Organic dyes containing furan moiety for high-performance dye-sensitized solar cells

Article abstract of DOI:10.1021/ol8025236

New metal-free dyes with a furan moiety in the conjugated spacer between the arylamine donor and the 2-cyanoacrylic acid acceptor have been synthesized, and high efficiency dye-sensitized solar cells were fabricated using these molecules as light-harvesti

Full text of DOI:10.1021/ol8025236

ORGANIC
LETTERS
2009
Vol. 11, No. 1
97-100
Organic Dyes Containing Furan Moiety
for High-Performance Dye-Sensitized
Solar Cells
Jiann T. Lin,*,† Pin-Cheng Chen,‡ Yung-Sheng Yen,† Ying-Chan Hsu,†
Hsien-Hsin Chou,† and Ming-Chang P. Yeh*,‡
Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, and Department of
Chemistry, National Taiwan Normal UniVersity, Taipei, Taiwan 115
jtlin@sinica.edu.tw; cheyeh@scc.ntnu.edu.tw
Received October 31, 2008
ABSTRACT
New metal-free dyes with a furan moiety in the conjugated spacer between the arylamine donor and the 2-cyanoacrylic acid acceptor have
been synthesized, and high efficiency dye-sensitized solar cells were fabricated using these molecules as light-harvesting sensitizers.
The increasing demand for power supply as well as
environmental concern for the consumption of fossil fuel
have triggered global research on the development of clean
and renewable energy sources.1 Among possible alternates
for fossil fuel energy, solar energy appears to be very
attractive: covering 0.16% of the Earth with 10% efficient
solar conversion systems would provide power nearly twice
the world’s consumption rate of fossil energy.2 Athough
silicon- and other semiconductor-based solar cells (known
as photovoltaic cells)3 have dominated the solar cell market
for decades, dye-sensitized solar cells (DSSCs) have also
attracted considerable interest ever since the breakthrough
was made by Gra¨tzel and co-workers.4 An efficiency record
of ∼11% has been achieved with ruthenium-based sensitizers
developed by Gra¨tzel and other groups.5 Athough being
developed later than ruthenium dyes, metal-free sensitizers
also attract much attention, and a high efficiency of ∼9%
has also been achieved for DSSCs using metal-free sensitizers.6
Various metal-free dyes have been used for the construc-
tion of DSSCs.7 Previously we reported metal-free sensitizers
(4) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Hagfeldt,
A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (c) Gra¨tzel, M. Nature 2001,
414, 338. (d) Gra¨tzel, M. J. Photochem. Photobiol. A 2004, 164, 3. (e)
Gra¨tzel, M. Chem. Lett. 2005, 34, 8.
† Academia Sinica.
(5) (a) Nazeeruddin, M. K.; DeAngelis, F.; Fantacci, S.; Selloni, A.;
Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc.
2005, 127, 16835. (b) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.;
Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (c) Gao, F.;
Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphrey-Baker, R.; Wang, P.;
Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Commun. 2008, 2629.
‡ National Taiwan Normal University.
(1) Armaroli, N.; Balzani, V. Angew. Chem., Int. Ed. 2007, 46, 52.
(2) Service, R. F. Science 2005, 309, 548.
(3) Goetzberger, A.; Hebling, C.; Schock, H.-W. Mater. Sci. Eng. R-Rep.
2003, 40, 1.
10.1021/ol8025236 CCC: $40.75
Published on Web 12/09/2008
2009 American Chemical Society

which consisted of an arylamine as the electron donor, a
2-cyanoacrylic acid as the electron acceptor, and a conjugated
bridge containing thiophene moieties.8 Compared with
benzenoid moieties, the thiophene can provide more effective
conjugation and lower the energy of the charge transfer
transition9 because of its smaller resonance energy (thiophene,
29; benzene, 36 kcal/mol).10 It is therefore interesting to
incorporate the furan ring with a smaller resonance energy
(16 kcal/mol) in the spacer. In this letter, we report new
metal-free compounds 1-4 (Figure 1) consisting of a
synthesized from Wittig reagents containing a triphenylamine
or a diphenylamino-2-fluorene moiety. Reactions of these
Wittig reagents with 2-furaldehyde provided intermediates
1b and 4b, which underwent formylation to form 1c and 4c.
The desired products were obtained from the subsequent
Knoevenagel condensation of 1c (or 4c) with cyanoacetic
acid. For the preparation of 2 and 3, 5-bromo-2-furaldehyde
was allowed to react with appropriate phenylboronic acid
via Suzuki coupling11 and with a stannyl compound via Stille
coupling,12 respectively, to form intermediates 2b and 3b.
Subsequent reactions of the intermediates with cyanoacetic
acid afforded the desired compounds.
The absorption of 1-4 recorded in THF solution and the
absorption spectrum of 1 in MeCN (vide infra) are displayed
in Figure 2, and the data are collected in Table 1. The
emission spectra are shown in Figure S1, Supporting
Information (see ESI). A prominent band at ∼400-600 nm
can be attributed to the superposition of π-π* and charge
transfer transitions. The charge transfer characteristic in these
dyes is also supported by the large Stokes shifts between
the absorption and the emission bands (3225-4895 cm-1).
A negative solvatochromism, i.e., blue shift of the charge
transfer band in more polar solvents, was noticed in these
dyes. For example, the absorption maxima (λmax) of 1 are
485 and 439 nm in toluene and acetonitrile, respectively.
This phenomenon can be attributed to the deprotonation of
the carboxylic acid, which decreases the strength of the
electron acceptor.7j,8d,e,13
Figure 1. Structure of the dyes.
diphenylamine donor, a 2-cyanoacrylic acid acceptor, and a
bridge with a furan moiety.
The dyes were obtained in moderate yields by the synthetic
protocol illustrated in Scheme 1. Compounds 1 and 4 were
A quasi-reversible wave (Eox in Table 1) observed for each
compound in the cyclic voltammetry measurements may be
attributed to the oxidation of the arylamine. Because of the
shorter spacers in 1 and 2, the electron-withdrawing acceptor
has greater influence on the arylamine and results in higher
oxidation potentials of the arylamines. The excited-state
potential (E0-0*) of the compounds (-0.74 to -0.80 V vs
NHE), deduced from Eox and the zero-zero excitation energy
(E0-0) from the absorption band edge, is more negative than
the conduction-band-edge energy level of the TiO2 electrode
(-0.5 V vs NHE)14 and assures that the electron injection
process is energetically favorable.
DSSCs were fabricated using these dyes as the sensitizers,
with an effective area of 0.25 cm2, nanocrystalline anatase
TiO2 particles, and the electrolyte composed of 0.05 M I2/
0.5 M LiI/0.5 M tert-butylpyridine in acetonitrile solution.
The device performance statistics under AM 1.5 illumination
are listed in Table 1. Figure 3 shows the photocurrent-voltage
(J-V) curves of the cells. The incident photon-to-current
conversion efficiencies (IPCE) of the dyes on TiO2 are plotted
in Figure 4. The cells exhibit very high conversion efficien-
cies (6.12 to 7.36%), and the best performance of the device
reaches ∼96% of a N719-based DSSC (7.69%) fabricated
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98
Org. Lett., Vol. 11, No. 1, 2009

Scheme 1
.
Synthesis of Dyes 1-4
Table 1. Optical, Redox, and DSSCs Performance Parameters of the Dyesa b
,
dye
λabs, nm (ε, M-1cm-1
)
λem, nm (ΦF)
Eox (∆Ep), mV
VOC, V
JSC, mA/cm2
ff
η, %
τR, ms
1
2
3
4
469 (33000)
456 (33500)
457 (44900)
459 (35500)
476 (45900)
533
496
526
534
500 (220)
570 (140)
460 (130)
440 (110)
0.69
0.68
0.68
0.68
0.64
0.73
16.59
14.16
14.08
13.44
14.61
16.77
0.64
0.66
0.65
0.67
0.65
0.63
7.36
6.30
6.20
6.12
6.09
7.69
4.9
5.8
4.3
3.1
4.3
D5
N719
10.4
a Absorption, emission, and electrochemical data were recorded in THF solutions. The absorption spectrum of D5 was recorded in MeCN. Scan rate: 100
mV/s; Electrolyte: (n-C4H9)4NPF6; ∆Ep: separation between the anodic and cathodic peaks. Potentials are quoted with reference to the internal ferrocene
standard (E1/2 ) +265 mV vs Ag/AgNO3). τR: recombination lifetime from the photovoltage measurements. b Experiments were conducted using TiO2
photoelectrodes with approximately 15 µm thickness and 0.25 cm2 working area on the FTO (7 Ω/sq.) substrates.
Figure 3. J-V curves of DSSCs based on the dyes.
Figure 2. Absorption spectra of dyes.
and measured under similar conditions. The device efficien-
cies are in the order of 1 > 2 > 3 > 4, and the difference
between structurally similar 1 and 2 may be rationalized by
the following factors: (a) the compound with better absorp-
tion in the longer wavelength region is expected to have
better light-harvesting, i.e., 1 > 2; and (b) the higher dye
density adsorbed on TiO2 (1, 4.5; 2, 4.1 × 10-7 mol/cm2) is
beneficial to light-harvesting, i.e., 1 > 2. The recombination
lifetimes (Table 1) of the photoinjected electron with the
oxidized dye (τR: 1, 4.9; 2, 5.8; 3, 4.3; 4, 3.1; N719, 10.4
ms) were also measured by transient photovoltage at open
circuit15 and found to be in parallel with the trend of the
cell efficiencies.
A comparison is made on 1 and the thiophene congener,
3-(5-4-(diphenylamino)styryl)thiophen-2-yl-2-cyanoacrylic
(15) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J.
J. Phys. Chem. B 2003, 107, 11307.
Org. Lett., Vol. 11, No. 1, 2009
99

under similar conditions. Faster recombination lifetimes of
D5 (τR ) 4.3 ms) may deteriorate the efficiency of the cell.
Possible nonbonded S-S interaction16 in D5 may be also
detrimental to electron injection of the excited dye into TiO2.
Another possibility for the lower efficiency of D5 may be
due to the slightly higher tendency of thiophene to trap
charge from the arylamine, based on our preliminary
computation results using time-dependent density functional
theory (TDDFT) at the B3LYP/6-31G* level of theory.
In summary, we have synthesized a new series of metal-
free sensitizers containing the furan entity for high-
performance DSSCs. The highest efficiency nearing to that
of N719 is intriguing. More detailed elucidation of the cell
performance and extension of this synthetic strategy to better
sensitizers are currently ongoing.
Figure 4. IPCE plots for the DSSCs using dyes 1-4
.
Acknowledgment. We thank Academia Sinica, National
Science Council (Taiwan), and Plasmag Technology, Inc.
for financial support.
acid (D5).7j,k Though D5 has slightly better light-harvesting
(λabs ) 476 nm (45 900 M-1cm-1) in MeCN (Figure 2); dye
density on TiO2 ) 4.8 × 10-7 mol/cm2) than 1 (λabs ) 439
nm (33 000 M-1cm-1) in MeCN; dye density on TiO2 )
4.5 × 10-7 mol/cm2), the performance of DSSC based on 1
is superior to that based on D5 (VOC ) 0.64 V; JSC ) 14.61
mA/cm2; ff ) 0.65; η ) 6.09%) fabricated and measured
Supporting Information Available: Synthetic procedures
and characterization for new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL8025236
(16) Yamada, K.; Okamoto, T.; Kudoh, K.; Wakamiya, A.; Yamaguchi,
S.; Takeya, J. Appl. Phys. Lett. 2007, 90, 072102.
100
Org. Lett., Vol. 11, No. 1, 2009