A new sensitizer containing dihexyloxy-substituted triphenylamine as donor and a binary conjugated spacer for dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra07939j

The synthesis and application to dye-sensitized solar cells (DSSC) of a new dihexyloxy-substituted triphenylamine-based organic dye is reported. The dye design, based on the push-pull concept, consists of dihexyloxy-substituted triphenylamine as an electr

Full text of DOI:10.1039/c5ra07939j

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M. Mihaila, C. Constantin, S. Chisca, B. Serban, C. Diaconu, O. Buiu, E. M. Pavelescu and M. Kusko, RSC
Adv., 2015, DOI: 10.1039/C5RA07939J.
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ARTICLE
A new sensitizer containing dihexyloxy-substituted
triphenylamine as donor and a binary conjugated spacer for dye-
sensitized solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
a*
b,c
a
a
DOI: 10.1039/x0xx00000x
Mariana-Dana Damaceanu , Mihai Mihaila , Catalin-Paul Constantin , Stefan Chisca , Bogdan-
b
b
b
c
c
Catalin Serban , Cristian Diaconu , Octavian Buiu , Emil Mihai Pavelescu , Mihaela Kusko
www.rsc.org/
The synthesis and application to dye-sensitized solar cells (DSSC) of a new dihexyloxy-substituted triphenylamine-based
organic dye is reported. The dye design, based on the push-pull concept, consists of dihexyloxy-substituted triphenylamine
as an electron donor and a cyanoacrylic acid as an anchoring and electron acceptor connected through a π-conjugated
bridge. The electron spacer containing furan and thiophene moieties were employed in the dye sensitizer for expansion of
the π-conjugated fragment as to adjust the absorption spectra and HOMO/LUMO levels of the dye. The relationship
between the dye chemical structure and the photophysical, electrochemical, and photovoltaic properties by spectral,
electrochemical, photovoltaic experiments, and density functional theory calculations was thoroughly investigated. The
photovoltaic performance of the dye as sensitizer was assessed in DSSC cells realized using electrolytes containing the
iodide/triiodide redox couple. The cells obtained with the new dye show a power conversion efficiency of 5.14 %, without
using any coadsorbant and optimization.
6-8
attainable structural tuning and less environmental issues
.
1
.Introduction
Many of these structures are of push-pull type featuring a
donor (D) and acceptor (A) segments connected through a π-
9 10,11
conjugated bridge (D−π−A ). Indoline , triphenylamine
In response to energy demands and environment pollution,
intensive efforts have been carried out to find and develop
sustainable energy solutions. Solar energy is one of the fastest
growing renewable energy sources: it does not produce
pollutants or harmful byproducts and is free of emissions.
Organic solar cells emerged as highly promising and cost-
,
1
2,13
14,15
coumarin
, carbazole
etc, were widely used as donor
moieties in the design of sensitizers, while cyanoacrylic acid
was by far the most commonly used anchoring and electron
acceptor unit. The structure of π-conjugated bridge can be
easily tuned and can comprise a variety of functional units
such as phenylenevinylene, benzothiadiazole, thiophene,
1
effective alternative for the photovoltaic energy sector .
Among them, dye-sensitized solar cells (DSSCs) have received
considerable attention due to their high performances and low
2
cost of production . An important feature of the DSSC is the
6
,16
dithienothiophene, EDOT, selenophene etc
.
For further development of high performance organic dyes
in DSSCs, these have been designed in order to absorb most of
the radiation of solar light in visible and near-IR regions and to
generate a substantial photocurrent response. In addition, the
charge transfer process between the electron donor and
acceptor units in the dye must allow the rapid electron
injection from the organic dye into the conduction band of the
2
dye anchored on the TiO surface, which act as light absorber.
Several metal-organic complexes have been widely
investigated as sensitizers of DSSCs. Among them, ruthenium-
based sensitizers exhibit the highest validated power
3
-5
conversion efficiency
.
Metal - free organic chromophores are considered as a
viable alternative to ruthenium dyes in DSSCs due to their well-
known advantages: high molar extinction coefficients, versatile
and relatively cheap synthesis (i.e., no noble metal is involved),
2
wide band semiconductor, i.e. TiO , which is a very important
issue which has to be addressed for obtaining efficient DSSCs.
Another distinguishable feature consists in the use of a binary
π-conjugated spacer, aside from the blocks of a lipophilic
alkoxy-substituted electron-donor (D) and
cyanoacrylic acid electron-acceptor (A).
a hydrophilic
a.
"
Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A,
Iasi-700487, ROMANIA. *Corresponding author, e-mail: damaceanu@icmpp.ro
Honeywell Romania, Sensors and Wireless Laboratory, 3, George Constantinescu,
BOC Tower, 020339 Bucharest, Romania
National Institute for Research and Development in Microtechnologies (IMT),
1
b.
Triarylamine moiety has been widely used in the design of
opto- and electro-active materials due to its good electron
donating and transporting ability. Therefore, the functional
materials incorporating triarylamine as electron donor unit
were intensively explored in the field of solar cells. These
c.
26A, Erou Iancu Nicolae Street, 077190, Bucharest, Romania
† Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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materials have significantly enhanced the conversion efficiency Vis absorption spectra, as well as the HOMO/LUMO levels of
1
7,18
of the solar cells, especially of DSSCs
. Also, the use of the dye. One C=C bond was introduced in the spacer between
thiophene moiety in the conjugated linker was expected to the two heterocycles and another one between the spacer and
provide a red-shift of the absorption spectra, thus broaden the the acceptor unit in order to maintain the extended
dye absorption region, which may lead to an appropriate conjugation level. The organic dye has a cyanoacrylic acid
lowest unoccupied molecular orbital (LUMO) energy. On the group as electron-withdrawing part and for anchoring onto the
2
other hand, it was proved that furan ring can be employed in TiO surface. The preliminary performances of the novel dye as
the dye-sensitized solar cells as a stabilizing unit, enlarging the photosensitizer in DSSCs are also reported.
selection range of building blocks for further dye design.
Moreover there is a trade-off between enhanced photocurrent
2
. Experimental Section
and reduced photovoltage along with the use of
electropositive heteroatoms as well as more conjugated
19
units .
To get a better insight into the relationship between dye
structure, photo-physical behaviour, electrochemical
2
.1. Starting materials
Details about starting materials are available in the ESI
-(Hexyloxy)-4-iodobenzene was prepared by the reaction of
p-iodophenol and 1-bromohexane in DMF, in the presence of
†
.
1
properties, and the performance of DSSCs based on metal-free
sensitizers, a new D-π-A organic dye based on dihexyloxy-
2
1
2
0
K
2
CO3,
according to literature data . Diethyl-[(3-
substituted triphenylamine moiety
(
BCS-1
)
has been
hexylthiophen-2-yl)methyl]phosphonate was synthesized
2
according to reference , starting from 3-hexylthiophene.
designed, synthesized, and fully characterized for the use in
DSSCs. The acronym was chosen as to concord the ones given
in the patent. The hexyloxy groups were attached to the
triphenylamine to enhance the extent of electron
2
2
.2. Synthesis
delocalization, thus the ability of electron donating of the The synthetic pathway to BCS-1 dye is outlined in scheme 1
sensitizer. An electron spacer containing furan and thiophene and the details are described as follows.
moieties were employed in the dye sensitizer for the
expansion of the π-conjugated fragment as to adjust the UV –
Scheme 1. Synthetic pathway of the BCS-1 dye (1: 4-bromo-N,N-bis(4-(hexyloxy)phenyl)aniline; 2: 5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)furan-2-carbaldehyde; 3: N,N-(bis(4-
hexyloxy-phenyl)-N-(4-(5-(2-(3-hexylthiophen-2-yl)vinyl)furan-2-yl)phenyl- aniline; 4: 3-{5-[2-(5-{4-[bis(4-hexyloxy-phenyl)-amino]-phenyl}-furan-2-yl)-vinyl]-3-hexyl-thiophen-2-yl}-
2
-carbaldehyde; BCS-1: 3-{5-[2-(5-{4-[bis(4-hexyloxy-phenyl)-amino]-phenyl}-furan-2-yl)-vinyl]-3-hexyl-thiophen-2-yl}-2-cyanoacrylic acid)
2
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4-bromo-N,N-bis(4-(hexyloxy)phenyl)aniline (
1)
phosphonate dissolved in 17 mL anhydrous THF were
In a 100 mL three-necked round-bottomed flask equipped with introduced dropwise in the flask under strong stirring, being
a magnetic stirring bar and nitrogen inlet and outlet, 1- accomplished by colour change to dark-red. Then, 1.168 g
(hexyloxy)-4-iodobenzene (9.12 g, 30 mmol), 4-bromoaniline (2.233 mmol) of 2 dissolved in 20 mL anhydrous THF was
(2.064 g, 12 mmol), 1,10-phenanthroline (432 mg, 2.4 mmol) added dropwise for 30 minutes under nitrogen stream while
and toluene (50 mL) were placed. The resulting solution was maintaining the flask in the ice bath. The temperature was
0
°
heated at 100 C under vigorous stirring and potassium allowed to reach 25 C and stirring was continued for another 3
hydroxide (5.376 g, 96 mmol) and cuprous chloride (237.6 mg, hours. The reaction was not quenched with water nor
2
.4 mmol) were added under nitrogen. The reaction mixture processed by extraction. The resulting mixture was brought to
0
was refluxed at 120 C overnight (21 h) when the solution dryness and further purified by column chromatography using
became darker (violet), followed by cooling to room successively hexane: ethyl acetate 9/1, v/v and hexane: ethyl
temperature. The reaction was quenched with 200 mL water. acetate 7/3, v/v, to yield a viscous red-brown oil (1.037 g,
1
The crude product was extracted three times with 67% yield). H-NMR (CDCl
3
, 400 MHz), δ (ppm): 7.50-7.48 (d,
dichloromethane (DCM), and the combined organic layer was 2H), 7.13-7.09 (d, 1H), 7.06-7.04 (d, 4H), 6.94 - 6.92 (d, 2H),
washed with water and dried over anhydrous sodium sulphate. 6.88 (s, 1H), 6.83-6.81 (d. 4H), 6.75 (s, 1H), 6.70-6.64 (d, 1H),
After removing the solvent under reduced pressure, the oil 6.50-6.49 (d, 1H), 6.35-6.34 (d, 1H), 3.94 (t, 4H), 2.55 (t, 2H),
residue was purified by column chromatography using hexane 1.77 (m, 4H), 1.60 (m, 2H), 1.46 (m, 4H), 1.36-1.24 (m, 14H),
successively hexane and hexane: ethyl acetate 30/1, v/v to 0.90 (m, 9H).
1
obtain a brown oil (3.1g, 51% yield). H-NMR (DMSO, 400 3-{5-[2-(5-{4-[bis(4-hexyloxy-phenyl)-amino]-phenyl}-furan-2-
MHz), δ (ppm): 7.31-7.28 (d, 2H), 7.01-6.99 (d, 4H), 6.90-6.88 yl)-vinyl]-3-hexyl-thiophen-2-yl}-2-carbaldehyde (4)
(d. 4H), 6.67-6.65 (d, 2H), 3.92 (t, 4H), 1.69 (m, 4H), 1.39 (m, In a Schlenk tube, Vilsmeier reactive was prepared under
4
H), 1.31 (m, 8H), 0.889 (t, 6H).
nitrogen stream and stirring by dropping 925.35 mg (6.04
mmol, 0.56 mL) POCl over 441.5 mg (6.04 mmol) anhydrous
DMF maintained in an ice bath for 5-10 minutes. After
.05 g (5.997 mmol) of 1 dissolved in 10 mL anhydrous THF, occurrence of a slightly opaque ice, 35 mL dichloroethane was
.148 mg (8.208 mmol) 5-formyl-2-furanboronic acid and 85 added to the Vilsmeier reactive and the stirring continued for 1
5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)furan-2-
3
carbaldehyde (2)
3
1
0
mL anhydrous THF were placed in a Schlenk tube under strong hour at 0 C, when a white opalescent solution was obtained.
nitrogen stream. The resulting solution was stirred at room To this solution, 842 mg (1.225 mmol) of 3 dissolved in 35 mL
temperature under nitrogen for several minutes, after that 35 dichloroethane was added dropwise under nitrogen stream for
0
mL aqueous K CO solution (2M) and 250 mg (0.216 mmol) 30 minutes. Then, the reaction was run 4 hours at 0 C and
2
3
Pd(PPh ) were added when the colour changed to green- quenched with cold water. Neutralization was done by
3
4
0
black. The reaction mixture was stirred at 78 C overnight, extracting the reaction mixture with saturated sodium
cooled down to room temperature, and quenched with 20 mL bicarbonate solution to turn the colour from purple to orange-
water. The crude product was extracted three times with red, when the organic phase was extracted with DCM and
DCM, and the combined organic layer was washed with water wash with water. After drying over anhydrous sodium sulphate
and dried over anhydrous sodium sulphate. After removing the and the solvent evaporation under reduced pressure the
solvent under reduced pressure, the residue was purified by residue was purified by column chromatography using hexane:
column chromatography (hexane: ethyl acetate 30/1, v/v and ethyl acetate 9/1, v/v, affording a reddish-brown clay (750 mg,
1
hexane: ethyl acetate 7/3, v/v) to afford a viscous yellow-green 86% yield). H-NMR (DMSO, 400 MHz), δ (ppm): 9.98 (s, 1H),
1
oil (1.114 g, 35.6 % yield). H-NMR (DMSO, 400 MHz), δ (ppm): 7.62-7.59 (d, 2H), 7.29 (s, 1H), 7.18 (s, 1H), 7.12 (s, 1H), 7.06-
9
.51 (s, 1H), 7.66-7.64 (d, 2H), 7.61-7.60 (d, 1H), 7.11-7.09 (d, 7.04 (d, 4H), 6.94 - 6.92 (d, 4H), 6.85-6.84 (d, 1H), 6.80-6.78 (d.
H), 7.04-7.03 (d, 1H), 6.96-6.93 (d. 4H), 6.78-6.76 (d, 2H), 3.95 2H), 6.77-6.76 (d, 1H), 3.95 (t, 4H), 2.93 (t, 2H), 1.71 (m, 4H),
4
(t, 4H), 1.71 (m, 4H), 1.40 (m, 4H), 1.32 (m, 8H), 0.90 (t, 6H).
1.63 (m, 2H), 1.42 (m, 4H), 1.36-1.24 (m, 14H), 0.89 (m, 9H).
N,N-(bis(4-hexyloxy-phenyl)-N-(4-(5-(2-(3-hexylthiophen-2-
3-{5-[2-(5-{4-[bis(4-hexyloxy-phenyl)-amino]-phenyl}-furan-2-
yl)vinyl)furan-2-yl)phenyl- aniline (
3
)
yl)-vinyl]-3-hexyl-thiophen-2-yl}-2-cyanoacrylic acid (BCS-1)
326 mg (2.903 mmol) t-BuOK were placed in a Schlenk tube, In a Schlenk tube, 633 mg (0.885 mmol) of 4 dissolved in 10 ml
dried under vacuum and purged with nitrogen, after that 17 chloroform and 225.9 mg (2.656 mmol) of cyanoacetic acid
mL dry THF were added and cooled in an ice bath. 851.6 mg were placed. This mixture was stirred for 10 minutes under
(2.68 mmol) of diethyl [(3-hexylthiophen-2-yl)methyl]- nitrogen stream, and then 527.6 mg (6.196 mmol, 0.61 mL)
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piperidine was added dropwise to the mixture and other 23 The electrochemical cell was equipped with three-electrodes:
mL of chloroform. The reaction mixture was kept under stirring a working electrode (ITO-coated glass covered with the dye or
0
and gentle reflux (65 C) for 12 h. The reaction was quenched platinum), an auxiliary electrode (platinum wire) and a
with 75 mL HCl 2M, when the colour turned from cherry-red to reference electrode (silver wire coated with AgCl).
purple. The organic phase was extracted 3 times with DCM and Tetrabutylammonium perchlorate (TBAP)/acetonitrile was
washed three times with water as the last washing water to used as supporting electrolyte for the dye coating samples and
have pH = 5.5. After drying over Na
evaporation, the residue was purified by column concentration of 1
chromatography using successively DCM and DCM: methanol respect to Ag/AgCl electrode under ambient conditions, at the
2 4
SO and solvent TBAP/DCM or TBAP/DMF for solution samples having the
−
3
×
10 M. All potentials were reported with
1
1
0/1, v/v, to afford a purple powder (470 mg, 61.8% yield). H- scan rate of 50 mV/s. Ferrocene was used as an external
1
NMR (DMSO, 400 MHz), δ (ppm): H-NMR (DMSO, 400 reference for calibration (+0.425 vs Ag/AgCl).
MHz), δ (ppm): 8.17 (s, 1H), 7.62-7.60 (d, 2H), 7.28 (s, 1H),
7
4
4
1
.24-7.00 (d, 1H), 7.20 (s, 1H), 7.05-7.03 (d, 4H), 6.93 - 6.91 (d, 2.5. Solar cell fabrication and characterization
H), 6.85-6.84 (d, 1H), 6.80 (s, 1H), 6.79-6.78 (d, 2H), 3.95 (t,
A 10 μm TiO
dense TiO barrier layer, grown on the FTO conducting glass
Solaronix, Aubonne, Switzerland). Then, a 4 μm scattering
layer was grown on the mesoporous layer. Both layers were
treated in a TiCl atmosphere. The TiO film was functionalized
2
mesoporous layer was screen printed on a
H), 2.71 (t, 2H), 1.71 (m, 4H), 1.58 (m, 2H), 1.42 (m, 4H), 1.33-
1
3
2
.24 (m, 14H), 0.90 (m, 9H).
3
C-NMR (CDCl , 100
(
MHz), δ (ppm): 168.66, 155.79, 155.50, 150.98, 148.69,
1
1
3
2
7
43.66, 140.33, 129.34, 128.06, 126.80, 124.99, 122.20,
20.02, 117.59, 115.42, 114.67, 106.21, 68.36, 67.95, 43.42,
1.93, 31.62, 30.90, 29.69, 29.35, 28.98, 25.77, 25.61, 22.67,
4
2
using a 0.3 mM solution of the dye. Different solvents or
solvent combinations have been used to prepare the
functionalization solutions; the immersion time has been
varied between 2.5 and 16 h. Samples with and without
2.60, 22.53, 14.05, 13.99. MS (ESI, m/z): calc for C34
¯
98.4066; found, 797.3412 [M - H] .
H
24
N
O
4 3
,
additive (chenodeoxycholic acid, CDCA) were prepared. In
0
film was re-fired at 450 C, for 30
2.3. Preparation of dye coatings
some experiments, the TiO
2
Very diluted dye solutions in DCM with concentration of 0.5-1 minutes, slowly cooled and immersed into the
0
weight were used to obtain thin coatings having the functionalization solution at 80 C. After functionalization, the
%
thickness in the range of nanometers onto glass and ITO- film was fully rinsed with ethanol and soaked in nitrogen flux.
coated glass substrates by drop-casting technique. The as- A platinized FTO was used as counter electrode (Solaronix).
2
prepared dye solutions were filtered through a 0.45 µm filter The functionalized TiO film was encapsulated using a platinum
before drop-casting. In order to remove the residual solvent, counter electrode with a Surlyn sealant (Solaronix). A hole
these coatings were gradually heated from room temperature practiced into the counter electrode was used to introduce the
up to 50°C, and kept at 50°C for 3 h and were used afterwards electrolyte. The hole was sealed by heating using a Surlyn
for UV-Vis, fluorescence and electrochemical properties sheet and a small glass cover. Two different commercial
investigations.
electrolytes (HI-30 and AN-50, Solaronix) were used, both
¯
¯
based on I /I
¯
3
redox couple, with two different I3
2
.4. Measurements
concentrations, 30 mM (HI-30) and 50 mM (AN-50), with ionic
liquid, lithium salt and pyridine derivative used as additives
1
The H- and
13
C-NMR spectra were recorded on a
2
and acetonitrile as solvent. The device area was 0.36 cm . The
BrukerAvance III 400 spectrometer, equipped with a 5 mm
multinuclear inverse detection probe, operating at 400.1 and
current-voltage (I-V) characteristics of the cells were recorded
using a Keithley I-V tracer, while samples were illuminated
with broad band ozone-free Xe source through a standard
1
13
1
00.6 MHz for H and C nuclei, respectively. H and
13
1
C
chemical shifts are reported in δ units (ppm) relative to the
residual peak of the solvent.
2
AM1.5G filter, at a power density of 1 Sun (100 mW ⁄ cm )
measured by a National Institute of Standards and Technology
traceable thermopile detector. The interfacial transfer
processes have been analyzed using the electrochemical
impedance spectroscopy (EIS) technique, under dark and
illumination conditions. The EIS measurements were
performed on a PARSTAT 2273 (Princeton Applied Research),
under constant light illumination of 0.14 Sun. The spectra were
recorded over a frequency range from 10 mHz to 500 kHz with
logarithmic point spacing, at a low AC amplitude of 10
mV (RMS) to maintain the response linearity in open circuit
conditions.
Mass spectral (MS) data were collected on an Agilent 6520
Accurate Mass ESI Q-ToF Liquid Chromatography/Mass
Spectrometer.
The infrared (FTIR) spectra were registered with a FTIR
Bruker Vertex 70 Spectrophotometer in transmission mode by
using KBr pellets.
The UV-Vis absorption and photoluminescence spectra
were recorded at room temperature on Specord M42 and
Perkin Elmer LS 55 instruments, respectively, by using very
−
5
diluted dye solutions (≈ 10 M) in DCM or DMF and very thin
coatings on glass plates.
Electrochemical properties of the dye were investigated by
2
.6. Computational calculations
cyclic voltammetry (CV) which was performed on
a
Bioanalytical System, Potentiostat-Galvanostat (BAS 100B/W).
4
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−
1
−1
A slightly modified molecular structure of BCS-1 was used for cm and 967 cm attributed to the =C-O-C= vibration or
simulations - BCS-1m (whose structure is shown in ESI , figure heteroatom ring deformation of the furan ring. The band from
S6), in which the hexyl groups were replaced by methyl units 3039 cm is assigned to the C–H stretching on phenylene
†
−
1
-1
to avoid geometry optimization difficulties due to the units, while the system band from 2954-2853 cm
floppiness of the long alkyl chains. The geometry of BCS-1m corresponds to aliphatic C-H stretching of the alkyloxy and
was optimized using the density functional theory (DFT) with alkyl lateral groups of the dye. The bands at 1601 and 1505
3,24
2
-1
and 6- cm were assigned to aromatic C=C stretching in thiophene,
the B3LYP exchange-correlation functional
2
5-27
-1
This approach for geometry furan and phenylene rings, while the band at 1239 cm were
3
11++G** basis set
.
2
optimization has been noted to be robust .
8
associated with the C-O-C stretching in hexyloxy units.
−
1
The UV-Vis spectrum of the dye was simulated by Vibrations at 1480, 1414, 1261 and 1164 cm originate from
computing the excited states using the time-dependent (TD) the stretching of C-C and C=C bonds in the thiophene ring.
DFT with the CAM-B3LYP functional and the 6-311++G** basis Further IR bands corresponding to the C-S bond in the
−
1
set using the B3LYP/6-311++G** optimized geometry. CAM- thiophene ring can be found at 825 cm . The FTIR spectrum of
2
B3LYP is a range-separated hybrid functional and it has been BCS-1 dye is available in ESI† (figure S1).
9
1
found to successfully describe charge-transfer excited
0,31
In the H-NMR spectrum (figure 1) the proton belonging to
the thiophene ring was evidenced by the singlet at 7.29 ppm
3
states
.
All DFT and TD-DFT calculations were performed using (H-3), whereas the protons of the furan unit (H-6,H-7) gave
3
Gaussian09 ; molecular and orbitals figures, along with UV-Vis splitted patterns that are partially overlapped with the signals
2
3
3
spectra were produced using GaussView Version 5 .
corresponding to the aromatic protons of the triphenylamine
H-11). The protons belonging to the phenylene units fall at
.62-7.60 ppm (d, H8), 7.06-7.03 (d, H-11), 6.94-6.91 (d, H-10)
(
7
and 6.78 ppm (H-9, overlapped signals). The most shielded
signal at 8.17 ppm was attributed to the proton H-2 of
cyanoacrylic acid. The two doublets from 6.85-6.84 and 6.80-
3
. Results and discussion
6
.79 ppm were assigned to the protons H-4 and H-5,
3
.1. Synthesis and structural characterization of BCS-1 dye
respectively, of the vinylene bridge between thiophene and
The new sensitizer combines some structural features furan rings. The protons belonging to the hexyloxy groups
which are usually encountered in the design of other dyes: the were evidenced by the triplet at 3.96-3.93 ppm (H-12) and the
triphenylamine containing two hexyloxy substituents as donor multiplets at 1.75-1.68 ppm (H-13) and at 1.44-1.41 ppm (H-
moiety, the linker containing both thiophene and furan rings 14), while the methylene protons H-17 and H18 of the hexyl
connected through a vinylene bridge, and the cyanoacrylic acid group appeared at 2.74-2.70 ppm (triplet) and 1.58-1.56 ppm
used both as anchoring group and electron acceptor. A long (multiplet), respectively. All the other aliphatic protons (H-15,
alkyl chain on the thiophene ring was attached in order to H-16, H-19, H-20) of these groups gave overlapped signals at
3
4
13
suppress the aggregation and minimize the recombination . 1.33-1.24 ppm and 0.91-0.86 ppm. In the C-NMR spectrum
The cyanoacrylic acid group is essentially coplanar with respect the presence of carboxylic group in the BCS-1 dye was
to the thiophene unit, reflecting the strong conjugation across evidenced by the most shielded signal at 168.66 ppm, while
the linker and the anchoring group. Thus, the electrons the nitrile group appeared at 117.59 ppm (figure S2, ESI
distribution is accomplished: they move from the
†
).
triphenylamine moiety through the π-conjugated furan-vinyl-
1
6
3
15 15 14 13 12
thiophene bridge to the cyanoacrylic acid acceptor by means
of photoexcitation of the dye (a π-π* transition), and then are
CH CH CH2CH CH CH O
11
H
2
2
2
2
1
H
0
9
H
8
H
7
H
6
H
2
injected to the conduction band of TiO via the anchoring
5
H
NC
1
carboxyl group. This photoexcitation-induced intramolecular
charge transfer occurring in the BCS-1 dye is analogous to that
tacking place in a Ru-complex photosensitizer and leads to a
N
COOH
S
O
4
H
1
0
H
2
H
20 19 19 19 18 17
CH CH CH CH CH CH
2
H
3
3
2
2
2
2
H
11
high electron-injection yield into the conduction band of TiO
2
C
6
H
13
O
and, consequently, to an efficient charge separation in the
system. Therefore, the molecular structure of our organic-dye
photosensitizer suggests that it is promising for use in DSSCs;
thus the preliminary results of its performance are reported.
a)
1
13
The BCS-1 dye was characterized by FTIR, H- and C-NMR
spectroscopy, thereby fully proving its chemical structure. In
FTIR spectrum the dye exhibits characteristic absorption bands
−
1
of the carboxylic functional group at 3443 cm (OH stretching,
−1
wide band) and 1712 cm (C=O stretching), as well as of the
−
1
CN group at 2211 cm . The incorporation of furan ring into
the dye was evidenced by the medium-strong bands at 1021
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b)
CAM-B3LYP/6-311++G**calculations proved that this visible
band originates intrinsically from the intramolecular charge-
transfer transition from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital (LUMO).
There is no important change observed between the DCM
solution and solid state absorption. In solid state, the UV-Vis
absorption spectrum of BCS-1 dye showed a broadening
compared with its solution, the maximum from the lower
energy region being red-shifted with 9 nm compared with the
absorption of isolated molecules (figure 2). The dye is likely in
an aggregated state in DCM solution, and therefore the
absorption does not shift an appreciable amount upon
evaporation of the solvent into a thin film. It seems that this
solvent favours strong π-π interactions between dye
molecules, the molecules being predominantly in planar
conformations. This statement is sustained by a strong blue
shift (45 nm) of the main absorption maximum of BCS-1 dye in
DMF solution, proving that the dye molecules lose conjugation
in this solvent (Supporting Information, figure S3). The solvated
dye molecules are more disordered, showing that solvent-
solute interactions are prevalent in DMF solution. In fact, this
is easily visible to the naked eye: the DCM solutions are deep
purple coloured, while DMF solutions are of orange-red colour.
This is a simple example to prove that the absorption spectra
of BCS-1 dye is highly solvent sensitive, demonstrating that the
solar cell engineering strategy will be the most important
feature in achieving good power conversion efficiency.
1
Figure 1. H-NMR spectrum of BCS-1 dye (a: aliphatic region, b: aromatic region)
3
.2. Photo-physical properties
The optical properties of the BCS-1 dye were assessed on the
basis of UV-Vis absorption and photoluminescence spectra
recorded for both solutions and coatings, after irradiation with
UV light of various lengths corresponding to each absorption
maxima, with a special concern on the solvent effect.
The prominent feature of this oligomer material is its
extended conjugated π system; such extension of conjugation
is reflected in the electronic absorption spectrum.
Figure 2 displays the absorption spectra of BCS-1 dye in DCM
solution and solid state in comparison with the simulated one
(
TD CAM-B3LYP/6-311++G**).
It was found that both in solution and in solid state, BCS-1
The absorption edge (λedge), being a long-wavelength wing
of the longest-wavelength band of the spectrum, moves to
longer wavelength (smaller energy) for the dye coatings when
compared with the absorption edge of solutions, as a
consequence of different molecule conformation in
exhibited a structured band in the lower energy region,
centred at 507 and 516nm, respectively, and a less intense
peak in the UV domain, at 366nm (DCM solution) and 368
nm (film). An additional absorption peak in solution was also
detected at 303nm. The absorption bands in the lower energy
region are assignable to the π-π* transition resulting from the
conjugation between the triphenylamine moiety and
cyanoacrylic unit through the π-conjugated furan-vinyl-
thiophene bridge.
dependence on the surroundings. The optical energy band gap
opt
(
ꢀg ) could be estimated from the following equation:
ꢂꢃ
opt
ꢀg
ꢁ
ꢄedge
where h is the Planck constant, c is the light velocity, and
λ
edge
is the wavelength of the absorption edges of the optical
absorption spectra. Thus, the optical energy band gap values
corresponding to this compound are of 1.81 eV for the coating,
and 1.95 eV and 2.24 eV, for DCM and DMF solutions,
opt
respectively. The ꢀg values of BCS-1 dye decrease with the
increasing of the conjugation level that is mirrored by the
nature and degree of aggregation.
The photoluminescence (PL) spectra were recorded for
both solutions and coatings, at the excitation wavelength
corresponding to each peak from UV-Vis spectra. Figure 3
displays the PL spectra recorded by excitation with the
wavelength corresponding to the absorption maximum from
the lower energy region (507 and 463 nm for the DCM and
DMF solution, respectively, and 516 nm for the coating).
2 2
Figure2. Recorded UV-Vis spectra of BCS-1 in CH Cl solution and solid state
compared with simulated UV-Vis spectrum of BCS-1m
The origin of this electronic absorption was confirmed by
calculating the singlet electronic transitions using TD-DFT. TD
6
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According to the optimized geometry of the BCS-1 dye
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
DCM sol.
DMF sol.
Solid state
(figure 4), the benzene rings in the triphenylamine core are
non-planar and could reduce the contact between molecules.
Triphenylamine has a propeller shape, with the dihedral angles
highlighted in figures 4b and 4c of ≈50°. The two hexyloxy
substituents of the head of the molecule act just like two
umbrellas, to maintain a certain distance between the dye
2
molecules when absorbed on TiO . Moreover, the long alkyl
chain attached on the thiophene ring is expected to disturb
the tight packing of the dye molecules in π-stacks, thus
suppressing the aggregation.
500
600
700
800
900
Wavelength (nm)
Figure 3. PL spectra of BCS-1 obtained by excitation with the wavelength corresponding
to the main UV-Vis maximum
A PL band in the red region, centred at 675 nm for the
DCM solution and 625 nm for the DMF solution was registered,
while a broad and weak emission without a clear maximum,
centred at ≈ 730 nm was obtained for the dye coating.
Irradiation with the wavelength corresponding to the middle
absorption peak (approx. 360 nm) resulted in PL spectra with a
maximum at 664 nm for DCM solution, and no peak for the
solid state. In DCM solution, a red shift of the emission
Figure 4. (a) Optimized geometry of BCS-1m; (b) and (c) two dihedral angles between
the benzene rings in the triphenylamine group showing the propeller shape (hydrogen
maximum was registered as compared to the same dye in DMF atoms hidden for clarity in b and c).
solution due to the better π-electrons delocalization between
the triphenylamine moiety and cyanoacrylic unit through the
Figure 5 shows the frontier orbital plots of the HOMOs and
LUMOs of BCS-1 dye. The HOMO orbital is delocalized in large
extent over the donor part dihexyloxy-substituted
π-conjugated furan-vinyl-thiophene bridge. Since the dye
bears electron donor and acceptor groups, a charge transfer
probably occurs in the excited state from the electron-donor
to the electron-acceptor group through the conjugation core.
The shape of the emission spectra undergoes a slight change
by excitation at 360 nm, suggesting that the change of solvent
polarity exerts an impact on the emissive center. It is
noteworthy to mention the splitting of the emission band in
DMF solution in two peaks, at 547 and 606 nm, due to the
coiled conformations gained by the dye in this solvent that
impart the conjugated core in two individual fluorophores. A
new emission peak appears, indicating that the charge transfer
process is suppressed in this solvent. Excitation with the
wavelength corresponding to the highest energy absorption
peak (approx. 300 nm for both solutions and film) produced no
fluorescence in DCM solution and film, and a weak emission
-
triphenylamine moiety and the conjugated bridge (furan-vinyl-
thiophene core), while LUMO is mainly located in the electron
withdrawing group - cyanoacrylate moiety through the π-
spacer, giving the strongest D-A strength. Thus, the HOMO to
LUMO excitation induced by light irradiation could move the
electron distribution from the whole molecule to the
anchoring moiety. As depicted above, it also revealed that the
heteroaromatic π-spacer is essentially coplanar with the
cyanoacetic acid group, demonstrating an increased
delocalization of the electrons between donor and acceptor
units. Furthermore, the location of the LUMO next to the TiO
2
surface and the HOMO at the opposite side of the molecule
dye causes an easier electron injection into the semiconductor
and prevents back regeneration of the dye with injected
electrons.
peak at 540 nm in DMF solution. Figure S4 (ESI†) provides the
PL spectra of the BCS-1 dye registered by excitation at 360 nm
in DCM and DMF solution.
3
.3. Theoretical calculations
The effect of molecular structure and electron distribution of
BCS-1 on the performances of dye-sensitized solar cells were
surveyed by optimization of its geometry and HOMO – LUMO
energies by using density functional theory (DFT) calculation at
the B3LYP/6-311++G** level with Gaussian09 (details in ESI†).
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The first oxidation peak, occurring at Eox1= 0.72 V, is caused by
the formation of the triphenylamine cation radicals. Due to the
strong electron releasing properties of hexyloxy groups toward
the tri-arylamine moiety, the BCS-1 dye exhibits the oxidation
process at
a much lower potential value than the
corresponding tri-arylamines without alkyl groups that usually
3
5
give one oxidation peak at 1-1.2 V . The redox process and
the resonance forms of BCS-1 are displayed in scheme 2.
6 13
C H O
Oxidation
.
NC
+
N
Figure 5. Frontier orbitals of BCS-1dye computed with CAM-B3LYP/6-311++G**
ClO-
S
COOH
COOH
COOH
O
6 13
C H
6 13
C H O
3
.4. Electrochemical properties
To judge the possibilities of electron transfer from the excited
BCS-1 dye molecule to the conductive band of TiO and the
resonance
C6H13
O
6 13
C H O
2
NC
- 1e
-
NC
.
+
N
dye regeneration, its redox behaviour in solution and solid
state was investigated by cyclic voltammetry (CV): (1)
in DCM or DMF solution using Pt as working electrode and (2)
as coatings deposited on glass substrates coated with ITO as
working electrode.
N
S
COOH
O
O
S
C
6
H
13
C6H13
C
6
H
13
O
C6H13O
-
1e-
6 13
C H O
The cyclic voltammogram of BCS-1 dye in the first CV cycle
recorded in DCM solution containing TBAP as electrolyte, in
ambient conditions, is shown in figure 6. By scanning the
potential in the anodic direction, four oxidation and two
reduction peaks were registered. The BCS-1 dye undergoes in
the first scan a reversible redox process arising from
triphenylamine unit followed by a series of quasireversible or
irreversible oxidations originating from π-bridged aromatic
moieties.
.
+
NC
N
S
O
.
+
6 13
C H
6 13
C H O
Reduction
C
6
H
13
O
C6H13O
NC
+ 1e
-
NC
.
- COOH
N
S
COOH
N
O
O
S
H
H
C
6
H
13
C6H13
C
6
H
13
O
C6H13O
resonance
6 13
C H O
-
-
-
-
-
5
5
5
5
6
2
2
1
1
5
.5x10
.0x10
.5x10
.0x10
.0x10
a)
NC
-
+
N
S
COOH
9 4
H (C H )
O
.
N
4
4
H
1.64
6 13
C H
6 13
C H O
1.43
Scheme 2.The simplified redox mechanism of BCS-1 from neutral to dication state
ox
1.16
0.72
When BCS-1 molecules were oxidized to their cationic state,
the molecules would possess positive charge and the
¯
red
0
.0
1
.03
corresponding anions from the electrolyte solution (ClO
4
)
0.64
-
6
-
5.0x10
would insert into the molecules and neutralize the charge.
After first step of oxidation, resonant reactions may occur and
0
.0
0.5
1.0
E(V) vs. Ag/AgCl
1.5
2.0
+·
the radical state of N would resonate to the phenyl group.
The further oxidation process revealed three anodic peaks
at Eox2= 1.16V, Eox3= 1.43 V, and Eox4= 1.64 V. The second peak
can be ascribed to the formation of dications resulted upon
subsequent oxidation, most probably of the furan unit, being
followed by the oxidation of the π-conjugated system (scheme
0
.0
b)
ox
-
1.53
-
-
-
5
red
-
-
-
2.0x10
4.0x10
6.0x10
-1.47
2
). The oxidation of thiophene ring cannot be taken into
5
consideration since it is substituted with the strong electron-
withdrawing cyanoacrylic group and is more susceptible for
3
6
the reduction process . Generally, by increasing the
conjugation and the number of electroactive sites, a higher
electronic delocalization is attained which may cause
differences in redox behavior of the derivatives due to the
5
-
2.0
-1.5
-1.0
-0.5
0.0
E(V) vs. Ag/AgCl
3
7
Figure 6.Cyclic voltammogram of BCS-1 in solution (a: anodic scan, b: cathodic generation of specific electrochemical species . Such feature
scan)
rd
is sustained by the absence of the 3 and 4 oxidation peaks
th
8
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when the CV was performed in DMF solution, in perfect radical cations during the oxidation process, were observed as
agreement with the UV-Vis data. In DMF, the conjugation of well. In the chatodic region, an inflection at -1.59V appeared
BCS-1 dye decreased and a lower electronic delocalization being associated with the reduction of cyanoacrylic moiety,
occurred, leading to less redox sites. Moreover, the oxidation without
a peak corresponding to its oxidation. The
peaks registered in DMF solution due to the formation of electrochemical stability of BCS-1 dye in solid state was not
radical cations and dications are positively shifted compared to possible to be evaluated since the films stripped off from the
DCM solution, at Eox1= 0.77V and Eox2= 1.19V, respectively. The electrode after the first CV wave, most probably due to the
positive shift of the anodic peak is associated with a distortion dissolution of non-covalently attached BCS-1 dye molecules
of the π-conjugated system, and is usually accompanied by a from the electrode surface after the current flow.
3
8
blue shift of the UV-Vis absorption maximum .
Knowing the HOMO and LUMO energy levels is crucial for
On the reverse scanning, two reduction processes occurred the choice of materials that can form an appropriate interface
in DCM solution, at Ered1= 1.03 V and Ered2= 0.64 V, whereas for configuration of the photovoltaic cells from the energetic
one clear reduction peak at Ered= 0.29 V was registered in DMF point of view. The energy levels of the highest occupied
solution. The intensity of the redox peaks decreased with molecular orbital (HOMO) and lowest unoccupied molecular
increasing the number of scans in DCM solution, indicating orbital (LUMO) of the investigated BCS-1 dye can be more
that the oxidation process is stable only at the triphenylamine precisely evaluated from the oxidation onset potentials
ox
red
moiety, while the other electrochemical species deactivated (E onset) and reduction onset potentials (E onset) versus
upon multiple scanning the potential in the anodic direction Ag/AgCl. The onset potentials were determined from the
(
figure 7). Instead, a very good stability of redox processes was intersection of the two tangents drawn at the rising current
observed in DMF solution (ESI , figure S5). In the cathodic and baseline charging current of the CV curves. The external
region, one reduction peak at -1.47V and -1.15V in DCM and ferrocene/ferrocenium (F
DMF solutions, respectively, was registered, being attributed versus Ag/AgCl. Assuming that the HOMO energy for the F
†
+
c
c
/F ) redox standard Eonset is 0.425 V
+
c
/F
c
to the formation of radical anions of the electron-withdrawing standard was 4.80 eV with respect to the zero vacuum level,
cyanoacrylic unit that undergoes subsequent oxidation at we can estimate the HOMO and LUMO energy levels, and the
-
1.53V only in DCM solution (scheme 2). During the repetitive energy bandgap (E
g
) (in electron volts, eV) according to the
potential swept the reduction step remained very stable in following equations:
red
ox
DCM solution since no change in the position of the cathodic
peak was observed.
E
E
LUMO = E onset + 4.37, EHOMO = E onset + 4.37
g
= EHOMO − ELUMO
The HOMO energy level was calculated to be 4.96 eV for BCS-1
dye in solution and 4.92 eV in solid state, while the LUMO
energy value reaches 3.34 eV for solution and 3.41 eV for
coating. Thus, the resulted values for the energy bandgap of
BCS-1 dye are 1.62 eV and 1.51 eV, for DCM solution and
coating, respectively. A small disparity was observed between
the band gap energy values measured by spectroscopic and
electrochemical methods. There is no conflict between these
values, since they describe distinct processes. The
spectroscopic measurements provide an estimation of the
energy difference of the frontier orbitals and the obtained
optical band gap corresponds to the energy required to form a
tightly-bound exciton, whereas the redox potentials give the
absolute energetic positions of the HOMO and LUMO levels
and the electrochemical or transport band gap corresponds to
-
5
2
2
1
1
5
.5x10
.0x10
.5x10
.0x10
.0x10
st
1
2
3
4
5
6
scan
scan
scan
scan
scan
scan
nd
rd
th
th
th
-5
-5
-5
-6
0
.0
0
.0
0.5
1.0
1.5
2.0
4
0
E(V) vs. Ag/AgCl
the energy required to form free charge carriers .
Figure 7.Electrochemical stability of BCS-1 in solution over sixsubsequent scans
The energy barriers between the BCS-1 dye and electrodes
can be estimated by comparing the edge of the conduction
¯
¯
2 3
Evaporation of a drop of diluted dye solutions in DCM with band of TiO and the potential of the redox couple (I /I ) with
concentration of 0.5 % onto an ITO electrode should result in its HOMO and LUMO energy levels. Thus, the hole injection
the formation of strong bonds between the carboxylate groups barrier is = EHOMO – 4.8 (eV) (4.8 eV is the redox couple of
vs. vacuum), whereas the electron injection barrier is
near the potentials of the solution waves after the evaporation = 4.0 – ELUMO (eV), (4.0 eV is the edge of the conduction band
ꢀE
h
3
9
¯
¯
of the dye and ITO . Characteristic surface waves are recorded I /I
3
ꢀE
e
4
1-43
step (ESI
†
, figure S5).Only two large oxidation waves were of TiO
2
vs. vacuum)
. The band structure of BCS-1 dye is
registered in the anodic region, being centred at 0.94 and 1.44 depicted in figure 8. It can be observed that the BCS-1 dye has
¯
¯
V. The first step was associated with the formation of radical a more negative HOMO energy than the I /I
3
redox couple,
cations of triphenylamine unit, while the second one may be leading to the regeneration of the oxidized dye. The more
assigned to the oxidation of furan moiety. Two reduction steps positive LUMO energy relative to the conduction band of TiO
at 1.38 and 0.63V, assigned to the reduction of the formed ensures an effective injection of excited electrons.
2
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important. Such a dependence of the efficiency on the
immersion time would indicate the presence of dye
4
4-49
aggregation
. Fig. 10 shows the action spectrum (IPCE,
Incident Photons to Current Conversion Efficiency) of the C23
cell. The IPCE, with an onset at ≈650nm and a maximum at 65
%, was recorded at 471nm. Apparently, the 36nm blue shift of
the ICPE peak does not support the hypothesis of dye
aggregation; therefore, another mechanism could be at play in
degrading the open circuit voltage. A small variation of
the VOC with the cheno content has been reported by Qu et
5
0
al. for dyes with triphenylamine moiety as donor and
diketopyrrolopyrrole as core. The explanation consists in the
reduction of the amount of the dye adsorbed by the
coadsorbate. For dyes with triphenylamine as donor and
hexyloxy chains tethered at the conjugated bridge, Chou et
5
1
al. found inferior efficiency when CDCA was added. This
might be associated with a competition between the dye and
5
CDCA absorption mechanisms on the TiO surface .
2
2
Figure 8. Band structure of BCS-1 dye vs. vacuum
Cell Soaking CDCA
time
Jsc
(mA/cm2)
8.388
Voc (V)
ff
η(%)
3
.5. Cell performance and discussion
C35
C32
C25
C23
C38
C39
3.5
3.5
5
yes
yes
yes
-
0.58
0.643 3.13%
0.623 3.89
The I-V characteristics of six cells, realized with and without
10.41
0.600
0.595
0.587
0.625
0.675
CDCA, different functionalization time and solvents are shown
in figure 9. Their complete electrical performance is presented
in the table 1. Four cells (C35, C32, C25, C23) have been
9.438
0.648 3.643
0.645 4.094
5
10.789
13.647
10.936
5
-
0.603
5.14
14
-
0.609 5.108
prepared by immersing the TiO
acetonitrile and tetrahydrofuran (6:4; v/v), while for two cells
C38 and C39), the TiO film has been functionalized in
acetonitrile:THF:ethanol (6:2:2, v/v/v). The first three cells
shown in table I have been realized with CDCA in the the absolute (Mulliken) electronegativity of the dye was
2
film in a dye solution of
Table 1. Cells’ electrical parameters
(
2
To identify the main recombination mechanism in our cells,
5
3,54
functionalization solution.
calculated with the formula (HOMO+LUMO)/2
. Using the
HOMO and LUMO data of the dye in solution from the figure 8,
the value of the electronegativity is -4.15eV. For the cells with
1
2
0
8
6
4
2
0
V
2
OC lower than 0.6V, the Fermi level in TiO is estimated to be
1
placed at about -4.2eV. Since it is slightly negative than the
dye’s absolute electronegativity, back electron transfer to the
dye is less probable. This is a strong hint that the dominant
mechanism of recombination is the electron interception by
electrolyte. Therefore, keeping the electrolyte apart from the
2
TiO surface by increasing the concentration of the adsorbed
dye would reduce the recombination with the electrolyte and,
consequently, would increase VOC. However, such an effect
C39: 14h, no CDCA, ethanol
C38: 5h, no CDCA, ethanol
C23: 5h, no CDCA
2
was not observed with the TiO functionalized in acetonitrile:
C25: 5h,
CDCA
THF (6:4; v/v), but it became visible when two parts of THF
were replaced by ethanol. One notes that for a high
C32: 3.5h, CDCA
C35: 3.5h,
0.2
CDCA, 450°C TiO2
5
5
0
0.1
0.3
0.4
0.5
0.6
0.7
performance dye (C217), Zhang et al. reported a strong
degradation of the efficiency when unsuitable solvent was
Voltage (V)
Figure 9. I-V characteristics of six cells realized with different solvent combinations and
soaking times, with and without chenodeoxycholic acid (CDCA), with (450°C) and
used. The I-V characteristics of two cells prepared with TiO
2
immersed in acetonitrile:THF:ethanol (6:2:2; v/v/v), with the
same dye concentration, no additive, but with 5 hours (C38)
and 14 hours (C39) soaking time, respectively, are shown in
without re-firing of the TiO
.88 Sun.
2
film; I-V characteristics of C38 and C39 were acquired at
0
figure 9. Both cells were measured at 0.88 Sun. For C38, both
2
OC=0.625V and ISC=12mA/cm increased and the efficiency
In the presence of CDCA, the increase of the functionalization
time (C25 vs. C32) decreases the injected current, with a minor
V
was 5.14%. The current increase can be associated with a
higher dye concentration on TiO surface, therefore a better
effect on the V . A similar effect was found in the absence of
OC
2
CDCA (C39 vs. C38), but in this case the effect on VOC is
injection, while the small VOC increase (25mV) cannot be
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unequivocally attributed to the electrolyte screening by the electrode)/electrolyte interface in high frequency domain (the
dye scaffold on TiO surface. That because a value of 25mV can first semicircle from the left); (ii) the reaction at
be easily associated with a Fermi level shift toward the edge of the TiO /electrolyte interface and the electron transport
the TiO conduction band produced by a better electron in TiO film (at intermediate frequencies, corresponding to the
2
2
2
2
injection into it. By contrast, for 14 hours soaking time, C39 main semicircle observed); (iii) the ion Nernst diffusion within
exhibited a consistent (75mV) increase of VOC=0.675V which the electrolyte (at low frequencies, partially overlapped by the
can be associated with the inhibition of the electron previous large semicircle when the measurements were made
interception by electrolyte. This is supported by the decrease under illumination and absent in dark).
of the short circuit current, a phenomenon attributable to dye
5
6
aggregation or/and specific dye-dye
or dye-electrolyte
5
interaction. In this respect, for a Ru-based dye, O’Regan et al.
7
reported that the electron recombination was promoted by
iodide binding to the oxygen atom in a hexyloxy chain tethered
at a phenyl ring. Replacing the oxygen atom by a sulphur one,
the dye properties do not change but the recombination
mechanism does, sulphur being a more efficient promoter
5
8
than oxygen. On the contrary, Robson et al. found that
oxygen on the hexyloxy chain is a more efficient promoter
5
than the sulphur. Our dye has an identical donor as C217 and
6
5
8
one of the dyes analyzed by Robson et al. , therefore a similar
5
9,60
behaviour could be expected. In addition, it was reported
that the electron lifetime in a cell manufactured with a highly-
conjugated triphenylamine-polyene dye, with a polarizability
3
of 106 Å is an order of magnitude lower than the one in a cell Figure 11. Nyquist diagrams of the C32 and C35 cells
3
realized with a dye of lower polarity (46 Å ). Since calculated
3
polarizability of our dye is 109.14 Å (details in ESI
effect would be expected.
†), a similar
Assuming for each of these interfaces a parallel resistance-
capacitor group, the resulting circuit used for EIS spectra fitting
is R (R C )(R C )(R C ). Therefore, several parameters such as
s
1
1
2
2
3 3
series resistance(Rs), charge transfer resistance at
Pt/electrolyte interface (R , R ), charge transfer resistance
1
ce
at TiO /dye/electrolyte interface (R , R ), or chemical
2
2
rec
capacitance of the TiO film (C ) can obtained from this
2
2
electrochemical model. The values obtained after fitting are
presented in the Table 2.
Cell no.
C32
R
S
R
1
C
1
R
2
C
2
R
3
C
3
8.56
10.52 6.64 253.5 41.3 41.37 19.6
C35
10.42 10.10 5.85 229.4 43.7 42.04 19.5
Table 2. Parameters obtained by fitting the experimental EIS data with
2
2
s 1 1 2 2 3 3
a R (R C )(R C )(R C ) equivalent circuit ([R] = Ω.cm , [C] = μF⁄cm )
Figure 10. Incident Photons to Current Conversion Efficiency (IPCE) of C23 cell
An increase in the series resistance is observed in C35 by the
re-firing of the film. The resistance corresponding to the
electron transfer at the counter electrode remains practically
the same, indirectly indicating that the transport of
Figure 9 also shows the I-V characteristic of a cell(C35) realized
2
with a TiO film re-fired at 450°C before functionalization. Both
JSC and VOC were degraded by re-firing. To clarify the
the I
3 2
¯ through the TiO pores is not affected by the film
degradation mechanism, both cells realized with
a 3.5h soaking time have been investigated by Electrochemical
heating. The only modification appears in the value of R
2
,
6
1-63
which decreases in the case of the heated film. It points out
that the charge recombination rate between the injected
electrons and the acceptors from electrolyte is enhanced in
the case of the heated film.
Impedance Spectroscopy (EIS)
and their Nyquist diagrams
are presented in the figure 11. The shape of both diagrams
suggests the existence of a quite strong recombination
6
Gerischer regime) in both cells. However, a diagram fit using
4
(
the ZSimWin 3.21 software did not give a good description
with the Gerischer model, especially for the non-annealed
sample (C32).
Conclusions
Therefore, we have used an equivalent circuit model which A new dye comprising dihexyloxy-triphenylamine substituted
takes into consideration: (i) charge transfer at Pt (counter moiety as donor group, furan-thiophene core as electron
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Journal Name
spacer and cyanoacrylic acid group as electron-withdrawing 11 L. Zhu, H. B. Yang, C. Zhong and C. M. Li, Dyes Pigm., 2014,
1
05, 97–104.
part has been synthesized for use in dye-sensitized solar cells.
The relationship between the new push-pull dye structure and
its photophysical behaviour, electrochemical properties and
the performance of solar cells based on it has been thoroughly
1
1
2 K. D. Seo, I. T. Choi, Y. G. Park, S. Kang, J. Y. Lee and H. K. Kim,
Dyes Pigm., 2012, 94, 469–474.
3 C. Zhong, J. Gao, Y. Cui, T. Li and L. Han, J. Power Sources,
2015, 273, 831–838.
investigated. The absorption spectra are highly solvent 14 A. Venkateswararao, K. R. Justin Thomas, C. T. Li and K. C.
Ho, RSC Adv., 2015, 5, 17953-17966.
sensitive, a strong blue shift of the main absorption maximum
being registered upon solvent polarity increasing. Occurrence
of the charge transfer in the excited state was evidenced by
1
5 K. S. V. Gupta, J. Zhang, G. Marotta, M. A. Reddy, S. P. Singh,
A. Islam, L. Han, F. de Angelis, M. Chandrasekharam and M.
Pastore, Dyes Pigm., 2015, 113, 536–545.
the dye photoluminescence band in the red spectral region, 16 J. Liu, X. Yang, J. Zhao and L. Sun, RSC Adv., 2013,
3, 15734-
for DCM solution and coating. Excitation at lower wavelengths
produced a splitting of the emission band in DMF solution in
two peaks, indicating that the charge transfer process might
be suppressed in this solvent. The electrochemical oxidation
mechanism of the dye involved the formation of very stable
15743.
1
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7 Z. J. Ning and H. Tian, Chem. Commun., 2009, 45, 5483–5495.
8 W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long and H. J.
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9 R. Li, X. Lv, D. Shi, D. Zhou,Y. Cheng, G. Zhang and P. Wang, J.
Phys. Chem. C, 2009, 113, 7469–7479.
1
radical cations at the triphenylamine moiety, while its cathodic 20 EU Pat., 13180787.7-1555, 2013.
2
1 H. Yamanishi, I. Tomita, K. Ohta and T. Endo, Mol. Cryst. Liq.
Cryst. 2001, 169, 47–61.
reduction resulted in stable radical anions of the cyanoacrylate
electron-withdrawing unit. TiO films functionalized with
various solutions of the new dye were used to test the
2
2
2 C. Wang and L. R. Dalton, Tetrahedron Lett., 2000, 41, 617–
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spectroscopy. The results correlate with the functionalization
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realized with the new sensitizer feature more than
2
2
5 R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, J. Chem.
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,
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% efficiency, without coadsorbant and optimization. Further 27 T. Clark, J. Chandrasekhar, G. W. Spitznagel and P. von Ragué
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research will be carried out to achieve better performance.
However, the nice balance of the properties encountered in
this dye provides an important guideline for enlarging the
2
8 C. J. Cramer, in Essentials of computational chemistry:
Theories and models, John Wiley & Sons, Chichester, 2nd
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selection range of building blocks for further dyes design that 29 T. Yanai, D. Tew and N. Handy, Chem. Phys. Lett., 2004, 393
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51–57.
3
3
3
0 I. Ciofini, T. le Bahers, C. Adamo, F. Odobel and D.
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ACKNOWLEDGMENTS
8
56.
2 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F.
Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E.Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
in Gaussian 09 Revision D.01,Gaussian Inc., Wallingford,
Honeywell Romania SRL acknowledges the financial support of
the Romanian National Agency of Scientific Research through
contract 274/2010 (European Union Structural Funds).
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Table of contents
A new D-π-A dye based on dihexyloxy-substituted triphenylamine moiety has been synthesized
and its preliminary performances in DSSCs are reported.