Indoline and benzothiazole-based squaraine dye-sensitized solar cells containing bis-pendent sulfonate groups: Synthesis, characterization and solar cell performance

Article abstract of DOI:10.1016/j.molstruc.2019.05.068

Two new symmetric squaraine sensitizers (SQTHZ and SQIND) carrying benzothiazole and indoline moieties as strong electron donating groups to inject the electron into the TiO₂ nanoparticles were tested as DSSC. The theoretical calculations and absorption results show that the electron density of LUMO of SQTHZ is delocalized in the whole chromophore, leading to strong electronic coupling between SQTHZ sensitizer and the conduction band of TiO₂. Furthermore, the presence of long alkyl chain with pendent bis-SO₃ ^- groups would inhibit recombination and decrease the dye aggregation. Interestingly, SQTHZ displayed UV–Vis and NIR absorption at a longer wavelength compared to SQIND. This structure feature, as well as optical properties, would lead to improved efficiency of dye-sensitized solar cell with overall better photovoltaic performance (η of 3.31%, Jsc of 7.65 mA/cm², Voc of 0.59, ff of 0.71 and IPCE of 47% at 674 nm) compared to SQIND.

Full text of DOI:10.1016/j.molstruc.2019.05.068

Accepted Manuscript
Indoline and benzothiazole-based squaraine dye-sensitized solar cells containing bis-
pendent sulfonate groups: Synthesis, characterization and solar cell performance
Sultan A. Al-horaibi, Abdullah M. Asiri, Reda M. El-Shishtawy, Suresh T. Gaikwad,
Anjali S. Rajbhoj
PII:
S0022-2860(19)30632-5
DOI:
https://doi.org/10.1016/j.molstruc.2019.05.068
MOLSTR 26575
Reference:
To appear in: Journal of Molecular Structure
Received Date: 13 March 2019
Revised Date: 15 May 2019
Accepted Date: 17 May 2019
Please cite this article as: S.A. Al-horaibi, A.M. Asiri, R.M. El-Shishtawy, S.T. Gaikwad, A.S. Rajbhoj,
Indoline and benzothiazole-based squaraine dye-sensitized solar cells containing bis-pendent sulfonate
groups: Synthesis, characterization and solar cell performance, Journal of Molecular Structure (2019),
doi: https://doi.org/10.1016/j.molstruc.2019.05.068.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Indoline and benzothiazole-based squaraine dye-sensitized solar
cells containing bis-pendent sulfonate groups: Synthesis,
characterization and solar cell performance
Sultan A. Al-horaibia,b, Abdullah M. asiric,d, Reda M. El-Shishtawyc,e, Suresh T. Gaikwada, Anjali S. Rajbhoja,*
aDepartment of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Maharashtra 431001, India
b
Department of Chemistry, Al-Baydha University, Yemen.
cDepartment of Chemistry, King Abdulaziz University, P.O BOX 80203, Jeddah, Saudi Arabia.
dCenter of Excellence for Advanced Materials Research.
eDyeing, Printing and Textile Auxiliaries Department, Textile Research Division, National Research Center, Dokki, Cairo, 12622,
Egypt.
* Corresponding author e-mail address: anjali.rajbhoj@hotmail.com
Abstract: Two new symmetric squaraine sensitizers (SQTHZ and SQIND) carrying benzothiazole
and indoline moieties as strong electron donating groups to inject the electron into the TiO2
nanoparticles were tested as DSSC. The theoretical calculations and absorption results show that the
electron density of LUMO of SQTHZ is delocalized in the whole chromophore, leading to strong
electronic coupling between SQTHZ sensitizer and the conduction band of TiO2. Furthermore, the
-
presence of long alkyl chain with pendent bis-SO3 groups would inhibit recombination and decrease
the dye aggregation. Interestingly, SQTHZ displayed UV-Vis and NIR absorption at a longer
wavelength compared to SQIND. This structure feature, as well as optical properties, would lead to
improved efficiency of dye-sensitized solar cell with overall better photovoltaic performance (η of
3.31 %, Jsc of 7.65 mA/ cm2, Voc of 0.59, ff of 0.71 and IPCE of 47 % at 674 nm) compared to
SQIND.
Keywords: Dye-sensitized solar cell, Symmetric squaraine dyes, Photoelectrochemical properties,
DFT.
1 Introduction
For the last four decades, conventional silicon solar cells have witnessed development rapidly,
with energy current-conversion efficiencies of more than 23%, by addressing the optical effects of
impurities, by incorporation of alloys and superlattices, and silicon-based tandem cells [1, 2]. Dye-
sensitized solar cells (DSSC) are one of the most important sources of renewable energy[3, 4]. It has
seen tremendous growth and a great deal of interest from scientists and economic researchers because

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of their low cost, eco-friendly and ease of fabrication compared to conventional silicon cells based.
Several organic dyes for example, coumarin [5], indoline[6], triphenylamine[7], hemicyanine[8, 9],
squaraine dyes [10], phthalocyanine [9, 11], and perylene [12], have been developed and used in
DSSC. In addition, the squaraine dyes (SQ) are particularly important in research as they are eco-
friendly, low-cost and give high-performance, this is resulting from photovoltaic and electrochemical
stability as well as high molar extinction coefficients in the visible region [13, 14]. A perusal revision
to photoconversion by potential dye-sensitizer used in dye-sensitized solar cells confirms that
photoreceptor efficiency can be achieved to 12% even with the photon harvest mainly in the visible
absorption area of 400-700 nm of the solar spectrum [15, 9]. This indicates an urgent need to improve
photovoltaic efficiency by developing high-capacity sensitizer’s dyes to harvest photons in NIR-IR
wavelength region [16]. Among the various organic dyes, squaraine dyes are considered the best
candidates for use in applications of dye-sensitized solar cells. They are low cost and have higher
molar extinction coefficients ≈3×105 M−1 cm−1, also squaraine dyes absorb the photons in the range
400 to 800 nm [17, 18]. Our previous studies indicated that the absorption bands of squaraine dyes
could be easily tuned to get into NIR region by structural modification, which closely matches the
spectral response of sunlight. In addition, theoretical calculations on squaraine dyes revealed their
suitability for DSCC, as their LUMO energy lie above the LUMO energy of TiO2 conduction band
and thus ease electron injection from the dye to TiO2 [19-23].
The purpose of the present work is to fabricate and develop an efficient photovoltaic system
capable of absorption at long wavelength of the spectrum through the application of two types of
benzothiazole and indoline based symmetric squaraine sensitizers containing bis-sulfonate groups
which act as anchors for DSSC.
We have synthesized, characterized and studied photovoltaic properties of two new squaraine
sensitizers. The SQTHZ and SQIND were calculated by Gaussian 09 packages to have a detailed
understanding of the ground, excited state electron density distribution and band gap [19, 24, 25].
These two types of SQTHZ and SQIND sensitizers exhibited a panchromatic response from the
-
visible region to the NIR region. The presence of long alkyl chain in SQTHZ with pendent bis-SO3
groups would inhibit recombination and decrease the dye aggregation and thus resulting in improved
dye-sensitized solar cell efficiency.
2. Materials, Instruments and Methods
2.1 Materials
All chemicals, 4-Bromophenyl hydrazine hydrochloride (98%), Phenyl hydrazine (97%),

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Squaric acid (98%), Isopropyl methyl ketone (98%), 1,4-butan sultone, 1,3-propane sultone, n-
Butanol, Toluene, Acetonitrile (98%), Methanol, Ethanol (99.9%), petroleum ether 60/80 and silica
gel 60-120 mesh for column chromatography (Vetec) were from Sigma-Aldrich. Synthesis processes
of SQTHZ and SQIND are showed in Scheme 1.
2.2 Instrumental analysis
All HNMR spectra were recorded on Bruker AVANCE III WM 400 spectrometers (Karlsruhe,
Germany) and 13C NMR at 150 MHz using DMSO-d6 and CDCl3. UV-vis spectra were recorded on a
Perkin Elmer UV-1800 in dimethyl sulfoxide. The oxidation/ reduction properties of the SQTHZ and
SQIND were examined by cyclic voltammetry (CV) on a Metrohm Autolab PGSTAT 128N in dried
dimethyl sulfoxide with three electrode systems consists of a saturated Ag/AgCl reference electrode, a
platinum wire auxiliary electrode and Pt electrode as working electrode. The scan rate was 100 mV/ s.
The optimization geometries for SQTHZ and SQIND were calculated at B3LYP/3-21G (d, p) level of
calculations using Gaussian 09 program package. Fluorescence spectra were conducted on a RF-6000
spectrofluorometer and LabSolutions RF software using methanol solution (0.5 ꢀM) for both dyes and
were corrected for wavelength-dependent instrument sensitivity. The dye solution was filled in a 1-cm
standard transparent quartz cell, and the sample was purged with argon for 15 min prior
measurements. Emission maxima were determined with an accuracy of ±1.0 nm.
2.3 Synthesis and characterizations
2.3.1 Synthesis of 2-methylbenzo[d]thiazole (1)
A mixture of 2-aminothiophenol (14.92 g, 0.1 mol) in acetic anhydride (40 ml) was refluxed
for 4 h at temperature 150 ◦C. It was then cooled to room temperature. The solvent was removed under
reduced pressure. Water was added to the product in addition to aqueous NaOH until pH=8. It was
then extracted with ethyl acetate (3-4×50 ml) to give compound (1) as an oil in 98.3% yield. The FT-
IR spectrum showed an absorption bands at γ(cm-1) 3062(st C-H), 1713(N=C), 1594(C=C), 1240(C-
1
N). HNMR (400MHz-CDCl3) spectrum recorded at δ(ppm ) 2.74(s, 3H, CH3), 7.28(t,1H, J=5.6 Hz,
Ar-H), 7.39(t, 1H, J= 9.2 Hz, Ar-H), 7.72(d, 1H, J=8.04Hz, Ar-H), 7.94(d, 1H, J=8.16 Hz, Ar-H). 13C-
NMR (CDCl3) 19.74 (CH3), 121.08, 122.04, 124.38, 125.60(carbon phenyl aromatic), 148.53, 153(C7,
C8 respectively), 166.62(C2).
2.3.2 Synthesis of 5-bromo 2, 3, 3-trimethyl -3H -indolenine (2)
In a round bottom flask p-bromophenylhydrazine hydrochloride (7.55 g, 33.85 mmol),
isopropyl methyl ketone (5.85 g, 67.5 mmol) and acetic acid (60 ml) were refluxed under N2

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atmosphere for 7 h and stirred at RT for 19 h. The solvent was removed and the residue was dissolved
in Et2O/H2O (3×5 ml). The combined organic phases were washed with aqueous KOH/H2O solution
(3×100 ml) and dried over MgSO4 for 1h with stirred, then filtered and washed with diethyl ether to
1
give compound (2) as brownish oil in 97% yield. HNMR (400MHz-DMSO-d6) spectrum recorded at
δ(ppm ) 1.21(s, 3H, CH3), 1.9(s, 6H, 2CH3), 7.45(d,1H, J=8.2 Hz ,Ar-H), 7.69(d, 1H, J= 8.6 8Hz, Ar-
H),7.74(s, 1H, Ar-H). 13C-NMR (DMSO-d6) 21.68, 21.76 (CH3, 2CH3), 54.52 (C3), 118.70, 119.43,
125.6, 139.90(carbon phenyl aromatic), 146.17, 153.26(C8, C9 respectively), 178.98 (C2).
2.3.3 Synthesis of 4-(2-methylbenzo[d] thiazol-3- ium -3 -yl) butane-1-sulfonate (3)
A mixture of 2-methylbenzothiazole (3 g, 0.02 mmol), 1, 4-butane sultone (2.74 g, 0.02 mmol)
°
and 1, 2 dichlorobenzene (20 ml, anhydrous) were heated at 110 C for 18 h. After cooling to room
temperature, the mixture was diluted with diethyl ether (350 ml). The product was collected by
vacuum filtration, washed with ether (3×100 ml) and then dried in vacuum to give compound (3) as
1
light orange solid in yield 64%. HNMR (400 MHz-DMSO-d6) spectrum recorded at δ(ppm)
1.77(pent, 2H, CH2-SO3), 2.07 (pent, 2H, CH2-SO3), 3.22(s,3H, CH3), 3.30(t, 2H, J=6.12Hz, N-
CH2),4.47(t, 2H, J= 5.4Hz, CH2-SO3), 7.79(t, 1H, J=7.12Hz, Ar-H), 7.88(t, 1H, J=7.08 Hz, Ar-H),
8.42 (overlapping doublets, 2H, J=8.36 Hz, Ar-H).
2.3.4 Synthesis of 3-(5-bromo-2, 3, 3-trimethyl- 3H- indol-1-ium-1-yl) propane-1-sulfonate (4)
A mixture of 5-bromo-2,3,3-trimethyl-3H-indole (2.5 g, 0.0105 mmol) and 1, 3-propane
sultone (2.5 g, 0.005 mmol) was refluxed for 6 h under N2 atmosphere in nitromethane (20 ml) and
cooled to room temperature, then diethyl ether was added and left for 1h. It was then filtered and
1
washed with diethyl ether to give compound (4) as light brown colour in 77% yield. HNMR
(400MHz-DMSO-d6) spectrum was recorded at δ(ppm) 1.45(s,6H, 2CH2), 1.89(pent, 2H, CH2), 2.26
(t, 2H, CH2-SO3), 4.47(t, 2H, CH2-N+), 7.78( d, 1H, J=6.84Hz, Ar-H), 7.87(d, 1H, J=8.56 Hz, Ar-H),
8.04(s, 1H, Ar -H).13C-NMR(DMSO-d6) 21.83, 23.78, 25.27, 26.83, 33.17, 47.74(C3), 54.32(CH2–
N+), 117.14, 123.11, 126.59, 131.94 (carbon phenyl aromatic), 139.97, 143.74 (C8, C9 respectively),
174.54(CO), 196.67 (C2).
2.3.5 Synthesis of SQIND
A mixture of 3-(5-bromo-2, 3, 3-trimethyl- 3H- indolium-1-yl) propane-1-sulfonate (3 g,
0.0069 mmol) and squaric acid (0.395 g, 0.00346 mmol) was taken 40 ml of dry BuOH/toluene 1:1
(v/v) was added and reaction mixture was refluxed for 8 h using Dean–Stark trap for azeotropic
removal of water. The product was purified by silica gel 60-120 mesh column chromatography with
hexane and methanol (9:1, 9:2, 9:3, 9:4 and 9:5 v/v) to give SQIND as blue dye in 78% yield. 1HNMR

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(400MHz-DMSO-d6) spectrum recorded at δ(ppm) 1.55 (s, 6H, 2CH3), 1.68(s, 6H, 2CH3), 1.98 (pent,
-
2H, CH2), 2.13(pent, 2H, CH2), 2.6-2.67(t t, 4H, CH2-SO3 ), 4.2(t, 2H, CH2-N), 4.6 (t, 2H, CH2-N+),
5.83(s, 1H, CH=C), 7.4(d, 1H, J=8.56Hz, Ar-H), 7.54(d, s, 2H, J=6.52Hz, Ar-H), 7.88(s, 1H, Ar-H),
13
8.0(d, 1H, Ar-H), 8.16(d, 1H, Ar- H). C-NMR (DMSO-d6) 13.73, 19.15, 24.61, 26.61, 26.50, 26.71,
27.06, 30.66, 34.00(C3), 43.65 (N-CH2), 49.39(N+-CH2), 64.31(C3), 87.15(C=CH), 110.68, 116.74,
125(carbon phen -yl aromatic), 130.76, 141.45(C8, C9 respectively), 169.55(C2), 173.43 (CO).
2.3.6 Synthesis of SQTHZ
A mixture of 4-(2-methylbenzo[d]thiazol-3-ium-3-yl) butane-1-sulfonate (3 g, 0.011 mmol)
with squaric acid (0.559 g, 0.0052 mmol) was taken and 40 ml of dry BuOH/toluene1:1 (v/v) was
added and reaction mixture was refluxed for 8 h using Dean–Stark trap for azeotropic removal of
water. Reaction mixture was cooled to room temperature, solvent was evaporated. The product was
purified by silica gel 60-120 mesh column chromatography with ethyl acetate and methanol (9:1, 9:2,
1
9:3, 9:4 and 9:5 (v/v)) to give SQTHZ as blue dye solid in 47% yield. HNMR (400MHz-DMSO-d6)
spectrum was recorded at δ(1.5 pent, 4H, 2CH2), 1.7(t, 4H, 2CH2), 3(t, pent, 4H, CH2-N), 7.5(t, t, 2H,
J=7.68 Hz, Ar-H), 7.7(tt, 2H, J=7.4Hz, Ar-H), 8(2dd, 4H, J=3.8Hz, Ar-H). 13C-NMR (DMSO-d6)
spectrum was recorded at 16.69, 22.10, 26.53, 48.92, 50.11, 117.03, 124.50, 128.05, 128.05, 129.02,
129.33, 140.88, and 177.07. Mass spectral (m/z): [M+] calcd for C26H22N2O8S4 = 618.03: found:
2-
618.41.

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2.4 Preparation, Fabrication and measurements of DSSC
A mixture of TiO2 powder ≤ 25 nm (3 g), nitric acid aqueous solution 0.5 ml in dimethyl
sulfoxide 3 ml, and deionized water 2 ml was taken and reagent of Triton X-100 (2 ml) was added.
The TiO2 paste was stirred by a rod glass for 10 min and then left it for 5 min and stored in a closed
bottle for later use. The conducting glasses substrate indium tin oxide glass slide were cleaned in a
detergent solution for 10 min by sonication and with acetone for 10 min, then with deionized water for
10 min and finally with ethanol for 10 min. The conducting glass was dried by air and then in oven.
The glass square (~8ꢁ) was measured by a multi-meter set to examine the conductive side, then strips
of Scotch Magic tape on were put on the edges of the glass square by the doctor blade technique. The
TiO2 paste was deposited in the middle of the glass square. The conducting glass TiO2 porous film
◦
was left in the air for 5 min, and then placed on hotplate for 30 min at 70 C. In the final step it was
placed in furnace for 1 h at 450 ◦C and immersed in squaraine dyes solution (SQTHZ and SQIND) for
25 h in the dark in ethanol with chenodeoxycholic acid (CDCA) (0.001 M). Counter electrode was
prepared as follows: A mixture of polyvinylpyrrolidone (PVP) 1 g, H2PtCl6 0.3 g was dissolved in
deionized water (2 ml) under stirring and NaBH4 solution was added to the mixture. The color of the
mixture was changed from orange to black, finally the counter electrode PVP-coated was placed in
◦
-
furnace at 450 C for 1 h. The solution iodide/tri-iodide (I-/I3 ) electrolyte was prepared as follows: A
mixture of iodine (0.03 M), lithium iodide (0.02 M), 1-butyl-3-methylimidazolium iodide (0.015 M),
and 4-tertbutylpyridine (0.5 M) were taken in CH3CN.
The photovoltaic performance of the fabricated SQTHZ & SQIND based DSSC were measured
under sunlight (AM 1.5, 100 mW/ cm2) solar simulator (Dayton, Instruments, USA) with a source
meter (Keithley, 2400) at 25 ◦C.
3 Results and Discussion
3.1 Synthesis and spectro-electrochemical properties of the SQTHZ and SQIND dyes
The synthesis details of squaraine sensitizers (SQTHZ and SQIND) are showed in Scheme1.
SQTHZ and SQIND based benzothiazole and indoline have two sulfonate groups in each dye to
facilitate electron injection to the conduction band of nanocrystalline TiO2.
Fig.1 shows the normalized UV-Vis absorption spectra on TiO2 along with the fluorescence
emission for SQTHZ and SQIND dissolved in methanol solution are summarized in Table1. The
bands maxima of SQTHZ and SQIND are observed at 655 and 669 nm, respectively, and their

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corresponding high molar extinction coefficient values 2.36×105 and 2.84×105 M−1 cm−1 are in
agreement with π-π* transitions.
0.4
SQTHZ
SQIND
0.3
0.2
0.1
0.0
550
600
650
700
750
800
Wavelength (nm)
Fig 1. The UV-vis absorption spectrum of SQTHZ and SQIND.
0.7
SQTHZ on TiO2
0.6
0.5
0.4
0.3
0.2
0.1
0.0
SQIND on TiO2
550
600
650
700
750
800
Wavelength(nm)
Fig 2. The UV-vis absorption spectrum of SQTHZ and SQIND on TiO2.
In Fig. 2 the absorption spectrum of the SQTHZ on TiO2 shift towards a longer wavelength
compared to the SQIND. The absorption spectra of SQTHZ and SQIND sensitizers on TiO2
semiconductor electrode are broad which strongly refer to the increased interaction between
sulfonate groups for SQTHZ and SQIND and nanopourus TiO2. The SQTHZ and SQIND
showed fluorescence emission at 685 and 676 nm, respectively in MeOH. The λmax of SQTHZ
shifts from 669 to 689 nm (ꢂλshift, SQTHZ=20 nm), while the λmax of SQIND shifts from 655
to 674 nm (ꢂλshift, SQIND=19 nm). The ꢂλshift, SQTHZ is one nm higher than ꢂλshift, SQIND
(see Fig .3). This refers to that SQTHZ sensitizer shows interaction electronic coupling with
the TiO2 better than SQIND. SQTHZ sensitizer has long alkyl chain with bis-sulfonate groups,

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which inhibit recombination and decrease the dye aggregation and thus resulting in an improved dye-
sensitized solar cell efficiency compared with SQIND and dye1 & dye 2. The E0-0 (band gap)
energy was estimated from the intersection of the absorption and emission spectra in the
MeOH solution for SQTHZ and SQIND to be 1.85 and 1.88 eV, respectively.
7x105
FL SQTHZ
FL SQIND
6x105
5x105
4x105
3x105
2x105
1x105
0
200
400
600
800
1000
Wavelength (nm)
Fig 3. Fluorescence intensity spectrum of SQTHZ and SQIND.
To investigate the electrochemical property of dye, cyclic voltammetry (CV) measurement
was performed on a Metrohm Autolab PGSTAT 128N with three electrode systems consists of
a saturated Ag/AgCl reference electrode, a platinum wire auxiliary electrode and Pt electrode
as working electrode. Cyclic voltammetry measurement was performed in DMSO using 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte and 0.001 M of
SQTHZ and SQIND at room temperature. The potential scan was taken from -1.5 to 1.0 V at
scan rate 25 mV/s (see Fig.4).

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5.0x10-6
4.0x10-6
3.0x10-6
2.0x10-6
1.0x10-6
0.0
SQTHZ
SQIND
-1.0x10-6
-2.0x10-6
-3.0x10-6
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential applied (mV/s)
Fig 4. Cyclic voltammograms of SQTHZ and SQIND.
We have also estimated the energies of frontier orbitals (HOMOs-LUMOs) in SQTHZ and
SQIND sensitizers in order to justify its candidatures as sensitizers in DSSC.
In Table 1, the photophysical and electrochemical properties of SQTHZ & SQIND. The
Eoxd of SQTHZ and SQIND was 0.63 and 0.61 V, respectively which estimated from the
oxidation potential of dyes molecules in CV graph using question (1). The values of SQTHZ
and SQIND were converted on an absolute scale of -5.06 and -5.04 eV, respectively. The
HOMO energy levels
onset
ox
− EFoncs/eFtc+ )− 4.8(eV)...(1)
were calculated using the
EHOMO = −(E
equation (1) [25].
onset
ox
onset
where
E
and
E
are the onset oxidation potentials for the squaraine sample and ferrocene
Fc/Fc+
against the Ag/AgCl reference electrode, while the value -4.8 eV is the HOMO energy level of
ferrocene against vacuum [25].
While LUMO energies were estimated by using following equation (2).
LUMO = HOMO – Eg ……………… .......(2)
The values of band gap (E0-0) for SQTHZ and SQIND were obtained based on the
intersection (685 nm for SQTHZ and 676 nm for SQIND) of the normalized absorption and
fluorescence spectra, respectively. The oxidation level of Eox (HOMO) for SQTHZ and
SQIND are more positive than the redox potential of the iodide/triiodide couple used as an
electrolyte. The energy reduction level of Ered (LUMO) for SQTHZ and SQIND is sufficiently

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negative to inject electrons into the conduction band of TiO2.
In Table 1, the data of experimental reveal that the benzohiazole-indoline based squaraine
sensitizers (SQTHZ and SQIND) have superior absorption wavelengths (λmax), fluorescence
emission spectrum (λem) and bands gap (Eg) compared to dye 1 & dye 2. The λmax of SQTHZ and
SQIND, are red shifted by 31, 40 nm and 16, 25 nm compared to dye 1 & dye 2 respectively.
SQTHZ and SQIND are red shifted more than dye1 & dye 2.
Table1.The comparative study of photophysical and electrochemical properties of SQTHZ and
SQIND
λmax on
TiO2
(nm)
Ref.
No
a
b
λmax
Ɛ max
λem E0-0
Eoxd
HOMO LUMO
Dye
(nm) (M-1 cm-1)
(nm) (eV) (V vs.NHE)
(eV)
(eV)
SQTHZ
669
236000
674
685 1.85
0.63
-5.06
-3.21
SQIND
Dye 1
Dye 2
654
638
629
284000
155000
287000
671
-
-
676 1.88
648 2.02
643 2.18
0.61
0.91
0.72
-5.04
-5
-4.98
-3.19
-2.98
-2.80
[26]
a The E0-0 was determined from the intersection of the absorption and emission spectra. bThe Eox was measured on 0.1
M TBAPF6 in DMSO (counter electrode (Pt), reference electrode (Ag/AgCl), working electrode (Pt)) and calibrated
with ferrocene/ ferrocenium (Fc/Fc+).
3.2 Frontier molecular orbitals calculations
In the charge density distribution patterns of the ground state highest occupied molecular orbitals
(HOMOs and HOMOs-1), lowest unoccupied molecular orbitals (LUMOs and LUMOs+1) of
(SQTHZ and SQIND) benzothiazole and indole-based squaraine dyes are illustrated in Fig. 5.
SQTHZ
-4.25 eV
-2.72 eV
-2.89 eV
-4.71 eV
SQIND
-2.96 eV
-2.93 eV
LUMO+1
-4.51 eV
HOMO
-4.93 eV
HOM-1
LUMO

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Fig 5. The charge density distribution of the frontier molecular orbitals (0.02 contour value) of SQTHZ and SQIND
based squaraine sensitizers at B3LYP/3- 21G (d, p) level of theory.
The HOMO-1 is distributed on center of squaric acid and on right is indoline moiety in SQTHZ
while it is distributed on center of squaric acid in SQIND. The charge density of HOMO is distributed
on center of squaric acid and only on right indoline moiety in SQTHZ as compared SQIND. The
charge densities of HOMOs are localized on squaraine moieties. The computed energies of HOMO
(EHOMOs), LUMO (ELUMOs), and LUMOs +1 (ELUMOs+1) and energy gaps (Eg) of SQTHZ and SQIND
benzothiazole-indoline based squaraine dyes at B3LYP/3-21G (d, p) level of theory.
In Fig. 5, the charge density distribution on LUMO is expected on SQTHZ to be more stable after
-
anchoring with the TiO2 surface. Furthermore, bis-SO3 groups would be a favorable site to transfer
electrons from dyes to the TiO2 surface [26]. The electron distributions for HOMO-1, HOMO-LUMO
and LUMO+1 are shown in Fig. 5.
3.3 Photovoltaic characterized of the SQTHZ and SQIND
The photocurrent short-circuit density-Photovoltage (J-V) characteristics were tested for SQTHZ
-
and SQIND of DSSC having I-/I3 as redox electrolyte under illumination with AM 1.5G solar
showed in Fig. 6 and summarized in Table 2.
In Table 2, lists the J-V characteristics of DSSC fabricated based benzothiazole-indoline squaraine
sensitizers (SQTHZ and SQIND) on TiO2 film with redox electrolyte I-/I3 . The SQTHZ showed
-
better photovoltaic performance than SQIND. The SQTHZ sensitizer exhibits a JSC at 7.65 mA/cm2,
VOC at 0.59 V, FF at 0.73 and η at 3.31 %, while SQIND sensitizer gives a JSC at 10.24 mA /cm2, VOC
at 0.53 V, FF at 0.53, and η at 2.94.
Table 2 shows the values of Jsc=7.65 mA/ cm2, Voc=0.59V,
FF=0.73, η= 3.31 % and Jsc =10.24 mA/ cm2, Voc=0.53V, FF =0.53, η =2.94 % respectively, for
squaraine sensitizers (SQTHZ and SQIND) which are superior to squarylium dyes (dye1 & dye2),
Jsc =0.62 mA/ cm2, Voc =0.27 V, FF =0.47, η =1.03% and Jsc = 0.03 mA/ cm2, Voc =0.03V, FF =0.10,
η=0.008 % respectively, revealing that the SQTHZ and SQIND sensitizers study of in the present
article would be efficient materials for DSSC and improved electron injection to TiO2 [26].
The Eq. (3) was used to measure the incident photon to current conversion efficiencies (IPCE) from
the measurements.
µ
A
1240× Jsc (
)
cm2
Wavelength(nm)× P (
IPCE(%) =
........(3)
watt
m2
)
in

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So, the IPCE of SQTHZ and SQIND base DSSC were 47% and 41% respectively.
Table 2. The comparative study of Photo-electrochemical characteristics for DSSC based
benzothiazole-indoline
illumination AM 1.5 (100
dye 2 as a standard.
squaraine dyes under the
Jsc
Voc
FF
η
mW/cm2) with dye 1 and
DSSC
(mA/cm2) (V)
(%)
(%)
3.31
2.94
1.03
0.008
SQ-5
7.65
10.24
0.62
0.59 0.73
0.53 0.53
0.27 0.47
0.03 0.10
SQ-6
a Dye 1
a Dye 2
0.03
aDetails can be found in reference [26].
The low-conversion efficiency in DSSC refer to charge π-aggregates on squaraine sensitizers,
causing charge recombination on TiO2 electrode leading to reduced cell efficiency. So, the addition
certain amount of CDCA enhances the dye sensitized solar cell reducing the π-aggregates of the
squaraine sensitizers, thus increasing the process of electron injection to nanoporus titanium dioxide
and increasing cell efficiency [27, 28, 25].
In Fig.7, shows the incident photon-to-current conversion efficiency (IPCE) spectrum of DSSC
based on SQTHZ and SQIND. The IPCE spectrum for SQTHZ exhibited better performance than
SQIND. The SQTHZ and SQIND sensitizer’s photoanodes without CDCA showed a poor
photocurrent because of the large amount of charge losses caused by charge recombination on the π-
aggregated squaraine dyes. The additions of CDCA act to enhance the photocurrent generation
significantly in DSSC based on SQTHZ and SQIND. In this regard, the DSSC based SQTHZ
sensitized solar cell generate larger photocurrents than SQIND sensitized ones though the whole
visible wavelength [29-31].
In SQTHZ sensitizer, the enhancement was due to the high charge transfer rate in the interfacial
region which is verified by J-V characterization and electrochemical impedance spectral (EIS) [32-
34]. It can be concluded that the strong electronic coupling between SQTHZ, SQIND and TiO2 leads
to a marked improvement in photovoltaic performance under solar radiation.

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12
10
8
SQTHZ
SQIND
6
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Potential (V)
Fig 6. Photocurrent density-voltage (J-V) characteristics for SQTHZ and SQIND.
100
90
80
70
SQTHZ
SQIND
60
50
40
30
20
10
500
550
600
650
700
750
800
Wavelength (nm)
Fig.7 The IPCE spectrum for DSSC based on SQTHZ and SQIND.
Fig. 8 Depicts Nyquist plots of DSSC based on SQTHZ and SQIND measured under AM 1.5G
illuminations (100 mW/cm2). The SQTHZ showed that the major semicircle in the electrochemical
impedance spectral (EIS) Nyquist plot corresponds to the resistance of electron transport at the
TiO2/dye/ electrolyte interface [30, 25]. It is clear from the Nyquist plot that SQTHZ showed a lesser

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recombination resistance because of the lower driving force from the electron injection. The smaller
semicircle diameters on the right are attributed to the diffusion of electrolytes which do not differ
significantly between SQTHZ and SQIND. The radius larger semicircle on the left contain R1 from
the charge transfer at the electrode and R2 from the electron transfer in the TiO2/dye/ electrolyte
interface, decreased in the order of dye1, dye2 > SQIND > SQTHZ. This result indicates that
electron generation and transport in DSSC based SQ-sensitizers (SQTHZ and SQIND) are similar
but batter than the dye1 and dye 2.
-2.0
SQTHZ
SQIND
-1.5
-1.0
-0.5
0
1
2
3
4
5
Z'(ohm)
Fig 8. Nyquist plots for SQTHZ and SQIND based DSSC.
4 Conclusions
Synthesis, characterization, and photovoltaic properties of two new symmetric benzothiazole and
indoline based squaraine dyes (SQTHZ and SQIND) were tested in DSSC with nanoporous TiO2.
The theoretical calculations and absorption results showed that the electron density of LUMO of
benzothiazole-based squaraine dye (SQTHZ) is delocalized in the whole chromophore, leading to
strong electronic coupling between SQTHZ sensitizer and the conduction band of TiO2. Additionally,
-
it has a long alkyl chain with pendent bis-SO3 groups that would inhibit recombination and decrease
the dye aggregation and thus resulting in improved dye-sensitized solar cell efficiency. Therefore, the
SQTHZ sensitizer exhibits better photovoltaic performance.

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Acknowledgements
The authors gratefully acknowledge Department of Chemistry, UGC SAP DRS II and DST FIST for
financial assistance and Sophisticated Analytical Instrumentation Facility (SAIF) Chandigarh.
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Highlight
•
Two new symmetric Benzothiazole and Indoline based squaraine dyes
SQTHZ and SQIND) were tested in DSSC with nanoporous TiO2.
The theoretical calculations and absorption results showed that the electron
(
•
density of LUMO of SQTHZ dye is delocalized in the whole chromophore,
leading to strong electronic coupling between SQTHZ sensitizer and the
conduction band of TiO2.
•
The SQTHZ sensitizer exhibits better photovoltaic performance.