New 2, 6-modified bodipy sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201100779

Three novel 2,6-modified Bodipy sensitizers were synthesized and evaluated for their use in dye-sensitized solar cells (DSSCs). Among them, dye B3, which carries a n-pentyl group at position 8, exhibits the best solar energy conversion efficiency (1.83 %). The results of this study provide a new strategy for the design of Bodipy derivatives as sensitizers for DSSCs.

Full text of DOI:10.1002/asia.201100779

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DOI: 10.1002/asia.201100779
New 2, 6-Modified Bodipy Sensitizers for Dye-Sensitized Solar Cells
Jian-Bo Wang,[a] Xia-Qin Fang,[b] Xu Pan,[b] Song-Yuan Dai,[b] and Qin-Hua Song*[a]
In recent years, dye-sensitized solar cells (DSSCs) have
received extensive attention in the field of photovoltaic ap-
plications because of their potential advantages such as low
cost and high solar light-to-electicity conversion efficiency
(h), as reported by Grꢀtzel.[1] Several factors, including sen-
sitizer properties, anode/cathode materials, and the electro-
lyte used determine the photovoltaic performance of
DSSCs.[2,3] Among these factors, the sensitizer plays a crucial
role, especially in the power conversion and cell stability.
Up to now, by using several RuII polypyridyl complexes, a
high conversion efficiency exceeding 11% has been ach-
ieved.[4] Currently, metal-free organic dyes, featuring higher
molar extinction coefficients, easier structural modification,
lower cost, and higher environmental friendliness in compar-
ison to ruthenium complexes, are under investigation in nu-
merous research laboratories. Donor–acceptor p-conjugated
(D-p-A) dyes possessing both electron-donating (D) and
electron-accepting (A) groups linked by p-conjugated
bridges are expected to be one of the most promising classes
of organic dyes due to their effective photo-induced intra-
molecular charge transfer.[4] In this context, many D-p-A
conjugated organic dyes have been designed and modified
by changing different moieties within D-p-A systems; modi-
fications mainly focused on the electron-donor unit by utiliz-
ing coumarin,[5] indoline,[6] N,N-dialkylaniline,[7] triphenyla-
mine,[8] phenothiazine,[9] tetrahydroquinoline,[10] and carba-
zole.[11] So far, DSSCs based on D-p-A conjugates have
reached an efficiency as high as 9–10%.
kuzumi et al.[13a] Thummel and co-workers[13d] modified a
Bodipy sensitizer by substituting fluoride with an ethynyl
group and by varying the styryl moiety in Bodipy units;
however, the efficiency of cells in which these dyes were
used has not been reported. Recently, Akkaya et al.[13b,f,h]
extended this line of research by synthesizing a series of
new Bodipy sensitizers in which different donors were intro-
duced at positions 3 and 5 of the Bodipy core, and acceptors
at position 8. This group then systematically studied the re-
lationship between the cell efficiency and the dye structure;
the highest efficiency obtained was h=2.46%.
In this work, we have modified the Bodipy core in an un-
precedented manner by covalently linking a triphenylamine
unit as an electron donor and a cyanoacetic acid moiety as
an electron receptor at positons 6 and 2, respectively
(Scheme 1). As a result, both moieties are well conjugated
with the Bodipy bridge, thus leading to an extended conju-
Scheme 1. Chemical structure of dyes B1–B3.
The use of boron dipyrromethene (Bodipy) derivatives[12]
as novel and recently widely exploited conjugate structure
with a strong absorption coefficient in the visible and near-
infrared ranges has been extended toward applications in
DSSCs. Currently, only a few reports on Bodipy derivatives
as sensitizers exist[13], with the first report being that of Fu-
gation system that should favor charge transfer from the
donor to the receptor. Based on the fact that substituents at
position 8 of Bodipy play an important role in the electron
density distribution,[13b] we introduced different substituting
groups at position 8, including methyl, phenyl, and n-pentyl,
to investigate the relation of the electron density distribu-
tion of the dye with cell performance. Accordingly, three
novel Bodipy-based dyes, B1–B3, have been synthesized and
applied as sensitizers in DSSCs.
The synthetic routes for B1–B3 are depicted in Scheme 2
and the detailed experimental procedure and characteriza-
tion data are listed in the Supporting Information. The
Bodipy derivatives 1a,[14a] 1b,[14b] 1c,[14c] and 4-(diphenylami-
no)phenylboronic acid 4[15a] were readily obtained using pre-
viously reported methods. The formylation of 1a–c was also
carried out according to a literature procedure.[15b] Subse-
quent monoiodation of the resulting compounds 2a–c using
ICl as an iodination reagent at room temperature resulted
[a] J.-B. Wang, Prof. Q.-H. Song
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (P.R. China)
Fax : (+86)551-3601592
E-mail: qhsong@ustc.edu.cn
[b] X.-Q. Fang, Dr. X. Pan, Prof. S.-Y. Dai
Key Laboratory of Novel Thin Film Solar Cells
Institute of Plasma Physics
Chinese Academy of Sciences
Hefei 230031 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100779.
696
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Chem. Asian J. 2012, 7, 696 – 700

Figure 1. Absorption spectra (open symbols) and fluorescence spectra
(closed symbols, excitation at 460 nm) of dyes B1 (squares), B2 (circles),
and B3 (triangles) in CHCl3. The fluorescence spectrum of B2 has been
amplified 150-fold. The corresponding spectra of 1a (solid and dashed
lines) are shown for comparison.
Figure 2 displays the absorption spectra of dyes B1–B3 in
the solid state. The dyes are absorbed on the surface of a
transparent mesoporous TiO2 film (6 mm thickness). The ab-
sorption of the blank TiO2 film has been subtracted from
the curve. Compared to the spectra in solution, the absorp-
tion of the dyes in the solid state is slightly blue-shifted in
the range of 400–650 nm. This effect is attributed to an H-
type aggregation of the dyes on the TiO2 surface and/or
dye–TiO2 interactions.[16] The absorption peaks of these
compounds are distinctly broadened in the region of 400–
650 nm, which is desirable for harvesting solar light. The ab-
sorption maximum of dye B3 has a smaller blue-shift com-
pared to those of B1 and B2.
Scheme 2. Synthetic routes for dyes B1–B3: a) POCl3, DMF,
(PPh3)4],
K2CO3 (aq), toluene, 808C, 24 h; d) cyanoacetic acid, piperidine, CH3CN/
ClCH2CH2Cl, 08C!508C; b) ICI, DMF/CH3OH, RT; c) [Pd
ACHTUNGTRENNUNG
CHCl3, reflux, 12 h.
To evaluate the feasibility of electron injection from the
excited-state molecules to the conduction band of TiO2, the
electrochemical characteristics of dyes B1–B3 were investi-
gated by cyclic voltammetry. The half-wave oxidation poten-
in the synthesis of 3a–c.[15c] Suzuki cross-coupling reactions
of 3a–c with 4 afforded compounds 5a–c. Finally, the target
products (B1–B3) were obtained through Knoevenagel con-
densation reactions of 5a–c with cyanoacetic acid in the
presence of piperidine.
The absorption and fluorescence spectra of dyes B1–B3 in
chloroform are shown in Figure 1. Strong absorption in two
regions, 280–400 nm and 450–620 nm, due to p–p* transi-
tions of the conjugated molecules were observed. Compared
to the spectra obtained with Bodipy bridge compounds 1a–c
(maximum at ca. 495 nm), the absorption maxima of B1–B3
are red-shifted by approximately 35 nm as a result of the
coupling interactions between Bodipy and the two substitu-
ents, triphenylamine and cyanoacetic acid. The absorption
spectra of dyes B1–B3 are only slightly different, thus indi-
cating that the substituents at position 8 of the Bodipy
bridge have only a small impact on the molecular conjuga-
tion system. The larger Stokes shifts (>100 nm) and weaker
fluorescence of B1–B3 compared with the unmodified
Bodipy compound 1a, as observed from the absorbance and
fluorescence spectra, can be attributed to a charge separa-
tion in the excited state, which is favorable to electron injec-
tion into the conduction band of TiO2 in solar cells.
Figure 2. Normalized absorption spectra of dyes B1–B3 adsorbed on a
nanocrystalline TiO2 film (6 mm thickness).
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697

Q.-H. Song et al.
COMMUNICATION
Table 2. Frontier molecular orbitals of dyes B1–B3 calculated at the
B3LYP/6-31G(d) level.
tials (Eox) can be obtained (Figure 3 and Table 1), and the
excited state oxidation potential (Eox*) can be estimated by
subtracting the 0,0 transition energy (E0,0) from Eox. The oxi-
dation potentials, Eox, corresponding to the HOMO levels of
these compounds, ranged from 1.08 to 1.14 V versus NHE
Dyes
HOMO
LUMO
B1
B2
B3
irradiation could redistribute the electron density from the
electron donor (triphenylamine) to the receptor part (cyano-
acetic acid). This demonstrates that electron injection by
means of intramolecular charge transfer across the Bodipy
bridge could be achieved.
Figure 3. Cyclic voltammograms of dyes B1–B3 measured in a solution of
CH2Cl2.
The measured photovoltaic parameters of cells
with dyes B1–B3 are summarized in Table 3 and
the corresponding photocurrent–voltage (J-V)
Table 1. Photophysical and electrochemical properties of dyes B1–B3.
curves are depicted in Figure 4. The power con-
version efficiency decreases in the order B3>
B1>B2, and the B3-sensitized cell exhibits a
better photovoltaic performance under AM 1.5 ir-
radiation (Jsc =4.38 mAcmꢀ2, Voc =0.58 V, FF=
0.72, and h=1.83%) than those fabricated with
B1 and B2. According to these data, we found
that the efficiencies of the DSSCs could be
Dye
lmax [nm][a]
lems [nm][a]
lmax [nm]
Eox[b] [V]
E0,0[c] [eV]
Eox*
[d] [V]
solution
solution
TiO2
B1
B2
B3
534 (6.45)
537 (6.65)
534 (7.44)
639
690
652
508
526
530
1.08
1.14
1.11
1.89
1.90
1.94
ꢀ0.81
ꢀ0.76
ꢀ0.83
[a] CHCl3 solutions, eꢂ10ꢀ4 mꢀ1 cmꢀ1 are given in parentheses. [b] Eox was measured in
CH2Cl2 and calibrated with ferrocene as an external reference. [c] E0,0 was estimated
from the absorption thresholds from absorption spectra of dyes absorbed on the TiO2
film. [d] Eox*=EoxꢀE0,0
.
Table 3. Photovoltaic parameters of DSSCs based on the dyes B1–B3
under AM 1.5m illumination (power density, 100 mWcmꢀ2).
and are thus more positive than the iodide/triiodide redox
couple (about 0.42 V vs. NHE), thereby indicating that the
oxidized dye formed after electron injection into the con-
duction band of TiO2 can be regenerated from the reduced
species in the electrolyte. The excited state oxidation poten-
tials (Eox*), corresponding to the LUMO levels, ranged
from ꢀ0.76 to ꢀ0.83 V versus NHE. These values are more
negative than the conduction band of TiO2 (about ꢀ0.5 V
vs. NHE), thus ensuring facile electron injection from the
excited-state molecule to the TiO2 conduction band. As a
result, the electron cycle is fully available in cells with dyes
B1–B3.
To gain insight into the electronic structures of dyes B1–
B3 at the molecular level, density functional theory (DFT)
calculations were performed at the B3LYP/6-31G(d) level
using the Gaussian 03 software; charge distributions are
listed in Table 2. At the HOMO level of the dyes, electrons
are almost delocalized on the triphenylamine moiety, where-
as they are mainly located on the Bodipy bridge and cyano-
acetic acid units at the LUMO level. Accordingly, these re-
sults reveal that the HOMO–LUMO excitation induced by
[a]
Dye
Jsc [mAcmꢀ2
]
Voc [V][a]
FF[a]
h [%][a]
B1
B2
B3
3.92
1.56
4.38
0.55
0.50
0.58
0.59
0.64
0.72
1.28
0.50
1.83
[a] Jsc is the short-circuit current, Voc is the open-circuit voltage, FF is the
fill factor, and h is the energy conversion efficiency.
strongly affected by the substituents at position 8 of the
Bodipy bridge in the dye molecules. The dye B2 with a
phenyl group at this position had the lowest cell efficiency,
0.5%, mainly as a result of a very low Jsc. This data indicates
that the phenyl group is detrimental to the electron trans-
port from the donor to TiO2. The dye B3, which contains an
n-pentyl chain, may suppress the aggregation of dye mole-
cules and reduce the charge recombination by blocking the
ꢀ
approach of the I3 ion to a certain degree, thus giving
higher Jsc and Voc values as compared to those of B1 and B2.
Moreover, we should note that we did not use chenodeoxy-
cholic acid (CDCA) as a coadsorbent to enhance the con-
698
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Chem. Asian J. 2012, 7, 696 – 700

Modified Bodipy Sensitizers
this position in dye B3 results in the best photovoltaic per-
formance, with the solar energy conversion efficiency in-
creasing to 1.83% under AM 1.5 irradiation. This result
may primarily be attributed to a suppression of dye aggrega-
tion and a reduction in charge recombination, and thus
offers a strategy for further development of more efficient
solar cells based on Bodipy derivatives. Research on im-
proving the performance of Bodipy-based DSSCs is current-
ly in progress in our laboratory.
Experimental Section
General methods
Figure 4. J-V characteristics of DSSCs based on B1–B3.
UV/Vis absorption measurements were performed at room temperature
using a Shimadzu UV-2450 UV/Vis spectrometer. Solid-state absorption
spectra were obtained using a 6-mm-thick transparent mesoporous TiO2
film that had been immersed for 24 h at room temperature in a solution
of dye (20 mm) in CHCl3. Cyclic voltammetry was performed at room
temperature using a CHI 620D Electrochemical Workstation. The work-
ing electrode was a glassy carbon electrode. The counter electrode was a
Pt wire, and the reference electrode was a saturated calomel electrode
(SCE). Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.05m) was
used as supporting electrolyte, with ferrocene/ferrocenium (Fc/Fc+) as an
external reference. The scan rate used was 10 mVsꢀ1. The synthetic pro-
cedures and characterization data of dyes B1–B3 are given in the Sup-
porting Information.
version efficiency. Hence, the efficiency could be improved
by performing some optimizations.
The incident photon-to-current conversion efficiency
(IPCE) of the DSSCs based on dye B3 was measured in the
visible region (350–750 nm), as shown in Figure 5. The IPCE
Fabrication of Dye-Sensitized Solar Cells and Measurements of Their
Characteristics
Colloidal TiO2 nanoparticles were prepared by hydrolysis of titanium tet-
raisopropoxide, as described elsewhere.[17] The TiO2 paste was printed on
transparent conducting glass sheets (TEC-8, LOF) by using a screen-
printing technique, and sintered in air at 4508C for 30 min to form a
nanostructured TiO2 electrode. The film thickness was about 10.5 mm,
which was determined by a profilometer (XP-2, AMBIOS Technology,
Inc., USA). After cooling to 508C, the films were immersed overnight in
a solution of dye B1–B3 (0.3 mm) in CHCl3. Subsequently, excess dye
was rinsed off the TiO2 film with anhydrous CHCl3 before assembly. The
counter electrode was platinized by spraying a solution of H2PtCl6 on
FTO glass and fired in air at 4108C for 20 min. Then, it was placed di-
rectly on the top of the dye-sensitized TiO2 film. The gap between the
two electrodes was sealed by thermal adhesive films (Surlyn, Dupont).
The electrolyte was filled from a hole made on the counter electrode,
which was later sealed by a cover glass and thermal adhesive films. The
cells had an effective area of 0.25 cm2 and contained an electrolyte com-
posed of 0.6m 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI),
0.1m LiI, 0.05m I2, and 0.5m 4-tert-butylpyridine in acetonitrile. The pho-
tocurrent–voltage (J-V) characteristics of the solar cells were determined
by using a Keithley 2420 SourceMeter controlled by Testpoint software
and a solar simulator (solar AAA simulator, Oriel, USA, calibrated with
a standard crystalline silicon reference cell). The incident photon-to-cur-
rent efficiency (IPCE) of the DSCCs was measured using a 300 W Xe
lamp light source with a monochromatic light in the range of 350–
750 nm.
Figure 5. IPCE spectrum of a B3-sensitized solar cell.
spectrum for dye B3 exhibits a high plateau in the range of
400 to 600 nm, with the IPCE reaching 22% at 485 nm, and
displays a decrease above 650 nm, corresponding to a de-
crease in the light-harvesting efficiency of dyes. This agrees
with the absorption spectrum of dye B3 on the TiO2 film.
In conclusion, we have synthesized three novel Bodipy-
based sensitizers, B1–B3, which carry the triphenylamine
moiety as an electron donor and the cyanoacetic acid unit as
an electron acceptor to form D-p-A conjugated systems,
and investigated their performances as sensitizers in DSSCs.
All three compounds absorb strongly in the visible region,
both in solution and in the solid state. Cyclic voltammetry
measurements and DFT calculations demonstrated the feasi-
bility of electron injection and intramolecular charge trans-
fer. The performance of the solar cells based on B1–B3
varies largely with the substituents at position 8 of the
Bodipy bridge. The introduction of the n-pentyl group at
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (Grant No. 30870581, 20972149).
Keywords: bodipy derivatives · charge transfer · dyes/
pigments · dye-sensitized solar cells · sensitizers
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Received: September 14, 2011
Published online: January 17, 2012
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Chem. Asian J. 2012, 7, 696 – 700