High spectral response heteroleptic ruthenium (II) complexes as sensitizers for dye sensitized solar cells

Article abstract of DOI:10.1007/s12039-011-0066-7

Heteroleptic ruthenium(II) bipyridyl complex, cis-Ru(II)(4,4'-bis(4-tert- butylstyryl)-2,2'-bipyridyl) (4,4'-dicarboxy-2,2'-bipyridyl) (NCS₂) (H112) was synthesized and characterized by 1H-NMR, MASS, Spectrofluorometer and UV-Vis spectroscopes. The photo-voltaic performance of the sensitizer was evaluated in Dye Sensitized Solar Cell (DSSC) under irradiation of AM 1.5 G solar light and the photovoltaic characteristics were compared with those of reference cells of HRS1 and N719 fabricated under comparable conditions. Compared to N719, H112 sensitizer showed enhanced molar extinction coefficient and relatively better monochromatic incident photon-to-current conversion efficiency (IPCE) across the spectral range of 400 to 800 nm with solar energy-to-electrical conversion efficiency (η) of 2.43% [open circuit photovoltage (V_OC) = 0.631V, short-circuit photocurrent density (J_SC) = 8.96 mA/cm², fill factor (ff) = 0.430], while η values of 2.51% (V_OC = 0.651 V, J_SC = 9.41mA/cm ², ff = 0.410) and 2.74% (V_OC = 0.705 V, J_SC = 8.62 mA/cm², ff = 0.455) were obtained for HRS1 and N719 sensitized solar cells respectively. The introduction of 4,4'-bis(4-tert-butylstyryl) moieties on one of the bipyridine moieties of N719 complex shows higher light absorption abilities, IPCE and J_SC. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0066-7

J. Chem. Sci., Vol. 123, No. 1, January 2011, pp. 37–46. * Indian Academy of Sciences.
High spectral response heteroleptic ruthenium (II) complexes
as sensitizers for dye sensitized solar cells
M CHANDRASEKHARAMa*, CH SRINIVASARAOa, T SURESHa, M ANIL REDDYa,
M RAGHAVENDERb, G RAJKUMARb, M SRINIVASUb, and P YELLA REDDYb
aInorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (IICT), Uppal
Road, Tarnaka, Hyderabad 500 607, India
bAisin Cosmos R&D Co. Ltd, Nanomaterials Laboratory, IICT campus, Hyderabad 500 607, India
e-mail: chandra@iict.res.in
MS received 21 July 2010; revised 2 November 2010; accepted 9 November 2010
Abstract. Heteroleptic ruthenium(II) bipyridyl complex, cis-Ru(II)(4,4’-bis(4-tert-butylstyryl)-2,2’-bipyridyl)
(4,4’-dicarboxy-2,2’-bipyridyl) (NCS2) (H112) was synthesized and characterized by 1H-NMR, MASS,
Spectrofluorometer and UV-Vis spectroscopes. The photo-voltaic performance of the sensitizer was evaluated
in Dye Sensitized Solar Cell (DSSC) under irradiation of AM 1·5 G solar light and the photovoltaic
characteristics were compared with those of reference cells of HRS1 and N719 fabricated under comparable
conditions. Compared to N719, H112 sensitizer showed enhanced molar extinction coefficient and relatively
better monochromatic incident photon-to-current conversion efficiency (IPCE) across the spectral range of 400
to 800 nm with solar energy-to-electrical conversion efficiency (η) of 2·43% [open circuit photovoltage
(VOC) = 0·631 V, short-circuit photocurrent density (JSC) = 8·96 mA/cm2, fill factor (ff) = 0·430], while
η values of 2·51% (VOC = 0·651 V, JSC = 9·41mA/cm2, ff = 0·410) and 2·74% (VOC = 0·705 V,
JSC = 8·62 mA/cm2, ff = 0·455) were obtained for HRS1 and N719 sensitized solar cells respectively. The
introduction of 4,4’-bis(4-tert-butylstyryl) moieties on one of the bipyridine moieties of N719 complex shows
higher light absorption abilities, IPCE and JSC
.
Keywords. Ru(II) complexes; extended π-conjugation; 4-tert-butylstyryl; nanocrystalline TiO2;
dye sensitized solar cell.
1. Introduction
cost and impressive conversion efficiencies.11 In
DSSC, sensitizers serve as solar light harvesters and
To replace fast depleting fossil fuels arising from their electronic properties influence the light harvest-
increasing global energy demands the search for ing efficiency and the overall photoelectric conversion
clean alternate energy sources is much needed.1–9 efficiency. The ideal sensitizer for DSSC should have
The conversion of solar light can be considered as the high molar extinction coefficient and absorb solar
important renewable energy source that can be tapped radiation strongly with absorption bands preferably
for the generation of mass energy. However, the covering a broad range of wavelengths across visible
unviable production of solar panels from expensive to near IR regions. In addition, the sensitizer should
traditional inorganic semiconductors necessitated the anchor strongly on semiconductor oxide (nanocrystal-
search for alternative cheaper solar energy technolo- line TiO2) surface, to have good electronic communi-
gies.10 In this context, since Michael Gratzel intro- cation and inject electrons efficiently into conduction
duced high surface area nanoporous TiO2 films as band of TiO2.
wide band gap semiconductor, a breakthrough in the
N719 sensitizer in conjunction with nanocrystalline
−
area of DSSC, the photovoltaic application of DSSC TiO2 film and I3 /I− redox couple based electrolyte
has been enormously explored because of their low give solar energy-to-electrical conversion efficiency
(η) of 10% at AM 1·5 solar radiation.12 Although
*For correspondence
Z907 sensitized solar cell resulted in low efficiency
37

38
M Chandrasekharam et al.
of around 7·3% better stability and long-term dura- 2. Experimental
bility in DSSC was achieved with quasi-solid state
ionic liquid electrolytes.13 The sensitizer showed 2.1 General
decreased molar extinction coefficient (ε) as com-
pared to N719, but the improved durability was 4-hexyl-thiophene-2-carboxaldehyde, 4-tert-butylbenzal-
assigned to the presence of hydrophobic long alkyl dehyde, tetrabutyl-ammoniumhydroxide (TBAOH), 2,4-
chains that increase the stability and compatibility of dimethyl-3-propyl-1 H-imidazol-3-ium iodide (DMPII),
the dye inhibiting traces of moisture to attack NCS 4-tert-butyl-pyridine (TBP) and titaniumtetraisopropox-
groups.13–14 Substituted bipyridines recently attracted ide were procured from Sigma-Aldrich. The other
special attention for their importance in biological chemicals, unless specified, are of analytical grade and
applications as Zn2+ specific fluorescent molecular used without further purification. 4,4’-Diethyl-2,2’-
probes.15–17 Ruthenium(II) complexes with extensive bipyridine was procured from Merck and dichloro (p-
modifications at 4,4’-positions of one of the two cymene) ruthenium(II) dimer was synthesized according
bipyridyl moieties with hydrophilic and hydrophobic to the reported procedures.37 Sephadex LH-20 was
substituted phenylenevinylene and thienylenevinylene procured from GE Healthcare Bio-Sciences AB, SE-
as extended π-conjugations have been studied for 75184, Uppsala. A fluorine-doped SnO2 conducting
DSSC applications.18–24 These variations in dye glass (FTO, 10Ω −1, Nippon Sheet Glass) was employed
structure not only improved molar extinction coeffi- as substrate to coat nanocrystalline TiO2 pastes, 18 NRT
cient, but also showed high stability to thermal stress, and PST-400C, which were procured from JGC Catal-
light soaking and compatibility with ionic liquid ysis and Chemicals Ltd, Japan. A 50 μm thickness
electrolytes that is attributed to the hydrophobic Surlyn tape, procured from DyeSol, 3 Dominion Place,
nature of these ligands. In addition, recent reports Queanbeyan NSW, Australia was employed as gasket to
indicate that changes like insertion of sulfur, replac- seal both the working and counter electrodes. Brucker
1
ing the heteroatom with other elements of the same 300 Avance H NMR spectrometer (300 MHz) was
1
group and change in alkyl position effect the energy employed to record the H-NMR spectra. Shimadzu
levels and electronic transitions of ruthenium(II) LCMS-2010EV model with ESI probe was employed to
complexes influencing the overall performance of record mass spectrum. The electronic absorption and
DSSCs.25–29 The need to develop high molar extinc- emission spectra of the complex were recorded in N,N-
tion coefficient sensitizers gained significant interest dimethyl-formamide (DMF) using Shimadzu UV-Vis
particularly when organic hole transport materials, spectrometer, model 1700 and Fluorolog 3, J.Y.Horiba
with limited charge-carrier mobility are used in order Fluorescence Spectrometer respectively.
to explore thin film solid state/semisolid state
DSSCs.19,20,22,23,30–33
2.2 Experimental procedures
We have been engaged in our laboratory for the
synthesis and evaluation of stable and efficient new 2.2a 2, 2’-Bipyridine-4, 4’-dicarboxylic acid (dcbpy): 4,
metal free organic, phthalocyanine as well as rutheni- 4’-Dimethyl-2, 2’-bipyridine (1·0 g, 5·43 mmol) was
um‐based dyes as sensitizers for DSSC applica- dissolved in con. H2SO4 (30 ml) at room temperature.
tion.29,34–36 In this endeavour, the photo-voltaic To this solution, K2Cr2O7 (6·2 g) was added in portions
performance of HRD1 and HRD2 heteroleptic ruthe- during 4 h. The reaction flask was heated to 80°C for
nium(II) bipyridyl sensitizers developed in our labora- 12 h with stirring. After cooling to room temperature,
tory were found to be encouraging and comparable the reaction mixture was poured into ice water (200 ml).
with that of K77 sensitizer. In an effort to develop new After keeping it for 12 h at 0°C, the suspension was
extended π-conjugated Ru(II) complex, H112, was filtered by suction. The greenish-yellow precipitate
designed with 4,4’-bis(4-tert-butylstyryl)-2,2’-bipyri- separated was washed thoroughly with distilled water
dine ancillary ligand and synthesized by Horner– (until the filtrate is colourless) and dried under vacuum.
Emmons–Wadsworth reaction of 4-tert-butylbenzalde- The pale yellow solid was suspended in HNO3 (50%,
hyde with tetraethyl 2,2’-bipyridine-4,4’-diyldiphosph- 18 ml) and refluxed for 8 h. After cooling to room
onate (scheme 1). In this paper, we present the temperature, the reaction mixture was poured into ice
synthesis and the photovoltaic performance of the water (180 ml) and stored at 0°C overnight. The white
sensitizer and the results are compared with that of solid obtained was collected on a sintered glass crucible
HRS1 and N719 sensitizers.
(G4), washed thoroughly with water until the filtrate is

High spectral response heteroleptic ruthenium (II) complexes as sensitizers for dye sensitized solar cells
39
EtOOC
COOEt
COOH
HOOC
N
N
N
N
N
N
dcbpy
1
Et2O3P
PO3Et2
CH2Br
BrH2C
HOH2C
CH2OH
N
N
N
N
N
N
3
2
4
Et2O3P
Et2O3P
N
N
N
N
CHO
N
Ru
N
Cl
Scheme 1. Synthesis route for H112 sensitizer.
colourless and dried under vacuum to give 2,2’ bipyr- in one portion to a suspension of 1 (3·0 g, 10·0 mmol)
idine -4,4’ dicarboxylic acid (1·12 g, yield 85%) as a in absolute ethanol (200 ml). The mixture was refluxed
1
white solid compound. H-NMR (D2O/NaOD) ppm: for 3 h and cooled to room temperature, and then
7·72 (2H, dd), 8·22 (2H, s), 8·62 (2H, d). ESI-MASS saturated ammonium chloride (200 ml) solution was
(C12H8N2O4) calculated 244, found 244.
added to decompose the excess borohydride. Ethanol
was removed under vacuum and the precipitate
dissolved in minimum quantity of water. The resulting
solution was extracted with ethyl acetate (5x200 ml)
and dried over sodium sulfate (1·74 g, yield 81%).
1H NMR (CD3OD, ä): 4·75 (4H, s, CH2); 7·43 (2H, d,
J=5·5Hz, aryl H on C5 and C5’); 8·21 (2H, s, aryl H
on C3 and C3’); 8·59 (2H, d, J=5·5Hz, aryl H on C6
and C6’); ESI-MASS (C12H12N2O2) calculated 216,
found 216.
2.2b 4, 4’-Di(ethoxycarbonyl)-2, 2’-bipyridine (1): To
a suspension of 4, 4’-dicarboxy-2,2’-bipyridine (5·0 g,
20·5 mmol) in absolute ethanol (400 ml) was added to
concentrated sulfuric acid (5 ml). The mixture was
refluxed for 4 h to obtain a clear solution and then
cooled to room temperature. Water (400 ml) was added
and the excess ethanol removed under vacuum. The
pH was adjusted to neutral with NaOH solution, and
the resulting precipitate was filtered and washed with
:
1
water (pH=7 0). The solid was dried to obtain (5·5 g, 2.2d 4, 4’- Bis(bromomethyl) – 2, 2’-bipyridine (3):
yield 90%). H NMR (CDCl3, δ): 3·95 (3H, s, CH3); 4,4’-Bis(hydroxy methyl)– 2, 2’- bipyridine (90·0 g,
4·05 (3H, s,CH3 ); 7·98 (2H, d, J=6Hz, aryl H on C5 4·2 mmol) was dissolved in a mixture of 48% HBr (20
and C5’); 8·88 (2H, d, J=6Hz, aryl H on C6 and C6’); ml) and concentrated sulfuric acid (6·7 ml). The
9·00 (2H, s, aryl H on C3 and C3’). ESI-MASS resulting solution was refluxed for 6 h and then allowed
(C16H16N2O4) calculated 300, found 300.
to cool to room temperature, and water (40 ml) was
added. The pH was adjusted to neutral with NaOH
2.2c 4, 4’-Bis(hydroxymethyl)-2, 2’-bipyridine (2): solution and the resulting precipitate was filtered,
:
Sodium borohydride (8·2 g, 0·215 mmol) was added washed with water (pH=7 0), and air-dried. The

40
M Chandrasekharam et al.
product was dissolved in chloroform (40 ml) and 4h. To the resulting dark green solution solid
filtered. The solution was dried over magnesium sulfate NH4NCS (867 mg, 11·4 mmol) was added and the
and evaporated to dryness, yielding a white powder of reaction mixture was further heated for 4 h at 140°C.
1
(1·2 g, yield 85%). H NMR (CDCl3, δ): 4·50 (4H, s, DMF was removed on a rotavapor under vacuum and
CH2); 7·38 (2H, d, J=5Hz, aryl H on C5 and C5’); water (220 ml) was added to get the precipitate. The
8·45 (2H, s, aryl H on C3 and C3’); 8·68 (2H, d, J=5 purple solid was filtered off, washed with water and
Hz, aryl H on C6 and C6’). ESI-MASS (C12H10Br2N2) ether, and dried under vacuum. The crude masses
calculated 339, found 339.
were dissolved in basic methanol (with TBAOH) and
further purified on the Sephadex LH-20 with methanol
as eluent. The main band was collected, concentrated,
and precipitated with acidic methanol (HNO3). This
purification was repeated two times to obtain pure H112
(350 mg, yield 65%).
2.2e 4, 4’-Bis(diethylmethylphosphonate)-2, 2’-bipyr-
idine (4): A chloroform (10 ml) solution of 3 (1·5 g,
4·4 mmol) and triethyl phosphite (15 ml) was refluxed
for 3 h under nitrogen. The excess phosphite was
removed under high vacuum, and then the crude mass
was purified by column chromatography on silica gel
using ethyl acetate/methanol (80/20) as eluent (1·6 g,
yield 80%). 1H NMR (CDCl3, δ): 1·23 (12H, t,
J=7Hz, CH3); 3·20 (4H, d, J=22Hz, CH2P); 4·05 (8
H, apparent quintet, J=7Hz, OCH2); 7·25–7·35 (2H,
m, aryl H on C5 and C5’); 8·34–8·37 (2H, m, aryl H
on C3 and C3’); 8·62 (2H, d, J=5Hz, aryl H on C6 and
C6’). ESI-MASS (C20H30N2O6P2) calculated 456, found
456.
1H-NMR (300MHz, 25°C, CD3OD+CDCl3) δ [ppm]:
9·49 (d, 1H), 9·33 (d, 1H), 8·95 (d, 1H), 8·38 (s, 1H),
8·22 (s, 1H), 8·05 (s, 1H), 7·72–7.40 (m, 2H), 7·32–
7·15 (m, 2H), 7·10–6·91 (m, 2H), 3·91 (m, 16H), 1·66
(m, 16H), 1·52–1·17 (m, 25H), 1·0 (t, 24H). ESI-MS:
RuC48H44N6O4S2 (C16H36N)2 calculated 1418, found
1418.
2.3 Preparation of nanocrystalline TiO2 dye sensitized
electrode
A compact layer was formed over 2×2 cm2 FTO glass
by spin coating of a solution of 0·5 M Ti(isoprop-
oxide)4 and 0·6 M HCl prepared using 98% absolute
ethanol. The glass plates were then kept in the
preheated oven at 100°C for 15 minutes. One film of
18 NRT with an active area of 7×10·5 mm2 was formed
on the glass plate using screen printing technique and
then heated in pre-heated oven at 100°C for 30
minutes. After that another film of 18 NRT was formed
over the 1st film and then heated in the pre-heated oven
at 100°C for 30 minutes. Then a film of scattering
layer was formed with PST-400C and then annealed in
air for 1 h at 500°C to give overall thicknesses of
9 μm. The films were cooled to around 100°C and
sensitized by immersing the TiO2 electrodes in dye
solution for 16 h at room temperature under dark. The
counter electrode was prepared by spin coating platinic
solution on fluorine-doped tin oxide glass. Spin coater
(Model WS-400B-6NPP/LITE), from Laurell Technol-
ogies Corporation was employed to coat hexachlor-
oplatinic acid/isopropanol (1/4) over FTO glass plate.
In order to determine dye loadings, the dye sensitized
photo-electrodes were immersed in 0·3M KOH meth-
anol solution to desorb the dye molecules. The
quantity of desorbed dye in the alkaline solution was
established by UV/Vis spectroscopy in a separate
series of experiments.
2.2f 4,4’-Bis(4-tert-butylstyryl)-2,2’-bipyridyl (H112
ligand): Sodium hydride (171 mg, 7·12 mmol) was
washed (3 times) with dry hexane and THF (40 ml)
was added. To this suspension, a THF solution of 4
(408 mg, 0·89 mmol) was added and the resulting
mixture was stirred at room temperature for 30
minutes. Then 4-tert-butlbenzaldehyde (2·49 mmol)
dissolved in THF was added drop-wise at room
temperature while stirring. The reaction mixture was
refluxed for 12 h. After cooling to room temperature,
the mixture was filtered through a short plug of silica
gel using diethyl ether or dichloromethane. The filtrate
was concentrated and methanol was added. The
product was precipitated, filtered and washed with
cool methanol. The solid was dried to obtain the
1
product (231 mg 55%). H-NMR (L2): 8·68(d, 1H),
8·52(s,1H), 7·53(d,1H), 7·51(s,2H), 7·46–7·37(m,3H),
7·10(d,1H), 1·34(s,9H); ESI-MASS (C34H36N2) calcu-
lated 472, found 472.
2.2gH112 Complex: 4,4’-bis(4-tert-butylstyryl)-2,2’-
bipyridyl (180 mg, 0·38 mmol) and dichloro (p-cym-
ene) ruthenium(II) dimer (116 mg, 0·19 mmol) in DMF
were heated at 60°C for a period of 4 h under nitrogen in
the dark. Subsequently, 4, 4’-dicarboxylic acid-2, 2’-
bipyridine (92·7 mg, 0·38 mmol) was added and the
reaction mixture was heated to 140°C for another

High spectral response heteroleptic ruthenium (II) complexes as sensitizers for dye sensitized solar cells
41
2.4 DSSC Assembling
benzaldehyde with a precursor, tetraethyl 2,2’-bipyri-
dine-4,4’-diylbis(methylene)diphosphonate, which is
DSSCs were fabricated following the procedures accomplished in 5 steps starting from 4,4’-diethyl-2,2’-
described in the literature.38–40 A 50μm thickness bipyridine (scheme 1).
surlyn was sandwiched between the counter electrode and
Reagents: (i) K2Cr2O7/H2SO4, for 12h at 80oC (yield-
dye sensitized TiO2 electrode. The assembly was then 85%). (ii) EtOH - H2SO4, reflux for 4h (yield-90%).
placed on a preheated hot plate where the top counter (iii) NaBH4-EtOH, reflux for 3h (yield-81%). (iv) HBr-
electrode was pressed gently to seal both electrodes. The H2SO4, reflux for 6h (yield-85%). (v) P(OEt)3-CHCl3,
iodide electrolyte solution was filled through the pre- reflux for 3h (yield-80%). (vi) NaH-THF, reflux for 12
drilled hole present on the counter electrode and drawn h (yield-55%). (vii) Ru-p-cymene complex, 2,2’-bipyr-
into the space between the electrodes by capillary action. idine-4,4’-dicarboxylic acid, NH4NCS, DMF, reflux for
Then the hole was sealed using surlyn tape. The DSSCs 12h (yield-65%).
thus fabricated in our laboratory have an active area of
The H112 complex was obtained from the reaction of
0·74 cm2, which is around 6 times higher than the regular [RuCl2p-cymene2]2 and the 4,4’-bis(4-tert-butylstyryl)-
test cells of active area of 0.16 cm2. Two electrolytes E6 2,2’-bipyridine followed by addition of 2,2’-bipyridine-
and Ei301 were employed to evaluate the performance of 4,4’-dicarboxylic acid and excess ammonium thiocya-
H112 sensitizer. E6 electrolyte is composed of DMPII nate. The dried crude complex was purified on
(0·6M), I2 (0·1M) LiI (0·5M) and TBP (0·5M) in Sephadex LH20 column chromatography to afford the
methoxypropionitrile, while Ei301 was prepared from ruthenium complex. The presence of a peak at 1418 in
LiI (1·0M) and I2 (0·05M) in γ-butyrolactone.
ESI-MASS spectrum corresponds to molecular ion peak
of H112 with 2 TBA. The reference sensitizers HRS1
and N719 were synthesized in accordance with the
procedures reported.12,21 The FTIR spectrum of H112
2.5 Characterization and measurements
Incident photon-to-current conversion efficiency was compared with that of N719 (figure S1 [Electronic
spectra were measured using illumination from a supporting information]). The bands at around 2100 cm-1
150 W Xe lamp/grating monochromator calibrated in FTIR spectrum of H112 complex corresponds to the
ithilicon photodiode (traceable (5% to NBS). The NCS stretching band indicating that NCS coordinated to
current-voltage characteristics of the cells were the center ruthenium through N-atom. The stretching
measured using a xenon arc solar simulator (ELL- bands at around 1610 and 1357 cm−1 are due to the
SOL 1000, model SVX 1450) with an AM 1.5 symmetric and asymmetric stretching modes of corbax-
spectral filter, and the intensity was adjusted to ylate groups. The stretching bands at around 2961 to
provide 1 sun (100 mW cm−2) using a calibrated 2865 cm−1 correspond to the aryl-tert-butyl groups of
GaAs solar cell. Based on I–V curve, the fill factor H112 sensitizer. The absorption bands at 1540 and 1463
(ff) is defined as ff = (Imax x Vmax) / (ISC x VOC), cm−1 are ascribed to bipyridyl modes, while the larger
where Imax and Vmax are photocurrent and photo- broad stretching band centered at 3397 cm−1 is due to
voltage for maximum power output (Pmax), ISC and the adsorbed moisture of the dye.
VOC are the short-circuit photocurrent and open-
circuit photovoltage, respectively. The overall energy 3.2 Electronic absorption, emission and redox properties
conversion efficiency (η) is defined as η = (ISC x VOC x
ff)/Pin, where Pin is the power of incident light.
The absorbance of 4,4’-bis(4-tert-butylstyryl)-2,2’-
bipyridine (H112 ligand) (0·1mM) was recorded in
ethanol and compared with that of 4,4’-bis((E)-2-(5-
hexylthiophen-2-yl)vinyl)-2,2’-bipyridine (HRS1 li-
gand) (figure S2a, ESI). The absorption spectrum of
H112 was recorded in DMF as a function of
3. Results and discussion
3.1 Synthesis of H112
The structure of the new heteroleptic ruthenium(II) wavelength and the spectrum was compared with those
bipyridyl complex, cis-Ru(II)(4,4’-bis(4-tert-butyl- of HRS1 and N719 sensitizers (figure S2b, ESI).
styryl)-2,2’-bipyridyl) (4,4’-dicarboxy-2,2’-bipyridyl)
The absorption bands at 326 and 355nm corresponds
(NCS2) is shown in figure 1 The ancillary ligand 4,4’- to π→π* of ligands of H112 and HRS1 (figure S2a
bis(4-tert-butylstyryl)-2,2’-bipyridine was prepared by ESI). The red shift absorption of 4,4’-bis((E)-2-(5-
Horner–Emmons–Wadsworth reaction of 4-tert-butyl- hexylthiophen-2-yl)vinyl)-2,2’-bipyridine by 29 nm

42
M Chandrasekharam et al.
C6H13
S
COONBu4
C6H13
HOOC
HOOC
S
N
N
N
NCS
NCS
N
N
N
NCS
NCS
N
N
Ru
Ru
Ru
N
NCS
NCS
N
N
N
HOOC
HOOC
COONBu4
COOH
COOH
H112
HRS1
N719
Figure 1. Structures of the new heteroleptic polypyridyl ruthenium(II) complex
H112 and reference sensitizers HRS1, N719.
along with the enhanced molar absorptivity could be states of the ligands were optimized using DFT-B3LYP
due to the increase of conjugation further by two double method with Gaussian 09W programme using 6-31g
bonds bridged with sulfur atom in thiophene moiety (d) basis function on C, H, N and S atoms. The
with pyridine moieties. H112 complex shows one metal HOMO-LUMO of H112 and HRS1ligands are depicted
to ligand charge transfer transitions (MLCT) absorption in figures 2a and b. The simulation studies showed
band in long wavelength region, while a relatively weak higher HOMO/LUMO energy band gap for H112
shoulder MLCT band was observed in short wavelength ligand by 0·31eV as compared to that of HRS1 ligand.
region (figure S2b ESI). Compared to N719, H112 with
The emission spectra of HRS1 and H112 sensitizers
(E)-4-(4-tert-butylstyryl) as π-conjugation extension on were recorded in DMF by exciting these molecules at
one of the bipyridine moieties of N719 have improved wavelengths of 555 and 545 nm respectively. The
molar extinction coefficients, indicating higher light emission spectra were analysed by Gaussian Program
harvesting ability. The ε-values and absorption maxima in Origin to calculate the emission maxima for these
of low energy absorption band of H112 measured in DMF complexes (figure S2 ESI). Electrochemical properties
are 16, 700 cm−1M−1 (at 532 nm), while HRS1 showed of H112 were scrutinized by cyclic voltammetry using
high ε-value of 18, 300 cm−1M−1 at 536 nm. The H112 tetra-butyl ammonium perchlorate (0·1M in acetonitrile)
sensitizer showed better sensitivity across the spectral and ferrocene as an internal standard at 0·42 V vs. SCE
range of 400 – 800 nm as compared to N719 ( ε value of and the cyclic voltammogram of H112 is shown in figure
13, 200 M−1cm−1 ) under similar measuring conditions. S3 (ESI). The oxidation potential obtained for H112 is
The enhanced molar extinction coefficient of high 0·73 V, while the reduction potential is −0·91 V vs. SCE.
energy absorption band of H112 could be due to the The more positive potential of the H112 sensitizer,
contribution of π–π* transition of 4,4’-bis(4-tert-butyl- relative to I−/I3− redox couple in the electrolyte, provide a
styryl)-2,2’-bipyridine to the corresponding metal→ large thermodynamic driving force for the regeneration
ligand charge transfer transition of H112 complex as of the dye by iodide. Based on absorption and emission
compared to that of 4,4’-dicarboxy-2,2’-bipyridine in spectra, the excitation transition energy (E0–0) of H112
N719 complex. Compared to HRS1, the lower molar was estimated to be 1·773 eV and the standard potentials
extinction coefficient of high energy absorption band of (f0(S+/S*)) calculated from the relation of [f0(S+/S) =
H112 could be due to the reduced intensity of π–π* f0(S+/S*) –E0–0] for this sensitizer is −1·043 V versus
transitions of 4,4’-bis(4-tert-butylstyryl)-2,2’-bipyridine SCE. So, f0(S+/S*) value is more negative (or higher in
ligand because of its increased HOMO/LUMO band energy) than the conduction band edge of TiO2 and
gap compared to that of 4,4’-bis((E)-2-(5-hexylthio- providing a thermodynamic driving force to inject
phen-2-yl)vinyl)-2,2’-bipyridine of HRS1.
electron from the dye to TiO2.
To get an insight into the electron distribution of the
The optimum conditions for colouring of these dyes on
highest occupied molecular orbital (HOMO) and TiO2 electrode are found to be 3×10−4 M dye in ethanol
lowest unoccupied molecular orbital (LUMO) of and 16 h under dark. Absorption spectra of dye
H112 and HRS1 ligands and the corresponding sensitized TiO2 electrodes with reference to identical
complexes, the geometries of the electronic ground dye free blank TiO2 electrodes used to correct the light

High spectral response heteroleptic ruthenium (II) complexes as sensitizers for dye sensitized solar cells
43
(a)
HOMO
LUMO
-5.67 eV
-1.63 eV
H112 (ΔEHOMO-LUMO = 4.04 eV)
-5.42 eV
-1.69 eV
HRS1 (ΔEHOMO-LUMO = 3.73 eV)
(b)
Δ
E(HOMO-1)-
(LUMO+1)
λ
max
F
(nm)
(eVs)
H112
4.10
3.79
330
1.66
HOMO-1 (-5.72)
HOMO-1 (-5.46)
LUMO+1 (-1.62 )
HRS1
353
1.96
LUMO+1 (-1.67)
Figure 2. (a) HOMO and LUMO orbital diagrams along with the HOMO, LUMO
and ΔEHOMO-LUMO values for H112 and HRS1 ligands. (b) HOMO−1 and LUMO +
1 orbital diagrams along with the HOMO−1, LUMO + 1, ΔE(HOMO−1) –(LUMO+1)
λ max and oscillatory strengths values for H112 and HRS1 ligands.
,
scatter phenomena were recorded. For comparison N719 sensitized TiO2 electrodes are red shifted, while
purpose, the absorption of the sensitizers are normalized their low energy absorption bands are blue shifted as
with their absorption maxima of low energy absorption compared to absorption spectrum recorded in DMF.
bands and the normalized absorptions for each sensitizer The shift in absorption spectra on TiO2 surface is due
are plotted as function of wavelength (figure S4 and to the fact that the carboxylate groups of the sensitizers
figure S5, ESI).
bind electronically to the TiO2 surface. In case of
The absorption spectra of dye sensitized mesopo- HRS1, the position of low energy MLCT band
rous TiO2 films shown in figure S4 [ESI] indicate not maxima is slightly red shifted and broadened as
only the presence of dye but also the anchoring of dye compared to that of N719. The high energy MLCT
on TiO2 surface. When attached on TiO2 surface, the bands of HRS1 and H112 sensitizers have higher
absorption spectrum of dyes tends to shift relative to sensitivity in the short wavelength region as these
the spectrum recorded in solution. As expected, the sensitizers contain extended π-conjugations on ancil-
high energy absorption bands of HRS1, H112 and lary bipyridyl ligands.

44
M Chandrasekharam et al.
E6 electrolyte
H112
HRS1
N719
10.0
45
30
15
0
HRS1
H112
N719
7.5
5.0
2.5
0.0
0.0
0.2
0.4
0.6
0.8
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Figure 5. J–V characteristics of DSSC devices fabricated
from 0·74 cm2 active area TiO2 electrodes sensitized in dye
solutions of ethanol measured under the irradiance of AM
1·5 G sunlight of 100 mW cm−2.
Figure 3. Photocurrent action spectra of devices fabricated
from 0·74 cm2 active area TiO2 electrodes sensitized in dye
solutions of ethanol and E6 electrolyte: DMPII (0·6 M), I2
(0·1 M) LiI (0·5 M) and TBP (0·5 M) in methoxypropionitrile.
3.3 Evaluation of DSC
incident photon-to-current conversion efficiencies of
DSSCs fabricated with E6 electrolyte.
To evaluate the photovoltaic performance of H112
sensitizer in test cell, E6 electrolyte was prepared in
accordance with the reported procedure keeping the
The IPCE spectra of HRS1 and H112 sensitizers
resemble their absorption spectra adsorbed on TiO2
electrode and the IPCEs reached about 46 and 44%
respectively while that of N719 sensitizer is almost equal
to that of H112. The close observation of the IPCE
spectra shows relatively higher and broader IPCE
spectrum for HRS1 compared to H112 sensitizer. In
DSC test cell, increasing LiI concentration of liquid
electrolyte increases the exchange reaction between I-
−
redox couple concentration ([I−]+[I3 ]) as 1.5 and the
ratio ([I−]/[I2]) as 4/1.41 The short-circuit photocurrent
plotted as function of redox couple concentration
showed maximum JSC with the above electrolyte.
Figure 3 shows the action spectra of monochromatic
−
and I3 which in turn increases charge transport rate and
80
short-circuit photocurrent density. Therefore Ei301 elec-
trolyte with a greater concentration of LiI was prepared,
which would increase the charge transport rate and
decreases VOC due to shifting band edge towards
positive. Hence Ei301 electrolyte has been explored to
investigate IPCE of these sensitizers. The IPCE spectra
of the DSSCs fabricated employing Ei301 electrolyte are
shown in figure 4 As it is seen from the figure, HRS1
Ei301 electrolyte
H112
HRS1
N719
60
40
20
Table 1. Detailed photovoltaic parameters of DSSCs fabri-
cated from 0·74 cm2 active area TiO2 electrodes sensitized in
dye solutions of ethanol medium using E6 electrolyte.
0
400
500
600
700
800
Wavelength (nm)
Dye
JSC (mA cm−2)
VOC (V)
ff
η (%)
Figure 4. Photocurrent action spectra of devices fabricated
from 0·74 cm2 active area TiO2 electrodes sensitized in dye
solutions of ethanol and Ei301 electrolyte: LiI (1·0 M) and
I2 (0·05 M) in γ-butyrolactone.
H112
HRS1
N719
8·96
9·41
8·62
0·631
0·651
0·705
0·430
0·410
0·455
2·43
2·51
2·74

High spectral response heteroleptic ruthenium (II) complexes as sensitizers for dye sensitized solar cells
45
shows higher IPCE of 72% as compared to 62% and response, a higher JSC value of 9·41 mA/cm2 was
57% obtained for N719 and H112 sensitized solar cells. obtained among these sensitizers.
In order to see whether the decreased spectral
Short-circuit photocurrent density (Jsc), open-circuit
response is due to differences in packing densities photovoltage (Voc), fill factor (ff) values of cell
of these dye molecules adsorbed on TiO2 surface, parameters specified are average of three DSSCs
the film absorption ratio versus normalized molar fabricated under identical conditions. Error in JSC
extinction coefficient plots were studied (figure S5, 0·5, VOC 0·03, ff 0·05.
ESI), in which the absorption maxima of each dye
was normalized with the absorption maxima of N719
while the molar extinction coefficients of these
4. Conclusions
sensitizers are normalized with that of the molar
In summary, the novel cis-Ru(II)(4,4’-bis(4-tert-butyl-
extinction coefficient of N719. The film absorption
styryl)-2,2’-bipyridyl) (4,4’-dicarboxy-2,2’-bipyridyl)
ratios of H112 and HRS1 sensitizers are close to
(NCS2) (H112) was synthesized and characterized
each other but are below to the hypothetical diagonal
using the spectroscopic techniques. The sensitizer
line indicating relatively lower packing densities of
these sensitizer as compared to that of N719. The
lower film absorption of these sensitizers could be
because of the literally larger diameter of these
complexes as compared to N719. The difference in
the film absorption ratios in-between H112 and
HRS1 is very small indicating that these molecules
have similar packing densities, which indicates that
decreased IPCE of H112 relative to HRS1 could not
be due to the higher surface unsaturation of TiO2
electrodes. Compared to H112 sensitized solar cell,
HRS1 showed broader and higher IPCE spectrum
resulting from its high light harvesting capability.
Furthermore, sulfur has greater radial extension in its
bonding than the second-row elements such as
carbon. Thiophene being a more electron-rich moiety,
incorporation of thiophene onto bipyridine ligands in
HRS1 can raise the energy levels of the metal center
and the LUMO of the ligands. As a consequence, the
absorption band corresponding to metal to anchoring
ligand transition is red shifted as compared to the
H112 ligand, which may contribute for broader IPCE
spectrum of HRS1.
showed improved molar absorptivity and reversible
one electron oxidation process as compared to N719
sensitizer. The DSSC constructed based on this
sensitizer gave comparable solar light to electricity
conversion efficiency. Synthesis of new ruthenium(II)
sensitizers based on variations in bipyridine ancillary
ligands is under progress.
Supporting information
The electronic supporting information can be seen in
www.ias.ac.in/chemsci.
Acknowledgements
The authors thank Aisin Cosmos R&D Co. Ltd. for
financial support. ST and ARM thank Council of
Scientific and Industrial Research (CSIR) for Junior
Research Fellowship.
References
The photocurrent-voltage (J–V) curves of three
DSSCs fabricated using H112, HRS1 and N719
sensitizers and the corresponding photovoltaic per-
formances are listed in figure 5 and table 1. The short-
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that of N719 based DSSC, the device with sensitizer
HRS1 shows little higher Jsc value, which is
consistent to its higher IPCE value. As HRS1
sensitizer showed broadened and increased IPCE
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