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Addressing the Critical Challenge of Preparing Materials Interfaces that Control Charge Transfer

General Overview: Researchers
at the Wisconsin MRSEC are working to make energy-related devices, such as light
emitting diodes (LEDs) and solar cells, more efficient, more stable, and easier
to fabricate.  The research focuses on
the critical challenge of developing new methods to create ultrathin, highly
organized layers of molecules, termed donors, that efficiently transfer charges
between the compounds that comprise these devices.  The Wisconsin MRSEC has synthesized organic
molecules (see the structure in the top figure) that can be pre-organized as a one
molecule-thick layer on the surface of water, and then transferred onto the
interfaces of devices.  This approach has
yielded remarkably well ordered, densely packed, and stable monolayers of donor
molecules that have provided new insights into how charge moves across
interfaces.  The graphs in the figure demonstrate
the precision with which the molecules can be manipulated on the surface of
water, as measured by the pressure that the molecules generate (top) and their degree
of organization (bottom) when they are tightly packed within the molecule-thick
film.  The exactness with which these novel,
ultrathin, materials can now be fabricated is enabling the design of devices
with improved performance that use a wider range of molecular materials than
was possible in the past.

Technical Description: The development of a detailed understanding
of the fundamental processes that regulate charge transfer at organic-inorganic
interfaces requires an unprecedented level of control over the structure and
dynamical properties of these interfaces.  Since conventional deposition (e.g. crystal
growth) techniques cannot be used to synthesize these materials, controlling
the packing and time-dependent orientations of donor molecules at these
interfaces has been an unresolved challenge. 
IRG2 of the Wisconsin MRSEC has addressed this challenge by leveraging directed
assembly methods based on Langmuir-Blodgett (LB) techniques to create
well-ordered, densely packed monolayers of large donor molecules (see structure
in top figure) on SiO2 and TiO2.  Results obtained by IRG2 show that
rhenium-bipyridine dye molecules form a closely packed monolayer at a
well-defined surface pressure of approximately 10 mN/m (see top figure).  Detailed characterization using atomic force
microscopy and x-ray reflectivity (see bottom figure) confirm that the dye molecules
are extended to their maximum lengths and oriented with the long axis of the
molecule normal to the interface. The monolayers were transferred to solid
substrates with high transfer ratio, thus opening up a path to study the effect
of specific molecular orientations or aggregation states on the charge
injection dynamics in organic/inorganic devices.  The degree of molecular orientation,
uniformity, and thickness of these films, when combined with unique ultrafast spectroscopic
characterization tools in the Wisconsin MRSEC, are enabling studies that have
the potential to greatly improve the performance, reliability, and cost of a
broad range of devices that involve charge transfer at interfaces.