A star-shaped sensitizer based on thienylenevinylene for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2012.11.041

A novel star-shaped dye based on highly conjugated oligothienylenevinylene (nTV) bearing pendant solubilizing hexyl chains and end-capped with three tetramethylammonium cyanoacetates as anchoring groups has been synthetized. Its photovoltaic performance,

Full text of DOI:10.1016/j.tetlet.2012.11.041

Tetrahedron Letters 54 (2013) 431–435
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A star-shaped sensitizer based on thienylenevinylene for dye-sensitized
solar cells
Maxence Urbani a, Eva M. Barea b, , Roberto Trevisan b, Ana Aljarilla a, Pilar de la Cruz a, Juan Bisquert b,
⇑
Fernando Langa a,
⇑
a Instituto de Nanociencia, Nanotecnología y Materiales Moleculares (INAMOL), University of Castilla-La Mancha., Campus de la Fábrica de Armas, Toledo 45071, Spain
b Photovoltaic and Optoelectronic Devices Group, Physics Department, Universitat Jaume I, Castelló 12071, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel star-shaped dye based on highly conjugated oligothienylenevinylene (nTV) bearing pendant sol-
ubilizing hexyl chains and end-capped with three tetramethylammonium cyanoacetates as anchoring
groups has been synthetized. Its photovoltaic performance, in mesoporous TiO2 dye-sensitized solar cell,
has been tested under different experimental conditions. The best performances were obtained at longer
adsorption times of 4 h using different electrolyte compositions, reaching up to 3.11% of efficiency.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 21 September 2012
Revised 7 November 2012
Accepted 9 November 2012
Available online 23 November 2012
Keywords:
Oligothienylenevinylene
Dye-sensitized solar cell
Metal-free organic dye
Photovoltaic
The global environmental issues and growing energy demand
have greatly encouraged scientists and governments to explore
and develop renewable and environmentally friendly energy
sources. Solar photovoltaic (PV) electricity keeps growing and rep-
resents now the third source of the world-wide renewable energy
in terms of installed capacity after hydro and wind power.1 How-
ever, mainly due to the high-purity required for the inorganic sili-
con semiconductors in actual solar panels,2 this technology still
remains expensive in terms of price-per-KWh.
Alternative photovoltaic systems based on cheap materials and
production processes such as dye-sensitized solar cells (DSCs) ap-
pear to be a promising route to produce competitive and cost-
effective, new photovoltaic devices. DSCs were initially developed
based on ruthenium(II)-polypyridyl complexes (such as N3, N719,
or black dyes) as the active material3 adsorbed on nanocrystalline
TiO2 with validated conversion efficiencies up to 11% under simu-
lated AM 1.5G illumination (100 mW cmÀ2). Metal-free organic do-
nor–acceptor (D–A) dyes have attracted increasing attention due to
their low cost, low-impact environmental pollution (‘green dye’),
high and tunable molar absorption coefficients, and easy modifica-
tion of the molecular structures by design strategies.4–6 A porphy-
rin dye has recently achieved 11% solar-to-electric power
conversion efficiency at simulated 1 sun solar irradiation in combi-
nation with a Cobalt (II/III) redox couple, which is the best-efficient
free-metal organic DSC to date.7 This significant breakthrough has
strongly renewed the interest and perspectives for free-metal or-
ganic based DSC.
Conjugated oligomers are target compounds for many applica-
tions in material science8 due to their interesting optical, electrical,
and optoelectronic properties. One type is star-shaped conjugated
systems,9 in which identical arms are attached to a central core.
Three-armed star with a benzene core (propeller shaped)10 has
been widely used in material science as hole-blocking materials,
in OLED,11 liquid crystal,12 as active materials for organic electronic
devices such as BHJ solar cells13 or combined with fullerene in
studies of photoinduced electron transfer.14 Finally, star-shaped
systems substituted with carboxylate groups were used to produce
metal-organic frameworks (MOF’s).15
Recently we described new dyes for DSC where thienylenevin-
ylene oligomers (nTV) of different lengths act as donor16 or
bridge17 in D–
p–A structures for DSCs. For instance, we have re-
ported a linear 4-thienylenevynylene dye functionalized with
cyanoacrylic acid as anchoring group (Chart 1) and its performance
tested in mesoporous TiO2 DSCs.18
We report herein on the successful synthesis and characteriza-
tion of a star-shaped dye containing a benzene central core and
three arms, each one formed by four thienylenevinylene oligomer
units bearing pendant solubilizing hexyl chains and end-capped
with three tetramethylammonium cyanoacetates as anchoring
groups (Chart 2).
So far, most of the free-metal organic sensitizers previously
studied and tested in DSCs are based on D–A, linear ‘push–pull’
type dyes. To date, very few works report on proof-to-principle
⇑
Corresponding authors.
E-mail address: Fernando.Langa@uclm.es (F. Langa).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.041

432
M. Urbani et al. / Tetrahedron Letters 54 (2013) 431–435
to handle in common organic solvents, dye 5 was converted into
C6H13
C6H13
C6H13
C6H13
NC
its (tetramethylammonium) cyanoacetate salt 6. The structure of
dye 6, as well as its precursors 3 and 4, was confirmed by MAL-
DI-TOF mass spectrometry as well as 1H and 13C NMR, FT-IR, and
UV–vis spectroscopies (see SI.). The thermal stability of dye 6
was studied by thermogravimetric analysis (TGA) under nitrogen,
with a heating rate of 10 °C/min. Decomposition temperature
(Td) was estimated as the temperature that is the intercept of
the leading edge of the weight loss by the baseline of the TGA
scans. Dye 6 is thermally stable with decomposition temperature
higher than 378 °C (Fig. SI. 14).
COOH
S
S
S
S
C6H13
C6H13
C6H13
C6H13
Chart 1. Molecular structure of
previously described and tested in DSCs in our group.
a
linear 4-thienylenevinylene dye sensitizer
efficiency of multi-component, -light harvesting, and/or -anchor-
ing dyes used in free-metal organic based DSCs, together with their
structure shape relationship, such as star-shapes. To our knowl-
edge only one example of a star-shaped molecule has been inves-
tigated as a dye for DSCs.19 The advantage (or disadvantage) to use
these kinds of multi-component systems in DSCs, covalently
assembled at the supramolecular level, still remains unclear. For
example we need to tune the electrolyte so that electron injection
to the TiO2 conduction band is optimal while the bulky dye struc-
ture prevents recombination by maintaining the oxidized species
in the electrolyte away from the titania surface, so that the photo-
voltage is not reduced. These features are investigated below by
determination of the electrochemical, optical, and photovoltaic
properties of this new star-shaped dye and its performance in DSCs
under different experimental conditions.
The synthesis of dye 6 is depicted in Scheme 1. Tris(phospho-
nate) 120 and 4TV-CHO21 2 were prepared according to the proce-
dure previously described in the literature. The first step involves
the reaction of phosphonate 1 and 4TV-CHO 2 in THF with an ex-
cess of t-BuOK under Wittig–Horner conditions, affording triad 3
in 84% of yield. Subsequently, 3 was formylated via a Vilsmeyer–
Haack reaction in the presence of an excess of POCl3/DMF at reflux
in DCE overnight, affording (tris)aldehyde 4 in a 71% yield. The final
step was a Knoevenagel condensation with cyanoacetic acid in the
presence of a catalytic amount of piperidine to convert tris–alde-
hyde 4 into tris–cyanoacrylic acid 5. Due to its dual amphipathic
behavior, a strong lipophilic central core induced by numerous side
alkyl chains of 4TV skeletons and the hydrophylic behavior in-
duced by the three surrounding cyanoacrylic acid polar tails, dye
5 has a strong tendency to precipitate in organic solvents such as
CH2Cl2. In order to increase its solubility and make it more easy
The UV–visible spectra of dyes 3, 4, and 6 in CH2Cl2 are shown
in Figure 1. They display the characteristic band of 4TV centered at
around 545 nm. The maximum of absorption of dye 6 (549 nm,
loge = 4.78) is slightly red-shifted by 11 nm compared to com-
pounds 3 and 4 (538 nm). This can be explained by the increase
of conjugation induced by the terminal cyanoacetate groups. More-
over, it can be seen a broadening of the 4TV band in dye 6, which is
accompanied with a decrease of its extinction molar coefficient
compared to 3 and 4 which suggests the tendency of dye 6 to form
aggregates in organic solution. It is interesting to note that the
absorption of 6 covers up to 700 nm, facilitating the absorption
in this region of the solar emission. Utilization of materials that ab-
sorb light extending into the infrared range is one of the strategies
to harvest more solar light and thereby to increase power-effi-
ciency.22 The high molar extinction coefficient absorption bands
are as well in favor of light harvesting and hence photocurrent
generation.
The fluorescence emission spectroscopy was employed to get
information about the excited state-solvent interactions. The emis-
sion maxima of dye 6 can be found at 750 nm (kexc = 549 nm)
(Fig. SI.13). The optical band gap for dye 6 is 1.96 eV, and it was cal-
culated by the intersection of absorption and emission spectra. The
large Stokes shift of dye 6 (201 nm) can be attributed to the signif-
icant structural reorganization upon photoexcitation or to the
charge transfer nature of the excited state. Upon adsorption onto
TiO2 the emission was totally quenched indicating an efficient pho-
toinduced electron transfer process from dye 6 to the TiO2
nanoparticles.
The redox properties of dye 6 were investigated through Cyclic
Voltammetry (CV) and Osteryoung Square Wave Voltammetry
N
COO
CN
H13C6
C6H13
S
C6H13
C6H13
S
H13C6
H13C6
S
C6H13
C6H13
S
C6H13
C6H13
S
H13C6
S
C6H13
S
C6H13
S
S
H13C6
H13C6
S
S
C6H13
C6H13
C6H13
S
H13C6
C6H13
CN
H13C6
N
H13C6
OOC
C6H13
N
H13C6
NC COO
Chart 2. Molecular structure of the novel star-shaped sensitizer based on 4-thienylenevinylene studied in this work (dye 6).

M. Urbani et al. / Tetrahedron Letters 54 (2013) 431–435
433
R
H13C6
S
PO(OEt)2
H13C6
C6H13
(EtO)2OP
S
PO(OEt)2
1
C6H13
H13C6
+
S
H13C6
CHO
C6H13
C6H13
C6H13
(i)
S
S
C6H13
H13C6
H13C6
C6H13
S
C6H13
S
H13C6
S
S
C6H13
C6H13
C6H13
C6H13
S
S
H13C6
H13C6
C6H13
C6H13
S
C6H13
S
H13C6
S
H13C6
H13C6
C6H13
3
4
; R = H
S
S
H13C6
(ii)
C6H13
R
R
C6H13
H13C6
2
; R = CHO
(iii)
(iv)
5
; R =
COOH
NC
COO
6
; R =
N
NC
Scheme 1. Synthesis of dye 6. Reagents and conditions: (i) t-BuOK, THF, 0° ? 20 °C, 3 h (84%), (ii) DMF, POCl3, DCE, reflux overnight (71%), (iii) Cyanoacetic acid, piperidine,
CHCl3, reflux 24 h (83%), (iv) N(CH3)4OH/H2O (quantitative).
determined as À5.28 eV. In the cathodic side, a reduction process
is observed at À1.96 V which we attribute to the cyanoacrylic moi-
eties (Fig. SI.16). The LUMO value was determined as À3.14 eV.
To gain insight into the geometry and the electronic structures
of the frontier orbitals, the more stable conformation and the elec-
tron density distributions of HOMO and LUMO have been calcu-
lated using DFT on B3LYP 6-31G level in the gas phase with
GAUSSIAN 03W. DFT optimized gas-phase geometries of 5 have a to-
tally planar backbone (Fig. 2) with a HOMO well delocalized over
the molecules (Fig. SI.17). LUMO coefficients are mostly concen-
trated on the cyanoacrylic groups.
Theoretical calculations suggest that each arm in the dye be-
haves as an isolated chromophore with little electronic interaction
between the arms as the occupied orbitals, the HOMO, HOMOÀ1,
and HOMOÀ2, are threefold degenerated as well as the unoccupied
orbitals (LUMO, LUMO+1, LUMO+2). The electronic distribution of
compound 5 should be a sum of all these orbitals.
1.00
0.75
0.50
0.25
0.00
6
4
3
300
400
500
600
700
800
Wavelenght (nm)
Figure 1. Normalized UV–visible spectra of dyes 3, 4, and 6 in dichloromethane.
(OSWV) at room temperature in o-dichlorobenzene:acetonitrile
(4:1). Dye 6 undergoes a broad oxidation process (Fig. SI.15); the
deconvolution of this broad process showed two stable and revers-
ible (according to CV) radical cations, at 0.18 and 0.32 V (vs Fc/Fc+,
Fig. SI.15) assigned to the triple oxidation of the three 4TV arms of
the dye by comparison with a sample of 4TV-COOH. The HOMO va-
lue (calculated with respect to ferrocene, HOMO: À5.1 eV)23 was
Figure 2. B3LYP/3-21G optimized structure of compound 5.

434
M. Urbani et al. / Tetrahedron Letters 54 (2013) 431–435
Table 1
14
Photovoltaic performance obtained for dye 6
a
Electrolyte
Adsorption time (h)
Voc (V)
Jb (mA/cm2)
FFc
gd
12
10
8
E5
E5
E5
1
2
4
0.42
0.45
0.47
5.83
9.24
11.50
0.60
0.58
0.58
1.48
2.43
3.11e
F1
F1
F1
1
2
4
0.60
0.61
0.64
0.59
0.67
0.78
0.77
0.76
0.78
0.27
0.31
0.39
N7
N7
N7
1
2
4
0.54
0.56
0.44
4.65
5.06
11.60
0.68
0.71
0.60
1.70
2.00
3.05e
6
a
b
c
Open circuit potential voltage.
Short circuit photocurrent density.
Fill factor.
Power conversion efficiency.
Best efficiences.
4
d
e
N7-1
N7-2
N7-4
2
0
0.0
-0.2
-0.4
-0.6
14
12
10
8
Figure 4. Current density-potential curves for DSCs at
1
sun with dye 6 as
sensitizers after 1, 2, 4 h of adsorption using N7 as the electrolyte. (À1, À2, À4 are
the adsorption times).
Absorption time, longer than 4 h, gives as a result a strong decrease
in the performance (data not shown), probably due to strong
aggregation problems. Even the use of a co-adsorbent (chenodeox-
ycholic acid) does not improve the performance of the cells,
obtaining similar results. Both electrolytes are quite similar, con-
taining N7 1-methylbenzimidazole (MBII) that keep the titania
conduction band lightly shift up, obtaining around 0.54–0.56 V cir-
cuit voltage (Voc) compared with the voltage values obtained for E5
electrolyte without MBII. For all the cells the efficiency increases
with longer absorption time (been four h the optimum time).
Surprisingly, extremely poor efficiencies (0.27–0.39%) were ob-
tained with F1 electrolyte (without LiI, and containing ionic liquid
and different solvents). We explain this result by a strong upward
displacement of the TiO2 conduction band respecting that in the
case of electrolyte F1, due to the ionic liquid and the lack of LiI,
which strongly disfavors the electron injection from the LUMO of
dye 6 and consequently very low values are obtained for Jsc.24 This
is also reflected by the higher Voc values obtained with F1 electro-
lyte (Voc ꢀÀ0.6 V) than with N7 and E5 (Voc ꢀÀ0.5 and À0.4 V
respectively).
In conclusion, we have successfully synthesized a star-shaped
dye containing a benzene central core and three arms, each one
formed by four thienylenevinylene oligomer units bearing pendant
solubilizing hexyl chains and end-capped with three tetramethyl-
ammonium cyanoacetates as anchoring groups. Its optical and
electrochemical properties were studied and a DSC device was
manufactured; the overall conversion of this device appeared to
be strongly dependent on the electrolyte used. The highest values
of photocurrent and consequently the highest efficiencies, were
obtained using E5 and N7 as electrolytes with a maximum of
3.11%.
6
4
E5-1
2
E5-2
E5-4
0
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
V(V)
Figure 3. Current density-potential curves for DSCs at
1 sun with dye 6 as
sensitizers after 1, 2, 4 h of adsorption using E5 as the electrolyte. (À1, À2, À4 are
the adsorption times).
The photovoltaic performance of solar cells based on dye 6 un-
der AM 1.5G illumination (100 mW cmÀ1), was tested using three
different electrolyte compositions (named E5, F1 and N7) and vary-
ing the adsorption time (1, 2 and 4 h). The results, obtained from an
average of five samples per experiment, are summarized in Table 1
and the J–V curves are shown in Figures 3 and 4 and Figure SI.18.
The composition of the different electrolytes used are: (1) Electro-
lyte E5: [LiI] = 0.5 M, [I2] = 0.05 M in 3-methoxypropionitrile. (2)
Electrolyte F1: [guanidine thiocyanate] = 0.1 M, [I2] = 0.03 M, [1-
butyl-3-methylimidazolium iodide] = 0.6 M, and [4-tert-butylpyri-
dine] = 0.5 M in acetonitrile:valeronitrile (85:15) v/v. (3) Electro-
lyte
N7:
[LiI] = 0.5 M,
[I2] = 0.05 M,
and
[1-
methylbenzimidazole] = 0.5 M in 3-methoxypropionitrile. The rea-
son to adopt a variety of electrolytes is to modify the conduction
band and the surface properties of titania in order to obtain opti-
mal injection and photovoltaic parameters for this specific dye.
In all our experiments the same trend has been observed for the
three electrolytes used: longer adsorption times result in a strong
increase of the current density and therefore of the overall effi-
ciency of the cells, due to large amount of adsorbed dye. The best
efficiencies were obtained after four hours of adsorption time using
E5 and N7 as electrolytes, reaching 3.11 and 3.05%, respectively.
Acknowledgments
Financial support from the Ministerio de Ciencia y Tecnología of
Spain (Projects and Consolider Project HOPE-CSD2007-00007,
MAT2010-19827 and CTQ2010-1749), JCCM and FEDER funds (Pro-
ject POII09-0078-923) is gratefully acknowledged. UJI work is also
supported by the ‘Institute of Nanotechnologies for Clean Energies’,
funded by the Generalitat Valenciana under project ISIC/2012/008
and PROMETEO/2009/058.

M. Urbani et al. / Tetrahedron Letters 54 (2013) 431–435
435
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Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
11.041.
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