Structure-property relationship of different electron donors: New organic sensitizers based on bithiazole moiety for high efficiency dye-sensitized solar cells

Article abstract of DOI:10.1039/c3ra00181d

Three metal-free bithiazole organic dyes (BTT-I-III) based on D-A-π-A building blocks were designed and synthesized for dye-sensitized solar cells (DSSCs) to study the influence of different electron donors on photovoltaic properties, in which the electro

Full text of DOI:10.1039/c3ra00181d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Structure-property relationship of different electron
donors: new organic sensitizers based on bithiazole
moiety for high efficiency dye-sensitized solar cells
Cite this: RSC Advances, 2013, 3,
15900
Fuling Guo, Jinxiang He, Sanying Qu, Jing Li,* Qiong Zhang, Wenjun Wu
and Jianli Hua*
Three metal-free bithiazole organic dyes (BTT-I–III) based on D–A-p-A building blocks were designed and
synthesized for dye-sensitized solar cells (DSSCs) to study the influence of different electron donors on
photovoltaic properties, in which the electron donors of BTT-I–III were carbazole, triphenylamine and
indoline moieties, respectively. The UV/Vis absorption spectra of BTT-III containing indoline as electron-
donor displayed red-shifted absorption compared to the other two dyes with an onset close to 700 nm.
The incident photon-to-current conversion efficiency (IPCE) spectra of BTT-III showed a wide region and
kept a value higher than 10–15% during 580–650 nm. Electrochemical measurement data indicated that
the HOMO and LUMO energy levels could be tuned through introducing different electron-donors in the
dye molecule. It was found that the overall conversion efficiency of indoline donor based dye BTT-III
showed the highest efficiency of 7.86% under AM 1.5 irradiation (100 mW cm22). The electron lifetime
calculated from electrochemical impedance spectroscopy (EIS) measurement demonstrated the reduced
charge recombination and the higher open-circuit voltage.
Received 14th January 2013,
Accepted 19th June 2013
DOI: 10.1039/c3ra00181d
www.rsc.org/advances
10%, which indicates that metal-free organic dyes are a
favored type of higher sensitizer for DSSCs.12
Introduction
In recent decades, organic solar cells have been developed
extensively. Among these devices, dye-sensitized solar cells
(DSSCs) have attracted increasing attention because of their
low cost, easy synthesis and relatively high efficiency. The
sensitizers are a very important part of DSSCs which harvest
sunlight and inject electrons into the semiconductor. At
present, DSSCs based on RuII-polypyridyl complexes1 have
achieved power conversion efficiencies of almost 11% at
standard global air mass (AM) 1.5.2 However, the large-scale
application of Ru-complexes in DSSCs has been limited due to
the shortage of Ru-resources and high cost of purification. In
comparison with the metal complexes, metal-free organic
dyes, such as cyanine,3 merocyanine,4 coumarin,5 indoline,6
hemicyanine,7 phenothiazine,8 porphyrin,9 phenoxazine10 and
diketopyrrolopyrrole et al. dyes11 have higher molar extinction
coefficients and can be prepared and purified at lower costs.
These advantages indicate that metal-free organic dyes are
suitable substitutes for large-scale application in DSSCs. Great
development has been made during these years. The highest
overall photo-voltage conversion efficiency has been above
Most efficient metal-free organic sensitizers are constructed
with donor-p bridge-acceptor (D-p-A) systems. The introduc-
tion of different p-conjugated units in dye molecules between
the donor and acceptor can expand the absorption spectrum
and red-shift to longer wavelengths. However, the stability of
these dyes is reduced by extending the p conjugated bonding
bridges. To overcome this defect, some electron-withdrawing
moieties such as quinoline,13 isoxazole,14 thiazole moieties15
and pyrimidine ring16 were reported to be introduced into the
molecular to form the D–A-p-A configuration. It was found that
these systems showed more red-shifted absorption and higher
stability than their analogues. The bithiazole unit is a well-
known electron deficient unit and has been widely applied in
thin film transistors, photovoltaic cells, and light-emitting
diodes. In our previous studies, we have identified that small
molecule DSSCs based on bithiazole sensitizers with the
construction of D–A-p-A configuration could achieve high
performance and long stability.17 The two long alkyl chains
attached on the thiazole moiety suppressed charge recombina-
tion and dye aggregation successfully, resulting in high open-
circuit voltage.
As reported, the donor group can not only affect the
absorption spectrum but also adjust the HOMO and LUMO
energy levels. Different electron donors can lead to diverse
positions of the HOMO and LUMO energy levels resulting in
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science and Technology, Shanghai, P.R. China.
E-mail: jlhua@ecust.edu.cn; lijhy@ecust.edu.cn; Fax: +86 21-64252758; Tel: +86
21-64252756
15900 | RSC Adv., 2013, 3, 15900–15908
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
The voltage step and delay time of the photocurrent were 10
mV and 40 ms, respectively. Action spectra of the incident
monochromatic photon-to-electron conversion efficiency
(IPCE) for the solar cells were obtained with a Newport-
74125 system (Newport Instruments). The intensity of mono-
chromatic light was measured with a Si detector (Newport-
71640).
Materials
Dichloromethane (DCM) was dried over calcium hydroxide
and then distilled under normal pressure. Conductive glass
substrates (F: SnO2, transmission > 90% in the visible, sheet
resistance 15 V square21) were obtained from the Geao
Science and Educational Co. Ltd. of China. 1,2-dimethyl-3-n-
propylimidazolium iodide (DMPII) were purchased from
Aldrich. 4-tert-butylpyridine (TBP) and lithium iodide were
from Fluka and iodine (99.999%) was from Alfa Aesar. Starting
materials
5-(59-bromo-4,49-dihexyl-2,29-bithiazol-5-yl)thio-
phene-2-carbaldehyde (Compound 1) was pre-pared according
to the literature method.17 All other solvents and chemicals
used in this work were of reagent grade and used without
further purification unless other-wise noted.
Fig. 1 The molecular structures of the dyes BTT-I–III.
Device fabrication
The nanocrystal TiO2 films were fabricated according to the
published procedure.17 These nanocrystal titania films were
treated with 40 mM TiCl4 aqueous solution at room
temperature overnight, following by sintering at 450 uC for
30 min. After cooling to 80 uC, these films absorbed the co-
absorption chenodeoxycholic acid (CDCA) for 2 h or 4 h in 10
mM CDCA in ethanol. After rinsing with ethanol and drying,
these films were dipped into a 3 6 1024 M solution of dyes
(BTT-I–III) in dichloromethane for 16 h at room temperature.
After this, the electrodes were rinsed with CH2Cl2 and ethanol
and then dried. The size of the electrodes was 0.25 cm2. The
Pt-counter electrodes were prepared following the literature.17
For assembly of DSSCs as a sandwich type, the TiO2 photo
electrodes and Pt-counter electrodes were sealed with a hot-
melt gasket of 45 mm thickness. The electrolyte of liquid-state
DSSCs was 0.1 M LiI, 0.05 M I2, 0.6 M DMPII, and 0.5 M TBP in
acetonitrile.
different driving forces for electron injection and tune short-
circuit photocurrent density (Jsc). The indoline units, which
have been reported in organic sensitizers, showed more
powerful donor abilities,18 red-shifted absorption spectrum
and less positive HOMO levels compared to traditional
triphenylamine donors. To enrich the research field of
bithiazole dyes for DSSCs and gain more information on the
structure-property relationships, we have synthesized two new
bithiazole-based sensitizers (BTT-I and BTT-III) in which the
carbazole and indoline units are acting as electron donors,
thiophene as a p spacer and cyanoacrylic acid as the acceptor
moiety. We have also synthesized the corresponding bithiazole
dye BTT-II (Fig. 1) for the purpose of comparison. All dyes were
fabricated for efficient DSSCs. Finally, the three sensitizers
were applied to the sensitization of nanocrystalline TiO2-based
solar cells. It was found that DSSCs based on BTT-III with an
indoline electron-donor exhibited the best overall conversion
efficiency of 7.86% under AM 1.5 irradiation (100 mW cm22).
The effect of different electron-donors on the photophysical
and electrochemical properties of the dyes and the solar cell
performance were demonstrated in detail.
Synthesis
5-(4,49-dihexyl-59-(9-octyl-9H-carbazol-3-yl)-2,29-bithiazol-5-
yl)thiophene-2-carbaldehyde (2). Compound 1 (190 mg, 0.36
mmol), Pd (PPh3)4 (42 mg, 0.036 mmol), and K2CO3 (5 mL, 2 M
aqueous solution) in 8 mL of THF were heated to 45 uC under
an argon atmosphere for 30 min. A solution of 9-octyl-
9H-carbazol-3-ylboronic acid (234 mg, 0.72 mmol) in cyclohex-
ane (5 mL) was added slowly, and the mixture was refluxed for
further 12 h. After cooling to room temperature, the mixture
was extracted with CH2Cl2. The combined organic layers were
washed with water and brine and dried with anhydrous
Na2SO4. The solvent was evaporated, and the residue was
purified by column chromatography on silica gel (PE/CH2Cl2 =
Experimental
Photovoltaic performance measurements
Photovoltaic measurements employed an AM 1.5 solar
simulator equipped with a 300 W xenon lamp (Model No.
91160, Oriel). The power of the simulated light was calibrated
to 100 mW cm22 by using a Newport Oriel PV reference cell
system (Model 91150 V). J–V curves were obtained by applying
an external bias to the cell and measuring the generated
photocurrent with a Keithley model 2400 digital source meter.
1
2/1–1/1, v/v) to give an orange solid (250 mg, yield: 95%). H
NMR (CDCl3, 400 MHz), d (ppm): 9.91 (s, 1H), 8.18 (s, 1H), 8.11
(d, J = 7.6 Hz, 1H), 7.73 (d, J = 4.0 Hz, 1H), 7.55 (d, J = 8.4 Hz,
1H), 7.51 (t, J = 7.6 Hz, 1H), 7.44 (t, J = 8.1 Hz, 2H), 7.27 (m,
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 15900–15908 | 15901

View Article Online
Paper
RSC Advances
2H), 4.32 (t, J = 7.2 Hz, 2H), 3.01 (t, J = 7.8 Hz, 2H), 2.90 (t, J =
7.6 Hz, 2H), 1.94–1.88 (m, 2H), 1.86–1.79 (m, 4H), 1.44–1.25
(m, 22H), 0.91–0.84 (m, 9H).
5-(4,49-dihexyl-59-(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclo-
penta[b]indol-7-yl)-2,29-bithiazol-5-yl)thiophene-2-carbalde-
hyde (4). The synthesis method resembles that of compound 2
synthesis method resembles that of BTT-I using compound 3
reaction with 2-cyanoacetic acid, and the compound was
purified by column chromatography on silica gel (CH2Cl2/
1
EtOH = 20/1, v/v) to yield red solid (133 mg, yield 82.6%). H
NMR (THF-d8, 400 MHz), d: 8.54 (s, 1H), 8.04 (d, J = 4.0 Hz,
1H), 7.62 (d, J = 3.2 Hz, 1H), 7.43 (d, J = 8.0 Hz, 2H), 7.36 (t, J =
8.0 Hz, 4H), 7.15–7.10 (m, 6H), 7.01 (d, J = 8.0 Hz, 2H), 2.98 (t, J
= 7.4 Hz, 2H), 2.80 (t, J = 7.4 Hz, 2H), 1.78–1.65 (m, 4H), 1.30–
1.23 (m, 12H), 0.85 (t, J = 6.8 Hz, 6H). 13C NMR (THF-d8, 100
MHz), 165.1, 162.0, 159.0, 158.7, 156.0, 150.6, 149.7, 147.5,
144.2, 140.6, 138.8, 137.6, 132.2, 131.6, 130.2, 128.6, 127.3,
126.9, 125.9, 124.6, 118.1, 117.9, 34.2, 34.0, 33.0, 32.0, 31.9,
31.8, 31.4, 30.4, 29.4, 27.9, 27.8, 24.9, 23.8, 15.8. HRMS (m/z):
[M + H]+ Calcd for C44H45N4O2S3, 757.2705; Found, 757.2714.
2-Cyano-3-(5-(4,49-dihexyl-59-(4-p-tolyl-1,2,3,3a,4,8b-hexa-
hydrocyclopenta[b]indol-7-yl)-2,29-bithiazol-5-yl)thiophen-2-
yl)acrylic acid (BTT-III). The synthesis method resembles that
of BTT-I using compound 4 reaction with 2-cyanoacetic acid,
and the compound was purified by column chromatography on
silica gel (CH2Cl2/EtOH = 20/1, v/v) to give a dark red solid (110
mg, yield: 85%) 1H NMR (THF-d8, 400 MHz), d: 8.34 (s, 1H), 7.65
(d, J = 4.0 Hz, 1H), 7.32 (s, 1H), 7.25 (d, J = 8.4 Hz, 1H), 7.16–7.06
(m, 3H), 7.04 (d, J = 8.4 Hz, 2H), 6.85 (d, J = 8.0 Hz, 1H), 4.78 (t, J
= 6.6 Hz, 1H), 4.15 (t, J = 7.0 Hz, 1H), 3.82 (t, J = 8.8 Hz, 1H), 3.01–
2.92 (m, 6H), 2.21 (s, 3H), 2.04–1.97 (m, 1H), 1.77–1.71 (m, 4H),
1.56–1.53 (m, 1H), 1.37–1.24 (m, 13H), 0.86 (t, J = 6.8 Hz, 6H).
13C NMR (THF-d8, 100 MHz), 159.7, 156.0, 155.7, 152.7, 148.3,
140.0, 137.8, 137.7, 136.6, 135.6, 131.5, 129.9, 129.6, 128.5,
127.6, 127.5, 126.0, 125.4, 120.8, 120.3, 116.1, 107.0, 69.1, 67.7,
53.9, 45.2, 35.1, 33.4, 31.7, 30.8, 29.6, 29.5, 29.1, 29.0, 27.0, 22.6,
22.5, 22.0, 19.9, 13.5. HRMS (m/z): [M + H]+ Calcd for
C44H49N4O2S3 761.3018, Found: 761.3014.
using
4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-
ylboronic acid reaction with compound 1, and the compound
was purified by column chromatography on silica gel (PE/
1
CH2Cl2 = 1/1, v/v) to give a red solid (150 mg, yield: 91%) H
NMR(CDCl3, 400 MHz), d (ppm): 9.91 (s, 1H), 7.73 (d, J = 4.0
Hz, 1H), 7.26 (d, J = 4.0 Hz, 1H), 7.22–7.16 (m, 5H), 7.12 (dd, J =
8.0 Hz, J = 1.2 Hz, 1H), 6.89 (d, J = 8.4 Hz, 1H), 4.84 (t, J = 6.6
Hz, 1H), 3.85 (t, J = 7.2 Hz, 1H), 3.74 (t, J = 6.2 Hz, 1H), 2.99 (t, J
= 7.8 Hz, 2H), 2.83 (t, J = 8.0 Hz, 2H), 2.34 (s, 3H), 2.12–2.03 (m,
1H), 1.95–1.87 (m, 2H), 1.83–1.76 (m, 4H), 1.63–1.59 (m, 1H),
1.45–1.29 (m, 13H), 0.91–0.86 (m, 6H).
2-cyano-3-(5-(4,49-dihexyl-59-(9-octyl-9H-carbazol-3-yl)-
2,29-bithiazol-5-yl)thiophen-2-yl)acrylic acid (BTT-I).
Compound 2 (100 mg, 0.138 mmol), 2-cyanoacetic acid (83
mg, 0.98 mmol) and ammonium acetate (200 mg) in acetic
acid (8 mL) at reflux for 6 h. After cooling the solution, the
mixture was poured into water to quench the reaction. The
precipitate was filtered and washed with water. The residue
was purified by column chromatography on silica gel (CH2Cl2
to CH2Cl2/EtOH = 20/1, v/v) to yield 83 mg of dark red solid
(yield 75.4%). 1H NMR (THF-d8, 400 MHz), d (ppm): 8.32 (s,
1H), 8.21 (d, J = 7.6 Hz, 1H), 7.80 (s, 1H), 7.69–7.63 (m, 2H),
7.56–7.48 (m, 4H), 7.23 (t, J = 7.2 Hz, 1H), 4.41 (t, J = 6.0 Hz,
2H), 2.96 (t, J = 7.4 Hz, 2H), 2.85 (t, J = 7.0 Hz, 2H), 1.76–1.71
(m, 6H), 1.30–1.17(m, 22H), 0.86–0.78(m, 9H).13C NMR(THF-
d8, 100 MHz), 165.2, 162.2, 161.7, 158.9, 146.5, 145.8, 143.4,
142.2, 142.1, 133.2, 131.7, 131.5, 128.8, 128.0, 127.2, 126.6,
126.4, 125.8, 125.7, 124.6, 114.6, 114.5, 84.2, 83.8, 83.5, 59.4,
48.2, 37.3, 37.2, 36.3, 35.2, 35.0, 34.9, 34.8, 34.7, 34.6, 34.5,
32.7, 28.1, 19.0. HRMS (m/z): [M + H]+ Calcd for C46H55N4O2S3
791.3487, Found: 791.3508.
Results and discussion
Synthesis
2-Cyano-3-(5-(59-(4-(diphenylamino)phenyl)-4,49-dihexyl-
2,29-bithiazol-5-yl)thiophen-2-yl)acrylic acid (BTT-II). The
The synthetic route to the bithiazole-based dyes (BTT-I–III)
containing carbazole, triphenylamine and indoline as elec-
Scheme 1 The synthesis of the dyes BTT-I–III.
15902 | RSC Adv., 2013, 3, 15900–15908
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
maxima in CH2Cl2 are at 456, 457, and 464 nm for BTT-I–III,
respectively. Compared to BTT-I and BTT-II, the dye BTT-III
exhibits absorption red-shifted for 8 and 7 nm, respectively.
Especially, the threshold wavelength of BTT-III is red-shifted
above 50 nm. This result reveals that the indoline donor is
beneficial to extend the light absorption spectrum in
comparison to carbazole and triphenylamine donors. The
dye BTT-I containing a weaker carbazole moiety as donor
exists slightly blue-shifted absorption compared with BTT-II.
The molar extinction coefficients (emax
) of BTT-I–III in
dichloromethane are 32 500, 34 400, and 28 100 M21 cm21
,
respectively. In comparison with conventional ruthenium
complexes (for example, 1.39 6 10 M21 cm21 for N719),19
the molar extinction coefficients of the dyes (BTT-I–III) are
obviously larger than that of N719, indicating that these dyes
have good light-harvesting ability. The greater maximum
absorption coefficients of the organic dyes allows for a
correspondingly thinner nano-crystalline film so as to avoid
the decrease in the film’s mechanical strength. This also
benefits the electrolyte diffusion in the film and reduces the
recombination possibility of the light-induced charges during
transportation.20
Fig. 2 The absorption spectra of BTT-I–III in dichloromethane.
tron-donor is depicted in Scheme 1. The hexyl chain on the
bithiazole moiety can improve the solubility and photo-voltage
performance through forming a tightly packed insulating
2
monolayer blocking I3 or cations approaching the TiO2. The
electron donors were connected to the intermediate molecule
In order to explain the experimental spectra and gain
insights into the structural properties of the chromophores,
density functional theory (DFT) calculations were performed
on a B3LYP/6-31G* with Gaussian 03. From Fig. 4, in the
ground state the electrons are distributed at the donor for the
dyes of BTT-II and BTT-III, obviously; but for BTT-I, the
electrons are distributed around the whole molecule in the
ground state, which is attributed to the weaker electron donor,
and not beneficial for the performance of DSSCs. Upon light
illumination, the electrons were exited to the LUMO orbitals,
which is delocalized across the accepter part (cyanide acid) for
all three dyes. On the other hand, the calculation absorption
maximums of these dyes BTT-I–III are 459.9, 467.6 and 481.5
nm, which is similar to the experimental absorption max-
imums. The weaker electron donor of carbazole presents a
blue-shifted absorption peak for BTT-I, and the stronger
electron donor of indoline decides the red-shifted absorption
for BTT-III.
1
through Suzuki coupling reaction, using 5-(59-bromo-
4,49-dihexyl-2,29-bithiazol-5-yl)thiophene-2-carbaldehyde with
9-octyl-9H-carbazol-3-ylboronic acid, 4-(di-phenylamino)phe-
nylboronic acid and 4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopen-
ta[b]indol-7-ylboronic acid to afford compounds 2–4,
respectively. In the next step, the target products (BTT-I–III)
were synthesized via the Knoevenagel condensation reaction of
aldehydes 2–4 with cyanoacetic acid in the presence of acetic
acid and ammonium acetate. All the intermediates and
compounds were characterized by 1H and 13CNMR spectro-
scopy and HRMS.
Absorption properties in solution and on TiO2 film
The UV/Vis absorption spectra in dichloromethane of the dyes
BTT-I–III are shown in Fig. 2 and the corresponding data are
summarized in Table 1. In the UV/Vis spectra, these three dyes
exhibit two major bands, appearing at 250–320 nm and 420–
550 nm, respectively. The absorption band at 250–320 nm is
ascribed to a localized aromatic p–p* transition typical of
carbazole, triphenylamine and indoline units, and the
prominent bands at 420–550 nm can be attributed to the
intramolecular charge transfer (ICT) between the donor and
the acceptor. For the charge-transfer band, the absorption
The absorption spectra of BTT-I–III on thin transparent TiO2
films (4 mm thickness) after 4 h adsorption in DCM solution
are shown in Fig. 3. The maximum absorption peaks for BTT-
I–III on the TiO2 film are located at 441, 433, and 436 nm,
respectively. The absorption peaks are blue-shifted by 15, 24,
and 28 nm, respectively, from the solution spectra. The blue
Table 1 The optical and electrochemical properties of dyes BTT-I–III and calculation absorption maximums
lamax/nm
lbmax/nm
Dye
(e 6 104 M21 cm21
)
Loadc(61028 mol cm22
)
E0-0e/eV
ED D21 d/V
ED*/D+f/V
lgmax/nm
+
BTT-I
BTT-II
BTT-III
308(1.79), 456(3.25)
300(2.39), 457(3.44)
464(2.81)
441(9.84)
433(8.43)
436(11.6)
2.25
2.42
2.08
1.14
1.06
0.68
21.11
21.36
21.40
459.5
467.6
481.5
a
b
c
Absorption maximum in CH2Cl2. Absorption maximum on TiO2 film. The dye loads are calculated from absorbance data of the sensitized
TiO2 electrodes.24 d ED D21 is measured by using an SCE as reference electrode and ferrocene/ferrocenium (Fc/Fc+) redox couple as an external
standard in dichloridemethane. E0-0 is estimated from the absorption thresholds in dichloromethane. ED*/D+ is estimated by subtracting
E0-0 to ED D21
+
e
f
g
.
+
The calculation absorption maximums.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 15900–15908 | 15903

View Article Online
Paper
RSC Advances
Electrochemical properties
The electrochemical properties and energy diagram of the
three dyes are listed in Table 1 and depicted in Fig. 4. The first
oxidation potentials (Eox), corresponding to the HOMO level of
these dyes, were obtained by cyclic voltammetry (CV) with 0.1
M tetrabutylammoniumhexafluorophosphate as the support-
ing electrolyte, Pt as the working electrode and counter
electrode, and a saturated calomel electrode (SCE) as the
reference electrode. The SCE reference electrode was cali-
brated by using a ferrocene/ferrocenium (Fc/Fc+). As shown in
Table 1 and Fig. 4, the HOMO levels of these dyes were 1.14 V
for BTT-I, 1.06 V for BTT-II and 0.68 V for BTT-III vs. NHE,
respectively. They are all more positive than the iodine/iodide
redox potential value (0.4 V vs. NHE), indicating that the
oxidized dyes, formed after electron injection into the
conduction band of TiO2, could accept electrons from the
electrolyte thermodynamically. The HOMO level of BTT-III was
less positive in comparison to the dyes BTT-II and BTT-I,
which is ascribed to the strong electron donor ability of
indoline unit. At the same time, BTT-I exhibited the most
positive HOMO level, which is attributed to the weakest donor
of carbazole unit. The LUMO levels of BTT-I–III were 21.11,
21.36 and 21.40 V vs. NHE, respectively. All the dyes’ levels
were more negative than conduction-band-edge energy level of
the TiO2 electrode (20.5 V vs. NHE). These results indicate that
the light-harvesting excited dyes can provide enough driving
force for electron injection into the TiO2 conduction band.
Compared to BTT-I and BTT-II, the dye BTT-III has a much
higher LUMO level, which suggests that the dye BTT-III has
larger electron driving force and affords a higher photocurrent
(Jsc) than BTT-I and BTT-II.
Fig. 3 The absorption spectra of BTT-I–III on 4 mm TiO2 films without CDCA and
with CDCA for 2 h.
shift of the absorption spectra of BTT-I–III on TiO2 film could
be ascribed to the formation of H-aggregates (extended head-
to-tail stacking).8 We tested the absorption spectra of the dye
sensitized TiO2 films with co-absorbent (CDCA) for 2 h to
explain the reason (Fig. 3). The maximum absorption peaks for
BTT-I–III with CDCA are located at 446, 447, and 448 nm,
respectively, in which these absorption peaks show smaller
blue-shifts compared to the spectrum on TiO2 film without
CDCA, revealing the formation of dyes aggregation on TiO2
film. Meanwhile, this result also proves that the co-adsorption
of CDCA can retard the aggregation of the dyes BTT-I–III.
Photovoltaic device performance
As we know, the dyes exhibit diversified interaction between
the dyes and solvents,21 which could cause changes of the
physical and chemical properties between the dyes and
semiconductor surface. Therefore, the suitable solvent for
semiconductor sensitization is important to obtain good
overall conversion efficiency (g), in which the effect of DCM
and THF solvents on photovoltaic performance of the dyes
BTT-I–III was explored. Fig. 5 shows the J–V curves for different
dye baths of BTT-I–III -based DSSCs, and the corresponding
photovoltaic data are collected in Table 2. The data show large
differences in the performance of BTT-I–III-based DSSCs
fabricated in DCM and THF dye baths. When DCM solutions
of BTT-I–III were introduced to sensitize the semiconductor
electrode, the DSSCs obtained the optimized g value of 5.53%,
6.19% and 6.63%, respectively; while BTT-I–III gives only
4.64%, 5.68% and 6.04% g values when the dye bath is
changed to THF. The solvent effect on DSSCs performance is
complicated. The possible explanation is the adsorbed amount
of BTT-I–III on the TiO2 surface and the excellent surface
binding modes of anchored dye in DCM. In fact, such a
phenomenon has been also found in other organic dyes.22
To reduce aggregation, the nonchromophoric adsorbents
chenodeoxycholic acid (CDCA) was used as a co-absorbent for
BTT-I–III to improve the photo-electric conversion efficiency,
as it has been proven that this can improve solar cell
Fig. 4 The cyclic voltammetry plots of bithiazole dyes (a) BTT-I, (b) BTT-III and
(c) the optimized structures and electron distribution in HOMO and LUMO levels
of the BTT dyes.
15904 | RSC Adv., 2013, 3, 15900–15908
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Fig. 5 The Current–voltage (I–V) characteristics of DSSCs based on BTT-I–III in solvents and co-adsorption with CDCA: (a) BTT-I, (b) BTT-II, (c) BTT-III; (d) BTT-I–III co-
absorbed with 10 mM CDCA in ethanol for 2 h were listed together for comparison between these dyes.
performance significantly.22 The semiconductor films were
deposited in 10 mM CDCA in ethanol for 2 h and 4 h before
sensitization in BTT-I–III DCM solution. The I–V performances
(Voc, Jsc, ff, g) and IPCE were shown in Table 2, Fig. 5 and 6.
Upon co-adsorption with CDCA for 2 h, the maximum plateau
of the IPCE curves were lifted from around 70% to 80% for
these three dyes (Fig. 5a, 5b and 5c), and the Jsc increased from
11.90 to 12.16 mA cm22, 13.04 to 13.37 mA cm22, and 14.04 to
15.26 mA cm22 for BTT-I–III, respectively (Fig. 6a, 6b and 6c).
The remarkable enhancement of IPCE was attributed to higher
injection efficiency resulting from relative independent dye
molecules arraying on semiconductor surface.18 Here, the
increased Voc from 0.671 to 0.728 V for BTT-I, 0.693 to 0.734 V
for BTT-II and 0.740 to 0.747 V for BTT-III was caused by the
retarded charge recombination because of co-adsorption. The
remarkable decrease of IPCE and Jsc could be found when the
films were absorbed with CDCA for 4 h. This may be attributed
to the substitution of CDCA molecular for dyes on films,
resulting in the reduction of the dyes’ adsorption amount.
Finally, upon co-adsorption with CDCA for 2 h, three dyes
yielded an increased overall conversion efficiency of 6.23%,
7.12% and 7.86%, respectively.
Table 2 Photovoltaic performance of DSSCs based on the dyes BTT-I–III in
various solvents and co-absorption with CDCA with a liquid electrolytea
Dye
Solvent CDCA Time/h Voc/V Jsc/mA cm22 ff
g/%
BTT-I
THF
CH2Cl2
0
0
2
4
0
0
2
4
0
0
2
4
0.677
9.62
0.71 4.64
0.69 5.53
0.70 6.23
0.69 5.04
0.73 5.68
0.69 6.19
0.73 7.12
0.70 6.07
0.69 6.04
0.64 6.63
0.69 7.86
0.70 6.62
0.671 11.90
0.728 12.16
0.627 11.67
0.726 10.65
0.693 13.04
0.734 13.37
0.667 13.09
0.677 12.80
0.740 14.04
0.747 15.26
0.686 13.80
BTT-II
THF
CH2Cl2
BTT-III THF
CH2Cl2
Among these dyes, Fig. 5d showed the current–voltage
characteristics of DSSCs fabricated with BTT-I–III as sensiti-
zers co-absorbed with 10 mM CDCA in ethanol for 2 h under
standard global AM 1.5 solar light condition. The similar open-
circuit voltages of these dyes were about 728–747 mV. The
a
The liquid electrolyte was 0.1 M LiI, 0.05 M I2, 0.6 M DMPII, and
0.5 M TBP in acetonitrile.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 15900–15908 | 15905

View Article Online
Paper
RSC Advances
Fig. 6 The IPCE spectra of DSSCs based on BTT-I–III in solvents and co-absorption with CDCA: (a) BTT-I, (b) BTT-II, (c) BTT-III, (d) BTT-I–III co-absorbed with 10 mM
CDCA in ethanol for 2 h were listed together for comparison between these dyes.
higher Voc was ascribed to decrease of electron recombination
because of two long alkyl chains attached on thiazole moiety.
The similar dark current of these dyes shown in Fig. 5d also
proved this phenomenon. In comparison with the dyes BTT-I
and BTT-II, the short-circuit current of BTT-III was higher,
which was attributed to a much broader absorption spectra
and the more negative LUMO levels due to a stronger electron
driving-force for excited dyes’ electron injection. Although the
molar extinction coefficient of BTT-III was lower than those of
BTT-I and BTT-II, BTT-III had the largest adsorption amount of
11.6 6 1028 mol cm22 (Table 1). The much greater adsorption
of BTT-III can counteract this influence, indicating that the
BTT-III can harvest sunlight more efficiently. The dyes BTT-I
and BTT-II had similar absorption spectra, but the dye BTT-I
had a lower short-circuit current than BTT-II. This could be
explained by the decreased driving-force for electron-injection.
The photocurrent improvement trend accorded very well with
the IPCE measurements shown in Fig. 6d.
BTT-III had a little lower IPCE value in the region between
350–520 nm compared to BTT-II, BTT-III exhibited a broader
spectrum and kept higher IPCE value during 580–650 nm. It
conformed very closely to the absorption spectrum on the film
with the co-absorbent. The region of the IPCE spectra of BTT-I
resembled that of BTT-II due to the similar absorption
spectrum on the film with the co-absorbent. However, the
maximum value of BTT-I was reduced compared to BTT-II,
which indicated the lowest photocurrent, as a result of the
weakest electron-injection driving-force of carbazole.
Electron lifetime and EIS measurements
In addition, electrochemical impedance spectroscopy (EIS)
was employed to study the electron recombination in DSSCs
based on these dyes upon co-adsorption with CDCA for 2 h
under 20.70 V bias applied voltages in the dark and analyzed
by the software ZSimpWin. The EIS measurement was shown
in Fig. 7, and the data was listed in Table 3. RS, Rrec, and RCE
represent series resistance and charge-transfer resistance at
the dye/TiO2/electrolyte interface and counter electrode (CE),
respectively. The value of RS and RCE (the first semi-circle in
the Nyquist plot) were almost the same for these three dyes
Fig. 6d showed the spectra of the incident photon-to-current
conversion efficiency (IPCE) based on these dyes upon co-
adsorption with CDCA for 2 h. All the dyes exhibited a high
plateau in the visible region between 350–580 nm. Although
15906 | RSC Adv., 2013, 3, 15900–15908
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
between the electrolyte and TiO2 was calculated following a
literature procedure.23 The Rrec for dyes BTT-I–III were 45.92,
50.51 and 53.73 V, respectively. The larger resistance of Rrec
indicated that the higher open-circuit voltage for these dyes.
The calculated electron lifetimes of BTT-I–III were 16.5, 17.3
and 19.1 ms, respectively. Among BTT-I–III, BTT-III-based cell
has longer electron lifetime, which leads to lower rate of
charge recombination and thus improved Voc.
Conclusions
In summary, three metal-free bithiazole-based organic dyes
containing different electron donors (carbazole, triphenyla-
mine and indoline) in the D–A-p-A configuration were
synthesized and characterized for application in DSSCs. The
results demonstrated that modification of the electron donor
is an effective method to control the electrochemical and
absorption properties. Notably, the HOMO and LUMO energy
levels can be tuned by introducing different electron donors in
the dye molecule. Co-adsorption of 10 mM CDCA for 2 h can
significantly enhance photovoltaic performance of these dyes.
Moreover, the power conversion efficiency was shown to be
sensitive to the structural modifications of electron-donor
units. Among the three dyes, DSSCs based on the BTT-III with
indoline donor exhibits the best overall conversion efficiency
of 7.86% (Jsc = 15.26 mA cm22, Voc = 747 mV, ff = 0.69) under
standard AM 1.5 (100 mW cm22). This research enriches the
application of bithiazole-based derivatives in DSSCs and
indicates that the donor of the dye molecules can efficiently
impact the optoelectronic properties.
Acknowledgements
This work was supported by NSFC/China (2116110444,
21172073 and 91233207), the National Basic Research 973
Program (2013CB733700) and the Fundamental Research
Funds for the Central Universities (WJ0913001 and
WJ1114050).
Fig. 7 Impedance spectra of DSSCs based on dyes BTT-I–III. (a) Nyquist plots, (b)
Bode phase plots, (c) the equivalent circuits. The lines of (a) and (b) show
theoretical fits using the equivalent circuits (c).
because of the same electrode material and same electrolyte.
The charge-transfer resistance was determined by the middle
semi-circle in Nyquist plot. From the EIS measurements, the
electron lifetime (te) expressing the electron recombination
References
1 (a) B. O’Regan and M. Gratzel, Nature, 1991, 353, 737–740;
(b) K. L. Wu, C. H. Li, Y. Chi, J. N. Clifford, L. Cabau,
E. Palomares, Y. M Cheng, H. A. Pan and P. T. Chou, J. Am.
Chem. Soc., 2012, 134, 7488–7496.
2 (a) F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang,
R. Humphry-Baker, P. Wang, S. M. Zakeeruddin and
Table 3 Parameters obtained by fitting the impedance spectra of the DSSCs
with sensitizersa
¨
M. Gratzel, Chem. Commun., 2008, 2635–2637; (b) J.
H. Yum, I. Jung, C. Baik, J. Ko, M. K. Nazeeruddin and
¨
M. Gratzel, Energy Environ. Sci., 2009, 2, 100–102.
Dye
RS/V
RCE/V
Rrec/V
te/ms
3 K. Funabiki, H. Mase, A. Hibino, N. Tanaka, N. Mizuhata,
Y. Sakuragi, A. Nakashima, T. Yoshida, Y. Kubotaa and
M. Matsui, Energy Environ. Sci., 2011, 4, 2186–2192.
4 K. Sayama, S. Tsukagoshi, T. Mori, K. Hara, Y. Ohga,
A. Shinpo, Y. Abe, S. Suga and H. Arakawa, Sol. Energy
Mater. Sol. Cells, 2003, 80, 47–71.
BTT-I
BTT-II
BTT-III
18.47
17.41
15.63
6.04
6.00
5.52
45.92
50.51
53.73
16.5
17.3
19.1
a
All the parameters were obtained from the software ZSimpWin,
using the equivalent circuits Fig. 7(c).
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 15900–15908 | 15907

View Article Online
Paper
RSC Advances
5 Z. S. Wang, Y. Cui, K. Hara, Y. Dan-oh, C. Kasada and
A. Shinpo, Adv. Mater., 2007, 19, 1138–1141.
6 (a) W. H. Howie, F. Claeyssens, H. Miura and L. M. Peter, J.
Am. Chem. Soc., 2008, 130, 1367–1375; (b) W. Q. Li, Y.
Z. Wu, X. Li, Y. S. Xie and W. H. Zhu, Energy Environ. Sci.,
2011, 4, 1830–1837.
7 E. Stathatos and P. Lianos, Chem. Mater., 2001, 13,
3888–3892.
8 (a) J. Y. Mao, F. L. Guo, W. J. Ying, W. J. Wu, J. Li and J.
L. Hua, Chem.–Asian J., 2012, 7, 982–991; (b) Z. Q. Wan, C.
Y. Jia, Y. D. Duan, L. L. Zhou, J. Q. Zhang, Y. Lin and Y. Shi,
RSC Adv., 2012, 2, 4507–4514.
15 C. H. Chen, Y. C. Hsu, H. H. Chou, K. R. J. Thomas, J. T. Lin
and C. P. Hsu, Chem.–Eur. J., 2010, 16, 3184–3193.
16 L. Y. Lin, C. H. Tsai, K. T. Wong, T. W. Huang, C. C. Wu, S.
H. Chou, F. Lin, S. H. Chenc and A. I. Tsai, J. Mater. Chem.,
2011, 21, 5950–5958.
17 (a) J. X. He, W. J. Wu, J. L. Hua, Y. H. Jiang, S. Y. Qu, J. Li, Y.
T. Long and H. Tian, J. Mater. Chem., 2011, 21, 6054–6062;
(b) J. X. He, F. L. Guo, X. Li, W. J. Wu, J. B. Yang and J.
L. Hua, Chem.–Eur. J., 2012, 18, 7903–7915; (c) L. F. Lai, C.
L. Ho, Y. C. Chen, W. J. Wu, F. R. Dai, C. H. Chui, S.
P. Huang, K. P. Guo, J. T. Lin, H. Tian, S. H. Yang and W.
Y. Wong, Dyes Pigm., 2013, 96, 516–524; (d) Y. Z. Wu and W.
H. Zhu, Chem. Soc. Rev., 2013, 42, 2039–2058.
18 (a) B. Liu, W. Zhu, Q. Zhang, W. Wu, M. Xu, Z. Ning, Y. Xie
and H. Tian, Chem. Commun., 2009, 1766–1768; (b) W. Zhu,
Y. Wu, S. Wang, W. Li, X. Li, J. Chen, Z. Wang and H. Tian,
Adv. Funct. Mater., 2011, 21, 756–763; (c) T. Horiuchi,
H. Miura, K. Sumioka and S. Uchida, J. Am. Chem. Soc.,
2004, 126, 12218–12219; (d) T. Horiuchi, H. Miura,
K. Sumioka and S. Uchida, J. Am. Chem. Soc., 2008, 130,
1367–1375.
9 (a) A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran,
M. K. Nazeeruddin, E. W. Diau, C. Y. Yeh, S.
¨
M. Zakeeruddin and M. Gratzel, Science, 2011, 334,
629–634; (b) J. M. Ball, N. K. S. Davis, J. D. Wilkinson,
J. Kirkpatrick, J. Teuscher, R. Gunning, H. L. Anderson and
H. J. Snaith, RSC Adv., 2012, 2, 6846–6853; (c)
D. Daphnomili, G. Landrou, S. P. Singh, A. Thomas,
K. Yesudas, Bhanuprakash K. G. D. Sharma and A.
G. Coutsolelos, RSC Adv., 2012, 2, 12899–12908.
10 H. N. Tian, X. C. Yang, R. K. Chen, A. Hagfeldt and L.
C. Sun, Energy Environ. Sci., 2009, 2, 674–677.
19 P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin
¨
and M. Gratzel, J. Am. Chem. Soc., 2005, 127, 808–809.
11 (a) S. Y. Qu, C. J. Qin, A. Islam, Y. Z. Wu, W. H. Zhu, J.
L. Hua, H. Tian and L. Y. Han, Chem. Commun., 2012, 48,
6972–6974; (b) S. Qu, W. Wu, J. Hua, C. Kong, Y. Long and
H. Tian, J. Phys. Chem. C, 2010, 114, 1343–1349; (c) F. Guo,
S. Qu, W. Wu, J. Li, W. Ying and J. Hua, Synth. Met., 2010,
160, 1767–1773; (d) S. Y. Qu, J. L. Hua and H. Tian, Sci.
China: Chem., 2012, 55, 677–697; (e) J. Y. Mao, N. N. He, Z.
J. Ning, Q. Zhang, F. L. Guo, L. Chen, W. J. Wu, J. L. Hua
and He Tian, Angew. Chem., Int. Ed., 2012, 51, 9873–9876.
12 (a) W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang,
F. Wang, C. Pan and P. Wang, Chem. Mater., 2010, 22,
1915–1925; (b) L. Li, Y. Hao, X. Yang, J. Zhao, H. Tian,
C. Teng, A. Hagfeldt and L. C. Sun, ChemSusChem, 2011, 4,
609–612.
20 (a) J. Tang, J. Hua, W. Wu, J. Li, Z. Jin, Y. Long and H. Tian,
Energy Environ. Sci., 2010, 3, 1736–1745; (b) J. Tang, W. Wu,
J. Hua, X. Li and H. Tian, Energy Environ. Sci., 2009, 2,
982–990.
21 (a) N. Robertson, Angew. Chem., Int. Ed., 2006, 45,
2338–2345; (b) H. Tian, X. Yang, R. Chen, R. Zhang,
A. Hagfeldt and L. Sun, J. Phys. Chem. C, 2008, 112,
11023–11033.
22 (a) K. Hara, Y. Dan-oh, C. Kasada, Y. Ohga, A. Shinpo,
S. Suga, K. Sayama and H. Arakawa, Langmuir, 2004, 20,
4205–4210; (b) Z. S. Wang, Y. Cui, Y. Dan-oh, C. Kasada,
A. Shinpo and K. Hara, J. Phys. Chem. C, 2008, 112,
17011–17012.
23 L. C. Zou and C. Hunt, J. Electrochem. Soc., 2009, 156,
C8–C15.
13 H. Choi, S. Paek, K. Song, M. S. Kang and J. Ko, Bull. Korean
Chem. Soc., 2010, 31, 125–132.
24 A. Dreuw and M. Head-Gordon, J. Am. Chem. Soc., 2004,
126, 4007–4016.
14 Y. T. Li, C. L. Chen, Y. Y. Hsu, H. C. Hsu, Y. Chi, B. S. Chen,
W. H. Liu, C. H. Lai, T. Y. Lin and P. T. Chou, Tetrahedron,
2010, 66, 4223–4229.
15908 | RSC Adv., 2013, 3, 15900–15908
This journal is ß The Royal Society of Chemistry 2013