Dihydrophenanthrene-based metal-free dyes for highly efficient cosensitized solar cells

Article abstract of DOI:10.1021/ol301374c

Metal-free dyes (BP-1 to BP-3) containing a 9,10-dihydrophenanthrene unit in the spacer have been synthesized. The dye with the highest cell efficiency, BP-2, was used in combination with SQ2 for cosensitized DSSCs. The cosensitized DSSC in which the ratio of BP-2 and SQ2 is 8:2 (v/v) has a record high efficiency of 8.14% among cosensitized systems using all metal-free sensitizers. Dye distribution along the TiO₂ film depth was analyzed by an Auger electron spectroscopy technique.

Full text of DOI:10.1021/ol301374c

ORGANIC
LETTERS
2012
Vol. 14, No. 14
3612–3615
Dihydrophenanthrene-Based Metal-Free
Dyes for Highly Efficient Cosensitized
Solar Cells
Ryan Yeh-Yung Lin,†,‡ Yung-Sheng Yen,† Yung-Tse Cheng,†,§ Chuan-Pei Lee,‡
Ying-Chan Hsu,† Hsien-Hsin Chou,† Chih-Yu Hsu,† Yung-Chung Chen,†
Jiann T. Lin,*,† Kuo-Chuan Ho,*,‡, and Chiitang Tsai§
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan,
Department of Chemical Engineering, National Taiwan University, Taipei,
10617, Taiwan, Department of Chemistry, Chinese Culture University, Taipei, Taiwan,
and Institute of Polymer Science and Engineering, National Taiwan University,
Taipei 10617, Taiwan
jtlin@gate.sinica.edu.tw; kcho@ntu.edu.tw
Received May 18, 2012
ABSTRACT
Metal-free dyes (BP-1 to BP-3) containing a 9,10-dihydrophenanthrene unit in the spacer have been synthesized. The dye with the highest cell
efficiency, BP-2, was used in combination with SQ2 for cosensitized DSSCs. The cosensitized DSSC in which the ratio of BP-2 and SQ2 is 8:2 (v/v)
has a record high efficiency of 8.14% among cosensitized systems using all metal-free sensitizers. Dye distribution along the TiO2 film depth was
analyzed by an Auger electron spectroscopy technique.
Searching for clean and sustainable energy is one of the
most important scientific and technological challenges in
the 21st century. Compared to other renewable energy
resources, solar energy is very attractive because of its
unlimited supply. Dye-sensitized solar cells (DSSCs) based
on organic materials have received considerable attention
in the past two decades. DSSCs based on Ru complexes
have achieved efficiencies of >11% at standard AM 1.5G
irradiation.1 The best efficiency of metal-free sensitizer-
based DSSCs reaches only ∼10%.2 Nonetheless, metal-free
dyes are very attractive in view of some disadvantages of
Ru sensitizers: (1) Ru metal is expensive and less environ-
mentally friendly; (2) the metal-to-ligand charge-transfer
† Academia Sinica.
‡ Department of Chemical Engineering, National Taiwan University.
§ Chinese Culture University.
Institute of Polymer Science and Engineering, National Taiwan
University.
ꢀ
(1) (a) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin,
S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.;
(2) (a) Zeng, W.; Cao, Y.; Bai, Y.; Wang, Y.; Shi, Y.; Zhang, M.;
Wang, F.; Pan, C.; Wang, P. Chem. Mater. 2010, 22, 1915. (b) Mishra,
A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474.
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Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M.
J. Am. Chem. Soc. 2001, 123, 1613. (b) Nazeeruddin, M. K.; De Angelis,
F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Bessho, T.;
(c) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem.
Rev. 2010, 110, 6595. (d) Yen, Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu,
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r
10.1021/ol301374c
Published on Web 07/09/2012
2012 American Chemical Society

(MLCT) band of the Ru dye normally does not have molar
extinction coefficients >25 000 Mꢀ1 cmꢀ1 and a thicker
TiO2 film is needed.
Al2O3-coated TiO2 to retard the electron recombination
rate.3j An interesting ultrafast cosensitization was reported
by Holliman et al., with the best efficiency of 7.2%.3k
Previously we developed thienylfluorene-based dyes
with high light-harvesting efficiencies.4 One of the dyes,
(E)-2-cyano-3-(5-(9,9-diethyl-7-(naphthalen-1-yl(phenyl)-
amino)-9H-fluoren-2-yl)thiophen-2-yl)acrylic acid (FL
dye), was used as a cosensitizer of either N719 dye or black
dye with improved cell performance.5 We also tested the
combination of the FL dye with an NIR-absorbing squar-
aine dye, SQ2.6 However, the performance of cosensitized
DSSC was inferior to that of DSSCs based on either FL or
SQ2. Since a slight change of molecular conformation may
have a significant impact on the cell performance,7 we set
out to develop congeners of the FL dye in which the
fluorene entity is replaced by a 9,10-dihydrophenanthrene
entity based on two reasons: (1) the skeleton of 9,10-
dihydrophenanthrene is similar to that of fluorene, and
the good light-harvesting character of the FL dye can be
retained; (2) a slight change of the dye conformation may
lead to better dye packing of the cosensitizers. A series
of 9,10-dihydrophenanthrene-based sensitizers (BP-1 to
BP-3) (Figure 1) were thus synthesized, and the synthetic
protocols are shown in Scheme S1 (see Supporting Infor-
mation (SI)).
A small molecule sensitizer normally does not have a
broad absorption covering most of the visible to near-
infrared (NIR) region of the solar spectrum. In order for a
DSSC to have panchromatic absorption, use of different
sensitizers absorbing in different regions of the solar
spectrum appears to be a convenient approach because
the short-circuit current density (JSC) and thus the conver-
sion efficiency (η) can be improved. Cosensitized DSSCs
thus received increasing attention in recent years, and
significant progress has been made.3a Cosensitizing sys-
tems using a metal-free dye and a ruthenium dye have been
proved to be successful since the former has intense
absorption at shorter wavelengths and the latter absorbs
at longer wavelengths extending into the red to NIR
region. For example, Ogura et al. used the black dye in
combination with D131, and the efficiency was increased
by ∼1% compared to the cell with the black dye only.3b
Ko et al. reported that the cosensitized DSSC based on a
ruthenium sensitizer (JK-142) and a metal-free dye (JK-62)
exhibited an efficiency (10.2%) surpassing that using only
a single sensitizer (JK-142, 7.28%; JK-62, 5.36%).3c More
recently Han et al. also reported a very high efficiency
(11.4%) cosensitized DSSC using a metal-free dye and the
black dye.3d A DSSC with record high performance
(12.3%) was demonstrated with the use of two metal-
containing porphyrin dyes as cosensitizers together with
the use of a Co(II/III)-based redox electrolyte.3e Diau
et al. and Kimura et al. independently reported cosensi-
tized systems of porphyrin with organic dyes exhibited
improved performance in 2012.3f,g In comparison, there
were also successful examples of cosensitized DSSCs using
solely metal-free dyes. Nazeeruddin et al. and Torres et al.
employed NIR dyes (SQ1 or TT1) together with the JK2
for the cosensitized solar cell.3h,i The action spectra (IPCE)
could extend up to 700 nm, and the device efficiency over
7% was achieved. Choi et al. achieved a high performance
(8.65%) cosensitized solar cellusingJK2 and SQ1 dyes and
Figure 1. Structures of the dyes.
All of the BP dyes exhibit two prominent bands in the
absorption spectra (Figure 2), the one at a longer wave-
length region (410ꢀ500 nm) and with a high molar absorp-
tion coefficient (ε > 36500 Mꢀ1 cmꢀ1) is attributed to
πꢀπ* transition with charge transfer character. The λabs
(3) (a) Yum, J.-H.; Baranoff, E.; Wenger, S.; Nazeeruddin, M. K.;
€
Gratzel, M. Energy Environ. Sci. 2011, 4, 842. (b) Ogura, R.-Y.; Nakane,
S.; Morooka, M.; Orihashi, M.; Suzuki, Y.; Noda, K. Appl. Phys. Lett.
2009, 94, 073308. (c) Fan, S.-Q.; Kim, C.; Fang, B.; Liao, K.-X.; Yang,
G.-J.; Li, C.-J.; Kim, J.-J.; Ko, J. J. Phys. Chem. C 2011, 115, 7747. (d)
Han, L.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S.;
Yang, X.; Yanagida, M. Energy Environ. Sci. 2012, 5, 6057. (e) Yella, A.;
Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.;
€
Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Gratzel, M. Science
2011, 334, 629. (f) Lan, C.-M.; Wu, H.-P.; Pan, T.-Y.; Chang, C.-W.;
Chao, W.-S.; Chen, C.-T.; Wang, C.-L.; Lin, C.-Y.; Diau, E. W.-G.
Energy Environ. Sci. 2012, 5, 6460. (g) Kimura, M.; Nomoto, H.;
Masaki, N.; Mori, S. Angew. Chem., Int. Ed. 2012, 51, 4371. (h) Cid,
J. J.; Yum, J. H.; Jang, S. R.; Nazeeruddin, M. K.; Martinez-Ferrero, E.;
€
Palomares, E.; Ko, J.; Gratzel, M.; Torres, T. Angew. Chem., Int. Ed.
2007, 46, 8358. (i) Yum, J.-H.; Jang, S.-R.; Walter, P.; Geiger, T.;
€
€
Nuesch, F.; Kim, S.; Ko, J.; Gratzel, M.; Nazeeruddin, M. K. Chem.
Commun. 2007, 4680. (j) Choi, H.; Kim, S.; Kang, S. O.; Ko, J.; Kang,
M.-S.; Clifford, J. N.; Forneli, A.; Palomares, E.; Nazeeruddin, M. K.;
€
Gratzel, M. Angew. Chem., Int. Ed. 2008, 47, 8259. (k) Holliman, P. J.;
Mohsen, M.; Connell, A.; Davies, M. L.; Al-Salihi, K.; Pitak, M. B.;
Tizzard, G. J.; Coles, S. J.; Harrington, R. W.; Clegg, W.; Serpa, C.;
Fontes, O. H.; Charbonneau, C.; Carnie, M. J. Mater. Chem. 2012, 22,
13318.
Figure 2. Absorption spectra of the dyes in THF.
Org. Lett., Vol. 14, No. 14, 2012
3613

decreases intheorderofFL(443 nm) > BP-2 (430nm) and
BP-1 (441 nm) > S1 (418 nm), which is in accordance with
the degree of planarity of the two phenyl rings in the
conjugated spacer (vide infra). Blue-shifted absorption
spectra of the dyes on TiO2 (Figure S1) is attributed to
the deprotonation of the carboxylic acid. The cyclic vol-
tammograms of the dyes are shown in Figure S2 (SI). The
more planar configuration of the BP dye than S1 allows
better electronic communication between the arylamine
and electron excessive heteroaromatic ring; consequently
the former displays a negatively shifted oxidation wave
(Table 1). Quantum chemistry computation was carried
out, and the results for the theoretical approach are included
in Table S1 (SI). Frontier orbitals, Mulliken charge shifts,
the corresponding energy states, and optimized molecular
conformation are shown in Figures S3ꢀS6 (SI), respectively.
Table 1. Optical, Redox, and DSSC Performance Parameters of
the Dyesa
λ
abs (ε ꢁ 10ꢀ4
Eox
(ΔEp)c
mV
VOC
JSC
(mA cmꢀ2
η
(%)
M )
ꢀ1 cmꢀ1 b
dye
S1
FL
(V)
)
FF
nm
0.59
10.94
4.22 0.66
418 (3.43)
305 (2.45)
443 (3.96)
570
(153)
509
(82)
510
BP-1 0.61
BP-2 0.65
BP-3 0.62
N719 0.72
13.61
14.16
13.81
18.33
5.33 0.64
5.95 0.65
5.65 0.66
8.61 0.65
441 (4.03)
308 (2.21)
430 (3.65)
335 (1.92)
432 (3.68)
345 (2.19)
(125)
520
(133)
520
(134)
a Experiments were conducted using TiO2 photoelectrodes with an
approximately 20 μm thickness and 0.16 cm2 working area on the FTO
(15 Ω/sq.) substrates. b Recorded in THF solutions at 298 K. c Recorded
in THF solutions. Eox = 1/2(Epa þ Epc), ΔEp = Epa ꢀ Epc where Epa and
Epc are the anodic and cathodic peak potentials, respectively. The
oxidation potential reported is adjusted to the potential of ferrocene
which was used as an internal reference. The values in parentheses are the
peak separation of cathodic and anodic waves. Scan rate: 100 mV sꢀ1
.
of BP-2 among all are likely due to the highest dye density
and lowest dark current density of BP-2 compared to other
dyes.
BP-2 with the highest cell performance was subjected to
cosensitization with SQ2 because the latter could compen-
sate for the deficit of the former in the NIR region. DSSCs
with the use of BP-2 and SQ-2 solutions of the same
concentration in different volume ratios were fabricated.
Figure 4a shows the absorption spectra of four TiO2 films
cast with various volume ratios of BP-2/SQ2.
Figure 3. (a) JꢀV curves and (b) IPCE spectra of DSSCs based
on the dyes. The lower half in (a) shows the dark current
densities of DSSCs.
DSSCs with an effective area of 0.16 cm2 were fabricated
using an electrolyte composed of 0.1 M LiI, 0.6 M DMPII,
0.05 M I2, and 0.1 M GuSCN in ACN/MPN. The cell
performance data under AM 1.5 illumination are collected
in Table 1. The photocurrentꢀvoltage (JꢀV) curves and
the incident photon-to-current conversion efficiencies
(IPCE) of the cells are shown in Figure 3. The IPCE values
of the BP dyes are higher than that of S1 by ∼10ꢀ20%,
which is consistent with the absorption spectra. The de-
vices exhibited moderate to good power conversion effi-
ciencies ranging from 5.33 to 5.95%. The adsorbed dye
densities (S1, 4.34 ꢁ 10ꢀ7; BP-1, 4.01 ꢁ 10ꢀ7; BP-2, 4.54 ꢁ
10ꢀ7; BP-3, 3.86 ꢁ 10ꢀ7 mol cmꢀ2) on the TiO2 surface
differed within 20%. Therefore, the higher JSC of BP-1
thanthatof S1 can beascribedtothebetter lightharvesting
of the former. The lower open-circuit voltage (VOC) of S1
than that of BP dyes may be due to the larger dark current
density (Figure 3a) in the former. The highest JSC and VOC
(4) (a) Thomas, K. R. J.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Chem.
Commun. 2005, 4098. (b) Chen, C.-H.; Hsu, Y.-C.; Chou, H.-H.;
Thomas, K. R. J.; Lin, J. T.; Hsu, C.-P. Chem.;Eur. J. 2010, 16, 3184.
(5) Lee, K.-M.; Hsu, Y.-C.; Ikegami, M.; Miyasaka, T.; Thomas,
K. R. J.; Lin, J. T.; Ho, K.-C. J. Power Sources. 2011, 196, 2416.
(6) Geiger, T.; Kuster, S.; Yum, J.-H.; Moon, S.-J.; Nazeeruddin,
Figure 4. (a) Absorption spectra of BP-2/SQ2 coadsorbed on
TiO2 film (thickness ∼2 μm) in different volume ratios. (b) IPCE
spectra of BP-2/SQ2 cosensitized cells. (c) JꢀV curves of BP-2/
SQ2 cosensitized cells.
€
€
M. K.; Gratzel, M.; Nuesch, F. Adv. Funct. Mater. 2009, 19, 2720.
(7) Pack, S.; Choi, H.; Lee, C.-W.; Kang, M.-S.; Song, K.; Nazeeruddin,
M. K.; Ko, J. J. Phys. Chem. C 2010, 114, 14646.
Strong absorption of the BP-2 dye at wavelengths below
550 nm and absorption dueto SQ2 inthered toNIR region
3614
Org. Lett., Vol. 14, No. 14, 2012

(600ꢀ800 nm) are evident. The IPCE spectra (Figure 4b)
of BP-2/SQ2 cosensitized DSSCs cover a fairly broad
range (400ꢀ750 nm). In accordance with the film spectra
on TiO2, high conversion efficiencies were noticed at
400ꢀ500 nm and 600ꢀ700 nm. The JꢀV curves for these
BP-2/SQ2 cosensitized cells under AM 1.5 condition are
shown in Figure 4c, and the performance parameters are
collected in Table 2. The efficiency enhancement is in
compliance with the increasing volume ratio of BP-2. It
is intriguing that the JSC values reach 18.15 and 21.33 mA
cmꢀ2 at the BP-2/SQ2 ratio of 6:4 (v/v) and 8:2 (v/v),
respectively. The highest efficiency was achieved with the
lowest amount of SQ2. This outcome can be rationalized by
two explanations: (1) the high molar extinction coefficient
of SQ2 leads to absorption saturation at a relatively low
concentration; (2) a low concentration of SQ2 avoids dye
aggregation. To our knowledge, the efficiency of 8.14%
(Table 2) is the highest among cosensitized DSSCs ever
reported using metal-free sensitizers and Al2O3-free TiO2.
Figure 5. AES analysis showing the amounts of BP-2 and SQ2
across the depth of the TiO2 film at a 8:2 (v/v) ratio of BP-2/SQ2
cosensitized cells.
Figure 5 shows variation of the amounts (see experi-
mental section) of BP-2 and SQ2 along the TiO2 film depth
(0 μm is the interface of the scattering layer and the active
layer of TiO2). The amount of the SQ2 decreased with the
increasing depth of TiO2 film, while BP-2 increased when
the TiO2 depth increased and reached maximum at ∼7 μm,
and then decreased. This can be rationalized by the more
efficient penetration of BP-2 into the TiO2 film. We
speculate that the higher molecular weight of SQ2 retards
its penetration rate through the TiO2 film. A similar
concentration gradient was also found in Ko’s cosensitized
TiO2 film via sequential adsorption of two different dyes.3c
In summary, we have synthesized new arylamine
sensitizers with a 9,10-dihydrophenanthrene entity in the
conjugated spacer. Thedyes have a veryintense absorption
(ε > 36 500 Mꢀ1 cmꢀ1) at ∼430 nm. High performance
cosensitized DSSCs with a conversion efficiency up to
8.14% can be achieved. This represents the highest cosen-
sitized DSSC using all metal-free sensitizers and with-
out the use of Al2O3-coated TiO2. Moreover, cosen-
sitization from the mixture of different dyes simplifies
the cell fabrication process. The concentration distri-
bution found in this study demonstrates that the strategy
can be extended to other systems if dyes are appropriately
chosen.
Table 2. Performance Parameters of Cosensitized DSSCsa
VOC
JSC
(mA cmꢀ2
η
(%)
dye
(V)
)
FF
BP-2
0.65
0.57
0.58
0.59
0.63
0.66
0.72
14.16
10.33
14.33
16.67
18.15
21.33
18.33
5.95
3.78
5.53
6.09
7.18
8.14
8.61
0.65
0.64
0.66
0.62
0.63
0.58
0.65
SQ2
BP-2:SQ2 = 2:8
BP-2:SQ2 = 4:6
BP-2:SQ2 = 6:4
BP-2:SQ2 = 8:2
N719
a Experiments were conducted using TiO2 photoelectrodes with an
approximately 20 μm thickness and 0.16 cm2 working area on the FTO
(15 Ω/sq.) substrates.
The photocurrent estimated from integrating the prod-
uct of the IPCE value at each wavelength and the photon
flux density data in the AM 1.5 solar spectrum (100 mW/
cm2)8 was ∼16.5 mA/cm2 for the BP-2:SQ2 (8:2) cell, which is
smaller than the experimental value. This discrepancy may be
attributed to the filtration of infrared radiation by a water
filter in our light source during measurement, while the real
sun spectrum8 covers the near-infrared portion.
The TiO2 filmadsorbedwith cosensitizersBP-2/SQ2in a
ratio of 8:2 (v/v) was subjected to Auger electron spectro-
scopy (AES) for probing the dye distribution across the
TiO2 film. This technique is similar to the Electron Probe
Micro-Analyzer (EPMA) method used by Park et al.9 and
Ko3c et al. for investigating the distribution of a specific
element across the TiO2 depth.
Acknowledgment. We acknowledge the support of the
Academia Sinica (AC) and NSC (Taiwan), and the Instru-
mental Center of Institute of Chemistry (AC).
Supporting Information Available. Synthetic proce-
dures and characterization for new compounds. This
materials is available free of charge via the Internet at
http://pubs.acs.org.
(8) Annual Book of ASTM Standards, ASTM International: West
Con-shohocken, PA, 2003, Vol. 14.04, pp G159ꢀ98.
(9) Lee, K.; Park, S.-K.; Ko, M.-J.; Kim, K.; Park, N.-G. Nat. Mater.
2009, 8, 665.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 14, 2012
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