Novel di-anchoring dye for DSSC by bridging of two mono anchoring dye molecules: A conformational approach to reduce aggregation

Article abstract of DOI:10.1016/j.dyepig.2011.09.003

Two mono anchoring dye molecules were bridged together to give a new di-anchoring bis-merocyanine dye which possessed a non-planar conformation on a TiO₂ surface, a feature that impedes intermolecular aggregation of the dye in the adsorbed stat

Full text of DOI:10.1016/j.dyepig.2011.09.003

Dyes and Pigments 92 (2012) 1132e1137
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel di-anchoring dye for DSSC by bridging of two mono anchoring dye
molecules: A conformational approach to reduce aggregation
,
Reenu Sirohia, Dong Hee Kimb, Soo-Chang Yub, Sang Hee Leeb
*
a Department of Chemistry, Vivekananda Institute of Technology, Jaipur, Rajasthan, India
b Department of Chemistry, Kunsan National University, Kunsan 573-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2011
Received in revised form
3 September 2011
Accepted 6 September 2011
Available online 17 September 2011
Two mono anchoring dye molecules were bridged together to give a new di-anchoring bis-merocyanine
dye which possessed a non-planar conformation on a TiO2 surface, a feature that impedes intermolecular
aggregation of the dye in the adsorbed state. This dye also showed enhanced molar absorptivity and
increased adsorption on TiO2. A dye sensitized solar cell based on the bis-merocyanine dye yielded
enhanced power conversion efficiency of 6.1%.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
DSSC
Di-anchoring dye
Bridging
Suppression of aggregation
1. Introduction
anchoring groups are present for an efficient electron transfer [7].
Recently, several researchers have developed double/multi electron
DSSCs have emerged as a prospective alternative to conven-
tional silicon-based photovoltaic devices due to their low cost and
versatility [1,2]. The most important part of a DSSC is the photo
sensitizer. Though Ru-based sensitizers have achieved power
conversion efficiencies up to 11% under AM 1.5 irradiation [3], their
high cost and toxicity diverted the attention of researchers towards
metal free organic dyes which are inexpensive, easily synthesized,
and environment friendly [4]. In recent years, a considerable
improvement has been made in the performance of DSSCs based on
organic dyes [5]. However, the low conversion efficiency and low
stability [5a] are important issues still to be resolved.
One of the major factors which affects the conversion efficiency
and stability of DSSC is the binding strength of dye on the TiO2
surface. Strong binding of the dye on TiO2 not only improves
adsorption but also induces efficient charge injection [6]. One of the
drawbacks of most of the organic dye molecules is the presence of
only one anchoring group which can be a reason for inferior
performance as compared to Ru(II) sensitizers, where 1e4
acceptor type organic sensitizers to address the above mentioned
problem [8e12]. However, consideration of conformation while
designing a molecule of double electron acceptor type has received
only scant attention [12]. Preferential conformation of the dye on
the TiO2 surface is important in terms of the improvement of charge
injection and reduction of the effect of back electron transfer
processes by maintaining the large physical separation between
photo-oxidized donor and photo-injected electrons [12].
Formation of aggregates on the TiO2 surface is another drawback
of organic dyes in DSSC. A common structural strategy for
suppression of dye aggregation is the introduction of bulky group
[13]. Another unique approach is bridging of two chromophores
into a spiro configuration similar to Ru-dyes [12].
Therefore, keeping in mind the requirement of strong binding
ability and reduced tendency towards aggregation, we designed
and synthesized a novel dye KS-5 by bridging of two L1 dye
molecules (Fig. 1). A unique feature of KS-5 is its near cross-shaped
conformation in the adsorbed state which minimizes aggregation.
The presence of two anchoring group is another advantageous
feature of KS-5 which increases the binding strength of dye on TiO2.
Photovoltaic properties of KS-5 were compared with those of L1
[14,15] to study the effect of bridging on the performance in a dye
sensitized solar cell.
* Corresponding author. Tel.: þ82 63 469 4578; fax: þ82 63 469 4571.
E-mail address: leesh@kunsan.ac.kr (S.H. Lee).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.003

R. Sirohi et al. / Dyes and Pigments 92 (2012) 1132e1137
1133
The organic layer was dried over anhydrous MgSO4 and evaporated
under vacuum. The product was purified by silica gel column
chromatography eluting first with hexane and then with hex-
ane:toluene (7:1). White solid of 2P was obtained (770 mg,
1.35 mmol, 70%). mp 228e229 ꢀC. IR: 3082, 3056, 3033, 2922, 2351,
1596, 1582, 1505, 1484, 1308, 1270, 1069, 1004, 833, 815, 745, 693,
N
N
N
S
S
S
529, 520, 508. 1H NMR (500 MHz, CDCl3)
d
7.32 (d, 4H, J ¼ 9 Hz), 7.27
CN
NC
NC
CO2H
CO2H
CO2H
(dd, 4H, J ¼ 8 Hz and 8 Hz), 7.08 (d, 4H, J ¼ 8 Hz), 7.02 (t, 2H, J ¼ 8 Hz),
6.96 (s, 4H), 6.95 (d, 4H, J ¼ 9 Hz). 13CNMR (125 MHz, CDCl3)
d147.3,
L1
KS-5
146.9, 142.7,132.1,129.4,125.5,124.8,124.1,123.1,114.6. HRMS (FAB)
Fig. 1. Schematic structure of L1 and KS-5.
m/z: calcd. for C30H22Br2N2, 568.0149; found, 568.0143.
2.2.2. Synthesis of 5,50-(4,40-(1,4-phenylenebis(phenylazanediyl))
bis(4,1-phenylene)) dithiophene-2-carbaldehyde (3P)
2. Experimental section
2.1. Materials
To a solution of 2P (500 mg, 0.876 mmol, 1 eq), PdCl2(PPh2)2
(60 mg, 0.08 mmol, 0.1 eq) and dppf (48 mg, 0.08 mmol, 0.1 eq) in
dry toluene (50 ml) was added a solution of 5-fomylthiophene-2-
yl-2-boronic acid (274 mg, 1.75 mmol, 2 eq) and K2CO3 (725 mg,
5.25 mmol, 6 eq) in dry methanol (15 ml). The mixture was heated
by microwave irradiation at 70 ꢀC for 12 min. The reaction was
quenched by the addition of water and extracted with ethyl acetate.
The organic layer was dried over anhydrous MgSO4 and filtered.
Solvent removal by rotary evaporation followed by column chro-
matography on silica gel eluting first with toluene and then with
hexane:ethyl acetate (4:1) gave 3P as a yellow solid (310 mg,
0.49 mmol, 56%). mp 245e246 ꢀC. IR: 3025, 2918, 2360, 2341, 1692,
The chemicals and solvents used in this work were obtained
from commercial suppliers and were used without further purifi-
cation unless otherwise noted. Parent dye, L1 was synthesized
according to procedure given in literature [14].
2.2. Synthesis
KS-5 was synthesized according to the synthetic route depicted
in Scheme 1. It includes four general steps; amination of p-dibro-
mobenzene with N,N0-diphenylphenylenediamine (1P) to give
bisbromophenyl derivative 2P, Suzuki coupling [14] of 2P with
5-formyl-2-thiophene-boronic acid to afford the corresponding
carbaldehyde 3P, conversion of 3P into its cyanoacrylate derivative
4P by condensation with t-butyl cyanoacetate and subsequent
hydrolysis of 4P using trifluoroacetic acid afforded KS-5.
1588, 1506, 1311, 1271. 1H NMR (500 MHz, CDCl3)
d9.84 (2H, s), 7.70
(d, 2H, J ¼ 4 Hz), 7.53 (d, 4H, J ¼ 8.5 Hz), 7.32 (dd, 4H, J ¼ 8 Hz and
7.5 Hz), 7.30 (d, 2H, J ¼ 4 Hz), 7.17 (d, 4H, J ¼ 8 Hz), 7.09 (t, 2H,
J ¼ 7.5 Hz), 7.08 (d, 4H, J ¼ 8.5 Hz), 7.05 (s, 4H). 13CNMR (125 MHz,
CDCl3) d182.6, 154.5, 148.9, 146.8, 142.7, 141.3, 137.8, 131.5, 129.5,
127.3, 126.1, 125.1, 123.9, 122.8, 122.1. HRMS (FAB) m/z: calcd. for
C40H28N2O2S2, 632.1592; found, 632.1586.
2.2.1. Synthesis of N1,N4-bis(4-bromophenyl)-N1,N4-diphenylbenzene-
1,4-diamine (2P)
A
mixture of N,N0-diphenyl-1,4-phenylenediamine (1P,
2.2.3. Synthesis of (2E,20E)-tert-butyl 3,30-(5,50-(4,40-(1,4-
phenylenebis(phenylazanediyl))bis(4,1-phenylene))bis(thiophene-
5,2-diyl))bis(2-cyanoacrylate) (4P)
A mixture of 3P (200 mg, 0.316 mmol, 1 eq), tert-butyl cya-
noacetate (180 mg, 1.26 mmol, 4 eq), ammonium acetate (96 mg,
1.26 mmol, 4 eq), and acetic acid (2 ml) in toluene was placed in
a flask. Air was removed and the solution was heated under reflux
under argon atmosphere for 45 min. After cooling, the reaction was
commercially available) (500 mg, 1.92 mmol, 1 eq), 1,4 dibromo-
benzene (1.81 g, 7.68 mmol, 4 eq), sodium tert-butoxide (554 mg,
5.76 mmol, 3 eq), dppf (111 mg, 0.2 mmol, 1 eq) and Pd(OAc)2
(18 mg, 0.0 8 mmol,1 eq) in dry toluene (80 ml) was placed in a flask.
Air was removed and it was heated under reflux under argon
atmosphere for 8 h. After cooling to RT, the reaction was quenched
by adding water, and then was extracted with ethyl acetate.
Br
N
OHC
B(OH)2
N
Br
S
N
N
N
H
N
H
PdCl2(dppf) 2, K2CO3
Pd(OAc)2, dppf
Na2CO3
S
S
Br
Br
CHO
CHO
1P
2P
3P
CN
CO2
t
Bu
NH4OAc, AcOH
N
N
N
N
CF3CO2H
S
S
S
S
CN
CN
NC
NC
CO2 Bu
t
CO2 Bu
t
COOH
COOH
4P
KS-5
Scheme 1. Synthetic route for KS-5.

1134
R. Sirohi et al. / Dyes and Pigments 92 (2012) 1132e1137
quenched by addition of water and extracted with ethyl acetate.
The organic layer was dried over anhydrous MgSO4 and evaporated
under vacuum. The product was purified by silica gel column
chromatography eluted with toluene. Orange solid of 4P was
obtained (248 mg, 0.284 mmol, 95%). mp 231e232 ꢀC. IR: 3031,
2978, 2924, 2981, 2360, 2335, 2216, 1715, 1584, 1499, 1281, 1155. 1H
the FTO plate followed by sintering at 400 ꢀC for 15 min. The
dye-anchored TiO2 electrodes and the Pt counter electrodes were
assembled in to a sealed sandwich type cell using a thin Surlyn
polymer transparent film (SX 1170-25, 25 mm) as a spacer between
the electrodes. The sandwich cells were lightly compressed at
110 ꢀC to seal the two electrodes. A thin layer of electrolyte was
introduced into the inter electrode space from the counter elec-
trode side through pre-drilled holes using vacuum backfilling
method. Electrolyte contained 0.6 M 1-butyl-3-methylimidazolium
iodide (BMII), 0.04 M I2, 0.025 M LiI, 0.05 guanidinium thiocyanate
NMR (500 MHz, CDCl3)
d
8.18 (s, 2H), 7.69 (d, 2H, J ¼ 4 Hz), 7.56 (d,
4H, J ¼ 9 Hz), 7.32 (dd, 4H, J ¼ 8.5 Hz and 7.5 Hz), 7.30 (d, 2H,
J ¼ 4 Hz), 7.17 (d, 4H, J ¼ 7.5 Hz), 7.09 (t, 2H, J ¼ 8.5 Hz), 7.08 (d, 4H,
J ¼ 9 Hz), 7.06 (s, 4H), 1.57 (s, 18H). 13CNMR (125 MHz, CDCl3)
d162.03, 154.3, 149.03, 149.01, 146.7, 145.7, 142.7, 138.9, 133.9, 129.5,
(GUNCS), 0.25
M
4-tert-butylpyridine (TBP) in valeroni-
127.3, 126.1, 125.1, 123.9, 123.02, 122.09, 116.5, 98.7, 83.2, 28.02.
HRMS (FAB) m/z: calcd. for C54H46N4O4S2, 878.2960; found,
878.2966.
trile:acetonitrile (15:85). The holes were sealed with Surlyn and
microscope cover slides to avoid leakage of the electrolyte solution.
2.2.4. Synthesis of (2E,20E)-3,30-(5,50-(4,40-(1,4-phenylenebis
(phenylazanediyl))bis(4,1-phenylene))bis(thiophene-5,2-diyl))
bis(2-cyanoacrylic acid) (KS-5)
Compound 4P (200 mg, 0.224 mmol) was stirred with tri-
fluoroacetic acid (20 ml) for 15 min 200 ml of water was added and
the resulting dark red solid KS-5 (163 mg, 0.213 mmol, 95%) was
collected by filtration. mp 288e289 ꢀC. IR: 3443, 3032, 2923, 2958,
2963, 2345, 2215, 1685, 1571, 1492, 1436, 1412, 1316. 1H NMR
2.5. Photovoltaic characterization
The currentevoltage characteristics of the devices were carried
out under simulated AM 1.5G irradiation using a xenon lamp-based
solar simulator (Oriel 300 W solar simulator) attached to Kiethley
2400 source metre and calibrated with a crystalline silicon refer-
ence cell (VLSI standards, PVM-495-KG5). The solar cell efficiency
(
h
) was obtained with the relation,
h
¼ (JSC$VOC$FF)/Pin, in which JSC
(mA cmꢁ2) is the current density measured at short circuit, VOC (V)
is the voltage measured at open circuit, FF is the fill factor and Pin is
the input radiation power (for 1 sun illumination (AM 1.5G),
Pin ¼ 100 mW cmꢁ2). A black mask was applied to the area
surrounding the TiO2 with an illuminated active area of 0.159 cm2
for all measurements.
(500 MHz, DMSO-d6),
d
8.45 (s, 2H), 7.98 (d, 2H, J ¼ 4 Hz), 7.68 (d,
4H, J ¼ 8.5 Hz), 7.63 (d, 2H, J ¼ 4 Hz), 7.38 (dd, 4H, J ¼ 8.5 Hz and
7.5 Hz), 7.15 (m, 6H), 7.09 (s, 4H), 7.01 (d, 4H, J ¼ 8.5 Hz). 13CNMR
(125 MHz, DMSO-d6) d164.2, 153.7, 149.08, 147.1, 146.6, 142.7, 142.3,
133.9, 130.3, 127.9, 126.8, 125.6, 125.4, 124.7, 124.3, 121.9, 117.08,
97.5. HRMS (FAB) m/z: calcd. for C46H30N4O4S2, 766.17085; found,
766.1713.
3. Results and discussion
2.3. Instruments and characterizations
3.1. Photophysical properties
1H and 13CNMR spectra were recorded on a Varian VNMRS
500 MHz NMR spectrometer. Mass spectra were obtained on
a JMS-700, JEOL instrument operating in fast atom bombardment
(FAB) mode. UVeVisible absorption spectra were measured on
a Shimadzu UV-2401PC spectrophotometer. Fluorescence spectra
were recorded on a SCINCO, FluoroMate FS-2 Fluorescence spectro-
photometer. FTIR spectra were taken on a JASCO, 6300FV þ IRT5000
spectrophotometer. Cyclic voltammogram was measured in
DMSO:EtOH (1:1) solution containing 0.1 M TBA(PF6) with a scan
rate of 50 mV sꢁ1 using GCE (glassy carbon electrode) as working
electrode, Pt wire as counter electrode and Ag/Agþ as reference
electrode.
UVeVisible absorption spectrum of KS-5 in DMSO:ethanol (1:1)
solution (Fig. 2) exhibits a strong absorption band in the visible
region. As expected, the absorption spectrum for KS-5 is broader
and red-shifted in comparison to that of L1 due to the increased
length of
p-conjugation through phenylenediamine in KS-5 (Fig. 2).
The molar extinction coefficient
3 of KS-5 is more than twice as high
as that of the parent dye L1 (Table 1). The above observations
indicate higher light absorption efficiency of KS-5 as compared to
parent dye L1.
2.4. Fabrication of DSSCs
A paste consisting of 20 nm sized TiO2 particles (CICC, PST-18NR)
was applied with a scalpel on a fluorine-doped SnO2 (FTO, Pil-
kington TEC-8 glass, 6e9 Ohms/sq with 2.3 mm thickness) con-
ducting glass and then air-dried for 2 h. This TiO2 film was gradually
heated at 325 ꢀC for 5 min, at 375 ꢀC for 5 min, at 450 ꢀC for 15 min,
and at 500 ꢀC for 15 min. Subsequently, a second scattering layer
made up of a paste containing 400 nm anatase TiO2 particles (CICC,
PST-400C) was coated onto the first layer to form a light-scattering
layer. The second layer was air-dried and then sintered in the same
way as the first layer. The TiO2 electrodes were immersed in 40 mM
aqueous TiCl4 solution at 70 ꢀC for 30 min, washed with water and
ethanol and heated at 500 ꢀC for 30 min. After cooling to 80 ꢀC, the
TiO2 electrodes were dipped into the dye solutions (0.4 mM for L1
and 0.2 mM for KS-5 in DMSO:ethanol (1:1) solvent) for 2 h. The
dye-coated electrodes were rinsed quickly with ethanol. To prepare
a Pt counter electrode, a small hole was drilled in a FTO glass and
a drop of H2PtCl6 solution (2 mg Pt in 1 ml of ethanol) was placed on
Fig. 2. Absorption spectra for L1 and KS-5 in DMSO:ethanol (1:1) solution and on TiO2
film.

R. Sirohi et al. / Dyes and Pigments 92 (2012) 1132e1137
1135
Table 1
Photophysical, electrochemical and photovoltaic performance data.
Dye
Absorption
labsa/nm ( /Mꢁ1 cmꢁ1
Emission
Potentials and energy levels
Phtovoltaic performance datad
a
3
)
labs/nm (on TiO2)
lem (nm)
Eobx (V)
Eoceo (V)
Eox ꢁ Eoeo (V)
Jsc (mA cmꢁ2
)
VOC (mV)
FF
h (%)
L1
KS-5
418 (23,760)
435 (50,483)
425
435
580
593
1.21
0.92
2.48
2.36
ꢁ1.27
9.3
13.4
616
646
0.73
0.70
4.2
6.1
ꢁ1.46
Performances of DSSCs were measured with a 0.159 cm2 working area.
a
Absorption and emission spectra were measured in DMSO:ethanol (1:1) solution.
The oxidation potentials were measured in DMSO:ethanol (1:1) solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) with a scan rate of
b
50 mV sꢁ1 (vs NHE) using GCE (glassy carbon electrode) as working electrode, Pt wire as counter electrode and Ag/Agþ as reference electrode.
c
Eoeo was determined from the intersection of absorption and emission spectra.
The concentration was 0.4 mM for L1 and 0.2 mM for KS-5 in DMSO:ethanol (1:1) solution, with 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.04 M I2, 0.025 M LiI,
d
0.05 GUNCS, 0.25 M 4-tert-butylpyridine (TBP) in valeronitrile:acetonitrile (15:85) as electrolyte and using 4.5 mm thick TiO2 film.
3.2. Molecular conformation of KS-5 on TiO2 surface
stretching band at 1685 assigned to the carboxylic acid groups of
free KS-5 was found to have disappeared in the IR spectrum of KS-5
adsorbed on TiO2 film, indicating the involvement of both carbox-
ylic acid groups in adsorption on the TiO2 surface; this result is
similar to the observation made by Abbotto et al. [10] and Heredia
et al. [12].
As shown in Fig. 2, absorption spectra of dyes adsorbed on TiO2
film are broadened which is a favourable spectral property for light
harvesting of the solar spectrum. In addition to this, a slight red
shift (418e425 nm) relative to that in solution, is observed for L1
indicating partial J-type aggregation [16]. However, no such shift is
observed for KS-5 implying negligible J-type aggregation. This
suppression of aggregation in KS-5 may be ascribed to the non-
planar geometry achieved by KS-5 upon adsorption on TiO2. In the
free state, KS-5 can have several conformations in equilibrium with
each other. However, owing to its double anchoring mode, it
possesses a non-planar conformation in the adsorbed state which
reduces the possibility of intermolecular aggregation. Preferred
conformation in double anchoring mode is presented in Fig. 3. As
3.3. Amount of adsorption
The amount of the dyes adsorbed on TiO2 film (4.5 mm) were
then estimated by measuring the absorbance of desorbed dye
solution. The concentration of KS-5 (3.94 ꢂ 10ꢁ5 mmol/cm2) was
lower than that of L1 (4.86 ꢂ 10ꢁ5 mmol/cm2) but KS-5 loaded
a greater amount of De
p
eA units (2 ꢂ 3.94 ꢂ 10ꢁ5 mmol/cm2)
because one molecule of KS-5 consists of two molecules of L1
(Fig. 1). This result shows that strong binding of the dye can
increase the adsorbed amount of dye on TiO2 which in turn can
increase the photocurrent. A common strategy to increase the
amount of adsorption is the use of the thick TiO2 films but it is well
established that Voc and fill factor decrease with increase in thick-
ness of TiO2 film [18]. Improved amount of adsorption for KS-5 was
due to strong binding and is therefore important in view of its
application in solid state cells (SSDSSCs), where a thin film
performs much better than a thicker film and optimum thickness
shown in Fig. 3b (side view), the orientation of the two DepeA
branches is not coplanar but twisted by an angle of 88.6ꢀ. Spatial
separation (9.2 Å) between donor group and TiO2 surface in KS-5 is
also sufficient to diminish the deleterious back electron transfer
process.
In case of mono anchoring dye molecules, the preferential
molecular orientation on the TiO2 surface is perpendicular,
however a fraction of the molecules may align horizontal next to
the TiO2 surface [17]. The possibility of this type of horizontal
configuration in the adsorbed state is reduced in the case of KS-5
due to the double anchoring behaviour, thereby slowing down the
charge recombination process.
lies in the range of 2e4 mm [19].
A clue for the double anchoring behaviour of KS-5 was obtained
by comparing the binding strength of KS-5 on TiO2 with that of L1.
Desorption of L1 from TiO2 surface was achieved within 5 s by
sonication in KOH methanol solution. However, complete removal
of KS-5 from TiO2 surface required more than 5 min under identical
conditions. The di-anchoring mode of KS-5 was further confirmed
by FTIR spectroscopy (Supplementary Material, Fig. S1). The C]O
3.4. Molecular orbital calculations
Frontier molecular orbitals obtained by theoretical analysis
(DFT, B3LYP/6-31G(d) level) (Fig. 4) showed that the HOMO of KS-5
is located mostly on the N,N0-diphenylphenylenediamine and the
LUMO is located on cyanoacrylic acid units, indicating an efficient
photo induced electron transfer from dye to the TiO2 electrode.
Fig. 3. Ground state geometry of KS-5 calculated at B3LYP/6-31G(d): (a) front view; (b) side view.

1136
R. Sirohi et al. / Dyes and Pigments 92 (2012) 1132e1137
Fig. 6. Photocurrentevoltage curves for DSSCs based on L1 and KS-5.
spectral region compared to KS-5, with maximum IPCE of 74% at
460 nm. Broad spectrum and higher IPCE values for KS-5 reflect
higher photocurrent and therefore better photovoltaic perfor-
mance as compared to L1.
Fig.
6 shows the photocurrentevoltage (JeV) curves and
photovoltaic performance data are listed in Table 1. Under standard
global A.M.1.5 solar condition, the KS-5 sensitized solar cell with an
active cell area of 0.159 cm2 displayed a short circuit photocurrent
density (Jsc) of 13.4 mA cmꢁ2, an open circuit voltage (Voc) of 0.646
and a fill factor of 70.2 corresponding to an overall conversion
Fig. 4. HOMOeLUMO spatial orientation of KS-5.
efficiency (h) of 6.1%, while the DSSC based on parent dye L1
showed lower Jsc, leading to a lower efficiency of 4.2%. Under
different conditions (solvent and thickness of TiO2 film) reported
3.5. Electrochemical properties
Redox potentials of dyes were measured by cyclic voltammetry
(Supplementary Material, Figs. S2 and S3). The HOMO level of KS-5
was found to be sufficiently lower than that of electrolyte pair Iꢁ/I3ꢁ
(0.4 V vs NHE) [20,21] and its LUMO level was sufficiently higher
than conduction band edge of TiO2 (ꢁ0.5 V vs NHE) [20,21], thereby
indicating a favourable exothermic flow of charges throughout the
photo-electronic conversion. A comparison of redox potentials of
KS-5 and L1 revealed that Eox and Eoeo of KS-5 were lower than that
of L1 showing higher electron donating ability [21] of KS-5 (Table 1).
value of
h for L1 sensitized solar cell are 2.75% (3 mm) [14] and 5.2%
(6 m) [15].
m
4. Conclusions
We have synthesized and characterized a novel dye KS-5 and
demonstrated that appropriate bridging of two mono anchoring
dye molecules can give several positive effects on photovoltaic
performance. The di-anchoring moiety in KS-5 caused strong
binding to TiO2 which not only increased the amount of adsorbed
dye but also forced the dye molecule to assume a non-planar
conformation, thereby minimizing aggregation. Furthermore,
extended conjugated framework (through conjugate bridging) of
KS-5 gave a red-shifted and broadened spectrum as well as
enhanced absorptivity as compared to parent dye L1. These factors
led to about 1.5 times higher efficiency of KS-5 (6.1%) compared to
parent dye L1 (4.2%).
3.6. Photovoltaic performance
The incident photon-to-current conversion efficiencies (IPCEs)
of L1 and KS-5 are presented in Fig. 5. A solar cell based on KS-5
showed the highest IPCE value, 87%, at 460 nm. It exhibited a broad
IPCE spectrum with high IPCE values (>70%) from 400 to 540 nm.
The IPCE spectrum of L1 was however lower over the whole
Appendix. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.dyepig.2011.09.003.
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