Highly efficient organic dyes containing a benzopyran ring as a π-bridge for DSSCs

Article abstract of DOI:10.1039/c3ra41583j

A series of novel organic dyes containing a benzopyran ring as a π-bridge have been designed and applied in dye-sensitized solar cells (DSSCs). This series of dyes show the excellent DSSCs' performance, due to their efficient light-to-photocurrent conversion in the region from 380 nm to 600 nm, with the highest IPCE values exceeding 90%. Through modification of the donor units, an efficiency as high as 7.5% has been achieved under standard light illumination (AM 1.5G, 100 mW cm^-2) by the dye CC103. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41583j

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Highly efficient organic dyes containing a benzopyran
ring as a p–bridge for DSSCs3
Cite this: RSC Advances, 2013, 3,
12688
Cheng Chen,a Xichuan Yang,*a Ming Cheng,a Fuguo Zhang,a Jianghua Zhaoa
and Licheng Sun*ab
A series of novel organic dyes containing a benzopyran ring as a p–bridge have been designed and applied
in dye-sensitized solar cells (DSSCs). This series of dyes show the excellent DSSCs’ performance, due to their
efficient light-to-photocurrent conversion in the region from 380 nm to 600 nm, with the highest IPCE
values exceeding 90%. Through modification of the donor units, an efficiency as high as 7.5% has been
achieved under standard light illumination (AM 1.5G, 100 mW cm22) by the dye CC103.
Received 2nd April 2013,
Accepted 9th May 2013
DOI: 10.1039/c3ra41583j
www.rsc.org/advances
thiophene derivatives,7–18 furan,19 selenophene,20 and pyr-
role21 moieties and electron-withdrawing systems, such as
Introduction
quinoline,22 isoxazole,23 and thiazole24 moieties have been
adopted as p-conjugated spacers. Herein, we report a series of
organic D–p–A dyes. Their structures are composed of
different tetrahydroquinoline donors and cyanoacrylic acid
acceptors, bridged by an electron-rich benzopyran ring. The
detailed structure is shown in Fig. 1. To clarify the effects of
the new p-bridge on the photophysics, electrochemical proper-
ties and performance of DSSCs, a dye C1-1 reported by our
group previously,25 in which a thiophene ring is employed as
the p-bridge, was chosen as a reference dye.
Dye-sensitized solar cells (DSSCs) have attracted much atten-
tion from researchers due to their low cost, easy fabrication,
and environmentally friendly nature since O’Regan and
1
¨
Gratzel reported them in 1991. Photosensitizers play a crucial
role for highly efficient DSSCs and have been extensively
investigated. Many different kinds of photosensitizers includ-
ing both metal complex sensitizers and metal-free organic dyes
have been designed and applied in DSSCs and the highest
conversion efficiency (g) value of 12.3% has been achieved in
full sunlight with the co-sensitization of porphyrin dye YD2-
o-C8 and metal-free dye Y123 by Gratzel et al. However, the
2
¨
porphyrin dyes (such as YD2-o-C8) are difficult to synthesize
and their yield is low. Compared to metal complexes, metal-
free organic dyes are a good choice due to their easier
synthesis, higher molar extinction coefficients and lower cost.
In order to gain effective photoinduced intramolecular charge
transfer characteristics, most of the efficient organic sensiti-
zers are modelled on the donor–(p-spacer)–acceptor (D–p–A)
system. For efficient solar energy conversion, it is ideal to use
dyes that possess high molar extinction coefficients and broad
absorption bands extending throughout the visible regions.
Organic sensitizers with long p-conjugated spacers had been
shown to augment the molar extinction coefficients as well as
in realize panchromatic light-harvesting, giving moderate
DSSC efficiency.3–6 Typically electron-rich systems, such as
Experimental section
Measurement
1H NMR spectra were taken with a Varian INOVA 400 MHz
spectrometer (USA) with the chemical shifts against tetra-
methylsilane. Mass spectrometry (MS) data were obtained with
GCT CA156 (UK), HP1100 LC/MSD (USA), and LC/MALDI TOF
aState Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and Research
Centre on Molecular Devices, Dalian University of Technology (DUT), 2 Linggong Rd.,
116024 Dalian, China. E-mail: yangxc@dlut.edu.cn; Fax: +86 411 84986250;
Tel: +86 411 84986247
bSchool of Chemical Science and Engineering, Centre of Molecular Devices,
Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30,
10044 Stockholm, Sweden. E-mail: lichengs@kth.se; Fax: +46-8-791-2333
Electronic supplementary information (ESI) available: [details of synthesis of
3
Fig. 1 Structure of sensitizers CC101, CC102, CC103 and C1-1.
This journal is ß The Royal Society of Chemistry 2013
sensitizers CC101–CC103]. See DOI: 10.1039/c3ra41583j
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MS (UK). The absorption spectra of the sensitizers in solution
and adsorbed on the TiO2 films were measured with HP8453
(USA). Electrochemical redox potentials were obtained by
cyclic voltammetry (CV) on electrochemistry workstation
(BAS100B, USA). The working electrode was a glass carbon
disk electrode; the auxiliary electrode was a Pt wire; and Ag/Ag+
was used as the reference electrode. LiClO4 was used as
supporting electrolyte in ethanol. The ferrocenium/ferrocene
(Fc/Fc+) redox couple was used as an internal potential
reference. The irradiation source for the photocurrent den-
sity-voltage (J–V) measurement is an AM 1.5G solar simulator
(16S-002, SolarLight Co. Ltd., USA). The incident light intensity
was 100 mW cm22 calibrated with a standard Si solar cell. The
tested solar cells were masked to a working area of 0.159 cm2.
The J–V curves were obtained by linear sweep voltammetry
(LSV) method using an electrochemical workstation (LK9805,
Lanlike Co. Ltd., China). The measurement of the incident
photon-to-current conversion efficiency (IPCE) was performed
a Hypermono-light (SM–25, Jasco Co. Ltd., Japan). The Jsc was
calibrated by integrating the IPCE value tuned light density of
AM 1.5G against wavelength. Electrochemical impedance
spectroscopy (EIS) was measured with an impedance/gain-
phase analyzer (PARSTAT 2273, USA).
Scheme 1 Synthesis of sensitizers CC101–CC103.
Fabrication of dye-sensitized solar cell
DSSCs with CC101, CC102, CC103 and C1-1 as sensitizers were
fabricated following the literature.26 A layer of 2 mm TiO2 paste
(TPP3, Heptachroma, China) was coated on the F-doped tin
oxide conducting glass (TEC15, 15V/%, Pilkington, USA) by
screen printing and then dried for 5 min at 125 uC. This
procedure was repeated six times (12 mm) and coated by a layer
of 4 mm titania paste (DHS–SLP1, Heptachroma, China) as
scattering layer. The double-layer electrode (area: 6 6 6 mm)
was sintered at 500 uC for 30 min in air. The sintered film was
further treated with 40 mM TiCl4 aqueous solution at 70 uC for
30 min, washed with water, then annealed at 500 uC for 30
min. After the film was cooled to room temperature, the film
was immersed into a 2 6 1024 M ethanol solution of dye and
kept under dark conditions for 4 h. The film was then rinsed
with ethanol and air dried. The hermetically sealed cells were
fabricated by assembling the dye-loaded film (25 mm, Dupont).
The electrolyte consisting of 0.6 M DMPII, 0.06 M LiI, 0.04 M I2
and 0.4 M TBP in CH3CN was introduced into the cell via
vacuum backfilling by the hole in the back of the counter
electrode. Finally, the holes were also sealed using Surlyn 1720
film.
Synthesis
The synthetic route of sensitizers CC101–CC103 is shown in
Scheme 1. The specific synthesis of the compounds is
described in the ESI.
3
Fig. 2 Absorption spectra of CC101–CC103 dyes in ethanol solution (a) and on
Results and discussion
TiO2 films (b).
Absorption properties
The absorption spectra of dyes CC101, CC102 and CC103 in
ethanol solution and on TiO2 film are shown in Fig. 2. The
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Table 1 Photophysical and electrochemical properties of CC101, CC102, CC103 and C1-1
E020 (V)c
EHOMO (V)d
(vs. NHE)
ELUMO (V)e
(vs. NHE)
a
b
Dye
lmax in ethanol
e at lmax
lmax on TiO2
(nm)
(M21 cm21
)
(nm)
CC101
CC102
CC103
C1-1
431
448
455
422
38 100
38 600
37 100
27 500
469
486
496
460
2.03
1.91
1.94
2.20
0.96
0.87
0.90
0.80
21.07
21.04
21.04
21.40
a
b
Absorption spectra were measured in CH3CH2OH solution (2 6 1025 M). Absorption spectra on TiO2 film were measured with dye-loaded
c
TiO2 films immersed in CH3CH2OH solutions. E020 was determined from the intersection of the tangent of absorption on TiO2 film and the
d
X axis by 1240/l. The oxidation potentials of the dyes were measured in CH3CH2OH solutions with LiClO4 (0.1 M) as electrolyte, and
ferrocene/ferrocenium (Fc/Fc+) as an internal reference. ELUMO was calculated by EHOMO 2 E0–0
.
e
corresponding spectroscopic parameters are summarized in
Table 1. Dyes CC101–CC103 have a strong absorption in visible
region with the absorption maxima (lmax) of 431 nm (38 100
M21 cm21), 448 nm (38 600 M21 cm21) and 455 nm (37 100
M21 cm21), respectively. All of these dyes show obvious red-
shifted absorption peaks (lmax) and higher extinction coeffi-
cients compared with that of reference dye C1-1 (422 nm,
27 500 M21 cm21) due to the stronger p-conjugated system,
which has been proved by DFT calculations (see Table 2). The
absorption maxima (lmax) of dyes CC102 and CC103 red-shift
17 nm and 25 nm compared with that of dye CC101,
respectively. This can be mainly attributed to the much
stronger donating ability of the donor part with the introduc-
tion of a methoxy group and long alkyl chains. When attached
to TiO2, the absorption spectra of CC101, CC102 and CC103
are broadened to 700 nm and obviously bathochromically
shifted (about 38–41 nm) compared to those in ethanol
solution. At the same time, much wider spectra are achieved
for dyes CC102 and CC103 compared to CC101. Therefore,
much better performance of DSSCs can be expected for the
sensitizers CC102 and CC103.
Electrochemical properties
The highest occupied molecular orbital (HOMO) levels of these
dyes are determined by cyclic voltammetry. The lowest
unoccupied molecular orbital (LUMO) is calculated from
EHOMO 2 E020. The corresponding data is collected in
Table 1. The HOMO levels of dyes CC101, CC102, CC103 and
C1-1 are 0.96 V, 0.87 V, 0.90 V and 0.80 V vs. NHE, respectively,
which are much more positive than the redox potential of I2/
I32 (0.42 V vs. NHE),27–29 indicating that the oxidized dyes can
be regenerated by the redox couple I2/I3 effectively. The
2
HOMO levels of dyes CC102 and CC103 are 0.09 V and 0.06 V
negative than that of dye CC101, respectively. This is mainly
due to the introduction of the 7-methoxy and long alkyl chains
in the molecules of CC102 and CC103, which can increase the
electron-donating ability for the donor part. The LUMO levels
of these dyes are sufficiently more negative than the conduc-
tion band (CB) level of TiO2, which implies that the electron
injection from the excited dyes into the CB of TiO2 is
thermodynamically permitted. Moreover, considering the big
energy gap between the LUMO levels of these dyes and CB
level, TBP can be added in the electrolyte to upshift the
conduction band edge for obtaining higher photovoltage.
Compared with the reference dye C1-1, all of these dyes show a
much narrower energy band gap and more positive HOMO
and LUMO levels.
Table 2 Frontier molecular orbitals (HOMO and LUMO) of the dyes calculated
with DFT on a B3LYP/6-31G(d) level
Dye
HOMO
LUMO
CC101
CC102
CC103
Theoretical calculation
To get a further insight into the molecular structures and
electron distribution, density functional theory (DFT) calcula-
tions were performed at a B3LYP/6-31G(d) level for the
geometry optimization. As shown in Table 2, for dyes
CC101–CC103, the HOMOs are mainly distributed along the
linear system from tetrahydroquinoline ring to benzopyran
ring, and the LUMOs of the dyes are mainly concentrated on
the acceptor group. Consequently, the HOMO–LUMO excita-
tion can efficiently transfer the electron from the donor part to
the acceptor part, giving fast electron injection from the LUMO
to TiO2. The absorption occurring below 400 nm is due to the
C1-1
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Table 3 Photovoltaic performance of DSSCs based on the CC101, CC102,
CC103 and C1-1 dyes
Dye
Voc (mV)
Jsc (mA cm22
)
FF (%)
g (%)
CC101
CC102
CC103
C1-1
702
700
746
696
12.72
13.29
13.45
12.33
74.1
74.8
75.2
69.7
6.6
7.0
7.5
6.0
photocurrent density (Jsc) of 12.72 mA cm22, an open-circuit
photovoltage (Voc) of 702 mV and a fill factor (FF) of 74.1%.
When a 7-methoxy group was introduced to the construct
Fig. 3 IPCE spectra of the DSSCs based on CC101, CC102 and CC103.
CC102, the efficiency of the DSSCs was improved to 7.0% (Voc
=
700 mV, Jsc = 13.29 mA cm22, FF = 74.8%). It is clear that the Jsc
value of CC102 was higher than that of CC101, due to the
better light harvesting in visible region resulted from the
electronic excitation from the S0 to S2 state, which is
insensitive to the cyanoacrylic acid units in the dye. Even with
the presence of acid or base, the position of this peak is almost
introduction of stronger electron donor. When both
a
no change (see supporting information Fig. S1, ESI ).30 The
3
7-methoxy group and long alkyl chains were introduced into
the CC103 structure, an efficiency of 7.5% for the CC103 based
DSSCs was obtained (Voc = 746 mV, Jsc = 13.45 mA cm22, FF =
75.2%). CC103 not only showed a higher Jsc than CC101, but
also much higher Voc due to the decreased electron recombi-
nation. This is caused by the effective steric hindrance
provided by the long alkyl chains. As a result, the higher Jsc
and Voc led to a higher efficiency for CC103 based device
compared to those based on CC101 and CC102. By compar-
peak occurring above 400 nm was predicted for dyes CC101–
CC103 with appreciable oscillator strengths (see supporting
information Fig. S2, ESI ). This originates due to the electronic
3
excitation from the HOMO to the LUMO.
Photovoltaic performance of DSSCs
The incident photon-to-current efficiency (IPCE) spectra and
the current- voltage curves of DSSCs based on the dyes CC101–
CC103 are shown in Fig. 3 and Fig. 4, respectively. Dyes
CC101–CC103 can efficiently convert light to photocurrent in
the region from 380 nm to 600 nm, with the highest IPCE
values of all these dyes exceeding 90%, indicating a promising
DSSCs’ performance. It is also shown that the IPCE spectra of
these dyes are exactly in agreement with the absorption
spectra of these dyes on TiO2 (in Fig. 2). The IPCE values of
dyes CC102 and CC103 is a little broader than that of dye
CC101, due to the introduction of stronger electron donors of
7-methoxy unit and long alkyl chains.
As the photovoltaic properties of DSSCs based on CC101–
CC103 are shown in Table 3 and Fig. 4. The device sensitized
by CC101 showed an efficiency of 6.6%, with a short-circuit
Fig. 5 The Nyquist (a) and Bode plots (b) of DSSCs based on CC101, CC102 and
CC103.
Fig. 4 J–V curves of the DSSCs based on CC101, CC102, CC103 and C1-1.
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(KF0805), the Program for Innovative Research Team of
Liaoning Province (Grant No. LS2010042).
Table 4 Parameters obtained by fitting the EIS spectra to an electrochemical
model
Dye
Rs (ohm)
Rrec (ohm)
Rce (ohm)
CC101
CC102
CC103
19.4
18.1
17.3
21.8
20.1
27.8
3.39
3.25
3.24
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containing a benzopyran ring as a p–bridge were designed
and developed for dye-sensitized solar cells (DSSCs) for the
first time. When attached to TiO2, the absorption spectra of
the dyes CC101–CC103 can extend to 700 nm. This series of
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of 380 nm–600 nm, with the highest IPCE values exceeding
90%. Through the modification of the donor units, an
efficiency as high as 7.5% has been achieved under standard
light illumination (AM 1.5G, 100 mW cm22), for CC103 based
DSSCs. Based on the detailed photoelectrochemical study, this
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