When 10:30 AM - 12:30 PM Dec 09, 2015
Where NCRC B10-G063
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Morphological design of conjugated polymer thin films for charge transport and energy conversion


Anton Li
Thesis Defense

Peter Green, advisor.

 

Conjugated polymers hold great promise as versatile materials for a wide range of optoelectronic applications, but unlocking their full potential requires a deeper understanding of relationships between their complex structure and physical properties. For polymer/fullerene mixtures used in solar cells, controlling the “bulk heterojunction” morphology is of paramount importance. To this end, a copolymer additive was used modulate the phase behavior and self-organization of the blend, which ultimately yielded a 20% increase in power conversion efficiency. This improvement is attributed to 1) superior charge collection pathways resulting from the enhanced nano-scale phase separation; 2) suppression of surface recombination due to copolymer enrichment of the electrode interface.

 

Even as pure components, conjugated polymers exhibit very diverse morphologies, which can be tailored to produce very different electrical properties. For example, by aligning the polymer chains, it is possible exploit their molecular anisotropy at the macroscopic level. Films consisting of uniaxially-orientated polymer fibers were incorporated into photovoltaic devices, and it was found that the alignment enabled photo-excited charges to migrate large distances over 400 µm. In a contrasting yet complementary study, a novel vacuum deposition technique was used to fabricate conjugated polymer films possessing highly disordered morphologies, which nonetheless exhibited comparable transistor mobilities to spin-cast analogues in the in-plane direction. Their out-of-plane mobilities, however, were nearly an order of magnitude lower. These seemingly contradictory results are rationalized in terms of the morphologies and carrier densities at the interfaces versus within the bulk of polymer films.

 

Through different approaches to exploring various aspects of structure-property relationships in conjugated polymers, the findings of this dissertation yield important insights for the future design and application of these materials.