Porphyrins bearing long alkoxyl chains and carbazole for dye-sensitized solar cells: Tuning cell performance through an ethynylene bridge

Article abstract of DOI:10.1039/c3ra40788h

Two novel porphyrin dyes (Q1 and Q2) bearing alkoxyl chains with a carbazole moiety as the electron donor have been synthesized and utilized as sensitizers for dye-sensitized solar cells (DSSCs). Compared with Q2, the molecule of Q1 has an additional ethynylene bridge between the carbazole moiety and the porphyrin framework. Photophysical and electrochemical properties of the two dyes were investigated by UV-vis, fluorescence spectroscopy and cyclic voltammetry. DFT calculations indicated that Q2 has a more twisted non-planar conformation associated with a smaller π conjugation size because of the absence of ethynylene bridge, which resulted in its better solubility and larger amount of adsorption on TiO₂. Compared with Q1, Q2 showed better photovoltaic performance, with a short-circuit photocurrent density (J _sc) of 11.3 mA cm^-2, an open-circuit photovoltage (V _oc) of 0.68 V, and a fill factor (ff) of 0.71, corresponding to an overall conversion efficiency of 5.51% under standard global AM 1.5 solar light conditions. The additional ethynylene bridge in Q1 extends the absorption bands to a longer wavelength region with the absorption threshold of 743 nm on the TiO₂ film compared with that of 681 nm for Q2, but the cell efficiency is decreased to 2.22%, which may be ascribed to the worse solubility and stronger aggregation tendency resulting from the better molecule planarity. These results indicate that the extension of the absorption bands to a longer wavelength region by the introduction of an additional ethynylene bridge may result in worse solubility and more severe aggregation, and thus decrease the cell efficiency. For the design of efficient DSSC sensitizers, these contradictory effects must be fully considered and well balanced. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra40788h

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PAPER
Porphyrins bearing long alkoxyl chains and carbazole
for dye-sensitized solar cells: tuning cell performance
through an ethynylene bridge3
Cite this: RSC Advances, 2013, 3,
14780
Yueqiang Wang,a Xin Li,b Bo Liu,a Wenjun Wu,a Weihong Zhua and Yongshu Xie*a
Two novel porphyrin dyes (Q1 and Q2) bearing alkoxyl chains with a carbazole moiety as the electron
donor have been synthesized and utilized as sensitizers for dye-sensitized solar cells (DSSCs). Compared
with Q2, the molecule of Q1 has an additional ethynylene bridge between the carbazole moiety and the
porphyrin framework. Photophysical and electrochemical properties of the two dyes were investigated by
UV-vis, fluorescence spectroscopy and cyclic voltammetry. DFT calculations indicated that Q2 has a more
twisted non-planar conformation associated with a smaller p conjugation size because of the absence of
ethynylene bridge, which resulted in its better solubility and larger amount of adsorption on TiO2.
Compared with Q1, Q2 showed better photovoltaic performance, with a short-circuit photocurrent density
(Jsc) of 11.3 mA cm22, an open-circuit photovoltage (Voc) of 0.68 V, and a fill factor (ff) of 0.71,
corresponding to an overall conversion efficiency of 5.51% under standard global AM 1.5 solar light
conditions. The additional ethynylene bridge in Q1 extends the absorption bands to a longer wavelength
region with the absorption threshold of 743 nm on the TiO2 film compared with that of 681 nm for Q2,
but the cell efficiency is decreased to 2.22%, which may be ascribed to the worse solubility and stronger
aggregation tendency resulting from the better molecule planarity. These results indicate that the
extension of the absorption bands to a longer wavelength region by the introduction of an additional
ethynylene bridge may result in worse solubility and more severe aggregation, and thus decrease the cell
efficiency. For the design of efficient DSSC sensitizers, these contradictory effects must be fully considered
and well balanced.
Received 15th February 2013,
Accepted 10th June 2013
DOI: 10.1039/c3ra40788h
www.rsc.org/advances
and environmental concern limit their practical applications.
Consequently, in recent years, increasing interests have been
Introduction
Employment of inexhaustive solar energy is a promising
choice to solve the energy shortage problem. In this respect,
dye-sensitized solar cells (DSSCs) have attracted much atten-
tion during the past years due to their low production cost,
easy fabrication and relatively high solar energy conversion
focused on the development of new efficient organic sensiti-
zers because of their advantages of low cost, high molar
absorption coefficient, and facile modulation of the properties
by the modification of the molecular structures.4 A typical
organic sensitizer has a D-p-A framework, with the electron
donor (D) and the electron acceptor (A) linked by a conjugated
efficiency.1 In 1991, Gratzel et al. reported the excellent
¨
performance of DSSCs utilizing ruthenium complexes as the
sensitizers.1a Since then, increasing efforts have been devoted
to the design and syntheses of efficient DSSC sensitizers.2 The
advantages of ruthenium complexes include broad absorption
bands and appropriate excited-state energy levels for electron
injection into the conduction band of TiO2, the most
commonly used semiconductor in DSSCs.3 Whereas, the cost
p
bridge (p).5 Thus, organic dyes based on coumarin,
triphenylamine, carbazole, thiophene, polyene, cyanine, hemi-
cyanine, squaraine and phthalocyanine moieties have been
explored as sensitizers for DSSCs.6–14 In this respect, we have
developed various efficient dyes incorporating indoline,
isophorone and benzothiadiazole units.15
Inspired by the efficient energy and electron transfer
behavior in the light-harvesting antenna of natural systems,16
porphyrin derivatives17 and analogues18 have been widely used
in photovoltaic devices. Porphyrin based compounds have
several intrinsic advantages such as good photostability and
rigid molecular structure with large absorption coefficients in
the visible region.19 Furthermore, due to the presence of
multiple reactive sites, including four meso and eight b-posi-
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science and Technology, Shanghai, P. R. China.
E-mail: yshxie@ecust.edu.cn; Fax: (+86) 21-6425-2758; Tel: (+86) 21-6425-0772
bDepartment of Theoretical Chemistry and Biology, School of Biotechnology KTH
Royal Institute of Technology, Stockholm, Sweden
Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and
3
HRMS spectra of intermediates and target porphyrins (Fig. S1 to S25). See DOI:
10.1039/c3ra40788h
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tions, their electrochemical and photophysical properties can
be easily modulated by peripheral substituents and inner
metal complexation.20 Thus, numerous porphyrins have been
synthesized and used as DSSC sensitizers.21 Among these, a
push–pull type porphyrin dye with diarylamine and ethynyl-
benzoic acid substituents at meso positions has been reported
to achieve a PCE of 12.3%,22 which remains the highest value
for DSSCs so far. In spite of these successful examples, high
performance porphyrin sensitizers are still rather scarce, and
correlations between cell performance and the porphyrin
structures still remain relatively unexplored.
In an attempt to create efficient porphyrin dyes and further
understand the performance–structure correlations, we have
synthesized a number of porphyrins with various electron
donors, and demonstrated that photovoltaic performance of
the DSSCs can be optimized by means of modulating the
molecular orbital energy levels by variation of the numbers
and types of electron donors.23 As an electron donor, carbazole
has been introduced to various organic dyes for the syntheses
of efficient DSSC sensitizers,8 but examples of carbazole
containing porphyrin dyes are extremely rare.24 In this work,
two porphyrin sensitizers, Q1 and Q2 (Scheme 1) were
synthesized by introducing a carbazole moiety as the electron
donor, and long alkoxyl chains were introduced with the
purpose to increase the solubility and suppress dye aggrega-
tion,25 and thus improve the cell performance. And in each of
the dyes, a 4-ethynylbenzoic acid group was introduced to one
of the meso-positions as the electron acceptor and anchoring
moiety.
It has been demonstrated that the incorporation of an
ethynylene bridge can increase conjugate lengths of the dyes,
thus the absorption bands can be broadened to a longer
wavelength region, which may improve the DSSC efficiency.21d
Compared with Q2, an extra ethynylene bridge was introduced
into the molecule of Q1, with the aim to investigate the effect
of the ethynylene unit on the cell efficiency.
Absorption spectra and electrochemical analyses of Q1 and
Q2 indicate that the HOMO and LUMO levels are suitable for
DSSC sensitizers. The extra ethynylene bridge in Q1 extends
the absorption bands to a longer wavelength region, but
decreases the DSSC efficiency. The overall photon-to-electron
conversion efficiencies (PCE) of the DSSCs based on Q1 and
Q2 are 2.22% and 5.51%, respectively.
Experimental section
Materials and reagents
All reagents and solvents were obtained from commercial
sources and used without further purification unless otherwise
noted. THF was dried over 4 Å molecular sieves, and distilled
under nitrogen from sodium benzophenone prior to use.
Tetrabutylammonium hexafluorophosphate (TBAPF6) was
vacuum-dried for 48 h. The transparent FTO conducting glass
(fluorine-doped SnO2, transmission .90% in the visible range,
sheet resistance 15V/square) and the TiO2 paste was pur-
chased from Geao Science and Educational Co. Ltd. The FTO
conducting glass was washed with a detergent solution,
deionized water, ethanol, and acetone successively under
ultrasonication for 20 min before use. 4-(9H-carbazol-9-
yl)benzaldehyde26 and 9-(4-ethynylphenyl)carbazole27 were
prepared according to reported procedures.
Equipment and apparatus
1H NMR spectra were obtained using a Bruker AM 400
spectrometer. HRMS measurements were performed using a
Waters LCT Premier XE spectrometer. UV-Vis absorption
spectra were recorded on a Varian Cary 100 spectrophotometer
and fluorescence spectra were recorded on a Varian Cary
Eclipse fluorescence spectrophotometer. The cyclic voltammo-
grams of the dyes were obtained in acetonitrile with a
Versastat II electrochemical workstation (Princeton Applied
Research) using 0.1 M TBAPF6 (Aldrich) as the supporting
electrolyte, the sensitizer attached to a nanocrystalline TiO2
film deposited on the conducting FTO glass as the working
electrode, a platinum wire as the counter electrode, and a
regular calomel electrode in saturated KCl solution as the
reference electrode. The scan rate was 100 mV s21
.
Photovoltaic measurements were performed by employing
an AM 1.5 solar simulator equipped with a 300 W xenon lamp
(model no. 91160, Oriel). The power of the simulated light was
calibrated to 100 mW cm22 using a Newport Oriel PV reference
cell system (model 91150 V). I–V curves were obtained by
applying an external bias to the cell and measuring the
generated photocurrent with a model 2400 source meter
(Keithley Instruments, Inc. USA). The voltage step and delay
time of the photocurrent were 10 mV and 40 ms, respectively.
Action spectra of the incident monochromatic photon-to-
electron conversion efficiency (IPCE) for the solar cells were
obtained with a Newport-74125 system (Newport Instruments).
The intensity of monochromatic light was measured with a Si
detector (Newport-71640). The electrochemical impedance
spectroscopy (EIS) measurements of all the DSSCs were
performed using
a Zahner IM6e Impedance Analyzer
(ZAHNER-Elektrik GmbH & CoKG, Kronach, Germany), with
the frequency range 0.1 Hz–100 kHz and the alternative signal
of 10 mV.
Fabrication of the solar cells
The procedure for preparation of TiO2 electrodes and fabrica-
tion of the sealed cells for photovoltaic measurements were
28
¨
adapted from that reported by Gratzel and co-workers.
A
screen-printed double layer of TiO2 particles was used as the
photoelectrode. The detailed procedure was reported in our
Scheme 1 Molecular structures of the porphyrin sensitizers.
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previous work.23 Then the photoelectrode was dipped into a
7.84 (d, J = 8.4 Hz, 2H, phenyl), 7.97 (d, J = 7.6 Hz, 2H, phenyl),
8.15 (d, J = 8.4 Hz, 4H, phenyl), 8.26 (d, J = 7.6 Hz, 2H, phenyl),
8.44 (d, J = 7.6 Hz, 2H, phenyl), 9.02 (s, 4H, pyrrolic), 9.07 (d, J =
4.4 Hz, 2H, pyrrolic), 9.34 (d, J = 4.4 Hz, 2H, pyrrolic), 10.22 (s,
1H, meso). HRMS (ESI, m/z): [M + H]+ calcd for C74H82N5O2,
1072.6469; found, 1072.6465. IR (KBr pellet, cm21): 3304(br),
2920(m), 2850(w), 1605(w), 1513(m), 1505(w), 1467(w), 1451(s),
1402(m), 1360(m), 1333(m), 1314(m), 1286(m), 1244(s),
1174(s), 1107(m), 1065(w). 1048(w), 1018(w), 977(w), 963(m),
843(s), 797(s), 748(s), 723(s), 689(m), 625(m), 540(w), 453(w).
5,15-Bis(4-(dodecyloxy)phenyl)-10,20-dibromoporphyrin
0.2 mM porphyrin/ethanol solution for 12
h at room
temperature. The counter electrode was also prepared accord-
ing to our previously reported procedure.23 Finally, the DSSCs
were assembled, with the electrolyte solution containing 0.1 M
LiI, 0.05 M I2, 0.6 M 1-butyl-3-methyl imidazolium iodide
(BMII), and 0.5 M 4-tert-butylpyridine (TBP) in a mixture of
acetonitrile and valeronitrile (volume ratio 1 : 1).
Synthesis of the dyes
5-(4-(9H-carbazol-9-yl)phenyl)dipyrromethane
(1).
4-
(9H-carbazol-9-yl)benzaldehyde (2.0 g, 2.37 mmol) was added
to 50 mL pyrrole. Trifluoroacetic acid (1.8 mL) was then added,
and the reaction mixture was stirred at room temperature for
15 min, saturated sodium bicarbonate aqueous solution was
added to quench the reaction, the mixture was extracted with
CH2Cl2, washed with water, and dried over Na2SO4. After the
solvent had been completely removed, the residue was purified
by column chromatography on silica gel using petroleum and
(3a). To a solution of porphyrin 2a (241 mg, 0.29 mmol) in
CH2Cl2 (0.5 L) was added NBS (103 mg, 0.58 mmol) at room
temperature. After 1 h, the reaction was quenched with
acetone (10 mL). The solvent was removed under reduced
pressure, and the product was isolated by silica gel columns
followed by recrystallization from CH2Cl2/CH3OH to afford 3a
(260 mg, 90%). 1H NMR (CDCl3, 400 MHz): d 22.71 (s, 2H,
NH), 0.79–0.98 (m, 18H), 1.27–1.50 (m, 18H), 1.55–1.68 (m,
6H), 1.96–2.03 (m, 4H), 4.57 (t, J = 6.4 Hz, 4H, OCH2), 7.30 (d, J
= 8.4 Hz, 4H, phenyl), 8.05 (d, J = 8.4 Hz, 4H, phenyl), 8.88 (d, J
= 4.0 Hz, 4H, pyrrolic), 9.61 (d, J = 4.8 Hz, 4H, pyrrolic). HRMS
(ESI, m/z): [M + H]+ calcd for C56H69N4O2Br2, 987.3787; found,
987.3792. IR (KBr pellet, cm21): 3415(br), 2920(s), 2852(s),
2360(s), 2339(m), 1653(m), 1617(w), 1607(w), 1569(w), 1557(m),
1508(s), 1467(s), 1456(w), 1381(m), 1338(m), 1287(s), 1247(s),
1175(s), 1148(w), 1107(w), 1073(w), 996(m), 982(w), 963(s),
848(m), 798(s), 736(m), 721(m), 628(w), 555(w), 538(w), 471(w),
457(w), 418(m).
1
CH2Cl2 as eluent to give 1 in 70% yield. H NMR (CDCl3, 400
MHz): d 5.59 (s, 1H), 6.00 (s, 2H, pyrrolic), 6.22 (q, J = 2.8 Hz,
2H, pyrrolic), 6.76 (d, J = 1.2 Hz, 2H, pyrrolic), 7.28 (m, 2H,
phenyl), 7.43 (m, 6H, phenyl), 7.51 (d, J = 8.4 Hz, 2H, phenyl),
8.03 (s, 2H, –NH), 8.13 (d, J = 7.6 Hz, 2H, phenyl). 13C NMR
(CDCl3, 100 MHz): d 107.59, 108.64, 109.89, 117.68, 120.04,
120.42, 123.41, 126.02, 127.12, 129.85, 132.25, 136.43, 140.85,
141.46. HRMS (ESI, m/z): [M + H]+ calcd for C27H22N3,
388.1814; found, 388.1816.
5,15-Bis(4-(dodecyloxy)phenyl)porphyrin (2a). To a solution
of 4-(dodecyloxy)benzaldehyde (0.90 g, 3.0 mmol), dipyrro-
methane (0.45 g, 3.0 mmol) in CH2Cl2 (1 L) was added
trifluoroacetic acid (0.3 mL). The solution was stirred at room
temperature for 3 h. Then, DDQ (2,3-dichloro-5,6-dicyanoben-
zoquinone) (1.5 g, 6.61 mmol) was added and stirred at room
temperature for another 1 h. After the reaction was quenched
by the addition of 6 mL triethylamine, the solvent was
removed in vacuo, and the residue was purified by column
chromatography (silica gel) using CH2Cl2 as the eluent.
Recrystallization from CH2Cl2/CH3OH gave 2a (450 mg,
40%).1H NMR (CDCl3, 400 MHz): d 23.08 (s, 2H, NH), 0.83–
1.03 (m, 22H), 1.39–1.68 (m, 20H), 1.93–2.06 (m, 4H), 4.24–4.29
(t, J = 6.4 Hz, 4H, OCH2), 7.31–7.34 (d, J = 8.4 Hz, 4H, phenyl),
8.15–8.18 (d, J = 8.8 Hz, 4H, phenyl), 9.10–9.12 (d, J = 3.6 Hz,
4H, pyrrolic), 9.37–9.39 (d, J = 3.6 Hz, 4H, pyrrolic), 10.29 (s,
2H, meso). HRMS (ESI, m/z): [M + H]+ calcd for C56H71N4O2,
831.5577; found, 831.5575. IR (KBr pellet, cm21): 3397(br),
2917(s), 2846(m), 2763(w), 2693(w), 1638(m), 1615(s), 1521(m),
1456(m), 1386(m), 1241(s), 1171(s), 963(m), 858(w), 797(s),
621(s), 473(m).
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-yl)phenyl)-
20-bromoporphyrin (3b). To a solution of porphyrin 2b (150
mg, 0.14 mmol) in CH2Cl2 (0.5 L) was added NBS (25 mg, 0.14
mmol) at room temperature. After 1 h, the reaction was
quenched with acetone (5 mL). The solvent was removed
under reduced pressure, and the residue was isolated by silica
gel columns followed by recrystallization from CH2Cl2/CH3OH
gave 3b (145 mg, 90%). 1H NMR (CDCl3, 400 MHz): d 22.69 (s,
2H, NH), 0.88–0.92 (m, 6H), 1.26–1.49 (m, 32H), 1.59–1.68 (m,
4H), 1.96–2.04 (m, 4H), 4.26 (t, J = 6.4 Hz, 4H, OCH2), 7.31 (d, J
= 8.4 Hz, 4H, phenyl), 7.41 (t, J = 7.2 Hz, 2H, phenyl), 7.58 (t, J =
8.0 Hz, 2H, phenyl), 7.83 (d, J = 8.4 Hz, 2H, phenyl), 7.97 (d, J =
7.6 Hz, 2H, phenyl), 8.10 (d, J = 8.4 Hz, 4H, phenyl), 8.26 (d, J =
7.6 Hz, 2H, phenyl), 8.41 (d, J = 8.0 Hz, 2H, phenyl), 8.92 (s, 4H,
pyrrolic), 8.95 (d, J = 4.4 Hz, 2H, pyrrolic), 9.68 (d, J = 4.4 Hz,
2H, pyrrolic). HRMS (ESI, m/z): [M
+
H]+ calcd for
C
74H81N5O2Br, 1150.5574; found, 1150.5565. IR (KBr pellet,
cm21): 3415(br), 2923(m), 2851(m), 1607(m), 1504(m),
1466(m), 1451(s), 1399(m), 1384(s), 1244(m), 1173(w),
1104(m), 993(w), 965(w), 800(s), 745(m), 720(m), 624(w),
547(w), 466(s).
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-yl)phenyl)
porphyrin (2b). The synthesis was performed following the
procedure similar to that for 2a starting from 4-(dodecylox-
y)benzaldehyde (1.74 g, 6.00 mmol), 5-(4-(9H-carbazol-9-
yl)phenyl)dipyrromethane (1.16 g, 3.00 mmol), and dipyrro-
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-yl)phenyl-
ethynyl)-20-bromoporphyrin (4a). 3a (0.20 mmol), 9-(4-ethynyl-
phenyl)carbazole (0.19 mmol), Pd2(dba)3 (0.10 mmol), AsPh3
(0.41 mmol) were placed in a Schlenk flask, flushed with
nitrogen and then charged with dry THF (30 mL) and Et3N (5
mL). The mixture was stirred at 55 uC for 4.5 h. Then, the
solvent was removed under reduced pressure, and the residue
was dissolved in CH2Cl2 and washed with water, dried over
anhydrous sodium sulfate, and evaporated. The residue was
1
methane (0.45 mg, 3.00 mmol) Yield: 225 mg, 6.7%. H NMR
(CDCl3, 400 MHz): d 22.94 (s, 2H, NH), 0.88–0.92 (m, 6H),
1.25–1.41 (m, 32H), 1.60–1.68 (m, 4H), 1.96–2.04 (m, 4H),
4.27(t, J = 6.4 Hz, 4H, OCH2), 7.31 (d, J = 8.0 Hz, 4H, phenyl),
7.41 (t, J = 7.2 Hz, 2H, phenyl), 7.59 (t, J = 7.6 Hz, 2H, phenyl),
14782 | RSC Adv., 2013, 3, 14780–14790
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purified on a silica gel column. Recrystallization from CH2Cl2/
CH3OH gave the target compound (50 mg, 21%). 1H NMR
(CDCl3, 400 MHz): d 22.28 (s, 2H, NH), 0.85–0.92 (m, 6H),
1.25–1.41 (m, 32H), 1.60–1.68 (m, 4H), 1.96–2.04 (m, 4H), 4.27
(t, J = 6.4 Hz, 4H, OCH2), 7.29–7.38 (m, 6H, phenyl), 7.49 (dt, J1
= 1.2 Hz, J2 = 7.6 Hz, 2H, phenyl), 7.59 (d, J = 8.0 Hz, 2H,
phenyl), 7.80 (dd, J1 = 2.0 Hz, J2 = 6.0 Hz, 2H, phenyl), 8.07 (dd,
J1 = 2.0 Hz, J2 = 6.0 Hz, 4H, phenyl), 8.20 (d, J = 8.0 Hz, 2H,
phenyl), 8.25 (d, J = 8.4 Hz, 2H, phenyl), 8.85 (d, J = 4.8 Hz, 2H,
pyrrolic), 8.92 (d, J = 4.8 Hz, 2H, pyrrolic), 9.60 (d, J = 4.8 Hz,
2H, pyrrolic), 9.71 (d, J = 4.8 Hz, 2H, pyrrolic). HRMS (ESI, m/z):
[M + H]+ calcd for C76H81N5O2Br, 1174.5574; found, 1174.5573.
IR (KBr pellet, cm21): 3418(br), 2921(s), 2851(s), 1635(w),
1616(w), 1556(w), 1514(s), 1470(m), 1452(s), 1398(w), 1384(w),
1359(w), 1329(w), 1283(w), 1246(s), 1175(s), 1107(w), 1071(w),
1004(w), 965(w), 925(w), 846(w).
1723(s), 1603(s), 1509(m), 1494(w), 1452(s), 1433(w), 1399(w),
1381(w), 1361(w), 1273(s), 1244(s), 1173(s), 1155(w), 1106(m),
966(w), 928(w), 798(s), 748(m), 723(m), 625(w), 474(w).
General procedure for zinc coordination
To a solution of the porphyrin (0.028 mmol) in CH2Cl2 (100
mL) was added a CH3OH (20 mL) solution of Zn(OAc)2?2H2O
(199.7 mg, 0.910 mmol). After the mixture was stirred at room
temperature overnight, the solvent was removed under
reduced pressure, and the residue was purified on a silica
gel column, followed by recrystallization from CH2Cl2/CH3OH.
Zinc(II)
5,15-bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-
yl)phenylethynyl)-20-(4-methoxycarbonylphenylethynyl) por-
phyrinate (6a). Yield: 29.9 mg, 81%. 1H NMR (CDCl3, 400
MHz): d = 0.85–0.94 (m, 6H), 1.32–1.45 (m, 28H), 1.50–1.56 (m,
4H), 1.63–1.72 (m, 4H), 2.00–2.08 (m, 4H), 3.81 (s, 3H, OMe),
4.31 (t, J = 6.4 Hz, 4H, OCH2), 7.29–7.36 (m, 6H, phenyl), 7.46
(t, J = 7.2 Hz, 2H, phenyl), 7.54–7.58 (m, 4H, phenyl), 7.72 (d, J
= 8.0 Hz, 2H, phenyl), 7.89 (d, J = 8.0 Hz, 2H, phenyl), 8.10 (d, J
= 8.0 Hz, 4H, phenyl), 8.15 (d, J = 8.0 Hz, 2H, phenyl), 8.84 (d, J
= 4.4 Hz, 2H, pyrrolic), 8.88 (d, J = 4.4 Hz, 2H, pyrrolic), 9.21 (d,
J = 4.0 Hz, 2H, pyrrolic), 9.44 (d, J = 4.4 Hz, 2H, pyrrolic). IR
(KBr pellet, cm21): 3415(br), 2921(s), 2851(s), 2187(s), 1713(m),
1681(w), 1639(m), 1615(m), 1604(m), 1508(s), 1449(s), 1398(w),
1382(m), 1340(w), 1303(w), 1245(m), 1203(m), 1169(s), 1106(w),
1057(w), 997(s), 846(w), 821(w), 793(s), 765(m), 742(s), 723(w),
621(w), 466(w).
General procedure for the syntheses of 5a and 5b
Bromoporphyrin (0.043 mmol), methyl 4-ethynylbenzoate
(20.5 mg, 0.128 mmol), Pd2(dba)3 (21.1 mg, 0.023 mmol),
AsPh3 (33.1 mg, 0.092 mmol) were placed in a Schlenk flask,
which was flushed with nitrogen and then charged with dry
THF (20 mL) and Et3N (3 mL). The mixture was stirred at 50 uC
for 2 h. Then, the solvent was removed under reduced
pressure, and the residue was dissolved in CH2Cl2 and washed
with water, dried over anhydrous sodium sulfate, and
evaporated. The residue was purified on a silica gel column.
Recrystallization from CH2Cl2/CH3OH gave the target com-
pound.
Zinc(II)
5,15-bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-
yl)phenyl)-20-(4-methoxycarbonylphenylethynyl) porphyrinate
(6b). Yield: 32.6 mg, 90%. 1H NMR (CDCl3, 400 MHz): d
0.88–0.92 (m, 6H), 1.25–1.44 (m, 32H), 1.60–1.67 (m, 4H), 1.95–
2.04 (m, 4H), 3.90 (s, 3H, OMe), 4.27 (t, J = 6.4 Hz, 4H, OCH2),
7.31 (d, J = 8.4 Hz, 4H, phenyl), 7.38 (d, J = 7.2 Hz, 2H, phenyl),
7.56 (t, J = 8.0 Hz, 2H, phenyl), 7.82 (d, J = 8.4 Hz, 2H, phenyl),
7.95 (d, J = 8.0 Hz, 2H, phenyl), 8.01 (d, J = 8.0 Hz, 2H, phenyl),
8.07 (d, J = 8.4 Hz, 2H, phenyl), 8.12 (d, J = 8.4 Hz, 4H, phenyl),
8.24 (d, J = 7.6 Hz, 2H, phenyl), 8.38 (d, J = 8.0 Hz, 2H, phenyl),
8.98 (d, J = 4.8 Hz, 2H, pyrrolic), 9.00 (d, J = 4.8 Hz, 2H,
pyrrolic), 9.06 (d, J = 4.4 Hz, 2H, pyrrolic), 9.76 (d, J = 4.8 Hz,
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-yl)phenyl-
ethynyl)-20-(4-methoxycarbonylphenylethynyl)porphyrin (5a).
1
Yield: 35.6 mg, 66%. H NMR (CDCl3, 400 MHz): d 21.98 (s,
2H, NH), 0.88–0.93 (m, 6H), 1.25–1.48 (m, 32H), 1.60–1.68 (m,
4H), 1.96–2.04 (m, 4H), 4.00 (s, 3H, OMe), 4.27 (t, J = 6.4 Hz,
4H, OCH2), 7.30–7.37 (m, 6H, phenyl), 7.49 (t, J = 7.2 Hz, 2H,
phenyl), 7.58 (d, J = 8.4 Hz, 2H, phenyl), 7.79 (d, J = 8.0 Hz, 2H,
phenyl), 8.00 (d, J = 7.6 Hz, 2H, phenyl), 8.09 (d, J = 8.4 Hz, 4H,
phenyl), 8.18–8.23 (m, 6H, phenyl), 8.87 (d, J = 4.4 Hz, 2H,
pyrrolic), 8.88 (d, J = 4.4 Hz, 2H, pyrrolic), 9.59 (d, J = 4.0 Hz,
2H, pyrrolic), 9.67 (d, J = 4.4 Hz, 2H, pyrrolic). HRMS (ESI, m/z):
[M + H]+ calcd for C86H88N5O4, 1254.6836; found, 1254.6842.
IR (KBr pellet, cm21): 3417(br), 2921(s), 2850(s), 1719(m),
1604(w), 1555(w), 1514(s), 1449(m), 1398(w), 1384(s), 1273(s),
1244(s), 1174(m), 1158(w), 1107(w), 969(m), 844(w), 793(s),
751(m), 719(m), 623(w), 472(w).
2H, pyrrolic). HRMS (ESI, m/z): [M
+
H]+ calcd for
C
84H86N5O4Zn, 1292.5971; found, 1292.5973. IR (KBr pellet,
cm21): 3417(br), 2922(s), 2851(m), 2187(m), 1723(s), 1604(s),
1524(w), 1509(s), 1494(w), 1452(s), 1384(m), 1334(m), 1273(s),
1245(s), 1173(s), 1105(m), 1064(w), 998(s), 851(w), 795(m),
766(w), 747(m), 722(m), 627(w), 474(m).
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-yl)phenyl)-
20-(4-methoxycarbonylphenylethynyl)porphyrin (5b). Yield:
31.7 mg, 60%. 1H NMR (CDCl3, 400 MHz): d 22.25 (s, 2H,
NH), 0.88–0.92 (m, 6H), 1.26–1.49 (m,32H), 1.60–1.68 (m, 4H),
1.96–2.04 (m, 4H), 4.02 (s, 3H, OMe), 4.27 (t, J = 6.4 Hz, 4H,
OCH2), 7.31 (d, J = 8.4 Hz, 4H, phenyl), 7.41 (t, J = 7.2 Hz, 2H,
phenyl), 7.58 (t, J = 8.0 Hz, 2H, phenyl), 7.83 (d, J = 8.4 Hz, 2H,
phenyl), 7.97 (d, J = 7.6 Hz, 2H, phenyl), 8.01 (d, J = 8.4 Hz, 2H,
phenyl), 8.12 (d, J = 8.4 Hz, 4H, phenyl), 8.26 (t, J = 8.4 Hz, 4H,
phenyl), 8.40 (d, J = 8.0 Hz, 2H, phenyl), 8.88 (d, J = 4.8 Hz, 2H,
pyrrolic), 8.91 (d, J = 4.8 Hz, 2H, pyrrolic), 8.99 (d, J = 4.4 Hz,
2H, pyrrolic), 9.73 (d, J = 4.8 Hz, 2H, pyrrolic). HRMS (ESI, m/z):
[M + H]+ calcd for C84H88N5O4, 1230.6836; found, 1230.6835.
IR (KBr pellet, cm21): 3416(br), 2922(s), 2851(m), 2187(w),
General procedure for hydrolysis of the esters
A mixture of the porphyrin carboxylate (0.019 mmol) and
LiOH?H2O (31.9 mg, 0.76 mmol) in THF (30 mL) and H2O (4
mL) was refluxed for 12 h under nitrogen. Then, the solvent
was removed in vacuo. The residue was dissolved in THF and
the precipitate was filtered off. The filtrate was concentrated in
vacuo to afford the crude product, which was purified on a
silica gel column, followed by recrystallization from CH2Cl2/
CH3OH.
Zinc(II)
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-
yl)phenylethynyl)-20-(4-carboxylphenylethynyl)
porphyrin
(Q1). Yield: 21.1 mg, 85%. 1H NMR (DMSO-d6, 400 MHz): d
0.84–0.88 (m, 6H), 1.22–1.43 (m, 32H), 1.51–1.58 (m, 4H), 1.86–
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1.90 (m, 4H), 4.22 (t, J = 6.4 Hz, 4H, OCH2), 7.33–7.38 (m, 6H,
phenyl), 7.52 (t, J = 8.0 Hz, 2H, phenyl), 7.60 (d, J = 8.0 Hz, 2H,
phenyl), 7.92 (d, J = 8.4 Hz, 2H, phenyl), 8.07 (d, J = 8.0 Hz, 4H,
phenyl), 8.17–8.26 (m, 4H, phenyl), 8.32 (d, J = 8.0 Hz, 2H,
phenyl), 8.41 (d, J = 8.0 Hz, 2H, phenyl), 8.85 (d, J = 4.4 Hz, 4H,
pyrrolic), 9.74 (d, J = 4.8 Hz, 2H, pyrrolic), 9.78 (d, J = 4.4 Hz,
2H, pyrrolic), 13.24 (s, 1H, COOH). HRMS (ESI, m/z): [M 2 H]2
calcd for C85H82N5O4Zn, 1300.5658; found, 1300.5623. IR (KBr
pellet, cm21): 3416(br), 3232(w), 2923(s), 2847(m), 2187(w),
1689(w), 1637(m), 1617(s), 1507(s), 1453(s), 1386(m), 1316(w),
1255(m), 1210(w), 1175(s), 1104(w), 1066(w), 995(s), 854(w),
790(m), 765(m), 749(m), 623(m), 472(w).
Zinc(II)
5,15-Bis(4-(dodecyloxy)phenyl)-10-(4-(carbazol-9-
yl)phenyl)-20-(4-carboxylphenylethynyl) porphyrin (Q2). Yield:
1
21.9 mg, 90%. H NMR (DMSO-d6, 400 MHz): d 0.83–0.88 (m,
6H), 1.23–1.32 (m, 32H), 1.53–1.60 (m, 4H), 1.86–1.92 (m, 4H),
4.27 (t, J = 6.4 Hz, 4H, OCH2), 7.35 (d, J = 8.0 Hz, 4H, phenyl),
7.41 (t, J = 7.2 Hz, 2H, phenyl), 7.60 (t, J = 8.0 Hz, 2H, phenyl),
7.85 (d, J = 8.4 Hz, 2H, phenyl), 7.04 (d, J = 7.6 Hz, 2H, phenyl),
8.08 (d, J = 8.0 Hz, 4H, phenyl), 8.20 (d, J = 8.4 Hz, 2H, phenyl),
8.25 (d, J = 8.0 Hz, 2H, phenyl), 8.37 (d, J = 8.0 Hz, 2H, phenyl),
8.42 (d, J = 8.0 Hz, 2H, phenyl), 8.82 (d, J = 4.4 Hz, 2H, pyrrolic),
8.92 (d, J = 4.4 Hz, 2H, pyrrolic), 8.94 (d, J = 5.2 Hz, 2H,
pyrrolic), 9.79 (d, J = 4.8 Hz, 2H, pyrrolic), 13.23 (s, 1H, COOH).
HRMS (ESI, m/z): [M + H]+ calcd for C83H84N5O4Zn, 1278.5815;
found, 1278.5811. IR (KBr pellet, cm21): 3414(br), 2917(s),
2851(m), 2353(w), 2183(w), 1718(m), 1640(w), 1618(s), 1511(m),
1450(m), 1399(m), 1279(s), 1241(s), 1179(s), 1101(m), 976(m),
800(s), 752(w), 723(w), 624(w), 473(m).
Results and discussion
Molecular design and syntheses
One of the major unfavorable factors for DSSCs is the
formation of dye aggregates on the semiconductor surface.21
4-(Dodecyloxy)phenyl groups were introduced to the porphyrin
meso positions to improve solubility of the dyes in organic
solvents and suppress dye aggregation. Generally, organic
DSSC dyes have a D-p-A structure.6–14 In the molecules of Q1
and Q2 (Scheme 2), carbazole and 4-ethynylbenzoic acid
moieties were introduced to the porphyrin meso-positions as
the electron donor and the electron acceptor, respectively. An
extra ethynylene bridge was incorporated into Q1 with the
purpose of investigating the effect of p framework extension
on cell performance.
Scheme 2 The synthetic routes for the dyes. Reaction conditions: (i) TFA; (ii) TFA,
DDQ, CH2Cl2; (iii) NBS, CH2Cl2; (iv) methyl 4-ethynylbenzoate or 9-(4-
ethynylphenyl)carbazole, AsPh3, Pd2(dba)3, THF, Et3N; (v) Zn(CH3COO)2?2H2O,
CH2Cl2, CH3OH; (vi) LiOH?H2O, THF, H2O.
ing peaks for Q2 were observed at 575 and 628 nm. Thus, the
strong Soret bands and medium Q bands of these two dyes
may fulfil the requirement for DSSC sensitizers. Compared
Intermediates 2a and 2b were prepared by TFA-promoted
condensation of corresponding dipyrromethanes and alde-
hydes in CH2Cl2, followed by bromination with various
equivalents of NBS. The ethynyl group was attached to the
meso-position via Sonogashira coupling reactions in satisfac-
tory yields. All the intermediates and target porphyrins were
1
fully characterized by H NMR and HRMS (Fig. S1–S24, ESI ).
3
Absorption spectra in THF and on TiO2 films
Absorption spectra of Q1 and Q2 in THF (Fig. 1a) show intense
Soret bands at 449 nm and 444 nm, respectively. The Q bands
of Q1 were observed at 609 nm and 660 nm, and correspond-
Fig. 1 (a) Absorption spectra and (b) emission spectra of the porphyrins
measured in THF.
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Table 1 Electrochemical properties of the porphyrin dyes
Porphyrins
HOMOa/V (vs. NHE)
E0–0b/V
LUMOc/V (vs. NHE)
Q1
Q2
0.87
0.84
1.67
1.82
20.80
20.98
a
HOMO levels were measured in acetonitrile with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte
(working electrode: FTO/TiO2/dye; reference electrode: SCE;
calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external
b
Fig. 2 UV-visible spectra of Q1 and Q2 adsorbed on TiO2 films (5 mm).
reference. Counter electrode: Pt). E0–0 was estimated from the
c
absorption threshold of dyes adsorbed on the TiO2 film. The
LUMO levels were calculated with the equation of LUMO = HOMO 2
E0–0
.
with Q2, the p-conjugation system in Q1 is enlarged by an extra
ethynylene bridge, which results in a pronounced red shift and
broadening of both Soret and Q bands, with a long wavelength
absorption threshold of 695 nm. In comparison with most
reported porphyrin dyes without an ethynylene bridge,21,23 Q1
shows obviously red-shifted absorption maxima and threshold
wavelengths. The fluorescence spectra of Q1 and Q2 were also
checked in THF (Fig. 1b), and the emission wavelengths also
vary with a trend similar to that of the absorption bands.
Fig. 2 shows the UV-vis absorption spectra of the dye-loaded
TiO2 films (y5 mm) with a bare TiO2 film as the reference.
Comparable absorption bands are observed for Q1 and Q2,
with the intense Soret bands at 452 nm and 448 nm,
respectively. These bands are red-shifted compared with
corresponding solution spectra. And the absorption of the
films of Q2 is significantly stronger than those of Q1, which is
indicative of a larger amount of Q2 adsorbed on the TiO2
films. It is noteworthy that the absorption bands of the films
are broadened and red-shifted compared with those observed
in the THF solutions, which will result in higher light
harvesting efficiencies. In addition, the presence of the blue
shoulders of the Soret bands was observed, which may result
from the solvent effect, deprotonation of the carboxylic acid
upon dye attachment to the TiO2 surface,29 and/or
H-aggregation of the dye molecules.30
of Q1 shows two oxidation waves at 0.87 and 1.31 V, and Q2
also shows two redox couples at 0.84 and 1.38 V, respectively.
The first oxidation process at the lower potential may be
attributed to the oxidation of the carbazole unit, and the
second one may correspond to the oxidation of the porphyrin
unit. Thus the HOMO energy levels observed for Q1 and Q2 are
0.87 and 0.84 V (vs. NHE), respectively. The absorption
thresholds for dyes Q1 and Q2 on the TiO2 films (Fig. 2) are
743 and 681 nm, respectively, corresponding to the band gap
energies (E0–0) of 1.67 and 1.82 V, respectively. Thus, the
estimated LUMO energy levels calculated from EHOMO 2 E0–0
,
are 20.80 and 20.98 V, respectively. The HOMO levels for
these two dyes are almost the same, which can be attributed to
the similar electron-donating ability of the donors in the two
dyes. And introduction of the additional ethynylene bridge
into Q1 decreases the HOMO–LUMO energy gap, which is
consistent with red shift of the absorption bands, and a more
negative LUMO level. As shown in Fig. 4, the LUMO levels of
these dyes are both more negative than the conduction edge of
TiO2, indicating that the electron injection from the excited
states of the dyes to the conduction band (CB) of TiO2 is
possible. In addition, the HOMO levels of Q1 and Q2 are both
more positive than the oxidation potential of the I2/I32 redox
couple, indicating that the energy levels are thermodynami-
cally favourable for the regeneration of the dyes from the
oxidized states by accepting electrons from I2 ions presented
in the electrolyte of the DSSCs.31 Hence, both of Q1 and Q2
Electrochemical properties
Suitable highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels are
required for DSSC sensitizers to ensure efficient electron
injection and dye regeneration processes. To evaluate the
possibility of electron transfer from the excited dye molecule
to the conduction band of TiO2, cyclic voltammograms (Fig. 3)
were measured for the dyes absorbed on the TiO2 films, and
the data are summarized in Table 1. The cyclic voltammogram
Fig. 3 Cyclic voltammetry plots of the dyes adsorbed to a nanocrystalline TiO2
Fig. 4 Schematic energy-level diagram of the porphyrins. LUMO is estimated by
film deposited on conducting FTO glass.
subtracting E0–0 from HOMO.
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Fig. 6 IPCE curves of DSSCs based on the porphyrin dyes.
Photovoltaic performance of DSSCs
From the above mentioned results, it can be anticipated that
Q1 and Q2 may be employed as DSSC dyes. Thus, DSSCs were
fabricated based on these two dyes. Fig. 6 shows the incident
photon-to-electron conversion efficiency (IPCE) as a function
of incident wavelength. These two porphyrin dyes have
moderate IPCE values in a large wavelength range of 400–
750 nm. Large gaps between the Soret and Q bands have been
observed in absorption spectra of the porphyrins in THF, but
this feature is not evident in the IPCE curves because the
absorption bands are broadened on the TiO2 film, and the
scattering by TiO2 nanoparticles increases the photocurrents
in the relatively weak absorption region. Compared with Q2,
the IPCE of Q1 is broadened to 750 nm, which is in good
agreement with the UV-vis absorption spectra on TiO2 (Fig. 2).
This result indicates that introduction of an additional
ethynylene bridge is a good approach for extending IPCE to
the near IR-region. In spite of this, the absorbance of Q1 on
TiO2 films is much smaller than that of Q2 in most of the
wavelength range, which may be ascribed to the worse
solubility and less adsorption amount of Q1. Consequently,
the maximum IPCE value of Q1 is much smaller than that of
Q2 which may result in lower Jsc of Q1 (vide infra).
Fig. 5 Frontier molecular orbital profiles of the dyes calculated by DFT and
chemical structures indicating distortion of the dye molecules.
may serve as DSSC sensitizers. Q1 and Q2 have very similar
HOMO levels, indicating that they have similar driving force
for the dye regeneration process. On the other hand, Q2 has a
more negative LUMO level than that of Q1, which will result in
a larger driving force for the electron injection process. Thus,
by considering the molecular orbital levels, it can be
anticipated that Q2 sensitized solar cells will have higher
efficiency than that of Q1. This is in agreement with the
photovoltaic performance results (vide infra).
DFT calculations
Fig. 7 shows the photocurrent–voltage (J–V) curves of the
DSSCs, and the photovoltaic parameters of short-circuit
current density (Jsc), open-circuit voltage (Voc), fill factor (FF),
and solar energy to electricity conversion efficiencies (g) are
listed in Table 2. The efficiencies were obtained to be 2.22%
and 5.51% for Q1 and Q2, respectively. The higher efficiency
observed for Q2 may be ascribed to the better solubility, larger
adsorption amount, suppressed dye aggregation and more
To get further insight into the molecular structure and
electron distribution of the dyes, their geometries were
optimized by density functional theory (DFT) calculations at
the B3LYP/6-31G* level with Gaussian 09 program package.32
From the optimized conformation (Fig. 5), we can find that the
dihedral angle between the phenyl unit and the porphyrin
macrocycle is 71.1u for Q2, but corresponding angle for Q1 has
a very small value of 0.3u, indicating that Q2 has a relatively
nonplanar structure, and Q1 is relatively planar. As is well
known, for
a nonplanar and distorted DSSC dye, dye
aggregation and charge recombination could be efficiently
suppressed, which will be favorable for enhancing cell
efficiency.33 Thus, compared to Q1, Q2 could be considered
as a better dye due to its relatively nonplanar structure (vide
infra). Corresponding molecular orbitals for Q1 and Q2 are
shown in Fig. 5. The HOMO orbitals are delocalized between
the donors and the porphyrin macrocycles, and the LUMO
orbitals are delocalized between the porphyrin macrocycles
and the carboxyl groups. Hence, excitation of the electron from
HOMO to LUMO can induce electron redistribution from the
donor to the anchoring moiety, thus enabling electron
injection into the conduction band of TiO2.
Fig. 7 Current–voltage characteristics of the DSSCs based on the porphyrin dyes.
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electrolyte is overlapped by the intermediate-frequency large
semicircle. This observation is indicative of fast electron
transport and long electron lifetime in the TiO2 film. The
radius of the second semicircle for Q1 is smaller than that of
Q2, indicating that charge recombination is suppressed in the
cell based on Q2.
In the Bode phase plots (Fig. 8b), the middle frequency peak
corresponds to charge transfer at the TiO2/dye/electrolyte
interface. Compared with the DSSC based on Q1, a lower
frequency peak was observed for Q2, indicative of a longer
electron lifetime.36 Thus, the corresponding lifetime values
were found to be 11.5 ms and 29.1 ms for Q1 and Q2,
respectively. The longer lifetime observed for Q2 is indicative
of a more effective suppression of the back reaction between
the injected electrons and the electrolyte, resulting in
improvement of Voc due to the reduced charge recombination
rate. The EIS results are in good agreement with the
photovoltaic performance as indicated in Fig. 7.
Table 2 Photovoltaic parameters of porphyrin-based dye-sensitized solar cells
under AM 1.5 illumination (power 100 mW cm22) with an active area of 0.25
cm2
Porphyrins
Jsc/mA cm22
Voc/mV
ff
g (%)
Q1
Q2
5.43
11.30
0.57
0.68
0.72
0.71
2.22
5.51
suitable energy levels as mentioned above. The addition of
chenodeoxycholic acid (CDCA) is one of the most efficient
approaches of suppressing the aggregation effect.34 Thus, we
used CDCA as a coadsorbant, but no increased efficiency was
obtained. This observation may be attributed to the weakened
absorption spectrum caused by the addition of CDCA (Fig. S25,
ESI ), which decreased the light harvesting ability of the dyes
3
and thus balanced the effect of CDCA on suppressing the
aggregation of the dyes.
Electrochemical impedance spectroscopy
Conclusions
For better understanding of the correlation between charge-
transfer processes and photovoltaic properties, electrochemi-
cal impedance spectral (EIS) analyses were measured in the
dark at a forward bias of 0.70 V. For Q1 (Fig. 8a), three
semicircles could be clearly observed. The first one is
associated with the charge transfer at Pt/redox (I2/I32) inter-
face, the second one is large and related to charge transfer at
dye-sensitized TiO2/redox (I2/I32) interface, and the third one
In summary, we have synthesized two novel porphyrin dyes Q1
and Q2 bearing alkoxyl chains, and a carbazole moiety was
introduced as the electron donor. In Q1, a 4-(9H-carbazol-9-
yl)phenyl moiety was introduced to a porphyrin unit through
an ethynylene bridge. In contrast, these two units are directly
linked to each other in the molecule of Q2. Compared with Q1,
Q2 has a more twisted non-planar conformation, which results
in better solubility and larger amount of adsorption on the
TiO2 films with suppressed dye aggregation. Thus, the DSSC
device based on Q2 has been demonstrated to achieve a higher
power conversion efficiency of 5.51%, compared with that of
2.22% for Q1. These results indicate that the extension of the
absorption band to a longer wavelength region by the
introduction of an additional ethynylene bridge may result
in worse solubility, lower adsorption amount and more severe
aggregation, and thus may decrease the cell efficiency. For the
design of efficient DSSC sensitizers, these contradictory effects
must be fully considered and well balanced.
2
is associated with the Nernst diffusion of I3 in the
electrolyte.35 However, only two semicircles can be observed
for Q2 (Fig. 8a), because the Nernst diffusion within the
Acknowledgements
This work was financially supported by NSFC (21072060), the
Program for Professor of Special Appointment (Eastern
Scholar) at Shanghai Institutions of Higher Learning,
Program for New Century Excellent Talents in University
(NCET-11-0638), Innovation Program of Shanghai Municipal
Education Commission, the Fundamental Research Funds for
the Central Universities (WK1013002), and SRFDP
(20100074110015).
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