Morphological Design of Conjugated Polymer Thin Films for Charge Transport and Energy Conversion.

Abstract

Conjugated polymers hold great promise as a versatile class of materials for a wide range of optoelectronic applications, but unlocking their full potential requires deeper understanding of relationships between their complex structure and physical properties at multiple length scales. For polymer/fullerene blends used for thin film photovoltaics, controlling the “bulk heterojunction” morphology is of paramount importance to solar cell performance. By incorporating a small amount of an interfacially-active copolymer, the nano-scale phase separation was enhanced, generating more favorable pathways for transport and collection of photo-generated charges. The copolymer also enriched the region near the electrode, shifting the interfacial work function and suppressing surface recombination. Together these effects yielded up to a 20% increase in power conversion efficiencies.

Even as pure components, conjugated polymers exhibit very diverse morphologies. By aligning the polymer chains, it is possible to borrow their molecular anisotropy and exploit it at the macroscopic level. Highly-aligned films were fabricated consisting of fibers with uniaxial orientation over centimeter-scale regions, and it was experimentally demonstrated that chain alignment could enable photo-excited charges to migrate distances over 400 µm. The measured anisotropy of optical properties, photocurrent migration, and carrier mobilities are all correlated to the morphology of the aligned films. As a contrasting yet complementary study, the effect of structural disorder on different transport mechanisms/regimes was investigated. To this end, a novel vacuum deposition technique was used to fabricate conjugated polymer films with unique globular morphologies. Despite being more disordered, vacuum-deposited thin film transistors (0.0083 cm^2/V*s) exhibited comparable in-plane mobilities to spin-cast analogues (0.0055 cm^2/V*s). Their out-of-plane mobilities, on the other hand, were nearly an order of magnitude lower. The seemingly contradictory results were rationalized in terms of the morphologies and carrier densities at interfaces versus within the bulk.

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