Influence of synthesized thiourea derivatives as a prolific additive with tris(1,10-phenanthroline)cobalt(II/III)bis/tris(hexafluorophosphate)/ hydroxypropyl cellulose gel polymer electrolytes on dye-sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2018.12.099

An exceptional approach of appending tris(1,10-phenanthroline)cobalt(II)/(III)bis/tris(hexafluorophosphate) and synthesized tetramethyl benzidine-based thiourea derivatives as novel additives with hydroxypropyl cellulose are used to construct a gel polyme

Full text of DOI:10.1016/j.electacta.2018.12.099

Accepted Manuscript
Influence of synthesized thiourea derivatives as a prolific additive with tris(1,10-
phenanthroline)cobalt(II/III)bis/tris(hexafluorophosphate)/ hydroxypropyl cellulose gel
polymer electrolytes on dye-sensitized solar cells
Pranavamoorthy Karthika, Shanmugam Ganesan, Anoopa Thomas, Tirupathy
Mathew Sheeba Rani, Muthuramalingam Prakash
PII:
S0013-4686(18)32806-8
https://doi.org/10.1016/j.electacta.2018.12.099
EA 33314
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 17 October 2018
Revised Date: 17 December 2018
Accepted Date: 19 December 2018
Please cite this article as: P. Karthika, S. Ganesan, A. Thomas, T.M. Sheeba Rani, M. Prakash,
Influence of synthesized thiourea derivatives as a prolific additive with tris(1,10-phenanthroline)cobalt(II/
III)bis/tris(hexafluorophosphate)/ hydroxypropyl cellulose gel polymer electrolytes on dye-sensitized
solar cells, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2018.12.099.
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ACCEPTED MANUSCRIPT
Influence of Synthesized Thiourea Derivatives as a Prolific Additive with
Tris(1,10-phenanthroline)cobalt(II/III)bis/tris(hexafluorophosphate)/
Hydroxypropyl Cellulose Gel Polymer Electrolytes on Dye-Sensitized Solar
Cells
a
a,*
a
Pranavamoorthy Karthika , Shanmugam Ganesan , Anoopa Thomas , Tirupathy
a
a
Mathew Sheeba Rani , Muthuramalingam Prakash
Graphical Abstract

ACCEPTED MANUSCRIPT
Influence of Synthesized Thiourea Derivatives as a Prolific Additive with
Tris(1,10-phenanthroline)cobalt(II/III)bis/tris(hexafluorophosphate)/
Hydroxypropyl Cellulose Gel Polymer Electrolytes on Dye-Sensitized Solar
Cells
a
a,*
a
Pranavamoorthy Karthika , Shanmugam Ganesan , Anoopa Thomas , Tirupathy
a
a
Mathew Sheeba Rani , Muthuramalingam Prakash
a
Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology,
Kattankulathur 603 203, Kancheepuram District, Tamil Nadu, India
Abstract
An exceptional approach of appending tris(1,10-phenanthroline)cobalt(II)/(III)bis/
tris(hexafluorophosphate) and synthesized tetramethyl benzidine-based thiourea derivatives as
novel additives with hydroxypropyl cellulose are used to construct a gel polymer electrolyte
for DSSC. The thiourea derivatives are synthesized using the cost-effective method. The
1
13
synthesized derivatives are confirmed by H, C NMR and mass spectroscopy. The
2
+/
3+
conductivity, surface and redox potential of thiourea derivatives doped Co Co redox
electrolytes are scrutinized by impedance spectra, FTIR, UV, XRD. A density functional
theory method with full geometry optimization is used to elucidate the adsorption of the
symmetrical thiocarbamides on TiO2 (101) surface progressing with electron
donating/accepting groups. All the organic compounds display a negative shift on the TiO2
Fermi level simultaneously by adsorption and rebutted as an escalated additive in stable
Co(II/III)phen redox electrolyte of the dye-sensitized TiO solar cell. This impulse effect
2
influences the sorbate dipole moment component normal to TiO surface plane; further, the
2
stable redox mediator and hydroxypropyl cellulose deliver a good performance in a dye-
1

ACCEPTED MANUSCRIPT
2
+/
3+
sensitized solar cell. The newly synthesized thiourea derivatives doped Co Co with
hydroxypropyl cellulose reports high solar to electric conversion, particularly 1,1'-(3,3',5,5'-
tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-(3,4,5-trimethoxy phenyl)thiourea) (additive-
6
) doped redox electrolyte shows an outstanding efficiency (ŋ) of 9.1% under the sunlight
2
irradiation of about 70 mW/cm .
Keywords
Tris(1,10-phenanthroline)cobalt(II)/(III)bis/tris(hexafluorophosphate) redox couple,
Hydroxylpropyl cellulose, Gel polymer electrolyte, Thiourea, TiO additive, DFT, Dye-
2
sensitized solar cells
*
Corresponding author.
Tel: +91-44-2741 7872 Fax: +91-44-2745 2343
E-mail: talentganesan@gmail.com, ganesan.sh@ktr.srmuniv.ac.in
1
. Introduction
Global energy consumption is expanding rapidly in recent times by the depletion of
natural sources. To eradicate this concern, much focus has been attributed to the adoption of
renewable energy resources, typically solar energy [1-3]. Conversion of solar energy into the
electricity and for the design of photovoltaic gadgets will make an articulate role for the
amending of energy consumption and a quick fix for a long-term energy crisis. Gratzel and
O’Regan developed such a device with low-cost and high efficiency (ŋ) i.e., 11% via simple
fabrication in 1991, the so-called dye-sensitized solar cell (DSSC) [4]. The DSSC is build of
working electrode as TiO coated with Ru based dye, electrolyte with iodide/iodine as a redox
2
couple and counter electrode as platinum [5-7]. Among all, the electrolyte takes the athletic
2

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portrayal in intensifying efficiency proportion in DSSC. Different types of electrolytes are
habituated so far on our planet. A broader aspect is taken up by the liquid electrolyte [8] but
there are some disadvantages like leakage, poor chemical and physical stability. To deracinate
from such issues, surrogate admittance is undoubtedly by the gel electrolyte [9]. Many
research works have been implemented so far using such electrolyte for attaining good
efficiencies and improvement of shelf time storage in DSSC. It has more superior assets like
non-volatile nature, high stability, high ionic conductivity responsive to novel packing, little
self-discharge and low vapour pressure [10,11]. Thereupon for the preparation of gel polymer
electrolyte, several studies have been done by earning higher efficiencies using cellulose [12].
Recently, Hydroxypropyl cellulose was used in gel polymer electrolyte to gain a maximum
efficiency of 5.79% [13]. HPC possess extraordinary physical, chemical, stability property,
and are highly preferable by their biodegradable property, so this favours the choice rather
than using general polymers like polyethylene glycol, polyvinylidene fluoride,
polycaprolactone, etc [9].
In the reign of semiconductor TiO , out of three phases, the anatase crystal is the more
2
photoactive. The anatase (101) surface tempted more attention by its lower surface energy
compared to (100) and (001) surface and it is scrutinized to adsorb small molecule on its
surface which is thermodynamically more stable [14]. These are prominent and evident by its
regularly perceived tetragonal bipyramidal growth habit. The interfaces amidst rutile and
anatase are further considered to increase charge carrier separation and facilitate the
photocatalytic activity. As a result, biphasic titanium dioxide is considered to contain
enhanced functionality as a photocatalyst [15].
-
-
Towards redox couple, I /I is the classic oxidized/reduced for many years by grating
3
iodide salts and iodine. The cations of the added iodide salts and the concentration of iodide
3

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with triiodide in the redox couple hike the photocurrent-voltage tone [16,17]. Nevertheless,
this redox pair has achieved valuable efficiencies, but it has many consequences like the large
difference between their energy level with HOMO of the sensitizer results in shrink of VOC,
slow electron self-exchange kinetics, high vapour pressure which give up the balance module
of DSSC. To heal up the disadvantages, various eminent scientists introduced the application
of bromine, nitroxide radicals and organic redox mediators[18]. But they limit the efficiency
of the solar cell due to the slow recombination of electrons in the cathodic part to the oxidized
redox specimens. Succeeding, transition-metal-based redox couples proved to be more
efficient and stable. Among which cobalt-based redox couple showed a colossal improvement
in the efficiency of a solar cell [19-23]. Mastermind in DSSC i.e., Gratzel and his co-workers
achieved 12.3% (ŋ) by using zinc porphyrin sensitizer and cobalt-based oxidized/reduced duo
[
24].
In the convention, additives and co-additives such as 4-tert-butylpyridine (TBP),
guanidinium thiocyanate (GuSCN) etc, are utilized to improve the efficiency to a certain
extent by shifting the potential to negative in TiO [25-29]. Sarcastically the conjugated
2
hetero-organic dopants pave the way for the architectural flow of electrons from them to the
polymer and redox pair which helps in accumulating prosperous ionic conductivity in the
entire DSSC set up. Recently, thioureas have gained importance as additives [30-33] in a
polymer matrix for improving the efficiency in DSSC and reached a whopping efficiency of
7
.17% [34]. Thus, we focussed our efforts on synthesizing thiourea derivatives using electron
rich tetramethyl benzidine at the core with thiourea at the periphery as a whole for
synthesizing thiourea derivatives with cost-effective methods. The synthesized thioureas
posses a very strong adsorption on the TiO surface and shifts the band edge to negative
2
potential rapidly. Further, blending these additives with the hydroxypropyl cellulose, ethylene
carbonate, propylene carbonate, as plasticizers and the redox pair i.e., tris(1,10-
4

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phenanthroline)cobalt(II)/(III) bis/tris(hexafluorophosphate) boosted up the photophysical and
photovoltaic properties of the electrolyte. The influence of the additives in the GPE has been
discussed by its electrochemical, photovoltaic, diffraction, calorimetric, and vibrational
characterization. And the synthesized thiourea derivatives are subjected to DFT calculations
to evaluate the adsorption energy of the molecules towards the TiO2.
2
2
. Experimental Section
.1 Chemicals and Reagents
Hydroxypropyl cellulose Mw~370000, ethylene carbonate, 2-amino-6-methylpyridine,
thiophosgene, propylene carbonate, methanol (anhydrous) and all the isothiocyanates were
purchased from Aldrich and the 3,3′,5,5′-tetramethylbenzidine, triethylamine, sodium
bicarbonate, dichloromethane was purchased from AVRA synthesis Pvt. Ltd. Fluorine-doped
tin oxide coated glass slide L x W x D 100mm x 100mm x 2.3mm surface resistivity ~7 ꢀ/sq
and Titanium(IV)oxide, anatase nanopowder, <25 nm particle size, 99.7% trace metals basis
were purchased from Aldrich.
2
.2 Synthesis of Organic additives
The thiourea additives are synthesized using a very mild and cost-effective method.
The general scheme is depicted in Figure 1. The corresponding isothiocyanate was stirred
with 3,3′,5,5′-tetramethylbenzidine in anhydrous methanol at room temperature for 2-3 days
to obtain the product as a white precipitate which was then filtered, washed with methanol
and dried under vacuum.
2
.2.1 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(4-butylphenyl)thiourea)-Additive-1
5

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1
-butyl-4-isothiocyanatobenzene 0.875g (2.2 equiv.) with 0.5g (1 equiv.) 3,3’,5,5’-
tetramethylbenzidine was stirred in 40ml of anhydrous methanol at room temperature for 2-3
days. The product obtained was a white precipitate which was then filtered, washed with
methanol and dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-
diyl)bis(3-(4-butylphenyl)thiourea) as a white solid with 75% yield. The corresponding
1
scheme is depicted in Fig. S1. H NMR (DMSO-d , 500 MHz)
δ (ppm): 9.84 (br s, 2H,
6
2
1
NH), 8.90 (br s, 2H, 2NH), 7.41–7.36 (m, 8H), 7.10 (s, 4H), 2.57-2.54 (m, 4H), 2.29 (s,
1
3
2H), 1.58-1.52 (m, 4H), 1.33-1.29 (m, 4H), 0.92-0.89 (m, 6H); C NMR (DMSO-d , 125
6
MHz)
δ
(ppm): 180.75, 139.15, 137.29, 129.09, 128.99, 128.93, 126.57, 126.45, 123.97,
+
3
4.81, 33.67, 22.21, 18.77, 14.28; HRMS (ESI): calcd. For C H N S [M+H] 623.3164
3
8
46
4 2
found 623.3238. (See Supp. data S1, S2, S3, S4)
2
.2.2 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(4-chlorophenyl)thiourea)- Additive-2
1
-chloro-4-isothiocyanatobenzene 0.776g (2.2 equiv.) with 0.5g (1 equiv.) 3,3′,5,5′-
tetramethylbenzidine was stirred in 40 ml of anhydrous methanol at room temperature for 2-3
days. The product obtained was a white precipitate which was then filtered, washed with
methanol and dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-
diyl)bis(3-(4-chlorophenyl)thiourea) as a white solid 75% yield. The corresponding scheme
1
is depicted in Fig. S5. H NMR (DMSO-d , 500 MHz)
δ (ppm): 10.03 (br s, 2H, 2NH), 9.08
6
1
3
(
br s, 2H, 2NH), 7.58-7.57 (d, 4H), 7.42-7.41 (d, 4H), 7.40 (s, 4H), 2.28 (s, 12H); C NMR
(
DMSO-d , 125 MHz)
δ
(ppm): 185.60, 143.67, 143.54, 141.98, 133.71, 132.97, 132.89,
6
+
1
5
31.20, 129.98, 23.49; HRMS (ESI): calcd. For C H Cl N S [M+H] 579.1132 found
3
0
28
2
4 2
79.1206. (See Supp. data S5, S6, S7, S8)
6

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2
.2.3 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(4-methoxyphenyl)thiourea) Additive-3
1
-isothiocyanato-4-methoxybenzene 0.756g (2.2 equiv.) with 0.5g (1 equiv.) 3,3′,5,5′-
tetramethylbenzidine was stirred in 40 ml of anhydrous methanol at room temperature for 2-3
days. The product obtained was a white precipitate which was then filtered, washed with
methanol and dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-
diyl)bis(3-(4-methoxyphenyl)thiourea) as a white solid with 78% yield. The corresponding
1
scheme is depicted in Fig. S9. H NMR (DMSO-d , 500 MHz)
δ (ppm): 9.71 (br s, 2H,
6
2
2
1
NH), 8.76 (br s, 2H, 2NH), 7.37-7.37 (m, 4H), 7.32-7.31 (m, 4H), 6.92 (s, 4H), 3.74 (s, 6H),
1
3
.26 (s, 12H); C NMR (DMSO-d , 125 MHz)
δ (ppm): 180.99, 157.15, 137.34, 137.27,
6
26.71, 126.47, 126.43, 126.32, 114.49, 55.72, 18.75; HRMS (ESI): calcd. For
+
C H N O S [M+H] 571.2123 found 571.2198. (See Supp. data S9, S10, S11, S12)
3
2
34
4
2 2
2
.2.4 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(naphthalen-1-yl)thiourea)-Additive-4
1
-isothiocyanatonaphthalene 0.847g (2.2 equiv.) with 0.5g (1 equiv.) 3,3′,5,5′-
tetramethylbenzidine was stirred in 40 ml of anhydrous methanol at room temperature for 2-3
days. The product obtained was a white precipitate which was then filtered, washed with
methanol and dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-
diyl)bis(3-(naphthalen-1-yl)thiourea) as a white solid with 70% yield. The corresponding
1
scheme is depicted in Fig. S13. H NMR (DMSO-d , 500 MHz)
δ (ppm): 9.95 (br s, 2H,
6
2
7
NH), 8.70 (br s, 2H, 2NH), 8.07-8.04 (m, 2H), 8.00-7.98 (m, 2H), 7.91-7.88 (m, 2H), 7.64-
.61 (m, 2H), 7.57-7.56 (m, 4H), 7.36-7.33 (m, 2H), 7.21 (s, 2H), 7.13 (s, 2H), 2.13 (s, 12H);
1
3
C NMR (DMSO-d , 125 MHz)
δ
(ppm): 181.81, 143.89, 139.07, 136.39, 135.45, 134.14,
6
7

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1
30.31, 128.17, 127.21, 126.27, 125.95, 125.67, 124.58, 122.98, 120.93, 18.48; HRMS (ESI):
+
calcd. For C H N S [M+H] 611.2225 found 611.2297. (See Supp. data S13, S14, S15,
3
8
34
4 2
S16)
2
.2.5 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(4-nitrophenyl)thiourea)-Additive-5
1
-isothiocyanato-4-nitrobenzene 0.824g (2.2 equiv.) with 0.5g (1 equiv.) 3,3′,5,5′-
tetramethylbenzidine was stirred in 40 ml of anhydrous methanol at room temperature for 2-3
days. The product obtained was a white precipitate which was then filtered, washed with
methanol and dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-
diyl)bis(3-(4-nitrophenyl)thiourea) as a white solid with 68% yield. The corresponding
1
scheme is depicted in Fig. S17. H NMR (DMSO-d , 500 MHz)
δ (ppm): 10.09 (br s, 2H,
6
2
1
1
NH), 9.52 (br s, 2H, 2NH), 8.24–8.22 (m, 4H), 7.98–7.96 (m, 4H), 7.43 (s, 4H), 2.30(s,
1
3
2H); C NMR (DMSO-d6, 125 MHz)
36.02, 126.12, 124.56, 120.88, 120.80, 18.22; HRMS (ESI): calcd. For C H N O S
4 2
δ (ppm): 180.13, 146.42, 142.03, 138.56, 136.69,
3
0
28
6
+
[
M+H] 601.1613 found 601.1687. (See Supp. data S17, S18, S19, S20)
2
.2.6 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(3,4,5-trimethoxyphenyl)thiourea)-Additive-6
5
-isothiocyanato-1,2,3-trimethoxybenzene 1.03g (2.2 equiv.) with 0.5g (1 equiv.)
3
,3′,5,5′-tetramethylbenzidine was stirred in 40 ml of anhydrous methanol at room
temperature for 2-3 days. The product obtained was a white precipitate which was then
filtered, washed with methanol and dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-
[1,1'-biphenyl]-4,4'-diyl)bis(3-(3,4,5-trimethoxyphenyl)thiourea) as a white solid with
1
7
5% yield. The corresponding scheme is depicted in Fig. S21. H NMR (DMSO-d , 500
6
8

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MHz)
δ
(ppm): 9.86 (br s, 2H, 2NH), 8.93 (brs, 2H, 2NH), 7.38 (s,4H), 6.81 (s, 4H), 3.77(s,
1
3
1
8H), 2.23 (s, 12H); C NMR (DMSO-d , 125 MHz) δ (ppm): 179.83, 152.78, 136.75,
6
1
36.31, 134.79, 134.57, 125.93, 121.19, 101.23, 60.03, 55.77, 18.33; HRMS (ESI): calcd. For
+
C H N O S [M+H] 691.2546 found 691.2622. (See Supp. Data S21, S22, S23, S24)
3
6
42
4
6 2
2
.2.7 Synthesis of 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-
(6-methylpyridin-2-yl)thiourea)-Additive-7
2
-isothiocyanato-6-methylpyridine was synthesized by treating 2-amino-6-methyl
pyridine (1 equiv.) with thiophosgene (1.1 equiv.) in a mixture of CH Cl and saturated
2
2
aqueous solution of NaHCO (1:1 v/v) at 0°C for 2 h. Then, the organic layer was separated
3
and evaporated to give the product as a pale yellow solid with 92% yield. Generally, the
isothiocyanates vary instability so it is used in the next step without further purification.
2
-isothiocyanato-6-methylpyridine 0.687g (2.2 equiv.) with 0.5g (1 equiv.) 3,3′,5,5′-
tetramethylbenzidine was stirred in 40 ml of methanol at room temperature for 2-3 days. The
product obtained was a white precipitate which was then filtered, washed with methanol and
dried under vacuum to obtain 1,1'-(3,3',5,5'-tetramethyl-[1,1'-biphenyl]-4,4'-diyl)bis(3-(6-
methylpyridin-2-yl)thiourea) with 85% yield. The corresponding scheme is depicted in Fig.
1
S25. H NMR (DMSO-d , 500 MHz)
δ (ppm): 13.13 (br s, 2H, 2NH), 10.88 (br s, 2H, 2NH),
6
7
.74-7.70(t, 2H), 7.44 (s, 4H), 7.07-7.05(d, 2H), 6.96-6.94(d, 2H), 2.42 (s, 6H), 2.29 (s, 12H);
1
3
C NMR (DMSO-d , 125 MHz)
δ
(ppm): 179.62, 154.71, 153.27, 139.51, 138.41, 136.31,
6
+
1
35.75, 126.13, 117.22, 109.57, 23.59, 18.45; HRMS (ESI): calcd. For C H N S [M+H]
30 32 6 2
5
41.2130 found 541.2206. (See Supp. data S25, S26, S27, S28)
2
.3 Preparation of thiourea doped gel polymer electrolytes (GPE)
9

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The GPE’s were prepared by blending hydroxylpropyl cellulose (0.300g), Co(II)phen
(0.030g), Co(III)phen (0.030g), propylene carbonate and ethylene carbonate (1:1 ratio) (1 g)
and synthesized organic additive (0.010g) at 100°C until homogenous mixture was obtained.
The mixture was cooled to ambient temperature to obtain the corresponding GPE. The
undoped GPE were tested for DSSC applications and the obtained data was compared with
the doped GPE electrolytes system blended with organic additives 1-7. The GPE images are
represented in Figure 2.
2
.4 Fabrication of DSSC
The DSSC using GPE were fabricated using the nanocrystalline TiO cast on
2
fluorinated tin oxide (FTO) conducting glass by the reported procedure [35]. The TiO coated
2
electrodes are drenched for 24 hours in
a
methanol solution containing
ditetrabutylammonium cis bis (isothiocyanato) bis (2,2′bipyridyl4,4′dicarboxylato)
ruthenium(II) N719 dye and it formed the working electrode of the DSSC. The active area of
2
DSSC was 1 cm (1cm x 1cm). The GPE with thiourea derivative was placed between the
N719 dye coated TiO (working-electrode) and platinum (counter-electrode). Decisively,
2
DSSC (sandwich type) was fabricated by holding both the electrodes together with the use of
alligator clips. Further, all the measurements were performed at room temperature.
2
2
.5 Computational Details and Methods
.5.1. Gas Phase Optimization
The thiourea derivatives of tetramethyl benzidine containing different functional
groups were studied. The experimentally synthesized organic compounds were analyzed
using the computational investigation to study the efficiency of the molecules for dye-
sensitized solar cell applications. The geometry optimizations (i.e. gas phase) at the ground
1
0

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state were done using density functional theory (DFT) approach, Becke’s three-parameter
hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) [36]. Earlier
studies have shown that the DFT-B3LYP method is more suitable to predict the electronic
properties of the large size organic molecules [37,38] and the same with 6-311++G** basis
set was employed for all molecule optimization [39,40]. All electronic structure calculations
were performed using GAUSSIAN 16 and GaussView 6.0 and Chemcraft software’s were
used for visualization [41].
Initial coordinates were built from GaussView 6.0 and gas phase optimization
followed by frequency analysis was done at B3LYP/6-31G* method. Then we used these
coordinates for the high-level electronic structure calculation. The energy gap (E ) between
g
the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) are calculated from optimized geometries. This analysis can help to understand the
electronic transition between HOMO-LUMO and the calculated value can be used to study
the effect of functional groups present in the side chain. The lower the E easy to transfer the
g,
electron from HOMO to LUMO for DSSC application [42].
2
.5.2. Organic Molecules at TiO Surface
2
Optimized geometries of organic molecules were used to study the adsorption behaviour of
different orientation of these molecules at TiO anatase (101) surface. Since the titania
2
normally used is foremost anatase. Thus the molecules were placed over the surface
particularly and were freely optimized at TiO surface to predict the different properties such
2
as adsorption energy (E ) and shortest interacting distances (hydrogen bonding, vdWs, π-π
ad
stacking to the surface). In addition, we have also calculated Fermi energies for TiO after the
2
adsorption of additives. All the geometry optimizations were done using CP2K/Quickstep
code [43]. All DFT calculations are based on hybrid GPW (Gaussian plane wave) [44] and
1
1

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PBE (Perdew- Burke- Enzerhof) [45] exchange-correlation functional. A plane wave cut off
of 600 Ry was used. Grimme D3 [46,47] method is employed for dispersion corrections to
study the effect of dispersion. For atoms Ti and O we have used DZVP (Double- Zeta
Valance Polarized) [48] and for all other atoms, we used TZVP (Triple-Zeta Valance
Polarized) [49] basis sets. After optimization, the adsorption energies and Fermi energies were
calculated using the same method.
The adsorption energies, shortest N-H····O hydrogen bond distances, the shortest
distance between the aromatic ring and TiO surface further Fermi energies were analyzed
2
and are given in Table 2. Adsorption energies are calculated using Equation 1,
E = - {E – [E
+ E(Surf)]}Equation (1)
ad
(TE)
(Additive)
Here E denotes the adsorption energy of organic molecule on TiO surface, E stands for
ad
2
TE
the total energy of the complex system, EAdditive and Esurf are the energy of individual organic
molecule and TiO surface, respectively.
2
2
.6 Characterization
2
+/
3+
The undoped and doped thiourea derivative GPE’s with Co Co are subjected to
electrical, vibrational, absorption, diffraction and photovoltaic characterization. The FTIR
spectrum was recorded using FTIR, SHIMADZU IR TRACER with the frequency range of
-1
4
00-4000cm . The absorption band due to Co(II/III)phen with GPE and organic additives in
DMF were measured by using UV-Vis spectrophotometer ANALYTIK JENA, SPECORD
10 plus. Further, conductivity was evaluated by the impedance analysis using FTO glass
2
slide as electrodes and rubber O-ring as separator under Biologic SP-300. The X-ray
diffraction patterns were recorded using PANalytical x’pert powder X-ray diffractometer
using CuKα radiation. The NETZSCH DSC 204 F1 Phoenix instrument was used to analyze
1
2

ACCEPTED MANUSCRIPT
the differential scanning calorimetry (DSC) plot and understand the thermal response of the
GPE. The current-voltage curves were measured under BAS100A electrochemical analyzer.
Under standard filter conditions by utilizing 150 W tungsten halogen lamp (OSRAM,
1
13
Germany) as light source under 1.5 AM illumination. H and C NMR spectra were recorded
using BRUKER 500 MHz. HR Mass spectra were recorded using LC/MS, 6230B Time of
Flight (TOF), Agilent technologies.
3
3
. Results and Discussion
.1.1 Ionic Conductivity
The examination of the resistance of the samples was done by potentiostatic
electrochemical impedance spectroscopy technique in the specific nyquist plot, at room
temperature. The scan range was fixed from 500 kHz to 300 mHz and it was observed in
single sine mode to set Ewe to E of GPE as 0.800V. The resistivity was obtained well enough
from the intercept on the real axis at a higher frequency. It is shown in Fig.S29 (See Supp.
data). Further, the conductivity was measured from the resistance since both are inversely
related.
The formula utilized is as follows:
σ = t / (R A)
b
Where,
t is the thickness of the gel polymer electrolyte,
A is the area of the sample’s surface,
R is the bulk resistance of the material
b
1
3

ACCEPTED MANUSCRIPT
Figure 3 provides the electrical conductivity of the undoped GPE and seven additives
-5
-1
with GPE and cobalt redox couple in this study were calculated to be of 5.04 x10 Scm ,
-
4
-1
-4
-1
-4
-1
-4
-1
-4
-1
1
1
.36 x10 Scm , 1.18 x 10 Scm , 1.37 x 10 Scm , 1.05 x 10 Scm , 1.02 x 10 Scm ,
-
4
-1
-4
-1
.43 x 10 Scm , 1.16 x 10 Scm . It is noted that the 1,1'-(3,3',5,5'-tetramethyl-[1,1'-
biphenyl]-4,4'-diyl)bis(3-(3,4,5-trimethoxyphenyl)thiourea) (additive 6) reached the
-4
-1
highest ionic conductivity of 1.43 x 10 Scm which is higher than the undoped GPE by
almost one degree of magnitude which clearly imply that due to the effective redox couple
pairing with the polymer and the additives. The ionic conductivity increased upon the addition
of the additives with the HPC under ambient temperature. Hence conductivity gets strongly
boosted by the presence of a well-conjugated system with thiocarbamide linkage group and
electron donating groups present in the organic additive which exalt the mobility and
elasticity of ions. Even though by the addition of the remaining additives, the conductivity
exalted due to the presence of highly electron donating groups followed by electron
withdrawing groups when compared with the undoped GPE.
3
.1.2 Electrochemical Impedance Spectroscopy
EIS spectroscopy is the predominant technique to analyse the interfacial and electrical
properties of the electrolytes. The nyquistic plot and equivalent circuit of the polymer
electrolytes are used to scrutinize the chemical capacitance and charge transfer resistance
which isobtained using the fitting from the EIS spectra. The Impedance spectra of DSCs are
measured in dark at a potential of 0.8mV close to V . Generally EIS of DSCs contains three
oc
distinct arcs, initial and the light independent high frequency arc corresponds to the
electrolyte and the Pt interface, the second arc and the light-dependent mid frequency arc
corresponds to the electrolyte interface with TiO and the dye which is one of the vital
2
characteristics to improve the DSCs performance which has direct effect on V of the cells.
oc
1
4

ACCEPTED MANUSCRIPT
The final low-frequency arc corresponds to the Warburg resistance. The EIS obtained using
DSC setup is shown in Figure 4. The equivalent circuit used for interpreting impedance is
shown in Figure 5. Where Rs is the series resistance and C1, R1 is the capacitance and
charge transfer resistance in the Pt-electrolyte interface. C2 and R2 is the capacitance and
charge transfer resistance in the TiO and electrolyte interface, W is the Warburg resistance.
2
The RCT value tends to increase on implementing additives to the pristine polymer
electrolytes indicating the reduction of interfacial recombination in the electrolytes and
improving the V of the cells rapidly. R and C were also obtained at different bias
oc
CT
µ
potential to evaluate the electrochemical properties of the electrolytes [50]. It is clearly shown
in Figure 6 and 7. The electron density of states of the TiO can be determined from Cµ
2
which is an indirect measure of the Fermi level and was found to be increasing with the
applied potential. The additives added gave an upward displacement in the TiO CB by
2
adsorption which was proved by the displacement in C values. The displacement in the CB
µ
was attributed by the electron-donating and withdrawing groups present in the molecules. As
discussed earlier in computational studies the molecule possessing more electron-donating
groups like additive 6 was adsorbed more on the TiO which resulted in better FF, V and J
2
oc
sc
of the cells. The shift in chemical capacitance and conduction band of the semiconductor will
reduce the electron diffusion rate and the electron injection efficiency which has a direct
effect on the J . The upward shift in the TiO can also be due to the usage of cobalt redox
SC
2
shuttle rather than potassium or sodium iodide salts which are well known for downward
displacement in TiO CB [51,52]. The J of the cell was also improved by the addition of
2
sc
additives due to the increase in the ionic conductivity and mobility in the HPC polymer
matrix. And thus the impact of additives on the solar cell parameters was clearly proved by
the means of theoretical and experimental studies.
3
.2 Differential Scanning Calorimetry and Diffraction data
1
5

ACCEPTED MANUSCRIPT
DSC characterization of the GPE’s was measured and the range was fixed between -
4
0°C to 300°C. The heating rate is 10 K/min. The thermogram shows decomposition
temperature (T ) of 186°C respectively for undoped GPE. Rather than it shows the blunt
dc
peak at 126°C and 60°C. As seen in Figure 8 (Additive 1-7) the decomposition temperature
varied 185, 189, 186, 200, 182, 181 and 183°C due to the incorporation of the organic
additives, cellulose and cobalt redox electrolyte. Even it attains the new decomposition
temperature in a distinct range like 144, 143, 142, 157, 171, 142 and 138°C.
2
+/
3+
X-ray diffraction peaks were examined for Co Co GPE doped with newly
synthesized thiourea derivatives and undoped GPE. It is depicted in Figure 9. It clearly
shows that electrolytes are more amorphous in nature due to its gel consistency. As seen in
Figure 9 (Additive 1-7), it shows a broader peak at 30° which confirms that the additive 1-7
well interacts with GPE and increases the electrolyte ability and conductivity. In addition, all
the eight GPE the crystalline peak is not observed even after the summation of organic
additives. It ensures that the organic additives are well jumbled with the Co(II/III)phen and
cellulose. This result also holds good with conductivity studies.
3
.3 Vibrational and Absorption spectra
The vibration spectrum of the cellulose, Co(II/II) redox with/without organic
additives were analyzed by the attenuated total reflectance infrared (ATR-IR) spectrum at
-1
room temperature. The FTIR spectrum scanning range is from 400 to 4000cm . It is shown
-1
in Figure 10. The broad transmission band at 3600-3200cm is due to the stretching of the
hydroxyl groups in HPC can be remarkably distinguished. It can be seen that the peak
intensity and the peak shape were clearly different for undoped GPE apart from the addition
of organic additives with HPC and these differences were induced by the different organic
compounds i.e., thioureas. The presence of hydrogen-bond could be inferred from the peak
1
6

ACCEPTED MANUSCRIPT
shape and intensity of the absorption band of the hydroxyl stretching vibrations in the FTIR
−
1
spectrum. C=S stretching is observed at 1200cm and the C=O from PC and EC were at
−
1
-1
1
650cm . The broadband in the undoped HPC is maximum at 3471 cm , was assigned to
−
1
stretching vibrations of the -OH groups and pyranose unit. The bands at 2930cm and
−
1
-1
2
863cm are due to CH and CH stretching vibration. Interestingly the peak at 2930 cm get
2
-1
sharpened and the peaks at 1799cm get shortened these changes clearly shows that organic
−
1
additive gets infused well with Co redox pair and HPC. The absorption band at 1503 cm is
−
1
due to C=C stretching vibration and at 1073 cm is due to C-O-C stretching vibration which
-1
is the most important part of cellulose. The peaks at 1090 cm get broadened in most of the
gel polymer blends which is due to the coordination between –C-O- in HPC and with the
-1
additives. In Figure 10, the vibrations for additive 1-7 with HPC between 640 to 880cm
region gets shrunk this shows that there is a good linkage of the synthesized organic dopant
with hydroxypropyl cellulose, Co(II) and Co(III).
UV-Vis absorption of the GPE’s was analyzed for all the eight samples in DMF at
room temperature. It is explicated in the Fig.S30 (See Supp. data). The absorption band at
2
68 is due to undoped GPE. Organic additives which are added to the undoped GPE got the
absorption peak as follows: 266, 273, 272, 267 and 269 for additive 1 to 4; 268, 310, 381 for
additive-5; 269 for additive-6; 266 and 300 is due to additive-7. The different absorption
peaks were observed for different functional group attached elucidated clearly the uniform
blending of the polymer with the additives. These results clearly imply that the synthesized
organic additives are based on transitions of n or π electrons to the π* excited state. Since the
experimentally obtained absorption peaks match with the general aromatic π-π* and n-π*
transition wavelength i.e., 200-700 nm. And it even shows worthwhile that there is no impact
of an unsaturated group in the molecules. It is further analyzed through DFT studies with
HOMO LUMO calculations typically for organic additives at gas phase.
1
7

ACCEPTED MANUSCRIPT
3
.4 Computational details of band edge shift and its adsorption process
Table 1 provides the HOMO-LUMO gap (Eg) for all additives calculated from DFT
method. It was found that experimental efficiency order data is in very good agreement with
our calculated Eg values. In order to check the charge transfer phenomena in organic
additives, we have studied frontier molecular orbital (FMO) analysis. Fig. S31 (See Supp.
data) shows the different FMOs of additives considered for the composite materials. This
clearly reveals that there is a long-range charge transfer through aromatic π-π* and n-π*
transition. It can be seen that HOMO orbital always localized in the –S atom irrespective of
the electron donating/withdrawing group in the side chain.
The FMO analysis is done using the same functional and basis set (B3LYP/6-
3
11++G**). Orbitals such as HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2 were
analyzed and electron density pictures were taken. From this, we get a clear picture of how
the charge transfer takes place in different orbitals. From the Eg (Table 1) it was found that
additive 5(-nitro) has the lowest energy gap (Eg) and hence will show a better charge
transferability when compared with other additives. The order of magnitude of Eg values
obtained from DFT methods is close agreement with experimental efficiency values. The
calculated Eg order with respect to the functional group present in the side chain is –
pyridine>–chloro>–butyl~>trimethoxy>–methoxy>–naphthyl>–nitro. Here we found that the
role of functional groups in organic linker plays a vital role in the charge transfer through π-
clouds.
From Table 2, it is clear that the adsorption is more for electron donating groups and
is decreasing for electron withdrawing groups. Adsorption of linker on TiO surface reveals
2
that the electron releasing groups were strongly adsorbed when compared with electron
withdrawing groups. It is found that –(OMe) , –butyl and –OMe groups are strongly adsorbed
3
1
8

ACCEPTED MANUSCRIPT
and the calculated E is varied from 418.0 to 450.0 kcal/mol. Other additives have less E ,
ad
ad
particularly additive 5 (nitro) have very less E (i.e.275 kcal/mol). Additive 2 (chloro) have
ad
moderate binding with TiO surface. It can be seen from the optimized geometries of the
2
complex that there are various types of non-covalent interactions present (such as hydrogen
bonding, vdWs, π-π stacking to the surface). These interacting distances are shown in Figure
1
1. The shortest H-bond distances between –NH group of additives and O of TiO surface is
2
shown in Figure 11 and also the shortest distance is given in Table 2. This H-bond shows the
effective interaction between the molecule and the TiO surface. Also, we found that
2
aromatic ring perfectly adsorbed on the surface, this is a favour for the complex stability.
From the optimization steps, it is clear that the aromatic ring and –NH group orient itself
towards the TiO surface irrespective of the functional group present in the side chain. It
2
helps the additives to have a better adsorption on the surface. But the adsorption energies
vary based on the functional groups present as discussed above. Also, the Fermi energies are
explained in Table 2. Additive 3 and additive 6 has lesser Fermi energies compared to other
additives and thus an easy charge transfer after adsorption on the surface. It is important to
note that these two additives are strongly bound to the surface. From the above-mentioned
results, it is clear that strongly adsorbed additive have more charge transfer.
3
.5 Photovoltaic effects
The current-voltage (I-V) curve of DSSC fabricated cell using GPE’s is measured by
2
its photovoltaic achievement at 1.5 AM under sunlight irradiation of about 70mW/cm and it
is illustrated in Table 3. The corresponding I-V curve was depicted in Figure 12. The
illustration of gel polymer electrolytes was carried away without any special sealing to value
its concordance. The short circuit current (J (mA)), open-circuit voltage (V (mV)), fill
sc
oc
factor (ff) and energy conversion (ŋ) data attained were listed in Table 3 clearly. Out of all
1
9

ACCEPTED MANUSCRIPT
GPE’s the one with more electron donating groups (OMe) i.e., additive 6 resulted with the
3
highest efficiency of about 9.1%. Since the overall order of efficiency for all the remaining
seven GPE’s goes on from electron donor groups ends up with an electron withdra4wing
group. Figure 13 denotes the error bar for the efficiency which shows the reproducibility of
GPE’s efficiency for all the samples. It visibly implies that all the GPE’s are stable well
enough and retain its liable reproducibility for several hours. The utilized cobalt redox pair
-
-
can attain a V of above 1V more than I /I and promoted the efficiency by its more positive
oc
3
redox potential. And furthermore, the importance of less light absorption and less disruptive
nature on the metal surfaces by cobalt redox couples in DSSCs can improve the efficiency
[
53,54].
4
. Conclusion
In brief, the results revealed that symmetrical thioureas with more electron donating
groups are the most stable and possess high adsorption energy towards the TiO . These
2
additives shifted Fermi level of TiO (101) anatase. In addition, Cobalt redox pair escalated
2
the efficiency by its predominating properties. HPC played the vital role in the electrolyte for
boosting up the performance of the DSC by its excellent ionic mobility and conductivity. The
results exemplified that the additives with more electron donating groups i.e., additive 6
(
OMe) seems to the best. It is believed that this opens the way to synthesize a variety of
3
thioureas with numerous donating groups and engage DSSC with a wise band shift values on
working electrode leads to high open-circuit voltage, low photocurrent density and even
higher efficiencies in GPE. Thioureas usually have very high thermal, chemical stability and
can be utilized as a robust additive. Thus employing these additives gave a very good
efficiency in the range of 5.59-9.1%.
Conflicts of interest
2
0

ACCEPTED MANUSCRIPT
There are no conflicts to declare.
Acknowledgement
The authors sincerely thank the Department of Science and Technology (DST)
Fast Track, New Delhi (CS-378/2013). The authors also thank SRM IST for providing HRMS
facility.
Appendix A. Supplementary Data
1
13
Supplementary data containing Schemes, H NMR, C NMR, HR MASS and UV data
related to this article can be found at doi.
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List of Figure captions:
Figure 1: General Scheme for the Synthesis of Organic Additives.
Figure 2: Samples of Gel Polymer Electrolytes
Figure 3: Ionic conductivity for Gel Polymer Electrolytes at Room Temperature
Figure 4: Electrochemical Impedance Spectra for the DSSCs Composed of Gel Polymer
Electrolytes
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