Molecular engineering and theoretical investigation of organic sensitizers based on indoline dyes for quasi-solid state dye-sensitized solar cells

Article abstract of DOI:10.1039/c1cp20484j

Novel indoline dyes, I-1-I-4, with structural modification of π-linker group in the D-π-A system have been synthesized and fully characterized. Molecular engineering through expanding the π-linker segment has been performed. The ground and excited state p

Full text of DOI:10.1039/c1cp20484j

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PAPER
Molecular engineering and theoretical investigation of organic sensitizers
based on indoline dyes for quasi-solid state dye-sensitized solar cells
Bo Liu,*a Wenjun Wu,b Xiaoyan Li,a Lei Li,c Shaofu Guo,a Xiaoru Wei,a
Weihong Zhub and Qingbin Liu*a
Received 23rd February 2011, Accepted 3rd March 2011
DOI: 10.1039/c1cp20484j
Novel indoline dyes, I-1–I-4, with structural modification of p-linker group in the D-p-A system
have been synthesized and fully characterized. Molecular engineering through expanding the
p-linker segment has been performed. The ground and excited state properties of the dyes have
been studied by means of density functional theory (DFT) and time-dependent DFT (TD-DFT).
Larger p-conjugation linkers would lead to broader spectral response and higher molar extinction
coefficient but would decrease dye-loaded amount on TiO2 electrode and LUMO level.
While applied in DSSCs, the variation trends in short-circuit current density (Jsc) and open-circuit
voltage (Voc) were observed to be opposite to each other. The internal reasons were studied
by experimental data and theoretical calculations in detail. Notably, I-2 showed comparable
photocurrent values with liquid and quasi-solid state electrolyte, which suggested through
molecular engineering of organic sensitizers the dilemma between optical absorption and charge
diffusion lengths can be balanced well. Through studies of photophysical, electrochemical, and
theoretical calculation results, the internal relations between chemical structure and efficiency
have been revealed, which serve to enhance our knowledge regarding design and optimization
of new sensitizers for quasi-solid state DSSCs, providing a powerful strategy for prediction
of photovoltaic performances.
Most organic sensitizers are based on the D-p-A system.
Among donor groups, indoline, possessing strong light-capturing
and electron-donating capabilities, is an excellent candidate.
Some sensitizers containing an indoline group as donor have
been synthesized and show very promising results. Uchida
et al. combined indoline derivatives with rhodanine ring and
obtained a series of dyes, which presented a relatively broad
action spectrum and high conversion efficiency up to 6.1%.9
Through optimization of DSSCs devices, the conversion
efficiencies using AcAN- and ionic-liquid-based electrolytes
were enhanced to 9.03% and 6.67%, respectively.10 Recently,
we have also reported an indoline-based sensitizer. Using a
5.0 mm TiO2 electrode, 7.41% conversion efficiency was obtained
while N719 produced 7.03% under the same conditions.11
However, there is still much room for further insight into inter-
facial engineering through molecular engineering.
Introduction
Dye-sensitized solar cells (DSSCs), as promising candidate for
solar-energy conversion device, has attracted considerable atten-
tion due to their high performance and low-cost of production.1–3
As the critical component of DSSCs, sensitizers including
ruthenium and metal-free organic dyes have been intensively
investigated in recent years. The classic ruthenium complexes
give a solar-to-electrical energy conversion efficiency of up to
11% under AM 1.5 irradiation with long time stability.4,5
Through molecular design, several ruthenium derivatives, such
as black dye,6 Z907,7 and K19,8 also gained remarkable efficiency
over 7%. However, the use of rare metals with heavy purification
brings major problems in cost and environmental issues. There-
fore, metal-free organic sensitizers, such as indoline,9–12 fluorine,13
triphenylamine,14–17 coumarin,18,19 and porphyrin20 have been
extensively explored and their performances characterized.
As we all know, the p-linker segment can not only affect
absorption spectra but also tune the HOMO and LUMO
levels and some other properties. However, the efficiency is
not always enhanced along with the expansion of p-conjugation
system. This phenomenon has been reported by several groups
but the understanding of the origins of such results is still
limited.15,16,18 Therefore, we can investigate the contributions
of different p-linker and build spectral response and energy level
libraries by alternating different p-linker segments. This screening
a College of Chemistry and Material Science,
Hebei Normal University, 113 East Yuhua Road, Shijiazhuang,
Hebei, 050016, P. R. China. E-mail: liubo@mail.hebtu.edu.cn,
qbinliu@yahoo.com; Tel: +86-311-86268311
b Key Laboratory of Advanced Material, Institute of Fine Chemicals,
East China University of Science &Technology, 130 Meilong Road,
Shanghai, 200237, P. R. China
c China National Petroleum Corporation Occupational Health Center,
51 Xinkai Road, Langfang, Hebei, 065000, P. R. China
c
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strategy helps us to optimize the TiO2-sensitizer-hole transport
material (HTM) system in terms of balance between spectral
response, driving forces, and photovoltaic performance for
future preparation of high efficiency dyes by matching suitable
p-linker groups in the D-p-A system.21 As the most developed
group, triphenylamine was used as donor group for a series of
dyes which were designed and synthesized by Sun et al. and
the energy levels were tuned efficiently.14 However, we can
not apply previous experiences to guide works on novel
systems directly due to their own characteristics of different
functional groups. With this in mind, we have introduced
thiophene into the framework to extend the conjugate system
of dyes and developed a series of dyes based on indoline
group. The chemical structures of new dyes were confirmed
by 1H NMR, 13C NMR, and HR-MS. The DSSC devices
based on liquid and ionic liquid electrolytes have been fabri-
cated. The absorption spectra, dye-loaded amounts, energy
levels, and photovoltaic performances were tuned and
studied in detail. Through these studies, the internal relations
between chemical structure and efficiency have been revealed,
which serve to enhance our knowledge regarding design and
optimization of new sensitizers for quasi-solid state DSSCs,
providing a powerful strategy for prediction of photovoltaic
performances.
Sintering was carried out at 450 1C for 30 min. Before
immersion in the dye solution, these films were immersed in
a 40 mM aqueous TiCl4 solution at 70 1C for 30 min and
washed with water and ethanol. Then the films were heated
again at 450 1C for 30 min followed by cooling to 80 1C and
dipping into a 3 Â 10À4 M solution of dye in acetonitrile for
12 h at room temperature. To prepare the counter electrode,
the Pt catalyst was deposited on the cleaned FTO glass by
coating with a drop of H2PtCl6 solution (0.02 M in 2-propanol
solution) with heat treatment at 400 1C for 15 min. A hole
(0.8 mm diameter) was drilled on the counter electrode using a
drill-press. The perforated sheet was cleaned with ultrasound
in an ethanol bath for 10 min. For the assembly of DSSCs,
the dye-loaded TiO2 electrode and Pt-counter electrode were
assembled into a sandwich type cell and sealed with a hot-melt
gasket of 25 mm thickness made of the ionomer Surlyn
1702 (DuPont). The size of the TiO2 electrodes used was
0.25 cm2 (i.e. 5 mm  5 mm). A drop of the electrolyte was
put on the hole in the back of the counter electrode. It was
introduced into the cell via vacuum backfilling. The hole in the
counter electrode was sealed with a film of Surlyn 1702 and a
cover glass (0.1 mm thickness) using a hot iron bar.
The electrolyte employed was a solution of 0.6 M PMII
(1-propyl-3-methylimidazolium iodide), 0.05 M I2, 0.10 M LiI
and 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile and
methoxypropionitrile (v/v, 7 : 3). The quasi-solid state electrolyte
consisted of 1-butyl-3-methylimidazolium iodide (BMII)/I2/
benzimidazole (BI)/GuSCN (w/w, 40/1.67/0.67/3.33).
Experimental
General
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 cmÀ2 using a Newport Oriel PV reference cell
system (Model 91150V). I–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.
The voltage step and delay time of photocurrent were 10 mV
and 40 ms, respectively. The cell active area was tested with a
mask of 0.158 cm2. The photocurrent action spectra were
measured with an IPCE test system consisting of a Model
SR830 DSP Lock-In Amplifier and a Model SR540 Optical
Chopper (Stanford Research Corporation, USA), a 7IL/PX150
xenon lamp and power supply, and a 7ISW301 Spectrometer.
Each value in this paper was an average of three samples.
The FTO conducting glass (fluorine doped SnO2, sheet
resistance o 15 O/square, transmission 4 90% in the visible)
was obtained from Geao Science and Educational Co. Ltd.,
China. Titanium (IV) isopropoxide, lithium iodide, and tert-butyl-
pyridine (TBP) were purchased from Aldrich. All other solvents
and chemicals used were produced by Sinopharm Chemical
Reagent Co., Ltd, China (reagent grade) and used as received.
1H NMR spectra were obtained with a Bruker AM-400 spectro-
meter. The mass spectra were conducted on a 4700 Proteomics
Analyzer spectrometer.
Physical measurements
UV-visible spectra were determined with a Varian Cary 500
spectrometer. Fluorescent spectra were recorded on a Varian
Cary Eclipse spectrometer. The cyclic voltammograms were
measured on a Versastat II electrochemical workstation (Princeton
Applied Research) using a normal three-electrode cell with a
dye-loaded TiO2 electrode as working electrode, a Pt wire
auxiliary electrode, a Ag/AgCl reference electrode in saturated
KCl solution, and 0.1 M tetrabutylammonium hexafluoro-
phosphoric acid was used as supporting electrolyte. After the
measurement, ferrocene was added as the internal reference
for calibration.
Synthesis
The chemical structures and synthetic routes of I-1–I-4
were shown in Scheme 1 as well as all the intermediates
(compound 1–9).
Trans-2-cyano-3-(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta-
[b]indol-7-yl)acrylic acid (I-1). A 30 mL acetonitrile solution of 1
(0.277 g, 1 mmol), cyanoacetic acid (0.11 g, 1.3 mmol), and
piperidine (0.1 g, 1.2 mmol) was refluxed for 6 h under nitrogen
atmosphere. The resultant mixture was poured into 0.1 M HCl
solution (100 mL). The product was filtered and recrystallized in
DCM/petroleum ether. I-1 was obtained as a dark purple solid
(0.26 g, 76%). 1H NMR (500 MHz, DMSO) d ppm: 13.3
(s, 1H), 8.05 (s, 1H), 7.92 (s, 1H), 7.70–7.72 (m, 1H), 7.24–7.29
(m, 4H), 6.75 (d, J = 8.5 Hz, 1H), 5.04 (t, J = 8.5 Hz, 1H),
Fabrication and measurement of solar cells
A screen-printed double layer of TiO2 particles was used as
photoelectrode. A 10 mm thick film of 13 nm sized TiO2 particles
(Ti-Nanoxide T/SP) was first printed on the FTO conducting
glass and further coated with a 4 mm thick second layer of
400 nm light-scattering anatase particles (Ti-Nanoxide 300).
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3.84 (t, J = 8.5 Hz, 1H), 2.32 (s, 3H), 2.01–2.10 (m, 1H),
1.60–1.78 (m, 4H), 1.32–1.41 (m, 1H). 13C NMR (125 MHz,
DMSO) d ppm: 165.2, 154.0, 153.1, 137.8, 136.4, 135.5, 134.2,
130.5, 127.2, 122.5, 121.7, 118.4, 106.7, 94.2, 69.8, 44.3, 35.6,
32.9, 24.2, 21.0. HR-MS (TOF MS ES+) m/z 345.1605
[M + H+] calcd for C22H21N2O2 (M + H+) 345.1603.
nitrogen atmosphere. N-Butyl lithium (0.8 mL, 2.2 M hexane
solution) was added dropwise over 5 min and the mixture was
stirred at À78 1C for 1 h. Then the mixture was allowed to
warm to 01 C and stirred for 30 min. The mixture was once
again cooled to À781 C and DMF (0.1 mL, 1.3 mmol) was
added. The reaction was allowed to warm to ambient tem-
perature and was stirred for 2 h. The reaction was quenched by
the addition of 0.5 M HCl (100 mL) and extracted with DCM.
After drying over Na2SO4, the combined organic extract was
concentrated by rotary evaporation and column chromato-
graphy over silica gel with petroleum ether/ethyl acetic mixture
(20 : 1). The trans-type isomer 7 was obtained as a red solid
(0.22 g, 58%). 1H NMR (500 MHz, CDCl3) d ppm: 9.81
(s, 1H), 7.62 (d, J = 4.0 Hz, 1H), 7.30 (s, 1H), 7.14–7.17
(m, 5H), 7.08 (d, J = 16.0 Hz, 1H), 7.04 (J = 4.0 Hz, 1H), 6.99
(d, J = 16.0 Hz, 1H), 6.81 (d, J = 8.5 Hz, 1H), 4.82 (t, J = 8.5 Hz,
1H), 3.81 (t, J = 8.5 Hz, 1H), 2.34 (s, 3H), 2.01-2.10 (m, 1H),
1.85–1.93 (m, 2H), 1.49–1.79 (m, 3H).
5-(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)-
thiophene-2-carbaldehyde (4). A 15 mL THF solution of 2
(0.328 g, 1 mmol), 3 (0.156 g, 1 mmol), Pd(PPh3)4 (0.1 g,
B0.1 mmol), and 2 M Na2CO3 (5 mL) was refluxed for 24 h
under nitrogen atmosphere. The resultant mixture was poured
into 50 mL water and extracted with CH2Cl2 (3 Â 30 mL). The
combined organic extract was dried over anhydrous Na2SO4
and filtered. Solvent removal by rotary evaporation and
column chromatography over silica gel with a petroleum
ether/ethyl acetate mixture (20 : 1) was followed by recrystal-
lizing in DCM/petroleum ether. Product 4 was obtained as a
yellow solid (0.19 g, 53%). 1H NMR (500 MHz, CDCl3) d
ppm: 9.81 (s, 1H), 7.67 (d, J = 4.0 Hz, 1H), 7.40 (s, 1H),
7.36–7.38 (m, 1H), 7.23 (d, J = 4.0 Hz, 1H), 7.16–7.19
(m, 4H), 6.83 (d, J = 8.5 Hz, 1H), 4.84 (t, J = 8.5 Hz, 1H),
3.84 (t, J = 8.5 Hz, 1H), 2.34 (s, 3H), 2.03–2.12 (m, 1H),
1.87–1.94 (m, 2H), 1.65–1.71 (m, 2H), 1.51–1.62 (m, 1H).
Trans-2-cyano-3-(5-((E)-2-(4-p-tolyl-1,2,3,3a,4,8b-hexahydro-
cyclopenta[b]indol-7-yl)vinyl)thiophen-2-yl)acrylic acid (I-3). A
30 mL acetonitrile solution of 7 (0.2 g, 0.52 mmol), cyanoacetic
acid (0.11 g, 1.3 mmol), and piperidine (0.1 g, 1.2 mmol) was
refluxed for 6 h under nitrogen atmosphere. The resultant
mixture was poured into 100 mL HCl solution (0.1 M). The
product was filtered and recrystallized in DCM/petroleum
ether. I-3 was obtained as a dark purple solid (0.18 g, 78%).
1H NMR (500 MHz, DMSO) d ppm: 8.20 (s, 1H), 6.67 (d, J =
4.0 Hz, 1H), 7.46 (s, 1H), 7.16–7.28 (m, 7H), 7.03 (d, J = 16.0 Hz,
1H), 6.79 (d, J = 8.5 Hz, 1H), 4.86 (t, J = 8.5 Hz, 1H), 3.79
(t, J = 8.5 Hz, 1H), 2.28 (s, 3H), 1.95–2.08 (m, 1H), 1.65–1.82
(m, 4H), 1.28–1.40 (m, 1H). 13C NMR (125 MHz, DMSO)
d ppm: 165.0, 150.6, 148.3, 142.6, 139.7, 137.9, 136.0, 134.4,
132.3, 131.5, 130.2, 128.5, 127.0, 126.1, 123.1, 120.3, 119.2, 118.7,
117.0, 107.2, 68.8, 44.9, 35.2, 33.5, 24.4, 20.9. HR-MS (TOF MS
ES+) m/z 453.1634 [M + H+] calcd for C28H25N2O2S
(M + H+) 453.1637.
Trans-2-cyano-3-(5-(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclo-
penta[b]indol-7-yl)thiophen-2-yl)acrylic acid (I-2). A 30 mL
acetonitrile solution of 4 (0.359 g, 1 mmol), cyanoacetic acid
(0.11 g, 1.3 mmol), and piperidine (0.1 g, 1.2 mmol) was
refluxed for 6 h under nitrogen atmosphere. The resultant
mixture was poured into 0.1 M HCl solution (100 mL). The
product was filtered and recrystallized in DCM/petroleum
ether. I-2 was obtained as a dark purple solid (0.33 g, 78%).
1H NMR (500 MHz, DMSO) d ppm: 8.35 (s, 1H), 7.88
(d, J = 4.0 Hz, 1H), 7.54–7.55 (m, 2H), 7.45 (s, 1H), 7.20–7.24
(m, 4H), 6.83 (d, J = 8.5 Hz, 1H), 4.93 (t, J = 8.5 Hz, 1H), 3.86
(t, J = 8.5 Hz, 1H), 2.30 (s, 3H), 2.03–2.11 (m, 1H), 1.62–1.83
(m, 4H), 1.35–1.44 (m, 1H). 13C NMR (125 MHz, DMSO)
d ppm: 164.6, 154.7, 149.4, 145.8, 141.6, 139.3, 136.4, 132.8,
132.2, 130.4, 126.9, 123.0, 122.8, 122.7, 121.0, 117.9, 107.4, 69.1,
44.8, 35.4, 33.4, 24.4, 20.9. HR-MS (TOF MS ES+) m/z 427.1479
[M + H+] calcd for C26H22N2O2S (M + H+) 427.1480.
Trans-7-(5-(2-(thiophen-2-yl)vinyl)thiophen-2-yl)-4-p-tolyl-1,2-,
3,3a,4,8b-hexahydrocyclopenta[b]indole (8). 5 (0.439 g, 1 mmol)
was dispersed in 15 mL dry THF followed by the addition of
NaH (0.03 g, 1.25 mmol). The mixture was stirred at ambient
temperature under nitrogen atmosphere for 1 h. 4 (0.359 g,
1 mmol) was added and the reaction mixture was allowed to stir
at ambient temperature for another 48 h. Then the mixture was
poured into 100 mL HCl solution (0.1 M) and extracted with
DCM. The organic phase was dried over Na2SO4, then filtered
through a plug of silica gel with petroleum ether/ethyl acetic
mixture (100 : 1), and a crude intermediate was obtained as a
mixture of isomers. The crude product 8 was used directly in the
next step without further purification.
Trans-7-(2-(thiophen-2-yl)vinyl)-4-p-tolyl-1,2,3,3a,4,8b-hexa-
hydrocyclopenta[b]indole (6). 5 (0.439 g, 1 mmol) was dispersed
in 15 mL dry THF followed by the addition of NaH (0.03 g,
1.25 mmol). The mixture was stirred at ambient temperature
under nitrogen atmosphere for 1 h. I-CHO (0.277 g, 1 mmol)
was added and the reaction mixture was allowed to stir at
ambient temperature for another 48 h. Then the mixture was
poured into 100 mL HCl solution (0.1 M) and extracted with
DCM. The organic phase was dried over Na2SO4, then filtered
through a plug of silica gel with petroleum ether/ethyl acetic
mixture (100 : 1), and a crude intermediate was obtained as a
mixture of isomers. The crude product 6 was used directly in
the next step without further purification.
Trans-5-(2-(5-(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta-
[b]-indol-7-yl)thiophen-2-yl)vinyl)thiophene-2-carbaldehyde (9). 8
was dissolved in THF (10 mL) and the solution was cooled to
À78 1C under nitrogen atmosphere. N-Butyl lithium (0.8 mL,
2.2 M hexane solution) was added dropwise over 5 min and the
mixture was stirred at À78 1C for 1 h. Then the mixture was
allowed to warm to 0 1C and was stirred for 30 min. The mixture
was once again cooled to À78 1C and DMF (0.1 mL, 1.3 mmol)
Trans-5-(2-(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]-
indol-7-yl)vinyl)thiophene-2-carbaldehyde (7). 6 was dissolved
in THF (10 mL) and the solution was cooled to À78 1C under
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was added. The reaction was allowed to warm to ambient
temperature and was stirred for 2 h. The reaction was quenched
by the addition of 0.5 M HCl (100 mL) and extracted with
DCM. After drying over Na2SO4, the combined organic extract
was concentrated by rotary evaporation and column chromato-
graphy over silica gel with petroleum ether/ethyl acetic mixture
(15 : 1). The trans-type isomer 9 was obtained as a red solid
(0.20 g, 43%). 1H NMR (500 MHz, CDCl3) d ppm: 9.83 (s, 1H),
7.63 (d, J = 4.0 Hz, 1H), 7.34 (s, 1H), 7.28–7.30 (m, 1H), 7.22
(d, J = 16.0 Hz, 1H), 7.15–7.19 (m, 4H), 7.05–7.78 (m, 3H),
6.93 (d, J = 16.0 Hz, 1H), 6.85 (d, J = 8.0 Hz, 1H), 4.81
(t, J = 8.5 Hz, 1H), 3.84 (t, J = 8.5 Hz, 1H), 2.34 (s, 3H),
2.03–2.11 (m, 1H), 1.87–1.93 (m, 2H), 1.75–1.83 (m, 1H),
1.64–1.70 (m, 1H), 1.51–1.59 (m, 1H).
unit, whose preparation has been successfully scaled up to a
hundred kilograms,11 was applied as donor group. Cyanoacetic
acid was used as the acceptor group. Thiophene is employed to
provide conjugation between donor and acceptor groups. The
Suzuki coupling reaction, Wittig reaction, and Knoevenagel
reaction were applied in the synthetic routes. It is noteworthy
that the synthesis of these dyes was quite straightforward
and capable of scaling up since the important intermediates
including indoline derivatives and 5-formyl-thiophene-2-boronic
acid are commercially available.
Photophysical and electrochemical characterization
Fig. 1a showed the absorption and emission spectra of the
dyes in THF. Absorption peaks were observed in the visible
region at 438, 492, 504, and 518 nm, respectively. The lmax was
red-shifted from 438 nm to 518 nm as the p-conjugation
expanded and the absorption spectra were broadened in
visible region, which would obviously favor light harvesting
in DSSCs. Also, the molar extinction coefficient was enhanced.
The molar extinction coefficient at lmax of I-4 was determined
to be 4.86 Â 104 MÀ1 cmÀ1 at 518 nm, which is about 4 times
the corresponding value of I-1 (1.22 Â 104 MÀ1 cmÀ1 at 438 nm)
in the visible region (Table 1).
Trans-2-cyano-3-(5-(trans-2-(5-(4-p-tolyl-1,2,3,3a,4,8b-hexa-
hydrocyclopenta[b]indol-7-yl)thiophen-2-yl)vinyl)thiophen-2-yl)-
acrylic acid (I-4). A 30 mL acetonitrile solution of 9 (0.467 g,
1 mmol), cyanoacetic acid (0.11 g, 1.3 mmol), and piperidine
(0.1 g, 1.2 mmol) was refluxed for 6 h under nitrogen atmo-
sphere. The resultant mixture was poured into 0.1 M HCl
solution (100 mL). The product was filtered and recrystallized
in DCM/petroleum ether. I-4 was obtained as a dark purple
solid (0.34 g, 64%). 1H NMR (500 MHz, DMSO) d ppm: 8.11
(s, 1H), 7.64 (d, J = 3.5 Hz, 1H), 7.43 (s, 1H), 7.27–7.31
(m, 5H), 7.17–7.22 (m, 4H), 7.10 (d, J = 16.0 Hz, 1H), 6.84
(d, J = 3.5 Hz, 1H), 4.87 (t, J = 8.5 Hz, 1H), 3.84 (t, J = 8.5 Hz,
1H), 2.29 (s, 3H), 2.01–2.09 (m, 1H), 1.69–1.83 (m, 3H),
1.58–1.65 (m, 1H), 1.34–1.43 (m, 1H). 13C NMR (125 MHz,
DMSO) d ppm: 164.3, 148.2, 147.8, 145.4, 141.3, 139.9, 138.8,
136.9, 136.1, 135.7, 131.4, 130.6, 130.3, 127.4, 125.6, 124.8, 124.1,
122.5, 122.3, 120.2, 119.8, 119.6, 107.6, 68.8, 45.0, 35.2, 33.6,
24.4, 20.8. HR-MS (TOF MS ES+) m/z 535.1519 [M + H+]
calcd for C32H27N2O2S2 (M + H+) 535.1514.
When anchored on TiO2, absorption peaks of four dyes
showed blue-shifts in different extents (Fig. 1b). Apparently,
along with the broader spectra for I-3 and I-4, the cells could
be expected to exhibit better IPCE performances. Further-
more, after anchoring on TiO2, the absorption threshold was
red shifted. Take I-2 for example, when adsorbed on TiO2, the
absorption threshold was red shifted by approximately 100 nm
from 600 nm to 700 nm due to the interaction between the
carboxylate group and TiO2.4
To evaluate the possibility of electron transfer from the
excited dye molecule to the conduction band (CB) of TiO2,
cyclic voltammograms were performed in acetonitrile. Table 2
summarizes their HOMO levels and the LUMO levels.
The LUMO levels of the dyes anchored on TiO2 film lie in
the order I-1 o I-2 o I-3 o I-4, and were determined to be À1.43,
À1.35, À1.30, and À1.26 V vs. NHE, respectively, which were
more negative than the CB edge of TiO2 (À0.5 V vs. NHE),
Results and discussion
Synthesis
The chemical structures and synthetic routes of four dyes were
shown in Scheme 1. In a typical D-p-A system, the indoline
Scheme 1 Chemical structures and synthetic routes of indoline dyes (a) cyanoacetic acid, AcAN, reflux; (b) 2-PPh3Br-methyl-thiophen (5), THF,
NaH, rt; (c) POCl3, DMF, reflux; (d) 5-formylthiophen-2-ylboronic acid (3), Na2CO3, Pd(PPh3)4, THF, reflux.
c
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implemented in the Gaussian 03 program.22 The calculated
LUMO energies for I-1, I-2, I-3 and I-4 are À2.28, À1.90,
À1.84, and À1.68 V (shown in Table 2 and Fig. 2), respec-
tively, which lie in the same order with the experimental
results. The energies were overestimated partially due to
the neglect of solvation effect. Consequently, taking both the
corresponding extinction coefficient and orbital levels into
consideration, I-2 and I-3 could be expected to possess better
photovoltaic performances to a certain extent compared with
I-1 and I-4.
TDDFT calculation (See Table 3) shows the lowest transition
of charge transfer character from the donor moiety to the
acceptor, which is favorable for the charge injection. The
overestimates of the lowest transition energy for the dyes are
related to the more extended charge-transfer nature of this
transition.23 This screening strategy helps us to optimize the
TiO2-sensitizer-HTM system in terms of balance between
spectral response, driving forces, and photovoltaic performance
for future preparation of high efficiency dyes by matching
suitable p-linker groups in the D-p-A system.14,21
Photovoltaic performance evaluation
Fig. 1 Normalized absorbance and emission of dyes (a) in THF and
The IPCE action spectra of DSSCs sensitized by four dyes
without light scattering layer on TiO2 electrode are presented
in Fig. 3, which matched well with the absorbance spectra of
dye-loaded TiO2 film. Due to poor absorption (Fig. 1b), I-1
showed very low IPCE values in the visible region and
decreased to zero at about 560 nm. I-2 gained the highest
IPCE value (84%) among four dyes at about 478 nm and
exceeded 70% in the range of 415–525 nm.
(b) normalized absorbance on TiO2 in visible region.
Table 1 Absorption and emission properties measured in THF and
on TiO2
lmaxa/nm
e/MÀ1 cmÀ1
Emax/nm
lmaxb/nm
Amountc
I-1
I-2
I-3
I-4
438
492
504
518
12 200
37 400
41 500
48 600
508
603
629
644
354
447
471
480
3.56
4.18
3.87
2.43
Compared with I-2, I-3 presented a slightly lower maximum
IPCE value (77% at 480 nm), but the action region was much
broader, that is, the threshold was red-shifted from 690 nm to
about 740 nm. Therefore, the slightly higher short-circuit
current density (Jsc) of I-3 was reasonable. In the case of I-4,
although it showed the broadest spectra, the values were
relatively low, which led to the lower Jsc (6.13 mA cmÀ2).
a
b
Absorption of the dyes in THF solution. Absorption of the dyes
adsorbed on TiO2. The dye-loaded amount (Â10À7 mol cmÀ2) was
c
determined by desorbing the dye from surfaces of TiO2 electrodes into
NaOH solution and was analyzed by UV-visible spectrometer.
ensuring that the excited electron injection to the CB of
TiO2 was thermodynamically favorable. It is noteworthy that
compared with I-4, I-2 and I-3 gained more negative LUMO
levels. As expected, the higher LUMO level is preferable in
that a larger gap between LUMO and the CB will result in a
larger possibility of electronic injection into the CB of TiO2.
The orbital energies were also calculated by hybrid density
functional theory (B3LYP) with the 6-31G* basis set as
Table 2 Electrochemical properties of dyes I-1, I-2, I-3, and I-4
Experimentala (V)
Calculatedc (V)
b
HOMO
E0À0
LUMO
HOMO
E0À0
LUMO
I-1
I-2
I-3
I-4
1.19
0.90
0.88
0.84
2.62
2.25
2.18
2.10
À1.43
À1.35
À1.30
À1.26
1.04
0.71
0.66
0.44
3.32
2.61
2.50
2.12
À2.28
À1.90
À1.84
À1.68
a
Electrochemical properties of the dyes adsorbed on TiO2 films.
E0À0 values were estimated from the intersection of normalized
b
absorption and emission curves from solution measurements. c Calculated
at the B3LYP/6-31G* level in vacuum.
Fig. 2 Calculated HOMO and LUMO for I-1–I-4.
Phys. Chem. Chem. Phys., 2011, 13, 8985–8992 8989
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Table 3 Calculated TDDFT excitation energies for the lowest transi-
tion (eV, nm), oscillator strengths (f) and experimental absorption
maximaa
Table 4 Photovoltaic performance of DSSCs based on I-1–I-4 with
liquid electrolyte
Efficiency,
Z (%)
E (eV, nm)
f
Exp. (eV, nm)
Jsc/mA cmÀ2
Voc/mV
Fill factor
I-1
I-2
I-3
I-4
2.74 (452)
2.43 (509)
2.31 (537)
1.98 (626)
1.1517
0.7768
1.2936
1.0996
2.83 (438)
2.52 (492)
2.46 (504)
2.39 (518)
I-1
I-2
I-3
I-4
4.63
568
725
651
632
0.67
0.62
0.64
0.65
1.79
4.58
4.46
2.53
10.38
10.63
6.43
a
TDDFT excited state calculation was performed at the B3LYP/
6-31G* level in vacuum with the B3LYP/6-31G* optimized ground-
state geometry.
theoretical calculations analysis, which is origin-dependent
for charged species.24 Generally, the Voc is dependent upon
the difference between the CB energy level of TiO2 and the
redox potential of the HTM. Thus, a CB energy level upshift is
necessary to enhance photovoltage. Moreover, the dark-current
generated from the recombination of electrons also has a great
effect on the open-circuit photovoltage. Gratzel and coworkers
¨
found that larger dipole moment along the direction from
sensitizer to TiO2 (vertical) could suppress the dark current
more effectively, thus resulting in an increase in open circuit
voltage.25 A favorable dipole moment of sensitizer can upshift
the CB energy level efficiently. The dipole moments of I-2 and
I-3 at their optimized geometry were calculated by hybrid
density functional theory (B3LYP) with the 6-31G* basis set
as implemented in the Gaussian 03 program22 and listed in
Table 4. As shown in Fig. 5, the z-axis extends out of the plane
of the page and the TiO2 surface plane is parallel to the y- and
z-axes. Considering their bidentate binding mode, the dye
molecules are positioned in such a way that the C2 axis of
the carboxylate is parallel to the x-axis. Therefore, their
orientations after anchoring on to the TiO2 are simulated.
The components of dipoles parallel to the z-axis are fixed at
zero.26
Fig. 3 IPCE action spectra of I-1–I-4 based on liquid electrolyte in
the visible region.
To further elucidate the performance of DSSCs, we compared
I–V curves of solar cells sensitized with I-1, I-2, I-3, and I-4,
respectively. The results under illumination of AM 1.5
(100 mW cmÀ2) simulated solar light were shown in Fig. 4
and summarized in Table 4. I-3 showed the highest short-circuit
current density (Jsc) (10.63 mA cmÀ2). Except for I-4, the Jsc of
I-1–I-3 were increased from 4.63 mA cmÀ2 to 10.63 mA cmÀ2
The vertical component of dipoles (x components) of two
sensitizers possess the same direction but with different magni-
tude (10.28 D and 7.07 D for I-2 and I-3, respectively,
Table 5). Once adsorbed on TiO2, the larger dipole moment
along the direction for I-2 can lead to more negative charges
located close to the TiO2 surface than that of I-3, resulting in a
larger CB energy level upshift.
as e was enhanced from 12 200 MÀ1 cmÀ1 to 41 500 MÀ1 cmÀ1
.
Single molecule I-4 possessed the highest light capture
capability (48 600 MÀ1 cmÀ1), but the amount of I-4 adsorbed
on TiO2 electrode was the lowest one (Table 1) due to its
relatively large molecular volume. Therefore, the Jsc of I-4 was
only 6.43 mA cmÀ2, which was lower than I-2 and I-3.
For I-2–I-4, the open-circuit voltage (Voc) reduced from
725 mV to 632 mV when the p-conjugated linker of the dyes
expanded. Compared with I-2, the sharp decrease of Voc of I-3
from 725 mV to 651 mV can be explained based on the
Meanwhile, the adsorption angle y was reduced from 50.201
to 35.681 when p system was expanded. Actually, the adsorp-
tion form will affect the interfacial charge recombination to a
considerable extent, which is another major factor influencing
photovoltage.27 While the adsorption angle y was reduced
along with the expansion of the p system, the blocking moiety
Fig. 4 I–V characteristics measured under an irradiance of 100 mW cmÀ2
Fig. 5 Optimized geometrical structures and calculated total dipole
moments of I-2 and I-3.
(AM 1.5G).
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Table 5 Calculated dipole components for I-2 and I-3
Dipole moments/D
x
y
Total
y
I-2
I-3
10.28
7.07
8.57
9.84
13.38
12.12
50.201
35.681
(indoline group in these dyes) could not cover the p-linker
segment and TiO2 surface anymore and left them open to the
electrolyte, which would cause charge recombination between
À
I3 and excited electrons in the CB of TiO2. Moreover, while
À
Fig. 6 IPCE action spectra of I-1–I-4 based on ionic liquid electrolyte.
the adsorption angle y was reduced, the bound I3 would be
brought very close to TiO2 by the low-lying binding site, which
would also cause this interfacial charge recombination.26,28–30
Therefore, the charge recombination dynamic can be adjusted
through optimizing the anchoring form which could be realized
by modifying chemical structures of sensitizers.
Conclusion
In summary, novel indoline dyes, I-1–I-4, with structural
modification of p-linker group in the D-p-A system have been
synthesized and fully characterized. The photophysical, electro-
chemical, and photovoltaic properties of the dyes were tuned
efficiently. The ground and excited state properties of the
dyes have been studied by the means of density functional
theory (DFT) and time-dependent DFT (TD-DFT). The
larger p-conjugation linker would lead to broader spectral
response and higher molar extinction coefficient but decrease
dye-loaded amount on TiO2 electrode and LUMO level. The
variation trends in Jsc and Voc were observed to be opposite to
each other. The internal reasons were studied by experimental
data and theoretical calculations in detail. Beside liquid electro-
lyte based devices, quasi-solid state devices based on ionic
electrolyte have been fabricated. Notably, I-2 showed com-
parable photocurrent values based on these two devices (10.38 and
9.39 mA cmÀ2 for liquid and quasi-solid state electrolyte,
respectively), which suggested through molecular engineering
of organic sensitizers the dilemma between optical absorption
and charge diffusion lengths can be balanced well. Through
studies of photophysical, electrochemical, and theoretical
calculation results, the internal relations between chemical
structure and efficiency have been revealed, which serve to
enhance our knowledge regarding design and optimization of
new sensitizers for quasi-solid state DSSCs, providing a powerful
strategy for prediction of photovoltaic performances.
Comprehensively, the more favorable dipole moment and
adsorption form of I-2 led to a larger CB energy level upshift
and more effective suppression of charge recombination,
which in turn results in a higher open-circuit photovoltage
than I-3.
Interestingly, the above disciplines for indoline series dyes
were different from the trends of triphenylamine dyes reported
by Liang et al.,26 which was caused by the characteristic of a
certain system (indoline-thiophen in this work). Just for this
reason, in order to design and synthesize sensitizer with high
photovoltaic performance, an independent optimization
through molecular engineering is doubtlessly necessary.
Overall, DSSCs based on liquid electrolyte sensitized by
I-2 and I-3 produced similar results, with Jsc of 10.40 and
10.63 mA cmÀ2, Voc of 725 and 651 mV, ff of 0.62 and 0.64 and
efficiency Z of 4.58% and 4.46%, respectively. Under the same
conditions, I-4 only produced an overall conversion efficiency
of 2.53% due to its relatively low amount of adsorbed dyes on
TiO2.
Also, we have tested the photovoltaic performance of quasi-
solid state DSSCs using ionic liquid electrolyte consisting of
BMII/I2/BI/GuSCN (w/w, 40/1.67/0.67/3.33) and the results
are listed in Table 6. The IPCE action spectra of quasi-solid
state DSSCs were quite similar with those based on liquid
electrolyte (Fig. 6). From 400–600 nm, I-2 possessed higher
IPCE values than the others and reached a maximum value of
81% at 480 nm. Remarkably, there was no obvious reduction
of photocurrent for I-2 compared with liquid electrolyte based
devices (9.39 vs. 10.38 mA cmÀ2), which means that the
dilemma between optical absorption and charge diffusion
lengths can be balanced through rational molecular engineering
of organic sensitizers.31
Acknowledgements
This work was supported by Science Foundation of Hebei
Normal University (L2009B08) and the Nature Science Founda-
tion of Hebei Province (B2010000372).
Notes and references
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737.
¨
2 M. Pagliaro, G. Palmisano, R. Ciriminna and V. Loddo,
Energy Environ. Sci., 2009, 2, 838.
3 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595.
4 Md. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
Table 6 Photovoltaic performance of DSSCs based on I-1–I-4 with
ionic liquid electrolyte
Jsc/mA cmÀ2
Voc/mV
ff
Z (%)
E. Mueller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am.
¨
Chem. Soc., 1993, 115, 6382.
5 J. H. Yum, I. Jung, C. Baik, J. Ko, Md. K. Nazeeruddin and
M. Gratzel, Energy Environ. Sci., 2009, 2, 100.
¨
6 Md. K. Nazeeruddin, P. Pechy and M. Gratzel, Chem. Commun.,
¨
1997, 1705.
I-1
I-2
I-3
I-4
3.07
9.39
7.76
6.13
552
698
640
630
0.61
0.51
0.51
0.56
1.04
3.37
2.38
2.34
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 8985–8992 8991

View Article Online
7 P. Wang, S. M. Zakeeruddin, J. E. Moser, Md. K. Nazeeruddin,
T. Sekiguchi and M. Gratzel, Nat. Mater., 2003, 2, 402.
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
J. A. Pople, GAUSSIAN 03 (Revision C.01), Gaussian, Inc.,
Wallingford, CT, 2004.
¨
8 P. Wang, C. Klein, R. Humphrey-Baker, S. M. Zakeeruddin and
M. Gratzel, J. Am. Chem. Soc., 2005, 127, 808.
¨
9 T. Horiuchi, H. Miuraa and S. Uchida, Chem. Commun., 2003,
3036.
10 S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet,
P. Comte, Md. K. Nazeeruddin, P. Pechy, M. Takata, H. Miura,
´
S. Uchida and M. Gratzel, Adv. Mater., 2006, 18, 1202.
¨
11 B. Liu, W. H. Zhu, Q. Zhang, M. Xu, Z. Ning, Y. Xie and H. Tian,
Chem. Commun., 2009, 1766.
12 T. Le Bahers, T. Pauporte, G. Scalmani, C. Adamo and I. Ciofini,
Phys. Chem. Chem. Phys., 2009, 11, 11276.
13 D. Kim, J. K. Lee, S. O. Kang and J. Ko, Tetrahedron, 2007,
63, 1913.
14 D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura,
P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt and L.-C. Sun, J. Org.
Chem., 2007, 72, 9550.
15 T. Marinado, D. P. Hagberg, M. Hedlund, T. Edvinsson, E. M.
J. Johansson, G. Boschloo, H. Rensmo, T. Brinck, L.-C. Sun and
A. Hagfeldty, Phys. Chem. Chem. Phys., 2009, 11, 133.
16 H. Choi, I. Raabe, D. Kim, F. Teocoli, C. Kim, K. Song,
23 A. Dreuw and M. Head-Gordon, J. Am. Chem. Soc., 2004,
126, 4007.
24 P. Chen, J. H. Yum, F. De Angelis, E. Mosconi, S. Fantacci,
S. J. Moon, R. Humphry Baker, J. Ko, Md. K. Nazeeruddin and
M. Gratzel, Nano Lett., 2009, 9, 2487.
¨
J.-H. Yum, J. Ko, Md. K. Nazeeruddin and M. Gratzel,
¨
Chem.–Eur. J., 2010, 16, 1193.
25 J. Kruger, U. Bach and M. Gratzel, Adv. Mater., 2000, 12,
¨
¨
447.
17 J. Tang, J. L. Hua, W. J. Wu, J. Li, Z. G. Jin, Y. T. Long and
H. Tian, Energy Environ. Sci., 2010, 3, 1736.
26 Y. L. Liang, B. Peng and J. Chen, J. Phys. Chem. C, 2010,
114, 10992.
18 K. Hara, Z. S. Wang, T. Sato, A. Furube, R. Katoh, H. Sugihara,
Y. Dan-oh, C. Kasada, A. Shinpo and S. Suga, J. Phys. Chem. B,
2005, 109, 15476.
19 Z. S. Wang, Y. Cui, K. Hara, Y. Dan-Oh, C. Kasada and
A. Shinpo, Adv. Mater., 2007, 19, 1138.
27 B. A. Gregg, F. Pichot and S. Ferrere, J. Phys. Chem. B, 2001,
105, 1422.
28 A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski and
M. Gratzel, Nano Lett., 2008, 8, 977.
29 Z. S. Wang and G. Zhou, J. Phys. Chem. C, 2009, 113, 15417.
30 D. P. Hagberg, J.-H. Yum, H. Lee, F. De Angelis, T. Marinado,
K. M. Karlsson, R. Humphry-Baker, L. Sun, A. Hagfeldt,
20 L. Y. Luo, C. J. Lin, C. S. Hung, C. F. Lo, C. Y. Lin and
E. W. G. Diau, Phys. Chem. Chem. Phys., 2010, 12, 12973.
21 S. Roquet, A. Cravino, P. Leriche, O. Ale
´
veque, P. Frere and
M. Gratzel and Md. K. Nazeeruddin, J. Am. Chem. Soc., 2008,
¨
130, 6259.
J. Roncali, J. Am. Chem. Soc., 2006, 128, 3459.
22 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
31 H. Qin, S. Wenger, M. Xu, F. Gao, X. Jing, P. Wang,
S. M. Zakeeruddin and M. Gratzel, J. Am. Chem. Soc., 2008,
130, 9202.
¨
c
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