One bipyridine and triple advantages: Tailoring ancillary ligands in ruthenium complexes for efficient sensitization in dye solar cells

Article abstract of DOI:10.1039/c2jm34558g

Four new ruthenium bipyridyl complexes denoted as MC103-MC106, with novel unsymmetrical bipyridines as ancillary ligands have been successfully synthesized as efficient sensitizers for application in dye-sensitized solar cells (DSC). The spectral, electro

Full text of DOI:10.1039/c2jm34558g

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COMMUNICATION
One bipyridine and triple advantages: tailoring ancillary ligands in ruthenium
complexes for efficient sensitization in dye solar cells†
a
M. Chandrasekharam, M. Anil Reddy,a Surya P. Singh,a B. Priyanka,a K. Bhanuprakash,a
*
M. Lakshmi Kantam,a A. Islamb and L. Hanb
Received 12th July 2012, Accepted 31st July 2012
DOI: 10.1039/c2jm34558g
absorption by the electrolyte in a black dye based device and achieved
a record DSC efficiency of 11.4% which was independently certified
by AIST, Japan.6 Recently, porphyrin-sensitized devices with a
cobalt(II/III) based redox electrolyte showed efficiency that exceeds
12%, the highest to date by increasing the Voc of the device by 16%
over cells containing an iodide-based redox couple.7a New DSC
architectures using energy relay dyes (ERD) attached to the sensi-
tizing dyes (SD) or coadsorbed on a TiO2 surface have been invented
for better light harvesting via efficient Forster resonant energy
transfer from ERD to SD and the device efficiencies have been
further improved by increasing the sensitizer dye loading with ERD
placed within the electrolyte.8 Further to avoid the possibility of
photoluminescence quenching of the ERD within the hole trans-
porter, a light absorbing conjugated polymer, poly(3-hexylthiophene)
(P3HT), has been used as both an ERD and hole transporter in a
novel solid-state DSC device structure.8f Improving the performance
of the device without addition of a new element that would otherwise
demand further optimisation is an important issue and hence N719
modification is still an attractive alternative.
Four new ruthenium bipyridyl complexes denoted as MC103–
MC106, with novel unsymmetrical bipyridines as ancillary ligands
have been successfully synthesized as efficient sensitizers for appli-
cation in dye-sensitized solar cells (DSC). The spectral, electro-
chemical and photovoltaic properties of the new sensitizers have
been investigated. Alkyl thiophene/alkyl bithiophene and trialkyl
phenyl as substituents on ancillary ligands improved the spectral
properties and hence better efficiencies of DSCs are achieved. The
higher efficiencies of the new sensitizers obtained are 9.56%, 9.58%,
8.34% and 8.32%, respectively, compared to 7.2% for the standard
N719 sensitizer fabricated and evaluated under similar conditions.
Dye-sensitized solar cells (DSCs) are attracting increased interest
among scientists worldwide as less expensive alternatives for the
photovoltaic conversion of solar energy.1 Among several sensitizers
used for DSC fabrication,2,4,5 the classical ruthenium complex, cis-
di(thiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylate) ruthenium(II),
N719 has shown a respectable h value (more than 10%).3 While one
of the dicarboxybipyridines (dcbpys) serves as an anchoring ligand
for efficient electronic communication with TiO2, modification of the
other dcbpy of the N719 sensitizer as an ancillary ligand has been the
trend to achieve better efficiencies. Efforts have been made by us and
several groups to improve the spectral properties of the sensitizer by
the selection of a suitable organic chromophore on the pyridyl rings
of the ancillary bipyridine.2g–p,5a–h Thiophene has been extensively
used in organic dyes as well as highly conjugated ancillary ligands in
ruthenium sensitizers to increase the light-harvesting ability and red-
shift the MLCT band.4 We have demonstrated the influence of
mesityl, di-tert-butyl and/or tri-iso-propyl phenyl moieties with or
without conjugation on ancillary bipyridyl ligands for improved
photovoltaic parameters of DSC test cells.5c–g We also effectively
utilized the coadsorbent concept for the prevention of competitive
As a part of our continued program on the design and synthesis of
novel, efficient ruthenium sensitizers for DSC applications,5 we found
that the examples of modification of the ancillary bipyridine with
different chromophores on each of the two pyridyl rings are scant.
Our research interest in the development of new C–C bond forming/
aryl–aryl coupling reactions coupled with the demand for diverse
bipyridines not only for ruthenium sensitizers but also as chelates for
one electron outer-sphere redox couples such as cobalt(II/III)
complexes inspired us to investigate the synthesis of unsymmetrically
substituted bipyridines for DSC applications.7,9 Using conventional
symmetric bipyridines on ruthenium sensitizers, one can either
increase the light-harvesting ability and red-shift the MLCT band by
incorporating p-conjugation or improve electron donating/hole
transport properties by derivatizing with a desirable chromophore,
whereas the unsymmetrical bipyridine ligand concept allows the
possibility of a large variety of bipyridine ligands with varied struc-
tural features as shown in Fig. 1. By careful selection of substituents
on bipyridines, the properties of ruthenium sensitizers can be tuned in
multi-directions such as solubility, extention of p-conjugation and
electron donating/hole transport properties to enhance their efficiency
of sensitization of nanocrystalline TiO2 without increasing the steric
bulk (in symmetrical bipyridines both the moieties responsible for
higher efficiency have to be appended on each pyridine) of the
aNetwork of Institutes for Solar Energy, CSIR-Indian Institute of Chemical
Technology, I&PC Division, Uppal Road, Tarnaka, Hyderabad-500 607,
India. E-mail: chandra@iict.res.in; Fax: +91 4027193186; Tel: +91
4027193186
bPhotovoltaic Materials Unit, National Institute for Materials Science, 1-
2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
† Electronic supplementary information (ESI) available: Experimental
procedures for the synthesis of new sensitizers, 1H and 13C NMR data
of compounds. See DOI: 10.1039/c2jm34558g
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18757–18760 | 18757

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Fig. 3 Absorption spectra of sensitizers MC103–MC106 in DMF
solvent.
Fig. 1 Unsymmetrically substituted bipyridines.
avoiding isomerisation (unlike in double bonds); (2) the hexyl chain
provides a hydrophobic environment for the sensitizer, increases the
solubility, and prevents the approach of the redox electrolyte in the
vicinity of TiO2, reducing recombination; (3) the alkyl groups at two
ortho and para positions of the phenyl ring facilitate electron
pumping and red-shift the absorption. The new ligands L1–L4 and
the corresponding Ru-complexes MC103–MC106 were successfully
synthesised and fully characterised by their electronic absorption,
electrochemical properties, IR, 1H NMR and mass spectra (ESI†).
The optical absorption characteristics of the new sensitizers
measured in dimethylformamide (DMF) solvent are shown in Fig. 3.
All of the new sensitizers exhibited broad absorption ranging from
400 nm to above 700 nm. The high energy ligand–ligand transitions
with higher extinction coefficients (3max) corresponding to the ancil-
lary ligands in the 400 nm region for MC105 and MC106 sensitizers
compared to those of MC103 and MC104 are the result of extended
conjugation arising from two thienyl moieties. This effect is also
reflected in the low energy metal to ligand charge transfer (MLCT)
transitions where the 3max (546 nm) of MC105 is 20 150 Mꢀ1 cmꢀ1
and that of the MC106 sensitizer is 17 890 Mꢀ1 cmꢀ1 (545 nm). The
sensitizers with only one thiophene moiety exhibited a blue shift in the
lower energy MLCT band associated with a lower extinction coeffi-
cient. Thus, MC103 showed an absorption band at 533 nm with a
molar extinction coefficient of 16 250 Mꢀ1 cmꢀ1 whereas MC104
absorbed at 532 nm with an 3max value of 13 800 Mꢀ1 cmꢀ1. The
electronic absorption properties, such as shift and molar extinction
coefficient, of the sensitizers in solution are in accordance with the
molecular structure. The simulated absorption spectra of MC103 and
MC105 obtained from the theoretical calculation are in good
agreement with the one obtained from experiments (Fig. S28, ESI†).
The emission patterns for all of the new sensitizers MC103–MC106
match their respective absorption spectra (Fig. S26, ESI†). The
fluorescence maximum of MC104 is red-shifted by 22 nm relative to
MC103. The sensitizers have been screened for electrochemical
properties using cyclic voltammetric studies (Table 1, Fig. S27, ESI†).
The lower oxidation potentials of MC104 and MC106 compared to
MC103 and MC105 are because of the higher electron donating
power of the isopropyl groups (compared to methyl) present in these
dyes.
molecule. A similar concept can be used to bring about desirable
properties in the cobalt(II/III) complexes as alternative redox electro-
lytes to prevent recombination in DSCs. Therefore this concept has
the potential to find a one stop solution for sensitizer as well as
electrolyte problems to enhance the efficiency of DSCs. However
there are difficulties such as long reaction routes, formation of
mixtures of products, lower yields etc., involved in the synthesis of
unsymmetric bipyridines.10
In this communication, we present the design and synthesis of four
novel multifunctional unsymmetrically substituted ancillary bipyri-
dine ligands L1–L4 for molecular engineering of highly efficient
ruthenium sensitizers in DSC applications.
The chemical structures of the new ruthenium sensitizers denoted
as MC103–MC106 that are chelated with the ancillary ligands L1–L4
respectively are shown in Fig. 2. Alkyl thiophene and trialkyl phenyl
are tethered on each pyridine ring of the ligands L1 and L2 in the
complexes MC103 and MC104 whereas alkyl bithiophene was teth-
ered in place of alkyl thiophene for the ligands L3 and L4 in the
complexes MC105 and MC106 for further extension of conjugation.
Thus three performance increasing features are integrated in one
bipyridine as the ancillary ligand. The triple advantages of these
ligands are: (1) the thiophene moiety serves as the source of two
double bonds in conjugation for the increasing light harvesting ability
and red-shift of the MLCT band, while providing aromatic stability
The ground state optimised geometry of the sensitizers MC103 and
MC105 is shown in Fig. 4 where the hexyl chains have been replaced
by simple methyl groups. To study the electron density distribution
within the frontier and other close-lying orbitals, we report the
important molecular orbital compositions (Table S4 and S5, ESI†)
and the isodensity surface plots (Fig. S29 and S30, ESI†) of some
Fig. 2 Structures of new sensitizers MC103 (R ¼ Me, n ¼ 1); MC104
(R ¼ iPr, n ¼ 1); MC105 (R ¼ Me, n ¼ 2); MC106 (R ¼ iPr, n ¼ 2).
18758 | J. Mater. Chem., 2012, 22, 18757–18760
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sensitizers is because of low dye loading arising from the large size
compared to MC103 and MC104. Though the incident photon to
current conversion efficiencies are comparable, the higher efficiencies
of cells fabricated from MC103 and MC104 than the N719 based cell
are because of the inhibition of recombination which is also sup-
ported by the large difference in Voc values. The comparison of J–V
curves for the devices constructed from MC103–MC106 sensitizers
with the N719 sensitizer is shown in Fig. 6 and their detailed
photovoltaic parameters are described in Table 1. The highest effi-
ciency values of 9.56% and 9.58% were obtained for the sensitizers
MC103 and MC104 with the corresponding short circuit current
densities (Jsc, mA cmꢀ2) of 18.084 and 19.023, open circuit voltages
(Voc, V) of 0.729 and 0.704 and fill factors (ff) of 0.725 and 0.716,
whereas the sensitizers MC105 and MC106 exhibited lower efficien-
cies of 8.34% and 8.32% with the corresponding Jsc (17.146, 16.786),
Voc (0.670 and 0.674) and ff (0.726 and 0.736) parameters. Under
similar fabrication and measurement conditions, the standard N719
exhibited Jsc, Voc and ff of 18.435 mA cmꢀ2, 0.582 V and 0.671,
respectively corresponding to overall light to electricity conversion
efficiency of 7.2%.
Fig. 4 Optimised geometry (ground state) of sensitizers MC103 and
MC105 (hexyl chain replaced with methyl).
Table 1 Detailed photovoltaic parameters of the devices constructed
from the sensitizers MC103–MC106
Fig. 5 Incident photon-to-electric current conversion efficiencies as a
function of wavelength for the sensitizers MC103–MC106.
labs/nm
(3/Mꢀ1 cmꢀ1
Jsc
(mA cmꢀ2
Dye
)
)
Voc (V)
ff
Eff. (%)
selected molecular orbitals. The calculations show that the highest
occupied molecular orbital (HOMO) is centered on Ru-d orbitals and
the NCS ligand, whereas the lowest unoccupied molecular orbital
(LUMO) is a p* orbital centered mainly on the anchoring ligand
dicarboxybipyridine (dcbpy).
MC103
MC104
MC105
MC106
N719
533(16250)
532(13800)
546(20150)
545(17890)
18.084
19.023
17.146
16.786
18.435
0.729
0.704
0.670
0.674
0.582
0.725
0.716
0.726
0.736
0.671
9.56
9.58
8.34
8.32
7.20
The photovoltaic performance experiments of the sensitizers
MC103–MC106 compared with N719 were conducted on the test
cells constructed from 0.25 cm2 active area TiO2 electrodes and a
thermally stable electrolyte composed of 0.6 M dimethylpropyl-imi-
dazolium iodide (DMPII), 0.05 M I2, 0.5 M TBP and 0.1 M LiI in
acetonitrile. The incident monochromatic photon-to-electric current
conversion efficiency (IPCE) as a function of wavelength for the
sensitizers MC103–MC106 compared with N719 is shown in Fig. 5.
The IPCE values for MC103 and MC104 are above 70% over the
visible range from 380 nm to 700 nm reaching up to a maximum of
90% at 550 nm. The sensitizers MC105 and MC106 achieved a
maximum of 70% IPCE values at 550 nm with above 60% in the
entire visible wavelength region starting from 380 nm to 674 nm.
Inspite of better light harvesting ability, the low IPCE of these
In summary four novel unsymmetric bipyridine ligands have been
designed and successfully synthesized. These multifunctional bipyr-
idines worked as excellent ancillary ligands in ruthenium complexes
for DSC applications. Alkyl thiophene tethered sensitizers perform
better than the sensitizers with alkyl bithiophenes. We successfully
demonstrated that by careful tailoring of bipyridine ancillary ligands,
the optical properties of sensitizer can be effectively fine tuned for
better light harvesting in DSCs. These bipyridine ligands also have
tremendous potential for the synthesis of a great variety of recom-
bination inhibiting cobalt(II/III) based redox electrolytes, which is in
progress. This potential strategy would be a single solution for
sensitizer and electrolyte problems for achieving highly efficient DSC
devices.
Acknowledgements
MC thanks the Alexander von Humboldt Foundation and Professor
Dr Karl Leo, IAPP, TU-Dresden, Germany for a renewed research
fellowship. MAR and BP thank CSIR, New Delhi for a Senior
Research Fellowship.
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