CSEM Scientific and Technical Report 2008

J-aggregates inside Mesoporous Metal Oxide Thin Films for Light Harvesting and
Sensing Applications
R. Pugin, V. Monnier, E. Scolan, R. Steiger
A new method for fabricating cyanine J-aggregates into mesoporous metal oxide thin films has been established at CSEM. The approach is based
on a self-assembly process and consists of organizing dye molecules into mesopores pre-functionalized with self-assembled dendrimer layers. In
doing this, highly stable and ordered J-aggregates have been obtained; their potential applications in light harvesting devices, photovoltaic cells or
light emitting diodes are currently being evaluated.
The loss-free propagation of optical excitation energy over a
large number of non-covalently bonded molecules has been
demonstrated for a particular kind of supramolecular assembly,
the “J-aggregates”. These aggregates arise from the
controlled molecular organization of cyanine dyes and form
structures which have a highly ordered crystalline nature.
The optical properties of J-aggregates include nonlinear
optical effects, dichroism, high extinction coefficients and
resonance fluorescence. This is paired with very narrow
absorption and emission spectra of all the dyes due to
delocalised coherent excitons. Numerous devices have been
proposed to exploit the unique properties of cyanine
J-aggregates. Examples are light harvesting systems for
photovoltaic cells (Figure 1) and sensor devices, components
for non-linear optics, electron- or energy acceptors for excited
donors (e.g. spectral shifts in PLED’s or OLED’s).
Figure 1: Dye sensitized solar cell (courtesy of Solaronix)
Until now J-aggregates could only be produced using
Langmuir Blodgett (LB) or Layer-by-layer (LBL) techniques.
However LB and LBL methodologies have been shown to be
unsuitable to organize large areas of defect free J-aggregates
needed for industrial applications. In addition, reproducible
J-aggregates without defects could not be realized in porous
metal oxides layers (e.g. TiO2), and therefore have not
entered the domain of dye-sensitised photovoltaic cells yet.
Recently CSEM has developed and patented a simple
preparation method for the manufacturing of near defect-free
J-aggregate layers of various cyanine dyes on flat solid
surfaces (Si, glass, Au etc.) [1] . The approach is based on the
molecular organisation of cyanine dyes onto self-assembled
dendrimer monolayers. This method has been shown to
create J-aggregates which have been investigated as ultrafast
optical switches [2] .
This year, a similar method for producing defect-free Jaggregates
with high optical absorption in mesoporous,
nanoparticulate metal oxide coatings (ITO, TiO2, SiO2, AlOOH)
on both solid and flexible large area substrates has been
developed and patented (cf. Figure 2) [3] .
Figure 2: Formal structure of a cyanine dye; absorption spectrum of
highly ordered TDBC-J-aggregate in a silica nanoporous layer.
Under optimized conditions, J-aggregates reproducibly
obtained by this method, show a very small Stokes shift
(≈ 3 nm) and an extremely narrow full width at half maximum
(FWHM ≈ 13 nm). These two values demonstrate the creation
of highly ordered J-aggregate with higher quality than those
obtained in the best LB-layers transferred to flat solid surfaces.
Moreover, the optical absorbance of J-aggregates embedded
within mesopores can reach values 40 times higher than on
flat surfaces and is extremely stable over time.
The ability of such mesoporous layers functionalized with high
quality J-aggregates to be integrated in fluorescence based
sensors, light harvesting devices, dye-sensitized photovoltaic
cells or light emitting diodes is currently being evaluated.
CSEM gratefully thanks Ilford Imaging Switzerland for
supplying the cyanine dyes and Dr. T. Meyer from Solaronix
for providing the solar cell picture.
[1] Patent US2008279500
[2] R. Eckert, J. Dintinger, T. Ebbesen, R. P. Stanley, “Plasmon
based all-optical switch”, in this report, page 47
[3] Patent pending
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