Effect of different acceptors on N-hexyl carbazole moiety for dye-sensitized solar cells: design, characterization, molecular structure, and DSSC fabrications

Article abstract of DOI:10.1007/s13738-020-02082-y

Hexyl carbazole derivatives are one of the most prominent dye scaffolds in the dye-sensitized solar cells (DSSCs). New substituted carbazole dyes such as DRA-HC, DCA-HC, and DTC-HC were synthesized for DSSCs. These dyes are containing hexyl moiety as electron donor and rhodanine-3-acetic acid, cyanoacetic acid and tetracyanoethylene as an electron acceptor linked to carbazole moiety. The relation between dye structures, photophysical/electrochemical, molecular structure and DSSC manufacturing had been discussed. All structures showed more positive ground-state oxidation potential than I^?/I^?₃ and more negative excited state oxidation potential than the conduction band edge of the semiconductor. The highest efficiency of the DSSCs was obtained in the case of DCA-HC dye (η = 1.41%, V_OC = 708?mV, FF = 0.81, and J_SC = 2.45?mA?cm^?2 with 100?mW?cm^?2) compared to other synthesized dyes.

Full text of DOI:10.1007/s13738-020-02082-y

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-02082-y
ORIGINAL PAPER
Efect of diferent acceptors on N‑hexyl carbazole moiety
for dye‑sensitized solar cells: design, characterization, molecular
structure, and DSSC fabrications
Moustafa S. Abusaif1 · M. A. Abu‑Saied2 · M. Fathy3 · Ahmed A. El‑Sherif4 · A. B. Kashyout3 · Mohamed R. Selim1 ·
Yousry A. Ammar1
Received: 18 July 2020 / Accepted: 30 September 2020
© Iranian Chemical Society 2020
Abstract
Hexyl carbazole derivatives are one of the most prominent dye scafolds in the dye-sensitized solar cells (DSSCs). New sub-
stituted carbazole dyes such as DRA-HC, DCA-HC, and DTC-HC were synthesized for DSSCs. These dyes are containing
hexyl moiety as electron donor and rhodanine-3-acetic acid, cyanoacetic acid and tetracyanoethylene as an electron acceptor
linked to carbazole moiety. The relation between dye structures, photophysical/electrochemical, molecular structure and
DSSC manufacturing had been discussed. All structures showed more positive ground-state oxidation potential than I−/I−3 and
more negative excited state oxidation potential than the conduction band edge of the semiconductor. The highest efciency
of the DSSCs was obtained in the case of DCA-HC dye (η=1.41%, VOC =708 mV, FF=0.81, and JSC =2.45 mA cm−2 with
100 mW cm−2) compared to other synthesized dyes.
Keywords N-hexyl carbazole · Photovoltaic performance · DSSCs fabrication · Optical and electrochemical · Properties ·
Rhodanine-3-acetic acid
Abbreviations
DRA-HC 2,2′-(((9-Hexyl-9H-carbazole-3,6-diyl)
bis(methanylylidene))bis(4-oxo-2-thioxothia-
zolidin-3-yl-5-ylidene))diacetic acid
DCA-HC 3-(6-(2-Carboxy-2-cyanovinyl)-9-hexyl-
9H-carbazol-3-yl)-2-cyanoacrylic acid
DTC-HC 2,2′-((((9-Hexyl-9H-carbazole-3,6-diyl)
Electronic supplementary material The online version of this
bis(methanylylidene))bis(hydrazin-
article (https://doi.org/10.1007/s13738-020-02082-y) contains
1-yl-2-ylidene))bis(4,1-phenylene))
bis(ethene-1,1,2-tricarbonitrile)
Dye-sensitized solar cells
Open-circuit voltage
supplementary material, which is available to authorized users.
* Ahmed A. El-Sherif
DSSCs
VOC
aelsherif72@yahoo.com
1
2
JSC
Short-circuit current density
Donor–(π-conjugated spacer)–acceptor
Tetracyanoethylene
Department of Chemistry, Faculty of Science, Al-Azhar
University, Nasr City, Cairo, Egypt
D–π–A
TCNE
TLC
KBr
SEM
CV
Polymeric Materials Research Department, Advanced
Technology and New Materials Research Institute, City
of Scientifc Research and Technological Applications
(SRTA-City), New Borg El-Arab City, Alexandria 21934,
Egypt
Thin-layer chromatography
Potassium bromide
Scanning electron microscopy
Cyclic voltammetry techniques
4-Tertbutylpyridine
3
4
Electronic Materials Department, Advanced Technology
and New Materials Research Institute, City of Scientifc
Research and Technological Applications (SRTA-City),
New Borg El-Arab City, Alexandria 21934, Egypt
TBP
ε
E0–0
ELUMO
τe
Molar absorptivity
Band gap
Lowest unoccupied molecular orbital
Electron lifetime
Department of Chemistry, Faculty of Science, Cairo
University, Giza, Egypt
Vol.:(0123456789)
1 3

Journal of the Iranian Chemical Society
IP
Ionization potential
Absolute hardness
have gained high conversion efciency [14, 15], they are
inefcient for large-scale applications due to their difcult
preparation and the Ru atom is seldom and costly. For this
cause, metal-free dyes have become more important when
opposed to metal complexes because of their high and low
cost, molecular extinction. In DSSCs, many types of sensi-
tizers, including carbazole [16], coumarin [17], phenothia-
zine [18], indoline [19], are introduced in the literature as
sensitizers for efciency improvement. In general, sensitiz-
ers are made up of donor–π-conjugated spacer–acceptor
(D–π–A) structure because of the efcacy photoinduced
intermolecular charge transfer (ICT) property [20]. Carba-
zole moiety acts as donor linked to acceptor groups such as
thiobarbituric acid has shown better light conversion ef-
ciency similar to phenothiazine moieties due to carbazole
containing a heterocyclic atom [21].
η
ω
Electrophilicity index
Separation energy
ΔE
S
Global softness
η
Power conversion efciency
Fill factor
FF
TiO2
ICT
TBAI
Titanium dioxide
Intermolecular charge transfer
Tetra-n-butylammonium iodide
DMSO-d6 Deuterated dimethyl sulfoxide
UV–Vis
EIS
Ultraviolet–visible
Electrochemical impedance spectroscopy
Fluorine-doped tin oxide
Dimethyl formamide
FTO
DMF
Eox
Redox potentials
EHOMO
J–V
Highest occupied molecular orbital
Current–voltage density
Tetra-methyl silane
In this study, we report the synthesis of three novel metal-
free organic dyes with a structure of (D–π–A) DRA-HC,
DCA-HC, and DTC-HC (Fig. 1). In the design of these dyes,
N-hexyl carbazole was used as the donor part, while rhoda-
nine-3-acetic acid, cyanoacetic acid, and tetracyano ethylene
were used as the withdrawal group in DRA-HC, DCA-HC,
and DTC-HC, respectively. The efects on photophysical,
electrochemical, and photovoltaic properties of these dyes
were studied.
TMS
σ
Absolute softness
EA
Electron afnity
ΔNmax
χ
Maximum amount of electron transfer
Absolute electronegativity
Introduction
No doubt that solar energy is widely recognized as a promis-
ing energy source that can contribute to the alleviation of the
global energy crisis; the development of solar cells is a key
research goal [1]. There are many types of solar cells such as
silicon solar cell [2], organic solar cell (OPV) [3], perovskite
solar cell [4], plasmonic solar cell [5], quantum dot solar cell
[6], and dye-sensitized solar cell (DSSCs) [7]. Dye-sensi-
tized solar cells (DSSCs) have arisen as a technically and
economically credible alternative to the p–n junction pho-
tovoltaic devices [8]. Due to the advantages of the DSSCs,
such as easy processing, low cost, ecologically friendly, and
long-term durability, DSSCs are great alternative to silicon
solar cells [9]. The sensitizers are usually designed to have
functional groups such as –COOH, –PO3H2, and –B(OH)2
for stable adsorption onto the semiconductor substrate [10].
Much of the current research focuses on improving the
range of spectral absorbance by modifying the dye [11], on
improving hole transport and cell stability by replacing the
liquid electrolyte with ionic solids or conducting polymers
and on improving electron transport by using alternative
wide-band-gap semiconductor materials or core–shell struc-
tures [12]. Recently in 2018, an efciency of up to 14.3%
was achieved by Sharma et al. [13].
Experimental
Instrumentation and characterizations
For chromatographic isolation, Merck silica gel (254 nm
plates) was used. The Gallen Kamp MFB-595 device is
used to record all melting points. For the IR spectrum (KBr,
cm−1), a Shimadzu 440 was used. The Bruker spectrometers
were used in deuterated dimethyl sulfoxide (DMSO-d6) to
1
investigate H NMR and 13C NMR spectra using TMS as
standard. Mass spectrum is measured by the Thermo Scien-
tifc ISQLT Spectrometer. UV–Vis spectra have been regis-
tered on the Shimadzu 260 solution spectrometer. The parti-
cle size of the organic dyes was analyzed with a micrometer
particle size analyzer (Beckman Coulter, USA). The sample,
dispersed in water at 20 °C, had a viscosity of 1.002 and a
refractive index of 1.33. The organic materials are inves-
tigated with high-resolution scanning electron microscopy
(JEOL, JSM-6360LA) to examine the morphology and the
homogeneity of the surface.
The recording of cyclic voltammograms was reported
with the AUTOLAB potentiostat/galvanostat operated
device and traditional three-electrode devices at room
temperature, platinum working electrode, Ag/AgCl ref-
erence electrode and carbon counter electrode. The
potential is recorded versus ferrocene as standard using
In general, there are two main kinds of sensitizers, metal-
complex and organic metal-free sensitizers. However, the
DSSCs based on metal-complex sensitizers as Ru complex
1 3

Journal of the Iranian Chemical Society
Fig. 1 Molecular structure of organic dyes
tetrabutylammonium iodide as supporting electrolyte and
scan rate of 0.1 Vs−1 and 0.1 M.
2H, N–CH2); 1.77 (m, 2H, CH2), 1.25 (m, 6H, CH2), 0.79
(t, 3H, CH3); 13CNMR: δ 146.15 (alip. CH=N), 140.83,
138.36, 129.52, 127.77, 124.05, 123.60, 122.73, 122.09,
119.01, 118.69, 118.65, 112.29, 111.80, 110.33, 43.33
(N–CH2), 31.05, 28.76, 26.55, 22.46, 14.43 (CH3); Anal.
Calcd. for C32H33N5 (487.27): C, 78.82; H, 6.82; N, 14.36;
Found: C, 78.45; H, 6.65; N, 14.30%.
Materials and synthesis
The starting material 9H-carbazole was purchased from
Aldrich. The reagents such as rhodanine-3-acetic acid,
cyanoacetic acid, phenylhydrazine, and tetracyanoethyl-
ene [TCNE] were purchased from Aldrich Chemicals. All
organic solvents were obtained from Fisher and used with-
out further purifcation. Also, literature process used was to
prepare 9-hexyl-9H-carbazole 1, and 9-hexyl- 9H-carbazole-
3,6-diformaldehyde 2, with melting point mp. =75–77 °C
and 79–80 °C, respectively [22, 23].
2′‑((((9‑hexyl‑9H‑carbazole‑3,6‑diyl)bis(methanylylidene))
bis(hydrazin‑1‑yl‑2‑ylidene))bis(4,1‑phenylene))
bis(ethene‑1,1,2‑tricarbonitrile) [DTC‑HC]
To a solution of phenyl hydrazone derivatives 3 (0.68 g,
1 mmol) in DMF (20 ml), then TCNE (0.25 g, 2 mmol)
was added, a dark color appeared. The solution boiled at
400 °C for 8 h, the solvent was removed, and the residual
solid was collected and recrystallized from toluene-pro-
ducing DTC-HC.
9‑Hexyl‑3,6‑bis((2‑phenylhydrazono) methyl)‑9H‑carbazole
[3]
N-hexylcarbazole-3,6-dicarbaldehyde 2 (0.30 g, 1 mmol) and
phenylhydrazine (0.21 g, 2 mmol) were added to absolute
ethanol (10 ml), catalyzed with acetic acid (1 ml), and heated
under refux for 5 h until the precipitate is formed, and the
reaction was monitored by TLC. The collected powders were
dried, ethanol-washed, and acetic acid-recrystallized.
Yellow solid; 93% yield; mp: 185–187 °C; IR: ν/cm−1:
3302 (NH), 3053 (arom. C–H), 2927, 2856 (alip. C–H),
1598 (C=N), 1488 (C–N); 1H NMR: δ/ppm: 10.18, 10.24
(s, 2H, NH; D2O exchangeable); 8.83 (s, 1H, H4); 8.40
(s, 1H, H5); 8.04 (s, 2H, methine-H); 7.83 (d, 1H, H7),
7.62 (d, 1H, H1), 6.70–7.60 (m, 12H, arom. C–H); 4.43 (t,
Red-brown solid; 60% yield; mp: 230–232 °C; IR:
ν/cm−1: 3319 (br. NH), 3059 (arom. C–H), 2929, 2858
(alip. C–H), 2216 (C≡N), 1604 (C=N), 1558 (C=C),
1
1489 (C–N); H NMR: δ/ppm: 10.18, 10.24 (s, 2H, NH;
D2O exchangeable); 8.74 (s, 1H, H4); 8.52 (s, 1H, H5);
8.13, 8.11 (s, 2H, methine-H); 6.50–7.58 (m, 12H, arom.
C–H); 4.36 (t, 2H, N–CH2); 1.76 (m, 2H, CH2), 1.23 (m,
6H, CH2), 0.80 (t, 3H, CH3); 13CNMR: δ 143.91 (alip.
CH=N), 134.78, 134.40, 132.09, 129.91, 122.74, 120.72,
114.88, 111.82, 110.23, 43.02 (N–CH2), 31.21, 28.81,
26.48, 22.39, 14.00 (CH3); MS: (Mwt.: 689): m/z (%),
689 [M + , (6.67)], 534 (20.34), 368 (67.22), 339 (100%);
1 3

Journal of the Iranian Chemical Society
Anal. Calcd. for C42H31N11 (689.28): C, 73.13; H, 4.53; N,
22.34, Found: C, 73.55; H, 4.61; N, 22.10%.
N–CH2), 31.06, 28.61, 26.56, 22.43, 14.20 (CH3); MS: (Mwt.:
653): m/z (%), 653 [M+, (3.69)], 468 (5.05), 479 (36.08), 380
(100%); Anal. Calcd. for C30H27N3O6S4 (653.08): C, 55.22; H,
4.88; N, 6.39, Found; C, 54.90; H, 5.01; N, 5.99%.
3‑(6‑(2‑Carboxy‑2‑cyanovinyl)‑9‑hexyl‑9H‑carba‑
zol‑3‑yl)‑2‑cyanoacrylic acid [DCA‑HC]
Fabrication of DSSCs
N-hexylcarbazole-3,6-dicarbaldehyde 2 (0.30 g, 1 mmol)
and the requested cyanoacetic acid (0.17 g, 2 mmol) were
dissolved in ethanol (20 ml) in the presence of piperi-
dine (0.3 ml) as a catalyst. Over 6 h, the reaction was
well refuxed, the precipitated was collected after cooling
through fltration and washed with water to remove excess
of cyanoacetic acid and recrystallized from ethanol to aford
DCA-HC.
Double distilled water and acetone were used to clean fuori-
nated tin oxide (FTO), where FTO is a conductive glass
(15–20 Ω cm−2). Thin layer of TiO2 paste was coated on the
FTO glass plates by Doctor Blade technique method, then
allowed to dry in air, and heat-treated at 450 °C for 30 min.
After cooling the TiO2 working electrodes to 80 °C [24], it
was immersed in dye solutions (4×10–5 M in DMF) of DTC-
HC, DCA-HC, and DRA-HC and kept at room temperature for
24 h. A platinized counter electrode was formed by depositing
Pt thin flm on FTO glass substrate using magnetron sputtering
instrument (Hummer 8.1, USA) (P=100 W RF, t=2 min) fol-
lowed by heating at 450 °C for 30 min [25]. The counter elec-
trode was placed on the top of the dye-coated TiO2 flm, and
few drops of electrolyte consisted of 0.5 M LiI, 0.05 M iodine
I2, 3-methoxypropionitrile, and 0.5 M 4-tertbutylate pyridine
TBP, which were inserted between the electrodes.
Light-yellow solid; 90% yield; mp: 230–232 °C; IR: ν/
cm−1: 3419 (COOH), 3000 (arom. C–H), 2931, 2862 (alip.
C–H), 2224 (C≡N), 1688 (C=O), 1578 (C=C), 1485 (C–N);
1H NMR: δ/ppm: 8.79 (s, 2H, COOH, D2O exchangeable);
8.45 (s, 1H, H4); 8.41 (s, 1H, H5); 8.28 (s, 2H, methine-H);
7.87 (d, 1H, H7), 7.82 (d, 1H, H1), 7.17–7.79 (m, 2H, arom.
C–H); 4.38 (t, 2H, N–CH2); 1.75 (m, 2H, CH2), 1.23 (m, 6H,
CH2), 0.77 (t, 3H, CH3); 13CNMR: δ 164.34 (C=O), 155.22,
144.86, 143.75, 129.38, 129.28, 125.70, 125.55, 124.18,
122.82, 120.82, 117.46 (C≡N), 116.85, 111.60, 100.42,
43.33 (N–CH2), 31.30, 28.76, 26.33, 22.22, 14.20 (CH3);
MS: (Mwt.: 441): m/z (%), 441 [M+, (35.70)], 397 (17.05),
353 (36.08), 297 (100%); Anal. Calcd. for C26H23N3O4
(441.17): C, 70.74; H, 5.25; N, 9.52; Found: C, 70.40; H,
5.45; N, 9.30%.
Photovoltaic characterization
The light current–voltage (J–V) curves of the DSSCs were
recorded by using solar simulator device (PET Photo Emis-
sion Tech., Inc. USA). AM 1.5 spectrum is provided by the
xenon lamp of the solar simulator. Based on the light J–V
characteristics, the efciency η, VOC open-circuit voltage, JSC
short-circuits, and FF fll factor were measured. Under dark
illuminations, an electrochemical workstation was used to
record electrochemical impedance spectrum (EIS) (AUTO-
LAB-potentiostat/galvanostat) over a frequency range from 10
to 105 Hz with a bias potential of 0.10 V. Nyquist plots were
analyzed using the nonlinear fts to a suitable equivalent circuit
model that will ft with Nyquist plots with negligible error.
2′‑(((9‑Hexyl‑9H‑carbazole‑3,6‑diyl)bis(methanylylidene))
bis(4‑oxo‑2‑thioxothiazoli‑din‑3‑yl‑5‑ylidene))diacetic acid
[DRA‑HC]
Dicarbaldehyde derivatives 2 (0.30 g, 1 mmol) and rhoda-
nine-3-acetic acid (0.38 g, 2 mmol) were mixed in acetic
acid (10 ml) and ammonium acetate (5 mol%) as a catalyst.
Over 5 h, the product combination was heated under refux,
the precipitate evaporated under pressure and dissolved in
distilled water. The aqueous solution was then acidifed
with concentrated HCl, and the product was fltered of and
recrystallized from acetic acid to obtain DRA-HC.
Molecular modeling studies
DFT analyses have been completed in Materials Studio pack-
ages [26] using the DMOL3 software [27, 28]. Calculations
for DFT semi-core pseudopods (dspp) were created with dual
numerical base sets and polarization properties (DNP) [29].
The RPBE model is focused on the (GGA) generalized gradi-
ent approximation as the best functional approximation [30].
Deep-red solid; 85% yield; mp: 170–172 °C; IR: ν/cm−1:
3450 (br. OH), 3054 (arom. C–H), 2928, 2856 (alip. C–H),
1
1706 (C=O), 1577 (C=C), 1478 (C–N); H NMR: δ/ppm:
10.06 (s, 2H, COOH, D2O exchangeable); 8.86 (s, 1H, H4);
8.48 (s, 1H, H5); 8.46 (s, 2H, methine-H); 7.51–8-25 (m, 4H,
arom. C–H); 4.70 (s, 4H, CH2COOH); 4.43 (t, 2H, N–CH2);
1.76 (m, 2H, CH2), 1.24 (m, 6H, CH2), 0.8 (t, 3H, CH3);
13CNMR: δ 193.89 (C=S), 167.72 (C=O-thiazole), 166.92
(C=O-acetic), 149.59, 144.67, 142.64, 142.44, 135.53, 129.67,
127.78, 125.72, 125.23, 123.60, 123.19, 122.48, 121.76,
118.90, 118.61, 111.65, 111.03, 45.75 (CH2COOH), 43.33 (
1 3

Journal of the Iranian Chemical Society
Results and discussion
Chemistry
protons and two singlet signals δ 4.70, 10.06 ppm assigned,
respectively, to –CH2 and COOH groups. Likewise, the
13CNMR spectrum displayed signals for hexyl group at δ
14.20, 22.43, 26.56, 28.61, 31.06 in addition to 2 CH2, 2
C=O and C=S groups at δ 43.33, 45.75, 166.92, 167.72,
193.89, respectively. The DCA-HC [32] was obtained
through refuxing of (3) with cyanoacetic acid in ethanol cat-
alyzed with piperidine. The pattern of its 1H NMR displayed
four signals related to hexyl protons at δ 0.77, 1.23, 1.75,
4.38, three signals related to methine-H, aromatic protons
and COOH at δ 8.28, 8.41, 8.79 ppm, respectively. 13CNMR
spectrum displayed signals for hexyl carbons at δ 14.20,
22.22, 26.33, 26.33, 28.76, 31.30, N–CH2 signal at δ 43.33,
aromatic carbon at δ 100.42–155.22, beside C≡N, C=O at δ
117.46, 164.34 ppm, respectively. The m/z=441 is in agree-
ment with the expected chemical formula C26H23N3O4.
Reaction of dicarbaldehyde (2) with phenyl hydrazine
and ethanol gave the desired phenylhydrazone derivative
(3), where the organic dye (DTC-HC) was obtained by
electrophilic substitution of phenylhydrazone (3) with tet-
racyanoethylene in DMF and heating under refux for 8 h.
Dye molecules have an essential role in DSSCs performance,
and the capability of the solar cell in sun power harnessing
depends on dye structure. The synthetic routes followed to
prepare the target organic dyes are presented in (Scheme 1).
According to the reported method, hexyl-carbazole-3,6-di-
carbaldehyde (2) was developed by 9H-carbazole alkylation
and accompanied by Vilsmeier–Haack formylation [22, 23].
In the present study, N-hexyl-carbazole-3,6-dicarbaldehyde
(2) was used as starting material to prepare the target organic
dyes (Scheme 1). The starting material (2) reacted as a sub-
strate for the corresponding DRA-HC [31] dye with rhoda-
nine-3-acetic acid, and spectral data and elemental analy-
sis confrmed the structure. Due to hydroxyl and carbonyl
groups, IR spectroscopy displayed absorption bands at 3450
and 1706 cm−1, respectively, and its 1 H NMR spectrum dis-
played four signals at δ 0.8, 1.24, 1.76 and 4.43 due to hexyl
Scheme 1. Synthesis of three required dyes DRA-HC, DCA-HC and DTC-HC
1 3

Journal of the Iranian Chemical Society
The absorption bands of the IR spectrum were 3319 and
2216 cm−1, respectively, opposite to NH and C≡N groups.
The spectrum of 1H NMR displayed four sharp singlet sig-
nals at δ 8.13, 8.11, 8.74, 9.32 ppm specifc for the two
methine-H, H4, H5-carbazole, two NH groups, respectively,
multiple signals at δ 6.50–7.58 ppm region owing to aro-
matic protons. Its mass spectrum showed a molecular ion
peak at m/z = 689, corresponding to a molecular formula
C42H31N11.
Properties of organic dyes
Particle size analyzer
The particle size distribution of DCA-HC, DRA-HC and
DTC-HC dyes were measured and displayed in Fig. 2. The
particle sizes of DCA-HC, DRA-HC and DTC-HC were
found to be 10 nm, 45 nm and 85 nm, respectively. This
indicated all organic dyes are in the range of nanoparticles
1–100 nm. Also, this result is useful in solar cell applications
due to increased surface area.
Scanning electron microscope (SEM)
Morphology of organic dyes DCA-HC, DRA-HC and DTC-
HC at three diferent magnifcations is shown in Fig. 3). The
fgure shows that no phase separation in the particle and the
homogeneity of the particles can be easily observed. Also,
the diameter of particles is small.
80
DTC- HC
70
60
50
40
30
20
10
0
DRA- HC
DCA- HC
Photophysical properties
UV–Vis absorption spectrum of the organic dyes in DMF
solution at concentration 4 × 10–5 M is given in Fig. 4,
and the corresponding photophysical data are stated in
Table 1. The dyes exhibited large absorption spectrums
of about 250–500 nm with two absorption bands. At
250–380 nm, short wavelengths can be allocated to the
π–π* transitions of the whole conjugated network, while
20
40
60
80
100
120
140
Size (nm)
Fig. 2 Particle size distribution of organic dyes
Fig. 3 SEM micrographs surface of DCA-HC at a ×1000, b ×3000, c ×5000, DRA-HC at d ×1000, e ×3000, f ×5000, and DTC-HC at g ×1000,
h ×3000, I ×5000
1 3

Journal of the Iranian Chemical Society
1000
800
600
400
200
0
80000
70000
60000
50000
40000
30000
20000
10000
0
DTC-HC
DCA-HC
DRA-HC
DRA-HC
DTC-HC
DCA-HC
250
300
350
400
450
500
550
600
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 4 Absorption spectra of DRA-HC, DCA-HC, and DTC-HC in
Fig. 5 Fluorescence spectrum of the organic dyes in DMF solution
DMF solution (4×10−5 M)
(4×10−5 M)
the longer wavelengths between the donor component and
the electron drawing part are between 400 and 550 nm,
emission of DCA-HC, DRA-HC and DTC-HC was 610,
545 and 525 nm, respectively. Emission of three dyes is
transferred to the longer wavelength due to the presence
of hexyl moiety as electron donor, Emission from DCA-
HC is better than the other two dyes; this result could be
due to the presence of two cyanoacetic acids anchoring
groups.
which relate to the ICT. The highest peaks with (λmax
)
absorption of DCA-HC, DRA-HC and DTC-HC appear
at 450, 400 and 395 nm, where the ε (molar absorptivity)
values were 7.1 × 104, 5.6 × 104 and 1.4 × 104 M−1 cm−1
,
respectively, which were higher than those of N3 dyes
(1.39 × 104), showing a good light harvest capability. It
should be remembered that the presence of rhodanine-
3-acetic acid as electron anchoring part into the DRA-
HC dye not only leads to peak red absorption, but also
improves the ε and expands the spectrum of molecular
absorption, which for large photocurrent are highly desir-
able, due to the presence of C=O and C=S functions
which have π–π* and π–π* transitions. But the introduc-
tion of cyanoacrylic acid into the DCA-HC dye and tet-
racyanoethylene into the DTC-HC as electron acceptor
unit leads to the absorption peak redshift lower than ε
compared to DRA-HC dye. The fluorescence emission
distribution of the substances analyzed is also shown
in Fig. 5. The λex of DCA-HC, DRA-HC and DTC-HC
was 450, 400 and 395 nm, respectively. The maximum
Electrochemical properties
The cyclic voltammetry (CV) curves given in Fig. 6 and
the corresponding data are listed in Table 1. Oxidation
potential of three dyes DRA-HC, DCA-HC and DTC-HC
vs NHE is 0.88, 0.84 and 0.80 V, respectively. It is more
positive than the standard potential of iodine/iodide (0.4 vs.
NHE) that the photo-oxidized dyes could be expected to be
reduced efciently by the iodine/iodide redox couple. Eox
to optical band gap E0–0 measured the possible reduction of
dyes E0–0* against NHE. From the onset of absorption, the
*
optical band-gap energy was calculated. The E0–0 values
measured (− 1.57, − 1.85, and − 1.91 vs. NHE) are more
Table 1 Optical and
a
e
f
Dye
aλmax (nm) εmax (M−1 cm−1
)
bEox (V) cHOMO (eV) dLUMO (eV) E0–0 (eV) E0–0* (V)
electrochemical properties of
organic dyes in DMF solutions
DRA-HC 450
DCA-HC 395
DTC-HC 400
68,300
56,000
31,100
0.88
0.84
0.80
−5.08
−5.04
−5.00
−2.63
−2.35
−2.29
2.45
2.69
2.71
−1.57
−1.85
−1.91
aMaximum absorption and maximum molar absorption coefcient of dyes in DMF solutions (4×10−5 M at
298 K)
bRedox potentials are reported with reference to the ferrocene internal standard
cDeduced from the oxidation potential using the formula, HOMO= –(Eox +4.2)
dObtained from the optical band gap and the electrochemically deduced HOMO value,
LUMO=E0–0 +HOMO
eBand gap E0–0, calculated from the observed optical edge
fExcited-state oxidation potential versus NHE
1 3

Journal of the Iranian Chemical Society
with their electronic properties. It is wanted to estimate the
applicability of quantum–mechanical outcomes to inves-
tigate the efciency of such dyes. The quantum outcomes
were estimated for the three dyes utilizing the DMOL3 pro-
gram. Calculations are used to achieve quantum–chemical
specifcations of synthesized organic compounds, such as
the higher occupied molecular orbital energy (EHOMO), lower
unoccupied molecular orbital (ELUMO). Also, the specifc
parameters, ionization potential (IP), absolute softness (σ),
absolute hardness (η), electron afnity (EA), electrophilicity
index (ω), maximum amount of electron transfer (ΔNmax),
separation energy (ΔE), absolute electronegativity (χ), and
global softness (S) were measured and calculated [33–37].
In calculations of molecular parameters, Eqs. (1–7) are used
as indicated below (Fig. 8):
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0.005
0.004
0.003
0.002
0.001
0.000
Bare Pt cyclic
voltamogram( without dye)
0.2
0.3
0.4
0.5
0.6
0.7
Potential ( V vs Ag / AgCl )
0.8
0.9
1.0
DTC-HC
DRA-HC
DCA-HC
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Potential (V vs Ag/ AgCl)
Fig. 6 Cyclic voltammograms of organic dyes recorded and bare
Pt cyclic voltammogram blank in DMF solutions (4×10−5 M) with
TBAI
(
)
ꢀ = −1∕2 ELUMO + EHOMO
,
(1)
negative than the TiO2 conduction band (− 0.5 vs. NHE)
edge; indication to the cycle of electron injection is power-
fully desired to TiO2, while the dyes are regenerated. As
mentioned in Table 1, a negative change between the two Eox
and E0–0* can be obtained for DTC-HC against DCA-HC by
0.03 V, which led to a similar band gap. Negative changes
were found between both Eox and E0–0* for DRA-HC against
DCA-HC, and it was observed that the Eox shift was greater
IP = −EHOMO
,
(2)
(3)
(
)
ꢀ = 1∕2 ELUMO − EHOMO
,
S = 1∕2ꢀ,
(4)
(5)
(6)
(7)
ꢀ = IP2∕2ꢁ,
*
than that of E0–0 which contributed to a decrease in the
HOMO–LUMO band gap.
ΔNmax = −IP/ ꢀ,
Molecular modeling
EA = −ELUMO
.
Without a crystal structure, to achieve a compound molecu-
lar conformation, minimizing energy studies were carried
out using DMOL3 calculations based on the DFT semi-
core pseudopods [27, 28]. Upon geometrical optimization
of the synthesized compounds, the molecular parameters
are defned in Table 2. The molecular structure of the com-
pounds together with the scheme for atom numeration is
shown in Fig. 7.
We can conclude the following from the data obtained
in Table 2:
a. The most common quantum–chemical parameters
include HOMO and LUMO orbitals. HOMO orbital
is the electron donor orbital, it is the highest occupied
energy orbital by electrons comprises electrons. LUMO
is the electron accepter orbital since it is the lowest
energy orbital that can accept electrons. The HOMO
and LUMO energies are negative indicating the stability
of the synthesized compounds [35].
Molecular parameters
To confrm the study results, theoretical methodology was
evaluated to connect the efciency for the three tested dyes
Table 2 Calculated quantum–
Dye
HOMO LUMO
x
η
σ
IP
ΔE
ω
ΔNmax
EA
S
chemical parameters of organic
dyes
DRA-HC −5.01
DCA-HC −5.17
DTC-HC
−3.21
−3.10
−3.67
4.11 0.90 1.11 5.01 1.80 13.94
4.14 1.04 0.97 5.17 2.07 12.91
4.11 0.44 2.27 4.55 0.88 23.53 −10.34 3.67 0.22
−5.57 3.21 0.45
−5.00 3.10 0.52
−4.55
All units are (eV)
1 3

Journal of the Iranian Chemical Society
Fig. 7 Electron distributions in the HOMOs and LUMOs and optimized structures of organic dyes
b. Absolute softness (σ) and hardness (η) are essential for
the calculation of molecular stability and reactivity. A
hard molecule has a large energy gap, while soft mol-
ecule has a small energy. The reactivity of soft mol-
ecules more than hard molecules may be attributed to
their ability to donate electrons to an acceptor more than
hard ones.
(η = 1.03 eV) > DRA-HC (η = 0.90 eV) > DTC-HC
(η=0.44 eV).
d. The electrophilicity index (ω) indicates the molecule’s
ability to accept electrons. The results of this parameter
obey this order: DCA-HC (ω = 23.53 eV) > DRA-HC
(ω=13.29 eV)>DTC-HC (ω=12.91 eV).
e. The energy gap (EHOMO–ELUMO) acts as an important
stability index. It can be used as an important parameter
for characterization of the chemical molecular reactivity.
The soft molecule is a more polarized molecule with a
small energy gap.
c. A small HOMO–LUMO gap means a high chemical
reactivity [38]. Pearson indicated that energy separation
between HOMO–LUMO represents chemical hardness
of the molecule [39]. In accordance with the principle
of maximum hardness, increased hardness (η) gives the
molecule increased stability [40, 41], which is listed in
Table 2. The chemical hardness values of compounds
are observed in accordance with this order: DCA-HC
f. For compound DCA-HC, it was observed that as given
in Table 2, It has the highest value of EHOMO, which
means the most efcient potential for donated electrons,
when the electron removed from its lowest molecular
orbital occupation HOMO.
1 3

Journal of the Iranian Chemical Society
Fig. 8 Out mol geometry opti-
mization of organic dyes by m.s
g. Electron afnity represents the released energy when an
electron is added to the smallest unoccupied molecu-
lar orbital LUMO. It is higher for DTC-HC (3.67 eV)
and lower for DCA-HC (3.10 eV). This confrms that
the DTC-HC has a higher tendency to accept an elec-
tron to form the anion more than DCA-HC molecule.
Thus, DTC-HC dye shows high efciency as an electron
acceptor than DCA-HC. This is confrmed by the low-
est hardness and highest dipole moment for DTC-HC,
which measures the electrical asymmetry of a molecule.
Photovoltaic performance of DSSCs
Photovoltaic performance of the three organic dyes as sen-
sitizers in DSSCs was calculated by measuring current den-
sity–voltage (J–V) as shown in Fig. 9, and the corresponding
data including short-circuit current (JSC), open-circuit volt-
age (VOC), fll factor (FF), and photocurrent efciency or η
are mentioned in Table 3. The system focusing on DCA-HC
displayed the highest short-circuit current (JSC), compared
to the device based on DRA-HC, which exhibits the moder-
ate JSC. The device based on DCA-HC displayed the highest
FF (0.81), which gives the highest VOC (0.708 V), Pmax of
(0.354 mW) and highest η (1.41%) due to the highest JSC
(2.46 mA/cm2). The device based on DRA-HC exhibits the
0.4
DTC-HC
3.0
DTC-HC
DRA-HC
DRA-HC
DCA-HC
2.5
DCA-HC
0.3
0.2
0.1
0.0
2.0
1.5
1.0
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage ( V )
Voltage ( V )
Fig. 9 J–V curves and P–V curves for the DSSCs of organic dyes. The plots were measured under the light intensity of 1.0 sun
1 3

Journal of the Iranian Chemical Society
Table 3 Photovoltaic
performance of DSSCs based
on organic dyes
Dye
VOC (V) JSC (mA cm−2
)
FF
η (%) Pmax (mW) aRs (Ω) bR1 (Ὠ) cR2 (Ὠ) τe (ms)
DTC-HC 0.544
DRA-HC 0.583
DCA-HC 0.708
1.67
2.24
2.45
0.63 0.58 0.143
0.81 1.06 0.266
0.81 1.41 0.354
31.11
30.70
31.30
5.10
5.60
3.20
93.78
76.80
46.90
2.52
2.92
3.37
aRS is FTO interface resistance
bR1 is due to the resistance at the interface between the counter electrode and the electrolyte
cR2 possibly originates from the backward charge transfer from TiO2 to the electrolyte and the electron con-
duction in porous TiO2 flm
moderate η (1.06%) and Pmax of (0.266 mW) due to the mod-
erate ISC (2.24 mA/cm2), VOC (0.583 V) and FF (0.81). The
DTC-HC device exhibits the lowest VOC (0.544 V), the low-
est JSC (1.67 mA/cm2), and moderate FF (0.63) which lead
to lowest η (0.58%) and Pmax of (0.143 mW). The sensitizer
has larger energy gap between its LUMO level and the TiO2
conductive band level with more connected anchor groups,
leading to increased electron into TiO2, increased JSC, and
contributing to increase in η percentage. As shown on the
chemical structure of the three organic dyes, the electron
donating group (D) and conjugating group (π) are the same,
but the diference is in the anchoring function. Acrylic acid
accepter-based DCA-HC dye is highly efcient among all
the dyes in the acceptor modifcation component. This high
efciency is due to strong electrons withdrawing the charac-
ter and well-bound acrylic acid in the component dye mol-
ecule. In the case of rhodanine-3-acetic acid nucleus-based
dyes DRA-HC, the involvement of the –CH2 group isolates
the dye molecules of carboxylic acid, thus preventing the
injection of an electron into the TiO2 conduction band, and
this may be the reason for the low performance relative to
the acrylic acid. DTC-HC displayed very low efciency for
tetracyanoethylene-based dyes, which could be due to the
absence of conjugation between nitrile groups and poor
electron-withdrawing dye structure.
60
50
40
30
20
10
0
DTC-HC
DRA-HC
DCA-HC
20
40
60
80
Z' (Ω)
100
120
140
Fig. 10 Nyquist plots for DSSCs based on organic dyes under dark
The results of Nyquist plots are in good agreement with
the results of VOC with the DSSCs based on these dyes:
DCA-HC (0.708 V) > DRA-HC (0.583 V) > DTC-HC
(0.544 V). Also, the major semicircles (R1) displayed in the
Nyquist plots (Fig. 10) of the devices were related to the
charge-transfer resistance between the counter electrode
and the electrolyte. Also, (R2) possibly originates from the
backward charge transfer from TiO2 to the electrolyte and
the electron conduction in porous TiO2 flm. In this case,
Rs value represents the FTO interface and the resistance
interface between counter electrode and electrolyte, but we
used the same Pt counter electrode and the same electrolyte
in our case. The Rs value of all sensitizers, which corre-
sponds to the intercept of the frst semicircles in each case,
displayed similar results. The results of ftting are listed
in Table 3 using the inset model which is simple Randles
model including Rs, R with CPE (constant phase element)
which is related to nonideal capacitance owing to roughness
[43–45]. The electrochemical analysis displayed that the R2
values changed in accordance with applied devices, which
the R2 values for sensitizers employing longer branched
dyes (93.78 Ω for DTC-HC) were indeed higher than those
moderate branched (76.80 Ω for DRA-HC) but decreased
in case of simple branched (46.90 Ω for DCA-HC) owing
to their increased charge transfer ability. Also, the highest
VOC of the device based DCA-HC can be explained by the
electron lifetime, calculated through the equation τe=1/2πf,
Electrochemical impedance spectroscopy (EIS)
EIS is an efective characterization technology in DSSCs of
the charge transportation carrier [42]. Usually, high open-
circuit voltage (VOC) can also be provided by a large number
of electrons in the TiO2 conduction band. To analyze the
comparative charge selection performance with the novel
dyes in the DSSC systems, DCA-HC, DRA-HC, and DTC-
HC in addition to both the circuit and the Nyquist plot are
shown in Fig. 10. Larger semicircles in low-frequency refect
resistance to recombination (Rrec) at TiO2/electrolyte inter-
face. As shown, the radius of the semicircles is increased in
this order DTC-HC<DRA-HC<DCA-HC, indicating that
the DCA-HC device had the largest resistance between the
TiO2 surface and the electrolyte which can be attributed to
the highest voltage obtained.
1 3

Journal of the Iranian Chemical Society
14. C.Y. Chen, M. Wang, J.Y. Li, N. Pootrakulchote, L. Alibabaei,
C.H. Ngoc-Le, J.D. Decoppet, J.H. Tsai, C. Grätzel, C.G. Wu,
S.M. Zakeeruddin, M. Grätzel, ACS Nano 3, 3103 (2009)
15. S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B.F.E. Curchod,
N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M.K. Nazeerud-
din, M. Grätzel, Nat. Chem. 6, 242 (2014)
where f is the peak frequency of lower frequency range in
bode plot. The value of τe is listed in Table 3. In order of
DTC-HC>DRA-HC>DCA-HC, the lower-frequency peak
frequency range is decreased, and the electron recombina-
tion lifetime is calculated in reverse with the calculated val-
ues of 2.52, 2.92 and 3.37 ms, respectively.
16. L.Q. Bao, P. Ho, R.K. Chitumalla, J. Jang, S. Thogiti, J.H. Kim,
Dyes Pigments 149, 25 (2018)
17. V. Venkatraman, S. Abburu, B.K. Alsberg, Phys. Chem. Chem.
Phys. 17, 27672 (2015)
18. Y. Hua, L.T.L. Lee, C. Zhang, J. Zhao, T. Chen, W.Y. Wong, W.K.
Wong, X. Zhu, J. Mater. Chem. A 3, 13848 (2015)
19. T. Horiuchi, H. Miura, K. Sumioka, S. Uchida, J. Am. Chem. Soc.
126, 12218 (2004)
Conclusion
In the present work, three efcient dyes (DTC-HC, DRA-
HC, and DCA-HC) with diferent electron acceptors (tetra-
cyanoethylene, rhodanine acetic acid and cyanoacetic acid)
attached to N-hexyl carbazole-3,7-dicarbaldehyde (2) have
been designed and characterized as photosensitizers. The
performance of the DSSCs based on the prepared dyes was
tested and analyzed. Moreover, introducing the cyanoacetic
group as anchoring moiety enhances the TiO2 conduction
band, so the VOC of DCA-HC is higher than other two dyes
and the overall conversion power efciency is increased to
1.41%. Due to the absorption of TiO2 flm and the energy
of adsorption, an electron acceptor of dyes including tricya-
noethylene group is not sufcient to produce highly efcient
solar cells. However, it is suggested that carbazole and alkyl
moieties could be applied more than hexyl as a bridge and
a donor moiety.
20. M. Liang, J. Chen, Chem. Soc. Rev. 42, 3453 (2013)
21. S.H. Kim, C. Sakong, J.B. Chang, B. Kim, M.J. Ko, D.H. Kim,
K.S. Hong, J.P. Kim, Dyes Pigments 97, 262 (2013)
22. A.S. Beni, M. Zarandi, B. Hosseinzadeh, A.N. Chermahini, J.
Mol. Struct. 1164, 155 (2018)
23. J. Ostrauskaite, V. Voska, J. Antulis, V. Gaidelis, V. Jankauskas,
J.V. Grazulevicius, J. Mater. Chem. 12, 3469 (2002)
24. A.B. Kashyout, M. Soliman, M. Fathy, Renew. Energy 35, 2914
(2010)
25. D. Kuang, S. Ito, B. Wenger, C. Klein, J.E. Moser, R. Humphry-
Baker, S.M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 128,
4146 (2006)
26. A. Kessi, B. Delley, Int. J. Quantum Chem. 68, 135 (1998)
27. P.S. Institut, C.-V. Psi, Int. J. 69, 423 (1998)
28. B. Delley, J. Chem. Phys. 113, 7756 (2000)
29. Z. Zhang, X. Liu, B.I. Yakobson, W. Guo, J. Am. Chem. Soc. 134,
19326 (2012)
30. B. Hammer, L.B. Hansen, J.K. Nørskov, Phys. Rev. B Conden.
Matter Mater. Phys. 59, 7413 (1999)
31. T.Y. Wu, M.H. Tsao, S.G. Su, H.P. Wang, Y.C. Lin, F.L. Chen,
C.W. Chang, I.W. Sun, J. Braz. Chem. Soc. 22, 780 (2011)
32. S.A. Kim, H.J. Jo, M.R. Jung, Y.C. Choi, D.K. Lee, M. Lee, J.H.
Kim, Mol. Cryst. Liq. Cryst. 551, 283 (2011)
References
33. M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E.
Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc.
115, 6382 (1993)
1. K. Ranabhat, L. Patrikeev, A. A. Evna Revina, K. Andrianov, V.
Lapshinsky, and E. Sofronova, J. Appl. Eng. Sci. 14, 481 (2016).
2. A. Blakers, N. Zin, K.R. McIntosh, K. Fong, Energy Procedia 33,
1 (2013)
34. P. Geerlings, F. De Proft, W. Langenaeker, Chem. Rev. 103, 1793
(2003)
3. P.A. Troshin, R.N. Lyubovskaya, V.F. Razumov, Nanotechnol.
Russ. 3, 242 (2008)
35. R. G. Parr and R. G. Pearson, J. Am. Chem. Soc. 105, 7512 (1983)
36. I. Fernández, E. Martínez-Viviente, P.S. Pregosin, Inorg. Chem.
43, 4555 (2004)
4. Z. Fan, J. Xiao, K. Sun, L. Chen, Y. Hu, J. Ouyang, K.P. Ong, K.
Zeng, J. Wang, J. Phys. Chem. Lett. 6, 1155 (2015)
5. K.R. Catchpole, A. Polman, Opt. Express 16, 21793 (2008)
6. T. Sogabe, Q. Shen, K. Yamaguchi, J. Photonics Energy 6, 040901
(2016)
37. A.A. Soliman, M.A. Amin, A.A. El-Sherif, S. Ozdemir, C. Var-
likli, C. Zafer, Dyes Pigments 99, 1056 (2013)
38. J.I. Aihara, J. Phys. Chem. A 103, 7487 (1999)
39. R.G. Pearson, Coord. Chem. Rev. 100, 403 (1990)
40. M. Aljahdali, A.A. El-sherif, Inorg. Chim. Acta 407, 58 (2013)
41. M.H. Helal, M.A. Salem, M.A. Gouda, N.S. Ahmed, A.A. El-
Sherif, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 147,
73 (2015)
7. A.N.B. Zulkifli, T. Kento, M. Daiki, A. Fujiki, J. Clean Energy
Technol. 3, 382 (2015)
8. C. Satheeshkumar, M. Ravivarma, P. Rajakumar, R. Ashokkumar,
D.C. Jeong, C. Song, Tetrahedron Lett. 56, 321 (2015)
9. L. Giribabu, V.K. Singh, M. Srinivasu, C.V. Kumar, V.G. Reddy,
Y. Soujnya, P.Y. Reddy, J. Chem. Sci. 123, 371 (2011)
10. M.K. Nazeeruddin, E. Baranof, M. Grätzel, Sol. Energy 85, 1172
(2011)
42. G. Zhu, L. Pan, J. Yang, X. Liu, H. Sun, Z. Sun, J. Mater. Chem.
22, 24326 (2012)
43. F.E.T. Heakal, A.M. Fekry, M.Z. Fatayerji, J. Appl. Electrochem.
39, 1633 (2009)
11. W. Wu, J. Wang, Z. Zheng, Y. Hu, J. Jin, Q. Zhang, J. Hua, Sci.
Rep. 5, 8592 (2015)
44. F. El-Taib Heakal, O.S. Shehata, N.S. Tantawy, A.M. Fekry, Int.
J. Hydrog. Energy 37, 84 (2012)
12. S. Bykkam, B. Kalagadda, V.R. Kalagadda, M. Ahmadipour, C.S.
Chakra, V. Rajendar, J. Electron. Mater. 47, 620 (2018)
13. K. Sharma, V. Sharma, and S. S. Sharma, Nanoscale Res. Lett.
13, 381 (2018).
45. F. El-Taib Heakal, M.M. Hefny, A.M. Abd El-Tawab, Int. J. Elec-
trochem. Sci. 8, 4610 (2013)
1 3