Synthesis of bianchored metal free organic dyes for dye sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.01.014

Two bianchored metal free organic dyes (Car-th-CN and Dpa-th-CN) were designed and synthesized for DSSC application, in which carbazole or diphenylamine moieties were used as the donor, cyano vinyl thiophene unit as the π-bridge and cyanoacrylic acid group as the electron acceptor. The structures of the synthesized dyes were confirmed by NMR, HR-Mass and elemental analysis. The optical, electrochemical, theoretical and photovoltaic properties of the synthesized dyes were investigated. Fabricated photovoltaic devices based on carbazole unit as a donor (Car-th-CN) showed a maximum current conversion efficiency of 4.04% under AM 1.5 illumination (85 mW cm^-2) and monochromatic incident photon to current efficiency (IPCE) of 38.1%. The reason for the higher efficiency of Car-th-CN is due to the planar nature of carbazole moiety and hence increases the capability of adsorbed dye amount and electron lifetime in TiO₂ surface.

Full text of DOI:10.1016/j.dyepig.2013.01.014

Dyes and Pigments 97 (2013) 397e404
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of bianchored metal free organic dyes for dye sensitized
solar cells
*
Sekar Ramkumar, Sambandam Anandan
Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli-620 015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two bianchored metal free organic dyes (Car-th-CN and Dpa-th-CN) were designed and synthesized for
DSSC application, in which carbazole or diphenylamine moieties were used as the donor, cyano vinyl
thiophene unit as the p-bridge and cyanoacrylic acid group as the electron acceptor. The structures of the
synthesized dyes were confirmed by NMR, HR-Mass and elemental analysis. The optical, electrochemical,
theoretical and photovoltaic properties of the synthesized dyes were investigated. Fabricated photo-
voltaic devices based on carbazole unit as a donor (Car-th-CN) showed a maximum current conversion
efficiency of 4.04% under AM 1.5 illumination (85 mW cmꢀ2) and monochromatic incident photon to
current efficiency (IPCE) of 38.1%. The reason for the higher efficiency of Car-th-CN is due to the planar
nature of carbazole moiety and hence increases the capability of adsorbed dye amount and electron
lifetime in TiO2 surface.
Received 21 November 2012
Received in revised form
12 January 2013
Accepted 16 January 2013
Available online 25 January 2013
Keywords:
DSSC
Bi-anchoring molecules
Carbazole
Ó 2013 Elsevier Ltd. All rights reserved.
Diphenylamine
Planarity
Impedance
1. Introduction
biphenyl) and acceptor (cyanoacrylic acid and rodanine-3-acetic
acid) with better efficiencies [14].
After the first revelation of a dye sensitized solar cell (DSSC) in
1991 by Gratzel and O’Regan [1e3], numerous metal dyes and
metal free organic molecules that exhibit conversion of light into
electricity have been found by many researchers. Among the metal
and metal free dyes, the metal free dyes have attracted a great deal
of interest nowadays, due to their high molar extinction coefficient,
simple synthesis procedure, and environmental friendliness, even
though their efficiencies are relatively low [4e7]. In general, the
efficiency of a metal free DSSC strongly depends on the choice of
functional groups and on the design of the device structure. That is,
The absorption wavelengths of organic dyes, the rate of internal
charge recombination and the electron injection to the TiO2 surface
will depends upon the
p-conjugated unit which bridged the donor
and acceptor moieties [15]. Hence the modification in
p-conjugated
unit plays a critical parameter for designing the organic dyes for
DSSCs [16e18]. That is, lowering of delocalization energy occurs
due to increase in the
p-conjugation, leads to lowering of the
HOMO and LUMO energy level band gap and thereby shift the
absorption band to the visible region which helps to increase the
solar cell performance [19]. Hence in this manuscript, we report the
synthesis of two novel organic dyes [Car-th-CN ((2E,20E)-3,30-(5,50-
(1E,1’E)-2,20-(9-hexyl-9H-carbazole-3,6-diyl)bis(1-cyanoethene-2,
1-diyl)bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid)), and Dpa-
th-CN ((2E,20E)-3,30-(5,50-(1E,1’E)-2,20-(4,40-(hexylazanediyl)bis(4,
1-phenylene))bis(1-cyanoethene-2,1-diyl)bis(thiophene-5,2-diyl))
bis(2-cyanoacrylic acid))] (Scheme 1) based on donor (carbazole
the D-p-A (Donor, p-conjugated unit/linker and acceptor) ambi-
polar system is the basic aspect for most metal free organic dyes [8].
Recently, bi-anchoring dyes have aroused considerable interests in
the area of DSSCs because they can provide more dye adsorption,
increases the light absorption efficiency and the photoinduced
intramolecular charge transfer (ICT) processes and thus enhances
the electron injection from dye to TiO2 nanoparticle [9e13]. We
have also reported a series of bi-anchoring metal free dyes based
and diphenylamine),
phene) and bi electron acceptor (cyanoacrylic acid). The reason
for choosing thiophene attached cyano vinyl as -linker in this
organic dyes instead of our earlier published work with biphenyl
attached cyano vinyl as -linker is due to the (i) effective con-
p-linker (double bridged cyano vinyl thio-
on donor (carbazole and diphenylamine),
p
-linker (cyano vinyl
p
p
jugation (ii) lower resonance stabilization energy (thiophene,
29 kcal/mol; benzene, 36 kcal/mol) [20], (iii) increasing the
* Corresponding author. Tel.: þ91 431 2503639; fax: þ91 431 2500133.
E-mail addresses: sanand@nitt.edu, sanand99@yahoo.com (S. Anandan).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.014

398
S. Ramkumar, S. Anandan / Dyes and Pigments 97 (2013) 397e404
calculated for each wavelength using a calibrated monocrystalline
silicon diode as reference. All reactions were monitored using TLC
plates. All chromatographic separations were carried out on silica
gel (60e130 mesh) and neutral alumina powder.
2.2. Synthesis
2.2.1. Synthesis of 9-hexyl-9-H-carbazole (1a)
1-Bromohexane (3.356 g, 0.02033 mol), carbazole (2.0 g,
0.01196 mol) and sodium hydroxide (4.0 g, 0.10046 mol) were
added in dimethylsulfoxide (DMSO) (30 mL), followed by heating at
110 ꢁC for 12 h. After cooling to room temperature the resulting
mixture was extracted with Ethyl acetate (EA)/water and then dried
with Na2SO4. The solvent was evaporated and the resulting crude
solid was purified by column chromatography on neutral alumina
by using hexane as solvent to give white solid with yield 90.4%
(3.1 g).
1H NMR (CDCl3, ppm):
d
8.18 (d, 2H, J ¼ 7.6 Hz), 7.56e7.54 (m,
2H), 7.52e7.46 (m, 2H), 7.31 (t, 2H, J ¼ 7.2 Hz), 4.33 (t, 2H, J ¼ 7.2 Hz),
1.96e1.89 (m, 2H), 1.54e1.37 (m, 6H), 0.96 (d, 3H, J ¼ 6.8 Hz). 13C
NMR (CDCl3, ppm):
d 140.35, 125.48, 122.75, 120.25, 118.61, 108.57,
42.90, 31.51, 28.84, 26.89, 22.48, 13.97.
2.2.2. Synthesis of 9-hexyl-9H-carbazole-3, 6-dicarbaldehyde (1b)
Freshly distilled POCl3 (23.1 ml, 25eq) was added drop wise to
DMF (17.6 ml, 23eq) under an atmosphere of N2 at 0 ꢁC, and then it
was stirred for 1 h. Compound 1a (2.5 g, 9.9 m mol) was added to
the above solution, and the resulting mixture was stirred for 4 h at
95 ꢁC. After cooling to room temperature, the mixture was poured
into a beaker containing ice-cube, and basified with 4 M NaOH.
Filtered the solid and extracted with EA/brine. After evaporating
the organic solvent the crude product was purified by column
chromatography on neutral alumina using a mixture of Ethyl ace-
tate/Hexane (1:4, v/v), to give a white solid (1.5 g, yield ¼ 49%).
Scheme 1. Synthetic pathway of the organic dyes. (i) 1-Bromohexane, NaOH, DMSO,
110 ꢁC. (ii) POCl3, DMF, 95 ꢁC. (iii) 2-(thiophen-2-yl)-acetonitrile, t-BuOK, CH3OH,
refluxed. (iv) POCl3, DMF, 95 ꢁC. (v) Cyano acetic acid, piperidine, CHCl3, refluxed.
1H NMR (CDCl3, ppm):
d
10.13 (s, 2H), 8.67 (d, 2H, J ¼ 1.5 Hz),
8.10e8.08 (m, 2H), 7.55 (d, 2H, J ¼ 8.5 Hz), 4.39 (t, 2H, J ¼ 7.5 Hz),
1.94e1.88 (m, 2H), 1.41 (d, 2H, J ¼ 7 Hz), 1.38e1.30 (m, 4H), 0.86 (m,
3H). 13C NMR (CDCl3, ppm):
d 191.43, 144.73, 129.61, 127.79, 124.19,
absorption wavelength towards visible [21] and (iv) negatively
decreases the LUMO levels [22]. Thus, synthesized dyes have been
well characterized by experimental and theoretical approach and
studied their photoelectrochemical and photovoltaic performance
by fabricating DSSC devices.
123.12, 109.72, 43.78, 31.39, 28.85, 26.81, 22.42, 13.90.
2.2.3. Synthesis of (2E,20E)-3,30-(9-hexyl-9H-carbazole-3,6-diyl)
bis(2-(thiophen-2-yl)acrylonitrile) (1c)
Freshly distilled methanol (100 ml) was taken in a 250 ml single
neck round bottom flask. The compound 1b (3 g, 1eq) and thio-
phene-2-acetonitrile (2.65 g, 2.2eq) were added to the methanol. A
catalytic amount of potassium tert-butoxide was added into this
mixture at room temperature. The reaction mixture was stirred for
12 h at 50 ꢁC. It was monitored by TLC. A bright yellow solid was
filtered after 12 h. It was recrystallized in dichloromethane and
methanol to give the product 2.6 g (yield ¼ 52%).
2. Experimental section
2.1. Material and methods
1H and 13C NMR spectra were obtained on Bruker 300 MHz and
400 MHz NMR spectrometer using tetramethylsilane as internal
standard. Mass spectra HR-MS (ITS þ ESI Scan spectra) (ESI-MS)
were recorded on a Micromass QUATTRO 11 spectrometer coupled
to a Hewlett Packard series 1100 degasser. UVevis and fluorescence
spectra were recorded on a T90 þ UV/VIS spectrometer and Shi-
madzu RF-5301 PC spectrofluorophotometer respectively. The cy-
clic voltammograms reported here were recorded with a computer
controlled AUTOLAB-potentiostat/galvanostat with a conventional
3-electrode system such as platinum working electrode, Ag/AgNO3
reference electrode and glassy carbon counter electrode at room
temperature. The potentials are reported vs ferrocene as standard
using a scan rate of 0.1 Vsꢀ1 and 0.1 M tetrabutylammonium per-
chlorate as a supporting electrolyte. Argon was bubbled for 10 min
before each measurement. The IPCE spectra were recorded using
Oriel 300 W Xe Arc lamp in combination with an Oriel Cornerstone
260¼ monochromator. The number of incident photons was
1H NMR (CDCl3, ppm):
d 8.60e8.50 (m, 2H), 8.12e8.09 (m, 2H),
7.51e7.45 (m, 2H), 7. 42e7.38 (m, 2H), 7.26 (s, 2H), 7.13e7.10 (m, 2H),
7.04e7.00 (m, 2H), 4.32 (t, 2H, J ¼ 7.2 Hz), 1.93e1.55 (m, 2H), 1.39e
1.25 (m, 6H), 0.89e0.87 (m, 3H). 13C NMR (CDCl3, ppm):
d 142.22,
140.77, 140.08, 128.21, 127.42, 126.47, 125.63, 125.60, 123.32, 122.93,
117.91, 109.87, 103.17, 43.79, 31.64, 29.14, 27.05, 22.66, 14.13.
2.2.4. Synthesis of (2E,20E)-3,30-(9-hexyl-9H-carbazole-3,6-diyl)
bis(2-(5-formylthiophen-2-yl)acrylonitrile) (1d)
DMF (7 ml, 23eq) was taken in a 100 ml 3 neck round bottom
flask. To this freshly distilled POCl3 (9 ml, 25eq) was added drop
wise under an atmosphere of N2 at 0 ꢁC, and then it was stirred for
1 h. 2 g of Compound 1c was added to the above solution, and the
resulting mixture was stirred for 4 h at 95 ꢁC. It was monitored by
TLC. After the completion of the reaction, it was cooled to RT, and

S. Ramkumar, S. Anandan / Dyes and Pigments 97 (2013) 397e404
399
then the mixture was poured into a beaker containing ice-cube, and
1H NMR (CDCl3, ppm):
d
9.87 (s, 2H), 7.88 (d, 4H, J ¼ 8.8 Hz), 7.71
basified with 4 M NaOH. It was extracted with dichloromethane/
brine. After evaporating the organic solvent the crude product was
purified by column chromatography on silica using a mixture of
Ethyl acetate/Hexane (1:4, v/v), to give an orange colour solid 1.04 g
(yield ¼ 47%).
(d, 2H, J ¼ 4 Hz), 7.47 (s, 2H), 7.41 (d, 2H, J ¼ 4 Hz), 7.15e7.13 (m, 4H),
3.84 (t, 2H, J ¼ 8 Hz), 1.73e1.62 (m, 2H), 1.38e1.34 (m, 2H), 1.32e
1.25 (m, 4H), 0.90e0.87 (m, 3H). 13C NMR (CDCl3, ppm):
d 182.56,
149.53, 149.01, 142.67, 142.45, 137.11, 131.74, 126.94, 126.48, 121.07,
116.80, 102.18, 52.57, 31.63, 27.72, 26.76, 22.72, 14.11.
1H NMR (CDCl3, ppm):
d 10.14 (s, 2H), 8.15e8.17 (m, 2H), 7.57 (s,
2H), 7.47 (d, 2H, J ¼ 8.8 Hz), 7.39 (s, 2H), 7.39e7.29 (m, 2H), 7.26 (s, 2H),
7.10e7.08 (m, 2H), 4.33 (t, 2H, J ¼ 7.2 Hz),1.92e1.88 (m, 2H), 1.55e1.25
(m, 6H),1.38e1.25 (m, 4H), 0.89e0.86 (m, 3H).13C NMR (CDCl3, ppm):
2.2.10. Synthesis of (2E,20E)-3,30-(5,50-(1E,1’E)-2,20-(4,40-
(hexylazanediyl)bis(4,1-phenylene))bis(1-cyanoethene-2,1-diyl)
bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid) (2e)
d
180.94, 147.37, 144.52, 140.54, 125.67, 122.92, 120.44, 118.79, 113.02,
It was synthesized according to the procedure of 1e. The product
obtained was dark red colour solid (Yield ¼ 32%).
108.76, 107.19, 90.92, 43.15, 31.71, 29.04, 27.09, 22.67, 14.15.
1H NMR (CDCl3, ppm):
d 8.93 (m, 2H), 8.60 (m, 2H), 8.20 (m, 2H),
2.2.5. Synthesis of (2E,20E)-3,30-(5,50-(1E,1’E)-2,20-(9-hexyl-9H-
carbazole-3,6-diyl)bis(1-cyanoethene-2,1-diyl)bis(thiophene-5,2-
diyl))bis(2-cyanoacrylic acid) (1e)
8.10 (d, 2H, J ¼ 8.2 Hz), 7.96 (m, 2H), 7.79 (m, 2H), 7.70 (d, 2H,
J ¼ 8.1 Hz), 7.48 (m, 2H), 4.36 (s, 2H),1.74e1.54 (m, 4H),1.24 (m, 4H),
0.78 (s, 3H). 13C NMR (CDCl3, ppm):
d 160.01, 151.81, 140.27, 139.16,
A chloroform solution of 1d (0.5 g, 0.8714 mmol) and cyano
acetic acid (0.3 g, 3.49 mmol) was refluxed in the presence of
piperidine (0.181 ml) for 12 h. After cooling to room temperature,
the solvent was removed by distillation. The residue was purified
by column chromatography using silica gel and CHCl3:CH3OH (10:1,
v:v) mixed as the eluent to give the dye as red colour solid (0.27 g,
yield ¼ 43.8%).
132.28, 129.54, 128.67, 128.34, 127.85, 126.03, 121.43, 121.19, 120.13,
117.70,108.21, 55.33, 31.39, 29.63, 23.31, 22.49,13.90. Anal. Calcd for
C40H31N5O4S2: C, 67.68; H, 4.4; N, 9.87. Found: C, 67.29; H, 4.51; N,
9.92. HR-MS (ITS þ ESI Scan spectra) Anal. Calcd. for C40H31N5O4S2:
709.18. Found: 710.3 [M þ H]þ.
3. Results and discussion
1H NMR (CDCl3, ppm):
d 7.81e7.78 (m, 4H), 7.25 (d, 2H,
J ¼ 4.2 Hz), 7.14 (d, 2H, J ¼ 6.0 Hz), 7.09e7.02 (m, 4H), 7.00 (d, 2H,
3.1. Synthesis of bi-anchoring dyes
J ¼ 4 Hz), 3.80 (t, 2H, J ¼ 8 Hz), 1.71e1.68 (m, 2H), 1.37e1.25 (m, 6H),
0.90e0.88 (m, 3H). 13C NMR (CDCl3, ppm):
d
163.56, 140.43, 139.86,
The synthetic pathways of organic dyes Car-th-CN and Dpa-th-
CN were depicted in Scheme 1. Alkylation of amine compounds
(carbazole (1), diphenylamine (2)) using 1-bromohexane in dime-
thysulfoxide afforded alkylated carbazole (1a) and diphenylamine
(2a). The Vilsmeier-haack formylation in alkylated carbazole and
diphenylamine gave compounds 1b and 2b. The aldehydes (1b, 2b
and 1d, 2d) and active methylene compounds such as thiophene-2-
acetonitrile and cyano acetic acid were condensed by Knoevenagel
Condensation to gave different products (1c, 2c; and 1e, 2e)
respectively. Synthesized compounds were well confirmed by
various analytical tools (See Supporting information).
138.27, 135.58, 133.80, 129.73, 129.29, 128.90, 127.98, 126.01, 118.39,
118.08, 107.03, 90.50, 50.23, 32.90, 27.99, 27.91, 24.18, 15.50. Anal.
Calcd for C40H29N5O4S2: C, 67.87; H, 4.13; N, 9.89. Found: C, 68.26;
H, 3.81; N, 9.82. HR-MS (ITS þ ESI Scan spectra) Anal. Calcd. for
C40H29N5O4S2: 707.16. Found: 708.1 [M þ H]þ.
2.2.6. Synthesis of N-hexyl-N-phenylaniline (2a)
It was synthesized according to the procedure of 1a. The product
obtained was colourless liquid (Yield ¼ 80.2%).
1H NMR (CDCl3, ppm): 7.27e7.23 (m, 4H), 6.98e6.96 (m, 4H),
6.94e6.91 (m, 2H), 3.67 (t, 2H, J ¼ 8 Hz), 1.66e1.62 (m, 2H), 1.35e
1.26 (m, 6H), 0.88e0.85 (m, 3H). 13C NMR (CDCl3, ppm):
d
148.05,
3.2. Absorbance and photoluminescence properties
129.15, 120.95, 120.82, 52.28, 31.59, 27.36, 26.71, 22.62, 13.99.
The UVevis spectra of the synthesized dyes in DMF solution
(3 ꢂ 10ꢀ4 M) exhibit two intense peaks, one in the ultraviolet region
and the other in the visible region as illustrated in Fig. 1. The higher
energy peaks 338 nm for Car-th-CN and 363 nm for Dpa-th-CN are
2.2.7. Synthesis of 4, 40-(hexylazanediyl)dibenzaldehyde (2b)
It was synthesized according to the procedure of 1b. The product
is brown colour liquid (Yield ¼ 94%).
1H NMR (CDCl3, ppm):
d
9.85 (s, 2H), 7.78 (d, 4H, J ¼ 8.4 Hz), 7.13
associated with a pep* transition localized in aromatic portion of
(d, 4H, J ¼ 8.4 Hz), 3.82 (t, 2H, J ¼ 8.0 Hz), 1.67 (d, 4H, J ¼ 8.4 Hz),
the molecules and the lower energy peaks 438 nm for Car-th-CN
and 458 nm for Dpa-th-CN are associated with the intramolecular
charge transfer (ICT) from the donor to the acceptor due to the
orbital mixing between the donor and acceptor fragments. How-
ever, the end absorption of Dpa-th-CN was red shifted by approx-
imately 20 nm and its molar extinction coefficients were 3000
times greater than the Car-th-CN. This is mainly due to the (i)
planar nature of carbazole decreases the conjugation between the
two aryl segments (i.e., in general two types of electronic transi-
1.33e1.26 (m, 6H), 0.85 (t, 3H, J ¼ 6.4 Hz). 13C NMR (CDCl3, ppm):
d
190.25, 148.89, 129.92, 121.95, 121.69, 52.31, 31.69, 27.42, 26.90,
22.69, 13.87.
2.2.8. Synthesis of (2E,20E)-3,30-(4,40-(hexylazanediyl)bis(4,1-
phenylene))bis(2-(thiophen-2-yl)acrylonitrile) (2c)
It was synthesized according to the procedure of 1c. The product
obtained was red colour solid (Yield ¼ 68.5%).
1H NMR (CDCl3, ppm):
d
7.79 (d, 2H, J ¼ 10.4 Hz), 7.36e7.34
tions (pe
p* and nep*) are possible in aromatic amines, of which
(m, 2H), 7.30e7.26 (m, 2H), 7.26e7.22 (m, 2H), 7.12e7.04 (m, 4H),
6.97e6.95 (m, 2H), 3.80 (t, 2H, J ¼ 8 Hz), 1.72e1.66 (m, 2H), 1.38e
the ne
p* transition is comparatively less in carbazole (C2v sym-
metry) moeity compared to diphenylamine (C2 symmetry) moiety
because of its planar nature) [23,24] and (ii) ionization potential of
carbazole moiety is comparatively higher than diphenylamine
moiety [25]. Further upon comparing the thiophene attached cyano
1.25 (m, 6H), 0.91e0.87 (m, 3H). 13C NMR (CDCl3, ppm):
d 148.82,
139.91, 139.26, 130.89, 128.22, 126.98, 126.54, 125.67, 120.93, 117.61,
103.21, 52.49, 31.69, 27.68, 26.82, 22.76, 14.14.
vinyl as p-linker and biphenyl attached cyano vinyl as p-linker [14],
2.2.9. Synthesis of (2E,20E)-3,30-(4,40-(hexylazanediyl)bis(4,1-
phenylene))bis(2-(5-formylthiophen-2-yl)acrylonitrile) (2d)
It was synthesized according to the procedure of 1d. The product
obtained was red colour solid (Yield ¼ 39%).
an anticipated bathochromic and hyperchromic shift was observed
by introduction of a thiophene bridging unit, due to the planar
nature of the thiophene group [26]. Fig. 2 shows the absorption
spectra of the dye loaded TiO2 films after 12 h adsorption which

400
S. Ramkumar, S. Anandan / Dyes and Pigments 97 (2013) 397e404
Fig. 3. Cyclic volammogram of dyes were measured in DMF solutions with TBAP (0.1 M)
Fig. 1. AbsorptionandemissionspectraofCar-th-CNandDpa-th-CNdyesrecordedinDMF.
as the electrolyte working electrode: Pt; reference electrode: Ag/Agþ; counter electrode:
Glassy carbon; calibrated with Fc/Fcþ as a standard reference; scan rate: 0.1 Vsꢀ1
.
shows the maxima absorption peak at 433 and 446 nm for Car-th-
CN and Dpa-th-CN. Both spectra are and blue shifted upon com-
paring with the absorption spectra of the corresponding dyes in
solution due to the strong interaction between the dyes and the
semiconductor surface; and as well as the formation of H-aggregate
(deprotonation of carboxylic acid) on the TiO2 electrode respec-
tively leads to efficient electron injection from the excited dye to
the TiO2 conduction bands [27,28]. The fluorescence spectra
recorded upon their excitation of absorption maxima value of Car-
th-CN and Dpa-th-CN exhibits strong luminescence maxima at
558 nm (lex ¼ 442 nm) and 582 nm (lex ¼ 462 nm) respectively.
highest occupied molecular orbital (HOMO) energy level of the
organic dyes, were determined from the intersection of two tan-
gents drawn at the rising current and background charging current
of a cyclic voltammogram [29]. The oxidation potential of Car-th-CN
is more positive than that of Dpa-th-CN is due to the weaker
electron donating nature of Carbazole segment (high ionization
potential) [30]. The HOMO energy levels were calculated based on
the relationship of HOMO (eV) ¼ ꢀe (E
V (vs Fc/Fcþ) þ 4.8 V) by
ox
onset
assuming the Ferrocene (Fc) energy level to be ꢀ4.8 eV below the
vacuum level [31] and the LUMO levels of the dyes were calculated
ox
by E
ꢀ E0-0, where E0-0 is the zeroezero energy of the dyes
onset
3.3. Electrochemical properties
estimated from the intersection between the absorption and
emission spectra [32]. The introduction of thiophene -bridge will
p
The cyclic voltammograms (CV) of synthesized organic dyes in
DMF are shown in Fig. 3 and their data are summarized in Table 1.
The voltammogram of dyes, Car-th-CN and Dpa-th-CN exhibits
single oxidation peaks with onset of 1.04, 0.85 V respectively with
respect to Ag/AgCl reference electrode which corresponds to the
negatively increases the LUMO energy levels of the dyes [22] and in
turn enhances the electron injection to the TiO2 conduction band
compared to previous reported dyes containing cyano vinyl
biphenyl as
p-linker [14]. For better electron injection, this LUMO
level should lie above the conduction band (CB) of the TiO2 semi-
conductor (ꢀ4.0 eV vs vacuum) and for effective dye regeneration
the HOMO energy level should lie below the Iꢀ/I3ꢀ redox electrolyte
(ꢀ4.6 eV vs vacuum) which is further improved negatively about
0.3 V by adding additives such as 4-tert-butyl pyridine (TBP) to the
Iꢀ/Iꢀ3 redox electrolyte [33]. Hence, the energy levels of the syn-
thesized dyes are suitable for electron injection and dye regener-
ation thermodynamically [34].
3.4. Exploring the electronic structure of dyes using computational
methods
The optimized electronic structures of the synthesized dyes are
calculated by density functional theory (DFT) and time-dependent
density functional theory (TDDFT) methods using B3LYP/6-31 G (d)
level in gas phase with Gaussian 09 program. Calculated HOMO and
LUMO energies of the compounds are listed in Table 1. The opti-
mized structure of dyes and their HOMO, LUMO characters are
shown in Fig. 4. According to the optimized molecular geometry of
the Dpa-th-CN, the dihedral angle in :CeNeCeC is ꢃ90.98ꢁ,
whereas in Car-th-CN it is 0ꢁ or 180ꢁ. As a result, the HOMO of the bi
branched Dpa-th-CN is mainly on the one side of the central elec-
tron donating amine and thiophene moieties and LUMO is localized
on the another side of the dye mainly on the electron withdrawing
Fig. 2. Absorption spectra of bare TiO2 film, Car-th-CN and Dpa-th-CN dyes adsorbed
on nanocrystalline TiO2 films.

S. Ramkumar, S. Anandan / Dyes and Pigments 97 (2013) 397e404
401
Table 1
Optical, electrochemical data and HOMO, LUMO energy levels of Car-th-CN and Dpa-th-CN dyes.
a
,b
Dye
labs/nm (ε/Mꢀ1 cmꢀ1
)
lem/nma
Eogpt (eV)c
Eox/V (vs. Fc)
EHOMO (eV)d
ELUMO(eV)d
HOMOe
LUMOe
Car-th-CN
Dpa-th-CN
438 (35,526)
458 (38,578)
558
582
2.54
2.36
1.04
0.85
5.39
5.20
2.85
2.84
ꢀ5.69
ꢀ5.33
ꢀ3.54
ꢀ3.94
a
Absorption and emission spectra were recorded in DMF solution (3 ꢂ 10ꢀ4 M) at room temperature.
b
c
Dyes were excited at their absorption maximum value (for Car-th-CN, lex ¼ 442 nm and for Dpa-th-CN, lex ¼ 462 nm).
Optical band gap calculated from intersection between the absorption and emission spectra.
The values of EHOMO and ELUMO were calculated with the following formula: HOMO (eV) ¼ ꢀe (E
d
ox
ox
V (vs Fc/Fcþ) þ 4.8 V); LUMO (eV) ¼ E
ꢀ E0-0, where E0-0 is the
onset
onset
intersection between the absorption and emission spectra of the sensitizers.
e
B3LYP/6-31G(d) calculated values.
cyano acetic acid and thiophene moieties. But, in Car-th-CN the
HOMO lobes are localized both in amine and thiophene moieties
and LUMO lobes are also localized in both cyano acetic acid and
thiophene moieties. The LUMO of the thiophene moiety in Car-th-
CN is similar to the HOMO of Car-th-CN, but the lobes do not align.
This strategy appears more like an anti-bonding orbital which
indicates that the intramolecular charge transfer induced electron
movement from the donor site to the acceptor moiety through the
dye sensitizers for 24 h. The photo cathode was prepared by
H2PtCl6 solution (50 mM in isopropyl alcohol) which is deposited
on the FTO glass by drop casting method and heated at 400 ꢁC for
15 min [37,38]. The device was fabricated by the following method:
the photo cathode was placed on top of the photo anode and was
tightly clipped together. Then, 0.05 M I2/0.5 M KI/0.5 M 4-tert-butyl
pyridine (TBP) in 2-methoxypropionitrile electrolyte was injected
in-between the two electrodes. Fig. 6 shows the photovoltaic
properties of the fabricated devices under illumination in simu-
lated AM 1.5 irradiation (85 mW cmꢀ2). The short-circuit photo-
current density (JSC), open circuit photovoltage (VOC), and fill factor
(FF) of Car-th-CN and Dpa-th-CN were tabulated (see Table 2).
Upon comparing the photovoltaic properties of Car-th-CN and Dpa-
th-CN, Car-th-CN based device shows better efficiency even though
Dpa-th-CN have the better absorbance in the visible region [8]. This
is because; according to DFT calculation the optimized structure of
Car-th-CN is more planar than the Dpa-th-CN which helps to
enhance the dyes adsorption and increases the high number of
injected electrons through bi-anchoring eCOOH groups. This is
thiophene
p-bridge. In addition, the theoretical absorbance spectra
calculated using TDDFT shows higher absorption maxima (lmax
)
and molar extinction coefficient (ε) for Dpa-th-CN when compared
to Car-th-CN similar to experimental absorbance spectra (Fig. 5).
However, the theoretical lmax values are highly red shifted com-
pared to experimental values due to the error in TDDFT for charge
transfer (CT) excited states and in addition the end absorption peak
of Dpa-th-CN is blue shifted compared to Car-th-CN. The reasons
for such blue shift in Dpa-th-CN is due to the planes are aligned
perpendicular in diphenylamine which prevents the orbital over-
lapping and in turn reduces the ICT band [35].
further confirmed by calculating the adsorbed amount (G) of dyes
on TiO2 surface by desorption of the dye from the TiO2 surface
using 0.1M NaOH in DMF/H2O (1:1) mixture [39]. From the results,
it is found that Car-th-CN adsorbs comparatively more than Dpa-
th-CN dye may be due to the controlled aggregation of planar
Car-th-CN dye on TiO2 surface [40]. The spectra of incident photon
to current conversion efficiency (IPCE) for DSSC based on Car-th-
CN and Dpa-th-CN are shown in Fig. 7 and their data are sum-
marized in Table 2 also supports the above facts. That is, the IPCE
spectra of both dyes show the broader response between 350 nm
and 600 nm with a maximum efficiency of about 38.1% for Car-th-
CN and 26.6% for Dpa-th-CN at its wavelength maximum but the
3.5. Fabrication and characterization of DSSCs
TiO2 nanoparticles were prepared according to the literature
[36] and it was coated on a FTO glass substrate in the area of 1 cm2.
This plate was immersed in a DMF solution containing 3 ꢂ 10ꢀ4 M
Fig. 4. The optimized structure and the frontier molecular orbitals of the HOMO and
LUMO levels calculated with B3LYP/6-31G (d) of synthesized dyes.
Fig. 5. The TDDFT absorbance spectra of dyes calculated with B3LYP/6-31G (d) level.

402
S. Ramkumar, S. Anandan / Dyes and Pigments 97 (2013) 397e404
Fig. 6. Current density-voltage characteristics for Car-th-CN and Dpa-th-CN based
devices for DSSCs under illumination of simulated solar light (AM 1.5, 85 mW cmꢀ2).
Fig. 7. IPCE spectra of the fabricated devices based on Car-th-CN and Dpa-th-CN dyes.
later have red shifted about 15 nm which is consistent with the
absorption spectra of dye. Further comparing the IPCE spectra of
the fabricated devices (Fig. 7) with the absorption spectra of dye
loaded TiO2 film (Fig. 2), both the spectra resembles same which
led to efficient electron injection from the excited dye to the TiO2
conduction band.
Further to understand the correlation between the improved
cell performance and the internal resistances of the fabricated
DSSCs the Electrochemical impedance spectroscopy (EIS) analysis
[41] was performed at their open circuit potential (OCP) over
a frequency range of 100e104 Hz under AM 1.5. The observed
Nyquist plots (Fig. 8) for the DSSCs fabricated using the both dyes
shows two semicircles; a small arc at high frequencies corresponds
to the impedance of the Pt/electrolyte interface and a large arc at
the low frequency regime corresponds to the impedance at the
TiO2/electrolyte interface [42]. The calculated radius of the semi-
circle in the low frequency regime indicates that the charge
transport resistance is high for Dpa-th-CN based DSSC [43].Further,
Bode phase plots were plotted in order to calculate the effective
lifetime (
seff) of electrons in TiO2 conduction band using the fol-
lowing equation [44].
seff ¼ 1=2
p
f
where the frequency (f) corresponding to peak in Bode phase plot
for Car-th-CN and Dpa-th-CN is 35.56 Hz and 130.84 Hz and their
electron lifetime (seff) calculated from the above equation is 4.47 ms
and 1.21 ms respectively. Comparing Car-th-CN and Dpa-th-CN dye
based device, Car-th-CN dye based device exhibits longer electron
lifetime (seff) in TiO2 conduction band because the generated rad-
ical cation of the sensitizer by photo excitation, is stabilized well in
carbazole dye by the delocalization of the
p- electrons compared
Table 2
DSSC performance parameters of the fabricated devices.
Dye
Jsc
Voc FF
(mV)
ƞ (%)a IPCE Surface
Electron
(mA cmꢀ2
)
(%) concentration lifetime
G
(mol/cm2)
(seff) (ms)
Car-th-CN 7.64
Dpa-th-CN 7.01
656 0.6848 4.04 38.1 2.84 ꢂ 10ꢀ7
4.47
1.21
612 0.6270 3.16 26.6 2.12 ꢂ 10ꢀ7
Illumination: 85 mW cmꢀ2 simulated AM 1.5 G solar light; electrolyte con-
Fig. 8. Electrochemical impedance spectra of DSSCs measured at VOC, 85 mW cmꢀ2. (a)
a
taining: 0.05 M I2/0.5 M KI/0.5 M 4-tert-butyl pyridine in 2-methoxypropionitrile.
Nyquist plots. (b) Bode phase plots.

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