New heteroleptic benzimidazole functionalized Ru-sensitizer showing the highest efficiency for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.inoche.2014.10.033

We designed and synthesized a new ruthenium complex using terpyridine as an anchoring ligand and BOC (tert-butyloxycarbonyl) protected bidentate benzimidazole derivative as an ancillary ligand, coded as GS7. The complex was characterized using ¹H NMR, FTIR, elemental analysis, UV-vis spectrophotometer, and cyclic voltammetry. We also tested photovoltaic performance of this complex for dye-sensitized solar cell (DSSCs). GS7 when used as a sensitizer for DSSCs with iodine triiodide electrolyte, showed a J_sc of 15.25 mA cm^{- 2}, a V_oc of 0.576 V, a FF of 0.691 and overall power conversion efficiency of (η) 6.07%.

Full text of DOI:10.1016/j.inoche.2014.10.033

Inorganic Chemistry Communications 51 (2015) 61–65
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
New heteroleptic benzimidazole functionalized Ru-sensitizer showing
the highest efficiency for dye-sensitized solar cells
T. Swetha a, S. Niveditha a, K. Bhanuprakash a, Ashraful Islam b, Liyuan Han b, Idriss M. Bedja c,
Reza Fallahpour d, Surya Prakash Singh a,
⁎
a
Inorganic and Physical Chemistry Division, CSIR—Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
CRC, Optometry Department, College of Applied Medical Sciences, King Saud University, Riyadh 11433, Saudi Arabia
Institute of Organic Chemistry, University of Zurich UZH, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2014
Received in revised form 28 October 2014
Accepted 30 October 2014
Available online 31 October 2014
We designed and synthesized a new ruthenium complex using terpyridine as an anchoring ligand and BOC (tert-
butyloxycarbonyl) protected bidentate benzimidazole derivative as an ancillary ligand, coded as GS7. The complex
was characterized using 1H NMR, FTIR, elemental analysis, UV–vis spectrophotometer, and cyclic voltammetry.
We also tested photovoltaic performance of this complex for dye-sensitized solar cell (DSSCs). GS7 when used as
a sensitizer for DSSCs with iodine triiodide electrolyte, showed a Jsc of 15.25 mA cm−2, a Voc of 0.576 V, a FF of
0.691 and overall power conversion efficiency of (η) 6.07%.
Keywords:
Benzimidazole
© 2014 Published by Elsevier B.V.
Photovoltaic performance
Dye sensitized solar cells (DSSCs)
Iodine triiodide electrolyte
Power conversion efficiency
Owing to their versatility and low-cost manufacturing, dye sensitized
solar cells (DSSCs) have been intensively investigated over the past de-
cade [1–8]. Extensive research has been focused on the development of
new and highly efficient sensitizers, since they play a vital role to the cell's
photovoltaic performance. Sensitizers should have absorption over a wide
range of solar spectrum and a high molar extinction coefficient for im-
proving the conversion efficiency of DSSC. In order to enhance these effi-
ciencies, various sensitizers such as ruthenium complexes, [8,9] non-
ruthenium metal complexes [10–12], and metal free organic sensitizers
[13–16] have been synthesized and tested in DSSCs. Among the various
sensitizers developed ruthenium based sensitizers were intensively in-
vestigated as potential sensitizers with efficient DSSC performance, be-
cause their photophysical and electrochemical characteristics can be
finely-tuned to achieve optimal material properties. Different strategies
have been employed on the anchoring and ancillary ligands in ruthenium
sensitizers to improve the photon-to-electricity conversion efficiency [8].
Among the various reported anchoring groups such as phosphate [17,18],
boronic acid [19], and carboxylic acid [20,21], carboxylic acid group
remained the most common and efficient anchoring group. Many efforts
were also made on ancillary ligands by structural modifications such as
extending the conjugation [22–28], with electron donating groups
[29–34], use of multi-chromophoric dyes [35–37] and recently a new
class of cyclometalated ligands [38–42] to enhance the device perfor-
mance. Key examples of ruthenium complexes for DSSCs include the
well studied N3 [42], N719 [43], and N749 [44] dyes. It has been reported
that, introduction of 4,4′,4″-tricarboxy-2,2′:6,2″-terpyridine (H3tctpy) ex-
hibits panchromatic sensitization up to near-IR region [43,45]. Although
most of the complexes showed satisfactory sensitization up to 900 nm,
they still possess two major drawbacks as follows: i) inferior absorptivity
in the visible region, due to the lack of effective auxochrome; and ii) the
presence of two or three thiocyanato ligands, which not only decrease
the synthetic yield [44,46] but also the stability of the complex decreases
owing to dye decomposition by weak Ru\NCS bond. Despite of new Ru
complexes for the enhancement, there is still a need for efficient sensi-
tizers to promote the commercialization of DSSCs. Benzimidazole deriva-
tives have been used as layer materials for hole blocking and electron
transport in OLED (organic light-emitting diode) devices [47]. Recently
E. W. G. Diau et al. reported the Ru complexes having benzimidazole de-
rivatives for DSSC applications [48].
In conjugation with current endeavors, we [49–53] and others have
examined terpyridyl Ru complexes in an attempt to optimize near-IR
sensitizers. Here we have designed and synthesized new heteroleptic
Ru sensitizer GS7 bearing terpyridine ligand with anchoring carboxylic
groups, and newly designed bidentate benzimidazole derivative as an-
cillary ligand and a thiocyanato ligand to tune the redox properties of
Ru center. The photovoltaic performances of fabricated DSSCs using
new sensitizer were evaluated and compared with N749 sensitizer.
⁎
Corresponding author.
E-mail address: spsingh@iict.res.in (S.P. Singh).
http://dx.doi.org/10.1016/j.inoche.2014.10.033
1387-7003/© 2014 Published by Elsevier B.V.

62
T. Swetha et al. / Inorganic Chemistry Communications 51 (2015) 61–65
Scheme 1. Synthesis of sensitizer GS7. Reagents and conditions: i) RuCl3·3H2O, anhydrous DMF, reflux under dark, 4 h; ii) L1, DMF reflux, 4 h then NH4NCS, reflux, 2 h then TEA, water,
reflux, 48 h.
Fig. 1. a. Previous work in Chem. Comm., [48]. b. Present work.
GS7 was synthesized using the stepwise procedures as shown in
Scheme 1. The compound (1) was prepared by refluxing RuCl3·3H2O
and 4,4′,4″-trimethoxycarbonyl 2,2′-6′,2″'-terpyridine in ethanol:CHCl3
solution. The final step was performed by heating compound (1) with an-
cillary ligand L1 at 120 °C for 4 h using DMF as solvent, followed by hydro-
lysis to afford the desired Ru-complex GS7 in good yield. Fig. 1a represents
reported benzimidazole derivative by other research group [48], while
Fig. 1b represents the GS7 complex designed by us.
observed at 519 nm and 591 nm respectively. Compared to solution
spectrum, the absorption spectrum on TiO2 film was blue shifted by
≤25 nm. In addition to MLCT band one shoulder peak was observed at
around 700 nm, which is an advantageous spectral characteristic for
light harvesting.
The UV–vis absorption spectrum of new sensitizer GS7 and N749 in
DMF solution is depicted in Fig. 2 and corresponding data is presented in
Table 1. Strong π–π* absorption is observed in the UV region corre-
sponding to the coordinating ligand. GS7 showed broad visible absorp-
tion band which is assigned to metal-to-ligand charge transfer (MLCT)
band. The UV–vis spectrum of GS7 exhibited broad and strong absorp-
tion in the range from 250 to 850 nm. The high energy absorption
band around 300 nm originates from intraligand π–π* transitions of
the terpyridine ligand.
The spectrum was characterized with strong intermediate energy
absorption band at 410 nm, mainly attributed to a combination of
benzimidazole based ligand (L1) transitions and ligand-to-ligand
charge transfer (LLCT) transitions as well as to some contribution
from MLCT transitions. The lower energy absorption band at
540 nm (4629 M−1 cm−1) and indistinct shoulder around 720 nm
are characteristics of the MLCT transitions. N749 dye, the UV–vis
spectrum of which is shown for comparison, presents MLCT transi-
tion with absorption maxima at 613 nm.
30000
25000
N749
GS7
20000
15000
10000
5000
0
300
400
500
600
700
800
900
Wavelength (nm)
The UV–vis absorption spectrum of sensitizers GS7 and N749 on TiO2
was recorded and shown in Fig. 3. The maximum absorption peak was
Fig. 2. Absorption spectra of GS7 and N749 sensitizers in DMF.

T. Swetha et al. / Inorganic Chemistry Communications 51 (2015) 61–65
63
Table 1
Optical and DSSC performance parameters of sensitizer GS7.
f.ff
Eff.
(%)
d
e
Dye
λ
max(ε)
HOMO
[eV]b
E0–0
LUMO
[eV]b
Jsc
Voc
[V]
(M−1 cm−1) [nm]a
[eV]c
[mA cm−2
]
GS7
N749
540 (4629) 727 (1288)
613 (6080)
−5.70
−5.46
1.70
1.53
−4.00
−3.93
15.25
13.74
0.576
0.675
0.691
0.680
6.07
6.31
a
Absorption spectra were recorded in DMF solutions at 298 K.
HOMO values were measured by photoemission yield spectrometer.
The band gap, E0–0, was calculated from the onset of the absorption spectra.
Current density.
Open-circuit voltage.
Fill factor.
b
c
d
e
f
In order to analyze the absorption spectra in detail TDDFT calculations
sufficiently above the conduction band edge (−4.2 eV) [55,56] of the
nano-crystalline TiO2. Therefore, an efficient electron injection into the
conduction band of TiO2 is expected to occur with sensitizer GS7 [57].
A comparison of HOMO and LUMO for GS7 and N749 is given in Fig. 4
and the results are summarized in Table 1.
have been carried out. The sensitizer has unit positive charge; hence we
used hexafluorophosphate (PF−6 ) counter anion to make it electrically
neutral [54] and B3LYP (Table S1), M06 (Table S2) and PBE0 (Table S3)
functionals were used. Among the four functionals, B3LYP could correlate
well with the experimental results. Major peaks observed in the range of
450 nm–800 nm along with orbital contribution to each excited state are
listed in Table S1. The lowest energy excitation (S0 to S1) is at 774 nm, in-
volves the transition from HOMO to LUMO, is a low intense (f = 0.0267)
charge transfer transition which shifts the electron density from rutheni-
um and NCS to terpyridine, and is a low intense peak (f = 0.0267). The
most intense peak (f = 0.1002) is observed at 498 nm, showing 40 nm
blue shift from experimental absorption maxima (540 nm), and this
peak corresponds to HOMO to LUMO + 3 transition. From molecular or-
bital pictures it can be inferred that the charge transfer transition in GS7 is
of MLCT and LLCT types.
The ionization potential of GS7 bound to a nanocrystalline TiO2 film
was determined using a photoemission yield spectrometer (Riken Keiki,
AC-3E, Fig. S1) to ensure that the highest occupied molecular orbital
(HOMO) of newly synthesized sensitizer GS7 matches the redox poten-
tial of the redox electrolyte (I−/I−3 ) for dye regeneration and to verify
whether the lowest unoccupied molecular orbital (LUMO) is suited for
efficient electron injection into the conduction band of TiO2 (Fig. S1).
An oxidation potential of −5.70 eV, which corresponds to the HOMO
level, was obtained for the sensitizer GS7 adsorbed on a TiO2 electrode
and was low enough for efficient regeneration of the oxidized dye
through reaction with iodide.
The photovoltaic performance of sensitizer GS7 on a 0.25 cm2 active
area nanocrystalline TiO2 electrode was studied under standard AM 1.5
irradiation (100 mW cm−2) using an electrolyte with a composition of
0.5 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, and 0.1 M
LiI in acetonitrile:n-butanol (1:1). The incident photon-to-current effi-
ciency (IPCE) of the sensitizer GS7 is shown in Fig. 5 and compared
with standard N749 dye; it exhibited efficient sensitization of nanocrys-
talline TiO2 solar cell over the entire visible range extending into near-IR
region consistent with the UV–vis absorption spectra of GS7 and N749
adsorbed on TiO2 film (Fig. 3). The onset of the IPCE spectra is close to
~900 nm, which is an optimal parameter for anticipated best DSSCs.
Excellent IPCE performance was observed from 400 to 740 nm.
The new sensitizer GS7 shows an IPCE value of 70% in the region of
445–570 nm. Taking into account the reflection and absorption losses
by the conducting glass the IPCE in this range reaches a maximum of
78% at 520 nm. The decrease in the IPCE above 550 nm toward the red
is caused by the decrease in the molar extinction coefficient of the sen-
sitizer in that region (Fig. 3). In the 400–640 nm range, the GS7 exhibits
better IPCE performance than N749 due to its higher light harvesting
efficiency in this wavelength region (Figs. 2 and 3).
Fig. 6 shows the photocurrent density–voltage curve of the device
employing GS7 and N749 under AM 1.5 G simulated sunlight at a light in-
tensity of 100 mW cm−2, while the short-circuit photocurrent density
(Jsc), open circuit voltage (Voc), fill factors (FF), and overall cell efficiency
(η) data are summarized in Table 1. The solar cell sensitized with GS7
showed Jsc 15.25 mA cm−2, a Voc 0.576 V, and a FF of 0.691, corresponding
The excited-state oxidation potential (LUMO) could be calculated
from HOMO–E0–0 where E0–0 is zeroth–zeroth transition value, calculat-
ed from the onset of the absorption spectra (Figs. 2 and 4). The excited-
state oxidation potential of the GS7 sensitizer was −4.00 eV, which lies
1.0
0.9
0.8
N749
GS7
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
400
500
600
700
800
Wavelength (nm)
Fig. 3. Absorption spectra of GS7 and N749 on TiO2.
Fig. 4. Energy level diagram and comparison between HOMO and LUMO of GS7 and N749.

64
T. Swetha et al. / Inorganic Chemistry Communications 51 (2015) 61–65
80
70
60
50
40
30
20
10
0
N749
GS7
300
400
500
600
700
800
900
1000
Wave length (nm)
Fig. 5. IPCE spectra of GS7 and N749 sensitizers.
to an overall conversion efficiency (η) of 6.07%. Under similar fabrication
conditions standard N749 dye exhibited an overall power conversion
efficiency of 6.31%.
Fig. 7. Voc as a function of electron density for DSSCs sensitized with N749 and GS7.
To further understand the lower Voc observed for DSSCs sensitized
with GS7 dye compared to N749, we first measured the relative conduc-
tion band position of TiO2 by means of a charge extraction method. As
shown in Fig. 7, DSSCs sensitized with both GS7 and N749 exhibit a sim-
ilar linear increase in electron density as a function of Voc, indicating
that these cells had the same conduction band edge of TiO2 regardless
of the different molecular structures. Since the conduction band edge
was not influenced by GS7 dye, the decrease in Voc compared to N749
must be ascribed to increase of charge recombination. A theoretical
model of intensity-modulated photovoltage spectroscopy (IMVS) for
DSSCs considers a charge trapping/detrapping during the charge diffu-
sion and electron transfer from the TiO2 conduction band. The electron
lifetime (τ) in the DSSCs is a central quantity to determine the dynamics
of charge recombination which is related to open circuit voltage (Voc) of
the cell. Fig. 8 shows the τ as a function of Voc. GS7 exhibited a shorter τ
than the N749 dye at the same electron density, implying that recombi-
nation rate between electrons on the TiO2 surface and an I−3 ion in the
electrolyte was higher for GS7. The shorter charge life time in TiO2 for
GS7, can be attributed to the electronic effect in the molecular structure
and the orientation of anchored dye on TiO2. It is likely that N749 allows
for better geometry orientation and a more compact layer of the an-
chored dye on the TiO2 film to take place compared to that of GS7. The
sensitizer GS7 shows a shorter life time, which might be due to incom-
plete dye coverage and/or dye aggregation on the TiO2 surface, which
increases the concentration of I3− ions in the vicinity of the TiO2 surface
and thus decrease the electron lifetime (τ).
To summarize, a new type of Ru-sensitizer GS7 was meticulously de-
signed and synthesized which is having tricarboxyterpyridine ligand as
an anchoring ligand and BOC-protected benzimidazol derivative (L1) as
ancillary ligand for DSSCs and characterized using electrochemical and
spectroscopic methods. The absorption spectra of GS7 on TiO2 film re-
veal that incorporation of ligand (L1) in the ruthenium sensitizer leads
to enhanced spectral response and red-shifted absorption maxima in
the visible region. A solar-to-electricity conversion efficiency of the
DSSC based on GS7 sensitizer reached 6.07%, which is consistent with
the IPCE values. The IMVS data disclosed that the sensitizer GS7 exhib-
ited a higher charge transport resistance compared to N749 at the
15
12
9
6
3
0
N749
GS7
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Volatge (V)
Fig. 6. J–V curves of GS7 and N749 sensitizers.
Fig. 8. Electron lifetime (τ) as a function of Voc for DSSCs sensitized with N749 and GS7.

T. Swetha et al. / Inorganic Chemistry Communications 51 (2015) 61–65
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found online at http://dx.doi.org/10.1016/j.inoche.2014.10.033. These
data include MOL files and InChiKeys of the most important compounds
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