Organic dyes with remarkably high absorptivity; All solid-state dye sensitized solar cell and role of fluorine substitution

Article abstract of DOI:10.1039/c0cc00808g

A series of new organic D-π-A dyes, A1, A2-H and A2-F, possessing a remarkably high absorption extinction coefficient of ε > 5.0 × 10⁴ M^-1 cm^-1 at peak wavelength were synthesized, among which, A2-F having a key fluorine substitution attains excellent all solid-state DSSC performance, with optimized parameters of η = 4.86%, J_SC = 7.52 mA cm^-2, V_OC = 0.91 V, and FF = 0.71.

Full text of DOI:10.1039/c0cc00808g

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Organic dyes with remarkably high absorptivity; all solid-state dye
sensitized solar cell and role of fluorine substitutionw
Dong-Yi Chen,a Yu-Yen Hsu,b Hui-Chu Hsu,b Bo-So Chen,a Yi-Tsung Lee,b Hungshin Fu,a
Min-Wen Chung,a Shih-Hung Liu,a Hsieh-Chih Chen,a Yun Chi*b and Pi-Tai Chou*a
Received 7th April 2010, Accepted 16th June 2010
First published as an Advance Article on the web 28th June 2010
DOI: 10.1039/c0cc00808g
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A series of new organic D–p–A dyes, A1, A2-H and A2-F,
possessing a remarkably high absorption extinction coefficient
15 of e 4 5.0 ꢀ 104 Mꢁ1 cmꢁ1 at peak wavelength were synthe-
sized, among which, A2-F having a key fluorine substitution
attains excellent all solid-state DSSC performance, with
optimized parameters of g = 4.86%, JSC = 7.52 mA cmꢁ2
,
VOC = 0.91 V, and FF = 0.71.
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Due to the growing demand for green energy, in combination
with latent depletion of fossil fuels, there has been an intensive
search for cost-effective photovoltaics since the first develop-
ment of solar cells in the 1950s.1 Among the solar cells that
have been under intensive investigation, the dye-sensitized
solar cells (DSSCs), in particular, with a power conversion
efficiency (over 11%) comparable to amorphous silicon solar
cells,2 seem promising to offer a solution to the low-cost and
large-area photovoltaic applications. The conventional DSSC
is mainly composed of mesoporous TiO2 coated with light-
absorbing dyes and filled with electrolyte made of Iꢁ/I3ꢁ redox
couple dissolved in acetonitrile.3 Such a DSSC configuration,
though having reached high efficiency especially with Ru(II)
based sensitizers4 and metal-free organic dyes,5 unfortunately
encounters some inevitable inferiority. Of prime concern are
the evaporation and leakage of liquid electrolytes, which
seriously limit device stability as well as long-term operation.
Alternatively, solid-state DSSCs, for which the electrolyte is
replaced by hole conductors such as 2,20,7,70-tetrakis-(N,N-di-
p-methoxyphenylamine)-9,90-spirobifluorene (spiro-MeOTAD),
CuSCN, or even conducting polymers, have received con-
siderable attention.6–9 Few solid-state DSSCs with metal-free
organic dyes have shown encouraging results with the power
conversion efficiency (Z) exceeding 4%.10–12
Scheme 1 Synthetic route to the sensitizers A1, A2-H and A2-F.
Reagents and conditions: (i) n-BuLi, DMF, (ii) NBS, DMF, (iii)
Pd(PPh3)4, K2CO3, THF, (iv) cyanoacrylic acid, piperidine, CH3CN,
(v) n-BuLi, Bu3SnCl, (vi) PdCl2(PPh3)2, DMF.
Herein, we report the strategic design, synthesis, and charac-
terization of a new series of donor–bridge–acceptor (D–p–A)
sensitizers, A1, A2-H and A2-F, (see Scheme 1) for fabricating
high performance solid-state DSSCs with hole-conductor
spiro-MeOTAD. The parent A1 employs triphenylamine,
4,40-dihexylcyclopenta[2,1-b : 3,4-b]dithiophene, and cyanoacrylic
acid as the electron donor, spacer and acceptor, respectively.13–15
The intention of attaching the dialkyl chain onto the cyclo-
pentadithophene spacer is to avoid aggregation of sensitizers
upon absorbing on the TiO2 surface. An additional phenylene
group is then incorporated, forming A2-H, with the aim of
further elongating the p-conjugation. We thus expect an
increase of absorption cross section and/or spectral red-shifting
for A2-H (cf. A1). More importantly, we further designed
A2-F, the structure of which resembles that of A2-H, except
for a fluoro substituent being intentionally introduced in close
proximity to the cyanoacrylic anchor. Thus, in addition to its
common function to act as an electron withdrawing group, we
anticipate the possible interaction among fluorine, TiO2 nano-
particles and the hole conducting layer. For this approach,
A2-H also can be exploited as a control, having the parent
phenylene moiety to eliminate any additional electrostatic
interaction. Despite fluorine-containing compounds having
been widely applied in numerous cutting-edge applications
(vide infra),16 the fluoro substitution acting as a key component
in DSSCs has not been reported, at least to our knowledge.
The synthetic routes to the titled sensitizers are schemati-
cally depicted in Scheme 1. The 4,40-dihexylcyclopentadi-
thiophene is used as the key starting material, in which the
cyclopentadithiophene moiety enforces a good planarity,
fulfilling the prime requirement of increasing the transition
As for the design of solid-state DSSC dyes, we expected it to
fulfil the general requirements such as strong absorption in the
visible-to-near infrared region, band-gap matching (e.g. type
II energy gap arrangement versus TiO2) and avoiding solid-
state packing, etc. One key factor is to increase the molar
extinction coefficient in the visible region. The associated
device can thus harvest solar irradiation efficiently, so that
thinner TiO2 films can be used to circumvent the inferiority of
electron/hole mobility in solid electrolytes.
a Department of Chemistry, National Taiwan University, Taipei 106,
Taiwan. E-mail: chop@ntu.edu.tw
b Department of Chemistry, National Tsing Hua University,
Hsinchu 300, Taiwan. E-mail: ychi@mx.nthu.edu.tw
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c0cc00808g
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Fig. 1 Absorption spectra of the sensitizers, A1 and A2-H, A2-F, in
acetonitrile/tert-butanol (volume ratio 1 : 1). Insert: absorption profile
in the TiO2 film.
Fig. 2 The J–V characteristics of solid-state DSSCs based on the A1,
A2-H and A2-F sensitizers.
sensitizers and with an effective area of 0.04 cm2. The optimized
characteristics (i.e., 1.6 mm thickness for TiO2 NP provided by
Eternal Chemical Co., Ltd.) under standard global AM 1.5
solar conditions are shown in Fig. 2. As can be seen, the cell
made with sensitizer A1, A2-H and A2-F shows an overall
conversion efficiency (Z) of 3.62%, 3.69% and 4.86%, a short-
circuit photocurrent density (JSC) of 6.37 mA cmꢁ2, 6.79 mA cmꢁ2
and 7.52 mA cmꢁ2, an open-circuit voltage (VOC) of 0.80 V,
0.83 V and 0.91 V, and fill factor FF of 0.71, 0.65 and 0.71,
respectively. Fig. 3 shows the respective IPCE photocurrent
action spectra under short circuit conditions. Note that in this
approach we follow a general protocol for preparation of
small cells (0.04 cm2). Upon increasing the cell surface area
the efficiency of cell performance should commonly decrease.
Upon insertion of a phenylene or fluorophenylene group
into A1, forming A2-H or A2-F, JSC is improved by 6.6% and
18.1% (cf. A1), respectively. Such enhancement can be ratio-
nalized by the rise of IPCE (see Fig. 3) as result of the higher
molar extinction coefficient of both A2 series at the UV-to-
near visible region (350–450 nm) and around the onset of
550–650 nm. This result manifests the role of phenylene
(or fluorophenylene) on harvesting solar irradiation via
broadened spectral coverage and higher absorptivity. While
VOC for A2-H is even slightly better than that of A1, VOC of
0.91V in A2-F is significantly higher than that of A1 and A2-H
by 13.8% and 9.6%, respectively. The combination of JSC and
moment, i.e. the absorption cross section. Details of the
synthetic procedures as well as selected spectral data such as
mass, NMR and elemental analysis data are provided in the
supporting information.w
The solid-state DSSCs are prepared using a modification of
literature procedures.10,17,18 Detailed fabrication of solid-state
devices and measurement of light-to-electricity conversion
efficiency values are summarized in the supporting informa-
tion.w Fig. 1 shows the absorption spectra of all dyes in a 1 : 1
solution of acetonitrile/tert-butyl alcohol, while the corres-
ponding absorption spectra on mesoporous TiO2 thin film are
displayed in the inset. The A1 sensitizer shows an absorption
maximum at 477 nm, with an unusually high extinction
coefficient e B 51 660 ꢃ 120 Mꢁ1 cmꢁ1, which is ascribed to
the pp* charge transfer transition (CT) originating from the
triphenylamine entity and part of cyclopentadithiophene to
the electron accepting cyanoacrylic acid. Supplementary
support of this viewpoint is provided in the computational
approach (see ESIw). On the other hand, A2-H and A2-F
exhibit a slightly higher energy absorption band maximized at
B450 nm and B460 nm, but with similar extinction coefficient
of 52 560 ꢃ 130 Mꢁ1 cmꢁ1 and 53 740 ꢃ 130 Mꢁ1 cmꢁ1
,
respectively.
Comparing A1, the blue shift of peak wavelength for A2-H
and A2-F seems against the supposed reduction of energy gap
upon phenyl p-elongation. This converse effect is especially
significant for A2-H, being blue shifted by B25 nm versus that
of A-1. The result may be rationalized by the non-coplanar
cyclopentadithiophene versus phenylene (or thiophene-phenylene)
configuration in A2-H (A2-F), resulting in a reduction of
p-conjugation in solution. For A2-F, such an effect is more
or less compensated by the addition of an electron withdrawing
fluoro substituent, giving a similar peak wavelength to that
of A1. Upon being deposited on the TiO2 surface, however,
A2-H shows an appreciable red-shift compared to that in
solution (see Fig. 1 and inset), indicating its preference toward
more planarity due to the space confinement in the solid film.
Owing to their remarkably high absorption extinction
coefficient, in view of practical application, we thus bypassed
liquid-electrolyte DSSCs and made attempts to prepare the
solid-state DSSCs, which were fabricated employing the title
Fig. 3 IPCE action spectra for the A1, A2-H and A2-F sensitized
solid-state DSSCs.
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1 VOC improves power conversion efficiency for A2-F by 34.2%
and 31.7% with respect to that of A1 and A2-H. Fluorine
substitution obviously provides a significant contribution for
promoting the cell performance. It is thus crucial to mull
5 possible mechanisms over such a ‘‘fluorine-substitution’’
effect. Firstly, the fluorine substituents are known to lower
both the HOMO and LUMO levels.19 Moreover, introduction
of the fluorophenylene group may increase the spatial separa-
tion between the TiO2 photoanode and the triphenylamine
10 unit of the sensitizer (cf. A1, A2-H, and A2-F). The net result
is to improve the electron–hole separation as well as reducing
spiro-MeOTAD (0.81 V vs. NHE), a theoretical maximum of
OC 1.2–1.3 V could be reached for these solid-state DSSCs.12
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Thus, via the ingenious design of fluorine substitution, further
increase of VOC should be of great importance. This may open
a new chapter for fluorine chemistry in the field of solar energy
relevant research.
Notes and references
1 D. M. Chapin, C. S. Fuller and G. L. Pearson, J. Appl. Phys., 1954,
25, 676.
2 Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and
L. Han, Jpn. J. Appl. Phys., 2006, 45, L638.
3 M. Gratzel, J. Photochem. Photobiol., A, 2004, 164, 3.
¨
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15
20
25
30
35
40
45
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55
the dark current, giving higher VOC
.
In yet another approach, the significantly different VOC
between A2-H and A2-F, in combination with high electro-
15 negativity on the fluorine atom, leads us to wonder whether
there exists any intimate F. . .H bonding between the fluorine
substituent in A2-F and proton absorbed on the TiO2 surface.
Supported by various spectroscopic methods such as IR and
Raman spectroscopy,20–22 it has been well established that the
20 carboxylic group, upon anchoring onto TiO2, still holds
proton in proximity. The positive charge of proton then
4 (a) F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing,
R. Humphry-Baker, P. Wang, S. M. Zakeerudin and M. Gratzel,
¨
J. Am. Chem. Soc., 2008, 130, 10720; (b) B.-S. Chen, K. Chen,
Y.-H. Hong, W.-H. Liu, T.-H. Li, C.-H. Lai, P.-T. Chou, Y. Chi
and G.-H. Lee, Chem. Commun., 2009, 5844; (c) K.-L. Wu,
H.-C. Hsu, K. Chen, Y. Chi, M.-W. Chung, W.-H. Liu and
P.-T. Chou, Chem. Commun., 2010, 46, DOI: 10.1039/c002220a.
5 (a) G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu and
P. Wang, Chem. Commun., 2009, 2198; (b) Y. Ooyama and
Y. Harima, Eur. J. Org. Chem., 2009, 2903.
6 (a) H. J. Snaith, A. J. Moule, C. Klein, K. Meerholz, R. H. Friend
and M. Gratzel, Nano Lett., 2007, 7, 3372; (b) J.-H. Yum, P. Chen,
decreases the conduction band of TiO2, resulting in the
23,24
¨
M. Gratzel and M. K. Nazeruddin, ChemSusChem, 2008, 1, 699.
7 B. C. O’Regan and F. Lenzmann, J. Phys. Chem. B, 2004, 108,
4342.
8 R. Zhu, C.-Y. Jiang, B. Liu and S. Ramakrishna, Adv. Mater.,
2009, 21, 994.
9 (a) J. Wu, S. Hao, Z. Lan, J. Lin, M. Huang, Y. Huang, P. Li,
S. Yin and T. Sato, J. Am. Chem. Soc., 2008, 130, 11568;
(b) M. Wang, S.-J. Moon, M. Xu, K. Chittibabu, P. Wang,
N.-L. Cevey-Ha, R. Humphry-Baker, S. M. Zakeeruddin and
M. Graetzel, Small, 2010, 6, 319.
reduction of VOC
.
We thus carried out the computation
simulation (see ESI for detailw) in an attempt to verify this
25 proposal. As a result, the geometry optimization of A2-F
anchoring on the TiO2 nanocrystallines that consist of 38 Ti
atoms and stoichiometry ratio of oxygen atom is shown in
Fig. S3 (see ESIw). Unfortunately, as indicated by wide separa-
tion between F atom and H+ ion of 44.5 A, the proposed
30 cooperative hydrogen bonding can thus be eliminated.
Alternatively, on the one hand, the electronegative F atom
and its small polarizability tends to avert the hole transport
molecules possessing high HOMO energy, i.e. the low electron
affinity. The net effect may consequently suppress charge
35 recombination between spiro-MeOTAD and TiO2. Moreover,
during the fabrication of solid-state DSSCs, addition of
lithium salt into the blend of hole conductor is inevitable.10
It is thus plausible that the ortho-fluoro substituent in A2-F
attracts this lithium ion to a position adjacent to the TiO2
40 surface. Accordingly, an increase of the electron diffusion
length in TiO2 is expected, resulting in better electron collec-
tion. The combination of these two factors should account for
the increase of VOC as well as the overall cell performance.
In conclusion, we thus report a series of new organic dyes
45 with remarkably high absorptivity suited for the fabrication of
all solid-state DSSCs. Among these dyes, A2-F possessing
adjacent cyanoacrylic acid and fluorophenylene renders the
conversion efficiency as high as 4.86%. Obviously, the fluoro-
substitution serves as the key factor for improving the device
50 performance, the role of which is to avert spiro-MeOTAD and
instead to attract Li+ toward the TiO2 surface, resulting in a
significant increase of e.g. VOC and hence the superior perfor-
mance. Determined by the gap between the quasi Fermi level
(ꢁ0.4/ꢁ0.5 V vs. normal hydrogen electrode, NHE) of
55 TiO2 under illumination and the first oxidation potential of
10 L. Schmidt-Mende, U. Bach, R. Humphry-Baker, T. Horiuchi,
H. Miura, S. Ito, S. Uchida and M. Gratzel, Adv. Mater., 2005, 17,
¨
813.
11 J.-H. Yum, D. P. Hagberg, S.-J. Moon, K. M. Karlsson,
T. Marinado, L. Sun, A. Hagfeldt, Md. K. Nazeeruddin and
M. Gratzel, Angew. Chem., Int. Ed., 2009, 48, 1576.
¨
12 P. Chen, J. H. Yum, F. D. Angelis, E. Mosconi, S. Fantacci,
S.-J. Moon, R. Humphry-Baker, J. Ko, Md. K. Nazeeruddin and
M. Gratzel, Nano Lett., 2009, 9, 2487.
¨
13 J. Kruger, R. Plass, L. Cevey, M. Piccirelli, M. Gratzel and
¨
U. Bach, Appl. Phys. Lett., 2001, 79, 2085.
14 Z. Ning and H. Tian, Chem. Commun., 2009, 5483.
15 (a) P. Coppo, D. C. Cupertino, S. G. Yeates and M. L. Turner,
J. Mater. Chem., 2002, 12, 2597; (b) P. Yan, A. Xie, M. Wei and
L. M. Loew, J. Org. Chem., 2008, 73, 6587; (c) F. Babudri,
G. M. Farinola, F. Naso and R. Ragni, Chem. Commun., 2007,
1003.
16 B. McMahon, S. Mitchell, H. R. Brady and C. Godson, Trends
Pharmacol. Sci., 2001, 22, 391.
17 L. Kavan and M. Gratzel, Electrochim. Acta, 1995, 40, 643.
18 U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel,
J. Salbeck, H. Spreitzer and M. Gratzel, Nature, 1998, 395, 583.
19 F. Babudri, G. M. Farinola, F. Naso and R. Ragni, Chem.
Commun., 2007, 1003.
20 P. Qu and G. J. Meyer, Langmuir, 2001, 17, 6720.
21 L. Ojamae, C. Aulin, H. Pedersen and P.-O. Kall, J. Colloid
¨
Interface Sci., 2006, 296, 71.
¨
22 J. R. S. Brownson, M. I. Tejedor-Tejedo and M. A. Anderson,
J. Phys. Chem. B, 2006, 110, 12494.
23 Z.-S. Wang and H. Sugihara, Langmuir, 2006, 22, 9718.
24 Md. K. Nazeeruddin, R. Humphry-Baker, P. Liska and
M. Gratzel, J. Phys. Chem. B, 2003, 107, 8981.
¨
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5258 | Chem. Commun., 2010, 46, 5256–5258