A supramolecular approach to lithium ion solvation at nanostructured dye sensitised inorganic/organic heterojunctions

Article abstract of DOI:10.1039/b306604e

A novel arylamine based hole transporting material (HTM) with tetraethylene glycol (TEG) side groups is reported. Lithium ions solubilised by the TEG groups are employed to modulate interfacial electron transfer reactions at a dye sensitised TiO₂

Full text of DOI:10.1039/b306604e

A supramolecular approach to lithium ion solvation at nanostructured
dye sensitised inorganic/organic heterojunctions†
Taiho Park,a Saif A. Haque,b Robert J. Potter,c Andrew B. Holmes*a and James R. Durrant*b
a Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Lensfield
Road, Cambridge, UK CB2 1EW. E-mail: abh1@cam.ac.uk; Fax: ++44 (0) 1223 334866; Tel: ++44 (0)
1223 334370
b Centre for Electronic Materials and Devices, Department of Chemistry, Imperial College of Science
Technology and Medicine, London, UK SW7 2AZ. E-mail: j.durrant@imperial.ac.uk; Fax: ++44 (0) 207
5945801; Tel: ++44 (0) 207 5945321
c
Innovations Group, Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, UK
RG4 9NH
Received (in Cambridge, UK) 10th June 2003, Accepted 30th September 2003
First published as an Advance Article on the web 22nd October 2003
A novel arylamine based hole transporting material (HTM)
with tetraethylene glycol (TEG) side groups is reported.
Lithium ions solubilised by the TEG groups are employed to
modulate interfacial electron transfer reactions at a dye
sensitised TiO2/HTM interface.
ions and control charge transfer at the dye sensitised nano-
crystalline TiO2/organic semiconductor heterojunction.
The chemical structure of the novel hole transporting
materials is shown in Scheme 1. The HTMs are based on
arylamine TPD structures (TDP = N,NA-diphenyl-N,NA-(m-
tolyl)-benzidine) and comprise tetraethylene glycol groups on
the 9-position of a fluorene unit (9 in Scheme 1) to solvate
lithium ions in the HTM domain. The novel compound 9 and its
analogue 8 having octyl side chains for comparison were
synthesized by employing a Pd-catalyzed C–N coupling
reaction.8
The dioctyl substituted fluorene 5 was synthesised by
substitution reactions of the fluorene 1 carbanion and octyl
bromides with 2 equivalents of n-BuLi, followed by bromina-
tion in the presence of bromine, employing iodine as a catalyst,
giving an 87% overall yield.9a Similar reactions were carried
out to synthesise the dibromofluorene bearing tetraethylene
glycol groups 6. In this case, two repeated substitution
reactions9b of 1 and the tosylated alcohol 4 gave the fluorene 3
with tetraethylene glycol groups in high yields (89%). The same
bromination as above gave the dibromo fluorene 6 with
tetraethylene glycol groups in 94% yield. Details of chemical
analyses of 8 and 9 are described in the ESI.†
Methoxy substituents have been introduced onto the TPD
structure to provide a sufficient driving force for a high hole
transfer from the Ru(L)2(NCS)2 dye cation to the highest
occupied molecular orbitals of the HTM. Oxidation mid-point
potentials for 8 and 9 were determined from cyclic voltammetry
(CV) in CH2Cl2 solution to be +0.42 and +0.41 V vs. Ag+/AgCl,
respectively, indicating the substitution of the octyl side groups
of the fluorene unit with TEG groups had negligible effect on
the HTM oxidation potential.
Dye sensitized nanocrystalline TiO2 films were prepared as
previously described.10 The dye–TiO2 film pores were filled
with the HTM by spin coating from a 0.17 M, 1 : 9 acetonitrile
: chlorobenzene solution. Variation in lithium ion concentration
in the HTM layers was achieved by adding an appropriate
amount of lithium triflamide in the spin-coating solution with
and without the co-addition of 0.11 M tBP.
Nanocomposites of inorganic and organic semiconductors are
currently receiving extensive interest for both OLED and
photovoltaic device applications.1–3 The efficient operation of
such devices ultimately depends on the ability to control the
charge transfer at the device heterojunction. Recently several
studies have focused on the utilization of mobile lithium ions in
active device layers to control interfacial charge transfer. Such
an approach has been successfully employed to control charge
transfer at metal/organic and inorganic/organic semiconductor
interfaces for light emitting electrochemical cells and dye
sensitised nanocrystalline semiconductor solar cells respec-
tively.4,5,6 However the use of this approach has been limited to
date since ionic salts are not soluble in most organic
semiconductors, and has therefore required the addition of co-
solvents such as polyethylene oxide or tert-butylpyridine
(tBP).5,6 Herein we report an alternative approach based on hole
transporting oligomers comprising ion solvating and transport-
ing tetraethylene glycol (TEG) side groups. Such materials have
recently been employed to control charge transfer at a metal/
organic semiconductor interface for light emitting electro-
chemical cells.7 In this paper we demonstrate that such type
materials can be used to control charge transfer at a nano-
structured dye sensitised inorganic/organic semiconductor het-
erojunction. †
A configuration of particular interest is the dye sensitised
nanocrystalline solar cell (DSSC). A typical DSSC comprises a
dye sensitised nanocrystalline mesoporous TiO2 film inter-
penetrated by an organic hole transporting material (HTM).
Visible light is absorbed by the sensitizer dye RuL2(NCS)2
(chemical name: cis-isothiocyanato)bis(2,2A-bipyridyl-4,4A-di-
carboxylato)ruthenium(II). Electron injection from the excited-
state of the dye into the conduction band of TiO2 is followed by
subsequent hole-injection from the photo-oxidised dye to the
HOMO of the HTM, regenerating the dye’s original state. This
double-charge injection process generates mobile electrons and
holes in the TiO2 and organic semiconductor phases re-
spectively, which are subsequently collected at the device
contacts. The efficient operation of the device requires both
high yields of charge separation and the minimization of charge
recombination between the photo-generated electrons in the
TiO2 and holes in the HTM. In this paper we demonstrate that
novel HTMs bearing TEG chains can be used to solvate lithium
Transient absorption spectroscopy was employed as pre-
viously10 to study the influence of lithium ions upon the yield of
Scheme 1 Reagents and conditions: a) i) n-BuLi, THF, 265 °C, 1 h; ii)
C8H17Br or Ts(CH2CH2O)4CH3 4, THF, 265 °C, 30 min; b) Br2, I2,
CH2Cl2, rt, overnight; c) di(p-methoxyphenyl)amine 7, Pd(OAc)2, P(tBu)3,
toluene, 90 °C, overnight.
† Electronic Supplementary Information (ESI) available: experimental
details and absorption spectra. See http://www.rsc.org/suppdata/cc/b3/
b306604e/
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CHEM. COMMUN., 2003, 2878–2879
This journal is © The Royal Society of Chemistry 2003

hole transfer from the dye cation to the HTM (reaction 1) and
the recombination dynamics between photo-injected electrons
and HTM cations (reaction 2).
Dye+ + HTM ? Dye + HTM+
(1)
HTM+ + e2TiO2 ? HTM
(2)
Transient absorption data were collected following optical
excitation of the Ru(L)2(NCS)2 sensitiser dye at 550 nm. In the
absence of HTM, a broad positive absorption band was
observed centred at 800 nm and a negative feature at 540 nm,
spectral features assigned as previously to the formation of the
Ru(L)2(NCS)2 cation following photoinduced electron injec-
tion.10 In the presence of the HTM a reduction in the photo-
induced dye cation signal at 800 nm is observed, along with the
appearance of a positive feature with an absorption maximum at
~ 520 nm, a spectral feature we have previously assigned to the
formation of the HTM radical cation.10
Fig. 1 Transient absorption data collected for TiO2/RuL2(NCS)2/HTM 9
films as a function of lithium ion concentration added to the HTM
deposition solution. Data collected at a probe wavelength of 520 nm,
monitoring the appearance and decay of the HTM cation absorption band,
assigned to the charge recombination reaction, eqn. (2).
The addition of Li ions to the HTM spiro-OMeTAD in the
t
The yield of the hole transfer (reaction 1) was determined as
a function of lithium ions in the presence and absence of t-butyl
pyridine in the HTM domain for both HTMs 8 and 9, as
summarized in Table 1. In the absence of Li+, the yield of hole
transfer from the dye cation to HTMs 8 and 9 was determined as
79 and 88% respectively, consistent with the large free energy
difference between the dye and HTM oxidation potentials, as
we have discussed previously.10 For HTM 8 in the absence of
tBP, it is apparent that an increase in Li+ from 0 to 0.04 M results
in a reduction in the hole transfer yield from 88 to 74%.
Increasing the lithium ion concentration beyond 0.04 M results
in a rapid loss of hole transfer yield from the dye cation to the
HTM 8 and an increase in the optical light scattering of the film.
This observation can most reasonably be rationalized in terms
of incomplete wetting of the dye sensitised TiO2 film pores by
HTM due to crystallization of the lithium triflamide salt during
spin-coating. The addition of tBP to the HTM 8 solution reduced
the sensitivity of the transient signal amplitude to lithium ion
concentration, with significant loss of hole transfer yield only
being observed for a Li+ concentration of 0.25 M, consistent
presence of BP has previously been shown to increase the
efficiency of dye sensitized photovoltaic cells.6 This increase
was attributed to a retardation of the interfacial recombination
reaction (reaction 2) due to electrostatic screening of the
photoinjected electron from the HTM cation. We consider
therefore the influence of the lithium ions upon the interfacial
recombination dynamics employing HTM 9 in the absence of
tBP, as shown in Fig. 1. It is apparent that the addition of lithium
ions results in a 5–10 fold retardation of the interfacial
recombination dynamics, consistent with desired ionic screen-
ing of the charge separated species.
In conclusion we have demonstrated that an organic hole
transporting material with TEG side chains can be used to
solvate lithium ions without co-solvents such as tert-butylpyr-
idine. We have employed such materials to control interfacial
charge transfer at a dye sensitized nanocrystalline TiO2/organic
HTM heterojunction. Specifically we have demonstrated that
lithium ion doped TEG-HTMs can be used to control interfacial
charge recombination and simultaneously achieve a high yield
of hole transfer from dye cations to HTM. Such materials
therefore provide a novel approach to achieving ion solvation in
organic HTM’s for a wide range of potential device applica-
tions.
This work was funded by the LINK Foresight Program,
Department of Trade and Industry, UK, and the Engineering
and Physical Sciences Research Council (EPSRC). TP would
like to acknowledge Johnson Matthey Ltd., the British Council
and the Cambridge Commonwealth Trust for financial support.
We would also like to acknowledge the EPSRC Mass
Spectroscopy Centre for mass spectroscopy characterisation.
t
with previous reports that the co-addition of BP increases the
solubility of Li+ ions in the HTM.1 We note however that the
addition of tBP is undesirable for technological applications due
both to its volatility and carcinogenic properties.
In contrast to the data we obtained for HTM 8, we find that
the hole transfer yield for HTM 9 (bearing TEG groups) is not
reduced by Li+ for all ion concentrations studied. Furthermore
no recrystallisation of the lithium triflamide salt was observed.
We thus conclude that HTM 9 solvates lithium ions readily;
resulting in no crystallization of the lithium triflamide salt
during the spin-coating procedure. It is also apparent from the
data in Table 1 that, in the absence of tBP, the addition of Li+ to
the HTM 9 actually results in a small increase in HTM cation
yield (from 79 to 96%). This small increase most probably
arises from faster electron injection dynamics from the
RuL2(NCS)2 excited state into the TiO2 conduction band,
consistent with our previous observations.11
Notes and references
1 U. Bach, D. Lupo, P. Compte, J. E. Moser, F. Weissortel, J. Salbeck, H.
Spreitzer and M. Gratzel, Nature, 1998, 395, 583.
2 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295,
2425.
3 S. Coe, W-K. Woo, M. Bawendi and V. Bulovic, Nature, 2002, 420,
800.
Table 1 HTM cation yields determined by transient laser spectroscopy for
TiO2/RuL2(NCS)2/HTM filmsa
4 T. M. Brown, R. H. Friend, I. S. Millard, D. J. Lacey, J. H. Burroughes
and F. Cacialli, Appl. Phys. Lett., 2001, 79()), 174; J. Kruger, U. Bach
and M. Gratzel, Adv. Mater., 2000, 12(6), 447.
5 Q. Pei, Y. Yang, G. Yu, C. Zhang and A. J. Heeger, Science, 1995, 269,
1086.
Li+
tBP
Yield (%)
[Cell]a
[Li+]/[HTM]bb [tBP]/[Li+]b HTM 9 HTM 8
6 J. Kruger, R. Plass, L. Cevey, M. Piccirelli, M. Gratzel and U. Bach,
Appl. Phys. Lett., 2001, 79(13), 2085.
7 J. Morgado, F. Cacialli, R. H. Friend, B. S. Chuah, H. Rost and A. B.
Holmes, Macromolecules, 2001, 34, 3094.
8 T. Yamamoto, M. Nishiyama and Y. Koie, Tetrahedron Lett., 1998, 39,
2367.
9 (a) M. Ranger and M. Leclerc, Can. J. Chem., 1998, 76, 1571; (b) Q. Pei
and Y. Yang, J. Am. Chem. Soc., 1996, 118, 7416.
10 S. A. Haque, Taiho Park, Andrew B. Holmes and J. R. Durrant,
ChemPhysChem, 2003, 1, 89.
[HTM]
—
—
—
—
—
7.3
2.2
1.1
79
96
96
91
77
78
81
88
74
—
—
73
71
22
[HTM + Li+]
0.04
0.13
0.25
[HTM + Li+ + tBP] 0.04
0.13
0.25
a The yield of hole transfer from the dye cation to the HTM was determined
by comparing the amplitude of the dye cation band at 800 nm in the presence
and absence of HTM, as detailed previously.11
11 Y. Tachibana, S. A. Haque, I. Mercer, J. R. Durrant and D. R. Klug, J.
Phys. Chem. B, 2001, 105, 7424.
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