A new familiy of heteroleptic ruthenium(ii) polypyridyl complexes for sensitization of nanocrystalline TiO2 films

Article abstract of DOI:10.1039/c0dt01417f

Two new heteroleptic ruthenium(ii) photosensitizers that contains 2,2′;6,2′′-terpyridine with extended π-conjugation with donor groups, a 4,4′-dicarboxylic acid-2,2′-bipyridine anchoring ligand and a thiocyanate ligand have been designed, synthesized and fully characterized by CHN, mass spectrometry, UV-vis and fluorescence spectroscopies and cyclic voltammetry. The new sensitizers have either 3,5-di-tert-butyl phenyl (m-BL-5) or triphenylamine (m-BL-6) groups, where the molar extinction coefficient of both the sensitizers is higher than the analogous ruthenium dyes. Both the sensitizers were tested in dye-sensitized solar cells using two different redox electrolytes.

Full text of DOI:10.1039/c0dt01417f

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PAPER
A new familiy of heteroleptic ruthenium(II) polypyridyl complexes for
sensitization of nanocrystalline TiO2 films†
Lingamallu Giribabu,*a Takeru Bessho,b Mareedu Srinivasu,a Challuri Vijaykumar,a Yarasi Soujanya,c
Veeranagari Gopal Reddy,d Paidi Yella Reddy,d Jun-Ho Yum,b Michael Gra¨tzelb and
Mohammad Khaja Nazeeruddin*b
Received 19th October 2010, Accepted 1st February 2011
DOI: 10.1039/c0dt01417f
Two new heteroleptic ruthenium(II) photosensitizers that contains 2,2¢;6,2¢¢-terpyridine with extended
p-conjugation with donor groups, a 4,4¢-dicarboxylic acid-2,2¢-bipyridine anchoring ligand and a
thiocyanate ligand have been designed, synthesized and fully characterized by CHN, mass
spectrometry, UV-vis and fluorescence spectroscopies and cyclic voltammetry. The new sensitizers have
either 3,5-di-tert-butyl phenyl (m-BL-5) or triphenylamine (m-BL-6) groups, where the molar extinction
coefficient of both the sensitizers is higher than the analogous ruthenium dyes. Both the sensitizers were
tested in dye-sensitized solar cells using two different redox electrolytes.
display stable operation for millions of turnovers.5 The high
Introduction
efficiencies are due to suitable ground- and excited-state energy
Owing to their versatility and cost-effective manufacturing, dye-
levels with respect to the nanocrystalline TiO2 conduction-
sensitized solar cells (DSSC) have long been considered as a
band energy, molar absorption coefficient (e), forcing the use of
thicker TiO2 and the HOMO level of the redox couple iodine–
feasible alternative to conventional solid-state p-n photovoltaic
devices.1 Dye-sensitized solar cells have reached the best effi-
triiodide. However, these dyes suffer from relatively thin layers,
ciencies, approaching those exhibited by conventional systems
which reduces the open-circuit potential compared to thinner
such as amorphous silicon modules.2 In DSSC the dye sen-
films.
sitizer absorbs light to transfer an electron at the interface
Progress in the optimization of sensitizers has been made
with the wide-gap TiO2 semiconductor and the resulting pos-
through increasing the molar absorption coefficient and expanding
the light absorption capability.6 With regard to the former, DSSC
could be made thinner and thus more efficient because of reduced
transport losses in the nanoporous environment.7 With regard to
the latter, dyes are designed to harvest solar energy across the
itive charge to the redox mediator with efficiencies reaching
a maximum of 11.2% (AM 1.5).3 In order to enhance these
efficiencies, several sensitizers based on ruthenium polypyridyl
complexes, tetrapyrrolic compounds and organic molecules have
been synthesized and tested in DSSC.4 The widely used charge-
transfer sensitizers in such cells are bis(tetrabutylammonium)-cis-
di(thiocyanato)-N,N¢-bis(4-carboxylato-4¢-carboxylic acid-2,2¢-
bipyridine)ruthenium(II) (the N719 dye) and trithiocyanato-
4,4¢4¢¢-tricaboxy-2,2¢:6¢,2¢¢-terpyridine ruthenium(II) (the black
dye), which have produced solar-energy-to-electricity conver-
sion efficiencies (h) of >11% under AM 1.5 irradiation and
entire visible and NIR regions of the spectrum with the aim of
8
improving the short-circuit current JSC
.
Taking into account the above requirements, we have designed
and synthesized two new heteroleptic ruthenium(II) polypyridyl
complexes i.e., m-BL-5 and m-BL-6 sensitizers which have electron
releasing groups on extended p-conjugation of a terpyridine
ligand to increase the molar extinction coefficient,9 a bipyridine
ligand with anchoring carboxyl groups to attach onto the TiO2
surface and a thiocyanate ligand to tune the redox properties of
the ruthenium centre. Both the sensitizers have been completely
characterized by elemental analysis, mass spectrometry, 1H NMR,
UV-vis and emission spectroscopies and cyclic voltammetry. The
photovoltaic performance of these sensitizers was studied by using
two different redox electrolytes.
aNanomaterials Laboratory, Inorganic & Physical Chemistry Division,
Indian Institute of Chemical Technology, Hyderabad, 500607, India.
E-mail: giribabu@iict.res.in; Fax: 91-40-27160921; Tel: +91-40-27193186
bLaboratory for Photonics and Interfaces, Institute of Chemical Sciences and
Engineering, School of basic Sciences, Swiss Federal Institute of Technology,
CH–1015, Lausanne, Switzerland. E-mail: mdkhaja.nazeeruddin@epfl.ch;
Fax: +41-21-6934111; Tel: +41-21-6936124
cMolecular Modelling Group, Indian Institute of Chemical Technology,
Hyderabad, 500067, India
dAisin Cosmos R&D Co. Ltd., HUDA Complex, Tarnamaka, Hyderabad,
500007, India
Experimental
All chemicals and solvents were procured either from Aldrich or
from BDH (India) and were purified prior to use.10
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0dt01417f
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Synthesis
was refluxed under a nitrogen atmosphere for 5 h. The excess
phosphate was removed under high vacuum, and then the
crude product was purified by column chromatography on silica
gel and eluted with ethyl acetate–methanol (80 : 20) and the
desired compound was obtained in 80% yield. Anal. calcd for
C30H44N3O9P3: C, 52.71; H, 6.49; N, 6.15. Found: C, 52.75; H,
6.50; N, 6.10%. MS: m/z 707 (M + Na)+ requires 683. 1H NMR
(CDCl3) (300 MHz) d ppm: 8.65 (d, 2H), 8.40 (d, 2H), 8.35 (d,
2H), 7.35 (d, 2H) 4.25 (m, 6H), 3.35 (m, 6H), 1.30 (m, 18H).
4,4¢-Dicarboxy-2,2¢-bipyridine and 4,4¢,4¢¢-tricarboxy-2,2¢-6¢,2¢¢-
terpyridine ligands were synthesized as per the procedures re-
ported in the literature.8,11
4,4¢,4¢¢-Tricarbmethoxy-2,2¢:6¢2¢¢-terpyridine (Tpy–ester)
To
a suspension of 4,4¢,4¢¢-tricarboxy-2,2¢:6¢,2¢¢-terpyridine
(1.00 g, 2.74 mmol) in 50 ml of methanol was added 10 ml of conc.
H2SO4. The mixture was refluxed for 6 h to obtain a clear solution
and then cooled to room temperature. The reaction mixture was
quenched by adding water (400 ml) and the excess methanol was
removed under vacuum. The aqueous solution pH was adjusted to
pH 7.0 using 0.2 M NaOH solution, and the resulting precipitate
was filtered and washed with water. The solid was dried to obtain
the desired product in 85% yield. Anal. calcd for C18H11N3O6: C,
59.18; H, 3.04; N, 11.50. Found: C, 59.20; H, 3.00; N, 11.50%. MS:
General procedures for the synthesis of 4,4¢,4¢¢-substituted-
2,2¢;6,2¢¢-terpyridine ligands
Sodium hydride (26 mg, 1.09 mmol) was washed thrice with dry
hexane and dried under vacuum and to this 20 ml of dry THF
was added. To this suspension, a solution of Tpy–phosphonate
(100 mg, 0.14 mmol) dissolved in THF was added. The resulting
reaction mixture was stirred at room temperature for 30 min.
To this reaction mixture was added corresponding aryl aldehyde
(0.54 mmol) dissolved in 20 ml of THF drop wise at room
temperature with stirring and the contents were refluxed for 12 h.
The reaction mixture was allowed to cool to room temperature and
then filtered. The filtrate was concentrated and the solid material
obtained was recrystallized from ethanol. 3,5-Di-tert-butyl (m-BL-
5 ligand): Yield: 90%. Anal. calcd for C63H77N3: C, 86.35; H, 8.86;
N, 4.80. Found: C, 86.40; H, 8.90; N, 4.80%. MS: m/z 899 (M +
Na)+ requires 876. 1H NMR (CDCl3) (300 MHz) d ppm: 8.71 (d,
2H), 8.63 (dd, 2H), 8.55 (d, 2H), 7.50–7.00 (m, 17H), 1.25 (m,
54H). Triphenyl amine (m-BL-6 ligand): Yield: 85%. Anal. calcd
for C75H56N6: C, 86.51; H, 5.42; N, 8.07. Found: C, 86.60; H, 5.45;
m/z 389 (M + Na)+ requires 389. H NMR (CDCl3) (300 MHz)
d ppm: 9.10 (d, 2H, J = 4.2 Hz), 9.05 (s, 2H), 8.85 (d, 2H, J =
3.8 Hz), 7.70 (d, 2H, J = 4.6 Hz), 4.00 (s, 9H).
1
4,4¢,4¢¢-Trihydroxymethyl-2,2¢:6¢,2¢¢-tripyridine (Tpy–alcohol)
4,4¢,4¢¢-trimethoxycarbonyl-2,2¢:6¢,2¢¢-terpyridine
(0.500
g,
1.5 mmol) was dissolved in 30 ml of absolute ethanol and to
this was added in one portion 1.2 g (30.7 mmol) of NaBH4.
The reaction mixture was refluxed for 4 h and then cooled
to room temperature. Then, 20 ml of saturated ammonium
chloride solution was added to decompose the excess NaBH4.
The solvent ethanol was removed under vacuum and the solid was
precipitated. The precipitate was dissolved in a minimum amount
of water and the resulting solution was extracted with ethyl
acetate (5 ¥ 20 ml) and dried over sodium sulfate. The solvent was
removed under vacuum and the desired compound was obtained
in 55% yield. Anal. calcd for C18H17N3O3: C, 66.86; H, 5.30; N,
13.00. Found: C, 66.90; H, 5.35; N, 13.03%. MS: m/z 323 (M)+
requires 323. 1H NMR (CD3OD) (300 MHz) d ppm: 8.65 (d, 4H,
J = 4.2 Hz), 8.40 (s, 2H), 7.40 (d, 2H, J = 3.8 Hz), 4.80 (s, 6H).
1
N, 8.00%. MS: m/z 1041 (M)+ requires 1041. H NMR (CDCl3)
(300 MHz) d ppm: 8.65 (d, 2H), 8.55 (d, 2H), 8.45 (s, 2H), 7.60–
6.90 (m, 50H).
Synthesis of m-BL-5 and m-BL-6 complexes
Ruthenium trichloride (260 mg, 0.993) and the corresponding
terpyridine ligand (0.99 mmol) were dissolved in a mixture of
C2H5OH–CHCl3 (75 : 25, 100 ml). The reaction mixture was
refluxed for 4 h under a nitrogen atmosphere. The progress of
the reaction was monitored by UV-vis, spectroscopy. The solvent
was removed under reduced pressure to yield the solid material.
The solid material was dissolved in 100 ml of dry DMF and stirred
at room temperature for five min. To this 240 mg (1.04 mmol) of
4,4¢-dicarboxy-2,2¢-bipyridine was added. The resultant reaction
mixture was refluxed for 4 h. Then the temperature of the reaction
mixture was reduce to 80 ◦C and to this was added aqueous
ammonium thiocyanate (1.2 g in 3 ml of water) and refluxing
was continued for further 2 h. The progress of the reaction was
monitored by UV-vis, spectroscopy. The solvent was removed
under reduced pressure and the obtained solid material was
washed with water. The solid material was dissolved in methanol
and to this about 2 ml of tetrabutyl ammonium hydroxide solution
was added. The resulting solution was subjected to a Sephadex
column and eluted with methanol. The middle reddish brown
coloured band was collected and the solvent was removed under
reduced pressure to give the desired compound. m-BL-5: Anal.
calcd for C76H85N6O4RuS: C, 71.33; H, 6.70; N, 6.57. Found:
4,4¢,4¢¢-Tribromomethyl-2,2¢:6¢,2¢¢terpyridine (Tpy–bromide)
4,4¢,4¢¢-tris(hydroxymethyl)-2,2¢:6¢,2¢¢-terpyridine
(2.0
g,
6.2 mmol) was dissolved in a mixture of 48% HBr (43 ml,
804 mmol) and conc. H2SO4 (14.5 ml, 272 mmol). The resulting
solution was refluxed for 6 h and then allowed to cool to
room temperature, and to this 40 ml of water was added.
The pH was adjusted to neutral with NaOH solution and the
resulting precipitate filtered, washed with water, and air-dried.
The obtained solid was dissolved in chloroform (40 ml) and
filtered. The solution was dried over magnesium sulfate and
evaporated to dryness, as a white powder of yield 85%. 1H NMR
(CDCl3–CD3OD) (300 MHz) d ppm: 8.70 (d, 2H, J = 4.0 Hz),
8.60 (s, 2H), 8.40 (s, 2H), 7.50 (d, 2H, J = 3.8 Hz), 4.60 (s, 6H).
4,4¢,4¢¢-Tridiethylmethylphosphonate-2,2¢:6¢,2¢¢-terpyridine
(Tpy–phosphonate)
4,4¢,4¢¢-tris(bromomethyl)-2,2¢:6¢,2¢¢terpyridine
1.96 mmol) and 10 ml (59 mmol) of triethyl phosphite was
dissolved in 100 ml of chloroform. The resulting reaction mixture
(1.00
g,
1
C, 71.30; H, 6.75; N, 6.60%. MS: m/z + 2TBA 1761. H NMR
4498 | Dalton Trans., 2011, 40, 4497–4504
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(CD3OD) (300 MHz) d ppm: 9.20 (d, 2H, J = 4.8 Hz), 8.95 (s, 5H),
8.80 (s, 5H), 8.40 (d, 3H, J = 3.6 Hz), 8.35 (s, 3H), 8.05 (m, 4H), 7.50
(m, 2H), 7.40 (m, 2H), 7.10 (m, 1H) 1.55 (m, 54H). UV-Vis., (lmax, e
mol-1 cm-1): 520 (10,003). m-BL6: Anal. calcd for C88H64N9O4RuS:
C, 73.16; H, 4.47; N, 8.73. Found: C, 73.00; H, 4.50; N, 8.69%. MS:
in the mixed solvent of acetonitrile and valeronitrile (v/v, 85 : 15)
(here after labelled as Z959), and 1.0 M 1,3-dimethylimidazolium
iodide (DMII), 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M
guanidinium thiocyanate (GNCS), and 50 mM LiI in a mixed
solvent of acetonitrile and valeronitrile (v/v, 85 : 15) (here after
labelled as Z960).
1
m/z + TBA 1686. H NMR (CD3OD+DMSO-d6) (300 MHz) d
ppm: 9.15 (d, 2H, J = 4.6 Hz), 8.65 (s, 6H), 8.50 (s, 6H), 7.85 (dd,
6H, J = 4.0 Hz), 7.30 (dd, 7H, J = 3.8), 7.20 (dd, 7H, J = 3.8),
7.00 (m, 10H), 6.70 (m, 16H). UV-Vis, (lmax, e mol-1 cm-1): 441
(25,418).
The photovoltaic performance of these devices was charac-
terized under a 450 W xenon light source (Osram XBO 450,
USA), which is equivalent to an AM 1.5 solar simulator and was
calibrated by using a Tempax 113 solar filter (Schott). In order
to reduce scattered light from the edge of the glass electrodes
of the dyed TiO2 layer, a light shading black mask was used
onto the DSSCs. For photovoltaic measurements of the DSSCs,
the irradiation source was a 450 W xenon light source (Osram
XBO 450, Germany) with a filter (Schott 113), whose power
was regulated to the AM 1.5 G solar standard by using a
reference Si photodiode equipped with a colour matched filter
(KG-3, Schott) in order to reduce the mismatch in the region
of 350–750 nm between the simulated light and AM 1.5 G to
less than 4%. The measurement of incident photon-to-current
conversion efficiency (IPCE) was plotted as a function of excitation
wavelength by using the incident light from a 300 W xenon lamp
(ILC Technology, USA), which was focused through a Gemini-
180 double monochromator (Jobin Yvon Ltd.). The measure-
ment settling time between applying a voltage and measuring
a current for the I–V characterization of DSSCs was fixed to
40 ms.
Characterization methods
The UV-Visible spectra were recorded with a Shimadzu model
1700 spectrophotometer. The steady state fluorescence spectra
were recorded using a Spex model Fluoromax-3 spectrofluo-
rometer for solutions having optical density at the wavelength
of excitation (l) ª 0.2. 1H NMR spectra were obtained at
300 MHz using a Brucker 300 Avance NMR spectrometer
running X-WIN NMR software. The chemical shifts are relative
to tetramethylsilane (TMS). The Fourier transform IR (FTIR)
spectra of all the samples were measured using a Thermo
Nicolet Nexus 670 spectrometer. Cyclic- and differential pulse
voltammetric measurements were performed on a PC-controlled
CH instruments model CHI 620 C electrochemical analyzer. Cyclic
voltammetric experiments were performed on 1 mM dye solution
in acetonitrile at scan rate of 100 mV s-1 using 0.1 M tetrabutyl
ammoniumperchlorate (TBAP) as the supporting electrolyte.
The working electrode was glassy carbon, the standard calomel
electrode (SCE) was used as the reference electrode and platinum
wire was an auxillary electrode. After a cyclic voltammogram
(CV) had been recorded, ferrocene was added, and a second
voltammogram was measured.
Results and discussion
The details of the synthetic strategy adopted for the synthesis of the
m-BL-5 and m-BL-6 complexes are shown in Schemes 1 and 2. The
Tpy-phosphonate was synthesized as per the reaction procedures
reported for bipyridine precursors in the literature.13 The C
C
double bond was introduced at the 4,4¢,4¢¢ positions of the terpyri-
dine ligands, starting from their corresponding Tpy- phosphonate
and condensed with either 3,5-di-tert-butyl benzaldehyde (m-BL-
5 ligand) or N,N¢-diphenyl amino benzaldehyde (m-BL-6 ligand)
using a Wittig–Horner reaction.14 Both the m-BL-5 and m-BL-
Dye cell preparation
The detailed TiO2 photoelectrode (area: ca. 0.740 cm2) preparation
was described in our earlier studies.12 Briefly, nanocrystalline TiO2
films of 8–8.5 mm thickness were deposited onto transparent
conducting glass (Nippon Sheet Glass, which had been coated
with a fluorine-doped stannic oxide layer, sheet resistance of 8–
10 X cm-2) over which ~4.5 mm thickness of 400 nm anatase
TiO2 particles (CCIC, HPW-400) as scattering layer by screen-
printing. These films were gradually sintered at 500 ◦C for 30 min.
The heated electrodes were impregnated with a 0.04 M titanium
tetrachloride solution in water saturated desiccator for 30 min at
70 ◦C and then washed with distilled water◦and rinsed with ethanol.
The electrodes were heated again at 500 C for 30 min and then
allowed to cool to 50 ◦C before dipping them into the dye solution
(3 ¥ 10-4 M in ethanol). The electrodes were dipped into the dye
solution for >18 h at 22 ◦C. The dye sensitized TiO2 electrodes were
assembled with Pt counter electrodes by heating with a hot–melt
Surlyn film (Surlyn 1702, 25 mm thickness, Du-Pont) as a spacer in
between the electrodes. A liquid electrolyte was filled through the
predrilled hole present on the counter electrode and then the hole
was sealed with a Surlyn disk and a thin glass to avoid leakage
of the electrolyte. The liquid electrolytes having the composition
1.0 M 1,3-dimethylimidazolium iodide (DMII), 30 mM I2, 0.5 M
tert-butylpyridine, and 0.1 M guanidinium thiocyanate (GNCS)
1
6 ligands were characterized by H NMR, Mass spectrometry
and CHN analysis (see Supplementary information for Mass
spectrometry and CHN data†). The synthesis of the Ru(II) com-
plexes of m-BL-5 and m-BL-6 was performed following standard
procedures; refluxing RuCl3·2H2O with the corresponding terpyri-
dine ligand in a mixture of C2H5OH–CHCl3 to give the chloro
complex. The chloro complex, 4,4¢-dicarboxy-2,2¢-bipyridine and
aq. ammonium thiocyanate were refluxed in DMF to give the
final m-BL-5 and m-BL-6 complexes after Sephadex column
purification. Both the complexes were completely characterized
by CHN analysis, Mass spectrometry, UV-vis, and fluorescence
spectroscopies as well as cyclic voltammetry.
Photophysical characteristics
Fig. 1 shows the absorption spectra of m-BL-5 in ethanol and
the corresponding data are presented in Table 1. The absorption
bands between 400–550 nm can be ascribed to the metal-to-ligand
charge-transfer transitions in the singlet manifold (1MLCT). The
absorption maximum of m-BL-5 is centered at 523 nm whilst there
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Scheme 1
is a shoulder band at 527nm. The molar extinction coefficient of
the 523nm band is 10 080 M-1 cm-1, whereas m-BL-6 absorption
maxima is centered at 441 nm (e = 25 418 M-1 cm-1). The absorption
maxima of m-BL-5 shows a bathochromic shift of 8 nm, when
compared to that of the standard sensitizer N719. Intraligand p–p*
transitions of the terpyridine ligand are located at 320 nm. In both
the ruthenium(II) complexes, the absorption of 4,4¢-dicarboxy-
2,2¢-bipyridine is located at 280 nm. Similar absorption properties
were observed in other ruthenium(II) polypyridyl complexes in
which bipyridine is an ancillary ligand and anchoring group(s)
either carboxylic or phosphonate were present on the terpyridine
ligand.15 Fig. 1 also depicts the absorption spectra of m-BL-5
adsorbed onto 6 mm thick TiO2 film. The absorption features of
both the complexes in solution as well as when anchored onto
TiO2 are identical except for a slight red shift in the absorption
maxima due to interaction of anchoring groups with the surface.11b
Table 1 UV-visible, emission and electrochemical data
E
1/2/V vs. SCEc
Sensitizer
l
max/nm, e/mol-1 cm-1a
l
em/nmb
Ox
Red
E
0-0/eVd
Eox*
m-BL-5
m-BL-6
523 (10,023)
441 (25,418)
527 (sh)
715
740
0.95
0.90
-1.02
-1.41
-1.67
-1.77
1.65
1.75
-0.70
-0.85
a Solvent: ethanol, error limits: lmax
,
1 nm, e 10%. b Solvent: ethanol, lmax 1 nm. c Solvent: DMF, error limits: E1/2 0.03 V, 0.1 M TBAP. d E0-0 values
,
were estimated from the 5% intensity level of the emission spectra at 77 K.
4500 | Dalton Trans., 2011, 40, 4497–4504
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Scheme 2
A similar absorption spectrum was also observed in the case of
m-BL-6 (see Supporting information†). The emission spectra of
both m-BL-5 and m-BL-6 were measured in ethanol solvent and
the m-BL-5 emission spectrum is shown in Fig. 1. Excitation
of a lower energy MLCT transition of the m-BL-5 and m-BL-6
sensitizers in ethanol produces an emission centered at 715 and
740 nm, respectively. Based on absorption and emission, the E0-0
energy of m-BL-5 and m-BL-6 is 1.65 and 1.75 eV, respectively.16
However, the emission of both the m-BL-5 and m-BL-6 sensitizers
was quenched when adsorbed onto the TiO2 film as a consequence
of electron injection from the excited state of Ru(II) into the
conduction band of TiO2.
Fig. 1 Absorption (—) and emission spectra in ethanol, and absorption
(—–) and emission ( ◊ ◊ ◊ ) spectra adsorbed onto a 6 mm thick TiO2 film of
m-BL-5.
Electrochemistry
Table 1 presents the electrochemical properties of the two sensi-
tizers m-BL-5 and m-BL-6, which were measured by cyclic and
differential pulse voltammetric techniques in DMF solvent with
0.1 M tetrabutyl ammonium perchlorate. Both m-BL-5 and m-BL-
6 undergo one-electron irreversible oxidation at 0.95 and 0.90 V
vs. SCE, respectively. The oxidation process can be assigned to the
Ru(II)–Ru(III) couple, coupled with donor 3,5-di-tert-butyl phenyl
(m-BL-5) or triphenylamine groups. Both the sensitizers show two
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Fig. 2 Molecular orbital energy diagram of m-BL-5 and m-BL-6 compared to that of a TiO2 nanoparticle model. HOMO–LUMO gaps (eV).
quasireversible reduction peaks around -1.4 V, which we assign
to the reduction of 2,2¢-bipyridine dicarboxylic acid. The excited
state oxidation potentials (Eox*) of the dyes (m-BL-5: -0.70 V vs.
SCE; m-BL-6: -0.85 V vs. SCE) are much more negative than the
conduction band of TiO2 at approximately -0.5 V vs. SCE.16
that of the standard sensitizer using thermogravimetric analysis.
Fig. 3 shows the thermal behaviour of m-BL-5. From the Figure it
is clear that the sensitizer m-BL-5 is stable up to 200 ◦C. The
initial weight loss between 200 to 250 ◦C is attributed to the
removal of the carboxyl group. In contrast, the standard sensitizer
◦
N719 is stable up to 100 C only.19 A similar thermal stability
Computational study
was also obtained in m-BL-6 and the black dye (see Supporting
information†).
To gain insight into the electronic structure of the m-BL-5 and
m-BL-6 dyes we performed DFT geometry optimizations and
time-dependent DFT excited state calculations at the B3LYP/3-
21G* level in the gas phase, by means of a C-PCM solvation
model implemented in the Gaussian03 package.17 The results are
compared in Fig. 2 with the corresponding data for the black
dye and for a model TiO2 nanoparticle. As seen from Fig. 2, the
first two HOMOs have essentially ruthenium t2g character with
sizable contribution coming from the NCS ligand orbitals for both
m-BL-5 and m-BL-6. HOMO-2 along with a contribution from
ruthenium and the NCS ligand, also has a fairly large probability
amplitude on the tri-phenylamine moiety for m-BL-6. On the other
hand, in the N719 dye the first three HOMOs have ruthenium
t2g character with sizable contribution coming from the NCS
ligands.18 For m-BL-5 and m-BL-6 the LUMO and LUMO+2
are found to be p* orbitals localized on the 4,4¢-dicarboxylic acid-
2,2¢-bipyridine ligands with different localization, in which m-BL-
5 show a favorable alignment with the TiO2 conduction band
edge, which is consistent with the high photocurrents measured
experimentally.
◦
Fig. 3 TG/DTG curves of m-BL-5 with a heating rate of 10 C min-1
under nitrogen.
Photovoltaic measurements
Both sensitizers, m-BL-5 and m-BL-6, were anchored onto a
double layer nanocystalline TiO2 film and their photovoltaic
properties were measured using two electrolytes, Z959 and Z960.
Table 2 illustrates the photovoltaic performance characteristics
Thermal stability
We have examined the thermal stability of the new ruthenium(II)
polypyridyl sensitizers and compared their thermal stability with
4502 | Dalton Trans., 2011, 40, 4497–4504
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Table 2 Photovoltaic performance of m-BL-5 and m-BL-6a
Sensitizer
Electrolyteb
Z959
Light intensity (mW cm-2)
JSC (mA cm-2)c
VOC (mV)c
ffc
h (%)
m-BL-5
100
50
10
100
50
10
100
50
10
100
50
10
8.10
4.10
0.82
10.40
5.20
1.05
6.70
3.40
0.68
10.00
5.10
1.03
704
681
623
684
658
598
664
644
591
632
612
559
0.79
0.80
0.80
0.77
0.78
0.79
0.80
0.80
0.80
0.78
0.79
0.79
4.50
4.50
4.10
5.40
5.40
4.90
3.60
3.50
3.20
4.90
5.00
4.60
Z960
Z959
Z960
m-BL-6
a Photoelectrode: TiO2 (8 + 4 mm and 0.158 cm2); b electrolyte: Z959: 1.0 M 1,3-dimethylimidazolium iodide (DMII), 30 mM I2, 0.5 M ter-butylpyridine,
and 0.1 M guanidinium thiocyanate (GNCS) in acetonitrile and valeronitrile (v/v, 85 : 15), Z960 : Z959 + 50 mM LiI. c Error limits: JSC 20 mA cm-2,
VOC 30 mV, ff = 0.03.
:
=
of the cells with the two sensitizers. Under standard global Air
Mass (AM) 1.5 solar conditions, the m-BL-5 sensitizer based
cell gave a short-circuit photocurrent density (iSC) of 10.40
0.20 mA cm-2, an open-circuit voltage (VOC) of 684 30 mV
and a fill factor (ff) of 0.77 0.03, corresponding to an overall
conversion efficiency h, derived from the equation: h = iSCVOC
ff/light intensity of 5.40%. Under the same conditions, the m-BL-
6 sensitized solar cell showed an iSC of 10.00 0.20 mA cm-2,
a VOC of 632 30 mV, an ff of 0.78 0.03, and an h value of
4.90% (see Table 2 and Fig. 4(b)). The high concentration of LiI
in Z960 decreases the VOC by 20–30 mV due to a positive shift of
band edge but increases the driving force for electron injection20
leading to a >30% increase in the photocurrent density. In conse-
quence, Z960 involving LiI yielded a higher performance for both
sensitizers.
Conclusions
In conclusion, we have synthesized two new heteroleptic ruthe-
nium(II) polypyridyl complexes with a bipyridine and terpyri-
dine ligands in the same complex. The presence of electron
releasing groups with extended p-conjugation of a terpyridine
ligand increases the molar absorption coefficient. Both the new
sensitizers m-BL-5 and m-BL-6 where tested in DSSC devices
using volatile and non-volatile liquid redox electrolytes. The m-
BL-5 sensitizer exhibited superior performance compared to the
m-BL-6 sensitizer.
Fig. 4 (a) Photocurrent action spectra and (b) current–voltage charac-
teristics of m-BL-5 (red) and m-BL-6 (black). The redox electrolyte was
Z-960.
Acknowledgements
of m-BL-5, m-BL-6 at different light intensities. Fig. 4a shows
the IPCE spectra of m-BL-5 and m-BL-6. We have observed
maximum IPCE value of 66% at 550 nm in both the m-BL-5 and
m-BL-6 sensitizers. The photocurrent action spectrum resembles
the absorption spectra except for a slight red shift by ca. 15 nm.
The photoresponse of thin films displays a broad spectral response
covering the entire visible spectrum up to 800 nm. From the overlap
integral of this curve one measures a short-circuit photocurrent
density of 10.40 and 10.60 0.20 mA cm-2 using the m-BL-5 and
m-BL-6 sensitizers. Fig. 4b shows current–voltage characteristics
The research leading to these results has received funding from
the European Community’s Seventh Framework Programme
(FP7/2007-2013) under collaborative project FP7-ENERGY-
2010-INDIA, grant agreement no. 261920, “ESCORT”. LG is
thankful to IICT-Aisin Cosmos collaborative project for financial
support of this work. Ch. V is thankful to Council of Scientific
and Industrial Research (CSIR) for a research fellowship. MKN
thanks the World Class University (WCU) program funded by the
Ministry of Education, Science and Technology (grant no. R31-
2009-000-10035-0).
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7 A. Islam, H. Sugihara, M. Yanagida, K. Hara, G. Fujihashi, Y.
Tachibana, R. Katoh, S. Murata and H. Arakawa, New J. Chem., 2002,
26, 966.
8 M. K. Nazeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin, R.
Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover,
L. Spiccia, G. B. Deacon, C. A. Bignozzi and M. Gra¨tzel, J. Am. Chem.
Soc., 2001, 123, 1613.
9 P. Wang, S. Zakeeruddin, J. E. Moser, R. Humphry-Baker, P. Comete,
V. Aranyos, A. Hagfeldt, M. K. Nazeeruddin and M. Gra¨tzel, Adv.
Mater., 2004, 16, 1806.
Notes and references
1 (a) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pattersson, Chem.
Rev., 2010, 110, 6595; (b) A. Jager-Waldau, Renewable Sustainable
Energy Rev., 2007, 11, 1414; (c) M. Gra¨tzel, Acc. Chem. Res., 2009, 42,
1788; (d) J. W. Bowers, H. M. Upadhyaya, S. Calnan, R. Hashimoto,
T. Nakada and A. N. Tiwari, Progr. Photovolt.: Res. Appl., 2009,
17, 265; (e) M. K. Nazeeruddin, and M. Gra¨tzel, in Semiconductor
Photochemistry and Photophysics (Molecular and Supramolecular Pho-
tochemistry), ed. V. Ramamurthy and K. S. Schanze, Marcel Dekker,
New York, 2003, vol. 10, pp. 287–331.
2 M. Gra¨tzel, Philos. Trans. R. Soc. London, Ser. A, 2007, 365, 993.
3 C. –Y. Chen, M. Wang, J. –Y. Li, N. Pootrakulchote, L. Alibabaei, C.
–H. Ngoc-le, J. –D. Decoppet, J. –H Sai, C. Gra¨tzel, C. –G. Wu, S.
Zakeeruddin and M. Gra¨tzel, ACS Nano, 2009, 3, 3103.
4 (a) M. V. Martinez-Diaz, G. De la Torre and T. Torres, Chem. Commun.,
2010, 46, 7090; (b) Z. S. Wang and F. Liu, Front. Chem. China, 2010, 5,
150; (c) H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42,
1809; (d) A. Mishra, M. K. R. Fischer and P. Bauerle, Angew. Chem.,
Int. Ed., 2009, 48, 2474; (e) L. M. Goncalves, V. Z. Veronica, H. A.
Ribeiro and A. M. Mendis, Energy Environ. Sci., 2008, 1, 655.
5 (a) B. O’Regan and M. Gra¨tzel, Nature, 1991, 353, 737; (b) M. Gratzel,
Nature, 2001, 414, 338; (c) M. K. Nazeeruddin, A. Kay, I. Rodicio, R.
Humphry-Baker, E. Muller, P. Liska, N. Valchopoulos and M. Gra¨tzel,
J. Am. Chem. Soc., 1993, 115, 6382.
6 (a) Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and P. Wang, J.
Phys. Chem. C, 2009, 113, 6290; (b) L. Giribabu, Ch. Vijaykumar, Ch. S.
Rao, V. G. Reddy, P. Y. Reddy, M. Chandrasekharam and Y. Soujanya,
Energy Environ. Sci., 2009, 2, 770; (c) F. Matar, T. H. Ghaddar, K.
Walley, T. DosSantos, J. R. Durrant and B. O’Regan, J. Mater. Chem.,
2008, 18, 4246; (d) Z. Jin, H. Masuda, N. Yamanaka, M. Minami, T.
Nakamura and Y. Nishikitani, ChemSusChem, 2008, 1, 901; (e) F. Gao,
Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P.
Wang, S. M. Zakeeruddin and M. Gra¨tzel, J. Am. Chem. Soc., 2008,
130, 10720; (f) P. Y. Reddy, L. Giribabu, Ch. Lyness, H. J. Snaith, Ch.
Vijaykumar, M. Chandrasekharam, M. Lakshmikantam, J. H. Yum,
K. Kalyanasundaram, M. Gra¨tzel and M. K. Nazeeruddin, Angew.
Chem., Int. Ed., 2007, 46, 373; (g) K.-S. Chen, W.-H. Liu, Y.-H. Wang,
C.-H. Lai, P.-T. Chou, G.-H. Lee, K. Chen, H.-Y. Chen, Y. Chi and
F.-C. Tung, Adv. Funct. Mater., 2007, 17, 2964; (h) P. Wang, C. Klein,
R. Humphry-Baker, S. M. Zakeeruddin and M. Gra¨tzel, J. Am. Chem.
Soc., 2005, 127, 808.
10 D. D. Perrin, W. L. F. Armarego, and D. R. Perrin, Purification of
Laboratory Chemicals, Oxford, UK, 1980.
11 A. R. Oki and R. J. Morgan, Synth. Commun., 1995, 25,
4093.
12 (a) L. Giribabu, Ch. Vijaykumar, P. Y. Reddy, J. –H. Hum, M. Gra¨tzel
and M. K. Nazeeruddin, J. Chem. Sci., 2009, 121, 75; (b) L. Giribabu,
Ch. Vijaykumar, V. G. Reddy, Ch. S. Rao, S. –R. Jang, J. –H. Hum,
M. K. Nazeeruddin and M. Gra¨tzel, Sol. Energy Mater. Sol. Cells,
2007, 91, 1611; (c) L. Giribabu, Ch. Vijaykumar and P. Y. Reddy, J.
Porphyrins Phthalocyanines, 2006, 10, 1007.
13 I. G. Gauthier, F. Odobel, M. Alebbi, R. Argazzi, E. Costa, C. A.
Bignozzi, P. Qu and G. J. Meyer, Inorg. Chem., 2001, 40, 6073.
14 W. W. Wadsworth and W. D. Emmons, J. Am. Chem. Soc., 1961, 83,
1733.
15 (a) S. Altobello, R. Argazzi, S. Caramori, C. Contado, S. D. Fre, P.
Rubino, C. Chone, G. Larramona and C. A. Bignozzi, J. Am. Chem.
Soc., 2005, 127, 15342; (b) S. M. Zakeeruddin, M. K. Nazeeruddin, P.
Pechy, F. P. Rotzinger, R. Humphry-Baker, K. Klaynasundaram and
M. Gra¨tzel, Inorg. Chem., 1997, 36, 5937.
16 (a) Excited state oxidation potentials are calculated by using Eox* =
E1/2(ox) - E0-0; (b) A. Hagfeldt and M. Gratzel, Chem. Rev., 1995, 95, 49.
17 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; J. S. Binkley, J. A. Pople
and W. J. Hehre, J. Am. Chem. Soc., 1980, 102, 939; (b) M. Cossi and
V. J. Barone, J. Chem. Phys., 2001, 115, 4708; (c) M. Cossi, N. Rega, G.
Scalmani and V. J. Barone, J. Comput. Chem., 2003, 24, 669.
18 M. K. Nazeeruddin, T. Bessho, L. Cevey, S. Ito, C. Klein, F. De Angilis,
S. Fantacci, P. Comte, P. Liska, H. Imai and M. Gra¨tzel, J. Photochem.
Photobiol., A, 2007, 185, 331.
19 M. K. Nazeeruddin, M. Amirnasr, P. Comte, J. R. Mackay, A. J.
McQuillan, R. Houriet and M. Gra¨tzel, Langmuir, 2000, 16, 8525.
20 S. E. Koops, B. C. O’Regarn, P. R. F. Barnes and J. R. Durrant, J. Am.
Chem. Soc., 2009, 131, 4808.
4504 | Dalton Trans., 2011, 40, 4497–4504
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The Royal Society of Chemistry 2011
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