Low-cost dyes based on methylthiophene for high-performance dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2010.03.029

Three donor-acceptor, π-conjugated (D-π-A) dyes containing methylthiophene or vinylene methylthiophene as π-conjugated spacer were utilised in dye-sensitized nanocrystalline TiO₂ solar cells. The relationship between the structure of the dyes and their photophysical, electrochemical and photovoltaic properties was investigated systematically. The vinyl unit, introduced as the π-conjugated spacer, leads to unfavorable back-electron transfer and decrease of the open-circuit voltage. A dye-sensitized solar cell based on 2-cyano-3-(5-(4-(diphenylamino)phenyl)-4-methylthiophenyl-2-yl) acrylic acid displayed the most efficient solar-to-electricity conversion efficiency of the dyes with a maximum η value of 8.27% (V_oc?=?0.72?V, J_sc?=?15.76?mA?cm^-2, FF?=?0.73) under simulated AM 1.5 G solar irradiation (100?mW?cm^-2).

Full text of DOI:10.1016/j.dyepig.2010.03.029

Dyes and Pigments 87 (2010) 181e187
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Review
Low-cost dyes based on methylthiophene for high-performance
dye-sensitized solar cells
,
a
,
b
,
a
a
Zongfang Tian a, Meihua Huang a , Bin Zhao
, Hui Huang , Xiaoming Feng ,
*
*
Yujuan Nie a, Ping Shen a, Songting Tan a
,
b
a College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan University, Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three donoreacceptor,
p-conjugated (DepeA) dyes containing methylthiophene or vinylene methyl-
Received 1 February 2010
Received in revised form
28 March 2010
Accepted 29 March 2010
Available online 7 April 2010
thiophene as
relationship between the structure of the dyes and their photophysical, electrochemical and photovoltaic
properties was investigated systematically. The vinyl unit, introduced as the -conjugated spacer, leads
p-conjugated spacer were utilised in dye-sensitized nanocrystalline TiO2 solar cells. The
p
to unfavorable back-electron transfer and decrease of the open-circuit voltage. A dye-sensitized solar cell
based on 2-cyano-3-(5-(4-(diphenylamino)phenyl)-4-methylthiophenyl-2-yl) acrylic acid displayed the
most efficient solar-to-electricity conversion efficiency of the dyes with a maximum
h value of 8.27%
Keywords:
(Voc ¼ 0.72 V, Jsc ¼ 15.76 mA cmꢀ2, FF ¼ 0.73) under simulated AM 1.5 G solar irradiation (100 mW cmꢀ2).
Ó 2010 Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells
Metal-free organic dyes
Triphenylamine dyes
Methylthiophene
Vinylene methylthiophene
Photovoltaic performances
1. Introduction
[15], hemicyanine- [16,17], phenothiazine- [18], phthalocyanine-
[19,20], tetrahydroquinoline- [21], dimethylfluorene- [22], carba-
The search for novel dyes in the context of dye-sensitized solar
cells (DSSCs) has gained much impetus since the first report in 1991
by O’Regan et al. [1]. As a key part of DSSCs, dyes play a crucial role
zole- [23], and triphenylamine-based compounds [24e27]. Most of
these dyes show impressive values in the range of 5e10.3%.
The donore( -spacer)eacceptor (De eA) structural system is
h
p
p
in high solar-to-electricity conversion efficiency (
h
) and have been
the basis of metal-free dyes owing to its effective photoinduced
intramolecular charge transfer properties. The photoelectric prop-
erties of such dyes can be finely tuned by alternating independently
or matching different groups within the DepeA structure. In order
to possess broad and intense spectral absorption in the visible light
region, one strategy for optimizing dye structure is to introduce
studied in depth by many researchers. There are two kinds of dyes,
namely, metal-complex and metal-free types. Important metal-
complex dyes are ruthenium complexes, such as N3/N719 [2,3] and
others [4], including C101 [5,6] and CYC-B1 [7], which display solar-
to-electricity conversion efficiencies of 11.3% under AM 1.5 irradi-
ation. In view of the limited availability and environmental issues
associated with ruthenium dyes, metal-free dyes are considered to
be an alternative for use in DSSCs because they have high molar
absorption coefficients and can be prepared more easily and
economically. Several metal-free dyes have been reported as
sensitizers in DSSCs in recent years, representatives including
coumarin- [8,9], indoline- [10e12], oligoene- [13,14], merocyanine-
more
thereby forming a De
may induce the recombination of the injected electrons with
triiodide ions, * stacking and intermolecular aggregation at the
p-conjugation segments between the donor and acceptor,
pepeA structure [21,28,29]. However, this
pep
semiconductor surface [29,30], resulting in lower photovoltaic
performance. Another strategy is to incorporate a bis-donor moiety
into the framework, this resulting in various dyes of DeDe
structure having been studied [27,31e35]. Compared with De
p
p
eA
eA
dyes, DeDe
tron-donating ability but also greater steric hindrance, which can
prevent unfavorable dye aggregation [31]. However, DeDe eA
dyes do not always show better photovoltaic performances than
their De eA counterparts [27,32] because two donor moieties
peA compounds not only have much stronger elec-
* Corresponding author at: College of Chemistry and Key Laboratory of Envi-
ronmentally Friendly Chemistry and Applications of Ministry of Education, Xiang-
tan University, Xiangtan 411105, PR China. Tel.: þ86 731 58292207; fax: þ86 731
58293264.
p
p
E-mail addresses: xtuhmh@xtu.edu.cn (M. Huang), xtuzb@163.com (B. Zhao).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.029

182
Z. Tian et al. / Dyes and Pigments 87 (2010) 181e187
that posses steric hindrance can twist the structure and thereby
reduce coplanarity between the electron donor and electron
acceptor, resulting in lower light harvesting and charge transfer
internal potential reference at the end of the experiments. The
potentials of dyes versus NHE were calibrated by addition of 0.63 V
to the potentials versus Fcþ/Fc [38].
ability [32]. Moreover, the synthesis of both De
DeDe eA dyes is usually more complicated and expensive than
that of De eA dyes.
The present researchers have employed triphenylamine dyes
based on -conjugated hexylthiophene for DSSCs, and the effects
of the length of the -conjugated unit and the structure of the
pepeA and
p
2.3. General procedure for preparation and test of solar cells
p
Fluorine-doped, SnO2 conducting glass (FTO) was cleaned and
immersed in an aqueous 40 mmol Lꢀ1 TiCl4 solution at 70 ꢁC for
30 min, then washed with water and ethanol and finally sintered at
450 ꢁC for 30 min. 20e30 nm TiO2 colloid was prepared from 12 g
P25 (Degussa AG, Germany) following the literature procedure [2]
and was added to 3.6 mL 0.1% magnesium acetate solution [39].
200 nm TiO2 colloid comprised 4.7% 200-nm-sized TiO2 and 9.5%
ethyl cellulose in n-butanol. Firstly, 20e30 nm TiO2 colloid was
coated on the FTO glass using a sliding glass rod method to obtain
p
p
electron donor unit on photocurrent characteristics were studied
[36,37]. In this paper, low-cost methylthiophene is used instead of
hexylthiophene in the synthesis of De
effect, on solar cell performance, of the spacers between the
electron-donating moieties and -conjugated methylthiophene,
both methylthiophene and vinylene methylthiophene were used
as -conjugated spacer (Fig. 1).
peA dyes. To evaluate the
p
p
a TiO2 film of 10e15
200 nm TiO2 colloid was coated on the electrode using the same
method, resulting in a TiO2 light-scattering layer of 4e6
mm thickness after drying. Subsequently, the
2. Experimental section
mm
thickness. The double-layer TiO2-coated FTO glass was sintered at
450 ꢁC for 30 min, treated with TiCl4 solution and calcined at 450 ꢁC
for 30 min. After cooling to 100 ꢁC, the TiO2 electrodes were soaked
in a mixture of 0.5 mmol Lꢀ1 dye and 1.0 mmol Lꢀ1 coadsorbent
(chenodeoxycholic acid, CDCA), and kept at room temperature in
the dark for 24 h; the ensuing dye-adsorbed TiO2 electrode was
washed with ethanol and dried. A drop of electrolyte solution was
deposited on the surface of the electrode and a Pt foil counter
electrode was clipped onto the top of the TiO2 electrode so as to
comprise a dye-sensitized solar cell for photovoltaic performance
measurements. The electrolyte consisted of 0.5 mol Lꢀ1 LiI,
0.05 mol Lꢀ1 I2, and 0.5 mol Lꢀ1 4-tert-butylpyridine (TBP) in 3-
methoxypropionitrile and the irradiated area of the cell was
0.196 cm2. Photocurrentevoltage curves were measured using
a Keithley 2602 Source meter under 100 mW cmꢀ2 irradiation
using a 500 W Xe lamp equipped with a global AM 1.5 filter for solar
spectrum simulation; the incident light intensity was calibrated
using a standard Si solar cell. The measurement of monochromatic
incident photon-to-current conversion efficiencies (IPCE) was
performed using a Zolix DCS300PA Data acquisition system.
2.1. Materials and reagents
All starting materials were purchased from commercial
suppliers (Pacific ChemSource and Alfa Aesar) and were of analyt-
ical grade. THF and toluene were dried and distilled over sodium
and benzophenone. DMF, CHCl3, CH3CN and POCl3 were dried over
by accustomed methods and distilled before use. All other solvents
and chemicals used in this work were analytical grade and used
without further purification. All chromatographic separations were
carried out on silica gel (200e300 mesh).
2.2. Analytical measurements
1H NMR and 13C NMR spectra were measured with Bruker
Advance 400 instrument. UVevis spectra of the dyes were
measured on a PerkineElmer Lamada 25 spectrometer. The PL
spectra were obtained using PerkineElmer LS-50 luminescence
spectrometer. Electrochemical redox potentials were obtained by
cyclic voltammetry (CV) using a three-electrode cell and an elec-
trochemistry workstation (CHI830B, Chenhua Shanghai). The
working electrode was a glassy carbon electrode, the auxiliary
electrode was a Pt wire, and saturated calomel electrode (SCE) was
used as reference electrode. Tetrabutylammonium perchlorate
(TBAP) 0.1 M was used as supporting electrolyte in dry acetonitrile.
Ferrocene was added to each sample solution and used as an
2.4. Synthesis
Pinacol(4-(N,N-diphenylamino)phenyl)boronate (1), 2-bromo-
3-methyl-thiophene (2) and 5-bromo-4-methylthiophene-2-car-
baldehyde (3) were synthesized according to the methods reported
in the literature [36,40]. The synthetic routes to the three dyes are
shown in Fig. 2.
2.4.1. 5-[2-(4-Diphenylaminophenyl)]-4-methylthiophene-2-
carbaldehyde (4)
Under an argon atmosphere, 1 (0.63 g, 1.69 mmol), 3 (0.47 g,
2.29 mmol), aqueous K2CO3 (2 mol Lꢀ1, 2 mL) and DMF (18 mL)
were placed in a 100 mL three-necked flask, and the mixture was
heated to 60 ꢁC. Pd(PPh3)4 was added quickly and the mixture was
stirred at 100 ꢁC for 36 h. The solution was extracted with
dichloromethane after it cooled to room temperature and the
combined organic layer was washed with water and brine several
times, dried over anhydrous MgSO4, filtered and evaporated. The
crude product was purified by column chromatography on silica gel
with hexane/dichloromethane (1:1, v:v) as eluent to obtain
a yellow solid. Yield: 0.34 g (54%). 1H NMR (CDCl3, 400 MHz,
ppm): 9.81 (s, 1H), 7.57 (s, 1H), 7.35e7.25 (m, 6H), 7.16e7.24 (d, 4H),
7.09e7.06 (t, 4H), 2.37 (s, 3H). 13C NMR (CDCl3, 100 MHz,
/ppm):
d/
d
182.5, 149.0, 148.4, 147.2, 140.2, 139.9, 134.0, 129.7, 129.5, 126.6,
125.3, 124.1, 122.2, 15.3.
Fig. 1. Molecular structures of the dyes TD1, TD2, and TD3.

Z. Tian et al. / Dyes and Pigments 87 (2010) 181e187
183
Fig. 2. Synthetic routes for the three new dyes. Reaction conditions: (a) (I) THF, ꢀ78 ꢁC, n-BuLi, 3 h; (II) 2-isopoxyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, ꢀ78 ꢁC for 3 h then to rt,
12 h. (b) NBS, THF/Acetic acid, rt. (c) DMF, POCl3, 60e70 ꢁC, 8 h. (d) DMF, K2CO3, Pd[P(Ph)3]4, 100 ꢁC, 36 h. (e) CNCH2COOH, piperidine, MeCN, 90 ꢁC, 10 h. (f) HCl, HCHO, ꢀ5 to ꢀ10 ꢁC,
2 h. (g) PO(OCH2CH3)3, 140 ꢁC, 6 h. (h) t-BuOK, THF, rt, 24 h. (i) n-BuLi, ꢀ40 ꢁC, DMF.
2.4.2. 2-Cyano-3-(5-(4-(diphenylamino)phenyl)-4-
2.4.5. 2-(4-Diphenylaminostyryl)-3-methylthiophene (9)
methylthiophenyl-2-yl) acrylic acid (TD1)
4-(Diphenylamino)-benzaldehyde 7 (1.54 g, 5.63 mmol) and 6
(1.44 g, 5.82 mmol) were dissolved in 15 mL of fresh dried THF and
t-BuOK (0.39 g, 3.48 mmol) dissolved in 10 mL THF was added
dropwise under a N2 atmosphere. After reacting for 24 h at room
temperature, the reaction mixture was poured into water and
extracted twice with chloroform. The combined organic extracts
were washed three times with water, dried over anhydrous MgSO4,
evaporated under vacuum, and purified with column chromatog-
raphy on silica gel with petroleum ether/dichloromethane mixture
(10:1, v:v) as eluent to obtain a yellow viscous oil. Yield: 1.71 g
In a 100 mL three-necked flask, a mixture of 4 (0.29 g,
0.79 mmol), cyanoacetic acid (0.13 g, 1.56 mmol), piperidine
(0.2 mL), and CH3CN (30 mL) was refluxed for 12 h under an Ar
atmosphere. After cooling to room temperature, the reaction
product was poured into
a mixture of distilled water and
2.0 mol Lꢀ1 HCl aqueous solution and extracted with chloroform.
The organic layer was washed with water and dried over anhydrous
MgSO4. After removal of solvent, the crude product was purified by
column chromatography over silica gel with a dichloromethane/
methanol mixture (10:1, v:v) as eluent to obtain a dark purple solid.
(83%). 1H NMR (CDCl3, 400 MHz,
7.27e7.23 (m, 4H), 7.13e7.01 (m, 10H), 6.82e6.78 (m, 2H), 2.29 (s,
3H). 13C NMR (CDCl3, 100 MHz,
/ppm): 147.6, 147.2, 136.8, 135.0,
d/ppm): 7.35e7.33 (d, 2H),
Yield: 0.21 g (62%). 1H NMR (CDCl3, 400 MHz,
d
/ppm): 8.25 (s, 1H),
7.64 (s, 1H), 7.31e7.25 (m, 8H), 7.15e7.05 (m, 6H), 2.36 (s, 3H). 13C
NMR (CDCl3, 100 MHz, /ppm): 149.9, 147.0, 144.1, 136.2, 132.7,
d
d
131.6, 130.8, 129.3, 127.4, 127.1, 124.5, 123.7, 123.0, 14.0.
129.5, 129.4, 125.1, 123.8, 123.4, 122.0, 121.6, 120.0, 119.8, 29.7.
2.4.6. 2-[2-[p-(Di-p-tolylamino)phenyl]vinyl]-3-
2.4.3. 2-(Chloromethyl)-3-methylthiophene (5)
methylthiophene (10)
Hydrochloric acid (35%, 10 mL, 113.9 mmol), formaldehyde (37%,
40 mL, 402.1 mmol) and 3-methylthiophene (7.04 g, 71.8 mmol)
were placed in a 100 mL three-neck flask and kept at ꢀ5 ꢁC to
ꢀ10 ꢁC. The stirred reaction mixture was added gaseous HCl over
2 h and the ensuing mixture was poured into 75 mL water and
extracted with ether. The organic layer was washed with water and
dried over anhydrous MgSO4. After removal of solvent, 2-(chlor-
omethyl)-3-hexyl-thiophene was obtained as yellow oil and used
without further purification.
The synthetic procedure for 10 was similar to that for 9, except
that 4-(di-p-tolylamino)benzaldehyde 8 (1.03 g, 5.15 mmol) was
used instead of 7. The crude product was purified by column
chromatography over silica gel with petroleum ether/dichloro-
methane mixture (10:1, v:v) as eluent to obtain a yellow viscous
oil. Yield: 1.32 g (81%). 1H NMR (CDCl3, 400 MHz,
d
/ppm): 7.32 (d,
2H), 7.11e6.98 (m, 12H), 6.82e6.78 (d, 2H). 2.32 (s, 1H), 2.30 (s,
1H). 13C NMR (CDCl3, 100 MHz,
/ppm): 147.7, 145.2, 137.0, 134.8,
d
132.7, 130.8, 130.7, 130.0, 127.6, 127.0, 124.8, 122.5, 122.3, 118.4,
20.9, 14.0.
2.4.4. 2-(Methylphosphonate acid ethyl ester)-3-
methylthiophene (6)
2.4.7. 5-(4-(Diphenylamino)styryl)-4-methylthiophene-2-
carbaldehyde (11)
Compound 5 (4 g, 27.4 mmol) and triethyl phosphite (4.98 g,
30 mmol) were placed in a flask and the ensuing mixture was
stirred for 6 h at 140 ꢁC. Excess triethyl phosphite was distilled
under vacuum and a sticky yellow coloured oil was obtained. The
crude product was purified on silica gel chromatography using
a petroleum ether/ethyl acetate mixture (2:1, v:v) as eluent to
obtain a yellow viscous oil. Yield: 5.7 g (84%). 1H NMR (CDCl3,
Compound 9 (1.5 g, 4.08 mmol) was dissolved in THF (35 mL)
and the solution was cooled to ꢀ40 ꢁC under a nitrogen atmo-
sphere. n-Butyllithium (3 mL, 2.5 mol Lꢀ1 in hexane solution) was
added dropwise over 25 min and the mixture was stirred at ꢀ40 ꢁC
for 1 h. After cooling to room temperature DMF (1.2 mL, 15.5 mmol)
was added dropwise; after 2.5 h, the ensuing solution was poured
into a saturated aqueous solution of ammonium chloride and
extracted with dichloromethane. The organic layer was dried over
400 MHz,
d/ppm): 7.04 (d, 1H), 6.79 (d, 1H), 4.15e4.01 (q, 4H),
3.33e3.25 (d, 2H), 2.19 (s, 3H), 1.36e1.24 (t, 6H).

184
Z. Tian et al. / Dyes and Pigments 87 (2010) 181e187
anhydrous MgSO4 and evaporated, then the residue was purified
with column chromatography on silica gel with petroleum ether/
dichloromethane mixture (3:1, v:v) as eluent to obtain a red viscous
oil. Yield: 1.13 g (72%). 1H NMR (CDCl3, 400 MHz,
d
/ppm): 9.78 (s,
1H), 7.47 (s, 1H), 7.37e7.35 (d, 2H), 7.28e7.25 (m, 4H), 7.12e6.96 (m,
10H), 2.32 (s, 3H). 13C NMR (CDCl3, 100 MHz,
/ppm): 182.4, 148.4,
d
147.6, 147.2, 139.8, 139.0, 135.8, 132.1, 129.9, 129.4, 127.8, 124.9,
123.5, 122.8, 117.4, 14.0.
2.4.8. 5-[p-(Di-p-tolylamino)styryl]-4-methylthiophene-2-
carbaldehyde (12)
The synthetic procedure for 12 was similar to that for 11, except
that 10 (0.89 g, 2.25 mmol) was used instead of 9. The crude
product was purified by column chromatography over silica gel
with petroleum ether/dichloromethane mixture (3:1, v:v) as eluent
to obtain a red viscous oil. Yield: 0.66 g (63%). 1H NMR (CDCl3,
400 MHz,
7.09e6.95 (m, 14H), 2.31 (s, 1H,), 2.29 (s, 1H). 13C NMR (CDCl3,
100 MHz, /ppm): 182.3, 148.8, 147.8, 144.7, 139.7, 138.8, 135.5,
d/ppm): 9.77 (s, 1H), 7.46 (s, 1H), 7.33e7.31 (d, 2H),
Fig. 3. Absorption and emission spectra of the dyes in CHCl3 solutions (10ꢀ5 mol Lꢀ1).
d
133.3, 132.3, 130.0, 129.0, 127.7, 125.1, 121.6, 116.9, 20.8, 13.9.
presence of piperidine. The chemical structures of all dyes were
characterized with 1H NMR and 13C NMR spectroscopy.
2.4.9. 3-(5-(4-(Diphenylamino)styryl)-4-methylthiophen-2-yl)-2-
cyanoacrylic acid (TD2)
The synthetic procedure for TD2 was similar to that for TD1,
Yield 76%, a red solid. 1H NMR (CDCl3, 400 MHz,
d
/ppm): 8.22 (s,
1H), 7.52 (s, 1H), 7.39e7.37 (d, 2H), 7.29e7.26 (m, 2H), 7.14e7.10 (m,
12H), 2.34 (s, 3H). 13C NMR (CDCl3, 100 MHz,
/ppm): 149.5, 147.1,
3.2. Optical properties
The UVevis absorption spectra and fluorescence emission spectra
of the dyes in diluted CHCl3 solution (10ꢀ5 mol Lꢀ1) are shown in
Fig. 3, and the corresponding data are collected in Table 1. In CHCl3
solution, all the dyes give two distinct absorption bands: one rela-
tively weak band is in the near-UV region (280e311 nm) corre-
d
137.9, 133.2, 129.7, 129.4, 129.3, 129.3, 128.0, 125.1, 125.0, 124.5,
123.7, 123.1, 122.5, 121.5, 29.7.
2.4.10. 3-(5-(4-(Di-p-tolylamino)styryl)-4-hexylthiophen-2-yl)-2-
cyanoacrylic acid (TD3)
sponding to the
pep* electron transitions of the conjugated
molecules, and the other is a strong absorption band in the visible
region (460e540 nm) that can be attributed to an intramolecular
charge transfer (ICT) between the donating units and the cyanoa-
crylic acid anchoring moiety [43]. It can be seen that the maximum
absorption of TD2 and TD3 is obviously red-shifted in comparison
The synthetic procedure for TD3 was similar to that for TD1,
Yield 78%, a dark purple solid. 1H NMR (CDCl3, 400 MHz,
d
/ppm):
8.22 (s, 1H), 7.52 (s, 1H), 7.36e7.34 (d, 2H), 7.11e6.96 (m, 12H), 2.34
(s, 6H), 2.33 (s, 3H). 13C NMR (CDCl3, 100 MHz,
/ppm): 149.7, 147.1,
d
144.5,136.3,133.5,131.1,130.1,129.0, 128.0,126.9,125.3,121.2,116.2,
29.7, 20.9.
with that of TD1, which is due to the extension of
p-conjugated
system by introduction of vinyl unit in the -spacer part. On the other
p
hand, the spectrum of TD3 is red-shifted in comparison with that of
TD2 since di(p-tolyl)phenylamine moiety is a stronger electron-
donating unit than triphenylamine moiety. As depicted in Table 1,
3. Results and discussion
the molar extinction coefficients
3
(>2.3 ꢂ 104 molꢀ1 L cmꢀ1) of
3.1. Synthesis
three as-synthesized dyes are higher than that of the standard N719
dye (1.4 ꢂ 104 molꢀ1 L cmꢀ1) [42], which indicates the dyes have
a better ability of light harvesting. Fig. 3 also shows the emission
spectra of the three as-synthesized dyes, we can see that the
maximum emission wavelength of the dyes in CHCl3 solution is in
the order of TD3 > TD2 > TD1, which is consistent with that of
the maximum absorption.
When the dyes are adsorbed on TiO2 surface, the absorption
spectra of these dyes are broadened and blue-shifted more or less
as compared to that in solutions (Fig. 4), The similar phenomenon
was also observed for other organic dyes [9,13]. The trend may be
The molecular structures of the three dyes are shown in Fig. 1. All
of these dyes have been prepared according to several classical
reactions, which involve the Suzuki coupling reaction [41],
WittigeHorner reaction [37] and Knoevenagel condensation reac-
tion [40]. The synthetic strategies are showed in Fig. 2. Suzuki
coupling of triphenylamine boronic acid pinacol ester 1 and bromi-
nated methylthiophene aldehydes 3 with tetrakis(triphenylphos-
phine)palladium(0) (Pd(PPh3)4) as the catalyst in a biphasic mixture
of K2CO3 and DMF afforded 5-[2-(4-Diphenylaminophenyl)]-4-
methylthiophene-2-carbaldehyde 4. Subsequently, Knoevenagel
condensation reactions with cyanoacetic acid in the presence of
piperidine gave the target dye TD1 in 62% yield. 2-(Chloromethyl)-3-
methylthiophene 5 was synthesized from 3-methylthiophene in the
presence of formaldehyde and hydrochloric acid, and then reacted
with triethyl phosphite to obtain intermediate 6. WittigeHorner
reaction of 6 and 4-(diphenylamino) benzaldehyde or 4-(di-p-tolyl-
amino)benzaldehyde give the corresponding intermediates 9, 10,
respectively. Then compounds 9, 10 were subjected to formylation
reaction in the presence of n-BuLi and DMF, affording aldehydes 11,
12 [42]. The final Knoevenagel condensation reactions of 11, 12
with cyanoacetic acid gave the target dyes TD2 and TD3 in the
Table 1
Maximum absorption and emission data of the as-synthesized dyes.a
Dye
labsa/nm (
3
/molꢀ1 L cmꢀ1
)
lmaxb/nm
lemc/nm
TD1
TD2
TD3
466(22 000)
517(23 000)
535(34 000)
308(21 000)
310(16 000)
311(18 000)
e
460
485
504
625
684
708
279(14 000)
280(18 000)
3
is the extinction coefficient at labs of maximum absorption.
a
Maximum absorption in CHCl3 solution (10ꢀ5 mol Lꢀ1).
b
Maximum absorption on TiO2 film.
c
Maximum emission of the dyes in CHCl3 solution (10ꢀ5 mol Lꢀ1).

Z. Tian et al. / Dyes and Pigments 87 (2010) 181e187
185
Table 2
Electrochemical data of the as-synthesized dyes.a
Dye
lint/nm
E0e0/V
Eox/V vs. NHE
Ered/V vs. NHE
Egap/V
TD1
TD2
TD3
553
605
625
2.24
2.04
1.98
1.19
1.06
1.01
ꢀ1.05
ꢀ0.98
ꢀ0.97
0.55
0.48
0.47
a
E0e0 values were calculated from intersection of the normalized absorption and
the emission spectra (lint): E0e0 ¼ 1240/lint. The first oxidation potential (vs. NHE),
Eox, was measured in acetonitrile and calibrated by addition of 0.63 V to the
potential versus Fc/Fcþ. The reduction potential, Ered, was calculated from Eox ꢀ E0e0
.
Egap is the energy gap between the Ered of dye and the CB level of TiO2 (ꢀ0.5 V vs.
NHE).
potential (vs. SCE) versus Fc/Fcþ by CV (Fig. 5). And the reduction
potential vs. NHE (Ered), which corresponds to the lowest unoccu-
pied molecular orbital (LUMO vs. NHE), can be obtained from the
first oxidation potential and the E0e0 value determined from the
intersection of absorption and emission spectra, namely, Eox ꢀ E0e0
.
Fig. 4. Absorption spectra of the dyes on TiO2 film.
Table 2 summarizes the electrochemical properties of the three as-
synthesized dyes.
To get an efficient charge separation, the LUMO of the dye has to
be sufficiently more negative than the conduction-band edge of the
TiO2 (Ecb, ꢀ0.50 V vs. NHE), and the HOMO has to be more positive
than the redox potential of iodine/iodide (0.42 V) to ensure the
regeneration of dyes [44]. As shown in Fig. 5, the HOMO levels of
TD1, TD2, and TD3 are 1.19, 1.06, and 1.01 V (vs. NHE), respectively.
All these HOMO levels are sufficiently more positive than the
iodine/iodide redox potential value, indicating that the oxidized
dyes formed after electron injection to TiO2 could accept electrons
from iodide ions thermodynamically. From these values, we can
ascribed to H-type aggregation of the dyes on the TiO2 surface [29].
According to the data collected in Table 1, the blue-shift values of
TD1, TD2 and TD3 are 6, 32, 31 nm, respectively. It can be seen that
TD2 and TD3 have a much higher blue-shift values compared to
TD1, which indicates that they have a stronger tendency to form
aggregation state on TiO2 surface. This stronger tendency of
aggregation should be attributed to the differences of electron
spacer parts of the dyes. In our early study, triphenylamine dyes
with a vinyl bridge unit in the
p-spacer part could present
considerable coplanar character [37]. It means that the coplanar
structure of TD2 and TD3 dyes, linked by vinyl unit between the
donor moiety and the spacer part, could lead to the stronger
tendency to form aggregation state on TiO2 surface comparing
with TD1.
find the introduction of the vinyl unit in the p-spacer part lead to
the comparatively higher-lying HOMO of the dyes, which decrease
the gap between the HOMO level and the redox potential of iodine/
iodide. Simultaneously, it seems that triphenylamine and di(p-
tolyl)phenylamine have little differences in affecting the HOMO
levels of these triphenylamine dyes. On the other hand, the LUMO
levels of these as-synthesized dyes range from ꢀ0.97 V to ꢀ1.05 V,
which are more negative than the conduction-band edge of the
TiO2. Therefore, the three dyes can be used as effective sensitizers
because the electron injection process from the excited dye mole-
cule to the conduction band of TiO2 is thermodynamically favorable
in DSSCs.
3.3. Electrochemical properties
To evaluate the thermodynamic possibility of electron transfer
from the excited dye molecule to the conductive band of TiO2, cyclic
voltammetry (CV) was performed. The first oxidation potential vs.
NHE (Eox), which corresponds to the highest occupied molecular
orbital (HOMO vs. NHE), was calibrated by addition of 0.63 V to the
3.4. Photovoltaic performance of DSSCs based on the dyes
The photovoltaic properties of the solar cells constructed from
these organic dye-sensitized TiO2 electrodes were measured under
simulated AM 1.5 irradiation (100 mW cmꢀ2). The open-circuit
photovoltage (Voc), short-circuit photocurrent density (Jsc), fill
factor (FF), and solar-to-electricity conversion efficiencies (h) are
compared with N719 and listed in Table 3. The incident photon-to-
current conversion efficiencies (IPCE) and current densityevoltage
Table 3
Photovoltaic performance of DSSCs based on the as-synthesized dyes and N719.a
Dye
Jsc (mA cmꢀ2
)
Voc (V)
FF
h (%)
TD1
TD2
TD3
N719
15.76
16.29
16.94
17.33
0.72
0.66
0.69
0.76
0.73
0.71
0.67
0.74
8.27
7.62
7.83
9.75
Light source: 100 mW cmꢀ2, AM 1.5 G simulated solar light; working area:
0.196 cm2; Titania paste was prepared from P25 (Degussa AG, Germany) and added
0.1% magnesium acetate solution; 1 mmol Lꢀ1 chenodeoxycholic acid (CDCA) was
added as a coadsorbent; Dye bath: CH3CN solution (5 ꢂ 10ꢀ4 mol Lꢀ1).
a
Fig. 5. Cyclic voltammograms of the three as-synthesized dyes: working electrode, Pt
ring; auxiliary electrode, Pt wires; reference electrode, Hg/Hg2Cl2; scanning rate is
100 mV sꢀ1. The ferrocene (Fc) is the internal standard.

186
Z. Tian et al. / Dyes and Pigments 87 (2010) 181e187
Fig. 8. JeV curves obtained with DSSCs based on TD1, TD2, and TD3 dyes under dark
condition.
Fig. 6. IPCE action spectra for DSSCs based on the three TD dye-sensitized transparent
TiO2 films with the electrolyte: 0.5 mol Lꢀ1 LiI, 0.05 mol Lꢀ1 I2 and 0.5 mol Lꢀ1 4-tert-
butylpyridine (TBP) in 3-methoxypropionitrile.
adsorbed on the TiO2 surface is necessary for efficient electron
injection from the excited dye to TiO2. The aggregation could lead
to unfavorable back-electron transfer, and decrease of the open-
circuit voltage as well as fill factor of the device [45,46]. To confirm
this judgment, a dark current test, a technique of testing the
capability of the charge recombination, was conducted and the
results are shown in Fig. 8 [47]. It is found that the onsets for TD2
and TD3 dyes are similar but evidently lower than TD1 dye.
This indicates that the back-electron-transfer process correspond-
ing to the reaction between the conduction-band electrons in the
TiO2 and Iꢀ3 in the electrolyte under dark conditions occurs more
easily in the DSSCs based on the dyes with vinyl units. The Jsc and
Voc values of TD3 are increased slightly in comparison with TD2,
this can be ascribed to the improvement of the electron-donating
ability by substitution of triphenylamine moiety for di(p-tolyl)
phenylamine moiety, and this improvement ultimately makes TD3
has a better photovoltaic performance than TD2.
(JeV) characteristics of devices based on the three TD dyes are
shown in Figs. 6 and 7, respectively.
As shown in Fig. 6, the DSSCs based on TD1 show higher IPCE
values in the spectra range of 350e495 nm than those of TD2 and
TD3, producing a maximum IPCE of w81% at 443 nm. Nevertheless,
the IPCE spectra for DSSCs based on TD2 and TD3 are broader
toward the red region than that of TD1, which is in accordance with
their absorption spectra on the TiO2 films (Fig. 4). According to the
data in Table 3,
h values of 8.27%; 7.62% and 7.83% were obtained by
DSSCs based on TD1, TD2 and TD3 dyes, respectively. It’s clear that
the Jsc values of TD2 and TD3 are higher than that of TD1, which is
in agreement with the IPCE results (Fig. 6) of the three dyes. The
TD1 shows narrower IPCE spectra and lower IPCE values in the red
region compared to TD2 and TD3, hence it have a lower Jsc value
than that of TD2 and TD3. However, the DSSCs based on TD1 show
higher solar-to-electricity conversion efficiency because the Voc
and FF values of TD1 are obviously higher than those of TD2 and
TD3. The lower performances of DSSCs based on TD2 and TD3
may result from the stronger aggregation of TD2 and TD3 on
the TiO2 surface due to the introduction of the vinyl unit into the
According to the foregoing results and analysis, it seems that the
vinyl unit in electron spacer is not a suitable constructional moiety
for this series of sensitizers to obtain higher
h values, although the
vinyl unit could increase the light harvesting capability. Under the
p-conjugated spacers. Generally, a monolayer of dye molecules
same conditions, the
h value of the DSSC based on the standard
N719 dye is 9.75% (Jsc ¼ 17.33 mA cmꢀ2, Voc ¼ 0.76 V, FF ¼ 0.74).
The result suggests that TD1 dye is a promising alternative to the
traditional metal-organic dyes used in DSSCs.
4. Conlusions
Three triphenylamine dyes based on low-cost methylthiophene
as the p-conjugated spacer were designed and synthesized as the
dye sensitizers for DSSCs applications. The high photovoltaic
performances of the DSSCs based on these as-synthesized dyes
were obtained. Though the introduction of vinyl unit in the
p-conjugated spacer can obtain red-shifted absorption spectra, it
does not give a positive effect on the photovoltaic performance of
the DSSCs due to unfavorable back-electron transfer and decrease
of the open-circuit voltage. On the basis of optimized conditions,
the DSSCs based on these three as-synthesized dyes exhibited the
efficiencies ranging from 7.83% to 8.27%, which reached 80e85%
with respect to that of an N719-based device. The high conversion
efficiency and easy availability of raw materials reveal that these
metal-free organic dyes are promising in the development of
DSSCs.
Fig. 7. Current densityevoltage (JeV) characteristics for DSSCs from the three TD dyes
under illumination of simulated solar light (AM 1.5, 100 mW cmꢀ2); The electrolyte
used was the same as that Shown in Fig. 6. The dye bath used was CH3CN.

Z. Tian et al. / Dyes and Pigments 87 (2010) 181e187
187
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Acknowledgments
This work was supported by the National Nature Science
Foundation of China (Project Nos. 50973092), Natural Science
Foundation of Hunan Province of China (Grant Nos. 09JJ3020 &
09JJ4005), Specialized Research Fund for the Doctoral Program of
Higher Education of China (No. 20094301120005), and Postdoctoral
Science Foundation of Xiangtan University (Project Nos.2008102).
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and dye-structures on the performance of dye-sensitized solar cells based on
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11023e33.
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