Organic dyes incorporating N-functionalized pyrrole as conjugated bridge for dye-sensitized solar cells: Convenient synthesis, additional withdrawing group on the π-bridge and the suppressed aggregation

Article abstract of DOI:10.1016/j.dyepig.2013.05.030

Two new N-functionalized pyrrole-based organic dyes were designed and synthesized for dye-sensitized solar cells. In addition to the normal acceptor of cyanoacetic acid moieties, another electron-withdrawing group, pentafluorophenyl or cyanophenyl one, wa

Full text of DOI:10.1016/j.dyepig.2013.05.030

Accepted Manuscript
Organic Dyes Incorporating N-Functionalized Pyrrole as Conjugated Bridge for Dye-
Sensitized Solar Cells: Convenient Synthesis, Additional Withdrawing Group on the
π-Bridge and the Suppressed Aggregation
Huiyang Li, Luying Yang, Runli Tang, Yingqin Hou, Yizhou Yang, Heng Wang,
Hongwei Han, Jingui Qin, Qianqian Li, Zhen Li
PII:
S0143-7208(13)00201-5
10.1016/j.dyepig.2013.05.030
DYPI 3967
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 February 2013
Revised Date: 23 May 2013
Accepted Date: 24 May 2013
Please cite this article as: Li H, Yang L, Tang R, Hou Y, Yang Y, Wang H, Han H, Qin J, Li Q, Li Z,
Organic Dyes Incorporating N-Functionalized Pyrrole as Conjugated Bridge for Dye-Sensitized Solar
Cells: Convenient Synthesis, Additional Withdrawing Group on the π-Bridge and the Suppressed
Aggregation, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.05.030.
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ACCEPTED MANUSCRIPT
Organic Dyes Incorporating N-Functionalized Pyrrole as
Conjugated Bridge for Dye-Sensitized Solar Cells: Convenient
Synthesis, Additional Withdrawing Group on the π-Bridge and
the Suppressed Aggregation
Huiyang Li,a Luying Yang,a Runli Tang,a Yingqin Hou,a Yizhou Yang,a Heng Wang,b
Hongwei Han,b Jingui Qin,a Qianqian Li*a and Zhen Li*a
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic
Materials, Wuhan University, Wuhan 430072, China
b Wuhan National Laboratory For Optoelectronics (WNLO), Huazhong University of Science
and Technology (HUST),1037 Luoyu Road, Wuhan 430074, China
Highlights
1. Two new N-functionalized pyrrole-based organic dyes were applied in dye-sensitized
solar cells.
2. Another electron-withdrawing group, pentafluorophenly or cyanophenly one, was linked
to the nitrogen atom of the pyrrole ring, to construct the dyes.
3. The device based on sensitizer LI-6 achieved the efficiency of 6.23% without CDCA.
Graphical abstract
27

ACCEPTED MANUSCRIPT
Organic Dyes Incorporating N-Functionalized Pyrrole as
Conjugated Bridge for Dye-Sensitized Solar Cells: Convenient
Synthesis, Additional Withdrawing Group on the π-Bridge and
the Suppressed Aggregation
Huiyang Li,a Luying Yang,a Runli Tang,a Yingqin Hou,a Yizhou Yang,a Heng Wang,b
Hongwei Han,b Jingui Qin,a Qianqian Li*a and Zhen Li*a
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic
Materials, Wuhan University, Wuhan 430072, China
b Wuhan National Laboratory For Optoelectronics (WNLO), Huazhong University of Science
and Technology (HUST),1037 Luoyu Road, Wuhan 430074, China
*Corresponding author. Phone: 86-27-62254108; Fax: 86-27-68756757; E-mail: lizhen@whu.edu.cn (Z. Li)
*Corresponding author. Phone: 86-27-62306885; Fax: 86-27-68756757; E-mail: qianqian-alinda@163.com (Q. Li)
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Abstract
Two new N-functionalized pyrrole-based organic dyes were designed and synthesized for
dye-sensitized solar cells. In addition to the normal acceptor of cyanoacetic acid moieties,
another electron-withdrawing group, pentafluorophenyl or cyanophenyl one, was linked to the
nitrogen atom of the pyrrole ring, with the aim to both suppress the aggregation of the dyes on
the TiO2 surface and expand the electronic absorption spectra. As a result, dye LI-6 with
pentafluorophenyl group as the auxiliary acceptor showed remarkable power conversion
efficiencies of up to 6.23%, under AM 1.5G simulated solar light (100 mW/cm2), while that of
N719 was tested to be 7.83% at the same measuring conditions.
Keywords
Pyrrole; Organic dyes; Electron-withdrawing group; Aggregation; Dye-sensitized solar cells;
Power conversion efficiency
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1. Introduction
Since Grätzel published the nature paper [1] in 1991, dye-sensitized solar cells (DSCs)
have come into sight to challenge the traditional silicon-based solar cells, due to their low cost
and high efficiency [2]. In these cells, the sensitizers play the important role of capturing solar
energy and generating electric charges, as a key component. Thus, many laboratories around
the world focus on the design of new sensitizers, including metal-complexes and metal-free
ones, to achieve high efficiency [3]. Specially, metal-free sensitizers attract rising interest for
their high molar absorption coefficient, higher structural flexibility, and easier preparation and
purification [4]. Generally, the metal-free sensitizers consist of a push-pull structure of
donor-π conjugated unit-acceptor (D-π-A) [5], in which amine derivatives [6] usually act as the
electron donor while cyanoacetic or rhodanine acid one [7-8] as the electron acceptor. In
addition to the importance of the donor and acceptor groups, the π bridge [9], which links the
donor and acceptor moieties, badly affects the efficiency of the intramolecular charge transfer,
and partially determines the DSC performance of the corresponding cells. To optimize the
properties of dyes, it is a good approach to modify the structure of the π bridges at the
molecular level [10].
One strategy was to use novel conjugated bridges, instead of traditional ones. In fact,
some electron-rich heteroaromtic rings, such as thiophene [11] and furan [12], have been
successfully used as bridges, and exhibit high conversion efficiencies. However, the pyrrole
ring [13], the analogue of furan and thiophene moieties, has seldom been noticed, possibly due
to its instability. Recently, our group found that its stability could be improved at a large
extent, once being incorporated between other aromatic blocks or after the linkage of various
aromatic groups to the nitrogen atom of the pyrrole ring [14-16]. Another strategy was to
extend the conjugated bridge [17] or incorporate an additional electron acceptor [18] into the
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conjugated system, to expand the absorption spectra in the solar irradiation region. The
extension of the conjugated bridge can efficiently prolong the spectral response, however, the
longer conjugated linker usually results in the unfavorably binding or orientation on the TiO2
surface, and increases the possibility of the recombination of electrons to the triiodide,
directly leading to the poorer injection efficiency and the decreased conversion efficiency.
Thus, a new method was proposed to introduce some low band gap, strong
electron-withdrawing units, which acted as an additional electron acceptor, to form the
“D-A-π-A” type of organic dyes [19]. These dyes show several favorable characteristics in the
areas of light-harvesting efficiency, such as a large responsive range of wavelengths, an
improvement in the electron distribution from the donor unit to the acceptor, and the increase
of the photo-stability of dyes. Tian and Zhu et.al have systematically studied the role of
incorporated electron-withdrawing acceptors in conjugated bridge, and many
electron-withdrawing building blocks, such as benzothiadiazole, benzotriazole, quinoxaline,
diketopyrrolopyrrole, thienopyrazine, thiazole, triazine, cyanovinyl, cyano- and
fluoro-substituted phenyl, have been introduced into the skeleton of dyes as an auxiliary
acceptor. Most of them show the higher conversion efficiency and better stability than the
similar D-π-A ones.
Based on the excellent work on the normal “D-A-π-A” dyes with linear structure, we are
wondering if it is possible to introduce another acceptor group on the π-bridge but
perpendicular to the whole molecule? Perhaps, this introduced acceptor group could not only
expand the absorption spectra of the resultant dyes, but also hinder the possible aggregation
on the TiO2 surface. Therefore, two new N-functionalized pyrrole-based organic dyes (LI-6
and LI-7) were synthesized, in which an electron-withdrawing unit, pentafluorophenyl or
cyanophenyl one, was linked to the nitrogen atom of the pyrrole ring (Scheme 1), which
would act as the additional electron acceptor on the side chain. Since the
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electron-withdrawing units were linked to the pyrrole ring through Carbon-Nitrogen single
bond, they were non-coplanar to pyrrole, and could also act as the isolation group to suppress
the possible aggregation of the dyes on the TiO2 surface in some degree. The dyes were
conveniently synthesized, and demonstrated relatively high conversion efficiencies. Herein,
we would like to report their syntheses, structural characterization, absorption properties,
theoretical calculations, and photovoltaic performance in detail.
2. Experimental
2.1 Materials
Tetrahydrofuran (THF) was dried over and distilled from K-Na alloy under an
atmosphere of dry nitrogen. N, N-Dimethylformamide (DMF) was dried over and distilled
from CaH2 under an atmosphere of dry nitrogen. 1,2-Dichloroethane was dried over and
distilled from phosphorus pentoxide. Phosphorus oxychloride was freshly distilled before use.
Compound 4 was prepared following the procedure reported in the literature [20]. All other
reagents were used as received.
2.2 Instrumentation
1H and 13C NMR spectroscopy study was conducted with a Varian Mercury 300
spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as internal standard. HR-ESI-TOF
mass spectra were recorded on a Waters Micromass LCT Premier XE.. UV-visible spectra
were obtained using a Shimadzu UV-2550 spectrometer. Electrochemical cyclic voltammetry
was conducted on a CHI 660 voltammetric analyzer with Pt disk, Pt plate, and Ag/Ag+
electrode as working electrode, counterelectrode, and reference electrode, respectively, in
nitrogen-purged anhydrous CH2Cl2 with tetrabutylammonium hexafluorophosphate (TBAPF6)
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as the supporting electrolyte at a scanning rate of 100 mV/s. The ferrocene/ferrocenium redox
couple was used for potential calibration.
General procedure for the synthesis of 2a and 2b. To a solution of 1a or 1b (1.00 equiv)
in acetic acid, 2,5-dimethoxytetrahydrofuran (1.50 equiv) was added, and the mixture was
heated under refluxing for 2-4 h. The solvent was removed under reduced pressure, poured
into water, and then extracted with chloroform for several times. After removing the solvent
under vacuum, the crude product was purified by column chromatography using
petroleum/chloroform (2:1) as an eluent.
2a: 1a (0.73 g, 4.00 mmol), 2,5-dimethoxytetrahydrofuran (0.79 g, 6.00 mmol). White solid
(0.69 g, 69.6%). 1H NMR (CDCl3) δ (ppm): 6.88 (s, 2H, ArH), 6.42 (s, 2H, ArH).
2b: 1b (2.36 g, 20.0 mmol), 2,5-dimethoxytetrahydrofuran (3.96 g, 30.0 mmol). White solid
(2.70 g, 80.3%). 1H NMR (CDCl3) δ (ppm): 7.72 (d, J = 7.2 Hz, 2H, ArH), 7.48 (d, J = 7.2 Hz,
2H, ArH), 7.14 (s, 2H, ArH), 6.41 (s, 2H, ArH).
General procedure for the synthesis of 3a and 3b. Dry DMF (2.00 equiv) was added to
freshly distilled POCl3 (1.50 equiv) at 0 oC in an ice water bath, and the resultant solution was
stirred until its complete conversion into a glassy solid. After the addition of compound 2a or
2b (1.00 equiv) in 1,2-dichloroethane dropwise, the mixture was stirred at 0 oC for 1 h, then at
room temperature for 6 h. After this, it was poured into an aqueous solution of sodium
acetate, and stirred for another 2 h. The mixture was extracted with chloroform for several
times, then the organic fractions were collected and dried over anhydrous Na2SO4. After
removing the solvent under vacuum, the crude product was purified by column
chromatography using chloroform as an eluent.
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3a: 2a (4.10 g, 17.2 mmol), DMF (2.52 g, 34.4 mmol), POCl3 (3.96 g, 25.8 mmol). White
solid (4.15 g, 92.4%). 1H NMR (CDCl3) δ (ppm): 9.59 (s, 1H, -CHO), 7.18 (s, 1H, ArH), 6.98
(s, 1H, ArH), 6.55(s, 1H, ArH).
3b: 2b (2.25 g, 13.4 mmol), DMF (1.96 g, 26.8 mmol), POCl3 (3.08 g, 20.1 mmol). White
solid (2.16 g, 82.3%). 1H NMR (CDCl3) δ (ppm): 9.61 (s, 1H, -CHO), 7.76 (d, J = 7.2 Hz, 2H,
ArH), 7.48 (d, J = 7.2 Hz, 2H, ArH), 7.20 (s, 1H, ArH), 7.11 (s, 1H, ArH), 6.48(s, 1H, ArH).
General procedure for the synthesis of 5a and 5b. NaH (2.00 equiv) was added to an
anhydrous tetrahydrofuran solution of compound 4 (1.20 equiv) under an atmosphere of dry
nitrogen, and the resultant mixture was stirred at room temperature for 10 min. After the
solution of 3a or 3b (1.00 equiv) in anhydrous tetrahydrofuran was added dropwise, the
reaction mixture was stirred at room temperature overnight, then poured into water. The
organic product was extracted with chloroform, and dried over anhydrous Na SO . After
2
4
removing the solvent, the crude product was purified by column chromatography using
petroleum/chloroform (10:1) as an eluent.
5a: 3a (1.31 g, 5.00 mmol), 4 (2.37 g, 6.00 mmol), NaH (0.34 g, 10.0 mmol). Orange powder
(1.69 g, 67.2%). 1H NMR (CDCl3) δ (ppm): 7.26-7.18 (m, 6H, ArH), 7.10-6.97 (m, 8H, ArH),
6.83 (d, J = 16.2 Hz, 1H, -CH=CH-), 6.70 (s, 1H, ArH), 6.65 (s, 1H, ArH), 6.43-6.35 (m, 2H,
ArH and -CH=CH-).
5b: 3b (1.18 g, 6.02 mmol), 4 (2.86 g, 7.22 mmol), NaH (0.41 g, 12.04 mmol). Orange
powder (2.30 g, 87.3%). 1H NMR (CDCl3) δ (ppm): 7.77 (d, J = 6.9 Hz, 2H, ArH), 7.48 (d, J
= 6.9 Hz, 2H, ArH), 7.27-7.20 (m, 6H, ArH), 7.10-7.02 (m, 8H, ArH), 6.88-6.83 (m, 2H, ArH
and -CH=CH-), 6.66-6.63 (m, 2H, ArH and -CH=CH-), 6.38 (s, 1H, ArH).
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General procedure for the synthesis of 6a and 6b. Dry DMF (2.00 equiv) was added to
o
freshly distilled POCl (1.50 equiv) under an atmosphere of dry nitrogen at 0 C in an ice
3
water bath, and the resultant solution was stirred until its complete conversion into a glassy
solid. After 5a or 5b (1.00 equiv) in 1,2-dichloroethane was added dropwise, the mixture was
stirred at room temperature overnight, then poured into an aqueous solution of sodium
acetate, and stirred for another 2 h. The mixture was extracted with chloroform for several
times, then the organic fractions were combined and dried over anhydrous Na SO . After
2
4
removing the solvent under vacuum, the crude product was purified by column
chromatography using petroleum/ethyl acetate (4:1) as an eluent.
6a: 5a (1.90 g, 4.61 mmol), POCl (1.06 g, 6.92 mmol), DMF (0.67 g, 9.22 mmol). Orange
3
1
powder (1.66 g, 67.8%). H NMR (CDCl3) δ (ppm): 9.47 (s, 1H, -CHO), 7.26-7.16 (m, 7H,
ArH), 7.11-7.08 (m, 7H, ArH and -CH=CH-), 6.98 (d, J = 7.2 Hz, 2H, ArH), 6.77 (s, 1H,
ArH), 6.26 (d, J = 15.0, 1H, -CH=CH-).
6b: 5a (0.70 g, 1.39 mmol), POCl (0.32 g, 2.09 mmol), DMF (0.20 g, 2.78 mmol). Orange
3
powder (0.47 g, 72.0%). 1H NMR (CDCl3) δ (ppm): 9.43 (s, 1H, -CHO), 7.80 (d, J = 8.7 Hz,
2H, ArH), 7.44 (d, J = 8.4 Hz, 2H, ArH), 7.28-7.23 (m, 4H, ArH), 7.17-7.13 (m, 3H, ArH),
7.09-7.04 (m, 7H, ArH and -CH=CH-), 6.96 (d, J = 8.4 Hz, 2H, ArH), 6.72 (d, J = 4.8 Hz, 1H,
ArH), 6.30 (d, J = 16.5 Hz, 1H, -CH=CH-).
General procedure for the Synthesis of LI-6 and LI-7. A mixture of 6a or 6b (1.00
equiv) and cyanoacetic acid (3.00 equiv) was vacuum-dried, then acetonitrile as a solvent and
piperidine (10 ꢀL) as a catalyst were added. The solution was refluxed for 6-8 h. After the
solution was cooled to the room temperature, the organic layer was removed under vacuum.
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The product was purified by recrystallization or column chromatography.
LI-6: 6a (250 mg, 0.47 mmol), cyanoacetic acid (120 mg, 1.41 mmol). Black solid (200 mg,
o
71.3 %). Mp: 225–226 C. 1H NMR (CDCl3) δ (ppm): 8.08 (s, 1H, -CH=), 7.38-7.17 (m, 8H,
ArH), 7.12-7.05 (m, 6H, ArH and -CH=CH-), 6.98 (d, J = 8.1 Hz, 3H, ArH), 6.20 (d, 1H, J =
15.9 Hz, -CH=CH-); 13C NMR (DMSO-d6) δ (ppm): 164.6, 148.5, 147.3, 142.8, 137.0, 134.7,
130.3, 129.2, 125.3, 124.4, 122.5, 120.9, 118.3, 112.7, 112.2, 96.6; HRMS (ESI) calcd for
C34H20N3O2F5 597.1476, found 620.1378 [M+Na]+.
LI-7: 6b (230 mg, 0.49 mmol), cyanoacetic acid (126 mg, 1.47 mmol). Black solid (215 mg,
81.7 %). Mp: 227–228 oC. 1H NMR (CDCl3) δ (ppm): 8.01 (s, 1H, -CH=), 7.91 (d, J = 7.5 Hz,
2H, ArH), 7.47-7.42 (m, 4H, ArH), 7.28-7.26 (m, 1H, ArH), 7.16 (d, J = 6.0 Hz, 4H, ArH),
7.10-7.07 (m, 6H, -CH=CH- and ArH), 6.97-6.91 (m, 4H, ArH), 6.28 (d, J = 16.8 Hz, 1H,
13
-CH=CH-); C NMR (DMSO-d6) δ (ppm): 165.4, 148.7, 147.6, 142.8, 139.7, 139.1, 135.1,
134.1, 130.1, 129.9, 129.1, 129.0, 125.6, 124.7, 123.0, 120.4, 119.0, 118.7, 114.1, 113.7,
112.1, 93.6; HRMS (ESI) calcd for C35H24N4O2 532.1899, found 533.1980 [M+H]+.
2.3 Preparation of Solar Cells
The dye-sensitized TiO2 electrode was prepared by following the procedure reported in
the literature [21]. A TiO2 colloidal dispersion was made by employing commercial TiO2 (P25,
Degussa AG, Germany) as material. Films of nanocrystalline TiO2 colloidal on FTO were
prepared by sliding a glass rod over the conductive side of the FTO. Sintering was carried out
at 450 oC for 30 min. Before immersing in the dye solution, these films were soaked in the 0.2
M aqueous TiCl4 solution overnight in a closed chamber, which can significantly increase the
short-circuit photocurrent. The thickness of the TiO2 film was measured to be about 13 ꢀm.
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After being washed with deionized water and fully rinsed with ethanol, the films were heated
o
o
again at 450 C, followed by cooling to 80 C and dipping into a 3 ×10-4 M solution of dyes
for 24 h at room temperature. For the coadsorption, chenodeoxycholic acid was added. The
adsorbed TiO2 electrode and Pt counter electrode were assembled into a sealed sandwich-type
cell by heating with a hot-melt ionomer film (Surlyn 1702, DuPont). The redox electrolyte
was placed in a drilled hole in the counter electrode by capillary force and was driven into the
cell by means of vacuum backfilling. Finally, the hole was sealed using a UV-melt gum and a
cover glass. The electrolyte was composed of 0.1 M lithium iodide, 0.6 M
butylmethylimidazolium iodide (BMII), 0.05 M I2 and 0.5 M 4-tert-butylpyridine (4-TBP) in
the mixed solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v).
2.4 Photoelectrochemical Measurements
Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a 300 W
xenon lamp (model no. 91160, Oriel). The power of the simulated light was calibrated to be
100 MW/cm2 using a Newport Oriel PV reference cell system (model 91150 V). J-V curves
were obtained by applying an external bias to the cell and measuring the generated
photocurrent with a Keithley model 2400 digital source meter. The voltage step and delay
time of photocurrent were 10 mV and 40 ms, respectively. The cell active area was 0.28 cm2.
Incident photon-current conversion efficiency (IPCE) was recorded on a DC Power Meter
(Model 2931-C, Newport Co.) under irradiation of a 300Wxenon lamp light source with a
motorizedmonochromator (Oriel). The xenon lamp was powered by an Arc Lamp Power
Supply (Model 69920, Newport Co.). The DSC was illuminated by light with energy of a 42
mW cm-2 from a 500 W Xe lamp. A computer-controlled Keithley 2400 source meter was
employed to collect the J-V curves. The photoelectric conversion efficiency was calculated
according to eq 1:
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V JscFF
oc
η
(%) =
×100
Equation 1
Pin
In the formulas, η is the global efficiency, Voc, Jsc, and FF are open circuit voltage, short
circuit current density, and fill factor, respectively. Pin is the power of the incident light.
3. Results and discussion
3.1 Synthesis and Characterization of the dyes
The whole synthetic route was shown in Scheme 2. Compound 2a and 2b were synthesized
from 2,5-dimethoxy tetrahydrofuran and the corresponding arylamines in high yields, and the
reaction was conducted in mild conditions. It should be pointed out that the reaction could be
conveniently handled, and various arylamine with electron-withdrawing properties could be
incorporated into the conjugated system to tune the absorption spectra of the corresponding
dyes easily. Under the normal Vilsmeier reaction conditions, N-arylpyrrole-2-carbaldehydes
(3a and 3b) were obtained with good yields. Then the followed Wittig reaction between 3a or
3b and 4 gave compounds 5a and 5b, which underwent another Vilsmeier reaction to yield the
aldehydes 6a and 6b. Finally, the dyes were produced from the aldehydes 6a and 6b through
the Knoevenagel condensation reaction with cyanoacetic acid in the presence of piperidine.
All the compounds were well characterized, with the data shown in the experimental section.
3.2 Absorption spectra
The dyes were soluble in common organic solvents, such as CH2Cl2, THF, DMF, and
DMSO. As shown in their UV-vis spectra tested in CH2Cl2 (Fig. 1, Table 1), a distinct
absorption peak around 490 nm appeared, corresponding to an intramolecular charge transfer
(ICT) between the TPA donor part and the acceptor end group. Notably, the molar extinction
coefficients (44733 and 26300 M-1 cm-1) of LI-6 and LI-7 were much higher than that of
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Ru-organic complex N719 (14100 M-1 cm-1 in ethanol), indicating their good abilities for light
harvesting. The absorption spectra of the dyes became broadened after adsorption on the TiO2
surface (Fig. 2), due to the increased delocalization of the π* orbital of the conjugated
framework caused by the interaction between the carboxylate group and the Ti4+ ions. As a
result, the energy of the π* level decreased, which should favor the light harvesting of the
solar cells and thus increase the photocurrent response region, leading to the high Jsc.
3.3 Electrochemical properties
Electrochemical properties were determined by cyclic voltammetry in dichloromethane
with TBAPF6 (tetrabutylammonium hexafluorophosphate) as supporting electrolyte (Fig. 3),
and the corresponding data was collected in Table 1. The oxidation potential vs NHE (Eox)
corresponded to the highest occupied molecular orbital (HOMO), while the reduction
potential vs NHE (Ered), which corresponded to the lowest unoccupied molecular orbital
(LUMO), could be calculated from Eox-E0-0. E0-0 was derived from the observed optical edge.
Notably, the voltammograms of the two dyes exhibited quasi-reversible behavior, and two
oxidation waves were observed. The first oxidation at lower oxidation potentials is attributed
to the triphenylamine, whereas the second oxidation potentials are from pyrrole. Since two
different with-drawing groups were linked to the pyrrole ring, the second oxidation potentials
of the two dyes varied much. Compared to LI-7, the higher second oxidation potentials of
LI-6 indicated that pentafluorophenyl unit presented stronger electron-withdrawing property
than that of cyanophenyl unit in this system. And the first oxidation of the two dyes just
changed a little, which hint that the electronic effect of the two with-drawing groups had
small influence on the donor group. The reduction potential of LI-6 (-1.21 V vs. NHE) was
higher than that of LI-7 (-1.32 V vs. NHE), with the same trend of the second oxidation
potentials, which further confirm the stronger with-drawing ability of pentafluorophenyl
group. Thus, with various electron-withdrawing groups being linked to the pyrrole ring by
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convenient synthesis, the molecular orbital energy level of the sensitizers could be finely
tuned.
The molecular orbital energy diagram was shown in Fig. 4. First, the HOMO levels were
-
more positive than the redox potential of I-/I3 (0.4 V vs. NHE). Thus, the oxidized dyes could
be regenerated from the reduced species in the electrolyte to give an efficient charge
separation; second, the LUMO levels of the dyes were much more negative than the
conduction band (CB) of the TiO2 level (-0.5 V vs. NHE). Thus, the electrons could be
efficiently injected into the CB of TiO2 from the excited dyes, assuming that energy gap of 0.2
eV was necessary for the efficient electron injection. The above experimental results indicated
that the dyes LI-6 and LI-7 could be used as qualified sensitizers for efficient DSCs.
3.4 Theoretical approach.
To gain further insight into the correlation between structure and the physical properties
as well as the device performance, quantum chemistry computation was conducted. The
geometries of the dyes have been optimized using time-depending DFT (TDDFT)
calculations with Gaussian 03 program [22]. The structures of the dyes were analyzed using a
B3LYP/6-31G* hybrid functional for full geometrical optimization. In the ground state, the
D-π-A system of the dyes possessed a nearly coplanar conformation to achieve a maximal
extent of π-delocalization. The electron-withdrawing units linked to the pyrrole ring through
Carbon-Nitrogen single bond were non-coplanar with the D-π-A system, which could block
the redox electrolyte close to the TiO2 surface (recombination of electrons in the
semiconductor with electrolyte species) and alleviate the aggregation of the dyes. The charge
distributions in the frontier molecular orbitals were depicted in Fig. 5. As shown in the graphs,
the HOMO was delocalized over the π system with the highest electron density centered at the
central nitrogen atom, and the LUMO was located in anchoring groups through the π-bridge.
The electron distributions of both donor and acceptor were heavily coupled with the orbital of
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the π-bridge, which was favorable for charge transfer.
3.5 Photovoltaic performance of DSCs
The photovoltaic characteristics of the N-functionalized pyrrole-based dyes were
measured with a sandwich DSC cell comprising 0.1 M lithium iodide, 0.6 M
butylmethylimidazolium iodide (BMII), 0.05 M I2 and 0.5 M 4-tert-butylpyridine (4-TBP) in
the mixed solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v) as the redox electrolyte.
Details of the device preparation and characterization were described in the experimental
section. The incident photon-collected electron conversion efficiency (IPCE) of the devices
based on LI-6 and LI-7 were presented in Fig. 6. The IPCE spectra reflect the differences
observed in the absorption spectra for the dyes, and their short circuit photocurrent generation
capability. Regarding the wavelength region where the dyes are active, a relative correlation
between photocurrent action spectra and the wavelength absorption maximum at TiO2
adsorbed state (Fig. 2) is maintained. The IPCE values of DSCs based on LI-6 and LI-7 have
comparable values in the region from 400 to 700 nm, and both of their highest values exceed
50%, which infer that the two dyes could efficiently convert the light to photocurrent in the
region from 400 to 700 nm.
The photocurrent-voltage (J-V) plots of DSCs fabricated with the dyes were shown in
Fig. 7. The detailed parameters, i.e. short circuit current (Jsc), open-circuit photovoltage (Voc),
fill factor (FF) and light to electricity conversion efficiency (η) under AM 1.5 solar light (100
mW/cm2) were summarized in Table 2. To our knowledge, the dyes in different solvents
exhibit diversified interaction between the dyes and solvents, which could cause changes of
the physical and chemical properties between the dyes and semiconductor surface. Therefore,
the suitable solvent for semiconductor sensitization is important to obtain prominent
solar-to-electricity conversion efficiency. Considering their solubility, we chose CH2Cl2,
mixed solution with DMF and ethanol as the solvents. From the data, one can find the big
14

ACCEPTED MANUSCRIPT
differences in the performance of DSCs. Taking the short circuit photocurrent density for
example, when the mixed solvents with DMF and ethanol as the bath solution, the J values
sc
of the two dyes were 5.13 and 8.64 mA cm-2(Table 2), respectively. However, when CH2Cl2
solution was introduced to sensitize the semiconductor electrode, their J values increased to
sc
12.9 and 13.7 mA cm-2, indicating that the existence of the CH2Cl2 solution could be helpful
to the arrangement and orientation of the two dyes on the TiO2 surface. Under this condition,
the DSCs based on LI-6 obtained the bigger η value, 6.23%, about 80% of the standard cell
from N719 (7.83%) tested under the similar conditions.
CDCA (chenodeoxycholic acid) is a transparent organic compound, which binds
strongly to the surface of nanostructured TiO2, being able to displace dye molecules from the
semiconductor surface and therefore hindering the formation of dye aggregates. In this study,
when CDCA was added to improve the efficiency, the conversion efficiencies of the DSCs
decreased dramatically, in comparison with those without CDCA (Fig. 7). For example, after
the coadsorption of 10 mM CDCA, the η value of the DSC based on dye LI-7 decreased from
5.71% to 2.82%. A possible explanation was that the coadsorption of CDCA could lead to the
decrease of the dye loading on the TiO2 surface, resulting in a loss of active light-harvesting.
This fact suggested that the electron-withdrawing units linked to the pyrrole ring through
Carbon-Nitrogen single bond could act well as the isolation groups to suppress the possible
aggregation of the dyes on the TiO2 surface, and there is no need for the addition of CDCA to
improve the performance of devices.
The electrochemical impedance spectra (EIS) was used to analyze the resistance of the
DSCs, which were measured in dark under a forward bias of -0.70 V with a frequency range
of 0.1 Hz to 100 kHz. Fig. 8a shows the Nyquist plots for the DSCs made with TiO2
electrodes dipped with the two dyes. The interfacial charge recombination reaction of
15

ACCEPTED MANUSCRIPT
-
electrons with the I3 ions can be described by a charge transfer resistance (Rct), which
corresponds to the charge transfer processes at the TiO2/dye/electrolyte interface (a larger
semicircle occurs in the lower frequency range). As shown in Fig. 8, the radius of the larger
semicircle in LI-6 is bigger than that of LI-7, indicating that the DSC based on LI-6 exhibited
the lower charge recombination rate and therefore a smaller dark current. This significant
enhancement of Rct suggests that the introduction of pentafluorobenzene to the side chain of
the dye molecules is more effective on the modification of TiO2/dye/electrolyte interface.
In the Bode phase plot (Fig. 8b), there are two peak features for the frequency
investigated. The one at higher frequency corresponds to charge transfer at the Pt/electrolyte
interface and meanwhile the other one at lower frequency corresponds to the charge transfer
at the TiO2/dye/electrolyte interface, which is related to the charge recombination rate and the
reciprocal is associated with the electron lifetime. The Bode phase plots likewise support the
differences in the Rct for TiO2 films derived with the two dyes. The low frequency peak of
LI-6 shows a lower frequency than that of LI-7, indicating that the DSC based on LI-6
possesses a longer recombination lifetime. The photoelectrons that possess longer lifetimes
have the more chances of entering the external circuit and contributing to the photoelectric
conversion efficiency. Therefore, the larger charge transfer resistance and enhanced electron
lifetime may be the intrinsic reason for the higher Voc value (0.73 V) of the DSC based on
LI-6 compared to the Voc value (0.69 V) for the LI-7-based DSC.
4. Conclusions
In summary, we have designed and synthesized two N-functionalized pyrrole-based organic
16

ACCEPTED MANUSCRIPT
dyes featuring different electron-withdrawing units bonded to the nitrogen atom of the pyrrole
ring. The electron-withdrawing units could enhance the photo-stability of the pyrrole-based
dyes and the light-harvesting abilities, alleviate the aggregation on the surface of TiO2. Also,
it was expected to form a barrier layer to block redox electrolyte close to the TiO2 surface.
The DSC based on the dye LI-6 demonstrated the overall conversion efficiency of 6.23%,
which was about 80% of the standard cell from N719 (7.83%).
Acknowledgements
We are grateful to the National Science Foundation of China (no. 21002075), and the National
Fundamental Key Research Program (no. 2011CB932702) for financial support.
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Table captions
Table 1: Absorbance and electrochemical properties of dyes LI-6 and LI-7.
Table 2: Performance parameters of DSCs based on the sensitizers LI-6 and LI-7.
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Table 1 Absorbance and electrochemical properties of dyes LI-6 and LI-7.
a
a
b
c
λmax
λmax
E0-0
Eox (V) d
vs NHE
0.95
Ered (V) e
vs NHE
-1.21
ε
at λmax
Dye
(nm)
493
490
(M-1 cm-1)
(nm)
448
458
(eV)
2.16
2.21
LI-6
LI-7
44733
26300
0.89
-1.32
[a] Absorption spectra of dyes measured in CH2Cl2 with the concentration of 3×10-5 mol/L.
[b]
[c]
Absorption spectra of dyes adsorbed on the surface of TiO2.
The bandgap, E0-0 was derived
from the observed optical edge. The oxidation potential (Eox) referenced to calibrated Ag/Ag+
[d]
[e]
was converted to the NHE.
E
red
was calculated from Eox - E0-0.
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Table 2. Performance parameters of DSCs based on the sensitizers LI-6 and LI-7.
CDCA [mM] Jsca [mA cm-2]
Vocb [v]
0.73
0.66
0.71
0.69
0.64
0.66
0.76
FF c
0.67
0.67
0.60
0.61
0.65
0.63
0.61
η d [%]
6.23
2.15
5.31
5.71
3.57
2.82
7.83
Sensitizer
0e
12.9
5.13
12.6
13.7
8.64
6.82
16.96
LI-6
0f
10g
0e
LI-7
0f
10g
0
N719
[a]
Jsc
=
short circuit photocurrent density. [b] Voc
=
open circuit voltage. =fill factor.
[c] FF
[d]
[e]
η
=
overall light to electricity conversion efficiency
.
The TiO2 electrode (0.28 cm2)
[f]
was stained by immersing it into the dye solution (0.3 mM) in CH2Cl2 for 24 h.
The TiO2
electrode (0.28 cm2) was stained by immersing it into the dye solution (0.3 mM) in the mixture of
DMF and ethanol (V/V=3/7) for 24 h. [g] The TiO2 electrode (0.28 cm2) was stained by immersing
it into the CDCA solution (10 mM) for 6 h, then to the dye solution (0.3 mM) in CH2Cl2 for 24 h.
30