Novel indole-based photosensitizers coupled with PEG-HEC quasi-solid-state electrolyte to improve energy conversion and stability of organic dyes based-dye sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2021.138771

A series of novel Indole based organic dyes were synthesized and coupled with a Poly-ethylene glycol-Hydroxy ethyl cellulose quasi-solid-state-electrolyte (PEG-HEC QSSE) polymer blended iodine electrolyte to obtain a metal-free dye-based dye-sensitized so

Full text of DOI:10.1016/j.electacta.2021.138771

Electrochimica Acta 389 (2021) 138771
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Novel indole-based photosensitizers coupled with PEG-HEC
quasi-solid-state electrolyte to improve energy conversion and
stability of organic dyes based-dye sensitized solar cells
Kamaraj Santhosh, Shanmugam Ganesan∗, Selvaraj Balamurugan
Advanced Organic Laboratory, Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel Indole based organic dyes were synthesized and coupled with a Poly-ethylene glycol-
Hydroxy ethyl cellulose quasi-solid-state-electrolyte (PEG-HEC QSSE) polymer blended iodine electrolyte
to obtain a metal-free dye-based dye-sensitized solar cell (DSSCs) with better stability. The newly syn-
thesized organic dyes were confirmed through 1H, 13 C NMR, CHN analysis. The optical and electro-
chemical properties of the dyes were determined using UV-visible spectrometry and cyclic voltammetry
techniques. A quasi-solid state (QSS) blended polymer PEG-HEC with iodide redox couple as electrolyte
was characterized via Field emission Scanning Electron microscope (FE-SEM), UV-visible spectroscopy,
XRD and DSC. A foremost comparative study was analysed on DSSC devices with the newly synthe-
sised organic dyes (DPC, DTC, DPCY, DTCY, DPTCY and DTTCY) coupled with the QSSE. The electrochem-
ical impedance spectrum was discussed for all the six dyes coupled with PEG-HEC QSSE, among that
DTTCY sensitized device exhibits good ionic conductivity (1.13 × 10−3 S cm−1), high chemical capacitance
(Cμ=17.01 μF), high recombination resistance (53.97 Ω) and lower charge transfer resistance (2.35 Ω) due
to the presence of a strong electron-withdrawing group in the organic dye and also QSS behaviour of
electrolyte. The longer electron lifetime (τn-58.51 ms) and better diffusion coefficient (4.25 × 10−7 cm2
Received 30 December 2020
Revised 21 May 2021
Accepted 10 June 2021
Available online 13 June 2021
Keywords:
Indole-based organic photosensitizer
Quasi-solid-state DSSCs
PEG-HEC blend-polymer
Interfacial studies
Better stability
s
−1) of the device prove the short circuit current (Jsc= 16.39 mA cm−2) and enhancement of power con-
version efficiency (PCE) of 6.24 % with better stability under AM 1.5 G illumination (100 mW cm−2). The
combination of new Indole based organic dyes with PEG-HEC QSSE paved a way to improve the PCE and
stability of organic dyes based DSSCs.
© 2021 Elsevier Ltd. All rights reserved.
1. Introduction
the DSSCs is a sandwich-like structure comprised of working elec-
trode, electrolyte and counter electrode. In general, DSSCs works
In the current scenario, energy utilisation in human life is a sig-
nificant part and there is a demand for available resources. So-
lar energy is one of the clean, renewable resource, eco-friendly
and thus played as alternative energy compared to all other tra-
ditional resources.[1] Among other photovoltaic technologies today
researchers are emerging towards DSSCs as an alternate for silicon-
based solar cell technologies.[2] DSSCs also termed as photo-
electrochemical solar cell, where photo-induced reaction held on
semiconducting electrode through an electrochemical modulate
cell.[3] DSSCs show advantages in utilising solar energy to convert
it to electrical energy at low-cost, simple fabrication, the ambient
condition is enough to harvest energy and environment-friendly
character under indoor and outdoor conditions.[4] The structure of
upon illumination of solar light, initially, the dye molecules got ex-
cited by absorbing photons and the excited electrons have injected
into the conduction band of the semiconducting surface (photo-
anode). The transported electrons flow to the counter electrode
(usually platinum coated on the conductive substrate) through the
external circuit. At this juncture, the oxidation-reduction reactions
also happened in the electrolyte mediator that contains R/R+ redox
couple. Through the redox reactions of the electrolyte, the elec-
trons were shuttled back to the oxidized dye molecules and re-
taining the oxidised dye to its original state and these processes
take place vice versa as a cycle [5].
The overall device performance mainly depends on the photons
absorbed by the dye molecules. (e.g., Natural dyes, metal complex
dye, organic metal-free dyes). [6,7,8] The light-harvesting sensitizer
induces the photocurrent, with a low energy gap and suitable en-
ergy level for the electron injection process. [9] Nevertheless, metal
complex dyes are lack in commercial application due to the less
∗
Corresponding author.
E-mail address: ganesans2@srmist.edu.in (S. Ganesan).
https://doi.org/10.1016/j.electacta.2021.138771
0013-4686/© 2021 Elsevier Ltd. All rights reserved.

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
availability in nature and cost-effective. On the other hand, metal-
free organic photosensitizers receive a huge interest and achieve a
maximum PCE of 14 % by using a co-sensitizer. [10,11,12] A push-
pull concept on the organic dyes readily shifts highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy level, which should match the redox potential of
electrolyte and conduction band of photo-anode (TiO2), which fa-
cilitate a dye regeneration and electron injection.
have many advantages such as high thermal and chemical stabil-
ity, better charge carrier mobility, tensile strength, flexibility, less
spillage, good fluidity, excellent dielectric constant, increasing ionic
conductivity and ion transfer rate [31].
In polymer electrolytes based DSSCs, many researchers promote
gel polymer blended techniques that enrich performance and en-
hance the stability by ionic transport of dissociating ions through
the blended polymer matrix. Highly admired polymers utilized
so far in blended gel electrolytes are poly-(vinylidene fluoride)
(PVDF), poly-acrylonitrile (PAN), poly-(ethylene oxide) PEO, poly-
urethane (PU), poly-(vinyl chloride) (PVC) and some other biopoly-
mers. [32,33]. The diffusion of ions into the polymeric chain would
alter the ionic conductivity of the polymer by intermolecular in-
teraction. The blending technique helps to reduce the energy bar-
rier of charge carrier ions, enhancement of dielectric constant, boil-
ing point, subdues melting point and viscosity [34,35]. Typically,
the blending of different polymer matrix favours the suppression
of back electron transfer, increase in electron diffusion length and
charge collection efficiency. [36]
The organic small molecules with Donor-Acceptor (D-A) com-
bination hold some excellent characteristics and intrinsic prop-
erty like molar extinction coefficient (ε), internal charge transfer
(ICT), low molecular weight, polarizability and structural tenabil-
ity. The D-A system generally configures as D-π-A structure (i.e.,
Donor-spacer-Acceptor) that makes some special interest due to
the following characteristics. (i) Facile and versatile in synthetic
approaches, (ii) absorption wavelength can be shifted via structural
modifications and (iii) capable of photo-induced charge separation.
Meanwhile, the metal free-organic dyes will facilitate an intense
light absorption, high extinction coefficient, rigidity and steric hin-
drances. They performed as a photo-anode after adsorbed on TiO2
and allows the injection of photo-excited electrons [13,14,15]. The
D-A system played a vital role in device performance by modifica-
tion in molecular structure with different moiety to fine-tune en-
ergy level and kinetic properties of sensitizers [16,17,18].
Herein, we used poly-ethylene glycol (PEG) and hydroxyethyl
cellulose (HEC) as polymer matrixes for blending due to their gela-
tion behaviour, polar nature and chemically stability, superior me-
chanical and electrochemical behaviour. The blended (PEG-HEC)
gel electrolyte influences the device performance, possesses excel-
lent physical and chemical stability. The long-term stability issue
has sorted out using a PEG-HEC blend polymer matrix with an
I−/I3−redox mediator because of its high charge mobility proper-
ties and unique photoelectrical properties. [37]
In DSSCs huge number of D−π−A) the framework has used
as metal-free organic dye sensitizers ([19,20]. The optoelectronic
property of the (D−π−A) system possesses a wide light-absorbing
capacity due to its donor moiety and the π-conjugation spacer sig-
nificantly alters the HOMO-LUMO energy level which facilitates the
charge transfer. [21] The acceptor moiety binds on semi-conductor
surface and transfer charge during electron injection [22,23]. While
altering the (D−π−A) systems the performance DSSCs was altered
and improved [24]. The incorporation of additional electron-rich
and electron-deficient moiety into the (D−π−A) system results
in efficient internal charge transfer, alter the absorption spectrum
(redshift) and increased open-circuit voltage (Voc) [25,26]. The ex-
tension of the π-conjugation bridge provides an effectual molar
extinction coefficient (є) and better electron delocalization. [27]
In this work, we designed and synthesized a series of novel
Indole based organic dyes in (2D-π-D’-A) and (2D-π-D’-A-π-A)
system that provides low lying band-gap and favourable molecu-
lar energy levels to enhance the PCE of DSSCs. The molecular en-
gineering of (2D-π-D’-A), (2D-π-D’-A-π-A) arrangement involves
(bis-N, N’-dimethylaniline unit is an isolated Group (IG) and a
planar structure. The molecular stretch between two conjugated
units via phenyl and thienyl group will result in excellent charge-
transfer properties and red-shifted absorption [28]. Further im-
provement in charge mobility, electron delocalization by indole
moiety, holding nitrogen atom as lone pair into the π-conjugated
system [29] and pushes the electrons towards acceptor end. The
effective charge separation acquired through electron-deficient ac-
ceptor units such as carboxylic acid, cyanoacrylic acid belong to
(2D-π-D’-A) architecture and (2D-π-D’-A-π-A) arrangement was
designed by altering anchoring group such as (E)-2-cyano-3-(5-(1-
cyanovinyl)thiophen-2-yl)acrylic acid.
The present work addresses improvising i) The PCE of DSSCs
using the well-organised structure of the indole-based small or-
ganic molecule as photo-sensitizer, ii) The stability of DSSCs by in-
corporating polymer matrix in the electrolyte portion. The simple
and systematic procedure were designed as a (2D-π-D’-A), (2D-π-
D’-A-π-A) arrangement at low cost and high yield. The proposed
structures of the indole based organic dye has labelled as DPC,
DTC, DPCY, DTCY, DPTCY and DTTCY respectively. These sensitizer
molecules are the combination of bis-N, N’-dimethylaniline unit
and indole unit as electron-donating nature and the electron re-
lays carried through of phenyl or thienyl spacer groups as neutral
groups at the para-position, lastly, the collected electron has trans-
mitted through an identical auxiliary group possess both electron-
withdrawing nature and anchoring tendency (i.e.) carboxylic acid
(labelled as DTC and DPC), cyanoacrylic acid (labelled as DTCY and
DPCY), (E)-2-cyano-3-(5-(1-cyanovinyl)thiophen-2-yl)acrylic acid)
(labelled as DTTCY and DTPCY). The QSSE has prepared by blend-
ing PEG and HEC polymer with iodide redox couple and additive.
The correlation study has carried on these six newly synthesized
organic dyes which coupled with an I−/I3−PEG-HEC QSSE. The pho-
tovoltaic performance, interfacial studies, electron lifetime and sta-
bility of all six dyes have determined to find a better performing
organic sensitizer coupled with polymer gel electrolytes.
2. Experimental
2.1. Materials
The electrolytes also played a major role in the improvement of
PCE of DSSCs via proper regeneration of dyes and suppression of
recombination reactions between dye and redox couples. In con-
ventional DSSCs mainly liquid electrolyte has been used, but it
has some major drawbacks like solvent leakage and sublimation of
All the required chemicals and reagents were purchased com-
mercially and used without further purification. The organic sol-
vents were dried with anhydrous sodium sulphate and collected
under reduced pressure. The solar cell setups were purchased com-
mercially from Solarnix, Switzerland.
−
I−/I3 redox couple. This liquid electrolyte has replaced by intro-
ducing non-volatile and long-term stable polymer gel electrolyte
through the incorporation of redox couple into any high conduct-
ing polymer matrix [30]. So far, among the gel polymer electrolytes
the blended polymer electrolytes emerging as a stable and efficient
gel electrolyte for DSSCs. These blended gel polymer electrolytes
2.2. Methods and instruments
The synthesized organic molecules were extracted and puri-
fied using column chromatography with silica gel (100-200 mesh)
2

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 1. Synthetic route to synthesize organic dyes (DTC, DPC, DTCY, DPCY, DTTCY and DPTCY).
packed in thick-walled glass columns using a mixture of petroleum
ether (PE) and ethyl acetate (EA) as eluent. The primary investiga-
tion on separating compound was performed by thin-layer chro-
matography (TLC). The eluent spot of the organic mixture was vi-
sualized using a UV chamber by the light source (λ = 254 and 365
nm). The microcontroller-based melting point apparatus was uti-
lized for recording melting points using open capillaries. Initially,
the functional group confirmations were analysed by IR-Tracer 100
FTIR spectrophotometer (ATR mode). 1H and 13C NMR spectra were
recorded using CDCl3 solvent in BRUKER spectrometer at 400 MHz.
The elemental composition percentage were analysed using the El-
ementar Vario EL III CHNS analyser used to predict carbon, hy-
drogen, nitrogen, and sulphur (CHNS) present in the newly syn-
thesized compounds. The absorption spectra were obtained by
the SHIMADZU UV-3600 spectrophotometer. The cyclic voltamme-
try (CV) and electrochemical impedance experiments were per-
formed within an identically constructed setup connected with a
Biologic SP-300 electrochemical workstation. The photovoltaic per-
formance and incident photocurrent conversion efficiency (IPCE)
was measured with Keithley 2400 source meter and a Science Tech
monochromatic solar stimulator used as light source illumination
of 100 mWcm2 at AM-1.5G to analyze current-voltage characteris-
tics of a testing cell.
pling reaction was held between 1H-indole-3-carboxylic acid and
(4a-4b) in presence of K2CO3 that yields DPC, DTC respectively.
To synthesis DTCY, DPCY, DTTCY, DPTCY molecules, the cou-
pling of 1H-indole-3-carbaldehyde to (4a-4b) by Ullmann coupling
to obtain the product (5a-b) respectively [39]. Moreover, the con-
densation 2-thiopheneacetonitrile along with 5a-b in presence of
base yields 6a-b subsequently undergoes a Vilsmeier-Haack reac-
tion to achieve desire products (7a-b). The resultant aldehyde com-
pounds (5a-b, 7a-b) were subjected to Knoevenagel condensation
using cyanoacetic acid to achieve a higher yield of final prod-
ucts (DTCY, DPCY, DTTCY, DPTCY). All the reactions proceeded ef-
ficiently, affording excellent yields. The intermediates and targeted
molecules were purified using column chromatography and char-
acterised. The structural characterisations of new compounds were
confirmed by infrared spectroscopy, proton-1 and carbon-13 NMR
techniques were shown in Figure ESI S1-S6. The elemental CHN
analysis was summarised in Table S1. The detailed synthesis pro-
cedures of all the organic dyes and their intermediates were given
the supporting information. The characterized results of the final
dye molecules were given as follows,
1-(4-(2,2-bis(4-(dimethylamino)phenyl)vinyl)phenyl)-1H-
indole-3-carboxylic acid (DPC) Brown solid (74 % yield); Mp:
138.8 - 139.8°C; Rf= 0.38 (30% EA/PE); FT-IR (ATR): 2947, 2840,
2347, 1723, 1683, 1545, 1449, 1225, 1112, 915, 774, 740, 664, 628,
527, 489 cm−1 1H NMR (400MHz, (CDCl3,δ in ppm): 3.07 (s,12H);
;
2.3. Synthetic route of the target molecules
6.75 (d, 4H, J = 4 Hz); 6.91 (s, 1H); 7.24-7.35 (m, 6H,); 7.52 (d,
2H, J = 4 Hz); 7.60 (d, 2H, J = 4 Hz); 7.87 (d, 1H, J =8 Hz); 8.40
(s, 1H); 8,95 (d, 1H, J = 4 Hz); 11.07 (s, 1H). 13C NMR (100 MHz,
CDCl3, δ in ppm): 41.3, 109.5, 111.0, 111.7, 119.8, 121.8, 126.0, 126.6,
127.0, 128.7, 128.8, 129.3, 129.5, 133.4, 137.3, 137.9, 145.5, 150.3,
169.3. CHNS analysis for C33H31N3O2: C: 79.01, H: 6.23, N: 8.38.
Found: C: 79.05, H: 6.17, N: 8.46.
The structures of indole based organic molecules are shown
in Fig. 2. We disclosed
a synthetic route of designs strategy
with an optimised procedure from literature. These molecules
were synthesised as shown in Fig. 1. These procedures were
purely based on the commercialisation with a simple structure,
curtail the number of steps, at higher yield and usage of low-
cost catalyst. The multi-step reactions have initiated from 4-
bromobenzaldehyde (1a) and 5-bromothiophene-2-carbaldehyde
(1b) and then transformed to phosphonium salt (3a-3b) through
primary alcohols as intermediates (2a-2b). Further, the phospho-
1-(5-(2,2-bis(4-(dimethylamino)phenyl)vinyl)thiophen-2-
yl)-1H-indole-3-carboxylic acid (DTC): Bright brown solid (78%
yield); Mp: 128.9 - 129.9°C; Rf= 0.42 (30 % EA/PE); FT-IR (ATR):
2768, 2755, 2389, 2195, 1703 1457, 1448, 1363, 1228, 1030, 826,
;
745, 673, 504 cm−1 1H NMR (400MHz, (CDCl3,δ in ppm): 3.07
(s,12H); 6.75 (d, 4H, J = 4 Hz); 6.91 (s, 1H); 7.20 (d, 4H, J = 4
Hz); 7.23-7.24 (m, 2H); 7.32-7.35 (m, 2H); 7.87 (d, 1H, J = 4 Hz);
ꢀ
nium salt (3a-3b) were subjected to Wittig olefination with 4,4 -
bis(dimethylamino)thiobenzophenone using K2CO3 as a base to ac-
quire cross-coupled intermediates (4a-b) [38]. The Ullmann cou-
3

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 2. Molecular structures of newly synthesized organic dyes (DTC, DPC, DTCY, DPCY, DTTCY and DPTCY).
8.40 (s, 1H); 8,95 (d, 1H, J = 4 Hz); 11.07 (s, 1H). 13C NMR (100
MHz, CDCl3, δ in ppm): 41.3, 108.6, 109.5, 111.7, 113.8, 119.8, 121.8,
125.0, 126.3, 126.6, 127.1, 128.3, 128.8, 129.5, 137.8, 143.3, 144.4,
150.3, 153.6, 169.2. CHNS analysis for C31H29N3O2S: C: 73.34, H:
5.76, N: 8.28, S: 6.32. Found: C: 73.98, H: 5.90, N: 8.23, S: 6.07.
(E)-3-(1-(4-(2,2-bis(4-(dimethylamino)phenyl)vinyl) phenyl)-
1H-indol-3-yl)-2-cyanoacrylic acid (DPCY) Red solid (91 % yield);
Mp: 162.3 - 163.3°C; Rf= 0.36 (40 % EA/PE); FT-IR (ATR): 3098,
2886, 2211, 1719, 1652, 1590, 1412, 1247, 1146, 1152, 1025, 974, 812,
MHz, CDCl3, δ in ppm): 41.3, 92.8, 109.5, 110.6, 111.7, 113.8, 117.7,
19.8, 121.8, 125.0, 126.3, 126.6, 127.1, 128.3, 128.8, 129.5, 137.8,
143.3, 144.4, 150.3, 157.4, 159.8. CHNS analysis for C34H30N4O2S:
C: 73.09, H: 5.41, N: 10.03, S: 5.74. Found: C: 73.14, H: 5.49, N:
10.25, S: 5.35.
E)-3-(5-((E)-2-(1-(4-(2,2-bis(4-
(dimethylamino)phenyl)
vinyl)phenyl)-1H-indol-3-yl)-1-cyanovinyl)thiophen-2-yl)-2-
cyanoacrylic acid (DPTCY) Purple red solid (81 % yield); Mp:
168.4 - 169.4°C; Rf= 0.36 (40% EA /PE); FT-IR (ATR): 2213, 1693,
1588, 1497, 1402, 1337, 1279, 1049, 950, 797, 697, 511 cm−1 1H
746, 688, 616, 515 cm−1 1H NMR (400MHz, (CDCl3, δ in ppm):
.
.
NMR (400MHz, (CDCl3, δ in ppm): 3.06 (s,12H); 6.75 (d, 4H, J = 4
Hz); 6.91 (s, 1H); 7.25-7.28 (m, 2H); 7.34 (d, 4H, J = 4 Hz); 7.52-
7.54 (m, 3H); 7.62 (d, 2H, J = 4 Hz); 7.78 (d, 2H, J = 4 Hz); 7.87
(s, 1H); 7.90 (s, 1H); 8,38 (d, 1H, J= 4 Hz); 8.60 (s, 1H); 11.01
(s, 1H). 13C NMR (100 MHz, CDCl3, δ in ppm): 41.3, 92.6, 109.5,
111.7, 113.0, 113.7, 117.7, 118.8, 119.8, 121.8, 126.0, 126.6, 127.0,
128.2, 128.8, 128.7, 129.3, 129.5, 130.0, 133.4, 137.3, 137.8, 141.3,
143.2, 145.3, 150.3, 153.5, 159.8. CHNS analysis for C43H35N5O2S:
C: 75.30, H: 5.14, N: 10.21, S: 4.68. Found: C: 75.02, H: 5.26, N:
10.83, S: 4.21.
3.07 (s,12H); 6.75 (d, 4H, J = 4 Hz); 6.91 (s, 1H); 7.26 -7.29 (m,
2H); 7.34 (d, 4H, J = 4 Hz); 7.51-7.56 (m, 3H); 7.62 (d, 2H, J = 4
Hz); 7.87 (s, 1H); 8.26 (s, 1H); 8,37 (d, 1H, J = 4 Hz); 11.07 (s,
1H). 13C NMR (100 MHz, CDCl3, δ in ppm): 41.3, 92.8, 109.5, 111.7,
113.0, 117.7, 119.8, 121.8, 126.0, 126.6, 127.0, 128.7, 128.8, 129.3,
129.5, 133.4, 137.3, 137.9, 145.4, 150.3, 157.4, 159.8. CHNS analy-
sis for C36H32N4O2: C: 78.24, H: 5.84, N: 10.14. Found: C: 78.16, H:
5.38, N: 10.39.
(E)-3-(1-(5-(2,2-bis(4-(dimethylamino)phenyl)vinyl)t
hiophen-2-yl)-1H-indol-3-yl)-2-cyanoacrylic acid (DTCY) Dark
brown solid (89 % yield); Mp: 205.6 – 206.6°C; Rf= 0.36 (40%
EA/PE); FT-IR (ATR): 3057, 2846, 2801, 2257, 2211, 2107, 1705, 1693,
1533, 1437, 1408, 1321, 1283, 1079, 965, 869, 732, 643, 569, 489
(E)-3-(5-((E)-2-(1-(5-(2,2-bis(4-(dimethylamino)phenyl)
vinyl)thiophen-2-yl)-1H-indol-3-yl)-1-cyanovinyl)thiophen-2-
yl)-2-cyanoacrylic acid (DTTCY) Dark red solid (84 % yield); Mp:
186.8 - 187.8°C; Rf= 0.39 (40 % EA/ PE); FT-IR (ATR): 3031, 2931,
2845, 2219, 1707, 1695, 1564, 1446, 1409, 1341, 1262, 1175, 1011,
937, 811, 752, 697, 626, 517, 469 cm−1 1H NMR (400MHz, (CDCl3,δ
in ppm): 3.07 (s,12H); 6.71 (d, 4H, J =. 4 Hz); 6.91 (s, 1H); 7.20 (d,
cm−1
;
1H NMR (400MHz, (CDCl3,δ in ppm): 3.07 (s,12H); 6.71
(d, 4H, J = 4 Hz); 6.91 (s, 1H); 7.21 (d, 4H, J = 4 Hz); 7.23 (m,
3H); 7.35 (d, 1H, J = 4 Hz); 7.52 (d, 1H, J = 8 Hz); 7.84 (s, 1H);
8.22 (s, 1H); 8,39 (d, 1H, J = 4 Hz); 11.03 (s, 1H). 13C NMR (100
4

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 3. Absorption spectra of DTC, DPC, DTCY, DPCY, DTTCY and DPTCY dye recorded a) in CHCl3 (0.04 mM) b) after absorption on TiO2 surface.
4H, J = 4 Hz); 7.23-7.24 (m, 3H); 7.33 (d, 1H, J = 4 Hz); 7.52 (d,
1H, J = 4 Hz); 7.74 (d, 2H, J = 4 Hz); 7.84 (s, 1H); 7.91 (s, 1H);
8.39 (d, 1H, J = 4 Hz); 8.64 (s, 1H); 11.01 (s, 1H). 13C NMR (100
MHz, CDCl3, δ in ppm): 41.3, 92.6, 109.5, 110.6, 111.7, 113.5, 113.8,
117.7, 118.5, 119.8, 121.8, 125.0, 126.3, 126.6, 127.1, 128.3, 128.4,
128.8, 129.5, 129.9, 137.8, 141.3, 142.3, 143.3, 144.4, 150.3, 153.3,
153.6, 159.8. CHNS analysis for C41H33N5O2S2: C: 71.18, H: 4.81, N:
10.12, S: 9.27. Found: C: 71.03, H: 4.76, N: 10.21, S: 9.50.
the high energy region 340 to 400 nm, attributes in π-π∗ elec-
tronic transitions of the conjugated molecule as a backbone. A
broad peak in low energy region 400 to 500 nm attribute due
to intramolecular charge transfer (ICT) and delocalisation of the
molecule. The absorption maximum (λmax) of dye can be fine-
tuned by replacing π-conjugation relay. While increasing the with-
drawing group on the dye molecules, a wide absorption spec-
trum was attained in the UV-Visible light region. A redshift oc-
curs for both the dyes DPTCY and DTTCY due to their delocali-
sation and better light-harvesting properties. [40]. The electronic
transition between HOMO and LUMO energy level was determined
as bandgap (Egopt) using the λonset value of the optical absorption
band
2.4. Preparation of QSSE
Initially, the liquid electrolyte was prepared via mixing of 0.1
M of potassium iodide and 0.05 M of iodine, 0.6 M 1-methyl-
3propyl imidazolium iodide as an ionic solvent and 0.5 M 4-tert-
butylpyridine as an additive under ambient temperature. Subse-
quently, the gel polymer electrolyte was prepared by in-situ blend-
ing of (PEG-HEC) matrix (total 0.3g in 1:1 ratio) in dimethylfor-
mamide with the prepared liquid electrolyte and further stirred at
60-70°C until homogenous mixture formation. The gelation of the
QSSE image is shown in Fig. 5a
1240
Egopt eV
=
(
)
λonset
According to Lambert-Beer law, the molar extinction coeffi-
cients (ε) of these sensitizers were calculated and tabulated in
Table 1. These organic sensitizer exhibits (ε) in the range of
28,000 to 45,000 M−1 cm−1 comparably higher than ruthenium
complex dye N3 and N719 dyes [41]. The analysis of molar ab-
sorptivity values reveals that DTTCY dye possesses the highest ε
(45177 M−1cm−1) among the series of dyes, due to its elonga-
tion π-conjugation system. Moreover, the optical property of the
sensitizer had investigated after absorbing on the TiO2 surface
and the plot was shown in Fig. 3b. They exhibited a wide ICT
band with a redshift in absorbance maxima on the TiO2 surface,
which attributes to J-aggregation [42]. It occurs due to aromatic
π-π stacking and orient within molecules like head to tail fash-
ion. The strong interactions held between anchoring moieties of
the molecule and TiO2 surface owing to the deprotonation of the
acid group and supports in the electron displacement and im-
proves the performance of DSSCs. An increase in the count of the
electron-withdrawing group (cyano) results in a powerful red-shift
in absorption edges (λonset) around 544 and 666 nm. The energy
bandgap (Egopt) of these indole based dye molecules were acquired
between 2.28 and 1.86 eV respectively. These synthesised dyes
show a broad absorption window towards the near IR region, ef-
ficient electron injection capability and thus beneficial in attaining
higher Voc and PCE for DSSCs.
2.5. Device fabrication
The solar cell setup was commercially purchased from Sola-
ronix in Switzerland, consist of TiO2 coated transparent FTO glasses
plates (2.0 × 2.0 cm2) with an active surface area of 0.36 cm2 as
a photo-anode and a platinum-coated FTO plate as a counter elec-
trode. The photo-anode (active area 0.6 × 0.6 cm2) was immersed
into 0.5 mM (DTC, DPC, DTCY, DPCY, DTTCY and DPTCY) solution
in ethanol for 24h at room temperature. After complete adsorp-
tion of sensitizers on the anode surface, the electrodes were rinsed
with ethanol and dried. The platinum-coated FTO glass plate was
sandwiched by surlyn film as a spacer with alligator clips. The cell
setup was completed by filling the electrolyte in between two elec-
trodes. A black mask with a suitable aperture was used on the de-
vices during the photovoltaic analyses.
3. Results and discussions
3.1. Photo-physical properties of sensitizer molecules
The light absorption ability of DTC, DPC, DTCY, DPCY, DTTCY
and DPTCY sensitizer were investigated using UV-visible absorp-
tion spectra. A linear intense absorption spectrum was recorded
using an aprotic solvent. These organic molecules are record in
C=4.0 × 10−5 M chloroform solutions displays in Fig. 3a and re-
sultant parameters were tabulate in Table 1. A stretched band in
3.2. Electrochemical studies of sensitizer molecules
The cyclic voltammetry studies have performed to determine
the HOMO and LUMO energy levels of newly synthesized or-
ganic dye. The energy levels of dyes were analyzed to find redox
potential suitability towards the TiO2 and electrolyte. A total of
5

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Table 1
Photo-physical and Electrochemical parameters of new synthesized DTC, DPC, DPCY, DTCY, DPTCY and DTTCY
molecule.
Photo-physical properties
Electrochemical properties
Sensitizer λmaxa/nm
εa /M−1cm−1
λ
max/λonsetb/nm
Egopt/eVc
Eox1/Vd
EHOMO /eVe
ELUMO /eVf
DTC
408
383
433
430
495
482
28347
28482
35605
33365
45177
43942
443/556
423/544
488/623
477/608
539/666
515/650
2.23
2.28
1.99
2.04
1.86
1.91
1.09
1.05
0.90
0.89
0.84
0.83
-5.48
-5.44
-5.29
-5.28
-5.23
-5.22
-3.25
-3.16
-3.30
-3.24
-3.37
-3.31
DPC
DTCY
DPCY
DTTCY
DPTCY
a
Absorption maximum wavelength and molar extinction coefficient of dye molecules measured in CHCl3 solu-
tion.
b
Absorption maximum wavelength of dye molecules sensitized on TiO2 surface.
opt
c
Bandgap was determined from λonset using, Eg
/eV = 1240/λonset
.
d
e
Eox1 is the first oxidation potentials respectively compounds;
EHOMO was calculated using the formula, EHOMO = -[(Eox1- E1/2(ferrocene)) + 4.8] eV, given that E1/2(ferrocene) = 0.41
V;
f
opt
Since the reduction process was not observed clearly, ELUMO was calculated by ELUMO = EHMO +Eg
;
Fig. 4. a) Cyclic voltammogram for synthesized organic dyes at a scan rate of 50 mV s−1. b) Energy level diagrams of synthesised organic molecules.
− an
molecules are lesser than the potential level of I−/I3
electrolyte
2.0 × 10−3 M solution of organic molecules in acetonitrile was
drop-cast onto a glassy carbon electrode surface which acts as a
working electrode. The platinum wire and silver wire was served
as a counter electrode and reference electrode respectively. The
measurements were performed using 0.1 M nBu4ClO4 supporting
electrolyte at a scan rate of 50 mV s−1. The calibration was done
using a standard ferrocene/ferrocenium as the redox couple show
in Figure ESI S7. We obtain E1/2(ferrocene). value of ferrocene as 0.48
V. The HOMO energy levels of all compounds were derived from
the first oxidation potential (Eox1) using the following Pommerehne
empirical relation [43,44].
that responsible for efficient the regeneration process of the dye
molecules. The electron transfer from LUMO of indole based or-
ganic molecules should be higher than the conduction band of
TiO2 which permits the efficient photo-generated electron injection
and suppress the back-electron transfer [45]. These results provide
a way to enhance the PCE of DSSCs [46,47]. Since these molecules
exhibit quasi-reversible behaviour, the cathodic process was forbid-
den, and the LUMO energy levels were obtained by the following
formula [48].
ꢀꢁ
ꢂ
ꢃ
ELU MO = EHOMO + Egopt
EHOMO = − Eox1 − E1/2(ferrocene) + 4.8 eV
The first oxidation potential of organic dyes falls in the order,
DTC (1.O9 V) < DPC (1.05 V) < DTCY (0.90 V) < DPCY (0.89
V) < DTTCY (0.84 V) < DPTCY (0.83 V) were shown in Fig. 4a.
These results prove that π-conjugation spacer of phenyl contain-
ing dyes DPC, DPCY and DPTCY showed low oxidation potentials
than the thienyl dyes DTC, DTCY and DTTCY. The shift in the oxida-
tion peak of organic dyes is due to their acceptor units. The lower
oxidation potentials acquired for DPTCY and DTTCY molecules be-
cause of their electron-withdrawing capability of (E)-2-cyano-3-(5-
(1-cyanovinyl)thiophen-2-yl)acrylic acid. The HOMO energy level
of DTC, DPC, DTCY, DPCY, DTTCY and DPTCY compounds were
calculated from the above equation. Their resultant EHOMO val-
ues are -5.48, -5.44, -5.29, -5.28, -5.23, and -5.22 eV respectively.
The low-lying HOMO energy levels of these indole based organic
The calculated LUMO energy levels from the above equations
are -3.25, -3.16, -3.30, -3.24, -3.36 and -3.32 eV corresponds to
opt
EHOMO and Eg
tabulated in Table 1. The energy level diagram of
the dyes was represented in Fig. 4b. The optical band-gap of this
indole based sensitizer lies in the range of 2.28 to 1.86 eV where
their HOMO energy level varied from - 5.48 to -5.22 eV, results
in a gradual increment in photo-current. The sub-electronic energy
levels of dye molecules have altered due to the change in the ac-
ceptor unit, π-configuration group. There is a trivial change in the
replacement of the phenyl to thienyl group spacer because of elec-
tron delocalisation. The change in acceptor nature had possessed a
remarkable improvement in the HOMO-LUMO energy level which
is quite useful in the enhancement of PCE. [49]
6

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 5. a) Photographic image, b) Absorption spectra, c) DSC Thermogram, d) FESEM image and EDAX characteristics, e) XRD spectra of Quasi-solid state gel electrolyte.
3.3. Properties of QSSE
ture, HEC polymer reports the glass transition temperature (Tg) at
120-127°C, crystallization temperature (Tc) peaks at approximately
240°C and the melting temperature (Tm) at 308°C. From DSC ther-
mographs displayed in Fig. 5c, for PEG-HEC QSSE, the phase tran-
sition Tg at 70°C can be identified and this lower Tg was responsi-
ble to attain a suitable electrolyte material. The endothermic peak
was considered as its crystallisation temperature Tc at 167°C, this
possesses free mobility of ions. The exothermic peak was deter-
mined as melting temperature Tm at 370 °C. These results sat-
isfy the lower crystallinity behaviour of GPE and therefore it sup-
ports better charge mobility and conductivity. The amorphous na-
ture of GPE has confirmed by XRD conducted at a scanning rate
of 5o/min over a 2h interval from 10 to 80 degree. In Fig. 5e, PEG-
HEC blended composite has diffraction peaks at 25.9 degrees. From
our observation, the characteristic peaks were amorphous for GPE
and this property may lead to having a higher ionic conductivity.
We introduced a new blending gel polymer electrolyte (GPE) to
improve the stability and PCE of organic dye-based DSSCs. Herein,
biopolymer HEC was blended with PEG to form a stable iodine
redox-based QSSE. The presence of polyethylene glycol in elec-
trolyte facilitates the crosslinking with HEC to form polymer gel,
whereas the presence of redox couple produces a polymer iodine
complex. This will contribute to enhancing the conductivity and
photovoltaic performance of prepared PEG-HEC QSSE. The ionic
conductivity values for GPE are lower than those for the liquid
electrolyte, due to the lower crystallinity that−limits the mobil-
ity of I2 in the polymer matrix. However, I−/I3 ion release from
the polymer iodine complex and can easily accomplish greater ef-
ficiency. The conductivity of PEG-HEC QSSE as 1.55 × 10−3 S cm−1
depends on their amorphous nature, structural morphology and it
may respond to the photovoltaic performance of the device. The
PEG-HEC QSSE. has subjected to further reliable investigation.
3.3.1. Photo-physical properties
A comparison of absorption spectra between Liquid electrolyte
(LE), Gel polymer electrolyte (GPE) and PEG-HEC gel polymer has
presented in Fig. 5b. The blank polymer contains no intense ab-
sorption peak. On other hand, we have observed a difference in
intensity between LE and GPE confirms the attachment of iodolyte
into the polymer matrix. An intense peak appeared at 208 nm 292
3.3.3. Morphology
The morphology of the prepared PEG-HEC QSSE was studied us-
ing a Field emission scanning electron microscope. From Fig. 5d,
we observed that different surface morphologies for bare PEG-HEC
polymer and I−/I3−/PEG-HEC based polymer gel electrolyte respec-
tively. The observed change was primarily due to crosslinking be-
tween the polymer gel and the iodine complex. The uniformity of
dispersed cubic crystal formation on the surface of PEG-HEC based
polymer gel electrolyte imparts improved electrical conductivity to
the electrolyte. The distribution of elements into the prepared gel
polymer matrix has characterized using EDAX. The spectrum shows
the wt% values of elements which are shown in Table and it also
confirms the gelation of the polymer electrolyte. [50]
−
(σ→σ∗) and 363 nm (π→σ∗) belongs to I3 ion of the gel elec-
trolyte. The in-situ reaction held within the GPE and iodine species
enriches the redox behaviour of electrolyte that avoids sublimation
of the iodine ions.
3.3.2. DSC and XRD
The crystallinity of blended polymer gel electrolyte has charac-
terised using differential scanning calorimetry (DSC). As on litera-
7

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 6. a) Rrec and Rpt Values b) Ionic Conductivity and Dapp Values c) Nyquist plots d) Bode phase angle plots for organic dyes/I−/I3−/PEG-HEC.
Table 2
3.4. Ionic conductivity
Electrochemical Impedance Spectroscopy data of DTC, DPC, DPCY, DTCY,
DPTCY and DTTCY organic photo-sensitizer with PEG-HEC polymer gel
electrolytes.
The ionic conductivity of devices with prepared DTC, DPC, DTCY,
DPCY, DTTCY and DPTCY dyes coupled with PEG-HEC QSSE was
calculated from the following formulae, [51]
Devices
Rs/ohm
Rrec/ohm
Rpt/ohm
Cμ/μF
DTC/I−/I3−/PEG-HEC
DPC/I−/I3−/PEG-HEC
DTCY/I−/I3−/PEG-HEC
DPCY/I−/I3−/PEG-HEC
DTTCY/I−/I3−/PEG-HEC
DPTCY/I−/I3−/PEG-HEC
22.78
24.35
22.74
22.57
24.62
24.13
14.32
10.95
26.33
17.73
53.97
49.22
2.09
2.01
2.33
2.23
2.46
2.35
7.61
I
σ =
6.29
11.84
8.15
RbA
Where I, is the distance between the two electrodes, A is the
active surface area of the fabricated device Rb is the bulk resis-
tance obtained from the impedance curve. The ionic conductivity
has calculated and tabulated in Table 3 and shown in Fig. 6a.
The ionic conductivity of GPE was varied from 1.23 × 10−3 to
1.13 × 10−3, upon the influence of organic dye used. The inter-
action between organic dyes /I−/I3−/ PEG-HEC QSSE was health-
ier due to a larger ion transport inside the polymer chains of the
gel electrolytes. These results prove that PEG-HEC blended gel state
electrolyte leads a vital role to maintain the conductivity of organic
dyes-based devices.
17.02
13.63
electrolyte referred to as (Cμ). The obtained Nyquist plots were fit-
ted with an equivalent circuit as shown in the inset of Fig. 6c us-
ing Z-fit software and tabulated in Table 2. The lower RPt resis-
tance values about (2.01-2.46 ohm) for organic dyes/I−/I3−/ PEG-
HEC QSSE implies that there is a lower resistance at Pt/electrolyte
interfaces which favours larger I− ions formation in the devices.
On other hand, especially the four organic dyes contain cyanoacetic
acid as an anchor group coupled with PEG-HEC QSSE attained the
higher Rrec about (17.73-53.97 ohm) and Cμ about (8.15-17.02 μF)
which attributes suppression in recombination reactions between
3.5. Electrochemical impedance spectroscopy studies
−
TiO2 and I3 ion of the devices. These EIS results satisfy that com-
The EIS studies for DSSC devices consist of organic dyes cou-
pled with PEG-HEC blended gel electrolyte were carried out un-
der AC amplitude of 10 mV in the frequency range of 102-106 Hz
at their open-circuit conditions. The resultant Nyquist plot con-
sists of two semicircles corresponds to (RPt) and (Rrec). The charge
transfer resistance (Rpt) is the resistance between the (Pt) counter
electrode/electrolyte interfaces. The resistance occurs during re-
combination TiO2 adsorbed dye molecules/electrolyte interfaces as
charge recombination resistance (Rrec). Rs referred to as the series
resistance. The chemical capacitance between photo-anode and
bination of novel Indole based organic dyes possess a strong ac-
ceptor group was the blended efficiently and reduce the recombi-
nation processes of devices for larger VOC and better stability.
3.6. Electron lifetime and diffusion coefficient
The recombination phenomenon between the novel indole
based organic dyes/I−/I3−/ PEG-HEC QSSE can be understood from
the electron lifetime (τ) which was calculated using Bode phase
8

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Table 3
Ionic conductivity, Diffusion co-efficient and electron lifetime data of DTC, DPC, DPCY, DTCY,
DPTCY and DTTCY organic photo-sensitizer with PEG-HEC polymer gel electrolytes.
Devices
σ (10−3)/S cm−1
Dapp (10−7)/cm2 s−1
fmax/Hz
τn/ms
DTC/I−/I3−/PEG-HEC
DPC/I−/I3−/PEG-HEC
DTCY/I−/I3−/PEG-HEC
DPCY/I−/I3−/PEG-HEC
DTTCY/I−/I3−/PEG-HEC
DPTCY/I−/I3−/PEG-HEC
1.22
1.14
1.22
1.23
1.13
1.15
1.25
1.10
3.20
2.35
4.25
3.51
2.976
3.161
2.838
2.940
2.719
2.778
53.46
50.33
56..06
54.11
58.51
57.27
plot Fig. 6d from the following equation [52],
redox couple, FF was maximum power related to the total resis-
tance of DSSCs. The overall light to electricity conversion efficiency
performance has estimated by the following equation,
1
τn =
.
2π fmax
Jsc × Voc × FF
η =
The fmax is the maximum phase-frequency in the low-frequency
region. The electron lifetime of the new organic sensitizer was dif-
fering from each other due to the properties of internal standard
and blending of PEG-HEC QSSE. The lifetime results were shown
in Table 2 and it reveals the order of electron lifetime is de-
creasing as DTTCY > DPTCY > DTCY > DPCY > DTC > DPC. The
shorter τ value observed for DPC/I−/I3−/ PEG-HEC QSSE indicates
a low charge recombination potential between TiO2/electrolyte
P
in
The overall PCE of a photo-electrochemical cell fabricated with
organic dyes and PEG-HEC QSSCs lies in the range of 1.22- 6.24%.
The ruthenium N3 dye has also fabricated under the same exper-
imental condition, it shows a better efficiency compared to our
synthesised organic dye due to less charge recombination between
the injected electron in the photoanode and the electrolyte. The
improvement in Voc depends on the structures of dyes, which
that the spacer carrying phenyl unit (DPCY, DPTCY) superior to
the thienyl unit contained dyes (DTCY, DTTCY) respectively. This
is mainly due to the low resonance energy of thienyl units com-
pared to phenyl unit carrying dye that results in a lower charge re-
combination rate. However, DSSCs device fabricated with sensitizer
having carboxylic acid as anchoring core DTC & DPC attained PCE
of 1.88 and 1.22 % which was lower while compared to sensitizer
having cyanoacetic acid as anchoring core DTCY & DPCY showed
PCE of 4.86 % and 3.72 % respectively. These results may occur
due to the presence of cyano moiety in the acceptor end, shows
a strong ICT absorption band and harvest higher Jsc. Herein, sensi-
tizer DTTCY & DPCY obtained higher PCE of 6.24 % and 5.92 % due
to the presence of strong electron-withdrawing moieties results in
the redshift of absorption spectra compared with DTCY, DPCY sen-
sitiser. These PCE enhancements were also due to increasing the
count of cyano moiety in the acceptor part of sensitizer readily
enrich electron delocalization between donor and acceptor ends.
The trend set observed in photon to the current conversion effi-
ciency of the synthesised indole based organic dyes with PEG-HEC
QSSCs electrolyte were in the order DTTCY > DPTCY > DTCY >
DPCY > DTC > DPC. The difference shown in PCE with variation
in the anchoring group was due to the binding abilities and charge
transport of synthesised dye on the TiO2 surface. The overall per-
formances of organic dye may occur due to the light-harvesting
capacity of dye and efficient electron injection. The ionic conduc-
tivity and diffusion coefficient values were reliable and justice in
improved Jsc values. The PEG-HEC QSSE associated with good gela-
tion and proper pores filling on the porous TiO2 layer, resulting
in better interfacial contact between the gel polymer electrolyte
and photoanode surface. The PEG-HEC QSSE helps in decreasing
−
interfaces, whereas DTTCY/I−/I3 PEG-HEC QSSE observed longer
τ value readily lowers recombination rate and reliable in anti-
aggregation on the anode surface that improves the Voc. It can be
understood from the lifetime results that the combination of indole
based organic dyes with PEG-HEC QSSE enhances the efficiency as
well as stability owing to the lower recombination process of the
devices [53]. Moreover, the combination of new organic dye ma-
terials with PEG-HEC QSSE increases the electron lifetime of the
device increases leads to the improvement in Jsc and Voc, which
yields significantly enh−anced PCE.
The diffusion of I3 ion into organic DTC, DPC, DTCY, DPCY,
DTTCY and DPTCY dyes towards the platinum electrode has an im-
portant role for DSSCs device. Thus, the Dapp (apparent diffusion
coefficient of Tri-iodide) parameter was listed in Table 3. The en-
hancements of PCE in DSSC are mainly for the QSSE. The calcu-
lated Dapp values for organic dyes/I−/I3−/ PEG-HEC QSSE using the
following equation, [54]
ilim
2nFC0
Where, n- Number of electron transfer from redox pair
L
−
DI
=
3
(n=2), F-faraday’s constant, L-Distance between two electrodes
(L=100μm), C0-Molar concentration of I− (C0=0.05 M) the diffu-
3
sion limiting current density has determined from cyclic voltam-
metry at scan rate 5mVs−1. The diffusion coefficient of QSSE has
−
varied due to the incorporation of I3 ions into polymer matric
that increases I− ion concentration that results in increasing the
Jsc of DSSCs
3.7. Photovoltaic performance
The photovoltaic performance (photocurrent-voltage (I-V) char-
acteristics) of fabricated DSSCs device using N3, DTC, DPC, DTCY,
DPCY, DTTCY and DPTCY as dyes with PEG-HEC based polymer gel
electrolyte has investigated under-stimulated light condition (AM
1.5G illumination of 100 mW cm−2) and demonstrated in Fig. 7a.
The PCE of solar energy to electrical energy has calculated from
short circuit current density Jsc, open circuit photo-voltage Voc, Fill
−
the concentration of I3 ion that suppresses the reaction with in-
jected electron to improve the Voc value of the device.
3.8. IPCE
The incident photon to current conversion efficiency (IPCE) is a
function of particular wavelengths which has studied for obtain-
ing the light-harvesting property and also analyzes external quan-
factor FF, and the incident photon flux (Pin) was 100 mW cm−2
.
The corresponding photovoltaic parameters have summarized in
Table 4. The JSC was directly proportional to the electron collec-
tion efficiency from electrodes, Voc was the potential difference be-
tween the fermi levels of dye molecules and redox potential of the
−
tum efficiency (EQE). The absorption band of organic dyes/I−/I3
/
PEG-HEC QSSE have absorbed in a wider range in the visible re-
gion about 600 nm which records a good photocurrent shown
9

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 7. a) Photo voltaic studies and b) IPCE spectra of sensitized dyes performance for organic dyes/I−/I3−/PEG-HEC.
Table 4
Photovoltaic performance of device by DTC, DPC, DPCY, DTCY, DPTCY and DTTCY organic photo-sensitizer
with PEG-HEC polymer gel electrolytes.
Devices
V
oc/V
Jsc/mA cm−2
FF /%
Ƞ /%
IPCE/%
DTC/I−/I3−/PEG-HEC
DPC/I−/I3−/PEG-HEC
DTCY/I−/I3−/PEG-HEC
DPCY/I−/I3−/PEG-HEC
DTTCY/I−/I3−/PEG-HEC
DPTCY/I−/I3−/PEG-HEC
N3/I−/I3−/PEG-HEC
0.66
0.52
0.68
0.78
0.69
0.75
0.81
0.008
0.002
0.010
0.007
0.006
0.001
0.001
4.82
0.14
0.15
58.84
55.56
59.19
53.40
55.5
0.95
0.18
0.63
0.36
1.88
1.22
4.86
3.72
6.24
5.92
8.08
0.05
0.05
0.03
0.01
0.06
0.05
0.05
14.33
24.01
23.54
26.34
39.84
32.05
—
4.26
12.37
9.08
0.10
0.16
0.21
16.39
13.56
17.73
1.14
0.11
0.51
58.02
56.03
0.93
0.23
The Average values of three different devices, with the corresponding standard deviations. Active area:
0.36 cm2, photo-anode thickness 5 mm.
in Fig. 7b. The QSSE showed the maximum EQE for DTTCY sen-
stability of the devices. The Voc of this setup has stabilised by
protecting the photo corrosion of dye molecules compared to LE.
The corrosive obtained on the counter electrode have reduced and
these synergistic effects inspire to focus more on the development
of stable QSSE DSSCs.
sitizer of 39.84% around 400 nm while others are comparatively
less. EQE of these organic dyes are ordered as follow DPC < DTC
< DPCY < DTCY < DPTCY < DYTCY. The sensitizer with higher
IPCE has maximum light-harvesting properties of DSSCs. The sen-
sitizer DPTCY & DPCY shows higher VOC may due to the pres-
ences of non-planar π-conjugation linkers that reduce the dark
current, improvise light-harvesting and quenched the back-electron
transfer. The lengthening of linear structure in DTTCY & DPTCY
dye may form a dense layer on the TiO2 surface in the presence
of a strong electron-withdrawing unit. Therefore (2D-π-D’-A-π-
A) arrangement has designed to protect electrolyte penetration on
photo-anode TiO2 and hold back charge recombination.
4. Conclusion
In summary, the indole-based organic dyes having a low ly-
ing bandgap (1.86-2.28 eV) with favourable HOMO (-5.22 to -5.48
eV) and LUMO (-3.16 to -3.36 eV) energy levels make them suit-
able candidates for metal-free organic dye materials in QSS-DSSCs.
A correlation study carried on differently structured organic dyes
coupled with a PEG-HEC blended QSSE, to analyse the vital im-
portance of dyes and electrolytes in DSSCs. Among the six organic
dyes, the DTTCY/I−/I3−/PEG-HEC shows an effective charge sepa-
ration associated with the charge transfer absorption influenced
in better photovoltaic performance PCE (η) of 6.24 % with Jsc of
3.9. Photo-stability test
The stability assessment for a fabricated device with organic
dyes/I−/I3−/ PEG-HEC QSSE maintained over a period of 120 h un-
der irradiation of 1 sun illumination. The continuous illumination
of light process on the fabricated cell to estimate Jsc, Voc, FF, and
ƞ values in an interval period at room temperature are correlated
in Fig. 8. The cell setup was quantified using the three devices re-
peatedly. Initially, the Jsc of the dyes is relatively constant, while
Voc are almost unchanged and FF was floated. After some time
photo-voltage got dropped may due break in the covalent linkage
of dye units [55]. As a result, the PCE maintained 90% of the initial
level after 120 h under light condition respectively.
16.39 mA cm−2 Voc of 0.697 V, FF of 55.5%. The PCE improvement
,
was mainly due to the high binding absorption of the anchoring
group on TiO2 and also the influence of QSSE which reduces re-
combination reaction. The influence of indole based organic dyes
with PEG-HEC QSSE was elaborately understood through interfa-
cial charge transfer kinetics (EIS), lifetime and diffusion coefficient
studies of the devices. The stability of the fabricated devices was
excellent under continuous 1 sun illumination radiation due to the
nature of electrolyte species. This research opens a way to replace-
ment of costly metal complex dyes and liquid electrolytes in the
DSSCs world by introducing devices comprised of low-cost novel
indole based derivative sensitizers with PEG-HEC QSSE.
The nature of the polymer matrix and in-situ blend protects io-
dine sublimation, harvest maximum conductivity and resist the di-
rect contact of the electrode. Effective progress has noticed while
using polymer matrix onto the DSSCs device setup. The polymer
blend with a multi-component matrix may have a small drop in
conductivity but results in enhance viscosity which improves the
10

K. Santhosh, S. Ganesan and S. Balamurugan
Electrochimica Acta 389 (2021) 138771
Fig. 8. Photo-stability test for organic dyes/I−/I3−/PEG-HEC.
Author statement
[7] S. Ganesan, S. Balamurugan, P. Karthika, M. Marimuthu, Influence of synthe-
sized pyridine and tetra ethylene glycol derivatives in poly (vinylidene fluo-
ride)/poly (ethylene oxide) with Ti coated back contact dye-sensitized solar
cells, Sol. Energy 188 (2019) 667–684.
The manuscript entitled “Novel Indole-Based Photosensitizers
Coupled with PEG-HEC Quasi-Solid-State Electrolyte to Improve
Energy Conversion and Stability of Organic Dyes Based-Dye Sen-
sitized Solar Cells’’ is new work in dye-sensitized solar cells field.
There are no potential conflicts of interest and hereby declare that
this article has been neither copyrighted, classified, published, nor
is being considered for publication elsewhere.
[8] D. Patil, M. Jadhav, K. Avhad, T.H. Chowdhury, A. Islam, I. Bedja, N. Sekar, A
new class of triphenylamine-based novel sensitizers for DSSCs: a compara-
tive study of three different anchoring groups, New J. Chem. 42 (14) (2018)
11555–11564.
[9] P. Karthika, S. Ganesan, M. Arthanareeswari, Corrigendum to Low-cost synthe-
sized organic compounds in solvent free quasi-solid state polyethyleneimine,
polyethylene glycol based polymer electrolyte for dye-sensitized solar cells
with high photovoltaic conversion efficiencies, Sol. Energy 160 (2018) 225–250.
[10] C. Li, L. Luo, D. Wu, R. Jiang, J. Lan, R. Wang, J. You, Porphyrins with intense
absorptivity: highly efficient sensitizers with a photovoltaic efficiency of up to
10.7% without a cosensitizer and a coabsorbate, J. Mater. Chem. A 4 (30) (2016)
11829–11834.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgement
˚
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phyrin dyes with a phenothiazine donor: a high efficiency of 11.7% achieved
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The authors sincerely thank the Department of Science and
Technology (DST) Fast Track, New Delhi (CS-378/2013).
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.electacta.2021.138771.
[15] K.W. Park, L.A. Serrano, S. Ahn, M.H. Baek, A.A. Wiles, G. Cooke, J. Hong,
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