A ruthenium complex with superhigh light-harvesting capacity for dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.200601463

Dyeing for sunlight: A new dye CYC-B1 (see picture), with the highest absorption coefficient known so far for Ru-based photosensitizers, was prepared. Solar cells sensitized with CYC-B1 have a very high current density, and the conversion efficiency of CYC-B1 is 10% higher than that of cis-di(thiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) under AM1.5 simulated sunlight. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200601463

Communications
highest efficiency was attempted to increase the molar
absorption coefficient and reduce the number of protons on
the complexes. 4,4’-Dicarboxylic acid-2,2’-bipyridine (dcbpy)
has been considered as the best anchoring ligand in Ru
sensitizers.[15] Finding new metal-complex sensitizers with
higher conversion efficiency was achieved by modifying one
of the anchoring ligands. Replacement of one of the dcbpy
anchoring ligands with a highly conjugated ancillary ligand
represents a molecular engineering approach for increasing
the absorption coefficient and therefore the photocurrent
density of the sensitizers as reported by Grätzel and co-
workers.[16–20] Herein, we report a new ruthenium photo-
sensitizer CYC-B1 in which one of the dcbpy ligands in N3
was replaced with abtpy, a bipyridine ligand substituted with
alkyl bithiophene groups. CYC-B1 has the highest absorption
coefficient among the Ru-based photosensitizers used in
DSSCs, and its power-conversion efficiency is 10% higher
than that of N3 under the same cell fabrication and measuring
procedures carried out in our laboratory.
Photosensitizers
DOI: 10.1002/anie.200601463
A Ruthenium Complex with Superhigh
Light-Harvesting Capacity for Dye-Sensitized
Solar Cells**
CYC-B1 was prepared in a typical one-pot synthesis,[20]
and its structure (Scheme 1) was identified from NMR
Chia-Yuan Chen, Shi-Jhang Wu, Chun-Guey Wu,*
Jian-Ging Chen, and Kuo-Chuan Ho
A dye-sensitized solar cell (DSSC) using Ru complexes as a
photosensitizer was first reported by OꢀRegan and Grätzel in
1991.[1] The low-cost, easy preparation make DSSC one of the
most promising photovoltaic cells for conversion of sunlight
to electricity. Numerous sensitizers have been prepared, and
their performance has been tested.[2–10] A conversion effi-
ciency of up to 11% was achieved by using cis-di(thiocyana-
to)bis(2,2’-bipyridyl-4,4’-dicarboxylate)ruthenium(II) (N3) as
a photosensitizer.[11–12] However, the conversion efficiency of
DSSCs is still lower than that of the silicon-based photovoltaic
cells. To obtain a high conversion efficiency, optimization of
the short-circuit photocurrent (Isc) and open-circuit potential
(Voc) of the cell is essential. The value of Voc depends on the
edge of conduction band in TiO2 and the redox potential of IÀ/
I3À, otherwise Isc is related to the interaction between TiO2
and the sensitizer as well as the absorption coefficient of the
sensitizer. The conduction band of TiO2 was known to have a
Nernstian dependence on pH.[13–14] Thus, the molecular
engineering of the ruthenium complexes for achieving the
Scheme 1. The structure of CYC-B1.
spectroscopy, mass spectrometry, and elemental analysis.
The electronic absorption spectra of the free dcbpy and
abtpy ligands, CYC-B1, and N3 in DMF are displayed in
Figure 1, and the optical data are summarized in Table 1. The
absorption maximum (lmax) assigned to the p–p* transition
for dcbpy and abtpy are 299 nm and 375 nm, respectively. The
absorption maximum of abtpy is red-shifted by 76 nm, which
is attributed to its longer conjugation length, compared to that
of dcbpy. The absorption spectrum of CYC-B1 shows three
bands centered at 553 nm, 400 nm, and 312 nm. Based on
comparison with the free abtpy ligand and the homoleptic
complex N3, the absorption band at 312 nm for CYC-B1 is
assigned to the intraligand p–p* transitions of dcbpy. The
band centered at 400 nm contains two components: the p–p*
transition of the abtpy ligand and one of the metal-to-ligand
charge transfer (MLCT) transitions for CYC-B1. The p–p*
transition of abtpy has a higher contribution since this band
has a much stronger absorption coefficient than the corre-
sponding band observed in N3. The absorption band at
553 nm is the characteristic MLCT transition for determining
[*] C.-Y. Chen,S.-J. Wu,Prof. C.-G. Wu
Department of Chemistry
National Central University
Jung-Li,Taiwan 32054 (Republic of China)
Fax: (+886)3-422-7664
E-mail: t610002@cc.ncu.edu.tw
J.-G. Chen,Prof. K.-C. Ho
Department of Chemical Engineering
National Taiwan University
Taipei,10617,Taiwan (Republic of China)
[**] This work was financially supported by the National Science
Council,Taiwan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Figure 1. UV/Vis absorption spectra of dcbpy,abtpy, N3,and CYC-B1
in DMF.
the conversion efficiency of CYC-B1. This MLCT band may
come from one of the two different conjugated ligands in
CYC-B1. However, the COOH substituents on the dcbpy
anchoring ligand have a higher electron-withdrawing ability
than the alkyl bithiophene substituents of the abtpy ancillary
ligand. The LUMO of the dcbpy ligand has a lower energy
than that of abtpy.[21] Therefore, the lower-energy MLCT
band in CYC-B1 corresponds to the charge transfer between
the metal center and the dcbpy anchoring ligand. The
absorption maxima of both MLCT bands in CYC-B1 are
larger than those of N3, thus suggesting that the abtpy ligand
has lifted the energy level of the metal center. Encouragingly,
the molar absorption coefficient of the lower-energy MLCT
band of CYC-B1 is very high, higher than all Ru-based dyes
used in solar cells.
To confirm the identity of the lower-energy MLCT band
in CYC-B1, the location of the HOMOs and LUMOs in
CYC-B1 were calculated with a semiempirical computational
method (ZINDO/1),[22–25] and the results are displayed in
Figure 2. It is known[26] that the HOMO and HOMOÀ1 of N3
have amplitudes on the ruthenium center and the NCS ligands
whereas its LUMO and LUMO + 1 are localized homoge-
neously on the two dcbpy ligands, thus facilitating the
injection of electrons from the excited Ru complex to TiO2.
The locations of HOMO and HOMOÀ1 in CYC-B1 are
similar to those of N3, mainly on the metal center and NCS
ligands. However, the LUMO is located only on the dcbpy
anchoring ligand through which the excited electrons are
directly transferred to TiO2. These results suggest that CYC-
Figure 2. Graphical representation of the frontier orbitals of CYC-B1
(O red,S yellow,C green,N blue,Ru gray).
B1 is better than N3 as a photosensitizer for DSSCs, since
both dcbpy ligands on N3 cannot bind to TiO2 particles
simultaneously and the excited electron in the moiety that is
not directly connected to TiO2 provides a very small
contribution to the conversion efficiency of the DSSCs.
Furthermore, for a high-efficiency sensitizer in DSSCs, the
energy level of its LUMO should be well-matched with the
lower limit of the conduction band in TiO2, and its HOMO
should be sufficiently low in energy to accept electrons from
the redox electrolyte such as IÀ/I3À. The energy level of the
frontier orbitals for CYC-B1 that was established from the
electrochemical and absorption data is also listed in Table 1. It
was found that the energy levels of the HOMO and LUMO
for CYC-B1 match well with the redox potential of IÀ/I3À and
the LUMO of TiO2. Therefore, the higher molar absorption
coefficient of CYC-B1 can be expected to have a better
conversion efficiency than N3.
The monochromatic incident photon-to-current conver-
sion efficiency (IPCE) and characteristic photocurrent den-
sity versus voltage curves for CYC-B1- and N3-based cells are
illustrated in Figure 3, and the detailed parameters of the
devices are also included in Table 1. The broad IPCE curve
for CYC-B1 covers almost the entire visible spectrum from
Table 1: Physical data and performance of CYC-B1- and N3-sensitized cells.
e [”104 mÀ1 cmÀ1] (lmax [nm])
Eox of RuIII/II
EHOMO
[eV]
ELUMO
[eV]
Cell performance
[a]
[b]
Complex
p–p*
p–p* or 4d–p*
4d–p*
[V vs. Fc/Fc+]
Isc [mAcmÀ2
]
Voc [V]
FF
h [%]
CYC-B1
N3
3.58 (312)
5.11 (311)
4.64 (400)
1.52 (385)
2.12 (553)
1.45 (530)
0.30
0.40
5.10
5.20
3.52
3.52
23.92
21.32
0.65
0.66
0.55
0.54
8.54
7.70
[a] The Ag/AgNO3 reference electrode was calibrated with a ferrocene/ferrocinium (Fc/Fc+) redox couple. The electrochemical experiments were
carried out in 0.1m [nBu4N]BF4/DMF solution. [b] The absorption onset of each complex was used to calculate the energy gap and the frontier orbitals.
The energy levels of the HOMO and LUMO were calculated with the following formulas: EHOMO =EoxÀEFc/Fc+ +4.8 eV; ELUMO =EHOMOÀEg; Eg =
absorption threshold of the Ru complexes.
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Communications
ligands.[29] As a consequence, the band resulting from charge
transfer from the metal center to the anchoring ligand is red-
shifted, and upon illumination of the sample the electrons on
the metal center are transferred to the anchoring dcdpy ligand
where electrons can move to the outer circuit through the
TiO2 particles more efficiently. These results account for the
high current density of the CYC-B1-sensitized cell.
In this study we found a way to improve the power-
conversion efficiency of DSSCs by molecular engineering of
an Ru-based photosensitizer. The CYC-B1 compound
reported herein is a representative ruthenium complex with
high current density and conversion efficiency. We believe
that ruthenium-based DSSCs with even higher efficiency can
be obtained by fine-tuning the structure of the ruthenium
complexes.
Figure 3. Current density vs. voltage characteristic of photovoltaic
devices with N3 and CYC-B1 as sensitizers under AM1.5 simulated
sunlight (100 mWcmÀ2) illumination; inset: the typical photocurrent-
action spectra of the photovoltaic devices (thickness of TiO2: 20 mm;
cell active area: 0.25 cm2).
Experimental Section
CYC-B1 was prepared in a typical one-pot synthesis[20,30] by using the
new ligand abtpy, which was devised and prepared in our laboratory.
LRMS (FAB): calcd m/z: 1170.17; found: 1170.2 (m) [M]+, 1112.2 (s)
[MÀNCS]+; 1H NMR (500 MHz, [D6]DMSO, atom-numbering
scheme is provided in the Supporting Information): d = 9.49 (d, H6,
J = 5.7 Hz), 9.15 (d, H6’, J = 5.7 Hz), 9.10 (s, H3), 8.96 (s, H3’), 8.94 (s,
H3’’), 8.80 (s, H3’’’), 8.32 (d, H5, J = 5.7 Hz), 8.19 (d, H6’’, J = 3.8 Hz),
8.11 (d, H5’, J = 5.7 Hz), 7.99 (d, H5’’, J = 3.8 Hz), 7.95 (d, H6’’’, J =
350 to 700 nm with a maximum of 77.5%. Unlike the N3 cell,
the IPCE curves for the CYC-B1 cell are not consistent with
its absorption spectrum in DMF. The band centered at
400 nm, which mainly comes from the abtpy intraligand p–p*
transitions, has a smaller contribution to the photon-to-
current conversion efficiency. Under AM1.5 sunlight illumi-
nation (100 mWcmÀ2), the CYC-B1-sensitized solar cell gave
a power-conversion efficiency (h) of 8.54%, which is 10%
higher than that of the N3-sensitized cell under the same cell
fabrication and measuring procedures. Both the Voc and
FF values of the CYC-B1-sensitized cell are close to those of
the N3-sensitized cell. The higher h value of the CYC-B1 cell
(compared to the N3 cell) comes from the higher Isc value,
which is mainly attributed to the high absorption coefficient
of the MLCT band for dcbpy. Furthermore, the objective of
this work is to find the best photosensitizer for DSSCs. The
alkyl group on the abtpy ancillary ligand can prevent the
complexes from water-induced desorption of the dye mole-
cules from the TiO2 surface.[27] We could anticipate that
DSSCs sensitized with CYC-B1 may also exhibit good
stability.
5.9 Hz), 7.64 (d, H5’’’, J = 5.9 Hz), 7.47 (d, H9’’, J = 3.7 Hz), 7.37 (d, H9’’’
,
J = 6.8 Hz), 7.35 (d, H10’’, J = 3.7 Hz), 7.31 (d, H14’’, J = 3.4 Hz), 7.28 (d,
H10’’’, J = 6.8 Hz), 7.21 (d, H15’’, J = 3.4 Hz), 6.86 (d, H14’’’, J = 3.4 Hz),
6.80 (d, H15’’’, J = 3.4 Hz), 2.79 (t, 2H), 2.74 (t, 2H), 1.61 (t, 2H), 1.57
(t, 2H), 1.23 (m, 20H), 0.85 (t, 3H), 0.83 ppm (t, 3H). Elemental
analysis (%) calcd for C56H56N6O4S6Ru: C 57.46, H 4.82, N 7.18,
S 16.44; found: C 55.14, H 5.05, N 6.81, S 15.44. The detailed proce-
dures for synthesis and characterization of the ligand and complex as
well as the DSSC fabrication and measuring conditions can be found
in the Supporting Information.
Received: April 13, 2006
Revised: June 7, 2006
Published online: July 25, 2006
Keywords: charge transfer · conjugation · dyes/pigments ·
.
photosensitizers · ruthenium
Enhancing the absorption coefficient of the MLCT band
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and band-gap tuning. Furthermore, sulfur has greater radial
extension in its bonding than the second-row elements such as
carbon. Therefore, thiophene is a more electron-rich moiety;
incorporation of thiophene onto bipyridine ligands can raise
the energy levels of the metal center and the LUMO of the
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