The improvement of photovoltaic performance of quinoline-based dye-sensitized solar cells by modification of the auxiliary acceptors

Article abstract of DOI:10.1016/j.jphotochem.2020.112936

Three new dyes containing diphenylamine as electron donor, benzene (BIM1), benzothiadiazole (BTD) (BIM2) and N-ethylhexylbenzotriazole (BTZ) (BIM3) as auxiliary electron acceptors, quinoline as π-bridge and cyanoacrylic acid as anchoring group were synthesized in D-A-π-A structure for use in dye-sensitized solar cells (DSSCs). The optical, electrochemical, theoretical and photovoltaic methods were performed to understand the auxiliary acceptor influence on the performance of these dyes. Compared to the other dyes, the DSSC with dye BIM3 slightly increases the open circuit voltage (V_oc) owing to the retardation of charge recombination by BTZ. However, replacement of benzene or BTZ by BTD unit causes a large red shift of the absorption spectra, leading BIM2 cell to produce the highest short circuit current density (J_sc). Thus, among the three D-A-π-A dyes, BIM2 is determined to be the most efficient dye, which reached a V_oc of 0.627 V and J_sc of 11.53 mA cm^–2, corresponding to an overall power conversion efficiency of 5.21 % in the presence of chenodeoxycholic acid (CDCA) as the coadsorbent. These results suggest that the insertion of benzothiadiazole as auxiliary acceptor into quinoline-based D-A-π-A dyes can effectively improve photovoltaic performance of DSSCs.

Full text of DOI:10.1016/j.jphotochem.2020.112936

Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The improvement of photovoltaic performance of quinoline-based
dye-sensitized solar cells by modification of the auxiliary acceptors
,
,c,d
Barıs¸ Seçkin Arslana *, Burcu Arkana, Merve Gezgina, Yavuz Derina, Davut Avcıb
,
a
a,
c,
d
a,c,d
˙
˘
Ahmet Tutar , Mehmet Nebioglu
, Ilkay Sisman
¸ ¸
a Department of Chemistry, Sakarya University, 54050, Sakarya, Turkey
b Department of Physics, Sakarya University, 54050, Sakarya, Turkey
c Department of Renewable Energy Systems, Sakarya University, 54050, Sakarya, Turkey
d Biomedical, Magnetic and Semiconductor Materials Research Center (BIMAS-RC), Sakarya University, 54050, Sakarya, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
Three new dyes containing diphenylamine as electron donor, benzene (BIM1), benzothiadiazole (BTD) (BIM2)
and N-ethylhexylbenzotriazole (BTZ) (BIM3) as auxiliary electron acceptors, quinoline as -bridge and cya-
noacrylic acid as anchoring group were synthesized in D-A- -A structure for use in dye-sensitized solar cells
Dye-sensitized solar cells
π
D-A-π-A organic dyes
π
Quinoline
(DSSCs). The optical, electrochemical, theoretical and photovoltaic methods were performed to understand the
auxiliary acceptor influence on the performance of these dyes. Compared to the other dyes, the DSSC with dye
BIM3 slightly increases the open circuit voltage (Voc) owing to the retardation of charge recombination by BTZ.
However, replacement of benzene or BTZ by BTD unit causes a large red shift of the absorption spectra, leading
Auxiliary acceptor
Benzothiadiazole
N-ethylhexylbenzotriazole
BIM2 cell to produce the highest short circuit current density (Jsc). Thus, among the three D-A-π-A dyes, BIM2 is
determined to be the most efficient dye, which reached a Voc of 0.627 V and Jsc of 11.53 mA cm–2, corresponding
to an overall power conversion efficiency of 5.21 % in the presence of chenodeoxycholic acid (CDCA) as the
coadsorbent. These results suggest that the insertion of benzothiadiazole as auxiliary acceptor into quinoline-
based D-A-π-A dyes can effectively improve photovoltaic performance of DSSCs.
1. Introduction
compounds containing various π-bridges such as dibenzonaphthyridine
[10], phenothiazine [11,12], anthracene [13], N-annulated perylene
[14], phenanthrocarbazole [15], phenylene [16], furan [17], thiophene
derivatives [9,18] and quinoline [19] were synthesized and used in
Metal-free organic dye sensitized solar cells (DSSCs) have low cost,
ease of production, environmental friendliness, a high molar absorp-
tivity, easily adjustable power conversion efficiency (PCE). Thanks to
these advantages, it has attracted attention as one of the most important
methods of converting solar radiation into electrical energy as an
alternative to silicon solar cells [1–6]. The molecular structure of dyes
used as sensitizers is one of the main factors that depend on the
photovoltaic performance of DSSCs [7]. A traditional organic dye is in
DSSCs. In most D-π-A dyes, triphenylamine, carbazole, dialkylamine or
diphenylamine electron-rich groups are used as electron donating
groups and carboxylic acid or cyanoacrylic acid electron-withdrawing
groups are used as acceptor/anchoring groups. The strong ester bond
formed between carboxyl groups and TiO2 surface improves electron
conduction [20]. To achive higher short-circuit photocurrent density
(Jsc) and open-circuit voltage (Voc), and thus superior PCE, various new
the form of a donor-π-bridge-acceptor (D-π-A) and the induced electron
is transported from the donor to the acceptor by intra-molecular charge
transfer (ICT) [8]. The main component of sensitizers is aromatic
type structure such as D-A-π-A, D-D-π-A, double D-π-A and
multi-anchoring dyes have been developed in recent years [6,21–23].
π
-bridge which affects absorption capacity of the molecule and ICT
Zhu et al. developed an efficient and stable new type of organic
process the most [9]. It has been found that
π-bridges with fused con-
sensitizers in the D-A-π-A structure by adding an electron-withdrawing
jugated systems increase photovoltaic performance by wide absorption
in the visible radiation region. The planar structure of these systems may
also promote charge transfer between D and A units [10]. Many
group to act as an auxiliary acceptor into the π-bridge of the D-π-A
structure [24]. In recent years, low-bandgap electron-withdrawing
moieties such as benzothiadiazole, benzotriazole, quinoxaline,
* Corresponding author.
E-mail address: barisseckin@gmail.com (B.S. Arslan).
https://doi.org/10.1016/j.jphotochem.2020.112936
Received 1 June 2020; Received in revised form 15 September 2020; Accepted 21 September 2020
Available online 24 September 2020
1010-6030/© 2020 Elsevier B.V. All rights reserved.

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
Fig. 1. Molecular structures of dyes BIM1–3.
phthalimide and diketopyrrolopyrrole have been added between the
-bridge and electron donor, and D-A- -A dyes have been obtained [25,
26]. It was confirmed that the D-A- -A system not only increased the
dark yellow solid. 0.106 g, 85 % yield. Mp; 170ꢀ 171 ◦C. 1H NMR
(600 MHz, DMSO) δ 8.82 (s, 1 H), 8.23 (s, 1 H), 8.21 (d, J =1.7 Hz, 1 H),
8.05 (dd, J = 8.7, 2.1 Hz, 1 H), 7.85 (d, J =8.7 Hz, 1 H), 7.76 (d, J
π
π
π
energy conversion efficiency and photostability of the dye but also
suppressed electron recombination and reduced aggregation [27].
In addition to medical applications, quinolines can also be used in
polymer chemistry, electronics, high efficiency electron transport ma-
terials (ETMs) and corrosion prevention [1]. They have superior thermal
and oxidative stability. Thanks to its exciting optoelectronic features, it
has been used in OLEDs [28–34]. Due to the unsaturated nitrogen atom
which increases electron affinity, these fused planar species are prom-
ising as light sensitizers in DSSCs [35]. There aren’t many notification
on the practice of quinoline derivatives for DSSCs [36]. Recently, our
=8.7 Hz, 2 H), 7.40–7.27 (m, 4 H), 7.20–6.98 (m, 8 H), 4.08 (s, 3 H). 13
C
NMR (150 MHz, DMSO) 163.24, 159.43, 147.39, 147.33, 145.86,
142.79, 138.26, 136.34, 133.22, 130.28, 130.06, 128.22, 127.47,
125.48, 124.98, 124.67, 124.54, 123.78, 123.58, 118.95, 118.53, 54.36.
IR (cmꢀ 1): 3035, 2948, 2223, 1711, 1587, 1490, 1352, 1268. HRMS-ESI
(+) m/z: [M+H]+ calcd for C32H24N3O+3 , 498.1818; found, 498.1837.
2.1.2. Synthesis of 2-cyano-3-(6-(7-(diphenylamino)benzo[c][1,2,5]
thiadiazol-4-yl)-2-methoxyquinolin-3-yl)acrylic acid. BIM2
Compound BIM2 was synthesized using compound 14 (0.122 g,
group have been reported quinoline or pyridocarbazole-based D-
dyes for coadsorbent free DSSCs [19].
π
-A
0.25 mmol),
instead
of
6-(4-(diphenylamino)
phenyl)-2-
methoxyquinoline-3-carbaldehyde by the identical method used for
compound BIM1. The crude product was purified from silica gel column
(methanol:dichloromethane (1:10)) to give the desired product as red
solid. 0.111 g, 80 % yield. Mp; 226ꢀ 227 ◦C. 1H NMR (300 MHz, DMSO)
δ 8.82 (s, 1 H), 8.53 (s, 1 H), 8.33 (dd, J = 8.7, 2.0 Hz, 1 H), 8.17 (s, 1 H),
7.93 (dd, J = 13.6, 8.3 Hz, 2 H), 7.34–7.22 (m, 5 H), 7.09–6.96 (m, 6 H),
4.08 (s, 3 H). 13C NMR (75 MHz, DMSO) δ 163.00, 160.17, 154.96,
151.79, 147.87, 146.38, 141.27, 139.37, 138.28, 134.02, 132.52,
130.08(x2), 128.98, 128.17, 127.31, 125.02, 124.72, 124.34, 124.04,
119.75, 119.44, 118.29, 54.65. IR (cmꢀ 1): 3042, 2954, 2214, 1643,
1543, 1490, 1350, 1268. HRMS-ESI(+) m/z: [M+H]+ calcd for
In this work, we synthesized three dyes (Fig. 1) containing diphe-
nylamine as donors, benzene, benzothiadiazole or N-ethyl-
hexylbenzotriazole as auxiliary acceptor, quinoline as
π-bridge and
cyanoacrylic acid as anchoring group. We report synthesis steps, pho-
tophysical characteristics, density functional theory studies and photo-
voltaic performances of compounds BIM1, BIM2 and BIM3 in structure
D-A-π-A. We compare power conversion efficiencies when benzene,
benzothiadiazole and N-ethylhexylbenzotriazole auxiliary acceptor is
added to the structure of the dyes.
C
32H22N5O3S+, 556.1443; found, 556.1458.
2. Experimental
2.1.3. Synthesis of 2-cyano-3-(6-(7-(diphenylamino)-2-(2-ethylhexyl)-2H-
benzo[d][1,2,3]triazol-4-yl)-2-methoxyquinolin-3-yl)acrylic acid. BIM3
Compound BIM3 was synthesized using compound 15 (0.146 g,
0.25 mmol), instead of 6-(4-(diphenylamino)phenyl)-2-methoxyquino-
line-3-carbaldehyde by the identical method used for compound BIM1.
The crude product was purified from silica gel column (methanol:
dichloromethane (1:10)) to give the desired product as orange solid.
0.135 g, 83 % yield. Mp; 229ꢀ 230 ◦C. 1H NMR (600 MHz, DMSO) δ 8.75
(s, 1 H), 8.57 (s, 1 H), 8.38 (d, J =8.0 Hz, 1 H), 8.17 (s, 1 H), 7.85 (d, J
=8.8 Hz, 1 H), 7.74 (d, J =8.0 Hz, 1 H), 7.26 (t, J =7.7 Hz, 4 H),
7.08–6.94 (m, 7 H), 4.52 (d, J =5.9 Hz, 2 H), 4.07 (s, 3 H), 1.91–1.81 (m,
1 H), 1.23–1.04 (m, 8 H), 0.81–0.71 (m, 6 H). 13C NMR (150 MHz,
DMSO) δ 162.96, 159.68, 147.42, 145.95, 143.86, 141.53, 140.47,
137.95, 137.08, 133.53, 131.33, 129.66, 127.49, 127.08, 125.72,
124.77, 124.15, 124.07, 123.64, 120.32, 119.24, 119.01, 117.30, 58.95,
54.38, 30.37, 28.38, 23.81, 22.71, 14.34, 10.84. IR (cmꢀ 1): 3035, 2926,
2217, 1624, 1587, 1493, 1337, 1262. HRMS-ESI(+) m/z: [M+H]+ calcd
The used materials and instruments, preparation and characteriza-
tion of DSSCs, theoretical calculations, synthesis of compounds 1-15, 1H
NMR, 13C NMR, HRMS, fluorescence and FT-IR spectra were supplied as
electronic supplementary information (ESI).
2.1. Synthesis
2.1.1. Synthesis of 2-cyano-3-(6-(4-(diphenylamino)phenyl)-2-methoxy-
quinolin-3-yl)acrylic acid. BIM1
Compound 13 (0.11 g, 0.25 mmol), 2-cyanoacetic acid (0.106 g,
1.25 mmol), 0.1 mL of piperidine were dissolved in 10 mL of chloroform
and 10 mL of acetonitrile in a 50 mL flask. This mixture was refluxed for
12 h. After the reaction was complete, the solvent was removed. The
residue was extracted using ethyl acetate-water mixture. Organic layers
were combined, dried over anhydrous calcium chloride and the solvent
was evaporated off. The crude product was purified from silica gel col-
umn (methanol:dichloromethane (1:10)) to give the desired product as
2

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
Scheme 1. Synthetic pathway for BIM1, BIM2 and BIM3.
(a):DCM, Na2CO3, 0◦C, Ac2O, rt, 3 h, (b):DMF, 0◦C, POCl3, 80 ◦C, 16 h, (c):KOH, MeOH, 3 h, reflux, (d):B2pin2, Pd(dppf)Cl2, KOAc, 1,4-dioxane, 18 h, reflux, (e):
DMF, NBS, 0 ◦C, 5 h, (f): DCM, TEA, SOCl2, 5 h, reflux, (g):NBS, H2SO4, 60 ◦C, 3 h, (h):Diphenylamine, Pd(OAc)2, PPh3, t-BuONa, toluene, reflux, 18 h, (i):NaBH4,
CoCl2.6H2O, EtOH, reflux, 4 h, (j):AcOH, NaNO2, H2O, rt, 30 min, (k):K2CO3, DMF, 2-ethylhexyl bromide, 70 ◦C, 2 h, (l):Pd(PPh3)4, toluene:THF (1:1), 2 M K2CO3,
reflux, 24 h, (m): cyanoacetic acid, piperidine, chloroform:acetonitrile (1:1), reflux, 12 h.
3

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
obtained by Knoevenagel condensation [47].
3.2. Optical characteristics
UVꢀ Vis absorption spectra of BIM1¡3 in DMSO (10ꢀ 5 M) and on
the TiO2 film are depicted in Fig. 2 and the corresponding parameters
presented in Table 1. It can be seen from Fig. 2a, the absorption band
below 300 nm is attributed to the πꢀ π* transition, while the band be-
tween 300ꢀ 600 nm is the intramolecular charge transfer (ICT) from the
diphenylamine donor to the cyanoacrylic acid acceptor through the
quinoline π-bridge. λmax for dyes BIM1¡3 are 342, 476 and 400 nm for
the ICT band, respectively. Compared to the reference dye BIM1, sig-
nificant red-shifts (58–134 nm) are observed for dyes BIM2 and BIM3.
This can be attributed to the presence of stronger electron-withdrawing
BTD and BTZ units on the dyes, which can efficiently improve electron
delocalization and broaden the spectral response range [48]. Similarly,
the λmax for dye BIM2 bearing BTD is clearly more red-shifted than that
of BTZ-based dye BIM3 since the stronger electron-withdrawing ability
of the sulfur than the nitrogen, which reduces the band gap [49,50].
However, the ε for the ICT band of BIM2 is lower than those of BIM1 and
BIM3. This could be ascribed to the low oscillator strength (f) of the
charge-transfer transition due to reduced electronic coupling (Table S1)
[51]. The emission maximum for BIM2 was observed at longer wave-
length than those of BIM1 and BIM3 (Fig. S45), this put forward that the
BTD group in the dye likewise influences both light absorption and
fluorescence. To determine the effect of benzothiadiazole group on
absorbance, BIM2 was compared with a compound containing quinoline
unit coded as BPA-Q in the literature [52]. The solution absorbance
bands are 466 nm for BPA-Q and 476 nm for BIM2 whereas the absor-
bance bands on the TiO2 surface are 460 nm for BPA-Q and 484 nm for
BIM2. This means both of the solution and solid absorbance bands of
BIM2 are more red shifted due to strong electron withdrawing ability of
benzothiadiazole moiety. Since BIM2 absorbs a wider wavelength of
light, the DSSC based on this dye has a higher power conversion
efficiency.
Fig. 2. Absorption spectra of BIM1–3 (a) in DMSO (1 × 10–5 M) and (b) on
the TiO2.
The normalized absorption spectra of the dyes adsorbed onto the
TiO2 are shown in Fig. 2b. It can be seen that the absorption spectra are
broadened and red-shifted when compared with the solution spectra,
indicating existence of J-aggregation of dye molecules and the in-
teractions between the anchoring groups and titanium ions [53]. In
order to explain the binding of dyes BIM1–3 on TiO2, FT-IR spectra of
pure powders and their adsorbed forms on TiO2 film were measured
(Fig. S46). Considering the FT-IR spectra of the dye adsorbed films, two
characteristic streching vibrations are observed at ~1592 and
~1355 cmꢀ 1, respectively, which relate to the asymmetric and sym-
metric streching of the carboxylate groups bonded to the titanium
atoms. In addition, the stretching vibrations of the free carboxylic acid
groups observed in the FT-IR spectra of the dye powders were
completely disappear when they were adsorbed onto the TiO2 surface
[54].
Table 1
Optical properties of the dyes BIM1–3.
Dye
λmax [nm]a(
ε
[x 104
λonset
λmax (TiO2)
E0ꢀ 0 (opt)
Mꢀ 1 cmꢀ 1]a)
[nm]a
[nm]b
[eV]c
BIM1
BIM2
BIM3
272 (6.72), 342 (5.86)
300 (9.78), 476 (1.56)
294 (6.74), 400 (4.18)
386
552
456
390
484
426
3.21
2.25
2.72
a
λmax: absorption maximum wavelength;
λobnset: absorption onset wavelength.
ε: molar extinction coefficient;
λmax(TiO2): absorption maximum wavelength on the TiO2 film.
c
E0–0 (opt): optical band gap, estimated using the equation E0–0 = 1240/λonset
.
for C40H39N6O+3 , 651.3084; found, 651.3087.
3. Results and discussion
3.3. Electronic absorptions spectra, FMOs and band gap analyses
The experimental UV ꢀ Vis absorption maximum peaks of BIM1–3
3.1. Synthesis of dyes
assigned as the n→π*, π→π* transitions were appeared at the range of
272–476 nm in DMSO solvent. It is seen from Table S1 that the elec-
tronic absorption bands were calculated at the range from 616.4–305.7
for dye BIM1, from 539.1–317.8 for dye BIM2 and from 652.8–323.2 for
dye BIM3 by using TDꢀ B3LYP/6ꢀ 311 G(d,p) level in gas phase.
The important FMOs contributes belonging to electronic transitions
associated with λabs were determined via using Q-CHEM//Chemissian
programs (see Table S1). Among dye compounds BIM1–3, it is clear that
The total synthesis of dyes BIM1, BIM2 and BIM3 is shown in
Scheme 1. Acetylation of commercially available 4-bromoaniline fol-
lowed by Vilsmeier reaction and the nucleophilic substitution reaction
gave compound 3 to form the π-bridge of the dyes [37–39]. This com-
pound was converted to the boronic acid ester for use in coupling re-
actions to give compound 4 [40]. For coupling reactions, firstly,
bromine-containing compounds 5, 8, and 12 were synthesized
[41–46]. Subsequently, by Suzuki-Miyaura coupling reaction of com-
pound 4 with compounds 5, 8 and 12, compounds 13, 14 and 15 were
obtained, respectively [40]. Finally, dyes BIM1, BIM2 and BIM3 were
the similar π-system, electron-donor and -acceptor groups, as well as
different auxiliary acceptor moiety affect remarkable on the theoretical
λabs results. The largest λabs obtained at 616.4, 539.1 and 652.8 nm in
gas phase were assigned as the electronic contributions of H→L(99.3 %)
4

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
Fig. 3. The HOMO and LUMO being the most active in electronic transition for dyes BIM1–3 at B3LYP/6–311 G(d,p) in gas phase.
[D(67 %)+AA(24 %)+
BIM1, H→L(94.5 %) [D(61 %)+AA(30 %)+
(5%)+ -sys(42 %)] for BIM2 and H→L(99.0 %) [D(58 %)+AA(32
%)+ -sys(10 %)→A(54 %)+AA(4%)+ -sys(43 %)] for BIM3. It could be
π
-sys(9%)→A(53 %)+AA(3%)+
π
-sys(43 %)] for
describes chemically soft molecule which can be easily polarizable since
they need small energy to excite one electron from ground state to
π
-sys(9%)→A(52 %)+AA
π
excited state. χ parameter demonstrates that electron will be partially
π
π
transferred from the one of low χ to that of high χ for any two com-
stated that the minor differences in these contributions demonstrate the
strength of the different auxiliary acceptor moiety in dyes BIM1–3.
The other remarkable peaks for dyes BIM1–3 emerged at 272, 300
and 294 nm in DMSO solvent were obtained at 305.7, 317.8 and
323.3 nm with different FMOs contributions, respectively. These FMOs
contributions for dye BIM1 were assigned as H-6→L(44.2 %) [D(73)+
pounds contrary to the electrons flow from high chemical potential to
low chemical potential [19,55]. In this context, the investigation of the
relationship between FMOs and energy band gap for BIM1–3 were ful-
filled at B3LYP/6-311 G(d,p) level by using FMO energies and related
parameters (the electronegativity (
ness (S) parameters). The
(IP + EA)/2, = (IP–EA)/2 and S = 1/η
χ
), chemical hardness (
and S values were computed by using
equations [55,56]. Theoretical
η) and soft-
χ
,
η
χ =
AA(26 %)+
π
-sys(1%)→A(53 %)+AA(3%)+
π
-sys(43 %)] and H-5→L
-sys(43
η
(36.1 %) [D(65 %)+AA(33 %)+
π
-sys(1%)→A(53 %)+AA(3%)+
π
energy band gaps of dyes BIM1–3 were obtained at 2.31, 2.45 and
2.23 eV, and the corresponding optical values were found to be 3.21,
2.25 and 2.72 eV, respectively. The band gap obtained from DFT
calculation nicely corresponds with optical band gap for compounds
BIM2 and BIM3. However, the optical band gap for BIM1 differs from the
theoretical one. It can be attributed to the interaction of dyes with sol-
%)]. As for dyes BIM2 and BIM3, the FMOs contributions with Hꢀ 8→L
(95.0 %) and Hꢀ 1→L+1(87.0 %) appeared at depending on between
different percent and group transition of A(12 %)+
π
-sys(87 %)→D
-sys(45 %)→D
-sys(24 %), respectively. As can be seen in
Table S1, the variations of the effect of -system and auxiliary group in
(6%)+AA(85 %)+
π-sys(8%) and D(30 %)+AA(24 %)+π
(10 %)+AA(65 %)+
π
π
vent molecules [57]. The χ, η and S values of BIM1–3 were found at the
dyes BIM2 and BIM3 are observed while the influence of the electro-
n–acceptor and –donor groups in dye BIM1 draw the attention.
Furthermore, Fig. 3 depicts the occupied and unoccupied molecular
orbitals for dyes BIM1–3 being the most active in electronic transitions
estimated at B3LYP/6-311 G(d,p) level. It is clearly concluded from
Fig. 3 that the noteworthy electronic transitions for dyes BIM1–3
depending on among the impact of the electron-accepting (cyano group)
and -donating (diphenylamine) strengths and the different auxiliary
groups (benzene for BIM1, benzothiadiazole for BIM2 and N-ethyl-
hexylbenzotriazole for BIM3)
range of 4.12–4.19 eV, 1.12–1.22 eV and 0.408ꢀ 0.448 eVꢀ 1, respec-
tively. The theoretically obtained small
η values display that the charge
transfer will be occurred in BIM1–3. Considering
η and S values, the
dyes BIM1–3 could be depicted as the soft molecules. Since dipole
moment is a quantitative measure of the polarization tendency of a
bond, the large dipole moment characterizes polar compounds.
Although it depends on the presence of the strength of electron donor
and acceptor groups between the compounds, it is clear that the elec-
tronic structure difference plays a leading role in the change of this
parameter. Moreover, to understand the variation of the dipole moment
in BIM1–3 structures, the ground, excited and transition electric dipole
The energy gap is a critical parameter and characterized by the
molecular stability and spectroscopic properties of the molecular sys-
tems between the HOMO and LUMO energies. It is also used to deter-
mine molecular electrical transport properties because of providing
measurement of electron conductivity. Furthermore, the chemical
moments (μg, μe, μeg, in Debye) of BIM1–3 were investigated by using
the same level. The μg and μe dipole moments for BIM1–3 were obtained
at the range from 2.40 to 5.49 and from 31.54 to 34.82. The differences
between ground and excited dipole moment for BIM1–3 range from
28.25 to 31.29 D. The values of μeg dipole moment for BIM1–3 were
calculated at the range of 1.28 and 2.82 D. All parameters display dyes
hardness (
η) owing to the depending the energy band gap parameter is a
good indicator of the chemical stability. The smaller energy gap
5

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
explored by cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) (Fig. 4) and the numerical data are listed in Table 2. The ioni-
zation potential (IP) and electron affinitiy (EA) values, correspond to the
HOMO and LUMO energy levels vs. vacuum, were estimated using the
oxidation (Eox onset) and reduction onset potentials (Ered onset) [21,58].
CVs show that all of the three dyes undergo irreversible oxidation and
reduction processes (Fig. 4). The band gaps obtained by the CV data are
in good agreement with the DPV data. Moreover, the trend of the elec-
trochemical band gaps nicely compares with the optical band gaps
(BIM2 < BIM3 < BIM1). On the other hand, the band gaps as well as the
HOMO and LUMO energy levels estimated from the CV results are
significantly different from the theoretical ones. This difference may be
due to the fact that theoretical analysis uses the molecular orbitals,
while CV corresponds to the ionization energy [59].
The ground state oxidation potentials (Eox) of the dyes are almost the
same (1.01 V for BIM1; 0.98 V for BIM2; 0.99 V for BIM3) due to the
same electron donor unit in these dyes [40]. The Eox values match well
that of the previously reported quinoline-based BPA-Q dye which con-
tained a butoxy modified diphenylamine as the donor chromophore
[52]. These values are also more positive than the potential of Iꢀ /I3ꢀ
redox couple (0.4 V vs NHE), ensuring the dye regeneration [41]. The
excited state oxidation potentials (Eox*) for BIM1–3 are located at
ꢀ 1.35, ꢀ 0.94 and ꢀ 1.10 V vs NHE, respectively. The Eox* values
decrease with increasing the electron withdrawing ability of the auxil-
iary acceptors [60]. The decreasing trend is also in accordance with the
red shift of their ICT band due to a decrease in the band gap [61]. The
E
ox* values for BIM2 and BIM3 are down-shifted compared to the
similar dye BPA-Q [52], which is attributed to the presence of electron
withdrawing BTD and BTZ groups in former dyes. The Eox* values of the
sensitizers are also above the conduction band of TiO2 (ꢀ 0.5 V vs NHE),
indicating enough driving force for electron injection [49].
3.5. Photovoltaic properties of DSSCs
To study the influence of the auxiliary acceptors on the photovoltaic
performance, DSSCs were constructed using dyes BIM1–3 and their
performance was measured under AM 1.5 illumination. The current
density-voltage (J–V) curves of DSSCs based on the three quinoline
sensitizers are presented in Fig. 5a and the detailed photovoltaic pa-
rameters of short–circuit current density (Jsc), open–circuit voltage
(Voc), fill factor (FF), power conversion efficiency (PCE) and dye loading
amount are all summarized in Table 3. As expected, the dye loading
amounts for BIM2 and BIM3 with larger size are lower than that of
BIM1. The PCEs of DSSCs increase in the trend of BIM1 (1.78 %) < BIM3
(3.26 %) < BIM2 (4.08 %), which is in accordance with their Jsc values.
The best PCE based on these DSSCs reaches about 61 % of the cell with
N719 (6.58 %) under the same condition. This is mainly due to their
narrow absorption profiles compared to N719 [62]. However, the PCE
for both BIM2 and BIM3 is high when compared to the similar dye
BPA-Q may be due to the presence of electron withdrawing groups in
former dyes [46]. Similarly, despite the lower dye loading amounts, the
Fig. 4. CV and DPV of dyes BIM1–3 in DMSO including 0.1 M TBABF4 at a scan
rate of 0.050 V s–1
.
J
sc of the DSSCs based on BIM2 and BIM3 are much higher than that of
BIM1 due to their wider absorption spectrum which arises from the
incorporation of BTD and BTZ auxiliary acceptors on the dyes [46]. The
Voc value of BIM3 (0.632 V) was slightly higher than those of BIM1
(0.602 V) and BIM2 (0.619 V). It suggests that the branched alkyl chain
at the nitrogen atom of the triazole unit (BTZ) can effectively suppress
the unwanted electron recombination between the TiO2 and the elec-
trolyte [60].
BIM1–3 are polar compounds owing to the high difference between
ground and excited dipole moments, and the low transition dipole
moment values. It could be said that these differences attribute to the
depending on the strength of the different auxiliary acceptor moiety
(benzene for BIM1, benzothiadiazole for BIM2 and N-ethyl-
hexylbenzotriazole for BIM3). By considering electronic parameters, it
could be concluded that dyes BIM1–3 exhibit the ICT and the polari-
zations depending on the strength of the different auxiliary acceptor
moiety.
CDCA has been broady utilized as a coadsorbent that not only sup-
presses the dye aggregation but also protects the semiconductor metal
oxide from the redox electrolyte, leading to enhanced Jsc and Voc after
DSSC fabrication [63]. In the presence of CDCA, the Jsc, Voc and FF of
3.4. Electrochemical properties
devices based on dyes BIM1-3 are in the range of 5.38–11.53 mA cm–2
,
0.611–0.634 V and 0.70–0.74, respectively, corresponding to an overall
PCE of 2.30–5.21 % (Fig. 5b, Table 3). The DSSC based on BIM2 has the
The electrochemical properties of BIM1–3 in DMSO solution were
6

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
Table 2
Electrochemical properties of the dyes BIM1–3.
Dye
Eox onset/Ered onset (CV) [V]a
E0ꢀ 0 (CV) [eV]b
Eox/Eox*(CV) [V]c
EHOMO/ELUMO (CV) [eV]d
Eox onset/Ered onset (DPV) [V]e
E0ꢀ 0 (DPV) [eV]f
BIM1
BIM2
BIM3
0.38/– 1.98
0.35/– 1.57
0.36/– 1.79
2.36
1.92
2.15
1.01/– 1.35
0.98/– 0.94
0.99/– 1.10
– 5.48/– 3.12
– 5.45/– 3.53
– 5.46/– 3.31
0.29/– 1.94
0.21/– 1.49
0.23/– 1.71
2.23
1.70
1.94
a
Eox onset/Ered onset (CV): oxidation and reduction onset potentials in the CVs, referred to the external Fc/Fc+ standard (0.34 V vs. Pt disk).
b
c
E0–0 (CV): electrochemical band gap, estimated using the equation E0–0 = Eox onset (CV) – Ered onset (CV).
Eox/Eox*: ground and excited state oxidation potentials vs NHE. Eox was obtained by addition of E0
= 0.63 V vs NHE to the Eox onset. Eox* was calculated from
Fc/Fc+
the Eox* = Eox ꢀ E0–0 (CV).
d
EHOMO/ELUMO: HOMO and LUMO energy levels vs vacuum were estimated by EHOMO = ꢀ 5.1 ꢀ Eox onset (CV) and ELUMO = ꢀ 5.1 ꢀ Ered onset (CV) [21].
e
f
Eox onset/Ered onset (DPV): oxidation and reduction onset potentials vs Fc/Fc+ in the DPVs.
E0–0 (DPV): electrochemical band gap, obtained from the equation E0–0 = Eox onset (DPV) – Ered onset (DPV).
best PCE (5.21 %) among all, reaching 66 % of the device with
N719-based DSSC fabricated in the presence of CDCA having 7.92 %
PCE. Compared with our previous quinoline or pyridocarbazole-based
D-π-A dyes [19], using the BTD as the auxiliary acceptor unit proved
to promote the Jsc resulting from the broad spectral response of BIM2.
The PCEs of DSSCs with BIM1 and BIM2 represent over 28 % of
enhancement with respect to the cell without the coadsorbent, implying
that the insertion of CDCA to the dye bath decreases the unfavorable dye
aggregation on the TiO2 surface [64]. However, the DSSC based on
BIM3 with CDCA only obtains 12 % enhancement with respect to the
DSSC without the coadsorbent, which demonstrates that the BTZ unit
retards the aggregation due to its branched alkyl chain structure [49].
To further confirm the difference among the Jsc values of the three
dyes, the IPCE curves are plotted in Fig. 6a and the corresponding data
are summarized in Table 3. In accordance with their spectral responses
on the TiO2 film [19], the thresholds of the IPCE responses of these
dyes-based DSSCs are 530 nm (BIM1), 660 nm (BIM2) and 600 nm
(BIM3), respectively. The IPCE values of dyes BIM1–3 are over 64 %
from 375 to 450 nm (maximum IPCE value of 69.7 % at 400 nm), from
425 to 590 nm (maximum IPCE value of 77.4 % at 490 nm) and from 400
to 525 nm (maximum IPCE value of 73.9 % at 440 nm), respectively. The
IPCE
J
values integrated from the IPCE spectra are in good consistence
sc
with the Jsc values measured in the J–V tests. Consequently, the cell with
dye BIM2 gives highest and broadest IPCE spectrum, confirming its
IPCE
highest Jsc [65]. In the presence of CDCA, the J
values of the DSSCs
sc
based on dyes BIM1–3 with enhanced by 12 %, 11 % and 4 %, respec-
tively, with respect to the DSSCs without CDCA (Fig. 6b). The improved
IPCE responses are consistent with the improved Jsc [66].
To understand the Voc difference among the DSSCs without CDCA,
electrochemical impedance spectroscopy (EIS) experiments were
recorded in the dark. The larger semicircles in the Nyquist plots are
related to the charge recombination resistance (Rrec) between the TiO2
and the electrolyte (Fig. 7a). Obviously, the Rrec values increased in the
order BIM1 (65 Ω) < BIM 2 (71 Ω) < BIM3 (80 Ω), this tendency is in
agreement with the Voc order of the DSSCs. In the Bode plots, the elec-
Fig. 5. J-V curves of dyes BIM1–3 based DSSCs (a) without and (b) with CDCA.
tron lifetime (τe) calculated by the equation τe = 1/2
πf (f signify the peak
frequency in the left side of the Bode plots) are 5.87, 9.03 and 13.90 ms
Table 3
Photovoltaic parameters of the DSSCs with dyes BIM1–3.
(mA cm–2
)
IPCE (%)b
JSC(mA cm–2
)
VOC (V)
FF
PCE (%)
Dye loading amount (mol cm–2
)
IPCE
a
Dye
J
sc
BIM1
4.79
5.36
69.7
71.4
77.4
80.6
73.9
74.7
–
4.77
0.602
0.611
0.619
0.627
0.632
0.634
0.721
0.738
0.62
0.70
0.63
0.72
0.67
0.74
0.65
0.71
1.78
2.30
4.08
5.21
3.26
3.65
6.58
7.92
3.37 × 10–7
2.69 × 10–7
3.09 × 10–7
2.53 × 10–7
2.81 × 10–7
2.62 × 10–7
–
BIM1 + CDCA
BIM2
5.38
10.43
11.52
7.62
7.89
–
10.46
11.53
7.69
BIM2 + CDCA
BIM3
BIM3 + CDCA
N719
7.77
14.03
15.12
N719 + CDCA
–
–
–
a
b
IPCE
J
sc
values were integrated from the IPCE spectra.
IPCE values were obtained λmax from the IPCE spectra.
7

B.S. Arslan et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112936
Fig. 7. (a) Nyquist plots and (b) Bode plots of the DSSCs in the absence
of CDCA.
Fig. 6. IPCE spectra of the dyes–based cells (a) without and (b) with CDCA.
Software, Formal analysis. Ahmet Tutar: Visualization. Mehmet
˙
˘
Nebioglu: Supervision, Conceptualization. Ilkay Sis¸man: Supervision,
¸
Writing - review & editing.
for BIM1–3, respectively (Fig. 7b). Therefore, longest τe and largest Rrec
for BIM3 revealed that the BTZ unit can effectively restrain the charge
recombination, leading to the higher Voc [67].
Declaration of Competing Interest
The authors reported no declarations of interest.
Acknowledgment
4. Conclusions
In summary, three new quinoline
π-bridge based D-A-π-A dyes
bearing benzene, benzothiadiazole (BTD) or N-ethylhexylbenzotriazole
(BTZ) as auxiliary electron acceptor, respectively, were synthesized for
DSSCs. As compared to BIM1 and BIM3, BIM2 shows 166 and 96 nm red
shift of the absorption onset wavelength, which is due to the stronger
electron-withdrawing ability of the BTD unit on the latter dye. Upon
coadsorption with CDCA as the anti-aggregation agent, the DSSC based
on dye BIM2 exhibited the PCE of 5.21 %, which is better than those of
2.30 % and 3.65 % for BIM1 and BIM3, respectively. PCEs of the DSSCs
based on dyes BIM1–3 with coadsorbent improved by 29 %, 28 % and 12
%, respectively, compared to the DSSCs without coadsorbent. It implies
BTZ moiety on dye BIM3 not only suppresses the intermolecular ag-
gregation but also retard charge recombination to a certain degree.
Based on the above findings, the incorporation of a suitable auxiliary
This work was supported by the Scientific Research Projects Fund of
Sakarya University (Project No: 2019-6-22-238)
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2020.
112936.
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