Novel D-A-π-A carbazole dyes containing benzothiadiazole chromophores for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.orgel.2014.04.016

New D-A-π-A carbazole dyes containing benzothiadiazole chromophores were designed and synthesized for application in dye-sensitized solar cells (DSSCs). The light-harvesting capabilities and photovoltaic performance of these dyes were investigated systematically through comparison of different π-bridges and acceptors. Compared with thiophene bridge, benzene bridge provides improved IPCE and V_OC, which leads to better photoelectricity conversion efficiency. Dyes with cyanoacetic acid acceptor display superior photovoltaic properties though with shorter absorption maximum and lower molar absorption coefficient compared with dyes with rhodanine acetic acid acceptor. Therefore, dye with benzene bridge and cyanoacetic acid acceptor shows the most efficient photoelectricity conversion efficiency and has the maximum η value of 5.40% (V_OC = 710 mV, J_SC = 10.99 mA/cm², and ff = 0.71) under simulated AM 1.5 irradiation (100 mW/cm²).

Full text of DOI:10.1016/j.orgel.2014.04.016

ORGELE 2542
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Organic Electronics xxx (2014) xxx–xxx
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Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
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Novel D–A–p–A carbazole dyes containing benzothiadiazole
chromophores for dye-sensitized solar cells
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⇑
Liang Han, Xiaoyan Zu, Yanhong Cui, Huabiao Wu, Qing Ye, Jianrong Gao
7 Q1
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College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, China
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a r t i c l e i n f o
a b s t r a c t
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Article history:
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New D–A–p–A carbazole dyes containing benzothiadiazole chromophores were designed
and synthesized for application in dye-sensitized solar cells (DSSCs). The light-harvesting
capabilities and photovoltaic performance of these dyes were investigated systematically
Received 12 December 2013
Received in revised form 20 March 2014
Accepted 12 April 2014
Available online xxxx
through comparison of different
p-bridges and acceptors. Compared with thiophene
bridge, benzene bridge provides improved IPCE and VOC, which leads to better photoelec-
tricity conversion efficiency. Dyes with cyanoacetic acid acceptor display superior photo-
voltaic properties though with shorter absorption maximum and lower molar absorption
coefficient compared with dyes with rhodanine acetic acid acceptor. Therefore, dye with
benzene bridge and cyanoacetic acid acceptor shows the most efficient photoelectricity
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Keywords:
Carbazole
Sensitizing dyes
Solar cell
Benzothiadiazole
Synthesis
conversion efficiency and has the maximum
mA/cm2, and ff = 0.71) under simulated AM 1.5 irradiation (100 mW/cm2).
Ó 2014 Published by Elsevier B.V.
g value of 5.40% (VOC = 710 mV, JSC = 10.99 -
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Benzene bridge
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1. Introduction
absorption spectrum and improve the photoelectrical
conversion properties [9–11]. Typical sensitizing dyes con-
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Dye-sensitized solar cells (DSSCs) own the advantages
of low material cost, flexibility, easy manufacturing pro-
cess and high efficiency, which have attracted considerable
and sustainable attention [1–3]. In DSSCs, the sensitizing
dye absorbs sunlight under sun illumination to excite an
electron from the HOMO to the LUMO, and then the gener-
ated photoelectron is injected to the conduction band (CB)
edge of TiO2 from the LUMO, which initiates subsequent
electron transfer process [4,5]. Therefore, sensitizing dye
is a key component in achieving high photoelectricity con-
version efficiency and durability of DSSCs.
Much effort has been exerted to develop metal organic
sensitizing dyes, especially functional ruthenium com-
plexes [6–8]. Moreover, various metal-free organic dyes
have been designed and investigated not only to replace
the expensive ruthenium sensitizer but also to widen
sist of electron donor,
acceptor and exist as donor–
(D– –A). This push–pull D– –A system allows efficient
intramolecular charge separation and photoelectron injec-
tion. In D– –A system, the design of longer -conjugation
p
-conjugate bridge and electron
p
bridge–acceptor system
p
p
p
p
dye molecules is generally given attention to achieve larger
absorption maximum and extend the absorption region.
However, long conjugation dye molecules are unstable
under irradiation with high-energy photons and prone to
unfavorable
the design methods involves the development of novel
organic dyes with D–A– –A configuration which differs
from the traditional D– –A system for the introduction
p-stacks [12]. To solve this problem, one of
p
p
of the additional acceptor. Different electron-withdrawing
heteroaromatics, such as benzothiadiazole [13], benzotria-
zole [14] and diketopyrrolo-pyrrole [15], were used as
another acceptor in D–A–
et al. reported a series of D–A–
incorporating benzothiadiazole into the triphenylamine
p–A system. For example, Tian
p–A structural organic dyes
⇑
Corresponding author. Tel.: +86 0571 88320891.
E-mail address: hanliang@zjut.edu.cn (J. Gao).
http://dx.doi.org/10.1016/j.orgel.2014.04.016
1566-1199/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: L. Han et al., Novel D–A–
p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

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framework, resulting in red-shift in absorption and weak-
ening the deprotonation effect on TiO2 film, which is ben-
eficial for light-harvesting [16]. Zhu et al. designed indoline
sensitizing dyes with benzotriazole as another acceptor
and realized a power conversion efficiency of 8.02% [14].
These investigations indicate that the incorporation of
cells were fabricated in air by clamping the different photo-
electrodes with platinized counter electrodes. The electro-
lyte OPV-AN-I used here is purchased from Yingkou
Opvtech New Energy Co. Ltd., which contains 0.07 mM/L
Iꢁ. The active area of DSSCs was 0.16 cm2.
141
another acceptor will not only extend the
p-conjugation
2.3. Photovoltaic characterization
system of the dyes and realize long wavelength absorption
but also help decrease the HOMO–LUMO energy gap and
favor the electron injection.
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The photovoltaic performance of the DSSCs was evalu-
ated at AM 1.5 illumination (100 mW/cm2; Peccell-L15,
Peccell, Japan) using a Keithley digital source meter (Keith-
ley 2601, USA). The incident light intensity was 100 mW/
cm2 calibrated with a standard Si solar cell (BS-520, Japan).
The action spectra of monochromatic incident photon-
to-current conversion efficiency (IPCE) for solar cell were
performed by using a commercial setup (PEC-S20IPCE
Measurement System, Peccell, Japan). Electrochemical
impedance spectroscopy (EIS) experiments were
conducted using a computer-controlled potentiostat (Zeni-
umZahner, Germany). The measured frequency ranged
from 100 mHz to 1 MHz while the AC amplitude was set
to 10 mV. The bias of all EIS measurements was set to
the VOC of the corresponding dyes. The cyclic voltammetry
were recorded with a computer controlled electrochemical
analyzer (IviumStat, Holland) at a constant scan rate of
100 mV/s. Measurement were performed with a three elec-
trode cell in 0.1 M Bu4NClO4 N,N-dimethylformamide solu-
tion, Pt wire as a counter electrode and an Ag/AgCl
reference electrode and calibrated with ferrocene.
Carbazole and its derivatives have been used as electro-
luminescent, non-linear optical (NLO) and photorefractive
materials due to their strong emission and adsorption prop-
erties, good hole-transporting capability, and wide band
gap [17,18]. Several studies have reported about carbazole
type sensitizing dyes with optimal power conversion effi-
ciency of 9.20% [19]. In addition, carbazole has been incor-
porated as an auxiliary donor into triphenylamine- or
coumarin-based DSSCs and achieved moderate to high effi-
ciencies of up to ꢀ5.53% [20]. However, investigations on
carbazole type sensitizing dyes with double acceptor have
rarely been reported. Herein, to obtain sensitizing dyes
with improved spectral and photovoltaic performance, we
introduced auxiliary acceptor benzothiadiazole into carba-
zole molecule and synthesized four carbazole sensitizing
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dyes with thiophene or benzene as
p bridge and cyanoace-
tic acid or rhodanine acetic acid as acceptor. With this
molecular design, a power conversion efficiency of 5.40%
was achieved with ZXY-3 sensitized solar cell.
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2.4. Syntheses of dyes
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2. Experimental sections
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2.4.1. Synthesis of compound I
2.1. Materials and characterization
3-Bromo-9-butyl-9H-carbazole (4.80 g, 15.9 mmol) was
dissolved in dry THF (80 mL) and cooled down to ꢁ78 °C.
The solution of n-BuLi in n-hexane (7.5 mL, 2.5 M) was
added under N2 and the mixture was stirred for 1 h. After
the addition of B(OCH3)3 (5.6 mL, 23.85 mmol), the reac-
tant was warmed to room temperature and stirred for
24 h. The clear yellow solution obtained was used directly
in the next Suzuki coupling without further treatment.
In a 250 mL 3-necked flask, a solution was prepared by
dissolving the 4,7-dibromo-benzothiadiazole (4.80 g,
16.4 mmol), Pd(PPh3)4 (458.8 mg, 0.4 mmol) and K2CO3
aqueous solution (12.3 mL, 2 M) in THF (120 mL). The
above clear yellow solution was added and the mixture
was refluxed 24 h under N2, after which it was treated with
water (200 mL) and extracted with EtOAc (50 mL ꢂ 3). The
combined organic layer was dried with anhydrous MgSO4
and evaporated to dryness. The residue was purified by col-
umn chromatography (PE:EA = 50:1) to obtain yellow solid
I (4.61 g, 66.7%). m.p. 98–101 °C; 1H NMR (500 MHz, CDCl3)
d: 8.62(s, 1H, ArH), 8.18(d, J = 7.7 Hz, 1H, ArH), 8.01(dd,
J = 1.7, 8.5 Hz, 1H, ArH), 7.88(d, J = 7.6 Hz, 1H, ArH),
7.60(d, J = 7.6 Hz, 1H, ArH), 7.54–7.51(m, 2H, ArH), 7.46(d,
J = 8.2 Hz, 1H, ArH), 7.28(t, J = 7.0 Hz, 1H, ArH), 4.35(t,
J = 7.2 Hz, 2H, CH2CH2CH2CH3), 1.94–1.88(m, 2H, CH2CH2-
CH2CH3), 1.49–1.41(m, 2H, CH2CH2CH2CH3), 0.99(t,
J = 7.4 Hz, 3H, CH2CH2CH2CH3); 13C NMR (125 MHz, CDCl3)
d: 153.77, 153.35, 140.80, 140.44, 134.72, 132.23, 127.53,
127.18, 126.80, 125.69, 123.09, 122.80, 121.16, 120.44,
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Nuclear magnetic resonance spectra were recorded on
Bruker Avance III 500 MHz and chemical shifts are
expressed in ppm using TMS as an internal standard. Mass
spectra were measured using 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.
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2.2. Fabrication of DSSCs
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The FTO glass was first cleaned in a detergent solution
using an ultrasonic bath for 15 min, and then rinsed with
water and ethanol. For the fabrication of photoelectrodes,
two different titanium nanoxide pastes were deposited by
screen-printing, resulting in the TiO2 electrodes (0.16 cm2,
with light scattering anatase particles). The TiO2 electrodes
were gradually heated under an air flow at 325 °C for 5 min,
at 375 °C for 5 min, and at 450 °C for 15 min, and finally, at
500 °C for 15 min. After being cooled to 80 °C, the TiO2 elec-
trode was immersed into the dye solution in a mixture of
MeOH and CHCl3 (VMeOH : VCHCl ¼ 1 : 10) and kept at room
3
temperature for 24 h to assure complete dye uptake, which
were then rinsed with MeOH to remove excess dye. Mean-
while, the counter electrodes were prepared by screen-
printing a 50 nm Pt layer on the cleaned FTO plates. Open
Please cite this article in press as: L. Han et al., Novel D–A–
p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

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119.07, 111.58, 108.58, 108.64, 42.85, 31.03, 20.46, 13.80;
HREIMS m/z 437.1884 [M + H]+, cacld C22H18BrN3S for:
436.3675.
solution was filtrated. The residue was washed with CH3CN
and recrystallized with the cosolvent of CH2Cl2 and MeOH
(VCH
: VMeOH ¼ 10 : 1) to obtain dark red solid (100 mg,
2Cl2
72%). m.p.: 268–269 °C; 1H NMR (500 MHz, DMSO-d6) d:
8.78(s, 1H, ArH), 8.50(s, 1H, CH@CCN), 8.27–8.19(m, 3H,
ArH), 8.08–8.04(m, 2H, ArH), 7.94(d, J = 7.5 Hz, 1H, ArH),
7.67(d, J = 8.6 Hz, 1H, ArH), 7.62(d, J = 8.2 Hz, 1H, ArH),
7.49(t, J = 7.4 Hz, 1H, ArH), 7.24(t, J = 7.4 Hz, 1H, ArH),
4.41(t, J = 6.7 Hz, 2H, CH2CH2CH2H3), 1.80–1.74(m, 2H, CH2-
CH2CH2CH3), 1.34–1.27(m, 2H, CH2CH2CH2CH3), 0.88(t,
J = 7.3 Hz, 3H, CH2CH2CH2CH3); 13C NMR (125 MHz,
DMSO-d6) d: 171.74, 163.45, 153.15, 151.79, 147.56,
146.11, 140.42, 140.11, 139.38, 136.23, 134.68, 127.76,
127.49, 127.23, 127.00, 125.90, 122.69, 122.24, 122.15,
121.07, 120.32, 118.98, 116.39, 109.40, 109.14, 98.75,
42.08, 30.57, 20.87, 13.54; HREIMS m/z 533.1113 [M–H]ꢁ,
cacld for C30H22N4O2S2: 534.6514.
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2.4.2. Synthesis of IIa
Compound I (2.50 g, 5.75 mmol) and (5-formylthio-
phen-2-yl)boronic acid (1.08 g, 6.90 mmol) was dissolved
in dry THF (60 mL) and K2CO3 aqueous solution (3 mL,
2 M) and the catalyst Pd(PPh3)4 (0.66 mg, 0.575 mmol)
were added under N2. The solution was refluxed for 24 h,
after which it was treated with water (200 mL) and
extracted with EtOAc (30 mL ꢂ 3). The combined organic
layer was dried with anhydrous MgSO4 and evaporated to
dryness. The residue was purified by column chromatogra-
phy (PE:EA = 10:1) to obtain yellow solid IIa (0.46 g, 17.2%).
m.p. 168–169 °C; 1H NMR (500 MHz, CDCl3) d: 10.0(s, 1H,
CHO), 8.73(s, 1H, ArH), 8.24(d, J = 4.0 Hz, 1H, ArH), 8.20(d,
J = 7.7 Hz, 1H, ArH), 8.13(dd, J = 1.7, 8.5 Hz, 1H, ArH),
8.10(d, J = 7.5 Hz, 1H, ArH), 7.88(d, J = 2.3 Hz, 1H, ArH),
7.87(d, J = 0.78 Hz, 1H, ArH), 7.58(d, J = 8.5 Hz, 1H, ArH),
7.53(t, J = 7.1 Hz, 1H, ArH), 7.47(d, J = 8.1 Hz, 1H, ArH),
7.30(d, J = 7.5 Hz, 1H, ArH), 4.39(t, J = 7.1 Hz, 2H, CH2CH2-
CH2CH3), 1.96–1.90(m, 2H, CH2CH2CH2CH3), 1.50–1.42(m,
2H, CH2CH2CH2CH3), 0.99(t, J = 7.3 Hz, 3H, CH2CH2CH2-
CH3); 13C NMR (125 MHz, CDCl3) d: 182.95, 154.13,
152.66, 149.06, 143.13, 140.95, 140.70, 136.76, 135.97,
127.69, 127.61, 127.52, 127.07, 127.05, 126.04, 123.72,
123.27, 122.98, 121.45, 120.59, 119.25, 108.99, 108.81,
42.99, 31.15, 20.58, 13.90.
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2.4.5. Synthesis of ZXY-2
Compound IIa (0.12 g, 0.26 mmol) and rhodanine acetic
acid (0.080 g, 0.42 mmol) were dissolved in the cosolvent
CHCl3 (2 mL) and CH3CN (4 mL). After 5 drop of piperidine
was added, the reactant solution was refluxed for 4 h under
N2 atmosphere. After cooling to room temperature, the
solution was filtrated. The residue was washed with CH3CN
and recrystallized with the cosolvent of CH2Cl2 and MeOH
(VCH
: VMeOH ¼ 10 : 1) to obtain dark red solid (102 mg,
2Cl2
61.3%). m.p.: 277–279 °C; 1H NMR (500 MHz, DMSO-d6) d:
8.86(s, 1H, ArH), 8.39(d, J = 7.2 Hz, 1H, ArH), 8.32(s, 1H,
ArH), 8.25–8.16(m, 3H, ArH), 8.03(d, J = 6.0 Hz, 1H, ArH),
7.92(s, 1H, CH@C), 7.76(d, J = 8.1 Hz, 1H, ArH), 7.66(d,
J = 8.2 Hz, 1H, ArH), 7.50(t, J = 7.8 Hz, 1H, ArH), 7.25(t,
J = 7.3 Hz, 1H, ArH), 4.74(s, 2H, CH2COOH), 4.46(t,
J = 6.1 Hz, 2H, CH2CH2CH2H3), 1.84–1.78(m, 2H, CH2CH2-
CH2CH3), 1.38–1.31(m, 2H, CH2CH2CH2CH3), 0.90(t,
J = 7.4 Hz, 3H, CH2CH2CH2CH3); 13C NMR (125 MHz, pyri-
dine-d6) d: 193.65, 169.70, 167.85, 155.14, 154.67, 148.61,
147.03, 142.18, 141.18, 140.35, 139.65, 133.31, 130.27,
129.50, 128.98, 128.52, 128.36, 128.12, 127.75, 122.84,
122.28, 121.70, 121.56, 120.47, 115.30, 110.46, 110.23,
46.97, 43.70, 32.07, 21.31, 14.59; HREIMS m/z 639.0637
[M–H]ꢁ, cacld C32H24N4O3S4 for: 640.8181.
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2.4.3. Synthesis of IIb
Compound I (1.20 g, 2.76 mmol) and 4-formylphenyl
boric acid (0.50 g, 3.31 mmol) was dissolved in dry THF
(40 mL) and K2CO3 aqueous solution (2 mL, 2 M) and the
catalyst Pd(PPh3)4 (0.32 g, 0.276 mmol) were added under
N2. The solution was refluxed for 24 h, after which it was
treated with water (50 mL) and extracted with EtOAc
(20 mL ꢂ 3). The combined organic layer was dried with
anhydrous MgSO4 and evaporated to dryness. The residue
was purified by column chromatography (PE:EA = 10:1) to
obtain yellow solid IIb (0.22 g, 12.6%). m.p. 185–187 °C;
1H NMR (500 MHz, CDCl3) d: 10.14(s, 1H, CHO), 8.73(s,
1H, ArH), 8.21(d, J = 8.1 Hz, 3H, ArH), 8.15(dd, J = 1.7,
8.5 Hz, 1H, ArH), 8.08(d, J = 8.2 Hz, 2H, ArH), 7.93(dd,
J = 3.1, 7.3 Hz, 2H, ArH), 7.60(d, J = 8.6 Hz, 1H, ArH), 7.53(t,
J = 7.1 Hz, 1H, ArH), 7.48(d, J = 8.1 Hz, 1H, ArH), 7.30(t,
J = 7.7 Hz, 1H, ArH), 4.39(t, J = 7.2 Hz, 2H, CH2CH2CH2CH3),
1.97–1.91(m, 2H, CH2CH2CH2CH3), 1.51–1.43(m, 2H, CH2-
CH2CH2CH3), 1.0(t, J = 7.3 Hz, 3H, CH2CH2CH2CH3); 13C
NMR (125 MHz, CDCl3) d: 191.85, 154.44, 153.69, 143.51,
140.98, 140.65, 135.64, 135.55, 130.48, 130.30, 129.69,
129.72, 129.08, 127.97, 127.84, 127.37, 127.11, 126.01,
123.28, 123.03, 121.43, 120.60, 119.20, 108.99, 108.81,
43.01, 31.18, 20.99, 13.91.
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2.4.6. Synthesis of ZXY-3
Compound IIb (0.10 g, 0.22 mmol) and cyanoacetic acid
(0.037 g, 0.44 mmol) were dissolved in the cosolvent CHCl3
(2 mL) and CH3CN (4 mL). After 5 drop of piperidine was
added, the reactant solution was refluxed for 4 h under
N2 atmosphere. After cooling to room temperature, the
solution was filtrated. The residue was washed with CH3CN
and recrystallized with the cosolvent of CH2Cl2 and MeOH
(VCH
: VMeOH ¼ 10 : 1) to obtain dark red solid (97.6 mg,
2Cl2
84%). m.p.: 258–262 °C; 1H NMR (500 MHz, DMSO-d6) d:
8.82(s, 1H, ArH), 8.42(s, 1H, CH@CCN), 8.27–8.20(m, 5H,
ArH), 8.14(d, J = 8.5 Hz, 1H, ArH), 8.09(d, J = 7.4 Hz, 1H,
ArH), 8.04(d, J = 7.4 Hz, 1H, ArH), 7.75(d, J = 8.6 Hz, 1H,
ArH), 7.65(d, J = 8.2 Hz, 1H, ArH), 7.49(t, J = 7.7 Hz, 1H,
ArH), 7.24(t, J = 7.4 Hz, 1H, ArH), 4.46 (t, J = 6.9 Hz, 2H, CH2-
CH2CH2H3), 1.83–1.77(m, 2H, CH2CH2CH2CH3), 1.37–
1.30(m, 2H, CH2CH2CH2CH3), 0.89(t, J = 7.4 Hz, 3H, CH2CH2-
CH2CH3); 13C NMR (125 MHz, DMSO-d6) d: 163.31, 153.58,
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2.4.4. Synthesis of ZXY-1
Compound IIa (0.12 g, 0.26 mmol) and cyanoacetic acid
(0.044 g, 0.51 mmol) were dissolved in the cosolvent CHCl3
(2 mL) and CH3CN (4 mL). After 5 drop of piperidine was
added, the reactant solution was refluxed for 4 h under N2
atmosphere. After cooling to room temperature, the
Please cite this article in press as: L. Han et al., Novel D–A–
p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

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404
405
406
407
408
409
410
411
412
413
414
153.17, 141.14, 140.48, 140.07, 130.77, 129.64, 129.48,
129.15, 127.13, 125.98, 122.28, 122.22, 121.12, 120.45,
119.03, 116.24, 109.60, 109.21, 103.60, 42.15,
30.71,19.80(CH2CH3), 13.70; HREIMS m/z 527.1539 [M–
H]ꢁ; calcd for C32H24N4O2S: 528.6237.
electron density and its strong electron-donating ability
generally leads to smaller energy gap and broader absorp-
tion spectrum. Fig. 1 indicates that with thiophene as
bridge, dyes ZXY-1 and ZXY-2 have larger kmax and onset
than dyes ZXY-3 and ZXY-4 with benzene -bridge, respec-
p-
p
tively. The replacement of benzene with thiophene leads to
about 50 nm red shift of the kmax, and dyes ZXY-1 and ZXY-
2 display the maximum absorption peak at 500 nm. More-
over, dyes ZXY-1 and ZXY-2 have broader absorption spec-
tra than dyes ZXY-3 and ZXY-4, and their onset also show
about 50 nm red shift as in the case of the kmax and fall at
600–650 nm. Generally, a longer conjugation system leads
to a larger maximum absorption wavelength. Therefore,
dye ZXY-2 with rhodanine acetic acid acceptor has the lar-
ger kmax than dye ZXY-1 because of the long conjugation
molecule formed by the rhodanine acetic acid. However
in the case of dyes ZXY-3 and ZXY-4, the presence of rhoda-
nine acetic acid acceptor has little effect on the kmax of dye
ZXY-4. Though with larger conjugation system, dye ZXY-4
shows a kmax similar to that of dye ZXY-3.
After dyes are absorbed on TiO2 film, deprotonation and
aggregation of dye molecules occur, which often affect UV–
vis absorption. Generally, deprotonation and H-aggregate
lead to absorption peaks hypsochromically, while J-aggre-
gate results in absorption peaks bathochromically. Carba-
zole dyes show 20–40 nm blue shift on TiO2 film when
compared with in solution, indicating the occurrence of
deprotonation and H-aggregate. Notably, the kmax of dyes
ZXY-1 and ZXY-2 shift 20–30 nm hypsochromically, while
those of dyes ZXY-3 and ZXY-4 move 40–50 nm to the
shorter wavelength. This result can be ascribed to different
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
2.4.7. Synthesis of ZXY-4
Compound IIa (0.13 g, 0.26 mmol) and rhodanine acetic
acid (0.067 g, 0.35 mmol) were dissolved in the cosolvent
CHCl3 (2 mL) and CH3CN (4 mL). After 5 drop of piperidine
was added, the reactant solution was refluxed for 4 h under
N2 atmosphere. After cooling to room temperature, the
solution was filtrated. The residue was washed with CH3CN
and recrystallized with the cosolvent of CH2Cl2 and MeOH
(VCH
: VMeOH ¼ 10 : 1) to obtain dark red solid (90.6 mg,
2Cl2
65%). m.p. 271–274 °C; 1H NMR (500 MHz, DMSO-d6) d:
8.83(s, 1H, ArH), 8.28–8.24(m, 3H, ArH), 8.17(dd, J = 1.7,
8.6 Hz, 1H, ArH), 8.11(d, J = 7.4 Hz, 1H, ArH), 8.07(d,
J = 7.4 Hz, 1H, ArH), 7.98(s, 1H, CH@C), 7.86(d, J = 8.5 Hz,
2H, ArH), 7.77(d, J = 8.7 Hz, 1H, ArH), 7.66(d, J = 8.3 Hz,
1H, ArH), 7.50(t, J = 7.3 Hz, 1H, ArH), 7.25(t, J = 7.5 Hz, 1H,
ArH), 4.77(s, 2H, CH2COOH), 4.47(t, J = 7.0 Hz, 2H, CH2CH2-
CH2H3), 1.85–1.79(m, 2H, CH2CH2CH2CH3), 1.39–1.31(m,
2H, CH2CH2CH2CH3), 0.92(t, J = 7.3 Hz, 3H, CH2CH2CH2-
CH3); 13C NMR (125 MHz, DMSO-d6) d: 192.97, 166.98,
166.26, 153.64, 153.20, 140.45, 140.03, 134.14, 133.12,
132.27, 130.80, 129.74, 129.40, 128.95, 127.53, 127.33,
127.02, 125.89, 122.16, 121.96, 120.99, 120.31, 118.93,
109.40, 109.18, 45.12, 42.10, 30.59, 19.66, 13.55; HREIMS
m/z 633.1098 [M–H]ꢁ; calcd for C34H26N4O3S3: 634.7904.
p-stacks caused by benzene and thiophene. On the other
339
340
3. Results and discussion
hand, longer onset and broader spectra can be observed
on TiO2 film than in solution, which favors light harvest.
However, dye ZXY-2 is an exception. Although dye ZXY-2
has a longer onset on TiO2 film, its spectrum is apparently
narrowed when compared with in solution.
3.1. Synthesis
341
342
343
344
345
346
347
348
349
350
351
The synthetic procedure of new carbazole dyes is
shown in Scheme 1. 3-Bromo-N-butyl-carbazole was con-
verted into the corresponding boronic ester by lithiation
and subsequent reaction with methyl borate. Through
Suzuki coupling, the acceptor 4,7-dibromobenzothiadizole
was introduced into N-butyl-carbazole donor to yield com-
pound I. The second Suzuki coupling between I and form-
yarylboronic acids afforded the intermediate aldehydes II.
New carbazole dyes were prepared through Knoevenagel
condensation of II with cyanoacetic acid or rhodanine ace-
tic acid in the presence of piperidine.
Cyclic voltammograms were performed to evaluate the
feasibility of the electron injection from the excited states
of the dye to the TiO2 conduction band and the possibility
of dye regeneration by means of the electrolyte redox cou-
ple and the related data were shown in Table 2. As we can
see, the lowest unoccupied molecular orbital (LUMO)
energy level of four carbazole dyes are higher than the
TiO2 conduction band energy level (ꢁ4.0 eV), which guar-
antees the feasible electron injection to TiO2 CB from the
excited dyes. In addition, the oxidized carbazole dyes can
be efficiently regenerated since the highest occupied
molecular orbital (HOMO) energy level of these dyes are
lower than the energy level of the redox couple Iꢁ=I3ꢁ
(ꢁ4.9 eV).
352
3.2. Optical characterization
353
354
355
356
357
358
359
360
361
362
363
The UV–vis spectra of carbazole dyes recorded in CH3-
OHACHCl3 cosolvent and on TiO2 film are shown in Fig. 1
and summarized in Table 1. The absorption spectra of car-
bazole dyes display two distinct absorption peaks. One
absorption peak occurs at the shorter wavelength region
415
3.3. Theoretical investigations
416
417
418
419
420
421
422
To obtain insight into the relationship between the
structure and the photophysical properties, quantum
chemical analysis was performed on carbazole dyes using
the B3LYP hybrid density functional method in conjunction
with the d-polarized 6-31G(d) basis set implemented in
the Gaussian 03 program. The fully optimized geometrical
structures of four dyes are shown in Fig. 2.
corresponding to p–
pꢃ electron transition. Another absorp-
tion peak at the longer wavelength region can be ascribed
to the intramolecular charge transfer (ICT) transition from
the N-butyl-carbazole donor to the carboxyl-containing
acceptor mixed with delocalized
Compared with benzene, thiophene possesses higher
p–
pꢃ transition.
Please cite this article in press as: L. Han et al., Novel D–A–
p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

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Scheme 1. Syntheses of carbazole dyes.
Fig. 1. UV spectra of carbazole dyes (a) in CH3OHACHCl3 (v:v = 1:10) and (b) on TiO2 film.
Table 2
Electrochemical properties of carbazole dyes.
Table 1
Experimentala (eV)
Calculatedd (eV)
Optical properties of carbazole dyes.
Comp.
ea (Mꢁ1 cmꢁ1
)
kmax (nm)
kem (nm)
HOMOa
E0–0
LUMOc
HOMO
E0–0
LUMO
a
b
c
b
Comp.
kmax (nm)
ZXY-1
ZXY-2
ZXY-3
ZXY-4
486
507
441
446
30,633
45,367
21,667
34,667
463
480
391
397
593
596
568
572
ZXY-1
ZXY-2
ZXY-3
ZXY-4
–5.547
–5.427
–5.527
–5.487
2.28
2.19
2.47
2.46
–3.27
–3.23
–3.06
–3.03
–5.35
–5.22
–5.42
–5.36
2.44
2.30
2.64
2.53
–2.92
–2.92
–2.78
–2.82
a
a
Maximum absorption wavelength kmax and molar extinction coeffi-
cient at kmax of dyes measured in CH3OHACHCl3 cosolvent ( :v = 1:10).
Maximum absorption wavelength kmax of dyes on sensitized TiO2
electrodes.
The oxidation potential Eox in DMF was determined from cyclic vol-
v
tammograms and used to describe the ground-state energy HOMO.
b
b
E0–0 was calculated from E0–0 = 1240/kint and kint was the intersection
of the normalized absorption and emission spectra.
c
c
Maximum emission wavelength measured in CH3OHACHCl3 cosol-
vent ( :v = 1:10).
ELUMO was calculated from Eox–E0–0.
Calculated at the B3LYP/6-31G(d) level in vaccum.
d
v
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p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

ORGELE 2542
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L. Han et al. / Organic Electronics xxx (2014) xxx–xxx
-1
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
Large deviation from planarity between the carbazole
donor and benzothiadiazole was observed in the four car-
bazole dyes. Different with ZXY-1 and ZXY-2, large dihe-
drals between the benzothiadiazole and benzene bridge
in ZXY-3 and ZXY-4 indicates the inferior planarity of the
whole molecule. Therefore, ZXY-3 and ZXY-4 has shorter
kmax than ZXY-1 and ZXY-2. And both the dihedrals
between the benzothiadiazole and the benzene bridge or
carbazole donor in ZXY-4 are large than those in ZXY-3.
Therefore, the kmax of ZXY-4 is increased by only 5 nm
compared with that of ZXY-3, although ZXY-4 with larger
conjugation system.
The HOMO energy level shown in Fig. 3 is located at
ꢁ5.41 to ꢁ5.22 eV, which is lower than the potential of
the redox couple Iꢁ=I3ꢁ (ꢁ4.9 eV). The LUMO energy level
is concentrated on ꢁ2.92 to ꢁ2.77 eV and higher than the
TiO2 CB energy level (ꢁ4.0 eV). This means that these dyes
can favorably inject the electrons into the CB edge of TiO2,
and the oxidized dyes can be reduced by the electrolyte
successfully. Compared with cyanoacetic acid acceptor,
rhodanine acetic acid acceptor offers a larger conjugation
system, in accordance with the smaller energy gap
between the HOMO and the LUMO (Egap) of dyes ZXY-2
and ZXY-4 compared with that of dyes ZXY-1 and ZXY-3,
respectively. The energy level of Fig. 3 indicates that the
replacement of benzene with thiophene can not only
improve the HOMO energy level but also lower the LUMO
energy level, which may be caused by the higher electron
density of thiophene. Therefore, dyes ZXY-1 and ZXY-2
-2
-3
-4
-5
-6
2.44 eV
2.53 eV
2.64 eV
2.30 eV
ZXY-2
ZXY-4
ZXY-1
ZXY-3
Fig. 3. The energy of the frontier molecular orbitals of carbazole dyes at
PCM-B3LYP/6-31G(d)//B3LYP/6-31G(d) in vacuo.
465
3.4. Photovoltaic performances
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
The IPCE action spectra of DSSCs based on carbazole
dyes were measured under AM1.5 irradiation (100 mW/
cm2) and are shown in Fig. 5. With cyanoacetic acid accep-
tor, dyes ZXY-1 and ZXY-3 have IPCE value superior to
those of dyes ZXY-3 and ZXY-4 with rhodanine acetic acid
acceptor and have maximum IPCE value exceeding 65%.
With benzene bridge, dyes ZXY-3 and ZXY-4 display
higher IPCE values than dyes ZXY-1 and ZXY-2 with thio-
phene bridge, respectively. However, the latter possess
the broader spectra than the former, which confirms that
the electron rich thiophene is beneficial to the broadening
of the spectrum [21]. Among the four carbazole dyes, dye
ZXY-2 shows the weakest monochromatic incident
photo-to-current conversion, although it possesses the lon-
gest kmax and the broadest absorption spectrum. We spec-
ulate that it may be caused by the narrowed absorption
spectrum on TiO2 film.
have lower Egap, and consequently larger kmax
.
The LUMOs in all the four dyes are same to each other
and localized over the whole molecules as shown in
Fig. 4. However, different with ZXY-1 and ZXY-2, benzene
bridges have small contribution to the HOMO in ZXY-3 and
ZXY-4. Therefore, ZXY-1 and ZXY-2 possess larger
maximum UV absorption than ZXY-3 and ZXY-4, respec-
tively, in accordance with the geometrical and experimen-
tal result. Due to the presence of methylene group in
rhodanine acetic acid acceptor, the anchoring carboxyl
groups has little contribution to the LUMOs in ZXY-2 and
ZXY-4, which weakens the electron injection from the dyes
into the CB edge of TiO2.
The J–V curves of DSSCs based on carbazole dyes are
shown in Fig. 6. Photovoltaic parameters such as JSC, VOC
ff, and are collected in Table 3. As expected from the
result of IPCE, JSC of dyes ZXY-1 and ZXY-3 exhibit a great
improvement compared with those of ZXY-2 and ZXY-4,
respectively. This difference between dyes with
,
g
Fig. 2. Optimized geometries (side view) of carbazole dyes at the B3LYP/6-31G(d) level.
Please cite this article in press as: L. Han et al., Novel D–A–
p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

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Fig. 4. Frontier molecular orbital (HOMO and LUMO) of carbazole dyes.
15
0.7
0.6
0.5
0.4
0.3
0.2
0.1
ZXY-1
ZXY-2
ZXY-3
ZXY-4
ZXY-1
ZXY-2
ZXY-3
ZXY-4
10
5
0.0
0
300
400
500
600
700
800
0.0
0.2
0.4
0.6
0.8
Volatge / V
λ/nm
Fig. 6. Current–potential curve (J–V) for DSSCs based on carbazole dyes.
Fig. 5. IPCE curve for DSSCs based on carbazole dyes.
Table 3
Parameters for DSSCs based on carbazole dyes.
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
cyanoacetic acid acceptor and those with rhodanine acetic
acid acceptor may be attributed to low photo-excited elec-
tron injection yield of the latter [22]. At the same time, rel-
atively low VOC is observed in dyes ZXY-2 and ZXY-4 for
the inferior overlap between the electron density of the
dyes and the semiconductor. This result is ascribed to the
presence of the nonconjugated methylene group which
isolates the LUMO electron from the carboxyl anchoring
group, as shown in Fig. 3. On the other hand, benzene
bridge favors the improvement of VOC, and dyes ZXY-3
and ZXY-4 obtain better VOC than dyes ZXY-1 and ZXY-4
respectively, which is consistent with the conclusion in
Ref. [23]. Therefore, with better JSC and VOC, dyes ZXY-1
and ZXY-3 have the higher photoelectricity conversion
efficiency. The best performance comes from DSSC based
Comp.
VOC (mV)
JSC (mA cmꢁ2
)
FF
g (%)
ZXY-1
ZXY-2
ZXY-3
ZXY-4
0.61
0.56
0.71
0.63
12.08
4.12
10.99
6.98
0.69
0.71
0.69
0.87
5.07
1.65
5.40
3.81
on ZXY-3, which exhibits JSC value of 10.99 mA/cm2, VOC
value of 0.71 V, and ff value of 0.69, resulting in an effi-
ciency of 5.40%.
504
505
506
507
3.5. Electrochemical properties
508
509
Electrochemical impedence spectroscopy can be used to
investigate charge transport phenomena and evaluate the
Please cite this article in press as: L. Han et al., Novel D–A–
p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016

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545
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547
the hardest charge recombination but smallest Jsc. There-
fore, we speculate that among these carbazole dyes, elec-
Rce
Rct
Rs
ZXY-1
ZXY-2
ZXY-3
ZXY-4
-60
-50
-40
-30
-20
-10
0
tron
injection
efficiency
rather
than
.
charge
CPE2
CPE1
recombination exert the main effect on VOC
548
4. Conclusion
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
Four novel D–A–
were synthesized with N-butylcarbazole as donor, thio-
phene or benzene as bridge, cyanoacetic acid or rhoda-
p–A carbazole type sensitizing dyes
p
nine acetic acid as acceptor and benzothiadiazole as
auxiliary acceptor. Their spectral, photovoltaic and electro-
chemical properties were investigated systematically.
Compared with benzene bridge, thiophene bridge provides
the sensitizers superior spectral properties and dyes ZXY-1
and ZXY-2 possesses long absorption maximum and
broader light harvesting region. However, the presence of
benzene bridge favors the improvement of VOC which play
the main role in the photovoltaic performance. Among
these carbazole sensitizing dyes, dye ZXY-3 with benzene
bridge and cyanoacetic acid exhibits the highest photoelec-
tricity conversion efficiency 5.40%.
0
20
40
60
80
100 120 140 160 180
Z' /Ω
Fig. 7. Nyquist plots of DSSCs based on carbazole dyes.
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
electron transfer and charge recombination at photoanode/
electrolyte interface. The electrochemical impedance of
DSSCs based on carbazole dyes was evaluated in the dark.
In the dark under forward bias, electrons are transported
through the mesoscopic TiO2 network and react with Iꢁ3 .
Simultaneously, Iꢁ is oxidized to Iꢁ3 at the counter electrode
(CE) [24]. Fig. 7 shows the Nyquist spectrum for DSSCs
based on carbazole dyes. The reaction resistance of DSSCs
was analyzed by software (ZSimpWin) using an equivalent
circuit (shown in the inset of Fig. 7) containing a constant
phase element (CPE) and resistance (R). In the equivalent
circuit, RS corresponds to the overall series resistance,
whereas Rct and Rce represent charge-transfer resistances
at the photoanode/electrolyte interface and CE, respec-
tively. These DSSCs show similar series resistance (about
564
Acknowledgments
565
Q45 566
567
The authors gratefully acknowledge for funding:
National Natural Science Foundation of China (21176223);
the Key Innovation Team of Science and Technology in Zhe-
jiang Province (No. 2010R50018-06) and Natural Science
Foundation of Zhejiang Province (LY13B020012).
568
569
570
References
7.2–11.1
X) due to the same electrolyte and electrode in
both materials and surface area. The largest semicircle at
1–1 kHz represents the charge-transfer resistances (Rct) at
the photoanode/electrolyte interface and larger Rct means
more difficult electron recombination from the conduction
band to the electrolyte. As illustrated in Fig. 7 and Table 4,
the semicircles of dyes ZXY-1 and ZXY-2 at 1–1 kHz pos-
sess the larger diameter than those of dyes ZXY-3 and
ZXY-4, indicating their longer charge recombination life-
time with thiophene as bridge. In our study, the influence
of acceptor on the charge recombination lifetime is chan-
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582
583
584
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p bridge. With thiophene bridge dye ZXY-1 bear-
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ing cyanoacetic acid acceptor is more prone to charge
recombination than dye ZXY-2 bearing rhodanie acetic
acid, while in the case of dyes ZXY-3 and ZXY-4 the situa-
tion is reversed. Though ZXY-3 has medium charge recom-
bination, it attains the best VOC due to with good electron
injection efficiency; while the weakest photoelectricity
conversion efficiency is observed in ZXY-2 which possesses
Table 4
Parameters obtained by fitting the impedance spectra of DSSCs based on
carbazole dyes using the equivalent circuit.
Comp.
Rs
(X
)
Rct (X)
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ZXY-1
ZXY-2
ZXY-3
ZXY-4
7.20
8.17
11.14
7.34
67.7
112.8
54.7
38.9
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p–A carbazole dyes containing benzothiadiazole chromophores for dye-sensi-
tized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.016