Synthesis of novel colorants for DSSC to study effect of alkyl chain length alteration of auxiliary donor on light to current conversion efficiency

Article abstract of DOI:10.1016/j.jphotochem.2019.03.043

Five (MA1-MA5) hemicyanine based sensitizer having N, N-diethyl aniline as a primary donor and hydroxy or alkoxy as an auxiliary donor have been synthesized to establish a correlation between amphiphilic nature of the sensitizer and charge recombination. A strong electron withdrawing 3-(carboxymethyl)-2-methylbenzo[d]thiazol-3-ium bromide has been explored as an acceptor. All the dyes were characterized by ¹H-NMR, ¹³C-NMR and CHN analysis. The photophysical properties of these dyes were recorded in seven different solvents which do not show any significant impact on absorption and emission maxima while molar absorptivity coefficient decreases with increase in alkyl chain length. These dyes show very poor emission in all the solvents. Nano-crystalline mesoporous TiO₂ based dye-sensitized solar cells were fabricated using MA1 to MA5 sensitizers to evaluate their photovoltaic performance. MA5 having six carbon alkyl chain shows maximum efficiency of 4.97% while MA1 without any alkyl chain shows the lowest efficiency of 3.40%. As the length of alkyl chain increase efficiency increase due to increment in short circuit current (J_sc) and retardation in the recombination process. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were explored to obtain vertical excitation, HOMO-LUMO energy and electron density distribution.

Full text of DOI:10.1016/j.jphotochem.2019.03.043

Accepted Manuscript
Title: Synthesis of novel colorants for DSSC to study effect of
alkyl chain length alteration of auxiliary donor on light to
current conversion efficiency
Authors: Manoj M Jadhav, Jayraj V. Vaghasiya, Dinesh Patil,
Saurabh S. Soni, Nagaiyan Sekar
PII:
S1010-6030(19)30004-8
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.03.043
JPC 11780
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
3 January 2019
13 March 2019
26 March 2019
Pleasecitethisarticleas:JadhavMM, VaghasiyaJV, PatilD, SoniSS, SekarN, Synthesis
of novel colorants for DSSC to study effect of alkyl chain length alteration of auxiliary
donor on light to current conversion efficiency, Journal of Photochemistry and amp;
Photobiology, A: Chemistry (2019), https://doi.org/10.1016/j.jphotochem.2019.03.043
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Synthesis of novel colorants for DSSC to study effect of alkyl chain length
alteration of auxiliary donor on light to current conversion efficiency
1
2,3
1
2*
1*
Manoj M Jadhav , Jayraj V. Vaghasiya , Dinesh Patil , Saurabh S. Soni , Nagaiyan Sekar
1Department of Dyestuff Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai,
Maharashtra 400019, India
2
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, 388120, India
3Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1,
117575, Singapore
*Authors for correspondence e-mail: nethi.sekar@gmail.com, soni_b21@yahoo.co.in
Graphical Abstract

Highlights






Five hemicyanine based donor-π-acceptor chromophore having different alkyl chain
length are made for DSSCs.
Photophysical properties were studied using spectroscopic techniques and correlated
with DFT studies.
Structural modification are carried out to study the impact of chain length on properties
of chromophores.
Chain length has no impact on absorption and emission and HOMO-LUMO energy
levels molar extinction coefficient decrease.
Longer chain length results in enhancement in recombination resistance which increase
the efficiency.
The chromophore MA5 shows the highest efficiency of 4.97%.
Abstract:
Five (MA1-MA5) hemicyanine based sensitizer having N, N-diethyl aniline as a primary donor and hydroxy or
alkoxy as an auxiliary donor have been synthesized to establish a correlation between amphiphilic nature of the
sensitizer and charge recombination.
A
strong electron withdrawing 3-(carboxymethyl)-2-
1
methylbenzo[d]thiazol-3-ium bromide has been explored as an acceptor. All the dyes were characterized by H-
13
NMR, C-NMR and CHN analysis. The photophysical properties of these dyes were recorded in seven different
solvents which do not show any significant impact on absorption and emission maxima while molar absorptivity
coefficient decreases with increase in alkyl chain length. These dyes show very poor emission in all the solvents.
Nano-crystalline mesoporous TiO
to evaluate their photovoltaic performance. MA5 having six carbon alkyl chain shows maximum efficiency of
.97% while MA1 without any alkyl chain shows the lowest efficiency of 3.40%. As the length of alkyl chain
2
based dye-sensitized solar cells were fabricated using MA1 to MA5 sensitizers
4
increase efficiency increase due to increment in short circuit current (Jsc) and retardation in the recombination
process. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were explored
to obtain vertical excitation, HOMO-LUMO energy and electron density distribution.

Keywords:
DSSCs, Alkyl chain elongation, recombination resistance, DFT, TD-DFT
1
. Introduction:
An organic based dye-sensitized solar cell (DSSCs) have become one of the most promising alternatives for the
conventional silicon-based solar cell[1]. In 1991 Grätzel and co-workers have reported first ruthenium based
DSSCs having good light to current conversion efficiency[2]. The sensitizer is an important aspect of DSSCs which
plays a very crucial role in light harvesting[3–6]. The traditional ruthenium based sensitizers have shown
mammoth success with high performance in DSSC[7–10]. These ruthenium based sensitizers suffer from heavy
metal toxicity[11], environmental issues[12], high cost[13], and difficulty in purification[14]. The metal free
organic sensitizer have been explored extensively in recent times[15–20] due to their low synthesis cost[21],
environmental friendliness[22], high molar absorptivity[23,24], easily tuneable electrochemical properties[25]
and established synthetic protocol[26]. Ideally, efficient organic sensitizer contains donor-π-acceptor (D-π-A)
kind of skeleton having LUMO energy level above the conduction band of TiO for efficient electron injection
2
and HOMO energy level below the redox potential of electrolyte for effective dye regeneration[27–31]. These
metal-free sensitizers show poor light to current conversion efficiency due to absorption in a narrow region,
poor dye regeneration, high recombination rate, and dye aggregation. The dye aggregation arises due to pi-pi
stacking of dye molecules. The drawback of dye aggregation is avoided by using alkyl chain which affects the
planarity of the molecule. The length and position of the chain can play a vital role in governing the efficiency of
the sensitizer in DSSCs. In recent years, researchers have systematically investigated the photoelectric
conversion properties of hemicyanine dyes due to their D-π-A type of configuration[32]. The hemicyanine dyes
have a strong acceptor group which can withdraw electron density from donor very efficiently due to positive
charge present on the nitrogen atom, have high molar extinction coefficient and low emission. [33] These dyes
show poor light to current conversion efficiency due to the narrow absorption spectrum, poor dye regeneration,
charge separation and high recombination. In this paper, we have explored five sensitizers having varying length

of alkyl chain on an auxiliary donor to study the effect of alkyl chain variation on the performance of DSSCs. 2-
Methylbenzo[d]thiazole is N-alkylated with bromoacetic acid is utilized as hemicyanine based acceptor over
conventional acceptor to get dyes having strong electron withdrawing group, high molar extinction coefficient
and low emission while N,N-diethylaniline along with hydroxy or alkoxy group are utilized as primary and
auxiliary donor respectively. The strong electron donating group with alkoxy auxiliary donor with varied alkyl
chain help to reduce dye aggregation and increase excited electron injection on TiO2. The newly synthesized
molecules have been characterized by H NMR, 13C NMR, and elemental analyses. The electronic and
photophysical properties of novel sensitizers have been studied and correlated with vertical excitation obtained
by DFT. These dyes have been used to DSSC fabrication and their photovoltaic performance was investigated
and a correlation between alkyl chain modification and its impact on cell performance is established. The
introduction of alkyl chain has helped to overcome problem related to poor dye regeneration and high
recombination.
1
<< Please insert Figure 1: Chemical structure of hemicyanine dyes MA1 to MA5.>>
<< Please insert Scheme 1: Synthesis of MA1 to MA5 >>
2
2
. Experimental Section
.1. Computational methods:
Density functional theory (DFT) was extensively utilized to optimize MA1 to MA5 in gas phase and six different
solvents. A well-known B3LYP function [34] which is a combination of Becke’s three parameter exchange
functional (B3)[35] with the nonlocal correlation functional by Lee, Yang, and Parr (LYP)[36] was employed for
optimization of molecules. The local minimum energy and vertical excitation of each compound is estimated
using time dependent density functional theory (TD-DFT) calculation [37] at 6-31G+(d) basis set. The polarizable
continuum model (PCM)[38,39] was employed for optimization in solvents. All electronic structure
computations were carried out using the Gaussian 09 [40] program.
2
.2. Chemicals and Instruments:
All the synthetic grade chemicals, reagents and solvents were procured from S. D. Fine Chemicals Pvt Ltd and
used without purification. The 3-methoxy propionitrile (MNP), titania (IV) isoporopoxide, LiI, Iodine (I ), 4-tert-
butyl pyridine (TBP), TiO nanoparticle (<25nm particle size), ethyl cellulose and α-terpineol were purchased
from the Sigma-Aldrich, India and used as it is without further purification. FTO doped transparent conducting
2
2

glass (12Ω, Solaronix, thickness 2.2mm, SA, Switzerland) and Meltronix tape (20µm) were used for the
fabrication of DSSC device. All the reactions were monitored by using 0.25 mm silica gel 60, F254 percolated TLC
plates and visualized under UV light. H-NMR and 13C-NMR spectra were recorded on VARIAN 500-MHz
instrument (USA) using TMS as an internal standard. Perkins-Elmer Lambda 25 spectrometer was used to record
the UV-visible absorption spectra of the compounds in different solvents. Fluorescence emission spectra was
recorded on Varian Cary Eclipse fluorescence spectrophotometer. Electrochemical Impedance Spectroscopy
1
(EIS) were executed by utilizing Solartron 1260+1287 interfaces. CorrView and Zplot software were used for
interpretation of data. EIS was measured by changing the frequency from 120 kHz to 0.1 Hz having an AC
amplitude of 10 mV. Potentiometric data of all the samples estimated by doing cyclic voltammetry (CV) of all
the samples electrochemical workstation (CHI660E, USA). A solution of 0.1 M tetra-n-butylammonium
hexafluorophosphate dissolved in oxygen-free acetonitrile employed as supporting electrolyte. Three electrode
system, in which porous glassy carbon electrode acting as a working electrode, thin wire of platinum acting as a
counter electrode and Ag/AgCl acting as a reference electrode were utilized to construct the electrochemical
cell. The photocurrent density to voltage (J–V curve) spectra were recorded by employing an external potential
bias to the device. The generated photocurrent was measured with a Keithley 2450 digital source meter. Xenon
-
2
lamp (solar simulator, PET, USA) calibrated with the light intensity 100mW.cm at AM 1.5 G solar light conditions
by a certified silicon solar cell was used as light source. The photovoltaic parameters such as short-circuit current
density (Jsc), open circuit voltage (Voc), fill factor (FF) and photo-conversion efficiency were calculated using
previously reported method (refer experimental section in supporting information)[41,42]. Electron lifetime
curves and Tafel polarization were obtained by using Solartron (1260) electrochemical analyzer with sweeping
potential at ±0.65V in dark with 25mV/s scan rate.
2
.3 DSSC device Fabrication:
TiO
fabrication of sandwich type DSSC device [43]. For TiO
Dyesol) with thickness 12µm was employed on FTO glass plate. The thickness of TiO
2
photoelectrode as a working electrode and platinized electrode as a counter electrode were utilized for
photoelectrode a thin transparent layer of TiO (18 NRT,
layer was measured by
2
2
2
using a profilometer of Veeco Dektak 150 Stylus surface profiler. The photoelectrode was immersed overnight
in an ethanol solution containing 3 mM dye solution for dye adsorption on the surface. The electrolytic solution
-
-
of I /I
3
prepared by dissolving 0.5M lithium iodide (LiI), 0.05 M Iodine (I ), 0.5 M tert-butyl pyridine (TBP), and
2

0
.5M guanidium thiocyanate (GSCN) in 3-methoxy propionitrile. The electrolyte was employed in the cell
through the predrilled back holes onto the Pt-counter electrode. An epoxy adhesive was used to seal the holes.
The DSSCs device was stored in dark at room temperature for 12 h before photovoltaic measurements. Active
2
areas of the DSSCs were set to 0.16 cm using a metal mask during the measurement of photovoltaic
characterizations.
3
3
. Results and Discussion
.1. Design and synthesis of dyes
In order to understand the effect of chain length elongation on photovoltaic properties, we have synthesized
five molecules MA1 to MA5 with varying the chain length from zero carbon chain in MA1 to Six carbon chain in
MA5. Hemicyanine based dyes show narrow absorption, poor dye regeneration and high rate of recombination,
but MA1 as reported in literature shows broad absorption spectrum when adsorbed on TiO
adsorption is due to the formation of aggregates and interaction of hydroxy group and TiO
which results in a red shift in absorption. It also shows a reduction in emission intensity in the presence of
colloidal TiO [44] which can be very useful for enhanced light harvesting. By taking this into consideration we
2
surface. This broad
2
to form new species
2
have synthesized MA1-MA5 where MA1 is used as the reference for comparing the impact of O-alkylation and
chain length elongation on photovoltaic properties specially kinetic associated with dye recombination. Scheme
1
demonstrates the synthetic approach for the five dyes. Synthetic details and spectra data of all the compounds
are given in supporting information. Synthesis of MA1 is carried out according to reported protocol[44,45]. The
O-alkylation of 4-(diethylamino)-2-hydroxybenzaldehyde was carried out by refluxing it with a respective alkyl
halide in toluene and sodium hydroxide using phase transfer catalyst (p-toluene sulphonic acid). 2-
aminobenzenethiol and acetyl chloride were condensed with each other in toluene at reflux temperature to get
2-methylbenzo[d]thiazole which is further refluxed with bromoacetic acid in toluene to get acceptor 3-
(carboxymethyl)-2-methylbenzo[d]thiazol-3-ium bromide (compound 6). Synthesized aldehyde and acceptor
were condensed in acetonitrile using piperidine as a base to get crude MA1 to MA5 which is further purified
using column chromatography. The systematic alteration of chain length on hydroxy group shows its impact on
photophysical and photovoltaic properties due to change in band gap, charge separation and charge
recombination.

3
.2. Photophysical properties
-
6
Figure 2-6 represents the absorption and emission spectra of MA1-MA5 (1*10 M) recorded in seven solvents
having different polarity and respective photophysical data is reproduced in Table 1-5. The quantum yield for
MA1-MA5 in seven solvents are estimated by using Rhodamine 6G as standard [46,47]. For MA1 lowest
absorption maxima of 516 nm observed in acetonitrile while highest absorption maxima of 560 nm have been
observed in DCM. Increase in solvent polarity shows random solvatochromism in case of MA1. MA1 shows
exceptional behavior in non-polar DCM solvent where it shows highest absorption maxima and the highest molar
-
1
-1
absorptivity (94000 mol cm ). MA1 shows lowest emission maxima at 577 nm in acetonitrile whereas the
highest emission maximum of 588 nm was observed in DMSO. MA1 shows a low Stokes shift ranging from 26
nm to 61 nm. The lowest Stokes shift of 26 nm is observed in DCM while the highest Stokes shift of 61 nm was
observed in acetonitrile. In MA2, where a hydroxy group is replaced by O-methyl shows a red shift in absorption
and emission. The redshift is due to the positive inductive effect caused by the methyl group which improves
the donating capacity of dye suggesting the role of methoxy as auxiliary donor[20]. MA2 shows lowest
absorption maxima of 531 nm in acetonitrile while highest absorption maxima of 564 nm was observed in
dichloromethane. The lowest emission maxima of 583 nm was observed in ethanol and acetone while the
highest emission maxima of 593 nm was observed in dimethyl sulfoxide (DMSO). MA2 shows poor molar
extinction coefficient in ethyl acetate which might be due to its poor solubility in these solvents. For MA3 where
O-ethyl group is incorporated, absorption maxima range from 524 nm to 558 nm while emission maxima range
from 578 nm to 607 nm. MA3 show poor molar extinction coefficient in ethyl acetate and acetone which is due
to its poor solubility. In case of MA4 where the o-butyl group is introduced and MA5 where the o-hexyl group is
introduced absorption maxima range from 530 nm to 559 nm and 525 nm to 554 nm respectively. For MA4
emission maxima lie in the range of 564 nm to 596 nm while for MA5 it lies in the range of 588 nm to 598 nm.
Nearly the same absorption and emission maxima of MA2-MA5 suggest the zero impact of chain length
elongation on photophysical behavior. The comparison of experimental absorption along with vertical excitation
obtained by TD-DFT in respective solvents is replicated in supporting Table S1-S5. The vertical excitation
obtained from TD-DFT calculation are comparable with observed absorption. The orbital contribution for all the
vertical excitation lies in the range of 98.8 to 99.5%. Table 6 represents the absorption-emission data and vertical
excitation of sensitizer MA1 to MA5 in acetonitrile. From Table 6 it is evident that the introduction of alkyl chain

shows a very minute bathochromic shift in absorption and emission while chain length elongation does not have
any significant impact on absorption and emission maxima. The results further suggest that as the chain length
increases molecular weight increase and molar absorptivity decrease as the alteration in chain length is not
contributing to electron conjugation. The absorption and emission spectra of dye adsorbed on nanocrystalline
TiO surface is given in supporting information (Figure S1). When anchored on titania, a slight blue shift of
2
absorption maxima and widening of the absorption spectrum was found as compared to absorption in
solution is observed. The increase in the chain do not show any significant impact on absorption maxima
but shows enhancement in intensity of absorbed light and widening of the absorption spectrum. MA1-
MA5 do not show any emission on TiO surface due to quenching of emission intensity.
2
<< Please insert Figure 2: Absorption and emission spectra of MA1 (1*10-6 M).>>
<< Please insert Figure 3: Absorption and emission spectra of MA2 (1*10-6 M).>>
<< Please insert Figure 4: Absorption and emission spectra of MA3 (1*10-6 M).>>
<< Please insert Figure 5: Absorption and emission spectra of MA4 (1*10-6 M).>>
<< Please insert Figure 6: Absorption and emission spectra of MA5 (1*10-6 M).>>
<< Please insert Table 1: Photo physical data for MA1. >>
<< Please insert Table 2: Photo physical data for MA2. >>
<< Please insert Table 3: Photo physical data for MA3. >>
<< Please insert Table 4: Photo physical data for MA4. >>
<< Please insert Table 5: Photo physical data for MA5. >>
<< Please insert Table 6: Photophysical properties of MA1-MA5 in Acetonitrile (1.0*10-6M). >>
3
.3. Electrochemical properties:
Cyclic voltammetry measurement was carried out to estimate oxidation and reduction potential of MA1 to MA5
sensitizer and cyclic voltammogram of MA1 to MA5 is reproduced in Figure 7. The pronounced oxidation wave
observed in cyclic voltammogram attributing to removal of an electron from the lone pair of nitrogen atom has
been utilized to estimate Eonset value. The first oxidation potential EHOMO value is calculated from the following
equation:

+
E
HOMO= - (Eonset - (Ag/Ag ) + 4.8) eV-------(a)
The excited state oxidation potentials (ELUMO) is obtained from the following eq:
-EHOMO --------(b)
is the optical band gap obtained from absorption spectra of the dye in acetonitrile. The
E
LUMO = E
g
Where, E
g
oxidation-reduction potential values obtained from cyclic voltammogram are replicated in Table 7. The
first oxidation potential EHOMO obtained from equation (a) is more positive than the redox potential of
-
redox couple of electrolyte (I /I
3
̄
oxidised dye to promote reduction of dye. The excited state oxidation potential ELUMO has more negative
values than the conduction band of TiO which suggest the possibility of transfer of electron from LUMO
level of oxidised dye onto the conduction band of TiO . Table 8 represents the HOMO-LUMO energy
2
2
levels obtained from optimised structures using DFT. These values shows very good correlation with
experimental values. Figure 8 gives the pictorial representation of HOMO-LUMO energy levels obtained
by DFT and cyclic voltammogram. From Figure 8 it is evident that elongation of alkyl chain is not having
any significant effect on HOMO-LUMO energy levels as alkyl chain is not altering the electron density as
evident from pictorial presentation of electron density at HOMO-LUMO energy level (Figure 8). Also
from UV-Visible study it is clear that the chain alteration dose not contribute to overall conjugation in
the sensitizers, hence no significant change in HOMO-LUMO energy level is observed.
<< Please insert Figure 7: Absorption and emission spectra of MA5 (1*10-6 M).>>
<< Please insert Table 7: Oxidation-reduction potential of MA1-MA5 in acetonitrile obtained by cyclic
voltammetry. >>
<< Please insert Table 8: HOMO and LUMO energy values in eV for MA1 to MA5 in acetonitrile obtained
from optimized geometry. >>
<< Please insert Figure 8: Comparative analysis of HOMO-LUMO energy levels obtained by Cyclic
voltammetry and DFT.>>
F
igure 9. represents the pictorial representation of electron density at HOMO-LUMO level in MA1 to MA5. For
MA1 at HOMO level electron density lies mainly on nitrogen and oxygen atom in donor moiety and a sulphur
atom in acceptor moiety while it shifts slightly to the nitrogen atom of benzothiazole upon excitation at LUMO
level. This signifies the charge transfer from donor to positively charge hemicyanine nitrogen. For MA1 to MA5

the electron density distribution at HOMO-LUMO level is similar which suggest the zero impact of alkyl chain
elongation on electron density.
<
< Please insert Figure 9: Pictorial presentation of electron density distribution at HOMO and LUMO in MA1
to MA5.>>
3
.4. Photovoltaic properties of DSSC
Preliminary results based on photophysical properties and cyclic voltammetry data shows that the dyes MA1 to
MA5 have suitable ground/excited state oxidation potentials for their application as sensitizers for conventional
DSSCs. LUMO energy levels for these dyes are sufficiently negative for electron injection of excited dye on TiO
2
and HOMO energy level are sufficiently lower than redox potential of electrolyte for dye regeneration. The molar
absorptivity of the sensitizer decreases from MA1 to MA5 as chain length increases. While cyclic voltammetry,
DFT analysis and electron density analysis show no impact on chain elongation. Based on these results one can
predict a decrease in light to photocurrent conversion efficiency from MA1 to MA5. The photovoltaic parameters
of the freshly fabricated cells containing MA1 to MA5 as sensitizer are measured under AM1.5 solar irradiation
-
2
(
100 mW cm ) and the corresponding J–V characteristics graph of all cell are replicated in Figure. 10 (a). The
photovoltaic parameters such as short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF) and
overall conversion efficiency (η) are tabulated in Table 9. The results clearly suggest the importance of alkyl
chain length on performance of dye as sensitizers in DSSCs. Taking MA1 as a standard prototype, MA2 to MA5
molecules were designed and synthesized in to understand the significance of alkyl chain modification of
auxiliary donor on light to the current conversion efficiency of DSSCs. MA1 shows the lowest light to the current
conversion efficiency of 3.40% in the series. The low efficiency of MA1 is due to low Jsc which is 10.20mA/cm2
which in turn has affected the Pmax. In MA2 we have carried out o-methylation at donor. This methoxy group
2
affects the short circuit current significantly leading to an increase in Jsc value to 11.96 mA/cm which leads to
improvement in efficiency to 4.17% which is 22.6% more than MA1. In case of MA3, the length of O-alkyl chain
has been increased by CH in comparison with MA2 which results in 5.3% enhancement in efficiency. The
2
photocurrent efficiency of MA3 is 4.39%. Subsequent enhancement in alkyl chain length in MA4 and MA5 has
resulted in enhancement in photocurrent efficiency due to steady growth in short circuit current. MA4 shows
2
the efficiency of 4.62% with Jsc of 12.54mA/cm while MA5 shows the efficiency of 4.97% with Jsc of
2
1
3.18mA/cm . In the year 2005, Bao-Wen Zhang and co-workers have reported 5.2% efficiency [46] of MA1

whereas we have got 3.31% efficiency in our lab. The lower efficiency might be due to the difference in cell
fabrication process employed in our lab. Under the same conditions, MA2 to MA5 sensitized solar cells gave
overall power conversion efficiencies (η) of 4.17%, 4.39%, 4.62%, and 4.97% respectively. This clearly suggests
that the experimental efficiencies can be higher under improved conditions. The light to current conversion
efficiency increases from MA1 to MA5 clearly suggesting the role of alkyl chain on the enhancement of efficiency.
<< Please insert Table 9: Photovoltaic properties of chromophores. >>
<
< Please insert Figure 10: (a) Photovoltaic characterization (I-V spectra) MA1-MA5 and (b) Nyquist plots of
DSSC devices fabricated using MA1-MA5 chromophores.>>
To clarify the UV Vis trend on TiO2 film and influence of the alkyl chain length of the MA series dye on
the device performance, we carried out IPCE (incident photo to current conversion efficiency) as a
function of wavelength. As shown in supporting information Figure S2, MA-5 dye efficiency converts the
light to current in the region from 300 to 710 nm. The highest IPCEs value of 49% at 510nm for MA5
dye. Meanwhile the IPCE for MA-1 and 2 are obviously lower at 400nm to 600nm, which is in accordance
with their absorption spectra on the nanocrystalline TiO film. To understand the impact of chain length
2
alteration on the photovoltaic performance we have carried out electrochemical impedance spectral
analysis. EIS was recorded with two-electrode alignment under forward bias in the dark light[47,48]. EIS
spectra are given in Figure 10(b) while high-frequency range arc (Rct) recombination resistance (Rrec) and
,
constant phase element-parameter (CEP-P) values are determined from EIS spectra are replicated in
Table 9. The high-frequency range arc is suppressed and can’t be visualized clearly. The Rct value lies in
the range of 21.9 to 22.9 Ω which is almost constant for MA1 to MA5. CPE-P value suggests the surface
unevenness and penetrability of the photoelectrode used in DSSCs is almost constant for all the devices
and lies in the range of 0.79 to 0.82. As the length of alkyl chain increase from MA1 to MA5 the
recombination resistance (Rrec) measured from EIS shows radical enhancement. MA1 shows lowest
recombination resistance (192 Ω) suggesting the rate of electron transport via external circuit is low and rate
of recombination is high which leads to regeneration of dye without transfer of electron through the external
circuit. For MA5 recombination resistance is highest (340 Ω). The high recombination resistance suggest the ease
of electron transport through an external circuit which generates a higher potential difference between two

electrodes. It is evident from EIS spectra that increase in alkyl chain length retards the electron recombination
kinetic process which leads to enhancement in efficiency.
<< Please insert Figure 11: (a) Open Circuit Voltage Decay and (b) Electron carrier lifetime.>>
The impact of chain length alteration on open circuit voltage decay (OCVD) measured and decay in Voc
V) against time (sec) replicated in Figure 11(a). Open circuit voltage decay measurement was carried
(
out by estimating steady state voltage under 1 sun radiation and then decay measurement was carried
out in dark by discontinuing the light radiation[49,50]. The steady state voltage is replicated in Table 9
which shows slight increment from MA1 to MA2. For MA1 Voc is lowest i.e. 0.682V which increase up to
0
.696V upon o-methylation. This further remains constant for MA3 and MA4 while for MA5 it increases
to 0.712V. Open circuit voltage is highest for MA5. When the light radiation source is discontinued the open
circuit voltage (Voc) of the cells exhibit a sharp decline because of electron recombination. The decay of MA1 is
very sharp in comparison with remaining dyes while MA5 shows steady decay in open circuit voltage. For the
rest of the dyes, decay curve remains in-between. From OCVD spectra it is evident that introduction of O-
alkylation has pronounced effect on open circuit voltage and increasing the chain length retards the open circuit
voltage decay which inhibits the back recombination process in dark leading to an improved photovoltaic
performance by improving the forward kinetic process. The electron-carrier lifetime (τe) for MA1 to MA5 is
estimated by using below given equation[51] and electron carrier lifetime spectra is given in Figure 11(b).
−
1
퐾 푇 푑푉
퐵
표푐
휏푛 =
(
)
푒
푑푡
The electron carrier lifetime for MA1 is lowest and shows sharp decay with an increase in potential while
for MA5 it is highest. For MA2, MA3 and MA4 electron carrier lifetime lies in-between MA1 to MA5.
CHI660E electrochemical workstation has been utilised to record Tafel polarization plot which is been
replicated in Figure 12. Tafel polarization plot was recorded by employing DC potential from ±0.65V by
-
1
-
3-
keeping 10 mV s scan rate in dark to understand charge transmission of I /I redox couple on the
-
-
titania surface[52–54]. The cathodic (αc) and anodic (αa) rate of reaction on TiO
2
electrode and I /I
3
electrolyte interface can be defined by the following equation[55]
훼퐶ꢂ퐹
훼푎ꢂ퐹
푗 = ꢀ푗 ꢁ푒푥푝
⁄
ꢃ퐸 ꢀ 퐸 ꢅ ꢀ 푒푥푝
⁄
ꢃ퐸 ꢀ 퐸 ꢅꢆ
ꢄ푞
0
ꢄ푞
푅푇
푅푇

Here, E is applied voltage while Eeq represents the equilibrium potential of the redox mediator. J
o
represents current exchange density. In dark condition equation that j = j as E = Eeq and J depends on
o
o
-
3-
the concentrations of I /I and reaction area on the titania surface. Figure. 11 shows Tafel polarization
curve plotted to gain insight into the electrode kinetics for DSSCs based on the redox shutter. For all the
chromophoric dyes the values of βc and βa current are depicted in Table 10. We can see the higher βc
value determine the reduced recombination reaction at counter/electrolyte interface, while lower
values of βa correspond to the enhanced reaction of iodide at the photoanode/electrolyte interface.
<< Please insert Figure 12: Tafel polarization plot.>>
4
. Conclusion:
In this paper, we have reported synthesis, characterization, DFT and TD-DFT of five hemicyanine based sensitizer
MA1 to MA5. All these compounds were successfully characterized using H NMR, 13C NMR and elemental
analysis. These sensitizers were explored for their potential application in DSSCs. The photo-physical behaviour
of these compounds were studied in seven different solvents and successfully correlated with vertical excitation
obtained by TD-DFT. Zero or negative impact of chain length elongation has been observed on photophysical,
cyclic voltammetry and DFT study. In spite of this photovoltaic efficiency shows increment in efficiency with
chain length elongation due to enhancement in recombination resistance as measured by EIS. This highlights the
impact of chain length elongation. The order of power conversion efficiency for all sensitizers increases from
MA1 to MA5.
1
Acknowledgement
Manoj M Jadhav is grateful to Department of Science and Technology (Government of India) and
Technical Education Quality Improvement Programme, Ministry of Human Resource Development for
financial support.
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Figure 1 Chemical structure of hemicyanine dyes MA1 to MA5.
-
6
Figure 2: Absorption and emission spectra of MA1 (1*10 M)
EA
EA
DCM
Absorption Spectra of MA1
Acetone
Emission Spectra of MA1
DCM
Acetone
DMF
DMSO
Acetonitrile
Ethanol
0.10
0.08
0.06
0.04
0.02
0.00
DMF
DMSO
Acetonitrile
Ethanol
200
1
50
00
1
50
350
400
450
500
550
600
650
700
540
560
580
600
620
640
660
680
Wavelength in nm
Wavelength in nm

-
6
Figure 3: Absorption and emission spectra of MA2 (1*10 M)
EA
DCM
Acetone
DMF
DMSO
EA
DCM
Emission Spectra of MA2
80
1
Absorption Spectra of MA2
Acetone
DMF
DMSO
Acetonitrile
Ethanol
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
160
140
Acetonitrile
Ethanol
1
20
00
1
80
60
40
20
0
350
400
450
500
550
600
650
700
560
580
600
620
640
660
680
Wavelength in nm
Wavelength in nm
-
6
Figure 4: Absorption and emission spectra of MA3 (1*10 M
EA
EA
DCM
Acetone
DCM
Acetone
Absorption Spectra of MA3
Emission Spectra of MA3
Emission Spectra of MA4
6
00
0
.08
.12
.06
.10
.08
E
D
A
MF
SO
nitrile
anol
DMSO
EADMF
DC MD MSO
Aceto
tonitrile
DM
thanol
Absorption Spectra of MA4
D
D
C
M
M
200
500
75
A
D
E
A
ce
c
t
e
o
t
n
o
e
Ace
ne
0
MF
th
E
F
1
1
1
0
0
DMSO
Acetonitrile
Ethanol
400
50
Acetonitrile
Ethanol
0
300
25
0
.04
0.06
100
2
00
0
.02
75
0
.04
.02
100
5
0
5
0
0
0.00
0
560
2
3
50
00
400
450
500
550
600
700
650
700
580
600
620
640
660
680
700
0
.00
Wavelength in nm
Wavelength in nm
3
400
500
600
800
500
550
600
650
700
750
Wavelength in nm
Wavelength in nm
-
6
Figure 5: Absorption and emission spectra of MA4 (1*10 M)

-
6
Figure 6: Absorption and emission spectra of MA5 (1*10 M)
EA
EA
DCM
Acetone
DMF
DMSO
Acetonitrile
Ethanol
DCM
Acetone
DMF
DMSO
Acetonitrile
Ethanol
0.10
0.08
0.06
0.04
0.02
0.00
500
Absorption Spectra of MA5
Emission Spectra of MA5
400
300
200
100
0
350
400
450
500
550
600
650
700
500
550
600
650
700
Wavelenght in nm
Wavelength in nm

Figure 7. Cyclic voltammogram of MA1-MA5 in acetonitrile.
Cyclic Voltammogram of MA1 to MA5 in Acetonitrile
-5
-5
-5
-6
2.0x10
1.5x10
1.0x10
5.0x10
MA5
MA4
MA3
MA2
MA1
0.0
-6
-5.0x10
-1.0
-0.5
0.0
0.5
1.0
Potential V
Figure 8. Comparative analysis of HOMO-LUMO energy levels obtained by Cyclic
voltammetry and DFT.

Figure 9. Pictorial presentation of electron density distribution at HOMO and LUMO in
MA1 to MA5.
Figure 10 (a) Photovoltaic characterization (I-V spectra) MA1-MA5 and (b) Nyquist plots of
DSSC devices fabricated using MA1-MA5 chromophores.

Figure 11 (a) Open Circuit Voltage Decay and (b) Electron carrier lifetime.
Figure 12. Tafel polarization plot.

Scheme 1: Synthesis of MA1 to MA5:

Table 1: Photo physical data for MA1
Transition
dipole
moments
for
Quantum
Yield
Solvent
Absorption
Emission
ϵ Max
Stokes shift
FWHM
IAC
f
absorption
-
1
(
cm
mol
ɸ
nm) va(cm ) (nm) vb(cm-1)
-
1
Δvc(cm-1) (nm)
(nm)
Ma (D)
(
-
1)
EA
529 18903.6 578 17301.0 49000 1602.55
560 17857.1 586 17064.8 94000 792.30
540 18518.5 582 17182.1 33000 1336.39
558 17921.1 586 17064.8 50000 856.30
541 18484.3 588 17006.8 58000 1477.49
516 19379.84 577 17331.0 79800 2048.82
536 18656.72 580 17241.4 85000 1415.34
49
26
42
28
47
61
44
102
4000.6 273728656.6 1.1825
1623.4 204749027.9 0.8845
3634.4 147363921.6 0.6366
3727.3 196775638.6 0.8501
3053.5 230172590.0 0.9943
3209.0 383864828.0 1.6583
2523.3 259317854.2 1.1206
11.54
0.011
0.032
0.014
0.036
0.143
0.009
0.052
DCM
50
101
109
85
10.27
8.56
Acetone
DMF
10.05
10.70
13.50
11.31
DMSO
Acetonitrile
Ethanol
87
71
Table 2: Photo physical data for MA2
Transition
dipole
moments
for
Quantum
Yield
Solvent
Absorption
Emission
ϵ Max
Stokes shift
FWHM
IAC
f
absorption
-
1
(
cm
mol
ɸ
nm) va(cm ) (nm) vb(cm-1)
-
1
Δvc(cm-1) (nm)
(nm)
Ma (D)
(
-
1)
EA
546 18315.0 587 17035.8 7000
564 17730.5 589 16977.9 55000
1279.24
752.57
41
25
43
55
52
56
46
102
4000.6 39104093.8 0.1689
1888.1 153081577.9 0.6613
3309.3 69818410.7 0.3016
2910.3 178855222.7 0.7727
3007.2 168040180.3 0.7259
3035.6 311678287.9 1.3465
2683.1 213607527.1 0.9228
4.43
0.010
0.038
0.010
0.061
0.112
0.015
0.053
DCM
58
89
79
83
82
75
8.91
5.89
Acetone
DMF
540 18518.5 583 17152.7 18000 1365.86
535 18691.6 590 16949.2 51000 1742.44
541 18484.3 593 16863.4 46000 1620.88
531 18832.39 587 17035.8 75600 1796.62
537 18621.97 583 17152.7 66000 1469.32
9.38
DMSO
9.15
Acetonitrile
Ethanol
12.34
10.27

Table 3: Photo physical data for MA3
Transition
dipole
Solvent
Absorption
va
Emission
ϵ Max
Stokes shift
FWHM
IAC
f
moments Quantum
for
absorption
Yield
vb
(cm-1)
(mol-1
cm-1) (cm-1)
Δvc
ɸ
(nm)
(nm)
(nm) (nm)
Ma (D)
(cm-1)
EA
524
19084.0
17921.1
18797.0
18552.9
18587.4
578 17301.0 18000 1782.93 54
597 16750.4 78000 1170.73 39
582 17182.1 19000 1614.86 50
592 16891.9 67000 1660.98 53
607 16474.5 53000 2112.90 69
107 3877.9 81667405.8 0.3528
53 1730.8 158474235.6 0.6846
83 3024.5 65926846.6 0.2848
117 4008.4 392006490.7 1.6935
83 3014.7 209245872.1 0.9039
79 2270.9 185527318.1 0.8015
76 2704.1 242768101.8 1.0488
6.28
0.373
0.235
0.390
0.332
0.125
0.373
0.122
DCM
558
532
539
538
9.02
5.68
Acetone
DMF
13.94
10.18
10.46
10.96
DMSO
Acetonitrile 536 18656.72 580 17241.4 68000 1415.34 44
Ethanol
538 18587.36 585 17094.0 65000 1493.34 47
Table 4: Photo physical data for MA4
Transition
dipole
moments
for
Quantum
Yield
Solvent
Absorption
Emission
ϵ Max
Stokes shift
FWHM
IAC
f
absorption
Vb
cm )
(mol-
cm
1
Δvc
(cm )
ɸ
-1
(
nm) va(cm ) (nm)
(nm)
(nm)
Ma (D)
-
1
-1)
-1
(
0
0
0
0
0
0
0
.039
.026
.038
.017
.121
.029
.031
EA
547 18281.5
587 17035.8 70857 1245.76
596 16778.5 116143 1110.56
588 17006.8 71143 1754.92
592 16891.9 74000 1976.03
596 16778.5 95143 1913.07
40
37
55
62
61
30
51
77
57
80
80
80
86
84
2669.0 215641252.7 0.9316
1865.5 258380777.7 1.1162
2897.2 234508242.2 1.0131
2914.9 244226763.6 1.0551
2864.3 304721563.1 1.3164
3117.3 207313624.0 0.8956
2982.6 213581151.3 0.9227
10.42
11.53
10.72
10.91
12.25
10.09
10.28
DCM
Acetone
DMF
559 17889.1
533 18761.7
530 18867.9
535 18691.6
DMSO
Acetonitrile 534 18726.59 564 17730.5 59429 996.10
Ethanol 538 18587.36 589 16977.9 63286 1609.43

Table 5: Photo physical data for MA5
Transition
dipole
moments
for
Quantum
Yield
Solvent
Absorption
Emission
ϵ Max
Stokes shift
FWHM
IAC
f
absorption
-
1
(
cm
mol
ɸ
nm) va(cm ) (nm) vb(cm-1)
-
1
Δvc(cm ) (nm) (nm)
-1
Ma (D)
(
-
1)
0
0
0
0
0
0
0
.209
.132
.120
.180
.310
.065
.215
EA
525 19047.6 588 17006.8 67000 2040.82 63
546 18315.0 598 16722.4 87364 1592.61 52
529 18903.6 590 16949.2 66333 1954.44 61
531 18832.4 594 16835.0 65917 1997.37 63
538 18587.4 598 16722.4 60167 1864.95 60
79 2944.2 243298439.6 1.0510
70 2413.5 265348862.5 1.1463
79 2893.6 235503926.6 1.0174
80 2925.0 239024916.6 1.0326
81 2874.2 211832864.9 0.9151
77 2841.3 194595152.7 0.8407
71 2468.3 219629030.3 0.9488
10.84
11.55
10.71
10.81
10.24
9.72
DCM
Acetone
DMF
DMSO
Acetonitrile 528 18939.39 589 16977.9 55857 1961.47 61
Ethanol 554 18050.54 591 16920.5 72917 1130.07 37
10.58
-
6
Table 6: Photophysical properties of MA1-MA5 in Acetonitrile (1.0*10 M).
Dyes
λ
abs
λ
ems
ε
max
Stokes shift
TD-DFT
-
1
(cm-1)
-1
-1
(cm-1)
(
nm)
(cm )
(nm)
(M cm )
(nm)
Vertical
excitation Strength
nm)
Oscillator
(
MA1
516
19379.84
577
17331.0
79800
2048.82
61
477
1.6435
MA2
MA3
MA4
MA5
531
536
534
528
18832.39
18656.72
18726.59
18939.39
587
580
564
589
17035.8
17241.4
17730.5
16977.9
75600
68000
59429
55857
1796.62
1415.34
996.10
56
44
30
61
483
480
479
480
1.6557
1.6096
1.5914
1.5835
1961.47
Table 7: Oxidation-reduction potential of MA1-MA5 in acetonitrile obtained by cyclic
voltammetry.
a
-1
-1
c
d(eV)
e
Sample
Code
Absorption
λ (nm)
ε (M cm )
E
0
onset b
E
HOMO (eV)
E
g
E
LUMO (eV)
a
MA1
516
531
536
534
528
79800
75600
68000
59429
55857
-0.227
-0.225
-0.218
-0.241
-0.265
-4.873
-4.875
-4.882
-4.859
-4.835
1.973
2.014
2.001
2.015
2.050
-2.899
-2.861
-2.880
-2.844
-2.785
MA2
MA3
MA4
MA5

a
Measured in acetonitrile. b Measured from cyclic voltammetry spectra. c All potentials were obtained
+
from cyclic voltammograms. EHOMO= - (E
e: Calculated from ELUMO = E
0
onset - (Ag/Ag ) + 4.8) eV. d: calculated from UV-Visible spectra.
g
+EHOMO.
Table 8: HOMO and LUMO energy values in eV for MA1 to MA5 in acetonitrile obtained
from optimized geometry.
MA1
MA2
MA3
MA4
MA5
LUMO+1
LUMO
-0.9938
-2.9497
-5.7215
-6.3672
2.7718
-0.9785
-2.9323
-5.6981
-6.2948
2.7658
-0.9796
-2.9130
-5.6766
-6.3618
2.7636
-0.9818
-2.9113
-5.6741
-6.3642
2.7628
-1.0161
-2.9064
-5.6698
-6.3585
2.7633
HOMO
HOMO-1
Band Gap
Table 9 Photovoltaic properties of chromophores.
Photovoltaic
parameters
MA chromophore
1
2
3
4
5
J
sc (mA)a
oc(V) a
10.20
0.682
49
11.96
0.698
50
12.36
0.698
51
12.54
0.696
53
13.18
0.712
53
V
FF (%) a
PCE (%) a
3.40
22.9
192
4.17
22.6
231
4.39
22.3
246
4.62
22.3
292
4.97
21.9
340
R
R
CT (Ω)b
rec (Ω)b
CPE-P
0.82
77.4
0.82
80.5
0.79
81.4
0.81
81.8
0.82
120.9
β
a
(mV/decade) c
β
c
(mV/decade) c
449.7
368.3
307.1
392.1
318.6
a
2
Performances of DSSCs were measured with 0.16 cm working area (measurements were performed under
b
c
AM1.5, 1 sun irradiation), Values obtain by fitting Nyquist plot[1,2], Estimated from Tafel polarization
curves[3,4].
[
1]
J. V Vaghasiya, K.K. Sonigara, T. Beuvier, A. Gibaud, S.S. Soni, Iodine induced 1-D
lamellar self assembly in organic ionic crystals for solid state dye sensitized solar cells,
Nanoscale. 9 (2017) 15949–15957. doi:10.1039/C7NR06128E.