Organic dyes incorporating oligothienylenevinylene for efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/ol302738k

Two new organic dyes incorporating triphenylamine as a donor and oligothienylenevinylene as a bridge have been synthesized. The new dyes cover the entire visible region and have a power conversion of up to 6.25%.

Full text of DOI:10.1021/ol302738k

ORGANIC
LETTERS
2012
Vol. 14, No. 22
5732–5735
Organic Dyes Incorporating
Oligothienylenevinylene for Efficient
Dye-Sensitized Solar Cells
†
†
†
,‡
ꢀ
Ana Aljarilla, Leticia Lopez-Arroyo, Pilar de la Cruz, Frederic Oswald,*
Toby B. Meyer,‡ and Fernando Langa*,†
´
Instituto de Nanociencia, Nanotecnologıa y Materiales Moleculares (INAMOL),
Universidad de Castilla-La Mancha, 45071-Toledo, Spain, and Solaronix SA,
Rue de l’Ouriette 129, 1170 Aubonne, Switzerland
Fernando.Langa@uclm.es; frederic.oswald@solaronix.com
Received October 4, 2012
ABSTRACT
Two new organic dyes incorporating triphenylamine as a donor and oligothienylenevinylene as a bridge have been synthesized. The new dyes
cover the entire visible region and have a power conversion of up to 6.25%.
Dye-sensitized solar cells (DSCs) represent one of the
mostpromising and environmentallyfriendlyphotovoltaic
devices because of their low material cost, flexibility and
ease of manufacture.1 The best efficiencies have been
recorded on ruthenium complexes (validated energy con-
version efficiencies up to11.7%), but the shortage and high
cost of ruthenium metal hampers their commercial applic-
ability. Furthermore, the low molar extinction coefficients
of these dyes have led to great efforts to substitute these
sensitizers with metal-free organic dyes based on a donorꢀ
acceptor (DꢀA) system;2 with the aim of preventing
charge recombination, a π-bridge can be introduced be-
tween the donor and the acceptor (D-π-A).3 In this respect,
many D-π-A-substituted organic dyes have been prepared
and used as sensitizers in DSCs.4 These dyes have large
molar extinction coefficients due to the allowed πꢀπ*
transitions, and they give rise to high conversion efficiency
(around 10%).5
Triphenylamine (TPA) has been widely investigated
as an electron donor in DSC devices4b because of its
electron-donating capabilities6 and because it stabilizes
(4) (a) Ooyama, Y.; Harima, Y. Eur. J. Org. Chem. 2009, 2903. (b)
Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Petterson, H. Chem. Rev.
2010, 110, 6595.
† Universidad de Castilla-La Mancha
‡ Solaronix SA
(5) (a) Zhang, M.; Cao, Y.; Bai, Y.; Wang, Y.; Shi, Y.; Zhang, M.;
Wang, F.; Pan, C.; Wang, P. Chem. Mater. 2010, 22, 1915. (b) Chiba, Y.;
Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl.
Phys., Part 2 2006,45, L638. (c)Bessho, T.;Zakeeruddin, S. M.;Yeh, C.-Y.;
€
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(1) (a) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (b) Gratzel,
M. Acc. Chem. Res. 2009, 42, 1788.
€
(2) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int. Ed.
2009, 48, 2474.
(3) Clifford, J. N.; Martinez-Ferrero, E.; Viterisi, A.; Palomares, E.
€
Diau, E. W.-G.; Gratzel, M. Angew. Chem., Int. Ed. 2010, 49, 6646.
(6) (a) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953. (b)
Ning, Z.; Tian, H. Chem. Commun. 2009, 5483.
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r
10.1021/ol302738k
Published on Web 11/07/2012
2012 American Chemical Society

the separated hole generated from the exciton, thus im-
proving hole-transport.7 Regarding the bridge, in recent
years we have developed a series of sensitizers in which
oligothienylenevinylenes (oTVs)8 with different lengths
were employed as donors9 or bridges10 between the donor
and the acceptor. These units were chosen because of the
excellent wire behavior of oTVs.11
bromide derivatives and 5-formylthiophene-2-yl-2-boro-
nic acid in 75% yield in both cases. Subsequent reduction
of the corresponding aldehydes 2a,b with NaBH4 and
reaction with triethylphosphite gave phosphonates 3a,b
in good yields. Careful stoichiometric HornerꢀWittig
reaction between 3a,b and 2TV-2CHO afforded 4a,b in
64 and 60% yield, respectively. The final dyes 1a,b were
obtained by Knoevenagel condensation of 4a,b with cya-
noacetic acid as dark blue solids in good yields (1a: 71%
1
and 1b: 85%). All compounds were characterized by H
and 13C NMR, FT-IR and MALDI-MS spectrometry (see
Supporting Information).
The optical and electrochemical properties of 1a and 1b
were analyzed by UVꢀvis absorption spectroscopy, fluo-
rescence emission spectroscopy and cyclic (CV) and Oster-
young square wave (OSWV) voltammetries, and the data
are displayed in Table 1.
In a dichloromethane solution, 1a,b showed strong
absorption over the entire visible region (up to 700 nm,
Figure 2) with broad peaks (and intense absorption) at
555 nm (log ε = 4.62) and 462 nm (log ε = 4.82),
respectively. The higher bathochromic shift observed for
1b in comparison to 1a can be attributed to better light
harvesting behavior, which improves the photocurrent
generation in the DSC (vide infra).
Figure 1. Molecular structures of dyes 1a,b.
We report here the synthesis of two new dyes with strong
absorption in the visible/near IR region of the spectrum
(Figure 1) and their application in DSCs. The new sensi-
tizers are based on TPA as a donor, a three-unit oTV,
bearing hydrophobic alkyl chains to impart solubility and
prevent aggregation (and thus to avoid self-quenching of
the dye excited state),12 as the π-connector, and 2-cya-
noacrylic acid as the electron acceptor.
Sensitizers 1a,b were obtained in good yields by the
synthetic protocol shown in Scheme 1.
Aldehydes 2a,b were obtained by palladium-catalyzed
Stille coupling of the corresponding triphenylamine
For both dyes 1a,b, a positive solvatochromism of the
maximum absorption band was observed on increasing the
solvent polarity (Figure S28, Supporting Information),
thus confirming the CT character of this band. Moreover,
a good correlation (correlation coefficient > 0.97) was
found between the absorption maxima and the Kamletꢀ
Taft constants (π*)13 (Figure S29 and Table S3ꢀS4,
Supporting Information). The slope (S) of the Kamletꢀ
Taft equation (E = E° þ Sπ*) is higher, in absolute value,
for 1b (S = ꢀ4.98) than for 1a (S = ꢀ4.52) (Table S5,
Supporting Information), indicating a higher polarizabil-
ity for 1b as a consequence of the higher electron donor
ability of the alkoxy-TPA moiety. The dyes exhibited
emission bands at 760 nm (1a, λexc= 555 nm) and 770 nm
(1b, λexc= 562 nm) (Figures S30ꢀS31, Supporting
Information). These bands were totally quenched after
adsorption onto TiO2, indicating an efficient photoin-
duced electron transfer process from TPA moieties to the
TiO2 nanoparticles. The oligothienylene-vinylene acts as
an electron wire, favoring the efficiency between donors
and acceptors.10
The redox properties of 1a and 1b were investigated by
cyclic (CV) and Osteryoung square wave (OSWV) voltam-
metries in o-dichlorobenzene:acetonitrile (4:1) (Table 1,
Figures S32 and S33, Supporting Information). In the
cathodic side, the two compounds show a first reversible
oxidation wave at 0.22 V (1a) and 0.08 V (1b); the presence
of the electron-donating alkoxy groups significantly re-
duces the oxidation potential of 1b in comparison to that
of 1a. A second reversible oxidation wave is observed at
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Cruz, P.; Marquez, L.; Langa, F.; Bisquert, J. J. Phys. Chem. C 2008,
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Mora-Sero, I.; Gros, D.; Dittereder, T.; Lutich, A. A.; Susha, A. S.;
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Org. Lett., Vol. 14, No. 22, 2012
5733

Table 1. Data for UVꢀvis and Fluorescence Emission Spectroscopies, OSWV for 1a,b
λ
max (nm)a
log (ε)a
λ
em (nm)a
E1ox (V)b
E2ox (V)b
EHOMO (eV)c
E0ꢀ0 (eV)d
1a
1b
555
562
4.62
4.82
760
770
0.22
0.08
0.31
0.23
ꢀ5.32
ꢀ5.18
1.91
1.92
a 10ꢀ5 M, in dichloromethane. b [10ꢀ3 M] in o-DCB:acetonitrile (4:1) vs versus Fc/Fcþ, glassy carbon, Pt counter electrode, 20 °C, 0.1 M Bu4NClO4,
scan rate = 100 mV sꢀ1. c Calculated with respect to ferrocene, EHOMO: ꢀ5.1 eV. d Estimated from the intersection between the absorption and emission
spectra.
Scheme 1. Synthesis of Dyes 1a,b
Figure 3. SEM cross section of mesoporous titania photo anode.
Figure 2. Normalized UVꢀvis absorption spectra of com-
The molecular geometries and frontier molecular orbi-
tals of these new dyes were optimized by density functional
theory (DFT) calculations at the B3LYP/6-31G* level.
The electron distributions of the HOMO and LUMO of
dyes 1a and 1b are shown in Figures S34ꢀS35, Supporting
Information. HOMOs are mainly delocalized from the
TPA moiety system to the oTV oligomeric part in both
compounds (Figures S34ꢀS35, Supporting Information).
In both dyes (1a,b) the LUMO level is localized predomi-
nantly on the cyanoacrylic unit and extended to the
thiophene rings. This distribution suggests the existence
of charge transfer from the TPA to the cyanoacrylic unit
pounds 1a (dotted) and 1b (solid) in dichloromethane.
0.31 V (1a) and 0.23 V (1b), and these are assigned to the
oTV oligomer.10a The EHOMO values (calculated with
respect to ferrocene as reference, EHOMO: ꢀ5.1 eV)14 were
determined as ꢀ5.32 eV (1a) and ꢀ5.18 eV (1b), and these
are less negative than the potential of I/I3ꢀ. This charac-
teristic made it possible to regenerate the dye.
(14) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan,
G. C. Adv. Mater. 2011, 23, 2367.
5734
Org. Lett., Vol. 14, No. 22, 2012

Table 2. Performance of Dye-Sensitized Solar Cells Fabricated
with Dyes 1a,b and References
dye
J (mA/cm2)
Voc (V)
FF
efficiency (%)
N719
D5
17.52
14.57
13.85
15.64
0.670
0.647
0.542
0.588
0.68
0.67
0.70
0.68
8.06
6.34
5.25
6.25
1a
1b
via the π-spacer group (the oTV oligomer), thus allowing
significant charge separation within the dye after photo-
excitation.
Figure 4. Comparison of JscꢀV curves under full AM 1.5 solar
Finally, a state of the art cell was prepared with several
titania photo anodes (0.36 cm2) printed on FTO glass
(2.2 mm thick and 15 Ohm/sq). The layering used was as
follows (see SEM cross section in Figure 3): around 11 μm
of 18 nm TiO2 nanoparticles (Solaronix’ Ti-Nanoxide
T/SP) and 3 μm of over 100 nm TiO2 nanoparticles
(Solaronix’ Ti-Nanoxide R/SP).
TiCl4 pre- and a post-treatments were performed to
optimize the adhesion, porosity and specific area of the
titania. The counter electrode was prepared by paint
brushing three layers of Solaronix’ Platisol T, leading to
the formation of a platinized catalytic layer after firing at
400 °C. The photoanodes were then sensitized using the
various dyes. Usually 0.5 mM solutions in ethanol in the
presence of chenodeoxycholic acid (5 mM) were prepared,
but in the cases of dyes 1a and 1b, chenodeoxycholic acid
was found to compete with dye adsorption. Solutions
containing dyes 1a,b were prepared in dichloromethane
without any coadsorbent.
The working and counter electrodes were joined to-
gether using a 25 μm spacer to minimize any mass trans-
port limitation. The cells were then backfilled with an
electrolyte based on acetonitrile containing 0.5 M of
dimethyl propyl imidazolium iodide, 0.5 M of 4-tert-
butylpyridine, 0.1 M of lithium iodide and 50 mM of
iodine. For accurate measurement the cells were masked.
The total active area was 0.36 cm2.
intensity.
fill factor and an improved voltage, which can be attrib-
uted to the difference in oxidation potential or to slower
recombination dynamics with oxidized electrolyte. The
JscꢀV performanceofthesecellsmeasured infullsun(1000
W/m2, AM 1.5 G) is represented in Figure 4, and the values
are listed in Table 2.
In summary, we have developed two new organic sensi-
tizers that incorporate triphenylamine or alkoxytripheny-
lamine as the donor, a cyanoacrylic acid moiety as the
acceptor and a three-unit oTV as a bridge. The introduc-
tion of alkoxy groups in the TPA donor enhances con-
siderably the efficiency of the cell, with over 6.25% power
conversion efficiency achieved. We have also shown that
shortening the bridge between the TPA moiety and an-
choring group leads to a dramatic increase in the overall
cell efficiency. We believe that the development of highly
efficient dyes with these types of components is possible
through molecular modifications. Work is in progress to
achieve this aim.
Acknowledgment. This work was supported by the
Ministry of Economy and Innovation of Spain,
(CTQ2010-17498, PLE2009-0038 and Consolider-Ingenio
Projects HOPE CSD2007-00007).
The characteristics of the cells made under the above
conditions and using dye 1b were found to be Voc = 588
mV, Jsc = 15.64 mA/cm2 and FF = 68%, with a photon to
current efficiency (PCE) of 6.25%, which is similar to the
organic dye D5 used as a reference in this study.
In comparison with the previously published11a dye
FL-7, shortening the bridge between the TPA moiety
and anchoring group led to a dramatic increase in the
overall cell efficiency. The increase is mainly due to a better
Supporting Information Available. General experimen-
1
tal methods, synthetic procedures, H and 13C NMR,
MALDI-MS, FT-IR and UVꢀvis spectra; computa-
tional details, details of cell construction and character-
ization. This material is available free of charge via the
Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 22, 2012
5735