N-fused carbazole-zinc porphyrin-free-base porphyrin triad for efficient near-IR dye-sensitized solar cells

Article abstract of DOI:10.1039/c0cc03306e

N-fused carbazole-zinc porphyrin-free-base porphyrin triad featuring an ethynyl-linkage was synthesized; efficient sensitization as long as 900 nm was demonstrated and an overall light-to-electricity conversion effciency of 5.21% was achieved under AM 1.5

Full text of DOI:10.1039/c0cc03306e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 4010–4012
www.rsc.org/chemcomm
COMMUNICATION
N-fused carbazole–zinc porphyrin–free-base porphyrin triad for efficient
near-IR dye-sensitized solar cellsw
Yizhu Liu,ab Hong Lin,b Joanne Ting Dy,a Koichi Tamaki,a Jotaro Nakazaki,a
Daisuke Nakayama,a Satoshi Uchida,a Takaya Kuboa and Hiroshi Segawa*a
Received 17th August 2010, Accepted 17th January 2011
DOI: 10.1039/c0cc03306e
N-fused carbazole–zinc porphyrin–free-base porphyrin triad
featuring an ethynyl-linkage was synthesized; efficient sensitization
as long as 900 nm was demonstrated and an overall light-to-
electricity conversion effciency of 5.21% was achieved under
AM 1.5 G one sun illumination.
macro-ring is only limited to diphenylamino groups, while
modification with N-fused heterocycles, like carbazole, which
has demonstrated novel antenna function in heteroleptic Ru
dyes9 and also serves independently as an efficient sensitizer,10
is still absent, in the present study, a novel N-fused carbazole–
zinc porphyrin–free-base porphyrin donor–donor–acceptor
triad featuring an ethynyl-linkage was designed and synthesized.
The new compound was coded as DTBC (3,6-di-tert-butyl-
carbazol-9-yl) and the molecular structure is illustrated in
Fig. 1.
Dye-sensitized solar cells (DSCs)1 have been widely regarded
as next-generation photovoltaics for providing electricity at
lower expense and with more versatility. Although an overall
light-to-electricity energy conversion efficiency above 11% has
been entrusted with heteroleptic ruthenium sensitizers,1b,c the
limited resource and the environmental issue concerning Ru
usage make it necessary to seek alternative dyes. Recently, a
series of donor-substituted porphyrins featuring direct fusion
of diphenylamino groups to the meso-position of the porphyrin
macro-ring has been developed.2 They have shown intriguing
potential to fabricate highly efficient DSCs and, by device
optimization, comparable energy conversion efficiency above
11% has been demonstrated for the first time using such
Ru-free dyes.2a However, although donor substitution significantly
enhanced the light-harvesting abilities of such porphyrin
monomer dyes, the light absorption range is still limited,
which leaves space for further performance promotion by
extending the absorption onset into the near-IR region.3–5
On the other hand, meso,meso-ethynyl-linked porphyrin
dimers6 show moderate electronic conjugation and strong
excitonic coupling, and sensitizers based on such building
blocks have been reported by both Prof. Diau et al. and our
group for efficient near-IR DSCs.7 Considering the energy
level difference between zinc porphyrin and free-base porphyrin,8
in the present study, we combined the two strategies above by
inserting the free-base porphyrin component as an extended
acceptor into the commonly used ethynylbenzoic acid anchor.2,7b
Meanwhile, as the donor-substitution of the porphyrin
The detailed synthetic procedure is shown in the ESIw.
Briefly, an unprecedented substitution of porphyrin macro-
ring with heterocycles directly at the N atom and, amination of
meso-Br-substituted zinc porphyrin (5, as coded in ESIw)
with carbazole was much more challenging than it seemed
with diphenylamines, due to the reduced donating ability of
the N lone electron pair participating in conjugation and
increased steric hindrance from the completely planar configuration
of the carbazole moiety.11 Fortunately, N-fused carbazole-
substitution was accomplished in a moderate yield by increasing
the base equivalence and extending the reaction time. The
carbazole-fused zinc porphyrin was coupled with free-base
porphyrin by Sonogashira reaction,12 and the resulted
1
target compound was identified by H NMR, high-resolution
ESI-mass, MALDI-TOF-mass and FT-IR spectroscopy.
UV-vis absorption spectra of DTBC in chloroform–
methanol solution and adsorbed on TiO2 film are shown in
Fig. 2 and corresponding properties are listed in Table S1
(ESIw). As expected, the triad DTBC shows dramatically
red-shifted Q-bands, the maximum of which is located at
722 nm, compared to those reported for porphyrin monomers2
due to ethynyl-bridged inter-porphyrin electronic coupling.6
a Research Center for Advanced Science and Technology,
The University of Tokyo, 4-6-1, Komaba, Meguro-ku,
Tokyo 153-8904, Japan. E-mail: csegawa@mail.ecc.u-tokyo.ac.jp;
Fax: +81-3-5452-5299; Tel: +81-3-5452-5295
b State Key Laboratory of New Ceramics and Fine Processing,
Department of Materials Science and Engineering, Tsinghua
University, Beijing 100084, China
w Electronic supplementary information (ESI) available: Synthesis,
cell fabrication, characterizations, supplementary Table S1 and
Fig. S1–S3. See DOI: 10.1039/c0cc03306e
Fig. 1 Molecular structure of DTBC.
c
4010 Chem. Commun., 2011, 47, 4010–4012
This journal is The Royal Society of Chemistry 2011

View Article Online
but also a significant promotion compared to zinc porphyrin
dimers ever reported with an even larger injection driving
force.7b Such an enhanced spectral responsivity induced by the
extended triad structure results in an overall photovoltaic
performance of Jsc 14.26 mA cmÀ2, Voc 547 mV, FF 0.67
and Z 5.21%, which is, to the best of our knowledge, the
highest value ever reported for an ethynyl-linked porphyrin
array used for DSCs. However, considering the small injection
driving force due to the narrow optical bandgap, a TBP-free
electrolyte was adopted to lower the TiO2 conduction
bandedge.16 Consequent promotion in electron injection renders
not only a better utilization of longer-wavelength light as
indicated by the red-shift of the action spectrum threshold,16b
but also a drastic elevation of IPCE values up to 80%, yielding
a corresponding Jsc of 18.16 mA cmÀ2. Such an excellent
panchromatic responsivity constrasts most of the long-wave-
length sensitizers even using TBP-free electrolytes despite their
superior light absorptivity5 and is also significantly improved
compared to our previously reported zinc porphyrin dimer
under the same electrolyte conditions,7a mainly attributed
to the extended donor–donor–acceptor triad structure.
Unfortunately, an enlarged aromatic conjugation system
causes severe charge recombination with electrolyte redox
species,17 and combined with positive conduction bandedge
shift, the Voc as obtained is around only 400 mV. However, by
interfacial engineering like Al2O3-coating18 or electrolyte
optimization, it is possible to result in considerable suppression
of charge recombination and thus remarkable photovoltaic
performance based on the efficient panchromatic photon-to-
electron conversion demonstrated above. Such a detailed
study will be reported in the next publication.
Fig. 2 UV-vis absorption spectra of DTBC in chloroform–methanol
(2/1) solution (solid line) and adsorbed on TiO2 film (dash dot line).
This value is also significantly bathochromic even with reference
to our previously reported ethynyl-linked zinc porphyrin dimer
in the same solvent composition,7a which can be attributed to the
synergetic influence of carbazole-substitution and extended
acceptor component by the free-base porphyrin.13 In the
short-wavelength region, characteristic Soret-band splitting
was observed, ascribed to strong axial Bx excitonic coupling
and rotation-determined By coupling, respectively.14 Spectrum
on TiO2 resembles that in solution in the Q-band region,
indicating negligible dye aggregation due to high coadsorbent
(chenodeoxycholic acid, CDCA) equivalence during the dye
loading process (ESIw). A new band related to the long-lived
dye cation at around 940 nm evolved, demonstrating its slow
recombination with injected TiO2 electrons.2e
The oxidation and reduction potentials of DTBC were
measured by differential pulse voltammetry (DPV, Fig. S1,
ESIw) and are also summarized in Table S1 (ESIw). Injection
potential (ES+/S*) was calculated by subtracting the excitation
energy (E0À0) from the first oxidation potential (ES+/S) of the
sensitizer. DTBC shows ES+/S of +1.04 V vs. NHE, which is
In order to further understand the structural characteristics
of this novel triad molecule, density-functional theory (DFT)
calculation was performed at the B3LYP/6-31G level of
theory, and orbital distribution of different energy levels were
illustrated in Fig. S2 (ESIw). Unsymmetrical spatial distribution
was observed for HOMOs and LUMOs as expected. However,
it is noteworthy that the location of HOMO orbital is absent
on the carbazole moiety. A similar situation was reported for
carbazole-fused heteroleptic Ru dyes9c which can be ascribed
to the lower HOMO energy level than that of metal complex
due to its attenuated electron density mentioned above. In our
case, despite such an absence, substantial electronic orbital
distribution on carbazole is still manifested for HOMO-1 and
HOMO-2, which, especially the former, may also be involved
and hybridized in the actual optical transition.19 This contrasts
some N-fused carbazole-derivative ruthenium dyes9 and gives
rise to a larger transition dipole moment and thus higher
oscillator strength, accounting for excellent light-harvesting
abilities.20 Besides, as a well-known hole carrier,21 there is also
an opportunity for the N-fused carbazole to accept the positive
charge generated after electron injection from the porphyrin
framework so as to facilitate dye regeneration. However, due
to its lower energy level compared to zinc porphyrin, this may
happen only after additional light absorption.9c
low enough below the redox potential of IÀ/I3 to assure
À
efficient regeneration. However, due to the small optical
bandgap, the injection potential (À0.67 V vs. NHE) is only
slightly above the conduction band edge of TiO2 (commonly
recognized as À0.5 V vs. NHE15). Such a position, although
thermodynamically feasible as well for electron injection, is
0.05 V even more negative than that ever reported for an
ethynyl-linked zinc porphyrin dimer,7b resulting from the
low-lying energy level of free-base porphyrin and the weak
electron-donating property of carbazole moiety, which will be
discussed below.
Photovoltaic performances and photocurrent action spectra
of DSCs incorporated with DTBC are shown in Fig. 3.
Relevant parameters are listed in Table S1. A detailed cell
fabrication procedure is presented in the ESI.w The electrolyte
was composed of 0.1 M LiI, 25 mM I2, 0.6 M 1,2-dimethyl-3-
propylimidazolium iodide (DMPII) and 0.2 M 4-tert-butyl-
pyridine (TBP) in acetonitrile. As illustrated in Fig. 3b, the
IPCE spectrum shows an onset as long as 900 nm and
steeply reaches almost a plateau value close to 60% from
around 800 nm until 400 nm, demonstrating efficient panchromatic
sensitization. This is not only an effective bathochromic
extension of the sensitization spectrum compared to
monomers where IPCE values at 800 nm were almost zero,2
In conclusion,
a N-fused carbazole–zinc porphyrin–
free-base porphyrin triad featuring an ethynyl-linkage has
been designed and synthesized. Based on already reported
highly efficient porphyrin monomers, by inserting a free-base
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4010–4012 4011

View Article Online
5 (a) H. Choi, J. Kim, K. Song, J. Ko, M. K. Nazeeruddin and
M. Gratzel, J. Mater. Chem., 2010, 20, 3280; (b) T. Greiger,
¨
S. Kuster, J. Yum, S. Moon, M. K. Nazeeruddin, M. Gratzel
¨
and F. Nuesch, Adv. Funct. Mater., 2009, 19, 2720; (c) L. Beverina,
¨
R. Ruffo, C. M. Mari, G. A. Pagani, M. Sassi, F. D. Angelis,
S. Fantacci, J. Yum, M. Gratzel and M. K. Nazeerddin,
¨
ChemSusChem, 2009, 2, 621; (d) H. Tian, X. Yang, R. Chen,
A. Hagfeldt and L. Sun, Energy Environ. Sci., 2009, 2, 674;
(e) L. Macor, F. Fungo, T. Tempesti, E. N. Durantini, L. Otero,
E. M. Barea, F. Fabregat-Santiago and J. Bisquert, Energy Environ.
Sci., 2009, 2, 529; (f) J. Cid, J. Yum, S. Jang, M. K. Nazeeruddin,
E. Martınez-Ferrero, E. Palomares, J. Ko, M. Gratzel and T. Torres,
´
¨
Angew. Chem., Int. Ed., 2007, 46, 8358; (g) S. Erten-Ela,
M. D. Yilmaz, B. Icli, Y. Dede, S. Icli and E. U. Akkaya, Org. Lett.,
2008, 10, 3299.
6 (a) V. S. Lin, S. G. DiMagno and M. J. Therien, Science, 1994, 264,
1105; (b) R. Kumble, S. Palese, V. S. Lin, M. J. Therien and
R. M. Hochstrasser, J. Am. Chem. Soc., 1998, 120, 11489;
(c) R. Shediac, M. H. B. Gray, H. T. Uyeda, R. C. Johnson,
J. T. Hupp, P. J. Angiolillo and M. J. Therien, J. Am. Chem. Soc.,
2000, 122, 7017; (d) I. V. Rubtsov, K. Susumu, G. I. Rubtsov and
M. J. Therien, J. Am. Chem. Soc., 2003, 125, 2687;
(e) N. P. Redmore, I. V. Rubtsov and M. J. Therien, J. Am. Chem.
Soc., 2003, 125, 8769; (f) T. Zhang, Y. Zhao, I. Asselberghs,
A. Persoons, K. Clays and M. J. Therien, J. Am. Chem. Soc.,
2005, 127, 9710; (g) J. A. N. Fisher, K. Susumu, M. J. Therien and
A. G. Yodh, J. Chem. Phys., 2009, 130, 134506.
7 (a) J. T. Dy, K. Tamaki, Y. Sanehira, J. Nakazaki, S. Uchida,
T. Kubo and H. Segawa, Renewable Energy 2010 Proceedings,
Yokohama, 2010, O-Pv-11-7; (b) C. Mai, W. Huang, H. Lu,
C. Lee, C. Chiu, Y. Liang, E. W. Diau and C. Yeh,
Chem. Commun., 2010, 46, 809.
8 (a) F. Li, S. Gentemann, W. A. Kalsbeck, J. Seth, J. S. Lindsey,
D. Holton and D. F. Bocian, J. Mater. Chem., 1997, 7, 1245;
(b) B. Ventura, F. Barigelletti, F. Lodato, D. L. Officer and
L. Flamigni, Phys. Chem. Chem. Phys., 2009, 11, 2166.
9 (a) J. Li, C. Chen, J. Chen, C. Tan, K. Lee, S. Wu, Y. Tung,
H. Tsai, K. Ho and C. Wu, J. Mater. Chem., 2010, 20, 7158;
(b) C. Chen, N. Pootrakulchote, S. Wu, M. Wang, J. Li, J. Tsai,
Fig. 3 Photovoltaic properties of DTBC in DSCs using electrolyte
containing 0.2 M TBP (solid line) and without TBP (dashed line).
(a) I–V curves of DSCs under AM 1.5 G one sun illumination
(100 mW cmÀ2) with an active area of 0.16 cm2; (b) corresponding
incident photon-to-current conversion efficiency (IPCE) spectra.
porphyrin as an extended acceptor and novel N-fused carbazole-
substitution, efficient sensitization in the near-IR region was
demonstrated and an overall energy conversion efficiency of
5.21% was remarked. The spectral responsivity can be further
enhanced by interfacial engineering to 80% with a broad
sensitization range above 900 nm, making it a promising
candidate for high-efficiency DSCs. Investigation for
fundamental understanding of the photophysical and photo-
electrochemical properties and efforts for further performance
promotion are now in progress.
This work was supported by the Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST Program) on the development of organic photo-
voltaics toward a low-carbon society, from the Japanese
Government.
C. Wu, S. M. Zakeeruddin and M. Gratzel, J. Phys. Chem. C,
¨
2009, 113, 20752; (c) C. Chen, J. Chen, S. Wu, J. Li, C. Wu and
K. Ho, Angew. Chem., Int. Ed., 2008, 47, 7342.
10 (a) C. Teng, X. Yang, C. Yuan, C. Li, R. Chen, H. Tian, S. Li,
A. Hagfeldt and L. Sun, Org. Lett., 2009, 11, 5542; (b) Z.
Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi,
A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008,
20, 3993.
Notes and references
1 (a) B. O’Regan and M. Gratzel, Nature, 1991, 353, 737;
¨
(b) M. K. Nazeeruddin, F. D. Angelis, S. Fantacci, A. Selloni,
G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Gratzel, J. Am.
¨
Chem. Soc., 2005, 127, 16835; (c) C. Chen, M. Wang, J. Li,
11 J. Hartwig, Nature, 2008, 455, 314.
12 R. Chinchilla and C. Najera, Chem. Rev., 2007, 107, 874.
´
N. Pootrakulchote, L. Alibabaei, C. Ngoc-Ie, J. Decoppet,
J. Tsai, C. Gratzel, C. Wu, S. M. Zakeeruddin and M. Gratzel,
13 J. T. Dy, K. Tamaki, Y. Sanehira, J. Nakazaki, S. Uchida,
T. Kubo and H. Segawa, Electrochemistry, 2009, 77, 206.
14 (a) M. Kasha, H. R. Rawls and M. A. El-Bayoumi, Pure Appl.
Chem., 1965, 11, 371; (b) D. Kim and A. Osuka, Acc. Chem. Res.,
2004, 37, 735.
¨
ACS Nano, 2009, 3, 3103.
¨
2 (a) T. Bessho, S. M. Zakeeruddin, C. Yeh, E. W. Diau and
M. Gratzel, Angew. Chem., Int. Ed., 2010, 43, 6646;
¨
(b) H. Imahori, Y. Matsubara, H. Iijima, T. Umeyama,
Y. Matano, S. Ito, M. Niemi, N. V. Tkachenko and
H. Lemmetyinen, J. Phys. Chem. C, 2010, 114, 10656; (c) S. Wu,
H. Lu, H. Yu, S. Chuang, C. Chiu, C. Lee, E. W. Diau and C. Yeh,
Energy Environ. Sci., 2010, 3, 949; (d) C. Hsieh, H. Lu, C. Chiu,
C. Lee, S. Chuang, C. Lun, W. Yen, S. Hsu, E. W. Diau and C. Yeh,
J. Mater. Chem., 2010, 20, 1127; (e) H. Lu, C. Tsai, W. Yen, C. Hsieh,
C. Lee, C. Yeh and E. W. Diau, J. Phys. Chem. C, 2009, 113, 20990;
(f) H. Lu, C. Mai, C. Tsai, S. Hsu, C. Hsieh, C. Chiu, C. Yeh and
E. W. Diau, Phys. Chem. Chem. Phys., 2009, 11, 10270; (g) C. Lee,
H. Lu, C. Lan, Y. Huang, Y. Liang, W. Yen, Y. Liu, Y. Lin,
E. W. Diau and C. Yeh, Chem.–Eur. J., 2009, 15, 1403.
15 H. H. Kung, H. S. Jarrett, A. W. Sleight and A. Ferretti, J. Appl.
Phys., 1977, 48, 2463.
16 (a) G. Boschloo, L. Haggman and A. Hagfeldt, J. Phys. Chem. B,
¨
¨
2006, 110, 13144; (b) G. Boschloo, H. Lindstrom, E. Magnusson,
A. Holmberg and A. Hagfeldt, J. Photochem. Photobiol., A, 2002,
148, 11.
17 B. C. O’Regan, I. Lopez-Duarte, M. V. Martinez-Diaz, A. Fomeli,
´
J. Albero, A. Morandeira, E. Palomares, T. Torres and
J. R. Durrant, J. Am. Chem. Soc., 2008, 130, 2906.
18 (a) E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz and
J. R. Durrant, J. Am. Chem. Soc., 2003, 125, 475;
(b) C. Prasittichai and J. T. Hupp, J. Phys. Chem. Lett., 2010, 1,
1611.
3 (a) Y. Ooyama and Y. Harima, Eur. J. Org. Chem., 2009, 2903;
(b) A. Mishra, M. K. R. Fischer and P. Bauerle, Angew. Chem.,
¨
Int. Ed., 2009, 48, 2474; (c) H. Imahori, T. Umeyama and S. Ito,
Acc. Chem. Res., 2009, 42, 1809.
19 E. J. Baerends, G. Ricciardi, A. Rosa and S. J. A. van Gisbergen,
Coord. Chem. Rev., 2002, 230, 5.
4 (a) H. J. Snaith, Adv. Funct. Mater., 2010, 20, 13;
(b) T. W. Hamann, R. A. Jensen, A. B. F. Martinson,
H. V. Ryswyk and J. T. Hupp, Energy Environ. Sci., 2008, 1, 66.
20 M. Klessinger and J. Michl, in Excited States and Photochemistry
of Organic Molecules, VCH, New York, 1995, ch. 1, pp. 21–27.
21 M. Biswas and S. K. Das, Polymer, 1982, 23, 1713.
c
4012 Chem. Commun., 2011, 47, 4010–4012
This journal is The Royal Society of Chemistry 2011