New benzoselenadiazole-based D–A–π–A type triarylamine sensitizers for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2017.02.013

Three new D–A–π–A configuration organic dyes (LC-6, LC-7 and LC-8) based on triphenylamine as the electron donor, benzoselenadiazole (BSD) as the auxiliary acceptor, either thiophene or benzene as the π spacer and cyanoacetic acid as the anchoring group h

Full text of DOI:10.1016/j.dyepig.2017.02.013

Dyes and Pigments 141 (2017) 161e168
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New benzoselenadiazole-based DeAe
peA type triarylamine
sensitizers for highly efficient dye-sensitized solar cells
,
b
,
,
,
, b, **
Futai Lu a 1, Shibo Qi c 1, Jie Zhang a, Guang Yang a, Bao Zhang a *, Yaqing Feng a
a School of Chemical Engineering and Technology, Tianjin University, No.135, Yaguan Road, Jinnan District, Tianjin, 300350, PR China
b Tianjin Co-Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin, 300072, PR China
c School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new DeAe
peA configuration organic dyes (LC-6, LC-7 and LC-8) based on triphenylamine as the
Received 15 December 2016
Received in revised form
9 February 2017
Accepted 10 February 2017
Available online 13 February 2017
electron donor, benzoselenadiazole (BSD) as the auxiliary acceptor, either thiophene or benzene as the
p
spacer and cyanoacetic acid as the anchoring group have been designed and synthesized for dye-
sensitized solar cells (DSCs). Introduction of octyloxy chain on the triphenylamine unit was found to
be able to redshift the absorption spectra and suppress the charge recombination. It was also found that
using the benzene instead of thiophene as
p spacers and using CDCA coadsorbent could help to improve
the Jsc and Voc values of the cell. Under standard global AM 1.5 solar light conditions, a DSC employing a
dye with a 1,4-phenylene unit with CDCA gave the best photovoltaic performance with a Jsc of
13.21 mA cmꢀ2, a Voc of 734 mV, a FF of 0.69 and an overall PCE of 6.72%. These results suggest that the
BSD unit can be a promising electron-withdrawing candidate in DeAepeA type sensitizers for further
exploration DSCs.
Keywords:
Dye-sensitized solar cells
Photovoltaic
Benzoselenadiazole
Triphenylamine
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
photophysical properties [12e17].
Most metal-free organic sensitizers are constructed with a
donor- bridge-acceptor (D- -A) architecture for the benefit of
intramolecular charge transfer (ICT) [18e20]. In 2011, the Tian
group proposed a novel D-A- -A configuration for designing a new
Dye-sensitized solar cells (DSCs) have attracted considerable
attention due to their distinctive advantages of low cost of pro-
duction, high power conversion efficiency (PCE) and facile struc-
p
p
p
€
tural modification since the seminal report by Gratzel and
generation of efficient and stable organic sensitizers. It was shown
coworkers [1,2]. As the key component of the DSCs, photosensi-
tizers play an important role in light harvesting. In the past decade,
researchers have devoted significant efforts to develop new and
highly efficient sensitizers. The highest PCE of ruthenium complex
[3], zinc porphyrins [4e10] and metal-free organic dyes [11] have
reached 11.5%, 13% and 13%, respectively. Many research groups
have focused on the metal-free organic sensitizers owing to their
high molar extinction coefficients, good flexibility of molecular
tuning, low synthetic and purification cost and tunable
that introducing an additional strong electron-withdrawing group
between the donor group and the p-bridge can facilitate the elec-
tron transfer, decrease the HOMO-LUMO energy gap, modulate the
absorption spectra and thus result in the enhancement of the
photovoltaic performance [21,22]. A series of electron-withdrawing
building blocks have been successfully applied to this type of
organic dyes, such as benzoxadiazole (BOD) [23,24], benzothia-
diazole (BTD) [25], benzotriazole (BTZ) [26,27], quinoxaline (QN)
[28,29], diketopyrrolopyrrole (DPP) [30,31], and other units. The
significant role of the additional electron-withdrawing group in
promoting the performance of the D-A-
solar cell has inspired us to search for new and more efficient
additional acceptors for use in D-A- -A dye structures.
To the best of our knowledge, benzoselenadiazole (BSe), an
effective electron-withdrawing unit due to the symmetric presence
of the two unsaturated nitrogen atoms, has been widely utilized as
an electron-deficient unit to construct alternating D-A polymers in
p-A type sensitizer-based
* Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, No.135, Yaguan Road, Jinnan District, Tianjin, 300350, PR China.
** Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, No.135, Yaguan Road, Jinnan District, Tianjin, 300350, PR China.
E-mail addresses: lufutaitiger@126.com (F. Lu), boschi8000@163.com (S. Qi),
zhjie@tju.edu.cn (J. Zhang), 176879931@qq.com (G. Yang), baozhang@tju.edu.cn
(B. Zhang), yqfeng@tju.edu.cn (Y. Feng).
p
1
These two authors contributed equally to this work.
http://dx.doi.org/10.1016/j.dyepig.2017.02.013
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

162
F. Lu et al. / Dyes and Pigments 141 (2017) 161e168
the field of bulk heterojunction organic photovoltaics [32e34].
Moreover, the BSe unit can effectively tune the polarizability and
optical and electronic properties of organic conjugated chains
[35,36]. However, its application as an auxiliary acceptor in DSCs
was seldom reported [37].
The samples were then cooled to room temperature and immersed
in a freshly prepared TiCl4 aqueous solution (20 mM) for 0.5 h. The
films were air dried and sintered again at 550 ꢂC for 30 min.
Furthermore, the films were put into the dye solution (~0.3 mM in a
mixture of CHCl3:EtOH ¼ 1:1) at room temperature and kept for
12 h. The sensitized electrode was rinsed with ethanol, and dried. A
sandwich cell consisting of the porphyrin-sensitized TiO2 electrode
as the working electrode and a Pt foil as the counter electrode was
To further investigate the structure-properties correlations for
BSe-based D-A-
designed and synthesized three organic sensitizers based on the
benzoselenadiazole unit, containing triphenylamine/octyl-
p-A type sensitizers for use in DSSCs, we have
fabricated. The electrolyte solution of 0.1
dimethylimidazolium iodide (DMPII), 0.5
M
1-propyl-2,3-
triphenylamine as donors and cyanoacrylic acid as acceptors
[38e41], as shown in Fig. 1. The introduction of alkoxyl chains into
the triphenylamine unit was used to prevent charge recombination
due to the steric hindrance. The influence of the variation of the
donor and the heterocyclic linkers (thiophene and benzene) on the
optical and electrochemical properties as well as the photovoltaic
performance of these dyes are systematically investigated.
M 1-butyl-3-
methylimidazolium iodide (BMII), 0.05 M I2, 0.1 M LiI, 0.1 M gua-
nidine thiocyanate (GuSCN), and 0.5 M tert-butylpyridine (TBP) in
an 85/15 mixture of acetonitrile and valteronitrile were used as the
redox electrolytes.
The three organic dye dye-sensitized TiO2 electrodes were
tested under simulated AM 1.5 irradiation (100 mW/cm2) and the
photocurrent density-voltage (J-V) characteristics were recorded on
Keithley 2400 Source meter (solar AAA simulator, oriel China,
calibrated with a standard crystalline silicon solar). The irradiated
area of the cell was 0.159 cm2, the active area of the cell is so small
that no mask can be employed during the measurement. Mono-
chromatic incident photon-to current conversion efficiency (IPCE)
for the solar cells was performed on a commercial setup (Q Test
Station 2000 IPCE Measurement System, CROWNTECH, USA). The
monochromator was incremented through the visible spectrum to
generate IPCE spectra. A white light bias (1% sunlight intensity) was
applied onto the sample during the testing with an AC model
(10 Hz). Electrochemical impedance spectra (EIS) were scanned in a
frequency range of 0.1e106 Hz and at an AC amplitude of 10 mV at
room temperature with a Chenhua CHI600E analyzer.
2. Experimental
2.1. Materials
The synthetic routes to LC-6, LC-7 and LC-8 are shown in
Scheme 1. All solvents and starting materials were commercially
available and used without further purification (unless specially
mentioned), and all reactions dealing with air- or moisture-
sensitive compounds were carried out under a nitrogen atmo-
sphere using standard Schlenk techniques. Silica gel for column
chromatography was 300e400 mesh. The fluorine-doped tin oxide
(FTO) conducting glass used was washed successively with deter-
gent solution, deionized water and ethanol under ultrasonication.
2.2. Measurement
2.3. Synthesis
1H NMR spectra were recorded on a Bruker AV400 MHz spec-
trometer in CDCl3, DMSO or THF-d8 with tetramethylsilane as the
internal standard. MALDI-TOF-MS was obtained with a Bruker
Autoflex Tof/Tof III instrument. The UVeVis spectra of dyes in THF
solution (3 ꢁ 10ꢀ6 M) were measured using Shimadzu UV-1800 in
10 mm quartz cell spectrophotometer. Cyclic voltammetry (CV)
measurements were carried out with a Chenhua CHI600E electro-
chemical analyzer at a scan rate of 50 mV/s at room temperature: a
glassy carbon electrode, a Pt electrode and an Ag/Agþ electrode
were used as the working electrode, counter electrode and refer-
ence electrode respectively; Fc/Fcþ (ferrocene/ferrocenium) redox
couple was used as an internal potential reference, and 0.1 M tet-
rabutylammonium perchlorate (TBAP) in anhydrous THF for the
supporting electrolyte.
Compound 2: 4,7-dibromobenzoselenadiazole (340 mg,
1.00 mmol), Pd(PPh3)4 (100 mg, 0.087 mmol), 4-(diphenylamino)
phenylboronic acid (288 mg, 1.00 mmol) and K2CO3 (1.00 g,
7.25 mmol) in toluene (40 mL) and H2O (5 mL) were heated at
reflux for 12 h under a nitrogen atmosphere. After cooling to room
temperature, the mixture was extracted with CH2Cl2 (3 ꢁ 50 mL).
The organic portion was combined and dried over MgSO4, and
volatile components were removed by rotary evaporation. The
residue was purified by column chromatography on silica gel
(CH2Cl2/petroleum ether ¼ 1:2, v/v) to give 2 as an orange solid
(200 mg). Yield: 41.6%. 1H NMR (400 MHz, CDCl3)
d 7.83 (d,
J ¼ 7.5 Hz, 1H), 7.73 (d, J ¼ 8.7 Hz, 2H), 7.39 (d, J ¼ 7.5 Hz, 1H), 7.33 (t,
J ¼ 7.6 Hz, 4H), 7.19 (dd, J ¼ 8.5, 1.9 Hz, 6H), 7.08 (t, J ¼ 7.3 Hz, 2H).
MALDI-TOF-MS: m/z calcd for C24H16BrN3Se, 504.97; found, 504.83
[Mþ].
Commercial FTO (7
with ~8 m thick transparent layer (DSL 18NR-T, ~20 nm, Dyesol,
Australia) and ~4 m thick scattering layer (WER2-O, 150e400 nm
U
cmꢀ2, Hartford Glass, USA) was coated
m
Compound 3: Compound 2 (110 mg, 0.22 mmol), Pd(PPh3)4
(100 mg, 0.087 mmol), 2-formylthiophene-5-boronic acid (31 mg,
0.22 mmol) and K2CO3 (1.00 g, 7.25 mmol) in THF (30 mL) and H2O
(5 mL) were heated at reflux for 12 h. After cooling to room tem-
perature, the mixture was extracted with CH2Cl2 (3 ꢁ 30 mL). The
organic portion was combined and dried over MgSO4, and volatile
m
Dyesol, Australia) used as working electrodes. After the films were
air dried for about 30 min, the samples were put into an oven for
sintering using a three-stage ramp-up scheme (8 ꢂC minꢀ1, 200 ꢂ
C
for 15 min, 350 ꢂC for 15 min, and 550 ꢂC for 30 min) in ambient air.
Fig. 1. Molecular structure of three organic sensitizers.

F. Lu et al. / Dyes and Pigments 141 (2017) 161e168
163
Scheme 1. The synthetic routes for the three dyes. Reaction conditions: i) Pd(PPh3)4, 2 M K2CO3 aqueous solution, toluene, Ar, 115 ꢂC; ii) 2-formylthiophene-5-boronic acid,
Pd(PPh3)4, 2 M K2CO3 aqueous solution, THF, Ar, 80 ꢂC; iii) cyanoacetic acid, ammonium acetate, acetic acid, Ar, 120 ꢂC; iv) 2-formylphenyl-5-boronic acid, Pd(PPh3)4, 2 M K2CO3
aqueous solution, THF, Ar, 80 ꢂC.
components were removed by rotary evaporation. The residue was
purified by column chromatography on silica gel (CH2Cl2/petro-
leum ether ¼ 2:1, v/v) to give 3 as red solid (54 mg). Yield: 46%. 1H
(d, J ¼ 8.9 Hz, 4H), 3.97 (t, J ¼ 6.5 Hz, 4H), 1.97e1.75 (m, 4H),
1.55e1.42 (m, 4H), 1.43e1.17 (m, 16H), 0.91 (t, J ¼ 6.7 Hz, 6H).
MALDI-TOF-MS: m/z calcd for C45H51N3O3SSe, 793.28; found,
793.01.
Compound LC-7: The synthesis method resembles that of
compound LC-6, but uses compound 6 as reagent; the compound
was purified by column chromatography on silica gel (CH2Cl2/
CH3OH ¼ 1:10, v/v) to give LC-7 a purple solid. Yield: 83%. Melting
point: 238e239 ꢂC. IR (KBr, cmꢀ1): 3428, 3037, 2924, 2853, 2224,
1699, 1592, 1502, 1468, 1421, 1320, 1280, 1237, 1187, 1029, 826, 768,
NMR (400 MHz, CDCl3)
d
10.00 (s, 1H), 8.12 (d, J ¼ 3.9 Hz, 1H), 8.00
(d, J ¼ 7.4 Hz, 1H), 7.85 (dd, J ¼ 10.0, 6.2 Hz, 3H), 7.63 (s, 1H), 7.33 (t,
J ¼ 7.6 Hz, 4H), 7.21 (d, J ¼ 6.6 Hz, 6H), 7.10 (s, 2H). MALDI-TOF-MS:
m/z calcd for C29H19N3OSe, 537.04; found, 537.07 [Mþ].
Compound LC-6: A mixture of compound 3 (54 mg, 0.1 mmol)
with cyanoacetic acid (85 mg, 1 mmol) in acetic acid (15 mL) was
heated at reflux in the presence of ammonium acetate (150 mg) for
12 h under argon. After cooling the solution, water was added to
quench the reaction. The precipitate was filtered and washed with
water. The residue was purified by column chromatography on
silica gel (CH2Cl2/ethanol ¼ 10:1, v/v) to give a LC-6 red solid
(36 mg). Yield: 60%. Melting point: 276e278 ꢂC. IR (KBr, cmꢀ1):
3429, 3031, 2217, 1682, 1584, 1562, 1492, 1415, 1382, 1322, 1247,
1196, 1108, 1082, 828, 756, 696, 510.1H NMR (400 MHz, DMSO)
713, 526.1H NMR (400 MHz, THF)
d
8.52 (s, 1H), 8.23 (t, J ¼ 5.4 Hz,
2H), 8.09 (d, J ¼ 4.0 Hz, 1H), 7.84 (d, J ¼ 8.6 Hz, 2H), 7.69 (d,
J ¼ 7.5 Hz, 1H), 7.08 (d, J ¼ 8.7 Hz, 4H), 6.93 (d, J ¼ 8.8 Hz, 4H), 6.86
(d, J ¼ 8.6 Hz, 2H), 3.94 (t, J ¼ 6.3 Hz, 4H), 1.75e1.61 (m, 4H),
1.53e1.37 (m, 4H), 1.35e1.15 (m, 16H), 0.85 (t, J ¼ 6.1 Hz, 6H). 13
C
NMR (101 MHz, THF)
d 173.46, 163.16, 159.02, 157.88, 156.09,
149.23,149.18, 145.53, 140.28, 137.34, 136.91, 135.99, 130.26, 129.04,
127.55, 127.34, 126.85, 125.87, 125.04, 119.01, 115.74, 115.10, 98.90,
67.83, 31.86, 29.62, 29.37, 29.27, 26.10, 22.57, 13.46. MALDI-TOF-MS:
m/z calcd for C48H52BrN4O4SSe, 860.29; found, 860.98 [Mþ]. HRMS-
ESI(m/z): calcd for [M-COOH]-, C47H51N4O2SSeꢀ 815.2903; found
815.2950.
d
8.53 (s, 1H), 8.28e8.22 (m, 2H), 8.10 (d, J ¼ 4.2 Hz, 1H), 7.92 (d,
J ¼ 8.6 Hz, 2H), 7.75 (d, J ¼ 7.5 Hz, 1H), 7.37 (t, J ¼ 7.8 Hz, 4H),
7.16e7.03 (m, 8H). 13C NMR (101 MHz, DMSO)
174.56, 167.29,
d
164.24, 158.60, 157.54, 147.96, 147.74, 147.30, 146.04, 145.35, 143.29,
143.12, 138.57, 137.57, 135.18, 132.20, 131.92, 131.29, 131.06, 130.08,
129.09, 128.03, 127.24, 125.53, 124.96, 124.02, 122.44, 118.96.
MALDI-TOF-MS: m/z calcd for C32H20N4O2Se, 604.05; found, 604.09
[Mþ]. HRMS-ESI(m/z): calcd for [MꢀCOOH]-, C31H19N4SSeꢀ
559.0501; found 559.0520.
Compound 5: The synthesis method resembles that of com-
pound 2, but uses compound 4 as reagent; the compound was
purified by column chromatography on silica gel (CH2Cl2/petro-
leum ether ¼ 1:2, v/v) to give 5 a red solid. Yield: 37%. 1H NMR
Compound 7: The synthesis method resembles that of com-
pound 3, but uses compound 5 and 2-formylphenyl-5-boronic acid
as reagents; The compound was purified by column chromatog-
raphy on silica gel (CH2Cl2/petroleum ether ¼ 2:1, v/v) to give 8 a
red solid. Yield: 54%.1H NMR (500 MHz, CDCl3)
d 10.11 (s, 1H), 8.08
(d, J ¼ 8.2 Hz, 2H), 8.04 (d, J ¼ 8.3 Hz, 2H), 7.75 (d, J ¼ 8.4 Hz, 2H),
7.69 (d, J ¼ 7.2 Hz, 1H), 7.60 (d, J ¼ 7.2 Hz,1H), 7.14 (d, J ¼ 7.9 Hz, 4H),
7.05 (d, J ¼ 8.1 Hz, 2H), 6.86 (d, J ¼ 8.8 Hz, 4H), 3.95 (t, J ¼ 6.3 Hz,
4H), 1.87e1.70 (m, 4H), 1.52e1.42 (m, 4H), 1.41e1.28 (m, 16H), 0.90
(t, J ¼ 6.8 Hz, 6H). MALDI-TOF-MS: m/z calcd for C47H53N3O3Se,
787.33; found, 787.05 [Mþ].
(400 MHz, CDCl3)
d
7.82 (d, J ¼ 7.5 Hz, 1H), 7.68 (d, J ¼ 8.7 Hz, 2H),
7.36 (d, J ¼ 7.5 Hz,1H), 7.14 (d, J ¼ 8.9 Hz, 4H), 7.03 (d, J ¼ 8.7 Hz, 2H),
6.87 (d, J ¼ 8.9 Hz, 4H), 3.96 (t, J ¼ 6.5 Hz, 4H), 1.89e1.71 (m, 4H),
1.58e1.04 (m, 16H), 1.01e0.82 (m, 6H). MALDI-TOF-MS: m/z calcd
for C40H48BrN3O2Se, 761.21; found, 761.92 [Mþ].
Compound LC-8: The synthesis method resembles that of
compound LC-6, but uses compound 8 as reagent; the compound
was purified by column chromatography on silica gel (CH2Cl2/
CH3OH ¼ 1:10, v/v) to give LC-8 as a red solid. Yield: 78%. Melting
point: 140e142 ꢂC. IR (KBr, cmꢀ1): 3438, 3036, 2922, 2852, 2220,
1682, 1568,1501,1469,1415,1323,1286,1240,1202,1104,1044, 820,
Compound 6: The synthesis method resembles that of com-
pound 3, but uses compound 5 as reagent; the compound was
purified by column chromatography on silica gel (CH2Cl2/petro-
leum ether ¼ 2:1, v/v) to give 6 a purple solid. Yield: 48%. [Mþ].1H
NMR (400 MHz, CDCl3)
d
9.99 (s, 1H), 8.10 (d, J ¼ 4.0 Hz, 1H), 7.97 (d,
767, 725, 530.1H NMR (400 MHz, THF)
d 8.31 (s, 1H), 8.18 (q,
J ¼ 7.4 Hz,1H), 7.84 (d, J ¼ 4.0 Hz,1H), 7.77 (d, J ¼ 8.7 Hz, 2H), 7.58 (d,
J ¼ 7.4 Hz, 1H), 7.15 (d, J ¼ 8.9 Hz, 4H), 7.05 (d, J ¼ 8.7 Hz, 2H), 6.88
J ¼ 8.7 Hz, 4H), 7.84 (d, J ¼ 8.8 Hz, 2H), 7.78 (d, J ¼ 7.3 Hz, 1H), 7.64
(d, J ¼ 7.3 Hz, 1H), 7.08 (d, J ¼ 8.9 Hz, 4H), 6.97 (d, J ¼ 8.8 Hz, 2H),

164
F. Lu et al. / Dyes and Pigments 141 (2017) 161e168
6.85 (d, J ¼ 8.9 Hz, 4H), 3.94 (t, J ¼ 6.4 Hz, 4H), 1.84e1.73 (m, 4H),
the maximum absorption band red-shifted by 19 nm and the molar
extinction coefficient is increased. The reason behind the absorp-
tion red-shift is that the octyloxy chains can increase the p-
1.56e1.43 (m, 4H), 1.43e1.30 (m, 16H), 0.90 (t, J ¼ 6.8 Hz, 6H). 13
C
NMR (101 MHz, THF)
d
161.12, 157.74, 157.39, 154.16, 151.18, 147.20,
140.82,138.55,133.81,129.95,129.35,128.75,128.32,128.09,127.56,
127.16, 124.93, 124.40, 117.29, 113.62, 113.24, 101.73, 65.97, 30.00,
27.78, 27.52, 27.41, 24.24, 20.71, 11.61. MALDI-TOF-MS: m/z calcd for
conjugation length via enhanced electron delocalization over the
entire electron-donating moiety, which results in the lower band
gap and red-shifted absorption spectra [44,45]. On the contrary, the
34 nm hypochromic shift of LC-8 compared to LC-7 is mainly
caused by the higher tendency of thiophene compared to benzene
to form quinoid structure, which is in favor of electronic resonance
[46,47]. The absorption spectra of the three sensitizers adsorbed on
the TiO2 film also display two major absorption bands, but the
bands are significantly broadened and red-shifted when compared
with the corresponding solution spectra, indicating the formation
of J-aggregation of the dyes on the TiO2 surface [48] (see Fig. 3).
C
50H5rN4O4Se, 854.33; found, 853.99 [Mþ]. HRMS-ESI(m/z): calcd
for [M-COOH]-, C49H53N4O2Seꢀ 809.3339; found 809.3430.
3. Results and discussion
3.1. Synthesis
The synthesis of the three organic sensitizers is depicted in
Scheme 1. The synthesis of the three target sensitizers started from
2,5-dibromobenzoselenadiazole which was firstly converted to
compounds 2 and 5 via stepwise Suzuki coupling with either 4-
(diphenylamino)phenylboronic acid (1) or 4-(octyloxy)-N-(4-
(octyloxy)phenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl)aniline (4), respectively. Further Suzuki coupling with
either 2-formylthiophene-5-boronic acid or 4-formybenzene-5-
3.3. Electrochemical properties
The electronic properties of the three organic sensitizers were
investigated by cyclic voltammery (CV). The corresponding data are
listed in Table 1 and Fig. S10. All the three organic sensitizers show a
quasi-reversible one-electron oxidation couple which can be
ascribed to the oxidation of the triphenylamine. The redox poten-
tials (Eox) of LC-6, LC-7 and LC-8 corresponding to the HOMO en-
ergy level are located at 1.16, 0.90 and 0.91 V, respectively. The
HOMO level of LC-6 is higher than that of LC-7 and LC-8 due to the
stronger electron-donating ability as the incorporation of the
alkoxy chains in the latter. As well known, the HOMO level lies
predominantly on the donor moiety of the molecule, so that LC-7
and LC-8 containing the same donor unit have the similar HOMO
levels. The values of HOMO levels are all more positive than the
energy level of Iꢀ/I3ꢀ redox (0.4 V vs NHE) [49] guaranteeing the dye
regeneration. The E0-0 (zero-zero transition energies) of the three
sensitizers are 2.04, 1.97 and 2.13 V, respectively, which are calcu-
lated from the onset wavelength of the absorption spectra. The
estimated excited-state potential corresponding to the LUMO levels
calculated from EHOMO to E0-0, are ꢀ0.88, ꢀ1.07 and ꢀ1.22 V,
respectively, which are more negative than the conduction band
edge energy level of the TiO2 semiconductor (ꢀ0.5 V vs NHE) [50],
ensuring the efficient driving force for electron injection from the
excited state of the dyes into the TiO2 electrode.
boronic acid introduced the
p-spacer. The precursors 3, 6 and 7
were then converted to the final sensitizers LC-6, LC-7 and LC-8 by
Knoevenagel condensation with cyanoacetic acid. Compared to LC-
6, octyloxy substituted triphenylamino group in LC-7 and LC-8 as
the electron donor not only provided stronger electron donating
capability, resulting in the enhancement of the light harvesting
capacity, but also form an aliphatic network, which increases the
distance between TiO2 surface and the electrolyte, thus suppressing
the adverse charge recombination between photo-injected elec-
trons and Iꢀ3 /I2, which may improve the Voc [42].
3.2. Photophysical properties
The absorption spectra of LC-6, LC-7 and LC-8 in EtOH and on
the TiO2 film are shown in Fig. 2. The corresponding data are
summarized in Table 1. The UVevis spectra of the three sensitizers
exhibit two major absorption bands: the former at 300e400 nm
can be ascribed to the
p-p* transition from the triphenylamine to
the BSD moiety, and the latter at 450e550 nm is attributed to the
intramolecular charge transfer (ICT) from the eletron-donating unit
of triphenylamine to the cyanoacetic acid acceptor unit [43]. A
comparison of the absorption spectra of LC-6 and LC-7 suggested
that upon introducing the octyloxy chains in the triphenylamine,
3.4. Theoretical approach
Density functional theory (DFT) calculations were carried out to
obtain further insight into the geometrical configuration and
electron distribution of the frontier orbitals of the three dyes.
Structural optimizations and subsequent frequency calculations for
LC-6, LC-7 and LC-8 were carried out by using the B3LYP functional
with the 6-311G* basis set as implemented in the Gaussian 09 suite
of programs [51]. The polarizable continuum model (PCM) was
employed in the optimization with THF as the solvent. All the
octyloxy groups have been replaced with methoxy groups to save
calculation time. As shown in the optimized conformation
(Fig. S11), the dihedral angles between the triphenylamine and the
additional acceptor BSD for LC-6, LC-7 and LC-8 are 33.76ꢂ, 32.73ꢂ
and 34.59ꢂ, respectively. The dihedral angles between the BSD unit
and the neighboring thiophene and phenyl ring are 1.13ꢂ, 0.97ꢂ and
34.28ꢂ, respectively. The relative smaller torsion angle for
thiophene-bridged LC-6 and LC-7 ensures a good molecular co-
planarity to keep high electronic communication with respect to
phenylene-bridged LC-8, leading to the larger red-shift in absorp-
tion band in solution and the additional absorption band at around
400 nm.
6.0
5.5
LC-6
LC-7
LC-8
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Figs. 4 and 5 show the calculated frontier molecular orbital and
their corresponding energies and the HOMO and LUMO gaps of the
Fig. 2. UVevisible absorption spectra of the three dyes in THF (3.0 ꢁ 10ꢀ6 M).

F. Lu et al. / Dyes and Pigments 141 (2017) 161e168
165
Table 1
Photophysical properties of the three sensitizers.
Dye
lmax (nm (ε/103 Mꢀ1 cmꢀ1))a
lmax/nmb
HOMO (V)c
E0-0(V)d
LUMO (V)e
LC-6
LC-7
LC-8
392(12,000),503(13,733)
389 (24,000),522(22,333)
332(55,000),488(19,000)
517
552
495
1.16
0.90
0.91
2.04
1.97
2.13
ꢀ0.88
ꢀ1.07
ꢀ1.22
a
Absorption data were obtained in THF solution (3 ꢁ 10ꢀ6 M).
Absorption maximum on TiO2 film.
b
c
HOMO potentials measured vs Fcþ/Fc were converted to those vs. the normal hydrogen electrode (NHE) by addition of þ0.63 V.
E0-0 values were calculated from the onset wavelength of the absorption spectra in solution.
The LUMO was calculated with the expression of LUMO ¼ HOMO ꢀ E0-0.
d
e
facilitates the oxidation. When the thiophene spacer was replaced
by phenylene, the LUMO energy level and the band gap of LC-8
were both increased. As a consequence, LC-7 exhibits the lowest
HOMO-LUMO energy gap (1.86 eV) among all dyes, resulting in the
red shift of the absorption spectra. The simulated absorption
spectra based on DFT calculation (Fig. S12) show qualitative
agreement with experimental results, albeit the excitation energies
are overestimated. The corresponding data are summarized in
Table S1. The results suggest that the former band at 300e400 nm
can be ascribed to the local excitation of the organic dye, and the
latter at 450e550 nm is attributed to the S0 / S1 excitation.
1.0
0.8
0.6
0.4
0.2
0.0
LC-6
LC-7
LC-8
3.5. Photovoltaic performance of DSSCs
The photovoltaic performance of DSSCs were measured under
standard AM 1.5 irradiation (100 mW cmꢀ2) and the detailed pa-
rameters of short-circuit current density (Jsc), open-circuit voltage
(Voc), fill factor (FF) and power conversion efficiency (PCE) are listed
in Table 2. The photocurrent density-voltage (J-V) curves are dis-
played in Fig. 6. The DSSC based on LC-6 exhibited photovoltaic
performance with a Jsc of 7.87 mA cmꢀ2, a Voc of 585 mV, and a FF of
0.72, corresponding to a PCE of 3.33%. Upon incorporation of the
alkoxy chains in the triphenylamine, the resultant LC-7-based cell
showed an improvement of Voc (598 mV) and finally gave a higher
PCE of 3.46% with a FF of 0.74. A plausible explanation is that the
two octyloxy chains substituted on the triphenylamine of LC-7 are
beneficial to form a blocking layer which limits the electron
recombination, and hence increases the electron lifetime and the
Voc. Notably, when the thiophene-bridge of LC-7 was replaced by
benzene, the LC-8-based DSSC displayed a higher PCE of 4.11% with
an increased Jsc (8.59 mA cmꢀ2) and Voc (687 mV). It should be
pointed out that less planar skeleton of benzene can effectively
suppress the dark current and reduce the electron recombination,
resulting in a higher Voc [52]. The measured dye loading values for
400
500
600
700
Wavelength (nm)
Fig. 3. UVeVisible absorption spectra of the TiO2 films.
LC-6, LC-7 and LC-8 are close (0.94
ꢁ
10ꢀ7 mol cm2,
0.82 ꢁ 10ꢀ7 mol cm2 vs 0.88 ꢁ 10ꢀ7 mol cm2), indicating that the
improved Jsc for LP-8 is not resulted from the dye loading amount. It
is believed that one reason might be that the replacement of ben-
zene has the more negative LUMO energy level than the conduction
band of TiO2, providing greater driving force for charge injection.
Furthermore, as we know, sensitizer aggregation at the TiO2 surface
could induce the excited state quenching, and thus the less planar
conformation of LC-8 will ease the aggregation, leading to more
efficient electron injection.
Fig. 4. Energy level (eV) of HOMO and LUMO molecular orbitals for the three
sensitizers.
three organic dyes LC-6, LC-7 and LC-8. It is found that the electron
density is predominately distributed at the triphenylamine unit for
the HOMO orbital, while for the LUMO orbital, electrons are located
on the BSD and cyanoacrylate units. Here, the electrons can suc-
cessively transfer from the donor to BSD, then transfer to the an-
chor (cyanoacetic acid) and finally inject into TiO2 film.
Furthermore, attaching the octyloxy groups significantly extends
In order to prevent the unfavorable aggregation of dye molec-
ulars and retard charge recombination, a common and effective
strategy is to use CDCA as coadsorbent due to its nonplanar and
bulky configuration. In this work, 20 mM CDCA were added during
dye soaking of the three sensitizers. From Table 2, we can find that
both the Jsc and Voc values increased, indicating that CDCA had
successfully disrupt the aggregation and retard charge recombi-
nation between TiO2 and electrolyte. The best PCE have increased to
the p-conjugation of the donor, which arises the HOMO energy and

166
F. Lu et al. / Dyes and Pigments 141 (2017) 161e168
Fig. 5. Molecular orbital distributions and energy levels of the three dyes.
Table 2
Photovoltaic parameters of porphyrin-sensitized solar cells.
Dye
Jsc/mA cm2
Voc/mV
FF
h/%
LC-6
7.87
9.98
7.84
10.82
8.59
585
620
598
628
687
734
770
0.72
0.74
0.74
0.73
0.69
0.69
0.67
3.33
4.59
3.46
4.99
4.11
6.72
7.65
LC-6 þ CDCA
LC-7
LC-7 þ CDCA
LC-8
LC-8 þ CDCA
13.21
14.89
N719a
a
As
a reference, the overall efficiency of N719 sensitized solar cells was
determined.
Fig. 7. Typical action spectra of IPCE obtained for solar cells sensitized by the three
dyes.
300e800 nm for LC-7 and 300e700 nm for LC-8, respectively.
Unfortunately, even though the IPCE response of LC-7 is the
broadest, IPCE values are the lowest with the plateau no more than
50%, which is consistent with the lowest Jsc. The IPCE values of LC-8
are much higher at the range of 400e600 nm and have reached
over 60% at 520 nm, which could explain the corresponding higher
Jsc value for LC-8-based cell in spite of the narrower IPCE response
compared to LC-6 and LC-7. Interestingly, the IPCE values of the
three dyes with CDCA coabsorbent are much improved compared
to those without CDCA, which indicated that the aggregation was
indeed suppressed, thus resulting in more charge collection and the
greater Jsc. The IPCE values of the cell based on LC-8 with CDCA
achieved over 70% from 350 to 600 nm with the highest IPCE value
of 82% at 520 nm, which ensured the highest Jsc values among all
the three dye-based cells.
Fig. 6. J-V curves of DSSCs based on the three dyes.
6.72% based on LC-8 sensitized cell, with a Jsc of 13.21 mA cmꢀ2, a
Voc of 734 mV, and a FF of 0.69, reaching about 87% of the value of
that for an N719-based cell fabricated and measured under the
same condition.
The incident photon-to-current conversion efficiency (IPCE) as a
function of incident wavelength for DSSCs based on LC-6, LC-7 and
LC-8 with or without CDCA are shown in Fig. 7. It can be clearly seen
that the IPCE responses are in the range of 300e730 nm for LC-6,
Electrochemical impedance spectroscopy (EIS) of DSSCs were
obtained under ꢀ0.6 V in the dark to investigate the electron
recombination. The Nyquist plots for DSSCs based on LC-6, LC-7
and LC-8 are shown in Fig. 8 and the Bode plots shown in Fig. 9. The
larger semicircle in the Nyquist plots, indicates the larger resistance
of charge transfer (Rct) from TiO2 to the electrolyte. As we know, a

F. Lu et al. / Dyes and Pigments 141 (2017) 161e168
167
4. Conclusions
In summary, three new D-A-p-A organic dyes with triphenyl-
amine/octyltriphenylamine as donor, either thiophene or 1,4-
phenylene as the linker, benzoselenadiazole (BSD) and cyanoa-
crylic acid as the acceptor, were successfully designed and syn-
thesized. The influence of the variation of the donor and the
p
spacer on the optical and electrochemical properties of these dyes
as well as the photovoltaic performance of the corresponding solar
cells is systematically investigated. The results showed that
attaching the octyloxy chain to the donor is proved to be an
effective strategy to suppress the adverse charge recombination
and improve the Voc values. Moreover, the use of phenyl
p-spacer
rather than thiophene moiety and adding CDCA coadsorbent could
help to improve the Jsc and Voc values of the cells. Hence, the LC-8
based DSSC with CDCA coadsorbent gave the best photovoltaic
performance with a Jsc of 13.21 mA cmꢀ2, a Voc of 734 mV, a FF of
0.69 and an overall PCE of 6.72%. These results reveal that BSD can
be a promising electron-withdrawing group and thus an additional
Fig. 8. Nyquist plots of electrochemical impedance spectra measured at a forward bias
of ꢀ0.6 V under dark conditions for the DSSCs.
acceptor to construct effective D-A-p-A type organic sensitizers for
use in DSSCs.
Acknowledgment
This work was supported by National Natural Science Founda-
tion of China (No. 21476162). The acknowledgment is also extended
to International S&T Cooperation Project of China (No.
2012DFG41980, 2016YFE0114900).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.02.013.
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