Novel BODIPY dyes with electron donor variety for dye-sensitized solar cells

Article abstract of DOI:10.1039/c7ra04402j

Four new donor-π-acceptor (D-π-A) structured organic sensitizers (CB1-4), with N-octyl or N-methoxyphenyl carbazole connected to a 2-cyanoacrylate BODIPY module through their 2 or 3-positions, were designed and synthesized for application in dye-sensitize

Full text of DOI:10.1039/c7ra04402j

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Novel BODIPY dyes with electron donor variety for
dye-sensitized solar cells†
Cite this: RSC Adv., 2017, 7, 33975
a
a
Junxu Liao,ab Hongbin Zhao, Yongjun Xu,
Weinan Zhou,a Fei Peng,a
*
*
Yong Wanga and Yutang Fangb
Four new donor–p–acceptor (D–p–A) structured organic sensitizers (CB1-4), with N-octyl or N-
methoxyphenyl carbazole connected to a 2-cyanoacrylate BODIPY module through their 2 or 3-
positions, were designed and synthesized for application in dye-sensitized solar cells (DSSCs). The
photophysical and electrochemical properties were systematically studied and their performances as
sensitizers in DSSCs were comparatively investigated. The structure–property relationship suggests that
the electron-donating ability and substitution positions of the donor have a significant effect on these
dyes' electronic and photovoltaic properties. Electron-rich N-methoxyphenyl carbazole as a donor and
substitution at the 2-position shows a positive influence on the efficiency of DSSCs. Consequently, CB4,
with the combination of these advantages, presents the broadest (300–650 nm) and strongest
absorption profile with a high extinction coefficient (1.71 ꢀ 105 Mꢁ1 cmꢁ1) and the best photovoltaic
performance with a short circuit photocurrent density of 10.20 mA cmꢁ2, an open circuit voltage of
600 mV, and a fill factor of 0.7, corresponding to a fairly good overall conversion efficiency of 4.28%
under AM 1.5 irradiation (100 mW cmꢁ2).
Received 19th April 2017
Accepted 16th June 2017
DOI: 10.1039/c7ra04402j
rsc.li/rsc-advances
Boron dipyrromethene (BODIPY) have attracted numerous
explorations in many potential applications owing to their
1. Introduction
intensive absorption/emission in the visible and near-IR ranges,
high uorescence quantum yield, long excited-state lifetime,
excellent thermal/photochemical stability and good solu-
bility.25–27 Possessing all these characteristics, BODIPY dyes have
emerged as promising photo sensitizers in DSSCs.28–31 However,
the power efficiency of BODIPY-based DSSCs (6.06%)31 has lagged
behind porphyrin-based counterparts (13%),7 progress is being
made in order to improve their efficiencies. Recent studies have
focused on the development of BODIPY dyes with a donor–
chromophore–acceptor (D–BODIPY–A) molecular architecture to
enhance the light-harvesting ability.32,33 In this system, an
appropriate donor and suitable connect manner are the key
factors for achieving high power efficiency. Mao and coworkers
demonstrated that the D–p–A type BODIPY dyes modied with
2,6-donor/acceptor groups are promising photosensitizers in the
DSSCs, with relatively high efficiency of nearly 5.31%.33 From the
donor attention, carbazole derivative is good compound with
excellent photoelectric properties. It has strong absorption and
emission properties of spectrum, high hole transport capacity
and wide band gap, have a wide range of applications in the eld
of photoelectric materials.34,35 In view of the photoelectric prop-
erties of this kind of materials to highlight, the combination of
BODIPY and carbazole and used in dye-sensitised cells caused
our research interest.
The solar cell has been considered as a green and effective
technology to address increasingly serious energy and envi-
ronment issues. DSSCs have become one of the most promising
low cost alternatives to conventional inorganic silicon-based
solar cells.1–4 DSSCs based on Ru(II) complexes have been
widely studied, with the most promising ones, such as N719 and
black dye, achieving remarkable light-to-power conversion effi-
ciencies above 10%.5–7 However, because Ru is expensive and
rare, the use of metal-free organic dyes offers greater practical
use. Furthermore, metal-free organic dyes, feature high extinc-
tion coefficients, easier structural modication, simple
synthesis process, and environmentally friendliness in
comparison to Ru(II) complexes.8 In recent years, various
organic dyes based on D–p–A molecular architecture, including
porphyrin,5–7,9–12 triarylamine,13–16 thiophene,17,18 coumarins,19,20
indoline21,22 and phenothiazine23,24 have been widely investi-
gated as sensitizers for DSSCs.
aSchool of Chemical Engineering and Energy Technology, Dongguan University of
Technology, Dongguan 523808, People's Republic of China. E-mail: zhaohbhanlf@
163.com; hnllxyj@126.com; Fax: +86-769-2286-2601; +86-769-2286-2605; Tel: +86-
158-1831-8389; +86-139-2583-6155
bSchool of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510641, People's Republic of China
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
spectra and mass spectra of all unknown compounds. See DOI:
10.1039/c7ra04402j
Base on the above consideration, to gain insight into the
inuence of molecular structure on their optoelectronic
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then washed with deionized water and ethanol, sintered at
450 ꢂC for 30 min. The TiO2 suspension was prepared from P25
(Degussa AG, Germany)39 and 1 wt% magnesium acetate solu-
tion40 on following a literature procedure. Then the paste was
deposited onto the FTO glass by blade coating. Subsequently,
a 3 mm thick 200 nm particle sized TiO2 scattering layer was
deposited again by blade coating. The TiO2-coated FTO glass
was sintered at 450 ꢂC for 30 min, then treated with TiCl4
ꢂ
ꢂ
solution at 70 C for 30 min and calcined at 450 C for 30 min
again. Aer the lm was cooled to room temperature, it was
immersed into 0.5 mM dye solution in the dark overnight. The
sensitized electrode was then rinsed with ethanol and dried. A
drop of electrolyte was deposited onto the surface of the elec-
trode and a Pt foil counter electrode was clipped onto the top of
the TiO2 electrode to assemble a DSSC for photovoltaic perfor-
mance measurements. The electrolyte consisted of 0.5 M LiI,
0.05 M I2, 0.1 M guanidinium thiocyanate, 0.6 M PMII and 0.5 M
4-tert-butylpyridine (TBP) in 3-methoxypropionitrile and the
efficient irradiated area of the cell was 0.2 cmꢁ2. The current
density–voltage (J–V) curves were measured by a Keithley 2602
Source Meter under 100 Mw cmꢁ2 standard AM 1.5 G spectrum
using a Sol 3A Oriel solar simulator. The incident light intensity
was calibrated using a standard Si solar cell. The power
conversion efficiency (h) of the DSSC is calculated from the
short-circuit photo-current (Jsc), the open-circuit photovoltage
(Voc), the ll factor (FF) and the intensity of the incident light
(Pin) according to the following equation:
Chart 1 Molecular structures of BODIPY dyes CB1-4.
properties and explore good carbazole–BODIPY conjugates for
photovoltaic applications, in this contribution, we designed and
synthesized four novel carbazole–BODIPY based dyes (CB1-4,
Chart 1) with carbazole derivatives unit as the electron donor and
cyanoacetic acid as an electron receptor at 2 and 6 positions of
BODIPY core, respectively. The optical properties, electrochemical
behavior, density functional theory calculations and photovoltaic
performances of these sensitizers were systematically investigated
and the impact of donor's electron donating ability and substitute
positions on their electronic and photovoltaic properties were
also comparatively studied. We expect to illuminate the structure–
property relationship of these BODIPY-based sensitizers and to
develop an excellent photoelectric material.
JscðmA cmꢁ2Þ ꢀ VocðVÞ ꢀ FF
2. Experimental
2.1. Materials and measurements
h ¼
PinðmW cmꢁ2Þ
All chemicals were purchased from Aldrich or Aladdin and used
without further purication. Tetrahydrofuran and 1,4-dioxane
was distilled over sodium. Other materials were common
commercial level and used as received. Compounds 3a–d were
synthesized according to the reported procedures.36–38 All
chromatographic separations were carried out on silica gel
column (300–400 mesh).
2.3. Synthesis procedures and characterization data of new
compounds
2.3.1. Synthesis of 2a–b
3-Bromo-9-octyl-9H-carbazole 2a. As the procedure shown in
Scheme 1, treated 9-octyl-9H-carbazole 1a (2.8 g, 10 mmol) in
dry DMF (50 mL) at ꢁ20 ꢂC under argon atmosphere with NBS
(1.96 g, 11 mmol), and the mixture was stirred at this temper-
ature for 4 h. Then, the reaction mixture was brought to room
temperature, and poured into water and extracted with ethyl
acetate. The organic layer was dried over anhydrous Na2SO4,
and evaporated to dryness. The residue was puried by silica gel
column chromatography using petroleum ether as eluent,
afforded 2a as white solid (2.56 g, yield: 72%). 1H NMR (400
MHz, CDCl3) d 8.21 (s, 1H), 8.05 (d, J ¼ 6.1 Hz, 1H), 7.55–7.52
(m, 1H), 7.48 (d, J ¼ 7.2 Hz, 1H), 7.42–7.39 (m, 1H), 7.27 (t, J ¼
12.0 Hz, 2H), 4.28–4.25 (m, 2H), 1.87–1.82 (m, 2H), 1.46–1.23
(m, 10H), 0.85 (t, J ¼ 6.1 Hz, J ¼ 6.8 Hz, 3H).
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance spectrometer (400 and 101 MHz, respectively) with TMS
as the internal standard. Matrix-assisted laser desorption/
ionisation time of ight mass spectra (MALDI-TOF-MS) were
acquired on a Bruker Ultraex-II spectrometer. Absorption
spectra were measured on an SHIMADZU UV-2550 UV-Vis
spectrophotometer and emission spectra were obtained by
using a Hitachi F-4500 instrument. Cyclic voltammetry (CV) was
determined with
a CHI660E electrochemical workstation
utilizing a three electrode cell consisting of a gold electrode
(working electrode), platinum wire (auxillary electrode) and Ag/
AgCl electrode (reference electrode). The experiment was
carried out in dry CH2Cl2 under Ar atmosphere condition using
0.1 M tetrabutylammonium hexauorophosphate (n-Bu4NPF6)
as supporting electrolyte.
3-Bromo-9-(4-methoxyphenyl)-9H-carbazole 2b. Treated 9-(4-
methoxyphenyl)-9H-carbazole 1b (2.71 g, 10 mmol) in dry DMF
ꢂ
(50 mL) at 0 C under argon atmosphere with NBS (1.96 g, 11
mmol), and the mixture was stirred at room temperature for
12 h. Then, the reaction mixture was poured into water and
extracted with ethyl acetate. The organic layer was dried over
2.2. Fabrication and characterization of DSSCs
Fluorine-doped SnO2 conducting glass (FTO) were cleaned and anhydrous Na2SO4, and evaporated to dryness. The residue was
immersed in aqueous 40 mM TiCl4 solution at 70 ꢂC for 30 min, puried by silica gel column chromatography using petroleum
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Scheme 1 The synthetic routes for dyes CB1-4. (i) NBS, DMF; (ii) B2Pin2, KOAc, Pd(dppf)Cl2, 1,4-dioxane; (iii) n-hexanoyl chloride, CH2Cl2, then
Et3N, BF3$Et2O, toluene; (iv) POCl3, DMF, ClCH2CH2Cl; (v) ICl, DMF, MeOH; (vi) Pd(PPh3)4, K2CO3, THF; (vii) cyanoacetic, piperidine, CHCl3/
CH3CN.
ether as eluent, afforded 2b as white solid (3.1 g, yield: 90%). 1H
9-(4-Methoxyphenyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
NMR (400 MHz, CDCl3) d 8.22 (d, J ¼ 21.4 Hz, 1H), 8.08 (d, J ¼
6.8 Hz, 1H), 7.48–7.39 (m, 4H), 7.33–7.27 (m, 2H), 7.19 (d, J ¼
7.2 Hz, 1H), 7.11 (d, J ¼ 7.0 Hz, 2H), 3.92 (s, 3H).
2-yl)-9H-carbazole 3b. White solid, yield: 84%. Rf ¼ 0.35, (PE : EA
1
¼ 20 : 1). H NMR (400 MHz, CDCl3) d 8.26–8.24 (m, 1H), 8.18
(d, J ¼ 6.8 Hz, 1H), 7.48–7.45 (m, 4H), 7.31–7.28 (m, 2H), 7.19 (d,
J ¼ 7.2 Hz, 1H), 7.11 (d, J ¼ 7.1 Hz, 2H), 3.92 (m, 3H), 1.36 (s,
12H).
2.3.2. General procedure for the preparation of 3a–d. To
the solution of bromo-compounds (10 mmol) and
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-di-oxaborolane)
(12
9-Octyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carba-
1
mmol) in diethylene dioxide (100 mL) was added Pd(dppf)Cl2
(0.5 mmol) and potassium acetate (30 mmol). The mixture was
stirred at 90 ꢂC under argon atmosphere for 24 h. The reaction
mixture was cooled to room temperature and poured to water
(100 mL), extracted with ethyl acetate, the organic layer was
washed with brine for several times, dried over anhydrous
Na2SO4, and evaporated to dryness. The corresponding boron
ester was obtained by column chromatography on silica gel
using petroleum ether and ethyl acetate.
zole 3c. Yellow oil, yield: 80%. Rf ¼ 0.35, (PE : EA ¼ 20 : 1). H
NMR (400 MHz, CDCl3) d 8.60 (s, 1H), 8.13 (d, J ¼ 7.6 Hz, 1H),
7.92 (d, J ¼ 8.1 Hz, 1H), 7.45 (d, J ¼ 7.3 Hz, 1H), 7.40 (t, J ¼ 9.5 Hz,
2H), 7.23 (d, J ¼ 7.0 Hz, 1H), 4.30 (t, J ¼ 6.9 Hz, 2H), 1.90–1.86 (m,
2H), 1.44 (s, 12H), 1.36–1.28 (m, 10H), 0.85 (m, J ¼ 13.6 Hz, 3H).
9-(4-Methoxyphenyl)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9H-carbazole 3d. White solid, yield: 79%. Rf ¼ 0.35, (PE : EA
¼ 20 : 1). 1H NMR (400 MHz, CDCl3) d 8.19–8.13 (m, 2H), 7.80–
7.72 (m, 2H), 7.46 (d, J ¼ 8.7 Hz, 2H), 7.41 (d, J ¼ 7.4 Hz, 1H),
7.30 (d, J ¼ 8.6 Hz, 2H), 7.13 (d, J ¼ 8.6 Hz, 2H), 3.94 (m, 3H),
1.36 (s, 12H).
9-Octyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carba-
1
zole 3a. Yellow oil, yield: 82%. Rf ¼ 0.35, (PE : EA ¼ 20 : 1). H
NMR (400 MHz, CDCl3) d 8.60 (s, 1H), 8.13 (d, J ¼ 7.6 Hz, 1H),
7.92 (d, J ¼ 8.1 Hz, 1H), 7.45 (d, J ¼ 7.3 Hz, 1H), 7.40 (t, J ¼ 9.5 Hz,
2H), 7.23 (d, J ¼ 7.0 Hz, 1H), 4.30 (t, J ¼ 6.9 Hz, 2H), 1.90–1.86 (m,
2H), 1.44 (s, 12H), 1.36–1.28 (m, 10H), 0.85 (t, J ¼ 13.6 Hz, 3H).
2.3.3. General procedure for the preparation of 7a–d. The
above boron ester (1.2 mmol), BODIPY 6 (1.0 mmol), K2CO3
solution (2 M, 6 mL), THF (20 mL), toluene (20 mL) and
Pd(PPh3)4 (100 mg) were added into a ask (100 mL). The
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mixture was heated under reux overnight under N2. Aer and CH3CN (20 mL), was condensed with 2-cyanoacetic acid
evaporating the solvent under reduced pressure, water (50 mL) (0.40 mmol) in the presence of piperidine (0.05 mL). The
and CH2Cl2 (100 mL) were added. The organic layer was sepa- mixture was heated under reux for 24 h with a nitrogen
rated and dried in MgSO4. The solvent was removed under atmosphere. Aer cooling to room temperature, the mixture
reduced pressure. The according aldehyde derivative was ob- was poured to CH2Cl2 (100 mL) and washed with water (3 ꢀ 50
tained by column chromatography on silica gel using petroleum mL). Organic layer was dried over MgSO4 and solvent was
ether and ethyl acetate as eluent.
evaporated and the residue was puried by silica gel column
Compound 7a. Red solid, yield: 80%. Rf ¼ 0.35, (PE : EA ¼ chromatography using CH2Cl2 and CH3OH as eluent.
1
3 : 1). H NMR (400 MHz, CDCl3) d 10.14 (s, 1H), 8.10 (d, J ¼
7.0 Hz, 1H), 7.91 (s, 1H), 7.49 (dd, J ¼ 16.4, 8.8 Hz, 4H), 7.31 (s,
Dye CB1. Dark red solid, yield: 78%. Rf ¼ 0.56, (CH2Cl2-
1
: CH3OH ¼ 10 : 1). H NMR (600 MHz, DMSO) d 8.19 (d, J ¼
1H), 4.33 (t, J ¼ 6.0 Hz, 2H), 3.15 (t, J ¼ 1.1 Hz, 2H), 2.81 (s, 3H), 7.5 Hz, 1H), 8.12 (s, 1H), 7.86 (s, 1H), 7.67 (d, J ¼ 7.7 Hz, 1H),
2.79 (s, 3H), 2.56 (s, 3H), 2.43 (s, 3H), 1.91 (d, J ¼ 6.3 Hz, 2H), 7.61 (d, J ¼ 8.1 Hz, 1H), 7.46 (t, J ¼ 7.6 Hz, 1H), 7.36 (d, J ¼
1.75–1.68 (m, 2H), 1.53 (d, J ¼ 3.9 Hz, 2H), 1.42–1.26 (m, 12H), 8.2 Hz, 1H), 7.19 (t, J ¼ 7.3 Hz, 1H), 4.40 (s, 2H), 3.18–3.06 (m,
0.94 (t, J ¼ 6.0 Hz, 3H), 0.87 (s, 3H). 13C NMR (101 MHz, CDCl3) 2H), 2.46 (s, 3H), 2.45 (s, 3H), 2.42 (s, 3H), 2.40 (s, 3H), 1.78 (d, J
d 186.30, 160.12, 155.21, 148.52, 140.80, 139.91, 127.61, 126.12, ¼ 5.7 Hz, 2H), 1.69–1.62 (m, 2H), 1.50–1.46 (m, 2H), 1.39–1.35
123.06, 122.65, 122.48, 121.97, 120.42, 119.14, 108.97, 108.84, (m, 2H), 1.32 (d, J ¼ 7.0 Hz, 2H), 1.20 (dd, J ¼ 14.4, 7.5 Hz, 8H),
43.29, 32.50, 31.83, 31.61, 29.41, 29.22, 29.06, 28.82, 27.37, 0.89 (t, J ¼ 7.2 Hz, 3H), 0.80 (d, J ¼ 7.0 Hz, 3H). 13C NMR (151
22.65, 22.55, 15.08, 14.12, 14.07, 12.97. MALDI-TOF-MS, m/z: MHz, DMSO) d 155.70, 150.65, 148.15, 140.65, 139.73, 138.37,
calcd for C39H48BF2N3O [M]+: 623.390, found: 623.393.
137.21, 130.93, 130.11, 129.03, 128.71, 128.24, 126.42, 125.79,
Compound 7b. Red solid, yield: 85%. Rf ¼ 0.35, (PE : EA ¼ 123.10, 122.77, 122.64, 122.43, 121.08, 119.29, 118.74, 113.72,
1
3 : 1). H NMR (400 MHz, CDCl3) d 10.15 (s, 1H), 8.15 (dd, J ¼ 112.59, 109.83, 109.77, 109.49, 56.47, 33.02, 32.34, 31.62, 29.20,
11.1, 7.9 Hz, 2H), 7.46 (dd, J ¼ 21.6, 7.5 Hz, 2H), 7.28 (s, 1H), 29.18, 29.09, 29.03, 27.01, 22.49, 22.31, 15.36, 14.84, 14.43,
7.21 (s, 1H), 7.04 (d, J ¼ 7.6 Hz, 1H), 4.34–4.27 (m, 2H), 3.14 (s, 14.38, 13.73, 13.20. MALDI-TOF-MS, m/z: calcd for
2H), 2.82 (s, 3H), 2.79 (s, 3H), 2.57 (s, 3H), 2.44 (s, 3H), 1.87 (d, J
¼ 6.6 Hz, 2H), 1.78–1.65 (m, 2H), 1.52–1.54 (m, 2H), 1.41–1.22
(m, 12H), 0.95 (t, J ¼ 7.0 Hz, 3H), 0.84 (d, J ¼ 6.6 Hz, 3H). 13C
C
42H49BF2N4O2 [M]+: 690.390; found: 690.389.
Dye CB2. Dark red solid, yield: 82%. Rf ¼ 0.55, (CH2Cl2-
1
: CH3OH ¼ 10 : 1). H NMR (600 MHz, DMSO) d 8.20 (d, J ¼
NMR (101 MHz, CDCl3) d 186.23, 155.51, 148.88, 140.93, 140.55, 6.5 Hz, 1H), 8.17 (d, J ¼ 6.8 Hz, 1H), 7.91 (s, 1H), 7.60–7.55 (m,
129.80, 126.06, 122.53, 122.48, 120.91, 120.48, 119.18, 110.29, 2H), 7.46 (t, J ¼ 7.3 Hz, 1H), 7.21 (t, J ¼ 7.1 Hz, 1H), 7.09 (d, J ¼
108.86, 43.24, 32.50, 31.76, 29.43, 29.20, 29.12, 27.50, 22.62, 6.7 Hz, 1H), 4.41 (s, 2H), 3.09 (s, 2H), 2.48 (s, 6H), 2.41 (s, 6H),
22.53, 15.01, 14.05, 14.03, 13.01. MALDI-TOF-MS, m/z: calcd for 1.76 (s, 2H), 1.65 (s, 2H), 1.48 (s, 2H), 1.37 (d, J ¼ 6.6 Hz, 2H),
C
39H38BF2N3O2 [M]+: 617.300, found: 617.324.
1.23–1.13 (m, 10H), 0.90 (t, J ¼ 6.8 Hz, 3H), 0.78 (m, J ¼ 6.6 Hz,
Compound 7c. Brown solid, yield: 78%. Rf ¼ 0.25, (PE : EA ¼ 3H). 13C NMR (151 MHz, DMSO) d 155.41, 151.10, 148.38,
1
3 : 1). H NMR (400 MHz, CDCl3) d 10.14 (s, 1H), 8.14 (d, J ¼ 140.81, 140.66, 139.06, 137.58, 135.96, 132.25, 131.06, 130.20,
7.6 Hz, 1H), 7.96 (s, 1H), 7.49 (d, J ¼ 8.6 Hz, 2H), 7.45–7.34 (m, 126.32, 126.03, 122.34, 121.75, 121.27, 120.83, 120.72, 119.31,
3H), 7.30 (t, J ¼ 7.3 Hz, 1H), 7.21 (d, J ¼ 8.3 Hz, 1H), 7.14 (d, J ¼ 111.52, 109.77, 51.60, 32.81, 32.35, 31.57, 29.26, 29.23, 29.11,
8.5 Hz, 2H), 3.94 (s, 3H), 3.24–3.07 (m, 2H), 2.81 (s, 3H), 2.79 (s, 29.08, 27.05, 22.50, 22.29, 15.40, 14.77, 14.35, 14.32, 13.92,
3H), 2.57 (s, 3H), 2.44 (s, 3H), 1.80–1.67 (m, 2H), 1.55–1.49 (m, 13.13. MALDI-TOF-MS, m/z: calcd for C41H39BF2N4O3 [M]+:
2H), 1.45–1.40 (m, 2H), 0.95 (t, J ¼ 7.1 Hz, 3H). 13C NMR (101 684.310, found: 684.334.
MHz, DMSO) d 186.29, 159.08, 148.64, 141.78, 140.87, 130.02,
Dye CB3. Dark solid, yield: 77%. Rf ¼ 0.52, (CH2Cl2 : CH3OH
128.55, 127.90, 126.39, 125.85, 123.70, 123.40, 122.77, 121.89, ¼ 10 : 1). 1H NMR (600 MHz, DMSO) d 8.31 (d, J ¼ 7.7 Hz, 1H),
120.34, 120.07, 115.21, 110.01, 55.67, 32.51, 31.63, 28.84, 22.56, 8.24 (s, 1H), 7.79 (s, 1H), 7.59 (d, J ¼ 8.7 Hz, 2H), 7.46 (t, J ¼
15.07, 14.07, 13.00, 12.61. MALDI-TOF-MS, m/z: calcd for 7.7 Hz, 1H), 7.37 (q, J ¼ 8.3 Hz, 2H), 7.34–7.29 (m, 2H), 7.25 (d, J
C
39H48BF2N3O [M]+: 623.390, found: 623.409.
¼ 8.8 Hz, 2H), 3.90 (s, 3H), 3.14 (s, 1H), 2.47 (s, 3H), 2.46 (s, 3H),
Compound 7d. Brown solid, yield: 82%. Rf ¼ 0.25, (PE : EA ¼ 2.44 (s, 3H), 2.43 (s, 3H), 1.72–1.66 (m, 2H), 1.55–1.49 (m, 2H),
1
3 : 1). H NMR (400 MHz, CDCl3) d 10.13 (s, 1H), 8.20 (d, J ¼ 1.40 (dd, J ¼ 14.7, 7.3 Hz, 2H), 0.92 (t, J ¼ 7.3 Hz, 3H). 13C NMR
7.9 Hz, 2H), 7.45 (dd, J ¼ 12.6, 8.3 Hz, 3H), 7.38–7.28 (m, 2H), (151 MHz, DMSO) d 166.51, 162.78, 159.11, 155.30, 148.24,
7.15–7.08 (m, 4H), 3.91 (s, 3H), 3.11 (d, J ¼ 5.6 Hz, 2H), 2.80 (s, 141.46, 140.37, 138.98, 137.39, 132.10, 130.16, 129.70, 128.78,
3H), 2.77 (s, 3H), 2.52 (s, 3H), 2.39 (s, 3H), 1.77–1.61 (m, 2H), 126.94, 126.30, 124.39, 123.24, 122.87, 120.41, 119.93, 119.53,
1.53–1.48 (m, 2H), 1.40 (dd, J ¼ 14.3, 7.1 Hz, 2H), 0.93 (t, J ¼ 115.83, 112.56, 110.11, 110.03, 109.82, 109.31, 55.97, 33.00,
7.1 Hz, 3H). 13C NMR (101 MHz, DMSO) d 186.26, 159.06, 32.37, 31.56, 22.33, 15.38, 14.82, 14.43, 14.39, 13.19. MALDI-
155.48, 148.87, 141.86, 141.48, 130.15, 129.97, 128.50, 126.33, TOF-MS, m/z: calcd for C42H49BF2N4O2 [M]+: 690.390; found:
122.76, 121.83, 120.39, 120.08, 115.26, 111.21, 109.90, 55.64, 690.365.
32.49, 31.56, 28.81, 22.53, 15.00, 14.06, 13.00, 12.65. MALDI-
Dye CB4. Dark solid, yield: 79%. Rf ¼ 0.50, (CH2Cl2/CH3OH ¼
TOF-MS, m/z: calcd for C39H38BF2N3O2 [M]+: 617.300, found: 10 : 1). 1H NMR (600 MHz, DMSO) d 8.35 (d, J ¼ 7.9 Hz, 1H), 8.30
617.305.
(d, J ¼ 7.6 Hz, 1H), 7.77 (s, 1H), 7.57 (d, J ¼ 8.8 Hz, 2H), 7.46 (t, J
2.3.4. General procedure for preparation of CB1-4. The ¼ 7.7 Hz, 1H), 7.32 (dd, J ¼ 16.0, 7.9 Hz, 2H), 7.22 (d, J ¼ 8.7 Hz,
aldehyde derivative (0.20 mmol), dissolved in CHCl3 (20 mL) 3H), 7.15 (s, 1H), 3.87 (s, 3H), 3.10 (s, 1H), 2.44 (s, 3H), 2.41 (s,
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6H), 2.38 (s, 3H), 1.65 (s, 2H), 1.47 (d, J ¼ 7.6 Hz, 2H), 1.38 (dd, J
¼ 14.7, 7.3 Hz, 2H), 0.90 (t, J ¼ 7.3 Hz, 3H). 13C NMR (151 MHz,
DMSO) d 162.63, 160.84, 159.03, 156.18, 151.35, 148.58, 141.52,
141.13, 137.77, 135.55, 130.84, 129.97, 129.60, 128.68, 126.90,
122.73, 122.53, 122.30, 121.12, 120.50, 115.87, 111.45, 110.10,
55.92, 32.34, 31.41, 29.50, 22.28, 15.46, 14.77, 14.37, 14.26,
13.89. MALDI-TOF-MS, m/z: calcd for C41H39BF2N4O3 [M]+:
684.310; found: 684.325.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic strategy employed to access the target compounds
CB1-4 is shown in Scheme 1. Compounds 2a–d were prepared
from N-substituted carbazoles with NBS, according to the
similar published procedure.41 Aerward, the Miyaura bor-
ylation reaction of the compound 2a–d and commercially
available bis(pinacolato)diboron was carried out using KOAc as
a base and Pd(PPh3)2Cl2 as the catalyst in toluene, resulting in
corresponding donor motif 3a–d.42 The key intermediates, 2-
iodo-6-formyl BODIPY 6, was prepared from 2,4-dime-
Fig. 1 Absorption spectra of dyes CB1-4 in CHCl3 (1 ꢀ 10ꢁ5 M).
comparision with the unmodied BODIPY 4, the absorption
maxima of CB1, CB2, CB3 and CB4 is bathochromic shied by
about 32 nm, indicating that 2,6-modicated with donor and
acceptor units, respectively, efficient extended the conjugation
length of these dyes, leading to the reduced gap between the
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) levels. In comparision with
CB1 and CB3, CB2 and CB4 exhibit broader absorption pattern
with the low energy edge of absorption bands red-shi about
20 nm (Table 1), which is due to the factor that the N-methox-
yphenyl carbazole is a more electron-rich donor relative to the
N-octyl carbazole counterpart, thus resulting in much intensive
ICT transitions. In the other hand, the molar extinction coeffi-
cients (3) of the maximum absorption bands of 4, CB1, CB2, CB3
and CB4 were 0.81 ꢀ 105, 0.95 ꢀ 105, 0.87 ꢀ 105, 1.09 ꢀ 105 and
1.71 ꢀ 105 Mꢁ1 cmꢁ1, respectively, which are higher than the
well-known dye N719 (1.42 ꢀ 104 Mꢁ1 cmꢁ1).44 Due to the
extended conjugation length, CB1-4 display higher 3 values than
BODIPY 4. Furthermore, in comparison with CB1 and CB2, dyes
thylpyrrole, through
a three steps conversion, including
condensation–acylation–halogenation reaction sequence. In
the next step, palladium-catalyzed Suzuki coupling of 3a–d with
iodo conjugated bridge 6 produce the corresponding aldehydes
7a–d in excellent yields, respectively. At last, the synthesis of
target dyes CB1-4 were achieved by the Knoevenagel conden-
sation of the corresponding 7a–d with cyanoacetic acid in the
presence of a catalytic amount of piperidine. These target
molecules were sufficiently soluble in toluene, CH2Cl2, CHCl3
and THF to permit their characterization in solution as well as
their application in DSSCs devices. The structural assignments
of these dyes were conrmed by a range of spectroscopic tech-
niques, including NMR and MALDI-TOF-MS.
3.2. Photophysical properties
The UV-vis absorption spectra of CB1-4 recorded in a diluted CB3 and CB4 exhibit stronger absorption ability with higher 3
solution of CHCl3 (1 ꢀ 10ꢁ5 M) at room temperature are shown values, indicating that carbazole unit bonding to BODIPY
in Fig. 1. For comparison, the reference BODIPY 4 without module with 2-position is advantageous to form effective
substitutions at 2,6-position was also tested in the same conjugation than that with 3-position. Generally, it is believed
conditions. The corresponding data of the measurements are that a strong absorption in a broad visible region is favorable for
listed in Table 1. As shown in Fig. 1. All these dyes CB1-4 exhibit photocurrent generation in DSSCs.45
broad absorption spectra with two distinct absorption bands:
CB1-4 were self-adsorbed on TiO2 nanostructured lm by an
one in the range of 280–400 nm, the other in the range of 420– electrode soaking in dry CH2Cl2 solution. In general, when
650 nm. The band in the high-energy region are attributed to sensitizers are anchored onto a nanocrystalline TiO2 surface,
interplay of lower-intensity BODIPY-based S0 / S2 and the deprotonation and aggregation of the dye molecules would
carbazole-based p–p* transitions. The highly intense absorp- affect on the UV-vis absorption proles. Deprotonation and H-
tion band in the low-energy region are partly characteristics of aggregation always result in blue-shied of absorption, while
the p–p* (S0 / S1) transition of the BODIPY moiety. In addi- J-aggregates mainly lead to the red-shied absorption.46 As
tion, CB1-4 show obviously broader and red-shi absorption shown in Fig. 2, the lmax of CB1, CB2, CB3 and CB4 were blue-
prole as compared to the reference BODIPY 4, suggesting that shied by 35 nm, 27 nm, 30 nm and 19 nm as compared to
there is an intense intermolecular charge transfer (ICT) transi- those in solution. This is ascribed to deprotonation of the dyes
tion in these D–p–A molecules.43 Consequently, the low-energy and/or the formation of H-aggregation on the semiconductor
absorption region are originated from the mixture of p–p* (S0 surface. CB2 and CB4 have much less blue-shi in comparison
/ S1) transition of BODIPY moiety and the ICT transitions with CB1 and CB3, indicating that CB2 and CB4 with N-
between the carbazole donor and cyanoacetic acid acceptor. In methoxyphenyl carbazole donor motif enable better dispersion
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Table 1 Photophysical and electrochemical characteristics for sensitizers
lmaxa (nm)/3 (ꢀ105 Mꢁ1 cmꢁ1
)
lmaxb (nm)
Eox (V) vs. NHE
E0,0 (eV)/lonset (nm)
Eo*x (V) vs. NHE
c
d
e
Dye
CB1
CB2
CB3
CB4
531/0.95
529/0.87
530/1.09
533/1.71
496
502
500
514
1.07
0.99
1.05
0.97
2.05/603
2.00/620
2.04/605
1.92/646
ꢁ0.98
ꢁ1.01
ꢁ0.99
ꢁ0.95
a
b
In CHCl3 solutions. Absorption maximum on TiO2 was obtained through measuring the dyes adsorbed on 3 mm TiO2 nanoparticle lms in
c
d
a CHCl3 solution. Eox was measured in CH2Cl2 and calibrated with ferrocene as an external reference. E0,0 was estimated from the
absorption thresholds from absorption spectra of dyes adsorbed on the TiO2 lm, E0,0 ¼ 1240/lonset
e
.
Computed from the formula Eo*x ¼ Eox ꢁ E0;0
.
and capable of better photovoltaic performance. On the other the carbazole donor variety and their connect manner have
hand, the spectra of dyes adsorbed on TiO2 surface show signicant affects on the Eox. In comparision with CB1 (1.07 V),
broader absorption bands and each with an obvious shoulder the oxidation of CB2 (0.99 V) shied to the lower oxidation
band (ꢃ550 nm) as contrast to those in CH2Cl2 solution, this potentials, which is due to the factor that the electron-rich
may be attributed to strong intermolecular interactions, such as methoxyphenyl could strengthen the electron donating ability
dipolar intermolecular interactions and nonbonding intermo- of carbazole. Similar phenomenon was observed in CB3 and
lecular interactions (e.g. –F/N–), between the individual CB4. In addition, the substitutional position of the electron
molecules in the solid state.
donor group also affects on the oxidation potential of the dyes.
Both 2-position carbazole substituted CB3 (1.05 V) and CB4
(0.97 V) are oxidized at a slightly lower potential than
3.3. Electrochemical properties
Energy levels of sensitizers are crucial to judge the possibilities
of electron injection from the excited dyes to the conductor
band of TiO2 and dyes regeneration by the iodide/triiodide
electrolyte. The electrochemical properties of these sensitizers
were measured using cyclic voltammetry (CV) in CH2Cl2 solu-
tion containing 0.1 M TBAPF6 as supporting electrolyte. The CV
curves of CB1-4 are depicted in Fig. 3 and the results are
summarized in Table 1. The electrochemical study reveals that
all the novel dyes are reversibly oxidized, consistent with the
formation of stable radical-cations, which is a key requirement
for efficient electronic processes in DSSCs. CB1-4 exhibit two
oxidation and two reduction waves corresponding to the
formation of dication and dianion radicals respectively. As
shown in Fig. 3, CB1-4 present differential rst onset oxidation
potential as 1.07, 0.99, 1.05 and 0.97 V, respectively, indicating
Fig. 2 Normalized absorption spectra of dyes CB1-4 anchoring on the
3 mm porous TiO2 nanoparticle film.
Fig. 3 Cyclic voltammograms of dyes CB1-4 measured in CH2Cl2.
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new carbazole–BODIPY based dyes are considered to have
proper electronic energy levels as a promising sensitizer in
DSSCs.
3.4. Density functional theory calculations
For a better understanding of the structural and electronic
features of these novel dyes CB1-4, the TD-DFT theoretical
calculations were further performed, and the optimized struc-
tures and the electronic distribution in HOMO and LUMO levels
are presented in Fig. 5. All calculations were carried out with the
Gaussian 09 program suite by using the B3LYP method and 6-31
G* basis set.49 From the optimized ground-state geometries of
the dyes in Fig. 5, it found that the dihedral angle between the
carbazole unit and the BODIPY framework are computed to be
61.8ꢂ for CB1, 52.3ꢂ for CB2, 60.4ꢂ for CB3, and 50.0ꢂ for CB4,
respectively, indicating that all these dyes exhibit relatively
nonplanar molecule structures, which may not only suppress
the aggregation but also increase the molar extinction coeffi-
cient of the organic dyes (105 Mꢁ1 cmꢁ1).50 Obviously, CB2 and
CB4, with N-methoxyphenyl carbazole as electron donating
unit, display more coplanar p-conjugate backbone compared to
CB1 and CB3. Moreover, CB3 and CB4, with carbazole unit
linked to BODIPY at 2-position, show slightly more coplanar p-
conjugate backbone relative to the 3-position linked counter-
parts, CB1 and CB2. Generally, the high degree of molecular
coplanarity could facilitate efficient ICT transition in the D–p–A
structural molecule and result in red-shied absorption, which
is consistent with the UV-vis absorption spectra observed in
solution.
Fig. 4 Schematic energy levels of dyes CB1-4 (figure adapted from
ref. 33).
corresponding 3-position carbazole substituted analogs, CB1
and CB2, due to single electronegative nitrogen atom of carba-
zole are closer to the electron acceptor unit in the former ones.
All the four potential Eox values are substantially more positive
about 0.59 V than Iꢁ/I3 redox couples (0.40 V vs. NHE), indi-
ꢁ
cating that driving forces for the dye regeneration to efficiently
compete with the recapture of injected electrons by the dye
cation are sufficient (Fig. 4).47 The zero–zero transition energy
values (E0,0), which are related to the band gap energy (Eg), were
estimated from the onset wavelength of the absorption spec-
trum in CHCl3 solution. The excited-state oxidation potentials
ðEo*xÞ reect the lowest unoccupied molecular orbital (LUMO)
levels of dyes. It can be calculated from subtraction between Eox
and E0,0 ðEo*x ¼ Eox ꢁ E0;0Þ, were ꢁ0.98 V for CB1, ꢁ1.01 V for
CB2, ꢁ0.99 V for CB3 and ꢁ0.95 V for CB4, which are all more
negative than the conduction band (Ecb) of the TiO2 electrode
(ꢁ0.50 V), suggesting a sufficient driving force for the electron
injection.48 On the basis of the Eox and Eo*x energy values, these
The electron distributions of CB1-4 indicate that p-electrons
in the HOMO are distributed along the carbazole unit, while
their LUMO orbitals are mainly localized on cyanoacrylic acid
Fig. 5 Optimized structures and electron distribution in HOMO and LUMO levels of CB1-4.
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Fig. 6 IPCE spectra of DSSCs based on CB1-4.
Fig. 7 J–V characteristics observed for the DSSCs fabricated by CB1-
4.
and its adjacent BODIPY motifs (Fig. 5). The result suggests that
the HOMO / LUMO excitation by light would induce a transfer
of the electron from electron-donating carbazole moiety to
electron-accepting BODIPY–cyanoacrylic acid moiety. These
electron distributions allow sufficient charge separation within
the dyes. Apparently, the electron transfer from the HOMO to
the LUMO can easily result in electron redistribution from the
carbazole unit to the cyanoacrylic acid moiety, which is highly
signicant in the electron injection from excited state dyes into
the CB band of TiO2 semiconductor. Since N-methoxyphenyl
carbazole unit possess stronger electron-donating ability than
that of N-octyl carbazole unit, dyes CB2 and CB4 with better
molecular coplanarity deservedly present more intense move-
ment of electrons, which in turn lead to more efficient injection
of electrons into the TiO2 photoelectrode.
sensitizers (CB1-4) are also adopted the same thickness of the
TiO2 lm.
The IPCE spectra of CB1, CB2, CB3 and CB4 for DSSCs are
plotted as a function of wavelength from 350 to 700 nm as seen
in Fig. 6. The IPCE values over 50% were attained for CB2 and
CB4 from 350 to 550 nm with tail off at 700 nm. The dye CB1 and
CB3 reached over 40% in the range of 350–520 nm. The IPCE
values of the dyes derived from Fig. 6 are in the order of CB4 >
CB2 > CB3 > CB1. Highest IPCE value was observed for the dye
CB4, which is due to its broader absorption and better light
harvesting capacity in the visible region aer adsorbed on the
TiO2 surface (Fig. 2). As expected, the trend of IPCE is also in
good accordance with Jsc data obtained in J–V measurements.
The DSSC based on the dye N719 under the optimized
conditions exhibits the h of 7.93% with a Jsc of 15.34 mA cmꢁ2
,
a Voc of 718 mV and an FF of 0.72. Under the same conditions,
the DSSCs sensitized with CB1, bearing N-octyl carbazole as the
donor and linked BODIPY conjugated bridge with 3-position,
showed a Jsc of 6.30 mA cmꢁ2, a Voc of 540 mV, and an high FF
value of 0.76, generating an overall conversion efficiency h of
2.58%. However, when the N-octyl carbazole was replaced with
N-methoxyphenyl carbazole, the dye CB2 exhibited a Jsc of 9.18
mA cmꢁ2, a Voc of 560 mV, an FF of 0.73, and a h of 3.74%. It can
be clearly found that the two dyes (CB1 and CB2) show similar
Voc values, but the Jsc increased by about 2.88 mA cmꢁ2 for CB2
with N-methoxyphenyl carbazole motif to CB1 with N-octyl
carbazole unit. The electron-donating N-methoxyphenyl groups
3.5. Photovoltaic performance
In order to investigate the photovoltaic performance of CB1-4,
a set of DSSCs were fabricated and tested under the standard
conditions (AM 1.5 G, 100 mW cmꢁ2). The incident photon-to-
current efficiency (IPCE) and photocurrent density–photo-
voltage (J–V) curves of cells based on these dyes were measured,
and the results are depicted in Fig. 6 and 7, respectively, and the
corresponding data are summarized in Table 2. The photovol-
taic performance of these dyes as sensitizers for DSSCs was
tested using 0.5 M LiI 0.05 M I2, 0.1 M guanidinium thiocyanate,
0.6 M PMII and 0.5 M 4-tert-butylpyridine (TBP) in 3-methox-
ypropionitrile as the redox electrolyte. As well known, the
thickness of the TiO2 lm is one of the important factors to
dominantly affect the performance of a cell. Generally, thicker
TiO2 lms can provide more dye molecules to adsorb on them.51
Meanwhile, the electron transporting path becomes longer with
thicker TiO2 lms, which may cause a higher probability of
recombination. As a result, the open-circuit voltage (Voc) for the
pertinent DSSCs may decrease with further increase in the
thickness of the TiO2 lm.52 The optimized thickness of 200 nm
particle sized TiO2 lm for the referenced N719 dye in our work
is about 3 mm. In order to make a comparison, the four
Table 2 Photovoltaic parameters of the DSSCs based on CB1-4
Dye
Jsc (mA cmꢁ2
)
Voc (mV)
FF
h (%)
CB1
CB2
CB3
CB4
N719
6.30
9.18
6.84
10.20
15.34
540
560
540
600
718
0.76
0.73
0.73
0.70
0.72
2.58
3.74
2.68
4.28
7.93
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attached on the carbazole donor could reduce the LUMO level of
CB2 (as showed in Fig. 4), resulting in an extension of the
absorption spectral range to a longer wavelength than that of
CB1, which could consequently contribute to an increased light-
harvesting efficiency and hence a rise of the Jsc value. The same
result was observed in CB3 and CB4. The cell based on CB3
provided a Jsc of 6.84 mA cmꢁ2, and a Voc of 540 mV, and an h of
2.68%, while the cell sensitized with CB4 presented a higher Jsc
of 10.20 mA cmꢁ2, a higher Voc of 600 mV and an overall
conversion efficiency h of 4.28%, which reached ꢃ54% of the
N719 cell efficiency and are of a high level in the BODIPY based
DSSCs. However the PCE is still slightly lower than the similar
dye UY11 (4.52%), reported by Mao et al.30 This is due to the fact
that U11 with two BODIPY units exhibit a higher Jsc (11.82 mA
cmꢁ2) than CB4, although CB4 presentes a higher Voc value. The
results demonstrated that the electron-donating abilities of
donors in these dyes signicantly effect on the parameters of Jsc,
and Voc in their dyes sensitized DSSCs. On the other hand, the
substitutional positions also play an important role in the
performance of their DSSCs. Compared to CB1 with carbazole
donor linked BODIPY at 3-position, the 2-position linked CB3
showed the same Voc and a similar FF, but a higher Jsc value,
leading to a slightly enhancement of h. Similar observations was
also found in CB2 vs. CB4. Benet for the strong electron-
donating donor advantage, along with an appropriate con-
necting format merit, CB4 sensitized cell displayed the best
photovoltaic performance among the four CBs based DSSCs.
The excellent device performance also originated in the effective
Fig. 8 EIS Nyquist plots for DSSCs based on CB1-4 measured in the
dark under ꢁ0.55 V bias.
dyes CB4 (0.60 V) and CB2 (0.56 V). The signicantly increased
Rrec values of CB4 and CB2 imply the retardationꢁof the charge
recombination between injected electron and I3 ions in the
electrolyte, with a consequent increase of Voc.
4. Conclusions
electron injection from photo-excited CB4 to the conduction In summary, we have synthesized and characterized a series of
band of TiO2 lm, leading to higher electron injection effi- D–p–A type organic dyes CB1-4, featuring an N-octyl or N-
ciency, which can be conrmed by IPCE and Jsc values. It worth methoxyphenyl carbazole as donor with 2 or 3-position
noting that all these dyes sensitized cells exhibit high FF values, substituted at central BODIPY p-bridge, and cyanoacetic acid as
indicating that the special spatial structure of these molecules terminal acceptor. The structure–property relationships show
effectively suppress of excessive dye aggregation on the surface that donor's electron-donating abilities, as well as donor's
of TiO2.
substituted positions have signicant inuence on the sensi-
tizer's optoelectronic and photovoltaic properties. In com-
paration with N-octyl carbazole based CB1 and CB3, the N-
methoxyphenyl based CB2 and CB4 displayed broader and more
3.6. Electrochemical impedance spectroscopy studies
Electrochemical impedance spectroscopy (EIS) is a powerful intensive absorption both in solution and on the surface of TiO2
technique of characterizing the important interfacial charge due to the extension of p-conjugated system and enhancement
transfer processes in a DSSC. The EIS was measured in the dark of electron-donating ability, efficiently lead to a decrease of the
to elucidate correlation of Voc with those dyes. The Nyquist plots optical gap by reduce the LUMO energy level. Benet for these
of DSSCs with CB1-4 are shown in Fig. 8. The small and large optoelectronic merits, the photovoltaic performances of DSSCs
semicircles respectively located in the high- and middle- based on CB2 and CB4 are obviously superior to CB1 and CB3
frequency regions, are assigned to the charge transfer at Pt/ not only in Jsc and Voc values, but also in the overall conversion
electrolyte and TiO2/dye/electrolyte interface, respectively.53 efficiency h. Moreover, the substituted positions of donor also
The charge recombination resistance (Rrec) at the TiO2 surface a key factor affect on the device performances. In particular,
can be deduced by tting curves from the range of the middle- carbazole 2-position substituted dyes (CB3 and CB4) showed
frequency using a Z-view soware. Rrec is related to the charge higher Jsc than their carbazole 3-position substituted counter-
recombination rate between injected electron and electron part (CB1 and CB2). Possessing both of two advantages, CB4
acceptor (I3ꢁ) in the electrolyte, estimated by the large semi- presents the best photovoltaic performance with a Jsc of 10.20
circle width. A large Rrec means the small charge recombination mA cmꢁ2, an Voc of 600 mV, and FF of 0.7, corresponding to an h
rate. The Rrec values for CB4 and CB2 were estimated to be 135 of 4.28% under AM 1.5 irradiation (100 mW cmꢁ2), which is an
and 117 U, respectively, which are better than the similar dyes moderate device performance in DSSCs but a fairly good value
ever reported.30 In contrast, CB2 and CB1 were found to have in BODIPY based DSSCs. The results indicate that well tailored
relatively smaller Rrec values at 69 and 56 U, respectively. The carbazole–BODIPY conjugates are promising candidate for
result appears to be consistent with the larger Voc values for the efficient DSSCs.
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sensitizers for dye-sensitized solar cell applications, J. Am.
Chem. Soc., 2008, 130, 6259–6266.
Acknowledgements
14 W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, et al.,
Efficient dye-sensitized solar cells with an organic
We acknowledge the nancial support from Characteristic
Innovation Program of Guangdong Colleges and Universities
(2015KTSCX136), Young Innovative Talents Program of
Guangdong Colleges and Universities (2016KQNCX164), Nature
Science Foundation of Guangdong Province (2016A030313131,
2016A030313128) and National Nature Science Foundation of
China (51476036).
photosensitizer
featuring
orderly
conjugated
ethylenedioxythiophene and dithienosilole blocks, Chem.
Mater., 2010, 22, 1915–1925.
15 J. Y. Mao, N. N. He, Z. J. Ning, Q. Zhang, F. L. Guo, L. Chen,
et al., Stable dyes containing double acceptors without
COOH as anchors for highly efficient dye-sensitized solar
cells, Angew. Chem., Int. Ed., 2012, 51(9), 873–876.
16 Y. J. Chang, Y.-J. Wu, P.-T. Chou, M. Watanabe and
T. J. Chow, Triarylene linked spacer effect for dye-
sensitized solar cells, Thin Solid Films, 2014, 558, 330–336.
17 A. Diac, D. Demeter, S. Jungsuttiwong, I. Grosu and
J. Roncali, ADDA and ADADA systems based on
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