Ruthenium(II)- Bipyridyl with extended π-system: Improved thermo-stable sensitizer for efficient and long-term durable dye sensitized solar cells

Article abstract of DOI:10.1007/s12039-011-0121-4

A new extended thermo-stable high molar extinction coefficient bipyridyl ruthenium(II) complex "cis-Ru(4,4'-bis(3,5-di-tert-butylphenyl)-2,2'- bipyridine)(Ln)(NCS)₂ H101", where Ln = 4,4'-dicarboxylic acid-2,2'-bipyridine; was synthesized and c

Full text of DOI:10.1007/s12039-011-0121-4

c
J. Chem. Sci. Vol. 123, No. 5, September 2011, pp. 555–565. ꢀ Indian Academy of Sciences.
Ruthenium(II)- bipyridyl with extended π-system: Improved
thermo-stable sensitizer for efficient and long-term durable dye
sensitized solar cells
M CHANDRASEKHARAMa,∗, G RAJKUMARc, CH SRINIVASA RAOa, T SURESHa,
P Y REDDYc and Y SOUJANYAb
aInorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Tarnaka,
Hyderabad 500 607, India
bMolecular Modelling group, Indian Institute of Chemical Technology, Uppal Road, Tarnaka,
Hyderabad 500 607, India
cAisin Cosmos R&D Co. Ltd, Nanomaterials Laboratory, Indian Institute of Chemical Technology,
Uppal Road, Tarnaka, Hyderabad 500 607, India
e-mail: chandra@iict.res.in
MS received 6 January 2011; revised 18 May 2011; accepted 19 May 2011
Abstract. A new extended thermo-stable high molar extinction coefficient bipyridyl ruthenium(II) complex
“cis-Ru(4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine)(Ln)(NCS)2 H101”, where Ln = 4,4’-dicarboxylic
1
acid-2,2’-bipyridine; was synthesized and characterized by H-NMR, FT-IR and ESI-MASS spectroscopes.
The H101 sensitized solar cell constructed with an active area of 0.54 cm2 in combination with an ionic
liquid electrolyte exhibited broader photocurrent action spectrum with solar-to-electric energy conversion
efficiency (η) of 5.89 (JSC = 12.14 mA/cm2, VOC = 690 V, fill factor = 0.699) under Air Mass (AM)
1.5 sunlight, while the reference ‘cis-Ru(4,4’-dinonyl-2,2’-bipyridine)(Ln)(NCS)2’, Z907 sensitized solar
cell exhibited η-value of 5.17% (JSC = 11.93 mA/cm2, VOC = 650 V, fill factor = 0.666). TGA anal-
ysis of H101 showed extended thermal-stability and under continuous light exposure and aging at 55◦C,
the DSSC retained 85% of its initial η-value, while under comparable conditions Z907 sensitized solar cell
retained 88%. As compared to 4,4’-dinonyl-2,2’-bipyridine in Z907, the new ancillary bipyridyl ligand ‘4,4’-
bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine’ in H101 shifts the absorption bands remarkably towards blue.
The Density Functional Theory (DFT) and Time-Dependent DFT excited state calculations of the new sen-
sitizer show that the first three HOMOs have t2g character with sizeable mixing from the NCS ligands with
π-bonding orbitals of 4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine. The LUMO is a π*-orbital localized on
the 4,4-dicarboxylic acid-2,2’-bipyridine and higher un-occupied frontier orbitals have π*-combinations with
4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine.
Keywords. Dye sensitized solar cells; extended π-system; thermo-stable; Polypyridyl Ru(II)-sensitizers.
1. Introduction
energy to cater everyday needs of the human race.
Efficient trapping of sunlight and directly converting
into electricity has been a challenge over the last few
decades. Researchers all over the world are focussing
on development of high efficient long-term durable
solar cells. In this regard Silicon Solar cells have been
Essentially the source of all forms of energy on all
planets in the solar system is coming from the Sun.
The energy radiated by the Sun is about 10’000 times
the currently needed energy on the Earth. Some of it
being reflected back into space, a lot being absorbed by well established as highly efficient and their uses are
limited to space applications owing to the high cost
involved in their manufacture. Recently among thin
film photovoltaics, organic solar cells and dye sensi-
tized solar cells gained special attention because of
their high photon-to-electricity conversion efficiency,
easy fabrication, low cost involved in mass produc-
tion compared to the traditional photo-electrochemical
cells.1–5 Among the various components employed
water surfaces, mountains and clouds. The irradiation
variations are also moving winds and waves and evap-
orating water for useful precipitations for the growth
of biomass and the river motions by gravity. There is
a lot of potential to use all these forms of renewable
∗For correspondence
555

556
M Chandrasekharam et al.
in the fabrication of DSSC, sensitizer plays a key
role in-terms of overall performance. Since Michael
Graetzel introduced the first efficient nano-crystalline
TiO2 solar cell sensitized with cis-bis(thiocyanato)
bis (2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II)
bis(tetrabutylammonium) (N719), a variety of ruthe-
nium(II) polypyridyl complexes, porphyrins, phthalo-
cyanines and metal free organic sensitizers have been
developed.6–17 Over the last two decades, there have
been considerable interest to improve the overall solar-
to-electrical energy conversion efficiency (η) of the
DSSC devices by modifying the structure of sensitiz-
ers and varying the other components of the device.
For instance, one of the bipyridyl ligand of N719 was
substitued with alkoxy styryl, triethyleneoxy (ion coor-
dinating) styryl, tert-butoxy styryl, etc. groups with the
concept of increase in the π-conjugation extension for
high molar extinction coefficient dyes or optimizing
the DSSC fabrication conditions with suitable elec-
trolyte and favourable texture of the nanocrystaline
TiO2 electrodes.16,18–20 But long-term device durabil-
ity is found to be one of the key limitations of DSSC
device for outdoor application. The leakage of liq-
uid electrolyte, desorption of loosely anchored dye
molecules, and photo-degradation of the dye anchored
on TiO2 as well as corrosion of the Pt counter electrode
by the triiodide/iodide couple have been suggested
as the factors responsible for limiting the long-term
performance of DSSC. To overcome this, new counter-
electrode materials,21–23 alternative redox couples24–26
such as solid state or quasi-solid-state elecrtolytes have
been thoroughly studied. In this regard Z907 and K77
dyes showed considerable long term durability with
moderate conversion efficiencies. However, achieve-
ment of high device conversion efficiency along with
long-term durability remains a major challenging
task.13,27–36 Substituted bipyridines recently attracted
special attention for their importance in biological
applications as Zn2+ specific fluorescent molecular
probes.37–39 We developed ruthenium(II) bipyridyl
C9H19
S
S
C9H19
N
C
C
N
N
C
C
N
N
N
N
N
N
Ru
Ru
N
S
S
N
N
HOOC
HOOC
H101
Z907
COOH
COOH
Figure 1. Structure of H101.
bipyridines as ancillary ligand have shown substantial
improvement in the solar-to-electrical energy conver-
sion efficiency16,44,45 and it seems to be reasonable to
explore further these non-vinylogous structures for
achieving better overall performance in photovoltaic
devices. In this regard, a new ancillary bipyridyl ligand
4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine (L1)
was designed (figure 1) and synthesised by coupling the
corresponding 3,5-di-tert-butylphenylboronic acid with
4,4’-dibromo-2,2’-bipridine under palladium catalysed
Suzuki conditions.46,47 This ligand was successfully
employed to synthesize the corresponding polypyridyl
ruthenium(II) complex ‘cis-Ru(L1) (4,4’-dicarboxylic
acid-2,2’-bipyridine)(NCS)2 H101’. In this paper, we
presented the synthesis and the photovoltaic character-
istics of H101 sensitized solar cell fabricated with an
active area of 0.54 cm2 dye sensitized TiO2 electrodes
in conjunction with ionic liquid electrolyte. The photo-
voltaic characteristics of the sensitizer were compared
with reference Z907 cell fabricated and measured
under identical conditions.
2. Experimental
2.1 General
complexes cis-Ru(4,4’-bis(3,5-di-tert-butylstyryl)-2,2’- 1-bromo-3, 5- di-tert-butyl benzene, n-BuLi, Pd(PPh3)4,
bipyridine)(Ln)(NCS)2 ‘HRD1’ and cis-Ru(4,4’- NH4NCS, 1,2-dimethyl-3-n-propylimidazolium iodide
bis(2,4,6-trimethylstyryl)-2,2’-bipyridine)(Ln)(NCS)2
(DMPII), 4-tert-butylpyridine (TBP), hexachloropla-
‘HRD2’ where Ln = 4,4’-dicarboxylic acid-2,2’- tinic acid, tetrabutylammoniumhydroxide (TBA) were
bipyridine, which have shown good conversion purchased from Sigma-Aldrich. 4,4’-dibromo 2,2’-
efficiencies along with device durability comparable bipyridine was procured from Heterocycles and
with K77 sensitized solar cells.40 The ruthenium (II) Catalysts, Gundeldingerstrasse 174, CH-4053 Basel,
complexes, H112,41 PTZ1,42 mLBD1 and mLBD243 Switzerland. Sephadex LH-20 was procured from
developed in our laboratory with π-conjugation GE Healthcare Bio-Sciences AB, SE-75184, Uppsala.
extension also showed moderate efficiency in DSSC Dichloro (p-cymene) ruthenium (II) dimer and 2,2’-
device. Recently ruthenium(II) complexes with the bipyridine-4,4’-dicarboxylic acid were prepared in
alkyl substituted aryl moieties directly connected to accordance to the reported procedures.48,49 All solvents

Ruthenium(II)- bipyridyl with extended π-system
557
and reagents, unless otherwise stated, were of Labo- a reduced pressure, the resulting residue was diluted
ratory Reagent Grade and used as received. Brucker with a small quantity of methanol. The precipitate
1
300 Avance H NMR spectrometer, Shimadzu LCMS- formed was immediately filtrated, dried and puri-
2010EV model with ESI probe, Shimadzu UV-Vis fied by column chromatography on silica gel with
spectrometer (Model: UV-1700) and Fluorolog 3, J.Y. dichloromethane/methanol (9/1) as eluent to afford the
Horiba Fluorescence spectrometers were employed corresponding ligand (447 mg, 55% yield).
to characterize 4,4’-bis(3,5-ditert-butylphenyl)-2,2’-
1H-NMR (CDCl3) 1.35 (s, 9H), 6.82 (d, 1H), 6.97 (s,
bipyridine, and the new polypyridyl ruthenium(II) com- 2H), 7.30 (s, 1H), 8.07 (s, 1H), 8.50 (d, 1H); Chemi-
plex (H101). 4, 4’-dinonyl-2,2’-bipyridine (L2) was cal formula C38H48N2, ESIMS: Calcd for (M+H)+: 532,
procured from Sigma-Aldrich.
found: 532.
2.1a Synthesis of 3, 5-di-tert-butylphenylboronic acid:
To a stirring cooled (–78◦C) solution of 1-bromo-
3, 5-di-tert-butyl benzene (500 mg, 1.858 mmol) in
dry tetrahydrofuran, n-BuLi (1.394 ml, 1.6 M in hex-
ane) was added drop-wise under nitrogen atmosphere.
After stirring continued for 1 h, triisopropyl borate
(0.642 ml, 2.787 mmol) was added at the same temper-
ature. The reaction mixture was further stirred for 2 h,
and quenched with water followed by 6M HCl solution
drop-wise until pH dropped below 7.0. The resulting
mixture was poured into water and then extracted with
dichloromethane. The combined organic layers were
dried over anhydrous sodium sulphate and after evap-
oration of dichloromethane under a reduced pressure,
the resulting solid was purified by column chromatogra-
phy on silica gel with dichloromethane/methanol (9/1)
as eluent to afford 3,5-di-tert-butylphenylboronic acid
(239 mg, 55% yield).
2.1c Synthesis of H101: Compound 4,4’- bis(3,5-di-
tert-butylphenyl) - 2,2’- bipyridine (0.902 mmol) and
dichloro (p-cymene) ruthenium(II) dimmer (276 mg,
0.451 mmol) in DMF were heated at 60◦C for
a period of 4 h under nitrogen in the dark. Subsequent-
ly, 4,4’-dicarboxylic acid-2, 2’-bipyridine (220 mg,
0.902 mmol) was added and the reaction mixture was
heated to 140◦C for another 4 h. To the resulting
dark green solution was added solid NH4NCS (2.060 g,
22.064 mmol) and the reaction mixture was further
heated for 4 hours at 140◦C. After evaporation of DMF,
water (250 ml) was added to get precipitate. The pur-
ple solid was filtered off, washed with water and ether,
and dried under vacuum. The crude compound was
dissolved in methanol and dichloromethane and puri-
fied by column chromatography on Sephadex LH-20
with methanol/dichloromethane (3/2) as eluent to afford
ruthenium complex (582 mg, 65% yield).
1H-NMR (δH/ppm in CDCl3): 7.45 (s, 1H), 7.11 (s,
2H), 2.00 (s, 2H), 1.40 (s, 18H). Chemical formula
C14H23BO2, ESIMS: Calcd for (M+H)+ : 234, found:
234.
1HNMR (CDCl3 + CD3OD); 1.31 (s,18H), 1.40 (s,
18H), 6.52 (d,1H), 6.89 (s,2H), 7.00 (d,1H), 7.15 (s,2H),
7.20 (d,1H), 7.36 (s,1H), 7.50 (s,1H), 7.59 (s,1H), 7.73
(d,1H), 7.88 (s,1H), 7.98 (d,1H), 8.20 (d,1H), 8.80
(s,1H), 8.91 (s,1H), 9.20 (d,1H), 9.72 (d,1H).
2.1b Synthesis of 4,4’-bis(3,5-di-tert-butylphenyl)-
2,2’-bipyridine (L1): In a 25 ml one-necked round
bottom flask equipped with a condenser were placed
corresponding boronic acid (1.528 mmol), barium
2.2 Fabrication and photocurrent-voltage
measurements of the nanocrystalline TiO2 solar cells
hydroxide octa-hydrade (1.5 g, 4.77 mmol) and The test cells were fabricated using 0.54 cm2 active area
palladium tetrakis triphenyl phosphine (146 mg, TiO2 electrodes in combination with I−/I3− redox cou-
0.127 mmol). The reaction flask was evacuated and ple electrolyte. Initially the fluorine-doped SnO2 (FTO)
filled with nitrogen gas, then 1, 4-dioxane/water (v/v, conducting glass plates (Nippon Sheet Glass, 4 mm
3:1, 8 ml) and 4,4-dibromo-2,2’-bipyridine (200 mg, thick, 8 ꢀ/sq) were cleaned with a detergent solution
0.636 mmol) were added. The reaction mixture was and then rinsed with water and ethanol to remove organ-
refluxed for 24 h under nitrogen gas and cooled to ics or any other contaminants, the dried glass plates
room temperature. The dioxane was removed and were then treated in a UV-O3 system for 20 minutes.
the contents were poured into dichloromethane, the To facilitate a good mechanical contact between the
precipitate formed was removed by filtration through nanocrystalline TiO2 and the conducting FTO matrix,
filter paper and the organic layer was washed with the cleaned plates were treated with a 40 mM TiCl4
1.0 m NaOH aqueous solution, and dried over sodium aqueous solution and then heated at 70◦C for 30 min-
sulphate. After evaporation of dichloromethane under utes. Over the glass plate, 9 μm thickness layer of 18 nm

558
M Chandrasekharam et al.
TiO2 particles (D18T) as transparent layer and then a a Gemini-180 double monochromator (Jobin Yvon
4.8 μm thickness layer of 400 nm anatase TiO2 particles Ltd.), was employed to plot incident photon-to-current
(CCIC, HPW-400) as scattering layer were successively conversion efficiency as a function of excitation
laid one over the other. The electrodes coated with the wavelength.
TiO2 pastes were heated gradually under an air flow for
5 minutes each at 325◦C and 375◦C and for 15 minutes
each at 450◦C and 500◦C. The TiO2 electrodes were
3. Results and discussion
then allowed to cool and when the temperature attained
around 100◦C, immersed in dye solutions of ethanol and
3.1 Synthesis
remained soaked for 16 hours under dark. After taking
out from the dye solutions, the electrodes were rinsed The new polypyridyl ruthenium(II) complex, H101
with ethanol to remove the un-adsorbed dye molecules (figure 1) was synthesized in accordance to the
and then dried under nitrogen gas. The counter elec- sequence of steps illustrated in Scheme 1. 1-bromo-
trodes were prepared by treatment of FTO glass plates 3,5-di-tert-butylbenzene was subjected to bromine-
(TEC 15, 2.2 mm thickness, Libbey-Owens-Ford Indus- lithium exchange reaction using n-butyllithium in
tries) with TiCl4 and further with a drop of H2PtCl6 the presence of tri-isopropyl borate to give 3,5-di-
solution (2 mg of Pt in 1 mL of ethanol) and heated at tert-butylphenylboronic acid, which was reacted with
430◦C for 15 minutes. The dye sensitized TiO2 elec- 4,4’-dibromo-2,2’-bipyridine under Pd(PPh3)2Cl2
trode and Pt counter electrode were assembled into a catalysed Suzuki conditions to afford ‘4,4’-bis(3,5-di-
sealed sandwich type cell by heating with a hot-melt tert-butylphenyl)-2,2’-bipyridine’(L1). The reaction
surlyn film (Surlyn 1702, 25 μm thickness, Du-Pont) of [RuCl2 p-cymene2]2 and L1 in DMF followed by
as a spacer in-between the electrodes. The ionic liquid addition of 2,2’-bipyridine-4,4’-dicarboxylic acid and
electrolyte was filled through the predrilled hole present excess ammonium thiocyanate finally afforded the
on the counter electrode using vaccum filling technique, ruthenium complex. The crude dried compound was
the hole was sealed with a Surlyn disk and a thin glass purified on Sephadex LH20 column chromatography
to avoid leakage of the electrolyte. The photo current- using methanol/dichloromethane (1/1 V/V) mixture
voltage measurements of the DSSC devices fabricated as eluent. The bipyridine ligand L1 and the ruthe-
were executed under the irradiation source of 450 W nium(II) complex, H101 have been fully characterised
1
Xenon light (Osram XBO 450, U.S.A.), whose power with HNMR, FT-IR and ESI-MASS Spectroscopies
was equivalent to AM 1.5 solar simulator. Prior to these (figures S1 to S5 in the Supporting Information). The
measurements, the xenon light was calibrated by using a FTIR spectra (figure S7) of H101 show the bands at
Tempax 113 solar filter (Schott) and the output power of around 1610 cm−1 and 1383 cm−1 for the asymmetric
the solar simulator was calibrated by using a reference and symmetric stretching modes of the carboxylate
Si photodiode equipped with a coloured matched IR– groups. The stretching bands at around 2100 cm−1
cut-off filter (KG-3, Schott) to reduce the mismatch in correspond to NCS groups indicating that NCS coor-
the region of 350–750 nm between the simulated light dinated to the ruthenium center through the N-atom.
and AM 1.5 to <2%. The measurement delay time of The stretching bands at around 2961 to 2865 cm−1
photo I–V characteristics of DSSCs was fixed at 40 ms. correspond to the aryl-tert-butyl groups, while the
The incident light from a 300 W xenon lamp (ILC absorption bands at 1540 and 1463 cm−1 are ascribed
Technology, U.S.A.), which was focused through to bipyridyl modes. The larger broad stretching band
Br
Br
N
i
iii
N
iv
N
N
Ru
H
101
ii
N
N
B(OH)2
Br
L1
Cl
Scheme 1. Synthesis of H101 sensitizer. i) Tri-isopropylborate, n-BuLi, THF, at
−78◦C, 2M HCl; ii) Ba(OH)2.8H2O, Pd(PPh3)4, Dioxane:H2O, reflux for 24 h; iii)
Ru-p-cymene complex, DMF, reflux for 12 h; iv) 2,2’-bipyridine-4,4’-dicarboxylic
acid, NH4NCS, DMF, reflux for 12 h.

Ruthenium(II)- bipyridyl with extended π-system
559
centered at 3397 cm−1 is due to the moisture adsorbed measurements of the dye coated on TiO2 electrodes
by the dye. The reference ruthenium(II) complex, were carried out by staining 7.0 μm thick TiO2 (18 NRT
cis-Ru(4,4’-dinonyl-2,2’-bipyridine)(4,4’-dicarboxylic
layered) films in the 0.3 mM dye solutions for a period
acid-2,2’-bipyridine))(NCS)2 (Z907) was synthesized of 16 h under dark. The absorption spectra of H101
in accordance to the reported procedure.18
dye coated TiO2 film is compared with that of Z907.
Figure S8 (Supporting Information) indicates that the
dye molecules are anchored on TiO2 surface. The film
absorbance ratio of H101 sensitizer (0.936) calculated
by dividing the absorption maxima of H101 (0.9299)
with the absorption maxima of Z907 sensitizer (0.9935)
is relatively lower and indicates lower packing density
of H101 on TiO2 surface. The substitution of 4, 4’-
bis(3,5-di-tert-butylphenyl) on the ancillary bipyridine
in H101 increases the diagonal size of the dye molecule
as predicted resulting in lower packing density of the
dye on the TiO2 surface.50
3.2 Electronic absorption, emission and
electrochemical properties of the dyes
Equi-molar dye solutions of H101 and Z907 were
prepared in ethanol and their comparative electronic
absorption spectra recorded as a function of wave-
length is shown in figure 2. The molar absorptions
were recorded at μM dye concentrations, which are
highly dilute as compared to those employed for stain-
ing the TiO2 electrodes. H101 shows well-defined
two intense metal-to-ligand charge transfer transition
(MLCT) absorption bands one in short wavelength
region and the other one in long wavelength region.
The molar extinction coefficient (ε) of low-energy
absorption band of H101 is 15000 M−1 cm−1 at 531 nm,
while the reference sensitizer, Z907, showed ε-value
of 10, 800 M−1 cm−1 at 540 nm. Although, the MLCT
absorption bands are blue shifted significantly in the
high-energy region, as compared to those of Z907,
the sensitizer showed relatively 1/3 improvement in
molar absorptivity across the spectral range of 400–
750 nm. Besides the molar extinction coefficient, the
light-harvesting ability of DSSCs are also influenced
by the optical absorptivity of sensitizer anchored on
mesoporous TiO2 electrodes, which indirectly depend
on molecular geometry and size. The absorption
In DSSCs, favourite energy offset between dye and
titania is an essential requirement for achieving high-
efficiency, in which the sensitizer’s immediate charge
generation yield from the excited state has a direct influ-
ence on the device operation. The redox potentials of
H101 were scrutinized with cyclic voltammetry using
tetra-butyl ammonium perchlorate (0.1 m in acetoni-
trile) as an electrolyte and ferrocene as an internal stan-
dard at 0.42 V vs SCE. Figure S9 (supporting infor-
mation) shows the cyclic voltammogram of H101 mea-
sured in DMF solvent. The oxidation potential obtained
for H101 sensitizer was 0.918, while the reduction
potential was −0.810 vs SCE. The more positive poten-
tial of this sensitizer, relative to I−/I3− redox couple in
the electrolyte, provide a large thermodynamic driving
force for the regeneration of the dye by iodide. Based
on absorption and emission spectra recorded in ethanol
by exciting the complex with its corresponding low-
energy absorbance maximum, the excitation transition
energy (E0−0) of H101 was estimated to be 1.85 eV
and the standard potential (φ0(S+/S*)), calculated from
(a)
1.5
ꢀ
ꢁ
the relation of φ0 (S + /S) =φ0 (S + /S∗) − E0 − 0 ,
was −0.9341 V vs SCE. So, φ0(S+/S*) value of the
H101 dye is more negative (or higher in energy) than
the conduction band edge of TiO2, providing a thermo-
dynamic driving force to inject electron from the dye to
TiO2.
1.0
0.5
0.0
15x103
(b)
4500
3000
6x103
1500
0
3.3 Computational studies
300
450
600
750
Wavelength (nm)
To understand the increase in molar extinction coeffi-
cient augment with extension of π-conjugation through
4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine (L1)
in H101, DFT geometry optimization and Time Depen-
dent DFT excited state calculations for electronic
Figure 2. (a) Equi-molar absorption spectra of (—) 4,4’-
bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine and (··−··) 4,4’-
dinonyl-2,2’-bipyridine in DMF. (b) Electronic absorption
spectra of (—) H101 and (··−··) Z907 sensitizers in DMF
medium.

560
M Chandrasekharam et al.
(a)
(b)
(c)
(d)
(e)
(h)
(f)
(g)
(i)
(j)
(k)
Figure 3. Transition involved molecular orbitals of ancillary ligand L1 of H101:
(a) LUMO+3; (b) LUMO+2; (c) LUMO+1; (d) LUMO; (e) HOMO; (f) HOMO-1;
(g) HOMO-2; (h) HOMO-3; (i) HOMO-4; (j) HOMO-5; (k) HOMO-6.
ground states of L1, 4,4’-dinonyl-2,2’-bipyridine (L2), absorptivity along with bathochromic shift by 6 nm for
protonated ruthenium(II) complexes (H101 and Z907) L1 relative to L2. The TD-DFT excitation calculations
were performed by using mPW(mPWx+PW91c) of these ancillary ligands showed lower oscillation
parameterization method. The combination of the strength for L1 with bathochromic shift by around
Perdew–Wang 1991 exchange functional and modified 10 nm (table S1, Supporting Information) which indi-
by Adamoand Barone (mPWx) with the Perdew and cate that the predictions of photophysical properties are
Wang’s 1991 gradient corrected correlation functional close to the experimental observations.
(PW91c) being developed along with mixed basis,
Ancillary ligands with various extended π-
LANL2DZ for ruthenium and 6-31G(d) basis for other conjugation systems in the ruthenium(II) complexes
atoms have been used in Gaussian 03.51 The transi- are known to improve ε-value along with signif-
tion involved frontier molecular orbitals (HOMO-6 to icant bathochromic shift. To predict the influence
HOMO and LUMO to LUMO+3) of L1 are shown in of 3,5-di-tert-butyl phenyl as π-conjugation exten-
figure 3, while those of L2 (HOMO-4 to HOMO and sion, the occupied and unoccupied frontier orbitals
LUMO to LUMO+3) are shown in figure S10 in the of H101 are shown in figure 4, while those of Z907
Supporting Information. The HOMO-6 and HOMO- are compared (figure S11, Supporting Informa-
7 orbitals of L1 resemble HOMO-1 and HOMO-2 tion). The first three occupied (HOMO to HOMO-2)
orbitals of L2 and their energy levels shown in figure 5 orbitals of H101 exhibit ruthenium t2g character,
are found to be close to one another. The π-orbitals a sizeable contribution resulting from the mixing
corresponding to HOMO-1 to HOMO-3 of L1 are of thiocyanate ligand with π-bonding orbitals of
delocalized among the phenyl units (π-system) and 4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine and
pyridines, while LUMO to LUMO+3 orbitals have 4,4’-dicraboxylicacid-2,2’-bipyridine. The delocalized
more π*-orbital delocalization and the corresponding π-orbitals in HOMO to HOMO-2 orbitals of 4,4’-
energy levels are depressed as compared to those of bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine favours size
L2. The electronic absorption spectra recorded at equi- mixing with ruthenium(II)-NCS in H101 as compared
molar concentration (figure 2a) showed lower molar to those in 4,4’-dinonyl-2,2’-bipyridine of Z907, which

Ruthenium(II)- bipyridyl with extended π-system
561
(a)
(e)
(i)
(b)
(f)
(j)
(c)
(g)
(k)
(d)
(h)
(l)
Figure 4. Frontier molecular orbitals of H101: (a) LUMO+5; (b) LUMO+4;
(c) LUMO+3; (d) LUMO+2; (e) LUMO+1; (f) LUMO; (g) HOMO; (h) HOMO-1;
(i) HOMO-2; (j) HOMO-3; (k) HOMO-4; (l) HOMO-5.
contributes to lift their energy levels. For Z907 and 3.4 Photovoltaic performance of DSSCs based
H101, the HOMO-3 orbitals are non-bonding com- on the dyes
bination localized on the NCS ligands. But relative
To evaluate the performance of the new ruthe-
nium(II) bipyridyl sensitizer, we employed a high-
quality double-layer titania film (9.0 + 4.8 μm) and
the most compatible electrolyte (Z580, containing
0.2 m I2, 0.5 m guanidinium thiocyanate and 0.5 m
N-metylbenzimidazole in a mixture of 1-propyl-3-
methylimidazolium iodide/1-ethyl-3-methyl imdazoli-
umtetracyanoborate (65/35, v/v)) to construct the test
cells. Iodide-based ionic liquids are more viscous than
conventional organic solvents. To reduce the viscos-
ity of these electrolytes to some extent a binary ionic
liquid electrolytes prepared by mixing iodide based
ionic liquids (which can act both as a solvent and an
iodide source) with low viscosity ionic liquids (hav-
ing weakly coordinating anion based ionic liquids).
The components in Z580 electrolyte meet the above
requirements and apart from that Guanidinium thio-
cyanate in the redox electrolyte increases VOC by slow-
ing recombination and also by inducing a downward
to Z907 sensitizer, the increased molar extinction
coefficient of H101 could be attributed to the lifted
HOMO energy level. The HOMO-4 and HOMO-5 of
H101 are combinations of two π-bonding orbitals of
L1 with significant size mixing from NCS ligands and
ruthenium centre, lift their energy levels. In case of the
LUMO orbitals, it is noted that there is no obvious vari-
ation because of the sole distribution moved towards
4,4’-dicarboxylic acid-2,2’-bipridine and assuming
similar molecular orbital geometry when adsorbed on
TiO2 surface, the close position of the LUMO to the
anchoring moieties will enhance overlap with the 3d
orbitals of TiO2 leading to facile electron injection. In
case of higher unoccupied molecular orbitals of ancil-
lary bipyridyl ligand, L1, the π*-orbitals move from
π-system to bipyridine moieties and these transfer in
H101 sensitizer significantly depress the LUMO+4 and
LUMO+5 orbitals and a moderate extending in case of
LUMO+1 orbital relative to those of Z907.

562
M Chandrasekharam et al.
L2
L1
Z907
H101
with that of Z907 sensitized DSSC fabricated and eval-
uated under identical conditions. A lower IPCE value
reaching 78.6% with broad plateau relatively red shifted
as compared to H101 sensitizer was observed for Z907.
Owing to the lower packing density of H101 over TiO2
surface, the H101 sensitized cell is expected to show
a lower performance when compared to that of Z907
sensitized solar cell but the higher JSC of H101 could
be probably favoured by more localized electron cloud
over the 4,4’-dicarboxylic-2,2’-bypyridine in LUMO of
H101 as compared to those in Z907 sensitizer. Figure 7
shows a typical photocurrent density–voltage curve of
H101 sensitized solar cell measured under AM 1.5 sun-
light illumination. The short-circuit photocurrent den-
sity (JSC), open-circuit voltage (VOC), and fill factor
(ff) are 12.14 mAcm2, 690 mV and 0.699, respectively,
yielding an overall energy conversion efficiency (η) of
5.89%, while the test device fabricated under identical
conditions with Z907 dye gave JSC of 11.93 mA/cm2,
VOC of 650 mV, and ff of 0.666 yielding an overall
energy conversion efficiency of 5.17%.
-2
-4
-6
0
-3
-6
∗
t2g-
NCS
π
π-π*
Figure 5. Energy diagram of occupied (HOMO to HOMO-
5) and unoccupied (LUMO to LUMO+5) frontier orbitals of
4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine (L1); 4,4’-
dinonyl-2,2’-bipyridine (L2), Z907 and H101 sensitizers.
shift of the TiO2 conduction band edge.52 The inci-
dent photon-to-current conversion efficiency (IPCE) of
H101 sensitized DSSC plotted as a function of excita-
tion wavelength (figure 6) showed broad plateau IPCE
spectrum with IPCE exceeding 92.5%. Considering the
light absorption and scattering loss by the conducting
glass, the absorbed photon-to-collected electron conver-
sion efficiencies (APCE) are close to unity over a broad
spectral range, suggesting a very high charge collec-
tion yield. To see the effect of light-harvesting enhance-
ment due to increased π-conjugation extension, the
photocurrent action spectrum of H101 was compared
One of the desirable parameters to retain the ini-
tial photovoltaic performance of the DSSC is the high
thermal stability of the ruthenium(II) sensitizer.18 To
see initially the influence of bis(3,5-di-tert-butylphenyl)
substitution on the thermal stability of the new ruthe-
nium(II) sensitizer, H101, TGA analysis was performed
using a TGA/SDTA 851e thermal system (Mettler
10
5
90
60
30
0
0
0.25
0.50
Voltage (V)
0.75
Figure 7. J–V characteristics of (—) H101 and (··−··)
Z907 cells (fabricated from 0.54 cm2 active area TiO2
electrodes), measured under an irradiance of 100 mWcm−2
AM1.5G sun light. Double layer films (9.0 + 4.8 μm).
The electrolyte composition: 0.2 m I2, 0.5 m guanidinium
450
600
750
Wavelength (nm)
Figure 6. Photocurrent action spectra of devices of (—) thiocyanate and 0.5 m N-metylbenzimidazole in a mixture
H101 and (··−··) Z907 fabricated from 0.54 cm2 active area of 1-propyl-3-methylimidazolium iodide/1-ethyl-3-methyl-
TiO2 electrodes sensitized in dye solutions of ethanol.
imdazoliumtetracyanoborate (65/35, v/v).

Ruthenium(II)- bipyridyl with extended π-system
563
Toledo, Switzerland) at heating rate of 10◦C/min in the at the desired temperature and were plotted as a func-
temperature range of 25–600◦C under N2 atmosphere tion of temperature. Figure 8 shows a slight change in
(flow rate of 30 ml/min). Film samples ranging from 8 both VOC and fill factor across the temperature range of
to 10 mg were placed in the sample pan and heated, 30 to 80◦C, while a gradual increase in photo-current
while weight loss and temperature difference were density, JSC was observed with increasing temperature
recorded as a function of temperature. The thermo-gram of the device, which ultimately translated to gradual
thus obtained was compared with that of Z907. Figure increase in the overall energy conversion efficiency. The
S12(a) in supplementary information shows the deriva- photostability studies for these ruthenium(II) bipyridyl
tive of per cent of conversion with respect to tempera- complexes were performed under a solar simulator at
ture, wherein both the thermo-grams of H101 and Z907 100 mWcm2 intensity. The H101 sensitized solar cell
initially follow similar trend and relatively extended covered with an ultraviolet absorbing polymer film
thermal stability by another 30◦C was observed for showed a nominal decrease in its initial η-value after
H101. In order to investigate further, the factors 1,000 h of light-soaking at 55◦C. The aging tests have
contributing for the increased thermal stability of the been performed at 55◦C for 1000 h (figure 9), the
ruthenium(II) complex, the TGA analysis of ancillary power conversion efficiency of H101 sensitized solar
ligands L1 and L2 are shown in figure S12(b) (supple- cell was sustained even under heating, maintaining
mentary information). The thermo-gram of L2 shows a 85% of its initial value after this time period, while
transition at 420◦C, while that of L1 shows at around Z907 device retained 88% of its initial performance,
470◦C yielding an increase in thermal stability by 50◦C, despite the fact that the sustainability of the cell ini-
which could be attributed to the conjugation of aromatic tial efficiency varies based on the type of electrolyte
phenyl units against nonyl moieties on bipyridine. Dur- employed in the fabrication of DSSC. The retainabil-
ing the metal complex formation, these aromatic alkyl- ity of initial efficiency for H101 sensitizer is slightly
substituted phenyl units have no effect on chelating lower as compared to that of Z907. To understand this,
bonds of ruthenium-bipyridine and thereby the substitu- a series of desorption studies of H101 and Z907 sen-
tion of 4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine sitized TiO2 films in KOH-solutions were carried out
in place of 4,4’-dinonyl-2,2’-bipyridine further stabi- separately to evaluate the desorption pattern of these
lizes. This extended thermal stability is expected to dyes from TiO2 films. Though the results showed that
improve the long term durability of photovoltaic perfor- desorption of H101 from TiO2 is quantitatively lit-
mance of the sensitizer in DSSC.
tle more compared to that of Z907, the retainabil-
The photovoltaic characteristics of H101 sensitized ity of H101 sensitized solar cell is comparable with
solar cell were measured at various temperatures by that of Z907 sensitizer and the relatively higher effi-
annealing the solar device in the pre-heated oven set ciency observed after complete aging of 1000 h at
15
15
0.75
0.75
10
5
10
5
0.50
0.50
0
200
400
600
800
1000
40
60
80
Temperature (0C)
Time (hour)
Figure 8. Detail of device parameter variations with tem- Figure 9. Details of device parameter variations for cells
perature; (ꢀ) Short-circuit photocurrent density; (ꢁ) energy during accelerated aging at 50◦C; (ꢀ) Short-circuit photocur-
conversion efficiency; (ꢀ) open current voltage; (
•
) fill rent density; (ꢁ) energy conversion efficiency; (ꢀ) open
factor.
current voltage; ( ) fill factor.
•

564
M Chandrasekharam et al.
55◦C is presumed to be resulting from the substitu-
tion of 4,4’-bis(3,5-di-tert-butylphenyl)-2,2’-bipyridine
against 4,4’-dinonyl-2,2’-bipyridine in Z907 sensitizer.
The main disadvantage of the π-extension (C=C dou-
ble bond) in ruthenium(II)-bipyridyl sensitizers is the
slight catalysis of the electron/electrolyte recombina-
tion, which could be resulting through the vinylene
spacers. The new ruthenium(II)-bipyridyl sensitizer,
H101 is designed with exclusion of ethenyl spacers and
the long-term device durability studies of the device
indicate that the four tert-butyl groups on phenyl units
of the ancillary bipyridyl ligand gives similar effect in
terms of compatibility with ionic liquid electrolyte as
observed with two n-nonyl groups of Z907.
2. Kumara G R A, Konno A, Shiratsuchi K, Tsukahara J,
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Langmuir 20 6514
8. Reddy P Y, Giribabu L, Lyness Ch, Snaith H J,
Vijaykumar Ch, Chandrasekharam M, Lakshmikantam
M, Yum J H, Kalyanasundaram K, Graetzel M,
Nazeeruddin M K 2007 Angew. Chem., Int. Ed. 46 373
9. Kuang D, Uchida S, Humphry-Baker R, Zakeeruddin S
M, Graetzel M 2007 Angew. Chem. Int. Ed. 46 1949
10. Tatay S, Haque S A, O’Regan B, Durrant J R, Verhees
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4. Conclusions
To summarise, a high molar extinction coefficient ruthe-
nium(II) sensitizer, exhibiting 5.89% power conver-
11. Giribabu L, Vijaykumar Ch, Reddy P Y, Yum J H,
Graetzel M, Nazeeruddin M K 2009 J. Chem. Sci. 121 75
sion efficiency for 0.54 cm2 active area DSSC under 12. Nazeeruddin M K, Angelis F D, Fantacci S, Selloni A,
Viscardi G, Liska P, Ito S, Takeru B, Gratzel M 2005
J. Am. Chem. Soc. 127 16835
AM 1.5 G conditions, has been synthesized using
4,4’-bis(3,5-ditert-butylphenyl)-2,2’-bipyridine as π-
conjugated ancillary ligand. The TGA analysis of H101
showed relatively extended thermal stability, when
compared to that of reference Z907 sensitizer. The
sensitizer showed better incident photon-to-collected
electron conversion efficiency and higher overall pho-
tovoltaic performance before and after completion of
aging studies.
13. Nazeeruddin M K, Pechey P, Renouard T, Zakeeruddin S
M, Humphry-Baker R, Comte P, Liska P, Cevey L, Costa
E, Shklover V, Spiccia L, Deacon G B, Bignozzi C A,
Gratzel M 2001 J. Am. Chem. Soc. 123 1613
14. Graetzel M 2005 Inorg. Chem. 44 6841
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Chem. Commun. 42 2460
16. Chen C Y, Wu S J, Li J Y, Wu C G, Chen J G, Ho K C
2007 Adv. Mater. 19 3888
17. Hum J, Jung I, Nazeeruddin M K, Gratzel M 2009
Energy Environ. Sci. 2 100
Supplementary information
18. Wang P, Zakeeruddin S M, Moser J E, Nazeeruddin M
K, Sekiguchi T, Gratzel M 2003 Nat. Mater. 2 402
19. Wang P, Klein C, Humphry-Baker R, Zakeeruddin S M,
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20. Kuang D, Klein C, Ito S, Moser J-E, Baker R H,
Zakeeruddin S M, Gratzel M 2007 Adv. Funct. Mater.
17 154
The electronic supplementary information can be seen
in www.ias.ac.in/chemsci.
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Ch S R thanks Indian Institute of Chemical Technology-
Aisin Cosmos (IICT–AIC) collaborative project for a
fellowship. TS thanks Council of Scientific and Indus-
trial Research (CSIR), New Delhi for senior research
fellowship. MC thanks Department of Science and
Technology (DST) New Delhi for the grant of the
project entitled ‘Advancing the efficiency and produc-
tion potential of Excitonic Solar Cells (APEX)’ under
UK–DST consortium.
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