyl cellulose and 73 wt.%
terpineol [33], which was printed on a conducting glass (Nippon
Sheet Glass, Hyogo, Japan, fluorine-doped SnO2 over layer, sheet
resistance of 10 U/sq) using a screen printing technique. The
2.2. Photophysical and electrochemical measurements
thickness of the TiO2 film was controlled by selection of screen
mesh size and repetition of printing. The film was dried in air at
120 ꢂC for 30 min and calcined at 500 ꢂC for 30 min under flowing
oxygen before cooling to room temperature. The heated electrodes
were impregnated with a 0.05 M titanium tetrachloride solution in
a water-saturated desiccator at 70 ꢂC for 30 min and fired again to
The absorption spectra of the dyes either in solution or on the
adsorbed TiO2 films were measured by HITACHI U-3310 spectro-
photometer. Adsorption of the dye on the TiO2 surface was done by
soaking the TiO2 electrode in
a mixture solution ethanol-
dichloromethane 3:1 solution of the dye (standard concentration
3 ꢁ10ꢀ4 M) at room temperature for 24 h. Fluorescence measure-
ment was carried with a HITACHI F-4500 fluorescence spectro-
photometer. FT-IR spectra were obtained with a Bio-Rad FTS 135
FT-IR instrument.
give a ca. 12
mm thick mesoscopic TiO2 film. The TiO2 electrode was
stained by immersing it into a dye solution containing 300
mM
dye sensitizers (ethanol-dichloromethane 3:1) for 48 h at room
temperature. Then the sensitized-electrode was rinsed with dry
ethanol and dried by a dry air flow. Pt catalyst was deposited on the
Scheme 2. Synthesis of organic dyes (XS28e30).

1294
X. Cheng et al. / Dyes and Pigments 92 (2012) 1292e1299
FTO glass by coating with a drop of H2PtCl6 solution (40 mM in
ethanol) with the heat treatment at 395 ꢂC for 15 min to give
photoanode. The dye-covered TiO2 electrode and Pt-counter elec-
trode were assembled into a sandwich type cell according to the
literature method [33]. The DSSCs had an active area of 0.16 cm2
and electrolyte composed of 0.6 M 1,2-dimethyl-3-n-propylimida-
zolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M tertbu-
tylpyridine in acetonitrile.
2.6.3. 6-Bromo-4,4-diethyl-4H-cyclopenta(2,1-b:3,4-b0)
dithiophene-2-carbaldehyde (4)
To a solution of compound 3 (2 g, 6.36 mmol) in anhydrous DMF
(12 mL, 152 mmol) at 0 ꢂC under N2 atmosphere was added POCl3
(3.75 mL, 25 mmol) dropwise and stirred for 1 h. Subsequently, the
mixture was heated at 50 ꢂC for 12 h. The mixture was cooled and
poured into an ice-water with vigorous stirring. After neutraliza-
tion with NaOH, the mixture was further stirred at 70 ꢂC for 1 h.
After cooling and extraction with ethyl acetate, the organic frac-
tions were combined and dried over with MgSO4. The resulting
solid was purified by column chromatography on silica gel (petro-
leum: ethyl acetate ¼ 5: 1 as eluent) to give a black oil (1.42 g,
2.4. Characterization of DSSCs
The photocurrentevoltage (JeV) characteristics of the solar cells
were carried out using a Keithley 2400 digital source meter
controlled by a computer and a standard AM1.5 solar simulator-
Oriel 91160-1000 (300 W) SOLAR SIMULATOR 2 ꢁ 2 BEAM. The
light intensity was calibrated by an Oriel reference solar cell. The
action spectra of monochromatic incident photon-to-current
conversion efficiency (IPCE) for solar cell were performed by
using a commercial setup (QTest Station 2000 IPCE Measurement
System, CROWNTECH, USA).
65.4%). IR (KBr): 1637, 1555, 1141, 788 cmꢀ1 1H NMR (400 MHz,
.
CDCl3):
d 9.85 (s, 1H), 7.57 (s, 1H), 7.03 (s, 1H), 1.97e1.92 (m, 4H),
0.62 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d 182.4, 160.4,
156.4, 147.0, 143.6, 136.3, 129.7, 124.9, 116.2, 55.6, 29.9, 8.9. HRMS
(ESI) calcd for C14H13BrOS2 (M þ Hþ): 340.9664, found: 340.9661.
2.6.4. Synthesis of compound 6
A mixture of compound 5 (0.183 g, 1.1 mmol), 4 (0.342 g,
1 mmol), Pd(PPh3)4 (50 mg, 0.042 mmol), aqueous 1 M Na2CO3
(3 mL), and 10 mL DME was refluxed for 18 h under N2. Ethyl
acetate was added before cooling down to room temperature. The
organic layer was separated and washed 3 times with water, dried
over anhydrous MgSO4, and filtered. After removing the solvent,
the resulting solid was purified by column chromatography on
silica gel (petroleum: ethyl acetate ¼ 5: 1 as eluent) as a yellow
powder (0.29 g, 75%). Mp: 167e169 ꢂC, IR (KBr): 1647, 1066,
2.5. Computational methods
The geometrical structures of the three dyes were optimized
by performed density functional theory (DFT) calculations and
time-dependent DFT (TDDFT) calculations of the excited states at
the B3LYP/6-31 þ G(d) level with the Gaussian 03W program
package.
952 cmꢀ1. 1H NMR (400 MHz, CDCl3):
d 9.80 (s, 1H), 7.53 (s, 1H), 7.51
(d, J ¼ 8.4 Hz, 2H), 7.04 (s, 1H), 6.73 (d, J ¼ 8.4 Hz, 2H), 3.01 (s, 6H),
2.6. The detailed experimental procedures and
characterization data
1.96e1.93 (m, 4H), 0.64 (t, J ¼ 7.3 Hz, 6H). 13C NMR (100 MHz,
CDCl3):
d 182.2, 163.1, 156.2, 151.0, 150.4, 149.1, 142.3, 132.9, 129.8,
126.6, 122.7, 115.3, 112.4, 55.0, 40.3, 30.1, 9.1. HRMS (ESI) calcd for
2.6.1. 4,4-Diethyl-4H-cyclopenta(2,1-b:3,4-b0)dithiophene (2)
C22H23NOS2 (M þ Hþ): 382.1272, found: 382.1283.
Compound 1 (2.0 g, 11.2 mmol) was dissolved in dimethyl
sulfoxide (50 mL). Bromoethane (2.4 g, 22.4 mmol) and potas-
sium iodide (50 mg) were added. The mixture was flushed with
nitrogen and cooled in an ice bath, and finely ground potassium
hydroxide (2.0 g) was added in portions. The resulting green
mixture was vigorously stirred overnight at room temperature.
The reaction was then cooled in an ice bath, and water (50 mL)
was added. The organic phase was extracted twice with ethyl
acetate, washed with water and brine. The organic phase was
dried with magnesium sulfate. The title compound was purified
by chromatography to give an oil (2.23 g, 85%). IR (KBr): 1636,
2.6.5. Synthesis of XS28
To a solution of compound 6 (0.176 g, 0.5 mmol) and cyanoacetic
acid (0.084 g, 1 mmol) in acetonitrile (10 mL) was added dichlor-
methane (5 mL) and piperidine (50 mL). The solution was refluxed
for 24 h. After cooling the solution, the solvent was removed in
vacuo. The pure product was obtained by silica gel chromatography
(CH2Cl2: MeOH ¼ 5: 1 as eluent) as a red powder (0.15 g, 68%). Mp:
164e167 ꢂC. IR (KBr): 3565, 2357, 1633, 1506, 903 cmꢀ1 1H NMR
.
(400 MHz, DMSO-d6):
d
8.28 (s, 1H), 7.79 (s, 1H), 7.54 (d, J ¼ 8.4 Hz,
2H), 7.41 (s, 1H), 6.77 (d, J ¼ 8.4 Hz, 2H), 2.96 (s, 6H), 1.92
1558, 1173, 788 cmꢀ1 1H NMR (400 MHz, CDCl3):
. d 7.20
(q, J ¼ 7.2 Hz, 4H), 0.57 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, DMSO-
(d, J ¼ 4.4 Hz, 2H), 6.98 (d, J ¼ 4.4 Hz, 2H), 1.94 (q, J ¼ 7.2 Hz, 4H),
d6): d 165.0,156.6,150.6,136.3,132.4,129.1,126.6,122.2,116.3,112.8,
0.65 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d
157.4, 136.9,
54.8, 44.1, 29.7, 9.4. HRMS (ESI) calcd for C25H24N2O2S2 (M þ Hþ):
124.5, 121.6, 54.3, 30.1, 9.1. HRMS (ESI) calcd for C13H14S2
449.1352, found: 449.1341.
(M þ Hþ): 235.0610, found: 235.0608.
2.6.6. Synthesis of compound 8
2.6.2. 2-Bromo-4,4-diethyl-4H-cyclopenta(2,1-b:3,4-
A mixture of compound 7 (0.317 g, 1.1 mmol), 4 (0.342 g,
1 mmol), Pd(PPh3)4 (50 mg, 0.042 mmol), aqueous 1 M Na2CO3
(3 mL), and 10 mL DME was refluxed for 18 h under N2. Ethyl
acetate was added before cooling down to room temperature. The
organic layer was separated and washed three times with water,
dried over anhydrous MgSO4, and filtered. After removing the
solvent, the resulting solid was purified by column chromatography
on silica gel (petroleum: ethyl acetate ¼ 5: 1 as eluent) as a yellow
powder (0.303 g, 60%). Mp: 198e199 ꢂC, IR (KBr): 1638, 1141, 1066,
b0)dithiophene (3)
Compound 2 (1.96 g, 8.34 mmol) and NBS (1.48 g, 8.34 mmol)
were dissolved in DMF (50 mL). After stirring at room tempera-
ture for 24 h, the mixture was poured into water (50 mL) and led
to precipitate yellow solid. The precipitate was filtered and
purified by chromatography on silica gel (petroleum: ethyl
acetate ¼ 10: 1 as eluent) to give a yellow solid (2.35 g, 90%). Mp:
47e49 ꢂC. IR (KBr): 1636, 1558, 1138, 787 cmꢀ1 1H NMR
.
(400 MHz, CDCl3):
d
7.18 (s, 1H), 6.96 (d, J ¼ 5.6 Hz, 2H), 1.85e1.92
950 cmꢀ1. 1H NMR (400 MHz, CDCl3):
d 9.82 (s,1H), 7.55 (s,1H), 7.50
(m, 4H), 0.61 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3):
(d, J ¼ 7.9 Hz, 2H), 7.30e7.22 (m, 5H), 7.14e7.12 (m, 5H), 7.08e7.06
d
156.6, 155.2, 136.8, 125.0, 124.5, 121.5, 111.1, 110.5, 55.2, 30.1,
(m, 3H), 1.99e1.93 (m, 4H), 0.66 (t, J ¼ 7.2 Hz, 6H). 13C NMR
29.6, 9.0, 8.9. HRMS (ESI) calcd for C13H13BrS2 (M þ Hþ):
(100 MHz, CDCl3): d 182.4, 162.9, 156.7, 149.6, 148.6, 147.9, 147.3,
312.9715, found: 312.9708.
142.9, 134.2, 129.7, 129.4, 129.1, 128.2, 126.4, 126.3, 124.7, 123.4,

X. Cheng et al. / Dyes and Pigments 92 (2012) 1292e1299
1295
116.7, 55.1, 30.1, 9.1. HRMS (ESI) calcd for C32H27NOS 2(M þ Hþ):
506.1607, found: 506.1589.
2.6.7. Synthesis of XS29
The product was synthesized according to the procedure for the
synthesis of XS28, giving
203e205 ꢂC. IR (KBr): 3566, 2357, 1634, 1506, 914 cmꢀ1
(400 MHz, DMSO-d6):
a
red powder in 70% yield. Mp:
.
1H NMR
d
8.26 (s, 1H), 7.72 (s, 1H), 7.59 (d, J ¼ 8.0 Hz,
2H), 7.49 (s, 1H), 7.35e7.30 (m, 4H), 7.10e7.03 (m, 6H), 6.98
(d, J ¼ 8.0 Hz, 2H), 1.92 (d, J ¼ 7.2 Hz, 4H), 0.51 (t, J ¼ 7.2 Hz, 6H). 13C
NMR (100 MHz, DMSO-d6):
d 165.2, 162.4, 157.1, 148.2, 147.5, 147.3,
145.2, 143.7, 137.4, 134.3, 130.1, 128.5, 126.7, 124. 8, 124.0, 123.5,
119.6, 118.2, 55.0, 29.8, 9.4. HRMS (ESI) calcd for C35H28N2O2S2
(M þ Hþ): 573.1665, found: 573.1652.
2.6.8. Synthesis of compound 10
To a solution of compound 9 (650 mg, 0.96 mmol) and 4
(300 mg, 0.88 mmol) in toluene (15 mL), Pd(PPh3)4 (50 mg) was
added. The mixture was refluxed for 6 h under argon. The crude
compound was extracted into ethyl acetate, washed with brine and
water, and dried over anhydrous sodium sulfate. After removing
solvent under reduced pressure, the residue was purified by
column chromatography (petroleum: ethyl acetate ¼ 10: 1 as
eluent) on silica gel to yield a red powder (0.352 g,ꢀ162%). Mp:
Fig. 1. Absorption and emission spectra of dyes XS28e30 in CHCl3 (1 ꢁ10ꢀ5 M).
Fig. 2 shows absorption spectra of the XS28e30 adsorbed on the
surface of transparent mesoporous TiO2 films (3 m, Fig. 2a; 12 m,
m
m
Fig. 2b). Absorption of the blank TiO2 film was subtracted from the
curve. Compared to the solution spectra of XS28e30, an evident
hypsochromic effect can be found for the dyes deprotonation
concomitant with the carboxylate grafting of sensitizers on TiO2
208e210 ꢂC. IR (KBr): 1653, 1493, 1397, 1311, 1083 cm
.
1H NMR
(400 MHz, DMSO-d6):
d
9.79 (s, 1H), 7.59 (d, J ¼ 8.7 Hz, 2H), 7.52 (s,
film (3 mm), which could be ascribed to a weaker electron-
1H), 7.27e7.23 (m, 4H), 7.12e7.02 (m, 9H), 4.41e4.34 (m, 4H), 1.94
(m, 4H), 0.63 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, DMSO-d6):
withdrawing capability of the carboxylatetitanium assembly than
that of the carboxylic acid. It is clearly demonstrated from Fig. 2b
that the absorption spectra of those adsorbed onto the similar
thickness of TiO2 films under the DSSC fabrication condition show
a markedly broad profile, which is beneficial to light-harvesting.
To gain insight into the geometrical, electronic, and optical
properties of the dyes XS28e30, DFT calculations and time-
dependent DFT (TDDFT) calculations of the excited states were
performed. The molecular structures of XS28e30 are optimized in
the vacuo, and the isodensity surface plots of HOMO (the highest
occupied molecular orbital) and LUMO (the lowest unoccupied
molecular orbital) are presented in Fig. 3. The TDDFT calculations
show that the lowest excitation of three dyes is a charge-transfer
transition of dominantly HOMOeLUMO character. Excitation
energies and oscillator strengths (f) for dyes XS28e30
are summarized in Table 2. The HOMO orbital of XS28e30 is of
d
182.3, 162.2, 156.7, 148.6, 147.4, 146.6, 142.7, 140.2, 138.7, 137.6,
134.2, 129.9, 129.3, 126.8, 126.5, 124.5, 123.4, 123.1, 116.5, 109.7, 65.0,
64.7, 55.0, 30.1, 9.0. HRMS (ESI) calcd for C38H31NO3S3 (M þ Hþ):
646.1466, found: 646.1462.
2.6.9. Synthesis of XS30
The product was synthesized according to the procedure for the
synthesis of XS28, giving a red powder of the product in 70% yield.
Mp: >300 ꢂC. IR (KBr): 3435, 1587, 1497, 1363, 1263, 1083 cmꢀ1. 1H
NMR (400 MHz, DMSO-d6):
d
8.1 (s, 1H), 7.58 (d, J ¼ 8.7 Hz, 2H),
7.33e7.29 (m, 5H), 7.09e6.99 (m, 9H), 4.45e4.40 (m, 4H), 1.94e1.92
(m, 4H), 0.86 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, DMSO-d6):
d
162.3, 156.2, 148.8, 147.3, 143.7, 141.3, 138.3, 135.2, 130.0, 129.6,
127.1, 124.6, 123.8, 123.6, 118.5, 110.3, 65.6, 64.8, 55.2, 29.8, 9.0.
HRMS (ESI) calcd for C41H32N2O4S3 (M þ Hþ): 713.1524, found:
713.1522.
p
-character and is delocalized over the entire molecule with
maximum components on the N,N-dimethylaniline/triphenyl-
amine units. The LUMO orbital is, on the other hand, a single
p
*
3. Results and discussion
orbital delocalized across the cyclopentadithiophene and cyanoa-
crylic acid groups with sizable contributions from the latter. This
distribution of HOMO and LUMO levels is separated in the molecule
of compound, indicating that transition from HOMO to LUMO can
be considered as a charge-transfer transition [34]. In addition, this
3.1. UVevis absorption/emission spectra
The UVevis and emission spectra of the dyes XS28e30 in CHCl3
are displayed in Fig. 1. The characteristic data are summarized in
Table 1. All the dyes display two strong absorption bands at around
300e350 nm and 400e600 nm, which mainly stem from the
intramolecular charge-transfer transition. The maximum absorp-
tion peaks of XS28e30 display at 521, 491, and 514 nm with molar
Table 1
Optical properties and electrochemical properties of four dyes.
extinction coefficient (ε) of 34 000, 36 000, and 42 000 Mꢀ1 cmꢀ1
,
a
Dye
lmax/nm (ε/103 Mꢀ1 cmꢀ1
)
lint/nmb
E0e0/eVc
Eox/Vd
Ered/Ve
respectively. XS28 possessed of N,N-dimethylaniline as donor shifts
the absorption peak bathochromically 30 nm compared to XS29
with triphenylamine as donor, indicating N,N-dimethylaniline
acted as donor unit favors the light harvesting. XS30 bearing 3,4-
ethyldioxythiophene and cyclopentadithiophene as binary spacer
shows obviously red-shifted absorption compared with XS29 due
to the extension of the p-conjugation. Emission maxima of the dyes
XS28e30 can be found at 500 to 700 nm when the dyes are excited
at their respective absorption bands at 400e600 nm.
XS28
XS29
XS30
276 (18), 521(34)
294 (19), 491(36)
305 (22), 519 (42)
605
576
576
2.04
2.15
2.15
0.77
0.91
0.74
ꢀ1.23
ꢀ1.34
ꢀ1.40
a
The maximum absorption peaks of dyes measured in CHCl3.
The intersect of the normalized absorption and the emission spectra.
E0e0 values were calculated from intersect of the normalized absorption and the
b
c
emission spectra (lint): E0e0 ¼ 1240/lint
.
d
Eox (vs NHE) of the dyes in acetonitrile were measured with cyclic
voltammogram.
e
Eox* (vs. NHE) was calculated from Eox* ¼ EoxeE0e0
.

1296
X. Cheng et al. / Dyes and Pigments 92 (2012) 1292e1299
Table 2
Calculated details of electronic transitions with the relative oscillator strengths
larger than 0.1 of the dyes.
Dye
State
Calculated
Oscillator
Transition assignmenta
energy (eV)
strength (f)
XS28
1
2
3
2.36
3.15
3.67
1.08
0.32
0.10
H / L (78%)
H-1 / L (75%)
H / L þ 1 (69%)
XS29
1
2
3
2.22
2.92
3.43
0.96
0.61
0.15
H / L (84%)
H-1 / L (78%)
H / L þ 1 (78%)
XS30
1
2
3
2.03
2.59
2.96
1.29
0.66
0.17
H / L (83%)
H-1 / L (80%)
H / L þ 1 (72%)
a
H means HOMO, and L means LUMO.
Fig. 2. Absorption spectra of XS28e30 on TiO2 transparent films (a: 3 mm; b: 12 mm).
XS28e30 are ꢀ1.23, ꢀ1.34, and ꢀ1.40 V vs NHE, respectively.
Replacement of triphenylamine with N,N-dimethylaniline shifts
the HOMO level negatively and the LUMO level positively, nar-
rowing the HOMOeLUMO energy gaps and resulting in red shift
of absorption spectra. Inserting an EDOT unit between the tri-
phenylamine and cyclopentadithiophene shifts the HOMO level
negatively due to the electron rich character of EDOT. The LUMO
levels for these dyes are more negative than the conduction band
of TiO2 (ECB, ꢀ0.5 V vs NHE), which provide sufficient driving
forces for electron injection. On the other hand, the HOMO levels
for these dyes are more positive than the iodine redox potential
(0.4 V vs NHE) [35]. Thus, these oxidized dyes can be regenerated
from the reduced species in the electrolyte to give an efficient
charge separation. Therefore, the HOMOeLUMO levels of dyes
XS28e30 are suitable for DSSCs.
spatially directed separation of HOMO and LUMO is an ideal
condition for dye-sensitized solar cells, which not only facilitates
the ultrafast interfacial electron injection but also slows down the
recombination of the injected electron with the oxidized dyes.
3.2. Electrochemical properties
To obtain and understand the molecular orbital energy levels,
cyclic voltammetry (CV) was employed to measure the ground-
state oxidation potential (Eox) of the dyes in acetonitrile solu-
tion. Representative cyclic voltammograms are shown in Fig. 4.
The corresponding electrochemistry data are given in Table 2.
XS28e30 exhibit two reversible processes in the oxidation scan.
The first quasi-reversible one-electron oxidation wave is attrib-
uted to the removal of electron from the N,N-dimethylaniline/
triphenylamine unit. The second quasi-reversible one-electron
oxidation wave at a higher potential is attributed to the oxidation
of cyclopentadithiophene/EDOT-cyclopentadithiophene since
oligothienyl ring is a potent electron donor.
3.3. Photovoltaic performance
Fig. 5 shows the incident photon-to-current conversion effi-
ciency (IPCE) as a function of the wavelength for the sandwiched
DSSCs based on the three dyes as sensitizers. The IPCE spectra for
XS28 and XS30 are broader than that of XS29, which is consistent
with the absorption spectra of the sensitizers on the similar
thickness of TiO2 films to the DSSCs fabrication condition (Fig. 2b).
XS29 generates high maximum IPCE summit with 84%. Compared
with XS29, XS28 with N,N-dimethylaniline in place of triphenyl-
amine gives lower IPCE with a maximum value of 67%. The IPCE of
For the three dyes, the first oxidation potential at the ground
state is taken as the highest occupied molecular orbital (HOMO)
level. The excited-state potential (Eox*), reflecting the LUMO level
of the dye, can be derived from the ground-state oxidation
potential (Eox
) and the zeroezero excitation energy (E0e0)
according to the following equation: Eox* ¼ EoxeE0e0. As depicted
in Table 1, the HOMO levels of the dyes XS28e30 are 0.77, 0.91,
and 0.74 V vs NHE, respectively. The LUMO levels of the dyes
Fig. 3. Isodensity surface plots of the HOMO and LUMO of XS28e30.
Fig. 4. Cyclic voltammogram of XS28e30 in acetonitrile.

X. Cheng et al. / Dyes and Pigments 92 (2012) 1292e1299
1297
Table 3
Photovoltaic performance of DSSCs sensitized with XS28-30.a
Dye
CDCA
JSC/mA cmꢀ2
VOC/mV
FF
h
/%
XS28
0 mM
1.5 mM
3.0 mM
12.5
13.7
13.4
584
606
598
0.68
0.69
0.67
4.9
5.7
5.4
XS29
XS30
0 mM
1.5 mM
3.0 mM
14.4
15.2
15.0
601
605
582
0.67
0.68
0.71
5.8
6.3
6.2
0 mM
1.5 mM
3.0 mM
13.4
14.1
13.2
595
608
589
0.68
0.70
0.72
5.4
6.0
5.6
N719
N719
16.8
692
0.68
7.9
a
Photovoltaic performances of DSSCs were measured under irradiation of AM
1.5 G simulated solar light (100 mW cmꢀ2) at room temperature with a 0.16 cm2
working area. The thickness of the TiO2 film was 12 mm. Dye bath: ethanoleCH2Cl2
3:1 solution (10ꢀ3 M).
Fig. 5. IPCE spectra for DSSCs based on XS28e30.
conditions. 5.8% of solar energy to electricity conversion efficiency
(h) based on XS29 is achieved with a short-circuit photocurrent
density (JSC) of 14.4 mA cmꢀ2, an open-circuit voltage (VOC) of
601 mV, and a fill factor (ff) of 0.67. The power conversion effi-
ciencies (h) for the XS28 and XS30 are 4.9% and 5.4%, respectively.
In comparison to XS28, the efficiency of the XS29-based device is
somewhat higher due to enhancement of JSC and VOC arising from
triphenylamine unit. The efficiency of XS30 is relative lower than
that of XS29 primarily due to the decreasing of JSC. For a fair
comparison, the N719-sensitized TiO2 solar cell showed an effi-
ciency of 7.9%, with a JSC of 16.8 mA cmꢀ2, a VOC of 695 mV, and a fill
factor of 0.68.
XS30 containing binary conjugated spacer of EDOT and cyclo-
pentadithiophene is evidently lower compared to the XS29, which
will result in a loss in JSC
.
The photocurrentevoltage (JeV) curves for DSSCs based on the
XS28e30 are demonstrated in Fig. 6, the corresponding photovol-
taic data are listed in Table 3. In agreement with the result of IPCE,
the XS29-sensitized cell exhibites better photovoltaic perfor-
mances than those of XS28, 30 under standard global AM 1.5 solar
To study the effect of an incorporated ethyl substituted cyclo-
pentadithiophene unit on dye aggregation, the JeV curves of
XS28e30 with a variety of chenodeoxycholic acid (CDCA) concen-
trations are measured (Table 3, Fig. 6bed). Generally, CDCA is used
as coadsorbent to dissociate the
p-stacked dye aggregates and
improves the electron-injection yield, thus affording higher JSC
value [23]. For the three dyes sensitized-DSSCs, the JSC values
increase with the addition of CDCA from 0 to 1.5 mM, then decrease
with further increase of CDCA (3.0 mM). The results indicate that
there was no significant p-aggregation [36] of dyes XS28e30 on the
surface of the TiO2 film, which may be due to the existence of the
two ethyl chains on the cyclopentadithiophene unit.
Electrochemical impedance spectroscopy (EIS) analysis is per-
formed to further elucidate the photovoltaic properties. Fig. 7a
shows the electrochemical impedance spectra for the DSSCs made
with TiO2 electrodes, dipped with XS28e30 sensitizers under
a forward bias of ꢀ0.6 V in the dark with a frequency range of 10 Hz
to 100 kHz. The larger semicircle at lower frequencies represents
the interfacial charge transfer resistances (RCT) at the TiO2/dye/
electrolyte interface. The RCT is related to the charge recombination
rate, e.g., a smaller RCT indicates a faster charge recombination.
The radius of the larger semicircle increases in the order
XS29 > XS30 > XS28, implying the same order of the electron
recombination resistance (RCT) for the three dyes. The electron
recombination lifetimes (s), extracted from the angular frequency
at the midfrequency peak in the Bode phase [37] plot Fig. 7b are in
the order of XS29 > XS30 > XS28. These results are in agreement
with the observed shift in the VOC value under standard global AM
1.5 illumination. The longer lifetime of XS29 relative to XS28 could
be attributed to the steric hindrance of bulky triphenylamine unit.
Recently, a better correlation was found between electron lifetimes
and polarizabilities [38]. In general, organic dyes with high polar-
izabilities show strong interaction with surrounding species and
induce an increase in the local concentrations of acceptor species.
Fig. 6. Currentepotential (JeV) curves for the DSSCs based on XS28e30 under AM 1.5
irradiation (100 mW cmꢀ2).

1298
X. Cheng et al. / Dyes and Pigments 92 (2012) 1292e1299
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