New Pyrrole-Based Organic Dyes for Dye-Sensitized Solar Cells: Convenient Syntheses and High Efficiency

Article abstract of DOI:10.1002/chem.200901150

Harvesting the sunshine : Two new pyrrole-containing organic dyes were rationally designed and easily prepared. Thanks to the special electronic properties of pyrrole moieties and the introduction of the non-coplanar aromatic rings through a single bond (

Full text of DOI:10.1002/chem.200901150

DOI: 10.1002/chem.200901150
New Pyrrole-Based Organic Dyes for Dye-Sensitized Solar Cells: Convenient
Syntheses and High Efficiency
Qianqian Li, Lanlan Lu, Cheng Zhong, Jing Huang, Qing Huang, Jie Shi, Xianbo Jin,
Tianyou Peng,* Jingui Qin,* and Zhen Li*[a]
As petroleum and coal are depleted, renewable energy
sources have attracted increasing attention in recent years,
and solar energy is considered as a good alternate for fossil
fuel energy. Due to the advantages of low cost, easy produc-
tion, flexibility, and transparency relative to conventional
crystalline silicon solar cells, dye-sensitized solar cells
(DSSCs) have emerged as an important class of photovoltaic
devices, since the breakthrough was made by Grꢀtzel and
co-workers.[1] Thanks to the enthusiastic efforts of scientists,
significant progress was made in terms of the performance
of DSSC devices: power conversion efficiencies above 11%
in standard global air mass (AM) 1.5 have been achieved by
several RuII–polypyridyl complexes;[2] while 6–9% was
shown by metal-free organic dyes.[3] Although the efficiency
of the latter was a little lower, they possessed some advan-
tages over the former: ease of synthesis, high molar extinc-
tion coefficient, tunable absorption spectral response from
the visible to the near infrared (NIR) region, as well as envi-
ronmentally friendly and inexpensive production techniques.
To further improve the efficiencies of pure organic-dye-
based DSSCs, the structure and physical properties of the
sensitizer are clearly important, especially the conjugation
across the donor and anchoring groups, and good electronic
coupling between the lowest unoccupied orbital (LUMO) of
the dye and the conduction band of TiO2, which is very im-
portant for high electron-transfer rates.[4] Thus, in order to
exploit such sensitizers, it is necessary to lower the energy of
the charge-transfer transition. In this respect, some electron-
rich heteroaromtic rings have been used as bridges between
electron donor and acceptor: some thiophene and furan-
containing dyes exhibited good device efficiencies,[5] due to
their smaller resonance energy in comparison with that of
benzene (thiophene, 29; furan, 16; benzene, 36 kcalmolꢀ1).
However, the analogue of furan and thiophene moieties,
that is pyrrole group, has seldom been used as a conjugated
bridge in organic dyes,[6] possibly due to its unstability, al-
though it possesses
a
smaller resonance energy
(21 kcalmolꢀ1), and could be easily funtionalized, not only
on the ortho-position to the nitrogen atom, but also through
[7]
ꢀ
the substituted reactions on the N H bond. Fortunately,
our recent research and some reports of other groups dem-
onstrated that its stability could be enhanced dramatically
by inserting it into a p system and substituting the H atom
on the N atom. Some exciting results have been obtained
from pyrrole-containing materials in several different re-
search fields (including photochemistry,[8] second-order non-
linear optical chromophores,[9] and two-photon absorption
materials[10]).
On the other hand, the aggregation of the dyes on the
semiconductor surface would sharply decrease the power
conversion efficiency of DSSC, as a result of the influenced
light absorption caused by the filtering effect.[11] Thus, to
achieve optimal performance, possible aggregation should
be avoided through appropriate structural modification. Ac-
cording to our experience in the research of aggregation-in-
duced emission (AIE) materials, the introduction of non-co-
planar aromatic rings through single bonds helps to avoid of
the aggregation.[12]
Therefore, taking all the above-mentioned points into
consideration, we designed two new pyrrole-containing dyes,
LI-1 and LI-2, in which the triphenylamine (TPA) group
was used as the electron donor and a cyanoacrylic acid
moiety as the anchoring group. Another TPA group in LI-1
was linked to the pyrrole ring through the carbon–nitrogen
single bond to suppress the aggregation of the dye on the
surface of TiO2. For comparison, a carbazolyl (Cz) group
with more planar structure was used instead of TPA in LI-2.
These two dyes were conveniently obtained, and demon-
[a] Q. Li, L. Lu, C. Zhong, J. Huang, Q. Huang, J. Shi, Prof. X. Jin,
Prof. T. Peng, Prof. J. Qin, Prof. Z. Li
Department of Chemistry,
Wuhan University, Wuhan 430072 (P.R. China)
Fax : (+86)27-68756757
E-mail: lizhen@whu.edu.cn
typeng@whu.edu.cn
jgqin@whu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901150.
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Chem. Eur. J. 2009, 15, 9664 – 9668

COMMUNICATION
Table 1. Some characterization data of LI-1 and LI-2.
[c]
[d]
[e]
[f]
[g]
lmax[a] [nm]
lmax[b] [nm]
E0-0 [eV]
Ep [V]
Eox [V] vs. NHE
Ered [V] vs. NHE
Egap [V]
LI-1
LI-2
448
415
449
431
2.34
2.40
0.86, 1.01
0.60, 0.82
1.06
0.80
ꢀ1.28
ꢀ1.60
0.78
1.10
[a] Absorption spectra of dyes measured in ethanol with the concentration of 3ꢂ10ꢀ5 m. [b] Absorption spectra of dyes adsorbed on the surface of TiO2.
[c] The bandgap, E0-0 was derived from the observed optical edge. [d] Ep is the peak of DPV (the DPV of the dyes were mearsured in CH2Cl2 with 0.1m
(n-C4H9)4NPF6 as electrolyte (scanning rate, 100 mVsꢀ1 working electrode and counter electrode, Pt wires; reference electrode, Ag/AgCl)). [e] The oxi-
dation potential (Eox) referenced to calibrated Ag/AgCl was converted to the NHE reference scale: Eox =Ep +0.197 V. [f] Ered was calculated from
EoxꢀE0-0. [g] Egap is the energy gap between the Ered of dye and the CB level of TiO2 (ꢀ0.5 V vs. NHE).
strated high conversion efficiencies (up to 91% of the stan-
dard cell from N719). Herein, we report their syntheses,
structural characterization, electrochemical properties, theo-
retical calculations, and photovoltaic performance.
The whole synthetic route is shown in Scheme S1 along
with the detailed preparation procedure is described in the
Supporting Information. 4-Bromophenylpyrrole (1), the im-
portant spacer between the electron donor and the electron
acceptor, was synthesized from 2,5-dimethoxy tetrahydrofur-
an and 4-bromoaniline with high yield,[13] then 4-bromophe-
nylpyrrole-2-carbaldehyde (2) was obtained through normal
Vilsmeier reaction. Compound 2 was treated with 3 under
Wittig–Hornor reaction conditions to yield compound 4.
Another Vilsmeier reaction led to the introduction of an al-
dehyde group was to the pyrrole ring, and was followed by a
Suzuki reaction with triphenylamine and carbazole-based
boronic acid to give compounds 6 and 7, respectively. Final-
ly, the two dyes were obtained through a Knoevenagel con-
densation reaction of 6 or 7 with cynaoacetic acid in the
presence of piperidine.
The reaction products were characterized by spectroscopic
analyses, and all give satisfactory data (some data listed in
Table 1) corresponding to their expected molecular struc-
tures (see Supporting Information for details). As shown in
their IR spectra (Figure S1), the strong peak of the cyanide
groups in the two dyes were observed at about 2217 cmꢀ1,
indicating the success of Knoevenagel condensation reaction
between 6 or 7 and cynaoacetic acid. The absence of the
peak derived from the aldehyde groups at about d=
10.0 ppm in their 1H NMR spectra further confirmed the
complete conversion of the aldehyde groups to cyanoacrylic
acid moieties as the anchoring groups. LI-1 and LI-2 (see
below) are thermally stable, and their initial decomposition
temperatures were found to be 248 and 2838C, respectively,
by thermal gravimetric analysis (TGA) under argon.
The two dyes are soluble in common organic solvents,
such as dichloromethane, THF, DMF, and DMSO. As shown
in their UV/Vis spectra tested in ethanol (Figure S2 in the
Supporting Information, Table 1), LI-1 and LI-2 give two
distinct absorption bands: a solvent-independent band at
lmax <400 nm derived from the localized p–p* transition and
another solvent-dependent band at lmax in the range of 400–
600 nm, which could be ascribed to an efficient charge-sepa-
rated state produced by an intramolecular charge transfer
(ICT) between the donor and the cyanoacrylic acid anchor-
ing moiety. In addition, the charge-transfer bands exhibit a
negative solvatochromism, that is, blue shift in more polar
solvents (Table S1and Figures S3, S4, and S5 in the Support-
ing Information). This could be attributed to the deprotona-
tion of the carboxylic acid, which decreases the pulling abili-
ty of the electron acceptor. Similar phenomena have been
reported in the case of black dye and other metal-free or-
ganic dyes.[2,14] LI-1 shows much higher extinction coeffi-
cients (e=40500mꢀ1 cmꢀ1) than that of LI-2 (e=
27900mꢀ1 cmꢀ1), both of which is higher than that of Ru–or-
ganic complex N719 (e=14100mꢀ1 cmꢀ1 in ethanol), indicat-
ing that they are beneficial to light harvesting. After being
adsorbed on the surface of TiO2, the maximum absorption
wavelengths do not show a big difference as compared to
those in ethanol (Figure S6 in the Supporting Information).
However, the absorption threshold is red-shifted by about
60–80 nm, owing to the increased delocalization of the p*
orbital of the conjugated framework caused by the interac-
tion between the carboxylate group and the Ti4+ ions, which
directly decreases the energy of the p* level. The amounts
of the adsorbed dye on the TiO2 films were estimated by de-
sorbing the dye with basic solution; the surface concentra-
tions were determined to be 1.08ꢂ10ꢀ7 and 1.40ꢂ10ꢀ7 for
LI-1 and LI-2, respectively.
To investigate the possibility of electron transfer from the
excited dye molecule to the conductive band of TiO2 and
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Z. Li, T. Peng, J. Qin et al.
also dye regeneration, the electrochemical characteristics of
the dyes were investigated by using differential pulse vol-
tammetric (DPV) methods (Figure S7 in the Supporting In-
formation and Table 1). The oxidation potential of the dyes
was determined from the peak potentials (Ep) by DPV; the
oxidation potential versus NHE (Eox) corresponds to the
highest occupied molecular orbital (HOMO), while the re-
duction potential versus NHE (Ered), which corresponds to
the lowest unoccupied molecular obtital (LUMO), can be
calculated from EoxꢀE0-0. As shown in Figure 1, both the
Figure 2. Frontier orbitals of the dyes a) LI-1 and b) LI-2 optimized at
the B3LYP/6-31+G (D) level.
AHCTUNGTREGaNNNU mine group exhibits a more bulky three-dimensional struc-
ture. The theoretical results give two or three transitions
with large oscillator strengths (f) for the dyes that are con-
sistent with the absorption spectra. A HOMO!LUMO
transition is centered in the visible light field with large os-
cillator strengths f=1.0–1.3 and a combined transition for a
band in the UV region ascribed to HOMOꢀ2!LUMO,
HOMOꢀ1!LUMO, and HUMO!LUMO+2 transitions
with a lower f value (Table S2 in the Supporting Informa-
tion). It should also be noted that the HOMO!LUMO ex-
citation induced by light irradiation could move the electron
distribution from the dyes to the anchoring moieties. As-
suming similar molecular orbital geometry when anchored
to TiO2, the position of the LUMO close to the anchoring
groups would surely enhance the orbital overlap with the ti-
tanium 3d orbitals and thus benefit the electron injection
from dyes to TiO2.[15]
Figure 1. Schematic representation of the band positions in DSSCs based
on LI-1and LI-2. The energy scale is indicated in electron volts using the
normal hydrogen electrode (NHE).
LUMO and HOMO levels of these dyes agree with the re-
quirement for an efficient sensitizer: First, their LUMO
levels are below the conductive band (CB) level (ꢀ0.5 V vs.
NHE) of TiO2, and the Egap values range from 0.78–1.10 V,
which are all larger than the gap from N719 (Egap =0.35 V).
Assuming that energy gap of 0.2 eV is necessary for the effi-
cient electron injection, these driving forces are large
enough for the injection; secondly, the HOMO levels of the
dyes are more positive than the iodine redox potential
(0.4 V). Thus, these oxidized dyes can be regenerated from
the reduced species in the electrolyte to give an efficient
charge separation and can be used as sensitizers that should
function well, since the electron transfer in DSSCs is feasi-
ble.
Figure 3 shows the current-density/voltage characteristics
of the DSSCs obtained with the two sensitizers based on
pyrrole, and the detailed photovoltaic performance parame-
To get an insight into the molecular structure and electron
distribution of the organic dyes, the geometries of LI-1 and
LI-2 were optimized by using time-depending DFT
(TDDFT) calculations with the Gaussian 03 program.[14e] In
particular, we used the B3LYP exchange-correlation func-
tional and a 6-31 g* basis set; solvation effects were included
by means of the polarizable continuum model. To model the
electronic state of the dyes adsorbed on the TiO2 surface,
we employed the dyeꢃs potassium salt, since the dye should
be bonded to the TiO2 surface in its carboxylate form. The
electron distribution of the HOMOs and LUMOs are shown
in Figure 2. It reveals that the dyes are completely planar
except for the triphenylamine and carbazole moieties, which
are linked with pyrrole through a single carbon–nitrogen
bond to avoid sterical clashes. In addition the triphenyl-
Figure 3. Current-density/voltage characteristics obtained with a nano-
crystalline TiO2 film supported on FTO conducting glass and derivatized
with monolayer of the sensitizers; c: LI-1; a: LI-2.
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Dye-Sensitized Solar Cells
COMMUNICATION
ters are list in Table 2, in comparison with those of the stan-
dard ruthenium dye N719 fabricated and measured under
the same conditions. The DSSC based on LI-1 shows better
Table 2. Performance parameters of DSSCs vonstructed by using LI-1
and LI-2.
Jsc[a] [mAcmꢀ2
]
Voc[b] [V]
FF[c]
h[d] [%]
tn[e] [ms]
LI-1
LI-2
N719
16.34
10.50
16.65
0.72
0.71
0.75
0.61
0.62
0.63
7.21
4.62
7.88
4.01
2.79
7.62
[a] Jsc =short circuit photocurrent density. [b] Voc =open circuit voltage.
[c] FF=fill factor. [d] h=overall light to electricity conversion efficiency.
[e] The lifetime (tn) of injected electrons in the films can be drawn by the
positions of the low-frequency peak in Figure 4 (bottom) through the ex-
pression tn =1/ACHTUNGTRENNUNG(2pf), in which f is the frequency of the superimposed ac
voltage.[16]
comprehensive properties with an open circuit voltage of
0.72 V, short circuit photocurrent density of
a
16.34 mAcmꢀ2, and a fill factor of 0.61, corresponding to an
overall light to electricity conversion efficiency of 7.21%,
which is 91% of the standard cell from N719 (7.88%). The
higher efficiency benefits from the highest short-circuit pho-
tocurrent density, which is about 6.0 higher than that of LI-
2. Also, the three-dimensional structure of the TPA moiety
linked to the nitrogen atom of pyrrole ring, in contrast to
the more planar Cz one, will surely contribute to the sup-
pression of the aggregation of the LI-1 on the surface of
TiO2 to a larger degree, resulting in the better power con-
version efficiency directly.
Figure 4. Electrochemical impedance spectroscopy (EIS) for DSSCs
based on the sensitizers. Top: Nyquist plots. Bottom: Bode phase plots.
*
*
: LI-1, : LI-2.
To increase the efficiency of DSSCs, an increase of the
electron lifetime is essential. The photoelectrons that pos-
sess longer lifetimes have the more chance of entering the
external circuit and contributing to the photoelectric conver-
sion efficiency.[17] As can be seen from Table 2, the injected
electrons have the longer lifetime (4.01 ms) from the device
based on the sensitizer LI-1, which is found to be in parallel
with the trend of the cell efficiencies.
In conclusion, two novel metal-free organic dyes (namely,
LI-1 and LI-2) with simple structures, were conveniently
prepared and characterized. Thanks to the special electronic
properties of pyrrole moieties and the introduction of the
non-coplanar aromatic rings through single bond, a solar-
cell device based on the sensitizer LI-1 yields an overall con-
version efficiency of 7.21% (91% of the standard cell from
N719, 7.88%). The syntheses of more pyrrole-containing or-
ganic dyes are still in progress in our laboratory.
This can be explained from the electrochemical impe-
dance spectroscopy (EIS) of the DSSCs. Generally, three
characteristic frequency peaks can be obtained from EIS
spectra (Bode phase plot) when DSSCs are controlled under
open-circuit conditions under light illumination. The low-
frequency peak (in the mHz range) is attributed mainly to
ꢀ
the Nernst diffusion of I3 within the electrolyte. The
middle-frequency peak (in the 10–100 Hz region) is related
to the transport process of the injected electrons within
TiO2 porous films and the charge-transfer process of the in-
jected electrons at the interfaces between TiO2 and the elec-
trolyte/dye coating. The high-frequency peak (in the kHz
range) is ascribed to the charge-transfer process at the inter-
faces between the redox couple and the platinized counter
electrode. Figure 4 (bottom) shows the Bode phase plots of
EIS spectra for the DSSCs made with TiO2 electrodes
dipped with different sensitizers. In our system, the low- and
high-frequency peaks are similar, as the DSSCs are under
the same conditions except for the sensitizer is different.
However, distinct changes appeared in the middle frequency
depending on the various sensitizers. The interfacial charge-
transfer resistance (RCT), calculated from the middle-fre-
quency of EIS based on various sensitizers, is 10.95 W and
15.54 W. As can be seen, the sensitizer LI-1 shows the lower
Experimental Section
The experimental details for the synthesis and characterization of the
dyes LI-1 and LI-2 are given in the Supporting Information.
Acknowledgements
We are grateful to the National Science Foundation of China (no.
20674059, 20871096 and 20772094), the Program for NCET, the National
Fundamental Key Research Program and Hubei Province for financial
support.
ꢀ
RCT value, which indicated that the catalytic I3 reduction
and electron transfer in DSSCs based on sensitizer LI-1 are
the most efficient.
Chem. Eur. J. 2009, 15, 9664 – 9668
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Z. Li, T. Peng, J. Qin et al.
Barlow, E. W. Van Stryland, D. J. Hagan, J. L. Brꢅdas, S. R. Marder,
J. Am. Chem. Soc. 2005, 127, 7282–7283.
Keywords: density functional calculations · dye/pigments ·
electrochemistry · pyrroles · solar cells
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Received: April 30, 2009
Published online: August 15, 2009
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