The effects of second electron acceptor group on the performance of tetrazole-based nanocrystalline TiO2 sensitizers in DSSCs

Article abstract of DOI:10.1016/j.saa.2017.01.061

Three new organic sensitizers with two electron acceptor groups were synthesized and applied to nanocrystalline TiO₂ solar cells. The ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-yl) acrylonitrile and 1H-tetrazole-5-acetic acid moieties w

Full text of DOI:10.1016/j.saa.2017.01.061

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
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Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The effects of second electron acceptor group on the performance of
tetrazole-based nanocrystalline TiO2 sensitizers in DSSCs
⁎
Zahra Jafari Chermahini, Alireza Najafi Chermahini , Hossein A. Dabbagh, Behzad Rezaei, Neda Irannejad
Department of Chemistry, Isfahan University of Technology, 84154-83111 Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new organic sensitizers with two electron acceptor groups were synthesized and applied to nanocrystal-
line TiO2 solar cells. The ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-yl) acrylonitrile and 1H-tetrazole-5-
acetic acid moieties were introduced to the triphenylamine as electron acceptor groups. The photophysical, elec-
trochemical and photovoltaic properties of the solar cells based on the synthesized sensitizers were studied and
compared with their counterparts of single electron acceptor type. Quantum chemical calculations were also car-
ried out to consideration of the electronic and optical properties of these dyes. The dye with the (2H-tetrazol-5-
yl) acrylonitrile electron acceptors showed the absorption maxima in the longer wavelength, compared to the
dyes with ethyl 2-(1H-tetrazol-5-yl) acetate and 1H-tetrazole-5-acetic acid. The solar cell based on the dye
with 1H-tetrazole-5-acetic acid showed the highest conversion efficiency of 3.53% (open circuit voltage =
569 mV, short circuit photocurrent density = 11.50 mA cm−2, and fill factor of 54% under AM 1.5G conditions).
The results also showed that the dyes with two electron acceptor groups gave the higher performance than the
dyes with single electron acceptor.
Received 8 November 2016
Received in revised form 8 January 2017
Accepted 27 January 2017
Available online 30 January 2017
Keywords:
Anchoring group
Dye sensitized solar cell
Electron acceptor group
Sensitizer
Tetrazole
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Among the metal-free organic dyes, triphenylamine (TPA) and its deriv-
atives have been investigated widely due to the prominent electron-do-
Dye sensitized solar cells (DSSCs) discovered by Grätzel in 1991 [1]
have attracted considerable attention as a low-cost and high-efficiency
alternative to inorganic semiconductor-based photovoltaic devices for
the conversion of sunlight into electricity [2]. Anode, cathode, electro-
lyte, and photosensitive dyes are four main factors that influence the
performance of the DSSCs [3]. Upon light absorption by the dye an-
chored on the TiO2 surface, the excited electrons are injected into the
conduction band of TiO2, generating an electric current. At the same
time, the oxidized dye is regenerated by the electrolyte [4]. Therefore,
the sensitizer (dye) is a crucial element, which significantly influences
on the power conversion efficiency as well as the stability of the devices
[5]. Conventional Ru-based sensitizers such as N3, N719 and the black
dyes are investigated widely for DSSCs [6]. However, the scarcity of
the Ru(II) metal, lack of good absorption in the red region of the visible
spectrum and the difficulty of obtaining purified dyes have limited the
practical application of these complexes for DSSCs [7]. Organic dyes,
due to their many advantages including lower cost, easier modification
and purification, environmental friendliness and high molar extinction
coefficient are suitable as sensitizers for DSSCs [8]. Most organic sensi-
tizers contain a dipolar structure with an electron donor (D) and an
electron acceptor (A) separated by a π-conjugated bridge (B) [9].
nating ability and the hole-transport properties of triphenylamine [10–
14]. The non-planar configuration of TPA also reduces the aggregation of
sensitizer and suppresses charge recombination process of redox cou-
ple–TiO2 system in DSSCs, which can lead to the higher overall conver-
sion efficiencies [15].
Recently, the first triphenylamine-based dye for DSSC with the tetra-
zole anchoring group has been synthesized by Massin et al. The opto-
electronic and photovoltaic properties of the tetrazole-based dye
demonstrated the effectiveness of the tetrazole group as a serious alter-
native anchoring group for organic dyes in DSSCs [16].
We have also reported the triphenylamine-based dyes for DSSCs
with the electron acceptor groups based on tetrazole [17] and investi-
gated the modification of these sensitizers by introducing electron
donor groups into the TPA moiety. In addition to the electron-donating
groups, the introduction of withdrawing groups into the dye structure
expands the π-conjugated system of the sensitizing molecules and
gives charge-transfer character of absorption and broad and high ab-
sorptivity [18]. According to these strategies in molecular design of sen-
sitizers, in this article, the modification of these triphenylamine dyes is
reported with the electron acceptor groups based on tetrazole by intro-
ducing additional electron acceptors on the phenyl groups of TPA moi-
ety (Fig. 1). The photovoltaic properties of these dyes were measured
to identify the effects of additional electron acceptor group on the per-
formance of DSSCs. Density functional theory (DFT) calculations were
⁎
Corresponding author.
E-mail addresses: anajafi@cc.iut.ac.ir, najafy@gmail.com (A. Najafi Chermahini).
http://dx.doi.org/10.1016/j.saa.2017.01.061
1386-1425/© 2017 Elsevier B.V. All rights reserved.

80
Z. Jafari Chermahini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
Fig. 1. Structure of the synthesized dyes with two electron acceptors.
also performed for consideration of the electronic structures and the op-
tical properties of the dyes.
used. The precursor compound P1 and the acceptor groups (Scheme
1) were synthesized according to the literature procedures [19,20].
2.3. Fabrication of Dye-sensitized Solar Cells
2. Materials and Methods
Fluorine-doped tin oxide (FTO) coated glass substrates were treated
with 40 mM TiCl4 aqueous solution at 70 °C for 30 min to form a
blocking layer. The photoanodes were prepared by doctor blade and ti-
tania pastes (Sharif Solar Co.). They are composed of two layers of nano-
crystalline TiO2 particles including a transparent layer, composed of
20 nm TiO2 particles with thickness of 7 μm and a scattering layer com-
posed of 150–300 nm with thickness of 5 μm. The role of second layer is
to enhance the light harvesting and improve the device performance.
The TiO2-coated FTO films were sintered at 375 °C (10 min), 450 °C
(15 min), and 500 °C (30 min). After reaching to environment temper-
ature, the obtained films were again treated with 40 mM TiCl4 aqueous
solution at 70 °C for 30 min, washed with distilled water and heated at
500 °C for 30 min. Then, the TiO2 electrodes were immersed into the dye
solutions (0.4 mM in C2H5OH) for 20 h in the dark before washing. The
counter electrode was prepared using FTO glass coated with a drop of
solution of H2PtCl6 and then allowed to stand at 500 °C for 30 min.
Two electrodes were assembled into a sandwich type cell with an io-
dine-based liquid electrolyte. The active area of each prepared DSSC
was 0.25 cm2. Solar cells were illuminated by AM 1.5G solar simulator
(Sharif Solar, SIM-1000) to give 100 mW/cm2 at the surface of the cell.
The photocurrent-voltage (I-V) measurements were performed using
a Bio-Logic SAS model SP-300. The action spectra of monochromatic in-
cident photon-to-current conversion efficiencies (IPCEs) for the solar
2.1. Materials and Reagents
All used solvents were obtained from Sigma-Aldrich and applied as
received. All reagents were commercially available and used without
new purification. Ethyl cyanoacetate and triphenylamine were pur-
chased from Merck and used as received without further purification.
2.2. Analytical Instruments and Measurements
The 1H and 13C NMR spectra were recorded by means of Bruker
400 MHz spectrometers. FT-IR absorption spectra were taken in KBr pel-
lets using a FT-IR Jasco 680 plus spectrometer. FT-IR spectrum of TiO2
nanoparticles was subtracted from the spectra of TiO2-adsorbed dyes.
Mass analysis was performed on a 5973 Network Mass Selective Detec-
tor (Agilent Technology (HP)) apparatus. UV–Vis spectra in solution and
adsorbed on TiO2 films were measured with a Jasco-570 UV/Vis spectro-
photometer. Cyclic voltammetry data were gained using a SAMA 500
electroanalyser system, SAMA research center, Iran. The three-electrode
system (Pt working electrode, Pt counter electrode and an Ag/AgCl ref-
erence electrode calibrated with ferrocenium/ferrocene as an internal
reference) with 0.1 M tetrabutylammonium hexafluorophosphate as
the salt support at a scan rate of 0.1 V/s at room temperature was
Scheme 1. Synthesis paths for the dyes D1-D3: (i) POCl3, DMF, 80 °C, 48 h, (ii) Ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-yl) acrylonitrile, or 1H-tetrazole-5-acetic acid, NH(Et)2,
CH3CN, reflux, 48 h.

Z. Jafari Chermahini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
81
cells were detected with a IPCE system (Sharif Solar, IPCE-015) using a
100 W halogen lamp.
(400 MHz, DMSO-d6, ppm): δ = 6.92 (d, J = 8.8 Hz, 4H), 7.08 (d, J =
8.4 Hz, 4H), 7.32 (d, J = 8.4 Hz, 2H), 7.49 (t, J = 6.8 Hz, 1H), 7.72 (m,
6H). 13C NMR (400 MHz, DMSO-d6, ppm): δ = 121.48, 124.02, 125.03,
128.20, 129.82, 129.86, 137.35, 146.46, 148.51, 164.81. MS: m/z =
79.1, 103.0, 133.1, 158.0, 194.1, 215.1, 236.2 (base peak), 264.2, 292.1,
313.3, 341.3, 362.3, 383.2, 407.3, 430.4, 467.4, 495.4, 521.4 (M+). Anal.
Calcd for C26H19N9O4: C, 59.88; H, 3.67; N, 24.17; Found: C, 59.84; H,
3.63; N, 24.12.
2.4. Synthesis of Dyes
The detailed synthesis paths to the dyes are shown in Scheme 1. The
intermediate P1 was synthesized via the Vilsmeyer-Haack reaction. The
dyes D1-D3 were synthesized by Knoevenagel condensations of the in-
termediate P1 with ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-
yl) acrylonitrile, or 1H-tetrazole-5-acetic acid.
2.5. Calculations
2.4.1. General Procedures for the Synthesis of D1-D3
The dyes considered in this work were subjected to quantum chem-
ical analysis by DFT using the B3LYP [21] method at the 6–31 + G(d)
basis set in the Gaussian 09 program [22]. The stable structures were
demonstrated by no imaginary frequency. Time-dependent-DFT excited
state calculations in ethanol solution using the B3LYP/6–31 + G(d)
method were done for simulation of the absorption spectra of the
dyes. Solvent effects were included via the polarizable continuum
model (PCM) [23].
To an aldehyde (1 equiv) solution in CH3CN (5 mL), diethylamine
(20 equiv) and ethyl 2-(1H-tetrazol-5-yl) acetate ((2H-tetrazol-5-yl)
acrylonitrile, or 1H-tetrazole-5-acetic acid) (2.5 equiv) were added
and refluxed for 48 h. After cooling and removing the solvent, the mix-
ture acidified with 5 M hydrochloric acid. The resulted precipitate was
filtered and washed by water, dried, and purified by column chromatog-
raphy (ethyl acetate/ethanol).
2.4.2. Diethyl 3,3′-((phenylazanediyl)bis(4,1-phenylene))(2E,2′E)-bis(2-
(1H-tetrazol-5-yl)acrylate) (D1)
3. Results and Discussion
The dye D1 was prepared according to the general procedure from
P1 (301 mg, 1 mmol), ethyl 2-(1H-tetrazol-5-yl) acetate (390 mg,
2.5 mmol) and diethylamine (2 mL, 20 mmol) in CH3CN (5 mL) to
give a deep yellow solid with 90% yield. FT-IR (KBr, cm−1): ν = 511
(w), 545 (w), 601 (w), 622 (w), 692 (w), 748 (w), 832 (w), 936 (w),
1027 (w), 1062 (w), 1118 (m), 1215 (m), 1265 (s), 1327 (m), 1369
(w), 1509 (s), 1585 (s), 1712 (s), 2757 (m, br).
3.1. UV–Vis Absorption Spectra
Fig. 2 presents the absorption spectra of the synthesized dyes in eth-
anol solution and Table 1 lists the corresponding data. The dyes D1-D3
reveal two separate absorption bands around 270 and 400 nm. The
band found at a shorter wavelength is assigned to an aromatic π-π*
transition and the band at a longer wavelength can be assigned to an in-
tramolecular donor → acceptor charge-transfer (ICT) transition from
the triphenylamine moiety to the acceptor moieties. The D1 data
shows the highest extinction coefficient (ɛ) of 51,950 L/M−1 cm−1 at
410 nm in compare to D2 and D3. The D2 dye bearing (2H-tetrazol-5-
yl) acrylonitrile electron acceptors shows the absorption maxima at
the longer wavelength in compare to D1 and D3. The dyes D1-D3
show the higher molar extinction coefficients than the corresponding
dyes with single electron acceptor reported in our previous work [17].
For comparison of the maximum absorptions of the dyes D1-D3 with
those of the dyes with single electron acceptor, the absorption spectra
of D1-D3 were also recorded in CH2Cl2 solutions. The absorption spectra
in CH2Cl2 solutions are shown in Fig. S1 in the Supplementary materials.
The absorption spectra of the dyes with single electron acceptor group
are also shown in Fig. S1. The dyes with two electron acceptors (D1-
D3) are red shifted in their absorption spectra in compare to the corre-
sponding dyes with single electron acceptor due to the extension of
electron delocalization over the whole molecule caused by additional
acceptor moiety. The degrees of the shift of D1, D2 and D3 are 23, 21
1H NMR (400 MHz, DMSO-d6, ppm): δ = 1.20 (t, J = 7.2 Hz, 6H), 4.21
(q, J = 7.2 Hz, 4H), 6.84 (d, J = 8.8 Hz, 4H), 6.95 (d, J = 8.8 Hz, 4H), 7.07
(d, J = 7.6 Hz, 2H), 7.23 (t, J = 7.6 Hz, 1H), 7.39 (t, J = 7.6 Hz, 2H), 8.01 (s,
2H). 13C NMR (400 MHz, DMSO-d6, ppm): δ = 13.99, 61.46, 121.31,
122.19, 125.97, 126.42, 126.71, 130.15, 130.80, 131.98, 144.89, 146.38,
148.54, 164.56. MS: m/z = 69.1, 98.1, 129.1, 171.1, 211.2, 236.2, 264.2,
313.2, 341.3, 368.3 (base peak), 395.3, 423.3, 467.3, 495.4, 523.4, 551.5,
577.5 (M+). Anal. Calcd for C30H27N9O4: C, 62.38; H, 4.71; N, 21.83;
Found: C, 62.35; H, 4.69; N, 21.80.
2.4.3. (E)-3-(3-((4-((E)-2-cyano-2-(1H-tetrazol-yl)vinyl)phenyl)(phe
nyl)amino)phenyl)-2-(1H-tetrazol-5-yl)acrylonitrile (D2)
The D2 dye was prepared according to the general procedure
from P1 (301 mg,
1 mmol), (2H-tetrazol-5-yl) acrylonitrile
(272.5 mg, 2.5 mmol) and diethylamine (2 mL, 20 mmol) in CH3CN
(5 mL) to give a deep orange solid with 90% yield. FT-IR (KBr,
cm−1): ν = 497 (m), 525 (m), 616 (w), 699 (m), 727 (w), 762
(w), 832 (m), 922 (w), 985 (w), 1027 (w), 1069 (w), 1181 (s),
1222 (w), 1292 (s), 1327 (s), 1431 (w), 1509 (s), 1564 (s), 2217
(m), 2988 (m, br). 1H NMR (400 MHz, DMSO-d6, ppm): δ = 7.19
(d, J = 8.8 Hz, 4H), 7.25 (d, J = 7.6, 2H), 7.31 (t, J = 7.2 Hz, 1H),
7.48 (t, J = 7.6 Hz, 2H), 7.98 (d, J = 8.8 Hz, 4H), 8.29 (s, 2H). 13C
NMR (400 MHz, DMSO-d6, ppm): δ = 116.33, 122.61, 125.09,
126.18, 126.34, 126.85, 130.28, 131.57, 144.96, 145.81, 149.14,
156.53. MS: m/z = 55.1, 83.1, 103.1, 123.1, 151.1, 178.0 (base
peak), 196.2, 220.1, 256.2, 290.2, 332.3, 360.3, 451.1, 482.4
(M+−1). Anal. Calcd for C26H17N11: C, 64.59; H, 3.54; N, 31.87;
Found: C, 64.57; H, 3.50; N, 31.85.
2.4.4. (2E,2′E)-3,3′-((phenylazanediyl)bis(4,1-phenylene))bis(2-(1H-
tetrazol-5-yl)acrylic acid) (D3)
The D3 dye was prepared according to the general procedure from
P1 (301 mg, 1 mmol), 1H-tetrazole-5-acetic acid (320 mg, 2.5 mmol)
and diethylamine (2 mL, 20 mmol) in CH3CN (5 mL) to give a pale or-
ange solid with 85% yield. FT-IR (KBr, cm−1): ν = 517 (w), 699 (m),
762 (w), 825 (m), 985 (w), 1062 (w), 1166 (s), 1222 (m), 1285 (s),
1327 (s), 1502 (s), 1585 (s), 1687 (m), 3029 (m, br). 1H NMR
Fig. 2. Absorption spectra of D1-D3 in C2H5OH solution (2 × 10−5 M).

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Z. Jafari Chermahini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
Table 1
Optical and electrochemical properties of the dyes D1-D3 in solutions and adsorbed on TiO2.
a
a
b
c
d
e
f
HOMOexpg (calc)
(eV)
LUMOexph (calc)
(eV)
⁎
Dye
λmax
εmax
λmax
λmax
E0-0
Eox
E0-0
(V)
(nm)
(LM−1 cm−1
)
(nm)
(nm)
(eV)
(V)
D1
D2
D3
410
430
380
51,950
44,150
48,750
445
455
390
440
444
398
2.34
2.37
2.64
1.13
1.18
1.38
-1.21
-1.18
-1.26
-5.33 (−5.73)
-5.38 (−5.86)
-5.58 (−5.80)
-2.99 (−2.87)
-3.02 (−3.09)
-2.94 (−2.98)
a
Absorption maxima and extinction coefficient at λmax of dyes D1-D3 in C2H5OH solutions (2 × 10−5 M at 298 K).
Maximum absorption wavelength λmax of dyes in CH2Cl2 solutions.
Absorption maxima of dyes adsorbed on TiO2.
The band gap, E0–0, was estimated from the observed optical edge.
Oxidation potential measured in CH2Cl2 containing 0.1 M Bu4NPF6 as supporting electrolyte, Ag/AgCl as a reference electrode, Pt as working electrode. Potentials were converted to
b
c
d
e
normal hydrogen electrode (NHE) by addition of +0.63 V [27].
f
⁎
E0–0: the excited-state oxidation potential vs NHE.
g
HOMO = − (Eoxonset + 4.2), [28].
LUMO = E0-0 + HOMO.
h
and 6 nm, respectively. This red shift in absorption spectra favors utili-
zation in solar light.
dyes calculated by Eox − E0-0, where the optical band-gap energy (E0-
0) was estimated from the absorption onset, ranging from −1.18 to
−1.26 V vs NHE, and are more negative than the conduction band
edge of TiO2 (−0.5 vs NHE) [29], thus indicating that the excited elec-
trons of the dyes can be efficiently injected into the conduction band
of TiO2.
The absorption spectra of the dyes D1-D3 adsorbed on
a
nanocrystaline TiO2 film are shown in Fig. S2 in the Supplementary ma-
terials. The absorption peak of all dyes on the TiO2 surface shows peak
broadening in compare to the solution, which indicates that TiO2 layer
is effectively photosensitized by the dyes. This peak broadening in com-
pare to the solution was also observed in the dyes with single electron
acceptor. In the case of D1 and D2, a blue shift of λmax is observed (5
and 11 nm, respectively), attributable to H-aggregation of the dyes
[24] or deprotonation upon adsorption over the TiO2 surface [25]. The
absorption spectra of D3 absorbed on TiO2 film is red shifted in compare
to the dye in solution, which is attributed to the formation of J-type ag-
gregate [26]. When the dyes with single electron acceptor group were
adsorbed on the TiO2 surface, the similar manners were observed, so
that the D1 and D2 counterparts of single electron acceptor (TD1 and
TD5, respectively) exhibited the blue shift of the absorption by 14 and
28 nm, respectively and the D3 counterpart of single electron acceptor
(TD6) presented the red shift of the absorption by 24 nm. As seen, the
dyes with two electron acceptors show the low shifts of λmax in compar-
ison with the single electron acceptor ones. It maybe due to less aggre-
gation of these dyes on the TiO2 surface.
The dyes with single electron acceptor also illustrate the important
properties of the reduction by the electrolyte and the electron injection
into the semiconductor. The addition of second electron acceptor group
causes a positive shift in both the oxidation and reduction potentials of
the dye. The positive shift of the HOMO level refers to the electron with-
drawing property of the second acceptor group and the positive shift of
the LUMO level is due to the lengthening of the molecule. Consequently,
more positive shift of the LUMO levels causes a decrease of the HOMO-
LUMO band gaps. The observed red shifting in the absorption spectra of
the dyes with two electron acceptor groups in comparison with the
mono anchoring dyes is due to the decrease of the band gaps. The oxida-
tion and reduction potentials of the dyes with single anchoring group is
shown in Fig. S3 in the Supplementary materials. The experimental and
theoretical values of the HOMO and LUMO levels of these dyes are also
presented in Table S1 in the Supplementary materials.
3.3. Photovoltaic Properties
3.2. Electrochemical Properties
The IPCE for the DSSCs based on the synthesized dyes as a function of
the wavelength is shown in Fig. 5. The onset wavelength of the IPCE
spectra for the DSSCs based on three dyes is 725 nm. The IPCE spectra
of the dyes D1-D3 are broader than those of their counterparts with sin-
gle anchoring group, so that the beginning wavelength of the IPCE spec-
tra for the dyes with one anchoring group was obtained about 500 nm. It
can be seen that the photocurrent response of D3 sensitized DSSC is the
best due to the IPCE exceeding 59% in the range of 385–580 nm. D1 and
D2 also show the maximum IPCE values of 42 and 47% at 510 nm,
The redox potentials of the synthesized dyes were obtained from cy-
clic voltammograms and the results are summarized in Table 1. The re-
corded cyclic voltammograms and the energy levels of the dyes are
provided in Figs. 3 and 4, respectively. The oxidation potentials of the
dyes D1-D3 ranging from 1.13 to 1.38 V vs NHE is more positive than
the standard potential of the iodine/iodide redox couple (0.45 V vs
NHE) [29], guaranteeing plenty driving force for regeneration of the
⁎
dye by the electrolyte. The reduction potential (E0–0) values of the
Fig. 3. Cyclic voltammograms of the dyes D1-D3 recorded in CH2Cl2 solutions. Potentials
are vs Ag/AgCl.
Fig. 4. Comparison of the oxidation potentials of the dyes D1-D3.

Z. Jafari Chermahini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
83
Table 2
Photocurrent-photovoltage characteristics of the DSSCs sensitized with D1-D3a and dye
loadings on the TiO2 surface and calculated dipole moments of D1-D3.
Jsc
Voc
ff
η
Dye loading
Dipole momentb
(debye)
Dye (mA cm−2
)
(mV) (%) (%) (×10−7 mol cm−2
)
D1 8.10
D2 8.36
D3 11.50
466
475
569
51 1.92 3.0
51 2.02 4.0
54 3.53 3.1
10.03
7.62
8.90
a
Measured under standard conditions (AM1.5G, 100 mW cm−2); 0.25 cm2 effective
area. Dyes were maintained at 0.4 mM in C2H5OH solution.
b
Measured on TiO2 film by means of UV–Vis absorption spectroscopy.
of D1. The open circuit photovoltage of D3 was 103 mV larger than
that of D1. As shown in Table 2, the dye loading values are similar in
two cells and the permanent dipole moment in the ground state of D3
is smaller than that of D1. The comparatively lower Eox (HOMO) of D3
is the likely reason for the higher Voc of D3 than that of D1, which offers
enough of a larger driving force for the reduction of the oxidized dye. In
order to gain more insight into the effect of the binding mode between
the dyes and the TiO2 surface on the photovoltaic properties, their FT-IR
spectra on the TiO2 surface was measured (Fig. S6 in the Supplementary
materials). The FT-IR spectrum of D3 shows the disappearing of the car-
boxylic acid stretching peak (1687 cm−1) upon adsorption on the TiO2
surface, indicating the participation of both carboxylic acid groups in the
anchoring process. The band appearing at 1058 cm−1 assigned to a vi-
brational stretching for the tetrazole N\\N bonds in the free D3 [16]
shifts to 1039 cm−1 after dye adsorption. Therefore, the tetrazole
rings are also involved in the binding with the TiO2 surface. For D1,
the carbonyl peak at 1712 cm−1 in the free dye shifts to a lower wave-
number region (1690 cm−1) upon grafting, showing the interaction of
both carbonyl groups with the TiO2 surface. The vibrational stretching
peak of the tetrazole N\\N bonds also shifts from 1055 cm−1 to
1036 cm−1 after binding with TiO2. As seen, the carboxylic acid groups
of D3 interact with the TiO2 surface, leading to the strong binding with
the surface. The higher IPCE and Jsc values observed for D3 in compari-
son with D1 are consistent with the stronger interaction with the TiO2
surface, leading to the faster electron injection into the TiO2 electrode.
Fig. 5. Spectra of monochromatic incident photon-to-current conversion efficiencies
(IPCEs) for DSSCs based on the D1-D3 dyes.
respectively. The comparison of two series of data show that the maxi-
mum IPCE values of D1 and D3 are higher than those of TD1 and TD6,
respectively, while the maximum IPCE value for D2 is lower than that
of TD5. Among TD dyes, TD5 showed the best IPCE value (Fig. S4 in
the Supplementary materials).
Fig. 6 shows the I-V curves of DSSCs based on the synthesized dyes
D1-D3 and Table 2 summarizes the detailed photovoltaic data. Consis-
tent with the result of IPCE, the photovoltaic data show the order of
D3 ˃ D2 ˃ D1. For the most efficient dye, D3, an overall conversion effi-
ciency of 3.53% is obtained with an open circuit voltage of 569 mV, a
short circuit photocurrent density of 11.50 mA cm−2, and a fill factor
of 54%. The DSSCs based on D1 and D2 show the η values of 1.92 and
2.02%, respectively. The efficiency of the DSSC based on TD5 was
about three times larger than that of TD1 due to more effectively ad-
sorption onto the TiO2 surface with a different adsorption mode and a
higher molar extinction coefficient. The Voc value of TD5 was 90 mV
higher than that of TD1 due to the larger dipole moment [16]. The Jsc
value of TD5 was also approximately three times larger than that of
TD1, whereas the dye loading value for TD5 was only
0.56 × 10−7 mol cm−2 higher than that of TD1. In comparison to D1,
the overall conversion efficiency of the D2 based DSSC is slightly higher
due to the slightly larger Jsc and Voc. Although the dipole moment of D2
(7.62 D) is smaller than that of D1 (10.03 D), the Voc value of D2 is larger
than that of D1 that can be attributed to the higher dye loading,
4.0 × 10−7 mol cm−2, compared to 3.0 × 10−7 mol cm−2 for D1. The
larger Jsc value of D2 is also correlated to the higher dye loading [8].
The previous study showed that the efficiencies of both the dyes TD1
and TD6 were similar with almost similar adsorption mode onto TiO2
surface. In this study, the efficiency of D3 is 1.84 times larger than that
For D2, the CN frequency is observed at the same region (2217 cm−1
)
before and after adsorption on TiO2, suggesting that the cyano group
isn't involve in the interaction with TiO2. After D2 is adsorbed on TiO2,
the changes are observed in the range of 977–1111 cm−1 in the spec-
trum corresponding to the tetrazole N\\N bond vibrational stretching.
The peaks also appearing at 1223, 1452 and 1435 cm−1 shifts to 1214,
1427 and 1396 cm−1, respectively. These peaks are probably related
to the N _N and C _N bonds of the tetrazole rings. Such changes aren't
observed in the spectra of D1 and D3 after adsorption on TiO2, showing
that the ring vibrations are little influenced through the adsorption.
Therefore, in the D1 and D3 dyes, the tetrazole rings may not completely
involved in the binding with TiO2 and these dyes probably grafts on TiO2
through only one nitrogen atom assisted by the ester carbonyl group
(D1) or the carboxylic acid group (D3). Consequently, because of the
stronger interaction of the COOH group with the surface in comparison
with the tetrazole group [16], the binding of D3 with TiO2 is stronger
than that of D1 and D2. The stronger binding with the surface affects
on the Jsc value and the overall efficiency.
The dyes D1-D3 with two electron acceptor groups show better pho-
tovoltaic performance in compare to single electron acceptor dyes (the
solar cells was based on two series of the dyes constructed under the
same conditions). The I-V curves and the overall efficiencies of the
DSSCs based on TD1, TD5, and TD6 are shown in Fig. S5 and Table S1, re-
spectively in the Supplementary materials. The overall efficiencies of
D1-D3 are higher than those of TD1, TD5, and TD6 due to the higher
Jsc values. The higher Jsc values of D1-D3 may be due to the broader
IPCE spectra of the DSSCs based on these dyes [30], so that the photocur-
rent response of D1-D3 based DSSCs is seen at wavelengths higher than
500 nm. As mentioned above, the IPCE onset of the DSSCs based on TD1,
Fig. 6. Current-potential (I-V) curves for the DSSCs based on D1-D3 under AM 1.5
irradiation (100 mW cm−2).

84
Z. Jafari Chermahini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
TD5, and TD6 is 500 nm. The obtained Voc values for the DSSCs include
D1-D3 are lower than those for the DSSCs based on TD1, TD5, and
TD6. The lower Voc values are probably resulted from transferring
more protons to the semiconductor surface upon grafting the dyes
with two anchoring groups, which causes the downshift of the conduc-
tion band of TiO2 [8]. The dye loading data also show that a more
amount of the dyes with two anchoring groups is adsorbed on the
TiO2 surface in comparison with the dyes with one anchoring group
(The dye loading and the dipole moment values of TD1, TD5, and TD6
are shown in Table S1 in the Supplementary materials).
3.4. Quantum Chemical Calculations
Fig. 7 shows the electron distribution of the calculated HOMO and
LUMO orbitals of the dyes D1-D3. The HOMOs of the dyes are
delocalized on the entire molecule and the LUMOs are concentrated
on the acceptor parts of the triphenylamine. The distribution of the
HOMO and LUMO orbitals of TD1, TD5 and TD6 was also similar to
D1-D3. The calculated HOMO and LUMO energies for D1-D3 are pre-
sented in Table 1 and the diagram of them is shown in Fig. 8. The theo-
retical data show that the HOMO levels are lower than the I−3 /I−
potential and the LUMO levels are higher than the TiO2 conduction
band. Therefore, from theoretical point of view, these dyes are also use-
able in DSSCs. The theoretical HOMO and LUMO levels are close to the
experimental data obtained from cyclic voltammograms (Table 1).
With the exception of the LUMO level of D1, the theoretical HOMO
and LUMO levels are more negative than the experimental results. It is
due to the effect of functional and basis set on the energy levels [31].
As seen, the experimental band gaps are lower than the theoretical
band gaps and are in the following order: D3 ˃ D2 ˃ D1, while the theo-
retical band gaps increase in the order D1 ˃ D3 ˃ D2. When we compare
the theoretical HOMO and LUMO levels of D1-D3 with those of TD1,
TD5, and TD6, a downshift of both the HOMO and LUMO levels of D1-
D3 is observed, which is agreed with the experimental results. The re-
sulted theoretical band gaps of D1-D3 are also lower than those of
their counterparts of single anchoring group.
Fig. 8. Comparison of the theoretical energy levels of the dyes D1-D3.
The lowest singlet-singlet excitation energies of the dyes were calcu-
lated with TD-DFT level of theory to gain insight into the excited states
of these dyes (Table 3). The calculations show that the lowest energy
transition of the dyes corresponds to HOMO → LUMO excitation. There-
fore, the bands seen in the visible region of the experimental absorption
spectra include the HOMO → LUMO transitions. The red shift of λmax
from theory to experiment is related to the self-interaction error in
TD-DFT arising through the electron transfer in the extended charge-
transfer state [32]. The other transitions involve HOMO → LUMO + 1,
HOMO-1 → LUMO, and HOMO-3 → LUMO corresponding to the π-π*
transitions and are summarized in Table S2 in the Supplementary mate-
rials. In the case of the dyes with one anchoring group, the first transi-
tions were also seen from the HOMOs to the LUMO levels. These
transitions occur with a higher energy and lower oscillator strength
compared to the D1-D3 dyes. In fact, the higher transition energy in
these dyes is due to the larger band gaps. The theoretical absorption
spectra of these dyes also are blue shifted as compared to those of D1-
D3. The theoretical absorption spectra of the dyes are shown in Fig. S7
in the Supplementary materials.
Fig. 7. Isodensity surface plots of frontier orbitals of the dyes.

Z. Jafari Chermahini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 79–85
85
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Table 3
Computed maximum absorption wavelengths (eV) and oscillator strengths (f) of the dyes
D1-D3 in C2H5OH.
Dyes
λ
max (nm)
Composition (%)
E (eV)
f
D1
D2
D3
505
520
511
H → L (70.4)
H → L (70.5)
H → L (70.5)
2.45
2.38
2.42
1.25
1.25
1.25
4. Conclusions
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Three new triphenylamine based organic sensitizers with two elec-
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gate the effects of an additional electron acceptor group on the
performance of DSSCs. The dyes with two electron acceptors have the
higher extinction coefficients and are red shifted of their absorption
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We thank the Isfahan University of Technology (IRAN) for the
financial support of this work. We gratefully acknowledge the Sheikh
Bahaei National High Performance Computing Center (SBNHPCC) for
the computational support.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
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