Synthesis of new dyes containing double tetrazole groups for sensitization of TiO2 nanoparticles in dye-sensitized solar cells

Article abstract of DOI:10.1007/s13738-017-1096-y

Di(1H-tetrazol-5-yl)methane is employed as a new electron acceptor group in the synthesis of two metal-free organic dyes containing triphenylamine donor group. Dye-sensitized nanocrystalline TiO₂ solar cell (DSSC) applying these novel dyes is c

Full text of DOI:10.1007/s13738-017-1096-y

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1096-y
ORIGINAL PAPER
Synthesis of new dyes containing double tetrazole groups
for sensitization of TiO nanoparticles in dye‑sensitized solar cells
2
1
1
1
Zahra Jafari Chermahini · Alireza Najafi Chermahini · Hossein A. Dabbagh ·
1
1
Behzad Rezaei · Neda Irannejad
Received: 24 December 2016 / Accepted: 13 March 2017
Iranian Chemical Society 2017
©
Abstract Di(1H-tetrazol-5-yl)methane is employed as a
new electron acceptor group in the synthesis of two metal-
free organic dyes containing triphenylamine donor group.
Dye-sensitized nanocrystalline TiO2 solar cell (DSSC)
applying these novel dyes is constructed for consideration
of their photovoltaic properties. The electronic properties of
the dyes are also considered with the aid of theoretical cal-
culations. The DSSC constructed from 4-(2,2-di(1H-tetra-
zol-5-yl)vinyl)-N,N-diphenylaniline (T1) shows a short-
Introduction
There is wide interest in dye-sensitized solar cells (DSSCs)
in the field of conversion of sunlight to electricity [1]. The
solar cell efficiency is mainly affected by the properties of
the sensitizer (dye) [2]. High molar absorption coefficient,
constitution diversity, and easy synthesis method are the
reasons for development of metal-free organic sensitizers
for DSSCs in comparison with the ruthenium complexes
that are the best dyes for DSSCs [3]. The electron donor,
linker, and electron acceptor groups are the parts of a usual
organic photosensitizer [4]. The electrons from the dye
oxidation should inject to the semiconductor through chan-
nels being the acceptor groups that connect the dyes to the
semiconductor surface [5]. The most universally applied
acceptor group for the dyes is carboxyl group (-COOH)
[6]. However, many alternative anchoring groups such
as the nitro group [7], 2-(1,1-dicyanomethylene)rhoda-
nine [8], pyridine [9], aldehyde [10], benzothienopyridine
[11], and 8-hydroxyquinoline [12] have been considered.
Recently, tetrazole ring has been used as a novel anchor-
ing group in organic photosensitizers [13]. There are simi-
lar physical properties between tetrazoles and carboxylic
acids that cause tetrazoles to act as the carboxylic acid
isostere in active molecules in biology, and further they are
resistant to many metabolic degradation conditions [14]. In
addition to be useful in different scientific aspects, involv-
ing explosives, devices for information recording, and pho-
tography, tetrazoles are polydentate ligands in relation to
the metals in coordination chemistry [15–17]. Recently,
we have reported the synthesis of photosensitizers with
anchoring groups based on the tetrazole ring [18]. The
ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-yl) acry-
lonitrile, and 1H-tetrazole-5-acetic acid compounds were
−2
circuit photocurrent density of 13.38 mA cm , an open
circuit voltage of 578 mV, and a fill factor of 0.54, with a
resulted solar energy-to-electricity conversion efficiency of
−2
4
.18% under simulated 1 sun irradiation (100 mW cm ).
This result reveals that the dye with the di(1H-tetrazol-
-yl)methane anchoring group injects more electrons to
5
the conduction band of TiO in comparison with its ana-
2
logs with single tetrazole ring in their anchoring group. It is
found that in spite of a red-shift of the absorption spectrum
resulted from the lengthening of the molecule, the dye with
two di(1H-tetrazol-5-yl)methane groups gives lower per-
formance than the dye with a single electron acceptor.
Keywords DSSC · Dye · Electron acceptor ·
Sensitization · Tetrazole
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-017-1096-y) contains supplementary
material, which is available to authorized users.
*
Alireza Najafi Chermahini
anajafi@cc.iut.ac.ir; najafy@gmail.com
1
Department of Chemistry, Isfahan University of Technology,
Isfahan 84154-83111, Iran
1
3

J IRAN CHEM SOC
applied as anchoring group in the reported dyes. These
anchoring groups include single tetrazole ring. Herein, we
discuss the synthesis and application of new sensitizers
with double tetrazole in their anchoring group (Fig. 1).
A SAMA 500 electroanalyser system, SAMA Research
Center, Iran, with Pt working and counter electrodes, and
an Ag/AgCl reference electrode was used for cyclic voltam-
metry experiments. The 0.1 M Bu NPF and ferrocene as
4
6
the salt support and the internal standard, respectively, in a
scan rate of 0.1 V/s at room temperature were also applied,
respectively. The precursors 1, 2, and di(1H-tetrazol-5-yl)
methane were synthesized according to the methods men-
tioned in the studies [19–21] (see Scheme 1).
Experimental
General information
All reactions were preceded using commercially available
solvents and chemical reagents without extra purification.
Triphenylamine was prepared from Merck, and other used
chemical reagents and solvents were obtained from Sigma-
Aldrich. A 5973 Network Mass Selective Detector [Agilent
Technology (HP)] apparatus and a Bruker 400 MHz spec-
trometers were applied for recording the mass analyses and
Fabrication of dye‑sensitized solar cells
A 40-mM aqueous TiCl solution was used for coating
4
fluorine-doped tin oxide (FTO) glass substrates with a
blocking layer at 70 °C for 30 min. Then, the substrates
were applied for fabrication of anode films using TiO2
pastes (Sharif Solar [22]) and doctor blade method [23,
1
13
H and C nuclear magnetic resonance (NMR) spectra,
24]. The TiO pastes were deposited on the substrates as
2
respectively. Fourier transform infrared (FTIR) absorption
spectral data of the dyes using KBr pellets were taken on
an FTIR Jasco 680 plus spectrometer. The absorption spec-
two layers. The first layer was composed of 20-nm TiO2
particle with thickness of 7 µm, and the second layer was
included of 400-nm TiO particle with a thickness of
2
tra of two dyes in solution and adsorbed on TiO films were
obtained by using a Jasco-570 UV/Vis spectrophotometer.
5 µm. These films were heated at 375 °C (10 min), 450 °C
(15 min), and 500 °C (30 min). The films were again
2
Fig. 1 Molecular structures of
synthesized dyes
T1
T2
Scheme 1 Synthesis process of the dyes with double tetrazole anchoring group: i POCl , DMF, 95–100 °C, 20 h, ii Di(1H-tetrazol-5-yl)meth-
3
ane, NH(Et) , CH CN, reflux, 24 h, iii POCl , DMF, 80 °C, 48 h, iv Di(1H-tetrazol-5-yl)methane, NH(Et) , CH CN, reflux, 48 h
2
3
3
2
3
1
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J IRAN CHEM SOC
soaked in the 40-mM aqueous TiCl solution at 70 °C for
4‑(2,2‑di(1H‑tetrazol‑5‑yl)
4
3
0 min and rinsed with water and heated at 500 °C for
vinyl)‑N‑(4‑(2,2‑di(1H‑tetrazol‑5‑yl)vinyl)
phenyl)‑N‑phenylaniline
another 30 min. The obtained electrodes were dipped into
dye solutions (0.4 mM in C H OH) for 20 h in the room
2
5
temperature and then washed. For preparing the cathode,
the FTO glasses were coated with Pt. The obtained cath-
odes were put on the anodes, and the iodine-based elec-
trolyte was introduced into the space between two elec-
trodes. Two cells were constructed with the active area
The dye T2 was prepared as said by the general method
from 2 (301 mg, 1 mmol), di(1H-tetrazol-5-yl)methane
(380 mg, 2.5 mmol) and diethylamine (2 mL, 20 mmol)
in CH CN (5 mL). Yield: 90%. Dark red solid. IR (KBr,
3
−1
cm ): ν = 525 (w), 692 (m), 804 (s), 916 (w), 1054
2
of 0.25 cm . The spectra of incident photon-to-current
(s), 1159 (s), 1215 (w), 1285 (m), 1327 (s), 1390 (s),
conversion efficiency (IPCE) were taken on an IPCE
instrument (Sharif Solar, IPCE-015 [25]) with a 100-W
halogen lamp. Photocurrent–voltage data were gained
with the aid of a Bio-Logic SAS model SP-300 apparatus
without mask. The irradiance of 100 mW/cm was pro-
vided by an AM 1.5G solar simulator (Sharif Solar, SIM-
1453 (s), 1502 (s), 1585 (s), 1627 (s), 1669 (s), 1850
1
(w), 2820 (s, br). H NMR (400 MHz, DMSO-d , ppm):
6
δ = 7.02 (d, J = 8.4 Hz, 4H), 7.08 (d, J = 8.4 Hz, 2H),
7.28 (t, J = 8.8 Hz, 1H), 7.46 (t, J = 8.0 Hz, 2H), 7.94
2
13
(d, J = 8.4 Hz, 4H), 8.03 (s, 2H). C NMR (400 MHz,
DMSO-d , ppm): δ = 119.07, 123.77, 124.99, 125.61,
6
1
000 [25]). Electrochemical impedance spectra were
taken on an impedance analyzer (EIS-25H, Sharif Solar
25]) at a constant voltage.
126.22, 129.42, 129.63, 132.49, 145.18, 146.73. MS:
m/z = 57.1 (base peak), 83.1, 111.1, 149.0, 178.1, 210.2,
[
238.2, 261.2, 293.2, 337.4, 365.4, 400.4, 430.4, 472.4,
+
5
00.8, 530.5, 569.7 (M ). Anal. Calcd for C H N : C,
2
6
19 17
5
4.83; H, 3.36; N, 41.81; Found: C, 54.82; H, 3.33; N,
General method for synthesis of the dyes T1 and T2
41.80.
The synthesis process included the following steps: mix-
ing the intermediate compounds (1 mmol), di(1H-tetra-
zol-5-yl)methane, diethylamine (20 mmol), and acetoni-
trile (5 mL), refluxing and evaporating the solvent of
mixture, keeping in room environment for cooling, acidi-
fying with hydrochloric acid (5 M), filtering the resulted
residue, and drying and purifying the residue by column
chromatography.
Computational
The computational considerations were done through
Gaussian [26] program. The optimized minima structures
demonstrated by no imaginary frequency were obtained at
the Becke-3-parameter-Lee–Yang–Parr (B3LYP) [27] level
using 6-31 + G(d) basis set. TD-DFT (time-dependent
density functional theory) calculations were carried out at
B3LYP/6-31 + G(d) method for calculating the theoretical
absorption spectra in ethanol solvent using the polarizable
continuum model (PCM) [28] solvation model.
4
‑(2,2‑di(1H‑tetrazol‑5‑yl)vinyl)‑N,N‑diphenylaniline (T1)
The dye T1 was prepared as said by the general method
from 1 (273 mg, 1 mmol), di(1H-tetrazol-5-yl)methane
Results and discussion
Synthesis
(
228 mg, 1.5 mmol), and diethylamine (2 mL, 20 mmol)
in CH CN (5 mL). Yield: 90%. Red solid. IR (KBr,
3
−
1
cm ): ν = 461 (w), 622 (s), 650 (s), 699 (m), 748 (w),
8
1
25 (w), 888 (w), 922 (m), 964 (m), 1006 (m), 1048 (m),
A two-step route including the Vilsmeier–Haack formyla-
tion at the first step and the Knoevenagel condensation at
the second step was applied for the synthesis of T1 and T2.
The synthesis process of the dyes is shown in Scheme 1.
076 (m), 1187 (w), 1585 (w), 1446 (s), 1578 (s), 2855
1
(
s), 2925 (s), 2960 (s), 3406 (w, br). H NMR (400 MHz,
DMSO-d , ppm): δ = 8.92 (d, J = 8.8 Hz, 2H), 7.21 (m,
6
6
8
H), 7.42 (t, J = 8.0 Hz, 4H), 7.89 (d, J = 9.2 Hz, 2H),
13
.19 (s, 1H). C NMR (400 MHz, DMSO-d , ppm):
Optical properties of the dyes
6
δ = 118.78, 123.94, 125.04, 125.37, 125.89, 126.17,
1
29.88, 129.98, 131.79, 132.43, 145.43, 147.05. MS:
By looking to the recorded UV–Vis (ultraviolet–visible)
absorption spectra of the dyes in ethanol in Fig. 2, we can
recognize two separate peaks at 190–325 and 380–375 nm
regions. The π–π* transitions and the intramolecular
charge-transfer (ICT) transition from the triphenylamine
electron donor to the di(1H-tetrazol-5-yl)methane electron
m/z = 53.1, 69.1 (base peak), 91.1, 109.1, 129.1, 149.0,
1
3
65.1, 183.1, 207.1, 225.2, 241.2, 259.1, 275.2, 295.1,
10.1, 326.2, 341.2, 368.3, 386.3, 407.5 (M ). Anal.
+
Calcd for C H N : C, 59.01; H, 3.71; N, 37.28; Found:
2
2
17
9
C, 58.96; H, 3.70; N, 37.26.
1
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J IRAN CHEM SOC
−
−
8
7
6
5
4
3
2
1
0000
0000
0000
0000
0000
0000
0000
0000
0
more positive oxidation potentials than the I /I redox
3
T1
T2
couple [0.45 V vs NHE (normal hydrogen electrode)]
32], and the successful electron injection from the dyes
to the semiconductor is proved by more negative reduc-
tion potentials than the conduction band edge of TiO2
[
(
−0.5 V vs NHE) [33]. We can also see a positive shift of
both oxidation (0.04 V) and reduction (0.14 V) potentials
of T2 in comparison with those of T1 due to the electron
withdrawing effect of the second electron acceptor group.
The lengthening of T2 also decreases the band gap. The
energy levels of the dyes are shown in Fig. 3.
1
90
290
390
490
Wavelength (nm)
Fig. 2 Absorption spectra of the dyes in C H OH solution
2
5
2 × 10− M)
5
(
acceptor moiety cause the appearance of peaks at these
regions, respectively [3]. As seen in Table 1, the molar
Photovoltaic properties
−
1
−1
extinction coefficient of T2 (29,640 LM cm ) is more
The incident photo-to-current conversion efficiency (IPCE)
images of T1 and T2 in Fig. 4 show expansive spectra. The
maximum IPCE for T1 occurs at 595 nm with the value of
−
1
−1
than that of T1 (27,300 LM cm ). The existence of two
electron acceptor groups in the T2 molecule enlarges the
molecule; therefore, T2 is also red-shifted by 15 nm com-
pared to T1.
6
7%. Obviously, T1 shows the IPCE value of 55% in the
range 465–625 nm. The maximum IPCE of 59% is also
The absorption spectra of the dyes adsorbed on the TiO2
obtained at 525 nm for the DSSC based on the dye T2.
surface illustrated in Fig. S1 in the supporting information
seem to be broader than the spectrum in ethanol solution. It
is also obvious that the adsorption on the surface shifts the
absorption peaks to the higher wavelengths that can be a
sign of the formation J-type aggregation [31].
Electrochemical properties
The oxidation and the reduction potentials of a molecule
are a criterion of the HOMO (highest occupied molecu-
lar orbital) and LUMO (lowest unoccupied molecu-
lar orbital) levels in that molecule. Therefore, the onset
potentials of oxidation obtained from the cyclic voltam-
mogram (Fig. S2) and the reduction potential calculated
from the onset potentials of oxidation and the absorp-
tion onset are summarized in Table 1. The reduction of
the oxidized dyes by the electrolyte is verified by having
Fig. 3 Energy levels of the dyes T1 and T2
Table 1 Optical and electrochemical properties of the dyes in solutions and adsorbed on TiO2
a
max
a
max
−1
−1
b
max
c
0-0
ox d
onset
*e
0-0
HOMOf (calc) (eV) LUMOg (calc) (eV)
exp exp
Dye
λ
(nm)
ɛ
(LM cm
)
λ
(nm)
E
(eV)
E
(V)
E
(V)
T1
T2
370
385
27,300
29,640
426
430
2.67
2.57
0.78
0.82
−1.89
−1.75
−4.98 (−5.60)
−5.02 (−5.71)
−2.31 (−2.62)
−2.45 (−2.88)
a
−5
Absorption peak maximum and extinction coefficient at λmax in C H OH solutions (2 × 10 M at 298 K)
2
5
b
c
d
Absorption peak maximum on TiO2
The band gap, E0–0, was calculated approximately from the optical edge
Onset potentials of oxidation measured in CH Cl containing 0.1 M Bu NPF as supporting electrolyte, Ag/AgCl as a reference electrode, Pt
2
2
4
6
as working and counter electrodes. Potentials were expressed as normal hydrogen electrode (NHE) by the addition of +0.63 V [29]
e
f
*
0
E
: The excited-state oxidation potential versus NHE
–0
ox
onset
HOMO = −(E + 4.2) [30]
LUMO = E0 + HOMO
g
–0
1
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J IRAN CHEM SOC
1
00
1200
000
T1
T2
T1
T2
1
80
60
40
20
0
800
600
400
200
0
0
500
1000
Z' (
1500
2000
2500
)
370
470
570
670
770
Wavelength (nm)
Fig. 6 Nyquist plots for DSSCs sensitized with T1 and T2 measured
at 10 mV ac bias in the dark
Fig. 4 Photocurrent action spectra of the DSSCs based on the dyes
T1 and T2
600
T1
T2
1
1
1
4
2
0
8
6
4
2
0
500
T1
T2
4
00
00
3
200
100
0
0
200
400
600
800
1000
0
0.2
0.4
0.6
0.8
Z' (
Potential (V)
Fig. 7 Nyquist plots of the DSSCs fabricated using the dyes T1 and
T2
Fig. 5 Current–voltage characteristics of DSSCs using T1 and T2
Table 2 Photovoltaic performance of DSSCs sensitized with the
dyes T1 and T2
Voc (open circuit potential), ff (fill factor), and η (over-
all power conversion efficiency)) are summarized in
2
Dye Jsc (mA cm 2) Voc [mV] ff (%) η (%) Dye loading (×10
−
a
7
Table 2. For the T1-based cell, a Jsc of 13.38 mA/cm ,
−
2
mol cm
)
Voc of 578 mV, and ff of 0.54 with a η of 4.18% were
observed. The values of both Jsc and V of the T2-based
cell are lower than those for T1, resulting in a η of
oc
T1
T2
13.38
12.05
578
470
54
50
4.18 3.52
2.83 1.70
2
.83%. Although the T2 dye has a longer π-conjugation
a
Dye adsorption on TiO was measured from the subtraction of the
system and red-shifted absorption spectrum, it shows a
lower Jsc value. To find the reason for lower Jsc value of
T2, the dye loading values were determined for T1 and
T2 because in addition to the dye quality, the dye quan-
2
UV–Vis absorption spectra of dye solution before and after dipping
of the cathode
The IPCE of the dye T1 is broader than that of the
dyes previously reported with the ethyl 2-(1H-tetra-
zol-5-yl) acetate, (2H-tetrazol-5-yl) acrylonitrile, or
tity on the TiO surface influences the photovoltaic prop-
2
erties of DSSCs [13]. The obtained dye loading value
−
7
−2
for T2 (1.70 × 10 mol cm ) is lower than that for T1
−
7
−2
1
H-tetrazole-5-acetic acid moieties as anchoring group
(3.52 × 10 mol cm ); therefore, T2-based cell pro-
[
18]. The DSSCs based on these dyes were constructed
duces a lower J value.
sc
under the same conditions. The IPCE spectrum of T1
is higher than that of the dyes with ethyl 2-(1H-tetra-
zol-5-yl) acetate (22%) and 1H-tetrazole-5-acetic acid
As seen, the V value of the T1-based DSSC is higher
oc
than that of the T2-based DSSC. To explain the correlation
between the Voc values and the dyes, the electrochemical
impedance spectroscopy (EIS) of DSSCs was measured in
the dark (Fig. 6). As we can see in the picture, there is a
large semicircle in the Nyquist plots of two DSSCs related
(
25%) anchoring groups, while the dye with (2H-tetra-
zol-5-yl) acrylonitrile (80%) shows higher IPCE than
T1.
The current–voltage features of solar cells constructed
of the T1 and T2 dyes are depicted in Fig. 5, and the
cell performance parameters (Jsc (short-circuit current),
to the charge-transfer resistance at the TiO /dye/electro-
lyte interface [4]. The radius of this semicircle is higher for
the T1 in comparison with that for T2, implying that the
2
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J IRAN CHEM SOC
electron recombination resistance is high for the T1-based
Table 3 Computed maximum absorption wavelengths (eV) and
oscillator strengths (f) of the dyes T1 and T2 in C H OH
2
5
DSSC. Therefore, the V value of DSSC constructed with
oc
T1 is improved by the prevention of electron recombina-
tion between the electrolyte and the injected electrons.
In order to consider the electron transport in the DSSCs,
the EIS of DSSCs was also measured under illumination.
The Nyquist plots presented in Fig. 7 are recorded at the
bias voltage with ac amplitude of 10 mV with frequency
range of 0.1 Hz–100 kHz. In these plots, the larger semi-
circle is related to the charge-transfer processes at the TiO2/
dye/electrolyte [4]. Obviously, the radius of larger semicir-
cle for T2 is higher than that for T1, implying that more
charge transport occurs in the T1-based DSSC.
Dyes
λ
(nm)
Composition (%)
E (eV)
f
max
T1
T2
467
512
H → L (70.4)
H → L (70.4)
2.65
2.42
1.04
1.29
electron injection. The higher electron injection leads to the
higher IPCE.
The study by Massin et al. [13] has shown that the
photovoltaic properties of tetrazole-based dye with an
enhanced photovoltage rival those of carboxylic acid-based
dye. In addition, both tetrazole- and carboxylic acid-based
dyes show similar stability under the same conditions.
Based on these results, their study has demonstrated that
tetrazole group is an effective alternative to common car-
boxyl group. As mentioned above, the dye with two tetra-
zole rings in its anchoring group (T1) shows the higher
overall efficiency in comparison with the dyes with single
tetrazole ring, revealing more efficient adsorption of the
It is demonstrated that the dipole moment of the dye
molecule with upward shifting of the conduction band
of TiO affects the V value [34]. The calculated dipole
2
oc
moments for the dyes T1 and T2 are 10.09 and 8.83 D,
respectively. According to these data, a larger dipole
moment can also improve the V value of T1.
oc
In comparison with DSSCs based on the dyes with
ethyl 2-(1H-tetrazol-5-yl) acetate, (2H-tetrazol-5-yl) acry-
lonitrile, and 1H-tetrazole-5-acetic acid anchoring groups,
the cell based on T1 exhibits the higher η value due to
the highest short-circuit photocurrent density, which can
be explained by its high IPCE value. The effects of the
anchoring group on the overall power conversion effi-
ciency are ranked di(1H-tetrazol-5-yl)methane > (2H-tetra-
zol-5-yl) acrylonitrile > ethyl 2-(1H-tetrazol-5-yl) ace-
tate ~ 1H-tetrazole-5-acetic acid. The di(1H-tetrazol-5-yl)
methane anchoring group has two tetrazole rings, while
other anchoring groups include one tetrazole ring. It seems
that the dye with two tetrazole units in its anchoring group
dye on the TiO surface. Therefore, ditetrazole anchoring
2
group (di(1H-tetrazol-5-yl)methane) may be a useful sub-
stitute for carboxyl group in sensitizers of solar cells and
may lead to the higher efficiency of the dye bearing this
anchoring group.
Theoretical approach
DFT calculations are applied for the study of the frontier
orbitals of the dyes. The images of the frontier orbitals of
these dyes in Fig. 8 appear that the HOMO of these dyes
is distributed allover the molecule and the LUMO being on
is able to interact strongly with the TiO surface and pro-
2
vides two charge transfer channels, which increases the
Fig. 8 Isodensity surface plots
of frontier orbitals (a: HOMO,
b: LUMO) of the dyes T1 and
T2
1
3

J IRAN CHEM SOC
Acknowledgements We thank the Isfahan University of Technol-
ogy for the financial support of this work. We gratefully acknowledge
the Sheikh Bahaei National High Performance Computing Center
(
SBNHPCC) for the computational support.
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Fig. 9 Calculated absorption spectra of the dyes in C H OH calcu-
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2
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the acceptor group along with the phenyl ring connected to
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The TD-DFT calculations are useful to understand
the electronic transitions in these molecules. These cal-
culations show that, in two dyes, the first singlet–singlet
excitation occurs from the HOMO to the LUMO level
with energy of 2.65 and 2.42 eV for T1 and T2, respec-
tively. The data obtained from these calculations are pre-
sented in Table 3, and the absorption spectra are shown
in Fig. 9. Although the theoretical spectra are red-shifted
in comparison with the experimental spectra due to the
self-interaction error in TD-DFT caused by the electron
transfer in the extended charge-transfer state [34, 35],
the spectroscopic behaviors are similar for both the the-
ory and experiment. The absorption maxima of T1 and
T2 are 467 and 512 nm, respectively. The extra electron
acceptor group in T2 causes a decrease in the HOMO–
LUMO gap, leading to the red-shift of its absorption
spectrum.
7
8
9
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In this paper, we discussed the properties of organic sensi-
tizers with novel anchoring group including two tetrazole
rings that will be applicable in DSSCs. The overall conver-
sion efficiencies of the DSSCs based on T1 and T2 were
obtained 4.18 and 2.83%, respectively. The dye T2 with
having two electron acceptor groups showed the lower Jsc
and Voc values due to the lower dye loading and electron
recombination resistance and the smaller dipole moment.
The DSSC based on T1 with the di(1H-tetrazol-5-yl)
methane electron acceptor was more efficient compared to
those based on the sensitizers with single tetrazole ring in
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