<p>To fully optimize devices
for solar energy conversion, a comprehensive understanding of how excitons
migrate in materials for solar cell devices is crucial. Understanding the
mechanisms behind exciton diffusion and energy transfer will enable the
fabrication of highly efficient devices. However to thoroughly study exciton
properties, techniques implementing high spatial (nm sizes) and temporal (fs
time scales) resolution is required. Herein, we utilize transient absorption
microscopy (TAM) with 50 nm spatial resolution and 200 fs temporal resolution
to elucidate exciton diffusion in polymeric materials for solar energy
conversion.</p>
<p>While organic devices are
inexpensive and require simpler fabrication procedures than inorganic
materials, their device efficiencies often suffer due to their semi-crystalline
nature, lending to short diffusion lengths which lead to trap sites and inevitably
recombination. It has been demonstrated that achieving long-range exciton diffusion
lengths is possible through coherence effects. Coherence can be found in an
intermediate electronic coupling region where delocalization and localization compete.</p>
<p>To exploit coherence effects,
we study polymeric systems in which polymer chains are highly aligned via
simple and scalable procedures; semiconducting fibers and solution coated films.
In studying the fiber, we first implement polarized photoluminesce (PL) to
determine optical ansitropy. From the polarized PL and PL images, it is
observed that emission intensities are largest when probing along the transition
dipole moment of the polymer. This suggests a type of Förester Resonance Energy Transfer mechanism
in which excitons hop from one polymer chain to another.</p>
<p>Solution coated polymer films
are also studied to understand exciton diffusion as a function of deposition
methods. By varying the solution concentration as well as coating rate, we are
able to tune the morphology of the film. We observe a strong dependence between
diffusion constant and deposition parameters, with diffusion constants of <i>ca.</i> 9, 13 and
33 cm<sup>2</sup>/s for three different films. The results obtained in this thesis are
preliminary steps in an effort to elucidate energy transfer mechanisms and rates.</p><br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8940275 |
| Date | 12 August 2019 |
| Creators | Sona N Avetian (6984974) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Energy_Transfer_and_Optical_Anisotropy_in_Semiconducting_Polymers/8940275 |
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