Triphenylamine-based dyes bearing functionalized 3,4- propylenedioxythiophene linkers with enhanced performance for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol902973r

Chemical Equation Presented Introduction of modified 3,4-propylenedioxythiophene units into triphenylamine-based dyes is found to enhance light capturing, suppress dye aggregation, and remarkably retard charge recombination in dye-sensitized solar cells. Open circuit voltages of the as-synthesized dyes (～800 mV) are much higher than that with a thiophene congener (720 mV) under similar conditions as a result of self-passivation benefiting from their three-dimensional branched structures.

Full text of DOI:10.1021/ol902973r

ORGANIC
LETTERS
2010
Vol. 12, No. 6
1204-1207
Triphenylamine-Based Dyes Bearing
Functionalized 3,4-Propylenedioxythiophene
Linkers with Enhanced Performance for
Dye-Sensitized Solar Cells
Yanliang Liang, Bo Peng, Jing Liang, Zhanliang Tao, and Jun Chen*
Institute of New Energy Material Chemistry and Key Laboratory of AdVanced
Micro/Nanomaterials and Batteries/Cells (Ministry of Education), Chemistry College,
Nankai UniVersity, Tianjin 300071, People’s Republic of China
chenabc@nankai.edu.cn
Received December 29, 2009
ABSTRACT
Introduction of modified 3,4-propylenedioxythiophene units into triphenylamine-based dyes is found to enhance light capturing, suppress dye
aggregation, and remarkably retard charge recombination in dye-sensitized solar cells. Open circuit voltages of the as-synthesized dyes
(∼800 mV) are much higher than that with a thiophene congener (720 mV) under similar conditions as a result of self-passivation benefiting
from their three-dimensional branched structures.
Dye-sensitized solar cells (DSCs) are one of the most
promising candidates for widespread solar energy utilization
owing to their low cost and relatively high photoenergy-
conversion efficiencies (η).1 Although Ru-based complexes
hold the record validated efficiency of over 11% under full
sunlight,2 they have encountered problems such as limited
resources and elaborated purification. On the other hand,
organic sensitizers with robust availability, ease of structural
tuning, and generally high molar extinction coefficients have
emerged as competitive alternatives to the Ru-based coun-
terparts.3
Although encouraging efficiencies of near 10% have been
reported,4 organic dyes need to overcome three major
drawbacks before they can outperform Ru dyes, namely, (1)
narrow photoresponse range resulting from their limited
spectral coverage, (2) formation of aggregates on the TiO2
surface that decreases electron injection,5 and, especially,
(3) relatively fast charge recombination of the injected
electrons in the TiO2 with the redox species in the electro-
lyte.6 After making extensive studies on molecular modifica-
tion based on the donor-π-acceptor configuration,3 effective
strategies for fine-tuning the energy levels of dyes to better-
(1) (a) O’Regan, B. C.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Gra¨tzel,
M. Nature 2001, 414, 338.
(4) (a) Zhang, G. L.; Bala, H.; Cheng, Y. M.; Shi, D.; Lv, X. J.; Yu,
Q. J.; Wang, P. Chem. Commun. 2009, 2198. (b) Ito, S.; Miura, H.; Uchida,
S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.; Pe´chy, P.; Gra¨tzel, M.
Chem. Commun. 2008, 5194.
(2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han,
L. Y. Jpn. J. Appl. Phys., Part 2 2006, 45, L638.
(3) Mishra, A.; Fischer, M. K. R.; Ba¨uerle, P. Angew. Chem., Int. Ed.
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S.; Arakawa, H.; Sugihara, H. J. Phys. Chem. B 2005, 109, 3907.
10.1021/ol902973r  2010 American Chemical Society
Published on Web 02/19/2010

match the solar spectrum are now availble,7 and aggregation
of the adsorbed dyes could be prevented by introducing bulky
nonplanar stuctures.8 In contrast, few established strategies
for retardation of interfacial charge recombination are known.
Involvement of multiple long alkyl chains4b,9 and starburst
structures7b,c,10 in dye backbones are found to improve open
circuit voltage (VOC) since these bulky “functions” or the
thick dye aggregates formed can block the redox species,
i.e. passivate the TiO2 surface. However, construction of such
complex structures is synthetically challenging, and the VOC
values of these dyes remain mostly lower than that of the
Ru complex N719. More groundwork toward the reduction
of charge recombination for organic dyes is in urgent need.
Recent studies suggested that the generally low VOC values
for organic dyes were most likely suffered from complex
Figure 1. Molecular structures of sensitizers bearing bisProDOTs
(OR1, OR3, and OR6) and bithiophene (TT).
impressive performances in DSCs.4a,7g In addition, ProDOT
have a tetrahedral central carbon in the propylene ring at
which a variety of functional groups could be installed to
form three-dimensional (3D) branched structures12 which will
not only disfavor π-π packing but also block other
-
formation between dye molecules and I2 or I3 , or from
indirect electrostatic attraction of I3- by negatively charged
atoms in the dyes, whichever pathway increased the local
concentration of electron acceptor species in the vicinity of
TiO2 surface.6,11 We therefore envisage the possibility that
retardation of charge recombination could be effectively
realized by introducing appropriate steric hindrance to the
sensitizer backbone in order to alleviate the strong interation
between sensitizers and acceptor species in the electrolyte,
that is, passivation of the sensitizers, rather than the entire
TiO2 surface.
-
molecules (e.g., I3 ) from approaching. On the basis of the
above considerations, we could reasonably expect improved
optical properties, suppressed dye aggregation, and, most
hopefully, hindered charge recombination all by simply
replacing the commonly employed thiophene linker by
ProDOTs. Therefore, OR dyes containing dimeric dialkylated
ProDOTs (bisProDOT-R2) are engineered and tested in
DSCs. In particular, the sizes of alkyl groups are systemati-
cally varied to examine the role they play in the dyes’
photovoltaic performace. A bithiophene congener, coded
TT,10,13 is also synthesized for a comparison.
The synthetic route of OR dyes is shown in Scheme 1.
ProDOTs 1 were constructed by transetherification of com-
mercially available 3,4-dimethoxythiophene with the corre-
sponding 1,3-propanediols (Scheme S1, Supporting Infor-
mation (SI)). They were transformed into aromatic boron
derivatives 2b,c and brominated aromatic aldehydes 4b,c,
repectively, which were cross-coupled under Suzuki condi-
tions to give bisProDOT aldehydes 5 (aldehyde 5a was
prepared with a different method; see Scheme S2, SI).
Herein we report on the facile and expedient multifunc-
tionalization of triphenylamine dyes by introducing rationally
modified ProDOT building blocks to their π-conjugated
systems (OR dyes, Figure 1). ProDOT shares attractive
properties with 3,4-ethylenedioxythiophene (EDOT) such as
high electron-richness and excellent coplanarity of their
oligomers,12 while sensitizers bearing EDOT have shown
(6) (a) O’Regan, B. C.; Lo´pez-Duarte, I.; Mart´ınez-D´ıaz, M. V.; Forneli,
A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres, T.; Durrant, J. R.
J. Am. Chem. Soc. 2008, 130, 2906. (b) Miyashita, M.; Sunahara, K.;
Nishikawa, T.; Uemura, Y.; Koumura, N.; Hara, K.; Mori, A.; Abe, T.;
Suzuki, E.; Mori, S. J. Am. Chem. Soc. 2008, 130, 17874.
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2007, 9, 1971. (b) Hagberg, D. P.; Yum, J.-H.; Lee, H.; De Angelis, F.;
Marinado, T.; Karlsson, K. M.; Humphry-Baker, R.; Sun, L.; Hagfeldt, A.;
Gra¨tzel, M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2008, 130, 6259. (c)
Ning, Z. J.; Zhang, Q.; Wu, W. J.; Pei, H. C.; Liu, B.; Tian, H. J. Org.
Chem. 2008, 73, 3791. (d) Velusamy, M.; Justin Thomas, K. R.; Lin, J. T.;
Hsu, Y. C.; Ho, K. C. Org. Lett. 2005, 7, 1899. (e) Erten-Ela, S.; Yilmaz,
M. D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya, E. U. Org. Lett. 2008, 10, 3299.
(f) Lin, J. T.; Chen, P. C.; Yen, Y. S.; Hsu, Y. C.; Chou, H. H.; Yeh, M. C. P.
Org. Lett. 2009, 11, 97. (g) Liu, W. H.; Wu, I. C.; Lai, C. H.; Chou, P. T.;
Li, Y. T.; Chen, C. L.; Hsu, Y. Y.; Chi, Y. Chem. Commun. 2008, 5152.
(h) Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Zakeeruddin,
S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 9202. (i) Wang, Z. S.;
Cui, Y.; Hara, K.; Dan-Oh, Y.; Kasada, C.; Shinpo, A. AdV. Mater. 2007,
19, 1138. (j) Tian, H. N.; Yang, X. C.; Chen, R. K.; Hagfeldt, A.; Sun,
L. C. Energy EnViron. Sci. 2009, 2, 674. (k) Hao, Y.; Yang, X. C.; Cong,
J. Y.; Tian, H. N.; Hagfeldt, A.; Sun, L. C. Chem. Commun. 2009, 4031.
(8) (a) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci,
S.; De Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am.
Chem. Soc. 2006, 128, 16701. (b) Heredia, D.; Natera, J.; Gervaldo, M.;
Otero, L.; Fungo, F.; Lin, C.-Y.; Wong, K.-T. Org. Lett. 2010, 12, 12.
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Hara, K. J. Am. Chem. Soc. 2006, 128, 14256. (b) Kim, S.; Kim, D.; Choi,
H.; Kang, M. S.; Song, K.; Kang, S. O.; Ko, J. Chem. Commun. 2008,
4951. (c) Ning, Z. J.; Zhang, Q.; Pei, H. C.; Luan, J. F.; Lu, C. G.; Cui,
Scheme 1. Synthetic Route of OR Dyes
Y. P.; Tian, H. J. Phys. Chem. C 2009, 113, 10307
(10) Justin Thomas, K. R.; 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
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.
Org. Lett., Vol. 12, No. 6, 2010
1205

Subsequent bromination and coupling of 5 with an appropri-
ate phenylborate afforded aldyhydes 7, which underwent
Knoevenagel condensation with cyanoacetic acid to form the
target dyes.
The optical and electrochemical properties of the four dyes
are summarized in Table 1. In THF solutions (Figure 2a),
2b and inset), indicating the formation of J-aggregates.5
Interestingly, as the substituents at the ProDOT units become
bulkier, i.e., from methyl (OR1) to n-propyl (OR3) to
n-hexyl (OR6), the spectra become narrower and less red-
shifted accordingly, reflecting systematically reduced inter-
molecular π-π interactions.
All four dyes exhibit two reversible oxidative waves in
the cyclic voltammogram (Figure S1, SI). The first oxidation
potentials of OR dyes (0.96-1.03 V vs NHE) are more
negative than that of TT (1.10 V vs NHE) because of the
presence of the electron-donating alkoxy substituents. They
are all more positive than the Nernst potential of I-/I3- redox
couple (0.4 V vs NHE), ensuring regeneration of the oxidized
dyes by I- after electron injection. The excited state redox
potentials of the four dyes (-1.23 to -1.12 V vs NHE) are
sufficiently more negative than the fermi level of TiO2 (-0.5
V vs NHE),1b providing enough driving force for electron
injection from the excited dyes to the conduction band of
TiO2.
Table 1. Optical and Electrochemical Properties of the Four
Dyes Shown in Figure 1
dye
λabsa/nm
εa/M-1 cm-1
Eoxb/V
Eox*c/V
OR1
OR3
OR6
TT
499
503
502
472
52800
51700
47500
42800
1.03
0.97
0.96
1.10
-1.12
-1.19
-1.19
-1.23
a The absorption spectra were measured in THF solutions (1 × 10-5
M). b First oxidation potentials (vs NHE) in CH2Cl2 internally calibrated
c
with ferrocene (0.63 V vs NHE). Eox* ) Eox - E0-0. E0-0 values
(zeroth-zeroth transition energies) were estimated from the intersections
of normalized absorption and emission spectra in THF.
Table 2. DSC Performace Parameters of the Four Dyes Shown
in Figure 1 with N3 as a Reference.a
dye
JSC/mA cm-2
VOC/mV
fill factor
η/%
OR1
OR3
OR6
TT
9.54
10.33
9.45
9.38
9.32
756
797
793
720
745
0.62
0.64
0.67
0.64
0.67
4.45
5.30
5.04
4.36
4.65
N3
a The DSCs had an active area of ∼0.13 cm2 and used an electrolyte
composed of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-
butylpyridine in acetonitrile.
Figure 2. Absorption spectra of OR dyes and TT in THF (a) and
on 4.6 µm TiO2 films (b). The inset shows the absorption spectra
of the deprotonated dyes in alkaline MeOH solutions.
they all exhibit strong absorption at 400-600 nm, which is
attributed to intramolecular charge transitions (Figure S3,
SI). Compared to TT, ca. 30 nm red-shifts in the absorption
peaks together with increases in molar extinction coefficient
(ε) are found for OR dyes. Theoritical calculation shows
that replacement of bithiophene with bisProDOTs dramati-
cally decreases the dihedral angle between the two thienyl
rings from 13.3° to 0.4° while only slightly increases that
between the phenyl and thienyl rings from 22.4° to 23.6°,
thus resulting in a more planar conjugating system. The
improved optical properties of OR dyes imply the merit of
applying such electron-rich and highly planar linkers to
sensitizer engineering. Upon adsorption onto TiO2, TT and
OR1, which possess relatively planar structures, show
significantly broadened and red-shifted absorption spectra
compared with that measured in alkaline solutions (Figure
Figure 3. Photocurrent density-voltage (J-V) curves (a) and IPCE
spectra (inset) of DSCs based on OR dyes, TT, and N3. Optimized
3D structures of OR3 (b) and TT (c) are also shown.
Photovoltaic characteristics of the DSCs based on the four
dyes as well as the standard N3 dye under AM1.5G condition
(100 mW cm-2) are displayed in Table 2 and the J-V curves
are plotted in Figure 3. To demonstrate the advantage of these
highly absorptive sensitizers in thin-film devices, relatively
thin TiO2 films (ca. 4.6 µm) were employed, which thickness
(12) (a) Welsh, D. M.; Kumar, A.; Meijer, E. W.; Reynolds, J. R. AdV.
Mater. 1999, 11, 1379. (b) Nielsen, C. B.; Bjørnholm, T. Macromolecules
2005, 38, 10379. (c) Hammond, S. R.; Clot, O.; Firestone, K. A.; Bale,
D. H.; Lao, D.; Haller, M.; Phelan, G. D.; Carlson, B.; Jen, A. K. Y.; Reid,
P. J.; Dalton, L. R. Chem. Mater. 2008, 20, 3425.
(13) Moon, S. J.; Yum, J. H.; Humphry-Baker, R.; Karlsson, K. M.;
Hagberg, D. P.; Marinado, T.; Hagfeldt, A.; Sun, L. C.; Gra¨tzel, M.;
Nazeeruddin, M. K. J. Phys. Chem. C 2009, 113, 16816.
(14) Yum, J. H.; Chen, P.; Gra¨tzel, M.; Nazeeruddin, M. K. ChemSus-
Chem 2008, 1, 699.
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Org. Lett., Vol. 12, No. 6, 2010

is proposed to be optimal for future solid state DSCs.14 All
three OR dyes exhibit higher short circuit current densities
(JSC) than TT owing to their enhanced incident photon-to-
current conversion efficiency (IPCE) (Figure 3, inset) in the
long-wavelength region. Although the adsorption amount of
TT (17.2 × 10-8 mol cm-2) is ∼1.5-fold higher than that of
OR dyes (7.1 × 10-8 mol cm-2 for OR1,15 7.8 × 10-8 mol
cm-2 for OR3, and 6.2 × 10-8 mol cm-2 for OR6) and more
intense light-absorption could be expected, the IPCE maxi-
mum of TT (68%) is just comparable to that of OR dyes
(e.g., 68% for OR3), which is attributable to disadvantageous
intermolecular energy transfer as a result of aggregate
formation.5 Though all three OR dyes share similar broad-
ness in their IPCE spectra, OR1 and OR6 display lower
plateaus and thus lower JSC values than OR3 due to dye
aggregation and poor dye uptake, respectively. The JSC for
N3 is lower than the organic dyes because of insufficient
light-harvesting in such thin film devices.
in VOC from TT to OR3 is closely associated with a gradually
decreased tendency of the electron recombination in the TiO2
with the I-/I3- redox couple in the electrolyte.9c The slightly
lower VOC of OR6 than OR3 suggests that the presence of
long alkyl chains itself is not responsible for the reduction
of the interfacial charge recombination, which agrees with
a previous study.16 The success of OR dyes might have
originated from their 3D branched features, which effectively
passivate the dyes by blocking the acceptor species from
being attracted to the dye molecules (cf. Figure 3b,c). One
exciting conclusion from our results is that as long as the
scaffolds of organic dyes are rationally designed, very bulky
structures are not indispensable for the achievement of high
VOC values. In fact, the four differently oriented alkyl chains
in OR6 lead to loose packing,17 leaving more voids between
-
dye molecules, which might facilitate I3 to penetrate the
chromophore layer and in turn aggravate charge recombina-
tion. The size of steric hindrance for passivating the dyes
seems to require careful design in the future.
The open-circuit photovoltage (VOC) values are in the
sequence TT < N3 ≈ OR1 < OR6 ≈ OR3. When the
bithiophene bridge is displaced by bisProDOT-Me2, the VOC
rises remarkably from 720 mV to 756 mV, a phenomenon
that was also observed by other research groups where 3,4-
dialkoxythiophenes were employed instead of thiophene.7g
For OR3 incorporating a more branched ProDOT-Pr2 unit,
a significantly higher VOC (797 mV) was achieved. Introduc-
tion of long alkyl chains to ProDOTs (OR6) does not lead
to a further enhanced VOC (793 mV) than OR3. The
respectable increase of VOC obtained by the delicate change
in molecular structure (e.g., from OR1 to OR3) is intriguing.
To scrutinize the origin of the improvement in VOC, measure-
ment of the dark current-voltage characteristics is per-
formed. From the dark J-V curves (Figure S4, SI), onsets
of dark current of the five DSCs are derived as TT (639
mV) < OR1 (660 mV) ≈ N3 (664 mV) < OR6 (706 mV) ≈
OR3 (720 mV). This sequence is in good accordance with
that of the VOC values, indicating that the gradual increase
In summary, we have successfully multifunctionalized
triphenylamine-based organic dyes through simple replace-
ment of the thiophene linker with various ProDOT-R2
building blocks. Besides their enhanced light-capturing
abilities and controlled dye aggregation, the OR dyes have
achieved respectably high VOC values probably benefiting
from their 3D branched structures. Our findings provide a
novel strategy for multifunctionalization of organic dyes, as
well as retardation of charge recombination in DSCs.
Acknowledgment. This work was supported by the
Research Programs from National MOST (2005CB623607
and 2009AA05Z421). We thank Professor Jin Qu at Nankai
University, China, for the help in compound characterization.
We also acknowledge the computational support by NK-
STARS provided by the Center of Theoretical and Compu-
tational Chemistry at Nankai University.
Supporting Information Available: Experimental details,
copies of 1H and 13C NMR spectra of synthetic compounds,
cyclic voltammogram, emission spectra, 3D structures,
molecular orbitals, and dark J-V characteristics of the dyes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Alhough with a smaller molecular size, the slightly lower adsorption
amount of OR1 than that of OR3 is understandable because the concentra-
tion of the optimized dye bath for OR1 is about 70% lower than that for
the other dyes in this study (see the SI).
(16) Mori, S. N.; Kubo, W.; Kanzaki, T.; Masaki, N.; Wada, Y.;
Yanagida, S. J. Phys. Chem. C 2007, 111, 3522.
(17) For a graphical illustration of the relationship between the 3D
structure of OR dyes and packing density, see Figure S5 in the SI.
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