Hexyl dithiafulvalene (HDT)-substituted carbazole (CBZ) D-π-A based sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1039/d0nj04147e

Metal-free dithiafulvalene (DTF) organic sensitizers for dye-sensitized solar cell (DSSC) applications have been greatly improved and expanded in the last decade. Herein, for DSSC applications, we modified a basic carbazole (CBZ) sensitizer (C0) by substituting additional excellent donor, hexyl dithiafulvalene (HDT), to synthesize two dithiafulvalene-carbazole (DTF-CBZ) compounds using the D-π-A concept, in which either cyanoacrylic acid (HDT-C1) or rhodanine-3-acetic acid (HDT-C2) acts as an acceptor or anchoring group. All the synthesized sensitizers were characterized by various spectroscopic and electrochemical techniques and were successfully applied in DSSCs using the I-/I3- liquid redox couple. UV-visible absorption, electrochemical studies and theoretical calculations (DFT/TDDFT) were used to determine the favoured energy levels for the sensitization of nanocrystalline TiO2. The power conversion efficiencies (PCEs) of these two dyes were compared with that of a simple carbazole sensitizer (C0); the PCE improved significantly from 3.96% (C0) to 7.38% (HDT-C1) and 6.90% (HDT-C2). The superior performance is due to the introduction of the strong electron-donating hexyl dithiafulavalene group, which leads to minimized aggregation and also favours rapid regeneration of the oxidized dye from the redox couple in DSSCs. The PCE of HDT-C2 is not drastically reduced compared to that of HDT-C1, probably due to their well-matched HOMO levels with redox potentials of the redox couple. These thermally stable DTF-CBZ organic dyes possess excellent photovoltaic properties and are anticipated to be alternatives to existing materials in DSSC applications; this will be an active methodology to fabricate DTF sensitizers in the broad and quickly growing area of metal-free organic sensitizers in solar cells.

Full text of DOI:10.1039/d0nj04147e

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Hexyl dithiafulvalene (HDT)-substituted carbazole (CBZ) D-π-A
based sensitizers for dye-sensitized solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Naresh Duvva,a Lingamallu Giribabu,a,b
*
DOI: 10.1039/x0xx00000x
Metal-free dithiafulvalene (DTF) organic sensitizers for dye-sensitized solar cell (DSSC) applications have
been greatly improved and expanded in the last decade. Herein, we modified basic carbazole (CBZ) sensitizer
(C0) by substituted an additional excellent donor hexyl dithiafulvalene (HDT) and synthesized two
dithiafulvalene-carbazole (DTF-CBZ) using D-π-A concept in which either cyanoacrylic acid (HDT-C1) or
rhodanine-3-acetic acid (HDT-C2) as acceptor or anchoring group for DSSC applications. All the synthesized
sensitizers were characterized by various spectroscopic, electrochemical techniques and successfully applied
www.rsc.org/
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in DSSCs using I-/I3 liquid redox couple. UV-Visible absorption, electrochemical studies and theoretical
calculations (DFT/TDDFT) suggest that favoured energy levels for the sensitization of nanocrystalline TiO2.
The power conversion efficiency (PCE) of these two dyes compared with simple carbazole sensitizer (C0), the
PCE improved significantly from 3.96% (C0) to 7.38% (HDT-C1) and 6.90% (HDT-C2). The superior
performance due to the introduction of strong electron donating hexyl dithiafulavalene group, it leads to
minimizing the aggregation and also favoured quick regeneration of oxidized dye from redox couple in
DSSCs. The PCE of HDT-C2 is not drastically reduced as compared to HDT-C1 probably due to well match
HOMO level with the redox potentials of redox couple. This thermally stable DTF-CBZ organic dyes possess
the excellent photovoltaic property and anticipated towards the alternatives of existing materials in DSSCs
applications and it will be active methodology would make DTF sensitizers in this broad and quickly growing
area of metal-free organic sensitizers in solar cells.
phenothiazine (PTZ),25-29 and diketopyrrolopyrrole (DPP) dyes,30-32
etc. The champion device efficiency obtained till now is based on
carbazole scaffold and indeed the sensitizer played a vital role along
Introduction
Dye-sensitized solar cells (DSSCs) have attracted a wide-range of
attention in both academic and industry after motivating work by
O’Regan and Grätzel in 1991(using Ru(II) polyridyl complex as a dye
with I-/I3- redox couple showed an efficiency of 7.1%.1 Over a period
of couple of decades the certified efficiency of DSSC has reached to
with redox mediator.4 Recently, academicians paid attention to
thermally stable strong electron donor dithiafulvalene (DTF) based
scaffold organic sensitizers mainly due to straightforward synthesis,
low cost, and easy to optimize its optical and electronic properties
to suit DSSC applications.
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11.9% with Ru(II) polypyridyl complex as dye with traditional I-/I3
redox couple and device efficiency has reached 14.3% using metal-
free organic dye i.e., ADEKA-1 sensitizer using Co(II/III) as redox
couple.2-4 Despite showing such fancy efficiencies, the metal-free
organic sensitizer should require easy to synthesize, low-cost, less
harmful materials, thermally and photo-chemically stable. Among
the metal-free organic sensitizers, the most proficient tactic was
based on D-π-A, leading to a broad and intense optical absorption
band in the visible spectral region due to effective intramolecular
charge transfer (ICT) from D to A units.5-10 Quite a few organic
sensitizers have been reported in the literature using D-π-A
approach for application in DSSCs, such as coumarin dyes11,12
indoline dyes,13-15 triarylamine dyes,16-19 carbazole dyes,20-24
Dithiafulvalene (DTF) as a strong terminal electron donor has been
used in high success DSSCs due to its non-planar molecular
configuration that facilitates to minimize the formation of
aggregates on the surface of nanocrystalline TiO2.33-35 DTF
sensitizers facilitate the ultrafast interfacial electron injection from
excited state of sensitizer to the conduction band (CB) of
semiconductor.36 Secondly, the recombination of injected electrons
with the redox couple can be concealed because of the propeller-
shaped DTFs molecular structure. Besides, the oxidized DTFs unit is
spatially conveniently placed to support the move of electrons
towards the redox couple species, ensuring fast dye regeneration.
Dithiafulvalene (DTFs) exhibit unique charge transport
characteristics due to their coplanar molecular structures with
strong π-π* and S–S interfaces.37
aPolymers & Functional Materials Division, CSIR-Indian Institute of Chemical
Technology, Uppal Road, Tarnaka, Hyderabad 500607, India,
E-mail : giribabu@iict.res.in
bAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad 20100, India.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: Full synthetic details See
DOI: 10.1039/x0xx00000x
Shihe yang and co-workers for the first time investigated the
sensitization of nanocrystalline TiO2 using metal-free organic DTF
sensitizers by adopting D-π-A concept.38 In all three sensitizers
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keeping the same donor (DTF) and acceptor or anchoring
cyanoacrylic acid group with varying π-bridges or spacers such as
phenyl, biphenyl and phenyl-thiophene-phenyl that displayed
photon conversion efficiency (PCE) from 3.15% to 8.29% with IPCE
of 85% using I-/I3- liquid redox couple, which is highest till now using
DTF based dyes. The improved efficiency is due to increase of both
short-circuit current density (Jsc) and open-circuit voltage (Voc) with
increasing auxiliary acceptor π-bridge length. These results suggest
that the increasing the π-bridge length not only shifts the
absorption maxima with enhanced molar extinction coefficients but
also improve overall efficiency of the device. Thereafter a few
groups have started working on DTF based sensitizers using D-π-A
concept in which the π-spacers are triphenylamine, phenothiazine
and cyclopenta[2,1-b:3,4-b’]dithiophene have been used but
efficiencies are in the range of 3 to 7%.39-44 In an extension of our
efforts towards the development of thermally stable and efficient
metal-free organic dyes, we realized that the introduction of
carbazole moiety as π-spacer, DTF as donor and either cyanoacrylic
acid or rhodanine-3-carboxylic acid as an acceptor moiety may give
better efficiency to DSSC devices. Carbazole derivatives are widely
used in several optoelectronic applications based on their optical
and electronic properties and still, no reports are available based on
HDT-CBZ sensitizers for DSSC applications to the best of our
knowledge. In this work, we have introduced strong electron donor
4,5-bis(hexylthio)-1,3-dithiole-2-thione (HDT) at C6 position of
carbazole and either cyanoacrylic acid (HDT-C1) or rhodanine-3-
acetic acid (HDT-C2) as an acceptor or anchoring group in these
sensitizers. In addition to this, we have also synthesized a sensitizer
having cyanoacrylic acid anchoring group at C3 position of carbazole
as pristine (C0) sensitizer for comparison. All three sensitizers are
characterized by various spectroscopic and electrochemical
methods. Finally, the new sensitizers applied for the sensitization of
2.1. Synthesis
View Article Online
DOI: 10.1039/D0NJ04147E
9-(2-ethylhexyl)-9H-carbazole (1), C9-(2-ethylhexyl)-9H-carbazole-3-
carbaldehyde (2), 9-(2-ethylhexyl)-9H-carbazole-3,6-dicarbaldehyde
(3), and 4,5-bis(hexylthio)-1,3-dithiole-2-thione (HDT) were
synthesized according to the literature procedure.7,45,46
2.2.1.
Synthesis
of
6-((4,5-bis(hexylthio)-1,3-dithiol-2-
ylidene)methyl)-9-(2-ethylhexyl)-9H-carbazole-3-carbaldehyde (4)
Was synthesized modified literature procedure.7 Compound (3) (0.7
g, 1.969 mmol) and 4,5-bis(hexylthio)-1,3-dithiole-2-thione (HDT)
(0.722 g, 1.969 mmol) were suspended in 15 mL of toluene and 10
mL of triethyl phosphite. The reaction mixture was refluxed for 2
hours. After cooling to room temperature, the solvent was removed
by rotary evaporator and the resultant residue dissolved in
chloroform. The mixture was washed with brine and dried with
anhydrous Na2SO4. Solvents were removed by rotary evaporation
and the residue was purified by silica gel column chromatography
with Hexane: DCM (10: 1, v: v) as eluent to afford (4) as yellow oil.
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Yield: 48.0%; H NMR (400 MHz, CDCl3) δ 10.09 (s, 1H), 8.55 (d, J =
1.4 Hz, 1H), 8.26 (d, J = 1.9 Hz, 1H), 8.08 – 8.01 (dd, J = 8.6, 1.6 Hz,
1H), 7.62 – 7.58 (dd, J = 8.7, 1.9 Hz, 1H), 7.48 – 7.44 (d, J = 8.6 Hz,
1H), 7.33 – 7.28 (m, 1H), 6.27 (s, 1H), 4.24 – 4.20 (d, J = 7.6, 2H),
2.86 – 2.75 (t, J = 7.1 Hz, 4H),1.68 – 1.42 (m, 4H), 1.37 – 1.18 (m,
21H), 0.95 – 0.89 (m, 6H), 0.86 – 0.75(m, 6H). MALDI-TOF: (m/z)
found 654.9619 (calculated 654.1).
2.2.2. General procedure for the synthesis of C0 and HDT-C1:
Under N2 atmosphere, the mixture of corresponding aldehyde (2 or
4) (1 mmol), cyan acetic acid (5 mmol) and ammonium acetate (3
mmol) were dissolved in 10 mL of acetic acid and refluxed for 18
hours. The reaction was monitored by TLC and at the end of
reaction, the reaction mixture poured onto ice-cold water and
extracted with chloroform. Collected organic layers were
evaporated and dried. Then they obtained solid compound was
purified by column chromatography using silica gel, stationary
phase with Hexane: THF (10:1) as the eluent to give corresponding
C0 and HDT-C1.
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nanocrystalline TiO2 using I-/I3 redox couple in acetonitrile solvent
and found efficiencies up to 7.38%. The structures of
Dithiafulvalene-Carbazole based (C0, HDT-C1 and HDT-C2)
sensitizers are shown in Chart1.
2.2.2.1.
(Z)-2-cyano-3-(9-(2-ethylhexyl)-9H-carbazol-3-yl)acrylic
1
acid (C0): Yield: 75%; H NMR (400 MHz, CD2Cl2) δ 8.53 (d, J = 1.2
Hz, 1H), 8.26 (d, J = 1.8 Hz, 1H), 8.08 – 8.01 (dd, J = 8.6, 1.6 Hz, 1H),
7.82 (s, 1H), 7.66 – 7.60 (dd, J = 8.7, 1.9 Hz, 1H), 7.55 – 7.49 (d, J =
8.6 Hz, 2H), 7.41 – 7.30 (m, 1H), 4.21 – 4.16 (d, J = 7.6 Hz, 2H), 1.45
– 1.15 (m, 9H), 0.97 – 0.77 (m, 6H). anal. calc. for C24H26N2O2
(374.48): C 76.98, H 7.00, N 7.48; found: C 76.95, H 7.01, N 7.50.
ESI-Mass: (m/z) found 375 (calculated 374.2).
N
C6H13
N
C6H13
O
S
S
COOH
S
N
COOH
CN
S
HDT-C2
HDT-C1
S
S
S
S
S
S
C6H13
C6H13
N
COOH
CN
2.2.2.2. (Z)-3-(6-((4,5-bis(hexylthio)-1,3-dithiol-2-ylidene)methyl)-
9-(2-ethylhexyl)-9H-carbazol-3-yl)-2-cyanoacrylic acid (HDT-C1):
Yield: 62%; 1H NMR (400 MHz, CDCl3) δ 8.48 (d, J = 1.4 Hz, 1H), 8.20
– 8.18 (d, J = 4.9 Hz, 1H), 7.96 (s, 1H),7.59 – 7.55 (d, J = 8.2 Hz, 1H),
7.46 – 7.39 (d, J = 8.4 Hz, 2H), 7.32 – 7.27 (m, 1H), 6.34 (s, 1H),4.48
– 4.43 (d, J = 7.8, 2H), 2.78 – 2.72 (t, J = 7.4 Hz, 4H), 1.57 – 1.32 (m,
4H), 1.29 – 0.98 (m, 21H), 0.82 – 0.73 (m, 12H). anal. calc. for
C40H52N2O2S4 (721.11): C 66.63, H 7.27, N 3.88; found: C 66.66, H
7.25, N 3.90. MALDI-TOF: (m/z) found 722.14 (calculated 721.1).
C0
Chart 1. Molecular structures of Dithiafulvalene-Carbazole based
(C0, HDT-C1 and HDT-C2) sensitizers.
2. Experimental Section
2.1. Materials and reagents. All chemicals, reagents and solvents
are procured either from Sigma-Aldrich or from TCI. Analytical
reagent (AR) grade solvents were used for the reactions while
laboratory reagent (LR) grade solvents were used for purifications
and column chromatography. All the reactions were carried out
under nitrogen or argon atmosphere using dry and degassed
solvents.
2.3.3. Synthesis of (E)-2-(5-((6-((4,5-bis(hexylthio)-1,3-dithiol-2-
ylidene)methyl)-9-(2-ethylhexyl)-9H-carbazol-3-yl)methylene)-4-
oxo-2-thioxothiazolidin-3-yl)acetic acid (HDT-C2): We have
adopted similar synthetic procedure of C0 and HDT-C1, but only
difference is that here we have taken rhodanine-3-acetic acid
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DOI: 10.1039/D0NJ04147E
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OHC
CHO
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COOH
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C6H13
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S
S
HDT
S
S
C6H13
(i) POCl3, DMF, 1,2-dichloreethane, (ii) HDT, P(OEt)3, Toluene (iii) Cyanoacetic acid, CH3COONH4, CH3COOH, (iv) Rhodanine-3-
acetic Acid, CH3COONH4, CH3COOH
Scheme 1. Synthetic route for C0, HDT-C1 and HDT-C2 sensitizers.
The detailed synthetic route of dithiafulvalene-carbazole based
instead of cyanoacrylic acid. Yield: 65%; 1H NMR (400 MHz, CDCl3) δ
8.49 (d, J = 1.2 Hz, 1H), 8.15 – 8.20 (d, J = 4.6 Hz, 1H), 7.92 (s, 1H),
7.62 – 7.58 (d, J = 8.4 Hz, 1H), 7.42 – 7.37 (d, J = 8.2 Hz, 2H), 7.32 –
7.27 (m, 1H), 6.46 (s, 1H), 4.78 (s, 2H),4.40 – 4.36 (d, J = 7.6,
sensitizers is showed in Scheme 1. The intermediate compounds 1,
2 and 3 were accomplished as per literature methods.7,45,46 The vital
intermediate i.e., compound (4) was synthesized by adopting our
previously reported procedure and we adopted Knoevengel
condensation of formyl derivates with either cyano acetic acid or
2H),3.02 – 2.88 (t, J = 7.2 Hz, 4H), 1.55 – 1.40 (m, 4H), 1.34 – 1.08
(m, 21H), 0.89 – 0.72 (m, 12H). anal. calc. for C42H54N2O3S6 (827.27):
rhodanine-3-acetic acid using ammonium acetate and acetic acid.
C 60.98, H 6.58, N 3.39; found: C 61.00, H 6.60, N 3.40. MALDI-TOF:
Finally the desired sensitizers (C0, HDT-C1 and HDT-C2) were
(m/z) found 826.95 (calculated 826.2).
obtained by silica gel column chromatography purification. All
intermediate compounds and three sensitizers were fully
characterized by various spectroscopic techniques and
electrochemical methods. The MALDI-TOF spectrum of HDT-C1
showed a peak at m/z = 722.14 (CHN) and HDT-C2 at m/z = 826.95,
which was assigned to their comparable molecular ion peaks (Figure
S1 and S8). H NMR spectra of C0, HDT-C1 and HDT-C2 sensitizers
2.3. Characterization
Proton (1H) NMR was recorded by using 400 MHz Bruker Avance
NMR spectrometer with X-WIN NMR software. The samples were
prepared either with CDCl3 or CD2Cl2 according to their solubility
1
and the concentration was 1.0 x 10-3 M. The spectra were analyzed
are shown in Figure S1 to S8 (ESI).
using Mestrenova software. ESI (Electrospray Ionization) mass
spectra were recorded on a Water Quattro micro (Water Inc, USA).
3.2. Optical and Electrochemical Properties
Matrix-assisted laser desorption ionization time-of-flight (MALDI–
TOF) mass spectrometry performed on Shimadzu Biotech Axima
Performance 2.9.3.20110624: Mode Reflectron-HiRes, Power: 85.
Major fragmentations are given as percentages relative to the base
peak intensity. Cyclic voltammetric measurements were performed
on
electrochemical analyzer using
tetrahydrofuran solvent using 0.1
a
system controlled CH instruments model CHI 620C
mM sample solution in
Tetrabutylammonium
1
M
perchlorate (TBAP) as a supporting electrolyte at a scan rate of 100
mV/s. The working electrode is glassy carbon, standard calomel
electrode (SCE) is used as a reference electrode, and platinum wire
is a counter electrode. After a cyclic voltammogram (CV) was
recorded, ferrocene was added, and other voltammogram was
measured. Mettler Toledo TGA/SDTA 851e instrument was used for
the Thermogravimetric measurements at heating rate 10 °C min-1
with 10 mg of sample. The detailed instrumentation
characterization (UV-Visible absorption, time-resolved fluorescence
measurements, theoretical calculations (DFT/TDDFT) and device
fabrication measurements of these dyes are available in supporting
information.
3. Results and Discussion
3.1. Synthesis
Figure 1. (a) Absorption (left) and emission (right) spectra in the THF
solvent (b) Absorption spectra on TiO2 of C0, HDT-C1 and HDT-C2
sensitizers.
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The optical properties are essential in order to know the sensitizers technique, which will be useful for the estimation of HOMO and
View Article Online
to absorb photons and that converts it into electrical energy. For LUMO positions of dyes and the data presented in Table 1. From
DOI: 10.1039/D0NJ04147E
this reason, we have measured the absorption properties of C0, Figure 2 and Table 1 it suggests that the sensitizer C0 has quasi-
HDT-C1 and HDT-C2 in THF solvent. Figure 1a demonstrates the reversible oxidation peak at 1.12 V vs. NHE whereas in case of HDT-
absorption spectra of all three sensitizers and their corresponding C1 and HDT-C2 the oxidation peak cathodically shifted to ~0.75 V
wavelength of absorption maxima and molar extinction coefficients vs. NHE. The first oxidation potential of these DTF donor based dyes
data are given in Table 1. The absorption spectrum of each of was used to calculate the HOMO position and the LUMO was
sensitizer consists of two prominent peaks in 300-500 nm and estimated by subtracting the band gap from its HOMO level. The
around 550 nm regions. The absorption peaks at 320 and 386 nm in found energy levels of these newly synthesized sensitizers are
the case of C0 sensitizer purely belong to π-π* transitions of appropriate for the TiO2 and electrolyte energy levels.47 The HOMO
carbazole moiety as in this sensitizer not having a DTF donor and LUMO energy levels (vide infra) calculated by both
whereas, in the case of HDT-C1, the peak at this region is a experimental and theoretical are in good agreement with each
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combination of both donor DTF and π-spacer carbazole. In contrast, other. Finally, we used the expression EOX = EOX – E0-0 is used to
the absorption peak red-shifted to 435 nm in case of HDT-C2 assess excited-state oxidation potentials (EOX*) levels of C0, HDT-C1
sensitizer because of the presence of powerful electron and HDT-C2 dyes, which were estimated to be 1.86, 2.23 and 2.06
withdrawing rhodanine-3-acetic acid. In addition to this, the eV, respectively.
absorption peak at longer wavelength region at ~550 nm is feasible
to intramolecular charge transfer (ICT) bands corresponding to
donor DTF to the acceptor (cyanoacrylic acid or rhodanine-3-acetic
acid) through carbazole π-spacer. Figure 1b illustrates the
absorption properties of these D-π-A acceptor dyes adsorbed on
surface of the 6μm thick TiO2 films. The absorption spectra of all
three sensitizers are exhibit red-shifted in comparison with solution
spectra in view of the fact that the interaction of the carboxylic acid
group with TiO2 and also possibly because of aggregation. In both
the dyes the onset of absorption extends up to 650 nm. The
emission spectra were measured in THF solvent at room
temperature for C0, HDT-C1 and HDT-C2 dyes, which show emission
maxima at 446, 460 and 498 nm, respectively. The emission
intensity almost quenched when adsorbed on TiO2. The singlet
excited-state lifetimes were measured in THF solvent, and they
found in the range of 0.86-0.21 ns (Figure S9), and the excited
lifetimes were quenched when adsorbed on TiO2. The quenching of
both steady-state emission and excited-state lifetime on TiO2 is
presumably because of photoinduced electron transfer from singlet
state of dye to the conduction band of TiO2. In the end, we
calculated the singlet excited energies (E0-0), which are useful in
calculating excited state oxidation potentials that will be discussed
in presiding section from the intersection of absorption and
emission spectra, and they are found 2.98±0.05, 2.99±0.05 and
2.61±0.05 eV for C0, HDT-C1 and HDT-C2, respectively.
Table 1. Photophysical properties of C0, HDT-C1 and HDT-C2 dyes.
b
Dye
λmaxa (nm)/
λ(abs.
λem
nm
E1/2 V vs. NHE
Lifetime
(τ)
ns), (A
%)
(ɛ x 104 M-1 edge
(in
d
*d
OXc
E0-0
EOx
cm-1)
(in on
TiO2)a
(nm)
(eV)
Solution)
C0
320 (4.157) 435
386(5.936)
446
1.12
2.98
2.99
1.86
2.23
0.89
HDT-C1
320 (3.176) 529
380 (3.382)
551 (1.150)
434,
460
0.76,
0.99,
1.14
0.34 (86)
1.52 (14)
HDT-C2
345 (2.689) 538
430 (5.699)
552 (1.012)
498
0.75,
0.99,
1.15
2.61
2.06
0.21 (89)
1.06 (11)
aAbsorption spectra were recorded in THF solution, Error limits: max, ±1 nm,  ±10%. bSolvent: THF,

max, ±1 nm. cSolvent: THF, Error limits: E1/2 ±0.03 V, 0.1 M TBAP. dEOx* = E0-0-OX; Error limits: ±0.05 eV.
All life times are in nanoseconds (ns), at λex = 458 nm.
3.2. Theoretical Studies
The optimized energy structures, electronic distribution, molecular
electrostatic potential (MEP) maps, theoretical absorption spectra,
HOMO-LUMO energy levels, and bandgap of sensitizers (Figure 3,
Table S1, S2) were theoretically calculated using the Gaussian 09
program package with the functional basis set of the B3LYP/6-31 G
(d,p) level. From Table S1, it suggests that both HRD-C1 and HRD-C2
dyes have similar optimized structures in which the hexyl group on
the sulfur atom of DTF donor and on the 9-N of carbazole are in the
same plane which will not only minimize the aggregation but
enhance the solubility of sensitizers in common organic solvents.
Figure 3 and Table S1 suggests that the HOMOs for C0, HDT-C1 and
HDT-C2 dyes are completely located on the donor and π-spacer
(DTF and carbazole) units whereas LUMO delocalized over the
carbazole and cyanoacrylic acid group in C0 dye. On the other hand,
the LUMO delocalized completely on anchoring cyanoacrylic acid
(HDT-C1) and rhodanine-3-acetic acid (HDT-C2) and partially on
carbazole moiety, which indicates an efficient intramolecular
electron injection from singlet excited state of the dye to
conduction band of TiO2. This has been proved even from MEP
maps, for both dyes, the positive electrostatic potential was at the
donor DTF and π-spacer connecting the carbazole; while negative
potential was concentrated at anchoring group either cyanoacrylic
acid or rhodanine-3-acetic acid. The HOMO-LUMO gap of these
dyes is 3.34, 2.74 and 2.57 eV for C0, HDT-C1 and HDT-C2,
respectively. Additionally, we subjected to single-point TDDFT
Figure 2. Differential voltammograms of C0, HDT-C1 and HDT-C2 in
THF using 0.1 M TBAP.
Subsequently, we measured the redox potentials of all three
sensitizers in THF solvent by using differential pulse voltammetric
4 | J. Name., 2012, 00, 1-3
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studies (first 15 vertical singlet-singlet transitions to obtain the IPCE rose to 78% and in case of HDT-C2 it further enhances to 80%.
absorption spectra) (see Figure S10) with THF as the solvent and However, the presence of rhodanine-3-acetDicOaI:c1id0.1a0n3Vc9ihe/woDrA0irnNtigcJl0eg4Or1on4lui7npEe
calculated vertical excitation energies for the singlet together with in HDT-C2 more broaden the IPCE spectra that will help in tapping
oscillator strengths, as listed in Table S2. The experimental more photons leads to high PCE than the remaining two dyes. But,
absorption properties with the theoretical data expose that the this true only when compared to C0 sensitizer and the reason might
results are in excellent agreement with experimental values.
be due to its non-conjugated structure between rhodanine ring and
carboxylic anchoring group whereas HDT-C1 sensitizer having
cyanoacrylic acid anchoring group has maintained the
conjugation.48 HDT-C1 dye with conjugated cyanoacrylic acid
anchoring group not only enhances the electron injection into TiO2
conduction band but also slows the charge recombination as was
the case in our previous reports. However, literature reports point
out that the device efficiency drastically reduce the dyes having
rhodanin-3-acetic acid when compared to cyanoacrylic acid group
using same scaffold. In the present study the efficiency has not
much reduced probably the charge regeneration is the same in both
HDT-C1 and HDT-C2 as HOMO is not much different.
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Figure 3. Energy diagram of C0, HDT-C1 and HDT-C2 sensitizers
calculated by Gaussian 09 program package with functional basis
set of B3LYP/6-31 G (d, p) level.
3.3. Photovoltaic performances
At first, we have calculated the amount of sensitizer adsorbed on
the surface of TiO2. This has been executed by desorbing the
adsorbed dye and the obtained amounts are listed in Table 2. The
loaded amounts of C0, HDT-C1 and HDT-C2 dyes are found to 4.13 x
10-7, 4.06 x 10-7 and 3.99 x 10-7 mol cm-2, respectively. The dye
loading amounts of HDT-C2 low, when compared to HDT-C1 dye,
probably due to the larger molecular size former dye and weak
interaction with nanocrystalline TiO2 surface compared to HDT-C1.
The photovoltaic accomplishment of all three sensitizers i.e., the
incident photo-to-current conversion efficiency (IPCE) and
photocurrent-voltage (J-V) curves, was evaluated in test devices
using TiO2 films of 8 µm thick with a volatile acetonitrile based
electrolyte composition of 0.5 M LiI, 0.05 M I2, 0.5 M TBP (4-tert-
butyl pyridine), and 0.5 M 1,2-dimethyl-3-propylimidazolium iodide
(DMPII). The detailed device fabrication was portrayed in our
preceding studies.7,25 Figures 4a and 4b illustrate both I-V and IPCE
spectra of all three sensitizers and respective photovoltaic data are
presented in Table 2. As seen in Table 2, the carbazole sensitizer
(C0) without DTF donor have a low short-circuit current (Jsc) of
7.22±0.37 mA/cm2 whereas in case of sensitizers are having DTF
donor i.e., HDT-C1 and HDT-C2 has enhanced to 14.12±0.43 and
13.19±0.46 mA/cm2, respectively. The open-circuit voltage (Voc) and
fill factor of all three sensitizers are in the range of 0.76±0.02 to
0.78±0.02 V and 0.67 to 0.72, respectively. These results clearly
suggest that the presence of donor DTF has improved photovoltaic
performance and device efficiency of sensitizer C0 is 3.96±0.14%
and that of DTF substituted sensitizers has raised to 7.38±0.32 and
6.90±0.23%. The difference in PCE of C0, HDT-C1, and HDT-C2 is
reflected in the IPCE spectra, as illustrated in Figure 4b. In case of
C0 sensitizer, it was observed maximum IPCE of ~60% at 410 nm.
On the other hand, the introduction of donor DTF in HDT-C1 dye
Figure 4. (a) J-V characterstics (b) IPCE spectra of C0, HDT-C1 and
HDT-C2 sensitizers.
The present DTF based D-π-A dyes are showed better device
performance over literature reported dyes in which the π-spacer is
either triphenylamine or phenothiazine scaffold. There are many
reasons for getting high efficiency of present dyes over literature
reported ones in which the onset of the IPCE extends up to 650 nm
and second one is that the present sensitizers HOMO level is very
close to the oxidation potential of redox couple so that the driving
force for the regeneration of oxidized dye is <0.2 eV.34,40,41,44 On the
other hand, the efficiency of HDT-C1 and HDT-C2 dyes lower that
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literature reported DTF-C3 (8.29%)38 probably due to more substituted carbazole (HDT-CBZ) hybrid donor using D-π-A concept
extended π-conjugation so that the onset of IPCE extends up to 700 for application in DSSCs. Two novel sDeOnIs:i1t0iz.e10rs39/hDa0vNeJ04b1e4e7nE
nm as a result in high efficiency. Further, the efficiency of HDT-C1 synthesized using HDT-CBZ donor in which either cyanoacrylic acid
and HDT-C2 may enhance by the introduction more π-conjugation (HDT-C1) or rhodanine-3-acetic acid (HDT-C2) as acceptor or
View Article Online
between carbazole π-spacer and anchoring group.
anchoring group. With the introduction of HDT donor, the
absorption peak red-shifted and the PCE of DSSC was enhanced
compared to simple carbazole sensitizer η of 3.96±0.14% (C0) to
7.38±0.32 (HDT-C1) and 6.90±0.23% (HDT-C2). The improved PCE
might be attributed to the HDT moiety by improving the electron-
donating capacity, decreasing dye aggregation, possible to
increasing intramolecular charge transfer (ICT) from donor to
acceptor. Also the driving force for the regeneration of oxidized dye
from redox couple is <0.2 eV and retarding the charge
recombination due to propeller structure of DTF moiety. These
results are strongly representing that to change the molecular
design approaches for DTF based DSSCs further careful
manipulations and device fabrication would develop DTF sensitizers
in this broad and rapidly rising area of metal-free DTF based organic
sensitizers in photovoltaic applications.
Table 2. Photovoltaic parameters of C0, HDT-C1 and HDT-C2
sensitizers.e
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Adsorbed
Dyes
C0
Voc (V)a
Jsc
FFc
ηd (%)
amount of
dye
(mA/cm2)b
(mol.cm-2)
0.76±0.02 7.22±0.37
0.72 3.96±0.14%
4.13 x 10-7
4.06 x 10-7
3.99 x 10-7
HDT-C1 0.78±0.02 14.12±0.43 0.67 7.38±0.32%
HDT-C2 0.76±0.03 13.19±0.46 0.69 6.90±0.23%
aopen-circuit photovoltage (Voc), bshort-circuit photocurrent density (Jsc), cfill
factor (FF), dconversion efficiency (η). eErrors were take an average of four
photovoltaic devices.
Supporting Information
Classification data of the compounds such as methods and
instrumentation,1H NMR, ESI-MS and MALDI-MS spectra (Figure S1-
8), fluorescence decay (Figure S9), theoretical and experimental UV-
Visible spectra (Figure S10), Optimized and ESP structures of dyes
(Table S1) and Comparison of the experimental optical properties
with the theoretical data (Table S2) are available in supporting
information.
3.4. Thermogravimetric Analysis
In addition about the efficiency of devices, the thermal properties
of dyes are also essential for the commercialization of technology.
For this reason, we have adopted thermogravimetric analysis (TGA)
to understand the thermal stability of newly synthesized dyes. As
reported in literature that the thiafulvalene derivatives thermal
stability is high.49 Figure 5 illustrates the TGA curves of sensitizers
from 50 to 300oC at a heating rate 10oC min-1 under an inert
atmosphere. As seen in Figure 5, both HDT-C1 and HDT-C2 dyes are
stable up to 150oC and the initial weight loss (~5%) observed 100 to
150oC is due to the removal of moisture. Among metal-free organic
dyes, dithiafulvalene class of compounds are good thermal stability
and also easy of synthesis; this allows one can think about its
commercialization of technology.
Acknowledgements
This work was financially supported by the Department of Science
and Technology under solar energy research initiative (DST-CERI,
DST/TM/CERI/C228 & DST/TMD/CERI D46) programme. Authors ND
thanks to CSIR for senior research fellowship. We thank Director
CSIR-IICT for the support (IICT/Pubs./2020/233).
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In conclusion, we have successfully introduced strong electron
donor HDT unit into a carbazole for the first time to form HDT-
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“Hexyl dithiafulvalene (HDT)-substituted carbazole (CBZ) D-π-A based
DOI: 10.1039/D0NJ04147E
sensitizersfor dye-sensitized solar cells”
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Naresh Duvva,a Lingamallu Giribabua,b*
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We have successfully modified basic carbazole (CBZ) sensitizer (C0) by substituted
anadditional excellent donor hexyl dithiafulvalene (HDT) at C6 position using D-π-A concept
inwhich either cyanoacrylic acid (HDT-C1) or rhodanine-3-acetic acid (HDT-C2) as
acceptor oranchoring group for the applied to DSSCs. The PCE of DSSC was
enhancedcompared to simple carbazole sensitizer (without HDT unit) 3.96% to 7.38%. Only
few researcharticles are reported in metal-free DTF based organic sensitizers based DSSCs
application until now, hence designs of diverse molecular structures pave the way for
enhanced efficiency anddurability of photovoltaic devices.