Ethoxy-substituted oligo-phenylenevinylene-bridged organic dyes for efficient dye-sensitized solar cells

Article abstract of DOI:10.1002/cjoc.201200091

Organic dyes with ethoxy-substituted oligo-phenylenevinylene as chromophores were synthesized for dye-sensitized solar cells (DSSCs), and the detailed relationships between the dye structures, photophysical properties, electrochemical properties, and perf

Full text of DOI:10.1002/cjoc.201200091

FULL PAPER
DOI: 10.1002/cjoc.201200091
Ethoxy-substituted Oligo-phenylenevinylene-Bridged Organic
Dyes for Efficient Dye-Sensitized Solar Cells
Shan, Yifana(单益凡)
Tang, Jiea(汤杰)
Lai, Huab(赖华)
Tan, Hongweic(谭宏伟)
Liu, Xiaofengd(刘晓峰)
Yang, Fan*,a(杨帆)
Fang, Qiang*,b(房强)
a Institutes for Advanced Interdisciplinary Research, East China Normal University, Shanghai 200062, China
b Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
c Department of Chemistry, Beijing Normal University, Beijing 100875, China
d Reata Pharmaceuticals, Inc. 2801 Gateway Drive, Suite 150, Irving, Texas 75063, USA
Organic dyes with ethoxy-substituted oligo-phenylenevinylene as chromophores were synthesized for dye-sen-
sitized solar cells (DSSCs), and the detailed relationships between the dye structures, photophysical properties,
electrochemical properties, and performances of DSSCs were described. The dye S3O showed broad IPCE spectra
in the spectral range of 350—750 nm, and the dye S1P showed solar energy-to-electricity conversion efficiency (η)
of up to 4.23% under AM 1.5 irradiation (100 mW/cm2) in comparison with the reference Ru-complex (N719 dye)
with an η value of 5.90% under similar experimental conditions.
Keywords dye-sensitized solar cell, organic dyes, ethoxy-substituted oligo-phenylenevinylene, photovoltaic,
synthesis
Introduction
ing layer and yielding an increased electron lifetime.
The alkyl chains are also believed to prevent aggregate
formation on the surface of the TiO2. The ethoxy groups
also have been introduced at the phenylenevinylene
moiety to enhance the electron donating character and
introduced into the π-conjugated donor-acceptor skele-
ton to increase the absorption wavelength.[28]
Herein, we report three new organic dyes which
contain a simple triphenylamine or dibutoxy-substituted
triphenylamine moiety or phenoxazine unit as the elec-
tron donor and cyanoacrylic acid as the electron accep-
tor. The donor and acceptor are bridged by ethoxy-sub-
stituted oligo-phenylenevinylene which serves as a co-
planar π-conjugated spacer to give three organic D-π-A
dyes S1P, S3O and S3P (Figure 1).
Dye-sensitized solar cells (DSSCs) based on Ru dyes
have been actively investigated since Grätzel and
co-workers reported the first photoelectric device with
highly efficient solar-energy-to-electricity conversion
efficiencies of 11% under AM 1.5 irradiation.[1-7] In ad-
dition to Ru complexes, metal-free organic dyes have
also been utilized as photosensitizers in DSSCs because
of their wide variety of the structures, facile modifica-
tion, high molar absorption coefficient, low cost, and
environment-friendliness.[8-26] And impressive device
efficiencies of 9%—10% have been achieved based on
organic dyes. It is very instructive that porphyrin-
sensitized solar cells with cobalt(II/III)-based redox
electrolyte with exceed 12.3% efficiency was reported
by Grätzel and co-workers in 2011.[27]
Experimental
Generally, organic dyes should contain a structure of
donor (D)-to-acceptor (A) bridged by a π-conjugation
system and possess a broad and intense spectral absorp-
tion. Appropriate use of electron-excessive chains as
π-conjugation between an electron donor and an elec-
tron acceptor was reported to be beneficial to red shift
the charge-transfer transition. It was suggested that the
efficiency of the DSSCs could be increased by intro-
ducing alkyl chains in the linker or donor part of the dye,
which prevents the recombination of electrons from the
semiconductor to the electrolyte by forming an insulat-
Absorption spectra in CH2Cl2 solutions were re-
corded in a quartz cell with 1 cm path length on Varian
Cary 100 Bio UV-Visible spectrophotometer. Fluores-
cent spectra were recorded on Hitachi F-4500. 1H NMR
(500 MHz) and 13C NMR (125 MHz) spectra were
measured with Bruker AM-500 MHz, with the chemical
shifts against TMS. MS data were obtained with Bruker
micrOTOF II. Electrochemical redox potentials were
obtained by cyclic voltammetry (CV) using a three-
*
E-mail: fyang@chem.ecnu.edu.cn (F. Yang), qiangfang@mail.sioc.ac.cn (Q. Fang); Tel.: 0086-021-62232764; Fax: 0086-021-62232100
Received February 7, 2012; accepted April 1, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200091 or from the author.
Chin. J. Chem. 2012, 30, 1497—1503
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Shan et al.
FULL PAPER
couple was used as an internal potential reference. The
potentials versus NHE were calibrated by addition of
0.55 V to the potentials versus Fc/Fc＋. Unless otherwise
stated, starting materials were used as commercially
purchased without further purification. The syntheses
route of S1P, S3O and S3P are shown in Scheme 1.
The intermediate 2,5-diethoxyterephthalaldehyde
was prepared according to literature procedures.[29,30]
O
N
COOH
O
CN
S1P
O
N
Synthesis of S1P-CHO
O
To a solution of 2,5-diethoxyterephthalaldehyde
(0.85 g, 3.85 mmol) and compound 1 (2.31 g, 3.85
mmol) in 35 mL THF, t-BuOK (0.65 g, 5.78 mmol) was
added slowly with vigorously stirring at room tempera-
ture. The reaction was completed within 3 h. The reac-
tion mixture was poured into water and filtered to col-
lect the orange solid (a mixture of E and Z isomers),
which was then dried in vacuo. Then the dry solid was
dissolved in 35 mL THF to reflux in the presence of
catalysis amount iodine for 8 h. The mixture was added
sodium bisulfite aqueous solution to remove iodine, then
extracted with dichloromethane. Organic layer was dried
with anhydrous sodium sulfate and removed the solvent,
and was purified by column chromatography using sil-
ica gel and dichloromethane-petroleum ether (1/1, V/V)
as the eluent to give S1P-CHO (yellow solid, E, 0.78 g,
yield 44%); 1H NMR (500 MHz, CDCl3) δ: 1.45 (t, J＝
7.0 Hz, 3H), 1.48 (t. J＝7.0 Hz, 3H), 4.09 (q, J＝7.0 Hz,
2H), 4.19 (q, J＝7.0 Hz, 2H), 7.04—7.06 (m, 4H),
7.11—7.13 (m, 4H), 7.16—7.19 (m, 2H), 7.26—7.27 (m,
4H), 7.29 (s, 1H), 7.31—7.34 (m, 1H), 7.41—7.43 (m,
2H), 10.43 (s, 1H); 13C NMR (125 MHz, CDCl3) δ:
189.0, 156.1, 150.4, 147.9, 147.3, 134.7, 131.7, 131.0,
COOH
O
CN
S3O
O
O
N
COOH
O
CN
O
S3P
Figure 1 The molecular structures of all dyes.
electrode cell and an electrochemistry workstation
(CHI600C). The working electrode was a glass carbon
electrode; the auxiliary electrode was a Pt wire and
Ag/AgI/I－ was used as reference electrode. 0.1 mol/L
tetrabutylammonium hexaflourophosphate (TBAPF6)
was used as supporting electrolyte in CH2Cl2. Ferrocene
was added to each sample solution at the end of the ex-
periments and the ferrocenium/ferrocene (Fc/Fc＋) redox
Scheme 1 Synthetic routes of S1P, S3O and S3P
R
R
R
O
N
(ii)
O
(i)
N
CHO
.
N
PPh3 Br
O
COOH
R
O
NC
O
R
R
S1P-CHO: R = H
S3P-CHO: R = OC4H9
Compound 1: R = H;
Compound 2: R=OC4H9
S1P: R = H
S3P: R = OC4H9
OHC
CHO
O
O
N
O
N
O
N
O
(ii)
(i)
O
CHO
.
PPh3 Br
COOH
O
S3O-CHO
O
O
NC
Compound 3
OHC
CHO
S3O
O
(i) t-BuOK, THF; I2, THF; (ii) cyanoacrylic acid , piperidine, ACN
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Oligo-phenylenevinylene-Bridged Organic Dyes for Efficient Dye-Sensitized Solar Cells
129.3 (4C), 127.8 (2C), 124.7 (4C), 124.4, 123.9, 123.3
(2C), 123.0 (2C), 120.9, 110.2, 110.1, 64.8, 64.5, 14.8
(2C). HRMS (ESI) calcd for C31H29NNaO3 [M＋Na]＋
486.2045, found 486.2056.
(s, 1H), 8.73 (s, 1H); 13C NMR (125 MHz, DMSO-d6) δ:
163.74, 152.91, 149.35, 146.33, 144.22, 143.83, 133.21,
133.07, 132.11, 131.54, 129.90, 124.00, 123.92, 121.03,
119.45, 117.09, 117.09, 115.01, 111.95, 112.03, 111.80,
111.32, 110.20, 107.82, 64.70, 64.28, 43.00, 28.19,
24.22, 21.83, 14.48 (2C), 13.78. HRMS (ESI) calcd for
C33H34N2NaO5 [M＋Na]＋ 561.2365, found 561.2377.
Synthesis of S1P
An acetonitrile (10 mL) solution of S1P-CHO (0.10
g, 0.22 mmol) and cyanoacetic acid (22 mg, 0.26 mmol)
was refluxed in the presence of piperidine (0.10 mL) for
2 h. After removing the solvent, the residue was purified
by column chromatography using silica gel and di-
chloromethane-methanol (10/1, V/V) as the eluent to
give product S1P (dark-red solid, 70 mg, yield 60%).
m.p. 248—252 ℃; 1H NMR (500 MHz, CDCl3) δ: 1.47
—1.49 (m, 6H), 4.12—4.19 (m, 4H), 7.05—7.07 (m,
4H), 7.12 (d, J＝7.1 Hz, 5H), 7.19 (d, J＝16.6 Hz, 1H),
7.26—7.30 (m, 4H), 7.36 (d, J＝16.3 Hz, 1H), 7.42 (d,
J＝8.4 Hz, 2H), 7.96 (s, 1H), 8.77 (s, 1H); 13C NMR
(125 MHz, CDCl3＋CD3OD) δ: 183.90, 164.81, 153.54,
149.98, 148.64, 147.89, 147.13 (2C), 134.17, 131.70,
130.77, 129.11 (4C), 127.69 (2C), 124.56 (4C), 123.14
(2C), 122.71 (2C), 120.49, 119.58, 116.90, 111.42,
Synthesis of S3P-CHO
Compound S3P-CHO was synthesized according to
the same procedure as that of S1P-CHO except that
compound 2 (2.86 g, 3.85 mmol) was used instead of
compound 1. Yellow solid of S3P-CHO was obtained in
1
48% yield. H NMR (500 MHz, CDCl3) δ: 0.97 (t, J＝
7.0 Hz, 6H), 1.42—1.51 (m, 10H), 1.74—1.77 (m, 4H),
3.92 (t, J＝5.0 Hz, 4H), 4.05—4.09 (m, 2H), 4.15—4.19
(m, 2H), 6.81 (d, J＝8.5 Hz, 4H), 6.88 (d, J＝8.5 Hz,
2H), 7.04 (d, J＝9.0 Hz, 4H), 7.12—7.15 (m, 2H), 7.28
—7.31 (m, 2H), 7.33 (d, J＝8.5 Hz, 2H), 10.41 (s, 1H);
13C NMR (125 MHz, CDCl3) δ: 198.07, 156.12, 155.71
(2C), 150.28, 148.96, 140.18 (2C), 135.04, 132.02,
128.93, 127.69 (2C), 126.83 (4C), 123.62, 119.75 (2C),
119.67, 115.24 (4C), 110.02, 109.91, 67.88 (2C), 64.77,
64.50, 31.35 (2C), 19.23 (2C), 14.80, 14.77, 13.83 (2C).
HRMS (ESI) calcd for C39H45NNaO5 [M ＋Na] ＋
630.3195, found 630.3254.
109.19, 64.82, 64.55, 14.48, 14.40. HRMS (ESI) calcd
＋
for C34H30N2NaO4 [M ＋ Na]
553.2098, found
553.2086.
Synthesis of S3O-CHO
Synthesis of S3P
Compound S3O-CHO was synthesized according to
the same procedure as that of S1P-CHO except that
compound 3 (2.34 g, 3.85 mmol) was used instead of
compound 1. Yellow solid of S3O-CHO was obtained in
Compound S3P was synthesized according to the
same procedure as that of S1P except that S3P-CHO
(0.13 g, 0.22 mmol) was used instead of S1P-CHO. Red
solid of S3P was obtained in 40% yield. m.p. 246—248
1
70% yield. H NMR (500 MHz, CDCl3) δ: 0.97 (t, J＝
1
℃; H NMR (500 MHz, CDCl3) δ: 0.98 (t, J＝7.6 Hz,
7.0 Hz, 3H), 1.42—1.44 (m, 4H), 1.48 (t, J＝7.0 Hz,
3H), 1.50 (t, J＝7.0 Hz, 3H), 1.67—1.73 (m, 2H), 3.50
(t, J＝8.5 Hz, 2H), 4.11 (q, J＝7.0 Hz, 2H), 4.20 (q, J＝
7.0 Hz, 2H), 6.45 (d, J＝8.3 Hz, 1H), 6.50 (d, J＝8.2 Hz,
1H), 6.66—6.70 (m, 2H), 6.81—6.84 (m, 1H), 6.91 (s,
1H), 6.94 (d, J＝8.2 Hz, 1H), 7.03 (d, J＝16.4 Hz, 1H),
7.14 (s, 1H), 7.25 (d, J＝16.4 Hz, 1H), 7.32 (s, 1H),
10.44 (s, 1H); 13C NMR (125 MHz, CDCl3) δ: 189.06,
156.07, 150.36, 145.05, 144.69, 134.72, 133.55, 132.59,
131.32, 130.17, 123.79, 123.69 (2C), 121.08, 120.06,
115.38, 112.46, 111.40, 111.02, 110.12, 110.00, 64.81,
64.51, 44.08, 29.04, 24.73, 22.49, 14.86, 14.79, 14.06.
HRMS (ESI) calcd for C30H33NNaO4 [M ＋Na] ＋
494.2307, found 494.2312.
6H), 1.46—1.54 (m, 10H), 1.75—1.80 (m, 4H), 3.94 (t,
J＝6.4 Hz, 4H), 4.13—4.19 (m, 4H), 6.83 (d, J＝8.8 Hz,
4H), 6.89 (d, J＝8.6 Hz, 2H), 7.06—7.11 (m, 5H), 7.18
(d, J＝16.2 Hz, 1H), 7.30—7.37 (m, 3H), 7.99 (s, 1H),
8.83 (s, 1H); 13C NMR (125 MHz, CDCl3＋CD3OD) δ:
177.56, 165.01, 155.49, 153.56, 149.84, 148.49, 140.05,
140.01, 134.47, 131.96, 127.57 (2C), 126.64 (4C),
119.52 (2C), 119.29 (2C), 117.00, 115.13 (4C), 112.01,
111.37 (2C), 108.92 (2C), 67.81 (2C), 64.76, 64.52,
31.10 (2C), 18.95 (2C), 14.43, 14.34, 13.45 (2C).
HRMS (ESI) calcd for C42H46N2NaO6 [M ＋Na] ＋
697.3248, found 697.3233.
DSSCs fabrication
Synthesis of S3O
Titania paste was prepared from P25 (Degussa,
Germany) following the literature procedure[2] and de-
posited onto the F-doped tin oxide conducting glass
(sheet resistance of 15 Ω/square, polyethylene naphtha-
late) by doctor-blading. The resulted layer photoelec-
trode of 12 μm thickness, was sintered at 500 ℃ for 30
min in air. After the film was cooled to 40 ℃, it was
Compound S3O was synthesized according to the
same procedure as that of S1P except that S3O-CHO
(0.10 g, 0.22 mmol) was used instead of S1P-CHO. Red
solid of S3O was obtained in 46% yield. m.p. 258—260
1
℃; H NMR (500 MHz, CDCl3＋CD3OD) δ: 0.93 (t,
J＝6.6 Hz, 3H), 1.40 (m, 4H), 1.45—1.49 (m, 6H), 1.66
(m, 2H), 3.49 (m, 2H), 4.10—4.16 (m, 4H), 6.43 (d, J＝
8.0 Hz, 1H), 6.48 (d, J＝8.2 Hz, 1H), 6.64 (m, 2H),
6.78—6.80 (m, 1H), 6.88 (s, 1H), 6.92 (d, J＝8.2 Hz,
1H), 7.03—7.08 (m, 2H), 7.23 (d, J＝17.0 Hz, 1H), 7.92
immersed into a 3×10－ mol/L dye solution in THF
4
and maintained under dark for 12 h. The electrode was
then rinsed with CH2Cl2 and dried. One drop of electro-
lyte solution was deposited onto the surface of the elec-
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Shan et al.
FULL PAPER
trode and penetrated inside the TiO2 film via capillary
action. The electrolyte consisted of 1.0 mol/L
1,2-dimethyl-3-n-propylimidazolium iodide (DMPII),
0.05 mol/L LiI, 0.03 mol/L I2, 0.1 mol/L guanidinium
thiocyanate (GuSCN), 0.5 mol/L 4-tert-butylpyridine
(TBP) in AcCN and valeronitrile (85∶15) for our dyes
and N719 dye. A platinized counter electrode was then
clipped onto the top of the TiO2 working electrode to
form our test cell.
The irradiance source was a 150 W NREL traceable
Oriel Class AAA solar simulator (Model 92250A-1000).
The output power was calibrated by a NREL traceable
monocrystalline silicon reference cell (PVM 191) cou-
pled with Newport Oriel PV reference cell system
(Model 91150). Current-voltage characteristics were
measured using a Keithley 2420 sourcemeter. The
measurement delay time was fixed to 40 ms with 100
measurement points scanning from VOC to JSC.[31] For
the setup used in our measurements, there was no hys-
teresis in the IV curves when reversing the scan direc-
tion.
Incident photon-to-current conversion efficiency
(IPCE) of the DSSCs was measured by a DC method.
The light source was a 300 W Xenon Lamp (Oriel 6285)
coupled with a flux controller to improve the stability of
the irradiance. The light passed through a monochro-
mator (Cornerstone 260 Oriel 74125) to select a single
wavelength with a resolution of 10 nm. The monochro-
matic light beam was then focused on the active region
of the device. Light intensity was measured by a NREL
traceable Si detector (Oriel 71030NS) and the short cir-
cuit currents of the DSSCs were measured by an optical
power meter (Oriel 70310). The IPCE was calculated by
the equation: IPCE (λ)＝1240×Jsc(λ)/λ×Pin(λ).
Figure 2 (a) Absorption spectra of S1P, S3O and S3P in
CH2Cl2 (1×10－5 mol/L); (b) Absorption spectra of S1P, S3O and
S3P on TiO2 films.
－
－
1
1
dm3•mol •cm (507 nm), respectively. These num-
bers are obviously larger than that of N719, indicating
that these dyes have good light-harvesting ability. The
absorption spectra of the modified donor series show
that the longer alkoxy moieties of S3P red shift the ab-
sorption maximum when compared to S1P. Due to
strong electron donating ability of phenoxazine (POZ)
unit, S3O dye exhibits λabs at 509 nm which is similar to
the absorption maximum of S3P. When S1P, S3O and
S3P were sensitized on TiO2 surface, the absorption
maxima of these dyes were blue-shifted by 24, 54 and
58 nm in comparison to those in solution respectively,
due to H-aggregation on semiconductor surface.
Results and Discussion
Upon condensation of SiP-CHO or S3O-CHO or
S3P-CHO with cyanoacetic acid, respectively, by Kno-
evenagel reaction in the presence of piperidine, the tar-
get dyes were obtained conveniently. The geometric
structure of these type of compound with cyanoacrylic
acid as the acceptor/anchor group, has been investigated
by Zhang et al.[33] through X-ray analysis.
Electrochemical properties
To further study the possibilities of electron injection
from the dye in excited state to conduction band (CB) of
the semiconductor and the dye regeneration, CVs of
these dyes were performed in CH2Cl2 to measure the
redox potentials (Figure 3). The first oxidation poten-
tials (Eox) corresponding to the HOMO levels of the
dyes were summarized in Table 1. The HOMO levels of
S1P, S3O and S3P dyes are 1.12, 0.73 and 0.77 V, re-
spectively, which are positive enough compared to that
of iodine/iodide (0.4 V),[32] therefore the oxidized dyes
could be reduced effectively by electrolyte and regener-
ated (Figure 4). Compared to S1P, the additional butoxy
groups in S3P result in a red-shift of the absorption and
Photophysical properties
The absorption spectra of S1P, S3O and S3P dyes in
CH2Cl2 and on TiO2 films are shown in Figure 2, and
the corresponding data were collected in Table 1. All of
dyes (S1P, S3O and S3P) have a relatively broad and
strong absorption in the visible region with a maximum
at 478, 509 and 507 nm, respectively. These absorption
bands at around 500 nm can be attributed to the in-
tramolecular charge transfer (ICT) between the donor
and the acceptor. The molar extinction coefficients of
－
－
1
S1P, S3O and S3P are 35 300 dm3•mol •cm (478
1
－
－
1
1
nm), 33 200 dm3•mol •cm (509 nm) and 39 500
1500
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Chin. J. Chem. 2012, 30, 1497—1503

Oligo-phenylenevinylene-Bridged Organic Dyes for Efficient Dye-Sensitized Solar Cells
absorption and emission spectra, namely, HOMO－E0-0.
To effectively inject the electron into the CB of TiO2,
the LUMO levels of the dyes must be much more nega-
tive than the conducing band energy (Ecb) of semicon-
ductor, －0.5 V (vs. NHE). From the LUMO values, it
is clear that all of the dyes can achieve the electron in-
jection and form the oxidized dyes.
Photovoltaic properties
The photovoltaic properties of these dyes for DSSCs
are shown in Table 2 and J-V curves of the dyes are
shown in Figure 5(a). An η value of 4.23% has been
achieved (with short-circuit photo current density JSC＝
9.16 mA/cm2, open circuit photovoltage VOC＝717 mV,
and fill factor ff＝0.64) for DSSCs using S1P. The dyes
S3O and S3P give η values of 3.94%, and 4.02%, re-
spectively. Under similar experimental conditions, the
N719 dye gives an η value of 5.90%. The dye S3P with
longer alkoxy moieties shows higher open circuit
photovoltage value than S1P, and S3O containing
phenoxazine (POZ) unit shows the lowest open circuit
photovoltage among the three dyes. The incident pho-
ton-to-current conversion efficiencies (IPCEs) of these
dyes in DSSCs are shown in Figure 5(b). S3O gives
broader IPCE spectra than S1P and S3P do in the spec-
tral range of 350—750 nm. However, S1P gives higher
IPCE value (up to 72% at 430 nm) than S3O and S3P do.
Although the IPCE value of S3P is still lower than S1P,
Figure 3 The CV curves of dyes in CH2Cl2.
Table 1 Absorption, emission, and electrochemical properties
of S1P, S3O and S3P
Absorption
Oxidation potential datac
λmaxa/nm [ε/(L• λmaxb/nm
Dye
E0-0d/V Eox/V (Eox－E0-0)/V
－
－
1
mol •cm 1)] on TiO2
S1P 478 (35 300)
S3O 509 (32 200)
454
455
447
2.17
2.35
2.22
1.12 －1.05
0.73 －1.62
0.77 －1.45
S3P 505 (39 500)
a
Maximum absorption was measured in CH2Cl2 solution (1×
10－5 mol/L) at room temperature; b Maximum absorption on TiO2
c
film; The oxidation potentials of the dyes were measured in
CH2Cl2 with 0.1 mol/L tetrabutylammonium hexafluorophosphate
(TBAPF6) as electrolyte (working electrode: glassy carbon; ref-
erence electrode: Ag/AgI/I－; calibrated with ferrocene/ferro-
cenium (Fc/Fc＋) as an internal reference, counter electrode: Pt);
d E0-0 was estimated from the onset point of absorption spectra.
Figure 4 Molecular orbital energy diagram of HOMO and
LUMO of dyes.
an increase of the 0-0 excitation energy (E0-0) estimated
from the absorption onset. The first oxidation potential
of S1P is 1.12 V, but it shifts negatively by ca. 350 mV
for S3P. The phenomena can be attributed to the elec-
tron-donating butoxy groups which raise the HOMO
potential and reduce the coulombic repulsion between
the two charges. The LUMO levels of these dyes can be
obtained by HOMO level and zeroth-zeroth energy (E0-0)
of the dyes estimated from the intersection between the
Figure 5 (a) Photocurrent density vs. voltage curves for DSSCs
based on S1P, S3O, S3P and N719 under irradiation of AM 1.5G
simulated solar light (100 mW•cm－2). (b) The incident pho-
ton-to-current conversion efficiencies spectra for DSSCs based on
S1P, S3O, S3P and N719.
Chin. J. Chem. 2012, 30, 1497—1503
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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Shan et al.
LUMO
FULL PAPER
Table 2 Photovoltaic performance of DSSCs based on S1P,
S3O, S3P and N719 dyesa
Dye
S1P
HOMO
JSC/(mA•cm－
)
VOC/mV
717
ff
η/%
4.23
3.94
4.02
5.90
2
Dye
S1P
9.16
0.64
0.64
0.62
0.60
S3O
S3P
8.96
691
9.05
721
N719
12.28
793
a Measured under irradiation of AM 1.5 G simulated solar light
(100 mW•cm－2) at room temperature, 12 μm film thickness, 0.25
cm2 working area; electrolyte: 1.0 mol/L 1,2-dimethyl-3-n-
propylimidazolium iodide (DMPII), 0.05 mol/L LiI, 0.03 mol/L I2,
0.1 mol/L guanidinium thiocyanate (GuSCN), 0.5 mol/L
4-tert-butylpyridine (TBP) in AcCN and valeronitrile (85∶15);
S3O
－
c
b The concentration of dyes is 3×10 4 mol/L in THF; The con-
centration of N719 dye is 3×10－4 mol/L in ethanol.
S3P
the IPCE spectrum of S3P is broadened by modification
with alkoxy groups. The results show that S3P and S3O
have a broader absorption spectrum than S1P in di-
chloromethane solution because of the larger electron
delocalization, and consequently lead to better photo-
electric conversion properties to some extent. However,
the photovoltaic conversion efficiency of S1P is higher
than that of S3P and S3O due to the lower short-circuit
current density. Therefore, it was found that the increase
of electron donating ability is not always beneficial to
improving the photoelectric conversion efficiency of
DSSCs. These results agree well with the corresponding
IPCE spectra.
Density functional theory (DFT) calculations were
performed at a B3LYP/6-31G level for the geometry
optimization to observe frontier molecular orbitals
(MOs) of the HOMO and LUMO. The frontier MOs of
all the dyes reveal that HOMO-LUMO excitation moves
the electron density distribution from donor moieties to
the cyanoacrylic acid acceptor (Figure 6). It is notewor-
thy that the LUMO electron density of all dyes is lo-
cated mainly on the cyanoacrylic acid moiety. Therefore,
the excited electron can be injected into the TiO2 elec-
trode efficiently.
Figure 6 The frontier molecular orbitals (MOs) of the HOMO
(left) and LUMO (right) of S1P, S3O and S3P.
Acknowledgement
We gratefully acknowledge the financial support of
this work from Shanghai Committee of Science and
Technology of China (No. 10520710100). We also
thank the Laboratory of Organic Functional Molecules,
Sino-French Institute of ECNU for support.
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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