Novel pyrenoimidazole-based organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol2006874

A novel class of organic dyes containing pyrenoimidazole donors, cyanoacrylic acid acceptors, and oligothiophene π-linkers has been synthesized and characterized. The electro-optical properties of these dyes can be tuned by changing the conjugation length

Full text of DOI:10.1021/ol2006874

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2622–2625
Novel Pyrenoimidazole-Based Organic
Dyes for Dye-Sensitized Solar Cells
Dhirendra Kumar,† K. R. Justin Thomas,*,† Chuan-Pei Lee,‡ and Kuo-Chuan Ho‡
Organic Materials Lab, Department of Chemistry, Indian Institute of Technology
Roorkee, Roorkee 247 667, India, and Department of Chemical Engineering,
National Taiwan University, Taipei 10617, Taiwan
krjt8fcy@iitr.ernet.in
Received March 19, 2011
ABSTRACT
A novel class of organic dyes containing pyrenoimidazole donors, cyanoacrylic acid acceptors, and oligothiophene π-linkers has been
synthesized and characterized. The electro-optical properties of these dyes can be tuned by changing the conjugation length of the π-linkers. A
dye containing terthiophene in the conjugation pathway exhibited a solar energy-to-electricity conversion efficiency of 5.65%.
There has been considerable interest for the synthesis
and characterization of novel organic and organometallic
dyes suitable for application in nanocrystalline TiO2-based
dye-sensitized solar cells (DSSCs) due to the demand for
the alternative energy sources.1 These photoelectrochem-
in the light-harvesting process. Similarly, the organic dyes
derived with a donor-π-acceptor architecture show charge
migration from the donor to the acceptor through the π-
bridge. The charge transfer transition of the organic dyes
generally exhibits a higher molar extinction coefficient in
the visible region than the MLCT transition of the ruthe-
nium complexes. Also, the position and intensity of the
charge transfer transition in organic dyes can be tuned by
simple structural modifications such as variation of donor
strength and nature of the conjugation pathway.3 Due to
these advantages, many chromophores such as coumarin,
indoline, cyanine, hemicyanine, merocyanine, perylene,
xanthene, triarylamine, oligothiophene, thienothiophene,
indolothiophene, benzothiadiazole, and imidazole have
been used for the development of organic dyes suitable
for DSSCs.4
€
ical cells were first reported by Gratzel using ruthenium
complexes as sensitizers and demonstrated solar energy-
to-electricity conversion efficiency up to 11% under
100 mW cmꢀ2 irradiation (AM 1.5 G simulated sunlight).2
The metal-to-ligand charge transfer (MLCT) transition
present in the ruthenium complex plays an important role
† Indian Institute of Technology Roorkee.
‡ National Taiwan University.
(1) Armaroli, N.; Balzani, V. Angew. Chem., Int. Ed. 2006, 46, 52.
€
(2) (a) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (b) Nazeeruddin
€
M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.;
€
Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (c)
€
Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (d) Nazeeruddin,
€
(3) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int. Ed.
2009, 48, 2474.
(4) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
ꢀ
M. K.; Pechy, P.; Renouard, T.; ZaKeeruddin, S. M.; Humphry-Baker,
R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.;
€
Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem. Soc. 2001, 123,
1613.
Chem. Rev. 2010, 110, 6595.
r
10.1021/ol2006874
Published on Web 04/20/2011
2011 American Chemical Society

Scheme 1. Synthetic Sequence Employed to Obtain the Dyes 5aꢀc
Recombination of electrons injected to the conduction
band of TiO2 with the oxidized dyes or electrolyte consti-
tuents (I3ꢀ) were identified as detrimental for the overall
efficiency of the DSSCs.5 To retard these side processes,
selected chromophores were incorporated in the structure
of the organic dyes. The probability of the recombination
of the ejected electron back with the oxidized dye may be
diminished by the addition of groups that are capable of
dissipating the positive charge via electronic delocaliza-
tion. The direct recombination of injected electrons with
In this communication, we report three new dyes 5aꢀc
containing pyrenoimidazole donors and cyanoacrylic acid
acceptors. They are linked via a π-bridge constituted by
oligothiophene. It is hypothesized that on oxidation the
cation radical generated at either imidazole and/or the
oligothiophene segment will be resonance stabilized by the
pyrene moiety and will disfavor the back electron transfer.
In an earlier work, imidazole unit has been demonstrated
as a two-way π-linker for the triarylamine donors and
cyanoacrylic acid acceptor.9 They have observed that
triarylamine donors are mandatory to obtain decent effi-
ciency from the DSSCs. In this work, we demonstrate that
due to the enhanced donor ability of pyrenoimidazole and
oligothiophene linker the electro-optical properties show
significant improvements which manifests in the overall
efficiency for the DSSCs.
ꢀ
the I3 may be reduced by the inclusion of hydrophobic
substituents.6 Triarylamines or fused amine segments were
invariably used in most of the organic dyes to achieve a
pushꢀpull structure and resonance stabilization of oxi-
dized dyes.7 Triarylamine-free organic dyes suitable for
DSSC are scarce and often lead to inferior performance.8,9
The pyrenoimidazole-based organic dyes were conveni-
ently obtained from pyrenodione in four steps involving
imidazole construction with a suitable aldehyde, phase-
transfer catalyzed N-alkylation, Stille coupling10 reaction
followed by acid hydrolysis to liberate the aldehyde and
Knoevenagel condensation11 with cyanoacetic acid as
illustrated in Scheme 1.
(5) Ning, Z.; Fu, Y.; Tian, H. Energy Environ. Sci. 2010, 3, 1170.
(6) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.;
Hara, K. J. Am. Chem. Soc. 2006, 128, 14256.
(7) (a) Hagberg, D. P.; Marinado, T.; Karlsson, K. M.; Nonomura,
K.; Qin, P.; Boschloo, G.; Brinck, T.; Hagfeldt, A.; Sun, L. J. Org. Chem.
2007, 72, 9550. (b) Thomas, K. R. J.; Hsu, Y.-C.; Lin, J. T.; Lee, K.-M.;
Ho, K.-C.; Lai, C.-H.; Cheng, Y.-M.; Chou, P.-T. Chem. Mater. 2008,
20, 1830. (c) Fischer, M. K. R.; Wenger, S.; Wang, M.; Mishra, A.;
€
€
Zakeeruddin, S. M.; Gratzel, M.; Bauerle, P. Chem. Mater. 2010, 22,
1836.
The absorption and electrochemical properties of the
dyes are presented in Table 1. Figure 1 shows the absorption
(8) Won, Y. S.; Yang, Y. S.; Kim, J. H.; Ryu, J.-H.; Kim, K. K.; Park,
S. S. Energy Fuels 2010, 24, 3676.
(9) (a) Tsai, M.-S.; Hsu, Y.-C.; Lin, J. T.; Chen, H.-C.; Hsu, C.-P.
J. Phys. Chem. C 2007, 111, 18785. (b) Velusamy, M.; Hsu, Y.-C.; Lin,
J. T.; Chang, C.-W.; Hsu, C.-P. Chem. Asian J. 2010, 5, 87.
(10) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(11) Knoevenagel, E. Chem. Ber. 1896, 29, 172.
Org. Lett., Vol. 13, No. 10, 2011
2623

Table 1. Electro-Optical Properties of the Pyrenoimidazole Based Dyes
λ
abs, nm (ε, ꢁ 103 Mꢀ1 cmꢀ1
)
dye
thf
dmf
TiO2
λ
em, nm Eox, Va Eox*, eVb HOMO/LUMO, eVc
5a
5b
405 (31.7), 354 (31.3), 289 (48.8)
389 (35.7), 364 (37.3), 290 (29.9)
485
544
597
626
0.80
0.63
ꢀ0.97
ꢀ0.83
5.60/3.06
5.43/3.20
452 (32.4), 384 (18.7), 354 (23.5), 423 (35.5), 380 (24.6), 289 (25.9)
287 (30.5)
5c
464 (44.9), 358 (24.2), 290 (31.7)
442 (45.1), 357 (20.5), 290 (27.9)
580
624
0.54
ꢀ0.87
5.34/3.16
a Oxidation potentials with reference to the ferrocene which was used as an internal standard. b Computed from the formula Eox* = Eox ꢀ E0ꢀ0 where
the band gap was derived from the optical edge c From HOMO = 4.8 þ Eox and LUMO = HOMO ꢀ E0ꢀ0 where E0ꢀ0 was estimated from optical edge.
observed for most the organic dyes generally originating
from the dye-TiO2 interaction leading to a partial depro-
tonation of the carboxylic acid unit.15 However, in more
basic solvents such as dimethyl formamide and in the
presence of triethylamine they showed a slight blue-shift
for the charge transfer transition (Figures S1ꢀS4, SI)
typical of the presence of deprotonated species.
Figure 1. Absorption and emission spectra of the dyes 5aꢀc
recorded in tetrahydrofuran.
spectra of the dyes recorded in THF solutions. All the dyes
possess three absorption peaks at around 280, 350 and
>400 nm. The two bands in the ultraviolet region are
present in all the dyes and are probably originating from
the electronic transitions localized within the pyrenoimi-
dazole segment. The third absorption occurring in the
visible region is sensitive to the nature of the conjugation
pathway and red-shifts on progressive addition of thio-
phene units.12 The dye 5c bearing terthiophene as linker
Figure 2. Computed orbital energies (observed solution data
shown in parentheses) and electronic distribution in the dyes.
To get further insight into the molecular structure and
electron distribution of dyes, the geometries of the dyes
were optimized by density functional theory (DFT) calcu-
lations at the B3LYP/6-31G (d, p) level. Figure 2 shows the
relative energies and electron distributions of the HOMO
and LUMO of the dyes 5aꢀc. The HOMO is delocalized
over the pyrenoimidazole π system with the highest elec-
tron density located at the two nitrogen atoms of the
imidazole moiety, while the LUMO is located in the
anchoring group through the π bridge constituted by the
benzene and/or thiophene moieties. Thus, the HOMOꢀ
LUMO excitation induced by light irradiation could move
the electron distribution from the pyrenoimidazole seg-
ment to the anchoring unit through the conjugation
pathway.
shows a significant red-shift and enhancement in extinc-
tion coefficient in the longer-wavelength band when com-
pared with that of the dyes 5a and 5b. Also, the peak
position of the charge transfer transition for 5a is much
shorter when compared with the other two dyes, probably
due to the electron-deficiency of the benzene linker relative
to the thiophene unit.13 All the dyes when adsorbed on
TiO2 exhibits red-shifted absorption profile (Figure S5, SI)
in comparison to that measured in solution. The red-
shifted absorption may be attributed to the intermolecular
interactions at the TiO2 surface which is likely to happen
due to the planar pyrenoimidazole π-system or to the
thicker TiO2 layer.14 This is contrary to the blue-shift
(12) Zhang, F.; Luo, Y.-H.; Song, J.-S.; Guo, X.-Z.; Liu, W.-L.; Ma,
C.-P.; Huang, Y.; Ge, M.-F.; Bo, Z.; Meng, Q.-B. Dyes Pigm. 2009, 81,
224.
(13) Qin, P.; Yang, X.; Chen, R.; Sun, L.; Marinado, T.; Edvinsson,
T.; Boschloo, G.; Hagfeldt, A. J. Phys. Chem. C 2007, 111, 1853.
(14) Cheng, H.-M.; Hsieh, W.-F. Energy Environ Sci. 2010, 3, 442.
(15) (a) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.;
Hagfeldt, A.; Sun, L. Chem. Commun. 2006, 2245. (b) Yen, Y.-S.; Hsu,
Y.-C.; Lin, J. T.; Chang, C.-W.; Hsu, C.-P.; Yin, D. J. J. Phys. Chem. C
2008, 112, 12557.
2624
Org. Lett., Vol. 13, No. 10, 2011

Table 2. Performance Parameters of the DSSCs Fabricated
Using the Dyesa
dye
JSC, mA cmꢀ2
VOC, mV
ff
η, %
τe, msb
5a
8.8
11.4
15.5
18.6
545
524
543
665
0.71
0.63
0.67
0.70
3.41
3.75
5.65
8.62
2.55
1.41
3.80
1.48
5b
5c
N719
a Measured under irradiation of 100 mW cmꢀ2 (AM 1.5 G simulated
sunlight) at room temperature. b Estimated with the aid of electroche-
mical impedance spectroscopy (EIS) measurements.
The oxidation potentials of the dyes fall in the range of
1.31 (5c) and 1.57 (5a) vs NHE and are more positive
ꢀ
than the oxidation potential of the Iꢀ/I3 redox couple
(0.4 V vs NHE).16 This indicates the existence of fair
driving force for the regeneration of the oxidized dye by
the Iꢀ ions. The excited state oxidation potentials
of the dyes are more negative (ꢀ0.83 to ꢀ0.97 V vs NHE)
than the conduction band of TiO2, which ensures facile
electron injection from the dyes into the conduction band
of TiO2.
The performances of the DSSCs fabricated using these
dyes are presented in Table 2 and the IꢀV curves plotted in
Figure 3. The terthiophene bridged dye, 5c showed pro-
mising efficiency of 5.65% when compared to the dyes
containing bithiophene (5b) and phenylthiophene (5a)
linkers. The higher efficiency observed for 5c mainly
originates due to the relatively larger short-circuit photo-
current density which is consistent with the intense red-
shifted charge transfer transition realized for this dye.17
For the dye 5c, highest IPCE (∼85%) was observed from
420 to 500 nm while the dyes 5a and 5b gave moderate
IPCE response (Figure S6, SI) in line with the trend in
molar extinction coefficients of these dyes (Figure 1). The
lower open-circuit photovoltage of 5b is attributed to the
relatively shorter recombination time estimated for this
Figure 3. IꢀV characteristics of the DSSCs fabricated using the
dyes.
device from electrochemical impedance measurements
(Figures S7 and S8, SI).
In summary, we have developed new pyrenoimidazole-
based organic dyes yielding up to 5.65% power conversion
efficiency in a conventional DSSC. The promising perfor-
mance of these dyes as sensitizers is intriguing as they do
not possess triarylamine-based donors. Our studies open
avenues for the development of organic dyes featuring
polyaromatic fused heterocycles as electron donors. By
appropriate structural modifications, electron-rich poly-
aromatic fused heterocycles can be developed, which may
serve as an efficient donor.
Acknowledgment. Financial supports from the Council
of Scientific and Industrial Research, India, and the National
Science Council of Taiwan are gratefully acknowledged.
Supporting Information Available. Experimental details,
characterization data of the compounds, copies of 1H, and
13C NMR spectra for new compounds, and the Cartesian
coordinates of the optimized structures. This material
is available free of charge via the Internet at http://
pubs.acs.org.
€
(16) Gratzel, M. Nature 2001, 414, 338.
(17) (a) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci,
€
S.; Angelis, F. D.; Censo, D. D.; Nazeeruddin, M. K.; Gratzel, M. J. Am.
Chem. Soc. 2006, 128, 16701. (b) O’Rourke, C.; Bowler, D. R. J. Phys.
Chem. C 2010, 114, 20240.
Org. Lett., Vol. 13, No. 10, 2011
2625