A novel amine-free dianchoring organic dye for efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/ol303121z

An amine-free oligothiophene-based dye (BTB) featuring a tailor-made dianchoring function, a spiro-configured central unit, and bulky end-capping TIPS groups to diminish intermolecular interactions and to suppress aggregation-induced self-quenching was synthesized to achieve efficient dye-sensitized solar cells with a high power conversion efficiency of 6.52%.

Full text of DOI:10.1021/ol303121z

ORGANIC
LETTERS
2012
Vol. 14, No. 24
6338–6341
A Novel Amine-Free Dianchoring
Organic Dye for Efficient Dye-Sensitized
Solar Cells
Hao-Chun Ting,† Chih-Hung Tsai,‡,§ Jia-Hong Chen,† Li-Yen Lin,† Shu-Hua Chou,†
Ken-Tsung Wong,*,† Tsung-Wei Huang,‡ and Chung-Chih Wu*,‡
Department of Chemistry, National Taiwan University, Taipei, 10617, Taiwan,
Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering
and Graduate Institute of Electronics Engineering, National Taiwan University, Taipei,
10617, Taiwan, and Department of Opto-Electronic Engineering, National Dong Hwa
University, Hualien 97401, Taiwan
kenwong@ntu.edu.tw; chungwu@cc.ee.ntu.edu.tw
Received November 12, 2012
ABSTRACT
An amine-free oligothiophene-based dye (BTB) featuring a tailor-made dianchoring function, a spiro-configured central unit, and bulky end-
capping TIPS groups to diminish intermolecular interactions and to suppress aggregation-induced self-quenching was synthesized to achieve
efficient dye-sensitized solar cells with a high power conversion efficiency of 6.52%.
€
Since Gratzel’s pioneering work in 1991, dye-sensitized
Very recently, DSSCs sensitized with complementary por-
phyrin-based and organic dyes have achieved a PCE as
high as 12.3%,3 making contemporay DSSCs highly com-
petitive with other thin-film photovoltaic technologies.
As such, metal-free organic dyes have attracted significant
attention due to their various advantages such as great
flexibility in structural tuning, low cost, resource avail-
ability, and generally large molar extinction coefficients.4
solar cells (DSSCs) have evolved to be a potential candi-
date for a next-generation photovoltaic device.1 Over the
past two decades, tremendous research has been focused
toward the search for efficient photosensitizers with de-
sired physical properties, such as low band gap, high molar
extinction coefficient, well-aligned energy levels, and sup-
pressed aggregation behavior. To date, DSSCs with a
remarkable power conversion efficiency (PCE) exceeding
11% have been realized using Ru-based sensitizers.2
(3) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
€
Gratzel, M. Science 2011, 334, 629.
(4) (a) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int.
Ed. 2009, 48, 2474. (b) Clifford, J. N.; Ferrero, E. M.; Viterisi, A.;
Palomares, E. Chem. Soc. Rev. 2011, 40, 1635. (c) Zeng, W.; Cao, Y.; Bai,
Y.; Wang, Y.; Shi, Y.; Zhang, M.; Wang, F.; Pan, C.; Wang, P. Chem.
Mater. 2010, 22, 1915. (d) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.;
Pettersson, H. Chem. Rev. 2010, 110, 6595. (f) Yen, Y.-S.; Chou, H.-H.;
Chen, Y.-C.; Hsu, C.-Y.; Lin, J. T. J. Mater. Chem. 2012, 22, 8734. (e)
Ooyama, Y.; Harima, Y. Eur. J. Org. Chem. 2009, 2903. (f) Lin, L.-Y.;
Tsai, C.-H.; Wong, K.-T.; Huang, T.-W.; Wu, C.-C.; Chou, S.-H.; Lin,
F.; Chen, S.-H.; Tsai, A.-I. J. Mater. Chem. 2011, 21, 5950.
† Department of Chemistry, National Taiwan University.
‡ Department of Electrical Engineering, National Taiwan University.
§ National Dong Hwa University.
€
(1) O’reagen, B.; Gratzel, M. Nature 1991, 353, 737.
(2) (a) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker,
R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M. J. Am. Chem.
Soc. 1993, 115, 6382. (b) Nazeeruddin, M. K.; Zakeeruddin, S. M.;
Jirousek, M.; Humphry-Baker, R.; Liska, P.; Vlachopoulos, N.;
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10.1021/ol303121z
Published on Web 12/10/2012
2012 American Chemical Society

In the designs of metal-free organic dye sensitizers, the
dipolar donorꢀ(π-conjugated spacer)ꢀacceptor (DꢀπꢀA)
system is the most widely adopted molecular architecture, in
which various arylamines such as triarylamine, carbazole,
phenothiazine, and indoline are generally utilized as elec-
tron-donating moieties. The incorporation of these strong
electron-donating groups in organic dyes facilitates effective
photoinduced intramolecular charge transfer (ICT) and is,
thus, beneficial in improving their light-harvesting abilities.
Despite the wide adoption of such conventional arylamine-
based donor structures, there were few reports of amine-free
organic sensitizers.5 The replacement of arylamines with
an electron-rich moiety bearing a relatively compact spatial
volume as the donor group enables the sensitizers to have
higher dye densities on TiO2 surfaces, rendering feasible
the fabrication of cocktail cells for panchromatic photon
harvesting.6 In addition to the core skeletons, tailoring the
structure and number of anchoring groups is also a research
focus for developing efficient organic sensitizers.7 For
Ru-based sensitizers, the number of anchoring groups
(carboxylic acid) can be fine-tuned from one to four to
control interfacial charge transfer.8 Recently, several or-
ganic sensitizers composed of two anchoring groups
have also been introduced,9 and these dianchoring organic
sensitizers were found to possess several advantages such
as extended π-conjugation, increased electron extraction
channels, multibinding abilities, higher photocurrent, and
enhanced stability over their monoanchoring counter-
parts.10 As remarkable examples, the DSSCs sensitized with
benzothiadiazole- and phenothiazine-based multianchoring
dyes had demonstrated PCEs exceeding 6%.11
Here, we report a new amine-free dianchoring organic
sensitizer, BTB (Scheme 1). This novel molecule was
designed with the following structural characteristics: (i)
itadoptsoligothiopheneasanelectron-donatingbackbone
to endow the dye with visible light absorption and a large
molar extinction coefficient; (ii) the anchoring groups
(cyanoacrylic acid) are introduced at γ-positions of the
terminal thiophene rings to match the distance between the
neighboring adsorption sites of the anatase TiO2 surface
and to ensure dianchoring adsorption (vide infra), which is
critical for efficient electron transfer and better stability for
dyes; (iii) it incorporates a spiro-configured central unit as
a steric bulky group to diminish intermolecular interac-
tions and to suppress aggregation-induced self-quenching
that is usually encountered in organic dyes;12 (iv) intro-
duced bulky end-capping TIPS groups together with the
central spiro-configured aryl group can form a hydropho-
bic shell to block the electrolyte diffusion and to reduce
dark currents.
Scheme 1. Synthesis of Amine-Free Dianchoring Dye BTB
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Y.-C.; Chou, H.-H.; Lin, J. T. Org. Lett. 2010, 12, 16. (c) Guo, K.; Yan,
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Scheme 1 depicts the synthesis of BTB. We established
a modified approach for synthesizing spiro-bridged bithio-
phene, which avoids the use of reported cyclopenta[2,1-
b:30,40-b0]dithiophen-4-one13 as the essential intermediate.
Our synthesis started from the selective lithium-bromo
exchange reaction of 3,30,5,50-tetrabromo-2,20-bithio-
phene14 with n-BuLi, followed by quenching with chloro-
triisopropylsilane (TIPS-Cl) to give TIPS-capped bithiophene
1 (46%).15 For the subsequent cyclization reaction, the
introduction of TIPS groups on the reactive R-position of
thiophene is essential for preventing possible intermolecular
(9) (a) Heredia, D.; Natera, J.; Gervaldo, M.; Otero, L.; Fungo, F.;
Lin, C.-Y.; Wong, K.-T. Org. Lett. 2010, 12, 12. (b) Yen, Y.-S.; Chen,
W.-T.; Hsu, C.-Y.; Chou, H.-H.; Lin, J. T.; Yeh, M.-C. P. Org. Lett.
2011, 13, 4930. (c) Jiang, X.; Karlsson, K. M.; Gabrielsson, E.;
Johansson, E. M. J.; Quintana, M.; Karlsson, M.; Sun, L.; Boschloo,
G.; Hagfeldt, A. Adv. Funct. Mater. 2011, 21, 2944. (d) Wu, T.-Y.; Tsao,
M.-H.; Su, S.-G.; Wang, H. P.; Lin, Y.-C.; Chen, F.-L.; Chang, C.-W.;
Sun, I.-W. J. Braz. Chem. Soc 2011, 22, 780. (e) Li, Q.; Shi, J.; Li, H.; Li,
S.; Zhong, C.; Guo, F.; Peng, M.; Hua, J.; Qin, J.; Li, Z. J. Mater. Chem.
2012, 22, 6689. (f) Sahu, D.; Padhy, H.; Patra, D.; Yin, J.-F.; Hsu, Y.-C.;
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J.-T.; Bhattacharya, D.; Lu, K.-L.; Wei, K.-H.; Lin, H.-C. Dyes Pigments
2012, 93, 1488. (i) Ramkumar, S.; Manoharan, S.; Anandan, S. Dyes
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Chem. B 1997, 101, 2583.
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€
Pastore, M.; Yum, J.-H.; Nazeeruddin, M. K.; Gratzel, M. Eur. J. Org.
(13) (a) Lucas, P.; Mehdi, N. E.; Ho, H. A.; Belanger, D.; Breau, L.
Synthesis 2000, 9, 1253. (b) Mitschke, U.; Bauerle, P. J. Chem. Soc.,
Perkin Trans. 1 2001, 740.
Chem. 2011, 6195. (b) Abbotto, A.; Manfredi, N.; Marinzi, C.; Angelis,
F. D.; Mosconi, E.; Yum, J.-H.; Xianxi, Z.; Nazeeruddin, M. K.;
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2006, 128, 9034.
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Org. Lett., Vol. 14, No. 24, 2012
6339

reactions. The crucial intermediate 2 was obtained via
a three-step process: (1) 1 was treated with n-BuLi and
subsequent protonation with water; (2) the resulting mono-
bromo intermediate was further treated with n-BuLi,
followed by addition of a fluorenone, which was prepared
by a SuzukiꢀMiyaura coupling reaction of 3,6-dibromo-
9H-fluoren-9-one16 and 4-tert-butylphenylboronic acid;17
(3) the corresponding alcohol intermediate was subjected to
acid-mediated intramolecular FriedelꢀCrafts cyclization in
acetic acid to give 2 (21%, three steps). Protodesilylation of
2 with tetrabutylammonium fluoride yielded spiro-BT (3),
which was then reacted with N-bromosuccinimide to give
dibromo-BT (4) in 96% yield. Compound 4 was in situ
transformed into a distannyl intermediate with n-BuLi,
followed by quenching with Bu3SnCl and then reaction
with 2-bromothiophene to give tetrathiophene 5 (70%).
The brominations at R- and γ-positions of the terminal
thiophenes of 5 were successfully achieved by treating with
bromine to give the tetrabromo intermediate, which was
reacted with n-BuLi followed by the treatment with TIPS-Cl
to produce 6 (32%, two steps). The bromo groups of 6
were converted to their corresponding carbaldehydes (7)
by lithiation with n-BuLi and subsequently quenching with
N,N-dimethylformamide. Finally, the dialdehyde 7 was
condensed with cyanoacetic acid in the presence of ammo-
nium acetate to afford the target dye BTB in 89% yield via
Figure 1. (a) Absorption (measured in THF solution 10ꢀ5 M) of
BTB. (b) Absorption spectra of BTB anchoring on the 7 μm
porous TiO2 nanoparticle film.
(2215 cmꢀ1) remained unchanged before and after inter-
acting with TiO2, indicating cyano groups were not in-
volved in anchoring functions. For BTB adsorbed on the
TiO2 film, the IR absorption band at 1700 cmꢀ1 corre-
sponding to the free carboxylic acid groups disappeared,
while the absorption band at 1620 cmꢀ1 (overlapped with
aromatic CdC stretching) appeared, which was identified
as asymmetric stretching of carboxylate adsorbed on TiO2.
The absence of free carboxylic groups as indicated by the
FTIRisa good indicationof the dianchoringadsorption of
BTB on the TiO2 surface, agreeing with the previously
reported behavior of dianchoring dyes.20
€
a Knoevenagel reaction.
The electronic absorption spectrum of BTB in solution
(10ꢀ5 M in THF) is shown in Figure 1a. The absorption
spectrum of BTB shows two major absorption bands at
λmax = 317 (ε, 30 800 Mꢀ1 cmꢀ1) and 490 nm (ε, 30 000
M
ꢀ1 cmꢀ1), respectively. The shorter wavelength band can
be attributed to the πꢀπ* transition of the local spiro-
configured 3,6-disubstituted fluorenyl group,16 whereas
the longer one can be assigned to the πꢀπ* transition of
the tetrathiophene-based chromophore. The absorption
spectrum of BTB anchoring on a 7 μm porous TiO2 nano-
particle film (Figure 1b) shows an absorption maximum
of 506 nm, which is red-shifted by 16 nm as compared to
that in THF solution. The red shift is possibly due to the
interaction between the TiO2 surface and dyes that increases
the delocalization of the π*-orbital of the entire conjugated
configuration and thus lowers the energy level of the
π*-orbital.18 The observed broad and red-shifted absorp-
tion of BTB on the TiO2 nanoparticle film will be beneficial
to improve the light-harvesting capability, resulting in the
higher photocurrent density and better PCEs.19
Figure 2. FTIR spectra of BTB measured as solid powders (b)
and as stained onto the 7 μm porous TiO2 nanoparticle film (9).
To gain more insight into the structural and electronic
features of BTB, DFT (density function theory) calcula-
tions wereperformed. The distancebetween two anchoring
carboxylic acid groups of BTB was calculated to be ca.
10.72 A (Figure S1), which nicely matches the distance
between two neighboring adsorption sites (10.23 A) on the
anatase TiO2 surface.21 This result confirms BTB posses-
ses a suitable molecular configuration for dianchoring
adsorption on the TiO2 surface. This dianchoring feature
is believed to benefit efficient surface adherence and
The interactions between carboxylic groups of BTB and
TiO2 were probed by FTIR. Figure 2 shows the FTIR
spectra of pristine BTB and BTB adsorbed on the TiO2
surface. Obviously, the cyano group absorption band
(16) Estrada, L. A.; Neckers, D. C. J. Org. Chem. 2009, 74, 8484.
(17) (a) Paul, V.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 1988,
1895. (b) Ashton, P. R.; Harris, K. D. M.; Kariuki, B. M.; Philp, D.;
Robinson, J. M. A.; Spencer, N. J. Chem. Soc., Perkin Trans. 2 2001,
2166.
(18) Tang, J.; Hua, J.; Wu, W.; Li, J.; Jin, Z.; Long, Y.; Tian, H.
Energy Environ. Sci. 2010, 3, 1736.
(19) Tang, Z.-M.; Lei, T.; Jiang, K.-J.; Song, Y.-L.; Pei, J. Chem.;
(20) Park, J.; Barolo, C.; Sauvage, F.; Barbero, N.; Benzi, C.;
€
Quagliotto, P.; Coluccia, S.; Censo, D. D.; Gratzel, M.; Nazeeruddin,
M. K.; Viscardi, G. Chem. Commun. 2012, 48, 2782.
€
Asian J. 2010, 5, 1911.
(21) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269.
6340
Org. Lett., Vol. 14, No. 24, 2012

Figure 4. (a) IPCE spectra of DSSCs based on BTB (red) and
N719 (blue). (b) Currentꢀvoltage curves for DSSCs incorpor-
ating BTB (red) and N719 (blue) under AM 1.5 G simulated
sunlight.
Figure 3. DFT HOMO and LUMO of BTB calculated at the
B3LYP/6-31G(d) level.
subsequent electron transfer. In addition, DFT calculations
suggest that the HOMO of BTB is mainly localized on the
spirofluorene-bridged oligothiophene, whereas the LUMO
is populated through the oligothiophene-conjugated back-
bone and cyanoacrylic acid fragments (Figure 3). This
spatially separated arrangement of HOMO/LUMO en-
sures not only effective intramolecular charge transfer
between donor and acceptor moieties but also efficient
photoinduced electron injection from BTB to the TiO2
electrode.
The electrochemical properties of BTB were investigated
by cyclic voltammetry. As shown in Figure S2, BTB
exhibits two reversibleoxidation waves. The first oxidation
potential (Eox, 1.25V vs normal hydrogen electrode, NHE)
is more positive than the iodide/triiodide (Iꢀ/I3ꢀ) redox
couple (0.4 V vs NHE), indicating oxidized BTB is able to
accept electrons from Iꢀ thermodynamically for effective
500 nm, indicating highly efficient DSSC performance.
Figure 4b shows the current densityꢀvoltage (JꢀV) curve
of the DSSC under standard global AM 1.5G solar
irradiation. The short-circuit photocurrent density (JSC),
open-circuit voltage (VOC), and fill factor (FF) of the
DSSC based on BTB are 12.51 mA/cm2, 0.74 V, and
0.70, respectively, yielding an overall PCE of 6.52%. For
a fair comparison, the N719-sensitized DSSC was also
fabricated and tested under similar conditions (see SI).
The efficiency of the BTB cell reaches ∼85% of the N719
cell efficiency.
In summary, a new tailor-made amine-free, dianchoring
DSSC sensitizer (BTB) featuring a tetrathiophene as the
donor skeleton and elaborately equipped with cyano-
acrylic acidanchoringgroupsatthe γ-positionsof terminal
thiophene rings has been synthesized and characterized.
The DFT calculation showed the distance (10.72 A)
between two carboxylic acid groups of BTB was close to
the two neighboring adsorption sites on the anatase TiO2
surface, rendering proper dianchoring adsorption, con-
firmed by the absence of free carboxylic acid in the FTIR
spectra. A DSSC sensitized with BTB exhibited a high
PCE of 6.52% with JSC = 12.51 mA/cm2, VOC = 0.74 V,
and FF = 0.70. Our study introduces a new approach to
developing amine-free, dianchoring sensitizers for efficient
and stable DSSCs.
dye regeneration. The zeroꢀzero excitation energy (E0ꢀ0
,
2.31 eV) estimated from the intersection of the absorp-
tion and emission spectra was combined with the ground-
state oxidation potential (Eox) to calculate the excited-
state oxidation potential (Eox* = Eox ꢀ E0ꢀ0). The more
negative Eox* (ꢀ1.06 V vs NHE) of BTB relative to the
conduction band edge of TiO2 (ꢀ0.5 V vs NHE)22 reveals
the electron injection from excited BTB to TiO2 should be
energetically favorable.
The photovoltaic characteristics of BTB as a sensitizer
for DSSCs were evaluated with a sandwich DSSC cell
using 0.6 M 1-butyl-3-methylimidazolium iodide (BMII),
0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M
guanidinium thiocyanate in a mixture of acetonitrileꢀ
valeronitrile (85: 15, v/v) as the redox electrolyte (details
of the device preparation and characterization are de-
scribed in the Supporting Information (SI)). The incident
monochromatic photon-to-current conversion efficiency
(IPCE) spectrum of the DSSC is shown in Figure 4a. The
DSSC based on BTB shows an IPCE of >70% from 330
to 570 nm and reaches an IPCE maximum of 91% around
Acknowledgment. The authors gratefully acknowledge
financial support from the National Science Council of
Taiwan (NSC 99-2221-E-002-118-MY3, 98-2119-M-002-
007-MY3).
Supporting Information Available. Synthesis, charac-
1
terization, copies of H and 13C NMR spectra, device
fabrication, and optimization of solar cells. This material
is available free of charge via the Internet at http://pubs.
acs.org.
(22) Klein, C.; Nazzeeruddin, M. K.; Censo, D. D.; Liska, P.;
€
Gratzel, M. Inorg. Chem. 2004, 43, 4216.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 24, 2012
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