Fluorene-based organic dyes containing acetylene linkage for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.06.004

New organic dyes based on diphenylaminofluorene donors, cyanoacrylic acid acceptors and either ethynylbenzene or ethynylthiophene π-spacers have been synthesized and characterized as sensitizers for dye-sensitized solar cells. The dye with thiophene in the conjugation pathway exhibited longer wavelength absorption due to the significant lowering of the LUMO level when compared to the phenyl analog. However, the dye with the phenylacetylene linker displayed promising DSSC characteristics such as short circuit current, open circuit voltage and fill factor indicative of efficient charge generation and injection. The solvatochromic behavior of the dyes were examined in solvents of different polarity and found to exhibit negative solvatochromism of the fluorescence emission suggestive of a nonpolar solvent stabilized excited state with a significant structural reorganization. The TDDFT computations were used to explain the optical properties of the dyes.

Full text of DOI:10.1016/j.dyepig.2012.06.004

Dyes and Pigments 95 (2012) 523e533
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorene-based organic dyes containing acetylene linkage for dye-sensitized
solar cells
,
b
b
Prachi Singh a, Abhishek Baheti a, K.R. Justin Thomas a , Chuan-Pei Lee , Kuo-Chuan Ho
*
a Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
b Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
New organic dyes based on diphenylaminofluorene donors, cyanoacrylic acid acceptors and either
ethynylbenzene or ethynylthiophene -spacers have been synthesized and characterized as sensitizers
Received 28 March 2012
Received in revised form
31 May 2012
Accepted 8 June 2012
Available online 16 June 2012
p
for dye-sensitized solar cells. The dye with thiophene in the conjugation pathway exhibited longer
wavelength absorption due to the significant lowering of the LUMO level when compared to the phenyl
analog. However, the dye with the phenylacetylene linker displayed promising DSSC characteristics such
as short circuit current, open circuit voltage and fill factor indicative of efficient charge generation and
injection. The solvatochromic behavior of the dyes were examined in solvents of different polarity and
found to exhibit negative solvatochromism of the fluorescence emission suggestive of a nonpolar solvent
stabilized excited state with a significant structural reorganization. The TDDFT computations were used
to explain the optical properties of the dyes.
Keywords:
Organic dyes
Dye-sensitized solar cells
Optical spectra
TDDFT calculations
Fluorene
Ó 2012 Elsevier Ltd. All rights reserved.
Acetylene
1. Introduction
[5e7], carbazole [8,9], phenothiazine [10,11], coumarin [12,13],
indoline [14,15], oligothiophene substituted polyaromatic hydro-
There is a growing interest for the development of metal free
organic dyes displaying broad spectral response and efficient charge
separation due to their potential application as sensitizers in the
nanocrystalline anatase TiO2 based dye-sensitized solar cells
(DSSCs) [1] and donors in bulk-heterojunction solar cells [2]. The
DSSCs based on organic or organometallic dyes have exhibited
several advantages such as easy fabrication process, cost reduction
and easy functional alternations by chemical modification over the
conventional silicon-based solar cells. Grätzel and co-workers have
developed several ruthenium complexes [3,4] based on polypyridyl
ligands and found to perform as efficient sensitizers in DSSCs.
Despite the superiority of the ruthenium-based dyes, there is rapid
progress in the development of metal free organic dyes [1]. Several
attempts have been devoted to optimize the functional properties
such as absorption, intramolecular charge transfer and redox
stability by simple molecular engineering. The molecular configu-
ration of the organic sensitizers suitable for dye sensitized solar cells
carbons [16e18] or imidazoles [19e21], as the electron donor. Dyes
with the triphenylamine segment as the electron-donorhave shown
long-lived charge-separated state and good hole-transporting
ability. In addition, trigonal geometry of triarylamine prevents the
formation of aggregates at the semiconductor surface. An ideal
p-
conjugated linker should promote the absorption of light over
a wide wavelength region and at the same time facilitate intra-
molecular charge transfer from donor to acceptor. Spacer units also
play a crucial role to hinder the rate of internal charge recombina-
tion. A slight tilt from planarity in the conjugation bridge has been
found to be beneficial for charge collection despite displaying blue-
shifted absorption [22]. Various conjugated segments such as
carbazole [23,24], fluorene [25e27], spirobifluorene [28], phenox-
azine [29], oligothiophene [30], dithienothiophene [31], and thie-
nothiophene [32] have been used as the
p-bridge between the
electron donor and electron acceptor for tuning the absorption
parameters and photovoltaic performance. Incorporation of an
can be divided into three components: donor,
p-linker and acceptor.
acetylene segment is an interesting approach to increase the p-
Most of the organic dyes possessed a substituted triphenylamine
conjugation in dyes that provide enhanced photovoltaic properties
and ensure an efficient intramolecular charge separation with high
photocurrent generation and slow recombination [33].
Fluorene-based triarylamines have been widely used as
donors in the construction of efficient organic dyes for DSSCs
* Corresponding author. Tel.: þ91 1332 285376; fax: þ91 1332 286202.
E-mail address: krjt8fcy@iitr.ernet.in (K.R.J. Thomas).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.06.004

524
P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
[34,35]. However, fluorene-based organic dyes possessing acet-
ylene linkers have not been explored. In continuation of our
efforts to develop organic dyes suitable for application in dye-
sensitized solar cells [36,37], we report in this paper metal
free organic dyes featuring N,N0-diphenylfluoreneamine donor,
thienylacetylene or phenylacetylene conjugation pathway and
cyanoacrylic acid or dicyanovinyl acceptor as potential materials
for the fabrication of photovoltaic devices. We suppose that
elongation of the conjugation pathway by aryl/heteroaryl acet-
ylene would benefit the absorption characteristics and intra-
molecular charge separation. We have demonstrated the use of
the new cyanoacrylic acid derivatives as sensitizers in the
conventional Grätzel DSSCs and found to exhibit efficiency up to
3.0% (vide supra).
2.3. 5-(2-(7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)ethynyl)
thiophene-2-carbaldehyde (2b)
This was synthesized from 9,9-diethyl-7-ethynyl-N,N-diphenyl-
9H-fluoren-2-amine and 5-bromothiophene-2-carbaldehyde by
following the procedure described above for 2a. Yellow solid. Yield:
75%. Mp 204e206 ꢂC. 1H NMR (CDCl3, 500.13 MHz): 0.35e0.38 (m,
6H), 1.88e2.00 (m, 4H), 7.03e7.06 (m, 3H), 7.11 (d, J ¼ 2.0 Hz, 1H),
7.13e7.15 (m, 4H), 7.26e7.29 (m, 4H), 7.33 (d, J ¼ 4.0 Hz, 1H), 7.48
(m, 1H), 7.51e7.53 (m, 1H), 7.57 (d, J ¼ 8.0 Hz, 1H), 7.61 (d, J ¼ 8.0 Hz,
1H), 7.79 (d, J ¼ 4.0 Hz, 1H), 9.88 (s, 1H). 13C NMR (CDCl3,
125.75 MHz)
d 182.47, 151.81, 150.05, 148.07, 147.83, 143.57, 142.86,
136.31, 135.33, 133.43, 132.20, 131.00, 129.28, 125.94, 124.15, 123.32,
122.85, 120.96, 119.13, 118.97, 118.73, 99.99, 99.56, 82.02, 56.23,
32.61 8.55. GC/TOF HRMS-EI (m/z): [M þ H]þ calcd for C36H30NOS
524.2048; found, 524.2041.
2. Experimental details
2.4. 3-(2-(7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)ethynyl)
benzaldehyde (2c)
2.1. Materials and methods
All reactions and manipulations were carried out under N2 with
the use of standard inert atmosphere and Schlenk techniques.
Solvents were dried by standard procedures. All column chroma-
tography was performed with the use of silica gel (100e200 mesh,
Rankem) as the stationary phase in a column of 30 cm long and
2.0 cm diameter. The 1H and 13C NMR spectra were measured by
using Bruker AV 500 FT-NMR spectrometer. Cyclic voltammetric
experiments were performed with a CH Instruments electro-
chemical analyzer. All measurements were carried out at room
temperature with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
auxiliary electrode, and a nonaqueous Ag/AgNO3 reference elec-
trode. The E1/2 values were determined as 1/2ðEpa þ Epc Þ, where Epa
and Epc are the anodic and cathodic peak potentials, respectively.
The potentials are quoted against the ferrocene internal standard.
This was synthesized from 9,9-diethyl-7-ethynyl-N,N-diphenyl-
9H-fluoren-2-amine and 3-bromobenzaldehyde by following the
procedure described above for 2a. Yellow solid. Yield: 70%. Mp
175 ꢂC. 1H NMR (CDCl3, 500.13 MHz): 0.48e0.51 (m, 6H), 2.01e2.10
(m, 4H), 7.15e7.17 (m, 3H), 7.21e7.25 (m, 5H), 7.36e7.39 (m, 4H),
7.59 (s, 1H), 7.62e7.73 (m, 4H), 7.90e7.92 (m, 1H), 7.95e7.97 (m,
1H), 8.17e8.18 (m, 1H), 10.16 (s, 1H). 13C NMR (CDCl3, 125.75 MHz)
d
191.69,151.73,150.00,147.90,147.85,142.19,137.02,136.53,135.62,
132.99, 130.92,129.26,129.15,128.69, 126.02, 124.83, 124.07,123.43,
122.75, 120.81, 119.86, 119.07, 118.94, 92.28, 87.77, 56.18, 32.63, 8.56.
GC/TOF HRMS-EI (m/z): [M þ H]þ calcd for C38H32NO 518.2484;
found, 518.2477.
2.5. (E)-2-Cyano-3-(4-(2-(7-(diphenylamino)-9,9-diethyl-9H-
fluoren-2-yl)ethynyl)phenyl)acrylic acid (3a)
The solvent in all experiments was dichloromethane (1 ꢀ 10ꢁ4
M
solutions) and the supporting electrolyte was 0.1 M tetrabuty-
lammonium perchlorate. Electronic absorption spectra were ob-
tained on a UV-1800 Shimadzu UVeVisible spectrophotometer
using 1 ꢀ 10ꢁ5 M solutions. The HRMS was done on Agilent 1200
series system equipped with an Agilent 6210 Time-Of-Flight (TOF)
mass detector. 9,9-Diethyl-7-ethynyl-N,N-diphenyl-9H-fluoren-2-
amine was prepared by following the literature procedure [38].
A mixture of the aldehyde 2a (1.0 g, 1.93 mmol), 2-cyanoacetic
acid (0.18 g, 2.32 mmol), ammonium acetate (20 mg) and acetic
acid (5 mL) was refluxed for 12 h. The resulting dark yellow solution
was poured into water. The solid separated was filtered and thor-
oughly washed with water and dried. Red solid. Yield: 68%. Mp
220e222 ꢂC. 1H NMR (DMSO, 500.13 MHz)
d 0.24e0.27 (m, 6H),
1.86e1.88 (m, 2H), 1.97e1.99 (m, 2H), 6.95e6.97 (m, 1H), 7.02e7.07
(m, 8H), 7.29e7.32 (m, 4H), 7.53e7.55 (m, 1H), 7.61 (s, 1H),
7.65e7.67 (d, J ¼ 8.5 Hz, 2H), 7.76e7.78 (m, 2H), 7.92e7.95 (m, 3H).
2.2. 4-(2-(7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)ethynyl)
benzaldehyde (2a)
13C NMR (DMSO, 125.75 MHz)
d 163.48, 151.76, 150.24, 147.85,
147.76, 146.86, 142.12, 135.67, 132.15, 131.39, 130.09, 129.99, 126.24,
125.15, 124.03, 123.63, 123.44, 121.97, 120.03, 119.95, 119.45, 119.01,
56.24, 44.41, 32.11, 23.12, 22.49, 8.84. GC/TOF HRMS-EI (m/z):
[M þ H]þ calcd for C41H33N2O2, 585.2542; found, 585.2541.
To a mixture of (1.0 g, 2.42 mmol) 9,9-diethyl-7-ethynyl-
N,N-diphenyl-9H-fluoren-2-amine, (0.45 g, 2.42 mmol) 4-
bromobenzaldehyde, (17 mg) Pd(PPh3)2Cl2, (13 mg) PPh3 were
added successively (5 mg) CuI and Et3N (25 mL) under N2 atmo-
sphere. The reaction mixture was refluxed at 95 ꢂC for 24 h. The
resulting reaction mixture was poured in water and extracted with
dichloromethane and further purified by chromatography on silica
gel with CH2Cl2:hexane (2:1) to give compound 2a as yellow solid.
2.6. (E)-2-Cyano-3-(5-(2-(7-(diphenylamino)-9,9-diethyl-9H-
fluoren-2-yl)ethynyl)thiophen-2-yl)acrylic acid (3b)
Yield 80%. Mp 181 ꢂC. 1H NMR (CDCl3, 500.13 MHz)
d
0.34e0.37 (m,
This was synthesized in 67% yield from 2b as described above for
3a.Orange solid. Mp 240e242 ꢂC. 1H NMR (DMSO, 500.13 MHz)
6H), 1.87e1.99 (m, 4H), 7.01e7.04 (m, 3H), 7.10e7.13 (m, 5H),
7.25e7.28 (m, 4H), 7.48 (s, 1H), 7.52 (d, J ¼ 7.5 Hz, 1H), 7.56 (d,
J ¼ 8.5 Hz, 1H), 7.60 (d, J ¼ 7.5 Hz, 1H), 7.69 (d, J ¼ 8.0 Hz, 2H), 7.87
(d, J ¼ 8.0 Hz 2H), 10.01 (s, 1H). 13C NMR (CDCl3, 125.75 MHz)
d
0.33e0.39 (m, 6H), 1.71 (s, 2H), 1.90e2.02 (m, 2H), 7.03e7.06 (m,
3H), 7.10 (s, 1H), 7.13e7.15 (d, J ¼ 7.5 Hz, 4H), 7.26e7.29 (m, 5H), 7.46
(s, 1H), 7.51 (d, J ¼ 7.0 Hz, 1H), 7.57e7.62 (m, 3H), 8.17 (s, 1H). 13C
d
182.62, 142.85, 141.11, 139.03, 138.96, 133.57, 126.60, 126.29,
NMR (DMSO, 125.75 MHz) d 151.82, 150.02, 147.95, 147.86, 142.53,
123.58, 123.10, 122.18, 122.12, 121.05, 120.77, 120.37, 117.19, 115.19,
115.08, 114.48, 113.89, 111.97, 111.74, 110.74, 110.20, 109.94, 91.08,
86.09, 79.67, 47.29, 23.73, 13.79. GC/TOF HRMS-EI (m/z): [M þ H]þ
calcd for C38H32NO 518.2484; found, 518.2482.
137.67, 135.47, 134.76, 131.89, 130.80, 130.15, 129.26, 125.89, 124.11,
123.36, 122.79, 120.89, 119.39, 119.11, 118.81, 56.19, 44.42, 32.60,
22.72, 22.56, 8.55. GC/TOF HRMS-EI (m/z): [M þ H]þ calcd for
C39H31N2O2S 591.2106; found, 591.2101.

P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
525
2.7. (E)-2-Cyano-3-(3-(2-(7-(diphenylamino)-9,9-diethyl-9H-
2.11. DSSC fabrication and characterization
fluoren-2-yl)ethynyl)phenyl)acrylic acid (3c)
A fluorine-doped SnO2 conducting glass (FTO, 7
U
sqꢁ1, trans-
This was synthesized from 2c in 79% yield as described above for
3a. Red solid. Mp 208e212 ꢂC. 1H NMR (DMSO, 500.13 MHz)
mittance w80%, NSG America, Inc., New Jersey, USA) was first
cleaned with a neutral cleaner, and then washed with deionized
water, acetone, and isopropyl alcohol, sequentially. The conducting
surface of the FTO was treated with a solution of titanium tetrai-
sopropoxide (1 g) in 2-methoxyethanol (3 g) for obtaining a good
mechanical contact between the conducting glass and TiO2 film, as
well as to isolate the conducting glass surface from the electrolyte.
TiO2 paste was coated onto the treated conducting glass by rolling
a metal strip over it (doctor blade technique). After coating each
TiO2 layer, the dried TiO2 film was gradually heated to 450 ꢂC in an
oxygen atmosphere, and subsequently sintered at that temperature
for 30 min. The TiO2 photo-anodes of the DSSCs employed in the
d
0.24e0.27 (m, 6H), 1.84e1.91 (m, 2H), 1.96e2.03 (m, 2H), 6.96 (dd,
J ¼ 8.5, 4.0 Hz, 1H), 7.00e7.08 (m, 7H), 7.28e7.32 (m, 4H), 7.53 (dd,
J ¼ 8.0, 1.0 Hz, 1H), 7.62e7.65 (m, 2H), 7.77e7.79 (m, 3H), 8.03 (d,
J ¼ 7.5 Hz, 1H), 8.16 (s, 1H), 8.31 (d, J ¼ 3.5 Hz, 1H). 13C NMR (DMSO,
125.75 MHz)
d 151.75, 150.26, 147.85, 147.77, 142.14, 135.69, 135.40,
133.19, 132.92, 131.36, 130.54, 130.23, 130.00, 126.22, 124.02, 123.71,
123.41, 122.06, 120.06, 119.82, 119.10, 92.06, 88.64, 79.62, 56.26,
32.10, 8.87. GC/TOF HRMS-EI (m/z): [M þ H]þ calcd for C41H33N2O2,
585.2542; found, 585.2540.
2.8. 2-((4-(2-(7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)
ethynyl)phenyl)methylene)malononitrile (4a)
experiments were composed of a 14
mm thick transparent TiO2
layer with a scattering layer of 4.5 m thickness. After sintering at
m
450 ꢂC and cooling to 80 ꢂC, the TiO2 film was immersed in
a 3 ꢀ 10ꢁ4 M solution of dye at room temperature for 24 h. N719
(Solaronix S.A., Aubonne, Switzerland) was dissolved in acetonitrile
(ACN) and tert-butyl alcohol (volume ratio of 1:1) and used as
a standard dye solution for device optimization. The solutions of
the dyes were prepared in a mixing solvent containing ACN, tert-
butyl alcohol and dimethyl sulfoxide (DMSO) (volume ratio of
1:1:3). Then the photoanode (FTO/TiO2/dye) was placed on a plat-
A mixture of the aldehyde 2a (1.0 g, 1.93 mmol), malononitrile
(0.19 g, 2.89 mmol), piperidine (catalytic amount) and ethanol
(10 mL) was refluxed for 12 h. The resulting dark red precipitate
was filtered and thoroughly washed with ethanol and dried. Red
solid. Yield: 69%. Mp 235e237 ꢂC. 1H NMR (CDCl3, 500.13 MHz)
d
0.35e0.38 (m, 6H), 1.86e2.01 (m, 4H), 7.02e7.05 (m, 3H), 7.09 (d,
J ¼ 1.5 Hz, 1H), 7.12 (d, J ¼ 7.5 Hz, 4H), 7.25 (m, 1H), 7.26e7.27 (m,
2H), 7.48 (s,1H), 7.51e7.54 (m,1H), 7.56 (d, J ¼ 7.0 Hz,1H), 7.60e7.62
(m, 1H), 7.65e7.70 (m, 2H), 7.74 (s, 1H), 7.87e7.92 (m, 2H). 13C NMR
(CDCl3, 125.75 MHz) 158.59, 148.08, 147.84, 132.32, 131.25, 130.77,
129.96, 129.27, 126.19, 124.15, 124.10, 123.31, 122.85, 120.93, 119.12,
118.74, 76.78, 56.20, 32.61, 8.54. GC/TOF HRMS-EI (m/z): [M þ H]þ
calcd for C41H32N3 566.2596; found, 566.2587.
inum-sputtered conducting glass electrode (ITO, 7
Corporation, Hsinchu, Taiwan), keeping the two electrodes sepa-
rated by a 25
m-thick SurlynÒ (SX1170-25, Solaronix S.A.,
Aubonne, Switzerland). The two electrodes were then sealed
by heating. mixture of 0.1 LiI, 0.6 1-propyl-2,3-
U
sqꢁ1, Ritek
m
A
M
M
dimethylimidazolium iodide (DMPII), 0.05 M I2, and 0.5 M tert-
butylpyridine (TBP) in 3-methoxypropionitrile (MPN)/ACN
(volume ratio of 1:1) was used as the electrolyte. The electrolyte
was injected into the gap between the electrodes by capillarity; the
electrolyte-injecting hole was previously made in the counter
electrode with a drilling machine, and the hole was sealed with
hot-melt glue after the injection of the electrolyte.
Surface of the DSSC was covered by a mask with a light-
illuminated area of 0.16 cm2 and then illuminated by a class-A
quality solar simulator (XES-301S, AM 1.5 G, San-Ei Electric Co.,
Ltd.). Incident light intensity (100 mW cmꢁ2) was calibrated with
2.9. 2-((5-(2-(7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)
ethynyl)thiophen-2-yl)methylene)malononitrile (4b)
This was prepared from 2b by following the procedure similar to
that described above for 4a. Red solid. Yield: 70%. Mp 258e260 ꢂC.
1H NMR (CDCl3, 500.13 MHz)
d
0.35e0.37 (m, 6H), 1.88e1.98 (m,
4H), 7.02e7.05 (m, 3H), 7.09 (d, J ¼ 2.0 Hz, 1H), 7.12 (dd, J ¼ 8.5,
1.0 Hz, 4H), 7.25e7.26 (m,1H), 7.26e7.27 (m,1H), 7.27e7.28 (m, 2H),
7.32 (d, J ¼ 4.5 Hz, 1H), 7.46 (d, J ¼ 1.0 Hz, 1H), 7.50e7.52 (m, 1H),
7.56 (d, J ¼ 8.0 Hz,1H), 7.61 (d, J ¼ 7.5 Hz,1H), 7.66 (d, J ¼ 4.0 Hz,1H),
a
standard Si Cell (PECSI01, Peccell Technologies, Inc.).
7.77 (s, 1H). 13C NMR (CDCl3, 125.75 MHz)
d
152.72, 151.95, 150.38,
Photocurrentevoltage curves of the DSSCs were obtained with
a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, The
Netherlands). The thickness of the TiO2 film was judged by scan-
ning electron microscopic images (SEM, NanoSEM 230, Novat). For
UV-absorption spectra, dye molecules were coated on the TiO2
films and the corresponding spectra were obtained using an
UVevisible spectrophotometer (V-570, Jasco, Japan) equipped with
a total integrating sphere. Electrochemical impedance spectra (EIS)
were obtained by the above-mentioned potentiostat/galvanostat,
equipped with an FRA2 module, under a constant light illumination
148.17, 147.72, 143.11, 141.47, 136.29, 135.36, 133.85, 133.25, 131.58,
130.01, 126.48, 124.16, 123.53, 122.23, 120.16, 118.85, 118.62, 114.19,
101.66, 82.57, 77.05, 56.32, 32.03, 8.85. GC/TOF HRMS-EI (m/z):
[M þ H]þ calcd for C39H30N3S 572.2160; found, 572.2156.
2.10. Computational methods
For theoretical dipole moment and electronic vertical transition
calculations, the structures of the dyes were optimized by applying
DFT with the hybrid B3LYP [39] functional and 6-31G (d,p) basis set.
With the optimized structures, the ground state dipole moments
were calculated with the DFT (B3LYP) models (vacuum) and CAM-
B3LYP [40] (Coulomb-attenuating method) (vacuum and in THF) as
an exchange-correlation functional and the 6-31G (d,p) basis set.
The electronic transitions were calculated using time-dependent
DFT (B3LYP) theory and the 6-31G (d,p) basis set. Even though
the time-dependent DFT method less accurately describes the
states with charge-transfer nature, the qualitative trends in the
TDDFT results can still offer correct physical insight. At least 20
excited states were calculated for each molecule. For all the dyes,
the first excited states were highly optically active as indicated by
the larger oscillator strengths.
of 100 mW cmꢁ2
.
The frequency range explored was
10 mHze65 kHz. The applied bias voltage was set at the open-
circuit voltage of the DSSC, between the ITOePt counter electrode
and the FTOeTiO2-dye working electrode, starting from the short-
circuit conditions; the corresponding AC amplitude was 10 mV. The
impedance spectra were analyzed using an equivalent circuit
model. Incident photo-to-current conversion efficiency (IPCE)
curves were obtained under short-circuit conditions. The light
source was a class A quality solar simulator (PEC-L11, AM 1.5 G,
Peccell Technologies, Inc.); light was focused through a mono-
chromator (Oriel Instrument, model 74100) onto the photo-voltaic
cell. The monochromator was incremented through the visible
spectrum to generate the IPCE (l) as defined by IPCE (
l
) ¼ 1240 (JSC
/

526
P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
l
4), where l is the wavelength, JSC is short-circuit photocurrent
density (mA cmꢁ2) recorded with a potentiostat/galvanostat, and 4
is the incident radiative flux (W mꢁ2) measured with an optical
detector (Oriel Instrument, model 71580) and a power meter (Oriel
Instrument, model 70310).
3. Results and discussion
3.1. Synthesis and characterization
The synthetic route for the new dipolar dyes containing
fluorene-based triarylamine donor, aryl (phenyl/thienyl) acetylene
p-linkers and cyanoacrylic acid or dicyanovinyl acceptor is depicted
in Scheme 1. The aldehydes (2ae2c) required were first assembled
from the terminal acetylene, 1 and the corresponding bromo-
substituted aryl aldehydes by utilizing a palladium-catalyzed
Sonogashira coupling reaction [41]. They were later converted to
the desired dyes (3 and 4) in good yields by Knoevenagel
condensation [42] with either cyanoacetic acid or malononitrile in
the presence of appropriate catalyst (ammonium acetate/piperi-
dine). The meta-substituted derivative (3c) and the dicyanovinyl
derivatives (4a and 4b) were synthesized to study the influence of
acceptor on the optical and electrochemical properties. The dyes
were thoroughly characterized by 1H, 13C and mass spectral
methods and the spectral data are consistent with the formulated
structures. The dyes are yellow to red in color and reasonably
soluble in common organic solvents such as toluene, dichloro-
methane, acetonitrile and methanol.
Fig. 1. Absorption spectra of the dyes 3ae3c, 4a and 4b recorded in dichloromethane
solutions.
interaction. Within a group, the thiophene containing dye (3b or
4b) exhibits red-shifted absorption when compared to the corre-
sponding phenylene derivative (3a or 4a). It is attributed to the
enhanced electronic communication between the donor and
acceptor entities in 3b and 4b due to thiophene which can provide
more efficient conjugation than a benzenoid moiety and lower the
energy of the charge transfer in conjugated dipolar molecules.
Similarly, the dicyanovinyl derivatives (4a and 4b) show a red-
shifted absorption profile when compared to the corresponding
cyanoacrylic acid derivatives (3a and 3b) illustrating the
pronounced electron withdrawing ability of the dicyanovinyl unit.
It is interesting to compare the absorption parameters of the
dyes with that of the closely related compounds known in the
literature [36,44]. The effect of the ethynylthiophene linkage
becomes clear by comparing the photophysical properties of dye 3b
with the previously reported dye (E)-2-cyano-3-(7-(diphenyla-
mino)-9,9-diethyl-9H-fluoren-2-yl)acrylic acid [36]. The absorp-
tion spectra of the dye 3b (462 nm) is significantly red-shifted
(>35 nm) when compared to the dyes (E)-2-cyano-3-(7-(diphe-
nylamino)-9,9-diethyl-9H-fluoren-2-yl)acrylic acid (424 nm) and
(E)-2-cyano-3-(5-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-
3.2. Photophysical properties
The absorption spectra of the dyes recorded in dichloromethane
(DCM) are displayed in Fig. 1, and the data summarized in Table 1.
All the dyes with the exception of 3a exhibited two major
absorption bands covering the wavelength region from 350 nm to
600 nm. The dye 3a displayed a single band peaking at 391 nm. The
shorter wavelength absorption appearing at ca. 360 nm is assigned
to a
pep* transition while the band appearing at lower energy
(>400 nm) may originate from an electronic excitation resulting in
charge transfer from the amine donor to the cyanoacrylic acid
acceptor. Absence of any such absorption in 3c supports this
assignment. In 3c, the acceptor group is delinked from the donor by
meta-disposition and may not exhibit
a
donoreacceptor
Scheme 1. Synthesis of the dyes.

P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
527
Table 1
Absorption data of the dyes in different solvents.
Dyes
lmax, nm (
3
max, ꢀ 103 Mꢁ1 cmꢁ1
)
TOL
TOL þ TEA
TOL þ TFA
DCM
DCM þ TEA
DCM þ TFA
THF
ACN
MeOH
3a
3b
391 (40.2),
314 (34.2)
453 (26.20),
368 (26.1),
307 (17.2)
378 (41.8),
306 (38.7)
442 (23.7),
367 (40.0)
453 (25.0),
368 (24.9),
308 (16.4)
388 (50.2),
312 (38.4)
410 (43.6),
312 (22.7)
426 (27.4),
366 (41.7), 313 (28.6)
467 (25.9),
390 (30.5),
315 (21.6)
462 (38.9),
368 (38.8),
307 (24.7)
373 (43.1),
298 (40.5)
443 (20.7),
367 (34.6)
479 (34.5),
371 (36.1),
306 (22.7)
390 (29.9),
315 (25.2)
401 (69.9),
310 (47.1)
433 (21.8),
368 (35.3), 313 (25.1)
462 (43.4),
384 (29.8)
382 (30.5),
309 (21.6)
425 (29.1),
304 (16.5)
387 (40.1),
312 (30.9)
415 (37.1),
307 (18.8)
396 (19.2)
370 (38.6), 306 (18.4)
371 (56.9), 306 (42.7)
3c
4a
4b
377 (42.9),
301 (38.2)
e
379 (44.2),
308 (40.6),
e
373 (37.0),
299 (35.4)
e
379 (39.2),
308 (40.4)
e
375 (42.77),
304 (28.0)
373 (37.8),
300 (38.2)
406 (27.3),
361 (32.8)
455 (32.4),
365 (28.4),
302 (18.4)
373 (43.1),
298 (40.5)
365 (13.2)
413 (22.4),
364 (32.23),
463 (14.46),
392 (26.0),
368 (29.23)
e
e
e
e
458 (21.1),
365 (19.8),
302 (12.9)
yl)thiophen-2-yl)acrylic acid (427 nm) [44] which supports the
elongation of conjugation due to the ethynyl bridge. Similarly,
conjugation extension by ethynylbenzene in 3a is expected to result
in a red-shift in absorption when compared to (E)-2-cyano-3-(7-
(diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)acrylic acid. But the
shorter wavelength absorption realized for 3a is against this
expectation. It is probably the twisted conjugation bridge is detri-
mental for the transmission of the conjugation effects. Similarly,
the dicyanovinyl derivatives (4a and 4b) also displayed blue-shifted
absorption when compared to the known dyes 2-((7-(diphenyla-
mino)-9,9-diethyl-9H-fluoren-2-yl)methylene)malononitrile
(479 nm) and 2-((5-(9,9-diethyl-7-(naphthalen-1-yl(phenyl)
amino)-9H-fluoren-2-yl)thiophen-2-yl)methylene)malononitrile
(492 nm) [43] suggesting a poor donoreacceptor interaction in 4a
and 4b attributable to the acetylene linkage.
The cyanoacrylic acid derivatives (3a and 3b) showed alterna-
tions in the absorption maxima due to solvent polarity while the
dicyanovinyl derivatives (4) were found to be less sensitive to the
nature of the solvent (Fig. 2). Particularly, they displayed a blue-
shifted absorption in the polar solvents such as tetrahydrofuran
(THF), acetonitrile (ACN) and methanol (MeOH) as compared to the
absorption peak realized in toluene (TOL) and DCM. The variation of
the absorption peak did not correlate well with the solvent
parameters such as orientation polarizability and the solvent
polarity parameter of solvents introduced by Dimroth and Reich-
ardt [45], ET(30) suggesting a specific interaction of the dyes with
the polar solvents (see below). It appears that the polar solvents
effectively solvate the dyes in the ground state probably due to the
presence of more quantities of deprotonated species. The shift of
acidebase equilibrium toward the deprotonated form is facilitated
by the solvents capable of coordinating with the carboxylic acid
unit which led to the deprotonation. Such a deprotonation may
diminish the electron-accepting ability of the acceptor moiety,
which in turn will cause a reduction in donor acceptor interaction
in the dye and a consequent blue shift in absorption [36]. This
hypothesis is further supported by the fact that addition of a base
such as triethylamine to the TOL/DCM solution of the dyes 3a and
3b (see Fig. 3 and Table 1) blue shifted the absorption peak and that
addition of an acid (trifluoroacetic acid) to the TOL/DCM solution of
the dye led to a red shift. These observations suggest the presence
of a protonationedeprotonation equilibrium for the dyes in solu-
tion. It is speculated that in TOL/DCM solution, the dyes are
predominantly in the protonated form (DH). However, addition of
triethylamine deprotonated the dye and shifted the equilibrium to
the deprotonated form (Dꢁ) side, while the addition of trifluoro-
acetic acid reversed the equilibrium and increased the amount of
protonated form in the solution.
Though the dyes 3ae3c, 4a and 4b exhibited weak fluorescence,
their emission properties were examined in solvents of different
polarity to qualitatively ascertain their behavior in the excited state.
The emission spectra recorded for the dyes in toluene are presented
in Fig. 4, representative examples of solvatochromic behavior of the
dyes 3b and 4b are illustrated in Fig. 5 and relevant data compiled
in Table 2. The emission peak wavelength decreases as the solvent
polarity increases. A clear negative solvatochromism observed for
the dyes in fluorescence further corroborates the presence of an
intramolecular charge transfer transition from the donor to
acceptor within the De
p spacer-A framework. Since the dyes are
dipolar in nature, polar solvents are expected to stabilize them in
the ground and excited states equally. However a significant blue
Fig. 2. Absorption spectra of the dyes (a) 3b and (b) 4b recorded in different solvents.

528
P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
Table 2
Emission data of the dyes in different solvents.
Dyes lem, nm
Stokes shift, cmꢁ1
Toluene DCM ACN THF MeOH Toluene DCM ACN THF MeOH
3a
3b
3c
4a
4b
568
555
535
569
577
455 467 449 452
473 468 509 454
456 460 501 455
511 494 502 483
512 510 546 468
7970
4057
7763
5050
4744
3663 4765 3770 3716
503 2162 5606 2070
4880 5017 6707 4832
3004 4388 4293 6693
1346 2370 3283 467
3.3. Electrochemical properties
In order to identify the feasibility of electron injection from the
dye into the conduction band of TiO2 and regeneration of the dye by
the conducting electrolyte, we have performed electrochemical
measurements for the dyes using cyclic voltammetry (CV) in
dichloromethane solution with 0.1
M tetrabutylammonium
perchlorate as the electrolyte at a scan rate 100 mV/s. The cyclic
voltammograms recorded for the selected dyes are shown in Fig. 6.
All the dyes displayed a quasireversible one electron oxidation
process at potentials higher than that observed for internal ferro-
cene/ferrocenium couple. It is attributed to the removal of electron
from the arylamine unit. The oxidation potential for the dyes shift
to lower values on changing the spacer from thienylacetylene (3b
and 4b) to phenylacetylene (3a to 4a) or decreasing the strength of
the acceptor moiety (compare 3a/3b against 4a/4b). The high
oxidation potentials observed for 3b and 4b when compared to 3a
and 4a respectively is attributed to the stronger electronic
communication between the donor and acceptor via the thienyla-
cetylene linker. Similarly, the difficulty in oxidation observed for
the dicyanovinyl derivatives (4a and 4b) when compared to the
corresponding cyanoacrylic acid derivatives (3a and 3b) is assigned
to the stronger electron-accepting ability of dicyanovinyl moiety
[46] which delocalizes the electron-density from the amine
segment toward the acceptor segment and makes it less prone for
oxidation. The dyes 3a and 4a have undergone more facile oxida-
tion when compared to the dyes without a phenylacetylene linkage
[36,37] which further supports the identification of relatively
weaker interaction between the donor and acceptor units in the
present dyes caused by the extension of conjugation pathway.
The HOMO and LUMO energy levels for the dyes were from the
optical and electrochemical parameters (Table 3) in an attempt to
understand the interfacial charge transport thermodynamics. The
excited-state oxidation potential (Eox*) of the sensitizers (3ae3c),
estimated [47] from their first oxidation potential (Eox) at the
ground-state and the zeroezero excitation energy (E0e0), is more
negative (Fig. 7) than the conduction band-edge energy level of
Fig. 3. Absorption spectra of the dye 3b recorded in dichloromethane and in the
presence of TEA or TFA.
Fig. 4. Emission spectra of the dyes (3 & 4) recorded in toluene.
shift observed for the emission spectra recorded for polar solvents
such as methanol and acetonitrile when compared to that measure
for toluene solution is interesting and indicate a less polar excited
state. The large Stokes shifts observed for the dyes reveal significant
structural reorganization on excitation.
Fig. 5. Emission spectra of the dye (a) 3b and (b) 4b recorded in different solvents.

P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
529
Fig. 7. Energy levels of the dyes and other materials used for DSSC fabrication.
Fig. 6. Cyclic voltammograms recorded for the dyes (3 and 4) in dichloromethane.
enhancing the thermal stability for the materials. Thienyl conju-
gation and dicyanovinyl acceptor have been identified to render
effective donoreacceptor interaction in the dyes from optical and
electrochemical measurements (vide supra). The dicyanovinyl dyes
exhibit improved stability than the dyes without acetylene linkages
reported earlier [43]. The rigid rod-like configurations resulting due
to the presence of acetylene linkages is beneficial for the thermal
stability of the compounds.
TiO2 (ꢁ0.5 V vs NHE) [48] and suggests that the electron injection
process from the excited dye molecules to the conduction band of
TiO2 is energetically feasible and all dyes can complete the process
of electron flow from dye to TiO2. The oxidation potential of the
dyes are more positive (Fig. 7) than that of the electrolyte redox
couple Iꢁ/Iꢁ3 (ca.w0.4 V vs NHE) [49]. This is favorable for the
regeneration of the oxidized dyes by the electrolyte. While the
HOMO energies remain largely unaltered for the dyes, the LUMO
showed significant alternations attributable to the nature of the
linker and electron acceptor. The presence of thienylacetylene
p
p
-
-
3.5. Theoretical interpretations
spacer and dicyanovinyl acceptor dramatically lowered the LUMO
of the dyes. However, the dyes without the phenylacetylene/thie-
nylacetylene bridges have possessed higher HOMO and LUMO
levels [27,36] attributable to the stronger donoreacceptor
interactions.
To understand the optical and electrochemical properties of the
dyes we have modeled the electronic structure by density func-
tional theory calculations. We have used B3LYP [39] or CAM-B3LYP
[40] exchange correlation functional in conjunction with 6-31G**
basis set to simulate the molecular orbital characteristics and the
vertical transitions. The electronic distributions in the frontier
molecular orbitals of the selected dyes are displayed in Fig. 8.
Typically, the highest occupied molecular orbitals (HOMO) of the
dyes are generally contributed by the diphenylaminofluorene unit
and the lowest unoccupied molecular orbitals (LUMO) by the
acceptor unit (cyanoacrylic acid or dicyanovinyl) and spread up to
the acetylene linkage toward the amine segment. From this it is
evident that the electronic excitation from HOMO to LUMO would
result in the migration of charge from amino segment to the
acceptor end. In case of the meta-substituted derivative, the longer
wavelength absorption originated from the HOMO to LUMO þ 1
3.4. Thermal properties
The thermal properties of dyes were determined by thermog-
ravimetric analysis (TGA) (Table 3). The dyes displayed high
decomposition temperatures (>450 ꢂC) and the thiophene con-
taining dyes (3c and 4b) possess higher decomposition tempera-
tures than the phenyl analogs (3a and 4a). In general, the
dicyanovinyl derivatives (4) display greater thermal robustness
than the cyanoacrylic acid derivatives (3). Also the meta-
substituted derivative (3c) showed poor thermal stability. All
these observations indicate the importance of dipolar character in
Table 3
Redox and thermal properties of the dyes.
Dye
lmax nm (
3
max, Mꢁ1 cmꢁ1 ꢀ 103)
Eox, mV, (
D
Ep)a
HOMO, eVb
E0e0, eVc
LUMO, eVd
E*ox, Ve
Tonset
,
ꢂCf
Td, ꢂCf
3a
3b
3c
4a
4b
390 (30.5), 315 (21.6)
462 (38.9), 368 (38.8), 307 (24.7)
373 (43.1), 298 (40.5)
443 (20.7), 367 (34.6)
479 (34.5), 371 (36.1), 306 (22.7)
416 (53)
431 (65)
421 (60)
430 (69)
438 (68)
5.216
5.231
5.221
5.230
5.238
2.495
2.214
2.924
2.366
2.130
2.721
3.017
2.297
2.864
3.108
ꢁ1.31
ꢁ1.01
ꢁ1.73
ꢁ1.17
ꢁ0.92
467
505
491
519
657
539
585
535
601
696
a
Obtained from cyclic voltammograms recorded using tetrabutylammonium perchlorate (TBAPC) as supporting electrolyte at a scan rate of 100 mV/s in dichloromethane
solution and the potentials are quoted with reference to the ferrocene which was used as potential marker.
b
Deduced from the equations HOMO ¼ Eox þ 4.8 [47].
c
Derived from the optical edge.
d
Deduced from the equations LUMO ¼ HOMO ꢁ E0e0
.
e
f
Excited state oxidation potential vs NHE.
Measured under a flow of nitrogen at a heating rate of 10 ꢂC/min.

530
P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
Fig. 8. Electronic distributions observed in the selected molecular orbitals of the dyes 3a and 3b.
Table 4
transition arising from the HOMO to LUMO excitation. TDDFT
computations using B3LYP gave significantly red-shifted absorption
peaks while CAM-B3LYP model resulted in slightly blue-shifted
absorption than the observed values. Overestimation of charge
transfer transitions in dipolar compounds at the B3LYP level is quite
expected.
Excitation wavelengths, oscillator strength, assignment and orbital energies of the
dyes in vacuum computed using B3LYP.
Dyes lmax
f
Assignment
mg (debye) HOMO LUMO
(eV) (eV)
(nm)
3a
3b
3c
4a
4b
583.0 0.5693 HOMO / LUMO (100%)
417.8 1.2807 HOMO-1 / LUMO (92%)
595.5 0.6547 HOMO / LUMO (100%)
433.3 1.1620 HOMO-1 / LUMO (94%)
418.1 1.0252 HOMO / LUMO þ 1 (84%)
325.0 1.0296 HOMO-1 / LUMO þ 1 (74%)
626.4 0.5232 HOMO / LUMO (100%)
431.8 1.2384 HOMO-1 / LUMO (93%)
632.3 0.6096 HOMO / LUMO (100%)
444.0 1.1816 HOMO-1 / LUMO (94%)
8.06
ꢁ5.01 ꢁ2.61
ꢁ5.02 ꢁ2.74
ꢁ4.98 ꢁ2.60
ꢁ5.07 ꢁ2.91
ꢁ5.10 ꢁ2.95
9.14
3.6. DSSC characteristics
8.36
The dye-sensitized solar cells (DSSCs) based on nanocrystalline
anatase TiO2 were constructed by using the dyes (3ae3c) as the
sensitizer. The DSSCs with an effective area of 0.16 cm2 were
fabricated with an electrolyte composed of 0.05 M I2/0.5 M LiI/0.5 M
tert-butylpyridine in 3-methoxypropionitrile/acetonitrile (1:1)
solution. The device performance statistics were obtained under
AM 1.5 solar irradiation and collected in Table 6. The
currentevoltage curves of the DSSCs sensitized by the dyes are
shown in Fig. 9(a). Among the dyes 3a showed better efficiency
contributed by the high values of photocurrent density, open-
circuit voltage and fill factor. The higher photoconversion effi-
ciency observed for 3a was interesting as it possess shorter wave-
length absorption with smaller molar extinction coefficient in the
10.73
11.87
excitation which is a diphenylaminofluorenylacetylene localized
* transition.
The computed vertical energies and their orbital contributions
are listed in Tables 4 and 5. The qualitative trends in the absorption
peak positions (3c 3a 3b; 4a 4b) and intensities
pep
<
<
<
(3c > 3b > 3a; 4b > 4a) corresponding to the charge transfer
transitions matched well with the first prominent vertical
Table 5
Excitation wavelengths, oscillator strength, assignment and orbital energies of the dyes in vacuum and in THF computed using CAM-B3LYP.
Dyes
Medium
lmax (nm)
f
Assignment
mg (debye)
HOMO (eV)
LUMO (eV)
3a
Vacuum
THF
Vacuum
THF
377.6
388.2
401.6
419.3
341.7
1.9987
2.1582
1.8294
1.9332
1.2484
HOMO / LUMO (52%), HOMO-1 / LUMO (27%), HOMO / LUMO þ 1 (14%)
HOMO / LUMO (52%), HOMO-1 / LUMO (30%), HOMO / LUMO þ 1 (12%)
HOMO / LUMO (51%), HOMO-1 / LUMO (36%), HOMO / LUMO þ 1 (7%)
HOMO / LUMO (48%), HOMO-1 / LUMO (39%), HOMO / LUMO þ 1 (6%)
HOMO / LUMO þ 1 (40%), HOMO / LUMO (30%), HOMO-1 / LUMO (12%),
HOMO / LUMO þ 2 (6%)
6.86
8.15
7.88
9.84
7.70
ꢁ6.19
ꢁ6.20
ꢁ6.19
ꢁ6.20
ꢁ6.19
ꢁ1.56
ꢁ1.62
ꢁ1.64
ꢁ1.75
ꢁ1.38
3b
3c
4a
4b
Vacuum
Vacuum
THF
Vacuum
THF
389.3
399.6
412.9
431.4
1.8807
2.0735
1.7710
1.9022
HOMO / LUMO (56%), HOMO-1 / LUMO (26%), HOMO / LUMO þ 1 (11%)
HOMO / LUMO (50%), HOMO-1 / LUMO (30%), HOMO / LUMO þ 1 (10%)
HOMO / LUMO (53%), HOMO-1 / LUMO (35%), HOMO / LUMO þ 1 (7%)
HOMO / LUMO (50%), HOMO-1 / LUMO (39%), HOMO / LUMO þ 1 (6%)
9.05
10.07
10.06
11.78
ꢁ6.25
ꢁ6.22
ꢁ6.26
ꢁ6.22
ꢁ1.81
ꢁ1.80
ꢁ1.89
ꢁ1.91

P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
531
Table 6
Performance parameters of the DSSC device using the cyanoacrylic acid dyes 3ae3c.
Dye
JSC, mA cmꢁ2
VOC, mV
ff
h, %
Rct2, ohm
Electron lifetime, se, ms
3a
3b
3c
N719
7.36
5.45
1.92
18.6
587
541
498
665
0.69
0.67
0.63
0.70
3.00
1.99
0.61
8.62
21.52
24.58
63.51
e
18.61
0.95
1.72
e
Fig. 9. (a) IeV characteristics and (b) IPCE plots observed for the DSSCs fabricated using the dyes.
series. Probably, the more negative LUMO observed for 3a (Fig. 7)
provides sufficient driving force for the electron injection into the
conduction band of TiO2 and leads to efficient charge collection. The
dye 3b exhibited slightly higher power conversion efficiencies
(IPCE) at longer wavelength incident light when compared to the
dye 3a (Fig. 9(b)). But >75% IPCE was realized for 3a in the shorter
wavelength region which is in accordance with the shorter wave-
length absorption maximum realized for this dye. This clearly
supports poor charge collection efficiency for 3b despite possessing
longer wavelength absorption with high molar extinction coeffi-
cient. It is speculated that the more planar conjugation in 3b
probably facilitates the intramolecular charge recombination [37b].
The Nyquist plot displayed in Fig. 10(a) shows semicircles cor-
responding to the charge transfer resistances at the at the counter
electrode and TiO2/dye/electrolyte interface. The radius of the
bigger semicircle for the dye 3c is significantly larger than the
other dyes (3a and 3b) and suggest a huge charge transfer resis-
tance for this dye. The smallest charge transfer resistance is
observed for 3a which is in agreement with the larger Voc observed
for the corresponding DSSC. In the Bode phase plot (Fig. 10(b)), the
prominent lower frequency peak of the dye 3a is shifted to lower
frequency when compared to those of other dyes (3b and 3c). This
corresponds to an increase in the electron lifetime for the 3a
sensitized DSSCs. The electron lifetime can be extracted from the
angular frequency (umin) [50] at the mid-frequency peak in the
Bode phase plot using se ¼ 1/umin. The electron lifetime measured
for the dyes 3a, 3b, and 3c are 18.61, 0.95 and 1.72 ms, respectively,
and follows the trend, 3a > 3c > 3b. This trend is reverse of that
observed for the ground state oxidation potentials of the dyes
(3a < 3c < 3b). Dye 3a exhibited the lowest redox potential and
highest electron lifetime. This indicates that the position of the
HOMO of the dye relative to the redox potential of electrolyte is
crucial to suppress the dark current. The increase in electron
lifetime for 3a leads to more effective suppression of the back
reaction of the injected electrons with the Iꢁ3 in the electrolyte and
contributes to the improvement in the photocurrent and photo-
voltage and to the substantial enhancement of the device effi-
ciency. The high-frequency peak observed in the Bode plot
corresponds to charge transfer at the counter electrode. The
position of this peak for the devices remained unaltered. This
suggests the absence of significant reaction at the counter elec-
trode/electrolyte interface.
The present dyes exhibited lesser efficiency [3a (3.00%), 3b
(1.99%) and 3c (0.61%)] than the dye, (E)-2-cyano-3-(7-
Fig. 10. (a) Nyquist and (b) Bode phase plots measured for the DSSCs fabricated using the dyes.

532
P. Singh et al. / Dyes and Pigments 95 (2012) 523e533
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In summary, we have synthesized new dyes containing an N,N-
diphenylaminofluorene donor, cyanoacrylic acid/dicyanovinyl
acceptor and phenylacetylene or thienylacetylene p-linkers. It has
been found that the presence of acetylene unit in the conjugation
pathway was found to be detrimental to the transmission of
conjugating effects. Consequently the present dyes exhibited
reduced donoreacceptor interactions which manifested in the
photophysical and electrochemical properties and provided
a method to fine tune the electronic properties. Thienylacetylene
served as an efficient p-linker when compared to phenylacetylene
unit and resulted in a red-shifted absorption in the dyes originating
from the lowered LUMO. The cyanoacrylic acid derivatives have
been demonstrated as successful sensitizers in the dye-sensitized
solar cells based on nanocrystalline TiO2. Favorable LUMO in the
phenylacetylene-based dye led to efficient electron injection into
TiO2 layer and generated high photocurrent in DSSCs and low-lying
HOMO reduced the dark current.
(b) Baheti A, Lee CP, Thomas KRJ, Ho KC. Pyrene-based organic dyes with
thiophene containing
p-linkers for dye-sensitized solar cells: optical, elec-
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sensitizers for dye-sensitized solar cells. Dyes Pigm 2012;94:224e32.
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benzimidazole comprising carboxyl group. Eur Phys J App Phys 2007;38:
227e30.
Acknowledgments
KRJT is thankful to Department of Science and Technology (DST/
TSG/ME/2010/27), New Delhi, India for financial support and K.-C.H
acknowledges
Taiwan.
a grant-in-aid from National Science Council,
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dyes for dye-sensitized solar cells. Org Lett 2011;13:2622e5.
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featured D-A-p-A structural indoline chromophores for coadsorbent-free and
Appendix A. Supplementary material
panchromatic dye-sensitized solar cells. Adv Energy Mater 2012;2:149e56;
(b) Kim JJ, Choi H, Lee JW, Kang MS, Song K, Kang SO, et al. A polymer gel
electrolyte to achieve ꢃ6% power conversion efficiency with a novel organic
dye incorporating a low-band-gap chromophore. J Mater Chem 2008;18:
5223e9.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.dyepig.2012.06.
004.
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