Novel D-π-A benzocarbazole dyes with simple structures for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2019.03.015

Three benzocarbazole dyes were synthesized with N-butylbenzocarbazole as the electron donor, benzene, furan and thiophene as π-bridge, and cyanoacrylic acid as the electron acceptor. Their photophysical properties and photovoltaic performance were investigated and compared among three π-bridges. The incorporation of thiophene or furan unit favors intramolecular charge transfer through reducing the energy gap and improving the molecular conjugation due to their electron-rich properties and small volume compared with benzene unit. On the other hand, weakening coplanarity of dye with benzene bridge contributes to the antiaggregation and accordingly increase V_OC, though decreases photo-generated current. Among three dyes, dye with furan bridge achieves the optimal photovoltaic performance 7.67% due to the highest J_SC 15.45 mA cm^?2.

Full text of DOI:10.1016/j.jphotochem.2019.03.015

JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Novel D-π-A benzocarbazole dyes with simple structures for efficient dye-
sensitized solar cells
Liang Han, Xiaozhou Meng, Yong’an Ke, Hongqiang Ye, Yanhong Cui⁎
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310017, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Benzocarbazole
Sensitizers
Photovoltaic performance
Dye-sensitized solar cells
Three benzocarbazole dyes were synthesized with N-butylbenzocarbazole as the electron donor, benzene, furan
and thiophene as π-bridge, and cyanoacrylic acid as the electron acceptor. Their photophysical properties and
photovoltaic performance were investigated and compared among three π-bridges. The incorporation of thio-
phene or furan unit favors intramolecular charge transfer through reducing the energy gap and improving the
molecular conjugation due to their electron-rich properties and small volume compared with benzene unit. On
the other hand, weakening coplanarity of dye with benzene bridge contributes to the antiaggregation and ac-
cordingly increase VOC, though decreases photo-generated current. Among three dyes, dye with furan bridge
achieves the optimal photovoltaic performance 7.67% due to the highest JSC 15.45 mA cm−2
.
1. Introduction
prepared from the expensive metal through tedious synthetic and
purification process [9,10]. Then the researchers have turned the at-
tention to metal-free organic sensitizers which feature high molar ex-
tinction coefficient, structure diversity and low cost [11,12]. The ty-
pical structure of metal-free organic sensitizers generally consists of the
electron donor (D), π-bridge and the electron acceptor (A) to form a D-
π-A system. A lot of electron-rich nitrogen-containing heterocycles have
been introduced into the sensitizers as the electron donor, such as
triarylamine [13–15], carbazole [16–18], indoline [19,20], phe-
nothiazine [21,22] etc. Except the firstly reported vinyl bond, various
heterocycles in which thiophene and its derivatives dominate have been
selected as π-bridge to help the intramolecular charge transfer from the
electron donor to the electron acceptor [23–25]. Cyanoacrylic acid is
the most used acceptor, which also play an anchoring role on photo-
anode. The appropriate combination of the electron donor, π-bridge
and electron acceptor offers various kinds of metal-free organic sensi-
tizers.
Carbazole and its derivatives feature electron-rich property and
good hole-transporting capability, which are the common candidates
for the electron donor of dye sensitizers. Carbazole donors bearing
different substitution group combined with a wide array of heterocyclic
π-bridges afford a lot of carbazole sensitizers with good photoelectric
conversion efficiency [26–28]. Researchers have found that fine ad-
justment of carbazole sensitizers may tune the optical physical and
electrochemical properties, thereby optimizing DSSC performance. For
example, when N-phenylcarbazole was attached to the C-2 of thiophene
The fast depletion of petroleum reserves along with the growing
problem related with their combustion leads to a surge in the demand
for clean energy alternatives recently [1]. As the most plentiful and
widely distributed renewable energy source, solar energy seems to be a
good candidate for the development of the energy production tech-
nologies [2]. Therefore, there are many photovoltaic devices developed
to convert solar energy into electricity power through solar cells [3].
Nowadays solar cells based on silicon dominate the commercial market,
however, their development have been hindered by the demand of
highly crystalline silicon and consequent high cost [4]. Therefore, the
hybrid solar cells characterized by low material cost and easy manu-
facturing process have aroused great interest [5].
In various hybrid solar cells, dye-sensitized solar cells (DSSCs) have
attracted extensive interests for their good commercial prospects de-
rived from its low fabrication cost, flexibility in the scaling process and
good photovoltaic performance [6–8]. In DSSCs, dye molecules absorb
the sunlight and inject the electron into the photoanode, which initiates
the whole current cycling. Therefore, dyes play an important role in
DSSCs. Better light-harvesting ability and electron injection efficiency
of dyes are beneficial for the promotion of photoelectric conversion
efficiency. A lot of efforts had been made to design various dye sensi-
tizers to optimize the photovoltaic performance of DSSCs.
Earlier studies about dye sensitizers were related to metal organic
sensitizers, especially functional ruthenium complexes which were
⁎ Corresponding author.
E-mail address: yhcuiarticle@163.com (Y. Cui).
https://doi.org/10.1016/j.jphotochem.2019.03.015
Received 5 December 2018; Received in revised form 23 February 2019; Accepted 7 March 2019
Availableonline08March2019
1010-6030/©2019ElsevierB.V.Allrightsreserved.

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
Scheme 1. Syntheses of benzocarbazole dyes.
bridge instead of C-3, the photoelectric conversion efficiency was in-
creased to 2.40% from 0.34% [29]. In our previous study, varying
linking position of 4-phenyl-2-(thiophen-2-yl)thiazole π-bridge to ben-
zene ring of carbazole donor from N atom favors the improvement of
the short-circuit current density and open circuit voltage, elevating the
photoelectricity conversion efficiency to over 4% from 2% [30]. We
also found that the benzene unit in N-phenylcarbazole sensitizers is not
coplanar with carbazole unit and helps little to the enlargement of
conjugation system [31]. On the other hand, we noticed that benzo-
carbazole or thieno[3,2-a]carbazole sensitizers achieved improved
photoelectric conversion efficiency [26,28]. Therefore, we changed the
benzene unit in N-phenylcarbazole to fuse with carbazole ring instead
of substitution of N atom and obtained benzocarbazole donor with
larger conjugative system and better electron-donating ability. The
common used thiophene and furan units were incorporated as π-bridge
to further enlarge the conjugative system while benzene π-bridge was
also introduced in order to improve the photovoltage. Hence, three
benzocarbazole dyes were synthesized with N-butylbenzocarbazole
donor, benzene, thiophene, furan π-bridge and cyanoacrylic acid ac-
ceptor(Scheme 1). The results show that the photoelectric conversion
efficiency has been boosted to 5.10˜7.67% from 2.40% through the
location variation of benzene ring (Fig. 1).
spectra were recorded with a Waters Xevo Q-Tof Mass Spectrometer.
Absorption spectra were measured with SHIMADZU (model UV2550)
UV–vis spectrophotometer. Fluorescence spectra were obtained on a
SHIMADZU RF-5301PC spectrofluorometer.
2.2. Fabrication of DSSCs
TiO2 electrodes and Pt electrodes were purchased from Yingkou
Opvtech New Energy Co. Ltd. The TiO2 electrode was immersed into the
dye solution in a cosolvent of MeOH and CHCl3 (VMeOH: VCHCl3 = 1:10).
After being kept at room temperature for 24 h to assure complete dye
uptake, the TiO2 electrode was rinsed with MeOH to remove excess dye.
The dye-loaded TiO2 electrode and Pt electrode were assembled into a
sandwich type cell filled with an electrolyte. The electrolyte used here
is composed of the CH3CN solution of 0.30 M 1,2-dimethyl-3-propyli-
midazolium iodide (DMPII), 0.03 M I2, 0.07 M LiI, 0.10 M guanidine
thiocyanate, and 0.40 M 4-tert-butylpyridine (TBP). The active area of
DSSCs was 0.36 cm2.
2.3. Photovoltaic characterization
A Keithley digital source meter (Keithley 2420, USA) was used to
evaluate the photovoltaic performance of the DSSCs at AM 1.5 G illu-
mination (100 mW/cm2; Sol3A, Newport, USA). The IPCE (monochro-
matic incident photon-to-current conversion efficiency) spectra were
recorded by a Hypermonolight (SM-25, Jasco, Japan). The cyclic vol-
tammetry were recorded at a constant scan rate of 100 mV s−1 with a
computer controlled electrochemical analyzer (IviumStat, Holland).
2. Experimental
2.1. Materials and characterization
Nuclear magnetic resonance spectra were measured on Bruker
Avance III 500 MHz with chemical shifts expressed in ppm. Mass
Fig. 1. Structures of N-phenylcarbazole and benzocarbazole sensitizers.
128

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
2.4. General procedure for the synthesis of compounds
=7.4 Hz, 1 H), 4.86 (t, J =7.5 Hz, 2 H), 2.16-2.09 (m, 2 H), 1.65-1.57
(m, 2 H), 1.08 (t, J =7.4 Hz, 3 H); HREIMS m/z 378.1852 [M+H]+
,
2.4.1. Synthesis of 6,11-dihydro-5H-benzo[a]carbazole I
cacld C27H24NO for: 378.1858.
3,4-Dihydronaphthalen-1(2 H)-one (0.44 g, 3 mmol) and phenylhy-
drazine (0.32 g, 3 mmol) were dissolved in EtOH (6 mL) and then
concentrated HCl (0.30 mL) was added. The mixture was refluxed for
4 h under N2. After cooled, the solvent was evaporated to dryness and
the residue was recrystallized with cyclohexane-toluene cosolvent
(Vcyclohexane : Vtoluene = 1:1) to obtain light yellow solid (0.60 g, 92%).
m.p. 130˜131 °C; 1H NMR (CDCl3, 500 M) δ: 8.24 (s, 1 H), 7.57 (d, J
=7.8 Hz, 1 H), 7.41 (d, J =8.0 Hz, 1 H), 7.36 (d, J =7.2 Hz, 1 H), 7.32-
7.25 (m, 2 H), 7.20 (td, J = 7.2, 0.9 Hz, 2 H), 7.14 (t, J =7.0 Hz, 1 H),
3.13-3.05 (m, 2 H), 3.05-2.96 (m, 2 H).
2.4.6. Synthesis
carbaldehyde Vb
of
5-(11-butyl-11H-benzo[a]carbazol-5-yl)furan-2-
Compound IV (0.71 g, 2 mmol), (5-formylfuran-2-yl)boronic acid
(0.34 g, 2.40 mmol) and Pd(PPh3)4 (0.23 g, 0.20 mmol) were dissolved
in THF (10 mL) and aqueous K2CO3 solution (4 mL, 2 M) was added
dropwisely. The reactant was refluxed for 10 h under N2 and then
quenched with water. The mixture was extracted with CH2Cl2 and the
organic layers were combined. After the solvent was evaporated to
dryness, the residue was purified through column chromatography
(VPE:
VEtOAc = 10:1) to give orange oil (0.50 g, 68%). 1H NMR
2.4.2. Synthesis of 11H-benzo[a]carbazole II
(500 MHz, CDCl3) δ: 9.76 (s, 1 H), 8.58-8.55 (m, 3 H), 8.20 (d, J
=7.8 Hz, 1 H), 7.70-7.67 (m, 1 H), 7.63 (t, J =8.0 Hz, 1 H), 7.58 (d, J
=8.2 Hz, 1 H), 7.54 (t, J =7.5 Hz, 1 H), 7.49 (d, J =3.7 Hz, 1 H), 7.37
(t, J =7.0 Hz, 1 H), 6.94 (d, J =3.6 Hz, 1 H), 4.78 (t, J =8.0 Hz, 2 H),
2.11-2.05 (m, 2 H), 1.62-1.55 (m, 2 H), 1.06 (t, J =7.4 Hz, 3 H);
HREIMS m/z 368.1643 [M+H]+, cacld C25H22NO2 for: 368.1651.
Compound I (0.22 g, 1 mmol) and 2,3-Dichloro-5,6-dicyano-1,4-
benzoquinone (0.30 g, 1.30 mmol) were dissolved in toluene (10 mL).
The mixture was stirred for 3 h at room temperature. Then the reactant
was washed with NaOH aqueous solution (5%) and saturated NaCl
aqueous solution. The solvent was evaporated to dryness and the re-
sidue was purified with column chromatography (VPE: VEtOAc = 10:1)
to give white solid (0.15 g, 70%). m.p. 230˜231 °C; 1H NMR (CDCl3,
500 M) δ: 8.83 (s, 1 H), 8.16 (dd, J = 8.0, 6.1 Hz, 3 H), 8.04 (d, J
=8.2 Hz, 1 H), 7.69 (d, J =8.5 Hz, 1 H), 7.65-7.60 (m, 2 H), 7.58-7.54
(m, 1 H), 7.49-7.44 (m, 1 H), 7.33 (t, J =7.5 Hz, 1 H).
2.4.7. Synthesis of 5-(11-butyl-11H-benzo[a]carbazol-5-yl)thiophene-2-
carbaldehyde Vc
Compound IV (1.06 g, 3 mmol), (5-formylthiophen-2-yl)boronic
acid (0.56 g, 3.60 mmol) and Pd(PPh3)4 (0.35 g, 0.30 mmol) were dis-
solved in THF (15 mL) and aqueous K2CO3 solution (6 mL, 2 M) was
added dropwisely. The reactant was refluxed for 10 h under N2 and then
quenched with water. The mixture was extracted with CH2Cl2 and the
organic layers were combined. After the solvent was evaporated to
dryness, the residue was purified through column chromatography
(VPE: VEtOAc = 10:1) to give yellowgreen liquid (0.76 g, 66%); 1H NMR
(500 MHz, CDCl3) δ:10.00 (s, 1 H), 8.59 (d, J =8.5 Hz, 1 H), 8.54-8.48
(m, 1 H), 8.38 (d, J =8.1 Hz, 1 H), 8.32 (s, 1 H), 8.16 (d, J =7.8 Hz,
1 H), 7.89 (d, J =3.7 Hz, 1 H), 7.69 (t, J =8.0 Hz, 1 H), 7.59 (t, J
=8.0 Hz, 2 H), 7.56 (d, J =4.0 Hz, 1 H), 7.37 (d, J =7.1 Hz, 1 H), 4.80
(t, J =7.5 Hz, 2 H), 2.12-2.06 (m, 2 H), 1.63-1.55 (m, 2 H), 1.07 (t, J
=7.5 Hz, 3 H); HREIMS m/z 384.1418 [M+H]+, cacld C25H22NOS for:
384.1422
2.4.3. Synthesis of 5-bromo-11H-benzo[a]carbazole III
Compound II (0.43 g, 2 mmol) was dissolved in DMF (8.50 mL) and
then the solution of NBS (0.36 g, 2 mmol) in DMF (3 mL) was added
slowly under ice bath. After stirred for 4 h, the reactant was diluted
with water and extracted with dichloromethane. The organic layers
were combined, dried with anhydrous MgSO4 and filtrated. The solvent
was evaporated to dryness and the residue was purified through column
chromatography (VPE:VEtOAc = 10:1) to give grey solid (0.53 g, 90%).
m.p.146˜147 °C; 1H NMR (CDCl3, 500 M) δ: 8.87 (s, 1 H), 8.47-8.39 (m,
2 H), 8.14-8.10 (m, 1 H), 8.08 (d, J =7.8 Hz, 1 H), 7.68-7.64 (m, 2 H),
7.59 (d, J =8.1 Hz, 1 H), 7.48 (d, J =7.0 Hz, 1 H), 7.34 (d, J =7.0 Hz,
1 H); HREIMS m/z 296.0067, 298.0034[M+H]+, cacld C16H11BrN for:
296.0075
2.4.8. Synthesis of 3-(4-(11-butyl-11H-benzo[a]carbazol-5-yl)phenyl)-2-
cyanoacrylic acid YHQ-B
Va (0.19 g, 0.50 mmol) and cyanoacetic acid (0.09 g, 1.05 mmol)
were dissolved in CHCl3 (6 mL) and piperidine (0.50 mL) was added.
The reactant was refluxed for 10 h under N2. Then the mixture was
washed with diluted HCl and then evaporated to dryness. The residue
2.4.4. Synthesis of 5-bromo-11-butyl-11H-benzo[a]carbazole IV
Compound III (0.59 g, 2 mmol) was dissolved in DMSO (2 mL) and
mixed with 50% aqueous NaOH solution (1.28 mL). After the slow
addition of n-butyl bromide (0.27 mL), the reactant was stirred over-
night. The reaction was quenched with water and the precipitate was
filtrated. The filtrate cake was purified through column chromato-
graphy (VPE: VEtOAc = 10:1) to give white solid (0.39 g, 55%). m.p. 161˜
162 °C; 1H NMR (500 MHz, CDCl3) δ: 8.55-8.46 (m, 3 H), 8.11 (d, J
=7.8 Hz, 1 H), 7.71-7.63 (m, 2 H), 7.59-7.50 (m, 2 H), 7.35 (t, J
=7.5 Hz, 1 H), 4.73 (t, J =7.5 Hz, 2 H), 2.07-2.01 (m, 2 H), 1.59-1.51
(m, 2 H), 1.04 (t, J =7.4 Hz, 3 H); HREIMS m/z 352.0700, 354.0674 [M
+H]+, cacld C20H19BrN for: 352.0701
was purified through column chromatography (VHAc
:
VMeOH:
VCH2Cl2 = 1:4:400) to give brown solid (0.12 g, 54%). m.p. 224˜226 °C;
1H NMR (500 MHz, DMSO) δ: 8.71 (d, J =8.7 Hz, 1 H), 8.30-8.27 (m,
2 H), 8.10-8.07 (m, 3 H), 8.01 (d, J =8.5 Hz, 1 H), 7.84 (d, J =8.4 Hz,
1 H), 7.75-7.70 (m, 3 H), 7.57 (t, J =7.5 Hz, 1 H), 7.51 (t, J =7.6 Hz,
1 H), 7.29 (t, J =7.4 Hz, 1 H), 4.93 (t, J =7.2 Hz, 2 H), 1.97-1.91 (m,
2 H), 1.49-1.42 (m, 2 H), 0.95 (t, J =7.4 Hz, 3 H); 13C NMR (125 MHz,
DMSO) δ: 163.82, 163.78, 147.81, 143.94, 140.49, 133.51, 131.57,
130.87(2C), 130.69, 129.57(2C), 126.72, 125.82, 125.11(2C), 122.46,
122.26, 121.96, 120.56, 119.77, 119.67, 119.03, 118.20, 112.46,
2.4.5. Synthesis of 4-(11-butyl-11H-benzo[a]carbazol-5-yl)benzaldehyde
Va
Compound IV (0.71 g, 2 mmol), 4-formyphenylboronic acid (0.36 g,
2.40 mmol) and Pd(PPh3)4 (0.23 g, 0.20 mmol) were dissolved in THF
(10 mL) and aqueous K2CO3 solution (4 mL, 2 M) was added
dropwisely. The reactant was refluxed for 10 h under N2 and then
quenched with water. The mixture was extracted with CH2Cl2 and the
organic layers were combined. After the solvent was evaporated to
dryness, the residue was purified through column chromatography
109.98, 45.00, 31.74, 19.54, 13.79; HREIMS m/z 445.1916 [M+H]+
,
cacld C30H25N2O2 for: 445.1916
2.4.9. Synthesis of 3-(5-(11-butyl-11H-benzo[a]carbazol-5-yl)furan-2-yl)-
2-cyano- acrylic acid YHQ-F
Vb (0.73 g, 2 mmol) and cyanoacetic acid (0.36 g, 4.20 mmol) were
dissolved in CHCl3 (10 mL) and piperidine (0.70 mL) was added. The
reactant was refluxed for 10 h under N2. Then the mixture was washed
with diluted HCl and then evaporated to dryness. The residue was
(VPE
:
VEtOAc = 10:1) to give yellow oil (0.53 g, 70%). 1H NMR
(500 MHz, CDCl3) δ: 10.16 (s, 1 H), 8.63 (d, J =8.5 Hz, 1 H), 8.17-8.16
(m, 2 H), 8.07-8.05 (m, 3 H), 7.79 (d, J =8.0 Hz, 2 H), 7.68 (t, J
=7.2 Hz, 1 H), 7.62 (d, J =8.3 Hz, 1 H), 7.56-7.49 (m, 2 H), 7.35 (t, J
purified
through
column
chromatography
(VHAc
:
VMeOH:
V
CH2Cl2 = 1:4:400) to give orange solid (0.34 g, 39%); m.p. 215˜217 °C;
129

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
1H NMR (500 MHz, DMSO) δ: 8.82 (s, 1 H), 8.73-8.69 (m, 2 H), 8.27 (d,
J =7.8 Hz, 1 H), 7.88-7.86 (m, 2 H), 7.79 (t, J =7.5 Hz, 1 H), 7.70 (t, J
=7.5 Hz, 1 H), 7.54 (t, J =7.3 Hz, 1 H), 7.40 (d, J =3.5 Hz, 1 H), 7.34
(t, J =7.4 Hz, 1 H), 7.28-7.24 (m, 1 H), 4.93 (t, J =7.5 Hz, 2 H), 1.97-
1.88 (m, 2 H), 1.48-1.39 (m, 2 H), 0.94 (t, J =7.5 Hz, 3 H); 13C NMR
(125 MHz, DMSO) δ: 164.16, 158.77, 147.75, 140.74, 136.17, 134.72,
129.26, 126.39, 126.10(2C), 125.52, 122.71, 122.26, 121.90(2C),
121.46, 120.25(2C), 119.56, 118.29, 118.25, 117.66, 112.40, 110.28,
45.10, 31.57, 19.47, 13.74; HREIMS m/z 435.1702 [M+H]+, cacld
C28H23N2O3 for: 435.1709
coplanarity leading to more efficient intramolecular charge transfer.
Therefore, YHQ-F and YHQ-S show the red-shifted ICT absorption and
absorb more photons from the visible region, indicating their superior
light-harvesting ability to YHQ-B. The optical parameters of OM1 were
also listed in Table 1 for comparison. YHQ-S exhibits a red shift of
42 nm when compared with OM1, indicating it’s more efficient in-
tramolecular charge transfer. This improved ICT may be owing to the
better electron-donating ability of benzocarbazole than N-phe-
nylcarbazole, since YHQ-S has the same thiophene bridge and cya-
noacrylic acid acceptor with OM1.
On TiO2, the carboxylic acid group in dye molecule deprotonates to
form carboxylate-TiO2 unit. Dye molecules form H-aggregates with
parallel arrangement or J-aggregates with head to tail arrangement on
TiO2. The formation of aggregates often broadens the absorption
spectra on TiO2 related to in solution. The absorption spectra of three
benzocarbazole dyes on TiO2 were measured and listed in Fig. 2(b). As
can be seen in Fig. 2(b), the absorption spectrum onsets of three ben-
zocarbazole dyes on TiO2 shift bathochromically. In particular, YHQ-F
exhibits a 50 nm broadening of the absorption spectrum on TiO2 and its
absorption spectrum onset shifts to 600 nm on TiO2 from 550 nm in
solution, which should be ascribed to its highest loading amount on
TiO2 (Table 1).
In addition, the ICT absorption shift is observed on TiO2 for three
benzocarbazole dyes. YHQ-B exhibits a red shift of 4 nm while YHQ-F
and YHQ-S show the blue shift of 16 nm and 46 nm, respectively. Due
to a large dihedral angle between benzocarbazole unit and benzene
bridge (vide infra), YHQ-B shows inferior coplanarity and tends to form
J-aggregates through head to tail connection, which leads to the bath-
ochromic shift of the ICT absorption. On the other hand, when six-
member benzene bridge was replaced by five-member furan or thio-
phene bridge, the dihedral angle between benzocarbazole and π-bridge
decreased and better coplanarity was achieved in YHQ-F and YHQ-S.
Thus, they are easy to arrange in H-aggregates inducing the hypso-
chromic shift of the ICT absorption.
2.4.10. Synthesis of 3-(5-(11-butyl-11H-benzo[a]carbazol-5-yl)thiophen-
2-yl)-2- cyanoacrylic acid YHQ-S
Vc (1.15 g, 3 mmol) and cyanoacetic acid (0.54 g, 6.30 mmol) were
dissolved in CHCl3 (15 mL) and piperidine (1 mL) was added. The re-
actant was refluxed for 10 h under N2. Then the mixture was washed
with diluted HCl and then evaporated to dryness. The residue was
purified
through
column
chromatography
(VHAc
:
VMeOH:
V
CH2Cl2 = 1:4:400) to give orange solid (0.40 g, 29%); m.p. 220˜221 °C;
1H NMR (500 MHz, DMSO) δ: 13.79 (bs, 1 H), 8.74 (d, J =8.6 Hz, 1 H),
8.62 (s, 1 H), 8.54 (s, 1 H), 8.36 (d, J =7.8 Hz, 1 H), 8.31 (d, J =8.4 Hz,
1 H), 8.17 (d, J =4.0 Hz, 1 H), 7.87 (d, J =8.5 Hz, 1 H), 7.80 (t, J
=7.3 Hz, 1 H), 7.68 (t, J =8.0 Hz, 1 H), 7.60 (d, J =3.6 Hz, 1 H), 7.54
(t, J =7.4 Hz, 1 H), 7.36-7.30 (m, 1 H), 4.95 (t, J =7.3 Hz, 2 H), 1.98-
1.88 (m, 2 H), 1.49-1.41 (m, 2 H), 0.94 (t, J =7.4 Hz, 3 H); 13C NMR
(125 MHz, DMSO) δ: 163.77, 163.46, 146.84, 146.77, 140.59, 140.42,
139.40, 136.34, 135.16, 134.39, 130.50, 129.33, 128.68, 126.30,
125.83, 125.44, 122.68, 122.12, 121.95, 120.08, 119.97, 118.03,
116.58, 110.16, 45.06, 31.63, 19.50, 13.76; HREIMS m/z 451.1476 [M
+H]+, cacld C28H23N2O2S for: 451.1480
3. Results and discussion
3.1. Syntheses
Though the blueshift of ICT peak was observed, YHQ-F and YHQ-S
still exhibit larger maximum absorption wavelength than YHQ-B on
TiO2. Broader absorption spectra in the visible region were observed for
YHQ-F and YHQ-S when compared with YHQ-B, indicating superior
light-harvesting ability of the former. Thanks for the best light-har-
vesting ability and the largest loading amount, YHQ-F shows the
broadest absorption spectrum on TiO2 among three benzocarbazole
dyes, suggesting its most efficient photon absorption and conversion.
Benzocarbazole II was prepared from 3,4-dihydronaphthalen-
1(2H)-one and phenylhydrazine through Fischer indole synthesis and
the following oxidation reaction. Benzocarbazole was brominated with
NBS to obtain intermediate III which was substituted by 1-bromobu-
tane on N atom to give N-butylbenzocarbazole bromide IV. IV was
coupled with formylarylboronic acid through Suzuki coupling to syn-
thesize aldehyde intermediates V. Knoevenagel condensation between
aldehyde intermediates and cyanoacetic acid affords three benzo-
carbazole dyes YHQ-B, YHQ-F and YHQ-S, respectively.
3.3. Theoretical calculation
3.2. Optical properties
To better understand the relationship between the molecular
structure and the photophysical properties, quantum chemical calcu-
lations of three benzocarbazole dyes were performed by using spin-re-
stricted DFT method with B3LYP, in conjunction with 6-311+G(d,p)
basis set. The singlet ground-state geometrical optimizations of three
dyes were performed in CHCl3 solvent, which has been modeled using
conductor-like polarizable continuum model. Based on the optimized
geometries, the molecular orbitals (MOs) of three compounds were
calculated at the same level. The HOMO energy level (EHOMO) of each
compounds are taken from the Kohn-Sham eigenvalues calculated from
the DFT. TDDFT calculation of the single excitation energies was also
performed at the ground states. In the calculation coulomb-attenuated
hybrid exchange-correlation functional (CAM-B3LYP) was used which
is a long range corrected version of B3LYP, and the basis set is 6-
31+G(d). Then the energy gaps (Eg) were estimated based on the
singlet-singlet electronic transition energies. The LUMO energy level
In CHCl3-CH3OH solution the light-harvesting abilities of three
benzocarbazole dyes were measured. The corresponding absorption
spectra and photophysical data were listed in Fig. 2 and Table 1, re-
spectively. Above 400 nm, YHQ-F shows the broadest absorption range
and exhibits a peak at 450 nm with a molar extinction coefficient 21366
M
−1 cm−1. This peak is ascribed to the transition absorption of in-
tramolecular charge transfer (ICT) from the benzocarbazole donor to
cyanoacrylic acid acceptor. A hypsochromic shift of 17 nm was ob-
served in the case of YHQ-S along with an evident attenuation of molar
extinction coefficient. This may be attributed to the smaller resonance
energy of the furan than thiophene, which ensures a more effective
conjugation in the π-bridge of YHQ-F. When π-bridge was changed into
benzene unit, YHQ-B exhibits an ICT absorption at 367 nm with a lower
molar extinction coefficient 10960 M−1 cm−1 than YHQ-S. As can be
seen, π-bridge has a considerable influence on the light-harvesting
ability of these benzocarbazole dyes. The electron-rich furan and thio-
phene units have lower electron delocalization energy and at the same
time their five-atom structures have smaller size related to six-member
benzene unit. This contributes to the better conjugation and molecular
(ELUMO
)
can be obtained according to the equation of
E
LUMO=EHOMO(DFT) + Eg(TDDFT). The calculated ELUMO of three
compounds are in good agreement with the experimental data. All the
calculations of both ground and excited states were performed within
the Gaussian 09 quantum chemical package.
130

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
Fig. 2. UV–vis absorption spectra of benzocarbazole dyes.
(a) in CHCl3-CH3OH solution (v:v = 10:1); (b) on TiO2.
Table 1
Optical properties of benzocarbazole dyes.
a
b
c
Compd λmax
/
ε
M
a/ (104
−1⋅ cm−1
λmax
(nm)
/
λem
/
Dye loading
amount/
(nm)
)
(nm)
(mol⋅ cm−2
)
YHQ-B 367
YHQ-F 450
1.10
2.14
1.29
3.83
371
434
387
–
550
576
578
–
1.73*10−7
2.17*10−7
2.00*10−7
–
YHQ-S
OM1
433
391
a
maximum absorption wavelength λmax and molar extinction coefficient at
λmax of dyes measured in CH3Cl-CH3OH cosolvent (V CH3Cl: V CH3OH = 10:1).
b
maximum absorption wavelength λmax of dyes on sensitized TiO2 elec-
trodes.
c
maximum emission wavelength measured in CH3Cl-CH3OH cosolvent (V
CH3Cl: V CH3OH = 10:1).
Fig. 4. Energy level of the HOMO and LUMO of benzocarbazole dyes at B3LYP/
6-31 G(d) in CHCl3 solvent.
The optimized geometrical structures of three dyes were listed in
Fig. 3. As can be seen from three geometrical structures, π-bridges are
coplanar with cyanoacrylic aicd acceptor while there are large dihedral
angles between the benzocarbazole donor and π-bridges. These dihe-
dral angles are 50.8°, 32.3°, and 45.6° for YHQ-B, YHQ-F and YHQ-S,
respectively, and rank in the order of YHQ-B > YHQ-S > YHQ-F. The
large dihedral angle between the donor and π-bridge may hamper the
conjugation action of the whole molecule and lead to inferior ICT.
Therefore, YHQ-F with the smallest dihedral angle is the easiest to
transfer the electron from the electron donor to the electron acceptor
leading to the largest ICT absorption wavelength.
Fig. 4 shows the energy level of the frontier molecular orbitals for
three benzocarbazole dyes. LUMO energy levels of three dyes are lo-
cated at -3.66˜-3.36 eV, which is higher than the TiO2 CB energy level
(-4.0 eV); while HOMO energy levels are observed at -6.85˜-6.75 eV and
lower than the potential of the redox couple I3−/I− (−4.9 eV). Thus,
the excited dyes can inject the electrons efficiently into the TiO2 CB and
meanwhile the oxidized dyes may accept the electrons from the elec-
trolyte to be reduced successfully. The introduction of electron-rich
thiophene or furan unit to replace benzene unit shifts both HOMO and
Fig. 3. Optimized geometries of benzocarbazole dyes at the B3LYP/6-311+G(d,p) level.
131

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
Table 2
I3−/I− electrolyte. Their related action spectra of the incident photo-to-
current conversion efficiency (IPCE) were measured and shown in
Fig. 6(a). The trend of IPCE spectra of three benzocarbazole dye sen-
sitized DSSCs are similar to that of absorption spectra on TiO2. In IPCE
measurement, as the TiO2 film thickness increases to 18 um from 1 um
when absorption spectra measured, the loading amount of dye con-
tinually increase, which extends the IPCE responsive range. YHQ-B
sensitized DSSC exhibits an IPCE onset at about 600 nm with an ex-
tension by 50 nm compared with the absorption spectrum on TiO2.
Larger extension of the onsets was observed in the case of the other two
benzocarbazole dyes: YHQ-F and YHQ-S sensitized DSSCs show the
IPCE response till 700 nm and 675 nm respectively due to their higher
loading amount.
Energy levels of benzocarbazole dyes.
a
Compd.
Experimental (eV)
Calculated d(eV)
a
b
c
HOMO
E0-0
LUMO
HOMO
E0-0
LUMO
YHQ-B
YHQ-F
YHQ-S
−5.58
−5.62
−5.44
2.76
2.43
2.48
−2.82
−3.19
−2.96
−6.75
−6.77
−6.85
3.39
3.11
3.32
−3.36
−3.66
−3.53
a
The oxidation potential Eox in DMF was determined from cyclic voltam-
mograms and used to describe the ground-state energy HOMO.
b
E0-0 was calculated from E0-0 = 1240/ λint and λint was the intersection of
the normalized absorption and emission spectra.
c
ELUMO was calculated from Eox-E0-0
Calculated in CHCl3 solvent.
.
All benzocarbazole dye sensitized DSSCs gave the maximum IPCE
value over 70%. Among three DSSCs, YHQ-F sensitized DSSC displays a
high plateau over 60% from 380 to 580 nm with a maximum IPCE value
of 76% at 466 nm, indicating its superior incident photo-to-current
conversion efficiency and accordingly better JSC. For the DSSC based on
YHQ-S, IPCE values higher than 60% were observed in the range of
385–556 nm with a similar maximum IPCE value 74% at 464 nm to that
of YHQ-F. However, DSSC based on YHQ-S exhibits a little narrower
IPCE responsive range than that of YHQ-F. Though it exhibits a com-
parable maximum IPCE value with DSSC based on YHQ-S, YHQ-B
sensitized DSSC shows the narrowest IPCE spectrum among three
DSSCs suggesting its least photon harvest.
d
LUMO down, especially the LUMO. The greatly downshifted LUMO
decreased the energy gap of YHQ-F and YHQ-S leading to the easy ICT
transition which was proved by their longer ICT absorption wavelength
in Table 1. Cyclic voltammograms were performed to obtain the ex-
perimental value of energy levels, which basically agree with the cal-
culated values (Table 2). Compared with that of OM1 (-5.65 eV) [29],
HOMO levels of YHQ-series dyes (-5.62˜-5.44 eV) have been upshifted,
which further prove the better electron-donating ability of benzo-
carbazole than N-phenylcarbazole.
Frontier molecular orbitals of benzocarbazole dyes were listed in
Fig. 5. The HOMO of three dyes mainly delocalize over the benzo-
carbazole donor and partially stretch out on the π-bridge. The LUMO
are mainly confined to the cyanoacrylic acid acceptor and π-bridge,
which exhibit a good overlap over π-bridge with the HOMO, favoring
the efficient electron excitation. Comparison of the HOMO population
shows that dye with furan bridge has more electron distribution on π-
bridges than dyes with thiophene or benzene bridge, which may result
in better overlap between HOMO and LUMO of YHQ-F. Better overlap
of frontier molecular orbitals improves intramolecular charge transfer,
which also favor the redshift of ICT absorption peak.
J-V curves of DSSCs based on three benzocarbazole dyes were
evaluated under standard conditions (AM 1.5 G, 100 mW∙cm−2) and
shown in Fig. 6(b). The corresponding parameters VOC (open-circuit
photovoltage), JSC (short circuit photocurrent density), ff (fill factor)
and η (solar energy-to-electricity conversion) were listed in Table 3. The
corresponding parameters of OM1 were also listed in Table 3 for
comparison. As can be seen in Table 3, YHQ-series dyes achieved higher
JSC (8˜16 mA·cm−2) than OM1 (5.85 mA·cm−2). Especially, JSC of YHQ-
S is more than double that of its N-phenylcarbazole counterpart OM1.
Moreover, higher VOC (above 0.7 V) was observed in YHQ-series dyes
when compared with OM1 (0.66 V). We presumed that better electron-
donating ability of benzocarbazole benefits the intramolecular charge
transfer and accordingly the electron injection into TiO2 CB, which
contribute to the increase of JSC and VOC of YHQ-series dyes.
3.4. Photovoltaic performance
Three benzocarbazole dye sensitized DSSCs were fabricated with
Fig. 5. Frontier molecular orbitals (HOMO and LUMO) of benzocarbazole dyes.
132

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
Fig. 6. IPCE curves (a) and current-potential (J-V) curves (b) for DSSCs based on benzocarbazole dyes.
cm−2, AM 1.5 G) conditions. In Fig. 7, YHQ-S sensitized DSSC shows
two semicircles: the right semicircle represents the electron transfer at
the TiO2/dye/electrolyte interfaces while the left semicircle corre-
sponds to the redox reaction at the platinum counter electrode. How-
ever, only one semicircle is observed in the case of YHQ-B and YHQ-F
sensitized DSSCs. The redox reaction at the platinum counter electrode
in DSSCs based on YHQ-B and YHQ-F may be too quick when com-
pared with the electron transfer at the TiO2/dye/electrolyte interfaces,
which leads to only the presence of one semicircle relates to the elec-
tron transfer at the TiO2/dye/electrolyte interfaces [33]. Using an
equivalent circuit (shown in the inset of Fig. 7), the corresponding Rct
which represents electron transport resistances at the TiO2/dye/elec-
trolyte interface was analyzed by software (ZSimpWin). Rct of three
DSSCs follows the order: YHQ-S (15.0 Ω) > YHQ-F (14.3 Ω) > YHQ-B
(12.6 Ω). Low electron transport resistance of YHQ-B sensitized DSSC
indicates better electron collection efficiency. This may be attributed to
its antiaggregation effect derived from its inferior coplanar structure,
which reduces the charge recombination rate and contributes to the
promotion of VOC. The results agreed with the variation trend of VOC in
Table 3.
Table 3
Parameters for DSSC based on benzocarbazole dyes.
Comd.
Voc (V)
Jsc (mA·cm−2
)
ff
η%
YHQ-B
YHQ-F
YHQ-S
OM1
0.78
0.76
0.73
0.66
8.42
0.78
0.65
0.68
0.62
5.10
7.67
6.58
2.40
15.45
13.30
5.85
JSC ranks by the order of YHQ-F > YHQ-S > YHQ-B, which agrees
well with their IPCE responsive range. Thanks to its highest IPCE
maximum value and broadest IPCE responsive range, YHQ-F sensitized
DSSC achieves the highest JSC of 15.45 mA∙cm−2. Lower IPCE response
of DSSC based on YHQ-S from 380 to 700 nm than that of YHQ-F
lowers its JSC to 13.30 mA∙cm−2. YHQ-B sensitized DSSC exhibits the
lowest JSC of 8.45 mA∙cm−2, which may impair its photoelectric con-
version efficiency. Similar with our previous report [31,32], with
benzene π-bridge, YHQ-B achieves the highest VOC of 0.78 V due to its
improved antiaggregation effect derived from its inferior coplanarity.
VOC follows the order of YHQ-B > YHQ-F > YHQ-S and YHQ-S sensi-
tized DSSC shows the smallest VOC among three DSSCs. Therefore, due
to its highest JSC and better VOC, YHQ-F achieves the best photoelectric
conversion efficiency 7.67%.
4. Conclusion
Electrochemical impedance spectroscopy (ESI) was usually used to
investigate the interfacial charge transfer process, which can help to
understand the recombination rate and the lifetime of injected electrons
on TiO2 in the DSSC. Fig. 7 listed the Nyquist diagram of three DSSCs
based on YHQ-series dyes measured at standard illumination (100 mW
Benzocarbazole core was constructed from 3,4-dihydronaphthalen-
1(2 H)-one and phenylhydrazine through Fischer Indole synthesis and
selected as the electron donor after the nucleophilic substitution with 1-
bromobutane. Then benzene, furan and thiophene π-bridges were in-
corporated through Suzuki coupling respectively. After the condensa-
tion with cyanoacetic acid, three D-π-A benzocarbazole sensitizers were
prepared. The absorption spectra in solution and on TiO2 were mea-
sured and thiophene or furan π-bridge endows the sensitizer with
broader absorption spectra and bathochromic ICT absorption peak.
Theoretical calculation based on DFT and TD-DFT shows that dyes with
thiophene or furan π-bridge exhibit better coplanarity, increased
overlap between HOMO and LUMO, and decreased energy gap, which
account for their better light harvesting ability than dye with benzene
bridge. Superior light harvesting ability brings about improved JSC and
cell fabricated with dye bearing furan bridge shows the highest JSC. On
the other hand, the presence of benzene bridge brings about weakening
coplanarity, which favors the antiaggregation and accordingly improve
VOC. Among DSSCs based on three benzocarbazole dyes, JSC pre-
ponderates over VOC in determining the photovoltaic performance,
consequently dye with thiophene or furan bridge exhibits higher pho-
toelectric conversion efficiency than dye with benzene bridge.
Compared with the similar N-phenylcarbazole sensitizer, three benzo-
carbazole sensitizers achieve distinctly higher JSC and better VOC, which
contributes to the significant promotion of photoelectric conversion
efficiency.
Fig. 7. Nyquist phase plots for the DSSCs measured under illumination.
133

L. Han, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry376(2019)127–134
Acknowledgement
for dye-sensitized solar cells, Photochem. Photobiol. Sci. 16 (2017) 1049–1056.
[16] G. Sathiyan, E.K.T. Sivakumar, R. Ganesamoorthy, R. Thangamuthu, P. Sakthivel,
Review of carbazole based conjugated molecules for highly efficient organic solar
cell application, Tetrahedron Lett. 57 (2016) 243–252.
We gratefully acknowledge the financial support by the funding of
Natural Science Foundation of Zhejiang province (LY19B020009).
[17] Y. Liu, J. He, J.R. Gao, L. Han, Influence of the auxiliary acceptor and p-bridge in
carbazole dyes on photovoltaic properties, J. Photochem. Photobiol. A 332 (2017)
283–292.
Appendix A. Supplementary data
[18] L. Han, J. Liu, Y. Liu, Y.H. Cui, Novel D-A-π-A type benzocarbazole sensitizers for
dye sensitized solar cells, J. Mol. Struct. 1180 (2019) 651–658.
[19] N. Ohta, K. Awasthi, K. Okoshi, K. Manseki, H. Miura, Y. Inoue, et al., Stark spec-
troscopy of absorption and emission of indoline sensitizers: a correlation with the
performance of photovoltaic cells, J. Phys. Chem. C. 120 (2016) 26206–26216.
[20] L. Han, J.G. Zhao, B. Wang, S.L. Jiang, Influence of π-bridge in N-fluorenyl indoline
sensitizers on the photovoltaic performance of dye-sensitized solar cells, J.
Photochem. Photobiol. A 326 (2016) 1–8.
[21] M.L. Han, Y.Z. Zhu, S. Liu, Q.L. Liu, D. Ye, B. Wang, et al., The improved photo-
voltaic performance of phenothiazine-dithienopyrrole based dyes with auxiliary
acceptors, J. Power Sources 387 (2018) 117–125.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.03.
015.
References
[1] A. Zendehboudi, M.A. Baseer, R. Saidur, Application of support vector machine
models for forecasting solar and wind energy resources: a review, J. Clean. Prod.
199 (2018) 272–285.
[2] V. Esen, S. Saglam, B. Oral, Light sources of solar simulators for photovoltaic de-
vices: a review, Renew. Sustain. Energy Rev. 77 (2017) 1240–1250.
[3] Z. Zeng, W.H. Shao, H. Chen, B.R. Hu, W.S. Chen, H. Li, et al., Changes and chal-
lenges of photovoltaic inverter with silicon carbide device, Renew. Sustain. Energy
Rev. 78 (2017) 624–639.
[4] T. Saga, Advances in crystalline silicon solar cell technology for industrial mass
production, NPG Asia Mater. 2 (2010) 96–102.
[5] W. Chen, M.P. Nikiforov, S.B. Darling, Morphology characterization in organic and
hybrid solar cells, Energy Environ. Sci. 5 (2012) 8045–8074.
[22] B.Y. Zhu, L. Wu, Q. Ye, J.R. Gao, L. Han, Asymmetric double donor-p-acceptor dyes
based on phenothiazine and carbazole donors for dye-sensitized solar cells,
Tetrahedron 73 (2017) 6307–6315.
[23] H.M. Fan, R.K. Huang, G.Y. Yan, Regulation of energy levels and kinetics in dye-
sensitized solar cells: synergistic effect of N,N-bis(9,9-dimethyl-fluoren-2-yl)- ani-
line and 3,4-ethylenedioxythiophene, J. Power Sources 397 (2018) 196–203.
[24] H.X. Ji, Z.S. Huang, L.Y. Wang, D.R. Cao, Quinoxaline-based organic dyes for ef-
ficient dye-sensitized solar cells: effect of different electron-withdrawing auxiliary
acceptors on the solar cell performance, Dyes Pigm. 159 (2018) 8–17.
[25] D. Unny, J. Sivanadanam, S. Mandal, I.S. Aidhen, K. Ramanujam, Effect of flexible,
rigid planar and non-planar donors on the performance of dye-sensitized solar cells,
J. Electrochem. Soc. 165 (2018) H845–H860.
[6] V. Sugathan, E. John, K. Sudhakar, Recent improvements in dye sensitized solar
[26] X. Qian, Y.Z. Zhu, W.Y. Chang, J. Song, B. Pan, L. Lu, et al., Benzo[a]carbazole-
based donor−π−acceptor type organic dyes for highly efficient dye-sensitized
solar cells, ACS Appl. Mater. Interfaces 7 (2015) 9015–9022.
[27] L.Q. Huang, P. Ma, G.W. Deng, K. Zhang, T. Ou, Y. Lin, et al., Novel electron-
deficient quinoxalinedithienothiophene- and phenazinedithienothiophene- based
photosensitizers: the effect of conjugation expansion on DSSC performance, Dyes
Pigm. 159 (2018) 107–114.
cells: a review, Renew. Sustain. Energy Rev. 52 (2015) 54–64.
[7] C.P. Lee, R.Y.Y. Lin, L.Y. Lin, C.T. Li, T.C. Chu, S.S. Sun, et al., Recent progress in
organic sensitizers for dye-sensitized solar cells, RSC Adv. 5 (2015) 23810–23825.
[8] A. Mishra, M.K.R. Fischer, P. Bäuerle, Metal-free organic dyes for dye-sensitized
solar cells: from structure: property relationships to design rules, Angew. Chem. Int.
Ed. 48 (2009) 2474–2499.
[9] A.S. Polo, M.K. Itokazu, N.Y.M. Iha, Metal complex sensitizers in dye-sensitized
[28] H. Jiang, K. Oniwa, A. Islam, J. Zhan, L.Y. Han, Y.J. Sun, et al., Efficient thieno[3,2-
a]carbazole-based organic dyes for dye-sensitized solar cells, Tetrahedron 71
(2015) 6534–6540.
[29] B.B. Ma, Y.X. Peng, T. Tao, W. Huang, A distinguishable photovoltaic performance
on dye-sensitized solar cells using ruthenium sensitizers with a pair of isomeric
ancillary ligands, Dalton Trans. 43 (2014) 16601–16604.
[30] T. Li, J.R. Gao, Y.H. Cui, C.J. Zhong, Q. Ye, L. Han, Novel D-p-A carbazole sensi-
tizers with 4-phenyl-2-(thiophen-2-yl)thiazole as p-bridge for dye-sensitized solar
cells, J. Photochem. Photobio. A 303 (2015) 91–98.
[31] L. Han, X.Y. Zu, Y.H. Cui, H.B. Wu, Q. Ye, J.R. Gao, Novel D-A-pi-A carbazole dyes
containing benzothiadiazole chromophores for dye-sensitized solar cells, Org.
Electron. 15 (2014) 1536–1544.
[32] L. Han, H.B. Wu, Y.H. Cui, X.Y. Zu, Q. Ye, J.R. Gao. Synthesis and density functional
theory study of novel coumarin-typedyes for dye sensitized solar cells, J.
Photochem. Photobio. A 290 (2014) 54–62.
[33] J. Qu, X.P. Gao, G.R. Li, Q.W. Jiang, T.Y. Yan, Structure transformation and pho-
toelectrochemical properties of TiO2 nanomaterials calcined from titanate nano-
tubes, J. Phys. Chem. C 113 (2009) 3359–3363.
solar cells, Coord. Chem. Rev. 248 (2004) 1343–1361.
[10] S.H. Kang, M.J. Jeong, Y.K. Eom, I.T. Choi, S.M. Kwon, Y. Yoo, et al., Porphyrin
sensitizers with donor structural engineering for superior performance dye-sensi-
tized solar cells and tandem solar cells for water splitting applications, Adv. Energy
Mater. 7 (2017) 1602117.
[11] C. Antonio, B. Fabio, C. Roberto, Research progress on photosensitizers for DSSC,
Front. Chem. 6 (2018) 481.
[12] J.-M. Ji, H. Zhou, H.K. Kim, Rational design criteria for D-π-A structured organic
and porphyrin sensitizers for highly efficient dye-sensitized solar cells, J. Mater.
Chem. A 6 (2018) 14518–14545.
[13] Y.K. Eom, S.H. Kang, I.T. Choi, Y. Yoo, J. Kim, H.K. Kim, Significant light absorption
enhancement by a single heterocyclic unit change in the π-bridge moiety from
thieno[3,2-b]benzothiophene to thieno[3,2-b]indole for high performance dye-
sensitized and tandem solar cells, J. Mater. Chem. A 5 (2017) 2297–2308.
[14] C.J. Zhong, J.R. Gao, Y.H. Cui, T. Li, L. Han, Coumarin-bearing triarylamine sen-
sitizers with high molar extinction coefficient for dye-sensitized solar cells, J. Power
Sources 273 (2015) 831–838.
[15] J. He, Y. Liu, J.R. Gao, L. Han, New D-D-π-A triphenylamine-coumarin sensitizers
134