Nitro group as a new anchoring group for organic dyes in dye-sensitized solar cells

Article abstract of DOI:10.1039/c2cc31516e

An organic dye JY1 bearing a nitro group was designed, synthesized and applied in DSCs. An unusual colour change was observed when the voltage applied to the device was reversed which was accompanied by a five-fold increase in the cell efficiency. We propose that applying a bias enabled the attachment of nitro groups to the TiO₂ surface.

Full text of DOI:10.1039/c2cc31516e

View Online / Journal Homepage
ChemComm
Accepted Manuscript
This is an Accepted Manuscript, which has been through the RSC Publishing peer
review process and has been accepted for publication.
Accepted Manuscripts are published online shortly after acceptance, which is prior
to technical editing, formatting and proof reading. This free service from RSC
Publishing allows authors to make their results available to the community, in
citable form, before publication of the edited article. This Accepted Manuscript will
be replaced by the edited and formatted Advance Article as soon as this is available.
Chemical Communications
www.rsc.org/chemcomm
Volume 46
| Number 32 | 28 August 2010 | Pages 5813–5976
To cite this manuscript please use its permanent Digital Object Identifier (DOI®),
which is identical for all formats of publication.
More information about Accepted Manuscripts can be found in the
Information for Authors.
Please note that technical editing may introduce minor changes to the text and/or
graphics contained in the manuscript submitted by the author(s) which may alter
content, and that the standard Terms & Conditions and the ethical guidelines
that apply to the journal are still applicable. In no event shall the RSC be held
responsible for any errors or omissions in these Accepted Manuscript manuscripts or
any consequences arising from the use of any information contained in them.
ISSN 1359-7345
COMMUNICATION
FEATURE ARTICLE
J. Fraser Stoddart et al.
Directed self-assembly of
ring-in-ring complex
Wenbin Lin et al.
a
Hybrid nanomaterials for biomedical
applications
1359-7345(2010)46:32;1-H
www.rsc.org/chemcomm
Registered Charity Number 207890

Page 1 of 4
ChemComm
View Online
Graphical Abstract
Applying a bias enabled the attachment of the nitro group to the TiO2 surface.

ChemComm
Page 2 of 4
Dynamic Article Links ►
Journal Name
View Online
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Nitro Group as A New Anchoring Group for Organic Dyes in Dye-
Sensitized Solar Cells
Jiayan Cong,a Xichuan Yang,*a Jing Liu,a Jinxia Zhao,a Yan Hao,a Yu Wanga and Licheng Sun*ab
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
An organic dye JY1 bearing a nitro group was designed,
synthesized and applied in DSCs. An unusual colour change
was observed when the voltage applied to the device was
reversed which was accompanied by a five-fold increase in
10 the cell efficiency. We propose that applying a bias enabled
the attachment of the nitro group to the TiO2 surface.
Since they were first reported by O’Regan and Grätzel in
19911, low-cost dye-sensitized solar cells (DSCs) have
attracted great attention in both academic communities and in
15 industry. The sensitizer is a crucial component in the device,
which absorbs the light and injects electrons into the
conduction band of the semiconductor. DSCs sensitized by
Ru-complexes have reached more than 11% efficiency.2-5
However, organic dyes could prove more cost effective in
20 commercial devices and an efficiency of 12% has now been
achieved with a ruthenium-free senzitizer6. In many cases,
organic dyes are based on a donor-(π conjugation)-acceptor
(D-π-A) system. Frequently cyanoacetic acid has been used as
the acceptor7 but in our recent work we found that the double
25 bond linking the cyanoacetic acid group to the dye can harm
the photostability of the cells. Moreover, the cyanoacetic
group dominates the redox properties, limiting the ability to
tune the HOMO-LUMO level. Therefore, an alternative
anchoring group is urgently needed for the next generation of
30 stable organic sensitizers.
Fig. 1 The molecular structure of dye JY1.
are provided in Table S1. These data support the suitability of
50 JY1 in DSCs. The highest occupied molecular orbital
(HOMO) level of dye JY1 is located at 0.83 V vs. NHE. This
is more positive than the reducing potential of triiodide/iodide
(0.354 V vs. NHE)8 pair, ensuring regeneration of the
oxidized dyes after electron injection. The oxidation potential
55 of the excited dye, E(JY1*/+), level of JY1 was estimated to
be
(extrapolated from the absorption spectrum of JY1 adsorbed
on the TiO2 film) This value is more negative than the
1.21 V vs. NHE, by subtracting E0-0 from Eox
conduction band edge (CB) of TiO2 (approximately 0.5 V
60 vs. NHE) and so JY1 is considered to have suitable electronic
energy levels for photoinduced electron injection into TiO2.9
The nitro group is
a well-known electron accepting
chromophor, which is synthetically straightforward to
introduce, making it a good candidate in D-π-A sensitizers for
DSCs. However, to date there have been only a few examples
35 of nitro-functionalized dyes for DSC applications,
efficiencies with such dyes have been low. In this article, the
design and synthesis of a new dye, JY1, which contains the 2-
cyano-4-nitrophenyl group as the acceptor is presented. A
possible reason for the poor performance of dyes
40 functionalized with a nitro electron-withdrawing group based
on the characterization data for JY1-sensitized DSCs is
suggested and the potential novel use of the nitro group to
anchor the dye to TiO2 is discussed.
(a)
(b)
(c)
Fig. 2 Photographs of the DSCs with Dye JY1. (a) prior to the efficiency
65 test, (b) during the efficiency test, and (c) after the efficiency stabilized.
An interesting phenomenon was found when measuring the
photoelectric properties of the DSCs fabricated with JY1
under simulated AM 1.5G illumination (100 mW cm 2).
Initially, the efficiencies were comparably low (ca. 0.15%).
70 After a period of device testing, the efficiencies of DSCs
increased approximately five-fold to 0.81% (Jsc = 2.12 mA
cm 2, Voc = 0.59 V, ff = 0.66), while the colour of the TiO2
electrodes changed from red to light yellow. The colour
changing processes of the DSCs sensitized by JY1 were
75 photographed, and shown in Fig. 2. The photocurrent density-
The absorption spectra of JY1 in acetonitrile and adsorbed
45 on the TiO2 film are shown in Fig. S1 in the Supplementary
Information and the photophysical and electrochemical data
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Page 3 of 4
ChemComm
View Online
5
10
Table 1. The optimized structures and electron distribution in HOMO and LUMO levels of JY1.
Dye
Optimized structure
HOMO
LUMO
JY1
carboxylic acid is located at 1707 cm-1, and the symmetric
50 nitro stretching band ((-NO2)s) is located at 1344 cm-1.
2.5
2.0
1.5
1.0
0.5
0.0
JY1-a
JY2-a
(
C=O)as is shifted to 1605 cm-1 when the JY1 is adsorbed on
TiO2, and overlaps with the aromatic stretching band
indicating that binding takes place through the carboxylic
group.10 (-NO2)as and (-NO2)s of JY1 adsorbed on TiO2 are
55 located at 1521 cm-1 and 1346 cm-1 respectively, which are
almost the same as unabsorbed dye. These two peaks are
absent in the FT-IR spectrum of the JY1-sensitized TiO2 film
that had been bleached indicating that the colour change is
mainly caused by a structural rearrangement of nitro group on
60 the surface of TiO2.
0.0
0.2
0.4
Voltage / V
0.6
Fig. 3 Photovoltaic performances of DSCs based on JY1 before (a) and
15 after (b) the colour changed
100
JY1
80
60
40
20
0
2227
voltage (J-V) curves are shown in Fig. 3; the characteristics
are tabulated in Table S2and the incident photo-to-current
conversion efficiency (IPCE) is plotted in Fig. S4.
1521
1707
The colour change was also observed when a reverse bias
20 of 700 mV was applied between the two electrodes of the
DSC. Typically, a bleaching of the sensitizer would cause a
decrease in device efficiency since less light would be
captured by the dye and thus fewer electrons would be
injected into the conduction band of TiO2. Unusually here the
25 DSCs with the lighter colour achieved higher efficiencies than
they did in their initial, unbleached state. To investigate this
1344
1200
2200
2000
1800
1600
1400
)
1000
Wavenumbers (cm-2
100
100
80
60
40
20
0
colour-changed JY1 on TiO
JY1 on TiO
2
2
2237
90
80
70
60
50
1521
1605
1346
1606
electro-induced phenomenon,
a
series of additional
experiments were performed, and the results are shown as
follows.
2200
2000
1800
1600
1400
1200
1000
2200
2000
1800
1600
1400
)
1200
1000
Wavenumbers (cm-2
)
Wavenumbers (cm-2
30
The absorption spectra of transparent TiO2 films sensitized
with JY1 before and after it was assembled in a DSC and
exposed to current-voltage experiments are shown Fig. S2 in
the Supplementary Information. A dramatic decrease in light
absorption between 400 and 500 nm can be observed after the
Fig. 4 The FT-IR spectra of JY1, JY1 adsorbed on TiO2 and JY1 on TiO2
after the colour changed.
65
The molecular orbitals of JY1 were calculated using
density functional theory (DFT) at B3LYP/6-31 G level with
full geometry optimization.11, 12 The optimized structures, and
the electronic distribution of the HOMO and LUMO are
shown in Table 1. The calculations indicate that on adsorption
35 film was tested in a DSC. Initially, we hypothesized that the
bleaching of the dye was a result of a structural change in the
molecule. To investigate the nature of this dye-desorption
from the TiO2 was attempted. More than 48 hours (exposure
to 0.1 M NaOH solution in a mixed solvent (water/ethanol =
40 1:1, v/v)) was required to desorb the bleached dye, which was
24 times longer than that required to desorb the original dye
under the same conditions. To our surprise, the molecular ion
peak in the mass spectra (MS) were the same, suggesting that
the structure of the dyes was not altered and, instead, nature
45 that the dye was coordinated to the TiO2 surface was different.
The FT-IR spectra of free JY1 and JY1 on TiO2 before and
after bleaching was observed are displayed in Fig. 4. For JY1
the asymmetric carbonyl stretching band ((-C=O)as) of the
70 of the dye to the TiO2 via the carboxylate group, the acceptor
is brought close to the TiO2 surface.. The HOMO of JY1 is
localized on the donor unit, and the LUMO is localized on the
benzene ring and nitro group. These results suggest that JY1
should be an efficient sensitizer for DSCs. However, the
75 current-voltage experiments revealed that the efficiency was
initially low, possibly because the nitro group does not
efficiently inject electrons into the conduction band of TiO2.
However, when the colour of the dye changed dye and became
lighter, the efficiencies improved, perhaps as a result of more
80 efficient electron injection through a new interaction between
the nitro group and theTiO2 surface.
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

ChemComm
Page 4 of 4
View Online
Fig. 5 Schematic description of the proposed mechanism for the structure change exhibited by JY1.
(Grant No. LS2010042), the Ministry of Science and Technology
(MOST) (Grant 2001CCA02500), the Swedish Energy Agency,
K&A Wallenberg Foundation, and the State Key Laboratory of
Fine Chemicals (KF0805).
5
On the basis of these results we propose a mechanism for
this process which is illustrated schematically in Fig. 5. It is
likely that there is a weak interaction between the nitro group
and the TiO2 surface when JY1 is adsorbed via the
carboxylate group which brings it in close to the
10 semiconductor surface.13 This could also be generalized from
55 Notes and references
a
the molecular calculation. When a bias was applied to the
State Key Laboratory of Fine Chemicals, DUT–KTH Joint
DSC electrons may be transferred from the TiO2 to the dye
2- 14
Education and Research Centre on Molecular Devices, Dalian
University of Technology (DUT), 2 Linggong Rd, 116024 Dalian,
China. Fax: +86 411 84986250; Tel: +86 411 84986247; E-mail:
60 yangxc@dlut.edu.cn.
molecule, reducing the nitro group to –NO2
.
These
electrons are available for coordination to the TiO2 surface.
15 Since the nitro group would then possess less electron
withdrawing character, the absorbance in the visible region of
the spectrum would be bleached. Nonetheless, reduced nitro
group provides a means of electronic coupling to the TiO2
surface increasing the ease of electron injection from the
20 sensitizer to the conduction band of TiO2. This would lead to
the increase in efficiency of the JY1-sensitized DSC with the
lighter colour. This dual anchoring to the surface of TiO2
strengthens the binding in agreement with increased time
required for desorption bleached dyes from the TiO2 films.
b
School of Chemical Science and Engineering, Department of
Chemistry, KTH Royal Institute of Technology, Teknikringen 30,
10044 Stockholm, Sweden. Fax: +46 8 791 2333; Tel: +46 8 790
8127; E-mail: lichengs@kth.se.
65 † Electronic Supplementary Information (ESI) available: [Details of the
synthesis and characterization of TMAS, procedures of fabricating the
DSCs, complete assignment and discussion of CV, and other detail data
of DSCs mentioned in the paper.]. See DOI: 10.1039/b000000x/
70
75
80
85
1
2
B. O'Regan and M. Grätzel, Nature, 1991, 353, 737-740.
M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G.
Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, Journal of the
American Chemical Society, 2005, 127, 16835-16847.
25
In conclusion, a dye functionalized with an electron
withdrawing nitro group was designed and synthesized. A
new phenomenon was observed where by higher efficiencies
of DSCs were obtained by applying a voltage in the reverse
direction. The results of our investigation indicate that the
3
4
Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and L. Han,
Japanese Journal of Applied Physics, 2006, 45, L638.
F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-
Baker, P. Wang, S. M. Zakeeruddin and M. Grätzel, Journal of the
American Chemical Society, 2008, 130, 10720-10728.
30 nitro group attaches to the TiO2 surface, causing a bleach of
the dye absorption but increasing the ability of the dye to
inject the electron into the conduction band of TiO2. This
caused an increase in the efficiency of DSCs with JY1.
Evidence for stronger binding between the dye molecules and
35 TiO2 particles was provided by the desorption experiments.
These results demonstrate that the possibility of using a nitro
substituent as anchoring group. We therefore anticipate a new
generation of highly efficient nitro-functionalized organic
dyes will be inspired by these results. Further structural
40 optimization, such as the introduction of stronger electron-
donating groups and auxiliary electron-withdrawing groups, ,
will provide a promising series of organic sensitizers where
the traditional used cyanoacetic acid anchoring and electron
acceptor unit is replaced.
5
6
C.-Y. Chen, M. Wang, J.-Y. Li, N. Pootrakulchote, L. Alibabaei, C.-
h. Ngoc-le, J.-D. Decoppet, J.-H. Tsai, C. Grätzel, C.-G. Wu, S. M.
Zakeeruddin and M. Grätzel, ACS Nano, 2009, 3, 3103-3109.
A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K.
Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M.
Grätzel, Science, 2011, 334, 629-634.
7
8
9
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chemical Reviews, 2010, 110, 6595-6663.
G. Boschloo, E. A. Gibson and A. Hagfeldt, The Journal of Physical
Chemistry Letters, 2011, 2, 3016-3020.
A. Hagfeldt and M. Grätzel, Chemical Reviews, 1995, 95, 49-68.
90 10 H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt and L. Sun, The
Journal of Physical Chemistry C, 2008, 112, 11023-11033.
11 A. Becke, The Journal of Chemical Physics, 1993, 98, 5648-5652.
12 R. Ditchfield, W. J. Hehre and J. A. Pople, The Journal of Chemical
Physics, 1971, 54, 724-728.
95 13 P. Hirva and M. Haukka, Langmuir, 2010, 26, 17075-17081.
14 P. Mořkovská, M. Hromadová, L. Pospíšil and S. Giannarelli,
Langmuir, 2006, 22, 1896-1902.
45 We would like to thank Dr. Elizabeth Gibson from University of
Nottingham for help in content and statement. We gratefully
acknowledge the financial support of this work from China
Natural Science Foundation (Grant 21076039), the National
Basic Research Program of China (Grant No. 2009CB220009),
50 the Program for Innovative Research Team of Liaoning Province
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3