Highly efficient N-heterocyclic carbene/pyridine-based ruthenium sensitizers: Complexes for dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201001628

A new generation: The incorporation of N-heterocyclic carbene/pyridine- based ruthenium sensitizers derived from benzimidazolium salts into dye-sensitized solar cells results in superior current densities, cell voltages, and photoelectric conversion effic

Full text of DOI:10.1002/anie.201001628

Angewandte
Chemie
DOI: 10.1002/anie.201001628
Photosensitizers
Highly Efficient N-Heterocyclic Carbene/Pyridine-Based Ruthenium
Sensitizers: Complexes for Dye-Sensitized Solar Cells**
Wei-Chun Chang, Huei-Siou Chen, Ting-Yu Li, Nai-Mu Hsu, Yogesh S. Tingare, Chung-Yen Li,
Yi-Cheng Liu, Chaochin Su,* and Wen-Ren Li*
Dye-sensitized solar cells (DSSCs) are being investigated
extensively for their use in renewable energy technologies
because of their low cost and high light-to-electrical energy
conversion efficiency.[1] Since their initial report in 1991 by
OꢀRegan and Grꢁtzel, DSSCs have attracted much attention
from researchers, resulting in the preparation of thousands of
ruthenium polypyridine complexes, non-ruthenium organo-
metallic dyes, and metal-free organic sensitizers.[2,3] Organic
dyes exhibiting high molar absorption coefficients and
presenting a variety of specific functional groups, allowing
fine tuning of the absorption spectra, have provided respect-
able incident photon-to-current conversion efficiencies
(IPCEs).[2] Coordination complexes of ruthenium, including
N3,[4,5] N719,[6,7] and black dyes,[8–10] have also received
significant attention because their photophysical and photo-
chemical properties provide DSSCs with excellent photo-
electric conversion efficiencies. The most common inorganic
dyes exploited in DSSCs are typically ruthenium(II) poly-
pyridine complexes.[3] Several attempts have been made to
enhance the efficiency and long-term stability of the dyes,
including increasing the conjugation length of the anchoring
or ancillary ligand using thiophene,[11,12] carbazole,[13] or
other[14–17] substituents. Long-term stability can be achieved
by modifying the architecture of amphiphilic bis(bipyridyl)
RuII dyes featuring alkyl,[18–20] alkoxy,[16] or other[21] substitu-
ent groups. Grꢁtzel and co-workers replaced the NCS ligand
of a ruthenium polypyridine complex with an anionic carbon
atom as an alternative approach to structural modifica-
tion.[22,23] Herein, we report a set of N-heterocyclic carbene
(NHC)/pyridine ruthenium complexes, in which one of the
nitrogen atoms in the traditional bipyridine framework has
been replaced by a carbon atom, for use in DSSCs exhibiting
enhanced solar cell performance.
Although there is much research on the preparation of
metal–carbene complexes and their roles as catalytic reagents
and luminescent emitters, their photovoltaic characteristics
remain relatively unknown. NHC–pyridine-based ligands,
with their unique set of electronic properties, are an excep-
tional class of donors; therefore, we expected them to make
excellent ancillary ligands. The essential requirement of a dye
that provides a DSSC with high efficiency is for the energy
level of the lowest unoccupied molecular orbital (LUMO) of
the sensitizer to be sufficiently high for efficient charge
injection into the TiO2 electrode, while the energy level of the
highest occupied molecular orbital (HOMO) must be suffi-
ciently low for efficient regeneration of the oxidized dye by
the hole-transport material. Several methods have been
reported to lower the HOMO energy level through tuning
of the ancillary ligands and their substituents.[3,11,13] The use of
ruthenium complexes bearing ancillary ligands functionalized
with NHC–pyridine units might be an alternative approach
toward tuning the frontier orbitals of the dyes. Sensitizers
using benzimidazole-based carbenes,[24,25] rather than imida-
zole-based carbenes,[26,27] should stabilize the HOMO energy
levels while leaving the LUMO energy levels favorable for
the injection of electrons into the conduction band of the TiO2
electrode. In this study, we developed two photosensitizers
(1a and 1b), functionalized with electronically unsymmetrical
benzimidazole–pyridine-based carbene units, which exhibited
excellent photoelectric conversion efficiencies (h).
Scheme 1 illustrates the stepwise synthetic protocol
toward the ruthenium(II) NHC–pyridine complexes CBTR
(1a) and CfBTR (1b). The ligands 3a and 3b for the synthesis
of these ruthenium complexes were obtained through treat-
ment of benzimidazole (2) with 2-fluoro-4-methylpyridine,
followed by alkylation with 1-bromooctane and 3,5-difluoro-
benzyl bromide, respectively.[28] Treating the benzimidazole
salts 3a and 3b with [{RuCl2(p-cymene)}2] in the presence of
lithium bis(trimethylsilyl)amide (LHMDS) in CH2Cl2 and
then with 4,4’-bis(methoxycarbonyl)-2,2’-bipyridine afforded
the corresponding ruthenium–chloride complexes.[29] The
chloride ligands were replaced with NCS units at 908C in a
mixture of H2O and DMF, thereby yielding the corresponding
ruthenium–thiocyanide complexes 4a and 4b.[11,16] The molec-
ular geometry of the ester 4b was confirmed unequivocally
through single-crystal structural analysis. Dark red crystals
were obtained after slow evaporation of a solution of the ester
4b in CH2Cl2. The molecular structure (see Figure S1 and
Table S1 in the Supporting Information) revealed that the
coordination geometry around the ruthenium atom could be
[*] W.-C. Chang, T.-Y. Li, N.-M. Hsu, Y. S. Tingare, C.-Y. Li, Prof. W.-R. Li
Department of Chemistry, National Central University
Chung-Li, Taiwan 32001 (ROC)
Fax: (+886)3-427-7972
E-mail: ch01@ncu.edu.tw
H.-S. Chen, Y.-C. Liu, Prof. C. Su
Institute of Organic and Polymeric Materials
National Taipei University of Technology
Taipei, Taiwan 10608 (ROC)
E-mail: f10913@ntut.edu.tw
[**] We thank the National Science Council, ROC, for financial support
(grants NSC 96-2113M-008-002-MY3 and NSC 96-2113M-027-005-
MY23) and Prof. H.-W. A. Fang (Department of Chemical Engi-
neering and Biotechnology, National Taipei University of Technol-
ogy) for measurement of film thicknesses.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001628
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Scheme 1. Synthesis of the NHC-pyridine-based ruthenium sensitizers
CBTR (1a) and CfBTR (1b).
Figure 1. Absorption spectra of a) 8.0ꢀ10À5 m solutions of CBTR (1a)
in CH3CN, CH3CN/tBuOH, and DMF and CfBTR (1b) in CH3CN and
CH3CN/tBuOH and b) transparent TiO2 films (thickness: 10 mm)
coated with 8.0ꢀ10À5 m solutions of CBTR in CH3CN/tBuOH, CH3CN,
and DMF, CfBTR in CH3CN/tBuOH, and N719 in DMF, CH3CN/
tBuOH, and CH3CN.
rationalized as a slightly distorted octahedron. The Ru–
C
carbene bond [Ru(1)–C(26)] of the ester 4b is located trans to
one of pyridine nitrogen atoms (N2) of the 4,4’-dicarboxylic
acid-2,2’-bipyridine (dcbpy) anchoring ligand. The Ru–Ccarbene
distance [1.973(3) ꢂ] indicates that this bond is a single bond
featuring back-donation; it is shorter than those reported for
typical Ru–NHC complexes.[26,27,29] Accordingly, in complex
4b, the Ru–Npyridine bond [Ru(1)–N(2), 2.119(2) ꢂ] that is
positioned trans to the NHC carbon atom is longer than that
[Ru(1)–N(1), 2.038(2) ꢂ] positioned trans to the NCS ligand.
decrease in the molar extinction coefficient of CBTR, which
bears a strong s donor, relative to that of N719 (535 nm; e =
1.5 ꢃ 104 mÀ1 cmÀ1 in C2H5OH or DMF).[30,31] The absorption
spectra revealed negative solvatochromism (i.e., a blue shift
from 535 nm in CH3CN to 469 nm in DMF) of the charge-
transfer bands of CBTR in more-polar aprotic solvents
(DMF). For example, the absorption maxima (lmax) of
CBTR (1a) in CH3CN, CH3CN/tBuOH, and DMF are 535,
529, and 469 nm, respectively.[10,32,33] In contrast, in the case of
N719, which features tetrabutylammonium units as counter-
cations, we observed only a slight blue shift.[30,34,35] The
absorption spectra of CBTR and N719 anchored onto TiO2
nanocrystalline electrodes from various solutions featured
broadened MLCT bands (Figure 1b) relative to those present
in the solution spectra. Such broadening has also been
reported for several other organic sensitizers deposited onto
TiO2 electrodes.[36,37] These changes could be ascribed to
aggregation or coordination of the dye molecules on the TiO2
surface or to hydrogen-bonding interactions between the
carboxylate groups of CBTR (1a) and the TiO2 surface.[38]
Comparing the absorption spectra of CBTR in solution and
on the film, the TiO2 film that had been immersed in CH3CN/
tBuOH exhibited wider absorption bands with higher absorp-
tion intensity over the region from 400 to 750 nm than those
À
It is the longest Ru N bond in the entire complex because of
the strong trans influence of the carbene carbon atom.
Hydrolysis of the esters 4a and 4b with LiOH in H2O/
EtOH yielded the desired acids CBTR (1a) and CfBTR (1b),
respectively.
To predict the behavior of the interfacial molecules when
used in devices, the optical properties of CBTR, CfBTR, and
N719 were investigated using UV/Vis absorption spectrosco-
py. Figure 1 displays the UV/Vis spectra of the dyes in various
solvents and adsorbed onto TiO2 films. The absorption
spectra of CBTR solutions (Figure 1a) in CH3CN, CH3CN/
tBuOH, and DMF (8.0 ꢃ 10À5 m) reveal that the maximum
absorptions were dependent on the solvent used. The
absorption spectrum of CBTR in CH3CN features two
bands centered at 442 nm (molar absorption coefficient e =
1.1 ꢃ 104 mÀ1 cmÀ1) and 535 nm (e = 9.0 ꢃ 103 mÀ1 cmÀ1) and a
distinct shoulder at around 630 nm extending to 750 nm;
these signals are all characteristic of metal-to-ligand charge-
transfer (MLCT) transitions. We observed a significant
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Table 1: Physical and performance data of CBTR-, CfBTR- and N719-sensitized cells.
2,2’-bipyridine anchoring ligand,
[a]
[b]
[c]
[c]
[d]
[d]
thereby favoring efficient electron
injection into the TiO2 electrode.
The components of the HOMO and
HOMOÀ1 and of the LUMO and
LUMO + 1 of the CBTR are similar
to those reported for N719 dye.[6]
Figure 2 presents the photocur-
Sensitizer Eox of EHOMO
ELUMO
[eV]
E0–0
ELUMO
[eV]
E0–0
Cell performance
RuIII/II
[V vs.
[eV]
[eV]
[eV]
Jsc
Voc
[mV]
FF
h
[mAcmÀ2
]
[%]
Fc/Fc+]
CBTR
CfBTR
N719
0.34
0.36
0.31
5.14
5.16
5.11
3.39
3.39
3.46
1.75
1.77
1.65
3.53[e]
3.55
1.61
1.61
1.63
20.0
19.2
18.6
730
710
700
0.664 9.69
0.663 9.04
0.690 8.98
3.48
[a] The Ag/AgNO3 reference electrode was calibrated with a ferrocene/ferrocinium (Fc/Fc+) redox
couple. The same value for Eox of RuIII/II was obtained when the electrochemical experiments were
performed using 0.1m [nBu4N]PF6 in DMF or CH3CN/tBuOH (1:1). [b] EHOMO =EoxÀEFc/Fc+ + 4.8 eV
[c] ELUMO =EHOMOÀE0–0. The band gap E0–0 was estimated from the onset of the absorption spectrum
measured in DMF. [d] Obtained in CH3CN/tBuOH (1:1). [e] The LUMO energy level of CBTR was
estimated to be 3.54 eV from the reduction potential (Ered) observed in the cyclic voltammetry (CV)
measurements.
rent density–voltage (I–V) curves
and IPCE spectra of DSSCs assem-
bled with multiple layered TiO2
films, anchored with CBTR,
CfBTR, and N719 sensitizers.
Table 1 summarizes the correspond-
ing short-circuit photocurrent den-
sities (Jsc), open-circuit voltages
obtained from CH3CN, presumably because more dye mol-
ecules had adsorbed. Unlike that of CBTR, the absorption
spectra of N719 adsorbed on the film from various solvents
featured distinct absorption bands, except from CH3CN
because of poor solubility. In comparison with that of N719,
the absorption spectrum of CBTR adsorbed onto the TiO2
film from CH3CN/tBuOH was broader—an advantageous
spectral property for light harvesting of the solar spectrum.
The energy levels of the frontier orbitals of the sensitizer
play a crucial role in affecting the dye-regeneration and
electron-transfer processes in DSSCs. Prior to making the
solar devices, we determined the HOMO and LUMO energy
levels of CBTR and CfBTR from their UV/Vis absorption
spectra and their cyclic voltammograms to ensure that they
were suitable for injecting electrons and for regeneration
after electron donation. Table 1 reveals that the difference
between the HOMO energy level of CBTR (or CfBTR) and
(Voc), fill factors (FFs), and solar-to-electricity conversion
efficiencies (h). The CBTR- and CfBTR-sensitized DSSCs
exhibited slightly higher open-circuit voltages (730 and
710 mV, respectively) than that of N719 dye (700 mV)
under the same fabrication conditions. A possible explanation
for these results is that carbene sensitizers have an effect
similar to that of N719 dye on the Fermi level of TiO2 when
they bind to the conduction band of the TiO2 electrode. The
DSSC based on the CBTR sensitizer achieved a cell efficiency
of 9.69%, which is approximately 8% higher than that
(8.98%) of the N719-sensitized DSSC prepared using the
same cell fabrication technique. The higher value of h of the
CBTR-sensitized cell (relative to that of the N719 cell) arose
from its higher value of Jsc, which we attribute to the presence
of the NHC ligand (a strong s donor). The cell efficiency
(9.04%) of the CfBTR-sensitized DSSC, however, was only
slightly higher than that of the N719-sensitized DSSC. The
values of Jsc of the CBTR- and N719-containing cells are
consistent with the IPCE data (inset to Figure 2). The onsets
in the IPCE spectra of the DSSCs sensitized with the CBTR
and N719 dyes were equal (reaching 800 nm). The IPCE
curves for the CBTR-, CfBTR-, and N719-containing DSSCs
exhibited common broad characteristics that covered almost
the entire visible spectrum (from 350 to 800 nm), with
the IÀ/I3 redox potential was greater than that of N719 dye.
À
The oxidation potentials of CBTR and CfBTR are shifted by
0.03 and 0.05 eV, respectively, relative to that of N719 dye,
indicating that the HOMO energy levels of both sensitizers
are stabilized by their ancillary ligands—presumably their
NHC moieties. The energy difference between the LUMO of
CBTR (or CfBTR) and the TiO2 film, measured in DMF, was
larger than that of N719 dye. Stabilization of the LUMO was
observed, however, when the measurements were performed
in CH3CN/tBuOH. The band gaps of the newly prepared
sensitizers were greater than that of N719 dye in DMF, but
similar to the value in CH3CN/tBuOH. Nevertheless, the
LUMO energy level of CBTR (or CfBTR) in solution was
higher than that of the TiO2 film, suggesting that it should be
possible to inject electrons into the conduction band of the
TiO2 electrode.
To gain further insight into the frontier orbital profiles, the
locations of the HOMOs and LUMOs of CBTR (1a) were
calculated using a semiempirical computation method. The
HOMO, HOMOÀ1, HOMOÀ2, and HOMOÀ3 of CBTR
(see Figure S2 in the Supporting Information) feature sig-
nificant contributions from the NCS ligands. It is noteworthy
that the HOMOÀ4 of CBTR features contributions from the
NHC–pyridine–carbene ligand, the NCS ligand, and the
ruthenium center. The electrons on LUMO and LUMO + 1
are localized homogeneously on the 4,4’-dicarboxylic acid-
Figure 2. Current density–voltage characteristics of photovoltaic devi-
ces incorporating CBTR, CfBTR, and N719 as sensitizers under
illumination with AM 1.5 simulated sunlight (100 mWcmÀ2). Inset:
Monochromatic IPCE spectra of the photovoltaic devices (TiO2 thick-
ness: 16 mm; cell active area tested with a mask: 0.16 cm2).
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maxima at 65.1, 65.8, and 64.1%, respectively. The enhanced
IPCE spectrum of CBTR suggests that this new sensitizer
might act as a more efficient electron donor to the conduction
band of the TiO2 film upon photoexcitation. Our results
suggest that NHC–pyridine-based ruthenium complexes have
promising applications and indicate the importance of
molecular design for tuning sensitizers to produce high-
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In conclusion, incorporation of novel NHC–pyridine
ruthenium sensitizers results in DSSCs exhibiting excellent
performance and sensitizing capability. We obtained these
complexes by using simple, convergent, and short synthetic
methods and inexpensive purification techniques, which are
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replacing the traditional bipyridine unit of ruthenium poly-
pyridine complexes; therefore, we suspect that such super-
sensitizers might complement or be substitutes for [Ru(bpy)2-
(NCS)2] in dye design and DSSC applications. Relative to the
DSSC incorporating the most common standard dye, N719,
the DSSC incorporating the CBTR complex featuring an
NHC–pyridine ancillary ligand exhibited superior values of
Jsc, Voc, and h. The absorption profile of the CBTR sensitizer
indicates that red-shifted absorption properties can be
enriched by increasing the conjugation length of the ancillary
ligand to increase the light-harvesting ability. The introduc-
tion of carbene as an anchoring ligand by incorporating
carboxylate groups on the NHC-pyridine ligand is currently in
progress.
Received: March 18, 2010
Revised: June 18, 2010
Published online: September 20, 2010
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Keywords: carbene ligands · dyes/pigments · ruthenium ·
sensitizers · solar cells
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