Oligothiophene bearing 1-hydroxy-1-oxodithieno[2,3-b:3′,2′-d] phosphole as a novel anchoring group for dye-sensitized solar cells

Article abstract of DOI:10.1246/cl.2010.448

An oligothiophene bearing l-hydroxy-l-oxodithieno[2,3-b:3′,2′- d]phosphole (TP) as a novel anchoring group has been synthesized for dye-sensitized solar cells (DSSC). Attenuated total reflectance-Fourier transform infrared and X-ray photoelectron spectroscopy measurements disclosed the bidentate binding of the phosphinic acid unit to a TiO₂ surface. A TPsensitized TiO₂ cell yielded a maximum incident photon-tocurrent efficiency of 66% and a power conversion efficiency of 1.8%, indicating that 1-hydroxy-1-oxodithienophosphole is a potential unit as a new type of anchoring groups for DSSC.

Full text of DOI:10.1246/cl.2010.448

doi:10.1246/cl.2010.448
448
Published on the web March 31, 2010
Oligothiophene Bearing 1-Hydroxy-1-oxodithieno[2,3-b:3¤,2¤-d]phosphole
as a Novel Anchoring Group for Dye-sensitized Solar Cells
Aiko Kira,1 Yuki Shibano,1 Soonchul Kang,1 Hironobu Hayashi,1 Tomokazu Umeyama,1
Yoshihiro Matano,1 and Hiroshi Imahori*1,2,3
1Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510
2Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510
3Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103
(Received February 8, 2010; CL-100134; E-mail: imahori@scl.kyoto-u.ac.jp)
RSnBu3,
Pd(PPh3)4
An oligothiophene bearing 1-hydroxy-1-oxodithieno[2,3-
R
R
HCl
HCl
b:3¤,2¤-d]phosphole (TP) as a novel anchoring group has been
synthesized for dye-sensitized solar cells (DSSC). Attenuated
total reflectance-Fourier transform infrared and X-ray photo-
electron spectroscopy measurements disclosed the bidentate
binding of the phosphinic acid unit to a TiO2 surface. A TP-
sensitized TiO2 cell yielded a maximum incident photon-to-
current efficiency of 66% and a power conversion efficiency of
1.8%, indicating that 1-hydroxy-1-oxodithienophosphole is a
potential unit as a new type of anchoring groups for DSSC.
TP
PP
S
S
S
P
O
NEt2
R =
X
X
S
S
S
3 (from 2)
S
S
R'SnBu3,
Pd(PPh3)4,
CuI
P
O
NEt2
R'
R'
1 (X = H)
2 (X = Br)
S
P
R' =
O
NEt2
4 (from 2)
Scheme 1. Synthesis of TP and PP.
Recently, studies on metal-free or inexpensive metal-based
organic chromophores for DSSC have grown rapidly.1 In this
regard it is also essential to improve the binding ability of
anchoring groups to a TiO2 electrode as well as electron-transfer
properties through them for highly efficient DSSC.2 Never-
theless, the variations of the anchoring groups are still limited to
carboxylic acid,3 sulfonic acid,4 phosphonic acid,5 and others.6
Herein, we report an oligothiophene bearing 1-hydroxy-1-
oxodithieno[2,3-b:3¤,2¤-d]phosphole (TP) as a novel anchoring
group for DSSC (Figure 1).
8
6
4
ε
The electron density of the LUMO of 1-hydroxy-1-
oxodithienophosphole is extended to the phosphinic acid group
due to the effective ·*-³* orbital interaction in the phosphole
moiety,7 as predicted by DFT calculations (Figure S1).15 In
addition, it is worth noting that phosphole changes its electronic
structure by the introduction of ³-conjugative substituents at the
P-connected cis-1,3-diene function.7 Thus, we can expect the
occurrence of fast electron injection from the dye excited state to
the conduction band (CB) of the TiO2 through the anchoring
group by introducing electron-donating ³-conjugative chromo-
phores (i.e., oligothiophene) into the dithienophosphole core,
resulting in the efficient photocurrent generation.
2
0
300
400
500
600
700
800
Wavelength/ nm
Figure 2. Absorption spectra of TP (solid line) and PP
(dashed line) and normalized emission spectra of TP (solid line
with circles) and PP (dashed line with circles) in THF.
-
ex = 403 nm for TP, 370 nm for PP.
The synthetic route to TP is shown in Scheme 1. Dithi-
enophosphole 1 was synthesized according to methods previ-
ously reported by Cristau et al.8 Then, dithienophosphole 1 was
treated with N-bromosuccinimide (NBS) to yield dibromodi-
thienophosphole 2. Terthiophene-linked dithienophosphole 3
was prepared by the Stille coupling of 2 with 5-tri(n-butyl)-
stannyl-2,2¤:5¤,2¤¤-terthiophene. Finally, hydrolysis of 3 afforded
TP. Phenylethynyl-linked 1-hydroxy-1-oxodithienophosphole
(PP) was also prepared by the Sonogashira coupling to compare
the light-harvesting, optical, and photovoltaic properties with
those of TP.
S
S
S
S
S
S
Figure 2 displays the UV-vis absorption and fluorescence
spectra of TP and PP in THF. The absorption band of TP
appears at 403 nm with a molar extinction coefficient (¾) of
4.80 © 104 M¹1 cm¹1, whereas the fluorescence of TP is
centered at 522 nm. The absorption maximum of TP is red-
shifted by 32 nm compared with that of PP. The fluorescence
S
S
S
S
P
P
O
OH
O
OH
TP
PP
Figure 1. Molecular structures of the TP and PP sensitizers.
Chem. Lett. 2010, 39, 448-450
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

449
quantum yield (Φf) of TP (10%) is larger than that of PP. These
results imply that the introduction of the electron-donating ³-
extended chromophores into the dithienophosphole core im-
proves the light-harvesting and fluorescence properties in the
visible region considerably. In addition, the absorption and
fluorescence maxima of TP are significantly red-shifted relative
to those of 1 (-abs = 345 nm, -em = 422 nm) and terthiophene
(-abs = 352 nm, -em = 428 nm), supporting effective ³-exten-
sion in TP (Figure S2).15 The first one-electron oxidation
potentials (EOX) of TP (1.25 V vs. NHE) and PP (1.52 V
vs. NHE) were determined by differential pulse voltammetry
8
6
4
2
0
(Table S1).15 From the EOX and the excitation energy (E0-0
)
−2
values, the excited-state oxidation potentials (EOX*) of TP
and PP were estimated to be ¹1.48 and ¹1.62 V vs. NHE
(Table S1).15 Taking into account the energy levels of the CB of
0
0.1
0.2
0.3
0.4
0.5
Voltage/ V
¹
¹
TiO2 (¹0.5 V vs. NHE)9 and I /I3 couple (0.5 V vs. NHE),9
electron injection from the dye excited singlet state to the CB of
TiO2 and charge shift from I to the resulting dye radical cation
Figure 3. Current-voltage characteristics of the TiO2/TP
(solid line) and TiO2/PP (dashed line) electrodes under an
irradiation of 100 mW cm AM 1.5G sunlight and TiO2/TP
¹
¹2
(1.25 V vs. NHE) are thermodynamically feasible both in the TP
and PP cells. DFT calculations were employed to gain insight
into the equilibrium geometry and electronic structures for the
HOMO and LUMO of TP and the SOMO of TP radical cation
(Figure S3).15 The optimized geometry has no negative fre-
quencies, implying that optimized geometries are in the global
energy minima. The LUMO is localized mainly over the
dithienophosphole including the anchoring group, whereas the
HOMO of TP and the SOMO of TP radical cation are
delocalized except for the anchoring group. For TP, 70% of the
electron density in the HOMO is located on the two terthiophene
units, and 10% of the electron density in the LUMO is
distributed on the phosphinic acid group. For PP, 37% of the
HOMO is localized on the two phenylacetylene units, and 14%
of the LUMO lies on the phosphinic acid group. These results
indicate that the HOMO-LUMO excitation of TP shifts the
electron distribution from the chromophores to phosphinic acid
group more obviously than PP, thus allowing an efficient
photoinduced electron transfer from TP to the TiO2 electrode
and slow charge recombination from the electron in the CB to
the dye radical cation due to the remoteness.10
TP and PP were sensitized onto mesoporous P-25 based
TiO2 electrodes with chenodeoxycholic acid (CDCA) (denoted
as TiO2/TP and TiO2/PP, respectively). Little shift and broad-
ening of the absorption peak of TP on the TiO2 are noted in
comparison with that in THF (Figure S4),15 showing little
aggregation of TP on the TiO2 electrode. On the other hand, 3
did not bind to a bare TiO2 electrode (Figure S5). Therefore, 1-
hydroxy-1-oxodithienophosphole acts as an anchoring group to
the TiO2 surface through its phosphinic acid unit. To get
information on the binding mode of the molecules adsorbed on
the TiO2 electrode, attenuated total reflectance-Fourier transform
infrared (ATR-FTIR) spectra were measured for powders of TP
and PP as well as TiO2/TP and TiO2/PP (Figure S6).15 ATR-
FTIR spectra of TP and PP reveal the characteristic bands of
¯(P=O) and ¯(P-OH) of the phosphinic acid group at around
1200 and 960 cm¹1, respectively.11 However, these bands
disappear and a peak corresponding to ¯(O-P-O) at around
1040-1060 cm¹1 emerges for the spectra of TiO2/TP and TiO2/
PP.11 These results demonstrate that the two equivalent P-O
bonds are formed through a bidentate coordination of the
phosphinate to the TiO2 surface.11 To further shed light on
(solid line with circles) and TiO2/PP (dashed line with triangles)
electrodes in the dark. Electrolyte: 0.5 M LiI and 0.01 M I2 in
CH3CN.
adsorption state of the dyes on the TiO2 surface, X-ray
photoelectron spectroscopy (XPS) measurements were per-
formed for TiO2/TP and TiO2/PP together with TP and PP
(Figure S7).15 The O1s XPS spectra of TP and PP were curve-
fitted into two chemically different O1s subpeaks (Figures S7a
and S7b and Table S2).15 The peaks arising at around 531 and
533 eV can be assigned to the oxygen atoms of P=O and of
P-OH in the phosphinic acid, respectively.11c,12 The spectra of
TiO2/TP and TiO2/PP also exhibit two different peaks,
respectively (Figures S7c and S7d and Table S2).15 The peaks
at 530.2 eV stem from the oxygen atoms in TiO2.9,12 The peaks
at 531.2-531.6 eV can be attributed to the oxygen atoms of
P-O-Ti bonds.12 It is noteworthy that the peaks derived from the
O atoms of P-OH at around 533 eV disappear in the spectra of
TiO2/TP and TiO2/PP. Consequently, we can conclude that the
two oxygens in the phosphinate bind to the TiO2 surface with the
same binding energy through a bidentate coordination, which is
in good agreement with the results of the ATR-FTIR measure-
ments (vide supra). Given the surface area of P-25 (54 m2 g¹1),9
the packing densities (Γ) of TP and PP on the actual TiO2
¹10
¹10
surface area are determined to be 1.9 © 10
and 2.0 © 10
mol cm¹2. Assuming that TP and PP molecules are densely
packed onto the TiO2 surface yielding a full monolayer without
CDCA, the calculated Γ values are 2.4 © 10¹10 and 3.2 © 10
¹10
mol cm¹2. The experimental Γ values are considerably lower
than the calculated Γ values, implying that the dye molecules
are isolated from each other by the intercalation of CDCA
molecules, thereby reducing dye aggregation.
Current-voltage characteristics of the TiO2/TP and TiO2/
PP electrodes were evaluated under standard AM 1.5 conditions
(100 mW cm¹2) (Figure 3). An © = 1.8% value of the TP cell
was obtained with a JSC of 7.4 mA cm¹2, a VOC of 0.46 V, and a
ff of 0.54, whereas an © = 0.56% value of the PP cell was
obtained with a JSC of 2.3 mA cm¹2, a VOC of 0.44 V, and a ff of
0.54. The large © value of the TP cell relative to the PP cell is
consistent with the difference in the light-harvesting properties
of TP and PP (vide infra).
Chem. Lett. 2010, 39, 448-450
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

450
This work was supported by Grant-in-Aids (No. 21350100
to H.I.) from MEXT, Japan. H.I. also thanks NEDO for financial
support. A.K. is grateful for a JSPS fellowship for young
scientists. Ms. Rika Harui (Thermo Electron Corporation) for
help in ATR-FTIR measurements.
70
60
50
40
30
20
References and Notes
1
a) A. Mishra, M. K. R. Fischer, P. Bäuerle, Angew. Chem., Int. Ed.
2009, 48, 2474. b) Y. Ooyama, Y. Harima, Eur. J. Org. Chem. 2009,
2903. c) H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009,
42, 1809. d) Z. Ning, H. Tian, Chem. Commun. 2009, 5483.
a) W. M. Campbell, A. K. Burrell, D. L. Officer, K. W. Jolley,
Coord. Chem. Rev. 2004, 248, 1363. b) R. Ernstorfer, L. Gundlach,
S. Felber, W. Storck, R. Eichberger, F. Willig, J. Phys. Chem. B
2006, 110, 25383. c) R. Chen, X. Yang, H. Tian, X. Wang, A.
Hagfeldt, L. Sun, Chem. Mater. 2007, 19, 4007. d) J. Wiberg, T.
Marinado, D. P. Hagberg, L. Sun, A. Hagfeldt, B. Albinsson, J.
Phys. Chem. C 2009, 113, 3881.
2
10
0
400
500
600
700
800
Wavelength/ nm
3
a) K. Hara, K. Sayama, H. Arakawa, Y. Ohga, A. Shinpo, S. Suga,
Chem. Commun. 2001, 569. b) T. Kitamura, M. Ikeda, K. Shigaki,
T. Inoue, N. A. Anderson, X. Ai, T. Lian, S. Yanagida, Chem.
Mater. 2004, 16, 1806. c) K. Tanaka, K. Takimiya, T. Otsubo, K.
Kawabuchi, S. Kajihara, Y. Harima, Chem. Lett. 2006, 35, 592. d)
S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee,
W. Lee, J. Park, K. Kim, N.-G. Park, C. Kim, Chem. Commun.
2007, 4887. e) S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka,
P. Liska, P. Comte, P. Péchy, M. Grätzel, Chem. Commun. 2008,
5194. f) G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P.
Wang, Chem. Commun. 2009, 2198.
a) T. Ma, K. Inoue, H. Noma, K. Yao, E. Abe, J. Photochem.
Photobiol., A 2002, 152, 207. b) Y.-S. Chen, C. Li, Z.-H. Zeng,
W.-B. Wang, X.-S. Wang, B.-W. Zhang, J. Mater. Chem. 2005, 15,
1654.
a) F. Odobel, E. Blart, M. Lagrée, M. Villieras, H. Boujtita, N. E.
Murr, S. Caramori, C. A. Bignozzi, J. Mater. Chem. 2003, 13, 502.
b) M. K. Nazeeruddin, R. Humphry-Baker, D. L. Officer, W. M.
Campbell, A. K. Burrell, M. Grätzel, Langmuir 2004, 20, 6514.
a) E. L. Tae, S. H. Lee, J. K. Lee, S. S. Yoo, E. J. Kang, K. B. Yoon,
J. Phys. Chem. B 2005, 109, 22513. b) C. Baik, D. Kim, M.-S.
Kang, S. O. Kang, J. Ko, M. K. Nazeeruddin, M. Grätzel, J.
Photochem. Photobiol., A 2009, 201, 168.
Figure 4. Photocurrent action spectra of the TiO2/TP (solid
line) and TiO2/PP (dashed line) electrodes.
The photocurrent action spectra of the TiO2/TP and TiO2/
PP electrodes are illustrated in Figure 4. The action spectra and
the absorption spectra are virtually similar for the electrodes,
implying the involvement of the dyes for the photocurrent
generation. The TiO2/TP electrode exhibits a maximum IPCE
(incident photon-to-current efficiency) value of 66% at 400 nm.
This IPCE value is comparable to that of N719-sensitized TiO2
cell (72%) under our experimental conditions,13 implying the
occurrence of efficient electron injection from the TP excited
singlet state to the CB of TiO2 due to sufficient strong electronic
coupling between 1-hydroxy-1-oxodithienophosphole and the
3d orbital of TiO2. In contrast, the TiO2/PP electrode reveals
much lower IPCE values in the visible region (43% at 400 nm).
Similar difference is noted for APCE (absorbed photon-to-
current efficiency) values of the TiO2/TP (66%) and TiO2/PP
(44%) electrodes.14 To address the effect of the charge
recombination, we measured the current-voltage characteristics
under dark conditions (Figure 3). The onset of the dark current
for the TP cell appears at a higher potential than that for the PP
cell, indicating that the degree of the charge recombination for
the PP cell is higher than that for the TP cell. The differences in
the APCE values and in the effect of the charge recombination
between TP and PP may be attributed to those in fluorescence
quantum yields as well as in the HOMO-LUMO distribution
(vide supra).
In conclusion, we have successfully synthesized oligothio-
phene-bearing 1-hydroxy-1-oxodithienophosphole as a novel
anchoring group for DSSC for the first time. The introduction
of the oligothiophene moieties into 1-hydroxy-1-oxodithieno-
phosphole was found to improve the light-harvesting property in
the visible region, leading to significant enhancement in the cell
performance relative to a reference device with a weak light-
harvesting unit. We have also disclosed the binding mode of
the novel anchoring group to a TiO2 surface. These results
corroborate the potential utility of 1-hydroxy-1-oxodithieno-
phosphole as a novel anchoring group for DSSC. Further
improvement of the cell performance may be possible by
introducing excellent light-harvesting and electron-donating
chromophores into the 1-hydroxy-1-oxodithienophosphole as
well as improving the device structure.
4
5
6
7
a) F. Mathey, Chem. Rev. 1988, 88, 429. b) L. Nyulászi, Chem. Rev.
2001, 101, 1229. c) T. Baumgartner, R. Réau, Chem. Rev. 2006,
106, 4681; Corrigendum: T. Baumgartner, R. Réau, Chem. Rev.
2007, 107, 303. d) Y. Matano, T. Miyajima, T. Fukushima, H. Kaji,
Y. Kimura, H. Imahori, Chem.®Eur. J. 2008, 14, 8102.
8
9
V. Vicente, A. Fruchier, M. Taillefer, C. Combes-Chamalet, I. J.
Scowen, F. Plénat, H.-J. Cristau, New J. Chem. 2004, 28, 418.
S. Eu, S. Hayashi, T. Umeyama, Y. Matano, Y. Araki, H. Imahori,
J. Phys. Chem. C 2008, 112, 4396.
10 M. Xu, S. Wenger, H. Bala, D. Shi, R. Li, Y. Zhou, S. M.
Zakeeruddin, M. Grätzel, P. Wang, J. Phys. Chem. C 2009, 113,
2966.
11 a) C. N. Rusu, J. T. Yates, Jr., J. Phys. Chem. B 2000, 104, 12292.
b) E. Bae, W. Choi, J. Park, H. S. Shin, S. B. Kim, J. S. Lee, J.
Phys. Chem. B 2004, 108, 14093. c) A. Raman, M. Dubey, I.
Gouzman, E. S. Gawalt, Langmuir 2006, 22, 6469.
12 a) N. Adden, L. J. Gamble, D. G. Castner, A. Hoffmann, G. Gross,
H. Menzel, Langmuir 2006, 22, 8197. b) M. Gnauck, E. Jaehne, T.
Blaettler, S. Tosatti, M. Textor, H.-J. P. Adler, Langmuir 2007, 23,
377. c) G. Zorn, R. Adadi, R. Brener, V. A. Yakovlev, I. Gotman,
E. Y. Gutmanas, C. N. Sukenik, Chem. Mater. 2008, 20, 5368.
13 H. Imahori, S. Hayashi, H. Hayashi, A. Oguro, S. Eu, T. Umeyama,
Y. Matano, J. Phys. Chem. C 2009, 113, 18406.
14 The APCE values are lower limits because we do not take into
account the collection losses (e.g., reflection) in the TiO2 films.
15 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2010, 39, 448-450
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/