Synthesis of cyanovinyl thiophene with different acceptor containing organic dyes towards high efficient dye sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2016.05.052

A series of donor-π-acceptor (D-π-A) metal-free organic dyes employing a diethylamine moiety as donor, phenyl with and without cyanovinyl thiophene unit as a π-spacer and acrylic acid, rhodanine-3-acetic acid, and benzimidazole-5-carboxylic acid as accept

Full text of DOI:10.1016/j.dyepig.2016.05.052

Accepted Manuscript
Synthesis of cyanovinyl thiophene with different acceptor containing organic dyes
towards high efficient dye sensitized solar cells
Subbaiah Manoharan, Jerry J. Wu, Sambandam Anandan
PII:
S0143-7208(16)30251-0
10.1016/j.dyepig.2016.05.052
DYPI 5284
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 March 2016
Revised Date: 5 May 2016
Accepted Date: 31 May 2016
Please cite this article as: Manoharan S, Wu JJ, Anandan S, Synthesis of cyanovinyl thiophene with
different acceptor containing organic dyes towards high efficient dye sensitized solar cells, Dyes and
Pigments (2016), doi: 10.1016/j.dyepig.2016.05.052.
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ACCEPTED MANUSCRIPT
Graphical Abstract

ACCEPTED MANUSCRIPT
Synthesis of Cyanovinyl Thiophene with Different Acceptor Containing
Organic Dyes towards High Efficient Dye Sensitized Solar Cells
Subbaiah Manoharana, Jerry J. Wub,*, Sambandam Anandana,,b,*,*
aNanomaterials and Solar Energy Conversion Laboratory, Department of Chemistry,
National Institute of Technology, Trichy-620 015, India
bDepartment of Environmental Engineering and Science, Feng Chia University, Taichung
407, Taiwan
* To whom the correspondence should be addressed: E-mail: sanand@nitt.edu;
sanand99@yahoo.com; Tel.: 91-431-2503639; Fax: 91-431-2500133
Abstract
A series of donor-π-acceptor (D-π-A) metal-free organic dyes employing a
diethylamine moiety as donor, phenyl with and without cyanovinyl thiophene unit as a π-
–
spacer and acrylic acid, rhodanine-3-acetic acid, and benzimidazole-5-carboxylic acid as
acceptor/anchor, have been designed and synthesized through simple synthetic procedures
and investigated as photosensitizers for the dye-sensitized solar cell (DSSC) application. The
effects of both the π-spacer and an acceptor modification upon the optoelectronic properties,
theoretical and photovoltaic properties of the dyes were studied. In general, DSSCs fabricated
with the dyes containing cyanovinyl thiophene unit as a π-spacer shows more than two times
higher efficiency than the DSSCs fabricated without cyanovinyl thiophene unit. The
maximum power conversion efficiency of 3.01 % was achieved for the cyanovinyl thiophene
unit in the π-spacer and acrylic acid acceptor dye based DSSC.
Keywords: cyanovinyl thiophene, π-spacer, Dye-sensitized solar cell, acceptor, organic dye,
optoelectronic properties
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1. Introduction
From the beginning of the last century, our energy demand has grown tremendously as
a result of the rapid population growth along with their higher living standards. The most
viable way for producing clean energy is through solar cell technology due to its
sustainability and abundant supply of solar energy. After seminal report by O’Regan and
Gratzel in 1991[1], Dye-sensitized solar cells (DSSCs) have attracted lots of attention for
their use in next-generation solar cell technologies owing to their multiple advantages, such
as mechanical flexibility, light weight, high efficiency, and cost-effective fabrication method
[2; 3]. As a critical component in DSSCs for improving the efficiency, the metal-free organic
chromophores have a great deal of interest in the last decades for many researchers due to
their simple synthesis procedures and easy purification, high molar extinction coefficients,
environmental concerns, and tunable structural modification. The extensively studied metal
free organic dyes usually contain donor (D) and acceptor (A) groups bridged by the π-
conjugated unit (D−π−A). This is because of their intramolecular charge transfer processes
and easy to design new dye structures for adjusting the energy levels and extend the
absorption spectra [4; 5]. In the past decades, a large variety of aromatic compounds
containing nitrogen atom were studied as a donors, such as coumarin, triarylamine, indoline,
carbazole, N,N-dialkylaniline, phenothiazine, etc. Compared to the donors, π-conjugated
units and acceptors were studied very limited in the metal free organic dyes. The π-
conjugated unit not only extends the absorption wavelength in the visible region, but also
affects the rate of internal charge recombination and the electron injection from excited dyes
to the semiconductor surface [6; 7]. Hence, the π-conjugated unit modification plays a crucial
role in the designing of metal free organic dyes for obtaining better DSSC performance.
Widely used π-conjugated units are thiophene derivatives because of their intrinsic charge
transport behavior, environmental and chemical stability as well as structural versatility [8;
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9]. However, the introduction of thiophene derivative is very complicated because their
synthetic procedure consumes very expensive metal catalysts and the reaction conditions are
very rigorously anhydrous and inert [10; 11]. To avoid such problems, thiophene-2-
acetonitrile was chosen as a starting material and by Knoevenagel Condensation reaction,
where the thiophene derivative is easily introduced. Further in such processes, it does not
need of expensive catalysts and also it follows simple reaction conditions. This synthetic
route has another merit, i.e. the cyanovinyl group introduced along with thiophene moiety
leads to extended π-conjugation and also reduces the gap between HOMO and LUMO energy
levels. In addition, the need of the acceptor modification in this cyanovinyl thiophene based
dyes were carried out for getting high solar cell performance. In this regard, acrylic acid,
rhodanine-3-acetic acid, and benzimidazole-5-carboxylic acid were chosen.
On the basis of the foregoing points, a series of six metal free organic dyes, based on
N,N-diethylamine as a donor, phenyl with and without cyanovinyl thiophene as a π-
conjugation unit and acrylic acid, rhodanine-3-acetic acid, and benzimidazole-5-carboxylic
acid as an acceptor were synthesized (as shown in scheme 1) which are namely 3-(4-
(diethylamino)phenyl)acrylic acid (EC), 3-(5-(1-cyano-2-(4-(diethylamino)phenyl)vinyl)
thiophen-2-yl)acrylic
acid
(ETC),
acid
2-(5-(4-(diethylamino)benzylidene)-4-oxo-2-
(ER), 2-(5-((5-(1-cyano-2-(4-(diethylamino)
thioxothiazolidin-3-yl)acetic
phenyl)vinyl)thiophen-2-yl)methylene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (ETR), 2-
(4-(diethylamino)phenyl)-benzimidazole-5-carboxylic acid (EB), and 2-(5-(1-cyano-2-(4-
(diethylamino)phenyl)vinyl)thiophen-2-yl)-1H-benzimidazole-5-carboxylic acid (ETB).
Therefore in this article, the effects of π-conjugated unit and acceptor towards metal free
organic dyes on the photophysical, electrochemical, theoretical properties, and DSSC
performance of the dyes have been investigated orderly. This result strongly implies that
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along with π-conjugated unit acceptor modification also plays an important role for
constructing the high efficient organic dyes for DSSCs.
2. Experimental Section
2.1. Materials and Methods
Thiophene-2-acetonitrile (Sigma-Aldrich), 4-diethylaminobenzaldehyde (Alfa Aesar),
3,4-diaminobenzoic acid (Alfa Aesar), Rhodanine-3-aceticacid (Sigma-Aldrich), Malonic
acid (Alfa Aesar), Copper (II) acetate (Cu(OAc)2) (Sigma-Aldrich), and
Tetrabutylammonium perchlorate (TBAP) (Sigma-Aldrich) were purchased and used without
further purification. Phosphorus oxychloride (POCl3), N,N-dimethylforamide (DMF),
Ammonium acetate, glacial acetic acid, piperidine and nitrobenzene were purchased from
Merck. 1H and 13C NMR analysis were measured on Bruker 500 MHz NMR Spectrometer in
deuterated chloroform or dimethylsulfoxide solution at 298K. Chemical shifts (ꢀ values)
were recorded in units of ppm relative to tetramethylsilane (TMS) as an internal standard.
High-resolution mass spectra were obtained by ESI using Orbitrap mass spectrometer. The
FT-IR spectra were obtained with a Thermo Scientific Nicolet iS5 FT-IR spectrometer. All
reactions were monitored by using TLC plates. All chromatographic separations were carried
out on silica gel (60ꢁ130 mesh).
2.2. Photophysical and electrochemical measurements
Absorption and fluorescence spectra were measured in DMF solution on a T90+ UV-
vis spectrometer and Shimadzu RF-5301 PC spectrofluorophotometer, respectively.
Electrochemical measurements were performed on a Metrohm Autolab PGSTAT
potentiostat/galvanostat-84610. All measurements were carried out at room temperature with
a conventional three-electrode configuration consisting of a glassy carbon (GC) working
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electrode, a platinum wire auxiliary electrode, and a silver wire used as the quasi-reference
electrode. The potentials were reported vs. ferrocene as standard using a scan rate of 0.1 Vs-1.
The CV experiments were performed with 3×10-4 M dye solution and 0.1 M
tetrabutylammonium perchlorate (TBAP) in anhydrous DMF as a supporting electrolyte
under Argon atmosphere. Electrochemical impedance spectroscopy (EIS) measurements were
done under 85 mW/cm2 light illumination with a frequency ranging between 10 kHz to 0.1
Hz at their open circuit potential (OCP) by using an Autolab PGSTAT
potentiostat/galvanostat-84610.
2.3. Synthesis
2.3.1. 3-(4-(diethylamino)phenyl)-2-(thiophen-2-yl)acrylonitrile (1a)
4-diethylaminobenzaldehyde (2 g, 11.3 mmol) and thiophene-2-acetonitrile (1.44 mL,
13.6 mmol) were taken in a two neck round bottom flask containing 30 mL freshly distilled
ethanol. 6 M NaOH (0.1 mL) was added to this reaction mixture at room temperature after
that the resulting mixture was refluxed for 2 h. After cooling to room temperature, the
mixture was poured into water and extracted with chloroform. The collected organic extracts
were dried by Na2SO4. The solvent was removed under vacuum and the residue was further
purified by column chromatography on silica gel by using a hexane as an eluent to give
1
orange yellow color solid with yield 78.40% (2.5 g). H NMR (CDCl3, ppm): δ 1.24 (t,
6H), 3.43-3.47 (q, 4H), 6.69-7.82 (ArH, 7H). 13C NMR (CDCl3, ppm): δ 12.62, 44.53,
111.18, 118.43, 120.45, 124.29, 127.89, 131.45, 140.50, 140.76, 149.37. IR (KBr pellet, cm-
1): 3096, 2970, 2897, 2207, 1605, 1575, 1406, 1341, 1272, 1196, 1153, 1076, 1015, 848, 808,
712. Anal. Calcd. for C17H18N2S: C, 72.30; H, 6.42; N, 9.92. Found: C, 72.18; H, 6.62; N,
9.87. HRMS (ESI): Anal. Calcd. for C17H18N2S: 282.1263. Found: 283.1259 [M+H]+.
2.3.2. 3-(4-(diethylamino)phenyl)-2-(5-formylthiophen-2-yl)acrylonitrile (1b)
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Freshly distilled DMF (4.4 mL, 57 mmol) and POCl3 (5.4 mL, 57 mmol) were taken in
a 3-neck round bottom flask under N2 atmosphere at 0 °C, and then it was stirred for 1 h. 2 g
(7.1 mmol) of compound 1a dissolved in 1,2-dichloroethane (20 mL) was added drop-wise to
the above solution and the resulting mixture was stirred for 3 h at 95 °C. After cooled to room
temperature, the mixture was poured into crushed ice and basified with 6 M NaOH. It was
then extracted by dichloromethane and the organic layer was dried under Na2SO4. After
evaporating the organic solvent, the crude product was purified by column chromatography
on silica gel using a mixture of Ethyl acetate/Hexane (v/v = 2/100), to give a maroon color
solid with yield 22.74% (0.5 g). 1H NMR (CDCl3, ppm): δ 1.24 (t, 6H, J=6.5 Hz, 7 Hz), 3.43-
3.48 (q, 4H, J=7 Hz), 6.71 (d, 2H, J=6.5 Hz), 7.32 (d, 1H, J=3.5 Hz), 7.39 (s, 1H), 7.69 (d,
1H, J=3.5 Hz), 7.85 (d, 2H, J=8 Hz), 9.84 (s, 1H). 13C NMR (CDCl3, ppm): δ 12.57, 44.79,
96.91, 111.43, 117.71, 125.17, 132.54, 137.35, 141.25, 143.54, 150.24, 150.72, 182.30. IR
(KBr pellet, cm-1): 3086, 2964, 2870, 2210, 1653, 1601, 1562, 1514, 1440, 1410, 1356, 1276,
1221, 1190, 1157, 1053, 1005, 942, 897, 810, 704. Anal. Calcd. for C18H18N2OS: C, 69.65;
H, 5.84; N, 9.02. Found: C, 69.50; H, 5.92; N, 9.00. HRMS (ESI): Anal. Calcd. for
C18H18N2OS: 310.1208. Found: 311.1213 [M+H]+.
2.3.3. 3-(5-(1-cyano-2-(4-(diethylamino)phenyl)vinyl)thiophen-2-yl)acrylic acid (ETC)
A 25 mL two-neck round bottom flask was charged with 0.15 g (0.48 mmol) of
compound 1b, 0.15 g (1.45 mmol) of malonic acid, and 3 mL pyridine. Then the reaction
mixture was refluxed for 5 h in the presence of piperidine (0.052 mL, 0.53 mmol). After
cooling to room temperature, the mixture was poured into ice water followed by acidified
with dilute HCl solution. The precipitate was filtered, washed with distilled water, and then
kept drying under vacuum for 10 h. After drying under vacuum, the precipitate was purified
by recrystallization from ethanol to afford pure red solid in 76.43 % yield (0.13 g). 1H NMR
(DMSO-d6, ppm): δ 1.13 (t, 6H, J=6.5 Hz, 6 Hz), 3.43-3.47 (q, 4H, J=7 Hz), 6.13 (d, 1H,
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J=15.5 Hz), 6.78 (s, 2H), 7.25 (s, 1H), 7.35 (d, 1H, J=3.5 Hz), 7.48 (s, 1H), 7.59 (d, 1H, J=3.5
Hz), 7.71 (d, 1H, J=16 Hz), 7.83 (s, 2H). 13C NMR (DMSO-d6, ppm): δ 12.91, 44.37, 111.73,
117.93, 118.61, 119.95, 123.82, 125.36, 133.15, 134.88, 136.78, 138.03, 142.52, 146.53,
150.19, 168.02. IR (KBr pellet, cm-1): 3424, 3086, 2978, 2928, 2208, 1679, 1608, 1565,
1517, 1408, 1356, 1277, 1195, 1155, 1080, 1014, 969, 807. Anal. Calcd. for C20H20N2O2S: C,
68.16; H, 5.72; N, 7.95. Found: C, 68.20; H, 5.75; N, 7.90. HRMS (ESI): Anal. Calcd. for
C20H20N2O2S: 352.1318. Found: 353.1317 [M+H]+.
2.3.4. 2-(5-((5-(1-cyano-2-(4-(diethylamino)phenyl)vinyl)thiophen-2-yl)methylene)-4-oxo-
2-thioxothiazolidin-3-yl)acetic acid (ETR)
0.15 g (0.48 mmol) of compound 1b and 0.097 g (0.50 mmol) of rhodanine-3-acetic
acid were added to 25 mL two-neck round bottom flask containing 20 mL of acetic acid. The
reaction mixture was refluxed for 3 h in the presence of ammonium acetate 0.088 g (1.14
mmol). After cooling to room temperature, the mixture was poured into ice water. The
precipitate was filtered and washed with distilled water. After drying under vacuum, the
precipitate was recrystallized from ethanol to yield red colour solid (yield: 0.142 g, 60.83 %).
1H NMR (DMSO-d6, ppm): δ 1.15 (t, 6H, J=7 Hz), 3.44-3.49 (q, 4H, J=7 Hz), 4.73 (s, 2H),
6.68 (d, 1H, J=9 Hz), 7.42 (t, 1H, J=3.5 Hz), 7.74 (s, 1H), 7.78 (d, 1H, J=3.5 Hz), 7.90 (d,
2H, J=9.5 Hz), 8.13 (s, 1H). 13C NMR (DMSO-d6, ppm): δ 12.85, 44.33, 45.62, 111.18,
118.51, 119.92, 123.71, 125.39, 128.73, 130.25, 131.71, 132.16, 135.69, 142.24, 143.43,
149.85, 150.06, 168.26. IR (KBr pellet, cm-1): 3388, 3095, 2972, 2928, 2208, 1708, 1606,
1567, 1517, 1433, 1406, 1351, 1240, 1190, 1156, 1108, 1066, 1049, 948, 891, 819, 795, 746,
622. Anal. Calcd. for C23H21N3O3S3 : C, 57.12; H, 4.38; N, 8.69. Found: C, 57.13; H, 4.40; N,
8.65. HRMS (ESI): Anal. Calcd. for C23H21N3O3S3 : 483.0818. Found: 484.0819 [M+H]+.
2.3.5.
2-(5-(1-cyano-2-(4-(diethylamino)phenyl)vinyl)thiophen-2-yl)benzimidazole-5-
carboxylic acid (ETB)
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A mixture of 3,4-diaminobenzoic acid (0.12 g, 0.81 mmol) and compound 1b (0.25 g,
0.81 mmol) were added to 25 mL two-neck round bottom flask containing 3.5 mL
nitrobenzene. The reaction mixture was heated at 155-160 °C for 20 h. After cooled to room
temperature, the crude product was diluted with diethyl ether (Et2O). The solids were filtered,
1
washed with Et2O and vacuum dried to get 0.28 g of red colour solid with 78.87% yield. H
NMR (DMSO-d6, ppm): δ 1.08 (t, 6H), 3.37 (q, 4H), 6.53-8.21 (ArH, 10H), 13.38 (s, 1H).
13C NMR (DMSO-d6, ppm): δ 12.85, 44.33, 111.18, 118.51, 119.92, 123.71, 125.39, 128.73,
130.25, 131.71, 132.93, 135.69, 142.24, 143.43, 149.85, 150.66, 168.26. IR (KBr pellet, cm-
1): 3443, 3082, 2979, 2934, 2208, 1671, 1608, 1560, 1517, 1409, 1355, 1272, 1192, 1155,
1077, 1013, 964, 891, 810, 715. Anal. Calcd. for C25H22N4O2S: C, 67.85; H, 5.01; N, 12.66.
Found: C, 67.90; H, 5.05; N, 12.60. HRMS (ESI): Anal. Calcd. for C25H22N4O2S: 442.1536.
Found: 443.1536 [M+H]+.
2.3.6. Synthesis of relevant dyes without cyanovinyl thiophene (EC, ER and EB)
3-(4-(diethylamino)phenyl)acrylicacid (EC), 2-(5-(4-(diethylamino)benzylidene)-4-
oxo-2-thioxothiazolidin-3-yl) acetic acid (ER) and 2-(4-(diethylamino)phenyl)benzimidazole-
5-carboxylic acid (EB) dyes were synthesized from 4-(diethylamino)benzaldehyde according
to the reported procedures available in the literature [12; 13].
2.4. Device fabrication and Photovoltaic measurements
A fluorine-doped SnO2 conducting glass (FTO; sheet resistance 10 ꢀ sq.cm–1; brought
from BHEL, INDIA) was cleaned with double distilled water and acetone. The TiO2 paste
was coated on the cleaned conducting glass substrate according to the described procedure
[14]. The active area was 1 x 1 cm2. After heating the TiO2 thin film up to 80° C, the film
was soaked in a DMF solution containing 0.3 mM sensitizers for 24 h at room temperature
for the dye adsorbed on the TiO2 surfaces. A platinized FTO was prepared by chemical
deposition of 0.05 M hexachloroplatinic acid in isopropanol which acts as a counter electrode
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[15]. After being rinsed with DMF, the photoanode was placed on top of the counter
electrode and tightly clipped it to form a cell. The electrolyte was composed of 0.5 M LiI,
0.05 M iodine (I2) and 0.5 M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile which was
injected in between the two electrodes.
The photoelectrochemical characterizations was measured by using modified light
source 300 W Xe lamp (Oriel, 6258) equipped with a AM 1.5 filter (Oriel, 81080 kit) and the
light intensity was 85 mW/cm2. The photocurrent-voltage characteristics of the DSSCs were
recorded with an Autolab potentiostat/galvanostat-84610. The spectra of monochromatic
incident photon-to-current conversion efficiency (IPCE) as a function of wavelength were
measured for the solar cells with an Enlitech QE-T spectral response measurement system.
An RC-S103011-E standard single crystal Si solar reference cell was used to calibrate the
IPCE measurement spectrum.
3. Results and discussion
3.1. Synthesis and Characterization
Molecular structures and synthetic protocol used to obtain the desired organic dyes
(EC, ER, EB, ETC, ETR and ETB) were shown in the Scheme 1. 3-(4-
(diethylamino)phenyl)-2-(thiophen-2-yl)acrylonitrile (1a) and 3-(4-(diethylamino)phenyl)-2-
(5-formylthiophen-2-yl)acrylonitrile (1b) were synthesized by well-known reactions, such as
Knoevenagel and Vilsmeier-Haack formylation reaction. Later compound (1b) was
successfully converted to the desired dyes (ETC and ETR) on treatment with active
methylene compounds, such as malonic acid and rhodanine-3-acetic acid by Knoevenagel
Condensation reaction. The dye ETB was synthesized by performing condensation reaction
of compound (1b) with 3,4-diaminobenzoic acid in the presence of nitrobenzene. Like-wise,
relevant dyes without cyanovinyl thiophene moiety (EC, ER and EB) were synthesized for
1
13
comparison purpose. Synthesized dyes were thoroughly characterized by FT-IR, H and C
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1
NMR, and mass spectral methods. In the H NMR spectra of compound (1a), the aldehyde
hydrogen peak (chemical shift value around 9-10 ppm) was disappeared and new peaks
appear at 7.82 and 7.81 ppm, which corresponds to the newly formed benzylic hydrogen and
hydrogen present in the thiophene unit. In the compound (1b) the new peak at 9.84 ppm
corresponds to the aldehyde hydrogen at thiophene unit. Thus, the NMR analysis conform the
1
compound 1a and 1b product formation. In the H NMR spectra of ETC and EC dyes, the
aldehyde hydrogen peak (chemical shift value around 9-10 ppm) was disappeared and new
peaks appeared at 6.2 and 7.7 ppm with coupling constant of 16 Hz which implies that the
newly formed double bond (C=C) exist in E-conformation. Similarly, in the 1H NMR spectra
of ETR and ER, a peak appear at 7.4 ppm region which corresponds to the hydrogen present
in the newly formed C=C double bond. Likewise in the 1H NMR spectra of ETB and EB dye,
a new peak was seen at chemical shift value around 13 ppm, corresponding to the –NH
13
hydrogen in the benzimidazole moiety. The C NMR results were also an effective support
for the 1H NMR results. Thus, the NMR analysis conform the desired end product formation.
In addition, the observed FT-IR and mass spectra of synthesized dyes also supports such
product formation.
3.2. Photophysical properties
To find out the π−spacer and acceptor effect on the optical properties of the synthesized
dyes, the electronic absorption and fluorescence emission spectral studies were performed in
DMF medium (5x10-5 M). The optical parameters deduced from these spectral studies were
listed in Table 1 and their corresponding spectra were displayed in Figure 1. All the
synthesized dyes exhibit two distinct absorption bands: one weak band in the ultraviolet
region (280 to 340 nm) corresponding to the π-π* electronic transitions and the strong
absorption band lying in the visible region (350 to 531 nm) that can be assigned to an
intramolecular charge transfer (ICT) between the donor unit and the acceptor end group. The
absorption spectra are red-shifted (~60 nm) and expanded when the cyanovinyl thiophene
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π−spacer is introduced to the dye molecules (ETC, ETR and ETB), compared to the relevant
dyes without such π−spacer unit (EC, ER and EB) which clearly indicates that the insertion
of cyanovinyl thiophene unit in between the donor and acceptor, increases the π-bridge length
and also the electron withdrawing nature of acceptor [16; 17]. Similarly, the molar extinction
coefficient of the cyanovinyl thiophene containing dyes shows ~10,000 M-1cm-1 higher than
that of the dyes without such group, which is also due to the lengthening of π-conjugation and
increasing the electron withdrawing ability of acceptor. This property may be responsible
towards enhancing the light harvesting ability and photocurrent generation in DSSC [18; 19].
Among the synthesised dyes, rhodanine-3-acetic acid acceptor based dyes (ER and
ETR) show high maximum absorption wavelength (λmax) with highest molar extension
coefficient (ε) compared to the corresponding dyes with carboxylic acid (EC and ETC) and
benzimidazole-5-carboxylic acid (EB and ETB) acceptor. This may be due to extension of
π−conjugated system with strong electron withdrawing ability were found when rhodanine-3-
acetic acid as acceptor [20; 21]. In the photoluminescence spectra, all the synthesized dyes
were excited at their absorption maximum. All the dyes exhibited strong luminescence
maxima in the wavelength range of 445–662 nm, respectively. The emission spectra also
follow the same trend like absorbance spectra. Stokes shift calculation of these two set of
dyes revealed that cyanovinyl thiophene unit containing dyes shows greater shift (in the range
of 80-131 nm) than dyes without such group (in the range of 69-80 nm), which indicates that
the insertion of cyanovinyl thiophene unit enhances the charge transfer nature of the excited
state [22; 23]. The insertion of cyanovinyl thiophene unit reduces the planar structure of the
dye molecule which may be the reason for the observed larger Stokes shifts [24]. The optical
band gap (E0-0) of synthesized dyes were derived from the intersection of normalized
absorption and emission spectra [25] and illustrated in Table 1. Due to the change in the
conjugation length and electron withdrawing nature of the acceptor, the band gap of the
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synthesized dyes get decreased which may be responsible for broadening of absorption peak
and in addition harvesting higher light efficiency [26; 27].
3.3. Electrochemical measurements
To find out the electrochemical behaviour of the synthesized dyes, the cyclic
voltammetry (CV) was carried out in a solution of 0.1 M TBAP in DMF medium in order to
illustrate the thermodynamic feasibility of electron injection from the excited dye molecule to
the TiO2 conduction band and dye regeneration through electrolyte. The summarized CV data
are presented in Table 1 and the representative cyclic voltammograms are shown in Figure 2.
The HOMO energy level can be calculated from the onset oxidation potential [Eox (onset)]
based on the reference energy level of ferrocene (4.8 eV below the vacuum level) and using
the relationship of HOMO = − [Eox (onset) − EFC + 4.8] eV, where in EFC is the potential of
the internal standard, the ferrocene/ferricenium ion (Fc/Fc+) couple [28]. The value of EFC,
determined under such experimental conditions, was about 0.53 V vs Ag electrode. The
LUMO energy was obtained by the addition of zero-zero energy (E0-0) and HOMO.
All the dyes show one irreversible peak arising from the oxidation potential around
0.60 to 1 V, which is due to the removal of electron from the donor amine moiety. Noticed
negative shift in the oxidation potential for cyanovinyl thiophene unit containing dyes (ETC,
ETR and ETB) upon compared to the corresponding dyes without such unit (EC, ER and EB)
could be attributed to the expansion of their π-conjugated units by linking a cyanovinyl
thiophene unit [29].
The schematic energy levels (HOMO and LUMO) of synthesized dyes, conduction
-
band of TiO2, and redox potential of I-/I3 are illustrated in Figure 3. Inserting the cyanovinyl
thiophene unit as the π-spacer between the donor moiety and anchoring group may be
beneficial in bringing down the LUMO energy level closer to TiO2 conduction band and lift
up the HOMO energy level closer to the electrolyte potential thus leads to band gap
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narrowing in the synthesized dyes. The calculated LUMO values of all dyes were found in
the range -1.93 to -3.00 eV, which is above than the conduction band edge of TiO2 (-4.0 eV
vs vacuum) [30]. This should be sufficient for down-hill electron injection process from the
excited dye molecule into the conduction band of TiO2. The HOMO values of dyes were also
-
found in between -4.90 to -5.23 eV (lower than the I-/I3 redox couple; -4.6 eV vs vacuum)
[31] which ensures favourable dye regeneration through combination between the oxidized
dye and I- ions in the electrolyte.
3.4. Molecular orbital calculations
To obtain a further insights and better understanding of the molecular geometrical
structures and electron distributions in the frontier orbitals of synthesized dyes, the density
functional calculations were carried out by using the B3LYP exchange-correlation functional
and the d-polarized 6-31G (d) basis set implemented in Gaussian 09 program [32-34]. Figure
4 shows the optimized structure and the frontier orbitals (HOMO and LUMO) of the dyes.
The optimized dye structures reveals that the introduction of cyanovinyl thiophene unit
in the synthesized dyes (ETC, ETR, and ETB) bent the geometry of the molecules, which
reduce the aggregation of dye molecules on TiO2 surface [35; 36]. The HOMO levels of
acrylic acid (EC and ETC) and benzimidazole-5-carboxylic acid (EB and ETB) acceptor
based dyes are homogeneously distributed on the electron donating amine moieties and
the π−spacer, whereas LUMO levels are delocalized through the π−spacer and acceptor.
Such spatial separation in HOMO and LUMO orbitals through π-bridge is an important
characteristic for D-π-A organic dyes in DSSCs. Such result indicates that upon illumination
of light the electron density may effectively move from the donor moiety to the acceptor
segment in the HOMO-LUMO excitation, which leads to the fast electron injection from
LUMO to the conduction band of TiO2 [37].
13

ACCEPTED MANUSCRIPT
However in the case of rhodanine-3-acetic acid as acceptor dye (ER and ETR), the
HOMO and LUMO levels were majorly populated especially on the sulfur and thiocarbonyl
group, and due to the presence of -CH2- group (broken conjugation), the anchoring -COOH
group in LUMO level gets isolated which may render anchoring group, and the π−spacer in
two different planes [38]. Consequently, it prevents electrons injected into the TiO2
conduction band via the carboxyl group, which lowers short circuit current density and in
turn solar cell efficiency.
3.5. Photovoltaic Performance
The typical investigation of photovoltaic characteristics of fabricated DSSCs were
sensitized with synthesized dyes on nanocrystalline TiO2 film and using an electrolyte consist
of 0.05 M I2/0.5 M LiI/0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile solution. The
photovoltaic performances of the solar cells, under AM 1.5 solar irradiation (85 mWcm-2)
with an effective light exposure area of 1 cm2, were shown in Figure 5a and the
corresponding solar cell parameters, such as short-circuit current density (Jsc), open-circuit
voltage (Voc), fill factor (FF), efficiency (η), and other relevant data are summarized in Table
2.
According to Figure 5a, it is clear that the efficiency of the fabricated DSSCs can be
affected by the acceptor and π−spacer modification in the synthesized dye molecules. DSSCs
fabricated with the synthesized dyes without cyanovinyl thiophene unit (EC, ER, and EB)
show lower efficiency, whereas the introduction of cyanovinyl thiophene unit in the
corresponding dyes (ETC, ETR, and ETB) could improve the power conversion efficiency
more than two times higher with the enhancement of Jsc and Voc. This is because of the
extension of conjugation and increase in the electron withdrawing nature of the dye
molecules, being capable for reduced band gap, broad and red-shifted absorption with high
molar extinction coefficient, and more negative LUMO, which provides sufficient driving
14

ACCEPTED MANUSCRIPT
force for the photo-induced electron transfer from the excited dye molecules to the
conduction band of TiO2 [39; 40]. Moreover, insertion of cyanovinyl thiophene unit reduces
the planar nature of the dye structure which suppresses the intermolecular interactions and
inhibits the back electron transfer. This may be the reason for the higher Voc values of the
cyanovinyl thiophene unit containing dyes compared to the parent dye molecules [19; 41].
The observed photophysical, electrochemical, and DFT results were well supporting for the
achieved solar cell performance.
In the acceptor modification part, acrylic acid acceptor based dyes (EC and ETC) show
high efficiency among all the dyes. Strong electron withdrawing nature and well conjugation
of acrylic acid in the entire dye molecule may be the reason for such a high efficiency. In the
case of rhodanine-3-acidic acid moiety based dyes (ER and ETR), the presence of −CH2−
group isolates the anchoring carboxylic acid group from the dye molecule, thus prevents
electron injection into TiO2 conduction band [42]. This may be reason for the low efficiency
compared to the corresponding acrylic acid acceptor based dyes. Among the synthesized
dyes, the benzimidazole-5-carboxylic acid acceptor based dyes (EB and ETB) show very low
efficiency, which may be due to their poor electron withdrawing nature.
Furthermore, the incident photon-to-current conversion efficiency (IPCE) for the
fabricated DSSCs was plotted in Figure 5b and their corresponding data were summarized in
Table 2. Fabricated DSSCs based on cyanovinyl thiophene containing dyes (ETC, ETR and
ETB) shows higher IPCE values compared to the dyes without cyanovinyl thiophene unit
(EC, ER, and EB), matching well with the J-V measurement study. In addition, dye loading
amount on the TiO2 surface also affects the solar cell performance which was estimated by
desorption study carried out by 0.1 M NaOH in DMF/water (1:1) mixture [30]. The surface
concentration (Γ) of the sensitizers was measured by UV-vis spectroscopy and the resulting
data are summarized in Table 2. As shown in Table 2, acrylic acid containing dyes possessed
15

ACCEPTED MANUSCRIPT
higher dye loading amount than corresponding dyes with rhodanine-3-acetic acid and
benzimidazole-5-carboxylic acid, which may be the reason for the bulky and more bend
structure responsible for decrease in the dye loading amount [43]. The introduction of
cyanovinyl thiophene unit also enhances the dye loading amount due to the presence of cyano
group which facilitates the dye adsorption on the TiO2 surface [44]. Therefore, fabricated
DSSCs with acrylic acid based dyes (EC and ETC) show higher efficiency and IPCE.
3.6. Electrochemical impedance Analysis
The effect of acceptor and π−spacer modification in the synthesized dye molecules for
DSSC was further elucidated by electrochemical impedance spectroscopy (EIS). The results
of EIS illustrate the kinetics of charge transfer processes at the interfaces of the fabricated
DSSCs, such as the charge recombination at the TiO2/dye/electrolyte interface, electron
−
transport in the TiO2 electrode, electron transfer at the counter electrode, and I3 transport in
the electrolyte [45]. The impedance spectra (Nyquist plots and Bode phase plots) of
fabricated DSSC based on EC, ER, EB, ETC, ETR, and ETB dyes were shown in Figure 6. In
the Nyquist plots, the observed semicircle corresponds to the electron transfer resistance at
the TiO2/dye/electrolyte interface. A larger radius semicircle indicates a huge charge transfer
resistance and slower electron recombination between TiO2 and electrolyte [46]. The radius
of the semicircles corresponding to the ETC, ETR, and ETB dyes based DSSCs exhibited
significantly larger than the EC, ER, and EB dyes based DSSCs. It implies that the insertion
of cyanovinyl thiophene unit increases the charge transfer resistance and slowed down the
electron recombination at the TiO2/dye/electrolyte interfaces which may be reason for the
achieved higher efficiency
In addition, the electron recombination lifetime (τ) can be calculated from Bode phase
plots (figure 6(b)) by using the following equation τ=1/2ꢂf, where f is the frequency of the
mid-frequency peak [47]. The calculated electron lifetime values are presented in Table 2.
16

ACCEPTED MANUSCRIPT
The relatively higher electron lifetime of ETC sensitized DSSC indicates that introduction of
the cyanovinyl thiophene unit in the π−spacer and carboxylic acid acceptor can effectively
–
suppress the injected electrons recombination with the I3 in the electrolyte, which may
enhance the photo voltage, photocurrent and the device efficiency [48].
4. Conclusions
In this article, a series of donor-π-acceptor (D-π-A) metal-free organic dyes having
diethylamine moiety as donor, phenyl with and without cyanovinyl thiophene unit as a π-
–
spacer and acrylic acid, rhodanine-3-acetic acid, and benzimidazole-5-carboxylic acid as
acceptor/anchor were synthesized as photosensitizers for the dye-sensitized solar cell
application. From the experimental and theoretical results, the incorporation of cyanovinyl
thiophene linker is superior in broadening of absorption band with high molar extinction
coefficient and negative shift of LUMO level for an effective electron injection. Due to such
reasons, cyanovinyl thiophene unit containing dye (ETC) shows maximum efficiency of
3.01%, which is more than two times higher than the corresponding dye without cyanovinyl
thiophene unit (EC). These results imply that the π-spacer group modification is a possible
alternative to produce excellent PCE value with longer recombination resistance and high
electron lifetime.
Acknowledgments
Author SA thank DST, New Delhi (Ref. No. EMR/2014/000009) and DST-FIST, New
Delhi (SR/FT/CSI-190/2008 dated 16th Mar 2008) for the sanction of research fund towards
development of new facilities. Author SM thank UGC, New Delhi for the sanction of Senior
Research Fellowship. In addition, support from the Ministry of Science and Technology
(MOST) in Taiwan under the contract number of 104-2221-E-035-004-MY3 is also
acknowledged.
17

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Supplementary material:
Spectral data and DFT data of synthesised compounds were presented here. This
material is available free of charge via the Internet.
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Table 1. Optical, electrochemical data and HOMO, LUMO energy levels of synthesised dyes.
Stokes EHOMO
shift
(nm)
E0-0
ELUMO
(eV)d
Eox/V
(Vs.Fc)
λabs/nm
λem
S.No
1
Dye
EC
(eV)b
(eV )c
(nm)a
(ε/M-1cm-1)a
367
(35550)
458
(35750)
352
(29014)
470
(45424)
531
(47163)
460
445
78
-4.98
3.05
-1.93
0.71
2
3
ER
EB
550
421
92
69
-5.09
-5.23
2.48
3.28
-2.61
-1.97
0.82
0.96
4
5
6
ETC
ETR
ETB
570
662
540
100
131
80
-4.90
-5.02
-5.08
2.37
2.02
2.45
-2.53
-3.00
-2.63
0.63
0.75
0.81
(41962)
aAbsorption and emission spectra were measured in DMF. The oxidation potential of the
dyes was measured under the following conditions: glassy carbon working electrode, 0.1 M
TBAP in DMF as an electrolyte, scan rate of 100 mV/s. Potentials measured vs Fc/Fc+ were
converted to NHE by addition of +0.53 V. cThe E(0-0) are estimated from the intercept of the
b
d
absorption and emission spectra. The excited state oxidation potentials were derived from
the equation: E(LUMO) = E(HOMO) + E(0-0).
Table 2. DSSC Performance Parameters of the synthesised dyes.
Electron
lifetime
Surface
Jsc
Voc
Efficience
(Ƞ) (%)
IPCE
(%)
Dye
FF
concentration
(mAcm-2) (mV)
( τ) (ms) (Γ) (mol/cm2)
1.68 x 10-7
0.98 x 10-7
1.12 x 10-7
2.35 x 10-7
1.35 x 10-7
1.57 x 10-7
23.13
(371 nm)
20.26
(450 nm)
15.50
(355 nm)
55.01
(450 nm)
32.36
(495 nm)
19.48
(440 nm)
EC
ER
2.83
2.27
670
529
0.62
0.59
1.38
0.82
9.04
2.79
EB
0.54
5.71
3.88
1.50
567
689
597
614
0.61
0.65
0.63
0.58
0.22
3.01
1.73
0.63
2.30
ETC
ETR
ETB
14.01
10.80
4.42
23

ACCEPTED MANUSCRIPT
Figure 1
1.0
0.5
0.0
1.0
0.5
0.0
ETR
750
300
350
350
350
400
450
500
550
600
550
550
650
700
800
700
700
650
550
550
1.0
0.5
0.0
1.0
0.5
0.0
ETC
300
300
400
450
450
500
600
650
650
600
1.0
0.5
0.0
1.0
0.5
0.0
ETB
400
500
600
1.0
0.5
0.0
1.0
0.5
0.0
ER
EC
300
350
400
450
500
550
1.0
0.5
0.0
1.0
0.5
0.0
300
350
400
450
500
500
1.0
0.5
0.0
1.0
0.5
0.0
EB
300
350
400
450
Wavelength (nm)
24

ACCEPTED MANUSCRIPT
Figure 2
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
0.000015
0.000010
0.000005
0.000000
-0.000005
-0.000010
ER
ETR
-0.00004
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
-1.0
-0.5
0.0
0.5
1.0
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
0.000020
0.000015
0.000010
0.000005
0.000000
-0.000005
-0.000010
EC
ETC
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
-1.0
-0.5
0.0
0.5
1.0
0.00004
0.00003
0.00002
0.00001
0.00000
-0.00001
0.00008
0.00006
0.00004
0.00002
0.00000
EB
ETB
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-1.0
-0.5
0.0
0.5
1.0
potential(v)
potential(v)
25

ACCEPTED MANUSCRIPT
Figure 3
26

ACCEPTED MANUSCRIPT
Figure 4
Dye
Optimized structure
HOMO
LUMO
EC
ER
EB
ETC
ETR
ETB
27

ACCEPTED MANUSCRIPT
Figure 5
6
(b)
EC
20
30
20
10
ETB
ETR
ER
EB
ETC
ETR
5
15
(a)
10
4
3
5
0
0
350
400
450
500
550
600
650
350
400
450
500
550
600
650
420
500
ETB
60
40
20
0
20
15
10
5
ETC
EB
2
1
0
350
400
450
500
550
600
340
360
380
400
25
20
15
10
5
0
20
16
12
8
ER
EC
-1
-2
4
0
0
0
100
200
300
400
500
600
700
800
440 460 480 500 520 540 560
350
400
450
Wavelength (nm)
Voltage (mV)
Wavelength (nm)
28