Effect of the bridge substitution on the efficiency of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2012.09.091

We have designed and synthesized a novel D-π-A dye based on zinc porphyrin as the donor, cyanoacrylic acid as the acceptor and trans-bis(3,4-ethylenedioxythiophene)vinylene as the bridge. Under standard global AM 1.5 solar conditions, the Zn-porphyrin-2-EDOT sensitizer based cell gave a J_SC of 7.28 mA cm^-2, V_OC of 0.438 V was obtained with a FF of 0.514, corresponding to an overall conversion efficiency η = 1.64%. The reasons for this low efficiency are analyzed to try to develop knowledge for future dye designing.

Full text of DOI:10.1016/j.tetlet.2012.09.091

Tetrahedron Letters 53 (2012) 6665–6669
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effect of the bridge substitution on the efficiency of dye-sensitized solar cells
Beatriz Pelado a, Pilar de la Cruz a, Victoria González-Pedro b, Eva M. Barea b, , Fernando Langa a,
⇑
⇑
a Instituto de Nanociencia, Nanotecnología y Materiales Moleculares (INAMOL), Universidad de Castilla—La Mancha, Campus de la Antigua Fábrica de Armas, Avda, Carlos III,
s/n 45071, Toledo, Spain
b Photovoltaic and Optoelectronic Devices Group, Physics Department, Universitat Jaume I, 12071 Castelló, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2012
Revised 19 September 2012
Accepted 21 September 2012
Available online 28 September 2012
We have designed and synthesized a novel D–p–A dye based on zinc porphyrin as the donor, cyanoacrylic
acid as the acceptor and trans-bis(3,4-ethylenedioxythiophene)vinylene as the bridge. Under standard
global AM 1.5 solar conditions, the Zn-porphyrin-2-EDOT sensitizer based cell gave a JSC of 7.28 mA cmꢀ2
VOC of 0.438 V was obtained with a FF of 0.514, corresponding to an overall conversion efficiency
= 1.64%. The reasons for this low efficiency are analyzed to try to develop knowledge for future dye
designing.
,
g
Keywords:
EDOT
Ó 2012 Elsevier Ltd. All rights reserved.
Porphyrin
Dye
DSSCs
Dye-sensitized solar cells (DSCs) based on nanocrystalline TiO2
electrodes have received considerable attention because of high
incident solar light-to-electricity conversion efficiency and low
cost of production.1 Recently, much research has focused on the
development of new metal-free organic dyes as a cheap alternative
to Ru dyes as the feasible modification of their structure, the large
molar extinction coefficients and moreover, their electrochemical
and photophysical properties can be tunable by synthesis. In this
sense, porphyrins are one of the most widely studied sensitizers
for DSCs because of their strong Soret (400–450 nm) and moderate
Q bands (550–600 nm). Elongation of the
p conjugation and loss of
symmetry in porphyrins cause broadening and a red shift of the
absorption bands together with an increasing intensity of the Q
bands relative to that of the Soret band. On the basis of this strat-
egy, the cell performance of porphyrin-sensitized solar cells has
been improved intensively2 and efficiencies up to 12.7% have been
achieved.3
As part of our research on developing new organic sensitizers,4
we recently described a new dye 1 (Zn-Por-2TV-COOH, Chart 1),5
carrying a Zn-porphyrin conjugated to a thienylenevinylene oligo-
mer (TV) carrying alkyl chains. The solar cell prepared with dye 1
Chart 1. Structures of dyes 1 and 2.
provided
a
threefold enhancement of the photocurrent
systems, due to a unique combination of strong electron donor
properties and self-structuring effects.6 EDOT unit has been used
for the design of polymers with low band gap or specific electro-
chemical properties and PEDOT occupies a prominent position
among conducting polymers due, among other things, to the
multiple well-established technological applications of its various
conducting forms.7 Finally, in the last years, EDOT has been
(g
= 4.77%) with respect to the parent dyes (Zn-Por-COOH and
2TV-COOH), as a consequence of the additional strong light absorp-
tion in the region of 400–650 nm leading to a IPCE of 60%.
3,4-ethylenethiophene (EDOT) has been widely studied as a
building block for the synthesis of functional
p-conjugated
employed as a spacer in D–
p–A push–pull systems, some of them
⇑
Corresponding authors. Tel.: +34 925 268 843; fax: +34 925 268 840 (F.L.).
E-mail address: fernando.langa@uclm.es (F. Langa).
applied in the development of DSCs.8
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.091

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B. Pelado et al. / Tetrahedron Letters 53 (2012) 6665–6669
Herein, we describe the synthesis of the new D–p–A dye 2
(Chart 1), in which, different to dye 1, the 3,4-dihexylthiophene
rings were substituted by 3,4-ethylenedioxythiophene (EDOT).
Their optical and electrochemical properties were studied and,
additionally, dye 2 has been tested in DSC.
The synthesis of dye 2 has been accomplished according to
Scheme 1. Phosphonate 3 was prepared by the quantitative reduc-
tion of 3,4-ethylenedioxythiophenecarbaldehyde9 with NaBH4 fol-
lowed by in situ reaction with triethylphosphite in the presence of
iodine. After removing in vacuo the excess of triethylphosphite, 3
was purified by column chromatography (silica gel, hexane:AcOEt,
1:1) and obtained in 95% yield, improving significantly the de-
scribed procedure.10 Horner–Wadsworth–Emmons (HWE) reaction
between 3 and 3,4-ethylenedioxythiophenecarbaldehyde, yielded
trans-bis-(3,4-ethylene-dioxythiophene)-vinylene (4) as a yellow
solid in 88% yield, showing that to be a better procedure than the
method previously described (49%) from a Grignard reagent.11
Double Vilsmeier formylation reaction of 4 afforded bisaldehyde
5, in quantitative yield. The crucial step was the HWE olefination
between the corresponding phosphonate-Zn-porphyrin12 and 4
under careful stoichiometry control giving monoaldehyde 6 in
42% yield as a red solid (mp >300 °C), purified by column chroma-
tography (silica gel, hexane:dichloromethane, 3:7). The measured
coupling constant (J) values in the 1H NMR spectrum of 6 clearly
show the E geometry of both, the double bond between the por-
phyrin ring and the thiophene (J = 16 Hz) and the double bond be-
tween both thiophene rings (J = 16 Hz). Finally, Knoevenagel
condensation of 6 with cyanoacetic acid in the presence of piperi-
dine yielded, after purification by column chromatography (chloro-
form:methanol 9:1) dye 2 as a purple solid (mp >300 °C) in 74%
yield. All new compounds were fully characterized by MALDI-
TOF mass spectrometry as well as 1H and 13C NMR, FT-IR, and
UV–Vis spectroscopies.13
Figure 1. UV–Vis spectrum of dye 2 (in dichloromethane).
characteristic of the porphyrin at 422 nm (loge = 4.99) and a con-
tinuous absorption in the visible region, up to 650 nm, due to the
presence of the oligothienylenevinylene moiety, with maxima cor-
responding to the Q bands of the porphyrin at 552 nm (log
and 595 nm (log = 4.09).
e = 4.32)
e
Figure 2 shows the steady-state fluorescence spectra of dye 2 in
dichloromethane (solid line) exciting at the Soret band of the por-
phyrin (kexc = 422 nm) and the typical porphyrin emission at
603 nm and 649 nm is observed. However, in the presence of
TiO2 (dotted line) the fluorescence of the porphyrin is totally
quenched indicating efficient electron injection from the sensitizer
into the conduction band of the TiO2 semiconductor through the
conjugated bridge; a similar quenching is observed when excited
at 554 nm wavelength where the absorption of the oligothienyl-
enevinylene bridge is predominant.
The optical and electrochemical properties of dye 2 have been
analyzed by UV–Vis absorption spectroscopy, fluorescence emis-
sion spectroscopy and cyclic (CV) and Osteryoung (OSWV)
voltammetries.
The UV–Vis absorption spectrum of dye 2, recorded in dichloro-
methane (Fig. 1 and Table 1), exhibits an intense Soret band
The electrochemical properties of dye 2 were investigated in
solution by cyclic and Osteryoung square wave voltammetries at
Scheme 1. Synthesis of dye 2.

B. Pelado et al. / Tetrahedron Letters 53 (2012) 6665–6669
6667
Table 1
Data of UV–Vis and fluorescence emission and electrochemical data (OSWV, V vs Fc/Fc+) for dyes 1, 2, and precursors 3 and ZnPor-Br
a
a,b
loga
(e)
kem (nm)
E1 (V)
oxc
E2 (V)
oxc
E3 (V)
oxc
EHOMO (eV)
d
kmax (nm)
1
2
4
293, 422, 553
292, 423, 552, 595
361
4.6, 5.4, 4.7
4.08, 4.99, 4.32, 4.09
4.47
602, 648, 705
603, 649, 712
422
0.25
0.24
0.28
0.26
0.45
0.37
0.61
0.66
0.65
0.63
—
ꢀ5.08
ꢀ5.04
ꢀ5.08
ꢀ5.06
ZnPor-Br
421, 550, 589
—
a
b
c
10ꢀ5 M, in dichloromethane.
10ꢀ5 M, in dichloromethane, kexc = 422 nm.
[10ꢀ3 M] in o-DCB-acetonitrile (4:1), Ag/AgNO3 (0.01 M in CH3CN) electrode was used as reference and checked against the ferrocene/ferrocenium coupled (Fc/Fc+), glassy
carbon, Pt counter electrode, 20 °C, 0.1 M Bu4NClO4, scan rate = 100 mV sꢀ1
.
d
Calculated respecting to ferrocene, EHOMO: ꢀ4.8 eV.
Under our experimental conditions, dye 2 undergoes two
reversible oxidation processes (see Fig. S13) at 0.24 and 0.63 V cen-
tered on the porphyrin moiety as revealed by the comparison with
model compound ZnPor-Br (0.26 and 0.66 V, see Table 1). An addi-
tional non-reversible oxidation is also observed at 0.37 V. The lat-
ter is centered on the bis-3,4-ethylenedioxythienylvinylene bridge.
In model compound 4 this oxidation process appears at 0.28 V. The
observed shift can be explained by the strong acceptor character of
the cyanoacrylic moiety attached to conjugated oligomer in com-
pound 2. Importantly, the HOMO orbital of dye 2 is located on
the ZnPorphyrin moiety showing that the compound posseses
the requested oxidation potential for an efficient photoinduced
electron transfer from the porphyrin to the semiconductor through
the conjugated bridge as reported for compound 1. In addition, the
first oxidation of 2 is perfectly reversible indicating the formation
of a stable radical cation. This is an important issue for the photo-
voltaic applications. In the reduction side, the voltammogram of
dye 2 showed two reduction processes, at ꢀ1.50 eV (reversible)
is attributable to the EDOT fragment (by comparison to that of
4); and another reversible wave, at ꢀ1.82 eV, corresponding to
the model ZnPor-Br, were observed.
Figure 2. Emission spectra of dye
absence () and presence (—) of TiO2.
2 (dichloromethane, kexc = 422 nm) in the
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 gas phase with Gaussian
03 W.14
room temperature in o-dichlorobenzene–acetonitrile (4:1) as the
solvent containing 0.1 M tetrabutylammonium perchlorate as the
supporting electrolite (Table 1). The comparison with those of
dye 15 and the model compounds 4 and Zn-Por-CH2-Br is shown
in Table 1.
Figure 3. Optimized structure by DFT on B3LYP 6-31G of dye 2.

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B. Pelado et al. / Tetrahedron Letters 53 (2012) 6665–6669
-2.12 eV
-2.77 eV
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
ΔE = 2.18 eV
-4.95 eV
-5.17 eV
Figure 5. Density current versus voltage response for dye 2 under 100 mW cmꢀ2
sun simulation.
Figure 4. Theoretical energy levels and electron density distribution of the frontier
orbital of dye 2.
hand, the alkyl chains, besides affording a better solubility to the
dye, should better prevent the aggregation of the dye (it is known
that the aggregation during the TiO2 sensitization time decreases
the injection of electrons and the overall performance of the
DSC).17 On the other hand, the non polar alkyl chains should pre-
vent, more than the polar and with lower size (3,4-ethylenedioxyl
moiety), the close proximity of the electrolyte to TiO2 surface
making more difficult the charge recombination between the
semiconductor and the electrolyte. Also the hydrophobic chains
block the possible interaction between dye cation molecule and io-
dine, preventing the electron back recombination process at the
titania/dye/electrolyte interface given better DSC performance.
In summary, we have successfully synthesized a novel dye with
Zn-porphyrin as the donor, bis-(3,4-ethylene-dioxythiophene)-
vinylene as the bridge and cyanocrylic acid as the acceptor. Its
optical and electrochemical properties were studied. A DSC device
was fabricated and under standard global AM 1.5 solar conditions,
the cell gave a Jsc of 7.28 mA cmꢀ2, Voc of 0.438 V was obtained
with a FF of 0.514, corresponding to an overall conversion
The phenyl ring of the porphyrin unit and the EDOT bridge are
almost in a coplanar conformation (calculated dihedral angle is
0.25°). Nevertheless, the electron donor fragment is in an orthogo-
nal conformation, the dihedral angle between the porphyrin and
the p-conjugated bridge is over 89.12° (Fig. 3).
The BLA (bond length alternation)15 value was calculated for the
dye 2 according with the theoretical bond distances. The small va-
0
lue of BLA (0.05 ÅA) suggests a partially quinoid character and a pos-
sible zwitterionic contribution to the ground state.16
Figure 4 shows the energy levels and the electronic distribution
of the HOMOꢀ1, HOMO, LUMO, and LUMO+1. The electron density
of the HOMO is located along the oligomeric fragment and the
HOMOꢀ1 (E = ꢀ5.17 eV), close in energy to the HOMO
(E = ꢀ4.95 eV), is located in the porphyrin. This fact is in sharp con-
trast with the experimental results obtained by electrochemistry,
where HOMO is located in the porphyrin fragment and the
HOMOꢀ1 in the oligomeric fragment; this difference can be ex-
plained by the fact that theoretical calculations were performed
in vacuo and the electrochemical analyses were carried out in solu-
tion. On the other hand, the LUMO (E = ꢀ2.77 eV) is located over
the cyanoacrylic unit and the oligomeric system, suggesting the
possibility of charge transfer from the donor (EDOT) to the accep-
tor (cyanoacrylate) units. The calculated LUMO energy levels lie
above the conduction band edge of TiO2 (ꢁꢀ4.0 eV vs vacuum),
which should ensure sufficient driving force for electron injection
from the dye into the conduction band of the semiconductor. Final-
ly, the calculated HOMO–LUMO gap was evaluated in 2.18 eV, sim-
ilar to that found for dye 1.
efficiency
g = 1.64% lower to that found for a similar dye with a
different substitution in the bridge. The importance of the substitu-
tion in the bridge was evaluated.
Acknowledgments
Financial support from the Ministerio de Economia y Competi-
tividad of Spain (Projects CTQ2010-1749, MAT2010-19827 and
Consolider Project HOPE-CSD2007-00007), JCCM and FEDER funds
(Project POII09-0078-923) is gratefully acknowledged. One of us,
B.P. thanks to the MICINN for a FPI grant. UJI work is also supported
by the ‘Institute of Nanotechnologies for Clean Energies’, funded by
the Generalitat Valenciana under project ISIC/2012/008 and PRO-
METEO/2009/058.
The performance of the cell fabricated with dye 2 was measured
under standard conditions (100 mW cmꢀ2, 1.5 AM G) a current of
Jsc = 7.28 mA cmꢀ2 and a voltage of Voc = 0.438 V were obtained
with a fill factor (FF) of 0.514 (Fig. 5).
These values afford an efficiency of
previously obtained for dye 1 (Jsc = 15.6 mA cmꢀ2, Voc = 0.63 V,
FF = 0.49 and = 4.77%) with a similar structure but carrying hexyl
g = 1.64%, lower than that
g
Supplementary data
chains instead of 3,4-ethylenedioxyl substituent in the thiophene
rings. Both, the voltage and, particularly, the current are lower
for dye 2 than for dye 1. The better behavior as sensitizer of dye
1, than that of dye 2, can be attributed to the presence of the alkyl
chains (1) instead of the 3,4-ethylenedioxyl bridge (2). On one
Supplementary data (detailed experimental procedures, synthe-
sis, and characterization of all compounds) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2012.09.091.

B. Pelado et al. / Tetrahedron Letters 53 (2012) 6665–6669
6669
J = 4.6 Hz), 8.72 (d, 2H, J = 4.6 Hz), 8.68–8.66 (m, 4H), 8.18 (d, 2H, J = 8.1 Hz),
7.83 (d, 2H, J = 8.1 Hz), 7.46 (d, 1H, J = 16 Hz), 7.27 (bs, 7H), 7.23 (d, 1H,
J = 15.6 Hz), 7.19 (d, 1H, J = 15.6 Hz), 4.38 (m, 8H), 2.59 (m, 9H), 1.82–1.80 (m,
18H); 13C NMR (CD2Cl2, 100 MHz): 179.1, 149.9, 149.8, 149.7, 142.3, 141.0,
139.8, 139.3, 139.1, 138.9, 138.2, 137.4, 136.2, 134.9, 132.0, 131.1, 131.0, 130.6,
127.6, 127.2, 124.5, 121.1, 119.7, 118.8, 118.6, 118.1, 117.9, 115.2, 114.9, 65.4,
65.1, 64.9, 64.6, 30.9, 29.7, 21.8, 21.7, 21.5, 14.1; UV–Vis (CH2Cl2), kmax (nm)
References and notes
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(log
1645, 1440, 1079, 993; MS (m/z) (MALDI-TOF): 1162.47; calculated for
70H58N4O5S2Zn: 1162.31. Compound 2: 1H NMR (C6D6, 400 MHz) d: 9.11 (d,
e): 422 (5.30); 478 (4.61); 550.5 (4.27); FT-IR (KBr) m
/cmꢀ1: 2980, 2914,
C
2H, J = 4.4 Hz), 9.00 (d, 2H, J = 4.4 Hz), 8.93 (d, 2H, J = 4.4 Hz), 8.89 (d, 2H,
J = 4.4 Hz), 8.79 (bs,1H), 8.03 (d, 2H, J = 16 Hz), 7.57 (d, 1H, J = 16 Hz), 7.51 (d,
1H, J = 16 Hz), 7.46 (d, 2H, J = 16 Hz), 7.26 (d, 1H, J = 16 Hz), 7.23 (d, 1H,
J = 16 Hz), 7.17 (s, 6H), 4.39 (bs, 8H), 3.43–3.37 (m, 8H), 2.41–2.39 (m,9H), 1.96
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13. Selected spectroscopic data: 5: 1H NMR (CDCl3, 400 MHz) d: 9.91 (s, 2H), 7.14 (s,
2H), 4.37 (m, 8H); 13C NMR (CDCl3, 100 MHz): 179.5, 148.4, 139.7, 126.7, 120.0,
116.6, 65.3, 64.7; UV–Vis (CH2Cl2), kmax (nm) (log
FT-IR (ATR)
/cmꢀ1: 1635, 1508, 1454, 1363, 1288, 1269, 1215, 1115, 1076,
955, 931, 845, 675; MS (m/z) (MALDI-TOF): 364.86 (M+); calculated for
16H12O6S2: 364.00. 6: 1H NMR (CD2Cl2, 400 MHz) d: 9.81 (s, 1H), 8.90 (d, 2H,
e): 427 (4.63), 451.5 (4.61);
m
C