Interface engineering for solid-state dye-sensitised nanocrystalline solar cells: The use of an organic redox cascade

Article abstract of DOI:10.1039/b513180d

We demonstrate the formation of a charge transfer cascade at a nanostructured TiO₂/dye/polymer/molecular hole transport multilayer interface. Charge recombination dynamics at this interface are shown to be retarded when the ionisation potential

Full text of DOI:10.1039/b513180d

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Interface engineering for solid-state dye-sensitised nanocrystalline solar
cells: the use of an organic redox cascade{
Narukuni Hirata,a Jessica E. Kroeze,a Taiho Park,b David Jones,c Saif A. Haque,a Andrew B. Holmes*ac and
James R. Durrant*a
Received (in Cambridge, UK) 16th September 2005, Accepted 8th November 2005
First published as an Advance Article on the web 14th December 2005
DOI: 10.1039/b513180d
of up to 4%.13 This efficiency, whilst promising, remains however
We demonstrate the formation of a charge transfer cascade at a
significantly below those reported for analogous devices employing
iodide/iodine-based redox electrolytes. A key factor limiting the
performance of devices of spiro-OMeTAD DSSCs is their fast
interfacial charge recombination dynamics relative to liquid
electrolyte devices.
nanostructured TiO2/dye/polymer/molecular hole transport
multilayer interface. Charge recombination dynamics at this
interface are shown to be retarded when the ionisation potential
of the polymer layer exceeds that of the molecular hole
transport layer.
Fig. 2 illustrates this point, showing typical recombination
dynamics for these two device types employing the sensitiser dye
RuL2(NCS)2 (cis-isothiocyanato bis(2,29-bipyridyl-4,49-dicarboxy-
lato)-ruthenium(II)). It is apparent that the charge recombination
dynamics for the spiro-OMeTAD device are two orders of
magnitude faster then those observed for the liquid electrolyte
device, consistent with previous observations. These faster
recombination dynamics limit the diffusion length of photogener-
ated charges within the device, and therefore limit the maximum
device thickness (and therefore light absorption) compatible with
efficient charge collection.
Nanocomposites of inorganic and organic semiconductors are
currently receiving extensive interest for both organic light emitting
diodes 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. We, and
others, have recently been focusing upon a range of ‘interface
engineering’ strategies to control electron transfer dynamics at
nanostructured inorganic/organic interfaces.4–8 Such dynamics are
of particular interest for the development of solid-state dye-
sensitised solar cells (DSSCs).9–12 In this study we demonstrate a
novel strategy to retard interfacial charge recombination in such
devices, employing an organic redox cascade to increase the spatial
separation of photogenerated electrons and holes, as illustrated in
Fig. 1.
Fig. 2 also shows the corresponding charge recombination
dynamics of injected TiO2 electrons with dye cations, observed in
the absence of any organic hole conductor or redox couple. It is
striking that charge recombination dynamics to dye cations and
spiro-OMeTAD cations are indistinguishable. We have previously
shown that recombination dynamics to dye cations are dependent
upon both the dynamics of electron transport in the TiO2 film and
Solid-state DSSCs employing the organic hole transporter spiro-
OMeTAD
(2,29–7,79-tetrakis(N,N-di-p-methoxyphenyl-amine)
9,99-spirobifluorene) have recently been reported with efficiencies
Fig. 1 Illustration of the organic redox cascade strategy addressed in this
study to retard interfacial charge recombination.
aCentre for Electronic Materials and Devices, Department of Chemistry,
Imperial College London, Exhibition Road, London, UK SW7 2AZ.
E-mail: j.durrant@imperial.ac.uk; Fax: +44 (0) 207 5945801;
Tel: +44 (0) 207 5945321
Fig. 2 Transient absorption data monitoring charge recombination
dynamics of (a) a RuL2(NCS)2-sensitised TiO2 film, (b) the same film
covered in redox electrolyte and (c) the same film following addition of
spiro-OMeTAD. Data collected monitoring TiO2 electron absorption at
1000 nm following optical excitation at 530 nm, with 25 mW cm22
continuous white light bias. The spiro-OMeTAD was spin coated from
chlorobenzene with no additives. The redox electrolyte comprised 0.1 M
lithium iodide, 0.8 M tetra butyl ammonium iodide, 0.5 M tert-
butylpyridine and 0.05 M iodine in acetonitrile.
bMelville Laboratory for Polymer Synthesis, Department of Chemistry,
University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW
cThe School of Chemistry, University of Melbourne, Parkville Vic 3010,
Australia. E-mail: aholmes@unimelb.edu.au; Fax: +61 3 8344 2384;
Tel: +61 3 8344 2344
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures and analytical data for P1–P4. See DOI: 10.1039/b513180d
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Chem. Commun., 2006, 535–537 | 535

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the spatial separation of the cation from the TiO2 surface.14,15 The
observation of indistinguishable recombination dynamics to both
the dye and spiro-OMeTAD cations indicates that hole transfer
from the dye to spiro-OMeTAD does not result in a significant
movement of the cation away from the TiO2 surface. In other
words, despite the spiro-OMeTAD largely filling the pores of the
mesoporous film, in these films the photogenerated spiro-
OMeTAD cations remain localised close to the dye-sensitised
TiO2 surface.
The localisation of the spiro-OMeTAD cations (‘holes’) on the
TiO2 surface can be readily understood in terms of local electric
fields at TiO2/dye/spiro-OMeTAD interface. Such electric fields
may derive both from interface dipoles and from coulombic
interactions of the photogenerated charges.16,17 One strategy to
overcome this localisation of the spiro-OMeTAD cations is the
introduction of mobile ions to the organic phase, such as lithium
triflamide salts, to screen these electrostatic interactions. The
addition of such salts results in a significant retardation of
recombination dynamics, consistent with an increased spatial
separation of the spiro-OMeTAD cations from the TiO2 surface,
and results in an improvement in device efficiencies by up to an
order of magnitude.9 One limitation to this approach is however
the requirement of the addition of volatile pyridine to the hole-
transport layer to solubilise the lithium salt. We have recently
demonstrated an alternative solubilisation strategy based upon the
attachment of tetraethylene glycol (TEG) side groups to the
organic hole transporter to ligate the lithium ions.12,18 We have
furthermore employed this strategy to achieve an organic multi-
layer structure, depositing the dual function ion ligating and hole
transport polymer between the sensitiser dye and spiro-OMeTAD
to achieve a localised ionic screening of the interfacial electric
fields.12 In this study we extend this multilayer approach,
modulating the ionisation potential of the polymer layer to achieve
an energetic redox cascade, as illustrated in Fig. 1, and resulting in
a further retardation of the interfacial charge recombination
dynamics.
Fig. 3 Chemical structures of the ion-ligating hole transport polymers
employed in this study, and corresponding ionisation potentials deter-
mined by cyclic voltammetry.
The ion-ligating, hole-transporting polymers employed in this
study are shown in Fig. 3. We have previously demonstrated that
polymer P1 can be adsorbed by dip-coating to form a conformal
monolayer on dye-sensitised nanocrystalline TiO2 films.
Subsequent spin-coating of a spiro-OMeTAD bulk hole transport
layer did not result in dissolution of this layer. In this study we
vary the substituents to the phenyl side group of the polymer,
modulating its ionisation potential from 4.73 to 5.05 eV (see Fig. 3).
It is apparent that this range overlaps the ionisation potential of
spiro-OMeTAD (4.77 eV).
Fig. 4 Transient absorption data monitoring charge recombination
dynamics of TiO2/dye/polymer/spiro-OMeTAD films as a function of
polymer employed. Data collected in the dark monitoring TiO2 electron
absorption at 1000 nm following optical excitation at 530 nm. All data
collected with lithium triflamide in the polymer deposition solutions, and
without any additives to the spiro-OMeTAD spin coating solution.
Control data collected without any polymer layer.
Fig. 4 shows typical charge recombination dynamics for
multilayer structures employing polymers P1 to P4 obtained, as
for Fig. 2, by monitoring the decay of photoinduced absorption of
electrons injected into the TiO2 electrode. In each case, the polymer
layer was deposited by immersing dye-sensitized nanocrystalline
TiO2 films in a 1 : 9 acetonitrile–chlorobenzene solution containing
1.5 mM of P1 to P4 and 18 mM lithium triflamide
(Li[CF3SO2]2N) at 70 uC for 30 min, followed by drying under
ambient conditions. Spiro-OMeTAD was subsequently deposited
by spin coating a 0.17 M solution of spiro-OMeTAD in
chlorobenzene. Control data were also obtained by omitting the
polymer layer. It is apparent that, depending upon which polymer
is employed, the deposition of the polymer layer can result in a
significant retardation of the interfacial charge recombination
relative to the polymer-free control.
The retardation of the recombination dynamics observed for
polymers P1 to P4 shows excellent correlation with the ionisation
potential of these polymers, as shown in Fig. 5. The recombination
dynamics are becoming progressively slower with increasing
polymer ionisation potential. This behaviour is consistent with
our redox cascade strategy, as illustrated in Fig. 1. For polymer
ionisation potentials close to that of spiro-OMeTAD, a significant
proportion of photogenerated holes are likely to remain on the
polymer layer. However, polymer ionisation potentials greater
than spiro-OMeTAD may allow the formation of a redox cascade
536 | Chem. Commun., 2006, 535–537
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polymer-based devices and light-emitting diodes. We report here
the successful fabrication of such a multilayer structure, employing
organic semiconducting materials with different ionisation poten-
tials to achieve an energetic redox cascade at a dye-sensitised
nanostructured interface. The application of this strategy to the
optimisation of solid-state dye-sensitised solar cells is currently
ongoing.
We acknowledge the Commission of the European Community
(Project MOLYCELL Contract No. 502783) and the EPSRC for
their financial assistance and provision of the Swansea National
Mass Spectrometry service. We thank Johnson Matthey Ltd. for
the award of a studentship (TP) and we thank ARC, CSIRO
and VESKI for financial support. We are very grateful to
Ms. Samantha Handa for performing the cyclic voltammetry
measurements.
Fig. 5 Recombination halftimes determined from transient absorption
data such as that shown in Fig. 4 as a function of polymer ionisation
potential. Data shown with the inclusion ($) and omission (#) of lithium
triflamide from the polymer dipping solution.
Notes and references
as illustrated in Fig. 1, increasing hole transfer from the polymer to
the spiro-OMeTAD. This would increase the spatial separation of
photogenerated holes from the TiO2 surface and consequently
result in slower charge recombination dynamics, as observed
experimentally.
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The solution processing of organic multilayer nanostructures is
a key challenge for a range of organic electronic and optoelectronic
devices, including not only dye-sensitised solar cells, but also
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 535–537 | 537